TEMPO-Modified Linear Poly(ethylenimine) for Immobilization-Enhanced Electrocatalytic Oxidation of Alcohol

Small molecule electrooxidation catalysts such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) are capable of electrochemically oxidizing short chain alcohols and various sugars to the corresponding aldehydes and carboxylic acids under physiological aqueous conditions. This makes them ideal alternatives to enzymatic catalysts when deeper oxidation of a fuel is desired in the anodic compartment of a biofuel cell. While enzymes are capable of very fast rates of electrochemical alcohol oxidation (when compared to that of either transition metal catalysts or small organic catalysts), their activity is often limited to a small range of substrates. Therefore multiple oxidative reactions with a single molecule of fuel requires that several redox enzymes be immobilized at an electrode surface simultaneously, which results in additional complexity as well as diffusional limitations as substrate travel between a specific sequence of active sites.

Alternatively, TEMPO has been recently shown to catalyze multiple sequential oxidations in situ of the biofuel glycerol thereby allowing for the collection of up to 12 electrons per molecule of fuel. However, in order to utilize TEMPO in a fuel cell application, the catalyst must be immobilized at the electrode surface, and previous methods for TEMPO immobilization have resulted in a dramatic loss of catalytic activity due to diffusional restrictions. Thus there is still a need for immobilization techniques that do not result in activity loss.

We have developed a method to immobilize a TEMPO catalyst in a cross-linked hydrogel film at the surface of a carbon electrode that allows for a dramatic increase in the rate of electrocatalytic oxidation of several sugars and short-chain alcohols. A TEMPO derivative was covalently attached to a linear poly(ethylenimine) backbone, and the modified polymer was cross-linked onto the surface of a glassy carbon electrode. The use of a hydrogel film allows for a porous network through which substrate can freely diffuse and the use of a polyamine provides a high localized concentration of base necessary for the catalytic cycle of TEMPO.