Gas Diffusion Layer Coated with a Microporous Layer Containing Hydrophilic CNTs to Enhance PEFC Performance without Humidification Using Anode Gas Recirculation

Abstract

Polymer electrolyte fuel cells (PEFCs) generally have external humidifiers to supply humidified fuel and oxidant gases, preventing dehydration of the membrane electrode assembly (MEA). However, if a PEFC could be operated without humidification, these external humidifiers could be removed, resulting in a simplified PEFC system with increased overall efficiency and reduced cost. One of the most important goals related to advancing the commercial viability of PEFCs has been the development of a high-performance PEFC that can operate without humidification. The humidification requirements of the anode and cathode are different. At the anode, humidified pure hydrogen gas that is not used for the electrochemical reaction can be recirculated, and it is therefore possible to remove an external humidifier. At the cathode, air is exhausted without recirculation and so fresh humidified air is supplied using an external humidifier. It is therefore important to find a method to enhance the performance of a PEFC that does not require humidification at the cathode. The present study was carried out to clarify the effect of anode gas recirculation on PEFC performance without humidification. The cell temperature was set at 75°C. Air utilization was set to 60% and the hydrogen flow rate supplied from the hydrogen tank was set so as to maintain a hydrogen utilization of 100%. The anode gas recirculation flow rate was varied between 0 and 100 cm3 min-1. The relative humidity (RH) of the cathode inlet gas was set at 0%. The RH was maintained at a very low value of 30% at the anode. The gas diffusion layer (GDL) used at the anode was a commercial SGL24BA GDL without a microporous layer (MPL). The MPL-coated GDLs were used at the cathode. The hydrophobic MPL consisted of an SGL24BA GDL coated with an MPL composed of 80 mass% carbon black and 20 mass% PTFE. The MPL containing hydrophilic carbon nanotubes (CNTs) was made from an SGL24BA GDL coated with an MPL composed of 4 mass% CNTs, 76 mass% carbon black and 20 mass% PTFE. The CNT surfaces were modified via an oxidation treatment. As a result, the contact angle of a CNT sheet was 30°, demonstrating that the CNT surfaces exhibited high hydrophilicity. Increasing the anode gas recirculation flow rate from 0 to 100 cm3 min-1 reduced the IR (ohmic) overpotential, which enhanced PEFC performance without humidification. Increasing the anode gas recirculation flow rate to more than 100 cm3 min-1 raised the gas velocity in the flow channel to over 2 m s-1. This is effective at promoting water transport from the anode gas to the MEA, thereby enhancing PEFC performance without humidification. In the case of the MPL without CNTs, decreasing the MPL mean flow pore diameter from 10 to 2 μm lowered the gas permeability. This effectively enhanced the ability of the MPL to prevent membrane dehydration, which improved PEFC performance without humidification. The PEFC performance obtained when employing the MPL with CNTs was higher than that for the MPL without CNTs. Because the pore diameter was constant at 2 μm both with and without CNTs, the difference in the gas permeability between these MPL-coated GDLs was negligible. Since the hydrophobicity of the MPL containing hydrophilic CNTs was reduced, its ability to retain humidity of the MEA was improved. This resulted in much higher PEFC performance compared with that obtained from the MPL without CNTs.Even when a PEFC is operated without humidification, it is also essential to prevent flooding due to water produced in the cell under high current density conditions. Therefore, oxygen transport resistances were also assessed based on the limiting current density values of polarization curves to evaluate the ability of the MPL-coated GDL to reduce flooding under high humidity conditions. In the case of an MPL without CNTs, decreasing the MPL pore diameter to 2 μm increased the water breakthrough pressure. This degraded the ability of the MPL to prevent flooding under high humidity conditions. The appropriate MPL pore diameter for enhanced performance was different under no and high humidity conditions. In the case of an MPL with CNTs, it was possible to decrease the MPL pore diameter to 2 μm without lowering PEFC performance under high humidity conditions. An MPL with CNTs allows much higher performance under both no and high humidity conditions compared to that obtained with a hydrophobic MPL-coated GDL. The authors would like to thank NITTA Corporation for supplying the hydrophilic CNTs used in this study.