Graphene Oxide Membrane Fuel Cells: Utilizing of a New Class of Ionic Conductor

Nafion, is the most common electrolyte for polymer electrolyte membrane fuel cells (PEMFCs) due to its high proton conductivity. However it has several disadvantages like high methanol permeability, loss of mechanical stability at temperatures > 100 °C and high cost,¹ which impede PEMFC commercialization at large scale. New alternatives with adequate proton conductivity are needed.

Graphene has been utilized in PEMFC electrocatalysts due to its high electronic conductivity, large surface area, and wide electrochemical potential window.² Graphene oxide (GO) is functionalized with e.g. epoxy and hydroxyl groups, and offers an interesting alternative electrolyte for PEMFCs, since it is strong, electronically insulating, gas tight, yet water-permeable.^3,4

We prepare free-standing GO membranes by vacuum-filtration onto Millipore filters, and investigated their suitability as alternative electrolytes in PEMFCs. We characterize their chemical, structural and mechanical properties. We measure their conductivity (by impedance spectroscopy), and use them as fuel cell membranes.

X-ray diffraction showed higher interlayer spacing compared to graphite due to the oxygen groups acting as spacers, thus also allowing incorporation of inter-laminar water between the GO sheets. Our GO membranes have nearly the double tensile strength of Nafion (although with higher stiffness). The water uptake is much higher, which could lead to a better water-retention capability under fuel cell conditions.⁵ Surface roughness analysis by 3D laser microscopy (Fig. 1) showed that GO has higher surface roughness compared to Nafion, which may potentially result in better interaction between the electrocatalyst and the membrane and therefore better fuel cell performance.

The conductivity was found to be ~1.5 mS/cm at 70 °C and showed a strong dependence on humidity and temperature, due to a Grotthuss-like water-mediated transport mechanism. Additionally, the capacitance decreased with decreasing humidity, due to loss of water, which acts as a dielectric spacer between GO sheets.

Membrane electrode assemblies, with an active electrode area of 0.5 mm² and 0.3 mg_Pt/cm² catalyst loading, were prepared by spraying catalyst ink (Pt/C 46.2 wt% Pt) followed by a hot-pressing process. The resulting device is essentially a graphene oxide membrane fuel cell (GOMFC). The maximum power density achieved for a GOMFC using a 49 µm thick membrane was ~6 mW/cm² at 30 °C and 100 % RH. A high OCV of 0.99 V indicates low fuel crossover and the pure protonic conducting nature of these GO membranes. However the performance decreases rapidly with increasing temperature (from 30 to 40 °C) from ~ 5.5 to 1.5 mW/cm². This is very likely to be due to a loss of oxygen-groups on the GO surface; after the measurements, the membrane showed a clear change in color on the anode side. Post-measurement XPS analysis confirmed a reduction on oxygen content of the membrane (anode 8.6 at% and cathode 15.4 at% oxygen, compared to 20.9 at% before measurement), accelerated by exposure to hydrogen fuel.

The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

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