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The acidic nature of proton exchange membrane (PEM) electrolysis coupled with the highly oxidizing conditions of the oxygen evolution reaction (OER) greatly limits viable options for catalysts that are both active and stable. Precious metal catalysts, such as IrOx, are high performing OER materials, but scarcity and high cost may limit widespread implementation. Pyrochlore catalysts of the form A2B2O7 (where A is typically a rare-earth or alkaline-earth metal and B is a transition metal) have emerged as promising active candidates with reduced precious metal content and intriguing stability. For example, Y2Ir2O7 has demonstrated excellent activity and stability under acidic OER conditions despite some leaching of the Y cation, potentially leading to the in situ formation of a new active phase during catalysis1. In this work, the influence of the A-site cation on the activity and stability of Ir-based pyrochlore materials (A2Ir2O7) was systematically explored. Using a series of rare-earth metals, activity and stability were experimentally correlated with changes in the physical, chemical, and electronic properties to better understand the origin of the enhanced performance. This experimental work was supported by density functional theory (DFT) calculations. The fundamental insight gained will help to design OER catalysts with improved activity and reduced precious metal content, all without compromising stability. Cyclic voltammetry, chronoamperometry, chronopotentiometry, and double layer capacitance were used to determine the electrochemical performance of the catalysts. Inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify leaching of the A-site cations and Ir under acidic OER conditions and establish a relationship between the loss of material and the change in catalytic performance. This relationship was further investigated with DFT calculations. Additionally, the crystal structure, surface composition, and morphology of the catalysts before and after electrochemical testing were probed using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). References: Lebedev, D. et al. Highly Active and Stable Iridium Pyrochlores for Oxygen Evolution Reaction. Chem. Mater. (2017). doi:10.1021/acs.chemmater.7b00766 |
