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

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 Ni 2+/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 Ni x+ (2<x<3) surface sites have been shown experimentally to form more amorphous surface structure 5 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 Ni x+ (2<x<3) active sites enable high turn-over-frequencies by decreasing the binding energy of the Ni-O 2 ads reaction 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). References: (1) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T.; Zhuang, L. Energy & Environmental Science 2014 , 7 , 3135. (2) Xiao, L.; Zhang, S.; Pan, J.; Yang, C.; He, M.; Zhuang, L.; Lu, J. Energy & Environmental Science 2012 , 5 , 7869. (3) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. Journal of the American Chemical Society 2013 , 135 , 8452. (4) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Nat Commun 2013 , 4 . (5) Louie, M. W.; Bell, A. T. Journal of the American Chemical Society 2013 , 135 , 12329. (6) Arruda, T. M.; Shyam, B.; Lawton, J. S.; Ramaswamy, N.; Budil, D. E.; Ramaker, D. E.; Mukerjee, S. The Journal of Physical Chemistry C 2009 , 114 , 1028. Figure 1