Single-atom surface anchoring strategy via atomic layer deposition to achieve dual catalysts with remarkable electrochemical performance

Pt-Ir catalysts have been widely applied in unitized regenerative fuel cells due to their great activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the application of noble metals is seriously hindered by their high cost and low abundance. To reduce the noble metals loading and catalyst cost, the atomic layer deposition is applied to selectively surface anchoring of Ir single atoms (SA) on Pt nanoparticles (NP). With the formation of SA-NP composite structure, the Ir SA -Pt NP catalyst exhibits significantly improved performance, achieving 2.0-and 90-times mass activity by comparison with the benchmark Pt/C catalyst for the ORR and OER, respectively. Density functional theory calculations indicate that the SA-NP cooperation synergy endows the Ir SA -Pt NP catalyst to surpass the bifunctional catalytic activity limit of Pt-Ir NPs. This work provides a novel strategy for the construction of high-performing dual catalyst through designing the single atom anchoring on NPs.


| INTRODUCTION
As a promising approach to produce hydrogen, the water splitting in an electrolyzer driven by renewable energy has emerged great attention.The clean energy device of proton exchange membrane fuel cell (PEMFC) uses regenerative hydrogen as fuel, which can produce and output the electricity by consuming the chemical energy.2][3][4][5][6] Suffer from the sluggish kinetics of oxygen electrode reactions including the oxygen reduction reaction (ORR) at PEMFC operation and the oxygen evolution reaction (OER) at water splitting mode, developing the high efficient ORR and OER bifunctional electrocatalysts to reduce the overpotential while improve the energy conversion efficiency is in highly demand for the widespread application of URFCs.
Many research efforts demonstrate that an ideal bifunctional oxygen electrocatalyst should show high activity toward both the ORR and OER, with low overpotential and outstanding stability.][9][10][11][12] However, simple assembling RuO 2 or IrO 2 with Pt for design the bifunctional oxygen catalysts, which exhibits greater than 1.0 V overpotential between OER and ORR, making them unsuitable for URFCs application.Not only that, the widespread use and application of noble metals is severely blocked due to their limited resources and ever-increasing price.Therefore, to solve the catalyst challenges of URFCs and make a tradeoff between performance and cost, it is of great significance to develop the advanced bifunctional oxygen electrocatalysts with optimum performance and minimum noble metal loadings.
4][15][16][17] For example, Kong and co-workers synthesized the catalysts of IrO 2 nanoparticles (NPs) supported Pt through a microwaveassisted polyol method. 18The Pt/IrO 2 NPs catalyst displayed higher active current in comparison to the pure IrO 2 and Pt supported on commercial IrO 2 catalysts, for the ORR and OER.The comprehensive investigation conducted by Ticianelli group suggests that the Pt:Ir molar ratio in Pt/IrO 2 catalysts shows great impact on their electrocatalytic performance, and the catalyst with Pt:Ir ratio of 1:9 exhibit good balance between the OER and ORR activities. 1 However, the high ratio of IrO 2 , which is necessary in achieving the competitive OER activity, causes the high-cost issue of URFCs.Moreover, when IrO 2 NPs are located on the Pt surface, the exposed active sites of Pt catalysts might be blocked, resulting in the decrease of ORR activity.Therefore, it is still a great challenge to investigate the advanced bifunctional oxygen catalysts with further improved metal atoms utilization and high performance to surpass the limitation of conventional Pt-Ir URFCs catalyst.
To innovative, the bifunctional oxygen Pt-Ir catalysts with utmost Pt exposed active sites and minimizing Ir loading, herein, we propose the Ir SA modified Pt NPs (Ir SA -Pt) catalyst supported on the nitrogen-doped carbon nanotubes (NCNT) substrate by an atomic layer deposition (ALD) approach.Experimentally, the chemical coordination structure and morphology of the Ir SA -Pt NPs are deeply investigated by the x-ray absorption spectroscopy and scanning transmission electron microscopy.This work indicates that the Ir SA -Pt catalyst with SA and NP cooperation structure shows enhanced catalytic activity, and excellent stability toward both the OER and ORR.Thanks to the surface modification of Pt NPs with isolated Ir atoms, the microelectronic structure and coordination environment of surface Pt atoms are well tuned while not blocking the active sites, which contribute to the Ir SA -Pt catalyst with enhanced ORR performance.Furthermore, the Ir SAs enable Ir SA -Pt catalyst with extremely high OER mass activity in contrast to the benchmark Pt-IrO 2 catalyst.More importantly, the introduction of Ir SAs on Pt NPs surface can effectively boost the water oxidation which helps relieve the carbon substrate oxidation under the high potential condition, thus ensuring Ir SA -Pt catalyst with both the excellent OER activity and long-term carbon corrosion resistant durability.The structure design of SAs surface anchoring on NPs is benefit in realizing the high metal atoms utilization and avoids blocking active sites from NP, which is expected to be a novel strategy for construction of highperforming dual catalyst applied in URFCs.

| RESULTS AND DISCUSSION
2.1 | Fabrication of Ir SA-modified Pt (Ir SA -Pt) NPs by the ALD We prepared N-doped carbon nanotubes (NCNTs) with an average diameter of 100 nm by an ultrasonic spray pyrolysis method, as reported previously. 19First, NCNTs used as catalyst substrate and were put into the ALD chamber, then Pt NPs deposition on the surface of NCNTs was conducted by executing 20 ALD cycles to obtain the sample of PtNPs-NCNTs.As shown in Figure S1, the scanning electron microscope (SEM) images of PtNPs-NCNTs indicate that Pt as NPs are uniformly dispersed on the surface of NCNTs.The typical transmission electron microscope (TEM) images show that the isolated Pt NPs in an average particle size of 2 nm are densely distributed on the NCNT substrate (Figure S2a).The further high-resolution TEM images clearly identify the periodic fringe space of Pt crystal as 0.23 nm, which is exactly consistent with the d value of the Pt (111) crystal plane (Figure S2b).
Subsequently, Pt NPs are modified by surface anchoring of Ir SAs through an ALD single pulse of iridium (III) acetylacetonate precursor, and the step-by-step synthesis of Ir SA-modified Pt NPs (Ir SA -Pt) is schematically depicted in Figure 1A.The typical SEM (Figure 1B) and TEM (Figure 1C,D) images demonstrate that the morphology and particle size of Pt NPs remains unchanged after the Ir SAs decoration.The (111) periodic fringe space of Ir SA -Pt NPs is found the same of 0.23 nm, suggesting the well-maintained Pt crystal structure.However, the highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Figure 1E clearly show that the Pt NPs surface of Ir SA -Pt sample become rougher in contrast to that of PtNPs-NCNTs, indicating the possible decoration and anchoring of Ir SAs on Pt NPs.To determine the presence of Ir atoms in Ir SA -Pt, the isolated Ir atom layer was further confirmed by Cryo-TEM images (Figure S3).HAADF-STEM-EDS spectroscopy and maps shown in Figure S4 demonstrate the homogeneous distribution of C, N, Pt, and Ir elements on the NCNTs substrate.More importantly, the HAADF-STEM-EDS profiles on composite metal NPs (Figure 1E,F) reveals that the Ir atoms are selectively deposited on the surface of Pt NPs, and no Ir can be detected on the NCNTs, which confirms the effective modification and incorporation of Ir SAs with Pt NPs.The inductively coupled plasma optical emission spectrometer (ICP-OES) detection shows that the mass loading of Pt and Ir in the as-prepared Ir SA -Pt sample is 15.0 and 2.2 wt%, respectively.

| X-ray absorption fine structure of Ir SA -Pt
To further understand the microstructure effect of Ir SAs on Pt, the XAFS is detected to reveal the electronic coordination environment of Pt and Ir in Ir SA -Pt, Pt NPs and reference samples.Figure 2A,B show the normalized xray absorption near edge structure spectra (XANES) at the Pt L 2 and L 3 edges.Evidently, the absorption edges of Pt in Ir SA -Pt and Pt NPs are close to the edge peak energy for Pt foil, suggesting a metallic state of Pt designed by the ALD.It is observed that Ir SA -Pt and Pt NPs exhibit the clearly increased white line (WL) intensity at both the Pt L 2 and L 3 edges in contrast to the Pt foil, and Ir SA -Pt appears to have the most intense WL.Meanwhile, the WL energy of E 0 determined by the first derivative of XANES spectra (Figure 2C) for Ir SA -Pt (11564.5 eV) is shifted to the higher energy than that of Pt NPs (11 564 eV).1][22][23][24][25] The results of increased WL intensity and high peak energy suggest that Pt is in a more oxidized chemical state when Pt NPs surface anchoring with Ir SAs.
Furthermore, to in-depth unveil the local coordination structure of Pt, the extended x-ray absorption fine structure (EXAFS) spectra at Pt L 3 -edge are evaluated using the first-shell fitting.Figure 2D displays the K 3 -weighted Fourier transforms (FT) EXAFS of Ir SA -Pt and reference samples.7][28] Quantitative coordination parameters of Pt centers are extracted from the EXAFS curve fitting analysis, and the characteristic index of obtained coordination number (CN) and bonding length are displayed in Figure S5 and Table S1.The fitting results show that the Pt-Pt for the Pt NPs and Ir SA -Pt NPs have lower CN (9.0 and 8.9) compared with Pt foil (12).Moreover, it is found that the bonding distance of Pt-Pt in the Ir SA -Pt sample decreases to 2.73 Å, which is lower than that of 2.76 Å for Pt foil.These results verify that the Pt electronic structure and coordination configuration can be successfully tuned by the modification of Ir atoms.
The local electronic structure of Ir at L 3 edge for the Ir SA -Pt and reference samples are further studied by the XANES and EXAFS.Qualitative examination of the Ir at L 3 -edge for Ir SA -Pt clearly shows the WL with a shift to the higher energy in comparison to the Ir foil.In addition, the Ir SA -Pt appears to have the more intense WL compared to Ir foil.An increase in the L 3 -edge WL intensity indicates a decrease in the number of electrons in the occupied d band.These results indicated that Ir exists in the Ir SA -Pt as a chemical oxidative state (Figure 2E).The FT-EXAFS region for Ir SA -Pt NPs and Ir foil is plotted in Figure 2F.The fitted microstructure parameters of Ir L 3 edge for Ir SA -Pt and Ir foil are analyzed and summary in Table S2.0][31][32][33] As shown in Figure S6a,b, the R space simulation curves can agree well with that of the experimental FT-EXAFS spectra.According to the FT-EXAFS fitting results, the coordination number of Ir-O is around 4, which is much smaller than that in IrO 2 .By combing the XANES and EXAFS results, it can be concluded that the Ir valence state is higher than that of Ir foils and lower than that of IrO 2 .From the Ir L 3 space fitting results, the Ir atoms have the CN of 2.3 for Ir-Pt scattering, further suggesting the successful coordination of isolated Ir atoms with Pt NPs.

| The ORR performance of Ir SA -Pt catalyst
The ORR performance of Ir SA -Pt NPs, Pt NPs and commercial 40wt%Pt/C catalysts was examined in a three-electrode system.Cyclic voltammograms (CV) were recorded in the N 2 -saturated 0.10 M aqueous HClO 4 at a scanning rate of 50 mV s À1 (Figure 3A).The polarization I-V curves in Figure 3B show that the Ir SA -Pt NPs demonstrate much better catalytic activity for the ORR than that of the Pt NPs and 40% Pt/C (Figure 3B).The kinetic current density ( j k ) derived from the Koutecky-Levich equation can be applied to calculate the mass activity via normalized to the Pt mass.The results in Figure 3C show that the Ir SA -Pt NPs catalyst display significantly enhanced specific activity (j k, specific ), with the j k, specific value of 0.67 mA cm À2 at 0.9 V versus RHE, which is higher than that of 0.51 mA cm À2 for the Pt NPs and 0.20 mA cm À2 for Pt/C catalyst.The mass activities of the Ir SA -Pt NPs, Pt NPs, and 40% Pt/C catalysts were obtained to be 0.27, 0.26, and 0.11 A mg À1 Pt , respectively, agree well with the specific activity trend (Figure 3D).The better ORR activity of Ir SA -Pt NPs catalyst is thought due to the shortened Pt-Pt bond distance by the Ir SAs modification, which contributes to the improve activity of the Ir SA -Pt catalysts for the ORR.
We evaluated the stability of Ir SA -Pt NPs catalyst by long-term test protocols (5000 cycles) under two different regions: the ORR condition (0.6-1.1 V vs. RHE) and the OER potential range (1.0-1.5 V vs. RHE), respectively.As shown in Figure S7, after cycling under the ORR condition, the ECSA maintained 92%, 86%, and 76% for the Ir SA -Pt NPs, Pt NPs, and Pt/C catalysts, respectively.The Ir SA -Pt NPs exhibited the mass activity of 0.25 A mg À1 Pt , which only reduced 8.0% after 5000 potential cycles (Figure S8).To mimic the carbon corrosion resistance for the as-prepared catalysts, the 5000 cyclic voltammetry sweeps were also carried out between 1.0-1.5 V versus RHE.In the presence of IrO 2 , the carbon substrate can avoid corrosion and the stability after cycling at the OER potential range can be significantly improved.The polarization curves indicate that the ECSAs of the catalysts significantly decreased after the 5000 durability cycles under the high potential region.The ECSA maintained 73%, 45%, and 52% for the Ir SA -Pt NPs, Pt NPs, and Pt/C catalysts, respectively (Figure S9).After the carbon corrosion ADT examination, the Ir SA -Pt NPs catalyst still maintained a mass activity of 0.21 A mg À1 at 0.9 V, 77% of its initial activity.While the mass activities of Pt NPs and Pt/C catalysts dropped to 54% and 50% of the initial performance (Figure 4).It has been reported that the carbon corrosion reaction causes Pt NPs detachment from the support, which is the main reason for the activity degradation at high potential region. 34The introduction of Ir SAs on the Pt surface can effectively boost the OER while avoid the carbon corrosion under high potential condition, thus resulting in the Ir SA -Pt NPs catalyst with better carbon corrosion resistance and excellent long-term stability.

| The OER performance of Ir SA -Pt catalyst
The OER activity of Ir SA -Pt catalyst is performed in the 0.5 M H 2 SO 4 by conducting linear sweep voltammetry and the results compared with Pt NPs and commercial IrO 2 catalysts are shown in Figure 5A.The polarization I-V curves demonstrate that the Ir SA -Pt NPs catalyst exhibit much better OER activity in comparison with the commercial IrO 2 and Pt NPs catalysts.The specific activity, which is applied to evaluate the catalytic activity for the OER, is calculated from the polarization curves by normalizing the kinetic current with the geometric area of the working electrode (Figure 5B).The Ir SA -Pt NPs exhibits a current density of 9.8 mA cm À2 at 1.60 V (vs.RHE), which is 14 times and 2.0 times greater than that of the Pt NPs and benchmark IrO 2 catalysts for the OER, respectively.The overpotential for the Ir SA -Pt NPs is 372 mV in delivering the current of 10 mA cm À2 , lower than that of 418 mV required by the commercial IrO 2 catalyst.Although the overpotential for the Ir SA -Pt NPs is not comparable with the transition metal-based catalysts in alkaline solutions, [35][36][37][38][39] the mass activity of the Ir SA -Pt NPs is 1434.4mA mg À1 at 1.55 V due to the formation of The ORR curves of Ir SA -Pt NPs catalyst, regular Pt NPs, and commercial Pt/C catalysts before and after accelerated durability tests between 1.0 and 1.5 V for 5000 cycles.The current densities were normalized to the geometric area of the RDE (0.196 cm 2 ).(D) The mass activity at 0.9 V of the catalysts before and after accelerated durability tests between 1.0 and 1.5 V for 5000 cycles.isolated atoms, which is more than 90 times higher than the commercial IrO 2 (15.53 mA mg À1 ).Furthermore, the mass activity of Ir SA -Pt NPs exceeded most of the other state-of-the-art Pt-Ir catalysts (Table S3).To evaluate the durability of the as-prepared Ir SA -Pt NPs catalyst under the OER conditions, the accelerate durability test (ADT) protocol is adopted from potential of 1.1 to 1.6 V (vs.RHE) at 100 mV s À1 with total 1000 cyclic voltammetry sweeps.As displayed in Figure 5C, the polarization curve of Ir SA -Pt NPs after ADT indicates a slightly reduced OER performance in comparison to the beginning of life, and it exhibited comparable stability in contrast to the commercial IrO 2 (Figure 5C,D).Furthermore, the ECSAs of the Ir SA -Pt NPs were also estimated from double layer capacitance (CDL) and specific capacitance (CS). 40,41The CS value was found to be 0.17 mF cm À2 in 0.5 M H 2 SO 4 electrolyte using a polished glassy carbon-rotating disk electrode (Figure S10a,b).The CDL values of the Ir SA -Pt NPs catalyst were estimated from the CV curves in Figure S10c,d.4][15] Furthermore, the TEM image results of post-testing Ir SA -Pt sample indicate that the Pt NPs are still uniformly dispersed on the NCNTs no big Ir SA -Pt particles aggregation can be detected (Figure S11a-c), suggesting the well stability of Ir SA -Pt catalyst under the OER operation condition.EDS mapping shows that the composition of Ir and Pt are well preserved, and are not dissolved in the electrolyte during the ADT condition (Figure S11d).In contrast, the Pt NPs are clearly observed aggregated on NCNT in Figure S12 after experiencing the same ADT protocol.These results indicate that the Ir SA -Pt NPs catalyst exhibits good durability during the high potential water oxidation condition.

| Origin of the enhanced electrocatalytic mechanism by DFT
We conducted DFT calculations to fully understand the original mechanism for the improved ORR and OER performance enabled by the synergistic effect of Ir SAs and Pt NPs (Figures 6 and S13-16).Two different possible models have been proposed for the single atoms on substrate.3][44] According to our atomic resolution STEM characterization and FT-EXAFS fitting results, the Ir atoms are anchored at the outer layer of Pt surface.In our case, the model is fabricated by adding Pt 10 into the graphene, followed by adding an Ir 1 O 2 cluster onto Pt 10 .Therefore, the reaction mechanism is similar with an adsorbate evolving mechanism, which occurs on SAC1.As shown in Figures 6A and S13 activity. 45In addition to the shortened Pt-Pt bond distance, the electronic structure of Pt might be affected by the neighboring Ir atoms.It has been reported that the single atom-modified Pt exhibits extremely high activity compared with pure Pt and Pt-based alloy structures. 46ccordingly, we speculated that the enhanced ORR activity of Ir SA -Pt NPs was attributed to the shorten Pt-Pt bond distance on the surface and the electronic structure adjustment, which were caused by the Ir surface modification.
The OER pathway involves proton-coupled electron transfer steps and the formation of intermediates of *OH, *O, and *OOH.As shown in Figures 6C and S15, The DFT calculations reveal the free energy diagrams of OER over regular IrO 2 sites and Ir single atomic sites on Pt NPs.For regular IrO 2 sites, the free energy changes for the four steps are +0.19,+1.19, +1.89, and +1.65 eV, respectively, during the OER pathway at 0 V (Figure S16).This result indicates that the ratedetermining step is the conversion of *O to *OOH.By contrast, for Ir single atomic sites on Pt NPs, the free energy changes for the four steps are +0.80,+1.66, +1.53, and +0.93 eV, respectively.The relatively lower energy change (+1.66 eV) implies that the atomic Ir sites on Pt can effectively promote the OER kinetics.In addition, the Gibbs free energies of the OER over regular IrO 2 sites and Ir single atomic sites on Pt NPs exhibited significantly different at 1.6 V.The OER pathway over Ir single atomic sites is almost all downhill with only one 0.06 eV thermodynamically uphill; while the third step for the regular IrO 2 sites is +0.29 eV (Figure 6D).This result agrees well with our experimental results.All the above DFT calculations revealed the superior ORR and OER behaviors of the as-prepared bifunctional catalyst is contributed from the synergistic effect of Ir SA and Pt NPs.

| CONCLUSION
In conclusion, the Ir SA surface anchoring on Pt NPs as a bifunctional catalyst is successfully fabricated by a selective ALD method.XAS results provide the evidence of the SA-NP cooperation and low coordinated Ir-Pt bonding configuration in the novel Ir SA -Pt NPs catalyst.With the formation of NP-SA structure, the excellent Ir SA -Pt NPs catalyst exhibits significantly improved performance for both the ORR and OER, which surpass the bifunctional activity limit of the benchmark Pt-Ir catalysts.The DFT calculation reveals that the Ir SA -Pt NPs catalyst have a small free energy for desorption of *OH in ORR.During the OER process, the energy change is low on the Ir SA -Pt NPs catalyst, which is the main contribution for the much-enhanced intrinsic catalytic activity.In addition, the single-atom surface anchoring on NPs can provide a new strategy for the rational design of high performing bifunctional oxygen catalysts.

F
I G U R E 1 (A) Schematic illustration of ALD synthesis of Ir SA -Pt NPs on nitrogen-doped carbon nanotubes (NCNTs).(B-D) Typical SEM, low-resolution TEM, and high-resolution TEM images of the Ir SA -Pt NPs.(E) Aberration-corrected HAADF-STEM images of Ir SA -Pt NPs catalyst NPs catalysts, showing the formation of Ir SAs on Pt particles.(F) The low resolution HAADF-STEM image.(G) HAADF-STEM-EDS elemental mapping of Ir SA -Pt NPs catalyst.F I G U R E 2 X-ray absorption studies of the Ir SA -Pt NPs and regular Pt NPs in comparison with Pt foil.(A, B) The normalized XANES spectra at the Pt L 3 -edge and L 2 -edge of the Ir SA -Pt NPs, regular Pt NPs and Pt foil.(C) The first derivative of the XANES spectrum at Pt L 3 edge.(D) Corresponding K 3 -weighted magnitude of Fourier transform spectra from EXAFS of I Ir SA -Pt NPs NPs, Pt NPs, and Pt foil.(E) The normalized XANES spectra at the Ir L 3 -edge of the Ir SA -Pt NPs, and Ir foil.(F) Corresponding K 3 -weighted magnitude of Fourier transform spectra from Ir L 3 -edge of Ir SA -Pt NPs, Pt NPs, and Pt foil.

F
I G U R E 3 (A) The CV curves in the potential region from 0.05 to 1.10 V recorded on Ir SA -Pt NPs, regular Pt NPs and commercial Pt/C catalysts.(B) ORR polarization curves of the Ir SA -Pt NPs catalyst in comparison with regular Pt NPs and commercial Pt/C catalysts.The current densities were normalized to the geometric area of the RDE (0.196 cm 2 ).(C) Specific and (D) mass activities given as kinetic current densities (j k ) normalized against the ECSAs of the catalysts and the mass of Pt, respectively.

F
I G U R E 5 (A) The OER polarization curves recorded on Ir SA -Pt NPs catalyst, regular Pt NPs, and commercial Pt/C catalysts in 0.5 M H 2 SO 4 at room temperature.(B) The current density at 1.6 V in acid solution.(C, D) The OER curves of the Ir SA -Pt NPs catalyst and commercial IrO 2 before and after accelerated durability tests between 1.1 and 1.6 V for 1000 cycles.
, the ORR pathway involves three stable adsorption models of different oxygenated intermediate species (*OOH, *O, and *OH) on Ir SA -Pt NPs and Pt NPs.During the first three steps of reaction on Pt NPs, the *O 2 is converted to *OH by reaction with proton-electron pairs and the free energy (ΔG) is downhill.The last step is the desorption of *OH from the Pt NP -NC surface and the free energy (ΔG) is thermodynamically uphill (endothermic).This result indicates that the desorption of *OH is the rate-determining step for the ORR on the Pt NP surface.Different from the pathway on the Pt NP surface, the free energy (ΔG) for formation and desorption of OH* on Ir SA -Pt NPs are both thermodynamically uphill.In addition, the results show that the ΔG value for desorption is 0.51 eV, which is higher than 0.14 eV for the formation of OH*.Therefore, the desorption of *OH is still the rate-determining step on the Ir SA -Pt NPs catalyst (Figure6B).Compared with Pt NP -NCNTs surface, the Ir SA -Pt NPs catalyst shows a relatively smaller ΔG, implying that the ORR electrocatalytic activity of Ir SA -Pt NPs with atomically Ir coordinated Pt is superior than that of Pt NP -NCNTs.According to the FT-EXAFS, fitting results in TableS2, the Pt-Pt average bonding distance for the Ir SA -Pt is slightly reduced due to the Ir atoms on the surface of Pt NPs.The shorten Pt-Pt bond distance could effectively enhance their ORR F I G U R E 6 The reaction mechanism and the corresponding free energy profiles for Ir SA -Pt NPs, Pt NPs, and IrO 2 catalysts.(A) The reaction mechanisms of ORR on Ir SA -Pt NPs.(B) The calculated free energy profiles of ORR steps on Ir SA -Pt NPs and Pt NPs under the applied potential of U = 0.9 V. (C) The reaction mechanisms of OER on Ir SA -Pt NPs.(D) The calculated free energy profiles of OER steps on Ir SA -Pt NPs and IrO 2 under the applied potential of U = 1.6 V.