Transition metal phosphides (TMP) are state-of-the-art electrocatalysts that can efficiently catalyze both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Exploring how the surface of TMP restructures under electrochemical water-splitting conditions is essential toward understanding the structure-properties correlations and rational design of high-performance catalysts. Quasi-in-situ X-ray Spectroscopy (XPS) is employed to unveil the changes in oxidation state and composition of the TMP surface induced by electrochemical conditions. As a model catalyst for water splitting, cobalt phosphide (CoP) is found to possess a naturally oxidized and phosphorus-rich surface. The oxidized species will be removed in acidic electrolyte under HER conditions, exposing a fresh CoP surface. In alkaline electrolyte under HER conditions, replacement of oxidized phosphorus by hydroxides and reduction of oxidized species will occur simultaneously, rendering a cobalt-rich surface. Under oxidizing conditions in alkaline electrolyte, the surface will be oxidized to an oxide/hydroxide-dominated one below the equilibrium potential of OER. In order to explore the dependence of catalytic reactivity on substitutional doping, cation- and anion- substituted CoP nanoparticles are also investigated. For Fe0.5Co0.5P, the Fe substituents are more easily oxidized than Co in the air; they are also more difficult to reduce under HER conditions in alkaline electrolyte and the remaining Fe-OH species on the surface hampers the activity for HER. For CoP0.5S0.5, the S substituents are less oxidized than P in the air; they are also more difficult to oxidize under OER conditions in alkaline electrolyte, and the remaining sulfate-like species enhances the activity for OER.

Surface restructuring of transition metal phosphide nanoparticles under electrochemical water-splitting conditions

2019 Spring National Meeting

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