This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. http://pubs.acs.org/journal/acscii Research Article Atomically Dispersed Iridium on Indium Tin Oxide Efficiently Catalyzes Water Oxidation Dmitry Lebedev, Roman Ezhov, Javier Heras-Domingo, Aleix Comas-Vives, Nicolas Kaeffer, Marc Willinger, Xavier Solans-Monfort,* Xing Huang,* Yulia Pushkar,* and Christophe Copeŕ et* Cite This: ACS Cent. Sci. 2020, 6, 1189−1198 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Heterogeneous catalysts in the form of atomically dispersed metals on a support provide the most efficient utilization of the active component, which is especially important for scarce and expensive late transition metals. These catalysts also enable unique opportunities to understand reaction pathways through detailed spectroscopic and computational studies. Here, we demonstrate that atomically dispersed iridium sites on indium tin oxide prepared via surface organometallic chemistry display exemplary catalytic activity in one of the most challenging electrochemical processes, the oxygen evolution reaction (OER). In situ X-ray absorption studies revealed the formation of IrVO intermediate under OER conditions with an Ir−O distance of 1.83 Å. Modeling of the reaction mechanism indicates that IrVO is likely a catalyst resting state, which is subsequently oxidized to IrVI enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo)electrochemical applications. ■ INTRODUCTION Atomically dispersed late transition metals (often referred to as We reasoned that employing SAC for the acidic OER offers multiple opportunities and advantages over classical catalysts. First, ultimate catalyst dispersion allows decreasing the noble single-atom catalysts, SACs) enable numerous processes metal content in the catalyst, which is critical for the including alkene hydrogenation,1 CO oxidation,2−5 elec- development of cost-effective acidic water electrolysis.22 tro-6−11 and photocatalysis,12,13 etc. They represent an Second, studying OER SAC can help shed light on the ultimate dispersion of a metal on a surface with all the reaction mechanism, which currently remains under debate atoms being exposed, which is critical for the expensive and due to its high complexity.23−25 The proposed OER scarce noble metal catalysts, such as Ir, Pd, and Pt. In addition, mechanisms differ by the nature of the OO bond formation SACs open specific reaction pathways12,14 and are well suited step: water nucleophilic attack (WNA) on a high-valent MO for a molecular-level understanding of the active-site structure intermediate versus radical coupling of two MO inter- and reaction mechanism by using a variety of element specific mediates.26−28 The nature of SACs eliminates the radical analytical tools, augmented by in situ/operando characterization coupling approach due to the physical separation of active sites coupled with computational modeling.1,2,7,9 on a support surface, and therefore, testing the catalytic activity Water splitting, widely considered the most sustainable of SAC in the OER and studying the reaction mechanism by in method for hydrogen production, is perhaps one of the situ spectroscopy can potentially lead to observing reaction ultimate platforms for exploiting SACs.15−20 Water splitting is intermediates and the discovery of novel reaction pathways. most efficiently carried out in either acidic or alkali conditions We thus generated isolated iridium sites on a conductive and consists of two half-reactions: hydrogen and oxygen oxide support (indium tin oxide, ITO), which, to the best of evolution reactions (HER and OER). Acidic water electrolysis our knowledge, is the first iridium SAC for the acidic OER. In offers a number of advantages over alkali water electrolysis including much larger operative current density and pressure,21 Received: May 12, 2020 yet it is strongly limited by the OER. A large overpotential and Published: July 1, 2020 harsh reaction conditions of the OER in an acidic environment result in high catalyst instability, limiting the pool of possible candidates to solely iridium- and ruthenium-based materials, utilized mostly in the form of films or nanoparticles. © 2020 American Chemical Society https://dx.doi.org/10.1021/acscentsci.0c00604 1189 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article Figure 1. (a) IrSAC−ITO preparation scheme with the proposed structure of the surface site (after exposure to ambient conditions). HAADF- STEM images of [Ir(COD)(IMes)(OH)] grafted on ITO (b) and IrSAC−ITO (c); scale bars are 2 nm. Bright dots highlighted with yellow circles are assigned to isolated Ir atoms. Inset shows the line intensity profile over the single Ir atom on the ITO surface. fact, previous studies of mononuclear and dinuclear Ir sequently calcined the sample at 400 °C to remove all electrocatalysts were limited to neutral conditions due to the organic ligands, yielding IrSAC−ITO (Figure 1a).35,36 We note catalyst degradation in acid.16 We further demonstrated that that after the 400 °C calcination step the exact coordination the activity of isolated Ir sites is on par with the state-of-the-art environment of the Ir site is not fully known; therefore, the catalysts, which raises the question of reaction mechanism. We IrSAC−ITO structure depicted in Figure 1a reflects the subsequently performed detailed electrochemical experiments proposed surface site after exposure to ambient conditions coupled with in situ X-ray absorption spectroscopy that (e.g., moisture) and is in line with the computational model revealed the formation of a high-valent IrVO intermediate, introduced below. High-angle annular dark-field scanning which so far remains elusive for the classical heterogeneous transmission electron microscopy (HAADF-STEM) studies IrOx catalysts.29,30 Our DFT computational studies further of the grafted sample (Figure 1b and Figure S2) show only the support the formation of IrVO as a resting state and its presence of crystalline ITO without any sign of iridium conversion into highly reactive IrVIO, which generates O2 deposition. After calcination, bright dots clearly appear on the upon reaction with water via the WNA mechanism. We also ITO surface (highlighted in dotted yellow circles in Figure 1c; show that at higher current density isolated Ir sites can readily Figure S3), which we attribute to single iridium atoms.16,17 migrate to form small clusters that open additional OER Note that no Ir or IrO2 clusters or nanoparticles are observed. reaction pathways. The line profile across the bright dot highlights the increase of ■ RESULTS AND DISCUSSION Generation and Characterization of Isolated Ir Sites. the intensity compared to the support which is due to strong scattering of Ir. The absence of bright dots on the grafted sample is likely due to the dangling of the surface grafted Most of the reported single atom electrocatalysts are prepared species over image accumulation time (smearing the HAADF- on carbon-based supports,8,20 which offers a number of STEM intensity), whereas upon the calcination Ir strongly advantages including high surface area and low support anchors to the ITO surface and becomes clearly visible in contrast in transmission electron microscopy. As these HAADF-STEM. carbon-based supports are prone to degradation under harsh In order to further make evident the formation of isolated Ir oxidizing OER conditions,31 we decided to utilize a conductive atoms on the ITO surface, we performed Fourier filtering of oxide (ITO), which in addition to higher OER stability the ITO lattice periodic contrast for the images of pristine ITO possesses surface hydroxyl groups allowing for controllable and IrSAC−ITO. While Fourier filtered ITO images show anchoring of molecular species.32−35 Using surface organo- uniform gray contrast, nonperiodic white dots for IrSAC−ITO metallic chemistry (SOMC),32−35 we grafted [Ir(COD)- further support the formation of atomically dispersed Ir (IMes)(OH)] (COD = cyclooctadiene; IMes = 1,3-dimesity- (Figure S4). We note that most likely the oxidative limidazol-2-ylidene) on porous ITO electrodes and sub- environment at an elevated temperature favors Ir dispersion 1190 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article Figure 2. Spectroscopic and electrochemical characterization of IrSAC−ITO. Ir 4f XPS (a) and EXAFS (b) data of pristine IrSAC−ITO and IrSAC− ITO after 2 h at 10 mA cm−2 showing no change in the catalyst structure. (c) Cyclic voltammetry of blank ITO, pristine IrSAC−ITO, and IrSAC− ITO after 2 h at 10 mA cm−2 (0.1 M HClO4, 10 mV s−1) revealing IrIV/III and IrV/IV redox waves. (d) 2 h 10 mA cm−2 chronopotentiometric measurement of IrSAC−ITO showing high stability of the catalyst. Table 1. Structural Parameters for Fitsa of the ex Situ and in Situ EXAFS Data for IrSAC−ITO sample fit no. shell R, Å Nb σ2 × 103 R-factor reduced Chi2 pristine IrSAC−ITO (ex situ); 3.2−10.68 k-space; 1.25−2.08 R-space 1 c Ir−O 2.04 6c 5.6 0.003 214 IrSAC−ITO after 2 h at 10 mA cm−2 (ex situ); 3.2−10.68 k-space; 1.25−2.08 R-space 2c Ir−O 2.03 6c 5.1 0.002 376 IrSAC−ITO at 1.46 V vs RHE (in situ); 3.2−10.68 k-space; 1.25−2.04 R-space 3 Ir−O 2.00 6 4.9 0.004 240 Ir−O 1.80 1 3.5b IrSAC−ITO at 1.46 V vs RHE (in situ); 3.2−10.68 k-space; 1.25−2.04 R-space 4 0.0006 40 Ir−O 1.99 5 3.5b IrSAC−ITO at 1.46 V vs RHE (in situ); 3.2−12.26 k-space; 1.25−2.04 R-space 5 Ir−O 2.01 6 7.0 0.005 40 Ir−O 1.83 1 2.3b IrSAC−ITO at 1.46 V vs RHE (in situ); 3.2−12.26 k-space; 1.25−2.04 R-space 6 0.0013 11 Ir−O 2.03 5 2.3b a Fits were done in the q-space. R is the Ir−backscatter distance. σ2 is the Debye−Waller factor. R-factor and reduced Chi2 are the goodness-of-fit parameters. S02 = 1.0 was used for all the fits. bDenotes that σ2 was set to be the same for multiple vectors. cNote that changing the number of neighbors (N = 5 or 7) and/or splitting the first coordination shell did not improve these fits. on the ITO surface, as grafting and calcination of multinuclear and atomic nature of the iridium species. The proposed Ir compounds also lead to the formation of single atom species catalyst design is thus suitable as a transparent OER (Figure S5). electrocatalytic layer for use in the sunlight-driven water Elemental analysis of the IrSAC−ITO electrodes gives an Ir splitting devices. loading of 0.86 wt %, corresponding to the loading of 110 ± 10 XPS studies of the IrSAC−ITO (Figure S18) show the nmol(Ir) cm−2electrode (21 ± 2 μg(Ir) cm−2electrode, 0.62 ± 0.06 presence of O, In, Sn, and Ir, consistent with the sample Ir nm−2ITO), which is significantly lower compared to the composition. The Ir 4f region (Figure 2a) consists of two loadings typically used for OER catalysts.37,38 Visible−near-IR peaks, which are assigned to the spin−orbit split Ir 4f transmission studies of the electrodes (Figure S17) show that components: Ir 4f 7/2 and Ir 4f 5/2. We find the Ir 4f 7/2 deposition of iridium does not lead to a significant increase of energy of 61.9 eV, which corresponds to IrIII.9 While XPS light absorption at the electrodes (on average 4% decrease of spectra of conductive Ir-containing compounds (Ir metal, IrO2, the transmission), which is in line with the low iridium loading Ir pyrochlores, etc.) always display asymmetrical line shapes 1191 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article (originating from the electron screening effects),39−41 Ir 4f 118 mV dec−1 reported for molecular Ir catalyst deposited on peaks of IrSAC−ITO are symmetrical and therefore were fitted ITO.44 The current density of 10 A gIr−1 is reached at an using a Gaussian line shape. The symmetrical line shape arises overpotential of 220 ± 5 mV, which is lower than 240 mV from the absence of the electron screening effects and is reported for Ir nanoparticles dispersed on ITO37 and high consistent with the absence of the iridium (oxide) bulk surface area IrO2 and IrO2−TiO2 catalysts (260−270 mV).39,45 metallic character, further supporting the presence of isolated IrSAC−ITO produces the current density of 49 ± 5 A g(Ir)−1 Ir sites. and 156 ± 13 A g(Ir)−1 at 1.48 and 1.51 V vs RHE In order to further probe the Ir oxidation state and respectively, which is higher than values reported for state-of- coordination environment, we performed ex situ and in situ the-art Ir and IrO2 catalysts42,46 and approaches the highest (vide inf ra) X-ray absorption spectroscopy (XAS) studies at performing mixed metal Ir−M catalysts (see Table S1 for a the Ir LIII edge (see details in the SI). X-ray absorption near- detailed comparison).47,48 The high activity of the IrSAC−ITO edge spectra (XANES, Figure S19) yielded an Ir edge energy is likely due to the ultimate Ir dispersion on the electrode (defined here as a maximum of the white line) of 11218 eV surface and hence optimal utilization of Ir. These results consistent with IrIII.38,39 Extended X-ray absorption fine demonstrate that isolated Ir surface sites can reach high structure (EXAFS) data in the R-space (Figure 2b) have one catalytic turnover in the 4-electron water oxidation without the major peak at approximately 2 Å, which corresponds to the need of adjacent Ir partners or underlying Ir/IrOx bulk (vide first coordination shell of iridium (Ir−O). Minor peaks at infra). ∼2.5−3.5 Å apparent distance likely originate from the The stability of IrSAC−ITO was investigated by carrying out contribution of the ITO support. The lack of strong Ir−Ir or a 3 h electrolysis test at 0.5 mA cm−2 (Figure S14) and a 2 h Ir−O−Ir interactions expected at ∼2.7 or ∼3.5 and ∼5.5 Å electrolysis test at 10 mA cm−2 (Figure 2d),24 which apparent distance, respectively, infers the absence of large corresponds to turnover numbers (TONs) of approximately iridium and iridium oxide aggregates (Figure 2b). We find that 130 and 1700 O2 molecules per Ir atom, respectively (see the best model for the first coordination sphere of Ir is 6 details in the SI). During these experiments, the IrSAC−ITO oxygen neighbors at a distance of 2.04 ± 0.02 Å (see Table 1 catalyst shows nearly no drift in potential and requires on and Figure S20), which is very close to 2.03 ± 0.01 Å found for average 255 ± 10 and 350 ± 20 mV overpotential to drive the IrIII in IrOx.39 Note that either changing the number of current of 0.5 and 10 mA cm−2, respectively. A similar neighbors (N = 5 or 7) or splitting the first coordination shell overpotential of 340 ± 20 mV at 10 mA cm−2 is found for Ir does not improve the fit (results in larger Chi2). nanoparticles dispersed on ITO37 at the expense of a 5 times Electrochemical Studies and OER Activity. Electro- higher Ir loading. chemical studies of IrSAC−ITO were performed in 0.1 M The use of electrodes with relatively high surface area HClO 4 electrolyte (pH 1) with the additional cyclic (hence large amount of catalyst) allows for postanalyses of the voltammetry measurements at pH 4. Cyclic voltammetry catalyst and, more importantly, in situ studies (vide infra).37 (Figure 2c and Figure S11) at pH 1 shows the presence of two We find that both Ir 4f XPS and XAS data of IrSAC−ITO after 2 redox waves located at 0.89 and 1.35 V vs the reversible h electrolysis at 10 mA cm−2 (Figure 2 and Figure S19) are hydrogen electrode (RHE), assigned to Ir IV/III and IrV/IV nearly identical to the data of the pristine sample, indicating transitions, respectively,42 followed by an increase in current that the Ir oxidation state relaxes to the original IrIII; there are corresponding to catalytic water oxidation. Measurements at no changes in the Ir local coordination environment and no pH 4 (Figure S12) produce the same shape voltammogram formation of large Ir or IrO2 nanoparticles. However, knowing with redox waves located at 0.85 and 1.37 V vs RHE. These that XPS and XAS analyses are not sensitive to formation of values are very close to those obtained at pH 1, which indicates ultrasmall clusters, we performed detailed HAADF-STEM that both Ir IV/III and IrV/IV redox transitions are proton characterization of IrSAC−ITO at different stages of our coupled electron transfer (PCET) steps. The linear depend- electrochemical tests. HAADF-STEM images of IrSAC−ITO ence of the IrIV/III peak current (pH 1, forward scan) with the recorded after just immersing the electrode in the electrolyte scan rate indicates that the redox event is associated with the for 5 min, after 10 CV cycles from 0.46 to 1.56 V vs RHE (25 electrode-bound species (Figure S13). Integration of the IrIV/III mV s−1), and after a representative activity test (chronoamper- couple results in electroactive Ir loading of 100 ± 10 nmol(Ir) ometry from 1.41 to 1.53 V vs RHE with 25 mV steps in 0.1 M cm−2, which is in a good agreement with the loading of 110 ± HClO4) show no difference compared to the pristine IrSAC− 10 nmol(Ir) cm−2 obtained from the elemental analysis. The ITO (Figures S6−S8). However, HAADF-STEM images of fact that most of the capacitive response from the voltammo- IrSAC−ITO after 3 h at 0.5 mA cm−2 show that part of iridium grams of IrSAC−ITO originates from the ITO support and not single sites aggregated into small clusters, with the effect being from Ir (which only contributes to the redox events and OER more pronounced after 2 h electrolysis at 10 mA cm−2 (Figures catalytic current, contrary to the CV studies of iridium S9−S10). Formation of these aggregates with a diameter of nanoparticles on ITO)37 confirms the high dispersion of the approximately 0.6 nm indicates mobility of the surface Ir electroactive species for IrSAC−ITO. species during prolonged OER tests at high potentials. Cyclic The OER catalytic activity of IrSAC−ITO was examined voltammetry studies of IrSAC−ITO after 2 h electrolysis at 10 using linear sweep voltammetry (Figure S16a), steady-state mA cm−2 (Figure S14) show no change of Ir loading, which chronoamperometric, and chronopotentiometric measure- excludes Ir leaching in the electrolyte. Broadening of the CV ments in 0.1 M HClO4. Using a Clark oxygen sensor, we redox waves indicates the presence of the distribution of show that IrSAC−ITO produces O2 with 99% Faradaic surface Ir species, most probably associated with the efficiency (H2O2 yield <1%, see Figure S15 and details in coexistence of Ir single sites and aggregates of different sizes the SI). The slope of the IrSAC−ITO Tafel plot (Figure S16b) as revealed by HAADF-STEM. The Ir mobility on the ITO is 46 ± 4 mV dec−1, similar to what is observed on iridium surface under OER conditions may be explained by ITO oxide nanoparticles39,42,43 and significantly smaller than 66− surface reconstruction under applied potential or the 1192 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article Figure 3. In situ studies of IrSAC−ITO. Ir LIII-edge XANES spectra (a) and their first derivatives (b) reveal the shift of the edge energy as a function of the applied potential. Inset shows assignment of the edge energy to a certain Ir oxidation state. Figure 4. Computed OER mechanism with the structures of IrSAC−ITO (1) and IrVO site (3) shown on top. The mechanism involves the PCET step of observed IrVO to proposed IrVIO followed by WNA. involvement of surface indium or tin atoms in the OER degradation pathways play a role, requires further detailed process, although the exact origin and mechanism of Ir studies. EXAFS of spent electrodes shows a similar spectrum to the mobility and aggregation remain unclear. Whether the Ir starting IrSAC−ITO, with a small contribution from the second clustering leads to the decrease in OER performance and is coordination shell as for the starting material (Figure S20). responsible for the slight increase of the potential required to This observation is in line with previous studies where no Ir−Ir maintain the 10 mA cm−2 current (Figure 2d), or other contribution was detected for an Ir dimer deposited on 1193 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article Fe2O 3.16 The absence of scattering from the second calculated to occur at 0.60 V vs RHE followed by subsequent coordination shell can be explained by the small size and PCET oxidations to IrV and IrVI at 1.34 and 1.55 V vs RHE, disordered nature of the Ir clusters, which results in the respectively. While the IrIV intermediate (2, Figure S24) distribution of Ir−Ir distances. Fitting the first coordination features three hydroxo groups, IrV species (Figure 4) bear an shell using the same model as for the initial electrode results in oxo group (IrO) at 1.80 Å, a distance that closely matches the Ir−O distance of 2.03 ± 0.02 Å, similar to the value the experimental in situ EXAFS data recorded at 1.46 V vs obtained for the pristine IrSAC−ITO (Table 1). RHE (1.83 Å for fit 6 of Table 1). In Situ XAS. In order to investigate the iridium oxidation Next, we explored two possible pathways for OO bond state and probe the coordination environment under OER formation: direct WNA on pentavalent oxo complex (3) and conditions, we exploited in situ XAS studies at the Ir LIII-edge on high-valent IrVI dioxo intermediate (4). While the WNA using a custom-designed electrochemical cell. The XANES step directly from the pentavalent complex (3) is highly spectra (Figure 3) show an increase of the edge energy with an endergonic (0.97 eV, 22.4 kcal mol−1, see Figure S26), we find increase of the applied potential (see Table 1). Based on the very feasible energetics for the second pathway (Figure 4). reference IrO2 (IrIV), the oxidation states of the Ir were Indeed, the PCET to form IrVI dioxo intermediate (4) can assigned to IrIII (0−0.9 V vs RHE), IrIV (0.9−1.35 V vs RHE), happen at 1.55 V vs RHE followed by a much less endergonic and IrV (above 1.35 V vs RHE), which corroborates the WNA step (0.16 eV, 3.7 kcal mol−1) to form a hydroperoxo assignment done from the electrochemical studies (Figure 2c). intermediate (5, Figure S24), with the subsequent steps Similar shifts in the position of the Ir LIII-edge were reported in requiring lower potentials. In summary, the latter pathway does the XAS studies of the submonolayer film of IrOx on RuO2, not imply any challenging chemical steps and predicts an although unknown catalyst morphology and structure did not overpotential of 0.32 V, in agreement with the low over- allow correlating the observed changes with the changes of the potential observed experimentally. The fact that the IrVI local Ir coordination environment.30 Moreover it is unclear intermediate is not observed in the experiment neither by whether the spectroscopic changes are associated with electrochemistry nor by spectroscopy suggests that IrV is the catalysis, as the submonolayer IrOx film is grown on highly resting state of the catalyst and that the IrVI species are short- OER active RuO2. lived. In this context, DFT calculations indicate that the WNA Performing in situ studies on single atom catalysts on an is too challenging on the site-isolated IrV species, and another OER inactive support allows the overcoming of these PCET step is required to make the experimentally observed limitations and controversy by reducing the complexity of surface IrVO more electrophilic. Interestingly, recent the system. This approach enables establishing the local computational studies on molecular Ir complexes50,51 found coordination environment of Ir and drawing of correlations that formation of the high-valent IrVI intermediates is plausible with the catalyst OER activity. We collected in situ EXAFS data in the experiments and facilitates the OER catalysis.47 It is also in the OER regime at a potential of 1.46 V vs RHE and a worth noting that such proposed IrVI intermediates are current of approximately 0.2 mA cm−2 (Figures S21 and S22). isoelectronic to RuV species, which are proposed to play Attempts to collect EXAFS data at higher potentials were important role in molecular OER catalysts.27,52−54 unsuccessful because of intense bubble formation due to the As partial aggregation of Ir was observed during prolonged OER. Fitting the first iridium coordination shell using the same electrochemical stability tests, we decided to perform cluster model as for the initial electrode (Figure S21 and fits 3 and 5 calculations on Ir dimeric species in order to access the redox of Table 1) results in the IrO distance of 2.00 ± 0.02 Å, potentials. We find that the potentials required for oxidizing which is shorter than the distance obtained for pristine IrSAC− the Ir(III)−Ir(III) species to Ir(V)−Ir(V) are not consistent ITO. We found that splitting the first coordination shell into 5 with the experimental CV and in situ XAS data (Figure S27), + 1 oxygen scatterers (Figure S21 and fits 4 and 6 of Table 1) suggesting that iridium dimers are highly unlikely present in improves the fit quality (lower R-factor and reduced Chi2) and the initial material. However, if formed during the catalytic results in the short IrO backscatterer at 1.83 ± 0.02 Å. We process as a consequence of Ir migration on the surface, our assign this short oxygen backscatterer to the IrVO calculations suggest that the dimeric species would also be intermediate, which can be a predominant species for IrSAC− active at reaction conditions (see further details in the SI). ITO under OER conditions (i.e., resting state of the catalyst).27 For comparison, the only known terminal-oxo IrV complex (oxotrimesityliridium),49 albeit in tetrahedral geom- ■ CONCLUSIONS We have described the synthesis and characterization of an etry, features a shorter IrO distance of 1.73 Å. atomically dispersed Ir OER electrocatalyst, highly active and DFT Calculations under Periodic Boundary Condi- stable in acidic conditions. High-resolution electron micros- tions. After establishing that OER activity of the site-isolated copy images, a unique symmetrical Ir 4f XPS line shape, and Ir on ITO is on par with the state-of-the-art Ir catalysts, we clear redox transitions detected both electrochemically and by carried out DFT periodic calculations with the VASP package in situ XANES make evident the formation of isolated IrIII sites (see the Methods section and the SI for the computational on ITO. The OER activity of the catalyst ranks among those of details) with the aim of gaining a further understanding of the the highest performing Ir and IrOx catalysts, which is achieved catalytic cycle. IrSAC−ITO was modeled as a tris-bound IrIII due to the ultimate Ir dispersion and demonstrates that site- isomer of a general formula of (Inμ 2 OH) 3 Ir- isolated mononuclear Ir centers can effectively catalyze the (OH)2(H2O), where (Inμ2OH) describes bridging OER. In situ EXAFS studies of the catalyst during the OER hydroxyl ligands bonded to Ir and In atoms of the oxide reveal the presence of IrVO species with the IrO distance surface (Figure 4). The IrO distances vary from 1.98 to 2.14 of 1.83 Å. Computational modeling of the reaction pathway Å with an average value of 2.06 Å (Table S2), which is close to with isolated Ir sites indicates that the IrVO species are the 2.04 Å obtained via EXAFS modeling of the pristine IrSAC− catalyst resting state that can further undergo a PCET step to ITO (Table 1). The first PCET oxidation to produce IrIV is form an IrVI dioxo intermediate, from which WNA is 1194 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article endergonic by only 0.16 eV. The feasible WNA step suggests Electrochemical Characterization. Electrochemical that the IrVI dioxo species are short-lived under electrocatalytic measurements were performed in a standard single-compart- conditions, explaining why they are not experimentally ment 3-electrode cell with Pt counter and Ag/AgCl reference detected. While the catalyst stays in the atomically dispersed electrodes. The porous ITO electrodes were electrically form during CV and chronoamperometry tests, high-resolution contacted using the uncoated FTO layer and masked to a electron microscopy studies indicate that part of the iridium geometrical surface area of 0.8−1.2 cm2 using nonconductive single sites aggregates in small clusters on a longer time scale epoxy (see Figure S12). 0.1 M HClO4 or 0.1 M acetate buffer under high applied potential. The mechanism of this process as (with total ionic strength adjusted to 0.5 M with NaClO4) was well as its impact on the catalyst activity remain unclear and used as the electrolyte and was saturated with Ar prior to the require future investigations. measurements. Polarization curves were obtained in 0.1 M Our studies show that surface organometallic chemistry is a HClO4 from the steady-state chronoamperometric measure- powerful approach to access atomically dispersed and efficient ments: the potential was gradually stepped from 1.41 to 1.53 V metal sites on oxide supports enabling detailed characterization vs RHE (with 25 mV steps) while holding for 1 min at each and mechanistic studies via state-of-the-art spectroscopic tools potential. Electrochemical impedance spectroscopy measure- and computational modeling. This approach is likely general ments were used for Ohmic drop compensation. Chronopo- and applicable to a broad range of heterogeneous trans- tentiometry (stability) measurements were performed in 0.1 M formations including partial oxidation of methane, electro- HClO4 at 0.5 or 10 mA cm−2 for 3 or 2 h, respectively. Both reduction of CO2, (photo)electrochemical water splitting, etc. vigorous stirring and argon bubbling of electrolyte were used ■ for chronoamperometric and chronopotentiometric measure- METHODS ments. The current expressed as [mA cm−2] refers to the geometrical surface area of the electrode, and [A gIr−1] refers to IrSAC−ITO Preparation. Grafting of [Ir(COD)(IMes)- the total mass of Ir on the electrode, determined using the (OH)]55 on porous ITO electrodes27,56 was performed elemental analysis. All reported measurements were repeated under inert conditions according to the following procedure: several times to ensure the reproducibility of the results. Unless 10 mg of [Ir(COD)(IMes)(OH)] were dissolved in 8 mL of specified otherwise, all the potential values are reported vs dry, air-free heptane, and the obtained 2 mM solution was reversible hydrogen electrode (RHE) scale. See the SI for more added to 10 porous ITO electrodes (1.5 cm2 of ITO each) in details and the description of the Faraday efficiency measure- the Petri dish in the glovebox. The Petri dish was sealed with ment. parafilm to avoid heptane evaporation and left overnight. The X-ray Absorption (XAS) Studies. XAS studies were electrodes were washed three times with heptane, calcined at performed at the Advanced Photon Source (APS) at Argonne 400 °C for 1 h (with the ramp of 60 °C h−1), and exposed to National Laboratory in the fluorescence mode. Ex situ XAS was ambient conditions to produce IrSAC−ITO. No unexpected or taken for reference IrO2, pristine IrSAC−ITO, and IrSAC−ITO unusually high safety hazards were encountered. after 2 h of electrolysis at 10 mA cm−2 in 0.1 M HClO4 General. High-angle annular dark-field scanning trans- (Figures S18 and S19). An electrochemical cell (10 mL) of mission electron microscopy (HAADF-STEM) imaging was custom design equipped with Ag/AgCl reference and Pt performed using aberration-corrected JEOL JEM-ARM 200F counter electrodes was used for the in situ measurements. All and JEM-ARM 300F Grand ARM transmission electron data were background-corrected, normalized, and deglitched microscopes operated at 200 and 300 kV, respectively. TEM (if necessary) and then converted to wave vector space (k- samples were prepared by scratching ITO with surface Ir space) and weighted by k3. k-space data were truncated near species from the fluorine doped tin oxide (FTO) surface zero crossings before Fourier transformation. See more details followed by the dry deposition of the obtained powder on the in the SI. EXAFS data were analyzed using the Athena software Cu TEM grid (200 mesh) covered with the ultrathin holey package.58 carbon layer. After shaking off the material that is loosely Density Functional Theory (DFT). Geometry optimizations attached to the carbon support, we mounted the grid into a were carried out using spin-polarized PBE-D259,60 calculations standard single-tilt TEM sample holder. Fourier filtered images as implemented in VASP code.61,62 We considered the (Figure S4) were obtained by masking the periodic component projector augmented wave (PAW) pseudopotentials and in the FT image followed by an inverse FT. X-ray expanded the valence electrons in plane waves with a kinetic photoelectron spectroscopy (XPS) was conducted on an XPS energy cutoff of 500 eV. The Brillouin zone was sampled with a Sigma 2 instrument (ThermoFisher Scientific, Loughborough, Monkhorst-Pack grid of (6, 6, 6) and (6, 6, 1) k-points for bulk Great Britain) equipped with a UHV chamber (pressure < 10-6 and slab calculations, respectively. The isolated Ir centers on Pa), a non-monochromatic 200 W Al Kα source, a hemi- ITO were represented with a 4-layer slab model of the (111) spherical analyzer, and a seven channel electron multiplier. The surface containing the Ir center coordinated to indium oxide analyzer-to-source angle is 50°, while the emission angle is 0°. support via three Ir−O(H)−In bonds. Water molecules were A pass energy of 50 and 25 eV was set for the survey and the included around the active site to represent a high local water narrow scans, respectively. XPS data were analyzed in CasaXPS coverage. The c value was set to 35 Å ensuring an interlayer Version 2.3.19PR1.0 software; the binding energies of the distance of at least 21 Å. Tin atoms were omitted in the acquired spectra were referenced to the C 1s line at 284.8 eV. calculation for simplification as we do not expect that the Background subtraction was performed according to Shirley57 presence of very few Sn in the model would influence the (Shirley background modified with linear to capture the rising electronic structure of Ir. The electronic energies for the OER In 4p line at approximately 80 eV). Elemental analysis was mechanism were corrected with single point calculations performed by the Mikroanalytisches Labor Pascher, Remagen, applying an ONIOM like scheme63 with two layers: (i) the Germany, using inductively coupled plasma (ICP)-based low level corresponds to the PBE-D2 periodic model methods. computed with VASP, and (ii) the inner layer corresponds 1195 https://dx.doi.org/10.1021/acscentsci.0c00604 ACS Cent. Sci. 2020, 6, 1189−1198 ACS Central Science http://pubs.acs.org/journal/acscii Research Article to the (H2O)(OH)Ir(OH)2(H2O) cluster (for the initial IrIII performed the computational studies. All the authors species). This inner part was computed with the highly interpreted the data and wrote the manuscript. accurate double-hybrid functional B2GP-PLYP64 as imple- Funding mented in ORCA.65 In the molecular calculation, atoms were This research is based upon work supported by InnoSuisse represented with the split-valence triple-z basis set with through the Swiss Competence Center for Energy Research polarization def2-TZVP.66 See the SI for more computational (SCCER) Heat & Electricity Storage and by the National details. ■ Science Foundation, Division of Chemistry CHE-1900476 (Y.P.). The use of the Advanced Photon Source, an Office of ASSOCIATED CONTENT Science User Facility operated by the U.S. Department of * Supporting Information sı Energy (DOE) Office of Science by Argonne National The Supporting Information is available free of charge at Laboratory, was supported by the U.S. DOE under Contract https://pubs.acs.org/doi/10.1021/acscentsci.0c00604. DE-AC02-06CH11357. The PNC/XSD (Sector 20) facilities Additional data, methods, and supplementary figures at the Advanced Photon Source and research at these facilities including HRTEM images, electrochemical and spectro- were supported by the U.S. Department of EnergyBasic scopic data, and details of the computational modeling Energy Science, the Canadian Light Source and its funding (PDF) partners, the University of Washington, and the Advanced Photon Source. J. H.-D. and X.S.-M. acknowledge financial ■ AUTHOR INFORMATION Corresponding Authors support from MINECO (CTQ2017-89132-P) and Generalitat de Catalunya (2017SGR1320). A.C.-V. acknowledges the financial support from the “Ramon y Cajal” Fellowship, Xavier Solans-Monfort − Departament de Quimica,́ Universitat funded by Spanish MEC and the European Social Fund Autònoma de Barcelona, Bellaterra 08193, Catalonia, Spain; (RyC-2016-19930) and and the Spanish “Ministerio de orcid.org/0000-0002-2172-3895; Email: xavier.solans@ Ciencia, Innovación y Universidades” (PGC2018-100818-A- uab.cat I00). Xing Huang − Department of Chemistry and Applied Biosciences Notes and Scientific Center for Optical and Electron Microscopy The authors declare no competing financial interest. ■ (ScopeM), ETH Zurich, CH-8093 Zurich, Switzerland; orcid.org/0000-0002-8700-0606; Email: xing.huang@ ACKNOWLEDGMENTS scopem.ethz.ch Yulia Pushkar − Department of Physics and Astronomy, Purdue X.H. and M.W. acknowledge Prof. Robert Schlögl from the University, West Lafayette, Indiana 47907, United States; Department of Inorganic Chemistry, Fritz Haber Institute of orcid.org/0000-0001-7949-6472; Email: ypushkar@ Max Plank Society for the use of JEOL JEM-ARM 200F TEM. purdue.edu We would like to acknowledge Athanasia Tsoukalou (ETH Zurich) for performing the XPS measurements. ■ Christophe Copéret − Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland; REFERENCES orcid.org/0000-0001-9660-3890; Email: ccoperet@ ethz.ch (1) Liu, P.; Zhao, Y.; Qin, R.; Mo, S.; Chen, G.; Gu, L.; Chevrier, D. 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