Mechanisms of Degradation of the Air Cathode in an Alkaline Fuel Cell

Abstract

Rechargeable zinc-air redox flow batteries have been reported as an energy storage technology that has high energy density, uses abundant low-cost raw materials, and has low environmental impact [1, 2]. Due to these characteristics, zinc-air batteries are attractive for both portable and smart-grid energy storage applications [1, 2]. However, the limited cycle life has been identified as a major problem for commercialization of zinc-air batteries [2].Flooding of the electrolyte within alkaline electrode pores is reported as one of the major causes of degradation and performance loss of the air cathode in these devices [1, 3]. To prevent the ingress of electrolyte in the zinc-air cathode, modifying the hydrophobicity and conductivity of the catalyst layer has been reported [4]. The ratio of carbon/binder has impact on ionic conductivity and gas transport in the electrode. In addition, it is observed that degradation of performance of the electrode without binder is faster than that with binder [4].In this study, degradation of the air cathode in the zinc air flow battery was investigated by measuring the variation in the cell voltage at a constant current density. The air cathode consisted of a sintered active layer (AL) composed of platinum nanoparticles supported on carbon (Pt/C), a carbon / PTFE backing layer (BL), and a nickel mesh current feeder. The experimental setup used a 6 M KOH electrolyte, with a zinc wire reference and nickel counter electrode. A control system for real time control of the operating temperature, pressure, and inlet air humidity was implemented to maintain stable operating conditions and record the data. Post-mortem (PM) analysis was conducted on the tested samples. The sample preparation method was carefully evaluated for the PM study to make sure that the KOH present in the pores is preserved during imaging.No significant degradation in the performance of the air cathode was observed after 148 hours at baseline operating conditions (current density of 200 mA cm-2 and 21% oxygen). In order to enable rapid evaluation of electrode durability, conditions for accelerated degradation of the air cathode were investigated. Under conditions of low oxygen stoichiometry (achieved by 50% drop in oxygen stoichiometry) and high current density (400 mA cm-2), the rate of degradation was accelerated and the lifetime of the air cathode was less than 24 hours. The lifetime of the air cathode was estimated based on significant increase in the potential loss ca. 0.3 V, while performing oxygen reduction from an air feed at baseline operating conditions. Polarization studies were performed on the air cathode after a significant performance loss during the accelerated degradation experiments. The results indicate that the air cathode degradation accelerated under conditions of oxygen mass transfer limitation. The increased potential loss is likely due to flooding of electrolyte inside the pores of the active layer of the electrode, which increases the mass transfer resistance for oxygen transport [5, 6].For evaluation of the impact of binder loading on air cathode degradation, three different loadings of polytetrafluoroethylene (PTFE) in the catalyst layer were used, and the performance and durability were compared. The morphology and microstructure of active and backing layers of the used air cathode was investigated by scanning electron microscopy (SEM). Cross sectional SEM images were carried out for PM analysis to investigate the penetration depth of KOH inside the active and backing layers after a significant potential loss was observed. Surface analysis of the active and backing layers were also conducted using Raman spectroscopy.