MEA Preparation And Performance Study For Direct Methanol Fuel Cell

Direct methanol fuel cell (DMFC) is an ideal portable power source in the future because of the simple system, high energy density and convenient fuel storage and transportation. However, the specific power of DMFC is too low for applications at current stage. In order to increase the power density, DMFC often works at high temperature and with high catalyst loadings. For example, the Pt loading for DMFC anode is typically 4~12mg/cm~2, and the temperature 60~130℃. High catalyst loading and high working temperature force DMFC to adopt special membrane-electrode structure, e.g., unsupported catalyst electrode; the traditional techniques for MEA preparation are also required to improve and the fuel cell operating conditions need to be optimized.The purpose of this thesis is to improve the performance and MEA preparation technique for DMFC. In this work, two new methods for MEA preparation are developed, one is for ultra-thin electrode with high catalyst loading, another enables the direct cast of an electrode onto the Nafion membrane. Furthermore, some key operating conditions for DMFC are optimized, and finally a preliminary verification for the feasibility of alkaline membrane DMFC was conducted.The main contents and conclusions of this thesis are summarized as follows:1. New preparation method for unsupported catalyst electrode for DMFCFor high power density, DMFC requires high catalyst loading in electrodes. If carbon supported catalysts are used in this case, the electrode will be too thick, resulting in low catalyst efficiency and large IR drop in the electrode. As for unsupported catalyst, it is difficult to maintain high dispersion during electrode preparation due to particle sintering and conglomerating. In this thesis, a new method was developed to prepare unsupported catalyst electrode for DMFC. The characters of this method are firstly casting a raw electrode using a solution containing polymer electrolyte and catalyst precursors, then reducing the precursors in the electrode into catalysts. The advantagesof this method are good contact between catalyst particles and polymer electrolyte, high dispersion of unsupported catalysts and ultra-thin electrode with high catalyst loadings. An unsupported PtRu anode thus prepared, with Pt loading of 3.7mg/cm2, gave catalyst particle size of 3~4nm and performance of 400mA/cm2@0.35V when working at 90°C, which is superior to the literature report with similar working conditions in 2003.2. Preliminary study of MEA preparation using Nafion emulsionAmong those MEA preparation methods, direct casting catalyst layer onto the Nafion membrane gives the best membrane-electrode contact. However, the solvent of Nafion solution used for catalyst ink preparation will cause severe swelling of Nafion membrane, which prevents from obtaining uniform catalyst layer on Nafion membrane. In this thesis, a new approach is proposed to realize this direct casting method. In this ideology, Nafion emulsion or microemulsion, prepared using none-polar solvents, is used instead of the Nafion solution for catalyst ink preparation. Using the catalyst ink thus prepared, a catalyst layer can be easily cast onto Nafion membrane without hot press. As shown in this preliminary study, MEA prepared using Nafion/cyclohexane emulsion exhibited similar performance in catalyst efficiency and power density than those prepared by traditional methods. This new method is promising and practical for large-scale MEA preparation.3. Performance studies on DMFC single cellThis part focused on the performance of DMFC single cell constructed using the unsupported catalyst electrode technique introduced above, and the influence of some key operating conditions. The major conclusions are: (1) an as-prepared fuel cell need be activated before normal operation, the activation mechanism may involve cleaning effect and constructing ionic channels inside the polymer electrolyte. (2) In order to achieve high power density, DMFC requires high catalyst loadings; but the catalyst loading has an upper limit due to the character of a porous electrode. Furthermore, "gas blocking" effect will appear in thick anode when CO2 produced by the anode reaction cannot easily escape, resulting in low catalyst efficiency. (3) Increasing the gas pressurein cathode will benefit the reaction kinetics, and adequate gas flow rate can prevent cathode flooding and thus guarantee stable power output. (4) In our operating conditions, 2mol/L methanol solution is necessary to avoid concentration polarization at high current densities. (5) The maximum power density in our work at current stage is 207mW/cm2 (0.35V, 90°C), which is close to the best record in the literature.4. Feasibility studies for alkaline membrane direct methanol fuel cellThe value of alkaline electrolyte membrane direct methanol fuel cell (AEMDMFC) is to broaden the catalyst scope; but the difficulty for AEMDMFC at the moment is to develop high performance alkaline polymer electrolyte. Although alkaline polymer electrolyte suitable for fuel cell use is still unavailable, feasibility studies for AEMDMFC using current available alkaline membrane are necessary. In this work, a home-made quaternary ammonium polymer and a "gel electrolyte" membrane were used to construct AEMDMFC, and the ionic conductivities of the two membranes turned out to be O.OlS/cm, which were measured under fuel cell operating conditions. However, because the mechanical and thermal stabilities of the two membranes were unsatisfied and the MEA preparations were far from optimization, the fuel cell performance is low. Basically, this feasibility study can be satisfied since the testing cells took on normal behavior.