Unveiling the Activity-Electrochemically Active Surface Area Relationship in PGM-Free Oxyphosphide OER Electrocatalysts for Alkaline Water Electrolysis

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The oxygen evolution reaction (OER) plays a pivotal role in various electrochemical processes, from water splitting to CO2 electroreduction and metal-air batteries. However, the sluggish kinetics of OER, involving multi-electron transfer, necessitate the development of catalysts with high activity and stability. While platinum group metal (PGM) catalysts like IrO2 and RuO2 exhibit high OER activity in proton exchange membrane (PEM) water electrolysis, their cost hinders widespread adoption. Anion Exchange Membrane Water Electrolysis (AEMWE) emerges as a promising technology with the potential to be both cost-effective and sustainable for hydrogen production. It relies on PGM-free OER catalysts, bridging the gap and merging the strengths of PEM and traditional alkaline electrolysis systems Transition metal (TM) oxides, layered double hydroxides, and oxyphosphides, particularly those incorporating Ni, Fe, and Co, emerge as promising OER catalysts for alkaline water electrolysis. Yet, comparing different catalysts remains challenging due to varied testing protocols that influence reported performances, often referenced to geometric surface area. Herein, we address this challenge by evaluating the intrinsic activity and electrochemically active surface area (ECSA) of PGM-free OER catalysts. Employing a colloidal synthesis approach, TM catalysts were synthesized and tested for OER activity and durability in 1 M KOH. Transmission electron microscopy (TEM) imaging revealed a unique porous structure surrounding a metallic core. To estimate the ECSA, cyclic voltammograms were recorded at various scan rates in a non-Faradaic region. The double layer capacitance (Cdl) was evaluated from the slope of current versus scan rate, and a specific capacitance of 40 μF cm-2 was used to calculate the ECSA. [1-4] Preliminary results demonstrate a direct correlation between OER activity and ECSA. The correlation between the BET surface and OER activity was also studied. Acknowledgments This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). Argonne is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under Contract DE-AC-02-06CH11357. 1. McCrory, CCL; Jung, S; Peters, JC; Jaramillo, TF. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977–16987. 2. Sarkar R, Farghaly AA, Arachchige IU. Oxidative Self-Assembly of Au/Ag/Pt Alloy Nanoparticles into High-Surface Area, Mesoporous, and Conductive Aerogels for Methanol Electro-oxidation. Chemistry of Materials. 2022, 34(13), 5874-87. 3. Khalafallah D, Farghaly AA, Ouyang C, Huang W, Hong Z. Atomically dispersed Pt single sites and nanoengineered structural defects enable a high electrocatalytic activity and durability for hydrogen evolution reaction and overall urea electrolysis. Journal of Power Sources. 2023, 558, 232563. 4. Onajah S, Sarkar R, Islam MS, Lalley M, Khan K, Demir M, Abdelhamid HN, Farghaly AA. Silica‐Derived Nanostructured Electrode Materials for ORR, OER, HER, CO2RR Electrocatalysis, and Energy Storage Applications: A Review. The Chemical Record. 2024, e202300234.

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