Enzymatic fuel cells membranes

An EFC consists of two electrodes, anode and cathode, connected by an external load (shown schematically in Figure 5.1). In place of traditional nonselective metal catalysts, such as platinum, biological catalysts (enzymes) are used for fuel oxidation at the anode and oxidant reduction at the cathode. J udicious choice of enzymes allows such reactions to occur under relatively mild conditions (neutral pH, ambient temperature) compared to conventional fuel cells. In addition, the specificity of the enzyme reactions at the anode and cathode can eliminate the need for other components required for conventional fuel cells, such as a case and membrane. Due to the exclusion of such components, enzymatic fuel cells have the capacity to be miniaturized, and consequently micrometer-dimension membraneless EFCs have been developed [7]. In the simplest form, the difference between the formal redox potential (F ) of the active site of the enzymes utilized for the anode and cathode determines the maximum voltage (A ) of the EFC. Ideally enzymes should possess the following qualities. 

Rincon RA, Lau C, Luckarifl HR, Garcia KE, Adkins E, Johnson GR, Atanassov P. Enzymatic fuel cells integrating flow-through anode and air-breathing cathode into a membrane-less biofuel cell design. Biosens Bioelectron 2011 27 132 136. 

Rengaraj S, Kavanagh P, Leech D. A comparison of redox polymer and enzyme coimmobilization on carbon electrodes to provide membrane-less glucose/02 enzymatic fuel cells with improved power output and stability. Biosens Bioelectron 2011 30 294-299. 

Rengaraj S, Mani V, Kavanagh P, RusUng J, Leech D. A membrane-less enzymatic fuel cell with layer-by-layer assembly of redox polymer and enzyme over graphite electrodes. Chem Commun 2011 47 11861-11863. 

Reaction rates per unit area or volume of enzymatic systems are usually faster than those based on whole cell systems because of the higher loadings of desired enzymes per unit area or volume and no cellular membranes, which are often rate-limiting for substrate assimilation or product secretion (Figure 4.3). For example, it is believed that EFCs will become the next generation of environmentally friendly micropower sources but not microbial fuel cells... 

Wang SC, Yang F, Silva M, Zarow A, Wang Y, Iqbal Z (2009) Membrane-less and mediator-free enzymatic bio fuel cell using carbon nanotube/porous silicon electrodes. Electrochem Commun 11 34-37... 

Methane is regularly used as a fuel in microbial fuel cells and traditional metal-catalyzed fuel cells, but it has yet to be studied with enzymatic BFC systems. Methane monooxygenase is a membrane-bound enzyme that reduces methane to methanol with low redox potentials (< 100 mV) [44]. It is weU characterized, and one source of the enzyme has a copper center that would enable DET at the eleetrode [45]. This enzyme could be added to a methanol anode, thus allowing the use of methane as a fuel in a gas-permeable fuel eell. 

A membrane cell recycle reactor with continuous ethanol extraction by dibutyl phthalate increased the productivity fourfold with increased conversion of glucose from 45 to 91%.249 The ethanol was then removed from the dibutyl phthalate with water. It would be better to do this second step with a membrane. In another process, microencapsulated yeast converted glucose to ethanol, which was removed by an oleic acid phase containing a lipase that formed ethyl oleate.250 This could be used as biodiesel fuel. Continuous ultrafiltration has been used to separate the propionic acid produced from glycerol by a Propionibacterium.251 Whey proteins have been hydrolyzed enzymatically and continuously in an ultrafiltration reactor, with improved yields, productivity, and elimination of peptide coproducts.252 Continuous hydrolysis of a starch slurry has been carried out with a-amylase immobilized in a hollow fiber reactor.253 Oils have been hydrolyzed by a lipase immobilized on an aromatic polyamide ultrafiltration membrane with continuous separation of one product through the membrane to shift the equilibrium toward the desired products.254 Such a process could supplant the current energy-intensive industrial one that takes 3-24 h at 150-260X. Lipases have also been used to prepare esters. A lipase-surfactant complex in hexane was used to prepare a wax ester found in whale oil, by the esterification of 1 hexadecanol with palmitic acid in a membrane reactor.255 After 1 h, the yield was 96%. The current industrial process runs at 250°C for up to 20 h. 

From the great number of oxidoreductases used to modify enzymatic BFC electrodes only a minority is capable of DET, which reduces the number of fuels and oxidants (Table 1). The substrate specificity of enzymes redners half-cell separation by e.g., membranes unnecessary. DET between enzyme and electrode also stops the need for soluble redox mediators to shuttle electrons between enzyme and electrode. This results in the possibility to design membraneless, non-compartmentalized enzymatic BFCs with a simple architecture. However, so far achieved DET currents are lower than MET currents, because usually only enzyme monolayers can be contacted. Strategies to improve the current density aim at the use of high surface area electrode materials like CNTs, AuNPs etc. or the layer-by-layer approach... 

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