Novel Materials and Architectures for the Gas Diffusion Layer of Polymer Electrolyte Membrane Fuel Cells.

Rapid population expansion and increased global development have led to a significant surge in energy demand. It is critical that the energy needs of this growing population are met in a sustainable way to provide energy security for future generations and to promote harmonisation between the built and natural environment. Global climate change caused by anthropogenic carbon dioxide emissions and greenhouse gases is single biggest threat facing humanity today. Energy efficient designs and the increased use of renewable energy sources and hydrogen energy systems are a realistic means for the mitigation of CO2 emissions. Polymer Electrolyte Membrane fuel cells are a key component in the hydrogen economy, and they offer significant potential as prospective replacement for fossil fuel-based technologies. PEM fuel cells, as they are known, benefit from high power density, efficiency and most importantly versatility. Although their commercialisation is in the developmental phase, the technology has a broad spectrum of use, being implemented in fuel cell vehicles (Honda Clarity, Toyota Mirai, Japan), residential combined heat and power (Enefarm, Japan), and portable electronics (Intelligent Energy, UK). There are, however, limitations affecting the wide scale deployment of PEM fuel cell technology. The gas diffusion layer and the microporous layer are essential components for maintaining the balance of liquid water in the fuel cell, and the removal of excess liquid water from the cathode porous media continues to pose problems for the PEM fuel cell operating in humid conditions and at high current densities. The microporous layer is a layer applied to the catalyst interface of the gas diffusion layer of the PEM fuels, thus it plays an integral role in the efficient operation of the cell. It is responsible for ensuring the effective transfer of heat, electrons, and mass, as well as preserving the balance of liquid water to simultaneously maintain membrane hydration and remove excess liquid water. In this thesis, we present three chapters on novel materials and architectures for the microporous layer of the Polymer Electrolyte Membrane Fuel Cell. Conventional microporous layers are produced from carbon black and a hydrophobic binder. Microporous layers were produced with inclusions of graphene in order to alter the microstructure and consequently, the ability to reduce liquid water flooding. The inclusion of graphene nanoplates in the microporous layer resulted in changes to the microstructure and the physical properties of layer. The graphene nanoplates imparted desirable characteristics onto the layer, notably increasing the hydrophobicity of the layer and its electron conductivity. These improved characteristics resulted in significant performance enhancements in high humidity operation (50% relative humidity and greater) for microporous layers containing ≤ 50 wt.% of graphene. Graphene foam was considered a novel MPL material, in order to do so graphene foams were synthesised from the pyrolysis of sodium ethoxide in different atmospheres, 100% N2 and 95% N2 and 5% H2 to produce N-GF and NH-GF foams respectively. XPS was used to quantify the carbon content and SEM revealed the open pores structure of the foams. These graphene foams then formed the basis of novel microporous layers which were characterised alongside microporous layers produced from conventional carbon black and graphene nanoplates. The N-GF exhibited better ex-situ characteristics, such as higher electron mobility, and produced greater power density in the single cell measurements. Alternative architectures were considered from the construction of bi-layer porosity-graded microporous layers. Whereby microporous layers were produced by applying two separate layers of MPL ink formed of carbon black and graphene nanoplates. Each layer exhibits its individual physical and microstructural properties, where the carbon black layer is microporous, the graphene nanoplates were characterised by large mesopores. These layers were characterised ex-situ in terms of their microstructural, physical, and electrochemical properties, and in single cell measurements where higher power densities and limiting current densities were achieved with the bi-layer microporous layers.