Investigations of electrolytes suitable for use in fuel cells
Adam, Henry Harper
The use of potassium carbonate-bicarbonate buffers as an alternative to strong acids as CO2 rejecting electrolytes for low temperature fuel cells has been investigated. The physical properties which determine the suitability of buffer electrolytes for fuel cells, conductivity and pu, have been measured for solutions of K2C03 (a) under N2 and (b) under CO2 at 1 atmosphere pressure; and the behaviour of fuel cell electrodes in these electrolytes has also been studied. These conditions were chosen to represent the best and worst situations likely to occur with potassium carbonate-bicarbonate electrolytes in fuel cells. Conductance measurements have been carried out at 25°C, 40°C, 60°C and 75°C, measurements at low concentrations were carried out with a conventional conductance cell, those at high concentrations were carried out with an 'electrodeless' conductance cell. Concentrations up to saturation have been studied. These experiments represent the first complete study of such solutions over the full concentration range and at temperatures other than 25°C. pH measurements on these solutions were made by use of a novel method in which e.m.f values of cells containing hydrogen ion responsive glass electrodes and potassium ion responsive glass electrodes were determined. These measurements covered the full concentration range at 25°C, 45°C and 65°C. (Attempts to obtain reproducible results at 85°C failed.) Density measurements were made, primarily to interpret trio conductivity results, by a pyknometric technique over the full range of concentration at 25°C, 40°C, 60°C and 75°C. The results obtained for the K2CO3 solutions under N2 are compared and contrasted with the only available data and the values reported in this work are shown to be preferable. The results obtained for solutions under CO2 are not available elsewhere. The behaviour of fuel cell electrodes showed that limiting current densities with hydrogen electrodes can be increased by the use of thinner electrode substrates (a quantitative analysis of these results is given). 'ohmic free' polarisation is reduced with oxygen electrodes as the ph of the buffer electrolyte is increased and a quantitative description of the variation at two current densities is given. 1mpirical expressions are given for the variaions of conductivity and density with concentration. i method of calculation of the ph values of the solutions under various conditions of temperature and partial pressure of CO2 is reported and is shown to agree well with the experimental results. The sum of this information provides the means to estimate the optimum conditions for the use of the potassium carbonate—bicarbonate electrolyte in fuel cells. New values of limiting conductivity and apparent molal volume have been obtained for the species K2CO3, KHCO3 and CO2 (for C02 limiting apparent molal volume only) in aqueous solution from the analysis of the results at low concentrations.