Studies on the start-up characteristics and performance improvement of self-humidified proton exchange membrane fuel cells

Abstract

In this study, the start-up characteristics of self-humidified PEMFCs under dry conditions were investigated to provide useful information and better understanding on self-humidified PEMFCs. In addition, the effects of structure design and hydrophobicity of GDBL were investigated to improve the start-up and operating performance of self-humidified PEMFCs. In PEMFCs, the proton conductivity of the membrane determines the cell performance and it depends on the hydration level of the membrane. In conventional system, external humidifiers are used to hydrate the membrane. However, an external humidification system introduces disadvantages such as increased complexity of the system, parasitic power loss, increased weight, large volume, and high manufacturing cost. Therefore, operating PEMFC without external humidifier is a tremendous issue in this field. Although there have been some researches on self-humidified fuel cell, the start-up characteristic under dry condition is not clear yet. In addition, although relatively low performance of self-humidified fuel cell has been improved, it should be improved further. In this study, we investigated the characteristics of dry start-up process of self-humidified PEMFCs. Specifically, we evaluate the hydrogen crossover rate across the membrane, the influence of the direct reaction of hydrogen and oxygen producing water on the dry start-up process. The effect of starting temperature was also evaluated with different flow arrangement. As a result, It was found that start-up performance with counter flow is effective than co-flow and the available operating temperature increases with counter flow. However, the dry start-up was failed at a high temperature of 45ºC. In order to solve this problem, an advanced dry start-up process was applied and the result was successful. The results showed that the WSP played an important role during the dry start-up and the initial cell performance was remarkably improved. Moreover, WSP made dry start-up possible at high cell temperatures, without the need for a long time to cool down the fuel cell after the previous shut-down. In order to improve the start-up and operating performance of self-humidified PEMFCs, the effect of structure design and hydrophobicity of GDBL on water management of GDL was investigated with an analytic model based on the capillary pressure–saturation relationship with the Leverett J-function. In this model, structure design and hydrophobicity of GDBLs were represented by the measurable parameters, porosity and contact angle, respectively. With this analytic model, the liquid water saturation distribution and the amount of water remaining in GDL were evaluated as a function of porosity, contact angle, and thickness ratio of each GDBL. As a result, it was concluded that porosity gradient in negative direction and hydrophobicity gradient in positive direction is effective on water retention. Based on the analytic results, start-up and steady-state performance of fuel cell was measured with the modified GDL containing double GDBL with different porosity in negative direction (GDBL with lower porosity near the flow channel). The result showed that shorter duration of WSP was required for successful start-up and high performance with modified GDL. In addition, the effect of stacking was also evaluated with the GDL containing double GDBL with the same porosity. The result showed that the structural design of the GDBL had a major effect on its water retention capacity, whereas stacking has negligible effect on the water retention capacity. In additions, the self-humidification effect and the performance of the fuel cell containing the structurally modified GDBL were found to be significantly improved over a wide range of operating conditions. Lastly, functionalized GDL is optimized for water retention and/or water removal. For this, gas diffusion coefficient is considered. Although there were many difficulties in manufacturing multi-layer GDBL in present time, with the advanced manufacturing techniques, it is expected that manufacturing functionalized multi-layer GDBL is possible in near future. Then, optimization of functionalized GDL could provide important information to manufacturer.