A Re-Examination of Acid-Base Flow Batteries

Abstract

The need for large scale energy storage is becoming more and more apparent as renewable energy sources such as wind and solar power gain a larger fraction of the electricity generation market. Redox flow batteries (RFBs) offer a promising chance to address that need at an acceptable cost, and the market for RFBs is expected to grow dramatically in the near future. However, none of the current flow battery technologies under development are ideal; generally, one or more deficiencies are found in areas of safety, cost, durability or efficiency. One concept for large scale energy storage that we are considering is an acid-base flow battery. In this device, the cell potential is developed from the proton concentration difference between a strongly acidic electrolyte (positive) and a strongly basic electrolyte (negative). Both electrodes employ the hydrogen oxidation/hydrogen evolution reactions. As hydrogen is evolved at one electrode, it is consumed at the other, thus only a small volume of hydrogen is required; the energy is stored in the changing concentrations of the acidic and basic electrolytes. Thus, an acid-base flow battery combines aspects of both a fuel cell/electrolyzer and a conventional redox flow battery. The advantages of this approach are 1) that no metal salts or redox active organic species are required, only the acidic and basic electrolytes, thus providing an advantage in terms of both cost and safety, 2) while the reactions require a catalyst, the kinetics are fast with relatively small amounts of catalyst required, and 3) unlike fuel cells and electrolyzers, strongly oxidizing conditions and the formation of peroxides are avoided, allowing for non perfluorinated membranes to be considered. The disadvantages include a relatively low cell voltage (≈0.8V) and the possibility of low coulombic efficiency due to reactant crossover, depending on the diffusion and migration of the active species (H+, OH-) through the membrane. The general concept of an acid-base flow battery is known (1,2), but relatively little work has been done in this area, and has not taken full advantage of the current state of the art in membranes, catalysis, and electrode/flow field designs. Significant advances have been made in these areas with regards to PEM hydrogen-air fuel cells, electrolyzers, and hydrogen/bromine flow batteries, which suggests that acceptable voltaic efficiency (over 80% at current densities greater than 100 mA/cm2) is possible, even with the relatively low cell voltage of this chemistry. As a result, a new investigation of the acid-base flow battery concept is warranted. In this presentation, we will focus on the coulombic efficiency of an acid-base flow battery using either cationic or anionic ion-exchange membranes and with various electrolyte compositions. An example is shown below in Fig. 1 for a cell with a Nafion 212 membrane. Initially both electrolytes were 1.5M Na2SO4. The cell was then charged at different current densities to a maximum of either 0.1M H+ / OH- or 1.0M H+ / OH-. The electrolyte volume was varied so that the same total time was required to reach full charge. The coulombic efficiency increases with increasing current density, suggesting that both migration and diffusion are significant factors in determining the coulombic efficiency. As expected, the efficiency is higher when lower concentrations of H+ and OH- are present at the end of charge. These conclusions suggest that an optimal concentration, that allows for high coulombic efficiency and an acceptable capital cost associated with the volume of electrolyte required may exist.