During the past few years, PNNL invented several aqueous RFB systems, including mixed acid vanadium flow battery (VRB)and zinc-polyiodide redox flow battery (ZIB). It is well known that high charge/discharge rate (charge/discharge current density) is prone to generate high power density, but the energy efficiency (EE) would be significantly declined. The low EE means that more active species are required for identical energy output, which eventually results in increased cost. In order to increase EE especially at higher current density, much attention has been paid on the improvement of electrodes. Commonly, carbon-based materials are used as electrodes in both VRBs and ZIBs because they are readily available, highly stable, corrosion resistant, economical and conductive. However, they were proved to show poor kinetic reversibility. Although noble metal catalysts such as Pt, Au, Ir, and Pd were coated or dispersed onto the carbon surfaces so as to improve the electrochemical activity of active species, the high cost still restricts their commercial application. Here, we will present our development at PNNL on low-cost and highly active catalysts, such as metal (Bi), metal oxide (Nb2O5), carbon oxygen functional groups as well as metal-organic frameworks, used in VRBs and ZIBs, greatly enhancing the electrode performances. The corresponding mechanisms will be discussed in detail.
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