ABSTRACT
A biological fuel cell is a device that directly converts biochemical energy into electricity. The driving force of a biological fuel cell is the redox reaction of a carbohydrate substrate such as glucose using an enzyme as catalyst. Working principle is similar to that of chemical fuel cells. The main differences are that catalyst in the biological fuel cell is microorganism or enzyme, therefore noble metal is not needed, and its working conditions are mild neutral solution and room temperature.
This paper throws light on conventional and modern methodologies involved in bio-fuel cells.
Among the conventional methods ‘ALGAL HYDROGEN PRODUCTION BY CONTROLLING THE MOLECULAR SWITCH OF Chlamydomonas Reinhardtii’ is projected in detail here. H2 produced by this process is used in ordinary fuel cells for electricity production.
The modernization of this method results in ENZYMATIC FUEL CELLS which makes both hydrogen production and electricity generation as a concomitant process. introduction :
A fuel cell is an electrochemical device that converts the chemical energy into electrical energy by oxidizing a fuel. In this process, hydrogen is the fuel and oxygen the oxidant. A fuel cell has a cathode side and an anode side separated by an ion selective membrane. The hydrogen flows into the anode side of the fuel cell where a catalyst facilitates the separation of hydrogen into a proton and two electrons. The proton is able to travel through the membrane where the catalyst again facilitates the formation of a water molecule using external oxygen. Unable to pass through the membrane, the electrons travel from the anode to the cathode via an external circuit bearing the electrical
load . The source for hydrogen may vary either chemical or biological .The chemical hydrogen fuel cells includes the following demerits as using platinum catalyst, use of alkalis or acids , production of co ,high temperature requirements which is overcome by
Biological fuel cells The main differences are that catalyst in the biological fuel cell is microorganism or enzyme, therefore noble metal is not needed, and its working conditions are mild: neutral solution ,low pressure and room temperature. This paper explains two aspects of biological hydrogen production viz
(1).Photo biological splitting of water as a source of hydrogen by chlamydomonas reinhardtii
(2) .Enzymatic reduction of biomass as hydrogen source.
1.CONVENTIONAL METHOD
PHOTOBIOLOGICAL SPLITTING OF WATER BY Chlamydomonas reinhardtii:
The hydrogen production by chlamydomonas reinhardtii has a significant advantage over other photobiological process because ATP production is not required, high theoretical efficiencies are possible, and water is used directly as the source of reductant. However, algal H2 Photoproduction is sensitive to O2, a co-product of photosynthesis, and this sensitivity is a major factor currently limiting the use of algal systems for H2 production. The major enzyme responsible for the hydrogen production is hydrogenase, which is encoded in nucleus of green alga but it functions in chloroplast stroma.
H2
Sun light
Ch.reinhardtii
O2
H2 fuel cell
H2O
Here light energy is essential for the production of hydrogen because light absorption causes the oxidation of water molecule into protons and electrons and the photosynthetic ferridoxin serves as a electron donar to Fe-hydrogenase which links it to the electron transport chain in the chloroplast of green alga.
USE OF MITOCHONDRIAL RESPIRATION:
ROLE OF Fe - HYDROGENASE
The electrons are driven upon light absorption by PSI to ferredoxin. The latter is an efficient
electron donor to the Fe hydrogenase, which efficiently combines these electrons with protons to generate molecular H2 .Sulfur deprived and sealed cultures of C. reinhardtii become anaerobic in the light due to a significant and specific slowdown in the activity of the O2-evolving PSII, which is followed by automatic induction of the Fe-hydrogenase and by photosynthetic H2 production.
The Mitochondrial respiration scavenges the small amounts of O2 that evolve due to the residual activity of photosynthesis and thus ensures the maintenance of anaerobiosis in the culture.Thus, the physiology of H2 production by S deprivation involves a coordinated interaction between:
(a) Oxygenic photosynthesis, i.e. the residual PSII activity for the generation of electrons upon oxidation of water. These electrons are transported through the photosynthetic electron transport chain and eventually feed into the Fe hydrogenase, thereby contributing to H2 production.
(b) Mitochondrial respiration :
This scavenges all oxygen generated by the residual photosynthesis and, thus, maintains anaerobiosis in the culture.
(c) Electron transport via the hydrogenase pathway and the ensuing release of H2 gas by the algae sustains a baseline level of photosynthesis and, therefore, off respiratory electron transport for the generation of ATP.
Molecular switch
Oxygen evolution
Hydrogen evolution
Limiting sulfur
PRESENT
ABSENT
NO SULFUR
ABSENT
PRESENT
SCHEMATIC DIAGRAM:
At Anode : H2 à 2H+ + 2e-
At Cathode : 2H+ + 2e- + ½ O2 à H2O
II.MODERN METHOD:
The modern methods have the option of concomitant H2 production and current generation inside a single tank itself.The recent biological fuel cell are
1.Microbial fuel cell,
2.Enzymatic fuel cells
Microbial biofuel cells require the continuous fermentation of whole living cells performing numerous physiological processes, and thus dictate stringent working conditions. In order to overcome this constraint, the redox-enzymes responsible for desired processes may be separated and purified from living organisms and applied as biocatalysts in biofuel cells rather the whole microbial cells. That is, rather than utilizing the entire microbial cell-apparatus for the generation of electrical energy, the specific enzyme that oxidizes the target fuel-substrate may be electrically-contacted with the electrode of the biofuel cell element. Enzymes are still sensitive and expensive chemicals, and thus special ways for their stabilization and utilization must be established.
Enzymatic fuel cells are far more advantageous then microbial fuel cells.
ENZYMATIC FUEL CELLS:
Upon utilizing enzymes as catalytically active ingredients in biofuel , one may apply oxidative biocatalysts in the anodic compartments for the oxidation of the fuel-substate and transfer of electrons to the anode, whereas reductive biocatalysts may participate in the reduction of the oxidizer in the cathodic compartment of the biofuel cell. Redox-enzymes lack, however, direct electrical communication with electrodes due to the insulation of the redox-center from the conductive support by the protein matrices.
FUEL CELL MODEL:
GLUCOSE FROM BIOMASS
OXYGEN INLET
LOAD
POTASSIUM PHOSPATE BUFFER
WATER FORMATION
(-VE)
(+VE)
1
2
In the diagram:1.Immobilized Glucose dehyrogenase . 2. Proton exchange membrane.
Structural components:
This type of cells are called as integrated fuel cells in which both the conversion of substrate and the current production is from the same anode chamber. The main parts of the integrated systems are
1.Anode chamber
2,Cathode chamber
3.Proton exchange membrane.
The various components in the enzymatic fuel cells are,
ANODE CHAMBER
The anode chamber usually consist of
1.Immobilized GDH,
2.Mediator
3.Electrolyte.
Immobilized GDH:
GDH may be immobilized using any one of the following substrate,
• Sol-gel
• Propylene glycol alginate-gelatin beads
• Polyacrylamide functionalized with azlactone groups
• On silicon chips using;
– poly-L-lysine
– gelatin (entrapment)
– epoxysilane (covalent)
Electrodes and electrolyte:
usually woven graphite is used because of its high surface area and mesh like structure. Phosphate buffer solution is used as electrolyte. Care should be teken that anode voltage is should be lesser than the redox potential of the enzyme used. The electron transfer phenonmena can be increased using electron mediators.
Mediator:
Mediators are the chemical compounds of transferring electron (H+) from a bounded state to the electrode. Here neutral red , is used as mediator. The covalent coupling of neutral red to self-assembled mono layers on graphite electrode surfaces has many advantages. Some of the salient features of neutral red are 1.Forms reversible redox couple at the anode.2.Easy link with nadp 3.Highly stable in reduced & oxidized conditions. usage of neutral red may increases the electron transfer rate by ten folds.
Temperature and pH control:
Since the enzymes used here is extracted from thermopiles , they are heat resistive to some extent. Temperature must be maintained between 40 -70oc and pH around 6-8.
The anode chamber should be anaerobic.
Cathode chamber:
In this chamber there is continuous supply of oxygen the electrons coming out from the anode(through load) reacts with O2,and H+ from the proton exchange membrane to form water.
Principle of reactions
The reactions in the anode chamber of the EFC are the oxidation of the substrate by the enzyme and the release of electrons at the anode itself. The enzyme converts methanol to carbon dioxide and hydrogen ions in the presence of NAD+ (nicotinamide adenine dinucleotide). NAD+, a biological coenzyme, is prerequisite for the enzymatic reaction and readily captures electrons. Release of electrons from the reduced form, NADH, becomes easier when a mediator is used. Nad+ and the mediator are not consumed in the reactions.
In the cathode chamber the hydrogen ions, which flow through an ion selective membrane and oxygen taken form the outside react and form water.
reactions involved:
At the anode
Glucose + nad+ Gluconic acid + nadh
NADH + Mediatorox NAD+ + H+ + Mediatorre
[ NAD+H- NAD+ + H- ; H- H+ + 2e-]
Mediatorre Mediatorox + n e- (flow as electricity)
at the cathode
4H+ + O2 + 4e- 2H2O
Hypothetical calculation for electric charge from one glucose molecule.
Formula: Q = F × ns × Ne/s
For glucose: Q=96485*(1/80)*24=13131.33 C.(for complete oxidation)
Where Q=charges produced, ns=moles/mol.wt, Ne/s = no.of electrons produced per substrare
CONCLUSION:
The main aim of this paper is to explain conceptual the basis of biological hydrogen production and its application in fuel cells. The two examples dealt here are ecofriendly and oriented towards industrial applications. The main objective which makes the fuel cells more efficient is, the introduction of immobilized enzyme and electron mediator in the reactor.
The configurations of the biofuel cells discussed in this paper can theoretically be extended to other redox enzymes and fuel substrates, allowing numerous technological applications.The production of electrical energy from biomasssubstrates using biofuels could complement energy sources from chemical fuel cells. .Thus the future fuel is hydrogen derived using boilogical methods.
REFERENCES:
“ENZYMES”-by Trevor Palmer
“Bio process engineering”-Michaelshuler and Fikret kargi
“Microbiology”-Lansing m. Prescott ,John P.Harley.
“An introduction to fuell cells”-Robert .k. Leiwing.
