Automotive Cold Start of a PEMFC System by Using a Methanol Solution as Antifreeze

Abstract

Proton exchange membrane fuel cell vehicles are highly competitive due to their zero emissions, high efficiency and power density, quick response, low refuelling time and great autonomy. However, the lack of a sufficient hydrogen infrastructure and their required startup ability and durability at extreme winter conditions are limiting factors in their commercialisation. At subzero temperatures, the produced water during operation as well as the remained water in the cells after shutdown will freeze, hindering the cold start and causing several damages (e.g. cracks, pinholes, catalyst delamination or loss of electrochemical surface area). Commonly, the fuel cell system is purged with dried gases during shutdown to avoid that it freezes during the cold storage. Even though the dry gas purge methods have been extensively studied, thus reducing the degradation due to F/T cycles, there are still remaining unsolved issues, especially at very low temperatures. An alternative method that can mitigate this degradation and hence lower the temperature is the use of an antifreeze solution during the cold storage. Yet, it has been only tested in a single cell1. These results were quite promising, as the use of the antifreeze strongly diminished the degradation because of subzero temperatures and it was possible starting at -10°C. But they showed fairly poor cold start ability, since the residual methanol decreased the power density during the cold start. Therefore, the use of an antifreeze solution for a cold start should be further investigated in a stack.The aim of this study is to prove the cold start capability of a PEMFC system by using a methanol solution as antifreeze. For that, several cold starts at -10°C were carried out with a 4 kW commercial stack, a 25 vol% methanol solution to ensure no ice formation at -10°C and following next test protocol. A conditioning test and a polarization curve were conducted at room temperature. Then, the shutdown procedure, which involved soaking the stack with the methanol solution, was carried out before cooling down the system to -10°C. Following the cold storage, the antifreeze was removed from the stack and the cold start test was run. Finally, another polarization curve at room temperature was carried out. The cold start system ability with the antifreeze was compared with previous cold starts, which only used a dry gas purge method.All cold starts with antifreeze were successfully, thus proving the cold start capability of the PEMFC system by using the methanol solution. Indeed, they did not take longer than with the dry gas purge method. During the cold start the current generation did not decrease, which shows that the produced water did not freeze. Moreover, polarisation curves corroborated no performance losses after every cold start. These results are in concordance with those obtained at single cell level, as in both cases the antifreeze avoided ice formation during the cold storage. This significantly improvement with regard to the above results obtained with a single cell, is due to the ability of the stack to generate much more heat power and to minimize thermal losses, warming up the stack faster than a single cell.The obtained results allow confirming that a self-start of an automotive PEMFC system at subzero temperatures by using antifreeze solution is possible. This alternative method avoids ice formation during the cold storage, which reduces the degradation due to freezing and enables starting at lower temperatures. The decline in degradation increases the stack lifetime, thus can be achieved the strict automotive sector demands. The advantages of the antifreeze for automotive cold start can be used for others mobile applications too, such as trucks, trains, maritime or aircraft. In the future, cold starts with the antifreeze should be further investigated, and at temperatures down to -40°C.1. F. Knorr, D. Garcia, J. Schirmer, P. Gazdizcki, and KA. Friedrich. Appl. Energy, 238, 1 (2019).