Enhanced Proton Conductivity of (3-mercaptopropyl)trimethoxysilane—Grafted Graphene Oxide Membranes for Hydrogen Fuel Cells

Abstract

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.