ELECTROCHEMICAL PROPERTIES OF PERFLUOROALKANE DISULFONIC [HSO3(CF2)NSO3H] ACIDS RELEVANT TO FUEL-CELL TECHNOLOGY

The electrochemical properties of a homologous series of perfluoroalkane-alpha, omega-disulfonic acids were measured. These acids are high molecular weight analogs of trifluoromethane sulfonic acid, having the general molecular formula [HSO3(CF2)nSO3H]. Properties measured included the solubility and conductivity of aqueous solutions, the extent of anion specific adsorption on platinum, and oxygen reduction kinetics on platinum, for acids having n = 2, 4, and 6. The following trends in properties with variation in n were observed: solubility in water and the conductivity of aqueous solutions decreased dramatically with increasing n; an increase in the extent of anion specific adsorption with increasing n; at constant pH, rate of oxygen reduction on Pt decreased-with increasing n; for a given acid (value of n), the rate of oxygen reduction decreased exponentially with increasing acid concentration. These trends in physical properties can be explained in a straightforward manner if it is assumed that there is successively weaker hydration of the anions with increasing n, i.e., size of the anion. Of the acids in this homologous series, the n = 2 acid appears to have the best combination of properties for a fuel cell electrolyte.