THIS PROJECT FORMS PART OF THE TECHNICAL UNIVERSITY OF DELFT'S RESEARCH PROGRAMME ON MOLTEN CARBONATE FUEL CELLS,WHICH IS BEING PROMOTED INTO A NATIONAL FUEL CELL PROGRAMME WITH THE ECN (NETHERLANDS CENTRE FOR ENERGY RESEARCH) AS MAIN IMPLEMENTING BODY.
THE TUD IS CONCENTRATING ON ELECTRO-CHEMICAL RESEARCH INTO ELECTRODES AND ELECTROLYTES,WITH A VIEW TO STUDYING AND,IF POSSIBLE,SOLVING A NUMBER OF FUNDAMENTAL PROBLEMS.
THE MOST IMPORTANT OF THESE PROBLEMS IS THE DISSOLUTION OF NICKEL OXIDE CATHODES AFTER LENGTHY USE IN A WORKING CELL.THE TUD'S RESEARCH DIFFERS FROM THAT OF THE ECN AND THE ITALIAN CISE (CENTRE FOR INFORMATION,STUDIES AND EXPERIMENTS) IN STUDYING THE (ELECTRO-)CHEMISTRY OF THE NICKEL OXIDE DISSOLUTION PROCESS -THIS WITH A VIEW TO IMPROVING THE PERFORMANCE AND ENDURANCE OF THE CATHODES IN QUESTION.THE RESULTS OF ITS RESEARCH WILL BENEFIT BOTH DUTCH AND ITALIAN NATIONAL FUEL CELL PROGRAMMES.
The lithiated nickel oxide (LiNiO) cathode of a molten carbonate fuel cell slowly dissolves into the matrix, reducing the cell lifetime. The nickel is deposited in a band in the matrix. Both the cathodic and anodic band edges move towards the anode with time. Deposition of nickel and movement towards the anode are explained by a flux model involving the transport of nickel ions, peroxide, superoxide and hydrogen into the matrix. The movement of the band edges towards the anode is explained by a change in the tortuosity of the paste type matrix due to compression.
The oxygen reduction in molten carbonate has been studied on a gold electrode submerged in a molten carbonate melt and on 3 different porous electrodes, made of lithiated nickel oxide, lithium ferrite (LiFeO2) doped with either magnesium or cobalt and lithiated cobalt oxide (LiCoO2).
Impedance measurements on a gold electrode have shown that 2 parallel reaction mechanisms are involved in the oxygen reduction. The reaction steps for one of these mechanisms have been resolved. Peroxycarbonate which is formed in the melt by the reaction of peroxide and carbon dioxide, is reduced at the electrode surface which is partially oxidised. The impedance of this mechanism is several orders of magnitude smaller than that of the parallel mechanism, therefore the current produced by it is several orders of magnitude greater than that produced by the parallel mechanism. The second mechanism has, therefore, negligible influence on the chronoamperometry, and quasistationary polarisation measurements. The results of chronoamperometry which show that the reduction mechanism is more complex than a simple charge transfer reaction, are in agreement with theory which predicts a mechanism involving a chemical reaction and product adsorption. This mechanism also explains the partial pressure dependencies of the limiting currents observed in the quasi-steady state polarisation measurements.
The difference in magnitude betwee n the linking currents and universe polarisation resistances in a lithium carbonate/potassium carbonate electrolyte (62/38% mol) with and without 1% barium carbonate indicates that either the diffusion constant or the concentration of the diffusing species is higher when the barium carbonate is present.
Both oxygen and carbon dioxide are the diffusing species, indicated by the partial pressure dependencies of the diffusion are observed in the impedance of the porous electrodes. 2 reduction mechanisms can explain all observed reaction orders of the kinetic arc. It is due to either the reduction of dissociated oxygen or peroxycarbonate. Dissociation of oxygen or the formation of peroxycarbonate occurs at the electrode surface. The electrode surface of lithiated nickel oxide and of lithiated cobalt oxide has an intermediate coverage of absorbed oxide. The surface of cobalt doped lithium ferrite has little or no coverage.
From comparison of the diffusion and kinetic arc, it is concluded that the catalytic activities of the 3 materials do not differ significantly. The larger impedance observed for cobalt doped lithium ferrite is due to the larger specific resistivity of this material.
THE CATHODE MATERIAL USED AT PRESENT IN THE MOLTEN CARBONATE FUEL CELL GRADUALLY DISSOLVES.THE PURPOSE OF THE PROJECT IS TO STUDY THE MECHANISM AND KINETICS OF THIS DISSOLUTION PROCESS IN RELATION TO MOLTEN CARBONATE CHEMISTRY AND PREVAILING TRANSPORT CONDITIONS.
IF THERE IS ENOUGH TIME,ALTERNATIVE OXIDES AND OXIDE MIXTURES WILL ALSO BE STUDIED.