An inorganic-framework proton exchange membrane for direct methanol fuel cells with increased energy density

Abstract

Despite the striking advantages of direct methanol fuel cells as portable power sources, their practical application is impeded by a technical issue known as methanol crossover. To address this issue, a composite membrane was developed that utilizes mesoporous silica film as the framework for a membrane and surface-coated proton-conducting monomolecular layer (SO₃H–(CH₂)₃–Si–(OCH₃)₃) to realize proton transfer. Unlike the conventional Nafion membrane, this membrane has vertically aligned 1D nanochannels with a tunable channel diameter, benefiting from the inorganic framework, which is able to block methanol crossover by size-selective separation. After shrinking the nanochannel size to ∼0.5 nm, experimental tests demonstrated that the methanol permeability of this membrane was only 1.64 × 10⁻⁹ cm² s⁻¹, which is three orders of magnitude smaller than that of the Nafion membrane. In addition, the use of this inorganic-framework membrane in direct methanol fuel cells increased the methanol operating concentration, which greatly increased the system energy density, suggesting that this type of membrane has the potential to resolve the crossover issue by further improvement.