Cobalt-Based Thin Film Nanostructures As Bifunctional Electrocatalysts for Overall Water Splitting

CoP thin-film nanostructures as bifunctional electrocatalysts for water electrolysis

Julian A. Vigil ^1,2 and Timothy N. Lambert ¹ *  

¹ Department of Materials, Devices, and Energy Technologies, Sandia National Laboratories; Albuquerque, New Mexico 87123

² Department of Chemical and Biological Engineering, University of New Mexico;

Albuquerque, New Mexico 87106

* Fax: 505-844-7786; Tel.: 505-284-6967; Email: tnlambe@sandia.gov

The desire to reduce anthropogenic carbon emissions has targeted hydrogen (H₂) as a realistic, widespread next-generation fuel. However, H₂ is not abundant in the Earth’s atmosphere and the majority of H₂ is generated by steam reforming hydrocarbons, which is just as environmentally taxing as burning fossil fuels. The “clean” generation of hydrogen is done by water electrolysis, but it is rather inefficient. Commercial electrolyzers currently operate around 1.8-2 V, much higher than the 1.23 V theoretical potential gap of the cell’s two half-reactions, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The HER and OER are multi-step, multi-electron electrochemical reactions that generally have very high overpotentials and slow kinetics. Electrocatalysts are used to improve the reaction kinetics and lower overpotential, but the most active materials are precious metal-based. Thus, non-precious metal catalysts are being actively pursued to replace such costly, rare materials and improve water splitting device efficiencies. In addition, HER/OER bifunctional catalysts, i.e. materials with the ability to catalyze both the HER and OER, could simplify and improve electrolyzers and lead to other useful realities such as a reversible fuel cell.

Here, we report on a HER/OER bifunctional electrocatalyst: a nanostructured CoP thin-film exhibiting high activity and stability for both the HER and OER. The CoP electrocatalysts were prepared by a simple electrodeposition-thermal annealing-phosphidation methodology, with versatility in substrate selection. The CoP films showed low overpotentials and high stability for the HER and OER in highly acidic and basic electrolyte. Finally, the bifunctionality was tested by assembling a symmetrical electrolysis cell, which operated at a low overpotential of ~ 0.4 - 0.5 V (~ 1.7 – 1.8 V, operating potential) in alkaline electrolyte.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.