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. 2025 Apr 8;147(16):13345–13355. doi: 10.1021/jacs.4c18390

Mesoporous Single-Crystalline Particles as Robust and Efficient Acidic Oxygen Evolution Catalysts

Yong Wang †,‡, Yunpu Qin †, Sijia Liu †, Yongzhi Zhao †,∥, Luan Liu †, Di Zhang ‡, Shangqing Zhao ‡, Jianfang Liu †, Jie Wang †, Yadong Liu †, Haoyang Wu †, Baorui Jia †,§,*, Xuanhui Qu †,⊥, Hao Li ‡,*, Mingli Qin †,⊥,#,*
PMCID: PMC12023039  PMID: 40196994

Abstract

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The scarcity of iridium (Ir) limits its widespread use in acidic oxygen evolution reaction (OER). Herein, mesoporous single-crystalline spinel Co3O4 with atomically dispersed low-valence-state Ir has been developed to enable Ir’s efficient and stable utilization. The surface Pourbaix diagram suggests that under acidic OER conditions, O* fully covers both Co3O4(111) and (110) surfaces, passivating Co sites but enhancing Co3O4’s structural stability, a benefit further improved by Ir doping. Mesopores offer numerous loading sites for Ir single atoms (13.8 wt %), which activate the originally O*-passivated Co3O4(111) surface by creating high-intrinsic-activity Co–Ir bridge sites; meanwhile, Ir and Co leaching rates are reduced to about 1/4 and 1/5, respectively, compared to conventional Ir/Co3O4 catalysts. Our catalyst exhibits a low η10 of 248 mV for over 100 h, showcasing its potential in water electrolysis.

Introduction

Hydrogen (H2), known for its high energy density and clean nature, has emerged as a highly promising alternative to fossil fuels.1−4 Electrocatalytic water splitting using renewable energy sources represents a sustainable way to produce clean hydrogen fuel.5−9 Acidic water electrolysis, such as a proton exchange membrane (PEM) electrolyzer, offers benefits including high current density, flexibility in responding to intermittent electricity from sources like wind or solar energy, minimal side reactions, and high purity of the produced hydrogen.10−13 The main challenge with PEM technology lies in the performance of the anode, which requires a highly active and stable acidic oxygen evolution reaction (OER) catalyst to improve overall hydrogen production efficiency.14−16 Currently, iridium (Ir) oxides are considered the only materials capable of withstanding the harsh acidic conditions of a PEM electrolyzer while maintaining high catalytic activity.17−20 However, Ir remains both scarce and costly.21−24 Therefore, there is a critical need to develop catalysts with rational structures to reduce the dependence on the precious metal Ir.25−29

Recently, single atom catalysts (SACs) have garnered significant research attention due to their ability to maximize material usage efficiency and adjust the electronic structure by tuning the coordination environment.30−34 Currently, there is a strong focus on preparing Ir or Ru SACs for acidic OER catalysis, which has demonstrated high turnover frequencies (TOFs).35−37 For example, Yan and co-workers prepared single-atom Ir-doped NiCo2O4 porous nanosheets, which could reach a TOF of 6.70 s–1 at an overpotential of 370 mV in acidic electrolyte.38 Peng et al. synthesized a single-atom Ru-doped Co3O4 acidic OER catalyst, achieving an impressive mass activity of 4012.11 A g–1Ru(anc) at 1.5 V.39 Despite these advancements, the challenge lies in developing suitable catalysts with high Ir loading to achieve both high efficiency and stability. Mesoporous single crystals combine the benefits of mesoporous materials and single crystals. The mesopores increase the number of active sites or sites for catalyst loading and facilitate mass diffusion during catalysis.40−42 The single crystal nature promotes fast electron transfer in electrocatalysis and reduces the risk of metal leaching compared to nanocrystals.43−45 These characteristics make mesoporous single crystals expected to overcome the activity-stability trade-off in acid OER catalysis.

In this study, we designed and synthesized spinel Co3O4 mesoporous single crystals doped with Ir single atoms as a robust and efficient acidic OER catalyst. Calculated surface Pourbaix diagrams show that both the (110) and (111) surfaces of spinel Co3O4 inevitably have ∼1 monolayer coverage of O* at the OER potential. This passivates the surface sites compared to the HO* surface coverage configuration but may contribute to the stability of Co3O4 in acidic OER environments. Therefore, we utilized Co3O4 mesoporous single crystals for loading Ir single atoms for acid OER. The single-crystalline nature confers resistance to metal leaching, especially after the doping with Ir single atoms, as confirmed by surface Pourbaix analyses and inductively coupled plasma mass spectrometry (ICP-MS) measurement. Ir and Co leaching rates are reduced to about 1/4 and 1/5, respectively, compared to conventional Ir/Co3O4 catalysts. Moreover, the presence of mesopores provides additional sites for achieving high Ir loading up to 13.8 wt %, and in particular, the doped Ir effectively activates the O*-passivated (111) surface of Co3O4, as revealed by OER descriptor (ΔGO* – ΔGHO*) calculations and confirmed by modified electronic structure through X-ray absorption spectroscopy (XAS). As a result, the catalyst shows a low η10 of 248 mV in acid media among the state-of-the-art acid OER catalysts, and it remains high activity for over 100 h, underscoring the exceptional stability.

Results and Discussion

First, β-Co(OH)2 hexagonal nanoplates (Figure S1) were synthesized through coprecipitation and then calcined to form spinel Co3O4 nanoplates (Figures S2 and S3). The exposed crystalline faces of a hexagonal Co3O4 nanoplate primarily consist of two (111) basal surfaces and six (110) edge surfaces.46,47 Chronopotentiometry curves of Co3O4 at an OER current density of 10 mA cm–2 were tested on a glassy carbon electrode in 0.5 M H2SO4 (Figure 1a). Initially, the OER potential is relatively stable between 1.7 and 1.8 V vs RHE, but within just 1000 s of testing, the potential rapidly exceeded 2.2 V vs RHE, suggesting a rapid and complete loss of OER catalytic activity. Following the OER testing, we characterized the Co3O4 by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 1b shows that the hexagonal morphology of the Co3O4 nanoplate remains intact despite having become entirely inactive for OER. Figure 1c and the corresponding selected area electron diffraction (SAED) patterns confirm the preserved single-crystalline nature of the Co3O4 nanoplate, with the exposed surface identified as the (111) surface. The XRD results of the sample after catalysis align well with the characteristic diffraction peaks of spinel Co3O4 (Figure S3). These results suggest that the rapid inactivation of Co3O4 during acidic OER is more likely due to the changes in the surface structure of spinel rather than dissolution caused by acidic corrosion.

Figure 1.

Figure 1

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Deactivation of Co3O4 acidic OER catalyst. (a) Chronopotentiometry curve of Co3O4 at a current density of 10 mA cm–2 tested on a glassy carbon electrode. (b) TEM and (c) HRTEM images of Co3O4 after chronopotentiometry testing, with the inset showing the SAED pattern. Calculated 1D surface Pourbaix diagrams for the (110) edge surface (d) and (111) basal surface (f) of Co3O4 as a function of potential (pH = 0; T = 298.15 K). Calculated 2D surface Pourbaix diagrams for the (110) edge surface (e) and (111) basal surface (g) of Co3O4 as a function of potential (vs RHE) and pH (T = 298.15 K). (h) Structural models of the Co3O4(110) edge surface and (111) basal surface covered by 1 ML O* (side view). (i) Location of the Co3O4(110) edge surface and (111) basal surface with different surface coverages on the volcano plot. All relevant computational structures are available in the Digital Catalysis Platform (DigCat) database: https://www.digcat.org/.

To study the reason for rapid loss of OER activity and the stability of the spinel phase at OER potential in acidic media for Co3O4, we conducted density functional theory (DFT) calculations to obtain one-dimensional (1D) and two-dimensional (2D) surface Pourbaix diagrams for the (110) edge surface and (111) basal surface of Co3O4. The initial structures of the Co3O4(110) and (111) surfaces are depicted in Figure S4. Our analysis revealed that the Co3O4(110) surface in 0.5 M H2SO4 can be covered by ∼1 monolayer (ML) HO* at 1.17–1.83 V vs RHE, which then transforms into a coverage configuration of ∼1 ML O* above 1.83 V vs RHE, as shown in Figure 1d,e. Similarly, the Co3O4(111) surface is covered by ∼1 ML HO* between 0.34 and 1.67 V vs RHE and then becomes covered by ∼1 ML of O* above 1.67 V vs RHE (Figure 1f,g). The corresponding coverage structures can be observed in Figures 1h and S5. The transformation from HO* to O* coverage may enhance the stability of the spinel phase during OER, and this transformation may be responsible for the rapid loss of OER activity.48,49

We then calculated the difference in adsorption energies between the OER intermediates HO* and O* (ΔGO* – ΔGHO*) for these surface coverage structures as a descriptor for OER activity,48,50 and predicted the OER performance based on the state-of-the-art OER volcano by considering the kinetics and thermodynamics of the key OER elementary steps.50 For the Co3O4 (110) surface, the ΔGO* – ΔGHO* values of the model covered by 1 ML HO* or 1 ML O* are 1.58 and 1.93 eV, respectively (Figure 1i). The value of 1.58 eV is close to the volcano vertex, indicating good OER activity for the HO* configuration. However, this configuration is not stable and can convert to O* coverage, causing the ΔGO* – ΔGHO* value to move from the volcano vertex (1.93 eV), which implies a decrease in OER activity. The ΔGO* – ΔGHO* for the Co3O4(111) surface covered by 1 ML O* is 2.34 eV, significantly distant from the volcano vertex, which means that the Co3O4(111) surface is also inactive after being covered by O*. The surface coverage transition of the (110) surface from HO* to O* is unavoidable, which contributes to the rapid deactivation of Co3O4 in acidic OER catalysis but benefits the phase stability of spinel in acid, making it a suitable supporter in acidic OER catalysis.

In this study, we synthesized a type of Co3O4 mesoporous single-crystalline nanoplates through a micelle-assisted method51 and then anchored Ir single atoms onto these Co3O4 particles for use in acidic OER catalysis. The presence of mesopores dispersed on the Co3O4(111) basal surface creates numerous edge sites, improving both the Ir loading amount and its distribution within the Co3O4 matrix, which is critical in PEM applications. The single-crystalline nature and large particle size approaching the micrometer scale of these particles ensure catalysis stability while maximizing the number of catalysis sites, making them superior to conventional nanoparticle supports. The synthetic scheme of our catalyst is illustrated in Figure 2a. First, we prepared honeycomb-like β-Co(OH)2 mesoporous single-crystalline nanoplates with about 20 nm mesopores uniformly distributed within the (001) surfaces of β-Co(OH)2 (referred to as H–Co(OH)2) through a strategy involving the self-assembly of copolymer micelles during the spiral growth of β-Co(OH)2 (Figures S6 and S7). Then, H–Co(OH)2 was treated in an ethanol/water solution of potassium hexachloroiridate (K3IrCl6) at 80 °C. During this process, the Co atoms in H-β-Co(OH)2, particularly those at the edge sites surrounding the mesopores, were exchanged by Ir3+ to achieve the single-atom loading. Finally, the resulting powder was calcined in air for the topotactic decomposition of β-Co(OH)2 into spinel Co3O4 (Figure S8), forming Ir-single-atom-doped Co3O4 mesoporous single-crystalline nanoplates (designated as H–Ir@Co3O4).

Figure 2.

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Morphological characterizations of H–Ir@Co3O4. (a) Schematic illustration of the atomic exchange method for producing H–Ir@Co3O4. (b) XRD patterns of Co3O4 and H–Ir@Co3O4. (c, d) Dark-field TEM images, (e) AC HAADF-STEM image, and (f) EDS mapping of H–Ir@Co3O4.

In the XRD patterns of H–Ir@Co3O4, the diffraction peaks at 2θ values of 19°, 31°, and 37° can be assigned to the (111), (220), and (311) diffractions of spinel Co3O4 (PDF #43-1003) (Figure 2b). The pattern is similar to that of pure Co3O4, and no diffraction peaks of Ir metal or oxide were observed. The dark-field TEM images (Figure 2c,d) reveal the hexagonal nanoplate morphology of H–Ir@Co3O4, with the uniformly distributed 20 nm mesopores clearly visible within the basal surfaces of the nanoplates. Since the transformation from β-Co(OH)2 to spinel Co3O4 is topotactic, the resulting Co3O4 nanoplate maintains the single-crystalline nature, exposing two (111) basal surfaces and six (110) edge surfaces (Figure S9). The presence of mesopores distributed with the (111) basal surfaces introduces more edge sites around the pores. To determine the distribution of Ir species at the atomic scale, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) images were collected. In Figure 2e, Ir single atoms with high contrast can be identified, dispersed throughout the Co3O4 and relatively concentrated at the pore edges. The EDS elemental mapping (Figures 2f and S10) also reveals the uniform distribution of Ir single atoms, appearing to be more abundant at the edges of the mesopores. The composition in H–Ir@Co3O4, as determined via ICP-MS, shows an atomic ratio of 6:94 of Ir to Co (Ir loading of 13.8 wt % in the catalyst).

The surface electronic states of Co3O4 and H–Ir@Co3O4 were analyzed using X-ray photoelectron spectroscopy (XPS). In the XPS Co 2p spectra of H–Co3O4, characteristic peaks of Co2+ at 781.0 eV (2p3/2) and 796.5 eV (2p1/2), as well as Co3+ peaks at 779.6 eV (2p3/2) and 794.7 eV (2p1/2), were observed. After Ir doping, there was a positive shift of about 0.2 eV in the binding energy of the Co 2p3/2 peak, indicating a change in the electronic state of Co (Figure S11). In Figure 3a, the Ir 4f XPS spectra of IrO2 exhibit a doublet at 61.5 eV (4f7/2) and 64.5 eV (4f5/2), characteristic of Ir4+, consistent with that of the product directly calcined from potassium hexachloroiridate (Figure S12). In contrast, the Ir 4f peak of H–Ir@Co3O4 shifted by 0.5 eV toward a higher binding energy, suggesting a valence state below +4.38,52 The O 1s XPS spectra can be deconvoluted into three peaks corresponding to lattice oxygen (M–O), oxygen in coordinatively oxygen vacancy (Ov) or hydroxyl group (M–OH), and oxygen in adsorbed water (OH2O) (Figure 3b). H–Ir@Co3O4 shows a positive shift of about 0.3 eV for M–OH compared to Co3O4. These results indicate an electron interaction between Co and Ir, which modulates their electronic state to each other.

Figure 3.

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Electronic structure of H–Ir@Co3O4. (a, b) Ir 4f and O 1s XPS spectra of H–Ir@Co3O4 and Co3O4. (c, d) Co K-edge XANES spectra and FT-EXAFS spectra of H–Ir@Co3O4, Co3O4, and Co foil. (e, f) Ir L3-edge XANES spectra and FT-EXAFS spectra of H–Ir@Co3O4, IrO2, and Ir foil.

To further probe the local coordination environment of H–Ir@Co3O4, XAS was carried out. In the Co K-edge X-ray near edge absorption fine structure (XANES) spectra, the right-shifted white line of H–Ir@Co3O4 compared to Co3O4 indicates a slight increase in the Co valence state (Figure 3c),52 in agreement with the Co XPS results. The Co K-edge X-ray absorption fine structures (EXAFS) of pure Co3O4 show three distinct peaks at 1.49, 2.43, and 2.93 Å, identified as Co–O, Cooct–Cooct, and Cotet/Cooct–Cotet bonds, respectively53 (“oct” indicates octahedral sites and “tet” indicates tetrahedral sites in spinel) (Figure 3d). For H–Ir@Co3O4, the characteristic peak associated with the Co–O bond shifts by 0.1 Å to the left compared to Co3O4, suggesting a contraction in the Co–O bond lengths due to the lattice distortion of Co3O4 caused by the Ir doping.52 In addition, the intensities of the Co–O and, in particular, Co–Co peaks become weaker in H–Ir@Co3O4 compared to Co3O4, suggesting that the doped Ir may occupy the original Co site in the spinel structure and alter the coordination environment of surrounding Co atoms.

In the Ir L3-edge XANES spectra in Figure 3e, the white line intensity of H–Ir@Co3O4 is lower than that of IrO2, indicating a valence lower than +4,52 consistent with the XPS results. Meanwhile, the lower integrated white line intensity for H–Ir@Co3O4 indicates that Ir in H–Ir@Co3O4 has fewer empty d orbital states than Ir in IrO2, owing to its electrostatic interaction with surrounding Co atoms.52 In the Ir WT-EXAFS spectra (Figure 3f), the Ir–Ir bond is absent in H–Ir@Co3O4, unlike in Ir foil, where it is evident at 2.5 Å,54 indicating the presence of Ir single atoms. Furthermore, compared to IrO2, the position of the Ir–O bond in the H–Ir@Co3O4 is shifted to the left by 0.03 Å, which is also attributed to the entry of Ir into the Co3O4 lattice, considering that the M–O bond length in Co3O4 is smaller than that in IrO2. The wavelet transform (WT) is highly sensitive in both the k and R ranges of EXAFS and can provide valuable insights into the structural changes occurring in the material. The WT contour plots of H–Ir@Co3O4 exhibit a peak at 1.6 Å–1 ascribed to the Ir–O bond, and the Ir–Ir contribution is not visible, further revealing the isolated, atomically distributed Ir species (Figure S13). The coordination number of the Ir–O bond in H–Ir@Co3O4 is lower than that in IrO2 (Table S2). This unsaturated coordination environment is likely to enhance the adsorption of oxygen-containing intermediates on the active sites.55 The EXAFS fitting results in both r and k spaces for different samples are shown in Figures S14–S19, Tables S1, and S2. The low R factors and Χ2 provide confidence in the accuracy of the fitting procedure.

The electrocatalytic OER performance of the synthesized catalysts was evaluated in 0.5 M H2SO4 at room temperature. Figure 4a shows the current and resistance (iR)-corrected polarization curves of H–Ir@Co3O4, nonporous Co3O4 with the same Ir loading amount (Ir@Co3O4), H–Co3O4, Co3O4, and commercial IrO2 (sourced from Premetek) (the current density was normalized based on the area of the glassy carbon electrode). H–Ir@Co3O4 displays the lowest overpotential of 248 mV at a current density of 10 mA cm–2, significantly outperforming Ir@Co3O4 (301 mV), H–Co3O4 (485 mV), Co3O4 (500 mV), and IrO2 (295 mV) (Figure 4b). The reaction kinetics for electrochemical OER on the catalysts were evaluated by calculating Tafel slopes (Figure 4c). H–Ir@Co3O4 shows the smallest Tafel slope value of 57.9 mV dec–1, implying more favorable OER kinetic compared to other catalysts.

Figure 4.

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Electrocatalysis OER of H–Ir@Co3O4in 0.5 M H2SO4. (a) iR-corrected polarization curves, (b) overpotentials required for j = 10 mA cm–2 and (c) the corresponding Tafel plots of the catalysts. (d) TOFs based on Ir calculated at overpotentials of 300 or 350 mV. (e) Chronopotentiometry curves of H–Ir@Co3O4, Ir@Co3O4, H–Co3O4, and IrO2 at a current density of 10 mA cm–2. (f, g) bright-field TEM images (the inset is the SAED spots) and (h) EDS mapping of H–Ir@Co3O4 after 100 h of OER testing. (i) Leaching mass of Ir and Co from H–Ir@Co3O4 and Ir@Co3O4 after 2 and 10 h of OER at a current density of 10 mA cm–2 (the catalysts were loaded on a glassy carbon electrode). (j) Ir 4f XPS spectra of H-IrO2@Co3O4 before and after OER. (k) η10 and stability comparisons of H–Ir@Co3O4 and state-of-the-art OER catalysts in acid electrolyte.

The electrochemical active surface area (ECSA) of H–Ir@Co3O4 is approximately 1.84 times that of Ir@Co3O4 (Figure S20), indicating that the mesoporous structure of H–Ir@Co3O4 exposes more catalytically active sites. The specific activities of the H–Ir@ Co3O4, Ir@Co3O4, and IrO2 catalysts were evaluated by normalizing the measured current at 1.53 V vs RHE to their ECSAs (JECSA). The results show that the JECSA of H–Ir@Co3O4 (0.072 mA cm–2) is approximately 2.48 and 5.14 times higher than those of Ir@Co3O4 (0.029 mA cm–2) and IrO2 (0.014 mA cm–2), respectively, highlighting the high intrinsic activity of H–Ir@Co3O4 (Figure S21).

TOF, which reflects the intrinsic per-site activity of a catalyst, is a critical merit. Assuming that all metal active centers are accessible, H–Ir@Co3O4 displays a high TOF of 0.219 s–1 at an overpotential of 350 mV for OER (Figure S22). In comparison, the TOFs of Ir@Co3O4, H–Co3O4, Co3O4, and IrO2 are 0.042, 0.001, 0.001, and 0.142 s–1, respectively. H–Ir@Co3O4 exhibits an activity normalized by Ir mass of 1465 A g–1 at 1.53 V vs RHE, which is 21.5 times higher than that of IrO2 (69 A g–1) (Figure S23). The TOFs based on the Ir were also calculated, and the results show that the value of H–Ir@Co3O4 is an order of magnitude higher than that of IrO2 (Figure 4d), suggesting efficient Ir utilization.

Chronopotentiometry testing (no iR correction) of H–Ir@Co3O4 reveals minimal change in potential during catalysis for over 100 h at a current density of 10 mA cm–2 (Figure 4e), demonstrating its superior stability compared to Ir@Co3O4, IrO2, and Co3O4. H–Ir@Co3O4 maintained its hexagonal morphology after OER catalysis (Figure 4f), with the mesoporous structure remaining intact (Figure 4g). The SAED spots indicate that the nanoplate remains a spinel single crystal. The HRTEM image also shows that no amorphous layer is formed on the catalyst surface after the OER (Figure S24). The EDS mapping shows a uniform distribution of Co, Ir, and O through the nanoplate (Figure 4h). The Raman spectra of H–Ir@Co3O4 before and after the OER demonstrate that the Co3O4 matrix can maintain excellent structural stability during the OER process (Figure S25). We collected the electrolytes after 2 and 10 h of OER testing at 10 mAcm–2 for ICP testing to study metal leaching. As depicted in Figure 4i, the Ir and Co leaching amounts from H–Ir@Co3O4 after 2 h of catalysis are 0.09 and 0.78 μg, respectively. These amounts increase to 0.19 and 1.07 μg, respectively, after 10 h. The calculated Ir dissolution rate is 12.5 ng h–1, and the Co dissolution rate is 36 ng h–1. For Ir@Co3O4 without mesopores, the Ir and Co leaching amounts after 2 h are 0.17 and 1.42 μg, respectively, which rise to 0.49 and 2.55 μg after 10 h. Its Ir and Co dissolution rates are 40 and 141 ng h–1, respectively, which are about 3 and 4 times higher than those of H–Ir@Co3O4. Additionally, we measured the dissolution of Ir and Co from H–Ir@Co3O4 loaded on 1 cm2 carbon paper during OER operation for 100 h. As shown in Figure S26, the increase in Ir content leached into the electrolyte became minimal after 20 h, indicating the high stability of the Ir species. Although Co did not completely stop dissolving like Ir over time, the dissolution rate of Co was greatly reduced after 20 h (Figure S27), indicating a structural integrity of Co3O4, which is consistent with the literature.56,57 This suggests that the mesopores create more edge sites for the uniform loading of Ir, enabling the highly efficient utilization of Ir while effectively stabilizing both Co and Ir to enhance dissolution resistance. The Ir 4f7/2 and 4f5/2 peaks in the XPS spectra show a negative shift after 100 h OER (Figure 4j), revealing the high stability of Ir in H–Ir@Co3O4. Additionally, the Ir L3-edge EXAFS spectra of H–Ir@Co3O4 after OER remain similar to the original, confirming that Ir remains in a single atom state (Figure S28). The Ir–O bond length and coordination number show minimal variation before and after the OER (Figure S29 and Table S2). The OER activity and stability of H–Ir@Co3O4 have surpassed most advanced Ir-containing OER electrocatalysts (Figure 4k and Table S3).

We performed DFT calculations to further explore the impact of single-atom Ir doping on the stability of Co3O4 and its OER activity. An Ir atom was introduced to replace the Co atom on the (111) surface or (110) surface of spinel Co3O4 to build the models. The calculated surface Pourbaix diagram reveals that the Co3O4(110) surface is initially covered by ∼1 ML HO* at 0.1–1.73 V vs RHE and then covered by ∼1 ML O* after 1.73 V vs RHE (Figure 5a,b). For the Co3O4(111) surface, the coverage of 1 ML O* occurs at potentials higher than 1.29 V vs RHE (Figure 5c,d). The transformation potentials from 1 ML HO* to 1 ML O* coverage become lower after Ir doping than before for both the Co3O4(111) and (110) surfaces (Figure 5f), which implies that both the Co3O4(111) and (110) surfaces become more stable. We also calculated the Crystal Orbital Hamilton Populations (COHP) of HO* adsorbed on the Co3O4 surfaces with and without Ir doping. After Ir doping, the H–O bond lengths on the Co3O4(110) and (111) facets increase from 0.978 and 0.982 to 0.979 and 0.998 Å, respectively, indicating enhanced deprotonation ability, which is consistent with the surface Pourbaix diagram. The 1 ML O* surface coverage configurations of Ir-doped Co3O4 are depicted in Figure 5e.

Figure 5.

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DFT calculations of H–Ir@Co3O4. Calculated 1D surface Pourbaix diagrams for the H–Ir@Co3O4(110) surface (a) and (110) surface (c) as a function of potential (pH = 0; T = 298.15 K). Calculated 2D surface Pourbaix diagrams for the (111) surface (b) and (111) surface (d) of H–Ir@Co3O4 as a function of potential and pH (T = 298.15 K). (e) Structural models (side view) of the H–Ir@Co3O4(111) and (110) surfaces covered by 1 ML O*. (f) Formation voltages of Co3O4 and H–Ir@Co3O4(111) and (110) surfaces covered by 1 ML O*. (g) Schematic of the structure and active site of H–Ir@Co3O4. (h) Location of Co3O4 and H–Ir@Co3O4(111) and (110) surfaces covered by 1 ML O* in the volcano model. Computational structures are also available in the Digital Catalysis Platform (DigCat): https://www.digcat.org/.58

To compare the electronic structures of Ir–Co3O4 and IrO2, we performed partial density of states (PDOS) calculations (Figure S30). The results show that the distances from the Ir d-band center to the O p-band center in Ir–Co3O4(111) and (110) surfaces are lower than that in IrO2. The enhanced proximity of the Ir d-band center to the O p-band center in Ir–Co3O4 strengthens the Ir–O covalency, which may facilitate electron transfer between adsorbates and the active site.59 The lower d-band center for Ir in Ir–Co3O4 (−3.07 eV for (111) and −3.22 eV for (110)) compared to IrO2 (−2.83 eV) suggests that the 3d–2p–4d orbital hybridization modifies the electronic environment of the Ir d-orbitals. Notably, the dz2-state energy level of Ir in Ir–Co3O4(111) is elevated to −1.91 eV, closer to the Fermi level compared to −2.16 eV for IrO2 (Figure S31). This shift reduces the electronic population in the antibonding states, implying a strengthened adsorption for OER intermediate HO*.60

To assess the OER activity of the Ir-doped Co3O4 surfaces covered with 1 ML O*, we computed the OER descriptor ΔGO* – ΔGHO*. We then used DFT to model the adsorption of OER intermediates on the O*-covered (111) and (110) surfaces of Ir-doped Co3O4 under OER potentials. Three potential adsorption sites were considered: Ir-top, Co-top, and Ir–Co bridge sites (Figures S32 and S33). The results indicate that for both Ir-top and Co-top sites, the intermediate O* adsorption is unstable and spontaneously optimizes to the Ir–Co bridge site, highlighting the Ir–Co bridge position as the most stable adsorption site for O*. By performing structural optimization, we determined that the bridge sites of the two metal atoms in the models serve as stable absorption sites for the reaction intermediates O* and HO*, suggesting the Co–Co and Co–Ir bridge sites as the active sites for Co3O4 and Ir-doped Co3O4, respectively (Figures 5g, S29, and S30). For both (111) and (110) surfaces, the GO* – GHO* values can be optimized upon Ir doping. Especially, the GO* – GHO* on the Co–Ir bridge site of the Ir–Co3O4(111) surface is 1.93 eV, which is near the peak of the volcano model (Figure 5h), in contrast to the 2.34 eV of the Co–Co bridge site of the Co3O4(111) surface, indicating a substantial enhancement in OER catalysis activity. Upon single-atom Ir doping, the theoretical OER overpotential of the Co3O4(111) surface decreases by about 120 mV. Therefore, in our catalyst, the presence of mesopores on the (111) surface of Co3O4 single-crystalline nanoplates not only creates more intrinsically high-activity (110) edge sites but also allows the Ir single atoms to activate the O*-passivated (111) basal surface of Co3O4. Moreover, the single-crystalline nature of our Co3O4 can play a stable support to reduce metal leaching, while the mesopores offer a large loading capacity for Ir. The Co K-edge XANES reveals a valence reduction for Co after Ir doping, implying a modified electronic structure and electron transfer between Co and Ir, which contributes to the enhancement of the instinct OER catalytic activity.

To evaluate the application potential of the H–Ir@Co3O4 catalyst, a membrane electrode assembly (MEA) was fabricated, and the performance of the PEM water electrolyzer (PEMWE) was assessed. H–Ir@Co3O4 and commercial Pt/C catalysts were spray-coated onto opposite sides of a PEM (Nafion 115) with an area of 5 cm2. Titanium felt and carbon paper were employed as gas diffusion layers for the anode and cathode, respectively. The titanium felt was pressed against the membrane side coated with H–Ir@Co3O4 catalyst, while the carbon paper was pressed against the side with the Pt/C cathode catalyst, completing the assembly of the MEA (Figure 6a). Figure 6b illustrates the polarization curves of the PEMWE at 80 °C, where current densities of 200 and 300 mA cm–2 were achieved at applied voltages of 1.54 and 1.59 V, respectively. The PEMWE operated continuously for 330 h at a current density of 200 mA cm–2, demonstrating the catalyst’s excellent stability (Figure 6c). These results underscore the potential of H–Ir@Co3O4 as a promising acidic catalyst in water electrolysis for hydrogen production.

Figure 6.

Figure 6

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PEMWE test of H–Ir@Co3O4. (a) Photo and (b) polarization curve of the PEMWE at 80 °C. (c) Chronopotentiometry curve of the PEMWE at 200 mA cm–2 at 80 °C.

Conclusions

In this study, it was observed that spinel Co3O4 rapidly loses OER catalytic activity under acidic conditions, yet it remains structurally intact, ruling out dissolution as the cause of deactivation. DFT calculations revealed that before or during acidic OER, the (110) and (111) surfaces of Co3O4 can be covered by 1 ML O*, which leads to a rapid loss in OER catalytic activity but may contribute to the stability of Co3O4 in acidic OER environments. To address this trade-off between activity and stability, we designed and synthesized Co3O4 mesoporous single crystals doped with Ir single atoms, resulting in a stable and efficient acidic OER catalyst. The presence of mesopores creates additional sites for high Ir loading of up to 13.8 wt %, and the doped Ir single atoms activate the O*-passivated (111) basal surface of Co3O4. Moreover, the single-crystalline nature of our material makes it suitable as a robust catalyst, resistant to metal leaching, especially after the doping of Ir single atoms. As a result, our catalyst achieved a low η10 of 248 mV and demonstrated stable OER catalysis in acid media for over 100 h. This strategy of using mesoporous single crystals may be extended to the design and synthesis of other catalysts.

Acknowledgments

The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (2022YFB3708800 and 2021YFB3701900), the National Natural Science Foundation Program of China (52474320, 52131307, 52130407, 52071013, 52104359, and 51774035), Guangdong Basic and Applied Basic Research Foundation (2023A1515140193, 2021A1515110202), the National Natural Science Foundation of China (No. 22309109), the Fundamental Research Funds for the Central Universities (QNXM20210046, FRF-IDRY-GD21-002, FRF-TP-19-003C2, FRF-IDRY-19-025, FRF-IDRY-20-022, FRF-TP-20-032A2, and FRF-TP-20-100A1Z), Scientific and Technological Innovation Foundation of Foshan (BK21BE007), JSPS KAKENHI (Nos. JP23K13703 and JP24K23068), the Hirose Foundation, the China Scholarship Council Foundation Program (202306460013), and the Postdoctoral Fellowship Program of CPSF (GZC20240012). We thank the Center for Computational Materials Science, Institute for Materials Research, Tohoku University for the use of MASAMUNE-IMR (Nos. 202412-SCKXX-0211 and 202412-SCKXX-0209) and the Institute for Solid State Physics (ISSP) at the University of Tokyo for the use of their supercomputers.

Data Availability Statement

All the data are available from the authors upon reasonable request. The key results of the materials, OER experiments, and the computational structures are also available in the Digital Catalysis Platform (DigCat) database: https://www.digcat.org/.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c18390.

  • Experimental details, calculation of the electrochemical active surface area and turnover frequency, additional schematic, and DFT calculation details (DOCX)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ja4c18390_si_001.docx (34.2MB, docx)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ja4c18390_si_001.docx (34.2MB, docx)

Data Availability Statement

All the data are available from the authors upon reasonable request. The key results of the materials, OER experiments, and the computational structures are also available in the Digital Catalysis Platform (DigCat) database: https://www.digcat.org/.

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