Because of their higher theoretical energy density, lower toxicity, less expensive electrode materials and non-affinity to thermal runaway like e.g. Li-based systems, metal/air batteries such as Al-Air and Zn-air are potential candidates for sustainable energy storage applications. However, large scale commercialisation is hindered by some inherent drawbacks related to the metal electrode such as poor reversibility, formation of passive layer such as Al(OH)
3, shape change and dendrite formation as well as low energy efficiency due to high overpotential and carbonate precipitation at the air electrode in alkaline electrolyte
[1, 2]. In this context, recent developments in room temperature ionic liquids (IL) based electrolytes such as 1-Ethyl-3-methylimidazolium (EMIm) Trifluoromethane-sulfonate (Triflate) open new perspectives especially with respect to suppression of zinc dendrite formation as well as of parasitic reactions such as hydrogen evolution
[3]. Most of the ILs, however, are highly viscous and consequently have a very low ionic conductivity that drastically limits reaction kinetics. Other crucial aspects are related to the low surface tension of pure ILs compared to water that commonly hinders 3-phase boundary formation and to the absence of proton donator for oxygen reactions in aprotic solutions. Addition of water to ILs seems to be a promising strategy to overcome these major obstacles. Development of highly active and stable bifunctional air electrode catalysts is still challenging as well. Some perovskites have already exhibited high activity for both oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during charge in alkaline solutions
[4, 5]. By nature most of the perovskite oxides are poor electronic conductors, so that addition of more conductive materials such as carbon/graphite is necessary, particularly for ORR. For long-term stability of the gas diffusion electrode, highly corrosion-resistant carbon material is required.
In this work, 5 commercial carbon materials have been tested regarding their stability in 7 M KOH by means of accelerated degradation tests (ADT): Ketjen Black EC 600 JD (AkzoNobel), Vulcan XC72R (Cabot), Super C65, HSAG 300 and KS6L (TimCal) with N2 BET surface-areas of 1325, 280, 250, 62 and 20 m2 g‑1, respectively. ADT procedure was carried out in a N2-saturated electrolyte and consisted of 4000 cycles in the potential range of -1.35 to +0.25 V vs. Hg/HgO between oxygen and hydrogen evolution at dE/dt=1V s−1. Carbon materials Super C65 and KS6L showed the highest stability. Super C65 has been chosen for GDE preparation for its lower density and better interconnection properties. La0.6Ca0.4CoO3 (LCCO) and Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) perowskite were synthesized by a sol/gel process and characterized by XRD and SEM. Optimization of catalyst/carbon ratio was performed by evaluation of electrochemical activity of catalyst/C65 systems towards ORR/OER by means of half-cell measurements with the rotating ring-disc electrode (RRDE) set-up. GDE fabrication was performed by calendering a powder/PTFE mixture onto a Ni current collector. Their charge/discharge cycling behaviours were evaluated under half-cell conditions in 7 M KOH and different IL/water mixtures in oxygen and synthetic air.
Literature:
[1] M. Xu, D.G. Ivey, Z. Xie, W. Qu; Journal of Power Sources 283 (2015) 358
[2] Z. Zhang et al., Journal of Power Sources 251 (2014) 470
[3] Z. Liu, S. Zein El Abedin, F. Endres; Electrochimica Acta 89 (2013) 635
[4] J. Suntivich et al., Science 334 (2011), 138
[5] X.-Z. Yuan, W. Qu, J. Fahlman, D. G. Ivey, X. Zhang; ECS Transactions, 53 (2013) 265
