A Conversion Type Iron Anode for High-Areal-Capacity Hybrid Redox Flow Batteries

Abstract

Redox flow battery (RFB) is one of the most promising technologies for energy storage owing to its advantages of high safety, high efficiency, and low cost.1 Hybrid RFBs such as Zn-based RFBs offer a much higher energy density compared to all liquid RFBs. However, they rely on metal dissolution/deposition electrodes, which suffer from severe dendrite issues and poor reversibility, especially at high areal capacity and high current density. These inherent challenges of metal electrodes preclude their practical deployment for large-scale and long-duration energy storage applications.To break from this intrinsic limitation, in this work, we exploited a high-loading conversion electrodes to replace metal deposition electrodes to achieve high cycling stability at high areal capacity/current density for hybrid RFBs. With this strategy, using Fe3O4/Fe(OH)2 conversion negative electrode as an example coupling with ferricyanide/ferrocyanide positive electrode, we demonstrated conversion type all-iron hybrid RFBs with an unprecedented high cycling areal capacity of 126.6 mAh cm–2 at 50 mA cm–2 for 200 cycles (1000 hours) and 215 mAh cm–2 at 60 mA cm–2 for 100 cycles (700 hours) without capacity decay.2 The conversion electrode eliminates the dendrite issues and limitations on metal areal capacity, demonstrating unprecedented high cycling areal capacity and current density compared to conventional dissolution/deposition zinc metal negative electrode. We will discuss the compositional/morphological stability of the Fe3O4/Fe(OH)2 conversion iron negative electrode during the hybrid RFB operation and the techno-economic analysis of developed conversion type all-iron hybrid RFB for practical applications. Acknowledgment: The work described herein was supported by a grant from the Research Grant Council (RGC) of the Hong Kong Special Administrative Region, China (project no. C1002-21G). Reference: (1) Yao, Y. X.; Lei, J. F.; Shi, Y.; Ai, F.; Lu, Y. C. Assessment methods and performance metrics for redox flow batteries. Nat. Energy 2021, 6 (6), 582.(2) Shi, Y.; Wang, Z.; Yao, Y.; Wang, W.; Lu, Y.-C. High-Areal-Capacity Conversion Type Iron-Based Hybrid Redox Flow Batteries. Energy Environ. Sci. 2021, 10.1039/D1EE02258J.