Optimizing performance of ultra-thin gas diffusion layer-less PEMFCs via tailored transition layer thickness between catalyst and metal foam

Abstract The drawbacks of conventional flow channel-rib flow fields and gas diffusion layers (GDLs) severely restrict mass transport and water management in proton exchange membrane fuel cells (PEMFCs), thereby limiting their volumetric power density. Our previous study proposed an ultrathin GDL-less PEMFC that uses metal foam to replace traditional flow fields and GDLs, significantly reducing mass transport distance and cell thickness while enhancing volumetric power density. To ensure contact and transition between the catalyst layer and metal foam, an ultrathin carbon nanofiber film (CNFF) is employed in this structure. This study systematically investigates the effect of CNFF thickness on the performance of ultrathin GDL-less PEMFCs. Results demonstrate that the protective effect of CNFF on the catalyst coated membrane (CCM) is strongly correlated with its thickness. Specifically, thinner CNFF offers less protection to the catalyst layer, resulting in an 30% difference in electrochemical active surface area (ECSA). A moderate increase in thickness reduces ohmic overpotential and enhances Knudsen diffusion within the oxygen catalyst layer, while excessive thickness leads to a decrease in oxygen molecular diffusion. Additionally, thicker CNFF provides better water storage and more effective water management under medium current densities, although performance degrades at ultrahigh current densities. Overall, the 25-μm CNFF balances these various factors to achieve the best integrated performance. These findings highlight that the optimal performance of GDL-less PEMFCs can be achieved by regulating the thickness of CNFF.