Electrodeposited Co-Mo-P-TiO2 Composite Electrocatalysts for the Hydrogen Evolution Reaction

Aiming to improve the efficacy of earth abundant electrocatalysts for water splitting, novel Co-Mo and Co-Mo-P composites with TiO₂ were electrodeposited and their HER performance studied. The composites were electrodeposited galvanostatically onto rotating cylinder electrodes from a sulfate-citrate electrolyte. Similarly, the HER was evaluated in a 1 M NaOH electrolyte. Using a pure Co electrode as an example electrocatalyst having a poor activity for HER as a lower limit, exhibiting a relatively high overpotential of 216 mV at -10 mA/cm², and Pt, as the other extreme, having a high activity for HER, with an overpotential of 40 mV at -10 mA/cm², the overpotential of electrodeposited Co-Mo thin films were in the middle of this range ~ 110 mV, followed by a further decrease with Co-Mo-TiO₂ composites to 68 mV, and then a remarkable decrease to 30 mV with the addition of phosphorus, even lower than Pt. The electrochemical active surface area was assessed, surface morphology was characterized by SEM, the deposit composition was examined by XRF and EDS, and the structure determined by XRD. The Tafel analysis followed a Volmer-Heyrovsky mechanism. The addition of TiO₂ to a Co-Mo matrix did not significantly change the Tafel slope, but did change the exchange current density. However, the impact of adding phosphorus changed both the Tafel slope and exchange current density. Also, an electrolysis for 48 hours confirmed the electrocatalysts' stability.