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 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 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).

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