Toleuova, A; (2015) Mechanistic Studies of Liquid Metal Electrode Solid Oxide Fuel Cells. Doctoral thesis , UCL (University College London).

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Abstract

Liquid metal electrode solid oxide fuel cells (LME SOFC) offer the advantage of high efficiency with the benefits of multi-fuel operation, the concept being particularly well-suited to the direct use of solid fuel. With respect to reported uncertainties in performance limitations of SOFCs with liquid metal anodes (LMA), in-depth understanding of the fuel oxidation mechanisms and transport processes within liquid metal electrodes is needed. This study provides improved understanding of the operation of LME SOFCs using novel experimental and modelling approaches. H2 oxidation in LMA SOFC in a specific potential range was chosen as the model system. A classification setting out four possible mechanisms of oxidation of H2 is proposed. Two models are developed based upon Electrochemical (E) and Chemical-Electrochemical (CE) modes of operation of H2-LMA SOFC under conditions that eliminate the detrimental effects of metal oxide layer formation at the electrolyte-electrode interface. The E mode model of operation was found to be inconsistent with literature knowledge of H2 solubility in liquid tin. A possible explanation considered is the generation of metallic foam effectively ‘storing’ hydrogen within its matrix, but this was not observed experimentally. Thus mode E (direct anodic oxidation of hydrogen in LMA SOFC) is not applicable to this system. A major contribution of this work is the development and validation of a model for the CE mode. This is based upon fast dissolution of hydrogen in a molten tin anode, rate-determining homogeneous reaction of hydrogen with oxygen dissolved in the liquid tin, followed by anodic oxygen injection under diffusion control to replace the oxygen removed by chemical reaction. A new key parameter, related to the Damkohler number, termed the dynamic oxygen utilisation coefficient, (z), evolved out of the model; its value is determined by geometric, mass-transport and kinetic factors in the cell, as well as the partial pressure of the supplied hydrogen fuel. Current output of the cell is proportional to the value of (z). This parameter is expected to have important implications regarding the design, development and commercialisation of the technology. Additional validation of the CE mode model included development and application of a method named anodic injection coulometry (with similarities to anodic stripping voltammetry) for determination of the parameter(z), as well as measurement of the oxygen solubility in liquid tin. Feeding H2 at 16 kPa partial pressure into the LME SOFC resulted in a (z) value of 0.83 under the chosen conditions. This is consistent with a separate estimate in this study using an unrelated method. The solubility of oxygen at 780 °C was found to be 0.10 at.%, which is comparable to literature values. The possibility of application of the liquid metal electrode / YSZ system for water electrolysis in solid oxide electrolysers (SOE) is explored. An electrochemical model is presented for interpretation of generated experimental results. Application of a glassy carbon rod as a low-cost current collector dipping into the liquid tin electrode was successfully pioneered in this work; it showed stable operation without corrosion throughout the whole project at the chosen operating temperature of 780 °C. A novel rotating electrolyte disc (RED) apparatus is proposed, which is an inverted arrangement of the well-known rotating disc electrode (RDE). The RED offers the prospect of measuring transport properties of active species within a liquid metal electrode. Initial studies towards the development of this technique have been undertaken.

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