R.A. Rudkin

Oxidation of methane in solid state electrochemical reactors

Abstract Preliminary experiments are reported on the complete oxidation of CH 4 to CO 2 and H 2 O in ceramic electrochemical reactors (CER). Potentiostatic and cyclic voltammetric studies have demonstrated that platinum anodes are poor electrocatalysts for the oxidation of CH 4 at 800°C. However selected oxide anode materials are much more effective electro-catalysts and able to convert CH 4 to CO 2 and H 2 O with reasonable efficiencies at potentials appropriate for fuel cell operation. Other oxide anodes can be used in a CER to promote partial oxidation reactions and preliminary results for the conversion of CH 4 to C 2 H 6 are described using Bi 2 O 3 Pr 6 O 11 anodes.

Pd-promoted La0.6Sr0.4Co0.2Fe0.8O3 cathodes

Porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrodes supported on Ce0.9Gd0.1O1.95 (CGO) electrolyte were impregnated with small amounts of Pd and the electrochemical impedances resolved by AC impedance spectroscopy. The optimum Pd loading resulted in 3–4 times lower cathodic impedance in the temperature range 400–750°C. Single cell Solid Oxide Fuel Cells (SOFCs) were fabricated by tape casting CGO electrolyte onto an anode support. Under d.c operation, the addition of Pd to the LSCF cathode was found to decrease the overall cell resistance by 15% at 650°C and 40% at 550°C. This suggests that the SOFC becomes more limited by the performance of the cathode at lower temperatures and hence the greater benefit of Pd promotion. Finally, Pd particles were deposited on dense smooth LSCF substrates and Isotope Exchange Depth Profiling (IEDP) and Secondary Ion Mass Spectrometry (SIMS) were carried out to determine the tracer diffusion (D*) and surface exchange (k) coefficients. D* was not significantly effected but k decreased by well over an order of magnitude with Pd loadings similar to those applied to the electrode.

Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La0.6Sr0.4Co0.2Fe0.8O3∥Ce0.9Gd0.1O1.95∥Ni/YSZ, incorporating dense film (5–10 μm) Ce0.9Gd0.1O1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm2 were obtained at 650°C, and the cell resistance was 0.8Ω cm2 at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.

Material science aspects of SOFC technology with special reference to anode development

A model is presented for the development of alternative oxide anodes that can be optimised either for the complete or partial oxidation of methane. The model requires information about the kinetics of oxygen surface exchange and diffusion, and relevant data using 18O/16O isotopic exchange and SIMS measurements are summarised. Materials selection requires further information about the relevant defect chemistry and electronic conductivity and La0.8Sr0.2FeO3−σ and CeO2−x are used as examples of anode materials that can be used to electrochemically oxidise CH4 to CO2 and H2O. The performance of these materials in solid oxide electrolyte fuel cells is analysed in terms of the parameters mentioned earlier, and a strategy is suggested for the development of alternative oxide anodes for both the complete and partial oxidation of CH4.

Oxygen diffusion and surface exchange in La0.8Sr0.2Fe0.8Cr0.2O3−δ under reducing conditions

Abstract Oxygen tracer diffusion ( D *) and surface exchange ( k ) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry, in La 0.8 Sr 0.2 Fe 0.8 Cr 0.2 O 3− δ as a function of temperature (700–1000°C) in dry oxygen, in water vapour and in a water vapour/hydrogen/nitrogen mixture. The apparent activation energies for D * are 2.4 eV in oxygen, 2.6 eV in water vapour and 0.71 eV in the water vapour/hydrogen/nitrogen mixture. In these reducing conditions, D * is approximately 10 −6 cm 2 s −1 at 1000°C. At all temperatures, D * increases with reducing oxygen activity reflecting an increase in the concentration of oxygen vacancies. k remains high even under reducing conditions indicating that exchange with oxygen in the H 2 O molecule occurs at a similar rate to that with the O 2 molecule at normal atmospheric pressure of molecular oxygen.

The process, structure and performance of pen cells for the intermediate temperature SOFCs

Abstract The paper is principally concerned with strategic investigations based on the development of planar supported thin film electrolyte (STEF) configurations for intermediate temperature SOFC applications. A novel and cost effective electrostatic assisted vapour deposition process has been used to deposit dense YSZ film onto the porous anode substrates. Several approaches were investigated to deposit a dense and crack-free YSZ film onto the porous anode substrate during the fabrication of PEN structure. The use of an amorphous YSZ thin film followed by a crystalline YSZ film deposition using the continuous EAVD process seem to be able to produce a dense and crack-free YSZ film onto the porous NiO–YSZ substrate with a well defined interface. Individual dense/porous films and PEN assembly were characterised using XRD, SEM, and I–V test techniques for structural examination and electrical measurements.

Fabrication and discharge characteristics of thin film polymer electrolyte cells

Abstract Electrochemical cells, designed for incorporation into integrated circuits, have been fabricated using thin film deposition techniques. The cells, based on the Liƒ(PEO)8LiClO4ƒV2O5 system, are approximately 5 μm thick. Interpretation of the discharge characteristics indicates that diffusion-limiting behaviour is present in the polymeric electrolyte at discharge currents above 25 μA cm−2.

Metal-Supported Solid Oxide Fuel Cells for Operation at Temperatures of 500–650°C

Research within the Centre for Ion Conducting Membranes at Imperial College, London, is aimed at developing an innovative Intermediate Temperature Solid Oxide Fuel Cell. The main features of this technology involve the fabrication of a thick film PEN structure supported on a ferritic stainless steel substrate. Use of a metal support enables a robust structure to be fabricated, better able to withstand stresses developed during operation. Research has shown it is possible to arrange a processing schedule that allows the deposited electrolyte powder to be sintered into an impermeable thick film (10–20 μm) at temperatures around 1000°C. This relatively low sintering temperature is compatible with the mechanical integrity of the stainless steel support. An anode film is initially deposited on the metal support followed by deposition of the electrolyte powder. Much of the initial development work has been carried out using ceria based electrolytes. The cell is completed by the deposition of a cathode. This paper presents results arising from this programme, and reports on the development and characterisation of both anode and cathode materials, as well as progress in cell development.Copyright