Electron+Transport

All enzymatic fuel cells depend on the same principles. If an enzymatic fuel cell consisted of 2β – D-glucose as the fuel and glucose oxidase as the catalyst, the general chemical equations would be:

Anode: 2β - D-glucose --> 2δ gluconolactone + 4e- + 4H+ Cathode: O2 + 4H+ + 4e- --> 2H2O

Generally, the anode oxidizes a fuel where the electrons travel to an outer circuit, creating an electric current. Electrons continue on the pathway towards the cathode to create the byproducts of the reaction [1].

= = =**Electron Transfer:**=

Electron transfer can occur in two ways for a EFC: Direct Electron Transfer (DET) or Mediated Electron Transfer (MET). Enzymes that are able to catalyze and transfer electrons to the electrode use DET. Mediators are redox active compounds that are introduced to the system to aid the enzyme in surmounting the electron transfer kinetic barrier. The mediator reacts with the enzyme or its cofactor to transfer electrons to and from the electrode by reducing or oxidizing, this process is MET [2].



Mediators act as a co-substrate in the enzymatic system. The enzyme catalyzes the reduction/oxidation of the mediator, where the mediator regenerates at the electrode through electron transfer. DET shuttles electrons directly from the substrate to the electrode using the enzyme's active site. This mechanism involves the electrode surface acting as the enzyme co-substrate. The efficiency of DET depends on the location of the active site. If it is located closer to the protein surface, and is oriented towards the electrode; the electron distance needed for the electron the travel is minimized [3].

Electron transfer rates are typically lower for DET due to the redox center of the enzyme having to be close in proximity to the electrode. This requires the enzyme to be directly or covalently bonded to the electrode surface; in turn reducing the activity of the enzyme. However, introducing a new species into the system (the mediator) creates additional issues such as stability and selectivity of the fuels oxidized and reduced form. Mediators also require a low over-potential due to the reversibility of the redox chemistry [3].

Mediators may provide a larger electric current; however, the complex issues with introducing a new species may not be worthwhile compared to DET's simplicity. Researchers turn to new advancements in Immobilization to further develop DET and enzymatic fuel cell stability [3].

=References:= 1. Ivanov I, Vidaković-Koch T, Sundmacher, K. 2010. Recent Advances in Enzymatic Fuel Cells: Experiments and Modeling. Energies. 3 :803-846. 2. Barton SC, Gallaway J, Atanassov P. 2004. Enzymatic Biofuel Cells for Implantable and Microscale devices. Chemical Reviews. 104 : 4867-4886. 3. Cooney MJ, Svoboda V, Lau C, Martin G, Minteer SD. 2008. Enzyme catalyzed biofuel cells. Energy and Environmental Science. 1 :320-337. 6. Barton SC, Gallaway J, Atanassov P. 2004. Enzymatic biofuel cell for implantable and microscale devices. Chemical Reviews. 104 :4867-4886.