Development of a sub-kW open-cathode type PEM fuel cell stack integrated with a chemical hydride hydrogen generator system.

• Main Authors: Zhen-Ming Huang, 黃振銘
• Other Authors: Ay Su
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/00216431982948286874

博士 === 元智大學 === 機械工程學系 === 102 === High purity hydrogen produced by hydrolysis of sodium borohydride can be used directly in proton exchange membrane (PEM) fuel cells for portable devices and automotive applications. The advantages of high storage capacity, quickly start, controllable reaction and mild condition, hydrogen generation by catalytic hydrolysis of chemical hydride, such as sodium borohydride, has attracted much attention for development recently. Development and optimal design of a hydrogen generator based on sodium borohydride (NaBH4) using Ru catalyst is reported in the present study. A multi-layer coating process of Ru on Ni foam was employed to enhance its capability for hydrogen generation cycles. The optimal deposition density of Ru particles on Ni foam was confirmed by the SEM morphology. The cycling generation for the steady state reaction can be maintained at least 10 times. The activity of Ru catalyst during a hydrogen generation process was examined in a continuous flow reactor. In general, NaBH4 solution cannot be fully utilized through the hydrogen generator, as a result, the effluent from the generator may continue to dissociate, which turns into hydrogen leakage. A secondary reactor was therefore installed after the primary generator to collect the remaining hydrogen. The reactant solution containing 20 wt.% NaBH4 and 3 wt.% NaOH was introduced into the structured catalyst at a constant flow rate using a liquid pump, and H2 rate of 1.72 L min-1 (0.43 L(H2) min-1g-1 catalyst). The conversion of NaBH4 from flow rates of hydrogen was about 92.2%. In this paper we report on the development of a sub-kW fuel cell stack integrated with a NaBH4-based hydrogen generator. The fuel cell stack was built with 15 open-cathode PEM fuel cells and employed two fans to supply air for electrochemical reactions as well as for cooling. The open-cathode design simplifies the system’s balance-of-plant. A NaBH4 hydrogen generator using Ru coated on Ni foam was developed to provide hydrogen for the integrated system. It is found that chemical hydrides are a good candidate of hydrogen generation materials at room temperature and hydrogen generation rate of such generator can be achieved by controlling the catalytic hydrolysis reaction. The performance of the present integrated system is evaluated and its feasibility as a portable power source is assessed in this work. The performance of the present YZFC-OA stack showed an optimum for air fan voltage at 9.0V. A maximum power 355W was recorded with H2 inlet temperature 40°C and stack voltage at 8V. Reactant solution containing 20 wt.% NaBH4 and 3 wt.% NaOH was introduced into the structured catalyst at constant flow rate. For 14-pcs Ru catalyst and H2 rate of 6.08 L min-1(0.43 L(H2) min-1g-1 catalyst), the conversion efficiency of hydrogen was about 87%. The performance of the present fuel cell stack using the hydrogen from the NaBH4 hydrogen generator was found to be essentially the same as that from a commercial grade gas cylinder at 0.53V (stack voltage = 8V). The performance and stability of the present YZFC-OA stack, and the feasibility of integrated stack and hydrogen generator have been demonstrated in this study. The integrated system appears to have great potential for residential, light-duty vehicle and stationary applications. In this study, a PEMFC stack rated at 300W a large unit cell for high stack performance was fabricated. The fuel cell stack and a Li-ion battery were then employed to build a hybrid power system. The fuel cell stack ('YZFC-ZM') was constructed with 18 open-cathode type of PEMFCs and employed two fans to supply air for electrochemical reactions as well as for cooling. The performance of the present YZFC-ZM stack showed an optimum for air fan voltage at 10.5V (air flow 410.6 m3 min-1). A maximum power 321W was achieved with H2 inlet temperature at room temperature and stack current at 30A. Humidification of hydrogen at anode inlet was shown to degrade stack performance and stability due to flooding in the MEA. The capabilities of the YZFC-ZM stack for Hybrid power system test platform were validated by successful dynamic loading tests. The hybrid power system consisted of the YZFC-ZM stack, Li-ion battery and DC/DC converters. The hybrid power system was tested under three different modes, i.e., the normal power demand mode, charge mode, and high power demand, to demonstrate the exchange of power flow between the two power sources. The stack was operated under dynamic-loading mode to simulate vehicle acceleration. The present test platform was found to function satisfactorily and it can be further developed to simulate actual vehicle driving cycles for hybrid power systems.