A Computational Analysis on the Operational Behaviour of Open-Cathode Polymer Electrolyte Fuel Cells

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) is a promising sustainable energy conversion technology to replace conventional fossil fuel based applications. The products of the fuel cell operation are electricity, water, and heat. A common strategy to achieve high power density is to operate at high current densities. However, the rate of heat generation is approximately proportional to the current density and waste heat removal is therefore a challenge for high power density systems. Overheating can cause drying of the conductive polymeric membrane inside the cell causing significant ohmic loss. It can also create local hot spots in the membrane electrode assembly (MEA) that are susceptible to degradation [1]. Hence, liquid coolant such as water or other thermal fluids is used in conventional PEMFCs by embedding separate cooling channels in the flow field plates. However, this type of cooling adds extra balance-of-plant (BOP) components into the system. This can be eliminated by using air as both oxidant and coolant, which is denoted as ‘open-cathode’ design. Even though open-cathode systems considerably reduce BOP cost [2], the open-cathode cell performance is limited by ambient air quality and temperature. The objective of the present work is to simulate the operational behaviour of open-cathode PEMFCs in order to better understand their performance limitations compared to conventional, liquid cooled cells. A comprehensive, 3D computational fuel cell model was developed for this purpose and utilized to study the operational and hygrothermal behaviour of an open-cathode PEMFC at various ambient conditions. Furthermore, the distribution of different cell parameters was analysed across the cell and a relationship was established that the temperature gradient across the cell guides the humidity distribution and hence the overall water management in the cell. These parameters contribute to the net membrane conductivity of the cell which dictates the actual cell performance. Overheating in such kind of a system was found to be key factor limiting the net cell performance while in operation at moderate and higher current densities. The results obtained were also compared with an equivalent liquid cooled cell and a cell operating in hypothetical isothermal condition at similar operating conditions as described in Figure 1. These three sets of modelling results clearly distinguish the operational behaviour of an open-cathode cell against that of a liquid cooled cell and also draws conclusion on factors affecting the low cell performance for an open-cathode system. The results from the study also suggests the inlet air flow rate to be crucial in terms of heat extraction. The net cell performance shifts upwards cushioning the gap between an open-cathode system and a liquid cooled system operating at similar conditions when the inlet air flow rate is increased optimally. AcknowledgementsThis work is supported by the funding provided by Indian Oil R&D and Simon Fraser University under the SFU-IOCL joint PhD program in clean energy. Funding from Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation and British Columbia Knowledge Development Fund is appreciated. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.