Development of Hybrid Fuel Cell / Li-ion Battery Systems

Electrochemical power systems are needed to de-carbonise the transport industry. Fuel cells and battery systems alone may not be able to meet the diverse set of requirements, but when hybridised, their applicability to this sector is vastly increased. This raises questions around the specific nature of hybridisation. This thesis aims to expand our understanding of fuel cell and lithium-ion battery hybridisation for automotive applications, through a combined experimental and computational approach. Prior to undertaking such research, an understanding of each individual system is required. This is perused along the themes of current heterogeneity, and applied to parallel battery cells in two common electrical configurations and across the active area of a 100 cm2 polymer exchange fuel cell. First, it is shown the electrical configuration of the parallel string has significant impact on the current distribution, impacting the charge throughput of each cell and the usable capacity of the module. Degradation modelling showed the lifetime of the module is reduced by 4.5% in the less optimal configuration. Secondly, the current and thermal distribution within a fuel cell is investigated for a range of operating conditions such as flooding, drying and cold start. Electrochemical impedance spectroscopy is used to understand the conditions of the membrane and reactant time constants in-situ. Results indicate how the design of fuel cells can be refined to improve performance in challenging operating conditions. Finally, the investigation of electrical and thermal hybridisation is conducted on a passenger sized vehicle. A common modelling framework is developed, using the models developed in the fuel cell and battery chapters, to assess electrical energy management systems. A novel fuzzy logic controller is developed which mutates the output membership functions based on the ‘state-of-degradation’, a parameter derived from an interconnected electrochemical surface area loss and system state model. The controller is able to extend the lifetime of the fuel cell by 32.8% in its presented configuration. The common framework is then developed to include dynamic thermal models of the fuel cell, battery pack, radiator and auxiliaries to investigate whether combining the battery pack and fuel cell stack onto a single coolant loop is feasible. The system is tested against a range of operating conditions and its performance is discussed. These findings are expected to aid the transport industry in the transition to a zero emission future