2, Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada
Stable and affordable electrocatalysts are urgently needed to accelerate the transition from conventional fossil fuels to sustainable energy resources such as hydrogen derived from renewable energy sources such as solar and wind. Converting said harvested energy electrochemically into chemical fuels and feed stocks is also highly desirable. Over the last two decades, the electrocatalytic splitting of water, and carbon dioxide reduction to hydrocarbons have received great attention, however approximately 90% of the electricity input is consumed in the anodic oxygen evolution reaction (OER)1 due to poor reaction kinetics. Precious metal based catalysts (Pt, Ir, Ru) are commonly used as OER catalysts, but prove uneconomic for large scale industrial deployment. New low cost, stable, earth-abundant catalysts such as those based on transition-metal alloys (Ni,Co) offer great promise to overcome these kinetic limitations.
In this work, we used a two-step novel milling process to produce NiCo-based amorphous nano-electrocatalysts. Cryo-milling (mechanical milling of precursors at cryogenic temperatures to achieve alloying) followed by surfactant-assisted ball milling (SABM for particle size reduction) create stable amorphous alloys with high surface areas and coordinatively unsaturated active sites for the reaction of OER intermediates. The improved stability of the amorphous alloys is yet another exciting feature that can significantly enhance the overall catalytic performance of the catalyst.
Using density functional theory (DFT) calculations, two potential elements V and Se were predicted to reduce the overpotential of the catalyst when incorporated in these structure. Alloys of NiCoV and NiCoSe were milled under various conditions and the structural evolution of the system monitored using x-ray diffraction (XRD) and electron microscopy. Our results confirm the production of two fully alloyed ternary systems after 6 hours of milling time. Furthermore, a wide composition range could be investigated using this synthesis technique to accordingly tailor the activity of the catalyst. The electrocatalytic activity and stability of the catalysts was evaluated by Tafel measurements obtained from linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments. We found that V in NiCo-based alloys helped the amorphous structures to stabilize by forming non-transitional clusters2 while Se significantly reduced the size of the produced particles. Operando X-ray Absorption Spectroscopy (XAS) was conducted to reveal the role of adding V and Se on the chemical-structural transformation and bonding environment of surface species during the reaction. This work suggests that milling can potentially be used to produce OER catalysts for industrial applications.
1. S. Verma, S. Lu, and P. J. A. Kenis, Nat. Energy (2019).
2. J.-F. Deng, H. Li, and W. Wang, Catal. Today, 51, 113–125 (1999).