| As an energy conversion device, fuel cell (FC) will play a key role in the future world energy field as the mandatory characteristics, for power plants will be high power density, high efficiency, low emission and low noise level. In this thesis, domestic and foreign literatures on FC systems, especially the low temperature FC systems, namely the proton exchange membrane fuel cells (PEMFC) and alkaline fuel cells (AFC), were exhaustively reviewed. The current technical development and existing problems of PEMFC and AFC were reviewed. The main characteristics and development of mischmetal-based hydrogen storage electrode alloys (HSA) were also reviewed and discussed. On the basis of the literature review, the possibility of substituting Pt/C with HSA MlNij esCoo gsA10 3Mn<).3 as the anode catalyst of PEMFC and AFC, was studied in this thesis. The potential of using HSA as the catalyst in FC anode is studied on the ground of the optimization of the structure of FC anode, the improvements of electrocatalytic activity and working stability of HSA using the material structure analysis, electrochemical analysis and anode performance tests.Before adopting a HSA as the PEMFC anode catalyst, three main factors that affect the performance of PEMFC membrane electrode assembly (MEA) using Pt/C as anode catalyst and the optimum preparation technology for MEA were investigated. The results showed that the three-phase reaction zone of MEA anode could be enlarged by impregnating the catalyst loaded electrode in the protonic conducting material Nafion. The process enlarged the catalytic zone in MEA. The most suitable temperature for the compression of the Nafion impregnated electrode with the PEM to optimize the proton transfer between the catalyst layer and PEM and also the most suitable thickness of catalyst layer to enhance the exchange current density of MEA were found through experimentation. By means of the optimized technology, the Pt/C MEA obtained the current density of 483 mA-cm-2 at 0.5 V of cell voltage.The electrochemical properties of PEMFC MEA using differently prepared HSA MlNi3 65Coo.85Alo.3Mno.3 alloy as the anode electrocatalyst instead of Pt/C were studied and developed. Both the modification of anode HSA including the ball-milling, immersing the alloy powder in a hot alkaline solution containing KBHj and surface chemical coating with Pd, and the modification of the structure of MEA anode including the composition of the anode material and the introduction of pore-forming substance to improve the electrocatalytic activity of MEA anode were all tested. Compared with the as-cast HSA, the ball-milled HSA in fy had a higher specific surface areas and better electrocatalytic activity. And HSA ball-milled with acetylene black enhanced the three-phase interfaceregion in the electrode and improved the electron transfer ability of the catalyst layer. As a result of immersion the alloy powder in a hot alkaline solution containing KBH4, the oxidation layer on the surface of the alloy was partly destroyed, some elements including Mn, Al and their oxides were dissolved, and some oxides such as the Ni oxide were reduced into metallic state which all led to an increase of the specific surface area and the electrocatalytic activity of the alloy.The experimental data indicated that the best electrochemical performance of the MEA experimented was obtained by using the HSA modified by immersing in a hot alkaline solution containing KBHt and then chemical coating with 3 wt% Pd. The MEA with the aforesaid modification showed an enhanced surface capability of hydrogen dissociation and adsorption, an improved electron transfer ability and an increased electrochemical activity, especially in the electrode reaction region of ohmic polarization and concentration polarization. After being optimized, the current density of the MEA with HSA anode reached 168 mA-cm-2 at 0.5 V and 232.4 mA-cm-2 at 0.2 V and its maximum output power density reached 84 mW-cm-2. After 24-hour working stability test on MEA at 40 mA-cm-2, the power...
|
PDF Full Text Request