Deposition of Nickel Phosphide Ni2P on Carbon Materials Forming a Versatile and Very Active Electrocatalyst for Application in Gas Diffusion Electrodes

Kurzfassung

In alkaline media, transition metal phosphides and in particular nickel phosphide Ni2P were proved to be very active electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). While originating from much more inexpensive and abundant elements the catalytic activity of Ni2P is comparable to precious metal catalysts like iridium or ruthenium for OER as well as platinum for HER. However, it lacks suitability for the oxygen reduction reaction (ORR) which is also essential for charging metal-air batteries. For later implementation in gas diffusion electrodes used in metal-air batteries, we worked on nickel phosphide by itself and even on nickel phosphide supported on carbonaceous materials such as carbon nanotubes and new-type aerogel. These carbons not only provide good electronic conductivity between reaction sites and current collector but also expose much more surface area where catalyst particles can be deposited. By uniform deposition over a large surface area, the efficient utilization of catalyst material will be increased evidently. We introduce a simple synthesis route which brings earlier studies together and leads to carbon supported catalysts showing higher current densities normalized per mass of catalyst and lower overpotential at well-defined anodic currents. OER activity and stability of the modified phosphide catalysts have been evaluated by electrochemical measurements like cyclic voltammetry and impedance spectroscopy on a rotating disk electrode and in gas diffusion electrodes as well. Furthermore, we present the structural properties of the catalyst powder and the corresponding gas diffusion electrodes. It seems to be evident to us to enable the carbonaceous support to catalyze also the ORR, since neither transition metal phosphides nor undoped carbons are capable of this. Thus the presentation ends with a prospect of bifunctional catalysts based on nickel phosphide supported on doped carbons.