Investigation Of Membrane Electrode Assembly And Stack For Direct Dimethyl Ether Fuel Cell

Direct dimethyl ether fuel cell (DDFC) is a direct type fuel cell using dimethyl ether (DME) as fuel. DDFC has been recently studied. Up to now, a lot of work was focused on electro-oxidation of DME. The membrane electrode assembly (MEA) has not been studied in detail. MEA is the key component of the DDFC, and the chemical energy is directly converted into electrical energy on the MEA. The performance of the DDFC is greatly dependent on the performance of MEA. In this paper, the MEA of the DDFC was investigated, and the performance of the MEA was increased. A small passive DDFC stack was fabricated for room temperature applications.We investigated the effects of compositions and structure of MEA on the cell performance. The MEA with Pt/C as the anode catalyst showed higher performance than that with PtRu/C at the low cell voltage regions, but at the high cell voltage regions, the PtRu/C yielded better performance than Pt/C. The Pt/MWNTs showed a higher catalytic activity for dimethyl ether electro-oxidation, compared with the Pt/Vulcan XC-72 catalyst. The performances of the MEAs was improved with the increase of Pt loading up to 3.6 mg cm-2, and then decreased with further increase of Pt loading. And 20 mass % PTFE content and 1 mg cm-2 carbon black loading were optimal compositions in the anode diffusion layer. In the cathode, MEA with 20 mass % Nafion content in the catalyst layer and 30 mass % PTFE content in the diffusion layer presented the better performance. The (Open circuit voltage, OCV) of the cell reduced with the decrease of the thickness of the membrane. The performance of the MEA with Nafion 112 membrane is the worst. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm-2 among the three MEAs with different thickness of Nafion membranes.The DDFC with 1.5 mol L-1 and 5 mL min-1 DME solution showed the best performance. The DDFC with full humidification and 5 mL min-1 DME gas presented higher performance. Compared with DME gas, the DDFC with DME solution showed higher power density and better long-term operation performance. The fuel crossover of DME in the Nafion 115 membrane was expected to be smaller than that of methanol. The DDFC showed higher performance than DMFC at the low current density regions, but at the high current density regions, the DMFC exhibited better results than DDFC. The maximum power density of the DDFC was 69 % of the DMFC.The solubility of DME in water decrease with the increase of the temperature. We presented the novel MEA for DDFC. The anode diffusion layer of the MEA consisted of hydrophilic region and hydrophobic region. The performance of novel MEA for DDFC was enhanced due to the promotion the mass transport of DME fuel at 50℃. The electrochemical impedance spectra analyses revealed that the mass transfer resistance of novel MEA was lower than that of completed hydrophilic or hydrophobic MEA. The constant current operation curves showed that the degradation rate of the novel MEA was lower than that of conventional MEAs. It indicated that the novel MEA benefited the long-term operation of DDFC. At low temperature, the novel MEA with larger hydrophilic region showed excellent performance. The performance of novel MEA with larger hydrophobic region was better at high temperature.A small passive DDFC stack consisting of 6 single cells was fabricated for room temperature applications. The performance of each single cell is uniform. The OCV of the stack is around 4 V, with a maximum power of 300 mW. The serial connection was benificial to the OCV of the stack, and the parallel connection was in favor of the discharge current. The total power density was independent to the connection pattern. The stack operated stably at either constant or varied currents. When the stack operated at a constant current of 100 mA at 21℃, the temperature gradually rose to 37℃after 60 min and remained at this temperature. The initial voltage was 1.4 V at a constant current of 300 mA. After 1100 min operation, the voltage dropped to 0 V and the fuel utilization was 57 %.