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Editorial Cite This: ACS Appl. Energy Mater. 2019, 2, 1−2
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ACS Applied Energy Materials: A Special Forum Issue on Solar Fuels he New Year of 2019 brings the first of many planned thematic “Forum” issues to ACS Applied Energy Materials. The motivation for this and future Forum issues is to bring together a collection of experts to contribute articles on a specific topic of particular interest and relevance to energy applications. The hope is that such a collection in a single issue will provide a clear picture of the state-of-the-art and identify research challenges that would help enable applications and move the field forward more rapidly. The theme of this Forum issue is “Solar Fuels” a form of energy that could be viewed as a futuristic energy source or as a very old one. The vast majority of present day energy consumption in the world is from fossil fuels, which are arguably the oldest sources of readily available energy. Planetary scientists generally agree that fossil fuels originated from plants and organic matter that were present in the Earth’s crust millions of years ago. Plankton and plant materials subjected to high temperatures and pressures over this enormous time span formed what we now call fossil fuels. Since the origin of the energy embedded within them was originally from the sun, fossil fuels are in fact photosynthetic materials and hence could be considered solar fuels. The incredible energy densities present in fossil fuels such as gasoline (464.4 MJ/kg) are orders of magnitude larger than what can be stored in state-of-the-art batteries or capacitors and speak to the power of storing energy from the sun in chemical bonds. Indeed, the high energy density inspires continued research in Solar Fuels. Despite the fact that fossil fuels are in fact photosynthetic in origin, most researchers in the field would not consider them to be Solar Fuels because their continued burning is not sustainable. Some hope to bioengineer natural photosynthesis to speed up the process. A dream of many others active in the Solar Fuels area described herein is to identify a method for converting sunlight, water, and (sometimes) CO2 into high energy density fuels on demand that can be utilized immediately or stored for future use. Such an approach would not require in time the millions of years required for fossil fuel generation or even a single growing season. As this dream has not yet been realized, Solar Fuels of this type are best viewed as a potential future source of energy. An early breakthrough in solar fuel production came in the 1970s by Fujishima and Honda,1 who demonstrated that light energy can be used to split water into hydrogen and oxygen gases. Equation 1 represents a balanced equation for water splitting that involves both oxidation and reduction of water.
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2H 2O + 4hν → 2H 2 + O2
applications. This Forum issue describes new classes of photoelectrosynthetic cells (Hanson et al., Meyer et al.), dye molecules (White et al.), and semiconductor nanocrystals (McClelland and Weiss) that may one day enable the sensitization of water splitting to visible light. A lesson in photochemistry is that light absorption by materials or molecules creates excited states that are both stronger one-electron reductants and oxidants than the ground state, but coupling multiphoton and multielectron transfer reactivity represents a tremendous challenge. Reaction 1 requires four photons. The identification of photochemical assemblies that drive multielectron transfer reactions (Glusac et al.) and are capable of rapid redox catalysis (Nocera et al.) are described in this issue. An additional challenge of water splitting is that the reaction involves management of both electrons and protons. While much attention is focused on the electrons, the protons are of significant importance. Indeed, the identification of materials and interfaces that mediate protoncoupled electron transfer or PCET reactivity, where the electron and proton are transferred in one mechanistic step, are of keen interest (Yang et al.). Another strategy for Solar Fuel generation is to utilize the protons and electrons from water oxidation for the reduction of CO2 to carbon-based fuels. Equation 2 shows an example of water oxidation coupled with CO2 reduction to yield methane (natural gas), although a more profitable goal would be liquid fuels such as gasoline. 2H 2O + CO2 + 8hν → CH4 + 2O2
Such CO2 reduction for solar fuel production is likely far more futuristic than water splitting and would require many future breakthroughs. The photoreduction of CO2 is extremely challenging and also requires advances in light driven multielectron transfer reactivity, PCET, and C−C coupling chemistry. New materials that can drive this reaction chemistry (Marinescu et al., Polyanski et al.), and CO2 reduction catalysts that provide insights into mechanism (Delcamp et al.) are described in this issue. Tandem solar cells that contain one photoelectrode for water oxidation and a second photoelectrode for CO2 (and/or proton) reduction have been envisioned since the time of Fujishima and Honda.1 Oxide semiconductors, such as TiO2, have proven to be competent for water oxidation, while useful p-type materials for CO2 (and/or proton) reduction are lacking. The identification of new materials (Farnum et al.) and a more detailed understanding of classical materials such as p-Si (Rose et al.) are described herein. Yet another approach to the generation of Solar Fuels is to utilize photovoltaics to generate electrical power that then drives water oxidation and CO2 (or proton) reduction catalysis
(ΔG° = 4.92 eV, n = 4) (1)
The Gibbs free energy change for the reaction is highly unfavored. Hence the reverse reaction, the burning of H2 in air to form water, releases a tremendous amount of energy that can be utilized for practical applications. The demonstration of Fujishima and Honda was inspirational to the field; however, the inefficiency of the photoelectrochemical cell they described and the need for ultraviolet light precluded practical © 2019 American Chemical Society
(ΔG° = 10.3 eV, n = 8) (2)
Special Issue: New Chemistry to Advance the Quest for Sustainable Solar Fuels Published: January 28, 2019 1
DOI: 10.1021/acsaem.8b02262 ACS Appl. Energy Mater. 2019, 2, 1−2
ACS Applied Energy Materials
Editorial
in a separate reaction vessile. Such decoupling of the electrical power generation from catalysis allows these two processes to be independently fine-tuned and optimized. This approach is more akin to that utilized in natural photosynthesis where light absorption and charge separation are separated from catalysis by spatially arranged electron donors and acceptors that provide a free energy gradient to vectorially translate redox equivalents to the catalytic sites. It is likely that the first practical solar fuel production will utilize water splitting driven by photovoltaics connected to traditional electrolysis cells by copper wires. This approach will benefit from lower cost and more efficient photovoltaics that may be provided by future embodiments of dye-sensitized (Andersson et al.), polycrystalline (Gaury and Haney), or perovskite (Hadipour et al., Matsushita et al.) solar cells. In summary, practical applications of Solar Fuel generation require some key scientific advances yet hold considerable promise with inspiration from natural photosynthesis. I am delighted that the New Year brings us this special Forum issue and the opportunity to look forward. With critical breakthroughs enabled by research advances such as those described herein, the practical applications of Solar Fuels may become a reality within our lifetimes providing future generations with a sustainable source of energy.
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Gerald J. Meyer, Deputy Editor AUTHOR INFORMATION
ORCID
Gerald J. Meyer: 0000-0002-4227-6393 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38.
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DOI: 10.1021/acsaem.8b02262 ACS Appl. Energy Mater. 2019, 2, 1−2
