Dilatometry studies of phosphotungstic acid pellets during hydration and dehydration processes and design of a room temperature fuel cell

Abstract

Phosphotungstic acid (PWA) is still a potential solid electrolyte for fuel cells due to its high conductivity at room temperature. In this work, PWA was synthesised and characterised by X-ray, IR spectroscopy and thermal analysis. Proton number was determined by potentiometry. Characterization results agreed with those already published. Water of crystallization was determined by weight loss method on heating to anhydrous form. Changes in pellet thickness and diameter were measured as a function of water of crystallization and time for dehydration/hydration processes in controlled humid atmospheres. Geometric change parameter Delta L/L, where Delta L is the change in the volume (either thickness or diameter, and L is the initial dimension) was determined for each sample. After an initial hydration or dehydration process, pellets were subsequently dehydrated or hydrated in order to study the reversibility of the geometric changes. Effects of presence: of polytetrafluoroethylene (PTFE) as a binder, pelletizing pressure and particle size of PWA powder are discussed. It was found that changes in pellet diameter were greater than the pellet thickness irrespective of the pelletizing pressure and particle size. Changes in pellet thickness were lower for pellets made with higher pelletizing pressure and greater particle size, whereas changes in the pellet diameter were almost identical. The presence of PTFE lowered both the dimensional changes and the rate at which water of crystallization was lost. Interestingly, steps in the changes in pellet thickness during hydration and dehydration were observed in line with the loss of water of crystallization. Possibility of pinhole generation in the dehydrated pellets was also observed. A new fuel cell was designed, which eliminated the need of sealing the electrolyte completely in the cell. The hydrogen and oxygen gases were prehumidified to 87 percent relative humidities, and there was no accumulation of water at the cathode side of the fuel cell. An open circuit potential of similar to 0.7 V was observed for about 40 h during the testing of this cell. Further testing and use of this design incorporating PWA and other hydrated forms of proton conductors such as polymer membranes is suggested.