Use of Polarization Curves to Determine Kinetics Parameters in K–O2 batteries

Abstract

Metal–O2 batteries have shown great potential as energy storage devices and are foreseen as one of the potential successors to lithium ion batteries. Unlike either batteries or fuel cells, however, metal-O2 batteries form an insoluble product that diminishes cathode capacity. Cathode capacity is related to the electrochemically-accessible surface area and product accumulation results in a loss of that surface area, which then contributes to the total overpotential during discharge. While typically used to determine the effect of varying load levels on the performance of energy producing devices (i.e. batteries, fuel cells), polarization, or current-potential, measurements are also an indirect measure of the different energy losses that contribute to the total overpotential associated with a particular redox process. Each region of the curve is affected by a specific overpotential that is dominant within a certain potential range. Polarization curves therefore allow for the determination of certain properties that are important for understanding and improving the performance of electrochemical energy devices. As shown in Figure 1, the short steep, but curving, slope at the start of the current-potential curve is attributed to the activation overpotential, which is the energy required to overcome the potential barrier associated with the discharge reaction. The overpotential responsible for the long sloping line is attributed to the ohmic resistance, which is dominated by electrolyte resistance. Finally, the limiting current—sharp decline in the cell potential at higher current densities—is generally attributed to mass-transfer limitations. Using derived analytical expressions, essential kinetic parameters have been determined for fuel cells from the data obtained from polarization analyses.1,2 Proceeding from that previous work involving fuel cells, analytical expressions were derived that can be applied to the data obtained from polarization analyses of metal-O2 batteries. Due to the unique single-electron redox process (K+ + O2 + e− → KO2) in the cathode (oxygen) electrode, the K–O2 superoxide battery provided an ideal case for developing and evaluating those expressions.3,4 Polarization analyses were performed on K–O2 batteries and the data obtained was evaluated using the derived expressions to determine kinetic values essential to the operation and performance of the K–O2 battery.