An Integrated, Systems Approach to the Development of Solar Generators by Nathan S. Lewis

wo major technological challenges are imposed on the components therefore for a fully integrated solar generator in the development of a sustainable, depend intimately on the design of the whole system that can simultaneously combine clean energy system are providing system. Hence, optimal progress toward a the three desired attributes of cost-effective T viable solar fuels technology mandates a scalability, robustness, and efficiency. massive grid-scale energy storage and an ample supply of carbon-neutral, high- holistic, systems approach, to identify and A fully integrated artificial energy-density, transportation fuels. The then solve the research and development system is a complex assembly that will development and deployment of massive, needs that historically have served as need to bridge many length scales, likely grid-scale energy storage is imperative for barriers to the development of a scalably- over as many as seven orders of magnitude reliably and robustly compensating for the manufacturable solar fuels generator. (Fig. 1). The requirements, outcomes, and intermittency involved with the utilization Natural photosynthesis provides a success of the R&D at each scale length are of very large amounts of wind energy and complex, but elegant, blueprint for the intimately dependent on the requirements, .1 Another challenge is that ~ production of fuels from sunlight. With success, and outcomes of the R&D needed 40% of current global transportation fuel is only water, , and sunlight to construct such a system at many other consumed in uses for which electrification is as the inputs, solar energy is stored in the scale lengths. For instance, the requirements technically difficult, if not impossible, such form of chemical bonds as the output on the materials used for capture on as in heavy-duty trucks, ships, and aircraft.2 of photosynthesis. However, natural the nanoscale depend significantly on the Exhaustive use of advanced might photosynthesis has significant performance form factor and architecture of the system possibly supply adequate carbon-neutral limitations at the systems level, such as developed as a prototype on the cm-to-m transportation fuel for these uses, but saturation at approximately one-tenth the length scale. Similarly, success on the µm could not then also fulfill the requirement peak intensity of sunlight; relatively modest length scale for the production of fuels from for long term, massive, grid-scale energy overall energy conversion efficiencies on sunlight will generally produce bubbles and storage.3 Chemical fuels are desirable for an annually averaged basis (1% or less); flows of product that would, if not controlled energy storage because fuels are the most the need to spend significant amounts or mitigated, degrade the performance of energy-dense storage medium known to of energy internally to regenerate the the system on the m length scale. Rapid man (other than the atomic nucleus), and unstable and to resynthesize the progress therefore is facilitated by vertically could simultaneously provide a means to highly exquisitely ordered and arranged integrated R&D efforts that simultaneously baseload at scale intermittent renewable molecular machinery of photosynthesis; address a multitude of critical bottlenecks energy resources while also fulfilling and in general the production of a fuel that at a multitude of length scales, in a parallel, gaps in the need for high energy-density, is not directly compatible with widespread spiral development type of structure. carbon neutral, sustainable, transportation use in existing energy systems.8 Production fuels.4 Hence a clear rationale exists to of fuels directly from sunlight is thus Technology Bottlenecks develop technology options that involve the inspired by natural photosynthesis, but in the Development conversion of sunlight, by far the largest has the mandate to provide far superior energy source, directly into chemical fuels. performance than photosynthesis. In this of an Integrated Solar Fuels One approach to address both of these respect, “performance” is measured by the Generator System technology development imperatives net annually averaged energy conversion involves the development of artificial efficiency to produce a useful chemical fuel System components.—Figure 2 identifies photosynthesis. In artificial photosynthesis, in a scalable, cost-effective fashion. A fully some of the key bottlenecks that currently sunlight is directly converted, without artificial photosynthetic system would also prevent the demonstration and construction the use of (or the limitations of) living not require arable land, potable water, or of a scalably-manufacturable solar fuels systems, into a useful chemical fuel.5 involve tradeoffs of land to be used either generator system. Clearly, suitable materials Artificial photosynthesis has been pursued for food or for fuel production. are needed for the capture and conversion in the laboratory for over 40 years, since the It is clearly possible to construct a of sunlight. In , current can observation by Fujishima and Honda that fuel-producing, man-made, solar energy- be traded for voltage, so materials with exposure of TiO2 to sunlight effects water conversion system that outperforms natural a relatively wide range of band gaps can splitting to produce, with high quantum photosynthesis on an efficiency basis. For be used to produce mutually similar net 6 yields, H2 and O2. However, to date, no single example, solar panels can be over 30% electrical power as the system output. light absorber for artificial photosynthesis efficient in conversion of sunlight into However, the production of fuels, such as has been shown to combine simultaneously .9 In turn, electrolyzers can take the prototypical example of splitting water three desired technological attributes: electrical energy and produce H and O 2 2 into H2 and O2, requires a minimum voltage efficiency, affordability, and robustness.7 from water at over 70% energy efficiency.10 of 1.23 V, below which, according to the A manufacturable artificial photosynthesis Hence, in combination, the sunlight to first law of thermodynamics, no substantive system also involves much more than a fuel (in this case solar-to-, STH) net products will be produced.11 Hence, single photoelectrode, necessitating the energy-conversion efficiencies of a modular materials that are well-suited for light incorporation of suitable catalysts, materials photovoltaic/electrolyzer combination capture for solar fuels production are not for the separation of the products, mitigation system can be over 10 times greater than fully contained in the set of materials that of undesirable effects of bubbles and flows, that of the fastest growing plants (on a are used for production of photovoltaics. methods to manufacture the system at scale, yearly average). A goal of research in and approaches to encapsulate the materials artificial photosynthesis is however not while maintaining facile reactant access only to demonstrate high efficiency, but and product egress. The requirements that to develop a technology that is the basis (continued on next page)

The Electrochemical Society Interface • Summer 2013 43 Lewis (continued from previous page) Once (actually -hole A suitable half-cell electrocatalyst alone pairs) with the required energetics are does not of course suffice to provide an produced, catalysts are generally needed adequate blueprint for the construction of a The presence of kinetic and concentration to facilitate the efficient production of fully operational solar fuels generator system. overpotential losses in electrochemical chemical fuels. The need for catalysis can be The oxidative and reductive electrocatalysts systems,11 as well as the difference between readily understood at the fundamental level need to work under mutually compatible the voltage of operation at maximum power because the sun is not a laser! Hence solar conditions of pH, temperature, etc. The production relative to the open-circuit strike a device one at a time, but two electrocatalysts also need to be interfaced voltage produced by a solar light absorber, or more electrons are needed at once to make with the light capture components, while imply that a single threshold system probably and/or break chemical bonds (2 electrons to retaining as an assembly the function of all requires a of at least 2.0 eV, and reduce water to H2, 4 electrons to oxidize of the individual pieces. The system must 12,13 more likely at least 2.2 eV. However, two molecules of H2O to produce one also be capable of operating safely with the solar spectrum contains relatively few molecule of O2, 6 electrons to reduce CO2 to minimal, if any, co-evolution of H2 and O2 to photons that are capable of providing such CH3OH, etc.). The required electrocatalysts produce an explosive gas mixture. Similarly 14a high excitation energies; therefore the must be highly active, stable, and, for global if CO2 is reduced to form methanol, for ultimate energy conversion efficiency of the scalability, must be either comprised of example, the methanol must not diffuse to resulting photosystem is limited to <10%. In earth-abundant elements or must minimally the oxidative region of the system, or it will contrast, the use of two light absorbers that utilize scarce metals such as Ru or Ir. be oxidized back to CO2 and the overall are connected electrically in series, each of Unfortunately, the most active catalysts for system efficiency will be unacceptably which contributes a portion of the needed (in acidic environments) are degraded. 7 photovoltage for the fuel-production process, Pt and IrO2, so a goal for the global solar Hence, a membrane, or some type of can provide ultimate solar-to-fuel energy fuels research and development community physical and chemical separation system, is conversion efficiencies of >25% (Fig. 3).14b is to discover, develop, and exploit suitable needed to prevent deleterious product back- This type of artificial photosynthesis design systems and architectures that can allow for diffusion or convention. But production therefore closely resembles the “Z-scheme” the replacement of large quantities of these of O2 from water will produce , employed in natural photosynthesis.15 scarce transition metals with more abundant whereas the reductive formation of a fuel metals, such as Mo, W, Co, Ni, Fe, or Mn.

Fig. 1. Development scale of an artificial photosynthesis system.

44 The Electrochemical Society Interface • Summer 2013 Fig. 2. Some key R&D bottlenecks in development of a solar fuels generator system. will consume protons. The system must performance in the process. A central focus in the half-cell reaction of interest. These therefore allow a facile, low-resistance, of such efforts will involve understanding effects can be mitigated, or in fact exploited path for ion (generally or hydroxide) the inefficient charge transport between light beneficially, through the use of optically conduction to neutralize the pH gradient, absorbers and catalysts and, in particular, transparent metal films and/or through else the net reaction to form products will between the sites of water oxidation and fuel- nanoscale control over the space-charge cease to occur. For this reason, a key R&D generating half-reactions. Typically, a large region of laterally inhomogeneous metal/ opportunity in the development of a solar fraction of the photogenerated electrons or liquid/ interfaces. Hence, fuels generator system is the development holes are diverted from productive paths by R&D efforts devoted to understanding, of suitable membranes or alternative back or side reactions, or are trapped at defect and controlling, the interfacial chemistry physical/chemical/mechanical product sites resulting in low photochemical quantum of electrocatalysts integrated onto light separation schemes for enabling a scalable, yields. absorbers is clearly an important component manufacturable solar fuels generator. A critical challenge is achieving acceptable of a systems approach to development of a Functional integration of components.— charge-transport efficiency at every stage solar fuels generator system. The best catalyst is of course not sufficient if, of the catalytic cycle, which reflects the Benchmarking.—Another important when combined with the best light absorber, inherent difficulty of reconciling one-, component of a global solar fuels R&D effort the catalyst poisons the light absorber single-charge generation with multi-electron is the development and implementation and in turn the light absorber poisons catalysis. In addition, interfacial reactions of standardized measurement methods the catalyst. Hence another goal for the during the deposition of metal catalysts on and techniques for assessing the activity global solar fuels R&D effort is to develop semiconducting light absorbers generally of electrocatalysts and photocatalysts for “interfaces” or “integrated systems,” which produce alloys or new compounds that solar fuels production. To date, a variety address strategies to link and interconnect introduce surface recombination sites in the of methods and procedures have been the individual components of a system light absorber and/or that reduce the catalytic used to assess the (photo)electrochemical in a robust fashion with minimal loss of activity of the metal electrocatalyst particles behavior of solar fuels generators, half cells, and catalysts. For example, the activities of heterogeneous electrocatalysts for water splitting are reported in their own, individual conditions of solvent, electrolyte solutions, and pH.7 Additionally, samples of catalysts are generally deposited, using various techniques, onto mutually different substrates, and the resulting surface topographies and morphologies differ substantially between research groups. Even the definition of catalytic activity varies substantially, with some workers quoting the exchange current density and others quoting the overpotential needed to reach a given current density. The quoted quantities often do not allow for systematic comparisons across investigators or from catalyst to catalyst, due for example to differences in quoting the geometrical, BET active, or electrochemically active surface areas of the system being measured. A very similar situation is present for photoelectrochemical systems and devices. Various investigators quote solar energy- conversion efficiencies using light sources that have mutually different spectral dependences, such as tungsten-halogen lamps, high- pressure Xe arc lamps, lasers, Hg lamps with or without filters that simulate Air Mass 1.0 or 1.5 conditions etc. The efficiency assessment is either

(continued on next page) Fig. 3. Energy diagrams for a tandem cell configuration with n-type and p-type photoelectrodes electrically connected in series.

The Electrochemical Society Interface • Summer 2013 45 Lewis (continued from previous page) larger surface area, as expressed by the however to systematically prepare very large measured roughness factor, when an activity libraries of compounds, in a standardized, is observed that is comparable to that of Pt. reference-able fashion, using ink jet printing, made by evaluation of the photocurrent- Consequently, the plot concisely reveals sputtering, and electrodeposition methods. voltage characteristics of the system or by that the catalytic activity per surface area of First-generation ink jet printing, using measurement of the production of fuels.7 In Ni-Mo is considerably smaller than that of drop-on-demand technology, has allowed addition, various methods have been used platinum. Population of such a plot by the the synthesis of 102 – 103 compounds per to calculate the efficiency of solar fuels global solar fuels community would greatly hour on FTO-coated glass. Reproducibility, production even from nominally the same facilitate inter-comparison between the mixing (sample compositional homogeneity) type of system output data.16 various global solar fuels R&D efforts. and thickness can be addressed by the A benchmarking effort should therefore Accelerated discovery.—Another replacement of layer-by-layer printing with involve the development and implementation important avenue for solar fuels R&D a bitmap process in which interlaid patterns of uniform standardized methods and would address the need to dramatically are created, and by surface pretreatments protocols for characterizing the activities of accelerate the process of discovery of that reduce the liquid surface tension and catalysts for the evolution, hydrogen new light absorbers, photocatalysts, and surface hydration. For example, a library evolution, and CO2 reduction reactions. One electrocatalysts. Traditionally, several of 1844 samples can be produced within goal is the development of an internationally years of time, and several person-years a few minutes at high resolution and low accepted facility that provides cross- of effort, are required to synthesize, print velocity (Fig. 5). Such methods allow comparable measurements on catalysts, purify, characterize, and then optimize an the synthesis of ~ 75,000 samples per hour half-cell systems and on devices, assessing individual light absorber or electrocatalyst. with a single machine, therefore enabling a their catalytic activity, solar conversion Although such directed research activities synthesis output of over one million samples efficiency, and stability. must still proceed with vigor and on a global per day assuming continuous operation of Figure 4 shows an example of scale, these activities can beneficially be such an instrument. catalyst evaluation in a multi-parameter complemented by a highly parallelized, High-throughput experimentation also representation. The abscissa displays the extremely rapid methodology that uses high- needs fast and reliable screening methods measured overpotential for the hydrogen- throughput experimentation methods. to comprise a full research pipeline system. evolution reaction obtained upon immersion A typical combinatorial approach For light absorbers, a suitable initial screen of the sample. These measurements have involves the design of libraries that produce would involve the determination of the been performed in acidic, neutral, and high information density in the discovery energy gap and of its nature with respect alkaline aqueous solutions. The ordinate process.17-19 The approach also includes a to direct versus indirect transitions. The displays the overpotential measured after the vectorial search component in the sense availability of several earth-abundant catalyst has passed 10 mA cm-2 of current that families and compositional fields of materials that can be used as photocathodes, density for a period of 2 h. The color code lead components are specifically examined such as Si and MX2 (M=W, Mo; X=S, displays the surface roughness factor that and selected. The combinatorial approach Se),23,24 reduces the materials search was determined by use of electrochemical mandates rapid screening methods of the predominantly to photoanodes that have impedance measurements. most relevant material properties at a speed band gaps between 1.6 eV and 1.9 eV.14 In acidic solution, a NiMo alloy appears that equals the rate of compound synthesis. Visual inspection of the prepared library to have an overpotential that is comparable to An additional area of concern involves the plates allows exclusion of materials that that of Pt. Both catalysts exhibit considerable differences between the (spatial) library are either transparent or opaque. For stability because the overpotential does not density and the synthesis speed, given determination of the optical properties of the change significantly after 2 h of operation. that the demands differ depending on the remaining samples, rapid diffuse reflection

In contrast, the overpotential for MoS2 application. spectroscopy, by use of an integrating increases upon operation, as indicated by For photoelectrochemical applications, sphere, has been developed (Fig. 6). Further the location of the MoS2 data point above combinatorial ink jet printing and sputtering screening consists of electrochemical the dashed line for acidic and neutral have already been used to generate some (electrocatalysts) and photoelectrochemical solution. The color code shows that the Ni- modestly sized libraries of compounds,20-22 (light absorbers) analyses. Stability under Mo electrocatalyst also has a substantially in rather small daily volume. It is possible operating conditions can then be assessed

Fig. 4. Benchmarking of electrocatalysts with regard to activity and stability.

46 The Electrochemical Society Interface • Summer 2013 in aqueous solutions and for different pH Understanding emergent phenomena solar fuels generator system will involve conditions, along with determination of on the mesoscale.—Even if suitable light the preferential alignment of such objects the composition and structure of entire absorbers, catalysts, and integrated systems over large areas, with the nanostructures collections of interesting compounds. The are developed, important phenomena also pointing predominantly towards the sun, resulting databases should optimally be exist on the mesoscale, i.e., on the length like blades of grass on a manicured lawn. made available for use by the global research scale of 100 nm-100 µm. A photovoltaic Similarly, the optical, chemical, transport, community, and also should be used to assembly that is constructed from a single and mechanical properties of an assembly provide feedback to direct subsequent more semiconducting nanoscale object is of refined high-throughput materials searches. great academic interest, but a practical (continued on next page)

ig F . 5. High-throughput plate with a materials library consisting of quaternary metal of Ni, Co, Fe, and TiOx. Synthesis conditions: the samples were aged at 100 ºC, and then calcined at 350 ºC for 6-8 h in air.

(a) (b)

Fig. 6. Screening of optical properties of an example materials composition field to determine suitable light absorbers: (a) relative absorption vs. photon energy, determined from evaluation of the diffuse reflection spectroscopy data; and (b) assignment of energy gaps and direct vs. indirect nature of the absorption edge.

The Electrochemical Society Interface • Summer 2013 47 Lewis (continued from previous page) components that will be needed to provide the acceptable, or even optimal system optimal stability and efficiency for the geometric parameters and system design of nanostructured objects are not generally prototype itself. Hence, R&D in solar fuels that would be needed to achieve acceptable predictable from the properties of a single generator systems also requires a concurrent operational performance. object by itself. Hence efforts are required prototyping effort, to design, debug, model, A prototyping project could beneficially to orient nanoscale objects of interest construct, and evaluate the performance of involve the use of electrochemical over large areas using self-assembly and a wide variety of physical embodiments engineering design tools to evaluate the scalably-manufacturable methods, as well of solar fuels generators, optimally even limitations and optimal operating conditions as to understand the optical, electrical, before the final components themselves are for several possible physical designs of an electrochemical, chemical, mechanical, specified in detail. integrated solar fuels generator system. and physical properties of macroscopic Some of the critical system design criteria Each design (Fig. 7) should be evaluated assemblies of systems of hard materials include: (1.) separation of the product with respect to the series resistance, the embedded into polymeric, soft materials. gases, to produce minimal recombination; ability to hold back pressure differentials, Membranes or other physical types of (2.) efficient neutralization of the pH gas crossover fluxes, and the fraction of barriers are needed to separate the products gradient that will be produced by a spatial the incident optical plane that is occupied while allowing for neutralization of the pH separation between the locations of water by photoactive material that can absorb the gradient that will be present otherwise as a oxidation and water reduction; (3.) a low incident sunlight. Resistance drops could result of the vectorial charge transport that gas crossover rate, for safety purposes, to be evaluated by use of a Poisson’s equation is involved in the production of a reduced avoid having a gas mixture above 4% H2 solver in Comsol Multiphysics. Comsol, as species (e.g., H2 or CH3OH, etc.) spatially in air (the flammability limit) and surely to well as analytical modeling where possible, separate from the production of an oxidized avoid having a gas mixture above 17% H2 in could be used to evaluate the gas crossover species (e.g., O2). Some membranes are air (the lower explosive limit); and (4.) the fluxes driven by either diffusion or available for separation of products with ability to hold back pressure differentials, convection for the different geometries and acceptable ionic conductivity under acidic to avoid gas and liquid flow in the system physical parameters of the different designs. or alkaline conditions, but few, if any, that will result from a pressure differential The modeling effort would aid in the membranes are available that can function between different regions of the system establishment of quantitative metrics for at neutral or near-neutral pH conditions.25-28 (to collect the gas or gases obviously success for the various components based Similarly, few membranes have acceptable requires a pressure gradient, but the 2:1 on their performance at the R&D level. For proton conductivities but low permeabilities H2:O2 stoichiometry of water instance, one prototype may only possibly to methanol and to other reduced carbon will produce, if not actively monitored and require an improvement in the light absorber 29 species from CO2. Hence R&D efforts controlled, a significant pressure differential band gap by 0.2 eV and then it is “done;” at the mesoscale are needed to develop between the H2- and O2-evolving and another prototype might have adequate light new types of membranes, with the needed collecting parts of the system). capture but require a H2 evolution catalyst properties, to support the functionality These design criteria are common to that is a factor of 15 better than the current required for a solar fuels generator system. both solar-driven electrolyzers as well as to state-of-the-art; yet another prototype may From components to constructs to conventional, electrical-driven electrolysis have adequate catalysts but the catalysts devices to prototypes.—At still larger scale systems. However, in the absence of optics operate at mutually different pH values or lengths, the reactor design, reaction inputs used to achieve solar concentration, the at a pH value where no suitable membranes and outputs, flow paths, assembly schemes, solar fuels generators will operate only at have yet been developed. In this way, both system integration issues, and physical a projected area current density of 10-20 the modeling and reduction to practice layout of the light absorbers relative to each mA cm-2, whereas to minimize the balance of the various prototypes of solar fuels other, relative to the incident light path, etc. of systems and the area of the membrane- generators is critical towards establishing need to be specified. In turn, the arrangement assemblies, present electrically rigid performance metrics and for insuring of the components in the prototype affects driven electrolysis systems operate at that the R&D program solves problems that the performance of the prototype as well projected area current densities of ~ 1 A are in fact real problems and does not solve as the requirements for the materials and cm-2. This difference potentially changes problems that are not problems.

(a) (b)

Fig. 7. Schematic illustrations of (a) a prototyping engineering model for design evaluations, (b) a prototyping design that integrates light absorbing materials and membranes.

48 The Electrochemical Society Interface • Summer 2013 Optionality in fuels produced.—A Acknowledgments 9. National Laboratory final emphasis of a balanced global R&D Best Research-Cell Efficiencies (2012). program on solar fuels generator systems Support for solar fuels R&D by the NSF 10. J. O. Bockris and A. K. Reddy, Modern should be to retain flexibility in the types Powering the Planet Center for Chemical . Vol. 2, Springer of fuels that can be produced from sunlight. Innovation, CHE-0947829 (development (2000). 11. 11. A. J. Bard and L. R. Faulkner, In one embodiment, water is split into H2 of electrocatalysts), by the DOE DE- and O2, but H2 is not necessarily the fuel Electrochemical Methods, FG02-03ER15483 (earth-abundant light nd that will be provided to the end-user. The absorbers), and by the Office of Science of Fundamentals and Applications, 2 ed., Wiley (2000). H2 could be converted into a liquid fuel the U.S. Department of Energy under Award by upgrading biofuels, for example, or No. DE-SC0004993 to the Joint Center for 12. J. R. Bolton, S. J. Strickler, and J. S. Connolly, Nature, 316, 495 (1985). could be combined with CO2 from flue gas Artificial Photosynthesis, a DOE Energy or otherwise using the reverse water-gas Innovation Hub (translational research in 13. A. J. Bard and M. A. Fox, Accounts shift reaction, in conjunction with Fischer- solar fuels generators), has enabled the Chem. Res., 28, 141 (1995). Tropsch reactions, to produce liquid fuels preparation of this article. 14. S. M. Sze, Physics of Semiconductor for use in transportation applications, Devices, 3rd ed. John Wiley and for example. An alternative is to directly About the Author Sons (1981); (b.) S. Hu, C. Xiang, S. reduce CO2 to methanol or , for Haussener, A. Berger, and N. S. Lewis, example.30 R&D should also therefore be Energy and Env. Sci., in press (2013). devoted to developing, discovering, and Nathan S. Lewis obtained his PhD in 15. J. M. Berg, J. L. Tymoczko, and L. studying catalysts that can promote the inorganic chemistry from MIT in 1981. He Stryer, Biochemistry, 5th ed., W. H. six-electron and eight-electron reduction of has been a Professor of Chemistry at Caltech Freeman (2002). since 1991; and Scientific Director of the CO2 to methanol or methane, respectively. 16. B. Parkinson, Accounts Chem. Res., 17, This process is envisioned as one in which Joint Center for Artificial Photosynthesis, 431 (1984). the prototypes will have the optionality to the DOE’s Energy Innovation Hub in Fuels 17. J. S. Wang, Y. Yoo, C. Gao, I. Takeuchi, produce either gaseous or liquid fuels, so from Sunlight, since 2010. His research X. Sun, H. Chang, X.-D. Xiang, and P. that when the catalyst development program interests include artificial photosynthesis G. Schultz, Science, 279, 1712 (1998). matures, the prototypes will already be and electronic noses. Dr. Lewis continues 18. D. Pei, H. D. Ulrich, and P. G. Schultz, developed and will be ready to receive this to study ways to harness sunlight and Science, 253, 1408 (1991). enhancement in functionality. generate chemical fuel by splitting water to 19. X. D. Xiang, X. Sun, G. Briceño, Y. generate hydrogen. He is also developing Lou, K.-A. Wang, H. Chang, W. G. Summary an electronic nose, which consists of a Wallace-Freedman, S.-W. Chen, and P. chemically sensitive conducting polymer G. Schultz, Science, 268, 1738 (1995). film capable of detecting and quantifying The development of a complex system, 20. M. Woodhouse and B. A. Parkinson, a broad variety of analytes. He may be Chem. Soc. Rev., 38, 197 (2009). such as a solar fuels generator, requires much reached at [email protected]. more than just the discovery of a catalyst, 21. T. F. Jaramillo, S. Baeck, A. K. or of a photocatalyst, or even of a water- Shwarstein, K. S. Choi, G. D. Stucky, splitting nanoscale construct. It requires References and E. W. McFarland, J. Comb. Chem. a full macroscale object that is embedded 7, 264 (2005). in, and forms the basis for, an article of 1. J. Long and M. John, California’s 22. J. E. Katz, T. R. Gingrich, E. A. Santori, manufacture that can be made at scale, and Energy Future: The View to 2050; and N. S. Lewis, Energ Environ. Sci., 2, that can operate safely, cost-effectively, Summary Report; California Council 103 (2009). and efficiently over all length and time on Science and Technology (2011). 23. S. W. Boettcher, E. L. Warren, M. C. scales of interest. A highly integrated effort, 2. Department of Energy (DOE) Report Putnam, E. A. Santori, D. Turner- involving individual research groups, teams on the First Quadrennial Technology Evans, M. D. Kelzenberg, M. G. Walter, of research groups, centralized, focused Review, Washington, DC (2011). J. R. 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