The Performance and Durability Evaluation of Amorphous and Rutile Iridium Catalysts in the Anode Catalyst Layer of Proton Exchange Membrane Water Electrolysis

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Proton exchange membrane water electrolysis (PEMWE) is a key technique in low temperature electrolysis for hydrogen production. For electrochemical water splitting, iridium (Ir)-based catalysts are often used for oxygen evolution reaction (OER) at the anode due to the moderate activity and stability in acid environment. Catalyst intrinsic activity and performance stability are crucial for developing next generation of anode catalyst and for enabling lower Ir loading, decreasing PEMWE capital cost, and expanding the adoption of this technology 1,2 . In this study, we evaluated the performance and durability of low loading anode catalyst layers (0.4 mg Ir /cm 2 ) with different crystallinity of IrO x catalysts (amorphous group: Alfa Aesar (AA), TKK77100 and Johnson Matthey (JM); rutile group: TKK77110, Pajarito and Furuya) in single cell testing for 2000 h. The catalysts with the majority of IrO x having an amorphous structure had lower kinetic losses, due to higher OER activity and higher surface area 3 . Compared to the baseline catalyst (AA), TKK77100 and JM had around 22 – 25 mV lower kinetic losses, while the cells with rutile catalysts had lower beginning of test performance. Under long term operation, however, rutile Ir catalysts displayed higher stability with lower decay rate of HFR-free voltages (9.75 – 10.9 µV/h at 0.1 A/cm 2 ). The decay rates of the amorphous group ranged from 20.4 – 21.8 µV/h at 0.1 A/cm 2 in the 2000 h test. By electrochemical impedance spectroscopy (EIS) analysis and modeling, the kinetic performance losses of amorphous Ir were correlated to the pronounced increase of polarization resistance and the significant decrease in equivalent capacitance, especially in the first 500 h, which suggested amorphous Ir tended to have higher losses in available site access and thus degraded faster. The catalysts with lower decay rates were tested for 4000 h to probe catalysts stability in longer duration via in situ and ex situ characterization methods. Understanding the kinetic features and long term behaviors of different Ir catalysts is important for developing high performing and durable anode catalyst layer and advancing PEMWE technology. Reference: S. M. Alia, Current Opinion in Chemical Engineering , 33 , 100703 (2021). K. Ayers et al., Annual Review of Chemical and Biomolecular Engineering , 10 , 219–239 (2019). V. Pfeifer et al., Chem. Sci. , 8 , 2143–2149 (2017). Figure 1

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