Substrate‐Driven Catalyst Reducibility for Oxygen Evolution and Its Effect on the Operation of Proton Exchange Membrane Water Electrolyzers

To increase the efficiency of hydrogen production by proton exchange membrane water electrolyzers (PEMWEs), the relationships between the specific activity and stability of the membrane–electrode assembly (MEA) must be clarified. Ir oxide electrodeposited on Ti substrate is used as an oxygen electrode, and its electronic properties and electrochemical behavior in PEMWE operation are observed. The electrode, fabricated through a facile strategy based on annealing in the air atmosphere, enhances the specific oxygen evolution reaction (OER) activity and stability in PEMWE operation. Furthermore, the electronic catalyst–substrate interactions associated with different reducibilities in the heteroatom system are studied. The morphology, electronic properties, and chemical state of the oxygen electrode are investigated through X‐ray spectroscopy, microscopy techniques, and computational analyses. Thermal treatment of the catalyst‐coated substrate decreases the bulk oxidation state of Ir and increases the surface oxidation state. According to the electrochemical and physical behavior analyses of PEMWEs, the oxygen content in the Ir oxide structure, which defines the OER activity and its stability, is influenced by the crystalline structure and formation of a stable interface between the catalyst and substrate. The outcomes can facilitate the development of strategies for enhancing the performance of PEMWEs and designing rational MEAs.


Introduction
Green hydrogen, which emits no carbon dioxide in its life cycle, has attracted global attention as an alternative energy source to fossil fuels.Green hydrogen is typically produced using water electrolyzers (WEs) powered by renewable sources such as solar and wind energy.However, to satisfy the carbon neutrality demands by 2050À2060, the production of green hydrogen must be increased.To this end, various WEs have been developed, such as conventional alkaline WEs, solid oxide WEs operating at high temperatures (>800 °C), proton exchange membrane (PEM) WEs, and anion exchange membrane (AEM) WEs. [1]Among these WEs, proton exchange membrane water electrolyzers (PEMWEs) can generate hydrogen at the highest rates owing to the higher ion mobility at a lower operating voltage and are thus, promising candidates to facilitate the realization of a hydrogen economy and society. [2]espite these advantages, several technical challenges remain to be overcome to mass produce hydrogen using PEMWEs.In particular, the kinetics of the oxygen evolution reaction (OER) at the anode is sluggish compared with that of the hydrogen evolution reaction at the cathode, which limits the choice of catalyst materials.Furthermore, the acidic environments at high anodic potentials can lead to the dissolution and/or oxidation of metal-based materials at the anode, which restricts the candidates for OER catalysts to noble metals.Ir is a promising OER catalyst in acidic media owing to its high OER activity and stability.However, the scarcity of Ir reserves limits its widespread use in PEMWEs. [3]Therefore, a potential solution for cost-effective green hydrogen production through PEMWEs is to decrease the loading of the Ir catalyst onto the anode.
Many researchers have attempted to enhance the specific activity and stability of Ir-based electrocatalysts, typically by 1) modifying the electronic and crystalline structures of Ir oxide (IrO x ) by tuning oxygen vacancies, [4][5][6][7][8] 2) forming various To increase the efficiency of hydrogen production by proton exchange membrane water electrolyzers (PEMWEs), the relationships between the specific activity and stability of the membrane-electrode assembly (MEA) must be clarified.Ir oxide electrodeposited on Ti substrate is used as an oxygen electrode, and its electronic properties and electrochemical behavior in PEMWE operation are observed.The electrode, fabricated through a facile strategy based on annealing in the air atmosphere, enhances the specific oxygen evolution reaction (OER) activity and stability in PEMWE operation.Furthermore, the electronic catalyst-substrate interactions associated with different reducibilities in the heteroatom system are studied.The morphology, electronic properties, and chemical state of the oxygen electrode are investigated through X-ray spectroscopy, microscopy techniques, and computational analyses.Thermal treatment of the catalyst-coated substrate decreases the bulk oxidation state of Ir and increases the surface oxidation state.According to the electrochemical and physical behavior analyses of PEMWEs, the oxygen content in the Ir oxide structure, which defines the OER activity and its stability, is influenced by the crystalline structure and formation of a stable interface between the catalyst and substrate.The outcomes can facilitate the development of strategies for enhancing the performance of PEMWEs and designing rational MEAs.
[15][16] Among the reported OER mechanisms for acidic media, especially, it is generally accepted that water molecules adsorb onto catalytically active sites to form *OH in the early stages, despite there being no consensus regarding the subsequent reaction steps. [17,18]Consequently, many attempts have been devoted to modify the electronic structures of catalysts to maximize the number of oxygen vacancies.For example, one effective approach is to leach transition metal components from the alloying structure, which promotes the generation of oxygen vacancies for neighboring elements through the straining effect. [8,19,20]23][24][25] These catalysts with modified electronic structures demonstrate outstanding intrinsic OER activity because of their improved kinetics at the active sites.By means of supports interaction, meanwhile, Oh et al. [13] prepared various supported Ir catalysts and investigated the electronic and physical properties of Ir modified by the interaction between Ir metal and Sb-doped SnO 2 .Puigdollers et al. [14] explored the reducibility of oxides to characterize the formation energy of oxygen vacancies and the corresponding influence of metal nanoparticles at the metal/ oxide interface.
Typically, the catalytic activity of Ir catalysts is assessed using a three-electrode cell consisting of a reference electrode, a counter electrode, and a thin film of catalyst-coated glassy carbon as the working electrode, immersed in a liquid electrolyte that readily interacts with nearly the entire catalyst surface.However, in PEMWEs, the diffusion electrode sandwiched between a membrane and flow field contains a thick catalyst layer (CL) with many catalyst/ionomer agglomerates with gas-phase voids and a porous transport layer (PTL) that facilitates reactant and product flow from the catalytically active sites in the CL.Consequently, the actual rate of electrochemical reaction is strongly influenced by various irreversible losses through PEMWE components and their contact interfaces as well as the decrease in the catalytically active area due to improper catalyst/ionomer interfaces.These phenomena are aggravated with increasing applied current density, which results in increased ohmic losses through the protonic and electronic pathways and each heterogeneous interface and intensified water concentration gradient. [26]Hence, to promote practical application, it is vital to evaluate the performance of catalysts in PEMWEs using various electrochemical methods.
Considering these aspects, we adopted commercially available Ti substrate (TS) that has high porosity, high electrical conductivity, no severe sintering by thermal stress, and strong corrosion resistance at highly oxidizing conditions and synthesized highly efficient IrO x catalysts on a TS through a facile electrodeposition process followed by thermal treatment. [27]We examined how the phase transition of IrO x , induced by TS during electrochemical and thermal processing, influences the intrinsic OER kinetics and durability with respect to the crystalline structure and active sites.Comprehensive analyses, involving high-resolution transmission electron microscopy (HRTEM), energy-dispersive spectroscopy (EDS), surface and depth X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffractometry (HRXRD), X-ray absorption spectroscopy (XAS), and computational analysis, were performed to clarify the surface and bulk properties of IrO x catalysts on the TS.The experimental and theoretical investigations had the following objectives: 1) exploring the oxide/metal interactions at the atomic level, 2) extending bulk CL/TS interactions using electrodeposited IrO x catalysts on the TS (EDITs) with thermal curing, and 3) overcoming limitations of half-cell testing using an actual single cell with a membrane-electrode assembly (MEA) consisting of a solid electrolyte membrane, a hydrogen electrode with a Pt-based catalyst, and an oxygen electrode with an Ir-based catalyst.The outcomes are expected to highlight the factors influencing the OER activity and stability of IrO x on Ti electrodes in PEMWE operations.

Results and Discussion
To gain fundamental insights into the OER activity of the fabricated catalysts at the electrode/electrolyte interface, their electronic configurations were comprehensively characterized.In general, the *OH binding energy (BE) is widely recognized as an OER descriptor, as it depends on the number of oxygen vacancies in the Ir structure.Before the physical and electrochemical characterizations, we performed density functional theory (DFT) calculations to predict the *OH BEs in various phases.As shown in Figure 1, a considerably high *OH adsorption energy was required for metallic Ir (111), which gradually decreased with the increasing number of oxygen vacancies on the surface of crystalline Ir oxide (110).This indicates that water molecules are more easily adsorbed by electrophilic attacks as more oxygen vacancies exist on the surface of Ir oxide than Ir metal.
Extensive theoretical and analytical investigations have been performed to clarify the correlation between oxygen vacancies and OER activity, and various hypotheses have been proposed.Pfeifer et al. [28,29] performed in situ XAS and attributed the enhancement in the OER activity with abundant oxygen vacancies to the preferred nucleophilic attack of water molecules on the electrophilic O IÀ species.The authors found that bulk O IÀ species were more weakly bound than O IIÀ species, resulting in the occurrence of a thermodynamically favorable reaction around the anionic defects.Focusing on the crystalline structure, Geiger et al. [20] investigated the role of oxygen vacancies as "active oxygen sites."The authors demonstrated that the OER activity depends on the ratio of corner-sharing oxygen atoms in the loose packing octahedral structure and accessibility to the active oxygen sites.In particular, the competition of covalent bonding between octahedral cations and oxygen p orbitals leads to the exposure of active catalytic sites. [30]Therefore, the exceptional OER activity of amorphous IrO x compared with crystalline IrO 2 may be explained by the lower activation energy for the adsorption of water molecules into the oxygen vacancies, as shown in Figure 1.However, the transient nature of oxygen vacancies, which may trigger catalyst dissolution instead of enhancing the OER activity, is a trade-off correlation that must be addressed in catalyst design. [20,31]o investigate the effect of annealing on the phase transition of Ir oxide, electrodeposited Ir oxide on the TS samples not subjected to any thermal treatment (EDIT-NT) and those subjected to additional heat treatment at 400 °C (EDIT-400) and 600 °C (EDIT-600) were prepared by focused ion beam (FIB) milling, and their cross-sections were characterized by transmission electron microscopy (TEM).Figure 2a,b shows the TEM images of the region near the interface of the Ir oxide layer (IOL) and TS for EDIT-NT.The microporous IOL homogeneously accumulated on the TS during electrodeposition, without any phase transition along the depth.][34] Upon annealing, the amorphous Ir oxide underwent crystallization.Furthermore, crystal growth occurred to form mesoporous nanocrystallites (EDIT-400), and a fully crystalline structure was achieved at the higher annealing temperature (EDIT-600).This transformation was confirmed by the thinning of the IOL during annealing, which led to the contraction of the atomic structure and the removal of hydrous species (Figure S1a-c, Supporting Information). [35]Notably, the Ir oxide underwent phase transition in the thickness direction during crystallization.Figure 2d,e shows that the Ir oxide in EDIT-400 crystalized, with higher crystallization near the IOL/TS interface than that in the bulk IOL.This result indicated that the TS altered the electronic structure of the Ir oxide, contradicting the assumption that crystallization preferentially proceeds from the outer surface to the bulk IOL.The HRTEM image in Figure 2f provides more details regarding the crystalline structure of Ir adjacent to the TS.The lattice spacing of these nanocrystallites near the interface was found to be 0.22 nm, in accordance with the (111) reflection of the metallic face-centered-cubic Ir facet, as indicated by the fast Fourier transform (FFT) and selected-area electron diffraction (SAED) results (inset of Figure 2f ).However, these lattices likely did not correspond to the metallic Ir because of the changes in the phase transition of Ir by the TS.Specifically, the interplanar spacings in the TEM images of EDIT-400 near the IOL/TS interface (Figure 1f ) were smaller than those of the EDIT-400 that exhibited rutile phase crystallinity due to the thermal oxidation at the IOL surface (Figure S1e, Supporting Information).Nevertheless, one could postulate that the bulk IOL between the surface and TS remained mostly unchanged, potentially because of the thermodynamic reaction conditions as the O 2 partial pressure at 400 °C in the thickness direction. [36]These findings suggest that the phase transition of Ir oxide occurred because of the interactions with adjacent Ti atoms, which led to the contraction of the Ir oxide lattice or with the O 2 molecules introduced during thermal treatment.Thermal annealing at 600 °C promoted the growth of the Ir oxide crystallite (Figure S1c,f, Supporting Information), and sintering reduced the surface area.The amorphous Ir oxide crystallized into the rutile phase.The SAED pattern in Figure 2i closely matched the lattice positions for the hkl indices as the polycrystalline rutile phase. [35]It is noted that a new layer appeared between rutile-type IOL and TS when subjected to thermal annealing at 600 °C, potentially because of the interplay between the IOL and TS.EDS mapping was performed to explore the mechanism by which TS transformed IOL to generate an intermediate layer during annealing.Figure 2j-o shows the elemental mapping images and atomic concentration profiles along the depth for EDIT-NT, EDIT-400, and EDIT-600.Figure 2j shows that the IOL and TS in EDIT-NT were clearly separated, with no remarkable migration of elements.The valence number of Ir was less than 4, which confirmed that the electrodeposited IOL (Figure 2c) was in the amorphous phase.In contrast, the atomic composition profile of EDIT-400 (Figure 2k) indicated an obvious shift in the element composition near the IOL/TS interface, accompanied by a significant increase in the oxygen concentration in the TS.Ti atoms were observed in the IOL, which slightly shifted the intersection between Ir and Ti upward, and the three elements exhibited a broad transition range of atomic concentrations.The Ti atoms were doped into the lattice of the Ir oxide, and thus, the Ti atom concentration could serve as the descriptor of the compositional changes.Interestingly, at higher annealing temperatures, an interlayer (assumed to be a Ti oxide layer) appeared between the IOL and TS.The Ir oxidation state in EDIT-600 was lower than that of pristine Ir oxide in EDIT-NT, even though its lattice parameters were consistent with the rutile phase observed in Figure 2i.The rutile phase of IrO 2 is typically associated with being an octahedral structure, where the coordination number of Ir can reach 6. [9,12,20] Hence, the lower oxidation state of  EDIT-600 suggests the possibility of structural distortion in the Ir oxide, induced by the surrounding elements modifying its electronic structure during annealing.
To further understand the characteristics of atomic migration observed during annealing in the TEM analysis, we conducted an XPS analysis to estimate the oxidation state of the constituent elements.Figure 3a-c shows the XPS spectra of Ir 4f and O 1s for the three EDITs.Notably, the Ti 2p BEs for the EDITs were not measured in the surface XPS analysis, as their Ir 4f spectra are considered to indicate the presence of Ir oxide when the Ir 4f 7/2 peaks are located at %62 eV. [37]However, in this study, the Ir 4f and O 1s peaks shifted to lower BEs as the annealing temperature increased, suggestive of clear differences between the chemical states and compositions of individual species in the EDITs.
The relative contents of Ir III and O IÀ in EDIT-NT were higher than those in the annealed samples and gradually decreased as the annealing temperature increased.These results confirmed that the Ir oxide phase transformed from amorphous to crystalline, as observed in the TEM analyses.Because the XPS analyses were performed using X-rays with a low kinetic energy of 180 eV, the results represented chemical states of the outer surfaces of EDITs. [38]To obtain the chemical states of the bulk, XPS analyses were conducted for the EDITs subjected to sputter etching with Ar þ ions.Figure S2a-f, Supporting Information, shows the XPS spectra of Ir 4f and O 1s before and after Ar þ sputtering for 100 s.Furthermore, the Ti 2p XPS spectra for the bulk surface after surface etching were obtained (Figure S2g, Supporting Information).According to the Ir 4f spectra, the Ir oxidation state in the EDIT-NT bulk remained unchanged, whereas the peak area at low energy increased with thermal treatment, as indicated by the negative shift in the Ir 4f 7/2 peaks.Notably, this shift was likely complicated by the surface area reduction during sputter etching. [39,40]These results demonstrated that thermal treatment changed the bulk oxidation state as well, depending on the annealing temperature.The Ti 2p spectra emerged in the bulk for both the EDIT-400 and EDIT-600 samples, but the dominant species in EDIT-400 and EDIT-600 were Ti III and Ti IV , respectively.These results indicated that Ir and Ti interacted with each other at their interface during annealing, and this interaction influenced their oxidation state in the bulk, unlike the outer surface.
HRXRD analyses were conducted to investigate the crystalline structure that led to the variation in the oxidation states of each element at different annealing temperatures.Figure 3d shows the HRXRD patterns of the bare TS, EDIT-NT, EDIT-400, and EDIT-600 samples.Each sample exhibited a prominent peak of metallic Ti, attributable to the substrate, and the peaks of other crystalline structures sparsely emerged with annealing.The EDIT-NT sample had nearly the same Ti peak as that of bare TS and a low peak of Ir metal at 40.5°, which was associated with its amorphous oxide structure (Figure 2c). [41,42]In contrast, the peak of crystalline IrO 2 was observed in the reflection of (110) for both EDIT-400 and EDIT-600.The TiO 2 diffraction at 25.5°was indexed to the (101) reflection for EDIT-600, which indicated that the TS and Ti atoms that diffused into the IOL were oxidized at 600 °C.Although EDIT-400 and EDIT-600 exhibited similar patterns, Ir underwent a structural change at 400 °C to reach the complete rutile phase.Each sample exhibited a different peak location near 40°.Specifically, for the EDIT-NT and EDIT-600 samples, reflections corresponding to metallic Ir (111) and (200) of crystalline IrO 2 appeared, respectively, whereas EDIT-400 exhibited the (110) reflection of Ir 0.8 Ti 1.2 alloy.These findings confirmed that the elusive nanocrystallites near the IOL/TS interface annealed at 400 °C were Ir and Ti alloys, consistent with the EDS results.Additionally, the DFT calculation results (Figure 3e) highlighted that the doping of the Ti atom into the lattice of IrO 2 (110) was the most favorable process during annealing.In other words, the small Ti atoms migrated to the IOL by the thermal driving force, as they could readily be incorporated into the IrO 2 crystalline structure and formed a novel structure with Ir atoms.These results provide insights into how Ti atoms lead to the transformation of the Ir phase near the IOL/TS interface and can thus define the crystalline structure.
To investigate the local atomic coordination environment of the Ir atoms, XAS studies were performed to observe changes in the metal oxidation states and chemical bond properties.Figure 3f,g shows the X-ray absorption near-edge spectroscopy (XANES) spectra of the Ir L III -edge and Ti K-edge of the three EDIT samples.For the Ir L III -edge spectrum, the Ir foil and IrO 2 were used as references, corresponding to the 2p 3/2 to 5d electronic transition of Ir.The number of empty d holes could be determined from the position of the 2p 3/2 to 5d white line.Hence, deconvolution fitting was performed to quantify the changes in the oxidation states and d vacancies (Figure S3, Supporting Information).Table 1 presents the oxidation state and d vacancy values for each sample.The average valence numbers of Ir atoms exhibited the following order, EDIT-NT > EDIT-400 > EDIT-600, consistent with the trend observed for the d vacancies.This is contrary to the result of Ticianelli and co-workers who reported that the bulk structure of unsupported IrO 2 calcined at 500 °C was more oxidized than amorphous IrO x through in situ XAS analysis. [31]In combination with the EDS profile results, we could conclude that the oxidation states of Ir atoms exhibited the opposite trend as the annealing temperature increased, even though the reported oxidation number for rutile IrO 2 is close to 4. [31,43,44] Assuming Ti to be a descriptor of the crystalline structure in the previous analysis, the Ti K-edge spectrum (top plot in Figure 3f ) was analyzed to examine this conflicting correlation.Pre-edge peaks at 4965 eV were observed, followed by linearly continuous white lines.The appearance of the pre-edge peak, resulting from the transition of the dipole forbidden core 1s to unoccupied 3d orbitals, suggested the presence of a centrosymmetric environment around the absorbers. [23,45]Changes in the pre-edge peak reflected the extent of distortion of the octahedral symmetry of the Ti species, and the consecutive steep white lines indicated the valence states of Ti atoms. [46,47]The main absorption peaks, which appeared after the white line at %4990 eV, were attributed to the transition of 1s to the outer 4p orbitals.The oxidation states of the Ti atoms could be estimated from these peaks as the outer p orbitals were more sensitive to changes in the electronic structure. [45]Furthermore, the intensity of the main absorption peak could be used to estimate the density of the Ir-O-Ti linkage in terms of the reverse electronic inductive effect of the surrounding Ir III species that play an important role in the coordination via heterogeneous atoms. [24,25]In the annealing process, the intensity of the pre-edge peak of EDIT-400 and EDIT-600 was lower and higher than that of EDIT-NT, respectively.This finding indicated that the local environment of the Ti IV species exhibited a more distorted nonsymmetrical octahedral structure in EDIT-400, whereas a more ordered structure appeared in EDIT-600, indicating an increase in the crystallite size of Ti clusters in the order of EDIT-400 < EDIT-NT < EDIT-600.Furthermore, the oxidation state of Ti gradually increased according to the variation in the white lines and main absorption peaks, which highlighted its tendency at various annealing temperatures.Overall, the TEM and XANES results indicated that the Ti atoms in the TS diffused into the IOL and interacted with the Ir and O atoms at 400 °C, thereby distorting the ordered IrO 2 structure.The Ti cluster growth progressed to the layer between the IOL and TS at 600 °C, leading to a symmetric octahedral structure.This observation can be considered reasonable because the coordination between Ir and Ti is the most abundant at 400 °C, due to the higher density of the edge-shared Ir-O-Ti linkages, attributable to the electronic inductive effect.Next, we conducted a Fourier-transform extended X-ray absorption fine structure (EXAFS) analysis to clarify the behavior of Ti in the IOL.Unfortunately, the FT curves for Ir could not be obtained due to the low amplitude associated with the thin-film structure.Instead, Figure 3h shows the FT curves of the Ti K-edge spectra of the EDITs.As the pristine EDITs (i.e., EDIT-NTs) were subjected to thermal treatment, the amplitude of predominant peaks related to Ti-Ti or Ti-M (Ir) increased along with a slight shift in the radial distance, depending on the annealing temperature.Thus, the observation of a feature at 2.5 Å indicated increased coordination via Ti atoms after annealing as discussed earlier.The increase in the Ti-Ti radial distance of EDIT-600 suggested that the oxidation of Ti atoms is in quantitative agreement with the reported anatase phase TiO 2 and consistent with the XRD results. [24]Another feature associated with Ti─O coordination emerged at a radial distance of %1.5 Å, indicating the formation of Ti oxide from small Ti clusters.Thus, the Ti─O bond found in the EXAFS spectra likely originated from the interlayer between the IOL and TS in EDIT-600 (Figure 2l).
We conducted comprehensive electrochemical analyses for EDITs using single cells including the PEM as the electrolyte, Pt/C catalyst and gas diffusion layer (GDL) cathodes, and EDIT anodes to elucidate how the IOL, which is influenced by the Ti atoms in the TS through electrodeposition and thermal annealing, regulates the OER kinetics.Figure 4a displays the polarization curves of the PEMWEs with EDITs operating at 80 °C and 1 bar.The overall performance of PEMWEs exhibits the following order: EDIT-400 > EDIT-NT > EDIT-600.In general, the polarization of PEMWEs is determined from various overpotentials, taking into account the kinetic, ohmic, and concentration effects.It is assumed with negligible cathodic contribution that the polarization behaviors of PEMWEs with different EDITs were primarily influenced by kinetic and Ohmic overpotentials at the anode because of no significant mass transfer limitation in high-current-density region. [48]The kinetic losses in PEMWEs primarily depend on the catalytically active area and intrinsic activity of the catalyst, especially at the anode, which requires a considerably higher overpotential than the cathode.The double-layer capacitance C dl , which characterizes the interface between the anode and electrolyte, can indicate the electrochemically active area available for both Faradaic and non-Faradaic processes, even though this interface is not entirely utilized for the OER.Thus, cyclic voltammetry (CV) experiments were conducted to assess the C dl of PEMWEs with EDITs at the anode.Figure 4b shows that C dl decreased with thermal oxidation and increasing annealing temperature.In other words, the catalyst area available for electrochemical processes decreased with increasing crystallinity due to the thermal treatment, and this observation was confirmed by the XRD and TEM results.Furthermore, an Ir III /Ir IV redox peak between 0.6 and 0.95 V was clearly observed for EDIT-NT, which broadened for EDIT-400 and almost disappeared for EDIT-600, indicating changes in the Ir oxidation state. [43,49,50]In addition, a more negative peak potential was found for EDIT-400, suggestive of its faster kinetics during the OER.The Tafel slope obtained in the kinetically controlled region, b k , may provide clues regarding the rate-determining step for the OER processes, which involve water adsorption, consecutive proton-coupled electron transfer, and oxygen liberation.Figure 4c shows the polarization data of PEMWEs with EDITs at low current densities, in addition to the b k estimated through linear approximation.[53][54] At cell potentials (E cell ) <1.5 V versus the reversible hydrogen electrode (RHE), nonelectrochemical adsorption of water, which is slower than electrochemically driven processes at higher potentials, occurs and controls b k .The value of b k can potentially be used to identify catalytically active sites in which the adsorption of water molecules is thermodynamically favorable. [51]Values of b k > 100 mV dec À1 at E cell < 1.5 V versus RHE may suggest that the adsorption regime corresponds to a Langmuir-type isotherm, which is responsible for the one-electron transfer rate-determining step. [53]The lower b k of EDIT-400 than that of EDIT-NT indicates that EDIT-400 had a highly effective surface area available for OER.In contrast, the higher b k of EDIT-600 indicates that thermal annealing reduced the catalytically active sites due to the higher degree of crystallization.
Electrochemical impedance spectroscopy (EIS) is a useful diagnostic tool for separately examining individual polarization sources in an electrode with different relaxation times.Figure 4d shows the iR-corrected AC impedance spectra obtained at 0.1 A cm À2 using the EIS technique.The high-frequency resistance R HF of the EDITs estimated from the first x-intercept (close to the origin) in the Nyquist plot is also shown in Figure 4d (inset).The R HF values differ across the EDITs.Typically, R HF depends on the proton conductivity through the membrane and CL.However, the proton conductivity across the membrane of the EDITs is assumed to be nearly identical in this study because the membrane was fully hydrated by liquid water as the feed.Thus, the R HF was likely attributable to the electrical resistance through the IOL and TS, especially at their interface.Specifically, alloying Ir with Ti at a moderate temperature (EDIT-400) promoted electron transfer at the interface, whereas the interlayer mostly composed of the TiO x layer (EDIT-600) considerably increased the electrical resistance.The mediumfrequency arc, R MF (i.e., the magnitude of the first circle at medium frequencies), mainly reflects the charge transfer rate at the catalytically active sites in which the water molecules are separated into oxygen molecules, protons, and electrons. [55]he trend observed for the R MF of the EDITs was qualitatively consistent with that of b k and the potential of the Ir III /Ir IV redox peak.This observation highlighted that the electron transfer rate combined with the adsorption of water molecules onto the surface of the catalysts depends on the Ir oxidation state and catalytically active area for the OER.Furthermore, the low-frequency arc, R LF (i.e., the magnitude of the second circle at low frequencies), followed by R MF and ending with the second x-intercepts, was also observed for the EDITs.The small R LF of EDIT-400 indicated faster removal of the oxygen bubbles grown on the surface of the catalysts, due to which, more active sites per unit time were available for the OER. [55]everal researchers have studied the OER activity depending on the annealing temperatures of the Ir oxide deposited on the TS. [50,56,57]However, these studies were not aimed at exploring the preeminent OER intrinsic activity at the atomic scale.In contrast, we considered the oxygen content in the IOL, determined in the EDS analysis, as a descriptor of the number of reaction sites to identify the reasons for the higher OER activity of EDIT-400.Figure 5 shows the energies required to transfer O atoms at the interface between the IOL and TS, determined through DFT interface calculations.The results indicate that the transfer of O from Ir oxide to Ti (oxide) is a more favorable reaction.The Ti III sites may be stronger Lewis acid sites than Ir III sites.Therefore, the O atoms required for Ti to be oxidized by thermal annealing were extracted from the Ir oxide structure.The transfer of O atoms was consistent with the results reported by Puigdollers et al., [14] who suggested that Ti interactions could indicate the reducibility of Ir oxide, as the oxygen atoms could be removed from the interface sites at low energies.Our XANES analysis confirmed this observation, as the oxidation state of Ir decreased with increasing annealing temperatures.Similar to the findings of Sun et al., [57] our DFT calculations showed that the O─O bond length decreased when the IrO 2 lattice was strained at the interface of TiO 2 lattice, which favored O 2 formation (Figure S8, Supporting Information).EDIT-400 exhibited a higher OER activity than EDIT-NT because the deprived O sites in the Ir oxide became electrophilic activated O sites.The lower activation energy for the water adsorption at the activated O sites likely contributed to the higher activity of EDIT-400.Furthermore, the crystallization at a mild annealing temperature likely provided a spacious mesoporous network, while the surface-confined densely packed edge-sharing structure formed at 600 °C decreased the number of reaction sites. [58]This finding demonstrated that the anomalous relationship between the OER activity and annealing temperature was attributable to the catalyst-substrate interaction and closely related to the volcano-type variation in the PEMWE performance.
The stability and activity of the EDITs were assessed in PEMWEs operating at 1.0 A cm À2 for 100 h.As shown in Figure 4e, the degradation rate of PEMWEs with EDITs exhibited a contrasting trend with the polarization behaviors shown in Figure 4a.Specifically, EDIT-400 (0.34 mV h À1 ) was 3.9 and 25.7 times more stable than EDIT-NT (1.33 mV h À1 ) and EDIT-600 (8.75 mV h À1 ), respectively.To gain a mechanistic understanding of the degradation of each MEA, electrochemical (i.e., CV and EIS) and physical (i.e., TEM and XPS) analyses were conducted alongside the stability test.The C dl of EDIT-NT (Figure S4a, Supporting Information) decreased after the MEA stability test, even though its initial C dl was the highest among the EDITs (Figure S4a-c, Supporting Information).In contrast, for EDIT-400, the initial and final CV curves exhibited similar C dl values, which suggested that the initial electrochemical surface area was retained during the high current hold for 100 h.Interestingly, as depicted in Figure S4d,e, Supporting Information, the values of R HF for EDIT-NT and EDIT-400 slightly decreased after the 100 h current hold.In the cases of EDIT-NT and EDIT-400, the Ohmic contact resistance rather than the ionic resistance across the membrane and/or specific electrical resistance through the layered structures including the IOL, TS, and flow field was likely reduced due to the modified interfaces (i.e., the membrane/IOL and the IOL/TS) that facilitated the proton and/or electron flows (Figure S7, Supporting Information).In addition, the increased R MF for EDIT-NT was ascribed to its sluggish OER kinetics, combined with the decreased catalytic activity and active sites and limited mass transfer attributable to the alteration of the triple-phase boundaries (TPBs) when subjected to electrochemical stress.Figure S5a,b, Supporting Information, shows the cross-sectional TEM images of MEAs using EDIT-NT and EDIT-400 after the 100 h current hold.A thin amorphous Ir film was observed at the interface between the IOL and membrane for both EDIT-NT and EDIT-400.However, the morphology and thickness of this Ir film differed for the two samples.These results could be related to the rate of Ir dissolution under high anodic potential, because the intensity of the normalized Ir 4f 7/2 peak of EDIT-NT before and after the 100 h current hold was substantially different, and the difference during the stability test was considerably larger than that of EDIT-400 (Figure S6a,b, Supporting Information).The aged MEA using EDIT-600 (Figure S6c, Supporting Information) exhibited not only a thin Ir film similar to that of EDIT-400 but also numerous aggregated Ir particles, which helped maintain the rutile crystalline structure.Furthermore, unlike EDIT-NT and EDIT-400, R HF for EDIT-600 in Figure S4f, Supporting Information, appreciably increased.The results suggest that the degradation processes of EDITs were dependent on the initially formed Ir phase.Specifically, the EDIT-NT not subjected to thermal annealing exhibited severe Ir dissolution, which led to the formation of a thick Ir film on the membrane.The Ir particles in EDIT-600 were further sintered in highly oxidizing environments, which deteriorated the Ohmic contacts and porous structure of TPBs.The strong robustness of EDIT-400 under electrochemical stress was attributable to Ti doping into the Ir lattice, which prevented Ir particles from dissolution and sintering under highly oxidizing environments.
Our findings provide valuable insights into the factors determining the performance and durability of PEMWEs, which are not limited to the electrocatalyst characteristics.Typically, the dissolution of electrocatalysts has been perceived as the main factor contributing to the potential loss in PEMWEs.However, we demonstrated that the most crystallized Ir oxide (i.e., EDIT-600) exhibited the fastest degradation during PEMWE operation.This observation suggests that the delamination between the CL and substrate, rather than the dissolution of electrocatalysts, was primarily responsible for the decrease in the TPBs, as indicated by the analyses after the tests.The stable interface between the catalyst and substrate alleviated the potential loss, as reflected by the low R HF and long-term stability of the catalyst-coated substrate (CCS).Furthermore, severe dissolution of the electrocatalysts was observed in the amorphous IrO x (i.e., EDIT-NT), consistent with the low-stability numbers suggested by Geiger et al., [20] who clarified the correlation between the intrinsic OER activity and stability of Ir according to the electronic structure.Our results are consistent with their correlation, in which the adsorbate evolution mechanism (AEM) is followed by a compact crystalline structure and the lattice oxygen participation mechanism (LOM) is followed by a defective amorphous structure.Nevertheless, the low activity associated with the fewer activated oxygen sites via AEM and the dissolution of Ir by the reaction through LOM may appear to be contradictory.In this study, we introduce the rational unified system of reaction mechanisms in MEA using CCS.We successfully modified the electronic structure of the thin Ir oxide film and clarified its physical properties and electrochemical behavior in PEMWE operation.The ideal structure corresponded to EDIT-400, in which the outer surface exhibited a crystalline rutile structure through AEM, whereas the bulk maintained an electrophilic active structure within the LOM reaction regime.This system preserved the corrosion resistance of Ir at the interface with the membrane, while compensating for the lower intrinsic activity of the surface with the bulk reaction sites, as discussed by Geiger et al. [20] This framework helped mitigate bulk dissolution because, in most cases, the oxygen vacancies could be refilled by the adsorption of water or bulk oxygen migration. [59]The high performance and durability of EDIT-400 in PEMWE operation likely originated from the superior utilization and stability of the electrocatalysts.

Conclusion
We studied the relationships between the electronic structure and electrochemical properties of electrodeposited Ir oxide on a TS in PEMWEs and found crucial evidence for electronic interactions between the electrocatalysts and substrate.Ti atoms derived from the substrate were doped into the lattice of Ir oxide through thermal annealing at 400 °C, and further oxidation of the TS at 600 °C led to the formation of a thick Ti oxide layer between the CL and substrate.The changes in the bulk Ir oxidation state with the annealing temperature were experimentally and theoretically interpreted in terms of the different reducibilities between the Ir oxide catalyst and TS, which induced the movement of oxygen sources from the oxide to the metal.The oxygen content in the Ir oxide, which depended on the catalyst-substrate interactions, significantly influenced the electrochemical behavior of the EDITs, and thus, it could be used as a descriptor of the OER activity.The electrophilic oxygen defective and mesoporous Ir oxide structure of EDIT-400 led to its higher intrinsic OER activity.In addition, the Ti-doped nonstoichiometric and crystalline Ir oxide was corrosion resistant and constructed a stable catalyst/substrate interface, resulting in high and stable PEMWE performance during extended operation.These results highlight that the catalyst-substrate interaction can control the synergistic bulk phase transition and interface tuning, offering a rational strategy for a variety of CCS designs.
Future research can be aimed at manipulating the electronic structure at the surface and bulk levels in the thin film CL.For example, other stable and conductive substrates that can mediate the electronic structure of the CL and lead to the formation of a stable interface must be identified.Interleaving materials that can balance the catalyst activity and crystallinity at the interface of the CL/substrate can be introduced to design a highly durable and active CCS.Accordingly, it is necessary to comprehensively clarify the operation mechanism of PEMWEs to facilitate green hydrogen mass production.

Figure 2 .
Figure 2. Morphological characterization of cross-sectional EDITs using HRTEM, SAED, FFT, and EDS images.a,d,g) TEM images of EDIT-NT, EDIT-400, and EDIT-600 at the interface between the IOL and TS, respectively.b,e,h) TEM images of EDIT-NT, EDIT-400, and EDIT-600 at the IOL, respectively.c,f,i) HRTEM, SAED, and FFT images of EDIT-NT, EDIT-400, and EDIT-600 at the IOL, respectively.HAADF STEM, atomic mapping images, and atomic composition profiles of j,m) EDIT-NT, k,n) EDIT-400, and l,o) EDIT-600.

Figure 3 .
Figure 3. Electronic and atomic coordination characterization of EDITs.a-c) Ir 4f and O 1s XPS results for EDIT-NT, EDIT-400, and EDIT-600, respectively.d) HR-XRD of EDITs.e) DFT calculation results of the energy required for Ti doping in IrO 2 (110).f,g) Ir L III -edge and Ti K-edge XANES results of the EDITs.h) Fourier-transform EXAFS spectra at the Ti K-edge of the EDITs.

Figure 4 .
Figure 4. Electrochemical characterization.a) Polarization curves of EDITs.b) Galvanostatic EIS Nyquist plots of EDITs.c) Tafel slopes of EDITs.d) CV curves of EDITs.e) Chronopotentiometries of EDITs.

Figure 5 .
Figure 5. DFT-based calculation of the difference in the oxygen vacancy formation energy to extract a single-oxygen atom at various locations in IrO 2 || TiO 2 .

Table 1 .
Electronic structures, oxidation states, and d-band hole values of EDITs.