Edward Brightman

Designing a miniaturised heated stage for in situ optical measurements of solid oxide fuel cell electrode surfaces, and probing the oxidation of solid oxide fuel cell anodes using in situ Raman spectroscopy

A novel miniaturised heated stage for in operando optical measurements on solid oxide fuel cell electrode surfaces is described. The design combines the advantages of previously reported designs, namely, (i) fully controllable dual atmosphere operation enabling fuel cell pellets to be tested in operando with either electrode in any atmosphere being the focus of study, and (ii) combined electrochemical measurements with optical spectroscopy measurements with the potential for highly detailed study of electrochemical processes; with the following advances, (iii) integrated fitting for mounting on a mapping stage enabling 2-D spatial characterisation of the surface, (iv) a compact profile that is externally cooled, enabling operation on an existing microscope without the need for specialized lenses, (v) the ability to cool very rapidly, from 600 °C to 300 °C in less than 5 min without damaging the experimental apparatus, and (vi) the ability to accommodate a range of pellet sizes and thicknesses.

Reduction Dynamics of Doped Ceria, Nickel Oxide, and Cermet Composites Probed Using In Situ Raman Spectroscopy

The redox properties of gadolinium doped ceria (CGO) and nickel oxide (NiO) composite cermets underpin the operation of solid oxide electrochemical cells. Although these systems have been widely studied, a full comprehension of the reaction dynamics at the interface of these materials is lacking. Here, in situ Raman spectroscopic monitoring of the redox cycle is used to investigate the interplay between the dynamic and competing processes of hydrogen spillover and water dissociation on the doped ceria surface. In order to elucidate these mechanisms, the redox process in pure CGO and NiO is studied when exposed to wet and dry hydrogen and is compared to the cermet behavior. In dry hydrogen, CGO reduces relatively rapidly via a series of intermediate phases, while NiO reduces via a single‐step process. In wet reducing atmospheres, however, the oxidation state of pure CGO is initially stabilized due to the dissociation of water by reduced Ce(III) and subsequent incorporation of oxygen into the structure. In the reduction process involving the composite cermet, the close proximity of the NiO improves the efficiency and speed of the composite reduction process. Although NiO is already incorporated into working cells, these observations suggest direct routes to further improve cell performance.

Degradation Study by Start-Up/Shut-Down Cycling of Superhydrophobic Electrosprayed Catalyst Layers Using a Localized Reference Electrode Technique

Degradation of a polymer electrolyte membrane fuel cell (PEMFC) with electrosprayed cathode catalyst layers is investigated during cyclic start-up and shut-down events. The study is carried out within a single cell incorporating an array of reference electrodes that enables measurement of cell current as a function of local cathode potential (localized polarization curves). Accelerated degradation of the cell by start-up/shut-down cycling gives rise to inhomogeneous performance loss, which is more severe close to the gas outlet and occurs predominantly during start-up. The degradation consists primarily of loss of cathode catalyst activity and increase in cell internal resistance, which is attributed to carbon corrosion and Pt aggregation in both anode and cathode. Cells with an electrosprayed cathode catalyst layer show lower degradation rates during the first 100 cycles, compared with those of a conventional gas diffusion electrode. This difference in behavior is attributed to the high hydrophobicity of the electrosprayed catalyst layer microstructure, which retards the kinetics of corrosion of the carbon support. In the long term, however, the degradation rate is dominated by the Pt/C ratio in the cathode catalyst layer.

Fuels and fuel processing for low temperature fuel cells

This chapter examines the role of the fuel in the operation, performance and degradation of fuel cells. The range of fuels and impurities that are of relevance to low-temperature fuel cells are discussed and the performance from a thermodynamic perspective is analysed. As a route to hydrogen, various fuel processing options are considered along with an overview of the major storage techniques. Issues associated with alternative fuels are covered along with the deleterious properties of fuels and their impurities.

In Situ Study of PEMFC Start-Up Degradation Using a Novel Through-Plate Reference Electrode Array

In order to meet cost and durability targets for polymer electrolyte membrane fuel cells (PEMFCs), there is an urgent need for improved in situ measurement techniques to enhance understanding of degradation mechanisms and to facilitate design optimisation. A highly innovative reference electrode design has recently been developed at NPL that makes use of a standard hydrogen electrode (SHE) connected to the active area of the cell via a fine Nafion tube salt bridge inserted through the end plates of the fuel cell. The ion conducting path through the gas diffusion layer is achieved by Nafion impregnation at the point of contact with the Nafion tubing. Use of an array of such electrodes has for the first time enabled mapping of the spatial distribution of electrode potential in an operating PEMFC while minimising the perturbation of the measurement that occurs with conventional fuel cell reference electrode designs. Here we demonstrate application of this technique to the investigation of catalyst support corrosion during fuel cell start-up, when fuel is introduced to the cell without a nitrogen purge. In this situation an air/fuel boundary progresses through the anode flow-field causing a transient mixed potential at the cathode of >1.4 V versus SHE, which enables carbon oxidation to occur. The NPL reference electrode array allows unprecedented time-resolved measurement of this potential spike as it progresses through the cell. The influence of flow-field design on the potential transient behaviour during start-up is investigated, supported by measurements of CO2 evolution from corrosion of the carbon catalyst support.