Direct methanol fuel cells (DMFCs) have received attention as a power source for portable devices. Especially for low power electronics, energy density, system efficiency, and life time are the important factors to consider. Improving the methanol conversion efficiency is the most significant challenge for achieving higher efficiency and long life time. The traditional polymer membranes need to be replaced with inorganic ones to reduce the methanol cross-over. Compliant and chemically stable electrodes are necessary for inorganic membranes. Moreover, new DMFC stack design is required for compact systems. The challenges to create a highly efficient DMFC for low power system are addressed in this work.
Inorganic glass membranes have been synthesized via a sol-gel reaction using 3-mercaptopropyl trimethoxysilane (3MPS), 3-glycidoxypropyl trimethoxysilane (GPTMS) and tetraethoxy orthosilcate (TEOS). The effect of oxidation time of the thiol group on the 3MPS, the mole fraction within the sol, and the water ratio in the reactant mixture were investigated. The particular behavior of conductivity and permeability for the glass membranes with respect to polymeric ones was observed. While the conductivity increased, the permeability decreased resulted in high selectivity membrane. The total energy loss of a DMFC was decreased by replacing a Nafion with a glass membrane.
Glass composite electrodes were prepared by incorporating the commercial catalyst nanoparticles (Pt/C or PtRu/C) into a silica-based matrix prepared by sol-gel reaction. A Leaman bath was used to electrolessly deposit Pt in order to both merge the catalyst islands and optimize the electrochemically active area of the electrode layer. For efficient methanol oxidation on the anode, PtxRu1-x bimetallic electrocatalysts have been prepared by modifying the Leaman bath. Formic acid was found to be a more efficient reducing agent than hydrazine to achieve 1:1 ratio of Pt and Ru deposition at lower temperature.
The blocking effect of the glass electrodes on methanol cross-over is of interest. The effect of the gelation time, curing temperature, the mole ratio of the sol components, and the ratio of catalyst to glass on reducing methanol cross-over were investigated using PtRu/SiO2 inorganic electrodes synthesized using 3TPS and GPTMS. The electroless deposition of PtxRu1-x improved both methanol permeability and the catalytic activity for methanol oxidation, since the additional metal not only decreased the cross-over but also increased the active catalytic surface area. As a result, the selectivity was increased leading to higher energy efficiency.
Anionic-cationic bi-cell stack design was proposed to achieve higher voltage from the limited volume. A bi-cell design consists of an alkaline exchange membrane (AEM) and proton exchange membrane (PEM) fuel cell in series using a common liquid fuel tank. The actual AEM cathode potential was essentially the same as the PEM anode potential making the bi-cell configuration viable, addressing the short circuit problem between adjacent anodes through the common fuel tank. The bi-cell system was demonstrated with the optimized AEM and PEM fuel cell in series operated from a single fuel tank with higher voltage (theoretically, 2.4 V) and reduced volume.
The current anionic-cationic bi-cell stack performance was limited by the immature alkaline electrode structure. The effect of hydrophobicity in alkaline electrodes was investigated using hydrophobic ionomer and polytetrafluoroethylene (PTFE). The cathode overpotential was improved by using hydrophobic ionomer even though the ionic conductivity was lower. The addition of PTFE in the catalyst layer does not only decrease the water content but also serves as binder, improving mechanical stability, and introduce porosity, improved mass transport. The ADMFC with optimized electrodes showed improved performance by three times compared to the electrode without PTFE.