Effects of testing conditions on the performance of carbon-supported bifunctional electrodes

Carbon-based electrodes with a bifunctional catalyst are being studied extensively for use in rechargeable metal-air batteries. In this study, the effects of various testing conditions on the electrochemical performance, cyclic lifetime, and degradation mechanism of the electrodes prepared using the bifunctional La0.6Ca0.4CoO3 catalyst mixed with the graphitized Vulcan XC-72 carbon black and polytetrafluoroethylene were evaluated in detail. The electrochemical performance and total cycle life of the electrodes were not noticeably affected by the KOH electrolyte concentration or rest time between the charge/discharge. The charge/discharge current density used for the cyclic test significantly influenced both the total cyclic lifetime and degradation mechanism of the electrode. At the higher current densities (±50 and ± 100 mA/cm2), carbon corrosion led to electrode flooding and the loss of oxygen reduction capability while the oxygen evolution reaction occurring at the catalyst was largely sustained. At a lower current density (±6 mA/cm2), with periodic replacement of electrolyte to avoid carbonate formation in the electrode pores, the loss of the anodic catalytic ability of the bifunctional catalyst presumably via surface oxidation was clearly evident from the increase in oxygen evolution reaction overpotential. With the unchanged electrolyte, the electrode gas transport pore could be blocked by carbonate formation, leading to a decreased catalytic capability of the electrode for oxygen reduction. The failure modes of these electrodes under various cycling conditions were also assessed using impedance spectroscopy and inductively coupled plasma optical emission spectrometry.