Translated Abstract
Water content and distribution in the PEMFC significantly affects the fuel cell performance, stability, durability as well as current distribution within the cells. Water management in PEMFC aims to balance the water content so that the membrane can get well hydrated to reduce the ohmic loss, and flooding is prevented to reduce the concentration loss. Furthermore, it also requires an even distribution of water to achieve even current and temperature distributions, avoiding a faster spot degradation and thus guarantee a long lifetime of the fuel cell. As a place where the oxygen is reduced and the water is generated, the cathode catalyst layer (cCL) needs water to enhance proton transfer within it and to improve oxygen reduction kinetics, at the same time water must be removed to avoid the triple-phase interfaces being covered and the reactant starvation. Thus, it is of vital importance to manage the water in the cCL. Structure and properties of the diffusion media, including the gas diffusion layer (GDL) and the micro-porous layer (MPL), have significant impacts on water management. However, most of the researches so far focus on the water distribution and the transfer mechanisms in the diffusion media and in the flow channels, lacking of its effect on water in the catalyst layer. Besides, effects on the overall performance rather than local performance are usually reported. This paper suggested and proved that the double layer capacitance can be an indicator of the water content in the cCL, thus giving a way of monitoring the water in the cCL. Using this indicator and the current distribution measurement technology, we studied the effects of the diffusion media structure on cell performance, current distribution and the water content in the cCL, and on the water transfer mechanism. The location in where flooding will first damage the cell performance was also discussed.
Charges redistribute on the interface when two conductors contact, forming a double layer which acts like a capacitor. Unlike the ohmic resistance, the double layer capacitance seperates the catalyst layer from the membrane. Thus, this paper came up with an idea that the double layer capacitance could be an indicator of the water content in the cathode CL. By designing cCLs with various carbon loadings and platinum loadings, the double layer capacitance was proved to be mainly formed at the Pt|ionomer interfaces. It was found that the double layer capacitance increased with the relative humidity due to two reasons. Higher water content enhanced the ionomer links with each other and therefore the effective Pt|ionomer interface areas increased. The increased free proton concentration within the ionomer caused an increase in the double layer capacitance per interface area. It was found that all the factors except for the structure and composition of CL had no direct influence on the double layer capacitance. As an application, the relative humidity can be estimated in a working fuel cell.
The functions and mechanisms of the MPL on water management were systematically studied based on the above method. The single cell performance, current distribution and the impedance were obtained. The study found that the effects of the MPL were quite different under different conditions, depending on whether it is easy to flood or to dehydrate. According to the current distribution results, the local condition could be rather different, thus the MPL could act quite differently at different locations. In the case vapor tended to condense and flooding would likely happen, introducing a MPL significantly reduced the water content in the cathode CL as vapor could hardly condense in the very tiny hydrophobic pores. In the case the membrane tended to dehydrate, the MPL conserved the water by its higher gas diffusion resistance. However, the positive effects of the MPL were negligible at elevated temperature or in extremely dry conditions.
Better dispersed and anti-settling slurry was prepared by ball milling. Thus, high solid content slurry was available and the penetration problem was greatly alleviated. Combining wire rod coating and ultrasonic spraying, a flat and crackfree MPL with low carbon loading was achieved. By GDL perforating and MPL fabricating, this paper studied the effects of the GDL perforation with and without MPL. The location in which the flooding limited cell performance was discussed. Without MPL, perforation significantly reduced the water content in the cCL, raising the limit current density. However, this improvement cannot compare with that brought by introducing a MPL. Perforation also showed negative effect on the uniformity of current distribution. With MPL, perforated or not didn’t show very significant differences. Therefore, it is believed that the cCL was the location in which flooding seriously damaged cell performance, thus more attentions should be paid to the water management within the CL.
Focusing on the water content at the CL|MPL interface, the effects of MPL morphology was studied. On the flat crack-free MPL, small and large lines of the competition was removed to simulate the cracks on it. The effects of the crack and its size on water transfer were studied by studying the single cell performance, current distribution and the water content in the cathode CL. The study showed that the cracks acted both as water transport paths and as water pools, the first one raising the limit current density, the later reducing the ohmic resistance and raising the double layer capacitance. The smaller the crack size was, the effect of being a water transport path became more important than of being a water pool.
Translated Keyword
[Diffusion media, Micro-porous layer, Proton exchange membrane fuel cell, Water content in catalyst layer, Water management]
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