Analysis of Performance Degradation and Durability of the Air Cathode in an Alkaline Fuel Cell

Abstract

Degradation of the air cathodes is one of the key issues affecting the lifetime and durability of alkaline fuel cells and metal-air batteries. To prevent the ingress of electrolyte in the air cathode, modifying the hydrophobicity and thickness of the AL has been reported [1,2]. It was reported that increased hydrophobicity/thickness of the AL resulted in a decrease in the air cathode performance [1,2]. In this work, experimental in-situ and post-test analyses of the air cathode were applied to investigate the mechanism of performance degradation. In-situ methods including performance / lifetime analysis using a half-cell setup [3, 4], polarization studies, and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical characteristics of the electrodes during operation. Post-mortem analysis methods included X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray computed tomography (X-ray CT), and Raman spectroscopy. The properties of pristine, conditioned, and failed air cathodes were characterized by these methods. Conditioning of air cathodes was performed by operating in half-cell utilizing 6 M KOH solution for 24 hours at 200 mA cm 2. The composition of the air cathode, operating conditions, and procedure for determining the performance and lifetime of the air cathode have been discussed in our previous work [3].Cross-sectional SEM-EDX analysis was conducted on the air cathode to determine the amount of flooding/penetration of the electrolyte inside the active layers for electrodes prepared with 15, 25, and 40 wt% PTFE in the AL. Air cathodes were operated in the half-cell battery at 200 mA cm -2. EDX maps of the air cathodes showed that increasing the PTFE content leads to increased hydrophobicity and decreased depth of KOH penetration inside the AL after 5 hours of operation. However, air cathodes with higher PTFE content of 40 wt% exhibited a lifetime of only 48 hours, compared to > 150 hours for those containing 15 wt% and 25 wt% (conducted in a limited test duration). This could be due to decreased electrolyte content in the AL or blocking of pore space and active reaction sites [5], leading to higher local current densities and more rapid degradation. Cross-sectional EDX analysis indicated that electrolyte penetration/flooding inside the macrostructure of the backing layer (BL) was not observed in any of the electrodes.XPS was performed to reveal further information of the chemical states of the conditioned and failed cathodes [6]. XPS analysis indicated changes in the surface functional groups, in particular increasing hydroxyl groups in the failed cathodes, which may be indicative of reduced hydrophobicity of the carbon support. XPS analysis also indicated other changes in the chemical states of the catalyst, oxygen, fluorine and carbon after conditioning and failure of the air cathode.In-situ galvanostatic EIS was conducted during long-duration and accelerated stress tests to determine ohmic resistance, charge transfer resistance, and mass transfer limitation. The EIS data indicates that mass transfer resistance increased significantly after failure of the air cathode, confirming that oxygen transport to the catalyst was the cause of the poor performance of failed cathodes. Raman spectra and mapping was carried out to obtain additional information about changes in the AL after degradation. X-ray CT were conducted on air cathodes to determine the changes of pore structure, distribution of catalyst, PTFE and potassium.