Improvement of PEFC Performance and Endurance By Employing Electrochemically Assisted Self-Assembly Porous Carbon Plate

Polymer-electrolyte fuel cells (PEFCs) are attracting enormous interest as a primary power source for automotive applications by virtue of its high power density, facility of operation, high efficiency and zero pollution. One of the critical issues associated with the high-power and endurance of PEFCs is water management, which needs a delicate water balance scheme between the membrane drying out and cathode flooding. Either of the two problems, if not solved, can result in mal-function of the power plant. A promising approach to deal with the two problem aforesaid is the usage of water transport plates (WTPs), designed by United Technologies Corporation (UTC). The WTPs perform two main functions. When the inlet gas is not saturated, the WTPs provide water to evaporate into the gas channels from water chamber to humidifying them. When the excessive water existed, the WTPs provide a passage for liquid water to escape to water chamber such that it does not clog the oxidant channel and inhibit access of oxidant gas to the catalyst.

In this study, a WTP was fabricated by electrochemically assisted self-assembly of silica film in the porous carbon plate, marked as HPCP. To make silica thin film generate in the pore of porous carbon plate uniformly, an electroplating device with pre-hydrolysed precursor flowing in the pores of porous carbon plates was designed, shown in Fig. 1(a), (b). The key properties of WTPs, the vapor water permeability and liquid water permeability, were measured by a home-made test apparatus. Moreover, the effects of HPCPs’ porosity, fraction of hydrophilic pore, tortuosity, wettability and hydrophilic pore size on water permeability are explored respectively. In order to reduce the complex physical problem of water permeability, which involved diffusion and convection process, to a simplest form, dimensional analysis was applied to gain a macroscopic correlation. The dimensional equation provided insight into the WTP fabrication, as well as optimizing the operating conditions of PEFC. Furthermore, the performance and endurance of PEFC with HPCP were evaluated under two conditions: one is internal-humidification with dry gas feeding, another is water drainage under dead-ended cathode.

Under the condition of dry gas feeding, due to the humidification function of WTP, the inlet gas could be moistened by the permeated water vapor from water chamber which achieved the improvement of membrane hydration level. By employing HPCP as the anode water transport plate, under no-humidity, the maximum power density of the cell was 699.3 mW cm^-2 higher than that with solid plate (SP), which was depicted in Fig. 2(a). Moreover, after 3000 min long-term operating at 800 mA cm^-2, only 13% energy dissipation of cell with HPCP was observed, while the cell with SP have no capacity to discharge at 800 mA cm^-2. Under the dead-ended cathode operation, contributed to the function of water drainage, by employing HPCP as the cathode water transport plate, the purge interval at 2000 mA cm^-2 was 103 s less than that with SP shown in Fig. 2(b). In comparison of SP, the loss of oxygen was reduced to 1/12.4 with HPCP. The favorable performance and endurance of the cell indicates that the HPCP is a promising candidate for water transport plate material.