Efficient Oxygen Reduction Electrocatalysis at Nickel-Iron Mixed Oxide Coated Microelectrode Arrays
Authors: Doyle Richard L 
Published: February 2020

The development of efficient, abundant, and inexpensive oxygen reaction catalysts is essential for renewable energy research. The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) play critical roles in the development of important energy conversion and storage devices - fuel cells and metal-air batteries - and in the generation of hydrogen as a renewable fuel via water splitting [1,2]. While significant advances in both the mechanistic understanding of these reactions and catalyst design have been achieved in recent years [3], materials based on expensive rare earth metals such as platinum, iridium and ruthenium are still the most commonly used commercial catalysts [4]. Optimizing the oxygen reactions using low cost materials is therefore the key to maximising the commercial potential of these devices.
In this respect, the oxides of non-precious metals such as nickel and iron, are attractive catalyst candidates [5-7]. Their high abundance and low cost make them economically viable alternatives to state-of-the-art noble metal catalysts, and the formation of a range of mixed metal oxides provides practical routes for catalytic enhancement. In particular, the beneficial effect of Fe impurities on the OER activity of Ni hydroxides was reported over 25 years ago [8] and since then Ni-Fe based oxide catalysts have been shown to be some of the most active OER catalysts in alkaline media [5-7,9]. However, this synergistic relationship has yet to be fully explored for the ORR. Although several Ni-Fe based ORR catalysts have been developed [10-12], these materials typically contain Pt as the active component with Ni and Fe acting simply to reduce the Pt loading rather than as the active catalysts.
In this work, we focus on the development and electrocatalytic activity of a range of Pt free Ni-Fe based mixed oxide catalysts for the ORR. The catalyst layers are prepared electrochemically using a simple and scalable potential cycling methodology on macro-scale substrates and chip-scale microelectrode arrays. The effect of substrate and oxide composition on the stability and electrocatalytic performance of the mixed oxides is highlighted and the mixed oxides are shown to exhibit high stability and reusability under catalytic conditions. The electrocatalytic properties of the catalyst layers are examined using a combination of hydrodynamic voltammetry and electrochemical impedance spectroscopy allowing the determination of kinetically significant parameters including Tafel slopes, electrochemical rate constants and reaction orders. Finally, these experimentally determined kinetic parameters have been combined with COMSOL simulations to provide a detailed understanding of the kinetics of oxygen reduction at Ni-Fe oxides.
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