Increasing energy demand and growing environmental concerns are driving the search for new, sustainable sources of energy. Global energy consumption, now running at 13.5 TWis estimated to reach 27 TW in 2050 with nearly 85% of today’s energy usage originating from fossil fuels. Imminent shortages of fossil fuels1 and CO2 induced globalwarming, is driving demand for new sustainable energy sources. While no single source is likely to satisfy all energy needs, solar energy, with120,000 TW of solar energy striking the surface of the earth at all times, isby far the most abundant clean energy source available. However, only 0.04% of these needs are presently generated by photovoltaics. An increase to 20 TW peak power would require covering 0.16% of the surface of the earth with 10% efficient solar cells,2 an area nearly equivalent to the entire states of Arizona, New Mexico and Utahcombined. Thus large scale solar cell deployment will only becomefeasible if it becomes possible to simultaneously satisfy the needs for very low cost, non toxic and easily recyclable materials coupled with reasonable energy conversion efficiencies and operating life times.
Nature has crafted elaborate photosynthetic machinery for harnessing solar energy, consisting of a number of photoactive proteins, which harvest solar energy and synchronize their function to make photosynthesis very efficient. In the last decades, solar cells, starting from silicon based materials have made slow, but gradual progress. An important milestone was the invention of excitonic solar cells, commonly called dye sensitized solar cells (DSSC).9 DSSC's are currently the most efficient third-generation solar technology (as high as 11%), while promising low cost and ease of manufacture. Instead of potentiallly costly and/or toxic semiconductors used in other thin-film technologies, DSSC utilizes nanocrystalline semiconducting metal oxides (SMO) such as TiO2, the low cost, inert pigment used in paint.
In DSSC, light active synthetic dyes (organic or organometallic) are bound to nanostructured wide band gap SMOs and used as photo-sensitizers to harvest the solar energy and generate excitons (see Fig. 1). Operationally, photon excitation of the dye results in transfer of electrons from the dye to the semiconductor with the holes remaining in the dye. Although energetically possible for electrons to recombine with holes in the dye, the rate at which this occurs is very slow compared to the rate that the dye regains an electron from the surrounding electrolyte. The oxidized species in the electrolyte are then reduced at the counter electrode with electrons supplied, via the outer circuit, from the photoanode. Due to the very low electron-hole recombination probability in the semiconductor, in contrast to e.g. Si-based solar cells, DSSC's work even in low-light conditions, e.g. under cloudy skies.
Bio sensitized solar cells (BSSC) derive their inspiration from the relatively less understood and appreciated phenomenon of photon triggered electron ejection by light activated proteins. DSSC and BSSC differ in the electron donor–synthetic (ruthenium II complex) dye in the former and light-activated biomolecules in the later. Similar to DSSC, in BSSC, the electron acceptor remains an SMO such as TiO2 or ZnO in various morphologies. Investigations on the possibility of employing naturally occurring organic molecular materials in energy conversion devices for charge generation and separation has been on going for some time, and especially focused on chlorophyll derivatives as the sensitizer in TiO2 based solar cells.10 An alternative to Ru based dyes, although highly effective, but potentially in short supply, is highly desirable.11 Importantly, the sensitizer should exhibit thermal robustness and maintain its functional activity long-term while absorbing solar radiation capable of producing charge generation/separation efficiently.
In considering alternative dyes, replacement of the Ru dye in a DSSC by environmentally friendly and lower cost light-harvesting proteins, would be most attractive. Chlorophyll and its derivatives are sensitive to extreme thermal conditions once extracted out of their source, in vitro condition. Alternatively, Bacteriorhodopsin (bR), a natural light activated protein, found in Halobacterium salinarum, is more robust, while holding high promise for solar energy conversion3 with high quantum efficiency. In particular, bR has remarkable functional stability against high concentrations of salt (up to 5 M NaCl) and exhibits thermal stability even up to 140 degrees Celsius, in the dry state. In addition, bR functionally tolerates a broad range of pH, 5–11 and is easy and inexpensive to clone and express. Re-engineered bio-macromolecules have already been used for various nanotechnology applications; they could provide transformative tools needed to produce a new generation of solar cells. A rational approach is to use a light-activated protein as the source of photo-generated carriers.
Previously published reports on bR films, deposited on metals or metal oxides, were targeted for bio-optoelectronic applications. We have recently demonstrated the construction of an excitonic solar cell based on bR triple mutant E9Q/E194Q/E204Q (3Glu) as photosensitizer. In preliminary studies, we examined the photoelectrical response of the constructed bio cell (bR/TiO2 film) in response to light illumination. Molecular dynamic calculations were carried out to describe the possible binding site(s) of genetically engineered bR mutant onto the surface of TiO2. While promising, the results to date demonstrate the need to pursue the study and development of such cells in earnest.
Towards this end, the main challenges for this new paradigm include:
1. Understanding and mastering interactions and charge transfer at the protein-SMO substrate interface.
2. Finding bR mutants that absorb light in the right parts of the solar spectrum, enhance charge separation and eject electrons to be captured by wide-gap semiconductors.
3. Optimizing the SMO to provide large active area and receptive surfaces for dye adhesion and charge transfer and a highly conductive pathway to the electrode collector.
4. Developing mathematical models to predict the current-voltage relation (and thus the solar energy conversion efficiency), taking into account non-radiative losses in the semiconductor/sensitizer/electrolyte microstructure.
Bio-fuel cells (BFC) are energy-conversion devices based on bio-electrocatalysts leveraging on enzymes or micro-organisms.1-4 Chemical reactions can proceed by direct electron transfer (DET), in which case the electron transfer occurs directly between enzymes and electrodes, or through shuttle mediated electron transfer (MET), in which electron transfer mediators shuttle the electron between enzymes and electrodes to reduce the kinetic barrier in the electron transfer between enzymes and electrodes. DET is desirable for efficient communication between enzymes and electrodes, and eliminating the need for mediators may simplify BFC construction.
(a) Principle of a low temperature H2/O2 fuel cell, (b) operating principle of a biofuel cell involving hydrogenase and laccase enzymes.
In terms of applications, BFCs will most likely be of use in miniature cells to derive power from biological macromolecules to power small devices. It may be possible to implant miniature BFCs within a human patient to power micro sensor/transmitter devices e.g. glucose sensors for diabetics, to monitor blood pressure, temperature, metabolite concentrations, etc. or to power a pacemaker or bladder control valve. It is also conceivable that these miniature BFCs may have defense applications. Several potential applications of BFCs have been reported or proposed in the literature for implantable devices, remote sensing and communication devices as a sustainable and renewable power source.However, there are no BFC design formats or templates that allow for the production of a working device with a size on the order of 1 cc, which are required for several “real world” applications. An enzyme based BFC is very attractive, however it has been shown that electron flow is too slow to make a viable fuel cell. This is due to the difficulty for enzymes to attain direct electron transfer with the electrodes of the cell and catalyze reactions effectively.
The two largest obstacles with BFCs that must be overcome are increasing the power density and preserving the structural integrity of enzyme-CNT adducts. In addition, understanding of the determinants governing the DET reaction and mutation of enzymes to tune the redox potential, to improve DET kinetics, or to reduce the enzyme size are also very important challenges facing the commercialization of BFCs. To address these key issues, various enzyme immobilization methods have been attempted for constructing BFCs, such as adsorption, entrapment, and covalent attachment. Recent advances in bionanotechnology are promising to improve the performance and stability of immobilized enzymes beyond the scope of these traditional approaches.The large surface area provided by nanomaterials for the attachment of enzymes will increase enzyme loading and possibly improve the power density of BFCs. Additionally, various nanostructured materials have shown great potential for stabilizing enzyme activity, which can be further employed in improving the lifetime/durability of BFCs.
Chemistry Chemical Biology Department,
Green Energy Technologies:
Challenges in Research and
Human Resource Development
Pro. Renugopalakrishnan from Northeastern University of USA Visited Guangzhou Institute of Energy Conversion
The 111st MANA Seminar
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