Co-Mo alloys are well known as excellent electrocatalysts for hydrogen generation.^1-3 In this work, nanoscale TiO₂ particles were embedded into Co-Mo alloys during galvanostatic electrodeposition from a citrate-boric acid electrolyte. Following deposition, the deposits were characterized for their HER performance. Enhancements of the exchange current density and of a lower overpotential with the composites compared to the alloy were observed, and interpreted with a Volmer-Tafel/Heyrovsky mechanism.

The HER kinetics depends on both the Mo and TiO₂ composition. For example, in an alkaline electrolyte, the exchange current density exhibited a volcano-type behavior with the Mo composition in the deposits. At the maximum value of the exchange current density, the addition of TiO₂ further increased its value nearly 3 times higher than the Co-Mo alloys, accounting for the difference in the electroactive surface area. Figure 1 shows polarization curves of deposits with nearly similar Mo:Co ratios with and without TiO₂ in a 1 N NaOH electrolyte. The overpotential at -10 mA/cm² decreased significantly when particles were present in the deposit by 37 mV, in this example, compared to the alloy in Figure 1. Other deposits having different Mo wt % compositions exhibited a variation in the overpotential reduction between 22-55 mV at similar exchange current densities. The Tafel slope absolute value of all deposits with TiO₂ having similar Mo:Co ratios were in the range of 70-75 mV/dec. The Tafel slopes of the alloy, without TiO₂, and having high Mo content, was comparable to the composite. When the alloy composition contained less Mo the Tafel slope absolute value increased. In acid, an experiment with a Co-Mo alloy verifies a further decrease of the overpotential, but corrosion of the metal and titania places limitations on the pH.

References

1. C. McCrory, S. Jung, I. M. Ferrer, S. M. Chatman, J. C. Peters, T. F. Jaramillo, J. Amer. Chem. Soc. 137, 4347 (2015).
2. P.R. Żabiński, H. Nemoto, S. Meguro, K. Asami, K. Hashimoto, J. Electrochem. Soc. 150, C717 (2003).
3. Y. Zhang, Q. Shao, S. Long, X. Huang, Nano Energy 45, 448 (2018).