(544gy) Electrode Engineering: Modifying the Hydrophilicity of Carbon Paper for Improved Cobalt Phosphide Hydrogen Evolution Catalysts

A sustainable energy supply depends on innovative breakthroughs in energy storage mediums that are able to minimize the curtailment of excess power generation during low load periods. Water electrolysis provides a suitable method to store excess energy in the form of hydrogen bonds that can later be fed to a proton exchange membrane (PEM) fuel cell for clean power generation. Recently, non-noble metal based catalysts such as transition metal phosphides (M_xP_x) have been identified as a promising family class of materials that have shown low overpotentials for the hydrogen evolution reaction (HER) while exhibiting excellent stability under acidic conditions. 3D support materials such as carbon fiber, paper, and cloth have been used to provide a conductive and porous structure in which metal phosphides can be directly deposited onto for the HER but in order to achieve low overpotentials (<50 mV) these electrodes have been reported with high loadings and thus low mass activities.

Various strategies including electrodeposition, nanoparticle synthesis, vapor phase phosphidations, and many others have been employed towards increasing the intrinsic activities and surface areas of M_xP_x but can be difficult and expensive to produce. In this work, we design and synthesize highly active CoP electrodes for the HER by tuning the hydrophobic and hydrophilic properties of the carbon support used during electrodeposition. Utilizing various oxidation methods, both physical and chemical, the surface O:C ratio is varied and is directly correlated to heterogeneity of cobalt phosphide deposition onto the surface of the carbon electrode. The degree of different oxygen-containing surface groups on carbon is tracked as a function of oxidation method and the most active electrode with a mass activity of >100 and >800 A/g at η=50 mV and 100 mV, respectively, which are among the highest shown in literature are correlated to a high concentration of carboxyl and hydroxyl functional groups on the surface as shown by X-ray photoelectron spectroscopy. Ultimately, we report a facile and translatable strategy to lower catalyst loading while maintaining extremely high mass activities on a 3D electrode.