Sterically Bulky Asymmetric Benzophenone Derivatives as Negolytes in a Non-Aqueous Redox Flow Battery

To increase renewable energy generation to the electrical grid, intermittent energy sources, such as wind and solar energy, need a long-term, large scale energy storage method. Redox flow batteries (RFB) are an attractive solution as they allow for long term storage in various redox states of an organic or inorganic compound. The ideal redox-active compound for a RFB should have a high energy density, which can be achieved through a large potential difference in redox events, high concentration of charge carrier, or by increasing the number of electrons transferred during a given redox event. In particular, nonaqueous RFB (NRFB) are promising for building a system with a large potential difference for redox events due the large electrochemical solvent window (~5 V for acetonitrile) as compared to water (~2 V). While metal complexes featuring redox active ligands have been thoroughly investigated as carriers in NRFBs, they are often too expensive for widespread use. This has led to increasing interest in organic carriers. Benzophenone is a promising candidate for an organic negolyte due to its high solubility in acetonitrile and the presence of a reversible reduction at -2.1 V vs. Ag/Ag+. Unfortunately, the durability of benzophenone during cycling is low. Here, we discuss the degradation products of benzophenone during cycling and show how the incorporation of sterically bulky groups ortho to the benzophenone carbonyl improves durability. We then evaluate the utility of these benzophenone derivatives as negolytes in a NRFB flow cell.