There has been a renewed interest in aqueous rechargeable metal-air batteries such as Li-air, Zn-air, Al-air, and Mg-air recently [1]. While most of the research on the air electrode of these batteries has focused on carbon-based electrodes, carbon can be severely attacked by the highly reactive oxygen generated from the oxygen evolution reaction (OER) during charging, which limits the battery cycle life [2,3]. To overcome this obstacle, it is imperative to develop carbon-free bifunctional air electrodes that exhibit improved stability and durability under charging and discharging.

Among different strategies adopted to eliminate carbon, utilization of an electrically-conductive, bifunctional oxides as both support and catalyst [4] to form an unsupported electrode bonded by a polymer binder (e.g. PTFE) has shown promising results. In addition to its decent electrical conductivity, the NiCo₂O₄ spinel has long been known to be an active bifunctional catalyst for both OER and oxygen reduction reaction (ORR) in alkaline media. Even though unsupported, PTFE-bonded, bifunctional NiCo₂O₄-based electrode has been explored by several research groups [5,6], none of these studies reported the cycle life. Furthermore, the effect of the NiCo₂O₄ crystallite size on the electrode performance has not been assessed.

In this study, the NiCo₂O₄ powders with different crystallite sizes were synthesized using two distinct processing routes. Carbon-free, unsupported, NiCo₂O₄-based air electrodes were prepared using these spinel powders and their electrochemical performance and cycle life were evaluated. For the first time, the dramatic effect of NiCo₂O₄ crystallite size on the cycle life was demonstrated and the promise of the carbon-free air electrodes for rechargeable metal-air battery application was highlighted.

Fig. 1. shows the XRD of the single phase NiCo₂O₄ catalyst powders with different particle size synthesized using the thermal decomposition (TD) and glycine-nitrate process (GNP) method. Fig. 2 compares the cycle life performance of the different electrodes measured at 50 mA.cm^-2. The carbon-free electrode with the 190-nm TD powder had a very short cycle life of about 30 cycles. However, the cycle life of the unsupported NiCo₂O₄-based electrodes was drastically increased, when the GNP powder with a smaller crystallite size (11-nm) was utilized. The cycle life of the NiCo₂O₄-based electrode prepared with the 11-nm spinel powder is still lower than that of the carbon-based electrode catalyzed by the same GNP powder. However, the inevitable degradation of the electrode due to carbon corrosion was completely eliminated and further improvement in cycle life of the carbon-free electrode is highly feasible.

References

[1] M. A. Rahman, X. Wang and C. Wen, J. Electrochem. Soc., 160, A1759 (2013).

[2] P. N. Ross, M. Sattler, J. Electrochem. Soc., 135, 1464 (1988).

[3] S. Velraj, J.H. Zhu, J. Electroanal. Chem., 736, 76 (2015).

[4] W.L. Fielder, J. Singer, NASA Technical Memorandum # 100947, Sept. 1988.E. J. M. Yeager, Catalysis, 1986, 38, 5.

[5] S. W. T. Price, S. J. Thompson, X. Li et al, Wills, J. Power Sources, 259, 43 (2014).

[6] M. Yuasa, H. Imamura, M. Nishida, T. Kida, K. Shimanoe, Electrochem. Commun., 24, 50 (2012).