Nickel phosphide: the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium

Transition metal phosphides possess novel, structural, physical and chemical properties and are an emerging newclass of materials for various catalytic applications. Electroplated or electrolessly plated nickel phosphide alloy materials with achievable phosphorus contents <15 at% P are known to be more corrosion resistant than nickel alone, and have been investigated as hydrogen evolution catalysts in alkaline environments. However, there is significant interest in developing new inexpensive catalysts for solid polymer electrolyte electrolysers which require acid stable catalysts. In this paper, we show that by increasing the phosphorus content beyond the limit available using electroplating techniques (similar to 12 at% P), the nickel based phosphides Ni12P5 and Ni2P with higher levels of phosphorus (29 and 33 at% P) maybe utilised for the hydrogen evolution reaction (HER) in acidic medium. Corrosion resistance in acid is directly correlated with phosphorus content - those materials with higher phosphorus content are more corrosion resistant. Hydrogen evolution activity in acid is also correlated with phosphorus content - Ni2P based catalysts appear to be more active for the hydrogen evolution reaction than Ni12P5. Electrochemical kinetic studies of the HER reveal high exchange current densities and little deviation in the Tafel slope especially in the lower overpotential regime for these nickel phosphide catalysts. The electrochemical impedance spectroscopy response of the respective system in acidic medium reveals the presence of two time constants associated with the HER.