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Graphene oxide membrane (GOM) is an excellent proton conductor under humidified conditions and can be suited to various electrochemical devices, including proton exchange membrane fuel cells (PEMFCs). Conventional PEMFCs are based on Nafion®, a commercial perfluorocarbon sulfonic acid (PSA) ionomer that has many disadvantages, such as high cost, strenuous synthesis process, and fuel crossover. To minimize these deficiencies, a low-cost, alternative, environmental friendliness, and highly proton conductive membrane was synthesized. Here, GOM is considered the base electrolyte of PEM. However, the proton conductivity of GOM shows a very large orientation dependence. In particular, due to the two-dimensional structure of graphene oxide, the through-plane direction of GOM is inevitably oriented in c-axis, and its conductivity is significantly lower than that of Nafion®. In this study, (3-mercaptopropyl)trimethoxysilane (MPTS, HS(CH2)3Si(OCH3)3) was reacted with the surface of graphene oxide particles and then oxidizes to provide a fast proton conduction path through the obtained sulfonic acid groups. That is, MPTS binds to the surface of graphene oxide (GO) through hydrolysis (Si–OCH3 to Si–OH) and condensation (Si–OH to Si–O–C), and the thiol group of MPTS is converted to sulfonic acid ligands through oxidation. FTIR and XPS spectra successfully confirmed the MPTS bound on the surface of GOM. The resulting MPTS-modified GOM (M-GOM) shows an enormous increase in proton conductivity (Th-plane) compared to GOM, but not much as high as Nafion®. The ion exchange capacity, proton conductivity, gas leakage, mechanical properties, thermal stability, electrochemical properties, and physico-chemical properties of M-GOM as electrolytes are measured and compared with GOM and Nafion®. Hydrogen fuel cells using M-GOM alone and a double-layered membrane composed of M-GOM and Pd thin films as electrolytes are built and their operating characteristics are reported.

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 and the best selectivity (methanol) of 4.10 × 105 S/cm3 at 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/cm2, higher than 590 mW/cm2 for 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

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