Bio fuel cells utilize biocatalysts for the conversion of chemical energy stored in glucose into electrical energy. Bio-catalyzed oxidation of glucose or other organic substances at two electrode interfaces provides a means for conversion of chemical energy into electrical energy. The obtainable power of a fuel cell (P cell) is the product of cell voltage (V cell) and the cell current (I cell)
The ideal voltage id affected by the difference in potential of the oxidizer and fuel as represented in the equation
(E◦1ox - E◦1fuel), with irreversible losses in voltage caused by kinetic limitations of electron movement, (η). Resistance and differences in concentration gradients of the energy source lead to decreased voltage values. As represented below
P cell = V cell × I cell (1)
V cell = E◦1ox - E◦1fuel – η (2)
The cell current is similarly controlled by the electrode sizes, the ion permeability and transport rates across the membrane separating the catholyte and the anolyte sections of the cell precisely the rate of transfer in relation to the electrode surface area. These factors influence the cell power.
These cells can utilize bio-catalytic enzymes in the reaction in either of two ways
- Participate in the electron transfer between the energy substrates and the electrodes
Most enzymes, however do not take part in direct electron transfers between the substrates and conductive electrodes. There are approaches however that utilize mono or multilayer of red-ox enzymes, electro catalysts and bio-electro catalysts that initiate electric transformation at the electrodes.
Enzymatic reduction of bio-fuels in an electric bio cell
These cells require the continuous fermentation of cells involved in numerous physiological processes thus requiring strict conditions in order to work.
Enzymes communicating with the electrodes can catalyse electrochemical oxidation of bio-fuels. Different enzymes e.g. oxidases and dehydrogenases are used with the help of molecular aids to establish this communication with the electrodes. Electron carriers’ between the enzyme active centers and the electrodes are used for effective functioning flavin adenine containing dinucleotide, (FAD- containing oxidases) such as glucose oxidase. NAD(P)+
Dependent dehydrogenasis eg lactate dehydrogenase utilize NAD(P)+ -co-factor and an electrode catalytically active for the oxidation of (NAD)PH and the regeneration of NAD(P)+ and for electrical contact to happen.
Anodes based on the bio-electro-catalyzed oxidation of NAD (P) H
The red-ox factors (NAD) + and (NADP) + are important players in the transport of electrons and activation of the bio-catalytic functions of dehydrogenases. For efficient utilization of the fuels in the bio-cells, there needs to be efficient regeneration of NADP+ in the anode cell compartment. The NAD (P) H in the anode functions transports electrons from the enzymes into the anode. The oxidation of the reduced co factors regenerates the bio-catalytic functions of the cell.
In aqueous solution at pH 7.0, the thermodynamic red-ox potential (E◦) for NAD(P)+/NAD(P)H H is ca. −0.56 V (vs. the saturated calomel electrode (SCE)) – sufﬁciently ne active for anode operation
The electro chemical reaction is highly irreversible and proceeds with over potentials of up to ca.0.4, 0.7 and 1Vvs SCE at carbon, Au and Pt electrodes in that order. The adsorption of NAD (P) H poisons the surface of the electrodes and makes it difficult for the oxidation process to occur.
The electro-catalytic regeneration of NAD+ at the anode utilizes a porous glass in the anode compartment. The enzyme with the production of the co-factor (NADH) oxidizes the substrase (glucose in this case). The reduced co-factor is oxidized at the anode after reaching it diffusion ally. The bio catalytic anode in the bio cell reduced water to hydrogen and the cell yielded Voc = 300mV and I sc =220µA cm−2after several hours of setup.
Most of the energy produced during the oxidation of bio fuels such as glucose is retained in the coenzymes NADH and FADH2 created by the glycolysis process and the citric acid cycle. Electrons are released during oxidation from the NADH and FADH2 and are transferred to the O2 making water in the process as indicated by the following reaction diagrams
The energy values for the highly exergonic reactions are −52.6 kcal/mol (NADH) and −43.4 kcal/mol (FADH2). The oxidation of these reduced co enzymes yields −613 kcal/mol [10(−52.6) + 2(−43.4)]. Thus potential energy stored in glucose bonds is (-680 Kcal/mol). Approximately 90 % energy is preserved in the reduced co enzymes.
Electron flow from FADH2 and NADH to O2 via multi-protein complexes
The final electron acceptor, O2 receives two electrons from both the NADH and the FADH2 these electrons reduce the oxygen atoms forming water
NADH ---------> NAD+ + 2H+ + 2e-
2e- + 2H+ + ½ O2 H2O
The electrons lose potential as they move from NADH to O2. Much of this energy is conserved at the three stages of electron movement as protons move from the inter-membrane space.
Reduction Potentials of Electron Carriers Favor Electron Flow from NADH to O2
Generally, an oxidized molecule becomes an oxidized molecule after the addition of an electron. This reaction is called a reduction reaction where molecules are reduced from a state of higher energy into a setae of lower energy with the release of energy. This flow of electrons in the bio cell causes a flow of current in the opposite direction as the electron flow.
Oxidized molecule + e- reduced molecule
NAD+ +H+ + 2e- NADH
2h + ½O2 +2e- -- H2O
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