Tuning the intrinsic catalytic activities of oxygen-evolution catalysts by doping: a comprehensive review

Abstract

Electrochemical water splitting produces clean hydrogen fuel as one of the pivotal alternative energies to fossil fuels in the near future. However, the anodic oxygen evolution reaction (OER) is a significant bottleneck that curtails large-scale applications of electrochemical water splitting technology, owing to its sluggish reaction kinetics. In the past few decades, various methods have been proposed to improve the OER kinetics. Among them, doping is a simple and efficient method to mold the OER kinetics of a catalyst by incorporating different or hetero atoms into the host lattice. These efforts are vital to design highly efficient OER catalysts for real-world applications. However, the OER mechanism of a doped catalyst varies, depending on the host lattice and the dopant. This review highlights different doping strategies and associated OER mechanisms of state-of-the-art catalysts, including oxides (noble metal oxides, perovskite oxides, spinel oxides, hydroxides and others), non-oxides (metal sulfides, metal selenides, metal phosphides, metal nitrides and metal carbides), and carbon-based catalysts (graphene, carbon nanotubes and others). Fundamental understanding of the doping effects on the OER from combined experimental and theoretical research provides guidelines for designing efficient catalysts.

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1. Introduction

The demand for transitioning the world's energy from fossil fuels to eco-friendly renewable energy is rapidly increasing, in order to cope with the depletion of fossil fuels and to curtail the associated CO₂ emission.¹ Currently, sunlight and wind are the main contributors to renewable energy, but they are highly intermittent.² To overcome this challenge, researchers have been working on new energy storage techniques that can sustain the erratic behavior of solar, wind and tidal energies.³ Instead, we can store these excess energies in the form of hydrogen through water electrolysis (eqn (1)),⁴ and the stored hydrogen is readily available as a fuel cell for energy production in a clean way without CO₂ emission (eqn (2)).⁵

2H₂ + O₂ → 2H₂O + heat (2)

Hydrogen production through water electrolysis is a thermodynamically infeasible process that requires additional energy to drive this reaction, and theoretically, 1.23 V (pH = 0) electric potential is required.⁶ However, the oxygen evolution reaction (OER) during the water electrolysis is associated with a sluggish reaction kinetics that comprises O=O bond formation with a loss of 4 e⁻/4 H⁺, requiring an even higher electric potential than the theoretical prediction.⁷ The excess potential above the theoretical prediction is known as overpotential (η), which is one of the major benchmarks for denoting OER activity, along with high current density and low Tafel slope values.⁸ Over the past few decades, significant research has been conducted to improve the OER kinetics through various strategies for economically viable hydrogen production.⁹ So far, RuO₂ and IrO₂ catalysts have been proven to be two of the most efficient types of electrocatalyst for the OER in both acidic and alkaline media. However, the high cost associated with these catalysts hinders their large-scale water electrolysis application.¹⁰ In alkaline media, researchers can replace these expensive catalysts with earth-abundant transition metal (TM)-based catalysts,¹¹ as they are stable in such alkaline environments, while being unstable in acidic media.^12,13 The catalyst stability is also an important factor for consideration.

The OER catalytic activity can be improved by enhancing the number of active sites on the surface of the catalyst, which is termed as the intrinsic activity of the catalyst.^14–22 The intrinsic activity of the OER catalyst can be augmented by introducing elemental dopants, creating vacancies, optimizing the morphology, decreasing the particle size, or via surface reconstruction during the OER process.²³ Among them, elemental doping is the most efficient way to engineer the activity of OER catalysts.⁹ However, a fundamental understanding of OER mechanisms is required in catalyst design to shift its efficiency closer to the thermodynamic limit.²⁴ In the past few decades, several reaction mechanisms for the OER have been unveiled to fabricate efficient OER catalysts.²⁵

To date, the current literature on OER catalysts lacks a comprehensive review on the doping effect in the OER mechanisms of catalysts. In this review, by corroborating experimental and theoretical results, we illustrate how particular elemental doping can improve the OER efficiency of a catalyst. In the beginning, OER intermediates by doping are listed. In particular materials, the OER mechanisms are presented, including the influence of dopant on improving the intrinsic activity through optimizing OER descriptors, such as e_g orbital occupancy, metal–oxygen covalency, and structural parameters like M–O bond binding energy, length, and angle. Various types of material that have exhibited efficient doping in the OER are reviewed. Understanding of these OER mechanisms will guide the design of new efficient OER catalysts.

2. OER mechanisms

Generally, in the OER, four electrons and protons are generated and combine on the surface of a catalyst. Several mechanisms are proposed for alkaline and acidic electrolytes; however, the research community primarily accepts the oxide²⁶ and electrochemical oxide²⁶ mechanisms. Karsil'shchikov²⁷ and Wade–Hackerman²⁸ pathways are the two known mechanisms in an acidic medium for the OER.²⁸ Here, the symbol * denotes an active catalytic site on the surface of the OER catalyst. In one of the reaction steps below, 2*O → 2* + O_2(g) in both oxide mechanisms (Fig. 1a and c) or the electrochemical oxide mechanism (Fig. 1b and d), oxygen gas is generated. In case of recombination of two *O intermediates or generation of oxygen from the *O intermediate, oxygen evolution is impossible because of a substantial thermodynamic barrier. The oxygen gas is generated from the active catalytic site after two steps via the formation of intermediate *O–OH, a step which is the most sluggish one out of all rate-determining steps (RDSs) in the OER.

Fig. 1 Schematic illustrations of oxide (a and c) and electrochemical oxide (b and d) mechanisms of the OER in acid (a and b) and base (c and d) electrolytes, respectively.

In acidic media, the cathodic reaction is expressed as

4H⁺ + 4e⁻ → 2H₂ (3)

and the anodic reaction is

2H₂O → O_2(g) + 4H⁺ + 4e⁻ (4)

The elementary steps of the oxide and electrochemical oxide mechanisms are described below:²⁶

(a) Oxide mechanism (Fig. 1a), including reaction steps of * + H₂O → *OH + H⁺ (aq) + e⁻, 2*OH → *O + H₂O, 2*O → 2* + O₂ (g), 2*O + H₂O → *O–OH + H⁺ + e⁻, and *O–OH + H₂O → * + O₂ (g) + H⁺ + e⁻.

(b) Electrochemical oxide mechanism (Fig. 1b), including reaction steps of * + H₂O → *OH + H⁺ (aq) + e⁻, 2*OH → *O + H⁺ (aq) + e⁻, 2*O → 2* + O₂ (g), 2*O + H₂O → *O–OH + H⁺ + e⁻, and *O–OH + H₂O → * + O₂ (g) + H⁺ + e⁻.

Likewise, in the alkaline medium, the cathodic reaction is

4H₂O + 4e⁻ → 2H₂ + 4OH⁻ (5)

and the anodic reaction is

4OH⁻ → 2O₂ (g) + 2H₂O + 4e⁻ (6)

The elementary steps of the oxide and electrochemical oxide mechanisms are described below:²⁶

(a) Oxide mechanism (Fig. 1c), including reaction steps of * + OH⁻ → *OH + e⁻, 2*OH → *O + H₂O, 2*O → 2* + O₂ (g), *O + OH⁻ → *O–OH + e⁻, and *O–OH + OH⁻ → * + O₂ (g) + H₂O.

(b) Electrochemical oxide mechanism (Fig. 1d), including reaction steps of * + OH⁻ → *OH + e⁻, *OH + OH⁻ → *O + H₂O + e⁻, 2*O → 2* + O₂ (g), *O + OH⁻ → *O–OH + e⁻, and *O–OH + OH⁻ → * + O₂ (g) + H₂O.

There are three other well-known mechanisms proposed by, namely, Karsil'shchikov,²⁷ Yeager,²⁹ and Bockris,³⁰ in addition to the primarily recognized oxide and electrochemical oxide mechanisms.^26,28

3. Why doping?

Besides the electrocatalyst type and chemical stability, the geometry and electronic structure are the key parameters for optimization in developing efficient OER catalysts.³¹

Geometrical surface area is the most fundamental and critical parameter, and it can be improved by synthesizing nano-forms of an electrocatalyst. By improving the specific surface area of the catalyst, the mass transfer rate and density of active catalytic sites can be accelerated.³² However, nanomaterials are often aggregated, resulting in poor stability during long-term service.⁹

Tuning the electronic structure is another critical factor in improving the OER activities, and elemental doping is one of the best ways of tuning the electronic structure.³³ The tuning of the electronic structure by doping influences the binding energy of OER intermediates, such as *O, *OH, and *O–OH. Hence, compared with other approaches for improving OER activities, doping provides an effective way to achieve large-scale activity and long-term stability under harsh applied anodic overpotentials.

The dopant is present in trace or small amounts. Doping generates ionizable species within the host material by incorporating impurities. The ionizable species could lead to the shift of the Fermi level (E_F), enabling carrier transport with superior catalytic activity. Doping can be performed through (a) gating for direct carrier injection; (b) antisites and vacancies as structural defects; and (c) photodoping through generation of transient carrier species via photo-excitation. Incorporation of foreign atoms that do not produce free carriers is not considered as doping.³⁴

In the following sections, we will elaborate on the influences of doping in the following common catalytic systems, including various oxides (noble metal oxides, perovskite oxides, spinel oxides, hydroxides and other oxides), non-oxides (metal sulfides, metal selenides, metal phosphides, metal nitrides, and metal carbides), and carbon-based catalysts. Fig. 2 shows the overpotential at 10 mA cm⁻² and Tafel slope of the representative types of catalyst (data collected from the following sections). Lower overpotential and lower Tafel slopes are desirable for high catalytic performance. The effects of doping and its impacts on OER activity in the various prescribed electrocatalysts are finely discussed hereafter in terms of accelerating OER kinetics with less energy input.

Fig. 2 Catalytic performance of representative types of catalyst: (a) overpotential @ 10 mA cm⁻²; (b) Tafel slope.

4. Doping of oxide electrocatalysts

4.1 Noble metal (Ir and Ru) oxides

Table 1 provides a summary of some representative doped noble metal oxides and their catalytic performances. Both IrO₂ and RuO₂ are considered as benchmark catalysts for the OER in basic and acidic electrolytes. Under OER conditions, IrO₂ is more stable than RuO₂, but both oxides are unstable at high anodic potentials. RuO₂ leaches out into the electrolyte in the form of RuO₄ at high anodic potentials.³⁵ Likewise, IrO₂ also dissolves in the electrolyte by forming IrO₃ at high anodic potentials.³⁶ However, IrO₂ shows considerably higher stability but lower OER performance, compared to RuO₂.

Table 1 Summary of representative doped noble metal oxides and perovskite oxides and their catalytic performances

Catalyst	Overpotential at a specific current density	Tafel slope (mV dec^−1)	Durability	Electrolyte
Noble metal oxides
Ce0.2–IrO2@NPC^45	224 mV @ 10 mA cm^−2	55.9	100 h @ 10 mA cm^−2	0.5 M H2SO4
Co-doped RuO2 (ref. 41)	169 mV @ 10 mA cm^−2	63	50 h @ 10 mA cm^−2	0.5 M H2SO4
Cr-doped IrOx (ref. 43)	250 mV @ 10 mA cm^−2	69	25 h @ 10 mA cm^−2	0.5 M H2SO4
Cr0.6Ru0.4O2 (ref. 44)	178 mV @ 10 mA cm^−2	56	10 h @ 10 mA cm^−2	0.5 M H2SO4
Cu-doped RuO2 (ref. 38)	188 mV @ 10 mA cm^−2	43.96	8 h @ 10 mA cm^−2	0.5 M H2SO4
Cu-doped IrO2 (ref. 39)	351 mV @ 10 mA cm^−2	63	6000 s @ 1.68 V vs. RHE	0.1 M HClO4
IrCoNi nanocrystals^40	309 mV @ 10 mA cm^−2	53.8	—	0.5 M H2SO4
Mg-doped RuO2 (ref. 47)	228 mV @ 10 mA cm^−2	48.66	30 h @ 1.15 V vs. RHE	0.5 M H2SO4
Mn-doped RuO2 (ref. 42)	158 mV @ 10 mA cm^−2	42.94	10 h @ 10 mA cm^−2	0.5 M H2SO4
W-doped IrO2 (ref. 46)	250 mV @ 10 mA cm^−2	56.6	—	0.1 M HClO4
Zn-doped RuO2 (ref. 48)	179 mV @ 10 mA cm^−2	36.9	20 h @ 10 mA cm^−2	0.5 M H2SO4
Perovskite oxides
Ba2CoMo0.5Nb0.5O6−δ^49	445 mV @ 10 mA cm^−2	77	2 h @ 10 mA cm^−2	0.1 M KOH
Ba0.5Sr0.5Co0.8Fe0.2O3−δ^50	340 mV @ 10 mA cm^−2	70	10 h @ 10 mA cm^−2	1 M KOH
CaMn0.75Nb0.25O3−δ^51	550 mV @ 10 mA cm^−2	98	7000 s @ 1.65 V vs. RHE	0.1 M KOH
Co-doped SrIrO3 (ref. 52)	235 mV @ 10 mA cm^−2	51.8	20 h @ 10 mA cm^−2	0.1 M HClO4
Pr0.5Ba0.5CoO3−δ^53	320 mV @ 10 mA cm^−2	60	120 min @ 5 mA cm^−2	0.1 M KOH
PrBa0.5Sr0.5Co1.5Fe0.5O5+δ^54	358 mV @ 10 mA cm^−2	52	12 h @ 10 mA cm^−2	0.1 M KOH
P-doped LaFeO3−δ^55	465 mV @ 10 mA cm^−2	50	—	0.1 M KOH
SCFW0.4-BM^56	357 mV @ 10 mA cm^−2	58	18 h @ 10 mA cm^−2	0.1 M KOH
Sr2Co1.5Fe0.5O6−δ^57	318 mV @ 10 mA cm^−2	44.8	10 h @ 10 mA cm^−2	1 M KOH
SrCo0.95P0.05O3−δ^58	480 mV @ 10 mA cm^−2	84	7000 s @ 1.65 V vs. RHE	0.1 M KOH
Sr-doped PrBaCo2O5+δ^59	420 mV @ 10 mA cm^−2	75.8	10 h @ 0.1 mA cm^−2	1 M KOH
SrCo0.9Ti0.1O3−δ^60	500 mV @ 10 mA cm^−2	88	10 h @ 10 mA cm^−2	0.1 M KOH
Yb-doped CaMnO3 (ref. 61)	330 mV @ 10 mA cm^−2	68	1500 s @ 520 mV	0.1 M KOH

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Researchers doped RuO₂ with a small amount of Ir (Ru_xIr_1−xO₂) and observed a significant improvement in stability without sacrificing the OER performance.³⁷ Another way of enhancing the OER activity with lower total noble metal content is doping with earth-abundant metals, such as Cu,^38,39 Ni,⁴⁰ Co,^40,41 Mn,⁴² Cr,^43,44 Ce,⁴⁵ W,⁴⁶ Mg,⁴⁷ and Zn.⁴⁸ For example, Cu-doped IrO₂ (Cu_0.3Ir_0.7O_δ) displays improved OER activity in acidic, neutral and basic media, compared to pristine IrO₂.³⁹ The enhanced OER activity of Cu_0.3Ir_0.7O_δ is primarily imputed to the Jahn–Teller distortion in IrO₆ octahedra induced by Cu, as well as oxygen vacancies induced by Cu doping. Density functional theory (DFT) calculations revealed the improved OER activity mechanism by the Cu doping. With the increase in Cu-doping, σ and π bonding regions become broader and shift to a lower energy level because of the occupancy of σ states. The partial density of states suggests that with an increase in Cu doping, the antibonding states of d_xy shift up; in contrast, d_z² shifts down. The d_xz and d_yz orbitals were crossed by the Fermi level E_F, while the d_x²−y² orbital is unchanged. Because of electrons hopping from d_xz and d_yz (t_2g) to d_z², the e_g level is partially filled as a consequence of enhanced degeneracy in Ir 5d orbitals by Cu doping. The strong interaction of e_g and O 2p orbitals facilitates bonding with oxygen intermediates during σ bond formation, which is more advantageous than the π bonding of t_2g orbitals.³⁹

Suntivch et al. postulated that the OER catalyst, specifically perovskite oxide with the e_g value close to unity, will exhibit superior OER performance.⁶² Later, other scientists found that the e_g filling values in some oxides also have a similar effect.⁶³ Site-specific deposition of a meager amount of Ir on facets of Ni nano-frameworks led to excellent OER activity in an acidic electrolyte and the catalyst even outperformed an Ir/C catalyst.⁶² The oxygen adsorption of TMs was effectively tuned by doping with Ir. Similarly, Feng et al. reported porous IrCoNi nanocrystals as an efficient OER catalyst in highly corrosive HClO₄ electrolyte.⁴⁰Fig. 3a shows the schematic illustration of IrCoNi porous hollow nanocrystals (PHNCs). A two-step process was followed to synthesize PHNCs. In the first step, IrCo single nanocrystals (SNCs) were prepared in oleylamine; and in the second step, IrCoNi PHNCs were obtained from SNCs by etching with Fe³⁺ in hexane/ethanol at room temperature. Fig. 3b shows a transmission electron microscopy (TEM) image of the as-synthesized IrCoNi PHNCs, presenting uniform hollow nanocrystals. The polarization curves of IrCo/C, IrCoNi/C, and IrNi/C PHNCs are compared with that of the Ir/C catalyst in Fig. 3c. The IrCoNi/C catalyst shows superior activity with an onset potential of 1.46 V and requires only 303 mV overpotential to achieve 10 mA cm⁻² current density. The projected density of states (PDOS) of the d-band of Ir and IrCoNi catalysts is displayed in Fig. 3d, which shows the left side shift of the d-band center of IrCoNi from the Fermi level. The shift in the Fermi level facilitates the desired charge and discharge of OER intermediates. The schematic representation of OER reaction paths, as shown in Fig. 3e, reveals that the adsorption of OER intermediates becomes weaker when Ir is alloyed with Co and Ni, compared to pristine Ir. In another study, porous W-doped IrO₂ nanodendrites were found to exhibit superior OER performance under all pH conditions.⁴⁶ DFT calculations suggested that W doping stabilizes the IrO₂ at harsh anodic potentials by lowering the binding energies of oxygen intermediates. Qu et al. synthesized Cr-doped IrO_x nanowires and evaluated their OER activity in 0.5 M H₂SO₄ electrolyte.⁴³ The catalyst exhibits excellent OER activity with 25 h durability without any decay in activity. This high activity was ascribed to an increase in active site number and facile adsorption of OER intermediates via electronic structure tuning.

Fig. 3 (a) Schematic illustration of the formation process of IrCoNi PHNCs; (b) TEM image of the as-prepared IrCoNi PHNCs; (c) polarization curves of IrCo, IrCoNi, and IrNi for the OER with 95% iR-compensation; (d) projected DOS of d bands of Ir (solid) and IrCoNi models (dotted) with the corresponding d-band center denoted by dashed lines; (e) schematic illustration of reaction paths for the OER, indicating that alloying Ir with M leads to weaker adsorption of oxygen-based intermediates, thus decreasing the barrier in the reaction path.⁴⁰ Reproduced from ref. 40 with permission, © 2017 John Wiley and Sons.

Like IrO₂, the OER performance and stability of RuO₂ were also enhanced by doping. Cu-doped RuO₂ porous nano-polyhedra, derived from organic frameworks, are probably one of the best examples for enhancing OER activity via the formation of oxygen vacancies.³⁸ Here, most oxygen vacancies formed near Cu, causing the adjacent Ru atom to become more negative, and thus shifting the O 2p band center towards the Fermi level for superior OER activity. Recently, Chen's group reported Cr-doped RuO₂ as an efficient OER catalyst in an acidic medium derived from a metal–organic framework (MOF).⁴⁴ The schematic representation of the synthetic approach of this catalyst is shown in Fig. 4a. Initially, RuCl₃ was incorporated into the MOF of Cr. The resulting composite was annealed between 450 and 600 °C for 4 h, and the composite which was annealed at 550 °C showed the formation of Cr_0.6Ru_0.4O₂. Among Cr_1−xRu_xO₂ catalysts, Cr_0.6Ru_0.4O₂ showed superior OER activity with a low overpotential of 178 mV at 10 mA cm⁻² current density, and at the same current density, it maintained its stability for 10 h. Chromium incorporation optimizes the DOS for RuO₂ and decreases the Fermi energy level from 2.01 to 1.07 eV, which stabilizes the crystal structure of Cr-doped RuO₂. Fig. 4b displays the Ru site, which is coordinated with five atoms that act as an adsorption site for OER intermediates, and the formation of *OOH is found to be a RDS. The free energy change in the RDS is calculated to be 1.87 eV for the doped one, which is nearly 0.1 eV less than the free energy on the RuO₂ surface.⁴⁴ It was predicted that Mn doping also has a similar effect on improving the OER activity of RuO₂. Very recently, Chen et al. reported Mn-doped RuO₂ as an OER catalyst in an acid with a record low overpotential of 158 mV at 10 mA cm⁻², which was stable up to 5000 cycles at a scan rate of 100 mV s⁻¹.⁴² Recently, Chen and co-workers demonstrated considerable improvement of OER activity of RuO₂ in an acid medium by doping with Mg in a one-step annealing of a Ru-exchanged MOF.⁴⁷ DFT calculations indicate the formation of oxygen vacancies on account of different oxidation states of Mg (2⁺) and Ru (4⁺), significantly bringing down the energy barrier for the RDS.⁴⁷

Fig. 4 (a) Schematic illustration of the preparation of Cr_0.6Ru_0.4O₂ electrocatalysts for OER application in acid media; (b) four-step OER process.⁴⁴ Reproduced with open access.

Creating defect-containing RuO₂via doping and dopant leaching is another way of improving the OER activity and stability in acidic medium. For example, Tian et al. doped RuO₂ with zinc and leached it out by aging in an acid, and the catalyst displayed 179 mV overpotential and 20 h stability at a current density of 10 mA cm⁻².⁴⁸ The high catalytic activity is accredited to the synergetic effect of active sites and tuning of the electronic structure. Recently, an oxygen-defect-containing Ru-based catalyst was synthesized by doping with cobalt, which showed 169 mV overpotential and 50 h endurance at 10 mA cm⁻² current density.⁴¹ The doping generated a large amount of oxygen defects, following a vacancy-related lattice oxygen oxidation mechanism (LOM) rather than an adsorbate evolution reaction mechanism (AEM). Both computational and experimental studies elucidate this mechanism. All these studies revealed the effectiveness of doping Ir and Ru oxides for enabling higher activity and longer stability.

4.2 Perovskite oxides

Perovskite oxides are a large family of catalysts with the general formula ABO₃, where A is either an alkaline earth metal or a rare earth metal, and B is a TM. Table 1 provides a summary of some representative doped perovskite oxides and their catalytic performances. The ideal perovskite oxide displays a cubic crystal structure. The TMs are in the oxidation states of +3, +4, or a mixture of them, governed by the metal on the A-site.^18,64 The OER activity can be tailored by doping at either the A-site, B-site, or both sites to create oxygen vacancies in the perovskite.¹⁸ The metal–oxygen hybridization depends upon the oxidation state of TMs, and the electronic properties of these perovskites rely on this parameter. The TMs with high electronegativity show high metal–oxygen hybridization, and the d-band center of the TM oxide moves closer to O 2p states as the electronegativity increases. However, besides the metal–oxygen hybridization, octahedral tilting, Jahn–Teller distortion, and displacement of B-site cations also play significant roles in governing the electronic properties of perovskite oxides.^18,65,66

4.2.1 A-site doping. The activity and durability of SrRuO₃ perovskite are enhanced mainly by doping the Sr site with Na (Sr_0.95Na_0.05RuO₃), which results in a decrease in octahedral distortion and an increase in the oxidation state.⁶⁷ Na-doping results in no distortion in octahedra because of the change in the oxidation state of Ru with the inclusion of Na. DFT studies reveal that Na doping weakens the bonding between OER intermediates and Ru, allowing release of oxygen freely.⁶⁷ The increase in the oxidation state of Ru in Na-doped SrRuO₃ further improves its stability at high anodic potentials in an acidic medium. Doping of the La site of LaCoO₃ with Sr induces the formation of cobalt species with higher oxidation states, which influence its crystal structure in such a way that O–Co–O is aligned at 180°. As a consequence, Co 3d and O 2p orbitals are mixed efficiently to enhance the intrinsic OER activity of the perovskite.⁶⁸

Grimaud et al. doped a series of lanthanides (Ln = Pr, Sm, Gd and Ho) at the A-site of BaCoO_3−δ to optimize the e_g filling values, thus improving the OER activity.⁵³ Among Ln-doped perovskites, Pr_0.5Ba_0.5CoO_3−δ shows superior activity. Estimated from X-ray absorption spectroscopy (XAS) and chemical titrations, the e_g filling values of all Ln-doped compounds are close to unity. However, DFT calculations suggest that hybridization of Co 3d and O 2p orbitals decreases from Pr to Ho, because of the decreased Co oxidation state with an increase in electronegativity of dopants from Pr to Ho. Similarly, when the A-site of CaMnO₃ is doped with Yb (Ca_0.9Yb_0.1MnO₃), the OER activity is found to be 100 times higher than that of the pristine sample.⁶¹ The excellent OER performance of Ca_0.9Yb_0.1MnO₃ is attributed to the increased Mn³⁺/Mn⁴⁺ ratio, which provides the double exchange of electrons and optimized e_g filling value of ∼0.81. The OER activity of double perovskites was also tuned by doping the A-site; for instance, Sr doping of the Ba site of PrBaCo₂O_5+δ improves the population of Co⁴⁺ ions on the surface.⁵⁹ The doping enhances the ease of adsorption and deprotonation of *OOH intermediates from the catalyst surface, thus improving the OER performance of the perovskite.

4.2.2 B-site doping. It is critical to tune the electronic structure of the B site to regulate the electrochemical behavior of perovskite oxides; hence, doping the B-site is the most facile way of governing the OER performance.^49,56 For instance, Shao's group demonstrated that B-site doping of SrTiO₃ with cobalt and iron elements resulted in an improved electrocatalyst for the OER.⁶⁰ Among doped perovskites, SrCo_0.9Ti_0.1O_3−δ displayed enhanced OER activity owing to its optimized e_g filling value of 1.16, and formation of redox-active oxygen deficiency. In another report, the Mn site of CaMnO₃ is doped with Nb (CaMn_0.75Nb_0.25O_3−δ), which enhances the OER performance in terms of current density by onefold, and mass activity by sevenfold, compared to the undoped perovskite.⁵¹ The significant enhancement of electrochemical activity is imputed to the reduction of the oxidation state of Mn from +4 to +3 caused by doping with Nb⁵⁺, which in turn tunes the e_g value close to unity.

Recently, researchers turned their attention to dope non-metals like phosphorus, sulfur, and silicon at the B-site to stabilize the crystal structure of perovskite oxides. An important reason for doping, specifically phosphorus, for improving electrocatalytic properties is that it has greater affinity towards combining with oxygen. Phosphorus doping at the B-site of SrCoO_3−δ (SC) perovskite results in tetragonal perovskite oxide SrCo_0.95P_0.05O_3−δ (SCP), which shows a considerable increase in OER performance in terms of activity and endurance relative to the parent compound.⁵⁸Fig. 5a and b show linear sweep voltammetry (LSV) curves of SC and SCP before and after the accelerated degradation test (ADT) conducted by repetition of 1000 CV cycles. The ADT results show a decrease in the activity of SC after ADT, while SCP shows an increase in OER activity after the ADT. The enhanced OER activity during the ADT of SCP is due to the presence of P dopant. The high-resolution TEM (HR-TEM) images presented Fig. 5c and d suggest that the decrease in the activity of SC is because of the formation of a 10–15 nm thick amorphous layer after the ADT, which curtails the diffusion by blocking active sites, while SCP shows an only 1–2 nm thick amorphous layer (Fig. 5d). Fig. 5e shows a schematic illustration of the OER mechanism on SCP during the ADT, which indicates improved interaction between active cobalt sites and electrolyte through leaching of either Sr²⁺ or P⁵⁺ from the surface of the catalyst.

Fig. 5 (a and b) LSV curves for the OER on a RDE (1600 rpm) for the (a) SC and (b) SCP samples before and after an ADT of 1000 cycles in O₂-saturated 0.1 M KOH solution; HRTEM images of the surface regions of (c) SC and (d) SCP after an ADT of 1000 cycles; (e) schematic illustration of the main origin of the OER activation for the SCP sample during the ADT.⁵⁸ Reproduced from ref. 58 with permission, © 2017 John Wiley and Sons.

Likewise, Liu's group demonstrated the universality of P doping at the B-site of LaFeO_3−δ, LaCoO_3−δ and LaNiO_3−δ perovskites for improving the electrochemical water oxidation capacity.⁵⁵ The superior activity is attributed largely to the generation of a large amount of O₂²⁻/O⁻ species, and DFT calculation suggests a change in the oxidation state of the B cation that optimizes the e_g value (∼1). Zou et al. doped a series of TMs at the B-site of 6H–SrIrO₃ for improving its OER activity in an acidic medium.⁵² Among them, cobalt doping was found to enhance the OER activity of 6H–SrIrO₃ significantly. Fig. 6a presents the crystal structure of 6H–SrIrO₃ having two types of IrO₆ octahedron: one is face sharing (red), and the other one is corner-sharing isolated (olive green). Thus, the doped TMs can replace the Ir present in either the face sharing octahedra or corner-sharing octahedra. Fig. 6b displays the SEM image of Co-doped 6H–SrIrO₃ having plate-like morphology and the thickness of these plates decreased after doping. Fig. 6c shows the polarization curves of pristine and Co-doped catalysts in 0.1 M HClO₄ electrolyte. The catalyst with a 0.2 : 1 ratio of Co and Ir shows superior activity with the lowest overpotential of 235 mV at 10 mA cm⁻², and the catalytic activity is 2.5 times higher than that of the pristine catalyst. A comparison between the mass activities of pristine and Co-doped catalysts is presented in Fig. 6d, which suggests that the catalyst with the 0.2 : 1 ratio of Co and Ir has 3 times higher mass activity than 6H–SrIrO₃. However, both catalysts show nearly the same OER kinetics with Tafel values of 51.8 mV dec⁻¹ for the catalyst with the 0.2 : 1 ratio of Co and Ir and 58.7 mV dec⁻¹ for the pristine sample. Computational studies suggest that in the pristine sample, a downshift of the Ir 5d band center away from the Fermi level is observed. In contrast, the O 2p band moves close to the Fermi level in the doped 6H–SrIrO₃, which thus results in enhanced covalency and increases the OER activity.⁵²

Fig. 6 (a) Crystal structure of 6H–SrIrO₃, and two kinds of Ir site for Co substitution. The iridium atom at face-sharing IrO₆ octahedra is marked by a red color and that at isolated IrO₆ octahedra is marked by an olive green color; (b) SEM image of Co doped 6H–SrIrO₃; (c) polarization curves of pure 6H–SrIrO₃ and Co-doped SrIrO₃ (Co : Ir = 0.1 : 1, 0.2 : 1, and 0.3 : 1) in 0.1 M HClO₄ solution with 85% iR compensation; (d) iridium amount-normalized mass activity of 6H–SrIrO₃ and Co-doped SrIrO₃ (Co : Ir = 0.1 : 1, 0.2 : 1, and 0.3 : 1); (e) Tafel slopes of 6H–SrIrO₃ and Co-doped 6H–SrIrO₃.⁵² Reproduced from ref. 52 with permission, © 2019 American Chemical Society.

Very recently, our group doped Fe on the B-site of SrCoO_3−δ and observed the formation of a tetragonal intermediate structure between perovskite and brownmillerite, with a stoichiometric formula of Sr₂Co_1.5Fe_0.5O_6−δ.⁵⁷Fig. 7a presents the OER activity of Sr–Co–Fe-oxides, which shows that Sr₂Co_1.5Fe_0.5O_6−δ exhibits superior OER activity among all doped oxides. The enhanced OER activity is ascribed to the unique stacking of octahedral (oh) and tetrahedral (th) layered units of the tetragonal structure, as shown in Fig. 7b, providing facile interactions between the electrode and electrolyte surface. The stability of the Sr₂Co_1.5Fe_0.5O_6−δ catalyst was evaluated at 1.55 V for 10 h as shown in Fig. 7c, and it presents good stability with a slight increase in overpotential of 16 mV as depicted in Fig. 7d. Furthermore, theoretical calculations suggest that the enhanced covalency of Co 3d and O 2p is another prime contributor to the augmented OER performance of Sr₂Co_1.5Fe_0.5O_6−δ. This tetragonal intermediate structure presents a model for developing advanced catalysts in other systems.

Fig. 7 (a) LSV plots of SrCoO_3−δ, Sr₂Co_1.75Fe_0.25O_6−δ, Sr₂Co_1.5Fe_0.5O_6−δ and SrCo_0.5Fe_0.5O_3−δ oxides; (b) structural model of the intermediate tetragonal phase containing a sequence of alternating octahedral (oh) and tetrahedral (th) layers (…oh–th–oh–th…), where only two randomly orientated tetrahedra are shown for clarity; (c) stability test of Sr₂Co_1.5Fe_0.5O_6−δ oxide at 1.550 V vs. RHE for 10 h; (d) LSV plots of samples before and after the stability test.⁵⁷ Reproduced from ref. 57 with permission, © 2021 American Chemical Society.

4.2.3 Both A- and B-site doping, and controlled formation of oxygen vacancies. Tuning the electrocatalytic activity of perovskites is also possible by simultaneously doping both A and B sites. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) is a well-studied dual-doped perovskite, and it displayed significant intrinsic activity as a catalyst for the OER, which is an order magnitude higher than that of the benchmark catalyst IrO₂.⁵⁰ The higher activity of BSCF originates from having the e_g value close to unity, which is revealed by Shao-Horn's group by comparing several perovskite catalysts.⁶⁵ The doping of the Ba site with Sr and Co site with Fe in the double perovskite PrBaCo₂O_5+δ crystal structure significantly improves the OER performance.⁵⁴ Specifically, the perovskite nanofibers with the composition of PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ displayed superior intrinsic activity that is way higher than that of the BSCF catalyst, and DFT calculations indicated that the O 2p band center was neither too close to nor too far away from the Fermi energy level. The adsorption scenarios of OER intermediates reveal that the *OH to *O conversion is the RDS, and the Fe site of Sr/Fe dual-doped PrBaCo₂O_5+δ requires a low overpotential (0.6 V) to initiate the reaction, while the Co site needs 0.7 V.⁶⁹ The best way to generate the oxygen vacancies in perovskites is doping a high valence A-site cation with a low valent dopant. For example, Sr doping at the A-site of LaCoO₃ results in ligand hole/oxygen deficiency. This deficiency increases with an increase in the Sr doping level.⁷⁰ The rise in oxygen vacancy leads to electroneutrality of the Co ion which improves the intermixing of Co 3d and O 2p orbitals. The evaluation of OER activity of a La_1−xSr_xCoO_3−δ perovskite series (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) suggests that with the increase in Sr content, the catalytic activity is improved. Furthermore, the authors evaluated ex situ electronic conductivity as a function of current density and found that the OER activity of the perovskite was enhanced with a large amount of Sr because of high electronic conductivity.

4.3 Spinel oxides

The general formula of spinel oxides is AB₂O₄. A and B are metal cations in general, which belong to the first row of TMs, or group 2 or group 13 in the periodic table. Table 2 provides a summary of representative doped spinel oxides and their catalytic performances. The structure of spinel oxides is complicated, because of the presence of two crystallographic sites, namely octahedral oh and tetrahedral th sites, respectively, in a single structure.²² Based on the arrangement of these crystallographic sites, they are classified into two types: one is spinel comprising A²⁺ on the th site and B³⁺ on the oh site, and the other one is inverse spinel containing A²⁺ on the oh site and B³⁺ on the th site. According to crystal field theory, the TM ions present in the oh and th sites show different d-orbital splitting, i.e., t_2ge_g for oh and e_gt_2g for th. This difference in d-orbital splitting plays a significant role in tuning the electronic properties of spinel oxides and optimizing OER performance.^71,72 Spinel oxides are highly stable in a basic medium with good electrical conductivity under harsh OER conditions. Mn-, Fe-, and Co-based spinel oxides are mostly reported as catalysts for the OER.⁷²

Table 2 Summary of representative doped spinel oxides, hydroxides and other oxides and their catalytic performances

Catalyst	Overpotential at a specific current density	Tafel slope (mV dec^−1)	Durability	Electrolyte
Spinel oxides
Ag-doped Co3O4 (ref. 73)	680 mV @ 10 mA cm^−2	219	10 h @ 370 mV	0.5 M H2SO4
Co2.25Cr0.75O4 (ref. 74)	350 mV @ 10 mA cm^−2	60	24 h @ 10 mA cm^−2	1 M NaOH
Cu-doped Co3O4 NAs/NF^75	230 mV @ 10 mA cm^−2	84	20 h @ 10 mA cm^−2	1 M KOH
Fe-doped Co3O4 (ref. 76)	262 mV @ 10 mA cm^−2	43	50 h @ 100 mA cm^−2	1 M KOH
Fe-doped Co3O4 (ref. 77)	160 mV @ 10 mA cm^−2	78	∼160 min @ 1.63 V vs. RHE	0.1 M KOH
Fe-doped NiCo2O4 (ref. 33)	201 mV @ 10 mA cm^−2	39	12 h @ 10 mA cm^−2	1 M KOH
LiMn0.25Co1.75O4 (ref. 78)	370 mV @ 10 mA cm^−2	60	—	1 M NaOH
Mn1.4Co1.6O4 (ref. 79)	327 mV @ 10 mA cm^−2	79	25 h @ 1.53 V vs. RHE	1 M KOH
Ni-doped Mn3O4 (ref. 80)	458 mV @ 10 mA cm^−2	74.4	21 h @ 10 mA cm^−2	3 M NaCl
NPCoO-UCSs^81	182 mV @ 10 mA cm^−2	54	50 000 s @ 1.42 V vs. RHE	0.1 M KOH
P8.6–Co3O4/NF^82	260 mV @ 20 mA cm^−2	60	25 h @ 1.53 V vs. RHE	1 M KOH
P–Co3O4 (ref. 83)	280 mV @ 20 mA cm^−2	51.6	40 000 s @ 280 mV	1 M KOH
Se/Ni–Co3O4 (ref. 84)	290 mV @ 20 mA cm^−2	62.9	—	1 M KOH
V-doped Co3O4 (ref. 85)	293.6 mV @ 10 mA cm^−2	53.3	30 h @ 5 mA cm^−2	1 M KOH
Hydroxides
Ce-doped NiFe LDH/CNT^86	227 mV @ 10 mA cm^−2	33	30 000 s @ 10 mA cm^−2	1 M KOH
Co0.54Fe0.46OOH^87	290 mV @ 10 mA cm^−2	47	—	0.1 M KOH
Cu-doped Co(OH)2 (ref. 88)	300 mV @ 10 mA cm^−2	47	36 h @ 1.55 V vs. RHE	1 M KOH
Fe-doped Co(OH)2 (ref. 89)	290 mV @ 10 mA cm^−2	69	12 h @ 10 mA cm^−2	1 M KOH
Fe–NiOOH^90	181 mV @ 50 mA cm^−2	43.2	56 h @ 20 mA cm^−2	1 M NaOH
Mn and F dual-doped Ni(OH)2 (ref. 91)	233 mV @ 20 mA cm^−2	56.9	10 h @ 20 mA cm^−2	1 M KOH
Ni-doped Co(OH)2 (ref. 92)	300 mV @ 10 mA cm^−2	47	36 h @ 1.55 V vs. RHE	1 M KOH
Ni-doped FeOOH^93	239 mV @ 10 mA cm^−2	90.4	—	1 M KOH
Ni0.83Fe0.17(OH)2 (ref. 94)	245 mV @ 10 mA cm^−2	61	10 h @ 10 mA cm^−2	1 M KOH
Se-doped FeOOH^95	348 mV @ 500 mA cm^−2	54	100 h @ 100 mA cm^−2	1 M KOH
W-doped Ni(OH)2 (ref. 96)	237 mV @ 10 mA cm^−2	33	—	1 M KOH
Other oxides
Ce-doped NiO–Au^97	244 mV @ 100 mA cm^−2		>24 h @ 50 mA cm^−2	1 M KOH
Co-doped CuO^98	299 mV @ 50 mA cm^−2	134	15 h @ 300 mV	1 M KOH
Co–MnO2/OV^99	279 mV @ 20 mA cm^−2	75	12 h @ 10 mA cm^−2	1 M KOH
Co-doped ZnO^100	450 mV @ 10 mA cm^−2	106	18 h @ 1.68 V vs. RHE	0.1 M phosphate-buffered saline
Fe–CoO/C nanofibers^101	362 mV @ 10 mA cm^−2	74.4	—	1 M KOH
NixCo1−xO^102	450 mV @ 10 mA cm^−2	66.8	—	1 M KOH
Ni-doped MnO2 (ref. 103)	330 mV @ 10 mA cm^−2	23.7	>3 h @ 10 mA cm^−2	1 M KOH
V-, Fe-, Co- and Ni-doped MnO2 (ref. 104)	390 mV @ 10 mA cm^−2	104.4	>27 h @ 20 mA cm^−2	1 M KOH
Y2−xBaxRu2O7 (ref. 105)	278 mV @ 10 mA cm^−2	40.8	5 h @ 10 mA cm^−2	0.5 M H2SO4
Zn-doped CoO^106	293 mV @ 10 mA cm^−2	63	24 h @ 10 mA cm^−2	1 M KOH

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4.3.1 MMn₂O₄. Mn₃O₄ is a normal spinel with Mn²⁺ on the th site and Mn³⁺ on the oh site, and its OER activity in the neutral medium is tuned by doping with various 3d-block elements such as Zn, Cu, Ni, Co, Fe, and Cr.⁸⁰ Among them, 5% Ni doping showed enhanced OER activity with better OER kinetics. Structural analysis suggests lattice distortion because Ni doping is a primary reason for improved OER activity.⁸⁰ Doping divalent metals such as Co, Zn, and Mg on the th site of Mn₃O₄ spinel results in good electrochemical properties. Specifically, Co doping is proven to be more effective in improving OER activity, and a tetragonal form of Co_xMn_3−xO₄ shows a superior activity compared to the cubic phase.¹⁰⁷ Enhanced OER activity of the tetragonal phase is made possible by optimizing the binding energies of OER intermediates with the defective Co (−0.26) and Mn (−0.60) sites, and the bonding energy is much lower than those of the cubic phase sites, with suppressed Jahn–Teller distortion. In LiMn₂O₄, lithium is on the th site, and doping of the Mn site significantly improves the OER activity. For instance, Manthiram et al. synthesized different morphologies (cubic, spherical, octahedral, and truncated octahedral) of LiNi_xMn_2−xO₄ and evaluated the electrochemical behavior for oxygen evolution.¹⁰⁸ LiNi_0.5Mn_1.5O₄ displayed enhanced OER activity with octahedral morphology because of the high surface area and fast reaction kinetics facilitated by the {111} crystal plane. Dismukes et al. doped Mg and Zn in the Li site and Co in the Mn site and found that spinel with the composition of LiMn_0.25Co_1.75O₄ exhibits augmented OER activity within this series.⁷⁸ From the mechanism, the superior OER activity is found to be linked with the reduction potential of the Mn(IV) cation, and greater reduction potential facilitates improved kinetics by directly driving surface oxidation.

4.3.2 MFe₂O₄. CoFe₂O₄ is an inverse spinel, in which one-eighth of the th sites are occupied by Fe³⁺ and one-half of the oh sites are occupied by Co²⁺ ions. The presence of such an arrangement provides good electrical conductivity as a consequence of electron hopping between valence states of cations on oh sites. It thus facilitates good electrochemical activity and produces longer stability in electrocatalytic reactions.^109–111 Doping of Co and Fe sites significantly influences the electrocatalytic activity of CoFe₂O₄. For example, Ni and Mn are doped in CoFe₂O₄ at the Fe site by stoichiometric mixing of Co₃O₄, Fe₂O₃, NiO, and MnO₂ using a solid-state synthesis.¹⁰⁹ The Ni-doped one displayed the lowest overpotential of 260 mV, which is attributed to Co(II/III) on oh sites along with Co(III) on th sites of the inverse spinel. Similarly, Ni doping at the Co site was also evaluated by synthesizing hollow nanospheres via a hydrothermal route. The inverse spinel Ni_0.75Co_0.25Fe₂O₄ showed the best OER activity, which is ascribed to the oxygen defects and cation distribution ratio on th and oh sites.¹⁰⁹ Singh et al. reported improved OER activity of CoFe₂O₄ through Cr doping at Fe site (CoFe_2−xCr_xO₄), and the catalyst with the CoFeCrO₄ composition shows high OER activity because of fast discharge of OH⁻.¹¹¹ Cr doping of NiFe₂O₄ at the Fe site also shows a similar result, and NiFeCrO₄ displayed superior activity within this series. Cr doping at the Fe site of MnFe₂O₄ and CuFe₂O₄ inverse spinels also showed improved OER activity. Doping pentavalent V into the lattice of NiFe₂O₄ significantly improves OER activity as well as its stability, which could be a consequence of the increased amount of Fe₂O₃ in the host lattice, along with enriched surface porosity.¹¹⁰

4.3.3 MCo₂O₄. Co₃O₄ is a normal spinel, whose Co²⁺ ions occupy th sites, while Co³⁺ ions occupy oh sites. It has been a well-known electrocatalyst for the OER in the past few decades in alkaline media, owing to its high activity, long stability and low cost. However, pristine Co₃O₄ suffers from poor electrocatalytic activity on account of its low electronic conductivity. Doping elements like V,⁸⁵ Cr,⁷⁴ Fe,^76,77 Ni,¹¹² Cu,⁷⁵ Zn,⁸¹ Ag,⁷³ and Li¹¹³ into the lattice of Co₃O₄ spinel tunes its electronic structure, thus significantly improving OER activity. Incorporation of vanadium into the lattice of Co₃O₄ through doping produces a highly disordered and defect-rich surface lattice, which essentially contributes to the OER and increases its stability in an extreme OER environment. Furthermore, DFT calculations and a methanol oxidation probing method revealed that decreased energy of the RDS barrier is key to the superior activity of V-doped Co₃O₄.⁸⁵ The DFT calculations were performed on pristine and doped Co₃O₄ (100) surfaces with different OER intermediates. After V doping, the Gibbs free energy change is decreased from 1.73 eV (pristine) to 1.57 eV for the RDS, which is in accordance with methanol oxidation probing.⁸⁵ McCrory's group demonstrated the augmentation of OER performance of Co₃O₄ by the inclusion of a discrete amount of chromium (Co_3−xCr_xO₄).⁷⁴ Co_2.25Cr_0.75O₄ is the most active one within the series, and its high activity is found to be caused by the increased electrophilic nature of Co²⁺ ions, thus enhancing the interaction between *O, *OH, and *OOH and active sites. Lou's group doped 13 metal atoms into the lattice of ultrathin nanosheets on the th sites of Co₃O₄via a cooperative etching–coordination–reorganization approach.⁷⁶ Among these 13 atoms, doping of a discrete amount of Fe greatly increases the electrocatalytic OER activity. Fig. 8a shows the schematic representation of the synthesis of Fe-doped Co₃O₄ hierarchically hollow NPs (Fe–Co₃O₄ HHNPs). Fig. 8b–e show the SEM, HR-TEM, high-angle annular dark-field scanning TEM (HAADF-STEM), and elemental mapping images of Fe–Co₃O₄ HHNPs, respectively, which reveal that NPs are uniform having an edge size of 900 nm and a uniform distribution of a small amount of Fe. From the DFT calculations, it was found that decreased adsorption energy from 1.90 to 1.54 eV for converting *OH to *O species is a major cause.

Fig. 8 (a) Schematic synthesis process of cross-channeled Fe–Co₃O₄ hierarchically hollow NPs (HHNPs); (b–d) TEM images of ZIF-67 NPs; (e) HAADF-STEM image and elemental mapping images of one Fe–Co₃O₄ HHNP.⁷⁶ Reproduced with open access. (f) Schematic illustration of the preference of Fe doping into tetrahedral sites of Ni–Co–O spinel oxides, and (g) adsorption energy of an H₂O molecule on the (001) surface of Ni–Co–O, Co–Fe–O, and Ni–Co–Fe–O spinels.³³ Reproduced from ref. 33 with permission, © 2019 American Chemical Society.

In another report, Swaminathan et al. demonstrated how minute changes in the amount of Fe doping influenced the OER activity of Co₃O₄.⁷⁷ They doped 3, 4.5, and 6% of Fe into the host lattice and observed that above or below the optimum level of Fe doping, fading of OER activity occurred. The 4.5% Fe doping caused superior activity, and the DFT study identified a considerable downshift of the Fermi energy level, originating from the optimized adsorption energy of reaction intermediates. By comparing Ni, Cu, and Li dopants, Vitanov et al. studied the effectiveness of doping in improving the OER activity of Co₃O₄ spinel and noticed that doping with Li (Li_xCo_3−xO₄) produced a promising activity.¹¹³ The raised activity of Li_xCo_3−xO₄ spinel is ascribed to faster reaction kinetics owing to the formation of active sites with high transfer coefficients, and a shortened reaction pathway to generate oxygen. Tian et al. revealed the Cu doping effect on improving the OER activity of Co₃O₄ nanosheets prepared on nickel foam, which outperforms the benchmark catalyst IrO₂ on nickel foam.⁷⁵ The high OER activity is found to have resulted from an increase in Co(III) species in the crystal lattice, along with highly uncovered active sites. Ding et al. demonstrated the improved OER performance of Co₃O₄ nanosheets by creation of oxygen vacancies upon Zn doping, which even surpassed that of the Ir/C catalyst.⁸¹

The stability and activity of Co₃O₄ spinel could be significantly enhanced in acidic electrolyte through Ag doping. Ag doping plays a crucial role in the growth of Co₃O₄ nanowire arrays, improving the surface area and stability under harsh acidic conditions and OER activity by providing shortened reaction paths.⁷³ Ni_xCo_2−xO₄ is an inverse spinel, in which Ni(II/III) is present in 16d oh sites and Co(II/III) is present in both 8a th sites and 16d oh sites. By tuning the Ni doping level, this inverse spinel's OER activity significantly increased, because the Ni²⁺/Ni³⁺ ratio plays a key role in governing the conductivity, surface roughness, and density of active sites. For example, Zhang et al. systematically studied the influence of an increase in the Ni dopant level to improve the overall OER performance of Ni_xCo_2−xO₄.¹¹² When x = 1, it shows superior OER activity. In this composition, Ni is in the +3 oxidation state. The enhanced OER activity is a result of the 0.27 eV upshifting of the valence band maximum toward the Fermi level, which creates a new hole. Such an electronic configuration favors the adsorption of OER intermediates.¹¹² The doping of Fe at the Ni site of NiCo₂O₄ induces oxygen vacancies, which impacts the OER activity considerably. The Fe_0.1Ni_0.9Co₂O₄ composition shows superior activity within the doped series, which is accredited to the presence of an optimum amount of oxygen defects for triggering electrocatalytic activity.¹¹⁴ Huang's group reported doping Fe into the th site of the NiCo₂O₄ lattice to improve its OER activity through a theoretical prediction.³³ As displayed in Fig. 8f, incorporation of Fe into the th site requires a lower energy (313.33 eV) compared to the oh site (313.58 eV) according to DFT calculations. Fig. 8g shows the DFT estimated H₂O adsorptive ability where the Fe-doped one is superior to the pristine one and CoFe₂O₄, which is highly favorable for improving OER performance to increase active sites. Furthermore, X-ray absorption spectroscopy confirms the successful incorporation of Fe into the th site of NiCo₂O₄, which enhances the OER performance by increasing the population of Ni³⁺ and Co²⁺ ions in oh sites. Mn_xCo_2−xO₄ is also an inverse spinel, and Mn cations preferably occupy oh sites. Lankauf et al. described Mn_xCo_2−xO₄ spinel by adjusting the composition of Mn (x = 0–2) using a soft chemistry method, and the spinel with a moderate content of Mn shows better OER activity.⁷⁹ The high activity was attributed to the improved oxidizing ability and better electronic conductivity. Chi et al. noticed that the change in copper content in the Cu_xCo_2−xO₄ spinel could influence its OER properties.¹¹⁵ When x was increased from 0.7 to 0.9, more Cu²⁺ ions displaced Co³⁺ ions from oh sites. They occupy th sites, and thus the Co³⁺ ion population on th sites was significantly increased, resulting in superior OER performance.

Li and coworkers reported an increase in the OER performance of the NiCo₂O₄ catalyst through double-exchange interaction (DEI).⁷² The DEI is induced by synergistic creation of oxygen vacancies (V_O) via doping with Fe and creating a heterojunction with MoS₂ (MoS₂/rFe–NiCo₂O₄) as shown in Fig. 9a. Fig. 9b and c show the SEM and TEM images of the MoS₂/rFe–NiCo₂O₄ catalyst, which is in the form of nanowires having a diameter of 50–60 nm, decorated with MoS₂ on Fe–NiCo₂O₄. Fig. 9d displays the comparative OER activity of NoCo₂O₄, Fe–NoCo₂O₄, MoS₂/Fe–NoCo₂O₄, rFe–NoCo₂O₄ and MoS₂/rFe–NoCo₂O₄ catalysts, which reveals that DEI has the most powerful effect on enhancing OER activity. Fig. 9e shows the schematic representation of the effect of DEI on enhancing the OER activity of MoS₂/rFe–NiCo₂O₄ through improving electrical conductivity, number of OER active sites, and reactivity of active centers. Recently, non-metal doping into Co₃O₄ was studied for improving electrochemical activity. For instance, Sun's group reported P-doped Co₃O₄ nanowires as an efficient OER catalyst in basic electrolyte.⁸² The P-doped Co₃O₄ nanowires on nickel foam were prepared by low-temperature annealing using NaH₂PO₂. First-principles calculations revealed that P-doped Co₃O₄ nanowires have a much lower free-energy value for the rate-determining step than pristine Co₃O₄.⁸² Wang et al. synthesized P-doped Co₃O₄ spinel by filling the in situ generated oxygen vacancies using an Ar plasma etching technique, and observed excellent OER activities after doping.⁸³ When oxygen vacancies were created in Co₃O₄, more electrons migrated into oh Co³⁺ 3d orbitals than th Co²⁺ 3d orbitals. With P doping, electrons are transferred out of the Co 3d states, resulting in more Co²⁺ on th sites than Co³⁺ on oh sites in P–Co₃O₄. Hence, the superior OER activity of the P-doped Co₃O₄ is linked with the presence of a greater number of Co²⁺ ions. Zhang et al. reported doping of Co₃O₄ with S by using facile solution-combustion and thioacetamide-sulfidation methods.¹¹⁶ The S-doping enhances the OER performance of Co₃O₄ in terms of activity as well as stability in an alkaline medium. The formation of S-substituted CoO layers on the surface of Co₃O₄ is a major reason for the superior activity. Theoretical calculations indicate that improving the electronic conductivity by S doping was beneficial for overall electrochemical activity. Dual doping of Co₃O₄ on both cation and anion sites with Ni and Se, respectively, resulted in a significant increase in its OER performance.⁸⁴ The Se doping enhances the active sites on the surface of a catalyst by inducing oxygen vacancies, and Ni doping helps in generating highly active Co⁴⁺ species.

Fig. 9 (a) Crystal structure of spinel oxides, and the creation of a double-exchange interaction (DEI); (b) SEM and (c) TEM images of MoS₂/rFe–NiCo₂O₄; (d) OER performance of the electrocatalysts NiCo₂O₄, Fe–NiCo₂O₄, MoS₂/Fe–NiCo₂O₄, rFe–NiCo₂O₄, and MoS₂/rFe–NiCo₂O₄ with respect to current density at 1.55 V RHE and the current density increasing factor; (e) schematic representation of the enhanced OER arising from the DEI effect.⁷² Reproduced from ref. 72 with permission, © 2020 American Chemical Society.

4.4 Hydroxides

Hydroxides, oxyhydroxides, and layered double hydroxides (LDHs) of oxides of TMs, such as Ni, Co, Fe, Mn, and V, are highly active in the electrocatalytic oxygen evolution reaction.^86,117,118 They exhibit typical layered stacking conformation with proton sandwiching between layers, and the metal is centered in the octahedron surrounded by oxygen anions at the corners as MO₆.¹¹⁷ Some representative hydroxides and their catalytic performances are listed in Table 2.

Fe doping of TM hydroxides is most widely reported in the literature for enhancing their OER performance.¹¹⁹ Specifically, Fe doping in Ni(OH)₂ (ref. 94 and 120–123) and Co(OH)₂ (ref. 89 and 124–126) has been widely studied. In 1901, Edison,¹²⁷ for the first time, noticed an increase in the OER activity of Ni(OH)₂ because of Fe impurity in the electrolyte in a Ni-based alkaline battery.^120,128 Because of oxygen evolution, the capacitance and cycle life of the battery were decreased considerably by the incorporation of Fe into the Ni(OH)₂ electrode. Later, researchers revealed that the OER activity of Ni(OH)₂ shortened the voltage window of the battery owing to the presence of Fe in the electrolyte, and they also found that the presence of a discrete amount of Fe (<1%) considerably contributed to augmentation of OER activity.¹²⁹ Corrigan demonstrated that the presence of 0.01% Fe impurity in electrolytes substantially decreased the discharge capacitance.¹³⁰

Boettcher's group studied the role of Fe doping in enhancing the OER performance of Ni(OH)₂ thin films and provided more insights on catalytic reactions at the electrode/electrolyte interface.^{122,124–126} They witnessed an increase in conductivity of thin films with systematically increasing the doping amount from 5 to 25%. Still, the CV curves show drastic changes, which signify the change in surface reconstruction with varying dopant levels.¹²² Klaus et al. studied the Fe inclusion by aging the thin films in an electrolyte with or without Fe impurities for 6 days.¹³¹ The sample aged in the electrolyte with Fe impurity shows an increase in OER activity gradually from the first day to the 6^th day. In contrast, the other one without Fe shows a decrease in activity gradually upon aging, and the enhanced OER activity is attributed to the increase in conductivity. Landon et al. noticed a rise in the coordination number of Fe in the OER environment using an in situ extended X-ray absorption fine structure (EXAFS) method.¹³² They proposed that forming a new phase (NiFe₂O₄) through reconstruction increased the coordination number of Fe. Nevertheless, the formation of NiFe₂O₄ is rejected by other researchers with the aid of in situ Raman spectroscopy.¹³³ They did not notice any signals for NiFe₂O₄ formation in the in situ Raman spectra (700 cm⁻¹), even after aging in KOH electrolyte with Fe at high OER potentials. In contrast, they observed two peaks which are characteristic of the NiOOH phase at 555 and 475 cm⁻¹, even in the presence of a high percentage of Fe.

Recently, Zhao's group studied the effect of doping Co and Fe into the lattice of Ni(OH)₂ for improving the OER performance, and they conducted electron energy-loss spectroscopy (EELS) characterization and Hubbard corrected density functional theory (DFT + U) calculations to understand the activity trend.⁹⁰ The Fe-doped sample shows superior OER activity to Co and Fe co-doped or Co-doped samples.⁹⁰ EELS analysis suggests electronic interactions between Ni, Co, and Fe dopants even after surface reconstruction during the OER process. A DFT + U study reveals that the d-band center of the Fe-doped sample is much closer to the Fermi level than that of the others, enhancing the binding energy values of OER intermediates to an optimal value. Wang et al. demonstrated an enhancement in OER activity of Ni(OH)₂ by co-doping Mn and F elements.⁹¹ A considerable increase in oxygen deficiencies was found after doping, which reduced the energy barrier for the key OER of *OH → *O. Ma's group doped a single atom of W⁶⁺ into the lattice of Ni(OH)₂ and studied its electrocatalytic oxygen evolution behavior.⁹⁶ Interestingly, W doping significantly enhances the OER activity compared to the pristine sample, and the superior activity is ascribed to the boosting of carrier migration at the interface of the electrode and electrolyte. Furthermore, a DFT study reveals that low spin W⁶⁺ (d⁰) stabilizes the O* and thus helps in the formation of O–O.

The augmentation of OER activity was also reported for the Co(OH)₂ catalyst by Fe doping. Boettcher's group reported the effect of Fe doping on the OER activity of electrodeposited films of Co(OH)₂.¹²⁴ They observed an enhancement of OER performance by 100 fold when the stoichiometric ratio of Co and Fe was 1 : 0.6–0.7. The reason for the superior OER activity is increased Co³⁺ population and improved thin-film conductivity with Fe doping. Unlike Ni(OH)₂, Co(OH)₂ requires a huge amount of Fe doping (60–70%) to display enhanced OER activity, while Ni(OH)₂ shows a significant effect even with 0.01% Fe inclusion. Above the 60–70% level, the OER activity started decreasing for Co(OH)₂. Researchers also doped other TMs into Co(OH)₂ to improve its OER performance. For example, Chen et al. doped Cu into the lattice of Co(OH)₂, which significantly enhances its OER activity and outperforms the state-of-the-art catalyst IrO₂. The considerable increase in the catalytic performance is attributed to an increase in Co³⁺ population after doping with Cu.⁸⁸ Koel's group studied the doping effect on the OER behavior of CoOOH nanowires by doping with Ni and Mn.⁹² Among them, Ni doping turned out to be fruitful compared to Mn doping because of Ni's ability to stabilize the OER intermediates. Similarly, FeOOH was also doped with different elements such as Ni, Co, and Se to enhance its OER performance. Tolstoy et al. doped Ni²⁺ into the lattice of FeOOH nanocrystals, and 25% Ni²⁺ doping was proven to improve the OER activity considerably.⁹³ Zhang et al. doped Co into FeOOH, and the Co_0.54Fe_0.46OOH composition showed superior activity to other doped samples because Co inclusion created more active sites at the edges of the three-dimensional hierarchical structures.⁸⁷ Recently, Niu et al. reported doping of Se into FeOOH via electrochemical oxidation of an FeSe/iron foam (IF) pre-catalyst by applying a constant 10 mA cm⁻² current density for 4 h.⁹⁵ XRD patterns, as shown in Fig. 10a, display the complete disappearance of diffraction peaks of FeSe which was converted into Se-doped FeOOH. However, the electrochemical oxidation did not disturb the nanosheet morphology as revealed by SEM and TEM images in Fig. 10b and c, respectively. Fig. 10d shows the polarization curves of the as-prepared catalysts, and among them, FeOOH(Se)/IF delivers industrial scale 500 mA cm⁻² current density at 348 mV overpotential. Tafel plots of these catalysts are presented in the inset of Fig. 10d, which shows that the FeOOH(Se)/IF catalyst possesses OER kinetics with the lowest Tafel slope value of 54 mV dec⁻¹. As a unary Fe-based catalyst, FeOOH(Se)/IF shows comparative activity and even outperforms Ni and Co-based OER catalysts as observed in Fig. 10e. Furthermore, DFT calculations revealed a considerable decrease in the energy barrier for the RDS from 3.20 eV to 1.45 eV after Se inclusion, which accounts for the superior OER activity.

Fig. 10 (a) XRD patterns of FeSe/iron foam (IF) and FeOOH(Se)/IF; (b) SEM image of FeSe/IF; (c) HRTEM image of FeSe/IF; (d) LSV curves of the as-prepared catalysts (inset: Tafel plots); (e) overpotential comparison of FeOOH(Se)/IF and state-of-the-art unary metal-based OER electrocatalysts.⁹⁵ Reproduced from ref. 95 with permission, © 2019 American Chemical Society.

4.5 Other oxides

TM oxides, such as NiO, CoO, and MnO₂, are widely reported as catalysts for the OER owing to their high durability and low cost.⁵ Some other representative oxides and their catalytic performances are listed in Table 2. The electrochemical activity of these oxides was further improved by tuning their electronic structure through the inclusion of various dopants. For instance, lithium-doped NiO (Li_xNi_1−xO) has shown unprecedented OER performance and even outperformed non-noble benchmark perovskite catalysts, such as BSCF and LaNiO₃.¹³⁴ Zhang et al. studied the role of lithium incorporation into the lattice of NiO.¹³⁴Fig. 11a shows the BET surface area normalized LSV curves of pristine and Li-doped NiO catalysts, and among them, the catalyst with a 1 : 1 ratio of Li and Ni shows 11 fold higher activity than pristine NiO. The inset of Fig. 11a presents cyclic voltammogram (CV) curves, which further reveals a positive shift in redox potential with an increase in Li content. A combinatorial study of the electronic structure of Li_xNi_1−xO was conducted using X-ray absorption spectroscopy and synchrotron-based photoemission spectroscopy. The schematic illustration of covalency of Ni 3d–O 2p from DFT calculations is shown in Fig. 11b, which reveals the formation of a new hole state near the Fermi level, and the increased covalency of Ni 3d–O 2p that ensured higher OER activity.

Fig. 11 (a) LSV polarization curves of the OER normalized by the BET surface area in 1 M KOH solution at a scan rate of 10 mV s⁻¹; the inset shows cyclic voltammogram curves at a scan rate of 10 mV s⁻¹; (b) schematic diagram for electronic structures of NiO (left) and LiNiO₂ (middle), and energy diagram at the oxide–liquid interface on the vacuum level scale at pH = 14 (right); the redox level for O₂/H₂O (red dashed line) is −4.84 eV, the conduction band minimum (CBM) level for NiO is −1.7 eV according to the formula E_CBM = E_bandgap − E_ionization (E_bandgap = 3.7 eV, E_ionization = 5.4 eV).¹³⁴ Reproduced from ref. 134 with permission, © 2019 American Chemical Society.

Enhancement in the OER activity of the NiO catalyst with Fe doping was first observed by Corrigan in 1987,¹³⁰ and Fe doping into the NiO thin film was found to be affected by the impurities present in KOH electrolyte.¹³⁵ Recently, Nocera et al. identified the essential role of Fe doping in improving the electrocatalytic activity of Ni oxides.¹³⁶ 5–10% doping of Fe increased the population of Ni⁴⁺ ions, as revealed by coulometric titrations. Consequently, O and Ni K-edge spectra indicated an increase in hybridization between Ni 3d and O 2p orbitals. Both experimental and computational studies showed that Fe³⁺ is a Lewis acid with pK_a = 2.2, and is the most acidic among TMs. Hence, the incorporation of Fe³⁺ into Ni oxides helps in enhancing the acidity of OH_x moieties coordinated with Ni, which increases the population of Ni⁴⁺ ions by lowering the Ni³⁺/Ni⁴⁺ reduction potential.¹³⁶ Ce-doped NiO_x deposited on a Au substrate shows significantly higher OER performance than pristine NiO_x.⁹⁷ The enhanced OER activity is accredited to the modification of the local environment favorable for binding of OER intermediates.

A nickel-doped rock-salt-structured OER catalyst, Ni_xCo_1−xO, was synthesized using a solid-state method.¹⁰² The inclusion of a discrete amount of Ni into the lattice of CoO stabilizes the crystal structure and tunes its e_g value. Within the doped series, the Ni_0.3Co_0.7O catalyst displays the highest OER activity with an e_g value of ∼1.3. Huo et al. reported the improved OER performance of CoO nanoflowers by doping with Zn.¹⁰⁶ A DFT study confirmed that OER active sites were increased by Zn doping, and the high surface area of nanoflowers was another contributor to the improved catalytic activity. Like in NiO, Fe doping of CoO could also greatly enhance its electrocatalytic performance by increasing the number of OER active sites.¹⁰¹ Ye et al. prepared V-, Fe-, Co-, and Ni-doped MnO₂ ultrathin nanosheets on carbon fiber paper (CFP) using an electrodeposition method as shown in Fig. 12a, and studied their OER activity in a basic medium.¹⁰⁴Fig. 12b and c show the doped MnO₂ ultrathin nanosheet/CFP composite electrode at low and high magnifications, which shows very dense nanoclusters with 5 nm thick nanosheets. Fig. 12d shows the LSV curves of CFP, pure MnO₂/CFP, metal-ion-doped MnO₂ ultrathin nanosheet/CFP, and IrO₂/CFP composite electrodes in 1 M KOH with a scan rate of 5 mV s⁻¹. The authors noticed that doping with these TM cations vastly enhances MnO₂ conductivity, resulting in higher catalytic activity compared to the pristine sample. The electrochemical stability of doped and pristine MnO₂ compared with IrO₂ evaluated at 20 mA cm⁻² is shown in Fig. 12e. Doped MnO₂ ultrathin nanosheet/CFP shows significant stability compared to the pristine sample with a slight increase in overpotential.

Fig. 12 (a) Schematic representation for the preparation of the metal-ion (Fe, V, Co, and Ni)-doped MnO₂ ultrathin nanosheet/CFP composite; (b and c) FESEM images showing the morphology of the metal-ion-doped MnO₂ ultrathin nanosheet/CFP composite electrode at different magnifications; (d) LSV curves of the CFP, the metal-ion-doped MnO₂ ultrathin nanosheet/CFP and the IrO₂/CFP composite electrodes measured in 1 M KOH with a scan rate of 5 mV s⁻¹, where the current is normalized by the geometrical area of the carbon fiber paper; (e) galvanostatic measurements at a current density of 20 mA cm⁻² for the different composite electrodes.¹⁰⁴ Reproduced from ref. 104 with permission, © 2017 John Wiley and Sons.

Yang et al. intercalated 3d block metal cations such as Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, and Fe³⁺ into MnO₂ nanosheets to enhance OER activity.¹⁰³ Among them, the inclusion of Ni²⁺ ions was more effective in enhancing the OER activity of MnO₂ nanosheets. The increased activity is associated with strengthening the Mn–O bond perpendicular to layered chains to promote the catalysis between two Mn sites. Oxygen vacancy formation in MnO₂ nanosheets by Co doping is demonstrated by Zhao et al.⁹⁹ Co doping enhances the OER activity of MnO₂ considerably by enhancing its conductivity and decreasing the energy barrier for adsorption of OER intermediates on the catalyst surface. Interestingly, a small amount of cobalt doping (3.47%) could activate the OER activity of ZnO. Theoretical studies suggest that Co activates inherently inactive adjacent Zn sites by increasing conductivity, which act as OER active sites for adsorption of OER intermediates.¹⁰⁰ Co doping also activates the OER activity in CuO. Sun et al. demonstrated a multifold enhancement of OER activity in a basic electrolyte after doping with Co on a Cu foam substrate,⁹⁸ which was related to a change in the electronic structure of CuO by Co doping. Meyer et al. reported the fabrication of an OER catalyst by modifying fluorine-doped tin oxide (FTO) by Co²⁺ single-atom doping.¹³⁷ X-ray photoelectron spectroscopy (XPS) and TEM reveal that doping Co²⁺ into FTO by replacing Sn⁴⁺ results in oxygen vacancies. The as-modified FTO electrode shows water oxidation activity at pH 7.2 with a turnover frequency of 29 000 per cobalt site, at a current density of 0.16 mA cm⁻² during an endurance test for 2 h. Furthermore, the current density was enhanced by 5 fold by utilizing nanoporous FTO in place of planar FTO. Wang et al. reported Ba doping of Y₂Ru₂O₇ pyrochlore oxide on the yttrium position (YBRO) as an efficient catalyst for the OER in an acid electrolyte.¹⁰⁵ Among the doped samples, YBRO-0.15 shows superior activity, and the OER activity decreases in the following sequence: YBRO-0.15 > YBRO-0.1 > YBRO-0.25 > YBRO-0.4 > YRO > RuO₂. The enhanced activity is attributed to the oxygen vacancy-rich surface because of partial replacement of Ba²⁺ with Y³⁺ and simultaneous formation of multivalent Ru (Ru⁵⁺/Ru⁴⁺) species.

5. Doping non-oxide catalysts

Non-oxide catalysts include metal sulfides, metal selenides, metal phosphides, metal nitrides, and metal carbides. These catalysts are pre-catalysts, and the actual catalysts are the oxides of the corresponding cations generated on the surface during electrochemical water oxidation.

5.1 Metal sulfides

Transition metal sulfides are widely explored in electrocatalysis due to their high stability in acidic and alkaline electrolytes and abundant redox chemistry.¹³⁸ Some doped metal sulfides and their catalytic performances are listed in Table 3. Like in metal oxides and hydroxides, Fe doping is effective in significantly improving the OER activity of TM sulfides.^139,140 For example, Zeng et al. synthesized Fe-doped CoS electrocatalysts via a simple hydrothermal method, and 20% Fe doping produced considerably the highest OER activity among them because of the high electrical conductivity by doping, which accelerated the charge transfer process by changing the local Co–S environment for easy adsorption of OER intermediates.¹⁴¹ Dong and coworkers synthesized nano-micro spheres of Fe-doped Co₉S₈ on nickel foam using a hydrothermal method; for comparison, they also synthesized Ni-doped Co₉S₈.¹⁴² The Fe-doped sample showed superior OER activity to the pristine and Ni-doped samples. It was found that the high activity was contributed by the synergetic effect of tuning the electronic structure of cobalt by an Fe dopant and high surface area induced by the nickel foam. Sun et al. synthesized Fe-doped NiS₂ nanoarrays on a Ti mesh through sulfidation of a NiFe layered hydroxide as a precursor, and they found that the catalyst with 10% Fe doping showed superior OER activity with an overpotential of 231 mV at 100 mA cm⁻² current density.¹⁴³ Wu's group reported Fe-doped Ni₃S₂ nanowires synthesized on a Ni foam and found that Ni₃S₂ nanowires with a 13.7% dopant level of Fe exhibit superior activity and only require 223 mV overpotential to achieve 200 mA cm⁻² in alkaline electrolyte.¹⁴⁴ The incorporation of iron into the matrix of Ni₃S₂ enhances the number of active site edges along with conductivity. Sugahara and coworkers prepared hollow iron-doped bimetallic CoMoS heterostructures, which show enhanced OER activity over the gold standard catalyst RuO₂.¹⁴⁵

Table 3 Summary of representative doped metal sulfides and selenides and their catalytic performances

Catalyst	Overpotential at a specific current density	Tafel slope (mV dec^−1)	Durability	Electrolyte
Metal sulfides
Al-doped Ni3S2/NF^146	223 mV @ 10 mA cm^−2	37	15 h @ 1.53 V vs. RHE	1 M KOH
Ce-doped Ni3S2 (ref. 147)	257 mV @ 50 mA cm^−2	81	24 h @ 0.6 V vs. Ag/AgCl	1 M KOH
Co-doped WS2 (ref. 148)	303 mV @ 10 mA cm^−2	79	—	1 M KOH
CoFeS/CNT-P 1000 (ref. 149)	309 mV @ 100 mA cm^−2	47	12 h @ 20 mA cm^−2	1 M KOH
Fe-doped CoS^141	290 mV @ 10 mA cm^−2	52.6	10 h @ 10 mA cm^−2	1 M KOH
Fe-doped Co9S8 (ref. 142)	270 mV @ 10 mA cm^−2	70	10 h @ 270 mV	1 M KOH
Fe-doped NiS2 (ref. 143)	231 mV @ 100 mA cm^−2	43	15 h @ 20 mA cm^−2	1 M KOH
Fe-doped Ni3S2 (ref. 144)	223 mV @ 200 mA cm^−2	55.7	14 h @ 223 mV	1 M KOH
Fe-doped Ni3S2/NF^139	249 mV @ 100 mA cm^−2	42	20 h @ 270 mV	1 M KOH
Fe2.1% doped Ni3S2/NF^140	213 mV @ 100 mA cm^−2	33.2	—	1 M KOH
Fe-doped H–CoMoS^145	282 mV @ 10 mA cm^−2	58		1 M KOH
Ni1.29Co1.49Mn0.22S4 (ref. 150)	348 mV @ 10 mA cm^−2	65	40 000 s @ 10 mA cm^−2	1 M KOH
N-doped Co9S8/G^151	409 mV @ 10 mA cm^−2	82.7	—	0.1 M KOH
Ni-doped FeS^152	228 mV @ 10 mA cm^−2	53	10 h @ 1.47 V vs. RHE	1 M KOH
N2–NiS2-500 (ref. 153)	270 mV @ 10 mA cm^−2		40 h @ 270 mV	1 M KOH
(N–Ni3S2@C)/NF^154	310 mV @ 100 mA cm^−2	75	20 h @ 1.70 V vs. RHE	1 M KOH
N-doped NiS/NiS2 (ref. 155)	270 mV @ 10 mA cm^−2	99	20 h @ 270 mV	1 M KOH
P-doped Co–Ni–S nanosheets^156	296 mV @ 100 mA cm^−2	61.1	16 h @ 10 mA cm^−2	1 M KOH
P–Ni3S2/NF^157	256 mV @ 10 mA cm^−2	30	30 h @ 1.525 V vs. RHE	1 M KOH
P–Ni3S2/NF^158	306 mV @ 100 mA cm^−2	99	10 h @ 1.54 V vs. RHE	1 M KOH
(P-(Ni,Fe)3S2/NF^159	196 mV @ 10 mA cm^−2	30	15 h @ 295 mV	1 M KOH
Zn-doped Ni3S2 (ref. 160)	330 mV @ 100 mA cm^−2	87	20 h @ 300 mV	1 M KOH
Metal selenides
Fe-doped CoSe2/NF^161	256 mV @ 100 mA cm^−2	35.6	10 h @ 231 mV	1 M KOH
Ag-doped CoSe2 nanobelts^162	320 mV @ 10 mA cm^−2	56	—	0.1 M KOH
B-doped Fe5Co4Ni20Se36 (ref. 163)	279.8 mV @ 10 mA cm^−2	59.5	10 h @ 10 mA cm^−2	1 M KOH
Co-doped NiSe^164	380 mV @ 100 mA cm^−2	111	>10 h @ 320 mV	1 M KOH
Co-doped nickel selenide^165	275 mV @ 30 mA cm^−2	63	24 h @ 1.5 V vs. RHE	1 M KOH
Co0.75Fe0.25(S0.2Se0.8)2 (ref. 166)	293 mV @ 10 mA cm^−2	77	—	1 M KOH
Cu-14-Co3Se4/GC^167	166 mV @ 10 mA cm^−2	111	12 000 s @ 166 mV	1 M KOH
Cu-doped FeSe2 (ref. 168)	193 mV @ 10 mA cm^−2	59.2	10 000 s @ 1.526 V vs. RHE	1 M KOH
FCS@N-CT^169	330 mV @ 10 mA cm^−2	74	—	1 M KOH
Fe-doped NiSe^170	291 mV @ 1000 mA cm^−2	43	22 h @ 217 mV	1 M KOH
Fe-doped NiSe2 (ref. 171)	231 mV @ 10 mA cm^−2	83	20 h @ 15 mA cm^−2	1 M KOH
Fe-doped Ni3Se4 (ref. 172)	225 mV @ 10 mA cm^−2	41	26 h @ 10 mA cm^−2	1 M KOH
Fe-doped Ni3Se2 (ref. 173)	225 mV @ 10 mA cm^−2	35.3	12 h @ 20 mA cm^−2	1 M KOH
Ni1.12Fe0.49Se2 (ref. 174)	227 mV @ 10 mA cm^−2	37.9	10 h @ 10 mA cm^−2	1 M KOH
Ni0.04Fe0.16Co0.8Se2 (ref. 175)	230 mV @ 10 mA cm^−2	39	15 h @ 1.5 V vs. RHE	1 M KOH
VSe–Ni0.70Fe0.30Se2 (ref. 176)	210 mV @ 10 mA cm^−2	61	20 h @ 10 mA cm^−2	1 M KOH
Zn-doped CoSe2 (ref. 177)	286 mV @ 10 mA cm^−2	37	24 h @ 10, 20, and 50 mA cm^−2	1 M KOH

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In addition to the Fe inclusion, doping other metals such as Ni,^152,178 Co,¹⁴⁸ Mn,¹⁵⁰ Zn,¹⁶⁰ Ce,¹⁴⁷ and Al¹⁴⁶ is also found to enhance the OER activity of metal sulfides. For instance, Lin et al. fabricated Ni-doped FeS nanosheets as an OER catalyst in alkaline electrolyte through a one-pot synthesis approach.¹⁵² The Ni-doped FeS nanosheets showed significant OER activity compared to commercial IrO₂ with 228 mV overpotential at 10 mA cm⁻² current density, by the consequent doping effect of exposing more active sites. Similarly, Kuila et al. synthesized binder-free self-standing Ni-doped FeS OER catalysts on Ni foam via an electrodeposition method.¹⁷⁸ The Ni doping amount is controlled by varying the electrodeposition time, and the electrode fabricated by electrodeposition at 0.9 V for 30 min shows superior activity among them. The change in the electronic structure and charge carrier density via the incorporation of Ni into the lattice domain of FeS significantly improves its electrocatalytic activity. Likewise, Wen and coworkers incorporated Ce into the lattice of Ni₃S₂via a one-step electrodeposition method.¹⁷⁹ Doping with Ce preserves the original structure and morphology. Still, it promotes the OER activity by regulating the active sites and chemical states on the surface of Ni₃S₂. Yang et al. significantly enhanced the electrocatalytic ability of WS₂ by incorporating nearly 1% Co into its lattice.¹⁴⁸ The overpotential was decreased from 492 to 303 mV after incorporation of Co, and the improved activity was ascribed to the presence of Co(II/III) species that improved the binding of reactants and facilitated the formation of the *OOH intermediate. Darr and coworkers optimized the OER activity of bimetallic cobalt–nickel sulfides by doping with various TMs, such as Ag, Fe, Mn, Cr, V and Ti.¹⁵⁰ Among them, Mn-incorporated cobalt–nickel sulfide (Ni_1.29Co_1.49Mn_0.22S₄) shows enhanced activity through tuning of the electronic structure of the host by Mn(III) cations. Zn-doped Ni₃S₂ nanoarrays on nickel foam grown by Sun et al. have better OER activity than their pristine counterpart, with excellent stability for 20 h at 300 mV overpotential.¹⁶⁰ The extended endurance and comparable activity were caused by modulation of the electronic structure of the Ni₃S₂ catalyst through Zn incorporation.

Recently, researchers reported incorporating non-metals, such as N, P, and F, into the lattice of metal sulfides to improve their electrocatalytic capability.^{149,151,153–159,180} N doping of a NiS₂ catalyst using NH₃·H₂O as a precursor considerably increased its water oxidation ability by overcoming the intrinsic limits of reaction kinetics through optimizing d-states.¹⁵³ 3D N-doped Ni₃S₂ nanoflakes grown on Ni foam show superior durability for 20 h at 1.7 V with ∼93% retention of their initial activity.¹⁵⁴ The experimental results indicate that N doping plays a key role in the quick discharging of OER intermediates and improved intrinsic activity.¹⁵⁵ Dou et al. found that a Co₉S₈/graphene hybrid, doped with N using NH₃ plasma etching, showed a significant improvement in its OER performance compared with a pristine sample.¹⁵¹ The synergetic effect of N-doping and surface etching considerably tuned the electronic structure of the Co₉S₈/graphene hybrid and thus enhanced the number of active sites on the surface. He et al. synthesized 3D self-supported and P-doped Ni₃S₂ on Ni foam by a one-step hydrothermal process by simultaneous phosphorization and sulfuration.¹⁵⁷ The as-fabricated electrode exhibits enhanced OER activity with an overpotential of 256 mV at 10 mA cm⁻². The inclusion of P anions optimizes the electronic structure and provides facile adsorption of reactants and thus enables a faster mass transfer. Similarly, P-doped bimetallic nickel–iron sulfide nanosheets on Ni foam were synthesized via a one-pot process.¹⁵⁹ The incorporated P considerably enhances the electrochemical surface area and electrical conductivity by optimizing the adsorption of OER intermediates. F-doped CoS₂ on Ni foam was synthesized using NH₄F as a fluorine source by a hydrothermal process.¹⁸⁰ The F dopant significantly enhances the wettability of the catalyst, and thus facilitates the maximum interaction with electrolyte in both alkaline and neutral media.

5.2 Metal selenides

Some doped metal selenides and their performances are listed in Table 3. The electrocatalytic behavior of metal selenides depends on the number of exposed edge sites.²¹ Edge sites are step-like apertures or defects generated at the basal plane because of doping.¹⁸¹ Besides the edge sites, the electrical conductivity and intrinsic activity of metal selenides are enhanced by doping; specifically, doping with TMs significantly improves the OER activity of metal selenides.^{169,171,173,174} For example, Zare et al. designed a self-supported electrocatalyst on Ni foam by doping Fe into the lattice domain of CoSe₂, which only requires 256 mV overpotential to attain 100 mA cm⁻² current density.¹⁶¹ Here, Fe improves the intrinsic electrical conductivity, and thus lowers the energy barrier for the OER. Zhao and coworkers improved the OER activity of CoSe₂ nanobelts by simultaneous Fe doping and Co vacancy creation, as shown in Fig. 13a.¹⁸² LSV curves of CoSe₂, CoSe₂-D_Fe, CoSe₂-D_Fe-V_Se, and CoSe₂-D_Fe-V_Co catalysts, normalized by electrochemical double-layer capacitance, are shown in Fig. 13b, and suggest that CoSe₂-D_Fe-V_Co has superior OER activity among the catalysts. Fig. 13c displays the FeCoOOH with Co vacancies and the OER mechanism occurring on its surface, which involves three OER intermediates (*OH, O*, and *OOH) and four proton-coupled electron transfer steps. The Gibbs free energy profiles of this catalyst show different catalytic mechanisms for Co and Fe sites. The transformation of OH* to O* potentially limits the activity of Co active sites owing to the large Gibbs free energy difference (ΔG₂). In contrast, the conversion of O* to *OOH is the RDS (ΔG₃) in case of Fe sites, due to the high free energy difference. Theoretical simulations identified that the cobalt dimer (Co₂) is the most active site for the OER, and is present adjacent to the Co vacancy near the surface Fe site. The combinatorial effect of Co vacancy and Fe doping significantly intensifies the OER performance by reducing the peak intensity near the Fermi level, so the binding of *OH with active sites is weakened. Zou et al. found that the minimal amount of Fe (7.4%) inclusion into the NiSe nanorod/nanosheet lattice significantly improved its OER activity.¹⁷⁰ The Fe-doped NiSe only required 291 mV overpotential to achieve 1000 mA cm⁻² current density. This superior activity is accredited to the increased electrical conductivity, active sites, and mass transfer. Xu et al. fabricated hierarchical Fe-doped Ni₃Se₄ on Ni foam using a hydrothermal method, and NiFe LDH was used as a precursor material.¹⁷² The doped sample shows superior OER activity than IrO₂ and pristine catalysts, because of its higher electrical conductivity. Mohanty et al. reported an increase in OER activity of nickel selenide, owing to the synergetic effect of Fe doping along with the creation of Se vacancies by chemical reduction with NaBH₄.¹⁷⁶ Computational studies suggest an increase in DOS near the Fermi level because of Fe doping and Se vacancies, which helps in decreasing the energy barrier for OER intermediates.

Fig. 13 (a) Schematic illustration of the synthetic methods for different catalysts; (b) LSV curves normalized by electrochemical double-layer capacitance; and (c) crystal structure of FeCoOOH-V_Co and the oxygen evolution reaction pathway on the (01−12) facets.¹⁸² Reproduced with open access.

Besides Fe, Co was also used as a dopant to improve the OER performance of nickel selenides,¹⁶⁵ and Co-doped NiSe nanoflowers grown on Ni foam exhibited considerable activity compared to their pristine counterpart. Here, Co doping generates more OER active sites via tuning the electronic structure of NiSe. Wang et al. showed MOF-derived Co-doped nickel selenide on Ni foam and analyzed its OER activity in alkaline electrolyte.¹⁶⁴ The as-designed catalyst exhibits an overpotential of 275 mV at 30 mA cm⁻², and the high activity is ascribed to an increase in electrical conductivity because of Co doping. Zhang et al. co-doped Ni and Fe into the matrix of CoSe₂, and noticed that the catalyst with a stoichiometric formula of Ni_0.04Fe_0.16Co_0.8Se₂ has remarkable OER activity among the prepared catalysts.¹⁷⁵ Ni doping does not alter the OER mechanism but reduces the energy barrier for the RDS and keeps Co-active sites intact, and Fe doping improves the charge transfer kinetics by transferring the active sites from Co to Fe. FeSe₂ was doped with various transition elements (Co, Ni, Mn, and Cu) by Wei et al.¹⁶⁸ Among these dopants, Cu is found to be more effective and shows enhanced OER activity. The theoretical and experimental results reveal that Cu doping optimizes the electronic structure of the FeSe₂ lattice to enable faster electron transfer and thus boosts the reaction kinetics of the OER. In cobalt selenide, the Cu doping results in a phase transformation from orthorhombic CoSe₂ to monoclinic Co₃Se₄, causing improved OER performance of cobalt selenide.¹⁶⁷ The electronic structure change in cobalt selenide by Cu doping is evident from X-ray photoelectron spectroscopy and theoretical simulations. DFT calculations further reveal that because of structural transformation from orthorhombic CoSe₂ to monoclinic Co₃Se₄, and the Gibbs free energy for the RDS was drastically decreased from 2.64 to 1.90 eV.

The minimal amount of Ag (1%) doping into the lattice domain of CoSe₂ nanobelts significantly increases its electrical conductivity without much loss of active sites (Co).¹⁶² Ag-doped CoSe₂ nanobelts exhibit high OER activity relative to pristine CoSe₂ nanobelts, because of high electrical conductivity and active site retention. Incorporation of low-valence Zn into the CoSe₂ lattice domain triggers subtle changes in the lattice and increases the number of active sites for the OER.¹⁷⁷ Theoretical simulations suggest that Zn inclusion decreases the kinetic energy barrier for the adsorption of OER intermediates by encouraging adsorption on Co sites and neighboring Zn sites. Boron doping into the lattice of ternary pyrite selenide (Fe₅Co₄Ni₂₀Se₃₆) decreased its overpotential from 543 to 279.8 mV at 10 mA cm⁻², with the Tafel slope lowered from 161 to 59.5 mV dec⁻¹.¹⁶³Fig. 14a and b show the schematic representation of NiSe₂ and Fe₅Co₄Ni₂₀Se₃₆B_x crystal structures, and the doped B atoms are incorporated into the interstitial positions and some B atoms even bond with metals (Ni, Co, and Fe). The proposed OER mechanisms on pristine and doped pyrite are shown in Fig. 14c and d, and reveal that B doping facilitates an easy discharge of *OOH from the active sites, as a consequence of lower Gibbs free energy. The PDOS reveals that B doping significantly reduces the total d electrons around metals, enhancing the interaction between them and thus weakening the bond between *OOH and active sites. Recently, anion doping, i.e., doping on the Se site, was also found to be beneficial in enhancing the OER activity. For example, Xu et al. reported doping of sulfur on the Se site, enhancing the OER activity of bimetallic cobalt iron selenide Co_0.75Fe_0.25(S_0.2Se_0.8)₂ as compared to non-doped Co_0.75Fe_0.25S₂ and Co_0.75Fe_0.25Se₂, since charge transfer and electrochemical surface area were increased by doping.¹⁶⁶

Fig. 14 (a and b) Schematic illustration of NiSe₂ and Fe₅Co₄Ni₂₀Se₃₆B_x crystal structures, and (c and d) proposed OER mechanisms for pyrite selenides before and after B dopant incorporation.¹⁶³ Reproduced from ref. 163 with permission, © 2019 American Chemical Society.

5.3 Metal phosphides

In the past five years, metal phosphides have emerged as one of the most promising types of catalyst for water oxidation, even though the first metal phosphide catalyst was reported as early as in 1990.¹⁸³ Some doped metal phosphides and their performances are listed in Table 4. Incorporation of a second element into the matrix of a metal phosphide could enhance the OER performance, in terms of both activity and stability.^184,185 For instance, Chen et al. doped Fe into the matrix of CoP ultrathin nanosheets grown on Ni foam, which exhibited a very low overpotential of 67 mV at 10 mA cm⁻² in 1 M KOH.¹⁸⁶ The excellent OER activity is attributed to the interference of Fe/Co/Ni in the low current density region. Post OER characterization revealed the complete absence of P because of the formation of CoOOH active species during the OER. Baek and coworkers synthesized Fe-doped porous CoP polyhedrons from MOFs by varying the Fe/Co ratio.¹⁸⁷ Among them, Co_0.68Fe_0.32P had better OER activity over its pristine counterparts, i.e., CoP and FeP. Fe doping accelerated the discharge of OER intermediates by optimizing their adsorption energy and stabilized the P species during contact with air. Peng's group initially conducted theoretical simulations to predict the Fe influence on the water oxidation capability of Ni₂P by constructing partially oxidized surfaces of Ni (111), Ni₂P (111), and Fe-doped Ni₂P (111) models.¹⁸⁸ The Gibbs free energy studies reveal that the Fe-doped sample requires an energy 0.14 eV lower than that of Ni₂P for the RDS (O* to *OOH). Experimental results were in accordance with the theoretical calculations, and they observed the formation of Fe, P co-doped γ-NiOOH during the electrochemical water oxidation reaction from the Fe-doped Ni₂P. Fe, P co-doped γ-NiOOH is a good catalyst for improved OER activity, and Fe doping primarily enhances the active sites for the OER. Xiang et al. tuned the electronic structure of vanadium phosphide by including Fe into its lattice domain, which demonstrated remarkable OER activity.¹⁸⁹ Among all the samples, the sample with a 1 : 3 ratio of Fe/V has the highest activity. XPS analysis identifies increased valency of V because of the incorporation of Fe, which optimizes the adsorption of *OH on active sites. Fe doping of bimetallic phosphides was also found to significantly contribute to improving electrochemical water oxidation.

Table 4 Summary of representative doped phosphides, nitrides, and carbides and their catalytic performances

Catalyst	Overpotential at a specific current density	Tafel slope (mV dec^−1)	Durability	Electrolyte
Metal phosphides
Al-doped CoP^193	360 mV @ 200 mA cm^−2	38	150 h @ 10 mA cm^−2	1 M KOH
Co0.68Fe0.32P^187	289 mV @ 50 mA cm^−2	64	3600 s @ 10 mA cm^−2	1 M KOH
Fe0.27Co0.73P/NF^184	251 mV @ 10 mA cm^−2	59.1	40 h @ 40 mA cm^−2	1 M KOH
Fe-doped CoP^186	166 mV @ 50 mA cm^−2	65.82	50 h @ 50 mA cm^−2	1 M KOH
Fe-doped Ni2P^192	330 mV @ 20 mA cm^−2	39	10 h @ 20 mA cm^−2	1 M KOH
Fe-doped Ni2P^188	300 mV @ 1000 mA cm^−2	52	—	1 M KOH
FexVy−xPC/NF^189	201 mV @ 10 mA cm^−2	75	24 h @ 10 mA cm^−2	1 M KOH
Mn-doped CoP^194	261 mV @ 10 mA cm^−2	44.9	24 h @ 10 mA cm^−2	1 M KOH
Mn-doped Ni2P^195	330 mV @ 100 mA cm^−2	116.7	18 h @ 320 mV	1 M KOH
NiCoFeP/NF^191	271 mV @ 200 mA cm^−2	45	10 h @ 1.485 V vs. RHE	1 M KOH
NiCoFexP/CC^190	275 mV @ 50 mA cm^−2	50	150 h @ 1.70 V vs. RHE	1 M KOH
NCNP^196	229 mV @ 10 mA cm^−2	78.6	100 h @ 100 mA cm^−2	1 M KOH
Ni1.85Fe0.15P NSAs/NF^185	270 mV @ 20 mA cm^−2	96	10 h @ 330 mV	1 M KOH
Ni2P/(NiFe)2P(O) NAs^197	530 mV @ 800 mA cm^−2	60	100 h @ 500 mV	1 M KOH
N-NiVFeP/NFF^198	229 mV @ 10 mA cm^−2	78.6	100 h @ 100 mA cm^−2	1 M KOH
O-doped Co2P/CuO NWs/CF P^199	270 mV @ 10 mA cm^−2	74.4	30 h @ 10 mA cm^−2	1 M KOH
S-doped CoP/NF^200	300 mV @ 10 mA cm^−2	82	20 h @ 350 mV	1 M KOH
Metal nitrides
Co-doped MoNi nitride^201	294 mV @ 10 mA cm^−2	73	10 h @ 317 mV	0.1 M KOH
Fe-doped cobalt nitride^202	200 mV @ 10 mA cm^−2	42.44	10 h @ 200 mV	1 M KOH
Mn0.15 Co0.85N^203	265 mV @ 10 mA cm^−2	48	30 h @ 280 mV	1 M KOH
Mn0.15 Co0.85N^203	285 mV @ 10 mA cm^−2	64	30 h @ 285 mV	1 M phosphate-buffered saline
P,F-Ni1.5Co1.5N^204	350 mV @ 100 mA cm^−2	66.1	—	1 M KOH
P-NiMo4N5@Ni^205	435 mV @ 100 mA cm^−2	65	24 h @ 50 mV	1 M KOH
Metal carbides
B, N:Mo2C@BCN^206	360 mV @ 100 mA cm^−2	61	20 h @ 350 mV	1 M KOH
Co3C-NB^207	354 mV @ 10 mA cm^−2	90	24 h @ 50 mA cm^−2	0.1 M KOH
Co-doped β-Mo2C^208	262.2 mV @ 10 mA cm^−2	28.8	16 h @ 1.49 V vs. RHE	1 M KOH
Co-doped Ni3C/Ni@C^209	325 mV @ 10 mA cm^−2	67.76	100 000 s @ 10 mA cm^−2	1 M KOH
FCC@CNOs/NF^210	271 mV @ 10 mA cm^−2	48.9	40 h @ 100 mA cm^−2	1 M KOH
Fe-doped Ni3C^211	275 mV @ 10 mA cm^−2	62	10 h @ 1.5 V vs. RHE	1 M KOH
V0.28Co2.72C/CNFs^212	210 mV @ 10 mA cm^−2	147	10 h @ 1.4 V vs. RHE	1 M KOH

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Like monometallic phosphides, bimetallic phosphides can also convert into their respective oxyhydroxides at high anodic potentials. Fe-doped NiCoP converts into Fe-doped NiOOH and CoOOH, and Fe doping improves the OER activity by accelerating the discharge of OER intermediates.^190,191 Simultaneously, the conductive metallic phosphide support improves OER kinetics by promoting the proton transfer. Tsang and coworkers studied the comparative doping effects of TMs (Co, Fe, Mn, and Mo) on the OER activity of NiP.¹⁹² Among them, the Fe doped sample was found to have more prominent activity since Fe is able to tune the electronic structure of Ni to interact optimally with OER intermediates.

Mn doping of cobalt phosphide and nickel phosphide improves their catalytic activity for electrochemical water oxidation. In cobalt phosphide, Mn doping enhances the electrochemical surface area by increasing the number of exposed active sites.¹⁹⁴ The optimal Mn/Ni ratio of 0.053 is found to provide superior activity, as the Ni₂P electronic structure is optimized.¹⁹⁵ Al doping was also used to enhance the electrochemical surface area of cobalt phosphide by dissolution during the OER. Al-doped cobalt phosphide shows a fourfold increase in electrochemical surface area compared to the pristine sample by the formation of Al/Co spinel oxide and partial leaching of Al.¹⁹³

Along with doping of metals, doping of non-metals such as N, O, and S can also enhance the electrocatalytic activity of phosphide catalysts for the OER.^{196,198–200} N doping in the anionic site (P) of NiCoP improved its OER performance, with better activity compared to NiCoP and NiCoN. The high activity was induced by the formation of abundant oxygen vacancies, improving the charge transfer during oxygen generation.¹⁹⁶ Likewise, oxygen and sulfur were also doped on the anionic sites to increase the OER activity of metallic phosphides. Oxygen doping of cobalt phosphide generates abundant defects and electroactive sites on the surface by optimizing the adsorption energy of reactants.¹⁹⁹ In the case of bimetallic nickel–iron phosphide, the OER activity is enhanced by oxygen doping on account of an efficient decrease in the energy barrier of OER intermediates and products.¹⁹⁷ Electronegative sulfur doping in the anionic site of cobalt phosphide significantly improved its OER activity, owing to the tuning of the electronic structure and increased number of active sites.²⁰⁰

5.4 Metal nitrides and carbides

Metal nitrides belong to an interstitial compound family, which contains nitrogen atoms on the interstitial sites of parent compounds. The presence of N atoms on the interstitial sites increases the distance between metal atoms; thus, it tunes the d band structure.²⁰ Some doped metal nitrides and carbides and their performances are listed in Table 4. Recent studies on the electrochemical activity of metal nitrides suggest that they are highly active for the OER owing to their high electrical conductivity.^204,205 However, the OER activity and stability were further improved by tuning their electronic structure by doping with TMs and non-metals. For instance, Mn-doped CoN nanowires grown on carbon cloth exhibit good OER activity in alkaline and neutral electrolytes.²⁰³ Among the doped catalysts, Mn_0.15Co_0.85N shows superior activity, and Mn doping considerably increases the charge transfer kinetics and facilitates the optimized discharge of OER intermediates. Zhou et al. investigated the doping effect of various metal atoms, such as Zn, Ni, Fe, and Mn, on the OER activity of cobalt nitride nanosheets.²⁰² The Fe doped one has better OER activity, and XPS analysis suggests more Co⁴⁺ OER active species because of the doping effect of Lewis acid Fe³⁺.

Cobalt doping into the lattice of bimetallic MoNi nitride nanowires shows a considerable increase in their OER activity by reduced charge transfer resistance and augmentation of the electrochemically active surface area.²⁰¹ The dual modulation in the electronic structure of Mo and Ni was induced by cobalt doping that improved the electrical conductivity for highly active OER sites. Liang et al. theoretically studied the doping effect of first-row TMs (Sc to Zn) for improving the OER activity of Al/GaN.²¹³ The inclusion of transition metals from Sc to Fe resulted in low OER activity because they bonded to OER intermediates too firmly and resulted in high overpotentials. Meanwhile, Cu and Zn also displayed no or low OER activity, as they attached to the intermediates very weakly. Only Ni doping had significant OER activity because it could switch from a high spin state to a low spin state and stabilized the OOH adsorbate. Tong and coworkers synthesized P and Co dual-doped vanadium nitride nanowires using a combined hydrothermal and annealing approach.²¹⁴ The dual-doped vanadium nitrate catalyst showed better activity and durability for the OER, compared to the single-doped or pristine sample, or IrO₂. Dual doping of Co and P synergistically tunes the number of active sites on the catalyst surface by moving the d states of vanadium closer to the Fermi level.

Zhang and coworkers demonstrated an increase in the OER performance of bi-metallic nitrides by co-doping with phosphorus and fluorine.²⁰⁴Fig. 15a shows the schematic representation of the fabrication process for P and F co-doped nickel–cobalt nitride (PF/Ni_1.5Co_1.5N) nanorods using an ionic liquid. The PF/Ni_1.5Co_1.5N prepared without using any ionic liquid results in a lamellar structure, as displayed in Fig. 15b. The SEM and elemental mapping analysis results of PF/Ni_1.5Co_1.5N nanorods are presented in Fig. 15c and d, which indicates the uniform distribution of N and F dopants all over the catalyst. Fig. 15e and f show the IR uncompensated LSV curves and comparison of η₁₀ values (potentials required to reach 10 mA cm⁻²) for PF/Co_xNi_3−xN catalysts. Electrochemical studies reveal that the PF/Ni_1.5Co_1.5N catalyst shows augmented OER activity as well as facile reaction kinetics among the prepared catalysts. DFT calculations suggest that co-doping of P and F atoms into the lattice of the Ni_1.5Co_1.5N catalyst considerably decreases the free energy barrier for OER intermediates, thus helps in improving the electrocatalytic performance.

Fig. 15 (a) Schematic illustration of the fabrication processes for PF/Ni_1.5Co_1.5N nanorods; (b) SEM; (c) highly magnified SEM; (d) HAADF-STEM elemental mapping results of the heteroatom doped Ni–Co (1 : 1) precursor; (e) LSV curves of PF/Co_xNi_3−xN catalysts for the OER (400 mg of ionic liquid, without IR compensation); and (f) comparison of η₁₀ values (potentials required to reach 10 mA cm⁻²) for various catalysts (Ni : Co = 1 : 1, 1 : 3, 1 : 0, 0 : 1, and 3 : 1).²⁰⁴ Reproduced from ref. 204 with permission, © 2017 John Wiley and Sons.

Recently, researchers found that metal carbides, specifically transition metal carbides, can be applied as electrocatalysts for the OER owing to their high stability.²¹⁵ Some doped metal carbides and their performances are listed in Table 4. The activity of TM carbides was improved by including one or more foreign dopants into their lattice domain. Yan et al. improved the OER activity of Ni₃C by doping 2% atomic iron, which yields an overpotential of 275 mV at 10 mA cm⁻².²¹¹ The optimized doping of Fe tuned the electronic structure of Ni and thus improved the number of active sites on the surface. Similarly, Jia et al. tuned the water oxidation ability of Ni₃C via incorporation of a small amount of Co into it using a MOF-derived method.²⁰⁹ The as-fabricated catalyst had better activity than the IrO₂ catalyst in the alkaline electrolyte, and the improved activity was associated with the active surface sites through optimization of electronic properties. In general, the Mo₂C catalyst is inactive for the OER, but doping 10% cobalt into the Mo₂C catalyst was found to increase its OER performance through significant enhancement in the density of active sites of the catalyst.²⁰⁸ Zhao et al. optimized the electronic structure of Co₃C by doping with low electronegativity vanadium element.²¹² Specifically, V_0.28Co_2.72C shows superior activity by forming local negative Co centers through the intake of electrons from vanadium dopants. Xu et al. doped TMs such as Mn, Co, and Ni into the matrix of Fe₃C and evaluated its OER performance.²¹⁰ They noticed that Co-doped iron carbide had enhanced activity, and the Gibbs free energy was estimated in the different steps in the OER process. The free energy profiles of the Co and Fe sites were compared, and the Gibbs free energy required for the Co site (2.35 eV) was lower than that for the Fe site (2.75 eV), for the conversion of *O to *OOH (RDS). From the comparison, Fe has a stronger affinity towards *O compared to Co, hence doping of Co into the Fe₃C facilitates the discharge of intermediates, thus lowering the potential.

Doping of metal carbides on the anionic site also improves their activity for electrocatalytic oxygen evolution. Generally, WC is a poor OER catalyst; however, when the carbon site is doped with nitrogen, it shows even better OER performance than IrO₂ in an acid electrolyte.²¹⁶ The N doping helps in accelerating the discharge of intermediates and thus improves activity, but its poor stability is still a concern. Dual doping of N and B on the anionic site of Co₃C considerably increases its water oxidation ability compared to the pristine or single-doped sample, or IrO₂. The synergetic effect of N and B dopants improves both activity and stability by boosting the charge transfer kinetics of intermediates.²⁰⁷ Likewise, Lee and coworkers synthesized dual-doped N and B into the matrix of Mo₂C embedded in the B, N doped carbon network (B,N:Mo₂C@BCN), which shows a considerable increase in OER activity compared to the mono-doped or pristine sample, or IrO₂.²⁰⁶ The significant improvement in electrocatalytic performance is accredited to the formation of active B- and N-doped molybdenum oxide at high anodic potentials.

6. Tuning carbon-based catalysts

Although metal-based catalysts are employed in electrocatalytic OER applications widely because of their promising capabilities to deliver efficient catalytic activity by variable oxidation states,²¹⁷ these catalysts suffer from relatively poor endurance, low selectivity, susceptibility towards potential gas poisoning, and hazardous environmental effects. In 2009, the first carbon-based catalyst was reported by Dai's group in fuel cells as an alternative to the platinum catalyst.²¹⁸ Since then, carbon-based catalysts, particularly those doped with various heteroatoms or metals, have received extensive attention and tremendous progress has been made in electrocatalysis.^219,220Table 5 provides a summary of some representative doped carbon-based electrocatalysts and their catalytic performances.

Table 5 Summary of representative doped carbon-based electrocatalysts and their catalytic performances

Catalyst	Overpotential at a specific current density	Tafel slope (mV dec^−1)	Durability	Electrolyte
20-NMWNT^221	320 mV @ 10 mA cm^−2	68	—	1 M NaOH
B, N-carbon^222	340 mV @ 10 mA cm^−2	84	—	1 M KOH
Co-C3N4/CNT^223	380 mV @ 10 mA cm^−2	68.4	—	1 M KOH
Co-N/GF^224	313 mV @ 10 mA cm^−2	84.1	30 h @ 1.6 V vs. RHE	1 M KOH
Co-TA^225	460 mV @ 10 mA cm^−2	66	—	0.1 M KOH
GO-PANi31-FP^226	300 mV @ 10 mA cm^−2	136	—	0.1 M KOH
N/C^227	380 mV @ 10 mA cm^−2	—	—	0.1 M KOH
NCHCs^228	290 mV onset potential	90	12 h @ 1.6 V vs. RHE	0.1 M KOH
NCNTFs^229	370 mV @ 10 mA cm^−2	93	10 000 s @ 1.70 V vs. RHE	1 M KOH
N-GRW^230	360 mV @ 10 mA cm^−2	47	24 h @ 10 mA cm^−2	1 M KOH
N-HC@G-900 (ref. 231)	350 mV @ 10 mA cm^−2	88.1	25 000 s @ 10 mA cm^−2	1 M KOH
N-HC@G-900 (ref. 231)	350 mV @ 10 mA cm^−2	297.5	7500 s @ 1 mA cm^−2	0.5 M H2SO4
NiFe@g-C3N4/CNT^232	326 mV @ 10 mA cm^−2	67	16 h @ 10 mA cm^−2	1 M KOH
Ni, Fe@PCN^233	310 mV @ 10 mA cm^−2	38	2 h @ 1.53 V vs. RHE	1 M KOH
Ni, N doped graphene^234	270 mV @ 10 mA cm^−2	59	40 h @ 1.40 V vs. RHE	1 M KOH
Ni-O-G SACs^235	224 mV @ 10 mA cm^−2	42	50 h @ 115 mA cm^−2	1 M KOH
N, S-CNT^236	360 mV @ 10 mA cm^−2	56	—	1 M KOH
N, S doped graphene^237	310 mV @ 10 mA cm^−2	65	25 h @ 1.55 V vs. RHE	0.1 M KOH
O-CNTs^238	360 mV @ 10 mA cm^−2	47.7	24 h @ 5 mA cm^−2	1 M KOH
O Doped MWCNTs^239	450 mV @ 10 mA cm^−2	72	10 000 s @ 5 mA cm^−2	0.1 M KOH
P-doped C3N4 (ref. 240)	400 mV @ 10 mA cm^−2	61.6	15 h @ 10 mA cm^−2	0.1 M KOH
P-doped graphene^241	330 mV @ 10 mA cm^−2	62	—	1 M KOH
S, N-Fe/N/C-CNT^242	370 mV @ 10 mA cm^−2	82	40 000 s @ 10 mA cm^−2	0.1 M KOH
S,S′-CNT^243	350 mV @ 10 mA cm^−2	95	75 h @ 10 mA cm^−2	1 M KOH

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6.1 Graphene

Graphene is a thin layer of single atoms packed with sp² hybridized carbon atoms in a hexagonal lattice. It is considered either a zero-bandgap semiconductor or conductor because of its symmetric band structure.²⁴⁴ Hence, pristine graphene shows low or no catalytic activity; however, the unique structural flexibility of graphene allows extensive functionalization via doping to improve catalytic activity. The dopants could interrupt the sp² hybridized network and induce the formation of sp³ in the lattice, resulting in an electronic density change that imparts catalytic activity to it.²⁴⁴ For instance, doping of N into the lattice of graphene disrupts the charge distribution and spin density of neighboring carbon atoms and, consequently, imparts OER activity. Yang et al. conducted a study to gain mechanistic insights into N-doped graphene in the OER.²³⁰ They identified the formation of electron-withdrawing pyridinic N functionalities, which is the key reason for the superior OER performance of N-doped graphene. Xia's group studied the active sites in N-doped graphene nanoribbons combined with other dopants, such as B, Cl, P, and S, using DFT calculations.²⁴⁵ Gibbs free energy analysis for the OER on doped graphene nanoribbons suggests that the conversion of *O to *OOH is the RDS. The co-doping shows better activity than mono-doping, and the synergetic effect of co-doping is mostly related to the distance between them. In another report, N doping was done by using polyaniline and graphene oxide as precursors, which produced nanoporous layered and N-doped graphene that exhibits enhanced OER activity.²⁴⁶ Nearly 51% quaternary N plays a significant role in the considerable increase in OER activity. Wang et al. demonstrated a significant improvement in the OER performance of graphene by P doping.²⁴¹ They found that the stronger oxygen adsorbing ability of P, which was present at the edges of graphene, played a crucial part in stabilizing OER intermediates.

Besides single atom doping of graphene, dual- and multi-atom doping were also found to deliver significant results in improving OER activity by their synergistic effects. Zhao and Xia studied several combinations of dual doping such as N–N, N–P, N–S, N–B, and N–Cl in optimizing the OER activity of graphene using first-principles modeling.²⁴⁷ They suggested that optimizing the distance between dopants, selecting the right combination of dopants, and efficient utilization of edge effect are the vital factors in designing a highly efficient electrocatalyst. For example, the co-doping of P and N into the graphene lattice results in graphene formation with P, N population at the edges, as a consequence of the large atomic size of P that always moves away from the planar surface. This co-doped catalyst exhibits a significant improvement in OER performance, and the elevated activity is attributed to the synergetic effect of both dopants, the high oxidation ability of the P-site that initially adsorbs O₂, and the N-site that stabilizes the OER intermediates and promotes desorption.²⁴⁸

Qiao's group synthesized N and S dual-doped graphene microwires using an ionic liquid assisted method, which showed an overpotential of 310 mV at 10 mA cm⁻², stability for 25 h and 95% faradaic efficiency.²³⁷ Moreover, this electrode was employed as an efficient cathode for Zn–air batteries, and this superior OER activity is attributed to the numerous active sites created by C–N–S spin density. Furthermore, Zhang and Dai demonstrated tri-doping of graphene with N, P and F atoms, yielding a multifunctional electrocatalyst.²²⁶ The N and P doping roles in improving OER performance were elaborated. Still, it is inconclusive how F inclusion into the graphene lattice contributes to the electrocatalytic activity. Li et al. demonstrated a significant improvement of the OER performance of graphene by including single nickel atoms.²³⁵ The single-atom doped graphene displayed 50 h of long-term stability at a very high current density of 115 mA cm⁻². This unusual activity is accredited to the bonding between single nickel atoms and oxygen sites of graphene. Chen et al. doped Ni single atoms on N-doped nano-porous graphene via a chemical vapor deposition (CVD) method, followed by chemical etching.²³⁴ The dual-doped catalyst showed superior OER activity to other catalysts, and even outperformed the IrO₂ catalyst. The DFT calculations suggest that Ni, N co-doping significantly decreased the stability difference between O* and *OH intermediates by 0.3 eV.

6.2 Carbon nanotubes (CNTs)

CNTs are 1D structural allotropes of carbon that consist of hexagons of carbons arranged in a concentric fashion. CNTs possess good electrical–thermal properties due to their unique hollow geometry and conjugated long-range π electron structure.²⁴⁹ However, CNTs in their pristine form exhibit poor electrocatalytic properties. To improve the electrocatalytic properties, CNTs need to be functionalized with several dopants.²⁵⁰ This functionalization can be done through two methodologies, i.e., covalent and non-covalent methods. The covalent functionalization proceeds via forming a covalent bond between the functional group and CNTs, which could disrupt the unique structure of CNTs. In the non-covalent method, the functionalization happens through physical processes such as electrostatic, hydrogen bonding, π stacking and van der Waals interactions, which leaves the CNT structure undisturbed.²⁴⁹ For example, Tian et al. doped nitrogen into CNTs and synthesized highly active CNT@NCNT for the OER as a catalyst.²⁵¹ The superior OER activity is ascribed to the highly exposed integrated N atoms present on the surface with the combined mechanical stability of the undisrupted inner walls of CNTs. Likewise, the Kallio group doped nitrogen into multiwalled CNTs (MWCNTs) and observed a considerable increase in OER performance after doping, compared with pristine MWCNTs.²²¹ The significant OER activity of N-doped MWCNTs is induced by pyridinic N functionalities that are electron-withdrawing. Wang's group designed a generalized approach to make highly efficient hollow N-doped CNTs as a catalyst for the OER from a MOF. The remarkable electrochemical activity is attributed to the synergistic effects from interconnected N-doped CNT structures and chemical compositions with high mechanical stability.²²⁹

Qiao's group synthesized dual-doped CNTs with nitrogen and sulfur inspired by the features of polydopamine (PDA), as depicted in Fig. 16a.²³⁶ N and S doped CNTs were prepared through a two-step “graft-and-pyrolyze” route using PDA. Initially, PDA was deposited on the surface of multiwalled CNTs and sulfur was incorporated at room temperature via chemical grafting, and subsequent pyrolysis at 700 °C resulted in a N and S uniformly co-doped carbon framework with enhanced S-doping efficiency. Even after doping and pyrolysis, the nanotube morphology of CNTs was preserved, as shown in Fig. 16b. The OER performance of pristine and doped CNT catalysts is displayed in Fig. 16c, which shows that the N, S-CNT catalyst exhibited superior OER activity among them with an overpotential of 360 mV and a Tafel slope of 56 mV dec⁻¹ (Fig. 16d). Theoretical studies show a considerable increase in spin-charge density difference among carbon atoms with doping; these carbon atoms act as electrocatalytically active sites. As a consequence, the apparent OER activity improved with dual doping compared to single doping. Liu's group has extensively studied the doping effect of oxygen on the OER performance of CNTs by synthesis via two different methods, i.e., oxygen plasma oxidation (O-CNTs) and thermal reduction (R-CNTs).²³⁸ O-CNTs exhibit enhanced OER activity over R-CNTs and pristine CNTs, and DFT calculations reveal that both O-CNTs and R-CNTs have the same active sites, i.e., carbon near C=O. The high-water oxidation ability of O-CNTs was thought to be possible due to the presence of a high density of active sites, with high E_a,app at the specific reaction temperature.

Fig. 16 (a) Fabrication of the N, S-CNT following a two-step “graft-and-pyrolyze” route. The oxidized CNT (ox-CNT) was mixed with dopamine (DA) in PBS buffer to obtain a CNT–PDA hybrid. After the addition of R-SH, CNT–PDA modified with thiol (CPS) was synthesized, which produced N, S-CNT after pyrolysis. R-SH = 2-mercaptoethanol; (b) TEM image of the N, S-CNT; (c) OER polarization curves; and (d) corresponding Tafel plots of CNT–PDA, ox-CNT, N-CNT, and N, S-CNT in 1 M KOH solution.²³⁶ Reproduced from ref. 236 with permission, © 2017 John Wiley and Sons.

Lu et al. oxidized the surface of MWCNTs through hydrothermal annealing and electrochemical activation, which results in a highly active OER catalyst.²³⁹ The superior OER activity originated from the formation of carbonyl functionalities on the outer wall, which altered the electronic structure of neighboring carbon atoms and facilitated adsorption of OER intermediates. Suib et al. fabricated a highly stable metal-free catalyst at high anodic potentials by successive two-step doping of sulfur into the MWCNTs, displaying two orders of magnitude higher turnover frequency (TOF) compared to the Ir/C catalyst.²⁴³ The enhanced OER activity was caused by the inclusion of heterocyclic sulfur into the hexagonal rings of CNTs. Wu and co-workers developed a single-atom doped catalyst for the water oxidation reaction by coating CNTs with Fe salt and 2,2 bipyridine, followed by pyrolysis and acid leaching, yielding FeN_x species on the S and N co-doped CNT catalyst.²⁴² This catalyst exhibits superior OER activity in alkaline medium, as a result of high exposure of Fe atoms and synergetic effects of S and N doping.

6.3 Other carbon-based catalysts

In addition to graphene and CNTs, other forms of carbon-based catalysts were also reported for OER activity. Graphitic carbon nitride (g-C₃N₄) has a C–N structure with graphite like sp² bonding, which exhibits semiconducting behavior due to π-conjugation.²⁵² Like graphene and CNTs, g-C₃N₄ is also inactive in its pristine form, and tuning its electronic structure by doping non-metal or metal elements significantly improves its water oxidation ability.^233,240 For instance, Huang et al. dual-doped N and B into 3D porous g-C₃N₄ and observed superior OER activity in alkaline solution, compared to N-doped or N and P co-doped 3D g-C₃N₄.²⁵³ It was found that the N and B co-doped g-C₃N₄ has a high population of pyrrolic and pyridinic nitrogen for enhanced OER.²²² Based on DFT calculations, Jiang and coworkers proposed doping single transition elements such as Pt, Pd, Co, Ni, and Cu into carbon nitride (CN) that could lead to a cost-efficient, highly efficient OER catalyst.²⁵⁴ Specifically, the electrocatalysts of non-noble metal Co/Ni@CN and Co–O@CN exhibit excellent activity and function with extremely low overpotential. Qiao's group conducted the theoretical study and experimental validation on cobalt inclusion on g-C₃N₄ for OER activity in alkaline medium.²²³ The combinative evaluation of theoretical and experimental methods reveals that metal–nitrogen M–N₂ coordination in the matrix of g-C₃N₄ is the origin of high electrocatalytic activity compared to the pristine sample. Song et al. studied the effect of dual metal doping in enhancing the OER activity, and doped Ni and Fe metals in tri-s-triazine units of g-C₃N₄ enveloped by CNTs.²³² Synchrotron-based X-ray absorption with combination of atomic microscopy indicated that the electronic configuration of both Fe and Ni was reconfigured, and the valence state of Ni was increased, and thus more OER active sites were created compared to single doping. Sun and coworkers dual-doped Ni and Fe into the framework of polymeric carbon nitride (Ni, Fe@PCN), in which superior activity was found.²³³ Computational studies suggest that the enhanced OER activity of NiFeOH@PCN is a consequence of the vastly reduced energy barrier (0.10 or 0.22 eV for U ¼ 1.58 V) through synergetic contribution of adjacent Ni and Fe atoms.

In 2013, Zhao et al. reported N-doped carbon nanomaterials as metal-free OER catalysts other than graphene and CNTs for the first time.²²⁷ In alkaline electrolyte, the catalyst exhibits 380 mV overpotential at a current density of 10 mA cm⁻², which is comparable to iridium and cobalt oxide catalysts. The pyridinic and quaternary nitrogen species shorten the diffusion path of OER intermediates. The N-doped carbon hollow cubes (NCHCs) for the OER were synthesized from biomass lysine using a NaCl-template method by Yang and coworkers.²²⁸ The NCHCs show comparable OER activity and high endurance compared with the RuO₂/C catalyst in alkaline medium. Synergetic effects of N-doping and high specific surface area were found. Ozkan et al. demonstrated the OER activity of N-doped carbon nanostructures (CN_x) in an acidic medium, which showed significantly higher OER performance than the Ir/C catalyst.²⁵⁵ XPS results suggested the presence of pyridinic N species for the enhanced OER activity of the CN_x catalyst. Zhao's group demonstrated a novel strategy to fabricate a highly efficient OER catalyst by synthesizing an ultra-thin N-doped holey carbon layer (HCL) on graphene through a two-step process as displayed in Fig. 17a.²³¹Fig. 17b presents the HR-TEM image of Zn@G-hydro annealed at 900 °C, which exhibits nanoholes with an approximate diameter of 2 nm, as a result of Zn evaporation (N-HC@G-900). The nano-holes are not observed when zinc gluconate is not used in the synthesis (N-C@G-900). N-HC@G-900 displayed enhanced OER activity, as observed in Fig. 17c, because of pyridinic N moieties at the edges of the HCL, and the sandwiched graphene sheet provided mechanical support.

Fig. 17 (a) Schematic representation of the N-HC@G@x synthesis procedure; (b) HRTEM image of N-HC@G-900 (nanoholes marked in red); (c) LSV curves of N–C@G-900, N-HC@G-900 and Ir/C in 1 M KOH.²³¹ Reproduced from ref. 231 with permission, © 2018 John Wiley and Sons.

The combination of N and transition metal doping into the carbon materials effectively improved the OER performance and stability by creating more active sites.^224,225,256 For instance, Mu et al. co-doped Co and N into the matrix of graphene-like porous carbon nanosheets (Co@N-PGCS), which possess a high specific surface area of 1716 cm² g⁻¹.²⁵⁶ The Co@N-PGCS catalyst showed comparable activity over the state-of-the-art catalyst IrO₂ by a synergetic effect of Co and N co-doping, and the porous surfaces open more diffusion channels. In another report, Tong and coworkers designed an efficient self-standing OER catalyst by dual doping Co and N into a graphite foam matrix (Co–N/GF).²²⁴ The Co–N/GF catalyst prepared at 700 °C could deliver significant OER activity, compared to the catalyst prepared at other temperatures in alkaline electrolyte. Initially, Co–N species were partly oxidized to CoOOH/Co–N. At higher anodic potentials, CoOOH was further oxidized to the most active CoO₂, which considerably promoted the oxidation of OH⁻ species. Wang's group, for the first time, fabricated a Co/N co-doped carbon composite for water oxidation from metal–polyphenol (Tanin) crystals, which are highly abundant in nature.²²⁵ Co/N co-doping in a carbon composite was found to produce a synergistic effect with a high specific surface area (180 m² g⁻¹), producing excellent OER activity in the alkaline electrolyte. Lee et al. demonstrated the synthesis of a tri-doped carbon catalyst for acidic OER.²⁵⁷ Here, they doped N, O, and TMs, such as Fe, Co, and Ni, onto the carbon, and they noticed that N/Fe/O and N/Ni/O doped carbon catalysts showed high OER activity, and N/Co/O displayed better stability.

7. Summary and concluding remarks

Searching for eco-friendly renewable energy that can replace carbon-based fossil fuels for future fuel is a prime goal of the research community. Electrochemical oxygen evolution plays a crucial role in energy storage by converting renewable energies, such as solar, wind, and tidal forms, into storable chemical fuel of hydrogen. Hence, the fundamental priority is to fabricate efficient OER catalysts that can be applied in the real world. This review summarized the research on the tuning of OER catalyst properties by doping. Doping of foreign elements can significantly alter the electrical conductivity and binding ability of OER intermediates by optimizing the host electronic structure. The considerable changes in the OER activity of metal oxides, non-metal oxides, and carbon-based catalysts via doping were reviewed. Experimental and theoretical studies were discussed to explain the OER mechanisms. Among the catalysts discussed above, doping in hydroxides, sulfides, selenides, and phosphides has been shown to be relatively more effective compared to the others discussed in this paper. Overall, this review outlines the systematic understanding of experimental and theoretical effects of doping on OER catalysts, which is associated with several challenges involving energy processes such as carbon dioxide reduction, nitrogen reduction, and production of hydrogen peroxide.

Considering the studies reported in this paper, an efficient OER catalyst should have optimum adsorption ability for OER intermediates, neither too weak nor too strong, and possess a good electrical conductivity. The optimized e_g filling value (∼1.2) can be obtained by tuning the electronic structure of TM ions present in metal oxides, and optimizing the covalency of metal and oxygen orbitals, including other metal ions along with TM ions. An optimized surface area can be achieved by preparing nanoforms. Furthermore, the catalyst should possess excellent resistance under extreme OER conditions with high anodic potentials and a good redox center that can lose or gain electrons.

In future, the development of advanced characterization techniques, such as in situ XPS, Fourier-transform infrared spectroscopy (FT-IR), X-ray absorption spectroscopy, XRD, etc., will provide more insights into OER intermediates on the surface of a catalyst. To design a highly stable catalyst for industrial-scale applications, it is essential to have the metric of long-term stability without dropping its initial activity evidently. Several catalysts display high activity, but a few of them can reach long-term endurance. Besides, large surface area and porosity are the other vital parameters in producing catalysts for large-scale use, which could provide high activity with a small catalyst. Most of the catalysts described in this paper were synthesized on a lab scale, while mass production of the same type of catalyst will be challenging in terms of cost effectiveness to fulfill the practical needs. Interdisciplinary collaborative research efforts are needed in science and engineering towards widespread applications of fuel cells.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation (DMR-1827731).

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