Metal-organic framework for high-performance catalyst layers in proton-exchange membrane fuel cells

Proton (H⁺)-exchange membrane fuel cells (PEMFCs) are considered effective power sources for fuel cell electric vehicles because of their clean emission, air-purification capability, and excellent durability. However, the performance degradation at low relative humidity (RH) remained to be addressed. Water back-diffusion, which refers to the diffusion of water from the cathode to the membrane by a concentration gradient, could lead to a remarkable improvement in the H⁺ conductivity of membrane/electrode assemblies (MEAs) at low RH. This effect can be achieved by introducing a functional material with a high water content into the cathode catalyst layer. Herein, we propose a Zr-based metal-organic framework (MOF; UiO-66(Zr)-(COOH)₂) as a water-rich additive material in the MEA and demonstrate that it increases the power performance of PEMFCs at low RH by promoting water back-diffusion. The high compositional and structural tunability of MOFs will expand the pool of materials to adjust the humidity level of PEMFCs.