Transition metal phosphides are emerging as promising materials for a range of catalytic processes, including water-splitting, hydrodesulfurization, and carbon dioxide reduction. Intriguingly, such materials perform well under a range of extreme conditions (pH = 0 or 14; large positive and negative potentials) despite limited stability of the “phosphide”, suggesting that, at least under certain conditions (i.e., water oxidation occurring at high pH and oxidizing potentials) the phosphide is a pre-catalyst and the actual catalyst has a “phosphide core” under an oxidized (oxide, hydroxide, phosphate) shell. We hypothesize that the phosphide core may assist in electrical transport to the electrically resistive surface, where processes such as proton-coupled electron transport may take over. In this context, a series of stability tests have been initiated employing discrete nanoparticles suspended in conducting ink and deposited onto the electrode surface, performing water oxidation at pH = 0 and 14. A variety of transition metal phosphides have been explored, including those with relatively low electronegativity (Mn) and those more inert to oxidation (Rh). We seek to compare, for similar particle sizes and loadings, the consequences of composition on activity (and the extent to which that activity is sustainable), and the effect of the electrocatalytic process on the transformation of the phosphide (morphology, surface oxidation, etc.). Specifically, we seek to understand whether the surface oxidation processes can be limited and/or reversed for long-term sustainable catalytic activity, motivating the design of 3-D porous phosphide electrocatalysts.<br/>

Accessing 3-D in transition-metal phosphide catalysts for water splitting: Challenges and opportunities

2019 Fall National Meeting

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