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eactions must produce the desired fuel (e.g., H2, CH3OH) and a desirable co-product (e.g. O2 from water oxidation). In so-called heterogeneous (insoluble) catalysts, requirements (1) and (2) are usually implemented by using semiconductor assemblies, while in homogeneous (soluble) catalysts these functions are performed by using molecular assemblies in solution. The critical issue for requirement (3) is the coupling of the onephoton light absorption events in (1) and (2) to the proton-coupled, multi-electron processes required for catalysis. Most previous efforts have used catalysts driven by one-electron reactions to avoid high-energy intermediates. However, fuel production requires multi-electron oxidation and reduction reactions, so new catalysts that couple single-photon events to the accumulation of multiple redox equivalents are essential. 44 hν hν * Photon Capture + - Charge Separation Energy Storage H H2O 2O H 2 CO 2 CH 3 OH Regeneration H H2O 2O O 2 + - Figure 11 Solar production of fuels by the following sequence: (1) light absorption, (2) charge separation, and (3) use of the separated charges to produce fuel (e.g., H2, CH3OH) and co-product (e.g., O2 from water oxidation). The reactions shown in the black box require efficient catalysts. For solar fuel production to be economically and environmentally attractive, the fuels must be formed from abundant, inexpensive raw materials such as water or CO2. The thermodynamics for generation of fuels such as H2, CH3OH, or CH4 by photodecomposition of water or CO2 from aqueous solutions are known (Arakawa et al. 2001; Sutin et al. 1997). Assuming a 100% charge separation efficiency and a catalyst coverage density of 1/nm 2 , the catalyst turnover rate must be about 100/s in order for the fuel generation/regeneration reactions to keep up with the solar production of electrons and/or holes. Currently available catalysts for CO2 reduction or water oxidation have turnover frequencies that are far below those required for a viable catalyst. Homogeneous Photocatalytic CO2 Reduction. Most systems studied to date generally use transition metal complexes (e.g., containing Ru and Ir) as catalysts for photoreduction of CO2 because they absorb a significant part of the solar spectrum, have long-lived excited states (about 1 μs), and can transfer electrons to or from small molecules. Typical systems also include a secondary metal complex (e.g., containing Co) as a co-catalyst to carry out the reduction of CO2. Metal hydride complexes are also important because bimolecular reactions of hydrides or their reactions with H2O/H3O + are responsible for the formation of H2. While both metal hydride and metal-CO2 complexes pertinent to some of the systems are known, in no case has the reduction mechanism been completely determined. For example, the best systems reported thus far show quantum efficiencies as high as 38% using a Re sensitizer, but have disappointing turnover frequencies of CO formation of
CATALYSTS FOR CO2 REDUCTION Photo-driven catalysts for CO2 reduction have made use of the versatile photochemistry and redox properties of Ru complexes. Photo-excitation of Ru(bpy)3 2+ (below, top panel) results in formation of an excited state that reacts readily with the sacrificial donor triethanolamine. The reduced Ru(bpy)3 + can then act as a source of electrons to drive a catalytic cycle of the type shown in the bottom panel below (Pugh et al. 1991). In this cycle, another Ru complex catalyzes the reduction of CO2 to formate ion. Input of electrons is required at two points in the overall cycle. The overall mechanism for the catalytic reduction of CO2 is complex, so that a great deal of work remains to find optimal catalysts that are wellcoupled to the photochemical agents that use solar energy to provide a source of electrons to drive the catalysts. H 2O, CO2 -. O hν N N O - N N e - N Ru N - CH3CN, HCO3 N N -. N N Ru CO NCCH3 CH 3CN 2+ 0 N(CH 2CH2OH)3 N N N N N Ru H + N(CH2CH2OH) 3 N -. CO N N Ru CO O H O decomposition products 0 + N N N N e - N Ru N N Ru H N N -. N CO + 0 CO 2 45 CATALYSTS FOR WATER OXIDATION Catalysts with sufficient oxidizing power to split water remain relatively rare. Yet, several recent catalysts based on Mn and Ru have demonstrated water oxidation following addition of strong oxidants to access the higher oxidation states of these metals. As an example, the top panel shows the structure of a Mn2 complex that generates O2 obtained from water using NaOCl as an added oxidant (Limberg et al. 1999). The mechanism of catalysis is shown in the lower panel. Thus far, the turnover frequencies and the stability of water oxidation catalysts remain low. In addition, coupling of a photo-driven oxidant to these catalysts has not been accomplished.
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On the Cover: One route to harvesti
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Revisions, September 2005 p ix, par
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CONTENTS (CONT.) Appendix 4: Additi
Page 8 and 9: OEC oxygen-evolving complex PCET pr
Page 10 and 11: viii
Page 12 and 13: thin films, organic semiconductors,
Page 14 and 15: genetic sequencing, protein product
Page 17 and 18: INTRODUCTION The supply and demand
Page 19 and 20: showing promise to overcome them. T
Page 21: GLOBAL ENERGY RESOURCES 7
Page 24 and 25: energy can be exploited on the need
Page 27 and 28: BASIC RESEARCH CHALLENGES FOR SOLAR
Page 29 and 30: CONVERSION OF SUNLIGHT INTO ELECTRI
Page 31 and 32: PHYSICS OF PHOTOVOLTAIC CELLS Inorg
Page 33 and 34: Figure 4 Learning curve for PV prod
Page 35 and 36: Needs of Direct-gap, Thin-film Phot
Page 37 and 38: Figure 6 Current record efficiencie
Page 39 and 40: devices. The molecules and material
Page 41 and 42: Figure 8 Structure for high-efficie
Page 43 and 44: One critical property of the photoa
Page 45 and 46: PHOTOELECTROCHEMICAL STORAGE CELLS
Page 47 and 48: BASIC RESEARCH CHALLENGES FOR SOLAR
Page 49 and 50: information about the proteins that
Page 51 and 52: convert it into H2 and liquid fuels
Page 53 and 54: charge separation ensures highly ef
Page 55 and 56: Efficient Photo-initiated Charge Se
Page 57: artificial RCs, have proven to be i
Page 61 and 62: Upon photoexcitation, the electron
Page 63 and 64: (5) determining the physical and ch
Page 65 and 66: following challenges must be met: (
Page 67 and 68: A.F. Heyduk and D.G. Nocera, “Hyd
Page 69: J. Seth, V. Palaniappan, R.W. Wagne
Page 72 and 73: THREE TYPES OF CONCENTRATED SOLAR
Page 74 and 75: Solar Thermal to Electric Energy Co
Page 76 and 77: Tm = mean temperature Z = measure o
Page 78 and 79: control and further diode developme
Page 80 and 81: An overview of solar thermochemical
Page 82 and 83: for electron and phonon band struct
Page 84 and 85: coefficient. The thermal conductivi
Page 86 and 87: J.J. Greffet, R. Carminati, K. Joul
Page 89 and 90: CROSS-CUTTING RESEARCH CHALLENGES B
Page 91 and 92: ange of time and length scales span
Page 93 and 94: Research Issues Advances in the syn
Page 95 and 96: Advances across several science fro
Page 97 and 98: Figure 19 Transparent conductive el
Page 99 and 100: Carrier generation, relaxation, and
Page 101 and 102: E. Hutter and J.H. Fendler, “Expl
Page 103: PRIORITY RESEARCH DIRECTIONS Revolu
Page 106 and 107: RESEARCH DIRECTIONS Several paths e
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Multiple Energy Level Solar Cells I
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Strain Relaxation. Growth of layers
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the relevant elastic and inelastic
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A.J. Nozik, “Quantum Dot Solar Ce
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these new organic structures, and t
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Figure 30 Schematic diagram (right)
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carrier injection between the indiv
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108
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Figure 32 One important example of
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interface for charge separation. Fo
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• Light harvesting, • Advanced
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116
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Multicomponent structures are often
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REFERENCES A.J. Bard and M.A. Fox,
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PLANT PRODUCTIVITY AND BIOFUEL PROD
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een made in identifying and charact
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126
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SUMMARY OF RESEARCH DIRECTION The k
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developing multi-electron catalytic
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coupling in multi-cofactor-containi
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134
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low toxicity, and processibility. T
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8 electrons) are extremely limited.
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largely unknown are the fundamental
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nanostructure design, and developme
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ealize an efficient artificial phot
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Figure 46 Defect-tolerant solar cel
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equired 20-30 years of operation to
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H2SO4 at 1,130K, and the University
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the sequestration step, these solar
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P. von Zedtwitz and A. Steinfeld,
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The continued advances in energy- a
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for the detailed understanding and
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160
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A shortcoming of this approach is t
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of ~2.4 at 300K and quantum-dot PbT
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High-throughput Experimental Screen
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Solar Concentrators and Hot Water H
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G. Chen, M.S. Dresselhaus, J.-P. Fl
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Materials and Processing Methods to
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materials and fabrication approache
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multi-electron H2O and CO2 activati
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178
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180
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and quantum dots. Quantum dots are
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184
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186
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Flux (mA/eV.cm2 Flux (mA/eV.cm ) 2
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Table 1 Worldwide PV Module Product
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Amorphous Silicon. From its discove
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Table 2 Are There Enough Materials
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passivating window and back-surface
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R.R. King, C.M. Fetzer, K.M. Edmond
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currently accounting for only a sma
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Anaerobic Digestion One gasificatio
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Electrolysis is the process for bre
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alkaline electrolyzers require a mi
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maintenance costs of the small prod
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P. Courty, P. Chaumette, and C. Rai
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212
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periods and extend the system opera
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When ηopt < 1, Equation (2) become
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Receiver & Power Conversion Unit So
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Figure 77 General concept of solar-
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Other areas whose development could
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THERMOPHOTOVOLTAICS Thermophotovolt
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N. Benz and Th. Kuckelkorn, “A Ne
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R.A. Sherif, H.L. Cotal, R.R. King,
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230
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Sub-panelists Edmond Amouyal, Centr
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Terry Tritt, Clemson University Joh
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236
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Plenary Closing Session — Wednesd
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Panel 1C (Photoelectrochemistry)
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Agenda for Solar Fuels Breakout Ses
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Tuesday, April 19, 2005, 8:00 a.m.
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Wednesday, April 20, 2005, 8:00 a.m
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Wednesday, April 20, 2005, 8:00 a.m
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250
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R. Corkish, S. Kettemann, and J. Ne
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T. Polivka and V. Sundstrom, “Ult
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256
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258
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microscopy, 81, 85, 101, 155-157, 1
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