Transition metal phosphides possess novel, structural, physical and chemical properties and are an emerging new class 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 (∼12 at% P), the nickel based phosphides Ni12P5 and Ni2P with higher levels of phosphorus (29 and 33 at% P) may be 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.

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

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Electroless deposition technique was used to coat Ni–P on graphite particles with high deposition rate and bath stability by activating the graphite powder in a furnace at 380 °C for 1 h. The effect of plating time, size of the powder and weight of the powder were studied. It was found that by a simple and controlled plating method a uniform and continuous layer of nickel could be deposited on the surface of graphite particles. Scanning electron microscopy images and EDS spectra before and after electroless nickel plating confirm that nickel is deposited on the surface of graphite particles. The rate of increase in weight percent nickel over graphite decreases with time and reaches a plateau. Further increase in weight is attained by plating the same powders in a fresh bath to obtain desired Ni–P alloy mass.

Electroless nickel–phosphorus plating on graphite powder

Graphite particles were coated with Ni–P by electroless deposition using a conventional bath consisting of a nickel salt and hypophosphite. After 15 min of electroless deposition, the graphite particles were covered with 10 wt% nickel and 0.7–1.0 wt% phosphorus as analysed by wet chemical method. Surface morphology was studied by scanning electron microscopy (SEM). Electrochemical characterisation for the catalytic activity was done by cyclic voltammetry. Pure Ni powder and electroless Ni–P coated on graphite were used as catalysts for the electro-oxidation of dextrose (1.8 × 10−3 to 4.5 × 10−3 M) in 0.1 M KOH solution. Comparative studies revealed that electroless Ni–P coated on graphite particles acted as a better catalyst than pure Ni powder for catalytic reaction.

G. Veera Babu

Papers

Electroless nickel–phosphorus plating on graphite powder

Electroless Ni-P coated on graphite as catalyst for the electro-oxidation of dextrose in alkali solution