Exploring the polarization performance of the polysulfide redox couple in a reference electrode-equipped single electrolyte flow cell

Flow batteries that employ polysulfide species are often limited by their sluggish redox reaction kinetics. While a number of catalyst/electrode options have been explored to lower activation overpotentials, progress has been challenged by the need to disentangle electrode losses from full cell performance as well as by the breadth of electrode, electrolyte, and operation conditions pursued. Here, we systematically characterize electrode performance for the polysulfide redox reaction as a function of electrode material (carbon, nickel, and nickel-sulfide) and temperature (~25 to 70 °C) using a reference electrode-equipped single-electrolyte flow cell with a set electrolyte composition. Our results highlight asymmetry in the polysulfide redox couple, where the different electrodes perform similarly for the faster oxidation, but distinguish themselves for the slower reduction. Furthermore, elevated temperature benefits both reduction and oxidation rates of polysulfide, with the greatest improvements observed for reductive overpotentials. Nickel(-sulfide) electrodes are found to have superior catalytic performance to carbon-based electrodes, while the efficacy of different carbon electrodes appears to depend on their electrochemically-active surface area and surface chemistry. Our reference-electrode based investigations, complemented with full cell polarization and electrochemical impedance spectroscopy, collectively show that the nickel(-sulfide) electrodes are susceptible to sulfidization over time (especially at elevated temperature) positively influencing their catalytic behavior, but negatively affecting cell resistance and mechanical integrity. In contrast, flow cells employing carbon electrodes maintain their kinetic and ohmic characteristics throughout the investigation. Overall, the presented methods are particularly effective for assessing the combined roles of electrode and temperature on kinetically-hindered redox flow cells.