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
Highlights
The shift to renewable sources of energy is beginning to expose a fundamental weakness in the world’s electricity grids
We have studied the influence of the operating parameters on the electrolysis energy efficiency of the highly concentrated F eCl2 solutions to maximize it
We measured the FeCl2 solutions in the concentration range from 0.5 to 4.5 mol L−1 at 25 °C and obtained the curve with a shape typical for the inorganic salt solutions
Summary
The shift to renewable sources of energy is beginning to expose a fundamental weakness in the world’s electricity grids. Wind, wave, and other forms of renewable energy are all, to some extent, subject to the vagaries of factors like the weather [1]. This means that we cannot match the amounts of energy available with demand, because we do not have an effective way to store and release this energy when we need it. The energy is stored in the oxidative-reductive potential change of iron (Fe2+/Fe). This is obtained by the electrolytic reduction of Fe2+ from a highly concentrated FeCl2 (aq) electrolyte yielding metallic iron that deposits on the cathode. For the optimum energy efficiency of the technology, optimization of the electrolytic process is crucial
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