Richard Dawson

Significance enhancement in the conductivity of core shell nanocomposite electrolytes

Today, there is great demand of electrolytes with high ionic conductivities at low operating temperatures for solid-oxide fuel cells. Therefore, a co-doped technique was used to synthesize a highly ionically conductive two phase nanocomposite electrolyte Sr/Sm–ceria–carbonate by a co-precipitation method. A significant increase in conductivity was measured in this co-doped Sr/Sm–ceria–carbonate electrolyte at 550 °C as compared to the more commonly studied samarium doped ceria. The fuel cell power density was 900 mW cm−2 at low temperature (400–580 °C). The composite electrolyte was found to have homogenous morphology with a core–shell structure using SEM and TEM. The two phase core–shell structure was confirmed using XRD analysis. The crystallite size was found to be 30–60 nm and is in good agreement with the SEM analysis. The thermal analysis was determined with DSC. The enhancement in conductivity is due to two effects; co-doping of Sr in samarium doped ceria and its composite with carbonate which is responsible for the core–shell structure. This co-doped approach with the second phase gives promise in addressing the challenge to lower the operating temperature of solid oxide fuel cells (SOFC).

An investigation into the use of additive manufacture for the production of metallic bipolar plates for polymer electrolyte fuel cell stacks

The bipolar plate is of critical importance to the efficient and long lasting operation of a polymer electrolyte fuel cell (PEMFC) stack. With advances in membrane electrode assembly design, greater attention has been focused on the bipolar plate and the important role it plays. Although carbon composite plates are a likely candidate for the mass introduction of fuel cells, it is metallic plates made from thin strip materials which could deliver significant advantages in terms of part cost, electrical performance and size. However, there are some disadvantages. Firstly, interfacial stability of the metal interconnect is difficult to achieve. Secondly, and the issue addressed here, is the difficultly and cost in developing new plate designs when there are very significant tooling costs associated with manufacture. The use of selective laser melting (SLM: an additive manufacturing technique) was explored to produce metallic bipolar plates for PEMFC as a route to inexpensively test several plate designs without committing to tooling. Crucial to this was proving that, electrically, bipolar plates fabricated by SLM behave similarly to those produced by conventional manufacturing techniques. This research presents the development of a small stack to compare the short term performance of metallic plates made by machining against those made by SLM. Experimental results demonstrate that the cell performance in this case was unaffected by the manufacturing method used and it is therefore concluded that additive manufacturing could be a very useful tool to aid the rapid development of metallic bipolar plate designs.

Finite element analysis and modelling of thermal stress in solid oxide fuel cells

Durability and reliability of anode supported solid oxide fuel cell stacks have proven unsatisfactory in large-scale trials, showing rapid failure, thermal cycling intolerance and step change in electrochemical performance most likely related to mechanical issues. Monitoring and understanding the mechanical conditions in the stack especially during temperature changes can lead to improvements of the design and of the operating regime targeting maximum durability. Within this project modelling and simulation of thermal stresses within the different parts of the cells and the stack and the validation of these models play a key role and were performed in this work. The modelling and simulation of stress and strain have been carried out using the FEA software ABAQUS™. Model variations documented the importance of exact knowledge of material properties like Young’s modulus, Poisson’s ratio, thermal expansion coefficient, thermal conductivity and creep viscosity. The benefit of literature data for these properties is limited by the fact that all these properties are highly dependent on the composition of materials but also on details of the fabrication process like mixing, fabrication technique and sintering temperature and duration. The work presented here is an investigation into the modelling techniques, which can be most efficiently applied to represent anode supported solid oxide fuel cells and demonstrates the temperature gradient and constraint on the stresses experienced in a typical design. Comparing different meshing elements representing the cell parts thin shell elements (S4R) provided the most efficiently derived solution. Tensile stress is most significant in the cathode layers reaching 155 MPa at working conditions. The stress relieving effect of creep led to a reduction of stress by up to 20% after 1000 h at 750 ℃, reducing the tensile stress in the cathode area to maximal 121 MPa. Constraint between bipolar plates increases the tensile stress, especially in the cathode layers leading to a peak value of 161 MPa.