| Proton exchange membrane fuel cells (PEMFC) have attracted much attention due to their advantages, such as high power density, zero or low exhaust and quick startup at low temperature et al. Membrane electrode assembly (MEA) is the key component of PEMFC, which has a great influence on fuel cell performance and is important for cost reduction to a commercially acceptable level. At present, carbon supported platinum is still the widely used electrocatalyst in MEA, which accounts for a large portion of PEMFC cost. In regard to there reasons, the study of membrane electrode assemblies with low or ultra-low platinum loading has always been one of the hot topics in the field of fuel cell. In addition, with the growth of energy demand in different applications, the development of self-humidifying MEA and the study of micro fuel cell have also received considerable attention.MEA with low and ultra-low platinum loadings and self-humidifying of MEA and miniaturization of PEMFC were studied in this thesis for cost reduction and improved cell performance. Firstly, we prepared a high performance MEA with ultra-low platinum loading by using a novel catalyst spraying technique. A cell performance of 0.7 A cm-2 at 0.7 V was achieved when the platinum loading of the anode and cathode was lowered to 0.04 and 0.12 mg cm-2 respectively. The effects of Nafion content, cell temperature and back pressures of the reactant gases on the cell performance were investigated. The optimal Nafion content in the catalyst layer was found to be ca. 25 wt.%, which was significantly lower than that for low platinum loading MEAs prepared by other methods, indicating adequate interfacial contact between the catalyst layer and membrane in our home-made MEAs. Scanning electron microscopy (SEM) observation and electrochemical impedance spectroscopy (EIS) measurements revealed that our home-made MEA possessed very thin anode and cathode catalyst layers which is in close contact with the membrane, resulting in low resistance and reduced mass transport limitations.Secondly, a novel double catalyst layer (DCL) cathode was prepared with different amounts of platinum at each electrode to maintain a dedicated balance between improved mass transfer and good platinum utilization: the catalyst with higher platinum loading was used in the inner layer to concentrate the platinum, and the catalystwith less platinum was used in the outer layer to maintain a suitable layer thickness. Polarization characteristics of cathode with this novel DCL, a conventional DCL, and a single catalyst layer (SCL) were obtained at ambient pressure in an H2/air PEMFC. The results showed a significant enhancement of cell performance with the novel DCL cathode. Compared with the SCL cathode, the current density of the novel DCL cathode at 0.6 V was increased by 35.9%, whereas that of the conventional DCL cathode was increased by 8.8% only.Thirdly, an ultra-low platinum loading MEA was prepared using above-mentioned novel DCL technique. Polarization characteristic of the MEAs with novel DCL, general DCL and SCL were evaluated in H2/air single cell system. The results showed that the novel DCL MEA performance was improved significantly, especially at high current densities. When the platinum loading of the anode and cathode was as low as 0.04 and 0.12 mg cm-2 respectively, the current density of the novel DCL MEA can reach 0.73 A cm-2 at a proposed working voltage of 0.65 V, which was comparable with that of the SCL MEA. In addition, the maximum power density of the novel DCL MEA reached 0.66 W cm-2 at 1.3 A cm-2 and 0.51 V, 11.9% higher than that of the SCL MEA, indicating mass transfer improvement for the novel MEA. EIS and cyclic voltammetry (CV) tests revealed that the novel DCL MEA possessed an efficient electrochemical active layer and good platinum utilization efficiency.Fourthly, we developed a novel self-humidifying MEA with Pt/SiO2/C anode composite catalyst to improve the performance of PEMFC operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, SEM and water uptake measurement. The optimal performance of the MEA was obtained with 10 wt.% silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H2/air PEMFC at ambient pressure under different relative humidity (RH) and cell temperature. The results showed that the MEA performance was almost unchanged when the RHs of both anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 oC, the MEA showed excellent stability for low humidity operating: the current density remained at 0.65 A cm-2 at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.Finally, a novel micro planar fuel cell power supplier, in which a six-cell PEM unitized regenerative fuel cell (URFC) stack was used as the power generator, was designed and fabricated. Six membrane electrode assemblies were prepared and integrated on one piece of membrane by spraying catalyst slurry on both sides of the membrane. Each cell was made by sandwiching a MEA between two graphite monopolar plates, and six cell units were mechanically fixed in two organic glass endplates. When the stack was operated in electrolysis mode, hydrogen was generated by water splitting and was stored using a hydrogen storage alloy; conversely, when the stack was operated in fuel cell mode, hydrogen was supplied by the hydrogen storage alloy and oxygen was supplied from air by self-breathing of the cathode. The open-circuit voltage (OCV) of the system reached 4.9 V at room temperature and standard atmospheric pressure; the system could discharge at a constant current density of 20 mA cm-2 for about 40 min at 2.9 V. The system showed good stability for 10 charge-discharge cycles. It suggests a potential orientation for the application of PEMFC in the field of portable devices.
|
PDF Full Text Request