Graphene-Based Electrolyte Membranes with Low Hydrogen Permeability
Authors: Lyth Stephen Matthew , Bayer Thomas , Sasaki Kazunari , CAN Enes Muhammet , Cunning Benjamin , Nishihara Masamichi , Mufundirwa Albert 
Published: December 2024

Electrolyzers and fuel cells are promising candidates for sustainable conversion of electricity to hydrogen and hydrogen to electricity, offering an energy storage solution with high efficiency and low environmental impact. However, the problem of hydrogen crossover in the polymer electrolyte membrane (PEM) presents a particular challenge, limiting the efficiency and ultimately hindering their widespread adoption. Meanwhile, graphene is a well-known nanomaterial that is reportedly practically impermeable to gases. Here, we present innovative approaches to tackle the issue of hydrogen crossover by using graphene in different forms and different cell architectures. First, we will explore pure graphene oxide (GO) as a new electrolyte material in both acid and alkaline configurations. Then we will look at thin films of GO sandwiched in Aquivion membranes as a gas barrier layer. Finally, we will demonstrate that monolayer graphene is an effective gas barrier layer when sandwiched in Nafion membranes.

Similar to the case of graphene, GO also exhibits remarkable materials properties including high strength, excellent hydrogen gas barrier properties, hydrophilicity, and proton conduction assisted by acidic functional groups. These factors potentially make GO an attractive material for electrolyte membranes. We demonstrate the preparation of pure GO membranes via vacuum-filtration, and measure hydrogen permeability (2x10-2 barrer) three orders of magnitude lower than conventional Nafion membranes (30 barrer). Furthermore, we observe a significant anisotropy in ionic conductivity, attributed to the lamellar structure of GO. The resulting fuel cell power density was relatively low, due to limited conductivity (0.3 mS cm-1). To compensate for this, extremely thin (3 µm) electrode-supported GO membranes were deposited via spray deposition, resulting in improved fuel cell power density up to 79 mW cm-2. This showcases the potential of GO-based membranes in enhancing fuel cell performance.

Furthermore, we present a novel class of anion exchange membrane (AEM) fabricated from KOH-modified multilayer graphene oxide paper. As with the case of acidic electrolyte membranes, these also display high tensile strength and low gas permeability. A hydroxide ion conductivity of 6 mS cm-1 was recorded and confirmed using ion blocking layers, and attributed to a water-mediated reverse Grotthuss-like mechanism. An alkaline fuel cell was assembled but a relatively low proton conductivity of 1 mW cm-2 was obtained, attributed to reaction with CO2 in the oxidant gas supply.

The above studies are promising, but the low ionic conductivity of pure graphene limits the ultimate achievable fuel cell power densities. Therefore, we explored the use of ultrathin GO layers (100 nm) sandwiched in Aquivion PEMs to combine the high ionic conductivity of Aquivion with the gas barrier properties of graphene oxide (as well as CeO2 as a radical scavenger). These novel multilayer sandwich PEMs were deposited via spray deposition, resulting in just 10 µm total thickness. This method resulted in extremely high-power densities (1.6 W/cm2) and very low hydrogen crossover current density (1 mA/cm2). As such the concept of adding hydrogen blocking interlayers is shown to be highly effective.

Finally, we investigate the potential of monolayer graphene as a barrier to hydrogen crossover in PEMs, since graphene reportedly presents minimal resistance to hydrogen ion transport through the basal plane. Graphene grown via chemical vapor deposition on a copper substrate was coated with Nafion via spray deposition. The copper was removed via etching in acid and then a second layer of Nafion was sprayed onto the freshly exposed graphene surface, forming a sandwich PEM. Again, this resulted in significantly reduced hydrogen crossover current, whilst having minimal impact on the power density.

In conclusion, graphene is an ideal material for incorporation into PEMs to minimise hydrogen crossover. This has the potential to enhancing the performance and durability of fuel cells and electrolysers.

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