Dual-Site Catalysis in Ni-Fe Phosphides: Understanding the Bifunctional Mechanism for Water Splitting.

Nickel-iron phosphides, as promising bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are still under debate due to their thermodynamic instability and unclear reaction mechanisms at multiple active sites. Here, highly exposed Ni2P-Fe2P heterostructures are synthesized, achieving rapid HER and OER with remarkably low and stable overpotentials of 86 (157) and 140 (251) mV at 10 (100) mA cm-2, respectively. The systematic analyses revealed two key beneficial mechanisms for the surface stability and kinetics: the self-adaptive hydrogenation (oxidation) of Ni2P-Fe2P surface under HER (OER) potentials and the cooperation between Fe2P and Ni2P. In the later, Fe2P aids in the water dissociation and Ni2P facilitates the forward transitions of hydrogen, hydroxyl, and hydroperoxyl intermediates, leading to the promoted gas evolutions. The realized superior bifunctional catalysis and established microscopic mechanisms here can both pave the way for designing stable and efficient phosphide catalysts.