Study On Fabrication And Performance Of Catalyst Coated Membrane For Direct Methanol Fuel Cell

As the key component of direct methanol fuel cell (DMFC), the membrane electrode assembly (MEA) plays an essential role in the cell performance. The conventional MEA is fabricated by hot-pressing the catalyzed diffusion medium (CDM) electrodes onto each side of a polymer electrolyte membrane. The CDM is subject to low Pt utilization due to insufficient contact sites between the electrolyte and the electrode. Recently, the catalyst coated membrane (CCM) has attracted more and more attention. The CCM is characterized by applying the electrocatalyst onto the electrolyte directly so as to to enhance the'reactant-catalyst-electrolyte'triple-phase boundary (TPB) and obtain a better Pt utilization. In this study, the CCMs with high electrochemical activity were prepared through dacal method. The performance of active and air-breathing DMFCs was increased by optimizing the CCM preparation technique and improving the MEA structure. Furthermore, the degradation mechanism of the MEA after long-term operation was investigated in detail.The CCMs prepared by the decal method showed a better contact between the electrolyte and the catalyst layer. The performance of the cell was improved by optimizing the hot-pressing condition for the CCMs. The maximum power density of the CCM hot-pressed at 185℃, 15 MPa for 90 s was 32 mW·cm-2, resulting from high electrochemical surface area (ESA) and porosity, as well as low internal resistance. High hot-pressing temperature and pressure were considered to be positive factors for the formation of triple-phase boundary and the reduction of the methanol crossover rate. However, the rise of hot-pressing temperature and pressure yielded the growth of Pt particles and the decrease of the porosity, which were unfavorable for the electro-catalytic activity and the mass transports. The suitable hot-pressing time not only contributed to the combination of the catalyst layer and the membrane, but also suppressed the agglomeration of the Pt partcles at high temperature.The electro-catalytic activity of Pt/C cathode was improved by introducing the pore-forming agent, NH₄HCO₃, to the catalyst layer. It was indicated by SEM test that the addition of NH₄HCO₃ prevented the agglomeration of the catalyst particles and promoted the oxygen diffusion to the active sites. The optimal mass ratio between NH₄HCO₃ and catalyst at the cathode was 1:1. As a result, the cathode ESA was enhanced from 379 cm2·mg–1 to 431 cm2·mg–1 and a 18.7 % increment in the peak power density was achieved at 30℃. A dual-functional composite anode catalyst layer was proposed. The inner sub-layer with a dense morphology can effectively suppress methanol crossover. On contrary, the outer sub-layer modified by the pore-forming agent, NH4HCO3 and the carbon nanotubes (CNT) can enhance the ESA and increase the catalyst utilization. The structural improvement of anode catalyst layer results in a 40 % increment in maximum power density during the single cell test at 30℃. The cell with CCM/CDM composite cathode catalyst layer displayed a better performance at high current density region due to the improved oxygen transportation to the catalytic active sites.A 240 h stability test of the cell was performed at 80℃and a constant current density of 80 mA·cm–2. The performance loss of the CCM and the invalidation of the diffusion layer were investigated individually. In the beginning, a short-term performance loss related to the starvation of the anode could be recovered after the methanol was replenished. The irreversible failure after a long-term operation was associated with the catalytic activity degradation of the cathode and the anode, and the increase in the ohmic resistance. The catalyst agglomeration, and the active metal dissolution, together with the worse contact between the catalyst layer and the membrane, led to the decrease in the cathode ESA and the increase in the reaction resistance. For the cathode MPL, The change of the pore information brought about two effects. One was that the cathode flooding was aggravated due to the decrease in the hydrophilic pores for water removal. The other was that the capillary pressure of hydrophobic pore reduced with the increase in the pore radius. In that case, the liquid water would permeate into the hydrophobic pores more easily and block part of the passages for the gas transports.The performance of air-breathing DMFC based on CCM technique was increased by improving the composition and the structure of the cathode diffusion layer. TGP-H-060 carbon paper was chosen as the cathode backing material. The optimal PTFE contents in cathode backing and cathode microporous layer (MPL) were 25 mass % and 30 mass % respectively. A novel cathode configuration which promotes the mass transports through the cathode of air-breathing DMFC was reported. This cathode, free of the support by conventional wet-proof carbon fibre backing, adopted a gold-plated Ni mesh as the air passages and the current collector. The cathode MPL, which acted as water management layer, was applied on the catalyst CCM through decal method. The mesh is combined with the MPL by hot-pressing procedure to form the cathode diffusion medium. The remarkable improvement of two-phase diffusions at cathode results in an increment in maximum power density of the single cell from 11.0 mW cm-2 to 15.5 mW cm-2 at 20℃. However, the water accumulation between the cathode flow field and the MEA due to the hydrophilicity of the mesh accelerated the degradation of the cell performance.