Translational Science for Energy and Beyond - Inorganic Chemistry

Sep 8, 2016 - She is an Inaugural ACS fellow and an AAAS fellow and has received a Senior Humboldt Award, a Japan Society of Promotion of Science Fell...
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Translational Science for Energy and Beyond James R. McKone,*,†,△ Debbie C. Crans,*,‡ Cheryl Martin,§ John Turner,∥ Anil R. Duggal,⊥ and Harry B. Gray*,# †

Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States § Harwich Partners LLC, formerly the U.S. Department of Energy (DOE), Advanced Research Projects AgencyEnergy, Washington, D.C. 20585, United States ∥ National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, United States ⊥ GE Global Research, Niskayuna, New York 12309, United States # Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ‡

ABSTRACT: A clear challenge for the coming decades is decreasing the carbon intensity of the global energy supply while simultaneously accommodating a rapid worldwide increase in power demand. Meeting this challenge of providing abundant, clean energy undoubtedly requires synergistic efforts between basic and applied researchers in the chemical sciences to develop and deploy new technologies. Among the available options, solar energy is one of the promising targets because of the high abundance of solar photons over much of the globe. Similarly, decarbonization of the global energy supply will require clean sources of hydrogen to use as reducing equivalents for fuel and chemical feedstocks. In this report, we discuss the importance of translational researchdefined as work that explicitly targets basic discovery as well as technology developmentin the context of photovoltaics and solar fuels. We focus on three representative research programs encompassing translational research in government, industry, and academia. We then discuss more broadly the benefits and challenges of translational research models and offer recommendations for research programs that address societal challenges in the energy sector and beyond.

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INTRODUCTION Energy is at the heart of human existence. The energy released by the oxidation of food and fuel has made it possible for humans to survive and prosper on our planet for millennia. Considering the last few hundred years, one can delineate “energy eras” according to the means by which we have obtained and used reducing equivalents from fuels (Figure 1) for energy. In the United States, for example, the years prior to 1850 were broadly based on burning wood and food crop byproducts for heat and light. Then came the industrial revolution, which brought technological innovations in mining, transportation, and other sectors, leading to the significant use of coal. Beginning around 1900, coal was gradually superseded by other fossil fuels, particularly petroleum and natural gas, giving rise to the modern energy era that continues today. The benefits of readily available energy from fossil fuels are clear in technological innovations in agriculture, water quality, building materials, and the like that help sustain a global population of over 7 billion people. However, some major disadvantages of the continued use of fossil fuels have become clear. The magnitude with which we now consume fossil energy is so great that the byproducts can no longer be considered as a small perturbation to the otherwise stable homeostatic global © XXXX American Chemical Society

ecosystem. Indeed, global climate change has been thoroughly documented and strongly linked to major fluctuations in the carbon cycle brought about by fossil fuel emissions.2 Similarly, ocean acidification from increased atmospheric CO2 concentration has already irreversibly modified ocean ecosystems, particularly through the degradation of habitats for certain species, like corals, that are sensitive to the local pH.3,4 Additional byproducts of fossil energy use are broadly classified as pollutants, such as sulfur and nitrogen oxides, particulates, and heavy metals. These harmful compounds associated with fossil fuel use are collectively responsible for diminishing the life expectancies of millions of people around the world, particularly in developing countries.5−7 Demand for energy is not likely to diminish in the near future; instead, it is projected to grow by nearly 40% of the current usage by 2040.8 Existing energy technologies could likely meet the projected demand, but there are significant downside risks associated with continuing to use the reducing equivalents readily available in fossil fuels as the primary source of our energy supply. Transitioning to a sustainable energy Received: May 5, 2016

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DOI: 10.1021/acs.inorgchem.6b01097 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Ternary diagram depicting three priorities in chemical science research: novelty, utility, and efficiency. Classifications of different types of research are highlighted as focusing on two or more of these factors.

Figure 1. U.S. total energy consumption, by source, between 1775 and the present day. Data were collected from the U.S. Energy Information Administration.1 Inset: Detail of minor contributors to the U.S. energy mix since 1990. The “other renewables” category comprises primarily solar, wind, and geothermal energy. The ordinate unit “Quads” refers to quadrillion (1015) British thermal units, where 1 Quad = 1.055 × 1018 J.

region of Figure 2, and we believe it represents the most interesting and fertile ground for rapid and impactful technological innovation in renewable energy research. The Manhattan Project was a clear and well-documented example of translational research.9 Meitner and Frisch first reported the phenomenon of nuclear fission in 1938.10 Proposals for producing a weapon based on fission followed shortly thereafter, but the intense wartime research effort was initiated in full awareness of the fact that a functional nuclear weapon would require major advances in fundamental and applied science and engineering. In fact, the Manhattan Project drove major innovations in physics, chemistry, materials science, mathematics, and computer science, to name a few. Thus, through a sustained translational effort that was driven by the urgency of the second World War, atomic bombs were tested and used within a decade. Translational innovations, however, do not necessarily require herculean development efforts, as was the case with the Manhattan Project. Interdisciplinary research teams of modest size and resources can be assembled with the explicit aim of solving challenging problems in a short time by combining the aspects of basic and applied research. Indeed, translational work is gaining popularity especially in the life sciences, thanks in part to strong financial support from the public and private sectors as well as broad recognition of the need to rapidly translate basic discoveries into applications for the treatment and prevention of disease. For example, in 2012 the U.S. National Institutes of Health established the National Center for Advancing Translational Science (NCATS) explicitly to support such efforts to treat and cure rare diseases. In the 2014 fiscal year, NCATS provided over U.S. $600 million to facilitate research in preclinical and clinical research.11,12 We emphasize that translational research, as we have defined it, is not the only route to technological innovation. Consider the development of a surface chemical analysis technique, X-ray photoelectron spectroscopy (XPS). This technology is fundamentally based on the photoelectric effect, which was first used by Einstein and his contemporaries to inform early work in quantum mechanics.13 The technology underlying XPS became possible only after considerable distributed fundamental and applied work related to understanding, controlling, and measuring electrons and X-rays.14

economy based on renewable sources will require significant cultural and political changes as well as major technological innovations. Science and engineering professionals can play many roles in facilitating these changes, but our biggest contributions are likely to be in the area of technology development. The chemical sciencesbroadly defined as encompassing chemistry, chemical engineering, materials science, biochemistry, and bioengineeringare centrally associated with the technological innovations needed to facilitate energy sustainability. The dominant paradigm for technology development in the chemical sciences involves synergistic, but largely uncoordinated, efforts between (a) fundamental researchers, traditionally in academia, who discover and measure the properties of matter in various forms and (b) applied researchers, traditionally in industry, who develop products and processes by making use of fundamental knowledge and practical design considerations. We argue in favor of a different paradigm for addressing major challenges, like energy security and sustainability, based on a coordinated combination of fundamental and applied work, which we call “translational research”.

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TRANSLATIONAL RESEARCH: DEFINITION AND CONTEXT Figure 2 illustrates the interplay among three crucial priorities in chemical science research: novelty, utility, and efficiency. A large proportion of the research landscape can be categorized according to the way it balances these three key priorities. For example, basic science researchers tend to prioritize novelty over all else, whereas applied scientists and engineers prioritize utility. However, all researchers are also impacted by the need to use time and financial resources efficiently. Therefore, basic and applied researchers traditionally seek opportunities along the left and right ascending legs of Figure 2, respectively. Translational research, conversely, involves the deliberate combination of fundamental and applied work within a single research program. This approach encompasses the central B

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Inorganic Chemistry Technological innovations can also follow rapidly from unexpected discoveries without the need for any explicit combination of fundamental and applied research. For example, according to historical records at the Corning Corp., in 1922 a Corning employee named William Woods observed that molten glass, when allowed to drop slowly through a hole in a shovel, tended to form a bubble that closely resembled the shape of the finished bulbs that were then being made by hand. This discovery led to the invention of the Corning ribbon machine for glass bulb blanks, which, in turn, facilitated rapid electrification of household lighting nationwide.15,16 Although there are many routes to technological innovation, we believe that translational research is particularly valuable as a means to advance a field where the needs are clearly defined, as is the case with sustainable energy and human health. In the following, we highlight examples of dedicated translational research efforts in the area of solar photovoltaics (PV) and solar fuels, taken from the experiences of the authors. We first review briefly the history of these fields for context and then describe research programs in government, corporate, and university laboratories. We conclude with a discussion of the requirements, challenges, and opportunities for this model of research in renewable energy and other fields.

Figure 3. Recent world record cell and module efficiencies for four dominant commercial solar photovoltaic technologies under nonconcentrated solar illumination, as reported by Green et al.22 Note that c-Si and poly-Si denote single-crystalline and polycrystalline silicon, respectively, and CIGS denotes CuInxGa1−xSe2.

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HISTORICAL OVERVIEW OF SOLAR TECHNOLOGIES One of the most promising opportunities for a fully sustainable energy supply involves massively increasing our direct use of abundant solar photons. The chemical sciences have played a crucial role in nearly every stage of solar energy science and technology development and will continue to do so in the future. In the following, we provide a brief historical overview of two solar technologiesPV and solar fuelsin the interest of informing the subsequent discussion of recent translational research efforts in these areas. Photovoltaics. The history of solar photovoltaic technologies is well-documented.17−19 It began with Becquerel, who reported the photovoltaic effect in 1839.20,21 Many years passed before the first practical solid-state solar cells were demonstrated at Bell Laboratories in the early 1940s. In the following years, Bell scientists developed methods to control the growth and crystallization of silicon, which led to the first photovoltaic cells with reasonable efficiencies in the mid-1950s. In the late 1950s and 1960s, the space program drove additional development, leading to highly efficient (albeit costly) cells based on silicon and III−V materials. Further innovations in PV were motivated by the worldwide oil crisis in the 1970s, and by the mid-1980s, photovoltaic modules for terrestrial use had matured to the point of commercial viability in niche applications, such as light-powered calculators. The intervening decades have seen continued maturation of photovoltaic technologies and the associated commercial infrastructure, yielding ever-higher cell and module efficiencies, greater stability, and lower manufacturing costs. Today, the commercial landscape of PV is dominated by three primary technologies: silicon (including single-crystalline, polycrystalline, and amorphous forms), thin films such as CdTe and CuInxGa1−xSe2 (CIGS), and III−V materials such as GaAs. Recently reported record efficiencies for cells (1−100 cm2 illuminated area) and modules (≥800 cm2 illuminated area) for each of these material types are summarized in Figure 3, as compiled by Green et al.22 Figure 4 shows the historical “learning curve” relationship between the module prices and

Figure 4. Compiled data on the global average module price per peak watt (Wp) for c-Si and thin-film CdTe solar modules versus cumulative production volume. Reproduced from analysis by the International Renewable Energy Agency (IRENA).23

cumulative production volume for silicon and CdTe thin-film solar modules.23 Except for a brief spike between 2004 and 2010 related to raw material availability, manufacturing costs for silicon modules have been decreasing at a consistent rate for over 3 decades, and thin-film technologies have kept pace in spite of their shorter development history. Extensive research continues in established and new photovoltaic technologies with the goal of achieving higher device efficiencies and lower costs for commercial solar modules. Materials that are of particular interest for fundamental research and early commercialization efforts include earth-abundant inorganic thin films such as Cu2ZnSnSxSe4−x,24,25 organic thin films,26,27 and dye-sensitized solar cells.28,29 Very recently, there has also been a surge of interest in hybrid organic−inorganic thin-film solar absorbers based on ammonium−lead halide perovskites.30−32 Solar Fuels. PV are rapidly approaching cost parity with fossil fuels as a source of electricity for the power grid.33 Researchers in the chemical sciences are becoming increasingly interested in converting solar energy not just to electricity but also to chemical fuels.34,35 Solar fuels are attractive as storage media to mitigate the variability of sunlight due to seasonal variations, diurnal cycles, and weather patterns. They are also attractive because the products can be used as feedstocks in the chemical industry or as fuels for large-scale thermal processes such as steelmaking. The most mature solar fuel technologies are biofuels such as ethanol and biodiesel, which make use of photosynthetic C

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Inorganic Chemistry organisms to capture and store solar energy.36−39 Many investigators are also interested in mimics of photosynthesis using nonbiological materials; this approach is broadly termed “artificial photosynthesis”.40,41 Early examples of artificial photosynthesis can be found in reports in the 1970s of solar photochemical water splitting by metal oxides.42,43 Interest in the area of photoelectrochemical water splitting quickly grew, and it has remained a major research emphasis in the chemical sciences for over 4 decades. Many excellent reviews are available on the concepts and history of artificial photosynthesis.44−58 Major research efforts in artificial photosynthesis have coalesced around various proposed device structures, including systems that incorporate fully functional PV cells paired with water electrolysis systems, monolithic photoelectrode systems that intimately integrate light absorbers with electrocatalysts, and molecular or colloidal systems intended to operate in aqueous solutions or suspensions (Figure 5).49,59,60 These approaches span a wide range of technologies with concomitant trade-offs between maturity and anticipated device costs.59−61

efficiency and stability of solar cells under operating conditions.67,68 Among its other roles, the NREL has worked closely with U.S. DOE offices to facilitate active communication and collaboration between the academic and commercial sectors, in part through analysis of future energy technologies and their economic implications. In this context, the NREL has been instrumental in facilitating translational research by providing an authoritative voice to help align basic research targets with commercial opportunities. We discuss here one future technological opportunity through the lens of ongoing NREL and DOE analyses: solar-driven hydrogen production for use as an energy-storage medium and chemical feedstock. Even aside from its nascent use in fuel-cell vehicles, hydrogen has a major role to play in any sustainable energy economy. Consider, for example, that one of the largest industrial uses of hydrogen is for the manufacture of ammonia. Ammonia production in the U.S. alone totaled over 9 billion kg in 2014, of which ∼88% was used domestically for fertilization.69 The magnitude and importance of this application demands that any proposed renewable energy economy in which hydrocarbons no longer dominate as the primary energy and chemical feedstocks must still provide massive quantities of hydrogen. Additionally, although storage of hydrogen on a small scale, such as for fuel cell vehicles, remains rather expensive, large-scale storage in tanks or natural salt caverns is not expected to be prohibitively costly.70,71 It follows that industrial use may be preferable even to transportation as the primary use for hydrogen derived from renewables such as solar. Technoeconomic assessments of hydrogen for use as a transportation fuel and chemical feedstock point toward cost targets of approximately $2/kg.72 Currently steam reforming of methane is the preferred method for the production of hydrogen, and manufactures using this method can readily achieve this cost target. Conversely, the NREL estimates that the final cost of hydrogen derived from water electrolysis using solar photovoltaic electricity input is not likely to meet the cost target without major advances in the associated technologies.61 In fact, a very recent technoeconomic analysis supported jointly by university, corporate, and philanthropic organizations placed the current cost of hydrogen derived from natural gas at ∼$1.40/kg and the lowest achievable cost for solar-derived hydrogen using known or near-term proposed technologies at ∼$10/kg.73 The primary reason for the high projected costs of solar hydrogen is the low (∼20%) capacity factor of solar energy, meaning that the average output of a solar PV system is only 20% of its peak output. As a result, the capital cost of a water electrolysis system sized for peak solar output is much larger than would be required for an equivalent average output from a more continuous (i.e., higher capacity factor) source such as a hydroelectric plant. Clearly, continued innovations in solar PV to produce technologies that are robust and lower in cost can be very helpful for enabling cost-effective solar hydrogen. However, there is also room for basic and applied research in other methods for generating solar hydrogen, including biomass thermolysis and integrated solar-driven water-splitting devices. Various pathways by which hydrogen can be derived from solar energy are given in Figure 6. Among these options, the dominant technologies will be those that offer the lowest final cost of hydrogen at the required scale.

Figure 5. Schematic depiction of the range of device architectures being actively considered for solar-driven water electrolysis. Reproduced from ref 59. Copyright 2014 American Chemical Society.

In the following, we summarize recent translational efforts in the development of solar energy technologies in three different research environments. First, we highlight ongoing work from the National Renewable Energy Laboratory (NREL) on envisioning a renewable energy economy based in part on hydrogen as an energy storage medium and chemical feedstock. Second, we describe work by GE Global Research on CdTe PV. Third, we discuss work on hydrogen production via solardriven water splitting in the CCI Solar program, a university consortium. Finally, we discuss the benefits and challenges of carrying out translational solar energy research in these various contexts and how lessons learned in this area might be applied more broadly to translational work in the chemical sciences.

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DIRECTION SETTING IN SOLAR TECHNOLOGIES: NREL Since it began operating in 1977 as the Solar Energy Research Institute, the NREL has been a major driver of solar energy research worldwide.62 The NREL has been involved in the development of every major photovoltaic technology, particularly thin-film PV and multijunction III−V cells.63 The NREL also serves a key role as a source of standards and measurements for renewables in the United States. The NREL manages the National Center for Photovoltaics,64 which maintains a research team dedicated to the measurement and development of standards and models for the terrestrial solar spectrum65,66 as well as a team focused on the testing D

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Inorganic Chemistry

absorbers for solar fuel systems. Rather, strategies to use oxides to protect otherwise unstable but highly efficient photovoltaic materials from harsh electrochemical environments are more likely to yield useful devices. Encouraging recent results have been obtained on this type of stabilization strategy.58,76−79 A third key conclusion is that, while raw material costs are important for scaling any solar technology, elemental abundance is much less important than energy conversion efficiency in laboratory efforts to demonstrate functional solar electrolysis devices. Indeed, CdTe thin-film modules currently enjoy some of the lowest module costs of any photovoltaic technology, in spite of the fact that tellurium is among the least terrestrially abundant elements. The analyses and conclusions discussed above are broadly representative of those that have come from research at the NREL and the other DOE-supported national laboratories. They are particularly important in helping to inform the community across the spectrum of basic and applied research as to the most promising ways forward.

Figure 6. Routes for solar energy conversion to hydrogen for use as a fuel and chemical feedstock. Adapted from ref 74. Copyright 1999 American Association for the Advancement of Science.

On the basis of the aforementioned cost requirements for hydrogen production, NREL analysis strongly suggests that viable solar fuel systems based on PV or artificial photosynthesis approaches need to be highly efficient: on the order of 15−25% solar energy conversion efficiency in terms of the thermodynamic energy content of the H2 and O2 electrolysis products relative to the incoming solar energy flux.61 This requirement clearly points in the direction of developing multijunction solar fuel systems. A broader implication of the need for high efficiency in solar fuel devices is that any new absorber materials discovered for artificial photosynthesis must have electronic properties comparable to those of semiconductors used in existing commercial PV devices. Research in this area is therefore a high-risk, high-reward proposition: extensive work has already been done in identifying promising PV light absorbers, making meaningful discoveries less likely, but plausible material candidates for solar fuel systems could also be viable for standalone PV. A recent encouraging example is work from Luo et al. making use of new hybrid lead halide perovskite absorbers for solar-driven water splitting at >10% energy conversion efficiency.75 Another implication of the need for high device efficiencies is that metal oxide materials with poor optoelectronic properties, in spite of their high stability, are not likely to be useful as

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INDUSTRIAL R&D: CdTe AT GE GLOBAL RESEARCH Owing to its long history of developing and implementing technologies for the electric power sector, the General Electric (GE) Corp. has had many opportunities to pursue promising new technologies in renewable energy. Beginning in the mid2000s, GE decided to deepen its involvement in the solar market by building its own manufacturing capability in PV. The following is a synopsis of GE’s research and development in this area between 2009 and 2013. On the basis of the initial internal analysis of the most promising PV technologies, GE decided to pursue an extensive R&D effort in thin-film photovoltaic modules. CdTe was deemed the most attractive technology because of the existing availability of low-cost manufacturing methods along with clear opportunities for gains to be made in module efficiency and reliability. As a result, GE adopted a translational R&D program intended to improve the underlying materials and methods

Figure 7. Schematic image of the various components of GE’s thin-film CdTe solar module. The annotations highlight design improvements that GE pursued in their efforts to develop a cost-competitive commercial CdTe manufacturing operation. Abbreviations used in the figure are as follows: TCO denotes “transparent conducting oxide”; HRT denotes “high-resistivity transparent” buffer layer; Voc denotes “open-circuit photovoltage”. E

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Inorganic Chemistry used to make CdTe solar cells and to immediately incorporate these developments into a solar module manufacturing facility. As part of its strategy, GE made a significant investment in expertise by purchasing PrimeStar Solar, which had already built manufacturing capabilities in CdTe solar cells. GE further leveraged its strong base of research capabilities and resources to assemble a team spanning the gamut in expertise from basic laboratory research to device design and engineering to scale-up and manufacturing. A unique aspect of the GE−PrimeStar Solar effort was the close coupling between basic laboratory research and manufacturing. Through this coupling, the company was able to adopt an approach that incorporated “Edisonian” elements of rapid experimental turnover as well as a more “scientific” approach based on detailed analysis of the most promising discoveries. The research team built a complete cell fabrication line, which produced its first functional CdTe solar cell in 2010. At its peak, this line was producing 200 unique parts per week, each of which could be extensively benchmarked and analyzed using a suite of characterization capabilities. Furthermore, the manufacturing capabilities and expertise enabled by the GE−PrimeStar partnership allowed GE to focus exclusively on innovations in cell design that could be applied to full solar modules, further streamlining the process. The CdTe R&D efforts at GE focused on a variety of technological advances, as highlighted in Figure 7. The work resulted in improved performance in nearly every metric, such as photovoltage, photocurrent, and fill factor. These improvements led the GE CdTe laboratory to break the world record for CdTe solar cell efficiency twice: first in October of 2012 at 18.3% and then again in June of 2013 at 19.6%.63,80,81 Notably, the first record was achieved only 2 years after GE produced its first cell. Over the same time period as GE was developing CdTe technologies, the makers of silicon solar cells were heavily investing in new manufacturing facilities, especially in Asia. The result was an unexpectedly steep decrease in the overall cost of silicon PV modules beginning around 2010 (see Figure 4). As a result, the environment in the solar sector shifted rapidly from competition between multiple emerging thin-film technologies to competition between thin-film and silicon-based technologies. This shift led GE to pivot away from the strategy of developing in-house CdTe manufacturing capabilities. They instead pursued a partnership with First Solar, the industry leader in CdTe PV module production, through technology licensing agreements beginning in 2013. First Solar, in a continuing partnership with GE, subsequently achieved record CdTe cell efficiencies of 22.1% and module efficiencies of 18.6% as of early 2016.22,82 These values represent a substantial improvement over the best available technology prior to GE’s entry into the solar market, as shown in Figure 8. These innovations have, in turn, allowed CdTe modules to remain competitive with state-of-the-art polycrystalline silicon in an increasingly competitive solar PV market.

Figure 8. Historical compilation of selected record CdTe cell (blue) and module (green) efficiencies as of early 2016, along with the associated cell/module makers. The timespan highlighted in gray encompasses the years in which GE Global Research invested heavily in CdTe R&D.

goal of CCI Solar is to enable discoveries in materials and methods for artificial photosynthesis via solar-driven water electrolysis. Research in CCI Solar has been organized into three broad areas encompassing the key aspects of artificial photosynthesis, namely, light absorption, catalysis, and charge transport (Figure 9).35 Light absorber research has primarily focused on

Figure 9. Schematic depicting the major research thrusts of CCI Solar.83

crystalline semiconductors. Catalyst research has focused on molecular and heterogeneous catalysts with a particular emphasis on earth-abundant components such as first-row transition metals and light main-group elements. Charge transport in artificial photosynthesis devices encompasses both electronic and ionic processes necessary to neutralize chemical and electrical potential gradients that develop as a result of oxidation and reduction reactions associated with fuel formation. Researchers within CCI Solar focus on basic and applied research innovations on these individual topics as well as their interconnections. One of the earliest impactful developments reported by CCI Solar was a cobalt oxide water oxidation catalyst that was found

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USE-INSPIRED ACADEMIC RESEARCH: CCI SOLAR The CCI Solar research program was established in 2005 as one of several U.S. National Science Foundation Centers for Chemical Innovation. Managed by faculty and staff members at the California Institute of Technology, CCI Solar encompasses collaborative efforts among researchers from over 20 different universities and research institutions worldwide. The primary F

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Inorganic Chemistry to be active and stable under pH-neutral aqueous conditions.84 Interestingly, in phosphate electrolytes, the catalyst was found to operate on the basis of continuous dissolution and redeposition of cobalt oxide, leading to a unique “self-healing” behavior of this phosphate-mediated cobalt oxide (Co−Pi) system.85 Subsequent work expanded the library of available materials and conditions to borate buffers and nickel-based oxides, where the nickel compounds were found to be more active per unit mass than the corresponding cobalt compounds.86,87 Extensive work has now been completed on understanding the mechanism of catalytic turnover in these heterogeneous water oxidation systems.88−90 This work on oxides further led to extensive applied research efforts on efficient full solar-driven water-splitting devices that operate under neutral conditions.58,91 Another major research focus of CCI Solar has been the discovery and development of catalysts for the hydrogen evolution reaction (HER) based on earth-abundant materials. Advances have been made on transition-metal alloys, sulfides, and phosphides. CCI Solar researchers focused initially on mixed nickel−molybdenum alloys, which compare favorably in HER activity to pure nickel and in sufficiently high mass loadings approach the geometric activity of platinum.92−94 Nickel−molybdenum HER electrocatalysts were synthesized and deposited on silicon light absorbers, and the composites were used to good effect for photoelectrochemical hydrogen evolution.94−96 More recent work has centered on the discovery and development of a library of earth-abundant heterogeneous transition-metal phosphides for aqueous hydrogen evolution.97−103 These materials are remarkable because they exhibit high mass-specific catalytic activities relative to other non-noble-metal catalysts, and many are stable under highly acidic conditions. A third research focus of CCI Solar has been the development of metal oxide absorbers for solar-driven water oxidation, particularly tungsten oxide (WO3) and bismuth vanadate (BiVO4). Although WO3 has been known for some time as a stable oxygen-evolving photoanode, it suffers from poor solar light absorption because of its large band gap and inefficient interfacial charge transfer and catalysis.104,105 Careful control over synthetic conditions led CCI Solar researchers to discover a way to incorporate nitrogen into the bulk crystal structure of WO3, leading to small perturbations in the crystal structure and greatly enhanced visible-light absorption.106 Analogous to work on combining hydrogen evolution electrocatalysts with semiconductor photocathodes, CCI Solar researchers have demonstrated functional photoelectrochemical oxygen-evolving devices based on metal oxide light absorbers coupled to earth-abundant water oxidation catalysts. Examples include deposition of oxygen-evolving catalysts such as Co−Pi on WO3 electrodes, which significantly improved the faradaic efficiency for water oxidation and greatly stabilized the system under operating conditions.107 More recent work focused on BiVO4 photoelectrodes, which have more favorable light absorption and electronic properties than WO3.108 CCI Solar researchers developed novel syntheses of BiVO4 photoelectrodes based on the electrodeposition of BiOI precursors and subsequent reaction with vanadium coordination complexes.109 They further found that multilayer catalysts based on nickel and iron oxyhydroxides can yield remarkably efficient and stable oxygen-evolving photoanodes under neutral aqueous conditions.110

Because of the efforts of CCI Solar and many others in the community, the past decade of research in artificial photosynthesis has been highly productive. Today there are earthabundant absorber and catalyst materials available for solardriven oxygen and hydrogen evolution that operate efficiently and stably under neutral and alkaline conditions. There are also clear absorber and catalyst candidates for hydrogen evolution under acidic conditions. However, a major fundamental challenge that remains is the discovery of an earth-abundant oxygen-evolving catalyst that is stable and active under acidic conditions.59,111 There is also a need for the development and demonstration of electrocatalysts that can efficiently convert CO2 into liquid fuels. Finally, there is a need for the further translation of basic science innovations to applied research aimed at the demonstration of artificial photosynthetic systems. The key challenges of photochemical CO2 reduction and artificial photosynthesis device demonstrations have been specifically targeted by the Joint Center for Artificial Photosynthesis, which is supported by the U.S. DOE.112

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DISCUSSION The research efforts described in the previous sections represent a cross section of recent and ongoing work on solar energy technologies from three different research sectors: government, industry, and academia. We believe that these stories make interesting case studies in translational research. In particular, each of the three research programs have aspired to bridge the gap between basic and applied research by heavily incorporating considerations of both novelty and utility into their work. The NREL practices translational research by actively engaging with members of the government, academic, and commercial research communities. Such interactions allow the NREL to function as a useful intermediary by providing technology validation and broad analysis to inform research goals in the renewable energy sector. GE’s impactful research efforts on CdTe also reflect the values of translational research in that the GE−PrimeStar team emphasized discovery-oriented laboratory science and detailed device-level characterization, while also structuring their R&D program to ensure that laboratory innovations were directly transferrable to module manufacturing. In so doing, GE was able to advance the stateof-the-art in CdTe PV technology and rapidly incorporate those developments into the manufacturing pipeline for deployment. The CCI Solar program practices translational research by focusing on a functional target (artificial photosynthetic devices) to motivate every aspect of its otherwise discoveryoriented research efforts. The result has been clear demonstrations of laboratory-scale functional devices that have been used for technoeconomic analysis and early commercialization efforts. Benefits and Challenges. Striking a balance between basic and applied research has both positive and negative implications. The largest and most obvious positive outcome is the potential for high research impact. Considering the three case studies discussed above, the GE CdTe effort had obvious tangible benefits in increased commercial viability for thin-film PV and lower prices for solar modules overall. The ongoing work at the NREL is paying dividends by driving discussions on the best ways to obtain renewable hydrogen for a sustainable energy economy. Finally, CCI Solar has discovered earthabundant materials that could be highly impactful for practical solar-driven water electrolysis devices. G

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multidiscplinary teams and, in many cases, targets very specific and challenging research outcomes. As a result, there are many pitfalls that might prevent the accomplishment of those goals, several of which are illustrated in Figure 10. Given this reality,

A second major benefit of translational research is that it promotes interdisciplinary cooperation through productive collaboration. Many scientists in every research sector are highly protective of their own methodologies, analytical approaches, molecular motifs, and the like. However, focusing research on solving a difficult problem can allow each member of a team to bring unique expertise to bear. GE initiated a collaborative research program through the acquisition of PrimeStar Solar that led to the production of world-record efficient devices in a very short period of time. The subsequent additional collaborative effort between GE and First Solar through licensing agreements has expedited the development of CdTe PV technology overall. Similarly, collaborations within CCI Solar between research groups focused on semiconductor synthesis and those focused on catalyst development have yielded functional components of solar-driven water electrolysis systems. A third benefit of translational research is that it is often easier for nonexperts to understand. Fundamental research programs undertaken for the sake of knowledge alone can be seen as esoteric and irrelevant. By contrast, highly applied work on incremental technology improvements can be seen as boring. Therefore, it is much easier to interest broad audiences by framing basic and applied research in the context of solving important problems. As an example, we cite the remarkable excitement engendered by the “Solar Army” outreach program run through CCI Solar, which to date has engaged thousands of nontechnical people in multiple demographics and age groups.113,114 Translational research also poses challenges, including intellectual constraints and competitive risk. In many cases, translational work demands a narrowing down of possible research topics to those that have opportunities for discovery as well as application. These constraints run counter to the common desire of academic scientists and engineers for complete intellectual freedom. However, the intellectual freedom enjoyed by academic researchers is arguably also diminished by the tendency for various subdisciplines to coalesce heavily around research fads that naturally drive funding cycles through peer evaluation. In any case, complete intellectual freedom is not a realistic expectation for researchers who hope to take on major challenges, such as energy sustainability. In this context, certain appropriate constraints, e.g., on material availability or minimum device efficiency, are necessary for genuinely advancing the field. A second challenge associated with translational research is increased risk. One type of risk is competitive risk, which is a particular concern for industrial or highly applied researchers hoping to make an impact through translational work. At the same time as one group may be making a broad translational effortperhaps to build a new and promising technology from the ground upcompetitors who are working along more traditional applied research pathways (i.e., the right leg of Figure 2) may find better or faster ways to the solve the problem. In fact, it was precisely this type of competition with c-Si manufacturers, who pursued incremental cost improvements through economies of scale, that motivated GE to shift away from implementing their original strategy of building an in-house CdTe manufacturing facility and instead to seek a licensing partnership with First Solar. Thus, there is real competitive risk in carrying out translational research. Another type of risk associated with translational research is that of missed targets. Translational work usually makes use of

Figure 10. Illustration of several of the most important pitfalls facing translational research efforts with ambitious targets for solving real problems by bringing products to market.

many of the most ambitious funding programs in energy, such as the U.S. DOE ARPA-E program, are designed to focus on specific technological targets, and they require detailed assessment of medium-to-long-term considerations such as potential markets, stakeholders, and customers.

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A BLUEPRINT FOR TRANSLATIONAL RESEARCH In view of the benefits and challenges outlined above, we provide a series of recommendations on how to build a successful translational research program. First, it is neither easy nor desirable to strike an ideal balance between applied and fundamental research approaches when building a translational research team. The previously discussed example of the Manhattan Project was certainly atypical with respect to how intimately it coupled (temporally, intellectually, and physically) discovery and functional implementation of new technologies, and it did so only at enormous financial and social cost. In many cases, it may be better to approach a problem predominantly from the perspective of basic research, with applied considerations included in the interest of guiding discoveries in fruitful directions. The opposite approach might be equivalently fruitful, with applied researchers pursuing opportunities for basic science innovations to enable step changes in a technology. Indeed, the CCI Solar and GE CdTe research efforts fit these respective descriptions, whereas the NREL’s work on envisioning future energy technologies strikes a more even balance. A second critical point of a successful translational research effort is building a good team. The specifics of what constitutes a good team depend heavily on the problem being addressed, but any successful team requires at least (1) visionary leadership, (2) excellent organizers, and (3) talented and motivated researchers. Research team leaders benefit from having deep subject knowledge and established reputations, together bring internal cohesiveness and external credibility. Just as importantly, the leaders must be visionary by defining a clear picture of success and a willingness to direct researchers away from attractive but comparatively unimportant lines of exploration. Organizers need to be excellent at translating the vision into clear implementation strategies and providing meaningful feedback between leaders and researchers. Finally, the researchers themselves make up the bulk of the team; they must be talented, motivated, and consistently aware of the H

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leadership role by incorporating well-conceived educational programs that include dedicated interdisciplinary research training. It is also important to ensure that young scientists and engineers are developing a breadth of knowledge and awareness of the context of their research in addition to the technical skills that they gain as undergraduates and postgraduates. Professional societies can also be helpful in this area by promoting involvement by youngsters in national meetings, training programs, and the like. We have discussed the concept of translational research and its implementation in the chemical sciences for the purpose of advancing a sustainable energy future. We foresee that the 21st century will continue the trend toward collaborative, translational work in many areas of science and engineering. Within the chemical sciences, these changes will be driven, in part, by the need to remain relevant in an age that is increasingly dominated by virtual, rather than physical, interactions. More importantly, the coming decades are sure to see their share of challenges for sustaining and advancing human society, many of which will demand innovations in the chemical sciences. Some of these are known already, and others will surely emerge. As a research community, we will be better prepared to address known and unknown challenges by breaking down boundaries among sectors and disciplines and working together toward credible solutions.

impact that their individual efforts are having on the outcomes defined by the leaders. Many of these recommendations are obvious, but implementation is never easy. We emphasize that cohesiveness of vision is particularly important and challenging in an interdisciplinary team. Adequate time and effort must be spent ensuring that every member is able to articulate his or her own role and its context in a common language. Indeed, the need for a common language is one reason for applying the term “translational” to this type of research. We encourage colleagues who are interested in pursuing translational work in any area to study the structure, approaches, successes, and failures of historical and modern translational efforts in related fields. The list of translational research programs worldwide continues to grow at a rapid pace. Academic institutions and industrial organization are increasingly organizing themselves around the reality that innovation occurs across multiple disciplines. Examples of designed translational research centers in academia include the German Fraunhoffer Institutes (which have operated since the 1950s), the King Abdullah University of Science and Technology in Saudi Arabia, and the Olin College of Engineering in the U.S. During its height of productivity, Bell Laboratories routinely practiced translational research that yielded incredible technological advances in electronics and other fields. More recently, technology companies such as Alphabet (the parent of Google) and Apple are increasingly turning toward translational research by devoting resources to basic science in the interest of informing new product development. More recently a group of philanthropists launched the Breakthrough Energy Coalition,115 which is well-positioned to support translational energy research by providing sustainable funding for early-stage renewable energy technologies. This trend of growth in translational research is likely to continue, and that growth would benefit from an active and open discussion in the community regarding effective strategies for fostering crossdisciplinary innovation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.R.M.). *E-mail: [email protected] (D.C.C.) *E-mail: [email protected] (H.B.G.). Present Address △

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261, United States Notes

The authors declare no competing financial interest.

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Biographies

SUMMARY Returning to the concept of energy eras in human history and considering current developments, we might make projections about how humans will use energy resources in the future. Alongside mainstay fossil fuel resources, the 20th century saw significant development of some low-carbon forms of energy, including hydroelectric, wind, and solar power. Interestingly, these alternative energy sources subvert the historic paradigm of combusting fuels for power because they rely rather on mechanical or electromagnetic forces to generate electricity with relatively little heat produced. Perhaps this distinction carries the seeds for what will become the next dominant energy era: one built primarily on electricity as an energy carrier, with chemical fuels and feedstocks produced and used only where they are needed.74,116−118 This outcome is certainly desirable for human health and environmental sustainability, and it appears increasingly likely with every advance in renewable energy R&D. A shift toward translational research brings a new set of challenges and opportunities, particularly in education. How do we train science and engineering students for involvement in these research efforts? Teamwork is already important in academia and industrial work, and it will only become more so with the increased breadth of expertise needed to make the next set of discoveries and products. Universities can take on a

James McKone received a Bachelor’s degree in chemistry and music from Saint Olaf College in Northfield, MN, in 2008. He holds a Ph.D. in chemistry (2013) from the California Institute of Technology (Caltech), where he was advised by Harry Gray and Nate Lewis. Upon graduating from Caltech, he was the recipient of several awards, including the Milton and Francis Clauser Doctoral Prize for original thesis research and the Demetriades−Tsafka−Kokkalis Prize in renewable energy. James is currently a postdoctoral researcher in the Department of Chemistry and Chemical Biology at Cornell University, advised by Héctor Abruña and Francis DiSalvo. His present research I

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Dr. Cheryl Martin is Head of the Centre for Global Industries at the World Economic Forum. She joined the Forum in March 2016 from the consulting firm Harwich Partners, which she launched to work with public and private sector entities to identify critical drivers that would accelerate the adoption of new technologies into markets. Previously, Dr. Martin served as the Acting Director of the Department of Energy’s Advanced Research Projects Agency−Energy (ARPA-E). In addition, she was the Deputy Director for Commercialization at the agency, where she developed the Technology-to-Market program, which helps breakthrough energy technologies succeed in the marketplace. Prior to joining ARPA-E, Dr. Martin was an Executive in Residence with the VC firm Kleiner Perkins Caufield and Byers, and the interim CEO of Renmatix, a startup company focused on renewable materials. She also spent 20 years with Rohm and Haas Company in roles ranging from technology to business and where, most recently, she had been the General Manager for the Paint and Coatings business in Europe, the Middle East, and Africa. Dr. Martin earned a B.A. in chemistry from College of the Holy Cross and went on to earn a Ph.D. in organic chemistry from the Massachusetts Institute of Technology. She is a non-resident Fellow at the Center on Global Energy Policy at Columbia University. Dr. Martin serves on the board of Enbala, an early stage company focused on making the electric grid more sustainable by harnessing the power of distributed energy.

Debbie C. Crans is a Professor of Organic and Inorganic Chemistry and is in the Chemical Biology and the Cell and Molecular Biology Programs at Colorado State University, where she has been since 1987. She is currently a Professor Laureate of the College of Natural Sciences. She was an undergraduate at the H. C. Ørsted Institute in Denmark in Biochemistry and Organic Chemistry and a graduate student at Harvard University and did her postdoctoral studies at the University of California, Los Angeles. She founded the International Vanadium Symposium in 1998 and the Zing Coordination Chemistry Conference in 2008 and will host the International Coordination Chemistry Conference 45 in Colorado in 2022. She was Program Chair and Chair for the Inorganic Division of the American Chemical Society and is currently one of their counselors and the Chair of the Colorado ACS section. She is a 2015 Arthur P. Cope Scholar, the Inaugural Vanadis Awardee (2004), and a 2011 awardee of the Colorado Sections Research Award. She is an Inaugural ACS fellow and an AAAS fellow and has received a Senior Humboldt Award, a Japan Society of Promotion of Science Fellowship, and a Japanese Coordination Chemistry Society Lectureship Award (2012). She is currently on the editorial boards of the New Journal of Chemistry and the Journal of Inorganic Biochemistry and is an Associate Editor of Coordination Chemistry Reviews. Dr. Crans has published over 180 original research papers in fundamental areas of inorganic and organic

John A. Turner is a Research Fellow at the National Renewable Energy Laboratory (NREL). He started his career in electrochemistry at Idaho State University as an undergraduate working on batteries. After receiving his B.S. degree in Chemistry, he moved to Colorado State University and worked on pulsed voltammetric techniques. He received his Ph.D. in Electroanalytical Chemistry and then moved to the California Institute of Technology (Caltech). It was at Caltech that he learned photoelectrochemistry and fell in love with the area. He joined NREL (then the Solar Energy Research Institute) in 1979 to

chemistry often applied to biological or medical problems. Her expertise includes metals in medicine, coordination chemistry, spectroscopy, and nanoscience, and she works on problems in diseases such as diabetes, cancer, Alzheimer’s, and tuberculosis. As a member of the Committee of Science, Crans initiated the efforts that led to the Symposium on Translation of Solar Energy research in 2015 and the preparation of this paper. J

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Letters, the Royal Swedish Academy of Sciences, the Royal Society of Great Britain, and the Accademia Nazionale dei Lincei.

work in Art Nozik’s group on hot-carrier processes at semiconductor electrodes. Shortly thereafter, he began research on photoelectrochemical water splitting for hydrogen production and has been at it ever since.

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ACKNOWLEDGMENTS We thank the American Chemical Society, Committee for Science, for sponsoring a session at the 249th ACS National Meeting in the Spring of 2015 highlighting the importance of translational research programs, where discussions leading to this manuscript began. We additionally acknowledge helpful input from Dr. Siddharth Dasgupta in the preparation of this manuscript.

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DOI: 10.1021/acs.inorgchem.6b01097 Inorg. Chem. XXXX, XXX, XXX−XXX