Oxidation of 2-propanol in alkaline electrolytes using platinum and ruthenium-based catalysts: prototype fuel cells and electrokinetics studies
- Author / Creator
- Markiewicz, Matthew Eugene Paul
Alcohols are an attractive alternative to hydrogen fuel in fuel cells. They are energy dense, easy to store, transport, and they are readily available. Alkaline fuel cells have several kinetic advantages over acidic fuel cells, but they are sensitive to carbon dioxide. The fuel most studied in direct alcohol fuel cells is methanol. The oxidation of methanol, however, produces carbon dioxide that will gradually carbonate alkaline electrolytes, degrading their performance.
Investigations into the electrochemical oxidation of 2-propanol to acetone in alkaline electrolytes over platinum, platinum-ruthenium, and ruthenium catalysts were performed in three-electrode experiments. At the low anodic potentials that are required for efficient direct alcohol fuel cell, the oxidation of 2-propanol gives higher current densities than methanol over platinum. In contrast with the oxidation of methanol, which forms a stable carbon monoxide or similar surface poisoning intermediate, the oxidation of 2-propanol to acetone is believed to occur in the absence of a strongly adsorbed intermediate that hinders the reaction. Consistent with the behaviour in three-electrode experiments, prototype fuel cells operating on 2-propanol gave higher power densities than when operated on methanol, and they were also more stable.
The oxidation of 2-propanol at low potentials is enhanced by surface ruthenium. Multidimensional regression of the potential-temperature-current relationship found that ruthenium reduces the activation enthalpy by an amount consistent with hydrogen bonding (9 kJ/mol). A new transition state complex where an adsorbed oxygen species on ruthenium hydrogen bonds to the alcoholic proton of an intermediate formed during the oxidation of 2-propanol is proposed to account for this stabilization. This new mode of the bifunctional mechanism is believed to be responsible for the increased rate observed during the oxidation of 2-propanol at low potentials using platinum-ruthenium catalysts.
In an operating fuel cell, ruthenium was found to increase the kinetics of the reaction and reduce its onset potential. Both these factors increase the power density of prototype alkaline direct 2-propanol fuel cells when a platinum-ruthenium anode is used as the catalysts compared to when platinum is used.