Development of semi-solid alkaline flow battery

Abstract

Flow battery is an attractive energy storage system due to its ability to decouple power and energy outputs, and its less stringent geometric requirement. However, commercial flow battery technology suffers from low energy density (~z40 Wh/Lcatholyte), high membrane cost (~$ 500/M 2 and species crossover. One elegant method to overcome all this problems is utilize flowable semisolid electrode. Although there have been several reports in the literature regarding performance of various semi-solid flow batteries, there is still lack of understanding on how to design the suspension electrode and its impact on flow cell design and performance. In this thesis, we emphasize on three main properties, namely stability, conductivity and flowability for high performing suspension electrode for Zn-Ni alkaline battery chemistry. Firstly, we recognized interplay of forces acting on particles in a suspension electrode and designed suspending medium with jammed Carbopol© microgel structures to prevent sedimentation of the large particles. Secondly, we determined percolation threshold of conductive additive and investigated its effect on electrochemical performance of the semi-solid electrodes in a closed static cell. We found that conductive additive concentration should be well above percolation threshold to obtain maximum discharge capacity of Ni(OH)2. Thirdly, based on the understanding from conductivity and stability of the suspension, we designed catholyte (~ 5 M Ni(OH)2) and tested its electrochemical performance in a static cell. We were able to achieve energy density of 137 Wh/Latholyte, about 3 times of commercial flow battery. We then assessed flowability of the suspensions. Catholyte and anolyte had yield stress of 204 Pa and 447 Pa respectively. We found that by choosing appropriate flow cell stack design energy loss due to pumping can be minimized to < 1% for continuous flow. Finally, flow cells were designed to test the electrochemical performance of these suspensions in an open system.