Few earth-abundant catalysts can catalyze the oxygen evolution reaction (OER) in acid with satisfactory durability, due to the significant dissolution of transition metals. Disclosing the deactivation mechanism is the prerequisite for the rational design of durable OER electrocatalysts. Here, we establish the link between catalytic deactivation and the crystal facet's evolution of an archetypical acidic OER catalyst of spinel Co3O4, by correlating electrochemical response with facet evolution during electrolysis. We establish a complete picture of the crystal facet's evolution throughout the whole course. Low-index crystal facets of {110}, {100}, and {211} are prone to dissolve, whereas the high-index ones including {311} and {402} emerge along with the concurrent transition of the catalytic mechanism. Combining electrochemical analysis, facet probing, and theoretical calculation, we unveil that the etching of the most unstable {110} facets (only medium active) significantly accelerates the dissolution of the most active {100} facets and the most durable {111} facets adjacent to them. Our discovery highlights the destructive effect of unstable facets with only moderate activity, which has always been overlooked before. Additionally, the decoupling of the most active facets from the most stable ones suggests that the activity-durability tradeoff could be addressed by facet engineering.
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