Facet and Phase Engineering of Single‐Crystalline Transition‐Metal Phosphide Electrocatalysts for Selective Electrochemical Reactions

Crystallographic facets and phase structures play a decisive role in determining interfacial electrochemical reactions by dictating surface characteristics. In electrochemical catalysis, conventional polycrystalline catalysts often obscure intrinsic structure–activity relationships due to their structural heterogeneity. Single‐crystal and facet‐controlled catalysts therefore serve as well‐defined model platforms for establishing direct correlations between atomic‐scale surface properties and electrochemical performance. This review highlights recent advances in facet‐ and phase‐engineered electrocatalysts, with a primary focus on transition‐metal phosphides (TMPs). Starting from the fundamental facet‐dependent phenomena revealed by noble metal single crystals, we bridge these insights to TMPs, clarifying how facets and phase structures modulate electronic and adsorption properties along with outlining the synthetic principles governing their structural evolution. Emerging synthetic platforms, including liquid metal–assisted synthesis, are reviewed as enabling routes to single‐crystalline catalysts with controlled facets and phases. Finally, we address key challenges in translating fundamental model surface insights into scalable electrode architectures and provide perspectives on future research directions.