How the crystal structures of ordered transition-metal phosphide catalysts affect the hydrogen-evolution reaction (HER) is investigated by measuring the anisotropic catalytic activities of selected crystallographic facets on large (mm-sized) single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP2). We find that different crystallographic facets exhibit distinct HER activities, in contrast to a commonly held assumption of severe surface restructuring during catalytic activity. Moreover, density-functional-theory-based computational studies show that the observed facet activity correlates well with the H-binding energy to P atoms on specific surface terminations. Direction dependent catalytic properties of two different phosphides with different transition metals, crystal structures, and electronic properties (FeP is a metal, while m-NiP2 is a semiconductor) suggests that the anisotropy of catalytic properties is a common trend for HER phosphide catalysts. This realization opens an additional rational design for highly efficient HER phosphide catalysts, through the growth of nanocrystals with specific exposed facets. Furthermore, the agreement between theory and experimental trends indicates that screening using DFT methods can accelerate the identification of desirable facets, especially for ternary or multinary compounds. The large single-crystal nature of the phosphide electrodes with well-defined surfaces allows for determination of the catalytically important double-layer capacitance of a flat surface, C dl = 39(2) μF cm-2 for FeP, useful for an accurate calculation of the turnover frequency (TOF). X-ray photoelectron spectroscopy (XPS) studies of the catalytic crystals that were used show the formation of a thin oxide/phosphate overlayer, presumably ex situ due to air-exposure. This layer is easily removed for FeP, revealing a surface of pristine metal phosphide.
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