THE INVESTIGATION OF IONIC AND MOLECULAR ADSORPTION ON PLATINUM IN TRIFLUOROMETHANESULFONIC ACID AND RELATED ALKANESULFONIC ACIDS

The nature of adsorption on platinum in the presence of trifluoromethanesulfonic acid (TFMSA) has been studied in an attempt to understand what factors make TFMSA a superior electrolyte to H(,3)PO(,4) under fuel cell conditions. The study has encompassed both molecular adsorption and ionic adsorption. From the molecular point of view, the adsorption of carbon monoxide has been investigated to determine why CO is the rate controlling species in the reaction of synthetic reformate fuels on platinum. Potentiokinetic and linear potential sweep techniques have been used to follow the adsorption and desorption processes. The study verified that, just as in the case of other acids, CO readily adsorbs on platinum but it is neither oxidized to CO(,2) nor desorbed at any appreciable rate. The rates of oxidation and desorption can be significantly increased by either increasing temperature or electrode potential. The absence of a strong interaction between the anion of TFMSA and the catalyst is believed to be one of the factors responsible for the superior performance of TFMSA. Using cyclic voltammetry the structure of hydrogen adsorption and desorption on platinum has been used as a tool for measuring anion adsorption. The results have shown that the adsorption/desorption structure in TFMSA is identical to the structures of other non-adsorbing acids such as HF or HClO(,4). In contrast, the hydrogen structure in H(,3)PO(,4) is significantly different from those of either TFMSA, HF or HClO(,4). The non-adsorbing nature of TFMSA has been attributed to the presence of a fully fluorinated hydrocarbon structure but when unfluorinated alkanesulfonic acids (C(,1)-C(,3)) were examined, only ethanesulfonic acid showed any indication of surface activity. While the presence of fluorine in the hydrocarbon structure has not been found to be critical at low acid concentrations (< .5N), at higher concentrations, fluorination will probably be necessary.