Eu-based anolytes for high-voltage and long-lifetime aqueous flow batteries

Aqueous flow batteries (AFBs) are among the most promising electrochemical energy storage solutions for the massive-scale adoption of renewable electricity because of decoupled energy and power, design flexibility, improved safety and low cost. The development of high-voltage AFB is, however, limited by the lack of stable anolytes that have low redox potential. Here we report Eu-based anolytes for high-voltage pH-neutral AFB applications. Eu 3+ has a reduction potential of −0.39 V vs. SHE, which can be dramatically lowered when forming stable complex with inexpensive organic chelates. A typical complex, EuDTPA, features a low redox potential of −1.09 V vs. SHE, fast redox kinetics, and a high water solubility of 1.5 M. When paired with ferrocyanide, the battery had an open-circuit voltage of 1.56 V and demonstrated stable cell cycling performance, including a capacity retention rate of 99.997% per cycle over 500 cycles at 40 mA cm −2 , a current efficiency of >99.9%, and an energy efficiency of >83.3%. A high concentration anolyte at 1.5 M exhibited a volumetric capacity of 40.2   Ah L −1 , which is one of the highest known for pH-neutral AFBs, promising a potent solution for the grid-scale storage of renewable electricity.