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. 2026 Jun 16;65(34):e4980930. doi: 10.1002/anie.4980930

Ultrahigh Density Ir Single‐Atom Catalysts With Synergistic Ir‐Ir Pairs for Efficient Acidic Oxygen Evolution

Junjie Zou 1,2, Jiankang Zhao 1, Kainan Mei 1, Zheng Liu 1, Ming Zuo 1, Kwun Nam Hui 3, Mingkai Liu 4, Zhirong Zhang 1,✉, Leonid Kustov 5, Jie Zeng 1,2,4,✉
PMCID: PMC13480679  PMID: 42299060

ABSTRACT

Constructing neighboring active sites in single‐atom catalysts (SACs) provides a new strategy for enhancing acidic oxygen evolution reaction (OER) performance. However, the OER activity of such systems is highly sensitive to their structural configuration. Due to the typically low density of single atoms, most synergistic effects originate from interactions between single atoms and the supports, while extensive synergy between neighboring single atoms remains scarce. In this work, we introduced ultrahigh density Ir single atoms onto Co3O4 support to construct numerous Ir‐Ir pairs for enhanced acidic OER performance. Electrochemical measurements revealed that the SACs achieved an overpotential of only 250 mV at a current density of 10 mA cm−2, 90 mV lower than that of the lower density SACs, and maintained stability over 3000 h at a current density of 50 mA cm−2. In a proton exchange membrane water electrolyzer, this catalyst required only 1.69 V to achieve a current densit of 1.0 A cm−2 and operated stably for 700 h. In situ spectroscopic characterization and density functional theory calculations confirmed Ir‐Co pairs in low‐density SACs exhibited excessively strong intermediate adsorption, whereas the Ir‐Ir pairs formed in ultrahigh density SACs optimized the adsorption strength, thus enhancing the synergistic efficiency of neighboring sites.

Keywords: acidic OER, neighboring sites, OPM mechanism, synergistic efficiency, ultrahigh density SACs

In this work, by precisely regulating the density of Ir single atoms on the Co3O4 support, the neighboring Ir‐Co sites and Ir‐Ir sites are constructed. Both types of sites catalyze the acidic OER via an O─O coupling mechanism. However, compared to Ir‐Co sites in low density SACs, the Ir‐Ir sites in ultrahigh density SACs exhibited optimized adsorption of intermediates, thereby enhancing acidic OER performance.

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1. Introduction

Proton exchange membrane water electrolyzer (PEMWE) has emerged as a promising green hydrogen production technology due to its rapid system response, high current density, and high hydrogen production purity [1, 2, 3, 4]. The performance of PEMWE is predominantly obstructed by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode [5, 6, 7, 8]. Although benchmark catalysts such as IrO2 and RuO2 exhibit high OER activity in acidic environments, their high cost and scarcity as precious metals hinder the widespread utilization of such catalysts [9, 10]. Spinel cobalt oxides are considered a highly promising candidate for acidic OER owing to their earth abundance and remarkable catalytic activity. Nevertheless, their practical application in PEMWE is hindered by insufficient performance under acidic operation [11, 12, 13]. Therefore, developing an effective strategy to enhance the activity of spinel cobalt oxides in acidic OER is urgently, but still a great challenge.

Spinel cobalt oxide‐supported single‐atom catalysts (SACs) represent a promising class of anode catalysts for PEMWE systems [14, 15, 16, 17, 18]. Nevertheless, the isolated atoms in SACs typically act as single active sites to catalyze the OER via the adsorbate evolution mechanism (AEM) [18, 19, 20]. This inherently constrains OER activity due to the scaling relationships, resulting in a relatively limited performance [21, 22, 23, 24]. Recently, several studies have demonstrated that neighboring‐site synergy in SACs offers an effective strategy to enhance oxygen evolution performance. In such systems, each site can serve as an active center for synergistically optimizing the adsorption of OER intermediates, facilitating a transition of OER mechanism from AEM to the oxide path mechanism (OPM). The OPM circumvents the limitations imposed by scaling relationships, thereby significantly improving OER performance [25, 26, 27]. However, due to the generally low density of single atoms, most synergistic effects originate from interactions between single atoms and atoms within the support material, while extensive synergy between neighboring single atoms remains scarce [28, 29]. Indeed, neighboring single atoms exhibit well‐defined site symmetry, which is advantageous for enhancing synergistic efficiency [30]. Therefore, precise design and configuration of neighboring single‐atom sites are highly desirable to maximize synergistic efficiency and boost OER performance.

In this work, we achieved efficient synergy between neighboring single‐atom sites by construction ultrahigh density SACs and significantly improved oxygen evolution performance. By precisely regulating the density of Ir single atoms on Co3O4 support, the neighboring Ir‐Co sites and Ir‐Ir sites were constructed. Electrochemical measurements revealed that 17.4%‐Ir1/Co3O4 with Ir‐Ir sites exhibited an overpotential of 250 mV at a current density of 10 mA cm−2, considerably lower than the 340 mV of 4.6%‐Ir1/Co3O4 with Ir‐Co sites, and long‐durable stability over 3000 h at a current density of 50 mA cm−2. Moreover, the 17.4%‐Ir1/Co3O4 achieved a current density of 1.0 A cm−2 at 1.69 V in a practical PEMWE device and demonstrated stable operation for 700 h at this current density. In situ spectroscopic characterizations and theoretical calculations confirmed that Ir‐Ir sites exhibited optimized intermediate binding affinity, enhancing synergistic efficiency compared to Ir‐Co sites. This work not only improved the synergistic efficiency of neighboring sites through single‐atom density engineering, but also provided new insights for designing high‐performance PEMWE anode catalysts.

2. Results and Discussion

2.1. Structural Characterization of Catalysts With Different Ir Single Atoms Densities

Single atoms with different densities supported on cobalt oxides were synthesized via a cation exchange‐pyrolysis strategy. By modulating the concentration of precursors, a series of Ir1/Co3O4 with different Ir single atom densities were obtained. Transmission electron microscopy (TEM) images showed that all prepared samples presented morphologies similar to those of the Co3O4 support (Figure S1). The x‐ray diffraction (XRD) patterns revealed that all characteristic peaks of the samples correspond to the structure of Co3O4 (PDF #25‐0270), suggesting the absence of detectable Ir‐based metals and metal oxides (Figure S2a). Furthermore, as the mass loading of Ir species increased, the XRD peaks gradually shifted toward smaller angles, indicating the Ir single atoms were incorporated into the lattice of Co3O4 (Figure S2b). The mass loadings of Ir species for Ir1/Co3O4 with different Ir single atoms densities were determined to be 4.6, 11.4, and 17.4 wt% by inductively coupled plasma‐atomic emission spectroscopy (ICP‐AES), denoted as 4.6%‐Ir1/Co3O4, 11.4%‐Ir1/Co3O4, and 17.4%‐Ir1/Co3O4, respectively. Remarkably, for 17.4%‐Ir1/Co3O4, the mass loading exceeds that of most reported SACs supported on cobalt oxides (Figure S3 and Table S1).

Aberration‐corrected high‐angle annular dark‐field scanning TEM (HAADF‐STEM) images of Ir1/Co3O4 with different Ir single atoms densities showed isolated bright spots against the Co3O4 support, confirming atomic‐level dispersion of Ir species (Figure 1a–c). Notably, the density of Ir single atoms rose with increasing Ir species loading. In the magnified HAADF‐STEM image of 17.4%‐Ir1/Co3O4, the line intensity profile along the white dashed rectangular region confirmed that Ir atoms were anchored at the lattice sites of Co3O4. Furthermore, multiple neighboring Ir‐Ir pairs were observed (Figure 1d,e). To accurately determine the distances between neighboring atoms, we measured the Ir‐Co and Ir‐Ir distances across multiple regions of HAADF‐STEM images from two samples. As shown in Figure S4, the Ir‐Co distance in 4.6%‐Ir1/Co3O4 was approximately 0.28 nm, while the Ir‐Ir distance in 17.4%‐Ir1/Co3O4 was about 0.26 nm (Figure 1d,e). This indicated that the interatomic distances indeed shorten after the formation of ultrahigh density single atoms. Statistical analysis revealed that the density of Ir single atoms was 9.7 atoms nm−2 for 17.4%‐Ir1/Co3O4, which was 3.6 times and 1.5 times higher than that of 4.6%‐Ir1/Co3O4 and 11.4%‐Ir1/Co3O4, respectively (Figures 1f and S5). Further analysis in three regions of the HAADF‐STEM images showed that the percentage of Ir‐Ir neighboring sites in 17.4%‐Ir1/Co3O4 was approximately 70.0%, which was substantially higher than that in 4.6%‐Ir1/Co3O4 (26.4%) and 11.4%‐Ir1/Co3O4 (42.2%) (Figures S6 and S7). Moreover, energy‐dispersive x‐ray (EDX) elemental mapping demonstrated homogeneous distribution of Co, O, and Ir species in 17.4%‐Ir1/Co3O4 (Figure 1g). The EDX elemental analysis showed a gradual rise in the intensity of the Ir element peak at about 2.0 keV, indicating an increase in Ir single atoms content (Figure 1h). Raman spectra displayed three characteristic peaks at about 474.1, 517.0, and 680.6 cm−1, corresponding to the E g, F 2g, and A 1g vibration of the spinel Co3O4 lattice (Figure 1i) [31]. Significantly, these peaks exhibited gradual red shifts in 4.6%‐Ir1/Co3O4, 11.4%‐Ir1/Co3O4, and 17.4%‐Ir1/Co3O4 compared to the Co3O4 support, which resulted from the increased density of Ir single atoms in the lattice.

FIGURE 1.

FIGURE 1

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Structural characterizations of catalysts with different Ir single atoms densities. (a–c) HAADF‐STEM images of Ir1/Co3O4 with different Ir single atoms densities. (d) Magnified HAADF‐STEM image of 17.4%‐Ir1/Co3O4. (e) Line intensity profile obtained from the selected atomic column in the magnified HAADF‐STEM image of 17.4%‐Ir1/Co3O4. (f) Density of Ir single atoms per nm2 on Ir1/Co3O4 with different Ir single atoms densities. (g) EDX elemental mapping of 17.4%‐Ir1/Co3O4. (h) EDX elemental analysis result of Co3O4 and Ir1/Co3O4 with different Ir single atoms densities. (i) Raman spectra of Co3O4 and Ir1/Co3O4 with different Ir single atoms densities.

The electronic structure and coordination environment of Ir1/Co3O4 with different Ir single atom densities were investigated by x‐ray absorption near‐edge spectroscopy (XANES) and extended x‐ray absorption fine structure (EXAFS) spectroscopy. In the Ir L 3‐edge XANES spectra, the white‐line intensity gradually declined with increasing Ir single‐atom density, which suggested a reduced valence state of the Ir species (Figure 2a) [32, 33, 34]. In the EXAFS spectra at the Ir L 3‐edge, two peaks were observed at about 1.5 and 2.6 Å, corresponding to first‐shell Ir‐O coordination and second‐shell Ir‐Co or Ir‐Ir coordination, respectively (Figure 2b). The appearance of second‐shell coordination confirmed that Ir single atoms were anchored at lattice sites of Co3O4. Furthermore, the Ir‐O bond length increased at higher Ir single atom densities (Figure 2c) [34, 35, 36]. Fitting results indicated that the coordination numbers (CNs) for both Ir‐O and Ir‐Co or Ir‐Ir decreased as Ir single‐atom density increased (Figure S8 and Table S2).

FIGURE 2.

FIGURE 2

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Characterization of electronic structure and coordination environment of catalysts with different Ir single atoms densities. (a, b) Normalized XANES (a) and EXAFS (b) spectra at the Ir L 3‐edge of 4.6%‐Ir1/Co3O4, 11.4%‐Ir1/Co3O4, 17.4%‐Ir1/Co3O4. IrO2 and Ir foil were used as references. (c) Bond length of Ir‐O of Ir1/Co3O4 with different Ir single atoms densities. (d) Normalized XANES spectra at the Co K‐edge of Co3O4 and Ir1/Co3O4 with different Ir single atoms densities. (e) Energy change of the Co K‐edge XANES absorption edge for Ir1/Co3O4 with different Ir single atoms densities relative to Co3O4 (top) and Co3+/Co2+ ratio of Ir1/Co3O4 with different Ir single atoms densities in Co 2p XPS spectra (down). (f) EXAFS spectra at the Co K‐edge of Co3O4 and Ir1/Co3O4 with different Ir single atoms densities. (g) XPS spectra of Ir 4f. (h) XPS spectra of O 1s.

In the Co K‐edge XANES spectra, the absorption edge slightly shifted to lower energy with the increased density of Ir single atoms, indicating a gradual reduction in the valence state of Co species (Figure 2d,e) [34, 37, 38]. This decrease in Co valence was further supported by Co 2p x‐ray photoelectron spectroscopy (XPS), which showed a reduction in the Co3+/Co2+ ratio as the Ir single‐atom density increased (Figures 2e and S9) [39]. The Co K‐edge EXAFS spectra displayed two prominent peaks at about 1.5 and 2.4 Å, corresponding to Co‐O and Co‐Co bonding, respectively (Figure 2f). Notably, the intensity of both the Co‐O and Co‐Co peaks decreased with increasing density of Ir single atoms, indicating a reduction in the CNs [40].

The electronic structure of SACs with different Ir atoms densities was further analyzed by XPS. It was observed that as the density of Ir single atoms increased, the Ir 4f peaks shifted toward lower binding energies, indicating a decrease in the valence state of Ir species. This observation was consistent with the XANES results (Figure 2g) [32, 41]. The O 1s XPS spectra of all samples exhibited three characteristic peaks at around 526.9, 528.1, and 529.7 eV, which were attributed to M‐O, oxygen vacancy (VO), and adsorbed H2O, respectively (Figure 2h) [42, 43]. It is noteworthy that the concentration of VO gradually increased with higher Ir single‐atom density, indicating that Ir single atoms induced VO generation in Co3O4. The increase in the concentration of these VO may have facilitated the adsorption of reaction intermediates at the active sites.

2.2. Electrocatalytic Evaluation of Acidic OER

To probe the role of Ir single atoms with different densities, we evaluated the catalytic performance of these catalysts in oxygen evolution under acidic media. Polarization curves were recorded in a standard three‐electrode system in 0.1 M HClO4 electrolyte. As shown in Figure 3a, these catalysts showed enhanced OER activities from Co3O4, IrO2, 4.6%‐Ir1/Co3O4, 11.4%‐Ir1/Co3O4, to 17.4%‐Ir1/Co3O4. Specifically, 17.4%‐Ir1/Co3O4 required only an overpotential (η) of 250 mV to deliver a current density of 10 mA cm−2, which was 140, 90, 90, and 30 mV lower than that of Co3O4, IrO2, 4.6%‐Ir1/Co3O4, and 11.4%‐Ir1/Co3O4, respectively (Figure 3b). For comparison, the overpotential at a current density of 10 mA cm−2 was lower than a number of the currently reported Co‐based acid OER catalysts (Figure S10 and Table S3). Taking into account the difference in mass loading of Ir species, we further compared the intrinsic activity of Ir single atoms. The results exhibited that 17.4%‐Ir1/Co3O4 delivered a mass activity of 125.3 mA mg−1 Ir at an overpotential of 300 mV, which was 2.5 and 1.7 times higher than that of 4.6%‐Ir1/Co3O4 (50.0 mA mg−1 Ir) and 11.4%‐Ir1/Co3O4 (73.3 mA mg−1 Ir), respectively (Figures 3c and S11). Similarly, the 17.4%‐Ir1/Co3O4 achieved a turnover frequency (TOF) of 0.1248 s−1 at an overpotential of 300 mV, which was 5.14 and 3.42 times higher than 4.6%‐Ir1/Co3O4 (0.0243 s−1) and 11.4%‐Ir1/Co3O4 (0.0365 s−1), respectively (Figure 3d). Moreover, the 17.4%‐Ir1/Co3O4 catalyst showed a specific activity of 0.89 mA cm−2 at an overpotential of 300 mV, which was 12.71 and 6.85 times higher than the pristine Co3O4 (0.07 mA cm−2) and 4.6%‐Ir1/Co3O4 (0.13 mA cm−2), respectively (Figures S12 and S13). We also normalized the current density against the surface area estimated from BET measurements to investigate the specific activity of these catalysts. The specific activity of 17.4%‐Ir1/Co3O4 reached 0.019 mA cm−2 at an overpotential of 300 mV, which was 2.7, 6.3, and 9.5 times higher than those of 11.4%‐Ir1/Co3O4, 4.6%‐Ir1/Co3O4, and Co3O4, respectively (Figure S14 and S15). These results demonstrated a strong correlation between OER performance and single‐atom density, with higher density of Ir atoms directly enhancing catalytic activity.

FIGURE 3.

FIGURE 3

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OER performance evaluation of catalysts with different Ir single atoms densities. (a) Polarization curves of catalysts towards oxygen evolution in 0.1 M HClO4 electrolyte. Commercial IrO2 was used as reference. (b) Overpotentials of Co3O4, IrO2 and Ir1/Co3O4 with different Ir single atoms densities at a current density of 10 mA cm−2. (c) Mass activity of Ir1/Co3O4 with different Ir single atoms densities at 300 mV overpotential. (d) TOFs of Ir1/Co3O4 with different Ir single atoms densities normalized to the number of Ir sites. (e) Chronopotentiometry curves of Co3O4 and 17.4%‐Ir1/Co3O4 towards OER at a current density of 50 mA cm−2. (f) Schematic of the PEMWE device. (g) Polarization curve of IrO2 and 17.4%‐Ir1/Co3O4 in the PEMWE electrolyzer. (h) Chronopotentiometry testing of 17.4%‐Ir1/Co3O4 at a current density of 1.0 A cm−2 in the PEMWE electrolyzer.

To gain a deeper insight into the kinetics of the OER, Tafel slopes and electrochemical impedance spectroscopy (EIS) were performed. The Tafel slope values for Co3O4, 4.6%‐Ir1/Co3O4, 11.4%‐Ir1/Co3O4, and 17.4%‐Ir1/Co3O4 decreased from 132, 118, 89, to 76 mV dec−1 (Figure S16). The lower Tafel slope value of 17.4%‐Ir1/Co3O4 suggested its faster OER reaction kinetics. The result was further reflected by the EIS measurements. For these samples, the semicircle diameter decreased as the density of Ir single atoms increased (Figure S17). The smallest semicircle diameter of 17.4%‐Ir1/Co3O4 suggested the fastest charge transfer at the interface, thus accelerating the OER kinetics.

The durability of 17.4%‐Ir1/Co3O4 was evaluated through long‐term chronopotentiometry test. As shown in Figure 3e, the catalyst maintained stable operation for 3000 h at a current density of 50 mA cm−2. After the stability test, the structure of the catalyst was characterized. The HAADF‐STEM image showed that the high density Ir atoms remained in isolated dispersion (Figure S18a). The EDX elemental mapping images revealed that the Co, O, and Ir elements maintained uniform distribution across the sample, indicating its excellent structural stability under acidic OER reactions (Figure S18b).

To further evaluate the industrial application potential of 17.4%‐Ir1/Co3O4, we fabricated a membrane electrode assembly (MEA) and assessed its performance in a proton exchange membrane (PEM) water electrolyzer (Figures 3f and S19). Specifically, the MEA was prepared by coating 17.4%‐Ir1/Co3O4 and Pt/C onto the both sides of Nafion 115 membrane as anode and cathode catalysts, respectively. Commercial IrO2 was used as the benchmark anode catalyst. The polarization curves measured at 80°C revealed that 17.4%‐Ir1/Co3O4 exhibited a cell voltage of 1.69 V at a current density of 1.0 A cm−2, which was 269 mV lower than that of the commercial IrO2 (Figure 3g). To evaluate the catalytic performance of 17.4%‐Ir1/Co3O4 at higher current densities, we assessed its catalytic activity at a current density of 2.0 A cm−2. The catalyst required 2.0 V to achieve a current density of 2.0 A cm−2, significantly lower than that of the commercial IrO2 (2.37 V), demonstrating its substantial potential for practical applications (Figure 3g). Moreover, the PEMWE operated continuously for 700 h at a current density of 1.0 A cm−2 (Figure 3h). The superior performance and excellent stability of 17.4%‐Ir1/Co3O4 in the PEMWE underscore its industrial potential for hydrogen production via water electrolysis. Furthermore, we monitored the leaching of catalyst components at various time points by inductively coupled plasma‐mass spectrometry (ICP‐MS). During the initial test stages, Co and Ir species were observed to leach rapidly. After that, the catalyst gradually stabilized over the next 100 h, and the amount of leached Co and Ir species remained below 4.4% of the total loading, indicating excellent stability (Figure S20).

2.3. In Situ Spectroscopic Analysis of Single Atoms With Different Densities

To probe the structural stability of ultrahigh density Ir single atoms under acidic OER conditions, a series of in situ spectroscopic tests were conducted. The in situ Ir L 3‐edge XANES spectra of 17.4%‐Ir1/Co3O4 revealed a slight positive shift in the white‐line position as the applied potential increased from the open circuit potential (OCP) to 1.45 V, indicating an increased valence state of the Ir species (Figures 4a and S21) [44, 45]. The EXAFS spectra of 17.4%‐Ir1/Co3O4 showed that the position and intensity of the peaks corresponding to Ir‐O and Ir‐Co/Ir bonds exhibited an insignificant change, demonstrating the structural stability of the catalyst throughout the acidic OER process (Figure 4b).

FIGURE 4.

FIGURE 4

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In situ spectroscopic characterizations of catalysts with different Ir single atoms densities. (a, b) In situ Ir L 3‐edge XANES (a) and EXAFS (b) spectra of 17.4%‐Ir1/Co3O4 at different applied potentials. (c–e) In situ Raman spectra of Co3O4 (c), 4.6%‐Ir1/Co3O4 (d), and 17.4%‐Ir1/Co3O4 (e) at different applied potentials. (f) In situ ATR‐SEIRAS of 4.6%‐Ir1/Co3O4 (left) and 17.4%‐Ir1/Co3O4 (right) at different applied potentials. (g, h) In situ DEMS signals of O2 products for 4.6%‐Ir1/Co3O4 (g) and 17.4%‐Ir1/Co3O4 (h) in the electrolyte using H2 18O as the solvent. (i) Ratio of 32O2/36O2 DEMS signals for 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 during four cycles.

The excellent stability was further demonstrated by in situ Raman spectroscopy. These measurements were conducted in a specially designed in situ reaction cell, maintaining each applied potential for 20 min. For Co3O4, as the applied potential increased from OCP to 1.45 V, the intensities of the E g, F 2g, and A 1g peaks gradually decreased. When the applied potential increased to 1.45 V, these peaks disappeared, indicating its structural dissolution during the acidic OER (Figure 4c) [31, 46, 47, 48]. For 4.6%‐Ir1/Co3O4, the intensities of these peaks also exhibited a decreasing tendency with increasing applied potential, suggesting that low‐density Ir single atoms provided a weaker stabilizing effect on the Co3O4 support (Figure 4d) [48]. Notably, for 17.4%‐Ir1/Co3O4, the intensities of the characteristic peaks remained essentially unchanged across the entire potential range, demonstrating its excellent stability (Figure 4e). The above results indicated that the structural stability of catalyst was strongly correlated with single‐atom density, and an increased density reinforced the structural integrity in acidic OER.

In situ attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) was employed to investigate the reaction mechanism. For Co3O4 support, the key *OOH intermediate peak located at about 1032.1 cm−1 was observed, indicating that the reaction proceeded via the AEM mechanism (Figure S22) [48]. In contrast, for 4.6%‐Ir1/Co3O4, two distinct absorption peaks emerged at about 1067.3 and 1103.6 cm−1 as the applied potential increased, corresponding to oxygen bridges between two adjacent sites and linearly linked metal‐O‐O* intermediates, respectively (Figure 4f). This characteristic demonstrated that the introduction of Ir single atoms transformed the OER mechanism from AEM to OPM [27, 49, 50]. For 17.4%‐Ir1/Co3O4, similar peaks also appeared at about 1125.6 and 1146.3 cm−1, confirming that it also followed the OPM pathway. Notably, compared to 4.6%‐Ir1/Co3O4, the absorption peaks for 17.4%‐Ir1/Co3O4 shifted to higher wavenumbers. This shift may result from the shorter distance of Ir‐Ir neighboring sites in 17.4%‐Ir1/Co3O4, which weakens the adsorption of intermediates, leading to a blue shift of the infrared peaks compared to the 4.6%‐Ir1/Co3O4 sample. To further confirm the assignment of the two absorption peaks for the 17.4%‐Ir1/Co3O4 catalyst, in situ ATR‐SEIRAS experiments with 18O isotope labeling were conducted. After 18O labeling, two absorption peaks appeared at 1069.5 and 1109.7 cm−1 as the applied voltage increased, both of which were red shifted compared to the positions measured in H2 16O (Figure S23). This shift was caused by the isotope effect, and similar phenomena were observed in other reports [30]. The results clearly confirmed that the 17.4%‐Ir1/Co3O4 catalyst followed the OPM pathway.

To further confirm the reaction mechanism, in situ differential electrochemical mass spectrometry (DEMS) experiments using 18O isotopic labeling were carried out in 0.1 M HClO4 electrolyte. For Co3O4, the absence of detectable 32O2 signal excluded the OPM pathway for the OER, consistent with prior in situ ATR‐SEIRAS confirmation that the reaction proceeded via the AEM pathway (Figure S24) [49, 50, 51, 52, 53]. For both 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4, three mass signals corresponding to 36O2 (18O18O), 34O2 (18O16O), and 32O2 (16O16O) were detected (Figure 4g,h) [27, 50, 52, 53]. The appearance of 32O2 indicated that these catalysts followed OPM pathway. Furthermore, as the number of CV cycles increased, the 32O2/36O2 ratio gradually decreased, attributed to the consumption of surface‐adsorbed 16O* species. This trend provided additional evidence that 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 followed the OPM pathway (Figure 4i). To further verify the OPM mechanism, the 18O‐labeled 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 catalysts were transferred into a 0.1 M HClO4 electrolyte containing H2 16O, and subjected to cyclic voltammetry (CV) for DEMS signal detection. The appearance of 36O2 (18O18O) confirmed the presence of 18O atoms coupled together on the surface of 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4, indicating these catalysts followed the OPM pathway (Figure S25).

2.4. Mechanistic Studies of Single Atoms With Different Densities

Density functional theory (DFT) calculations were performed to investigate the influence of the neighboring synergistic effects on OER. Structural models of isolated Ir single atoms and neighboring Ir‐Ir pairs on Co3O4 (311) were constructed to simulate 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4, respectively (Figure S26). Projected density of states (PDOS) was employed to elucidate the distinct electronic structures of Ir species in 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 (Figure 5a,b). The d‐band center of Ir species in 17.4%‐Ir1/Co3O4 was located at −1.97 eV, shifted downward by 0.23 eV relative to that in 4.6%‐Ir1/Co3O4 (−1.74 eV). The downshift of the d‐band center originated from the reduced inter‐atomic distances for the neighboring Ir‐Ir or Ir‐Co sites from 2.89 to 2.59 Å, which may modulate the adsorption strength of reaction intermediates (Figure 5c).

FIGURE 5.

FIGURE 5

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Mechanistic studies of catalysts with different Ir single atom densities. (a, b) Ir 5d PDOS plots of 4.6%‐Ir1/Co3O4 (a) and 17.4%‐Ir1/Co3O4 (b). (c) Ir 5d‐band center of 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 (left) and active sites distance of 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 (right). (d, e) Free‐energy diagrams of 4.6%‐Ir1/Co3O4 (d) and 17.4%‐Ir1/Co3O4 (e) toward OER. The red, blue, and green spheres represent O, Co, and Ir atoms, respectively. (f) Comparison of the ΔG of the RDS of 4.6%‐Ir1/Co3O4 (left) and 17.4%‐Ir1/Co3O4 (right). (g) Charge density differences of 4.6%‐Ir1/Co3O4 (left) and 17.4%‐Ir1/Co3O4 (right). The cyan and yellow regions indicate electron depletion and accumulation, respectively. (h, i) Schematic illustration of the synergistic effect of different neighboring atoms on the reaction activity of 4.6%‐Ir1/Co3O4 (h) and 17.4%‐Ir1/Co3O4 (i). The white spheres represent H atoms.

To elucidate the reaction mechanisms on the Co3O4, 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 in acidic media, the free energy diagrams of the potential OER pathways including AEM and OPM were evaluated. For Co3O4, the rate‐determining step (RDS) was the deprotonation of *OH to form *O for both pathways, with an energy barrier of 1.82 eV. Additionally, the Gibbs free energy for single‐site adsorption of the *OOH intermediate was 3.40 eV, lower than that for dual‐site adsorption (3.77 eV), indicating that single‐site adsorption is more favorable for *OOH on the Co3O4 surface (Figures S27 and S28). For both 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4, the RDS was the formation of *OOH intermediate from *O intermediate, regardless of AEM or OPM pathways (Figure 5d,e). Notably, the energy barriers for RDS at dual sites in both catalysts were lower than those at single sites, indicating that both 4.6%‐Ir1/Co3O4 and 17.4%‐Ir1/Co3O4 preferably followed OPM pathway (Figures S29 and S30). This finding was consistent with the results from in situ ATR‐SEIRAS and in situ DEMS. Specifically, the theoretical overpotential (η) of RDS for the reaction via dual sites in 17.4%‐Ir1/Co3O4 was 0.38 V, significantly lower than that of 4.6%‐Ir1/Co3O4 (0.53 V) (Figure 5f). This suggested that 17.4%‐Ir1/Co3O4 exhibited faster reaction kinetics, as confirmed by the experimental results.

To investigate the origin of the reduced RDS energy barrier, the adsorption of *OOH intermediates was analyzed. The Ir─O bond distance was elongated from 1.91 Å on Ir‐Co sites to 1.97 Å on Ir‐Ir sites (Figure 5g). The charge density difference further demonstrated that the longer Ir‐O bond distance results in less electrons accumulating in the bonding region between Ir and O on 17.4%‐Ir1/Co3O4. Meanwhile, the Bader charge analysis suggested the Ir‐Ir sites transfer 0.35 |e| electrons to *OOH species, more than that (0.30 |e|) on the Ir‐Co dual sites. The excess electrons were injected to the O‐O bond and increased the O‐O bond distance from 1.52 to 1.57 Å, decreasing the electrophilicity of *OOH on Ir‐Ir sites (Figure 5g) [54, 55]. Consequently, the adsorption strength of *OOH on 17.4%‐Ir1/Co3O4 was significantly weaker than that on 4.6%‐Ir1/Co3O4, indicating that Ir‐Ir pair sites optimized the adsorption of key intermediates. The above results demonstrated individual Ir atoms engaged in synergistic catalysis with adjacent Co atoms via Ir‐Co dual sites, which exhibited strong intermediate adsorption, leading to compromised OER activity (Figure 5h). When high‐density single atoms were introduced into the support, neighboring Ir atoms formed active Ir‐Ir pairs that optimized intermediate binding affinity, thus enhancing OER activity (Figure 5i).

3. Conclusion

In conclusion, we enhanced the synergistic efficiency of neighboring sites for acidic OER by modulating single‐atom density. Through incorporating Ir single atoms with different densities into the Co3O4 support, distinct neighboring sites were constructed. Electrochemical measurements confirmed that the catalysts with Ir‐Ir pairs exhibited superior acidic OER performance compared to those with Ir‐Co pairs. Both in situ spectroscopic characterizations and mechanistic studies revealed the absorption of key intermediates on Ir‐Ir pairs was weaker than that on Ir‐Co pairs, resulting in a reduced energy barrier for the RDS. This work not only provided insight into the configuration effect of neighboring sites on synergistic efficiency, but also paved the way for the rational design of high‐performance PEMWE anode catalysts.

Author Contributions

Z.Z. and J.Z. designed the study. J.Z., K.M., Z.L., and M.L. conducted the experiments. J.Z., Z.Z., L.K., J.Z., and K.H. wrote the paper. J.Z. carried out DFT calculations. M.Z. conducted HAADF‐STEM analysis. All authors discussed the results and contributed to the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Additional experimental details, materials, methods and characterization data, TEM, XRD, single atoms mass loadings evaluation data, experimental and fitting EXAFS spectra, XPS, OER evaluation data, polarization curves, CV curves, OER intrinsic activity, EIS, HAADF‐STEM, EDX elemental mapping, in situ XANES, in situ ATR‐SEIRAS, in situ DEMS and DFT data (PDF). The Supporting Information is available free of charge at

Supporting File: anie73230‐sup‐0001‐SuppMat.doc.

ANIE-65-e4980930-s001.doc (33.1MB, doc)

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2021YFA1500500), the CAS Project for Young Scientists in Basic Research (YSBR‐051), NSFC (22525021, 22202192, 22221003, 22250007, 22361162655), the Science and Technology Development Fund (FDCT) of Macao S.A.R (0070/2023/AFJ), Fundamental Research Funds for the Central Universities, the Joint Fund of the Yulin University and the Dalian National Laboratory for Clean Energy (YLU‐DNL Fund 2022012), the State Key Laboratory of Catalysis (2024SKL‐A‐011), and the International Partnership Program of Chinese Academy of Sciences (123GJHZ2022101GC). J.Z. acknowledges support from the New Cornerstone Science Foundation through the XPLORER PRIZE. This work was partially carried out at the Instruments Center for Physical Science, University of Science and Technology of China.

Contributor Information

Zhirong Zhang, Email: zzhirong@ustc.edu.cn.

Jie Zeng, Email: zengj@ustc.edu.cn.

Data Availability Statement

Research data are not shared.

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Associated Data

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

Supplementary Materials

Additional experimental details, materials, methods and characterization data, TEM, XRD, single atoms mass loadings evaluation data, experimental and fitting EXAFS spectra, XPS, OER evaluation data, polarization curves, CV curves, OER intrinsic activity, EIS, HAADF‐STEM, EDX elemental mapping, in situ XANES, in situ ATR‐SEIRAS, in situ DEMS and DFT data (PDF). The Supporting Information is available free of charge at

Supporting File: anie73230‐sup‐0001‐SuppMat.doc.

ANIE-65-e4980930-s001.doc (33.1MB, doc)

Data Availability Statement

Research data are not shared.

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