Atomic iron in nitrogen-doped carbon electrocatalysts for proton exchange membrane fuel cells

Abstract

Atomic Fe in Nitrogen-doped Carbon (Fe-N-C) is investigated for the O2 reduction reaction in proton exchange membrane fuel cells (PEMFCs). Chapter 1 introduces the research motivation. Chapter 2 provides a literature review, scoping out gaps in the literature and thus the aims and objectives for the thesis. Chapter 3 covers the methods and characterisation techniques used in this thesis. Chapter 4 focusses on the development and characterisation of a novel Fe-N-C with high surface area and porosity, produced by an Mg-templating approach. It is found this synthesis leads to record in situ electrochemical active site (FeNx) utilisation in a rotating disc electrode. The FeNx sites exhibit an axial coordination with low turnover frequency . Therefore, the impact of axial and N-coordination is investigated via DFT. Chapter 5 investigates the operando Fe dissolution mechanism under inert (Ar) and active (O2) conditions of the Fe-N-C developed in Chapter 4, using both flow-cell and gas diffusion electrode (GDE) coupled to an online ICP-MS. During O2 reduction at 20oC in GDE, a mechanism of Fe dissolution and reprecipitation into FexOy occurs due to pH increasing in the catalyst layer. This is supported by post-mortem characterization and kinetic modelling. Chapter 6 examines the impacts of increasing the ionomer to catalyst ratio in H2-O2 fed PEMFCs. The Fe-N-C optimisation is probed using in situ electrochemical techniques. A uniquely high ionomer:Fe-N-C content is required for maximum performance due to the Fe-N-C’s high mesoporosity. Chapter 7 investigates the life cycle and techno-economic analysis of Fe-N-C versus Pt/C for PEMFCs. Fe-N-C production leads to lower predicted endpoint environmental impacts. Monetized environmental impacts are found to provide higher cost than the catalyst production. Based on the techno-economic analysis, the power density for Fe-N-C to compete with Pt/C is calculated. Chapter 8 provides conclusions and research directions.