A review of modulation strategies for improving the catalytic performance of transition metal sulfide self-supported electrodes for the hydrogen evolution reaction

Abstract

Electrocatalytic water splitting is considered to be one of the most promising technologies for large-scale sustained production of H₂. Developing non-noble metal-based electrocatalytic materials with low cost, high activity and long life is the key to electrolysis of water. Transition metal sulfides (TMSs) with good electrical conductivity and a tunable electronic structure are potential candidates that are expected to replace noble metal electrocatalysts. In addition, self-supported electrodes have fast electron transfer and mass transport, resulting in enhanced kinetics and stability. In this paper, TMS self-supported electrocatalysts are taken as examples and their recent progress as hydrogen evolution reaction (HER) electrocatalysts is reviewed. The HER mechanism is first introduced. Then, based on optimizing the active sites, electrical conductivity, electronic structure and adsorption/dissociation energies of water and intermediates of the electrocatalysts, the article focuses on summarizing five modulation strategies to improve the activity and stability of TMS self-supported electrode electrocatalysts in recent years. Finally, the challenges and opportunities for the future development of TMS self-supported electrodes in the field of electrocatalytic water splitting are presented.

---

Introduction

In the era of increasing energy crisis and environmental pollution, finding efficient, clean and environmentally sustainable energy has become an urgent issue.^1–3 Hydrogen energy, as a kind of green source of energy with high energy density, wide sources and non-polluting nature, is regarded as an important carrier of the future energy structure.^4–11 Hydrogen from electrolysis of water is a method of converting H₂O to H₂ under mild conditions using electricity generated from renewable energy sources (solar, hydro, wind, ocean, etc.).^12–16 The process is non-polluting and the hydrogen produced is called “green hydrogen”; moreover, water is the most abundant renewable “hydrogen source” on Earth, providing a guaranteed source of feedstock for sustainable hydrogen production. Therefore, coupling renewable power generation with electrocatalytic water splitting to produce H₂ (“Green Power + Green Hydrogen”) is the best choice for the healthy development of the hydrogen industry.^17–22 Electrocatalytic water splitting (2H₂O → 2H₂ + O₂) is mainly composed of the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Theoretically, a voltage of 1.23 V applied under standard conditions is sufficient to drive the water splitting reaction. However, in the actual electrocatalytic water splitting process, the applied voltage is usually greater than the theoretical voltage. The excessive potential severely hinders the occurrence of the HER and OER, leading to unsatisfactory water splitting efficiency.^23–29 Hence, it is a feasible energy conversion strategy to reduce the potential barrier and energy consumption by adding an electrocatalyst to promote the electrolysis of water. Currently, noble metal Pt-based electrocatalysts are the most efficient catalysts for the HER.^30–34 Nevertheless, the scarcity and high cost of precious metal materials have been the main obstacles to their commercial application.^35–37 Accordingly, exploiting efficient, cheap, and abundant reserve-based non-noble metal-based electrocatalysts is greatly significant for the development of water splitting.

Transition metal sulfides (TMSs), with their abundance of valences, tunable metal centers, high electrical conductivity and structural tunability, have great potential for the HER.^38–47 However, the practical application of TMSs in water splitting is limited by their relatively rare catalytic sites and slow reaction kinetics. There are generally two strategies for increasing the intrinsic activity and reaction rate of an electrocatalyst: (1) increasing the number of sites and the intrinsic activity of each active site on the catalyst surface and (2) improving the overall electrical conductivity of the catalyst. Of course, these two strategies are not mutually exclusive and can be used simultaneously to increase catalytic activity.^17,48–51 Self-supported electrodes are a new class of electrodes developed in recent years, and they usually grow catalysts on conductive substrates (e.g., nickel foam, copper foam, carbon cloth, etc.). Compared with traditional powder electrocatalysts, self-supported electrodes have obvious advantages:^52,53 (1) the three-dimensional porous conductive substrate can provide a large electrochemical surface area and conductivity, which is favorable for the penetration of the electrolyte and the release of bubbles; (2) a variety of mature synthesis techniques have been used for the controllable conformation of nanoarray structures on the substrate, which can enrich the catalytically active sites and promote the material and electron transport rates; and (3) the use of adhesives is avoided, which improves the electrical conductivity and reduces the cost. Based on the above advantages, researchers have developed a series of TMS self-supported electrodes and achieved remarkable results, with catalytic activities comparable to or even exceeding the noble metal catalyst benchmarks.

In this paper, we review the progress of research on TMS electrode materials for the electrocatalytic HER. First, we briefly discuss the catalytic mechanism of the HER for the electrolysis of water in alkaline and acidic media. Then, we outline the effective strategies proposed by researchers in recent years to improve the activity and stability of TMS self-supported electrodes by modifying the intrinsic activities and increasing the number of active sites, which specifically include chemical doping, facet modulation, heterogeneous engineering, defect engineering, and morphology modulation. Finally, the challenges and opportunities of TMS self-supported electrodes in the future hydrogen production technology from electrolytic water splitting are summarized.

Electrochemical water splitting

The mechanism of the hydrogen evolution reaction (HER)

HER is a multi-step reaction consisting of adsorption, reduction and desorption processes. It is a dual electron transfer process and depends on the pH of the electrolyte. The HER can be carried out by either the Volmer–Tafel mechanism or the Volmer–Heyrovsky mechanism. The total reaction, elementary reaction and reaction mechanism of water splitting under acidic and alkaline conditions are shown in Table 1.⁵⁴

Table 1 HER mechanism under acidic and alkaline conditions

	Acid	Alkaline	Tafel slope
* represents the surface active sites of the catalyst.
Overall	2H^+ + 2e^− → H2	2H2O + 2e^− → H2 + 2OH^−	–
Volmer	H3O^+ + * + e^− → H* + H2O	H2O + e^− + * → H* + OH^−	120 mV dec^−1
Heyrovsky	H3O^+ + H* + e^− → H2 + H2	H* + H2O + e^− → H2 + OH^−	40 mV dec^−1
Tafel	H* + H* → H2	H* + H* → H2	30 mV dec^−1

---

The value of the Tafel slope can be used as a criterion to judge the mechanism of HER and the speed determination step. If the first Volmer step is the rate-limiting step, the Tafel slope is 120 mV dec⁻¹; if the second Heyrovsky step is the rate-limiting step, the Tafel slope is 40 mV dec⁻¹; if the third Tafel step is the rate-limiting step, then the Tafel slope is 30 mV dec⁻¹. What's more, the smaller the value of Tafel slope, the faster the kinetics of the HER.

No matter which mechanism is used for the HER, hydrogen atoms must be adsorbed onto the surface of the catalyst to react, so the speed of the whole reaction depends on the free energy of hydrogen adsorption (ΔG_H^*).^55–57 If the adsorption is too weak, the Volmer step will limit the overall reaction rate. If the adsorption is too strong, the desorption step (the Heyrovsky step/the Tafel step) will be the determining step. In the experiment, it is difficult to directly measure the exact value of the bond energy between the catalytic surface and the adsorbed intermediate. According to the free energy phase diagram of the HER, the ΔG_H^* value calculated by the density functional theory (DFT) is correlated with the catalytic activity measured by an experiment to form a “volcano curve” (Fig. 1).⁵⁸ ΔG_H^* can be used as an ideal descriptor to describe the activity trend of the catalyst. As a result, the hydrogen adsorption energy of an excellent HER catalyst should be close to ΔG_H = 0.¹⁷

Fig. 1 Schematic diagram of a HER volcanic curve. Reproduced with permission from ref. 40, copyright 2015, Wiley-VCH.

Modulation strategies for improving HER catalytic performance

Currently, studies on TMS electrocatalysts are mainly focused on metal sulfides composed of the first-row transition metal elements (Fe, Co, Ni, Cu, Mn, and Zn), the fifth subgroup elements (V, Nb, and Ta), and the sixth subgroup elements (Mo and W) according to the positional division. TMSs usually exhibit high intrinsic catalytic activity due to the interaction between the empty or unpaired p orbitals of the nonmetallic element S and the d orbitals of the metal elements that can stabilize the intermediates in the water splitting reaction. In recent years, researchers have reported a series of strategies to improve the catalytic activity of TMSs for water splitting, such as chemical doping, facet engineering, morphology modulation, heterogeneous engineering and defect engineering (Fig. 2). Next, we will give a detailed introduction to some typical representative metal sulfide materials.

Fig. 2 Summary diagram of strategies for improving the intrinsic activity of TMS electrocatalytic water splitting.

Morphology modulation

In recent years, modulating the morphology of materials is one of the effective ways to improve the water splitting performance of electrocatalysts. In recent years, researchers have reported transition metal sulfide electrocatalysts with different morphology structures, for example, one-dimensional (1D) CoS₂ nanowires⁵⁹ and Co₉S₈ nanowires,⁶⁰ two-dimensional (2D) Co-MoS₂/NiS₂ nanosheets,⁶¹ and three-dimensional (3D) coral-like NiS/FeS₂.⁶² Therefore, by modulating the morphology and structure of the electrocatalysts, the active specific surface area of the catalysts can be effectively increased to expose more active sites; at the same time, the diffusion resistance of the electrolyte can be reduced to enhance the diffusion of ions and the rapid transfer rate of electrons.

MoS₂ as a typical layered transition metal sulfide electrocatalyst has attracted much attention from researchers. Hinnemann et al. first recognized that it is the edge structure of MoS₂ that is the active site for the HER, while the basal surface is electrochemically inert.⁶³ Jaramillo et al. reported that the HER activity of MoS₂ is proportional to its edge length and independent of the surface area.⁴¹ Since then, in order to maximize the catalytic activity of MoS₂, researchers have regulated it through both the activation of the inert basal surface or the optimization of the edge active sites. Among them, one of the most effective ways to increase the number of edge-active sites of MoS₂ is to nanosize MoS₂ to obtain ultrathin or monolayer MoS₂ nanosheets.^64–66 In addition, the metallic state of VS₂ is similar to that of MoS₂ in that its edge structure directly affects the HER catalytic activity. Xu et al. grew self-assembled VS₂ micron flower self-supported electrodes with a few layers of MoS₂ nanoribbons embedded in Mo foil (VS₂/MoS₂/MF). This unique micron flower can expose abundant edge structures and increase the number of active sites of the catalyst, and the strong electronic interactions between the heterogeneous interfaces of VS₂ and MoS₂ greatly contribute to the HER catalytic performance of VS₂/MoS₂/MF.⁶⁷ Shi et al. transformed CoMoO₄ nanosheet arrays grown on nickel foam into 3D flower-like CoS₂/MoS₂ heterogeneous nanosheet arrays (CoS₂/MoS₂ HNSAs) composed of ultrathin nanosheets by in situ topological vulcanization (Fig. 3a). The 3D flower-like structure provides a large specific surface area and more active sites, which is favorable for creating a contact between the electrocatalyst and the electrolyte and the degassing of gas bubbles. CoS₂/MoS₂ HNSAs exhibit excellent HER activity and reaction kinetics at 1.0 M KOH, with overpotentials as low as 50 mV and a Tafel slope of 76 mV dec⁻¹ at 10 mA cm⁻².⁶⁸

Fig. 3 SEM images of (a) CoS₂/MoS₂ HNSAs; reproduced with permission from ref. 50, copyright 2020, Royal Society of Chemistry. (b) A-MoS₂-Ni₃S₂-NF; reproduced with permission from ref. 53, copyright 2023, Wiley-VCH. (c) CS–NS/NF; reproduced with permission from ref. 54, copyright 2022, Royal Society of Chemistry. (d) V-Ni₃S₂@NiO/NF; reproduced with permission from ref. 56, copyright 2020, American Chemical Society.

The development of hierarchical nanostructured electrocatalysts is one of the effective ways to maximize the efficiency of water splitting. Yang et al. prepared Ni₃S₂/VS₄ nanohorn-array electrocatalysts with rhinoceros horn-like hierarchical structures via a self-driven synthesis strategy and achieved a synergistic optimization of the catalytic material geometry and the surface/interface, which greatly improved the kinetics of substance transport, electron transfer, and intermediate adsorption in the water splitting.⁶⁹ Yang et al. grew a multilayered vanadium-doped Cu₂S nanowall array (V-Cu₂S-Nanowires NW) self-assembled from nanowires in situ on copper foam. Notably, the nanowires were assembled from nanoparticles. Due to its unique hierarchical structure that fully exposes the catalytically active sites and provides abundant electron/ion transport pathways, V-Cu₂S-Nanowires NW exhibits excellent HER activity and reaction kinetics.⁷⁰ A sea urchin-like MoS₂–Ni₃S₂ heterostructure self-supported electrode (A-MoS₂-Ni₃S₂-NF) was reported by Hu et al. (Fig. 3b). The coupling of the amorphous MoS₂ nanoshells with the highly conductive Ni₃S₂ cores resulted in an electron-rich state on the electrode surface, which has a strong binding energy for H_ads and can promote the dissociation of water. The overpotentials of the A-MoS₂-Ni₃S₂-NF electrode at a current density of 10 mA cm⁻² were 95 mV (1.0 M KOH) and 145 mV (0.5 M H₂SO₄).⁷¹

In addition, core–shell structured catalysts are a very promising class of materials for water electrolysis. On the one hand, core–shell heterogeneous materials have strong electronic interactions and abundant active sites, which improve the catalytic activity of the materials. On the other hand, the external shell can protect the internal nucleus, thus improving the stability. Liu et al. generated a superhydrophilic core–shell structured nanorod integrated electrode (CS-NS/NF) in situ on the surface of nickel foam (Fig. 3c). The nanorod was composed of amorphous VO_x nanoshells and crystalline Ni₃S₂ cores. It was demonstrated that the metallic Ni₃S₂ core improved the conductivity of the external VO_x, while the amorphous VO_x exhibited good hydrophilicity, thereby facilitating water adsorption, which significantly improved the electron transfer efficiency and electrocatalytic activity of the HER.⁷² Li et al. prepared a cable-like 1D core–shell nanostructured catalyst (Ni₃S₂@NGCLs/NF) consisting of an internal Ni₃S₂ nanowire and an external N-doped graphene carbon layer (NGCLs) on Ni foam. The existence of highly conductive NGCL shells not only improves the electrical conductivity of the Ni₃S₂ nucleus, but also compresses the lattice of Ni₃S₂ to reduce the adsorption energy of hydrogen, thus increasing the hydrogen evolution activity of Ni₃S₂@NGCLs/NF.⁷³ Liu et al. proposed a V doping-triggered self-assembly strategy to prepare a novel dendritic V-Ni₃S₂@NiO/NF core–shell nano-array self-supported electrode (Fig. 3d). The 3d multistage structure can not only provide more catalytic sites but also facilitate the contact with the electrolyte. In addition, the strong hydrophilic V-NiO surface is conducive to the adsorption of H₂O and OH⁻ by the catalyst, thus accelerating the adsorption/dissociation kinetics of water.⁷⁴

Chemical doping

In recent years, a typical method has been proposed to improve the overall water splitting of TMSs by introducing heteroatoms. Chemical doping includes the regulation of the energy band structure, d-band center, valence state of the active site and charge distribution. This method can effectively improve the intrinsic density of active sites of electrocatalysts and optimize the generation energy of intermediates, thus promoting the evolution of H₂ and O₂.

Qu et al. reported that V doping in Ni₃S₂ greatly increases the free carrier density near the Fermi level, thereby reducing the initial potential and increasing the conductivity (Fig. 4a).⁷⁵ Jian et al. prepared a tremella-like Sn-doped Ni₃S₂ ultra-thin nanosheet on Ni foam by a mild one-step hydrothermal method. It can be seen from the XPS results that the introduction of Sn reduces the content of Ni⁰ in Ni₃S₂, which is attributed to the fact that the introduction of Sn changes the coordination structure of S and transfers electrons from Ni to S, resulting in more electron vacancy structures at the Ni site, which is conducive to rapid electron transfer. The optimized Sn–Ni₃S₂/NF exhibits excellent hydrogen production performance (279 mV@300 mA cm⁻²) and long-term stability over 60 h at 200 mA cm⁻²(Fig. 4b and c).⁷⁶ Zhang et al. reported that Fe doping can not only increase the electrochemical active area of Ni₃S₂, but also improve the adsorption energy of H₂O and H* on the catalyst surface, thus optimizing the HER kinetics and catalytic activity of Ni₃S₂. The optimal Fe_17.5%–Ni₃S₂/NF showed the best HER activity in 1 M KOH electrolyte (47 mV@10 mA cm⁻²) (Fig. 4d–f).⁷⁷ Yan et al. grew Cd-doped Ni₃S₂ nanosheet arrays on Ni foams (Cd-Ni₃S₂/NF-x) by a simple aqueous method. It is worth noting that during the HER process, Cd doping did not change the active site of Ni₃S₂, and the H atom was still adsorbed onto the Ni site to form a Ni–H bond, but the length of the Ni–H bond increased from 1.476 Å to 1.546 Å, and the adsorption energy of hydrogen decreased, thus improving the HER catalytic activity of Ni₃S₂.⁷⁸

Fig. 4 (a) Relaxed bulk structure of VO-co-doped Ni₃S₂ in a 221 supercell with one Ni–S pair replaced by a VO pair and the calculated partial density of states of (a₃) p-Ni₃S₂, (a₄) O-doped Ni₃S₂, (a₅) V-doped Ni₃S₂, (a₆) VO–Ni₃S₂ and (a₇) V–O–Ni₃S₂; reproduced with permission from ref. 57, copyright 2017, American Chemical Society. (b) iR-correction LSV curves of NF, Ni₃S₂/NF, Sn-Ni₃S₂/NF, and 20 wt% Pt/C/NF. (c) i–t test of Sn-Ni₃S₂/NF for 60 h at −0.223 V; reproduced with permission from ref. 58, copyright 2018, American Chemical Society. (d) iR-corrected LSV curves of Fe–Ni₃S₂/NF with various Fe doping levels, Fe–Ni₃S₂/NF and Pt/C/NF for the HER in 1 M KOH. (e) Calculated free energy diagram of the HER over the {−210} plane of Ni₃S₂ and Fe-Ni₃S₂. (f) Calculated water adsorption energy of Ni₃S₂ and Fe-Ni₃S_2; reproduced with permission from ref. 59, copyright 2018, American Chemical Society.

In addition to metal atom doping, the introduction of more electronegative non-metallic atoms into the catalysts can effectively replace the anionic sites to modulate the electronic structure and weaken the metal–anion interactions, thus optimizing the hydrogen adsorption/desorption energies on the surface of the electrocatalysts, and thus increasing the HER activity. Kou et al. prepared N-doped Ni₃S₂ nanosheet self-supported electrodes (N-doped Ni₃S₂/NF) using an ammonia treatment strategy. Experimental results and theoretical analysis showed that the introduction of N not only increased the number of active sites but also optimized the H adsorption energy on the Ni₃S₂ surface.⁷⁹ He et al. reported an F-doped Ni₃S₂ nanosheet array self-supported electrode (F-Ni₃S₂/NF) by introducing F (3.98), which has an electronegativity greater than that of S (2.58), into Ni₃S₂. The introduction of F not only accelerated the dissociation of water molecules from the surface of the catalysts, but also optimized the ΔG_H^*, which resulted in an excellent HER (38 mV@10 mA cm⁻²) activity for the F-Ni₃S₂/NF electrode at 1 M KOH solution, which is superior to that of commercial Pt/C catalysts (50 mV@10 mA cm⁻²).⁸⁰ Qian et al. prepared a P-doped biphasic (1T/2H) MoS₂ electrocatalyst (P-BMS) using a mild hydrothermal method. The optimized P-BMS exhibits low overpotentials (60 mV@10 mA cm⁻² and 72 mV@10 mA cm⁻²) in H₂SO₄ (0.5 M) and KOH (1.0 M), respectively. The DFT results confirmed that the introduction of P modulated the electronic structure of MoS₂ and optimized the adsorption of H* during the HER, which accelerated the catalysis kinetics.⁸¹ In addition, Xiang et al. synthesized an N,P co-doped Co₉S₈ self-supported electrode (N,P-Co₉S₈) on cobalt foam. N,P-Co₉S₈ exhibits excellent HER activity with overpotentials of 80 and 189 mV at current densities of 10 and 100 mA cm⁻² in 1.0 M KOH solution. N and P introduction significantly improved the wettability of the electrode and promoted water adsorption and bubble detachment, thus accelerating the HER kinetics.⁸²

Although the introduction of heteroatoms can effectively regulate the structure and properties of TMSs, not all doping is positive, and so the choice of doping atoms is very important. In addition, different doping methods and doping amounts lead to different results, and the internal mechanism after doping needs further in-depth study.

Heterogeneous engineering

It is well known that heterostructured catalysts consisting of two or more component catalysts exhibit superior catalytic activity compared to single-component catalysts. In addition, heterogeneous structures can play an important role in more novel functions. Therefore, heterogeneous engineering is an effective strategy to promote the industrial application of water splitting.

The electronic interactions between heterogeneous structures can change the electron density distribution of the electrocatalysts, thereby optimizing H adsorption and desorption. Ji et al. designed and synthesized an ultrathin porous δ-FeOOH-modified Ni₃S₂ 3D heterostructured composite catalyst, which exhibits highly efficient water splitting performance in alkaline electrolytes through synergistic adsorption at the interfacial structure. δ-FeOOH has a good ability to adsorb H₂O molecules, while the bridging S atoms at the composite interface have a better ability to adsorb H, which makes H₂O molecules easy to dissociate, thus improving the performance of the HER in alkaline solutions (Fig. 5a). The results showed that the three-dimensional δ-FeOOH/Ni₃S₂ composite catalysts exhibit excellent HER performance in alkaline electrolytes (106 mV@10 mA cm⁻²).⁸³ Metal sulfides are prone to form S–H_ads bonds on their surfaces during the HER, which are favorable for the adsorption of H intermediates to the catalyst. However, the S–H_ads bonds on the surface of metal sulfides are usually very strong, which is not favorable for the conversion of H_ads to H₂. Feng et al. reported a Cu nanodot-modified Ni₃S₂ nanotube catalyst loaded on carbon fibers (Cu NDs/Ni₃S₂ NTs/CFs). The interaction of electrons between the metal Cu and Ni₃S₂ in Cu NDs/Ni₃S₂ NTs/CFs resulted in the transfer of electrons from Cu to Ni₃S₂, attenuating the S–H_ads bonds and optimizing the adsorption and desorption of H (Fig. 5b).⁸⁴ Wu et al. prepared Ni_xCo_3−xS₄/Ni₃S₂/NF electrode materials by the cation-exchange reaction, and both experimental results and theoretical calculations demonstrated that the heterostructured materials possessed high intrinsic activity, metallicity, and abundance of reactive heterointerfaces, which synergistically improved the overall electrolytic performance of the materials in the electrolysis of water.⁸⁵ Chen et al. constructed a one-dimensional heterostructured Ni₃S₂/NiS electrocatalyst on nickel foam (NF) via a solid-state phase transition strategy. Thanks to the strong electronic interactions at the Ni₃S₂/NiS heterogeneous interface, the intrinsic activity and durability of the catalyst are significantly improved (compared to a single phase). In addition, the downward shift of the d-band center of Ni₃S₂/NiS optimized the adsorption of H at the Ni sites, thus improving the HER reaction kinetics.⁸⁶

Fig. 5 (a) DFT+U calculation results for Ni₃S₂ and δ-FeOOH/Ni₃S₂; structure model and schematic illustration of the HER pathway of Ni₃S₂/FeOOH. Color scheme for chemical representation: grey for Ni, brown for Fe, red for O, yellow for S, and white for H atoms, respectively. Free energy diagram of the HER process for Ni₃S₂ and δ-FeOOH/Ni₃S₂; reproduced with permission from ref. 65, copyright 2020, Royal Society of Chemistry. (b) Schematic illustration of water adsorption, water activation, and hydrogen generation processes of Cu/Ni₃S₂; reproduced with permission from ref. 66, copyright 2018, American Chemical Society. (c) TEM images of the products obtained in the presence of AHM for 24 h; reproduced with permission from ref. 69, copyright 2017, American Chemical Society. (d) HER polarization curves of Ni₃S₂@NiV-LDH/NF, NiV-LDH/NF, Ni₃S₂/NF, NF and 20%Pt/C/NF in 1.0 M KOH; reproduced with permission from ref. 70, copyright 2019, Royal Society of Chemistry.

In addition, the active edges can be increased between the heterogeneous interfaces, and the strong electron interactions are conducive to the production of higher catalytic activity. Yang et al. prepared a MoS₂–Ni₃S₂ HNRs/NF heterogeneous nanorods self-supporting electrode using an environment friendly one-step hydrothermal method. The electrode is a hierarchical nanostructure uniformly composed of 1D Ni₃S₂ and 2D MoS₂, which can give full play to the catalytic activity of Ni₃S₂ and MoS₂. More importantly, the heterogeneous interface between them also provides a large number of active sites. As expected, MoS₂–Ni₃S₂ HNRs/NF serves as both the anode and cathode for overall water splitting, and it can provide a 10 mA cm⁻² current density with only 1.50 V cell voltage and has remarkable durability (>48 h) (Fig. 5c).⁸⁷ Liu et al. prepared a novel Ni₃S₂@NiV-LDH heterostructured nanosheet array using an in situ anion-exchange reaction. It was explored and verified that the active edge state length and surface chemical state of this heterostructure of Ni₃S₂@NiV-LDH can be well regulated by the heterogeneously coupled interface, which greatly facilitates the water splitting kinetics. The optimized Ni₃S₂@NiV-LDH/NF has excellent HER properties (126 mV@10 mA cm⁻² and 256 mV@100 mA cm⁻²), and its activity at a high current density is higher than that of the precious metal Pt/C. (Fig. 5d).⁸⁸

The construction of heterogeneous catalysts effectively optimizes the kinetic process of electrolytic water splitting, mainly due to the electron transfer, interfacial bonding and lattice strain at the interface, which leads to the optimization of the electronic structure of the catalysts, and the synergistic effect between different components greatly improves the intrinsic activity of the catalysts. Before the structural design, theoretical calculation can help us understand the structural design of the catalyst more effectively, but it is very important to narrow the gap between the model and the actual structure, which can help us further understand the mechanism of improving the electrocatalytic performance more accurately and deeply.

Facet engineering

As we all know, the electrocatalytic reaction takes place on the surface of the catalyst, so the atomic structure and arrangement on the surface of the electrocatalyst directly affect the catalytic activity. The high-index crystal planes of materials are usually rich in low coordination atoms and active sites, which can show higher catalytic activity for water splitting. Therefore, designing and synthesizing materials with high-index crystal planes exposure is one of the effective methods to improve the electrocatalytic performance.

In 2015, Feng et al. prepared controllable Ni₃S₂ nanosheet array materials with different exposed crystal surfaces grown on nickel foam (Ni₃S₂/NF) through an in situ sulfurization strategy and proposed that the HER activity of high-index faceted Ni₃S₂ was better than that of low-index faceted Ni₃S₂, and the theoretical calculations confirmed that the exposed high-index facets could reduce the coordination number of Ni and S sites and decrease the ΔG_H^*, which is conducive to the HER performance (Fig. 6a and b).⁸⁹ Li et al. reported ultrathin and abundant CuFeS₂ nanosheets (NSs) with high index exposed {02̄4} facets. The CuFeS₂ nanosheets have excellent HER reaction kinetics due to the high density of S₂-active sites on the exposed high-index {02̄4} facets with good free energy for the adsorption of hydrogen atoms.⁹⁰ Deng et al. reported a TiO₂@Ni₃S₂ self-supported electrode synthesized under low-temperature vulcanization conditions. The electrode is composed of atomic layer deposited (ALD) TiO₂ cores and Ni₃S₂ nanobranches with the high-index crystal plane {210}, abundant pore structures and high active sites (Fig. 6c). As a result, TiO₂@Ni₃S₂ exhibits excellent electrocatalytic activity in alkaline media, with an overpotential of only 112 mV for the HER at a current density of 10 mA cm⁻². More importantly, when TiO₂@Ni₃S₂ is used as the anode and cathode respectively to form an electrolytic cell, its water splitting activity is better than many reported catalysts (Fig. 6d).⁹¹ Dong et al. synthesized (003) facet-exposed Ni₃S₂ nanoporous films on nickel foil (Ni₃S₂ NTFs). The (003) facet exposure favored the formation of Ni₃ triangles, which not only provided more metallic Ni–Ni bonds but also adsorbed water molecules and OH⁻ more readily, improving the electrical conductivity of Ni₃S₂ NTFs while promoting water splitting.⁹² Huang et al. synthesized the high-index crystal plane (400) exposed NiCo₂S₄ electrocatalyst by a one-pot hydrothermal method using polyvinylpyrrolidone (PVP) as a surfactant. By regulating the amount of PVP added, the transition of NiCo₂S₄ from the (200) crystalline faceted exposed lamellar structure to a high-exposure (400) crystalline surface to a microsphere structure was observed. Thanks to the unique submicrospheric structure, NiCo₂S₄ has abundant active sites and fast mass/electron transfer efficiency. In addition, the (400) crystalline facet exposure can provide more S²⁻ and effectively stabilize the HER intermediates, thus improving the catalytic activity and stability of the HER.⁹³ Shi et al. designed a NiO nanolayer-coated NiWO₄-coupled Ni₃S₂ nanofiber self-supporting electrode (NiWO₄-Ni₃S₂@NiO/NF) by a two-step method. DFT calculations confirmed that the increase in electron density on the high-index crystal plane {123} and Ni₃S₂ at the Ni site induces a more suitable H adsorption energy and leads to a positive shift of the d-band center of Ni₃S₂ towards the Fermi energy level, which is responsible for the promoted activity.⁹⁴

Fig. 6 (a) Most stable terminations of the (2̄10) and (001) surfaces of Ni₃S₂. (b) Calculated free-energy diagram of HER over the (2̄10) and (001) surfaces at equilibrium potential. The blue and yellow spheres represent Ni and S atoms, respectively; reproduced with permission from ref. 71, copyright 2015, American Chemical Society. (c) Schematic illustration of the core/branch structure of the TiO₂@Ni₃S₂ arrays. (d) LSV curves of the Ni₂(OH)₂CO₃, Ni₃S₂, and TiO₂@Ni₃S₂ electrodes; reproduced with permission from ref. 73, copyright 2019, Springer.

Therefore, the synthesis of exposed TMSs with specific crystal planes can enrich the low coordination metal atoms and S vacancies on the catalyst surface, thus improving the catalytic activity. However, the synthesis method of this kind of sulfide is still challenging, mainly because the surface energy of nano-materials is high, and it is difficult to control the exposure of crystal facets during the preparation process.

Defect engineering

The existence of defects can break the periodic crystal structure of a material, which can not only provide more active sites and improve the conductivity of the material, but also adjust the electronic structure of the surface of the material through local electron redistribution to achieve the purpose of improving the performance of the catalyst.

Liu et al. obtained V-doped NiS₂ nanosheets by sulfurization with NiV-LDH as the precursor. Notably, experimental results and DFT calculations showed that the engineering of the V substitution defects enabled the electronic structure remodeling of NiS₂ to change from semiconducting to metallic properties, improving the electrical conductivity of NiS₂ (Fig. 7a and b).⁹⁵ Guo et al. obtained Ni–Co bimetallic sulfide (NiCo₂S₄) nanosheet arrays with controlled sulfur vacancies by multiple vulcanization of Ni–Co hydroxide precursors grown on carbon cloth. Thanks to the rapid electron transfer, NiCo₂S₄ with the best sulfur vacancy content exhibits better HER reaction kinetics in alkaline media with a Tafel slope of 82.5 mV dec⁻¹ (Fig. 7c and d).⁹⁶ MoS₂ and Fe-MOF with abundant active sites were grown in situ on nickel foam (Fe-MOF@MoS₂-6 h) by a two-step solvothermal method by R. Velayutham et al. The formation of oxygen/sulfur vacancies by partial oxidation of Fe/Mo surfaces resulted in a significant enhancement of the HER activity.⁹⁷ An et al. successfully constructed Ni₃S₂/MoS₂ catalysts with sulfur vacancies and non-homogeneous phase interfaces (VS-Ni₃S₂/MoS₂) on Ni foam by hydrothermal and hydrogen reduction methods. The abundant sulfur vacancies and heterogeneous interfaces provided a large number of active sites for the VS-Ni₃S₂/MoS₂ catalysts, which improved the electron and ion transport and thus enhanced the catalytic activity of electrolytic water.⁹⁸

Fig. 7 (a) Schematic diagram of the overall water splitting of 10% VNS. (b) Calculated DOS of 10% VNS; reproduced with permission from ref. 77, copyright 2017, American Chemical Society. (c) EPR spectra of NiCo₂S₄-1/CC, NiCo₂S₄-2/CC, and NiCo₂S₄-3/CC. (d) Tafel plots of three samples; reproduced with permission from ref. 83, copyright 2020, Royal Society of Chemistry. (e) Schematic illustration for the preparation of TaS_x nanosheets and (f) HRTEM of TaS_x NSs; reproduced with permission from ref. 78, copyright 2022, American Chemical Society.

In addition, the use of defect engineering to regulate the electronic structure of materials is expected to be an effective strategy to improve the electrocatalytic activity. Du et al. reported that secondary in situ sulfurization of Ni–Co hydroxide precursors yielded Co₉S₈/Ni₃S₂ nanowire-array electrocatalysts with a foamy shape. A large number of defects existed between the interfaces of the two phases, and both experimental results and theoretical calculations proved that they could optimize the electronic structure of the material, thereby reducing the chemisorption energy for H and OH.⁹⁹ A multicomponent 1T-MoS₂/Co₃S₄/Ni₃S₂ nano-array self-supported electrode was reported by Chen et al. The abundant interfaces and defects among the multicomponents provided abundant active sites, and the strong interactions among different components optimized the electronic structure of the catalyst, which synergistically enhanced the catalytic activity of 1T-MoS₂/Co₃S₄/Ni₃S₂ for electrolytic water splitting.¹⁰⁰ Xiao et al. pyrolyzed the bimetallic CuCo₂S₄ to construct the S vacancy-rich heterostructure of Cu_1.96S/Co₉S₈. DFT theoretical calculations demonstrated that the S vacancies and heterointerfaces in Cu_1.96S/Co₉S₈ were able to optimize the electronic structure of the catalyst and the adsorption of HER intermediates. The symmetric two-electrode cell of Cu_1.96S/Co₉S₈ has a current density of 10 mA cm⁻² at a low voltage of 1.43 V, which is far superior to most of the hydrolysis electrocatalysts reported so far, and has a long-term stability of 200 h.¹⁰¹ Cao et al. first used quaternary ammonium cations to intercalate TaS₂ crystals to expand their lattice and then obtained TaS_x nanosheets (TaS_x NSs) enriched with S vacancies by electrochemical desulfurization (Fig. 7e and f). TaS_x NSs exhibited remarkable HER activity (99 mV@10 mA cm⁻²) and fast HER kinetics (39 mV dec⁻¹). Theoretical calculations demonstrated that the S vacancies in TaS_x NSs could facilitate the electron transfer between H and TaS_x, thus lowering the overpotential and promoting hydrogen generation.¹⁰²

Conclusion and perspective

In summary, TMSs are a class of low-cost electrocatalysts with excellent water splitting activity and stability. Based on the research results in recent years, this paper summarizes different modification methods to improve the intrinsic activity of TMSs, including chemical doping, facet engineering, morphology modulation, heterogeneous engineering and defect engineering. In order to design more active TMSs and collaboratively improve the conductivity and the number of active sites of the electrocatalysts, different optimization strategies can be combined to maximize the efficiency of water splitting.

Therefore, when exploring new and efficient TMS catalysts, the following principles should be considered: (1) the intrinsic activity of the material, which determines the innate catalytic activity advantage of the material; (2) increasing the number of active sites of the material, which helps the catalyst to adsorb water molecules and intermediates to increase the catalytic activity; (3) increasing the electrical conductivity of the material, which is conducive to the transfer of electrons during the catalytic process and the reduction of the overpotential; (4) giving full consideration to the size of the catalyst reaction energy barrier and the adsorption/desorption capacity of the reaction intermediates; and (5) optimizing the substance and mass transport of the material to promote the penetration of the electrolyte and the release of gas bubbles. In addition, more simple and easy-to-scale synthesis methods should be designed and developed to reduce the cost of catalysts if TMSs are to be further commercialized. Currently, many in situ characterization methods have been applied to the electrocatalytic HER under laboratory conditions, such as electrochemically compatible in situ X-ray absorption spectroscopy and in situ Raman spectroscopy. However, there is a huge difference between the laboratory size and industrial conditions. Under strict industrial test conditions (higher temperatures, electrolyte concentrations, etc.), dynamic changes in the structural stability, catalytic activity, and ion mobility rate of the electrolyte can limit the in situ characterization techniques. Therefore, the further development of advanced characterization techniques at high current densities is helpful for the optimal design of electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22179074, 52174151, and U22A20144), the Natural Science Foundation of Shaanxi Province (2023-JC-QN-0497), the High-level Talents Foundation for Scientific Research of Xi'an University of Science and Technology (2050122011), the Jiangsu Provincial Key Laboratory of Micro-nano Biomedical Devices and Manufacturing Open Research Fund (No. KF201801), and the Young Talent Fund of Xi'an Association for Science and Technology.

References

1. Q. Yu, Z. Zhang, S. Qiu, Y. Luo, Z. Liu, F. Yang, H. Liu, S. Ge, X. Zou, B. Ding, W. Ren, H.-M. Cheng, C. Sun and B. Liu, Nat. Commun., 2021, 12, 6051.
2. X. Du, J. Huang, J. Zhang, Y. Yan, C. Wu, Y. Hu, C. Yan, T. Lei, W. Chen, C. Fan and J. Xiong, Angew. Chem., Int. Ed., 2019, 58, 4484–4502.
3. J. Kim, H. Kim, G. H. Han, S. Hong, J. Park, J. Bang, S. Y. Kim and S. H. Ahn, Exploration, 2022, 2, 20210077.
4. T. He, P. Pachfule, H. Wu, Q. Xu and P. Chen, Nat. Rev. Mater., 2016, 1, 16059.
5. C. J. Winter, Int. J. Hydrogen Energy, 2009, 34, S1–S52.
6. S. Chu and A. Majumdar, Nature, 2012, 488, 294–303.
7. Y. Liu, J. Wu, K. P. Hackenberg, J. Zhang, Y. M. Wang, Y. Yang, K. Keyshar, J. Gu, T. Ogitsu, R. Vajtai, J. Lou, P. M. Ajayan, B. C. Wood and B. I. Yakobson, Nat. Energy, 2017, 2, 17127.
8. L. Yu, B. Y. Xia, X. Wang and X. W. Lou, Adv. Mater., 2016, 28, 92–97.
9. X. Liang, K. X. Zhang, Y. C. Shen, K. Sun, L. Shi, H. Chen, K. Y. Zheng and X. X. Zou, J. Electrochem., 2022, 28, 1–30.
10. M. Zhang, Q. Liu, W. Sun, K. Sun, Y. Shen, W. An, L. Zhang, H. Chen and X. Zou, Chem. Synth., 2023, 3, 28.
11. Q. Chen, J. Huang, X. Li, M. Niu, K. Kajiyoshi, Y. Zhao, Z. Cheng, T. Liu, L. Cao and L. Feng, Surf. Interfaces, 2023, 42, 103406.
12. Y. Z. Wang, M. Yang, Y. M. Ding, N. W. Li and L. Yu, Adv. Funct. Mater., 2022, 32, 2108681.
13. L. Du, H. Xiong, H. Lu, L. M. Yang, R.-Z. Liao, B. Y. Xia and B. You, Exploration, 2022, 2, 20220024.
14. P. Krishnamurthy, A. Kumar, S. A. Alqarni, S. Silambarasan and T. Maiyalagan, Surf. Interfaces, 2024, 44, 103694.
15. R. He, C. Wang and L. Feng, Chin. Chem. Lett., 2023, 34, 107241.
16. Y. Song, K. Ji, H. Duan and M. Shao, Exploration, 2021, 1, 20210050.
17. J. D. Benck, T. R. Hellstern, J. Kibsgaard, P. Chakthranont and T. F. Jaramillo, ACS Catal., 2014, 4, 3957–3971.
18. Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev., 2015, 44, 2060–2086.
19. X. Wu, S. Zhao, L. Yin, L. Wang, L. Li, F. Hu and S. Peng, Chin. Chem. Lett., 2023, 34, 108016.
20. Y. Sun, S. Xu, C. A. Ortíz-Ledón, J. Zhu, S. Chen and J. Duan, Exploration, 2021, 1, 20210021.
21. K. K. Joshi, P. M. Pataniya, V. Patel and C. K. Sumesh, Surf. Interfaces, 2022, 29, 101807.
22. Z. Zhang, Y. Lei and W. Huang, Chin. Chem. Lett., 2022, 33, 3623–3631.
23. K. Zeng and D. Zhang, Prog. Energy Combust. Sci., 2010, 36, 307–326.
24. X. Li, L. Zhao, J. Yu, X. Liu, X. Zhang, H. Liu and W. Zhou, Nano-Micro Lett., 2020, 12, 131.
25. B. Liu, Y. Cheng, B. Cao, M. Hu, P. Jing, R. Gao, Y. Du, J. Zhang and J. Liu, Appl. Catal., B, 2021, 298, 120630.
26. X.-P. Li, C. Huang, W. K. Han, T. Ouyang and Z.-Q. Liu, Chin. Chem. Lett., 2021, 32, 2597–2616.
27. A. A. Yadav, Y. M. Hunge and S. W. Kang, Surf. Interfaces, 2021, 23, 101020.
28. M. Du, D. Li, S. Liu and J. Yan, Chin. Chem. Lett., 2023, 34, 108156.
29. C. Meng, X. Chen, Y. Gao, Q. Zhao, D. Kong, M. Lin, X. Chen, Y. Li and Y. Zhou, Molecules, 2020, 25, 1136.
30. C. Tang, L. Zhong, B. Zhang, H. F. Wang and Q. Zhang, Adv. Mater., 2018, 30, 1705110.
31. A. Kumar, V. Q. Bui, J. Lee, L. Wang, A. R. Jadhav, X. Liu, X. Shao, Y. Liu, J. Yu, Y. Hwang, H. T. D. Bui, S. Ajmal, M. G. Kim, S.-G. Kim, G.-S. Park, Y. Kawazoe and H. Lee, Nat. Commun., 2021, 12, 6766.
32. J. Cao, Y. Zhang, C. Zhang, L. Cai, Z. Li and C. Zhou, Surf. Interfaces, 2021, 25, 101305.
33. C. Lin, Y. Liu, Y. Sun, Z. Wang, H. Xu, M. Li, J. Feng, B. Hou and W. Yan, Chin. Chem. Lett., 2023, 34, 108265.
34. D. He, L. Cao, L. Feng, S. Li, Y. Feng, G. Li, Y. Zhang, J. Li and J. Huang, Chin. Chem. Lett., 2022, 33, 4781–4785.
35. J. Li, G. Du, X. Cheng, P. Feng and X. Luo, Chin. J. Catal., 2018, 39, 982–987.
36. M. Huynh, D. K. Bediako and D. G. Nocera, J. Am. Chem. Soc., 2014, 136, 6002–6010.
37. P. Wang, X. Zhang, J. Zhang, S. Wan, S. Guo, G. Lu, J. Yao and X. Huang, Nat. Commun., 2017, 8, 14580.
38. B. You, M. T. Tang, C. Tsai, F. Abild-Pedersen, X. Zheng and H. Li, Adv. Mater., 2019, 31, 1807001.
39. C. C. Yang, S. F. Zai, Y. T. Zhou, L. Du and Q. Jiang, Adv. Funct. Mater., 2019, 29, 1901949.
40. Q. Xu, H. Jiang, H. Zhang, Y. Hu and C. Li, Appl. Catal., B, 2019, 242, 60–66.
41. T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100–102.
42. Z. Qiu, Y. Ma and T. Edvinsson, Nano Energy, 2019, 66, 104118.
43. B. You and Y. Sun, Acc. Chem. Res., 2018, 51, 1571–1580.
44. H. Sun, W. Zhang, J.-G. Li, Z. Li, X. Ao, K.-H. Xue, K. K. Ostrikov, J. Tang and C. Wang, Appl. Catal., B, 2021, 284, 119740.
45. S. Sultan, J. N. Tiwari, A. N. Singh, S. Zhumagali, M. Ha, C. W. Myung, P. Thangavel and K. S. Kim, Adv. Energy Mater., 2019, 9, 1900624.
46. J. Shan, T. Ling, K. Davey, Y. Zheng and S.-Z. Qiao, Adv. Mater., 2019, 31, 1900510.
47. Z. Yang, C. Zhao, Y. Qu, H. Zhou, F. Zhou, J. Wang, Y. Wu and Y. Li, Adv. Mater., 2019, 31, 1808043.
48. J. Duan, S. Chen, M. Jaroniec and S. Z. Qiao, ACS Catal., 2015, 5, 5207–5234.
49. Y. Gu, X. Wang, A. Bao, L. Dong, X. Zhang, H. Pan, W. Cui and X. Qi, Nano Res., 2022, 15, 9511–9519.
50. H. Sun, X. Xu, H. Kim, Z. Shao and W. Jung, InfoMat, 2023, e12494.
51. C. Wu, S. Xue, Z. Qin, M. Nazari, G. Yang, S. Yue, T. Tong, H. Ghasemi, F. C. R. Hernandez, S. Xue, D. Zhang, H. Wang, Z. M. Wang, S. Pu and J. Bao, Appl. Catal., B, 2021, 282, 119557.
52. J. Wang, N. Zang, C. Xuan, B. Jia, W. Jin and T. Ma, Adv. Funct. Mater., 2021, 31, 2104620.
53. H. Sun, Z. Yan, F. Liu, W. Xu, F. Cheng and J. Chen, Adv. Mater., 2020, 32, 1806326.
54. H. Jin, C. Guo, X. Liu, J. Liu, A. Vasileff, Y. Jiao, Y. Zheng and S.-Z. Qiao, Chem. Rev., 2018, 118, 6337–6408.
55. R. Parsons, Trans. Faraday Soc., 1958, 54, 1053–1063.
56. J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov and U. Stimming, J. Electrochem. Soc., 2005, 152, J23.
57. E. Skúlason, V. Tripkovic, M. E. Björketun, S. Gudmundsdóttir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jónsson and J. K. Nørskov, J. Phys. Chem. C, 2010, 114, 18182–18197.
58. Y. Zheng, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2015, 54, 52–65.
59. M. S. Faber, R. Dziedzic, M. A. Lukowski, N. S. Kaiser, Q. Ding and S. Jin, J. Am. Chem. Soc., 2014, 136, 10053–10061.
60. C. Zhang, S. Bhoyate, P. K. Kahol, K. Siam, T. P. Poudel, S. R. Mishra, F. Perez, A. Gupta, G. Gupta and R. K. Gupta, ChemNanoMat, 2018, 4, 1240–1246.
61. M. Wu, X. Meng, M. Zhou and Y. Zhou, New J. Chem., 2023, 47, 9492–9500.
62. X. Yu, J. Mei, Y. Du, X. Cheng, X. Wang and Q. Wu, Nano Res., 2023 DOI:10.1007/s12274-023-5740-9.
63. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308–5309.
64. H. Ma, Z. Shen and S. Ben, J. Colloid Interface Sci., 2018, 517, 204–212.
65. R. Lei, F. Gao, J. Yuan, C. Jiang, X. Fu, W. Feng and P. Liu, Appl. Surf. Sci., 2022, 576, 151851.
66. P. M. Pataniya and C. K. Sumesh, J. Electroanal. Chem., 2022, 912, 116270.
67. R. Xu, J. Huang, L. Cao, L. Feng, Y. Feng, L. Kou, Q. Liu, D. Yang and L. Feng, J. Electrochem. Soc., 2020, 167, 026508.
68. M. Shi, Y. Zhang, Y. Zhu, W. Wang, C. Wang, A. Yu, X. Pu and J. Zhai, RSC Adv., 2020, 10, 8973–8981.
69. D. Yang, L. Cao, L. Feng, J. Huang, K. Kajiyoshi, Y. Feng, Q. Liu, W. Li, L. Feng and G. Hai, Appl. Catal., B, 2019, 257, 117911.
70. D. Yang, L. Cao, J. Huang, Q. Liu, G. Li, D. He, J. Wang and L. Feng, Scr. Mater., 2021, 196, 113756.
71. M. Hu, Y. Qian, S. Yu, Q. Yang, Z. Wang, Y. Huang and L. Li, Small, 2023, 2305948.
72. Q. Liu, J. Huang, K. Liu, H. Du, L. Kang, D. Yang, M. Niu, G. Li, L. Cao and L. Feng, Dalton Trans., 2022, 51, 7234–7240.
73. B. Li, Z. Li, Q. Pang and J. Z. Zhang, Chem. Eng. J., 2020, 401, 126045.
74. Q. Liu, J. Huang, L. Cao, K. Kajiyoshi, K. Li, Y. Feng, C. Fu, L. Kou and L. Feng, ACS Sustainable Chem. Eng., 2020, 8, 6222–6233.
75. Y. Qu, M. Yang, J. Chai, Z. Tang, M. Shao, C. T. Kwok, M. Yang, Z. Wang, D. Chua, S. Wang, Z. Lu and H. Pan, ACS Appl. Mater. Interfaces, 2017, 9, 5959–5967.
76. J. Jian, L. Yuan, H. Qi, X. Sun, L. Zhang, H. Li, H. Yuan and S. Feng, ACS Appl. Mater. Interfaces, 2018, 10, 40568–40576.
77. G. Zhang, Y. S. Feng, W. T. Lu, D. He, C.-Y. Wang, Y.-K. Li, X.-Y. Wang and F.-F. Cao, ACS Catal., 2018, 8, 5431–5441.
78. H. Yan, R. Deng, S. Zhang, H. Yao, J. Duan, H. Bai, Y. Li, R. Liu, K. Shi and S. Ma, J. Alloys Compd., 2023, 954, 170072.
79. T. Kou, T. Smart, B. Yao, I. Chen, D. Thota, Y. Ping and Y. Li, Adv. Energy Mater., 2018, 8, 1703538.
80. W. He, L. Han, Q. Hao, X. Zheng, Y. Li, J. Zhang, C. Liu, H. Liu and H. L. Xin, ACS Energy Lett., 2019, 4, 2905–2912.
81. Y. Qian, J. Yu, Z. Lyu, Q. Zhang, T. H. Lee, H. Pang and D. J. Kang, Carbon Energy, 2023, e376.
82. C. Xiang, D. Zeng, B. Du, X. Huang, H. Lin, P. Zhang, Z. Zhang, D. Chen, W. Li and Y. Meng, Inorg. Chem. Front., 2023, 10, 6964–6975.
83. X. Ji, C. Cheng, Z. Zang, L. Li, X. Li, Y. Cheng, X. Yang, X. Yu, Z. Lu, X. Zhang and H. Liu, J. Mater. Chem. A, 2020, 8, 21199–21207.
84. J.-X. Feng, J. Q. Wu, Y. X. Tong and G.-R. Li, J. Am. Chem. Soc., 2018, 140, 610–617.
85. Y. Wu, Y. Liu, G.-D. Li, X. Zou, X. Lian, D. Wang, L. Sun, T. Asefa and X. Zou, Nano Energy, 2017, 35, 161–170.
86. M. Chen, Q. Su, N. Kitiphatpiboon, J. Zhang, C. Feng, S. Li, Q. Zhao, A. Abudula, Y. Ma and G. Guan, Fuel, 2023, 331, 125794.
87. Y. Yang, K. Zhang, H. Lin, X. Li, H. C. Chan, L. Yang and Q. Gao, ACS Catal., 2017, 7, 2357–2366.
88. Q. Liu, J. Huang, Y. Zhao, L. Cao, K. Li, N. Zhang, D. Yang, L. Feng and L. Feng, Nanoscale, 2019, 11, 8855–8863.
89. L. L. Feng, G. Yu, Y. Wu, G.-D. Li, H. Li, Y. Sun, T. Asefa, W. Chen and X. Zou, J. Am. Chem. Soc., 2015, 137, 14023–14026.
90. Y. Li, Y. Wang, B. Pattengale, J. Yin, L. An, F. Cheng, Y. Li, J. Huang and P. Xi, Nanoscale, 2017, 9, 9230–9237.
91. S. Deng, K. Zhang, D. Xie, Y. Zhang, Y. Zhang, Y. Wang, J. Wu, X. Wang, H. J. Fan, X. Xia and J. Tu, Nano-Micro Lett., 2019, 11, 12.
92. J. Dong, F.-Q. Zhang, Y. Yang, Y.-B. Zhang, H. He, X. Huang, X. Fan and X. M. Zhang, Appl. Catal., B, 2019, 243, 693–702.
93. L. Huang, X. Lin, J. Zhang, Z. Sheng, Q. Wang, F. Zhang, S. Qu and Y. Wang, Appl. Catal., A, 2022, 643, 118803.
94. R. Shi, J. Yang and G. Zhou, Chem. Eng. J., 2023, 457, 141188.
95. H. Liu, Q. He, H. Jiang, Y. Lin, Y. Zhang, M. Habib, S. Chen and L. Song, ACS Nano, 2017, 11, 11574–11583.
96. X. Guo, Z. Liu, F. Liu, J. Zhang, L. Zheng, Y. Hu, J. Mao, H. Liu, Y. Xue and C. Tang, Catal. Sci. Technol., 2020, 10, 1056–1065.
97. R. Velayutham, C. J. Raj, H. M. Jang, W. J. Cho, K. Palanisamy, C. Kaya and B. C. Kim, Mater. Today Nano, 2023, 24, 100387.
98. Z. Y. An, H. Xue, J. Sun, N. K. Guo, T. S. Song, J. W. Sun, Y. R. Hao and Q. Wang, Chin. J. Struct. Chem., 2022, 41, 2208037–2208043.
99. F. Du, L. Shi, Y. Zhang, T. Li, J. Wang, G. Wen, A. Alsaedi, T. Hayat, Y. Zhou and Z. Zou, Appl. Catal., B, 2019, 253, 246–252.
100. S. Chen, Z. Cao, F. Gao, H. An, H. Wang, Z. Zhou, L. Mi and Y. Li, Colloids Surf., A, 2023, 661, 130930.
101. Y. Xiao, Y. Shen, D. Su, S. Zhang, J. Yang, D. Yan, S. Fang and X. Wang, J. Mater. Sci. Technol., 2023, 154, 1–8.
102. Y. Cao, L. Li, X. Yu, M. Tahir, Z. Xiang, W. Kong, Z. Lu, X. Xing and Y. Song, ACS Appl. Mater. Interfaces, 2022, 14, 56725–56734.

---