2, Advanced Battery Center, KAIST Institute for NanoCentury, Daejeon, , Korea (the Republic of)
Redox flow batteries (RFBs) are an exciting target for storing renewable energy at a large scale. Currently available commercial RFBs relying on vanadium as the redox material have suffered from high cost to be scale to a grid-level size. Such a challenge could be reduced if redox-active organic molecules could be used instead of vanadium. In addition, the design of redox molecules with various functionalities can tune the molecular properties, which eventually allows for improving the solubility and controlling the redox potentials to optimize their performance. However, there are very few candidates that could serve as organic redox materials and almost all of them act as a single-electron carrier. For increasing the energy density, the effective strategies are (1) increasing voltage gap as the redox materials reversibly respond more negative/positive potentials for negolyte/posolyte, respectively, (2) increasing solubility, and (3) designing redox materials capable of two (or more)-electron storage, which instantly doubles the energy density. Here we present some research results which have been done in my laboratory for the target of increasing energy density. We show a new class of organic material of naphthalene diimide (NDI) that can reversibly store two electrons at neutral pH in aqueous solution. By decorating with glycinate, the solubility of n-type organic semiconducting NDI was improved in aqueous solution. We studied the fundamental two-electron redox behavior from experimental and computational analyses and displayed a prototype RFB containing [K2-BNDI] negolyte and 4-OH-TEMPO posolyte with reasonable cyclability, energy efficiency and excellent stability. To improve the stability of redox materials, we also attempted to use redox-active organometallic molecules such as pseudo-octahedral Co-polypyridyl complexes, and developed rational strategies for enhancing the robustness, namely, the spin-crossover between low and high-spin states and the chelation effect emerging from replacing three bidentate ligands with two tridentate analogues.