Distance effect of single atoms on stability of cobalt oxide catalysts for acidic oxygen evolution

Developing efficient and economical electrocatalysts for acidic oxygen evolution reaction (OER) is essential for proton exchange membrane water electrolyzers (PEMWE). Cobalt oxides are considered promising non-precious OER catalysts due to their high activities. However, the severe dissolution of Co atoms in acid media leads to the collapse of crystal structure, which impedes their application in PEMWE. Here, we report that introducing acid-resistant Ir single atoms into the lattice of spinel cobalt oxides can significantly suppress the Co dissolution and keep them highly stable during the acidic OER process. Combining theoretical and experimental studies, we reveal that the stabilizing effect induced by Ir heteroatoms exhibits a strong dependence on the distance of adjacent Ir single atoms, where the OER stability of cobalt oxides continuously improves with decreasing the distance. When the distance reduces to about 0.6 nm, the spinel cobalt oxides present no obvious degradation over a 60-h stability test for acidic OER, suggesting potential for practical applications.

atoms leads to the collapse of crystal structure, which limited their application in PEMWE [20][21][22] .Recently, some pioneering works have reported that the introduction of acid-resistant heteroatoms such as Pb, Mn, and Sb into cobalt oxides could increase the stability of these OER catalysts in acid 6,18,23,24 .It was proposed that the heteroatoms would strengthen metal-oxygen bindings 6 , activate the self-healing process 23 , or stabilize the lattice 24 .However, atomic-level insight into the stabilizing effect induced by the acid-resistant heteroatoms is still lacking, which is crucial for designing highly efficient earth-abundant catalysts for acidic OER.
Herein, combining theoretical and experimental studies on singleatom Ir-introduced spinel cobalt oxides, we provided an in-depth understanding of the heteroatom's role on OER stability at the atomic level.Our density functional theory (DFT) calculations revealed that the introduction of Ir single atoms can significantly increase the migration energy of the nearest Co atoms, whereas has less influence on the distant ones.Accordingly, we further studied the inter-site distance effect of adjacent Ir atoms, i.e., Ir-Ir distance, on the stability of Co atoms between them.We found that the stability of the inbetween Co atom was continuously enhanced until the Ir-Ir distance was lowered down to the limited level of 0.56 nm where the Co atom was sandwiched by two Ir atoms.Experimentally, we successfully synthesized a series of single-atom catalysts with different Ir-Ir distances by adjusting the density of Ir single atoms in spinel cobalt oxides.Electrochemical measurements further demonstrated that the stabilizing effect induced by Ir single atoms was closely related to the Ir-Ir distance.With the decrease of the Ir-Ir distance, the electrochemical durability of these catalysts in acid exhibited a gradual improvement, accompanied by a significant decrease in the dissolution of Co species, which well confirmed the theoretical results.When the Ir-Ir distance reached about 0.6 nm, the long-term durability test at 10 mA cm geo −2 revealed that the spinel cobalt oxide remained stable under pH = 1 with just a small increase in the potential of about 20 mV after a 60 h continuous operation.Our work offered essential guidance for the precise design of highly stable earth-abundant OER catalysts in acid media.

Exploration of the distance-related stabilizing effect
We began with DFT calculations to study how the introduction of acidresistant heteroatoms influences the stability of Co atoms in spinel oxides.Since the dissolution of cobalt oxides in acid involves the migration of Co atoms at the surface 6,25 , we calculated the migration energies of these lattice atoms to evaluate their stabilities.Taking spinel Cu 0.3 Co 2.7 O 4 as a model (Fig. 1a), we proposed a migrating process of the Co atom at octahedral sites as shown in Fig. 1b, where the Co atom moves away from the (110) face via a transition state with an energy barrier of 1.58 eV.After Ir single atoms were introduced into the octahedral sites of spinel oxide (Fig. 1c), we first calculated the migration energy of the nearest Co atom (labeled as A) to the Ir single atom.We found that the energy increased to 1.70 eV (Supplementary Fig. 1a), indicating that the introduction of the Ir single atom can significantly enhance the stability of the nearest Co atom.Subsequently, we further assessed the influence of Ir single atom on the migration of distant Co atoms, i.e., the next nearest Co atom as labeled B in Fig. 1c.The calculated migration energy of B-site Co was 1.63 eV (Supplementary Fig. 1b), which suggested that the B-site Co atom was less stable than the A-site Co but more than the Co atom without introducing Ir (Fig. 1d).In other words, the stabilizing effect induced by Ir single atoms became weaker as the distance between Ir and Co atoms increased.This result strongly suggested that it is necessary to control the distance of adjacent Ir single atoms by adjusting the density of introduced Ir single atoms for improving the stability of the cobalt oxide catalysts.
To further explore the distance-dependent stabilizing effect of Ir single atoms, we constructed various structural models of Ir atomically Figure 2a-c showed the cases with d = 1.14, 0.87, and 0.56 nm, in which the number of Co atoms between two adjacent Ir atoms was 3, 2, and 1, respectively.For d = 1.14 nm, the calculated migration energy of the centered Co atom was 1.63 eV.For d = 0.87 nm, the energy was 1.75 eV (Fig. 2d).Both the energies are essentially the same as those for B-site and A-site Co atoms in Fig. 1c, respectively.For d = 0.56 nm, it took 1.83 eV to enable the migration of the Co atom sandwiched by two Ir single atoms (Fig. 2d).The increased migration energies indicated that the shorter the distance between adjacent Ir single atoms, the more stable the Co atoms on the catalyst surface.The above results indicated that Ir single atoms can stabilize their neighboring lattice but have limited effect on distant ones.When the distance between adjacent Ir single atoms was too large, the stabilizing effect of Ir single atoms was localized.Once the distance between Ir single atoms was reduced to a specific value, the stabilizing effect would cover the spinel oxides effectively, thus stabilizing the entire spinel oxides under acidic conditions (Fig. 2e).
Identifying Ir  2).X-ray diffraction (XRD) patterns revealed that all the samples were in the spinel-type structures with a Fd-3m space group (Supplementary Fig. 3).Moreover, the Raman spectra displayed four characteristic peaks located at 190, 470, 510, and 682 cm −1 for these oxides, which were assigned to F 2g , E g , F 2g , and A 1g vibration originated from the spinel lattice, respectively (Supplementary Fig. 4) 18,26 .These results demonstrated that the introduction of Ir heteroatoms into the spinel cobalt oxides not only formed no detectable impurity phase but also unchanged the spinel structure.Figure 3a-c showed the aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) images for three Ir 1 / Cu 0.3 Co 2.7 O 4 samples with different Ir-Ir distances.Individual bright spots in contrast to the Cu 0.3 Co 2.7 O 4 were found, which indicated that the Ir species were atomically dispersed in the spinel cobalt oxides.Moreover, by averaging the Ir-Ir distances of more than two hundred Ir-Ir pairs in the HAADF-STEM images, the value of d was estimated to be about 1.1, 0.8, and 0.6 nm, respectively (Fig. 3d-i).Energy dispersive X-ray (EDX) elemental mapping images revealed that the Ir atoms were uniformly distributed in all three Ir  (Supplementary Fig. 5).The quantitative analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) showed that the contents of Ir elements were 1.2, 2.1, and 3.6 wt% for Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 1.1, 0.8, and 0.6 nm, respectively.The electronic structure and coordination environment of Ir 1 / Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances were further characterized by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy.The Co K-edge XANES spectra showed that the absorption edges of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances overlapped with that of Cu 0.3 Co 2.7 O 4 , suggesting similar valence states for Co species in all the oxides (Fig. 4a) 27,28 .The Co K-edge EXAFS spectra presented three similar characteristic peaks at about 1.4, 2.3, and 3.0 Å, corresponding to Co-O, Co oct -Co oct (octahedral site), and Co tet -Co tet (tetrahedral site) (Fig. 4b) 29,30 , which implies that the coordination environment of Co sites exhibited no obvious change after the introduction of Ir single atoms.Moreover, the Co and Cu L-edge X-ray absorption spectroscopy (XAS) also revealed similar valence states of Co and Cu ions in these asprepared samples (Supplementary Fig. 6a) [31][32][33][34][35] .The above results indicated that no visible changes in the crystal and electronic structures of the cobalt oxides were found after the introduction of Ir single atoms with different distances.Figure 4c shows the Ir L 3 -edge XANES spectra of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances, where IrO 2 and Ir foil were used as references.The intensity of the white line was found to decline gradually with decreasing the Ir-Ir distances, which suggested a decrease in the valence state of Ir species 36,37 .The Ir L 3 -edge EXAFS spectra exhibited two characteristic peaks at about 2.0 Å and 3.0 Å, which were ascribed to first-shell Ir-O coordination and secondshell Ir-Co coordination, respectively (Fig. 4d) 38,39 .By fitting the experimental EXAFS spectra, the Ir-O and Ir-Co coordination numbers of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances were determined to be about six and three, respectively (Supplementary Fig. 7 and Table 1).The fitting results confirmed that the Ir single atoms were incorporated into the octahedral sites of Cu 0.3 Co 2.7 O 4 .

Electrocatalytic evaluation towards acidic oxygen evolution
To evaluate the stabilizing effect of Ir single atoms with different distances on Cu 0.3 Co 2.7 O 4 , we recorded the polarization curves in a standard three-electrode system under acidic media.For the pristine Cu 0.3 Co 2.7 O 4 , the current density showed an obvious decrease as the number of scans increased (Fig. 5a).After 1000 scan cycles, the current density decreased by 87.5%.As Ir single atoms with d = 1.1 and 0.8 nm were introduced into Cu 0.3 Co 2.7 O 4 , the current density decreased by 76.3% and 44.2% after 1000 scan cycles, respectively (Fig. 5a, b).When the Ir-Ir distance was further reduced to 0.6 nm, the current density showed an inconspicuous decrease during the sequential scans, indicating excellent OER stability for this sample in acidic media (Fig. 5a,  b).The dissolution of Co species under different scan cycles was also measured to explore the stability of Cu  5d, the durability of these catalysts was improved as the Ir-Ir distance decreased.For d = 0.6 nm, the catalyst remained stable with just a small increase in the potential of about 20 mV after a 60 h continuous operation.Notably, the Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 0.6 nm exhibited a better stability in comparison to that of commercial IrO 2 .The above results demonstrated that the stabilizing effect induced by Ir single atoms was strongly dependent on the distance of adjacent single atoms.When Ir single atoms with d = 0.6 nm were introduced into the Cu 0.3 Co 2.7 O 4 , the stabilizing effect was superimposed on each other, thus stabilizing the entire catalysts.
To exclude the possible influence of carbon corrosion and gas bubbles on the OER stability during the above electrochemical measurements, we further recorded the polarization curves in a threeelectrode system with the catalysts loading on Ti felt (Supplementary Figs. 9 and 10) and in a flow-cell setup (Supplementary Fig. 11).Similar improvement in the OER stability by decreasing the Ir-Ir distance can be found for both measurements, which indicated that the distance effect of Ir single atoms on the stability of cobalt oxide catalysts is intrinsic.Moreover, considering the difference in the content of Ir species on the electrode may influence the stability of the catalysts, we also carried out the electrochemical evaluation for these catalysts by fixing the Ir loadings on the electrode at 25 µg cm −2 .For the Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 1.1 and 0.8 nm, the current density showed an obvious decrease as the number of scans increased (Supplementary Fig. 12a, b, d).For the Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm, the current density showed a negligible decrease during the sequential scans, indicating excellent OER stability for this sample in acidic media (Supplementary Fig. 12c, d).The above results further proved that the stabilizing effect induced by Ir single atoms was only related to the distance of adjacent Ir single atoms.
The excellent stability of Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm was also supported by the HAADF-STEM image, EDX elemental mapping images, and XAS after the durability test.As shown in the HAADF-STEM image, Ir single atoms preserved their isolated dispersion on the Cu 0.3 Co 2.7 O 4 (Supplementary Fig. 13a).The EDX elemental mapping image showed that Ir species were still evenly distributed across the catalyst (Supplementary Fig. 13b).The Co L 3 -and L 2 -edge peaks displayed negligible change compared with that before durability test, suggesting an unchanged valence state of Co (Supplementary Fig. 13c).In addition, the O K-edge XAS also showed ignorable changes in the characteristic peaks, indicating a stable structure of the Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 0.6 nm during the durability test (Supplementary Fig. 13d).
To further prove the distance effect of Ir single atoms on the stability of Cu 0.3 Co 2.7 O 4 during acidic OER, in-situ XAFS were performed (Supplementary Fig. 14a, b).As shown in Supplementary Fig. 14c, the absorption edge of Co K-edge XANES spectra for Cu 0.   15b).The results indicated that the reduction of the distance between Ir single atoms significantly enhanced the catalytic activity under the acidic condition.To elucidate the reaction kinetics, we analyzed the Tafel slopes of these catalysts, where the Tafel slope of Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 0.6 nm gave the lowest value of 71 mV dec −1 (Supplementary Fig. 16).This suggested its fastest kinetics among this catalysts 41 .The reaction kinetics was further reflected by the electrochemical impedance spectroscopy (EIS) measurements.A remarkable decrease of charge transfer resistance (R ct ) was found in the Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 0.6 nm compared with other catalysts, confirming its fastest charge transfer (Supplementary Fig. 17) 42,43 .The improved catalytic performance and accelerated reaction kinetics of Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm may be attributed to the optimized electronic structure of Co atoms after the introduction of Ir single atoms.Comparatively, the overpotential at a current density of 10 mA cm −2 of Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm were on par with the best records of currently reported Co-based OER catalysts (Supplementary Fig. 18 and Table 2).
To test the generality of distance effect on the stability of cobalt oxide catalysts for acidic OER, we further introduced Ir single atoms into other spinel cobalt oxides such as Co 3 O 4 and Mn 0.3 Co 2.7 O 4 .Under similar preparing conditions, we successfully obtained the single-atom catalysts, Ir 1 /Co 3 O 4 and Ir 1 /Mn 0.3 Co 2.7 O 4 , with d = 0.6 nm (Supplementary Figs.19-21).The electrochemical measurements revealed that both Ir 1 /Co 3 O 4 and Ir 1 /Mn 0.3 Co 2.7 O 4 also exhibited a high OER stability in acid (Supplementary Fig. 22).This proved that the introduction of Ir single atoms with an appropriate distance into cobalt oxides was a universal strategy to stabilize its structure during acidic OER.

Discussion
In conclusion, we understand the distance effect of single atoms on the stability of cobalt oxide catalysts for acidic oxygen evolution.Both theoretical calculations and electrocatalytic measurements revealed the stabilizing effect was strongly dependent on the distance of adjacent Ir single atoms.As the distance of adjacent Ir single atoms was reduced to 0.6 nm, the stabilizing effect could cover the Cu 0.3 Co 2.7 O 4 , stabilizing entire spinel oxides under acidic conditions.In addition, the introduction of Ir single atoms with an appropriate distance was a universal strategy to stabilize other spinel cobalt oxides during acidic OER.Our work not only provided insight into the distance effect of single atoms on the stability of cobalt oxide catalysts at the atomic level but also pointed toward a direction to the rational design of highly stable catalysts applied in PEMWE.

XAFS measurements
XAFS spectra at Ir L 3 -edge were obtained at the BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai) operated at 3.5 GeV under 'top-up' mode with a constant current of 220 mA.The XAFS data were recorded under fluorescence mode.The energy was calibrated according to the absorption edge of pure Ir foil.XAFS and in-situ XAFS spectra at Co K-edge were obtained at the BL11B beam line of SSRF.The energy was calibrated according to the absorption edge of pure Co foil.Athena software was used to extract the data.For the in-situ XAFS, we performed the experiments in a specialized cell by using a three-electrode standard electrochemical workstation.The catalyst on the carbon substrate was cut into 1.5 × 1.5 cm 2 pieces and then sealed in a cell by Kapton film.Before the experiments, a series of potentials (OCP ~1.7 V) were applied to the electrode for 2 min, respectively.All XAFS data were collected during one period of beam time and each spectroscopy was recorded for 12 min.XAS spectra at Co L-edge, Cu L-edge, and O K-edge were measured at the beamline BL12B of the National Synchrotron Radiation Laboratory (NSRL, Hefei).

Electrochemical measurements
An electrochemical workstation (CHI 660E, Shanghai CH Instruments) was used to measure the electrocatalytic performance of the samples.The electrocatalytic measurements were conducted in a standard three-electrode system at room temperature.The carbon paper (1 × 0.5 cm −2 ) loaded with the as-obtained catalysts was used as the working electrode.The mass loadings of catalysts on the carbon paper were 2 mg cm −2 .A carbon rod was used as the counter electrode.A Hg/ Hg 2 SO 4 electrode was used as the reference electrode.The polarization curves of OER were obtained in 0. to the electrolyte to replenish the volume of the electrolyte.Finally, the dissolution fraction of Co species was obtained by comparing the mass of the remaining Co species to the mass of Co species in pristine catalysts.The experimental procedure for testing dissolved Ir species was similar to that for testing dissolved Co species, except that our experimental method is inductively coupled plasmamass spectrometry (ICP-MS).

DFT calculations
Spin-polarized DFT calculations were carried out with Perdew, Burke, and Ernzerhof (PBE) functionals using the Vienna ab initio simulation package (VASP) 45,46 .The projector augmented wave (PAW) method was adopted to describe the ions-electrons interaction 47,48 .A k-point mesh of 3 × 2 × 1 was used to sample the Brillouin zones.The kinetic energy cutoff was set to 400 eV for plane-wave expansion.During structural optimizations, the tolerances of total energy and force were set to 10 −5 eV and 0.05 eV/Å, respectively.To accurately describe the 3d electrons, a correlation energy (U) of 3.0 eV was used for Co and Cu atoms, and 2.0 eV was used for Ir atoms, values that have been tested by previous experimental and theoretical studies [49][50][51] .

Instrumentations
XRD patterns were recorded using a Philips X'Pert Pro Super diffractometer with Cu-Kα radiation (λ = 1.54178Å).HAADF-STEM images were taken on a JEOL ARM−200F field-emission transmission electron microscope operating at an accelerating voltage of 200 kV using Mo- based TEM grids.EDX elemental mapping images were taken on an FEI Talos F200X high-resolution transmission electron microscope using Mo-based TEM grids.ICP-MS (Atomscan Advantage, Thermo Jarrell Ash, USA) analyses were used to determine the mass loadings of Ir single atoms and the dissolved amount of Co species.The distance between Ir single atoms was measured on HAADF-STEM images by Nano Measurer software.

BFig. 1 |
Fig. 1 | Optimized structural models and calculated migration energies of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 .a Optimized structural model of Cu 0.3 Co 2.7 O 4 .Red, blue, and brown spheres represent O, Co, and Cu atoms, respectively.b Calculated migration energies of Co atoms on Cu 0.3 Co 2.7 O 4 .Pink spheres

Fig. 2 |
Fig. 2 | Optimized structural models and calculated migration energies of Ir 1 / Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Optimized structural models of Ir 1 / Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 1.14 nm (a), d = 0.87 nm (b), and d = 0.56 nm (c).Red, blue, brown, and green spheres represent O, Co, Cu, and Ir atoms, respectively.d Calculated migration energies of Co atoms on Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.The inset structures represent the initial state (IS), transition state (TS), and final state (FS), respectively.Pink spheres represent migrated Co atoms.e Schematic illustration of the distance effect of Ir single atoms on the stability of cobalt oxide catalysts.Blue and green spheres represent Co atoms and Ir single atoms.

Fig. 3 |
Fig. 3 | Structural characterizations of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.HAADF-STEM images of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 1.1 nm (a), d = 0.8 nm (b), and d = 0.6 nm (c).Distance distribution of Ir single atoms (d) and intensity profile (e) of atoms located at the square frame in (a).Distance distribution of Ir single atoms (f) and intensity profile (g) of atoms located at the square frame in (b).Distance distribution of Ir single atoms (h) and intensity profile (i) of atoms located at the square frame in (c).

3 Fig. 4 |
Fig. 4 | Electronic structure characterizations of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Normalized XANES (a) and EXAFS (b) spectra at the Co K-edge of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Co foil was used as a

Fig. 5 |
Fig. 5 | Stability evaluation towards acidic oxygen evolution.a Polarization curves of catalysts at different scan cycles in 0.1 M HClO 4 electrolyte.The displayed polarization curves are the 1, 200, 400, 600, 800, and 1000 cycles, respectively.b Decrease in current densities under different scan cycles of Cu 0.3 Co 2.7 O 4 and Ir 1 / Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.The selected overpotential was 1.60 V (E vs RHE) for all catalysts.c Dissolution of Co species under different scan cycles of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.d Chronopotentiometry curves of Cu 0.3 Co 2.7 O 4 , Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances, and commercial IrO 2 towards acidic OER at 10 mA cm −2 .

The distance between Ir single atoms was modulated by adjusting the density of Ir single atoms on Cu 0.3 Co 2.7 O 4 . Transmission electron microscopy (TEM) images showed that the as-prepared Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances presented similar morphologies
1 /Cu 0.3 Co 2.7 O 4 with different Ir single atoms distance Inspired by the theoretical calculations, we synthesized spinel oxide Cu 0.3 Co 2.7 O 4 and a series of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances through a high-temperature pyrolysis method (see Methods).
(Supplementary Fig. 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.For Co species in Cu 0.3 Co 2.7 O 4 , they were gradually dissolved with increasing the number of scan cycles.Specifically, 76.7% of Co species were dissolved after 1000 scan cycles (Fig. 5c).When Ir = 1.1 and 0.8 nm were introduced into the Cu 0.3 Co 2.7 O 4 , the dissolution rate of Co species slowed down, indicating the Ir single atoms could stabilize the Cu 0.3 Co 2.7 O 4 .When Ir = 0.6 nm were introduced into the Cu 0.3 Co 2.7 O 4 , Co species were merely dissolved (just 3.6%) after 1000 scan cycles, indicating high stability of Cu 0.3 Co 2.7 O 4 with d = 0.6 nm.For Ir species, the results demonstrated that as the Ir-Ir distance decreases, the dissolution rate of Ir species on the catalyst surface slows down (Supplementary Fig. 8).Notably, the Ir species in Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm were just dissolved 2.1% after 1000 scan cycles, indicating the high stability of this sample during acidic OER condition.Long-term chronopotentiometry at a constant current density of 10 mA cm −2 geo was also carried out to estimate the durability of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.As shown in Fig.
Co structure covered the entire Cu 0.3 Co 2.7 O 4 , making the stabilizing effect superimposed on each other, thus significantly stabilizing the entire spinel oxides.The electrocatalytic activity of Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances towards acidic OER was also evaluated.For comparison, Cu 0.3 Co 2.7 O 4 was measured under the same conditions.Supplementary Fig. 15a displayed that the introduction of Ir single atoms improves the OER activity.Moreover, the current density of Ir 1 /Cu 0.3 Co 2.7 O 4 increased with decreasing the distance of adjacent Ir single atoms.Specifically, Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm only required an overpotential of 290 mV to deliver a current density of 10 mA cm −2 , which was 120, 70, and 30 mV lower than those of Cu 0.3 Co 2.7 O 4 , Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 1.1 and 0.8 nm, respectively (Supplementary Fig. 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Normalized XANES (a) and EXAFS (b) spectra at the Co K-edge of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.Co foil was used as a reference.Normalized XANES (c) and EXAFS spectra (d) at the Ir L 3 -edge of Ir 1 / Co structure.As the distance of adjacent Ir single atoms narrowed to a suitable value, the Ir-O- 44Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances.dChronopotentiometrycurves of Cu 0.3 Co 2.7 O 4 , Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances, and commercial IrO 2 towards acidic OER at 10 mA cm −2 .Cu 0.3 Co 2.7 O 4 was synthesized through high-temperature pyrolysis of metal-organic frameworks with modifications44.Typically, 497.7 mg of Co(NO 3 ) 2 •6H 2 O, 70.0 mg of Cu(NO 3 ) 2 •3H 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. 9.1 g of 2-methylimidazole was dissolved in 140 mL of H 2 O to form solution B. The mixed solution was formed by adding solution A to solution B and mixed for 2 h under magnetic stirring.The resulting solution was centrifuged and washed with EtOH three times to obtain metal-organic frameworks.The product was dried in a vacuum oven overnight.Finally, the above solid was calcined at 350 °C for 4 h in the air to obtain the desired Cu 0.3 Co 2.7 O 4 .Synthesis ofIr 1 /Cu 0.3 Co 2.7 O 4 with different Ir single atoms distance Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir single atoms distances were synthesized using similar procedures as synthesizing Cu 0.3 Co 2.7 O 4 except for changing the composition of solution A. For Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 1.1 nm, 492.7 mg of Co(NO 3 ) 2 •6H 2 O, 69.3 mg of Cu(NO 3 ) 2 •3H 2 O, 6.7 mg of IrCl 4 •xH 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. For Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.8 nm, 487.7 mg of Co(NO 3 ) 2 •6H 2 O, 68.6 mg of Cu(NO 3 ) 2 •3H 2 O, 13.4 mg of IrCl 4 •xH 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. For Ir 1 /Cu 0.3 Co 2.7 O 4 with d = 0.6 nm, 482.7 mg of Co(NO 3 ) 2 •6H 2 O, 67.9 mg of Cu(NO 3 ) 2 •3H 2 O, 20.0 mg of IrCl 4 •xH 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. Synthesis of Co 3 O 4 and Mn 0.3 Co 2.7 O 4 Co 3 O 4 and Mn 0.3 Co 2.7 O 4 were synthesized using similar procedures as synthesizing Cu 0.3 Co 2.7 O 4 except for changing the composition of solution A. For Co 3 O 4 , 580.0 mg of Co(NO 3 ) 2 •6H 2 O and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. For Mn 0.3 Co 2.7 O 4 , 497.7 mg of Co(NO 3 ) 2 •6H 2 O, 52.9 mg of Mn(NO 3 ) 2 •6H 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. Synthesis of Ir 1 /Co 3 O 4 and Ir 1 /Mn 0.3 Co 2.7 O 4 with d = 0.6 nm Ir 1 /Co 3 O 4 with d = 0.6 nm was synthesized using similar procedures as synthesizing Co 3 O 4 except for changing the composition of solution A. For Ir 1 /Co 3 O 4 with d = 0.6 nm, 482.7 mg of Co(NO 3 ) 2 •6H 2 O, 20.0 mg of IrCl 4 •xH 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A. Ir 1 /Mn 0.3 Co 2.7 O 4 with d = 0.6 nm were synthesized using similar procedures as synthesizing Mn 0.3 Co 2.7 O 4 except for changing the composition of solution A. For Ir 1 /Mn 0.3 Co 2.7 O 4 with d = 0.6 nm, 482.7 mg of Co(NO 3 ) 2 •6H 2 O, 51.3 mg of Mn(NO 3 ) 2 •6H 2 O, 20.0 mg of IrCl 4 •xH 2 O, and 30.0 mg of CTAB were dissolved in 20 mL of H 2 O to form solution A.
1 M HClO 4 electrolyte, using a linear sweep voltammetry method at a potential range from 1.02 to 1.82 V with a sweep rate of 5 mV s −1 .All potentials mentioned in this work were measured against the Hg/Hg 2 SO 4 electrode and converted to reversible hydrogen electrode (RHE) scale by the equation: E (V vs RHE) = E (V vs Hg/Hg 2 SO 4 ) + 0.656 V + 0.0591 pH V.In the given equation, 0.656 V was obtained by calibration with respect to the RHE.The ohmic electrolyte resistance on carbon paper was measured to be 6 Ω.The potentials were corrected to compensate for the effect of solution resistance, which were calculated by the following equation:E iR-corrected = E (V vs RHE) − iR,where i is current, and R is the uncompensated ohmic electrolyte resistance.Tafel slope (b) was determined by fitting polarization curves data to the Tafel equation: η = a + b log | j | , where η is the overpotential for the OER, and j is the current density at the given overpotential.EIS measurements were conducted at 1.55 V.The amplitude of the sinusoidal wave was 5 mV.The frequency scan range was 100 kHz-0.01Hz.The dissolved Co species were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).We first dissolved the pristine Cu 0.3 Co 2.7 O 4 and Ir 1 / Cu 0.3 Co 2.7 O 4 with d = 1.1, 0.8, and 0.6 nm in aqua regia to obtain the mass of the original Co species in catalysts.Then, we conducted electrocatalytic measurements of the pristine Cu 0.3 Co 2.7 O 4 and Ir 1 / Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances in 100 mL 0.1 M HClO 4 electrolyte.Subsequently, we collected 5 mL electrolytes of Cu 0.3 Co 2.7 O 4 and Ir 1 /Cu 0.3 Co 2.7 O 4 with different Ir-Ir distances at 200, 400, 600, 800, and 1000 scan cycles to obtain the dissolved Co species by conducting ICP-AES test, respectively.Afterward, we added 5 mL of 0.1 M HClO 4