Carbon corrosion in proton exchange membrane fuel cells (PEMFC) causes not only kinetic degradation, but also damages to electrode microstructure and hydrophobicity, which can lead to increases in mass transport resistance. While much attention has been paid to catalyst degradation and kinetic losses, the increases in mass transfer loss is also a very serious problem and thus it is the focus of this work. To induce carbon corrosion, accelerated stress test (AST) by holding the cell potential at 1.4 V is used. The AST procedure is interrupted periodically to record cell performance after each period of AST at three different current densities. Experiment results show that at low current density, the decrease in cell voltage is linear with time of AST, but the rate in cell voltage decrease accelerates after some period of AST at medium and high current densities. It is hypothesized that such an accelerated voltage decrease is from the increase in mass transfer loss due to water flooding in the MEA. Further experiments with either reduced inlet air humidification or reduce air flow rate confirm that water flooding in the MEA is the cause for the sharp decline in cell voltages. A phenomenon that the cell voltage increases, or the rate in cell voltage reduction decreases as cell degradation progresses is repeatedly observed. It is determined that such a phenomenon is mainly caused by the enhanced phase-change-induced flow (PCI) due to the higher heat generation rate in a more severely degraded cell. The experimental results also provide some insights on how to optimize operating conditions for degraded fuel cells.
Impurity Nitrogen and water accumulations in fuel cells with dead-ended anode can cause severe cell performance decline and fluctuations. In this work, both overall and local effects of fuel cell operating parameters, i.e., cathode humidity, air stoichiometry, hydrogen pressure and operating current density, have been experimentally studied under galvanostatic mode. A purge at the anode is automatically triggered when the cell voltage has decreased by 0.1V and the mean purge interval, defined as the average time between two purges, is recorded as a characteristic parameter. Local current densities are measured to study the local effects and detailed local characteristics of the fuel cell. The experimental results show that mean purge intervals decrease with cathode inlet humidity and operating current density, and increase with inlet hydrogen pressure and air stoichiometry. The experimental results also show that the local current densities change very differently at different locations and impurities first accumulate near the end of the anode channel and then gradually progress upstream.
Inkjet printing technology is widely used in the manufacture of conformal structures, such as load-bearing antennas or frequency-selective surface radomes. It is particularly promising for preparing conductive patterns on non-developable surfaces. Existing printing technologies employ a single nozzle and a five-axis linkage technique for printing, which is time-consuming. In this study, a conformal plane printing technology based on the arrayed nozzle was developed to prepare conductive patterns on a non-developable surface. The technique actualizes fast printing of passive circuits on a conformal surface, such as a microstrip antenna. Compared to printing techniques employing a single nozzle, the proposed method greatly improves the printing efficiency on conformal surfaces. Specifically, we first developed a model for the driver waveforms and the printing injection parameters via simulation. Subsequently, the accuracy of the computational fluid dynamic simulation results was validated by comparing them with the experimental measurements of droplet trajectory captured using a camera. Next, a droplet spreading model was established, considering energy conservation principles. Finally, a conformal surface printing technology using arrayed nozzles was developed based on the injection parameter and droplet spreading models. The effectiveness and feasibility of the proposed printing method were further validated via simulation and experimental tests of return loss.
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