Development of Stable Bifunctional Air Electrodes for Zinc-Air Flow Batteries
Authors: Pichler Birgit , Weinberger Stephan , Reščec Lucas , Hacker Viktor 
Published: February 2020

The increasing installation capacities of renewable but fluctuating electricity generators such as wind and solar power systems have raised the need for energy storage with short response times. Out of the many different types of electrochemical energy storage systems, the stationary zinc-air flow battery is promising due to its use of the safe, abundant and inexpensive active storing metal zinc [1]. In this system the zinc is dissolved into the flowing alkaline electrolyte during discharge and can be stored as zincate ion outside the electrochemical cell (see schematic in left figure). It is redeposited during charging, whereby compact zinc morphologies are preferred in order to avoid internal short-circuiting and loss of active material [2]. As in other metal-air batteries, the reactions of the second electrode are the oxygen reduction reaction (ORR) during discharging using oxygen from the air and the oxygen evolution reaction (OER) during charging [1,3].
In practical applications, the recharge capability of the zinc-air cell is dependent on the compact zinc deposition and the mechanical and chemical long-term stability of the air electrode. Furthermore, the low cycling efficiencies of around 50% resulting from the slow ORR/OER reaction kinetics are challenging to overcome and rely on the activity of the chosen catalysts. In air electrodes non-precious metal oxides such as perovskites (e.g. La0.6Sr0.4Co0.2Fe0.8O3, LaNiO3) and spinels (e.g. NiCo2O4) can be utilized, due to their chemical stability in alkaline electrolyte [3,4]. Although many oxide catalysts exhibit bifunctional catalytic activity toward ORR and OER, they are often not equally active toward both reactions. Hence, a combination of two catalysts in one electrode is beneficial, as was investigated in this work. Furthermore, their implementation into a stable and porous electrode structure with balanced gas diffusivity and electrolyte wettability is important in order to obtain a high number of three-phase zones between gas, electrolyte and catalyst. This was achieved by selecting suitable additives such as carbon nanofibers and nickel powder for electrical conductivity and PTFE as hydrophobic binder material [4]. These materials need to be resistant toward the highly oxidative potentials of around 2 V vs. Zn/Zn2+ and the mechanical stress by the vigorous oxygen bubble formation during charging. In addition, nickel foam was found to be an applicable current collector material, which functions at the same time as mechanical backbone of the air electrode.
In-house manufactured electrodes were electrochemically characterized in a half-cell set-up in 8 M KOH electrolyte with the addition of 0.5 M ZnO, whereby one air electrode was operated as working electrode and a second air electrode as counter electrode. They were referenced against zinc foil. Discharging was performed at a constant current density of 50 mA cm-2 for 1 h per cycle. Because of the beneficial effects for the deposition of compact zinc layers, charging was conducted with the pulse charging method by applying tripled current densities during a short pulse of 50 ms followed by a pause of 100 ms (24 000 pulses in 1 h). The long-term cycling experiments, as depicted in the right figure, were performed for several hundred hours until the lower cut-off potential of 0.8 V vs. Zn/Zn2+ was reached. Although a decrease of ORR discharge potential of 0.1 V down to 1.0 V was observed within the first 50 hours of cycling, the OER potentials during charging were stable throughout the measurement. All in all, constant long-term performances over the course of more than 200 cycles with maximum charge/discharge potential gaps (ΔV) of 1.03 V were achieved [4].
[1] J. Fu, Z.P. Cano, M.G. Park, A. Yu, M. Fowler, Z. Chen, Electrically Rechargeable Zinc-Air Batteries: Progress, Challenges, and Perspectives, Adv. Mater. (2016).
[2] C. Zelger, J. Laumen, A. Laskos, B. Gollas, Rota-Hull Cell Study on Pulse Current Zinc Electrodeposition from Alkaline Electrolytes, Electrochim. Acta. 213 (2016) 208–216.
[3] L. Jörissen, Bifunctional oxygen/air electrodes, J. Power Sources. 155 (2006) 23–32.
[4] B. Pichler, S. Weinberger, L. Reščec, I. Grimmer, F. Gebetsroither, B. Bitschnau, V. Hacker, Long-term behavior of bifunctional air electrodes with high stability during pulse charging for zinc-air flow batteries (submitted).

Figure 1