TY - JOUR

T1 - Gas diffusion electrode activity measurements of iridium-based self-supported catalysts produced by alternated physical vapour deposition

AU - Jiménez, Pablo Collantes

AU - Sievers, Gustav

AU - Quade, Antje

AU - Brüser, Volker

AU - Pittkowski, Rebecca Katharina

AU - Arenz, Matthias

N1 - Funding Information: The GDE half-cell (Fig. 1) and a glass bubbler were placed inside an insulating glass chamber during the measurements (See Fig. S1 in SI). Precise temperature control (±0.1 °C) was achieved through a constant flow of distilled water recirculated in between the double glass walls with a water heating system (Lauda RC6 SC). The GDE half-cell was placed in the middle of the chamber, supported on an aluminium laboratory jack (Laborboy, Sigma Aldrich) and insulated with a PTFE plate in between. Before the start of the measurements, the system was allowed to equilibrate at a constant temperature for at least 30 min. All the temperature references correspond to the set point defined in the water heating system. To prevent any shifts in reference potential due to contaminations on the RHE electrode, the RE was protected in a glass frit manufactured by an in-house technical glassblower. In addition, the RHE electrode was calibrated before each measurement in a separate GDE cell against a Pt GDE with the same molarity and electrolyte as the testing GDE cell, i.e., 1 M HClO4 electrolyte. The H2 gas was supplied through an in-house electrolyzer, connected to the gas flow through lines of the GDE cell. The RHE offset was measured by cyclic voltammetry in a potential interval between −0.005 and 0.005 V at 100 mV s−1 for 200 cycles. The acceptable range for initial RHE values was defined as ± 0.003 VRHE. In case of a larger deviation, the RHE was remade, and the calibration procedure was repeated to avoid large iR-correction errors. Before the measurements, Ar was purged through the flow field as a conditioning step and cyclic voltammograms were recorded at a scan rate of 100 mV s−1 in a potential range between 0.025 and 1.2 VRHE until a stable cyclic voltammogram could be observed (ca. 30 cycles). The ECSA of the catalyst (Table 3) was determined by integrating the Hupd area in the potential window of 0.025–0.25 VRHE of the last CV acquired using a fixed conversion coefficient of 176 μC cm−2 [11] according to the following formula:In this study, we applied the GDE method to perform activity measurements of PVD-produced catalysts for the OER. First, three series of Ir–Co catalysts with equal 250 μg/cm2 Ir loading were sputtered on carbon substrate using different Co:Ir weight ratios (Ir28Co72, Ir45Co55 Ir75Co25). To create a self-supported nanoporous structure with increased ECSA, Co was removed in an acid-leaching step. This is rendering a distinct dendritical surface morphology with Ir-rich clusters and slight changes in crystallinity. During the process, a mixed metallic and oxide structure with local Ir–Co coordination is formed. A higher initial Co content leads to larger surface areas after leaching, outperforming the OER activity of a commercial IrOx catalyst benchmarked at 30 °C and 60 °C. Overall, the performance followed the Co:Ir series Ir28Co72 > Ir45Co55 > Ir75Co25 > IrOx, where the best-performing catalyst at 60 °C reached more than a tenth-fold increase in mass activity over the commercial sample. The performance increase as compared to the benchmark catalyst, accounting for loading and preparation differences, can be due to higher dispersion in addition to a ligand effect. The latter is supported by the specific activity trend correlation with the remaining Co after acid leaching and XAS coordination data. A strain effect, by comparison, was not supported by the XAS data. The temperature increase and dynamic surface activation due to oxidation of metallic Ir, both observed by CV and the OER activity, had a positive influence on the catalyst activity. The authors acknowledge that the complex mechanisms behind the influence of the Co content and the electrochemical performance may not be fully explained from the measurement results, but also remain beyond the scope of this study. On the other hand, it was demonstrated that the flexible and reproducible characteristics achievable from the nanostructured PVD-produced catalysts in combination with the three-electrode GDE setup can reveal further insights into the electrode evolution under more realistic conditions than traditional methods such as RDE, helping to fast-track OER catalyst experimental research.The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) in the framework of the VIP + Projekt. 03VP06451 (3DNanoMe). The authors thank Adam Clark from the SuperXAS beamline X10DA at the Paul Scherrer Institute (PSI) for measuring the XAS data via mail-in service. MA and RKP acknowledge funding from the Swiss National Science Foundation (SNSF) via project No. 200021 184742 and the Danish National Research Foundation Center for High Entropy Alloys Catalysis (CHEAC) DNRF-149. Funding Information: The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) in the framework of the VIP + Projekt. 03VP06451 (3DNanoMe). The authors thank Adam Clark from the SuperXAS beamline X10DA at the Paul Scherrer Institute (PSI) for measuring the XAS data via mail-in service. MA and RKP acknowledge funding from the Swiss National Science Foundation (SNSF) via project No. 200021 184742 and the Danish National Research Foundation Center for High Entropy Alloys Catalysis (CHEAC) DNRF-149 . Publisher Copyright: © 2023 The Authors

PY - 2023/6

Y1 - 2023/6

N2 - The scarce supply of Ir used to catalyze the sluggish oxygen evolution reaction in acidic water electrolysis calls for unconventional approaches to design more active catalysts with minimal resource usage for their commercial scaling. Industrial-ready production methods and laboratory scale tests that can reflect the catalyst behaviour realistically need to be included in this process. In this work, we benchmarked three series of self-supported Ir–Co catalysts with low Ir loading produced by physical vapour deposition under relevant current densities in a gas diffusion electrode setup. It was seen that after selective acid leaching of the Co, a nanoporous structure with a high electrochemically active surface area and a mixed oxide and metallic character was formed. Depending on the initial Co:Ir deposition ratio over ten times higher oxygen evolution mass activities could be reached as compared to a commercial, unsupported IrOx nanoparticle catalyst used as a benchmark in the same setup configuration. The presented integrative catalyst design and testing strategy will help to facilitate bridging the gap between research and application for the early introduction of next-generation catalysts for water splitting.

AB - The scarce supply of Ir used to catalyze the sluggish oxygen evolution reaction in acidic water electrolysis calls for unconventional approaches to design more active catalysts with minimal resource usage for their commercial scaling. Industrial-ready production methods and laboratory scale tests that can reflect the catalyst behaviour realistically need to be included in this process. In this work, we benchmarked three series of self-supported Ir–Co catalysts with low Ir loading produced by physical vapour deposition under relevant current densities in a gas diffusion electrode setup. It was seen that after selective acid leaching of the Co, a nanoporous structure with a high electrochemically active surface area and a mixed oxide and metallic character was formed. Depending on the initial Co:Ir deposition ratio over ten times higher oxygen evolution mass activities could be reached as compared to a commercial, unsupported IrOx nanoparticle catalyst used as a benchmark in the same setup configuration. The presented integrative catalyst design and testing strategy will help to facilitate bridging the gap between research and application for the early introduction of next-generation catalysts for water splitting.

KW - Gas diffusion electrodes

KW - Oxygen evolution reaction

KW - PEM Water electrolysis

KW - Physical vapour deposition

KW - Self-supported catalysts

U2 - 10.1016/j.jpowsour.2023.232990

DO - 10.1016/j.jpowsour.2023.232990

M3 - Journal article

VL - 569

JO - Journal of Power Sources

JF - Journal of Power Sources

SN - 0378-7753

M1 - 232990

ER -