Quantifying Chlorine Gas Evolution from Mixed-Acid Vanadium Redox Flow Batteries

Mixed-Acid (MA) Vanadium Redox Flow Batteries (VRFB) improve on standard VRFBs’ energy density via the addition of hydrochloric acid (HCl) to the standard sulfuric acid (H₂SO₄) electrolyte. This improves the stability of the vanadium (V) in the solution, which allows for electrolytes with higher concentrations of V ions and increased operating temperatures. However, the addition of HCl to the system introduces a new safety risk, the evolution of chlorine gas (Cl₂) from the catholyte.<br/> In this study, we cycled an MA VRFB under a range of conditions to understand the fundamental causes of gas evolution. We constructed a single cell MA VRFB using a standard electrolyte composition of 2M H₂SO₄, 5M HCl and 2M V, with a gas analyzer sampling the head space of the catholyte chamber. Cl₂ gas quantity was measured under systematically varied cycling rates and potential holds. The mechanism of gas evolution was determined to be an electrochemical side reaction that occurs when charging the cell beyond 1.5 volts. Depending on the duration the cell was held at high voltage and the maximum voltage, headspace Cl₂ concentrations between 1 and 4.2% were observed. Overcharging the cell beyond 1.7 volts (the standard VRFB upper limit) generated gas concentrations near 10%. These concentrations represent a significant safety risk that may occur during system operation and need to be mitigated.<br/> Previously it was assumed that the 360 millivolts potential difference between the V⁴⁺/V⁵⁺ (1.00 volts vs SHE) charging reaction in the posolyte and the 2Cl^-/Cl₂ (1.36 volts vs SHE) gas evolution reaction would prevent Cl₂ gas evolution through electrochemical means. We will discuss how solution chemistry and cell inefficiencies make Cl₂ generation a viable side reaction when charging a cell above 1.5 volts. This new understanding of gas evolution will aid development of safer MA VRFB systems that mitigate Cl₂ formation during operation.<br/> <br/> <br/>Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.