Effects of Ni2+/3+ Redox Peak Potential on Oxygen Evolution Activity of Mixed-Transition-Metal-Oxides in Alkaline Electrolyte

Abstract

High-performance, low-cost electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes are vital for the commercialization of water electrolysis systems. Modular water electrolyzers could be the building blocks of a “plug & play” hydrogen highway infrastructure which would allow on-demand hydrogen production for re-fueling fuel-cell electric vehicles. Current state-of-the-art water electrolysis systems are based on proton-exchange-membrane (PEM) technology. The highly acidic environment of these PEM systems requires the use of rare & expensive platinum group metals (PGMs). Recent break-throughs in anion-exchange-membrane (AEM) technology have re-ignited the development of non-PGM catalysts for operation in alkaline electrolytes.1 Recent literature has shown that PGM-free AEM-electolyzers can achieve performance on par with PEM systems.2 However the mass-loading of the catalysts in this proof-of-concept study was quite high. Further increases in HER & OER activity & stability will move these systems closer to commercial deployment. Recent studies have shown that Ni-Fe hydroxides exhibit the highest OER activity of any non-PGM electrocatalysts.3 Our research has investigated binary & ternary Ni-X-(Y) mixed metal oxides. Results indicate that amorphous (non-stoichiometric) oxy-hydroxide surfaces exhibit the greatest OER activity. Ni-Fe-Mo mixed oxide has shown OER activity on par with perovskites.4 We have identified that mixed metal oxide films exhibit shifts in the Ni2+/3+ redox peaks. In-situ XAS studies are correlated with CV data to show how mixing Ni-oxides with Co or Fe can stabilize or inhibit (respectively) formation of Ni in the 3+ oxidation state. These observations indicate that the OER enhancement from Ni-Fe-oxides is a result of charge-transfer from Fe to the Ni active sites, thus providing a lower (non-integer) Ni oxidation state. The resulting Nix+ (2<x<3) surface sites have been shown experimentally to form more amorphous surface structure5 and presumably behave as more reversible active sites. It is likely that the amorphous structure stabilizes the active oxy-hydroxide network, thus stabilizing the OER intermediates and decreasing the OER on-set potential. In addition, the more reversible Nix+ (2<x<3) active sites enable high turn-over-frequencies by decreasing the binding energy of the Ni-O2 adsreaction product. Acknowledgements: Authors acknowledge the financial support from Proton On-Site as part of an ARPA-E grant and use of the National Synchrotron Light Source (NSLS) (beamline X3B), Brookhaven National Lab (BNL).