Flow-field/electrode unified membrane-electrode assemblies using graphene foam in polymer electrolyte membrane fuel cells

Abstract

Fuel cells are electrochemical energy conversion devices that directly convert chemical energy to electrical energy. Among various kinds of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are attractive for transportation and portable electronics due to low operating temperature, high power density, and short start-up time. However, high cost components, scarcity of catalyst, and poor durability have limited the commercialization. Therefore many reports have focused on two ways: one is to develop inexpensive and durable electro-catalyst by reducing the use of platinum or replacing platinum with other nonprecious catalysts. The other is to enhance performance of single cell by improving structure of catalyst layer or modifying flow field of bipolar plate and gas diffusion layer. Recent researches have shown substituting conventional flow field for porous metal materials such as metal powder, metal coil, metal mesh, and metal foam, which have novel structures, increased cell performance by improving mass transport of reactant and product. Furthermore, some reports eliminated gas diffusion layer (GDL) from membrane-electrode assembly (MEA) by using these materials as multifunctional materials of GDL and flow field. These design reduced reactant pathway from bipolar plate to catalyst layer, and mass transport resistance, resulting in increased performance of single cell. This thesis focused on the enhancement of performance of PEMFCs by substituting graphene foam with cell components. Graphene foam is three-dimensional graphene-based material having advantage of graphene and structural characteristics of metal foam. Various kinds of the metal materials have drawbacks in PEMFCs operating condition. Metal is highly susceptible to corrosion in acidic condition and metal ion can contaminate MEA. Therefore, graphene foam is promising material that has novel structures without the use of metal. The ultimate objective is to fabricate unified MEA by using graphene foam as GDL and flow field. Compressed graphene foam can play role in GDL as well as flow field. This design reduces reactant pathway from bipolar plate to catalyst layer, mass transport resistance, and thickness of MEA, resulting in increased performance of single cell and volume power density of stacks. To prepare unified MEA, the effect of graphene foam on flow field and GDL were investigated. Firstly, cell test, oxygen gain, and electrochemical impedance spectroscopy (EIS) were conducted to examine of the effect of graphene foam as flow field to enhanced mass transport. These results have shown that graphene foam as flow field distributed uniformly reactants on entire area and removed generated water by inhibiting water flooding. Secondly, single cell that integrated gas diffusion layer (GDL) with flow field was conducted to investigate the effect of unified MEA on cell performance. Eliminating GDL in MEA by using graphene foam as multifunctional material of GDL and flow field reduced electrical and mass transport resistance, and thickness of MEAs, leading to enhanced performance and increased volume power density. As a result, GDL-less MEA increased power density by 50% and decreased thickness of MEA by 90%, resulting in enhanced volume power density. Therefore, graphene foam can play role in GDL and flow field.