Electrolysis energy efficiency of highly concentrated FeCl2 solutions for power-to-solid energy storage technology

Abstract

An electrochemical cycle for the grid energy storage in the redox potential of Fe involves the electrolysis of a highly concentrated aqueous FeCl2 solution yielding solid iron deposits. For the high overall energy efficiency of the cycle, it is crucial to maximize the energy efficiency of the electrolysis process. Here we present a study of the influence of electrolysis parameters on the energy efficiency of such electrolysis, performed in an industrial-type electrolyzer. We studied the conductivity of the FeCl2 solution as a function of concentration and temperature and correlated it with the electrolysis energy efficiency. The deviation from the correlation indicated an important contribution from the conductivity of the ion-exchange membrane. Another important studied parameter was the applied current density. We quantitatively showed how the contribution of the resistance polarization increases with the current density, causing a decrease in overall energy efficiency. The highest energy efficiency of 89 ± 3% was achieved using 2.5 mol L−1 FeCl2 solution at 70 °C and a current density of 0.1 kA m−2. In terms of the energy input per Fe mass, this means 1.88 Wh g−1. The limiting energy input per mass of the Fe deposit was found to be 1.76 Wh g−1. Graphical abstract

Back to the future with emerging iron technologies

“…Suitable for long-term high-capacity energy storage technology. 40,41 Integrating iron chemical cycles within the iron industry and renewable energy storage systems for supporting the grid, as presented in Fig. 2, the potential of iron as a sustainable fuel source is highlighted.…”

“…Thermo-electrochemical cycles for grid energy storage and examples of thermo–electrochemical cycles based on the reduction of Fe 2+ ions in the ferrous chloride aqueous solution were patented 40 and presented by Luin and Valant. 41 In the reductive segment of this cycle, the grid energy is used to electrolyze concentrated FeCl 2 solution and deposit the metallic Fe on the cathode. The iron metal becomes the energy carrier and can be easily stored.…”