Flow batteries are receiving wide attention for electrochemical energy storage that can be combined with renewable energies due to their perfect combination of high efficiency, high reliability and long cycle life.1 Alkaline zinc-iron flow battery owns the characteristics of low cost and easy upscale, together with two-electron-redox properties resulting in high capacity.2 However, further work on this type of flow battery has been broken off, because of the technological reasons, e.g. poor coulombic efficiency (76%) and electrochemical efficiency (61.5%) at low working current density (35 mA cm-2) because of zinc electrode etc.3 And since then, the alkaline zinc-iron flow battery has been rarely reported. Here we present an alkaline flow battery system that uses earth-abundant zinc and iron redox pairs as redox couples in combination with self-made, low cost membranes and 3D carbon felts electrode. The alkaline zinc-iron single cell with a coulombic efficiency of 99.49%, an energy efficiency of 82.78% is demonstrated at a current density of 160 mA cm-2, along with a stable long term cycling capacity (more than 150 cycles) and a well-defined open cell voltage of 1.83 V at 50% SOC. The performance is up to now the highest value for reported flow battery system. Most importantly, the practical application of this battery system is well confirmed by assembling a kW stack, affording an average CE of 98.84%, an average EE of 84.17% and an average output power of 1.127 kW at the current density of 80 mA cm-2. These fabulous results indicated that the alkaline zinc iron flow battery shows very promising prospect for stationary energy storage applications.
1. W. Lu, Z. Yuan, Y. Zhao, H. Zhang, H. Zhang and X. Li, Chem. Soc. Rev., 2017, 46, 2199-2236.
2. K. Gong, X. Ma, K. M. Conforti, K. J. Kuttler, J. B. Grunewald, K. L. Yeager, M. Z. Bazant, S. Gu and Y. Yan, Energy Environ. Sci., 2015, 8, 2941-2945.
3. J. McBreen, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1984, 168, 415-432.