Reduction of methanol crossover in direct methanol fuel cells (DMFC).

摘要

NafionRTM 117 membrane is the most studied polymer electrolyte membrane for direct methanol fuel cell (DMFC) applications. However, this membrane material suffers from its affinity for methanol, and any fuel that crosses the membrane is wasted. Consequently, for better efficiency in a DMFC, the flow of methanol through the membrane needs to be reduced. Using the diffusion method, it was found that methanol crossover can be reduced by simply washing the membrane using known pretreatment solutions. Moreover, it was demonstrated that the diffusion of methanol was much more significantly reduced through a NafionRTM 117 membrane doped as-received with cesium cations than a NafionRTM 117 membrane doped with cesium cations after being pretreated. In addition to the diffusion method, the potentiodynamic method was used to confirm these findings. Energy dispersive X-ray (EDX) technique was used to investigate this curious behavior, and residual potassium was observed in the as-received NafionRTM 117 membrane. The presence of potassium was further confirmed by analysis of the pretreatment solutions with inductively coupled plasma-mass spectrometry (ICP-MS). Also, the presence of potassium cations in NafionRTM 117 membrane was shown to enhance the uptake of cesium cations. Consequently, it was found that the proton conductivities of doped as-received membranes were lower than those of doped pretreated membranes: the latter was investigated using electrochemical impedance spectroscopy (EIS).; In an attempt to reduce methanol crossover and keep good proton conductivity. NafionRTM 117 membrane was doped with CsHSO4. Both the methanol crossover and the proton conductivity of the CsHSO4-doped membrane were measured, and a high ratio of proton conductivity to methanol diffusion was observed. Finally, pretreated, doped, and used as-received membranes were evaluated in a hydrogen fuel cell for their efficiencies. Results show that NafionRTM 117 membrane used as-received performs as well as NafionRTM 117 membrane pretreated in 10% H2O2 and 1 M H2SO 4. Finally, it was found that a direct methanol fuel cell loaded with CsHSO4-doped NafionRTM 117 membrane performs better than when it was loaded with NafionRTM 117 membrane pretreated in 1 M HNO3. Consequently, with appropriate catalysts, CsHSO 4-doped NafionRTM 117 membrane is a promising candidate for DMFC applications.