Directly integrated membrane-electrode assembly with a macroporous-carbon functional layer for the flexible operation of fuel cells under varying humidity

In this study, a directly integrated membrane-electrode assembly (MEA) with porous-carbon functional layer (PCFL) was developed. The PCFL contains Nafion ionomers as binder to enable the flexible operation of fuel cells under diverse humidity conditions. The direct integration of PCFL onto the catalyst layer (CL) of the MEA was fabricated by using a spray-coating process that avoids the need for high-temperature annealing, which is required to form a conventional microporous layer. This approach results in a reduction of the interfacial contact resistance between the CL and the single-layer-type gas-diffusion layer (S-GDL). Furthermore, the PCFL-MEA exhibits enhanced water retention, which reduces the detrimental impact of membrane and ionomer dehydration during low-humidity operating conditions. Introducing an additional PCFL with 1-μm-sized macropores (an mPCFL) creates a gradient in the pore-size profile from the CL to the GDL. This facilitates water drainage and minimizes oxygen-transfer resistance under high-humidity conditions while maintaining the water-retention capability under low-humidity conditions. The optimized MEA, which consists of 50 μm PCFL and 30 μm mPCFL, exhibits superior performance, achieving maximum power density that is 31.3 % higher at 50 °C and 35 % relative humidity (RH) (11.4 % higher at 70 °C and 100 % RH) than that of a reference MEA with conventional double-layer GDL.