Review on Dimensionless Numbers Relevant for Polymer Electrolyte Fuel Cells

Abstract

With the concern of global warming, air pollution and energy security, the use of polymer electrolyte fuel cells (PEFCs) has achieved substantial momentum for future sustainable and renewable energy conversion systems. The fuel cell (FC) is not a new invention and its principle dates back to 1838. FC science and technology cut across multiple disciplines, including (i) materials science, (ii) chemistry, (iii) (iv) mechanical/chemical engineering and (v) catalysis. The PEFC has emerged as the leading fuel cell type for automotive and some portable applications, and also as back-up power due to its operation at low temperature, comparative simplicity in construction, high power density, and ease of operation. In spite of tremendous scientific advance as well as engineering progress over the last decades, the commercialization of PEFCs has been delayed, due primarily to the following aspects: (1) technical problems mainly concerning water management, (2) economic viability associated with high prices for materials and components, (3) membrane fragileness and (4) membrane hydration. The difficulty in understanding water management lies mostly in the two phase multi-component flow involving phase-change in porous media, coupled heat and mass transfer, interactions between the porous layers and the gas channel (GC), and the complex relationship between water content and cell performance. In PEFCs the electrochemical reactions are strongly coupled to the transport of gas phase and liquid phase species, momentum, charge and heat. The transport, phase-change and reaction processes within PEFCs occur at different length and time scales simultaneously. Due to the low-temperature of the operation, water generated by the electrochemical reactions often condenses into liquid phase, potentially flooding the catalyst layer, gas diffusion layer (GDL), micro porous layer (MPL) and GC (see Fig. 1). Insight into the fundamental processes of liquid water evolution and transport is still lacking, preventing further enhanced fuel cell development. Diffusion media characterization and development still rely heavily on in-situ testing because of well-established correlations between in-situ performance results and ex-situ characterization data are not yet available. This limitation has resulted in the development of detailed computational fluid dynamics (CFD) based models where the ability to predict local and global characteristics such as voltage, current density, temperature and concentrations have been demonstrated. However, research combining experimental and modeling activities in a systematic iterative way is still missing. Some models assumes the GDL (and MPL) water transport as singe phase only. CFD models make it possible to reduce the number of experiments needed for cell design and development, and a decreased amount of tests are then required to validate the accuracy of the models. Modeling can also be used to confirm experimental results and conclusions. Various assumptions are made in CFD models, e.g., the gas phase, liquid phase, ionic and electronic tortuosities as well as the contact angles are normally treated as fitting parameters, used in the respective governing equations, thus unrealistic values may be assumed, or the property values might not be representative for the corresponding microstructures.