Phase Diversity of Nickel Phosphides in Oxygen Reduction Catalysis

Abstract

Motivated by the challenge to find a low-cost catalyst with high activity for the oxygen reduction reaction (ORR), transition metal phosphides (TMPs) appear to be one of the most burgeoning alternatives to noble metal based electrocatalysts. In addition to the low cost, TMPs have numerous interesting features such as high conductivity and chemical stability. Since, the catalytic activity of TMPs is highly dependent on the metal/phosphorous ratio, herein, we report the investigation of nickel-phosphide/carbon composites of different stoichiometries (NixPy/C) with tractable nickel phosphide phases as promising electrocatalyst for ORR under alkaline and acidic conditions. The NixPy/C composites are obtained by carbonization of a Ni-struvite (NiNH4PO4 ⋅ H2O) coated phenol-formaldehyde resin at different temperatures, resulting in variations of the formed NixPy phases. As expected, it is found that the electrocatalytic ORR performance of the NixPy/C composites highly depends on the predominant phase of nickel phosphide formed. The highest catalytic activity for ORR in alkaline as well as acidic media was found for the NixPy/C composite with the highest proportion of Ni2P as a predominant phase, obtained at 800 °C. For composites with NiP2 and Ni12P5 as the predominant phase, obtained at lower and higher temperatures, respectively, a lower catalytic activity was found. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim