Proton exchange membrane fuel cells (PEMFCs) have the potential to be a vital part of a decarbonized future. However, a number of critical issues must be resolved before mass adoption. Hydrogen crossover, in which molecular hydrogen “crosses over” the membrane without dissociating, robs the cell of two electrons for each molecule that crosses over, degrading cell efficiency and lifetime.

As proton conduction in a PEMFC occurs through the Grotthuss mechanism, one strategy of increasing efficiency through crossover reduction is applying a material that has a pore size between the kinetic diameters of water (0.265 nm) and molecular hydrogen (0.289 nm). 2D materials are a promising class of candidates for this application, able to sieve molecules with angstrom scale selectivity.

This research focuses on a number of 2D materials grown by CVD, namely graphene, hBN, and MoS₂ as hydrogen barriers in a fuel cell. All materials were transferred to a commercial proton exchange membrane and tested for proton conductivity, hydrogen crossover, and electrochemical surface area (ECSA) to identify any effects on the catalyst. Furthermore, all membranes’ conductivities were assessed at relative humidities of 20%, 50%, 80% and 100%.

All the applied 2D materials show the ability to impede hydrogen crossover to varying degrees; however, many also affect the proton conductivity, with few exceptions. In particular, MoS₂ coated membranes showed superior performances, particularly at lower relative humidities.

Interestingly, the results also illustrate high areal proton transport in 2D materials that have been previously unexplored in literature, on the order of 20-40 S/cm². This exploration of 2D materials tested in-situ informs intelligent design of proton conductive hydrogen barriers with applications to fuel cells, electrolyzers and membrane electrode assembly-based equipment.

Isabel Caldino-Bohn

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