Janus structure Ir–Ir₃Ce@IrO₂ nanocrystals as excellent bifunctionality catalysts for acidic overall water splitting

Abstract

Enhancing the catalytic performance of iridium (Ir)-based catalysts for water electrolysis under acidic conditions presents a significant challenge. This difficulty primarily arises from the substantial energy barriers associated with the four-electron–proton coupled oxygen evolution reaction (OER) at the anode and the two-electron transfer hydrogen evolution reaction (HER) at the cathode. This study successfully synthesized a novel type of Ir–Ir₃Ce@IrO₂ nanoparticles supported on defective carbon materials (DCMs; Ir–Ir₃Ce@IrO₂/DCMs) using freeze-drying and conversion methods. Notably, the catalyst core features a unique Janus structure comprising metal and alloy components. This catalyst demonstrates exceptional acidic OER activity, overall water-splitting catalytic performance, and high stability. Experimental results indicate that the Ir–Ir₃Ce@IrO₂/DCMs electrocatalyst delivers an ultralow overpotential of 210 mV at 10 mA cm⁻² for OER in 0.5 M H₂SO₄. Both structural characteristics and theoretical calculations suggest that Ir–Ir₃Ce@IrO₂/DCMs facilitate charge redistribution owing to various factors, including the alloying of precious metals and rare earth alloys, the Janus structure, and heterogeneous interfaces. The Ir–Ir₃Ce@IrO₂/DCMs || Ir–Ir₃Ce@IrO₂/DCMs electrolyzer can operate in acidic electrolytes for >40 h. This study presents a viable strategy to address the issues of instability and low efficiency associated with Ir-based OER electrocatalysts for acidic overall water splitting.

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1. Introduction

Because of the environmental crisis caused by fossil fuel combustion, there is growing awareness among the public regarding this issue, and clean energy sources such as hydrogen are increasingly gaining interest.^1,2 Hydrogen, recognized as a clean energy alternative, offers benefits such as affordability and a lack of pollution. Hydrogen production through the electrolysis of water is a promising technological solution, given the abundance of water resources on earth.^3,4 Additionally, using wind or solar power to generate electricity allows water to be electrolyzed to produce hydrogen and store it, providing another method of storing solar and wind energy.^5–7 However, the four-electron–proton coupling of the oxygen evolution reaction (OER) occurring at the anode presents significant energy barriers, severely limiting the kinetics of the electrochemical water-splitting process.^8,9 Furthermore, the acidic environment can accelerate the corrosion of the electrode.^10–12 There is an urgent need to develop bifunctional catalysts that can effectively improve the efficiency of the OER and hydrogen evolution reaction (HER) and optimize the electrochemical water-splitting process.

The noble metal iridium (Ir) and its oxide forms serve as catalysts in the electrochemical water-splitting process because of their excellent stability for OER.¹³ However, their performance in the OER still needs improvement compared with ruthenium-based catalysts, and their HER performance also needs enhancement compared with platinum-based catalysts. Moreover, the high cost of precious metals restricts the large-scale industrialization of catalysts.^14–16 Alloying other metals with Ir has been commonly used in recent years to reduce catalyst costs and improve performance. The charge transfer between two atoms alters the electronic structure of Ir and the adsorption energy of reaction intermediates.^17–19 The most widely used strategy is alloying 3d transition metals with precious metals, but dealloying often results in poor stability.^20–24 However, lanthanide series in the rare earth elements are often overlooked.²⁵ Among rare earth elements, cerium (Ce) is commonly used in catalysts for overall water splitting due to its unique electronic structure, and rare earth elements also have lanthanide contraction, which can control the lattice spacing of the alloy.^26–28 However, the alloying of rare earth elements with precious metals for overall water splitting has rarely been reported. If Ir and Ce form an alloy, the 4f orbital of Ce will be located near the Fermi level, which reduces the energy barrier for d–d electron transfer and the 5d orbital of Ir, also close to the Fermi level, will rearrange electrons owing to d–d and d–f coupling. The adsorption strength of species on the catalyst surface is influenced by the energy level at the center of the d-band, thereby impacting the catalytic performance.¹⁸ The displacement of the d-band center optimizes the adsorption energy of intermediates on the catalyst, improves the stability of the alloy to a certain extent, and can exhibit electron-rich features to support faster electron transfer, thus protecting the Ir site in acidic media.^25,29,30 However, because the trivalent Ce is easily oxidized to form oxides in air, there is an urgent need to develop an effective method to synthesize the alloy of Ir and Ce.^31,32

Janus nanocrystals have asymmetric structures on both sides and were recently discovered as a new bifunctional material that can optimize functions beyond what traditional structures can achieve.^33,34 Since the concept of “Janus” was proposed by Gennes in 1991,³⁵ it has been applied in various electrocatalysis fields and has demonstrated excellent performance owing to its unique structure. The preparation of isotropic nanoparticles has been well developed, and further advancements in structure and properties are limited,^36,37 prompting scientists to turn their attention to anisotropic nanoparticles. Janus nanoparticles possess two distinct regions that are independent yet interact with each other.³⁸ The asymmetric structure of Ir–Ir₃Ce forms the interface between Ir and Ir₃Ce, significantly enhancing the electron transfer capability of the nanoparticles compared to Ir nanoparticles or Ir₃Ce alloy nanoparticles. A substantial amount of electron transfer occurs at the interface, optimizing the chemisorption of precursor molecules and reaction intermediates, while the electron cloud density adjusts to reconstruct the active center and increase the active site density.^39,40

As previously mentioned, alloying Ir and Ce can significantly reduce the dissolution of precious metals at oxidation potentials; however, the extent of this improvement remains limited. The formation of soluble Irⁿ⁺ species readily occurs under oxidation potential,³⁰ and the development of IrO_x species on metal or alloy surfaces acts as a barrier that hinders the further oxidation and dissolution of these materials.^41,42 The incomplete oxidation of Ir is crucial for preserving the remarkable electrical conductivity of the composite throughout the electrocatalytic process.^41,43 The robust Ir–O bond considerably increases the theoretical dissolution potential of Ir during electrochemical oxidation, thereby enhancing the stability of the catalyst under acidic conditions.⁴⁴ Furthermore, as the outer layer of Janus nanocrystals, IrO₂ establishes two distinct interfaces with the nanocrystals: the Ir and IrO₂ interface and the Ir₃Ce and IrO₂ interface. These two interfaces can significantly influence interfacial effects and optimize the adsorption energy of the reaction intermediates. Therefore, adding IrO₂ can enhance the durability of the catalyst and improve its catalytic performance.^45,46

Identifying new carriers with superior electrical conductivity and enhanced corrosion resistance compared with carbon powder presents a significant challenge in the development of catalysts for overall water splitting.⁴⁷ The combination of a carbon atom with a nearby, more electronegative nitrogen atom alters the charge distribution.^48,49 However, it is often overlooked that structural defects in carbon with an electron-conjugated structure can fulfill this requirement and may exhibit better electrocatalytic activity than heteroatom doping. Modifying the electronic structure can significantly enhance the adsorption energy associated with the OER and facilitate electron transfer.^50–53 By directly calcining g-C₃N₄/C to introduce numerous defects as active sites, the loss of N atoms can adjust the electronic structure and enhance electron transfer in electrocatalysis.^54–58 Regarding specific surface area, structures with porous features and highly exposed surfaces can reveal more active sites, thereby improving the catalytic performance of the catalysts.⁵⁹ Consequently, defective carbon-based materials have emerged as the preferred choice for electrocatalyst carriers owing to their environmentally friendly properties and adjustable structures.

Inspired by the aforementioned perspectives, this study synthesized Ir–Ir₃Ce Janus nanocrystalline nanoparticles loaded with IrO₂ on defective carbon using freeze-drying and conversion methods, which were then applied to overall water splitting under acidic conditions. Compared with traditional techniques, this approach creates multiple interfaces that facilitate significant electron transfer. Furthermore, the alloying effect of Ce, which forms an alloy with Ir, enhances the inherent activity of the catalyst. As anticipated, the Ir–Ir₃Ce@IrO₂/DCMs demonstrate exceptional catalytic performance for overall water splitting in acidic electrolytes, requiring an overpotential of only 210 mV at 10 mA cm⁻² for the OER. The distinctive core–shell architecture exhibits remarkable stability. Additionally, theoretical calculations indicate that the transfer of charge among Ir, Ir₃Ce, and IrO₂ can influence the adsorption characteristics of crucial intermediates in the reactions, thereby lowering the energy barrier for both the OER and HER, which contributes to the impressive performance of the catalyst in overall water splitting.

2. Results and discussion

2.1 Morphology and structure

The Ir–Ir₃Ce@IrO₂/DCMs nanocrystal was synthesized using freeze-drying and conversion methods (Fig. 1a). The synthesized Ir–Ir₃Ce@IrO₂/DCMs exhibited a porous coral morphology, as observed in scanning electron microscope images (Fig. S1), providing many exposed active sites on these coral-like structures for the electrocatalytic reaction. The structure of Ir–Ir₃Ce@IrO₂/DCMs was further investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD results (Fig. 1b) showed that the prepared Ir–Ir₃Ce@IrO₂/DCMs corresponded well with the diffraction peaks of Ir and IrO₂, while the diffraction peaks of Ir₃Ce were not detected, which can be attributed to the low content of this alloy in the catalysts. Nevertheless, the diffraction peak of Ir in Ir–Ir₃Ce@IrO₂/DCMs is significantly shifted compared to that of Ir@IrO₂/DCMs, which may be due to the formation of the alloy. The TEM images of Ir–Ir₃Ce@IrO₂/DCMs (Fig. 1c) reveal that the nanoparticles were uniformly loaded on the coral-like carriers, with no signs of agglomeration. The sizes of the nanoparticles are highlighted in yellow circles in Fig. 1d for statistical purposes, and the size of the synthesized nanoparticles is clearly shown in Fig. 1e to be 9.5 ± 1.5 nm, indicating a small size. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 1f) of Ir–Ir₃Ce@IrO₂/DCMs shows continuous and distinct Ir₃Ce lattice fringes at 0.115 and 0.111 nm. Their existence is further confirmed by the presence of two distinct Debye rings in the selected electron diffraction (SAED) diagram, corresponding to the Ir₃Ce (101) and (012) crystal faces respectively, indicating that the Ir₃Ce alloy exists in nanocrystals, and the continuous diffraction rings indicate that it has a polycrystalline structure. Fig. 1g shows the lattice fringes of the (111) crystal face of Ir and the (220) crystal face of IrO₂, which clearly demonstrates that the structure of Ir–Ir₃Ce@IrO₂/DCMs consists of Janus structure nanocrystals formed by IrO₂-coated Ir and Ir₃Ce alloys loaded on the defective carbon-based materials.

Fig. 1 (a) Schematic diagram of the synthesis method, (b) XRD patterns of Ir@IrO₂/DCMs, Ir–Ir₃Ce/DCMs and Ir–Ir₃Ce@IrO₂/DCMs, (c) and (d) TEM image of Ir–Ir₃Ce@IrO₂/DCMs, (e) particle size distribution of Ir–Ir₃Ce@IrO₂/DCMs, (f) and (g) HRTEM images of Ir–Ir₃Ce@IrO₂/DCMs, and (h) HRTEM and EDS element mapping of the as-prepared Ir–Ir₃Ce@IrO₂/DCMs.

Inductively coupled plasma spectrometry/mass spectrometry (ICP-AES/MS) (Table S1) indicates that Ir and Ce comprise only a tiny percentage of the catalyst, yet they outperform commercial Pt/C with a 20% loading. ICP-AES/MS (Table S2) of the centrifuged supernatant revealed that many Ir and Ce ions were present in the supernatant, which were not incorporated into the precursor interlayer during vacuum stirring and can be recovered and reused. Energy dispersive spectroscopy (EDS) mapping (Fig. 1h) demonstrates that Ir, Ce, C, and O elements are uniformly distributed in the clusters, with no accumulation observed. We also conducted TEM on Ir–Ir₃Ce/DCMs without secondary calcination and on Ir@IrO₂/DCMs without Ce doping (Fig. S2 and S3). The Ir–Ir₃Ce Janus structure remains observable in the Ir–Ir₃Ce/DCMs without secondary calcination. Similarly, in the Ir@IrO₂/DCMs without Ce doping, the apparent core–shell structure of Ir and IrO₂ can be seen. This suggests that IrO₂ is formed during secondary calcination, which is an effective method for creating metal–metal oxide core–shell structures.

The introduction of glucose into the synthesis of g-C₃N₄/C increases the carbon source, contributing to the formation of a substantial amount of porous defective carbon-based materials after subsequent calcination. This porous defective carbon-based materials enhance the electron transfer capacity of metal nanoparticles and improves the accessibility of active sites. To investigate the pore size and pore type of Ir–Ir₃Ce@IrO₂/DCMs, we conducted Barrett–Emmett–Teller (BET) tests on g-C₃N₄/C and Ir–Ir₃Ce@IrO₂/DCMs (Fig. S4a). The N₂ adsorption isotherm curves of Ir–Ir₃Ce@IrO₂/DCMs exhibited typical type III isotherm characteristics. We noted that the desorption curves of g-C₃N₄/C and Ir–Ir₃Ce@IrO₂/DCMs are nearly identical to the adsorption curves, with no hysteresis loops or inflection points,^60,61 indicating predominantly well-connected open pores with a large pore volume. This leads to a significantly larger specific surface area of Ir–Ir₃Ce@IrO₂/DCMs compared to g-C₃N₄/C, thereby exposing more active sites to enhance catalytic activity. The pore size distribution diagram (Fig. S4b) indicates that both g-C₃N₄/C and Ir–Ir₃Ce@IrO₂/DCMs are primarily mesoporous. Ir–Ir₃Ce@IrO₂/DCMs exhibit a significantly higher pore count than g-C₃N₄/C, attributed to multiple high-temperature calcination processes; however, the pore size is considerably smaller than that of g-C₃N₄/C, owing to metal loading. It can be concluded that Ir–Ir₃Ce@IrO₂/DCMs are crucial for better exposure and utilization of active sites, thereby improving catalytic activity.

To characterize the chemical state and valence state of Ir–Ir₃Ce@IrO₂/DCMs, an X-ray photoelectron spectroscopy (XPS) was carried out. The full spectrum of Ir–Ir₃Ce@IrO₂/DCMs (Fig. S5), compared with Ir–Ir₃Ce/DCMs without secondary calcination and Ir@IrO₂/DCMs without Ce incorporation, can clearly show the coexistence of Ir, Ce, C and O elements, in which the peak of N element is found in Ir–Ir₃Ce/DCMs. It may be the residual N element in g-C₃N₄/C. Ir 4f spectra at Ir–Ir₃Ce@IrO₂/DCMs (Fig. S6a) shows that Ir mainly has obvious peaks at 66.6 eV and 64.7 eV, and 63.8 eV and 61.7 eV. They correspond to Ir⁴⁺ 4f_5/2, Ir⁰ 4f_5/2, Ir⁴⁺ 4f_7/2 and Ir⁰ 4f_7/2 respectively, and the peak area of Ir⁰ is larger.^20,62 The predominant form of Ir in Ir–Ir₃Ce@IrO₂/DCMs is still Ir⁰. Compared with Ir@IrO₂/DCMs without Ce (Fig. 2a), we find that the 4f peak of Ir has a slight positive shift, indicating electron transfer between Ir and Ce. Compared with Ir–Ir₃Ce/DCMs without secondary calcination (Fig. 2b), the 4f peak of Ir has a significant negative shift, indicating that electrons have been transferred from Ir–Ir₃Ce Janus nanocrystals to IrO₂. Janus nanocrystals and IrO₂ also form electron transfer at the interface. In the XPS spectra of Ce 3d for Ir–Ir₃Ce@IrO₂/DCMs, one can identify eight prominent peaks corresponding to Ce 3d_3/2 and Ce 3d_5/2(Fig. S6b). Cerium exists in the oxidized state as Ce³⁺ and Ce⁴⁺, but more often as Ce³⁺ (Fig. S7a).^63,64 Furthermore, O 1s spectra for Ir–Ir₃Ce@IrO₂/DCMs reveal three significant peaks: the metal–oxygen bond at 530.3 eV, oxygen vacancies at 532.5 eV, and the hydroxyl group at 533.8 eV (Fig. S6c). Compared with Ir–Ir₃Ce/DCMs without secondary calcination (Fig. S7b), we found that the M–O bonds of the catalyst after secondary calcination increased, which may be due to the formation of the IrO₂ shell (Fig. 2c). The C 1s spectra of Ir–Ir₃Ce@IrO₂/DCMs exhibit three peaks at 283, 284.8, and 285.8 eV, which belong to C=C, C–C and C–O, respectively (Fig. S6d).⁶⁵ XPS further confirmed the incorporation of Ce and the formation of IrO₂ shells. Electron transfer existed at various interfaces formed, optimizing intermediates’ adsorption energy.⁶⁶

Fig. 2 (a) and (b) Fitted Ir 4f peaks of Ir@IrO₂/DCMs, Ir–Ir₃Ce@IrO₂/DCMs and Ir–Ir₃Ce/DCMs; (c) fitted O 1s peaks of Ir–Ir₃Ce/DCMs and Ir–Ir₃Ce@IrO₂/DCMs, (d) Ir L₃-edge XANES and (e) Ir L₃-edge EXAFS spectra of Ir–Ir₃Ce@IrO₂/DCMs, IrO₂ and Ir foil; (f) structural illustration of Ir–Ir₃Ce@IrO₂/DCMs, and (g) wavelet transform of Ir L₃-edge EXAFS data of Ir foil, IrO₂ and Ir–Ir₃Ce@IrO₂/DCMs.

The local electronic structure of Ir–Ir₃Ce@IrO₂/DCMs electrocatalysts was probed by X-ray absorption near-edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) with Ir foil and IrO₂ as reference samples. The line intensities at the Ir L₃ edge were lower for Ir–Ir₃Ce@IrO₂/DCMs than for IrO₂ and higher than for Ir foils (Fig. 2d), indicating that the valence of Ir in Ir–Ir₃Ce@IrO₂/DCMs is intermediate between that of Ir foils and IrO₂. Subsequently, the coordination environments of Ir atoms were analyzed by EXAFS (Fig. S8), and the Ir–Ir₃Ce@IrO₂/DCMs showed three prominent peaks at ≈1.62, 2.64, and 3.50 Å from Ir–O bond, Ir–Ir bond, and Ir–Ce bond, respectively (Fig. 2e), with coordination numbers of 2.3, 7.2, and 4.3 for the three bonds, respectively (Table S3). Wavelet transform analysis (Fig. 2g) was next performed to gain more insight into the coordination information of the iridium species. Contour plots of Ir–Ir₃Ce@IrO₂/DCMs clearly show the Ir–O (k ≈ 5.5 Å) and Ir–Ir (k ≈ 11 Å) coordination of both intensities, similar to IrO₂ (k ≈ 5.3 Å) as well as Ir foil (k ≈ 11.7 Å), with Ir–Ce (k ≈ 6.7 Å) coordination being relatively less pronounced, and further combined with the EXAFS data and fitting results, the presence of Ir–Ce coordination in the samples can be determined. These results suggest that Ir atoms in Ir–Ir₃Ce@IrO₂/DCMs coexist in IrO₂, metallic Ir, and Ir₃Ce alloy crystal structures (Fig. 2f).

To verify that the nanocrystals synthesized with a core–shell structure are loaded onto defective carbon-based materials, the XRD pattern of the g-C₃N₄/C precursor synthesized via freeze-drying is shown in Fig. S9a. The prepared g-C₃N₄/C exhibits a g-C₃N₄ structure with typical characteristic diffraction peaks at 13.1° and 27.1°.^67–69 Compared with g-C₃N₄ synthesized via the direct calcination of dicyandiamide, the rapid dissolution and recrystallization of dicyandiamide result in more oligomers and smaller particle size, thus forming dicyandiamide nanocrystals favorable for the production of reticulate 3D g-C₃N₄.^69–71 The Raman spectrum (Fig. S9b) shows characteristic D and G bands at 1350 and 1588 cm⁻¹, respectively, where the D band corresponds to the disordered nature of the carbon material and the G band corresponds to its ordered nature. In addition to these two characteristic peaks, the Raman spectrum includes the D′′ band at 1496 cm⁻¹ and the T band at 1200 cm⁻¹, corresponding to the five-membered ring and the free state of carbon in the material, respectively. From the peak area fitting in Fig. S9b and S9c, it is evident that the proportion of the D′′ band and T band is smaller in the g-C₃N₄/C precursor after only 550 °C calcination compared with that of the carbon material after 550 °C calcination followed by 750 °C calcination and after 750 °C calcination followed by 400 °C air calcination. This indicates that the five-membered rings and free carbon states in the carbon material increase after several calcination cycles. The relative intensity ratio between the D band and G band (I_D/I_G) is often used to determine the degree of disorder in carbon materials. We found that carbon materials calcined at 750 °C and then air-calcined at 400 °C exhibited a significantly increased I_D/I_G ratio, suggesting that more carbon defects were created during the high-temperature calcination and subsequent air-calcination processes.^72–74 To determine whether these carbon defects persist after metal loading, we conducted electron paramagnetic resonance spectroscopy (EPR) tests on g-C₃N₄/C and Ir–Ir₃Ce@IrO₂/DCMs (Fig. S9d). We observed that compared with g-C₃N₄/C, Ir–Ir₃Ce@IrO₂/DCMs clearly show a stronger peak at 2.002 G, indicating that metal loading introduces additional carbon defects into the material.⁷⁵

2.2 Acidic OER and HER activity

The test used a standard three-electrode system in 0.5 M H₂SO₄.⁷⁶ Analysis of polarization curves of the OER (Fig. 3a) shows that Ir–Ir₃Ce@IrO₂/DCMs exhibit better OER performance than commercial IrO₂ and Pt/C. Additionally, the catalyst outperforms the same structure loaded on carbon black (Ir–Ir₃Ce@IrO₂/C; Fig. S10a). Specifically, the Ir–Ir₃Ce@IrO₂/DCMs electrocatalyst demonstrates a lower overpotential of 210 mV at 10 mA cm⁻² compared with Ir@IrO₂/DCMs and Ir–Ir₃Ce/DCMs, which do not form the Ir₃Ce alloy or a heterojunction with IrO₂ (Fig. 3b). We observed that almost no OER performance was detected in Ce/DCMs, indicating that Ir and its oxide are the primary catalytic sites in the OER. The alloying of Ce and Ir plays a role in optimizing performance, but Ce itself possesses no OER catalytic properties. Ir–Ir₃Ce@IrO₂/DCMs also exhibit a lower Tafel slope (25.73 mV dec⁻¹; Fig. 3c) and demonstrate a significant advantage in reaction kinetics.⁷⁷ Moreover, C_dl values (Fig. 3d) were obtained from the cyclic voltammetry (CV) curves (Fig. S11). Ir–Ir₃Ce@IrO₂/DCMs have an electrochemically active surface area (EASA), indicating that the alloying of Ce and the formation of a heterojunction with IrO₂ can enhance the activity of this system and increase the number of active sites.^13,78 Electrochemical impedance spectroscopy (EIS) (Fig. 3e) reveals that Ir–Ir₃Ce@IrO₂/DCMs have a minimum polarization resistance of only 2.5 ohms, a rapid electron transfer rate, and high activity.^79,80 When comparing Ir–Ir₃Ce@IrO₂/DCMs with Ir- and Ru-based catalysts reported in recent years (Fig. 3f and Table S4), the catalytic performance for OER is found to be exceptional. The OER performance of catalysts synthesized with different calcination temperatures and feed ratios was also tested (Fig. S12), and none of these catalysts exhibited inferior catalytic performance compared with Ir–Ir₃Ce@IrO₂/DCMs.

Fig. 3 OER catalytic performances. (a) Polarization curves of Pt/C, Ir/C, Ir–Ir₃Ce/DCMs, Ir–Ir₃Ce@IrO₂/DCMs, Ir@IrO₂/DCMs, Ce/DCMs, IrO₂/DCMs, and IrO₂, (b) comparison of overpotential (10 mA cm⁻²), (c) corresponding Tafel plots; (d) ECSA plots, (e) EIS, (f) comparison of OER properties with other electrocatalysts. HER catalytic performances, (g) polarization curves, (h) ECSA plots, and (i) EIS.

We further evaluated long-term operation stability of Ir-Ir₃Ce@IrO₂/DCMs electrocatalyst in acidic mediium for OER. At a current density of 10 mA cm⁻², after 72 h of continuous operation, there was only a minimal increase in the potential of Ir–Ir₃Ce@IrO₂/DCMs electrocatalysts compared to the initial measurements (Fig. S13a). On the other hand, Ir–Ir₃Ce/DCMs were rapidly deactivated in a short period, and TEM and HRTEM (Fig. S13b and c) showed that the structure of Ir–Ir₃Ce@IrO₂/DCMs did not collapse before and after catalysis. The characteristic peaks can also be clearly observed in the XRD pattern and XPS spectra after the durability test (Fig. S14a). However, in order to ensure the accuracy of the test, the working electrode is directly tested in the XRD and XPS tests at this time, and the electrocatalyst is loaded on the carbon paper, so the carbon peaks in the XRD pattern are more obvious. These results indicate that IrO₂ as a shell plays a significant role in preventing the dissolution of Ir and Ce during the catalytic process, ensuring the high structural stability of the acidic OER reaction.

For HER reaction in acidic media, Ir–Ir₃Ce@IrO₂/DCMs also show excellent catalytic performance, as shown in Fig. 3g. Ir–Ir₃Ce@IrO₂/DCMs and Ir–Ir₃Ce/DCMs exhibit slightly better performance than Pt/C. At an overpotential of 74 mV, a current density of 100 mA cm⁻² can be achieved. This performance is also superior to catalysts of the same structure supported on carbon black (Fig. S10b). Notably, after secondary air calcination to form precious metal oxides, a prominent hydrogen adsorption peak was observed at a potential of −0.07 to 0.1 V, with the current density fluctuating significantly due to the strong adsorption of *H on the catalyst surface. Therefore, we conclude that the oxide formed by secondary calcination, or the interface created between the oxide and the alloy or metal, contributes to the strong adsorption of *H on the catalyst surface. After overcoming the kinetic energy barrier to desorb *H, we observe that the polarization curve gradually stabilizes.^81–83 However, we found that Ir–Ir₃Ce@IrO₂/DCMs and Ir–Ir₃Ce/DCMs exhibited relatively similar catalytic activities, indicating that the active center for HER is Ir–Ir₃Ce. As a shell oxide, OER optimization does not compromise the HER performance. As shown in Fig. 3h, the C_dl value obtained by Ir₃Ce@IrO₂/DCMs from the CV curves (Fig. S15) is also relatively higher than that of other catalysts, demonstrating the largest EASA. Simultaneously, the lower impedance (EIS) value (Fig. 3i) indicates its strongest electron transfer rate.⁷⁸ The low Tafel slope (Fig. S16a) indicates that it has good reaction kinetics. Compared with some other Ir-based and Ru-based HER catalysts reported, the performance of Ir–Ir₃Ce@IrO₂/DCMs and Ir–Ir₃Ce/DCMs is also more outstanding (Fig. S16b).

At a current density of 10 mA cm⁻², in the 40 h stability test (Fig. S17a), high stability was nearly maintained, and the catalyst structure before and after the experiment (Fig. S17b and c) showed no significant differences. The HER performance of catalysts synthesized at various calcination temperatures and feed ratios was also tested in the experiment (Fig. S18), and the catalytic performance of these catalysts was lower than that of Ir–Ir₃Ce@IrO₂/DCMs and Ir–Ir₃Ce/DCMs. This evidence indicates that Ir–Ir₃Ce@IrO₂/DCMs is a bifunctional catalyst with excellent OER performance and high HER activity.

2.3 Acidic overall water splitting application

As the Ir–Ir₃Ce@IrO₂/DCMs catalyst exhibits strong catalytic performance for both OER and HER reactions in an acidic environment, it is used as the catalyst for both the cathode and anode of the electrolyzer.^84,85 The cell performance was observed to further evaluate the catalytic effect of the catalyst on overall water splitting (Fig. 4a). The Ir–Ir₃Ce@IrO₂/DCMs || Ir–Ir₃Ce@IrO₂/DCMs electrolyzer demonstrates better overall hydrolysis performance compared with the IrO₂ || Pt/C electrolyzer, requiring only a low voltage of 1.42 V to achieve a current density of 10 mA cm⁻² compared with 1.54 V for the IrO₂ || Pt/C electrolyzer. Only 1.68 V is needed to reach a current density of 100 mA cm⁻² (Fig. 4b). The Ir–Ir₃Ce@IrO₂/DCMs || Ir–Ir₃Ce@IrO₂/DCMs electrolyzer can operate continuously for >40 h, while the IrO₂ || Pt/C electrolyzer shows significant performance degradation even in the initial phase. In terms of durability as a measure of catalytic performance, Ir–Ir₃Ce@IrO₂/DCMs also hold a clear advantage (Fig. 4c). These results undoubtedly reaffirm that the Ir–Ir₃Ce@IrO₂/DCM catalyst is an excellent bifunctional electrocatalyst for overall water splitting in a 0.5 M H₂SO₄ electrolyte.

Fig. 4 Overall acidic water splitting of Ir–Ir₃Ce@IrO₂/DCMs || Ir–Ir₃Ce@IrO₂/DCMs and IrO₂ || Pt/C electrolyzer in 0.5 M H₂SO₄. (a) Schematic diagram, (b) polarization plots, and (c) chronopotentiometric curves.

2.4 Study of the electrocatalytic mechanism

To gain a more intuitive understanding of the OER and HER electrocatalytic mechanisms of Ir–Ir₃Ce@IrO₂/DCMs, density functional theory (DFT) calculations and in situ characterization were employed. First, charge density difference analysis was performed on Ir–Ir₃Ce@IrO₂/DCMs (Fig. 5a) to reveal charge redistribution among Ir₃Ce, Ir, and IrO₂.^86,87 The calculations indicated that 0.396 eV of electrons were transferred from the catalyst's nucleus (Janus nanocrystals) to the shell (IrO₂), suggesting that charge redistribution between this metal oxide and the metal and alloy is likely to regulate the adsorption behavior of critical intermediates. Many investigations have reported that surface electron-deficient precious metal sites can serve as active centers for various electrocatalytic reactions.^{39,40,46,88–91} Therefore, charge redistribution can generate more active sites. In the process of water splitting, a standard measure of whether the electrocatalyst is suitable is its ability to reduce the energy barrier of the reaction, thus accelerating the reaction rate. For the OER mechanism, there are three standard models: the adsorption evolution mechanism (AEM), the lattice oxygen evolution mechanism (LOM), and the oxide path mechanism (OPM).^75,92,93 Attenuated total reflection in situ surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) detected a gradually increasing signal peak at approximately 1090 cm⁻¹ as the voltage increased (Fig. 5b). The AEM path was chosen for DFT calculations of the Ir–Ir₃Ce@IrO₂/DCMs electrocatalysts. As shown in Fig. 5c, the d-band center of Ir 5d in Ir–Ir₃Ce@IrO₂/DCMs shows a significant positive shift, reaching −1.098 eV. Compared with Ir–Ir₃Ce/DCMs (−1.804 eV) and Ir@IrO₂/DCMs (−2.015 eV) (the catalyst structure is shown in Fig. 5d), this shift indicates an increased adsorption strength for various OER intermediates, which may enhance OER kinetics.^88,92,94 Consequently, we calculated the Gibbs free energy profiles of Ir–Ir₃Ce@IrO₂/DCMs, Ir–Ir₃Ce/DCMs, Ir@IrO₂/DCMs, Ir/C and IrO₂ for OER (Fig. 5e) and constructed the adsorption processes of relevant OER intermediates on each catalyst (Fig. S19). The Ir–Ir₃Ce@IrO₂/DCMs exhibit an OER energy barrier of 2.38 eV, which is superior to 2.71 eV for 2.71 eV for Ir–Ir₃Ce/DCMs, 2.57 eV for Ir@IrO₂/DCMs, 2.89 eV for Ir/C, and 3.17 eV for IrO₂, demonstrating the more favorable OER kinetics on the Ir–Ir₃Ce@IrO₂/DCMs, with an AEM path of the OER is shown in Fig. 5g. The conversion from *O to *OOH is the rate-limiting step catalyzed by Ir–Ir₃Ce@IrO₂/DCMs. The energy barrier of only 1.59 eV. This is lower than the 1.70 eV of Ir–Ir₃Ce/DCMs, 1.88 eV of Ir@IrO₂/DCMs, 2.09 eV of Ir/C, and 2.23 eV of IrO₂(Fig. 5f), further confirming that Ir–Ir₃Ce@IrO₂/DCMs exhibit more favorable OER dynamics. In terms of HER, the process of HER intermediate H* adsorption on each catalyst is shown in Fig. S20, Ir–Ir₃Ce@IrO₂/DCMs has the lowest HER energy barrier (0.17 eV; Fig. 5h and i). These findings indicate that Ir–Ir₃Ce@IrO₂/DCMs have high theoretical activity for both OER and HER and can be applied to acidic overall water splitting.

Fig. 5 (a) Differential charge density distribution and the electron transfer process, (b) ATR-SEIRAS measurements under various potentials for the Ir–Ir₃Ce@IrO₂/DCMs electrocatalyst during the OER process, (c) calculated PDOS plots of the Ir d-band for Ir–Ir₃Ce/DCMs, Ir@IrO₂/DCMs, and Ir–Ir₃Ce@IrO₂/DCMs with aligned Fermi levels, (d) structural illustration of Ir–Ir₃Ce/DCMs and Ir@IrO₂/DCMs, (e) free energy profiles of the OER process, (f) energy barrier of the rate-limiting step (conversion from *O to *OOH; U = 0 V), (g) schematic illustration of the adsorption evolution mechanism (AEM), (h) free energy profiles of the HER process, and (i) corresponding theoretical HER overpotentials.

3. Conclusion

In conclusion, we have successfully synthesized Ir–Ir₃Ce@IrO₂/DCM electrocatalysts loaded on defective carbon with Janus nanocrystals as cores, and they exhibit highly efficient and robust acidic water splitting performance. XRD, electron microscopy and XAFS have demonstrated the formation of alloys and a more complex structure that forms multiple interfaces to facilitate electron transfer. The electrocatalyst provides a low overpotential of 210 mV for OER at 10 mA cm⁻² and outstanding stability under the protection of metal oxides. More importantly, its application in an acidic electrolyzer shows that only 1.42 V is required to reach a current density of 10 mA cm⁻² as well as having a long-term stability of more than 40 hours, which outperforms the benchmark IrO₂ || Pt/C. DFT calculations confirm a significant reduction in the conversion energy barrier from *O to *OOH, with lower OER and HER energy barrier compared to other catalysts, which improves the OER and HER catalytic activity of the Ir–Ir₃Ce@IrO₂/DCMs electrocatalysts. This work explores the effects of interfacial effects, alloying effects, and metal oxide shell layer on the overall water splitting performance, providing ideas for future design of more novel overall water splitting catalysts.

4. Experimental section

4.1. Electrocatalyst synthesis

Synthesis of g-C₃N₄/C composites. The g-C₃N₄/C composite was obtained by mixing 1 g of dicyandiamide, 10 g of glucose, and 300 mL of deionized water in a beaker at room temperature (≈20 °C) for 12 h, followed by freeze-drying for 24 h and calcining under Ar gas at a heating rate of 3 °C min⁻¹ at 550 °C for 4 h.

Synthesis of Ir–Ir₃Ce@IrO₂/DCMs. 139 mg of IrCl₃·H₂O, 8 mg of Ce(NO)₃·3H₂O, 30 mL of glycol, and 50 mL of ethanol were placed into a beaker and stirred under vacuum for 8 h. Once dissolved, 400 mg of g-C₃N₄/C was added and subjected to vacuum for 30 min. The solids were collected via centrifugation to remove the Ir and Ce ions that were not incorporated into the precursor interlayer. The precipitate was then redissolved in a mixture of 70 ml of ethanol and 30 mL of glycol. At 180 °C under 300 rpm oil bath for 1 h and after cooling to room temperature, the mixture was washed with ethanol three times, vacuum-dried at 60 °C for 8 h, calcined at 750 °C at 3 °C min⁻¹ heating rate under Ar gas for 4 h, resulting in Ir–Ir₃Ce/DCMs. Further calcination at 400 °C at a heating rate of 3 °C min⁻¹ under air for 4 h produced Ir–Ir₃Ce@IrO₂/DCMs. The comparison samples Ir@IrO₂/DCMs and Ce/DCMs were prepared using the same method, except that Ce(NO)₃·3H₂O and IrCl₃·H₂O were not included. IrO₂/DCMs were prepared in the same manner, but they were not calcined at 750 °C. Ir–Ir₃Ce@IrO₂/C is obtained by replacing the added g-C₃N₄/C with Vulcan XC-72R carbon black.

Conflicts of interest

The authors report no declarations of interest.

Data availability

Data for this article are available at mendeley data at https://data.mendeley.com/datasets/nyjbjdkx38/1.

Supplementary information: original data from SEM tests, TEM tests, XPS tests, XRD tests, BET tests, EPR tests, Raman tests, ATR-SEIRAS tests, XANES and EXAFS tests, electrochemical tests, as well as DFT theoretical calculations. See DOI: https://doi.org/10.1039/d5qi00479a.

Acknowledgements

We would like to thank the National Natural Science Foundation of China (Project No. 22062019, 22362028), the Key Laboratory of Infinite-dimensional Hamiltonian System and Its Algorithm Application (Inner Mongolia Normal University), the Ministry of Education (2023KFYB01), and the Natural Science Foundation of Inner Mongolia of China (2022QN02002). Supercomputing facilities were provided by Hefei Advanced Computing Center and Computing Center in Xi'an.

References

1. D. Li, Y. Yu, W. Zhao, F. Wang, Y. Su and L. Cao, Rechargeable zinc-water battery for sustainable hydrogen generation, Nano Energy, 2024, 128, 109806.
2. J. Guo, J. D. Butson, Y. Zhang, G. Hu, X. Fan, G. K. Li, Y. Zhang, G. Hu, X. Fan and G. K. Li, Hydrogen generation from atmospheric water, J. Mater. Chem. A, 2024, 12, 12381–12396.
3. H. Xie, Z. Zhao, T. Liu, Y. Wu, C. Lan, W. Jiang, L. Zhu, Y. Wang, D. Yang and Z. Shao, A membrane-based seawater electrolyser for hydrogen generation, Nature, 2022, 612, 673–678.
4. L. Miao, W. Jia, X. Cao and L. Jiao, Computational chemistry for water-splitting electrocatalysis, Chem. Soc. Rev., 2024, 53, 2771–2807.
5. G. Singh, K. Ramadass, V. D. B. C. DasiReddy, X. Yuan, Y. Sik Ok, N. Bolan, X. Xiao, T. Ma, A. Karakoti, J. Yi and A. Vinu, Material-based generation, storage, and utilisation of hydrogen, Prog. Mater. Sci., 2023, 135, 101104.
6. J. J. T. Shibuya, D. E. Macphee and A. Cuesta, Putting stored hydrogen to work without consuming it: A flexible system for energy conversion and water desalination, J. Power Sources, 2024, 613, 234906.
7. S. Ghotia, T. Rimza, S. Singh, N. Dwivedi, A. K. Srivastava and P. Kumar, Hetero-atom doped graphene for marvellous hydrogen storage: unveiling recent advances and future pathways, J. Mater. Chem. A, 2024, 12, 12325–12357.
8. W. Wang, C. Li, C. Zhou, X. Xiao, F. Li, N. Y. Huang, L. Li, M. Gu and Q. Xu, Enrooted–type metal–support interaction boosting oxygen evolution reaction in acidic media, Angew. Chem., Int. Ed., 2024, 63, e202406947.
9. W. Jia, X. Cao, X. Chen, H. Qin, L. Miao, Q. Wang and L. Jiao, γ–MnO₂ as an electron reservoir for RuO₂ oxygen evolution catalyst in acidic media, Small, 2024, 2310464.
10. J. Chen, Y. Ma, T. Huang, T. Jiang, S. Park, J. Xu, X. Wang, Q. Peng, S. Liu, G. Wang and W. Chen, Ruthenium–based binary alloy with oxide nanosheath for highly efficient and stable oxygen evolution reaction in acidic media, Adv. Mater., 2024, 36, 2312369.
11. Z. Zhang, C. Jia, P. Ma, C. Feng, J. Yang, J. Huang, J. Zheng, M. Zuo, M. Liu, S. Zhou and J. Zeng, Distance effect of single atoms on stability of cobalt oxide catalysts for acidic oxygen evolution, Nat. Commun., 2024, 15, 1767.
12. H. Liu, Z. Zhang, J. Fang, M. Li, M. G. Sendeku, X. Wang, H. Wu, Y. Li, J. Ge, Z. Zhuang, D. Zhou, Y. Kuang and X. Sun, Eliminating over-oxidation of ruthenium oxides by niobium for highly stable electrocatalytic oxygen evolution in acidic media, Joule, 2023, 7, 558–573.
13. L. Quan, H. Jiang, G. Mei, Y. Sun and B. You, Bifunctional electrocatalysts for overall and hybrid water splitting, Chem. Rev., 2024, 124, 3694–3812.
14. H. Sun, X. Xu, H. Kim, Z. Shao and W. Jung, Advanced electrocatalysts with unusual active sites for electrochemical water splitting, InfoMat, 2023, 6, e12494.
15. L. Tian, Z. Li, X. Xu and C. Zhang, Advances in noble metal (Ru, Rh, and Ir) doping for boosting water splitting electrocatalysis, Mater. Chem. A, 2021, 9, 13459–13470.
16. B. Lu, C. Wahl, R. dos Reis, J. Edgington, X. K. Lu, R. Li, M. E. Sweers, B. Ruggiero, G. T. K. K. Gunasooriya, V. Dravid and L. C. Seitz, Key role of paracrystalline motifs on iridium oxide surfaces for acidic water oxidation, Nat. Catal., 2024, 7, 868–877.
17. F. Lv, W. Zhang, W. Yang, J. Feng, K. Wang, J. Zhou, P. Zhou and S. Guo, Ir–based alloy nanoflowers with optimized hydrogen binding energy as bifunctional electrocatalysts for overall water splitting, Small Methods, 2019, 4, 1900129.
18. J. Song, C. He, C. Ma, J. Xia, F. Feng, X. Ma, S. Han, H. Zhang, Y. Yang, B. Li, Q. Lu, W. Cao and L. Zhu, Atomically ordered Ir₃Ti intermetallics for pH-universal overall water splitting, Mater. Chem. A, 2024, 12, 18167–18174.
19. Y. Xie, Y. Feng, S. Pan, H. Bao, Y. Yu, F. Luo and Z. Yang, Electrochemical leaching of Ni dopants in IrRu alloy electrocatalyst boosts overall water splitting, Adv. Funct. Mater., 2024, 2406351.
20. Z. Zhang, S. Liu, Y. Zhou, J. Li, L. Xu, J. Yang, H. Pang, M. Zhang and Y. Tang, Engineering ultrafine PtIr alloy nanoparticles into porous nanobowls via a reactive template-engaged assembly strategy for high-performance electrocatalytic hydrogen production, Mater. Chem. A, 2024, 12, 10148–10156.
21. X. Wang, Z. Qin, J. Qian, L. Chen and K. Shen, IrCo nanoparticles encapsulated with carbon nanotubes for efficient and stable acidic water splitting, ACS Catal., 2023, 13, 10672–10682.
22. D. Cao, J. Wang, H. Zhang, H. Xu and D. Cheng, Growth of IrCu nanoislands with rich IrCu/Ir interfaces enables highly efficient overall water splitting in non-acidic electrolytes, Chem. Eng. J., 2021, 416, 129128.
23. W. H. Lee, J. Yi, H. N. Nong, P. Strasser, K. H. Chae, B. K. Min, Y. J. Hwang and H.-S. Oh, Electroactivation-induced IrNi nanoparticles under different pH conditions for neutral water oxidation, Nanoscale, 2020, 12, 14903–14910.
24. H. Mou, Q. Chang, Z. Xie, S. Hwang, S. Kattel and J. G. Chen, Enhancing glycerol electrooxidation from synergistic interactions of platinum and transition metal carbides, Appl. Catal., B, 2022, 316, 121648.
25. Y. Jiang, Z. Liang, J.-C. Liu, H. Fu, C.-H. Yan and Y. Du, Stimulating electron delocalization of lanthanide elements through high-entropy confinement to promote electrocatalytic water splitting, ACS Nano, 2024, 18, 19137–19149.
26. Y. Cheng, L. Zhu and Y. Gong, Ce doped Ni(OH)₂/Ni-MOF nanosheets as an efficient oxygen evolution and urea oxidation reactions electrocatalyst, Int. J. Hydrogen Energy, 2024, 58, 416–425.
27. W. Zhao, F. Xu, J. Yang, X. Hu and B. Weng, Ce single-atom incorporation enhances the oxygen evolution reaction of Co₃O₄ in acid, Inorg. Chem., 2024, 63, 1947–1953.
28. Z. Wan, K. Yang, P. Li, S. Yang, X. Wang, R. Gao, X. Xie, G. Deng, M. Yang and Z. Wang, Atomically dispersed Au single sites and nanoengineered structural defects enable a high electrocatalytic activity and durability for hydrogen evolution reaction and overall urea electrolysis, Electrochim. Acta, 2024, 499, 144685.
29. S. Zhu, L. Yang, J. Bai, Y. Chu, J. Liu, Z. Jin, C. Liu, J. Ge and W. Xing, Ultra-stable Pt₅La intermetallic compound towards highly efficient oxygen reduction reaction, Nano Res., 2022, 16, 2035–2040.
30. S. Zhang, M. Sun, L. Yin, S. Wang, B. Huang, Y. Du and C.-H. Yan, Tailoring the electronic structure of Ir alloy electrocatalysts through lanthanide (La, Ce, Pr, and Nd) for acidic oxygen evolution enhancement, Adv. Energy Sustainability Res., 2023, 4, 2300023.
31. J. Li, Z. Li, Z. Zheng, X. Zhang, H. Zhang, H. Wei and H. Chu, Tuning the product selectivity toward the high yield of glyceric acid in Pt−CeO₂/CNT electrocatalyzed oxidation of glycerol, ChemCatChem, 2022, 14, e202200509.
32. X. Zhang, D. Zhou, X. Wang, J. Zhou, J. Li, M. Zhang, Y. Shen, H. Chu and Y. Qu, Overcoming the deactivation of Pt/CNT by introducing CeO₂ for selective base-free glycerol-to-glyceric acid oxidation, ACS Catal., 2020, 10, 3832–3837.
33. X. Chen, J. Pu, X. Hu, Y. Yao, Y. Dou, J. Jiang and W. Zhang, Janus hollow nanofiber with bifunctional oxygen electrocatalyst for rechargeable Zn–Air Battery, Small, 2022, 18, 2200578.
34. M. Lattuada and T. A. Hatton, properties and applications of Janus nanoparticles, Nano Today, 2011, 6, 286–308.
35. P. G. Gennes, Soft matter, Rev. Mod. Phys., 1992, 64, 645–648.
36. S. K. T. Aziz, M. Ummekar, I. Karajagi, S. K. Riyajuddin, K. V. R. Siddhartha, A. Saini, A. Potbhare, R. G. Chaudhary, V. Vishal, P. C. Ghosh and A. Dutta, A Janus cerium-doped bismuth oxide electrocatalyst for complete water splitting, Cell Rep. Phys. Sci., 2022, 3, 101106.
37. H. Lou, K. Qiu and G. Yang, Janus Mo₂P₃ Monolayer as an electrocatalyst for hydrogen evolution, ACS Appl. Mater. Interfaces, 2021, 13, 57422–57429.
38. B. Tang, Y. Zhou, Q. Ji, Z. Zhuang, L. Zhang, C. Wang, H. Hu, H. Wang, B. Mei, F. Song, S. Yang, B. M. Weckhuysen, H. Tan, D. Wang and W. Yan, A Janus dual-atom catalyst for electrocatalytic oxygen reduction and evolution, Nat. Synth., 2024, 3, 878–890.
39. S. W. Yu, S. Kwon, Y. Chen, Z. Xie, X. Lu, K. He, S. Hwang and J. G. Chen, Construction of a Pt–CeO_x interface for the electrocatalytic hydrogen evolution reaction, Adv. Funct. Mater., 2024, 34, 2402966.
40. X. Mu, Y. Zhu, X. Gu, S. Dai, Q. Mao, L. Bao, W. Li, S. Liu, J. Bao and S. Mu, Awakening the oxygen evolution activity of MoS₂ by oxophilic-metal induced surface reorganization engineering, J. Energy Chem., 2021, 62, 546–551.
41. M. M. Hosseini, M. G. Hosseini, I. Ahadzadeh and R. Najjar, IrO₂-ZrO₂-SiO₂ ternary oxide composites- based DSAs: Activity toward oxygen evolution reaction with long-term stability, J. Taiwan Inst. Chem. Eng., 2024, 161, 105548.
42. M. Li, J. Qi, H. Zeng, J. Chen, Z. Liu, L. Gu, J. Wang, Y. Zhang, M. Wang, Y. Zhang, X. Lu and C. Yang, Structural impacts on the degradation behaviors of Ir-based electrocatalysts during water oxidation in acid, J. Colloid Interface Sci., 2024, 674, 108–117.
43. Y. Shan, J. Pan, Y. Song, Y. Zhu and T. Li, Engineering the orbital splitting and electronic reoccupation at alkali-metal-doped perovskites with face-shared IrO₆ octahedral dimers, Chem. Phys., 2024, 582, 112292.
44. S. Zhao, S.-F. Hung, L. Deng, W.-J. Zeng, T. Xiao, S. Li, C.-H. Kuo, H.-Y. Chen, F. Hu and S. Peng, Constructing regulable supports via non-stoichiometric engineering to stabilize ruthenium nanoparticles for enhanced pH-universal water splitting, Nat. Commun., 2024, 15, 2728.
45. J. Zhang, Q. Zhang and X. Feng, Support and interface effects in water–splitting electrocatalysts, Adv. Mater., 2019, 31, 1808167.
46. Z. Lu, J. Wang, P. Zhang, W. Guo, Y. Shen, P. Liu, J. Ji, H. Du, M. Zhao, H. Liang and J. Guo, Directed electron transport induced surface reconstruction of 2D NiFe-LDH/Stanene heterojunction catalysts for efficient oxygen evolution, Appl. Catal. B Environ. Energy, 2024, 353, 124073.
47. C. Z. Yuan, H. Zhao, S. Huang, J. Li, L. Zhang, W. Zhao, Y. Weng, X. Zhang, S. Ye and Y. Chen, Designing and regulating catalysts for enhanced oxygen evolution in acid electrolytes, Inorg. Chem., 2023, 2, 467–493.
48. Y. Chen, T. Jiang, C. Tian, Y. Zhan, A. Kempf, L. Molina-Luna, J. P. Hofmann, R. Riedel and Z. Yu, Single–source–precursor–derived binary FeNi phosphide nanoparticles encapsulated in N, P Co–doped carbon as electrocatalyst for hydrogen evolution reaction and oxygen evolution reaction, Energy Technol., 2023, 11, 2300233.
49. Y. Xu, J. Wang, S. Wang, Z. Zhai, B. Ren, Z. Tian, L. Zhang and Z. Liu, Lattice defective and N, S co-doped carbon nanotubes for trifunctional electrocatalyst application, Int. J. Hydrogen Energy, 2023, 48, 34340–34354.
50. L. Tao, Q. Wang, S. Dou, Z. Ma, J. Huo, S. Wang and L. Dai, Edge-rich and dopant-free graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction, Chem. Comm., 2016, 52, 2764–2767.
51. D. Bi, N. Shen, Z. Tang, Z. Yang, S. A. Grigoriev, P. He, Q. Lai and Y. Liang, Defective carbon derived using a dissolution–recrystallization strategy for oxygen reduction electrocatalysis, ACS Appl. Mater. Interfaces, 2023, 15, 30179–30186.
52. X. Yan, Y. Jia, J. Chen, Z. Zhu and X. Yao, Defective–activated–carbon–supported Mn–Co nanoparticles as a highly efficient electrocatalyst for oxygen reduction, Adv. Mater., 2016, 28, 8771–8778.
53. A. Kostuch, E. Negro, G. Pagot, S. Zoladek, I. A. Rutkowska, O. Siamuk, A. Chmielnicka, V. Di Noto and P. J. Kulesza, Importance of defective structure of ceria additive on selectivity and stability of carbon-supported low-Pt-content-catalysts during oxygen reduction, Catal. Today, 2024, 441, 114889.
54. W. Wang, L. Shang, G. Chang, C. Yan, R. Shi, Y. Zhao, G. I. N. Waterhouse, D. Yang and T. Zhang, Intrinsic carbon–defect–driven electrocatalytic reduction of carbon dioxide, Adv. Mater., 2019, 31, 1808276.
55. Y. Wang, S. Zhao, Y. Zhang, J. Fang, Y. Zhou, S. Yuan, C. Zhang and W. Chen, One-pot synthesis of K-doped g-C₃N₄ nanosheets with enhanced photocatalytic hydrogen production under visible-light irradiation, Appl. Surf. Sci., 2018, 440, 258–265.
56. L. Yang, X. Liu, Z. Liu, C. Wang, G. Liu, Q. Li and X. Feng, Enhanced photocatalytic activity of g-C₃N₄ 2D nanosheets through thermal exfoliation using dicyandiamide as precursor, Ceram. Int., 2018, 44, 20613–20619.
57. Q. Wu, J. Gao, J. Feng, Q. Liu, Y. Zhou, S. Zhang, M. Nie, Y. Liu, J. Zhao, F. Liu, J. Zhong and Z. Kang, A CO₂ adsorption dominated carbon defect-based electrocatalyst for efficient carbon dioxide reduction, J. Mater. Chem. A, 2020, 8, 1205–1211.
58. R. Kumar, S. Kumar, S. G. Chandrappa, N. Goyal, A. Yadav, N. Ravishankar, A. S. Prakash and B. Sahoo, Nitrogen-doped carbon nanostructures embedded with Fe-Co-Cr alloy based nanoparticles as robust electrocatalysts for Zn-air batteries, J. Alloys Compd., 2024, 984, 173862.
59. R. Daiyan, X. Tan, R. Chen, W. H. Saputera, H. A. Tahini, E. Lovell, Y. H. Ng, S. C. Smith, L. Dai, X. Lu and R. Amal, Electroreduction of CO₂ to CO on a mesoporous carbon catalyst with progressively removed nitrogen moieties, ACS Energy Lett., 2018, 3, 2292–2298.
60. Z. Li, D. Liu, Y. Cai, Y. Wang and J. Tengi, Tunable subradiant mode in free-standing metallic nanohole arrays for high-performance plasmofluidic sensing, Fuel, 2019, 257, 116031.
61. Y. Zhu, S. Zhao, K. Deng, P. Wu, K. Feng and L. Li, Integrated mRNA and small RNA sequencing reveals a microRNA regulatory network associated with starch biosynthesis in lotus (nelumbo nucifera gaertn.) rhizomes, Int. J. Mol. Sci., 2022, 23, 7605.
62. H. Hu, F. M. D. Kazim, Z. Ye, Y. Xie, Q. Zhang, K. Qu, J. Xu, W. Cai, S. Xiao and Z. Yang, Electronically delocalized Ir enables efficient and stable acidic water splitting, J. Mater. Chem. A, 2020, 8, 20168–20174.
63. D. Rathore, S. Ghosh, A. Gupta, J. Chowdhury and S. Pande, Ce-doped NiSe nanosheets on carbon cloth for electrochemical water-splitting, ACS Appl. Nano Mater., 2024, 7, 9730–9744.
64. Y. Liao, X. Lu, X. Jin, H. Chen, X. Huang, Y. Li and J. Li, Doping CeO₂/Ce(OH)CO₃/CF nanohybrids with Gd for structural tuning and oxygen evolution reaction performance enhancing, Int. J. Hydrogen Energy, 2024, 72, 288–296.
65. J. Chen, P. Cui, G. Zhao, K. Rui, M. Lao, Y. Chen, X. Zheng, Y. Jiang, H. Pan, S. X. Dou and W. Sun, Low–coordinate iridium oxide confined on graphitic carbon nitride for highly efficient oxygen evolution, Angew. Chem., Int. Ed., 2019, 58, 12540–12544.
66. R. Qin, G. Chen, X. Feng, J. Weng and Y. Han, Ru/Ir–based electrocatalysts for oxygen evolution reaction in acidic conditions: from mechanisms, optimizations to challenges, Adv. Sci., 2024, 11, 2309364.
67. S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan and H. Fu, Phosphorus-doped carbon nitride tubes with a layered micro-nanostructure for enhanced visible-light photocatalytic hydrogen evolution, Angew. Chem., Int. Ed., 2016, 55, 1830–1834.
68. J. Chen, Y. Xiao, N. Wang, X. Kang, D. Wang, C. Wang, J. Liu, Y. Jiang and H. Fu, Facile synthesis of a Z-scheme CeO₂/C₃N₄ heterojunction with enhanced charge transfer for CO₂ photoreduction, Sci. China Mater., 2023, 66, 3165–3175.
69. C. Zhang, J. Wang, Y. Liu, W. Li, Y. Wang, G. Qin and Z. Lv, Electrocatalytic HER enhancement of C₃N₄ in NiCo₂O₄, Chem. - Asian J., 2022, 17, e202200377.
70. J. Tian, Z. Chen, J. Jing, C. Feng, M. Sun and W. Li, Enhanced photocatalytic performance of the MoS₂/g-C₃N₄ heterojunction composite prepared by vacuum freeze drying method, J. Photochem. Photobiol., A, 2020, 390, 112260.
71. C. Hu, W.-L. Chiu, C.-Y. Wang and V.-H. Nguyen, Freeze-dried dicyandiamide-derived g-C₃N₄ as an effective photocatalyst for H₂ generation, J. Taiwan Inst. Chem. Eng., 2021, 129, 128–134.
72. J. E. Weis, J. Vejpravova, T. Verhagen, Z. Melnikova, S. Costa and M. Kalbac, Surface–enhanced Raman spectra on graphene, J. Raman Spectrosc., 2017, 49, 168–173.
73. L. Li, J. Liu, X. Liu and W. Wu, Preparation of mesocarbon microbeads by thermal polymerization of the blended coal tar and biomass tar pitch as anode material for sodium-ion batteries, Int. J. Electrochem. Sci., 2022, 17, 221150.
74. Q. Lai, J. Zheng, Z. Tang, D. Bi, J. Zhao and Y. Liang, Optimal configuration of N-Doped carbon defects in 2D turbostratic carbon nanomesh for advanced oxygen reduction electrocatalysis, Angew. Chem., Int. Ed., 2020, 59, 11999–12006.
75. Q. Wu, Y. Jia, Q. Liu, X. Mao, Q. Guo, X. Yan, J. Zhao, F. Liu, A. Du and X. Yao, Ultra-dense carbon defects as highly active sites for oxygen reduction catalysis, Chem, 2022, 8, 2715–2733.
76. Y. Wang, X. Ge, Q. Lu, W. Bai, C. Ye, Z. Shao and Y. Bu, Accelerated deprotonation with a hydroxy-silicon alkali solid for rechargeable zinc-air batteries, Nat. Commun., 2023, 14, 6968.
77. O. van der Heijden, S. Park, R. E. Vos, J. J. J. Eggebeen and M. T. M. Koper, Tafel slope plot as a tool to analyze electrocatalytic reactions, ACS Energy Lett., 2024, 9, 1871–1879.
78. G. Gao, Z. Sun, X. Chen, G. Zhu, B. Sun, Y. Yamauchi and S. Liu, Recent advances in Ru/Ir-based electrocatalysts for acidic oxygen evolution reaction, Appl. Catal. B Environ. Energy, 2024, 343, 123584.
79. S. Wang, J. Zhang, O. Gharbi, V. Vivier, M. Gao and M. E. Orazem, Electrochemical impedance spectroscopy, Nat. Rev. Methods Primers, 2021, 1, 41.
80. F. Ciucci, Modeling electrochemical impedance spectroscopy, Curr. Opin. Electrochem., 2019, 13, 132–139.
81. Z. Li, K. Werner, K. Qian, R. You, A. Płucienik, A. Jia, L. Wu, L. Zhang, H. Pan, H. Kuhlenbeck, S. Shaikhutdinov, W. Huang and H. J. Freund, Oxidation of reduced ceria by incorporation of hydrogen, Angew. Chem., Int. Ed., 2019, 58, 14686–14693.
82. C. Copéret, D. P. Estes, K. Larmier and K. Searles, Isolated surface hydrides: formation, structure, and reactivity, Chem. Rev., 2016, 116, 8463–8505.
83. K. Deng, Z. Lian, W. Wang, J. Yu, Q. Mao, H. Yu, Z. Wang, L. Wang and H. Wang, Hydrogen spillover effect tuning the rate-determining step of hydrogen evolution over Pd/Ir hetero-metallene for industry-level current density, Appl. Catal. B Environ. Energy, 2024, 352, 124047.
84. A. Zhou, D. Song, Z. Hui, J. Yang, W. An, X. Zuo, X. Ye and M. Jiang, Coupling NiFe-MOF with antiperovskite nickel-based nitrides as efficient electrocatalysts for overall water splitting, Results Phys., 2024, 59, 107611.
85. X. Qian, L. Jiang, J. Fang, J. Ye, G. He and H. Chen, Constructing a self-supported bifunctional multiphase heterostructure for electrocatalytic overall water splitting, Inorg. Chem., 2024, 4c01963.
86. A. Esmaeili, F. Keivanimehr, M. Mokhtarian, S. Habibzadeh, O. Abida and M. Moghaddamian, 2D Ni₂P/N-doped graphene heterostructure as a Novel electrocatalyst for hydrogen evolution reaction: A computational study, Heliyon, 2024, 10, e27133.
87. F. Yang, X. Zhu, B. Jia, J. Hao, Y. Ma, P. Lu and D. Li, Trifunctional electrocatalysts of single transition metal atom doped g-C₄N₃ with high performance for OER, ORR and HER, Int. J. Hydrogen Energy, 2024, 51, 195–206.
88. Y. Jiang, J. Leng, S. Zhang, T. Zhou, M. Liu, S. Liu, Y. Gao, J. Zhao, L. Yang, L. Li and W. Zhao, Modulating water splitting kinetics via charge transfer and interfacial hydrogen spillover effect for robust hydrogen evolution catalysis in alkaline media, Adv. Sci., 2023, 10, 2302358.
89. X. Zheng, L. Li, M. Deng, J. Li, W. Ding, Y. Nie and Z. Wei, Understanding the effect of interfacial interaction on metal/metal oxide electrocatalysts for hydrogen evolution and hydrogen oxidation reactions on the basis of first-principles calculations, Catal. Sci. Technol., 2020, 10, 4743–4751.
90. K. K. Mandari and M. Kang, Ni₂P/Co₂P as a highly efficient catalyst for enhancing the electrochemical performance of P-NiMoO₄ for oxygen evolution reaction, J. Alloys Compd., 2024, 976, 172973.
91. P. Li, S. Luo, Z. Xiong, H. Xiao, X. Wang, K. Peng, X. Xie, Z. Zhang, G. Deng, M. Yang and C. Wang, Ru_xMoS₂ interfacial heterojunctions achieve efficient overall water splitting and stability in both alkaline and acidic media under large current density exceeding 100 mA cm⁻², Mol. Catal., 2025, 570, 114710.
92. Z. Dong, C. Zhou, W. Chen, F. Lin, H. Luo, Z. Sun, Q. Huang, R. Zeng, Y. Tan, Z. Xiao, H. Huang, K. Wang, M. Luo, F. Lv and S. Guo, Cerium dioxide as an electron buffer to stabilize iridium for efficient water electrolysis, Adv. Funct. Mater., 2024, 2302358.
93. R. Ram, L. Xia, H. Benzidi, A. Guha, V. Golovanova, A. G. Manjón, D. L. Rauret, P. S. Berman, M. Dimitropoulos, B. Mundet, E. Pastor, V. Celorrio, C. A. Mesa, A. M. Das, A. Pinilla-Sánchez, S. Giménez, J. Arbiol, N. López and F. P. G. D. Arquer, Water-hydroxide trapping in cobalt tungstate for proton exchange membrane water electrolysis, Science, 2024, 384, 1373–1380.
94. J. Zhu, Y. Guo, F. Liu, H. Xu, L. Gong, W. Shi, D. Chen, P. Wang, Y. Yang, C. Zhang, J. Wu, J. Luo and S. Mu, Regulative electronic states around ruthenium/ruthenium disulphide heterointerfaces for efficient water splitting in acidic media, Adv. Funct. Mater., 2021, 60, 12328–12334.

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