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Self-Powered Systems Chemie a number of other aspects require proper attention, such as the mechanical properties of the materials at the nanoscale, modulation of the electrical properties of the materials, and the feasibility of manufacturing the materials on a large scale. 3.5. Biofuel Cells (BFCs) for the Harvesting of Chemical/ Biochemical Energy A fuel cell is an electrochemical-energy scavenger that converts the chemical energy of a fuel, such as hydrogen or methanol, into electricity through a chemical reaction with an oxidizing agent, such as oxygen or air.[117] In contrast to batteries, which chemically store electrical energy, fuel cells extract chemical energy from reactants and convert the extracted chemical energy into electricity as long as the reactants are available. Although it is a mature technology that finds numerous practical applications, mostly at the macroscale, conventional fuel-cell technology has several inherent disadvantages, such as the materials used, the fabrication cost, and size restrictions, for the cost-effective powering of MNSs in emerging applications, such as implanted biomedical sensors. A biofuel cell (BFC) is simply a fuel cell which uses biological enzymatic substances, rather than precious metals, to catalyze the anode and/or cathode reactions. BFCs have conventionally been classified into two categories: microbial fuel cells (MFCs) if the catalytic enzymes involved are in living cells, and enzymatic BFCs if the catalytic enzymes involved are located outside of living cells[118] (Figure 11 a). Chemical reactions in BFCs can occur by either direct electron transfer (DET), whereby the electrons transfer directly between enzymes and electrodes, or mediated electron transfer (MET), whereby mediators promote the transfer of electrons between enzymes and electrodes by reducing the kinetic barrier.[118a] In the case of DET, the enzyme needs to be strongly adsorbed onto the electrode surface through either physical or covalent bonding, and the redox center of the enzyme also has to be located adjacent to the electrode for efficient electron tunneling. Such a configuration is difficult to implement as well as detrimental to the enzyme activity. Furthermore, the transfer rate and hence the resultant current densities are low for DET. A direct path for electron transfer between the redox center of the enzyme and the electrode can be provided by immobiliz- ing the redox species of the enzyme on the electrode through conducting linkers or nanostructured conducting paths to improve DET efficiency.[119] However, these methods are either expensive or unreliable for long-term applications. The rate of electron transfer between the active sites of the enzyme and the electrode, and hence the current densities, can be significantly improved by redox mediation in MET. However, the output voltage is lower in the MET process as a result of the mediated transfer, and the range of appropriate mediators is limited to those with redox potentials close to those of the enzymes used in the BFC. As an emerging technology for the generation of elec- tricity from renewable biomass, MFCs can convert biode- gradable organic materials into electricity through microbial catalysis.[120] In contrast to conventional fuel-cell systems, MFCs employ live microbes to efficiently catalyze the degradation of organic substrates through metabolism under mild conditions. Hence, the materials used in MFCs are mostly abundant, nontoxic, and relatively inexpensive.[120b] Although the output power produced by MFCs is still insufficient to drive most electronic devices used nowadays, significant advances have been witnessed in MFC technology during the past decades, including enhanced power densities, improved reliability, and diversified functionality.[121] Recently, new platforms, such as microfluidics, have been introduced in the development of novel MFCs.[122] It has also been demonstrated that the electricity harvested in MFCs can be used in situ to drive other reactions integrated in the same system.[123] Moreover, MFCs can generate electricity renew- ably while treating waste water simultaneously;[124] they thus exhibit potential as a power source that could be used to drive self-powered sensors in environmental monitoring and pos- sibly in vivo biomedical applications. Although MFCs exhibit unique features unmatched by those of enzymatic BFCs, such as long-term stability and fuel efficiency, the power densities associated with MFCs are typically lower owing to inefficient mass transfer across cell membranes;[118a] for this reason the application of MFCs in miniaturized electronic devices might be limited. Unlike MFCs, enzymatic BFCs use isolated enzymes derived from living cells for the catalytic generation of electricity. These enzymes can be mass-produced readily and cheaply. Enzymes used in enzymatic BFCs can also process common inexpen- sive organic compounds that cannot be used in conventional fuel cells with metal catalysts owing to a poisoning effect.[125] In addition to simple BFC structures with enzymatic electro- des immersed in a buffer solution, enzymatic BFCs with more sophisticated designs to improve the conversion efficiency have also been reported, such as the integration of micro- fluidics[126] or an air-breathing cathode.[127] In enzymatic BFCs, higher catalyst concentrations are possible than in MFCs, and mass-transfer barriers can be readily removed to facilitate the production of higher current and power densities in the sub- mW cm2 range. These fuel cells thus exhibit the potential to power miniaturized devices and systems.[118a] Enzymatic BFCs with a power density of 1.25 mW cm3 were recently created through the use of multistacked structures and were demon- strated to be suitable for driving personal electronics.[127a] On one hand, the operation principle of enzymatic BFCs provides their potential as biocompatible and sustainable power sources for MNS-based in vivo biochemical/biomedical applications through the harvesting of biochemical energy directly from the human body. On the other hand, current BFCs normally suffer poor stability due to the limited lifetime of extracellular enzymes and their inability to fully oxidize fuels. 3.6. Hybrid Cells for the Concurrent Harvesting of Multiple Energy Types Rationally designed materials and technologies have been developed in the past decades for the conversion of various types of energy, such as solar, thermal, mechanical, and Angew. Chem. Int. Ed. 2012, 51, 2 – 24 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org &&&& These are not the final page numbers! Angewandte Ü ÜPDF Image | Self-Powered Nanosystems
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