A Route to Complex Materials Consisting of Multiple Crystalline Phases Ir‐Ru‐IrxRu1‐xO2 as Multifunctional Electrocatalysts

The synthesis of catalytic materials containing active sites of various oxidation states (metal, oxide, etc.) for the dual use in reduction and oxidation reactions remains challenging because most of the reported methods for the metallic state are often incompatible with those for the oxide state. A new methodology is reported for the synthesis of a library of bimetallic metal‐oxide materials containing three crystalline phases Ir‐Ru‐IrxRu1‐xO2 by combining the calcination under air and the polymerization of aniline in the presence of IrCl3 and RuCl3 precursors. Morphology, structure, and surface oxidation state studies (XRD, SEM/EDX, S/TEM, XPS) confirm the hypothesis that IrCl3 evolves to Ir while RuCl3 evolves to RuO2 during the calcination under air. An Ir:Ru atomic ratio of 50:50 can be converted into a heterogeneous nanostructure composed of Ir, Ru, and IrxRu1‐xO2 to date, with remarkable catalytic activity for both hydrogen evolution reaction (HER) with a small overpotential of 40 mV and oxygen evolution reaction (OER) with a small overpotential of 290 mV at the metric current density of 10 mA cm−2 in 0.5 m H2SO4. The results can serve as a platform for the development of efficient multifunctional materials for practical use in both catalytic oxidation and reduction reactions.


Introduction
[3][4] For example, a two-phase bifunctional electrocatalyst is required to perform O 2 reduction/evolution (ORR/OER) for the reversible/regenerative fuel cell/electrolyzers or rechargeable metalair batteries. [1,2,4][19][20] However, the synthesis of such materials remains challenging because the reaction atmosphere for the metallic state [reducing conditions to transform M (II/III) to zero valence M (0) (M = Ru, Ir, Pt, etc.)] [3,4,7,14] is often incompatible with that for the oxidized state [oxidizing conditions to transform M (II/III) to MO x (M = Ru, Ir, etc.)]. [4,6,7,21]ecause iridium-ruthenium are the state-of-the-art catalysts for the acidic water electrolyzers, [5] it is fundamental interesting to know whether M and MO 2 (M = Ir, Ru) can be synthesized and stabilized within the same material as a dual electrocatalyst for both HER and OER.For the synthesis of Ir-Ru nanostructures, reducing conditions are used in the liquid phase (NaBH 4 , hydrothermal, etc.) or in the solid-state by calcination in the presence of H 2 (absence of O 2 ). [3,13,22,23]For Ir x Ru 1-x O 2 , pyrolysis/calcination in the presence of air or O 2 is mainly used, and the presence of iridium metal (Ir) has been considered as a minor or a by-product during the decomposition of Ir(III)-Ru(III) precursors. [24,25]While calcination favors the diffusion of atoms to form an alloyed oxide Ir x Ru 1-x O 2 , [24,26,27] the formation of Ir instead of IrO 2 is intriguing and not well understood.While this formation of metallic species during calcification under O 2 is counterintuitive, we have recently observed that the presence of IrCl 3 during the polymerization of aniline leads to the formation of Ir particles after calcination at 250-400 °C under air and RuCl 3 produces RuO 2 particles in the same experimental conditions. [28]he prepared polyaniline (PANI)-based materials exhibit ultrafast electrocatalytic kinetics for HER (PANI-Ir: 36 mV overpotential to reach 10 mA cm −2 at 21 mV dec −1 ), OER (PANI-Ru: 240 mV overpotential to reach 10 mA cm −2 at 47 mV dec −1 ) and water splitting (starting exactly at the thermo-neutral cell voltage of 1.45 V). [28] An open question is whether a multiple-component metaloxide catalyst Ir-Ru-Ir x Ru 1-x O 2 , capable of HER and OER with intermediate activity, can be synthesized by the same calcination methodology, that is, controlling: i) the atom diffusion to limit the formation of a single phase, ii) the opposite kinetics of the reduction of Ir (III) to zero valence Ir (0) and the oxidation of Ru (III) to four valence RuO 2 .Herein, we effectively synthesized for the first time, to our knowledge, a library of complex materials consisting of three crystalline phases Ir-Ru-Ir x Ru 1-x O 2 by a two steps polymerization-calcination methodology.We have investigated the elemental distribution, structure and electrocatalytic properties of the obtained heterogeneous materials through a multivariate study (SEM, EDX, XRD, S/TEM, XPS, voltammetry).

Results and Discussion
While the redox properties of the iridium and ruthenium species, the availability of O 2 in the furnace, and the reducing/oxidizing atmosphere created by the decomposition of polyaniline are the key factors, [24,26,28] we hypothesized that the coexistence of metallic and oxidized states could be achieved by at least one of the scenarios shown in Figure 1a: i) initial presence of IrCl 3 and RuCl 3 during polymerization followed by calcination (referred to as "chemical mixing" ("CM")), and ii) separate polymerization prior to mechanical mixing of polymerized monometallic materials for calcination ("physical mixing" ("PM")).For this purpose, Ir:Ru atomic ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were studied.For electrochemical measurements, the potentials were scaled against a reversible hydrogen electrode (RHE, Figure S1, Supporting Information).
The oxidative polymerization of aniline to polyaniline (PANI) was performed in hydrochloric acid as the doping acid and in the presence of ammonium persulfate as the oxidizing agent, producing mainly the emeraldine form of PANI. [29,30]The calcination program under air consisted of heating at 2 °C min −1 up to 250, 350, and 400 °C (1 h dwell each, Figure S2 and Scheme S1, Supporting Information).While such calcination under air is expected to lead to the formation of oxides, [24,26] preliminary thermogravimetric and differential thermogravimetric analysis (TGA-DTA, Figure S2, Supporting Information) showed a lower metal content for PANI-Ir compared to PANI-Ru, which may indicate a different nature of the products.
X-ray diffraction (XRD) was used to determine the structure and composition of the materials.Figure 1b,c and Figure S3 (Supporting Information) (and Table 1, Tables S1 and S2 Bimetallic materials have the diffraction peaks of Ir x Ru 1-x O 2 , Ru, and Ir, validating our hypothesis of successful synthesis of separate phases rather than a single Ru x Ir 1-x O 2 by at least one of the two strategies of Figure 1a. Figure 1c clearly highlights that the majority of Ir and RuO 2 are formed after the calcination, starting with monometallic precursors.Indeed, the diffraction peaks are mainly below 40°for the non-heat treated materials, assigned to PANI and metals salts. [29,31,32]Furthermore, although IrO 2 and RuO 2 have the same structure (rutile) and the cation radius of Ir 4+ and Ru 4+ is similar (≈0.62 Å), [24][25][26]33] we ruled out IrO 2 and RuO 2 phase segregation because IrO 2 does not form in the absence of ruthenium. The ormation of metallic species in addition to oxides was reported, [24,25] however, the 61-65, 33-36, and 22-24 wt% Ir+Ru achieved with Ir:Ru atomic ratios of 75:25, 50:50, and 25:75, respectively, are the highest values to date.Based on these results, we can hypothesize that bimetallic materials will be active for both HER (benefiting from the metallic character) and OER (benefiting from the oxide character).
To further characterize the morphology and precisely localize the elemental distribution at both the micro and atomic scales, we next performed a serie of electron microscopy analysis.The atomic scale elemental maps of the different elements by scanning transmission electron microscopy coupled energydispersive X-ray spectroscopy (STEM-EDX) from Figure 2a-d are in agreement with bulk analysis by scanning electron microscopy   S3 and Figure S6, Supporting Information), which is in agreement with the STEM-EDX line profiles (Figure S32a,b, Supporting Information).The presence of oxygen species in PAN-Ir after calcination results from the natural oxidation of a metal when exposed to air, since quantitative XRD analysis revealed no significant quantity of IrO 2 and, as we will later see, electrochemistry confirmed the metallic character.for bimetallic, whereas the commonly reported values are in the range of 20-40 m 2 g −1 . [24,26,33]e next determined the oxidation state of the different elements by X-ray photoelectron spectroscopy (XPS).We used the NIST standard reference database 20 version 4.1 (Table S4, Supporting Information) for the assignment and the Ru 3d core-level to fit ruthenium. [25,34]Survey XPS spectra (Figure S38, Supporting Information) confirm the elements identified by EDX.Highresolution XPS (Figure 3a-f; Figure S39 and Table S5, Supporting Information) confirms that PANI-Ru leads to RuO 2 , while PANI-Ir produces Ir (the oxidized species are simply due to natural oxidation upon exposure to ambient air, as no oxide peak was detected in the XRD patterns).The increased intensity of the metallic component of Ir 4f in Figure 3c compared to Figure 3e (in agreement with Table 1 for Ru:Ir = 50:50) can be explained by the limited diffusion of atoms that are randomly distributed when the material results from mechanical mixing, referred to as "PM".
The previous results show the ability to synthesize complex materials consisting of three crystalline phases Ir-Ru-Ir x Ru 1-x O 2 .The formation of such materials could be explained by several factors, such as the discrepancy in the redox properties of the cationic species of iridium and ruthenium, the availability of oxygen in the calcination furnace, and the presence of polyaniline in the starting material, the combustion of which could provide a mixture of oxidative and reductive environments.Further study is needed for the origin of the selective heat treatment in air, for example by coupling TGA-DSC to mass spectrometry (MS) and testing different iridium and ruthenium precursors, possibly at different valences.XRD results showed a shift in the position of the main Ir(111) peak to higher 2 values in the bimetallic materials compared with monometallic PANI-Ir as the amount of ruthenium increased.For the main oxide peak, that is, Ir x Ru 1-x O 2 (110), the shift was not significant, but the intense main peak of the hexagonal Ru phase shifts, which could be attributed to the difference in crystallographic parameters of  the structures.As a result, the structural parameters of the metal and oxide species are markedly different as a function of phase fraction, suggesting a possible positive impact on catalytic activity due to the presence of electronic and synergistic interactions as well as the heterogeneous interface between the different phases.
Finally, the electrochemical properties were accessed (Figures 4 and 5; Figure S40, Supporting Information).We first used cyclic voltammetry (CV) in a 0.5 m H 2 SO 4 electrolyte to check whether the synthesized materials behave like metals or oxides.,34] On the other hand, Figure S40b (Supporting Information) demonstrates that PANI-Ir has the behavior of a Pt-like metal with a reversible proton adsorption/desorption processes in the 0.05-0.40V RHE region. [12,22,35]aken together, these results means that PANI-Ir material should be active for HER, while PANI-Ru would be suitable for OER.Furthermore, consistent with the previous physico-chemical characterization, the bimetallic materials exhibit mixed voltammetry profiles of both oxides and metals.This behavior was further confirmed by the double layer capacitance (C dl ) measurements (Figure S41, Supporting Information) for accessing the electrochemically active surface area (ECSA = C dl /C s , C s = reference capacitance of the monolayer, which value depends on the nature of the electrode material, [36][37][38] ranging from 11 to 130 μF cm −2 ).Specifically, C dl = 4.4, 1.1, and 1.6-4.1 mF for PANI-Ru, PANI-Ir, and PANI-Ru-Ir, respectively.
To verify that the bimetallic materials are active for both HER and OER, while being intermediate between RuO 2 (OER) and Ir (HER), we next performed linear sweep voltammetry (LSV, Figure 4), chronopotentiometry (Figure S42, Supporting Information), and electrochemical impedance spectroscopy (EIS, Figure 5; Figure S43 and Table S6, Supporting Information).Overall, the performance is analogous to that of reference materials (Pt/C and Ir for HER, and RuO 2 for OER) and the literature (Tables S7 and S8, Supporting Information).Specifically, LSV of HER (Figure 4a) shows that the overpotential needed to reach the metric current density of |j| = 10 mA cm -2 is 20 and 134 mV for PANI-Ir and PANI-Ru, which confirms the Pt-like characteristic of the synthesized PANI-Ir while PANI-Ru that is mainly composed of RuO 2 is not much active for HER.For OER, PANI-Ru is more efficient than PANI-Ir, with overpotentials of 0.27, and 0.35 V at 10 mA cm -2 , respectively.These data are substantiated by the stability tests performed by chropotentiometry at the current density of |j| = 10 mA cm -2 (Figure S42) and EIS at different electrode potentials (Figure 5a-c).To evaluate the Tafel slope, we have implemented two independent methods, the LSV (Figure 4c) and the EIS at different applied potentials (Figure 5c).The fitted EIS data by R Ω +Q CPE //R ct are gathered in Table S6 (Supporting Information); R Ω , Q CPE and R ct represent the uncompensated ohmic resistance, the constant phase element, and the charge transfer resistance, respectively. [39,40]The method of the charge transfer resistance (R ct ) determined by EIS enables to probe only the electron transfer ability at different electrode potentials, [34,41,42] which allows to eliminate the possible contribution of non-faradaic processes to LSV curves.We note that the experimental values of the Tafel slope can deviate from the theoretical metrics for several reasons, the thickness/compactness of the catalytic layer, the contribution of mass transport, the faradic processes associated with electrocatalyst itself, etc. [34,[43][44][45][46][47][48] Mechanistically, the main three steps of HER and the corresponding Tafel slopes at 25 °C are described by Equations (1-3), the overall reaction is Equation (4).For HER, when Ir is the main constituent, the reaction seems to be under H 2 evolution limitation by both Heyrovsky step (H (ads) + H + (aq) + e − → H 2 ) and Tafel step (H (ads) + H (ads) → H 2 ).When Ru predominates, the Tafel slope is >40 mV dec -1 , suggesting that the HER might be under both hydrogen adsorption limitation (Volmer step, H The elementary steps of OER are described by Equations (5-8), the overall reaction is Equation ( 9) (Tafel slopes correspond to 25 °C).
For OER, both methods lead to Tafel slopes of 45-60 mV dec -1 , which indicate that the deprotonation of the adsorbed hydroxyl species (from the first step of water splitting) is the limiting step. [34,49,50]Taking advantage of the structural characteristics (morphology, structure and coexistence of 20-26 wt% Ir, 7-15 wt% Ru and 64-67 wt% Ir x Ru 1-x O 2 ), PANI-Ir 50 Ru 50 has a reduced resistance, leading to a low overpotential for both HER (40 mV) and OER (290 mV) at the metric current density of 10 mA cm −2 .The main reason for such behavior could be the heterogeneous composition exhibiting a metallike behavior for ca.35 ± 5 wt% of Ir+Ru that is appropriate for HER and an oxide-like behavior for ca.65 5 wt% of Ir x Ru 1-x O 2 that is suitable for OER, as sketched in Figure 5d.The previously found Ir x Ru 1-x O 2 (110) by the electron microscopy could explain the high activity for OER because seminal density functional theory (DFT) calculations by Rossmeisl et al. [51] suggested that the optimal binding energy of the oxygenated and hydroxyl intermediates of OER is reached at MO 2 (M = Ru, Ir) surface exposing (110) surface.The heterojunction between the different phases could contribute to the catalytic activity.We point out that the catalytic ink needs to be strengthened before considering the real application, as the ink stability was not satisfactory (Figure S42, Supporting Information).Nevertheless, our results can inspire the development of multifunctional catalysts for use in both oxidation and reduction reactions.

Conclusion
In summary, we have demonstrated a new methodology that combines polymerization and calcination to synthesize a library of metal-oxide materials containing three phases, Ir, Ru, and Ir x Ru 1-x O 2 , referred to as Ir-Ru-Ir x Ru 1-x O 2 .The nature of MCl 3 (M = Ir, Ru) precursor during the polymerization results in a difference in composition after the calcination as well as a tunable catalytic performance.Mechanical mixing prior to calcination limits atom diffusion resulting in a heterogeneous, Ir-Ru-Ir x Ru 1-x O 2 , which is active for both HER and OER.This report will serve as a platform for the development of efficient synthesis methods for multifunctional catalysts.Ongoing research focuses on optimizing catalytic ink formation to improve stability in a membraneelectrode-assembly (MEA), as well as characterizing the origin of the selective heat treatment under air.
Synthesis: Ruthenium and iridium materials modified polyaniline (PANI) with different atomic ratio of Ru:Ir were synthesized according the method illustrated in Scheme S1 (Supporting Information) and inspired by our previous oxidative aniline polymerization and calcination. [28,52]Briefly, an aqueous solution containing 0.4 m aniline, 0.5 m HCl and appropriate metal precursor(s) was prepared (50 mL, 10.7 mg (total metal) per mL) and placed in a double-jacket reactor at 5 °C.A second solution containing 0.5 m HCl and 0.2 m APS was prepared (50 mL).Then, under vigorous stirring, the second solution was added to the first one at 5 mL per minute through a two-syringe infusion pump (KD Scientific).The mixture begins to change color until a stable color was obtained (the nature of which depends on the type of metal precursor), indicating the polymerization of aniline. [29]The reaction was continued for 15 h.The solvent was removed from the mixture by rotavap and the solid phase was dried in an oven at 80 °C for 12 h.For the final thermal treatment under air (ashes furnace, Vecstar), the temperature program was a heating at 2°C min −1 up to 250, 350, and 400 °C (1 h dwell each, see Scheme S1, Supporting Information) by adopting two strategies to synthesize a library of eight (8) samples: 1) Two monometallic-based materials referred to as a PANI-Ru and PANI-Ir, which means that the polymerization was performed in presence of single monometallic precursor, 2) Three bimetallic-based materials referred to as a PANI-Ru 75 Ir 25 -CM, PANI-Ru 50 Ir 50 -CM, and PANI-Ru 25 Ir 75 -CM; "CM" stands for "chemical mixture", which means that the polymerization was performed in presence of both monometallic precursors, 3) Three bimetallic-based materials referred to as a PANI-Ru 75 Ir 25 -PM, PANI-Ru 50 Ir 50 -PM, and PANI-Ru 25 Ir 75 -PM; "PM" stands for "physical mixture", which means that the polymerization was performed separately for each monometallic precursor followed by grinding in mortar the appropriate mixture of Ru:Ir molar ratio.
Physicochemical Characterization: Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) were performed with SDT Q600 TA Instruments using aluminum crucibles in the temperature range of room temperature to 400 °C at 2 °C min −1 under air flow of 100 mL min −1 (Scheme S1 , Supporting Information for further details).X-ray diffraction (XRD) was performed on a PANalytical Xpert-PRO diffractometer (Malvern Panalytical, Almelo, the Netherlands) (40 kV, 20 mA) equipped with a copper anode at (CuK  ) = 1.54 Å, and in Bragg-Brentano mode (with 2 = 20°-80°).A quantitative phase analysis was performed using the Fullprof program, [53] interfaced by WinPlotR.The atomic structures of Ru (54236-ICSD), Ir (640730-ICSD), and RuO 2 (172178-ICSD) were retrieved from the Inorganic Crystal Structure Database [54] and kept fixed during the subsequent calculations.Pseudo-Voigt profile functions were used to model the peak shapes.During the first cycles of the refinements, the scale factors and profile parameters were optimized and also an overall zero-shift parameter, after which the cell parameters were refined as well.The refined cell parameters are less reliable for the minority phases.The structural parameters are calculated on the basis of Rietveld method.Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses were carried out on Hitachi S-4800 FEG, and ZEISS EVOHD 15 microscopes, respectively.Scanning transmission electron microscopy (STEM), High-angle annular dark-field (HAADF)-STEM and EDX mapping with line scanning analysis were performed on FEG JEOL 2200FS microscope operated at 200 kV accelerating voltage.N 2 adsorption-desorption measurements were conducted on Micromeritics ASAP 2020 instruments.X-ray photoelectron spectroscopy (XPS) characterization was performed on a Thermo Electron ESCALAB 250 spectrometer equipped with a monochromatic radiation source Al Mono (Al ka = 1486.6eV) operating at 15 kV and 6 mA (survey at a step of 1 eV for transition energy of 150 eV and high-resolution at 0.1 eV for transition energy of 20 eV).The binding energies were corrected on the basis of the energy of C1s at 284.4 eV by using the AVANTAGE software for peaks fitting.The quantification was carried out from the peak area after correction with a suitable sensitivity factor.
Electrochemical Measurements: All the electrochemical tests were carried out at controlled temperature of 25 °C in a conventional threeelectrode cell (single compartment) using the potentiostat Autolab PG-STAT 128N (Metrohm, Netherland).The electrochemical system consists of: i) the working electrode was a glassy carbon part of rotating disk electrode (RDE, Metrohm Autolab, 0.196 cm 2 ), ii) the counter electrode was a glassy carbon slab (12 cm 2 ), and iii) the reference electrode was Ag|AgCl|(KCl, 3 m), referred to as "Ag/AgCl".In this work, all the electrode potentials were referenced to reversible hydrogen electrode (RHE) using the calibration method illustrated by Figure S1 (Supporting Information), and described by Equation (10).E (V vs RHE) = E (V vs Ag∕AgCl) + ΔE, ΔE = 0.217 V (10)  The catalytic ink composition of 260 μL water, 100 μL isopropanol, 50 μL Nafion suspension (5 wt% in alcohols, Sigma-Aldrich) and 4 mg catalyst.First, only the liquid phase is sonicated in a water bath for 15 min.The catalytic powder was then added and the suspension was sonicated for 30 min to obtain a homogeneous catalytic ink.Before dropping the catalyst onto glassy carbon electrode, the latter was polished with alumina powder of different size (1, 0.3, and 0.05 μm) until a mirror-like surface was obtained.The glassy carbon electrode was then sonicated and washed three times, alternating between water and ethanol.Finally, a volume of 4 μL of the catalyst ink was dropped on the glassy carbon electrode and dried at 100 rpm for 5 min and 400 rpm for 10 min to evaporate the solvent and obtain a homogeneous thin-film.The metal loading was estimated to be 0.2 mg cm −2 for all the materials prepared and 0.1 mg cm −2 for the benchmark Pt/C.The electrolyte was 0.5 m H 2 SO 4 .Cyclic voltammetry (CV) experiments were carried out over a potential range from 0.05 to 1.20 V versus RHE in an Ar-saturated electrolyte solution, at different scan rates (from 200 to 10 mV s −1 ).Linear sweep voltammetry (LSV) was performed at 1600 rpm in H 2 -satured electrolyte for HER and in O 2 -saturated electrolyte for OER, at 5 mV s −1 .Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range from 100 kHz to 1 Hz at different electrode potentials (see main text).Chronopotentiometry (CP) was performed at |j| = 10 mA cm −2 for 0.5 h.LSV curves of HER and OER were iR-drop corrected by ohmic resistance determined from EIS.

Figure 1 .
Figure 1.a) Developed methodology to access multi-phase Ir-Ru-Ir x Ru 1-x O 2 materials.b) XRD patterns."CM": "chemical mixture" during the polymerization."PM": "physical mixture" of the polymerized materials before the calcination.c) Effect of the calcination on the XRD patterns."CM" referred to as "chemical mixture" during the polymerization and "PM" means "physical mixture" by mechanical mixing the polymerized monometallic-based materials before the calcination.
coupled energy-dispersive X-ray spectroscopy (SEM-EDX) after calcination (Figures S4-S19, Supporting Information).Control SEM-EDX analysis prior to calcination (Figures S20-S27, Supporting Information) only show the initial precursors, which are IrCl 3 and RuCl 3 (at least the Ir, Ru and Cl signals overlap).The typical (HR)TEM images of the materials after the calcination are reported in Figures S28-S31 (Supporting Information).For monometallic materials, the Ir and O signals do not overlap for PANI-Ir (Figure 2a; Figure S4, Supporting Information), while those of Ru and O overlap in PANI-Ru where the atomic ratio O/Ru is ≈2.0 (Figure 2b; Table For PANI-Ir the d-spacing values in Figure S33 (Supporting Information) of 2.3 and 2.0 Å correspond to, respectively, (111) and (200) of Ir.For PANI-Ru, the d-spacing value of 3.2 Å (Figure S34, Supporting Information) corresponds to (110) of RuO 2 .These outcomes confirm the previous XRD results that PANI-Ir leads to Ir while PANI-Ru leads to RuO 2 .For bimetallic materials, Figure 2c,d and Figure S32 (Supporting Information) suggest that some Ru, Ir and O coexist at the atomic scale, which is supported by the STEM-EDX line profiles (Figure S32c,d, Supporting Information) and d-spacing values of Figures S35-S36 (Supporting Information) (3.0-3.3Å corresponds to (110) of Ru x Ir 1-x O 2 and 2.2-2.3Å corresponds to (111) of Ir).Conclusively, these electron microscopy analyses are consistent with XRD results and support the formation of heterogeneous materials composed of Ir, Ru, and Ir x Ru 1-x O 2 .The specific surface area (BET) from the N 2 adsorption-desorption isotherms for representative materials (Figure S37, Supporting Information) shows that the current materials have a high surface area, 103 m 2 g −1 for PANI-Ru (RuO 2 structure) and 60-84 m 2 g −1

Figure 5 .
Figure 5. a) Nyquist impedance (normalized by the electrode area) at E appl = −0.02V versus RHE (iR-drop uncorrected) for HER in 0.5 m H 2 SO 4 at 25 °C: inset the equivalent electrical circuit R Ω + Q CPE //R ct .b) Complex-plane Nyquist impedance (normalized by the electrode area) at E appl = 1.47 V versus RHE (iR-drop uncorrected) for OER in 0.5 m H 2 SO 4 at 25 °C: inset the equivalent electrical circuit R Ω + Q CPE //R ct .c) R ct -based Tafel plots for OER in 0.5 m H 2 SO 4 at 25 °C.d) Sketch of the operation of a multifunctional HER-OER electrocatalyst.