Monoalkyl viologens are effective carbohydrate O2-oxidation catalysts for electrical energy generation by fuel cells

Dialkyl viologens are known to be effective catalysts for carbohydrate oxidation by O2 under the mild conditions of room temperature and pH 9–12 and have been used to construct a simple carbohydrate fuel cell. The mechanism of carbohydrate oxidation proceeds by a stepwise, dialkyl viologen-mediated, sequential oxidation starting at the anomeric carbon and continuing down the carbohydrate chain producing carbonate and formate as products without loss of dialkyl viologen catalytic function. At pH values >13 the dialkyl viologens undergo a complex hydrolysis reaction forming monoalkyl viologen. The hydrolysis mixture catalytically oxidizes carbohydrate and monoalkyl viologen was confirmed as the active catalysts using independently synthesized monoalkyl viologens. Monoalkyl viologens are stable and catalytically active toward carbohydrate oxidation from pH 9.0 to conditions of 3.0M KOH. Monoalkyl viologens react at a rate approximately 50% of that of dialkyl viologens, but are more stable at high pH, making them versatile carbohydrate oxidation catalysts for fuel cell use. Because of a more negative redox potential, monoalkyl viologens more efficiently convert the chemical energy of carbohydrates into electrical energy. The catalytic reactivity of monoalkyl viologens reported here should be of use as homogeneous and immobilized catalysts in alkaline carbohydrate fuel cells.