Carbon-Monoxide Assisted Synthesis of Cobalt-Nickel Phosphides for Hydrogen Evolution Reaction

Abstract

Alkaline water electrolysis is a method for production of hydrogen as an alternative source for renewable energy. This method can be completely free of greenhouse gas emissions if it is paired with another alternative energy source, such as solar energy or wind power, to provide the necessary electricity to drive the reaction. Water electrolysis requires catalysts to make that necessary amount of electricity low enough for competitive hydrogen production in the current market. Vast numbers of studies have been performed in an attempt to fill this need for cost-effective catalysts for water electrolysis. This work focuses on the development of synthesis methods for cobalt-nickel phosphide catalysts for the hydrogen evolution reaction (HER) for water electrolysis in alkaline media. The methods for preparation of the catalysts are explored for cobalt, nickel, and cobalt-nickel alloy phosphides in the presence of carbon monoxide as a shape-controlling capping agent. Depending on the condition of carbon monoxide addition, the reaction results in high vs. low-aspect ratio nanorods or nanospheres for mono- or bimetallic cobalt-nickel phosphide nanostructures. These phosphides are then subjected to evaluation for HER via cyclic voltammetry and linear sweep voltammetry to study the effects of morphology on their HER activity. Materials characterization such as XRD, TEM, and ICP-MS are performed before the HER electrochemical evaluation for better understanding of the structure-activity relationship. The catalytic activities of these phosphides are also compared to the noble metal benchmark HER catalyst Pt/C in the same environment at their equal mass loadings. The best-performing phosphide is further tested for HER at increased mass loadings to study the effects of catalyst loadings on the mass transport and thus the activity. Further TEM characterization is performed on the most active catalyst after HER to evaluate morphological stability. It is found that in order to achieve an overpotential at the current density of 100 mA/cm2 comparable to Pt/C for 50 μg/cm2, the necessary loading for the phosphide is 250 µg/cm2.