TiO₂ nanotubes modified with cobalt oxyphosphide spheres for efficient electrocatalytic hydrogen evolution reaction in alkaline medium

Electronic and surface modulation through coupling between oxygen and phosphorous in cobalt oxyphosphide (CoOP) is identified as an effective approach to enhancing the electrocatalytic hydrogen evolution reaction (HER) in an alkaline medium. This work demonstrates a one-step and safe electrodeposition method to modify the surface of ordered titania nanotubes (TNTs) with CoOP spheres. The effect of electrodeposition charge density, pH of the electrodeposition bath, and presence of citric acid on the electrocatalytic HER performance of CoOP-modified TNTs was investigated. The Ti/TNTs/CoOP electrode requires only 130 mV overpotential to deliver 10 mA cm⁻² in a 1.0 M KOH electrolyte. The Tafel slope is estimated to be 72.5 mV dec⁻¹. Both the HER performance and Tafel value of Ti/TNTs/CoOP are superior to CoOP deposited on bare Ti foil (165 mV and 78.4 mV dec⁻¹), FTO (185 mV; 88.1 mV dec⁻¹), and glassy carbon (214 mV; 93 mV dec⁻¹). The electrochemically active surface area (ECSA) and impedance spectroscopy findings revealed that the synergistic coupling between the CoOP and TNTs enhances the surface area and density of accessible active sites while reducing the charge transfer resistance at the electrode/electrolyte interface. Such a synergistic effect is ascribed to the effective electronic interaction between CoOP and TNTs, and the faster electron transfer kinetics because of the unidirectionality of the 1-D TNTs.