Review—Functionalized Graphene Oxide Membranes as Electrolytes

Proton exchange membrane fuel cells (PEMFCs) typically use Nafion®, which has many drawbacks, such as high cost, fuel crossover, and strenuous synthesis processes. As such, an alternative Nafion®-ionomer free proton conductor has drawn significant interest. Graphene oxide membrane (GOM) is a promising alternative due to its hydrophilic nature and attractive proton conductivity under humidified conditions. However, pristine GOMs have drawbacks, including fuel crossover, a high reduction rate of negatively oxygenated functional groups during fuel cell operation, and proton conductivity showing excessive orientation dependence. We focused on nanocomposite-GOM (N-GOM) based on PFSAs, hydrocarbon polymers, synthetic polymers, inorganic-organic polymers, biopolymers, metal-organic frameworks, and micro- and nano-engineered surfaces. GO nanosheets have outstanding dispersion rate and compatibility with ionomer matrices that can be functionalized by sulfonation, polymerization, phosphorylation, cross-linking, incorporated inorganic nanoparticles, and blending with matrix, microscale-nanoscale fabrication. The N-GOM exhibits high-performance fuel cells with improved proton conductivity, physicochemical properties, and low fuel crossover compared to Nafion®. For instance, SCSP/SF membranes with 3% functionalized GO (FGO) content displayed the highest conductivity of 26.90 mS cm⁻¹and the best selectivity (methanol) of 4.10 × 10⁵S cm⁻³at room temperature. Moreover, a new scalable, efficient chitosan (CA)-based composite membrane (CA/GO) was fabricated. In addition, surface-patterned nanostructures in thin films increased the PEMFC output power to 950 mW cm⁻², higher than 590 mW cm⁻²for non-patterned Nafion®. Finally, we report on the optimal composition ratio for each material of the N-GOM-based membrane. This review discusses the most crucial developments in proton conductivity and outlines the current progress for the N-GOM as a revolutionary form of PEM. The general objective of this research is to review all possible modifications of N-GOM from the perspective of their practical application as electrolytes in fuel cells.