Composition-controlled synthesis of Ni2−xCoxP nanocrystals as bifunctional catalysts for water splitting

Abstract

Ni_2−xCo_xP (0 ≤ x ≤ 2) nanocrystals (NCs) with good control of composition are synthesized by a facile economical approach. Low-cost air-stable triphenylphosphine is used as phosphorus source and NaBH₄ is added to facilitate the reaction, which helps to carry out the reaction at relatively low temperature (250 °C). By modulating the Ni/Co precursor ratios, the composition of Ni_2−xCo_xP NCs can be tuned over the whole range. A comprehensive study of the key role of NaBH₄ on the metal phosphide NCs formation reveals that NaBH₄ can efficiently activate the triphenylphosphine, lowering the thermodynamic barrier for the P–C bond broken and thus reducing the reaction temperature. As-prepared Ni_2−xCo_xP NCs show Ni/Co-composition dependent catalytic performance on both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In particular, OER catalytic activities follow the volcano relationship due to the synergistic effect. The Ni_1.0Co_1.0P NCs exhibit the highest OER catalytic activity with the overpotential of 0.34 V at a current density of 10 mA cm⁻², among the top tier of water oxidation catalysts. In addition to providing a new green low-cost route to composition controllable synthesis of ternary phosphides, this work also contributes to fundamental guideline on rational design of low-cost high-efficient catalysts for water splitting.