Catalyst: Chemistry’s Role in Providing Clean and Affordable Energy for All

Abstract

Leif Hammarström is a professor of chemical physics at Uppsala University, Sweden. He is one of the leaders of the Swedish Consortium for Artificial Photosynthesis, founded in the mid-1990s. He is chair of the Swedish Solar Energy Platform and represents Uppsala University as a core member of the Solar Fuels Institute. Leif Hammarström is a professor of chemical physics at Uppsala University, Sweden. He is one of the leaders of the Swedish Consortium for Artificial Photosynthesis, founded in the mid-1990s. He is chair of the Swedish Solar Energy Platform and represents Uppsala University as a core member of the Solar Fuels Institute. Life on our planet depends on energy to sustain its development. However, the conditions and availability of different energy sources have changed dramatically during the history of Earth. One of the major changes was the development of photosynthesis billions of years ago. This process produces energy-rich compounds from low-energy substrates by tapping into the largest available energy source: the sun. Photosynthesis allows the biosphere to produce the energy it needs in a sustainable fashion instead of feeding on energy-rich compounds that were once created and that will eventually become scarce. More recently, the industrial revolution and modern welfare state have been based on fossil fuels. These fuels are the remains of biomass produced from hundreds of millions of years of photosynthesis. Today, we are burning fossil fuels at a very high rate, depleting our fossil resources. At the same time, we release large amounts of carbon dioxide into the atmosphere, which has potentially drastic effects on Earth’s climate and our living conditions. We need another transformation of the way global energy is supplied, and this time it is up to us humans to realize it. The United Nations (UN) has recently set up a number of Sustainable Development Goals, which came into force on January 1, 2016. One of these goals is to ensure clean and affordable energy for all, given that energy is central to nearly every major challenge and opportunity the world faces today.1United Nations. Goal 7: Ensure access to affordable, reliable, sustainable and modern energy for all. http://www.un.org/sustainabledevelopment/energy/.Google Scholar Many countries have high energy consumption per capita and are responsible for most of our greenhouse gas emissions. Energy savings can then be an important complement to the introduction of renewable energy in phasing out fossil fuels. At the same time, the UN estimates that 2.8 billion people have to rely on simple fires for cooking and heating, leading to over 4 million premature deaths per year as a result of indoor air pollution. Also, more than a billion people have no access to an electric grid. These societies cannot be asked to save energy. Instead, their sustainable development requires increased access to energy: for children to have light after sunset to do their homework, for refrigeration of vaccines, and for women and girls who could spend hours each day on collecting firewood and water, etc. This UN goal must thus be achieved with new energy solutions, and it must be done on a large scale. The sun is by far the largest renewable-energy source available to us. In about 1 hr, our planet receives the same amount of solar energy that humankind consumes in a year. It is a very evenly distributed resource, given that the annual irradiation over most populated areas differs by less than a factor of two. This makes it possible for most countries to produce energy for their own needs. Today, we are harvesting only a small fraction of the incoming solar energy. To form a sustainable future, it is imperative that we develop technologies to make use of this resource. Chemists have an important role to play in that effort. Photovoltaic solar cells are currently the most common technology for the production of electricity by solar energy. The solar cell market has developed strongly: the rate of new installations has increased by around 40% every year for more than a decade.2Fraunhofer Institute for Solar Energy Systems (2016). Photovoltaics Report. https://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report-in-englischer-sprache.pdf.Google Scholar Germany and Italy have led this development for some time, but since 2013 most of the installed solar cells have been in China, Japan, and the US. The price of photovoltaic panels has dropped by a remarkable 80% in just a few years. Some countries have even reached a so-called grid parity, which is when electricity from solar cells can be produced at a cost equal to or lower than that of electricity from the grid.3International Energy Agency Photovoltaic Power Systems Programme (2015). Trends 2015 in Photovoltaic Applications, Report IEA-PVPS T1-27:2015. http://www.iea-pvps.org/fileadmin/dam/public/report/national/IEA-PVPS_-_Trends_2015_-_MedRes.pdf.Google Scholar The increased diffusion of photovoltaics into the energy system demonstrates in practice the power of the sun—but also some of the problems that must be addressed. More than 90% of the solar cells installed in the last few years are based on silicon.2Fraunhofer Institute for Solar Energy Systems (2016). Photovoltaics Report. https://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report-in-englischer-sprache.pdf.Google Scholar Their present success makes it hard for other photovoltaic technologies to compete. Still, several alternative technologies at different stages of maturity could eventually become advantageous. One motivation is that further system price reductions will be an important driver for market growth. Silicon cell production needs very large capital investments, and cheaper production technologies of alternative cells could give these an advantage. Another important parameter is the energy required for producing the solar panel, which affects the energy payback time. Thus, alternative solar cells based on thin films2Fraunhofer Institute for Solar Energy Systems (2016). Photovoltaics Report. https://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report-in-englischer-sprache.pdf.Google Scholar, 3International Energy Agency Photovoltaic Power Systems Programme (2015). Trends 2015 in Photovoltaic Applications, Report IEA-PVPS T1-27:2015. http://www.iea-pvps.org/fileadmin/dam/public/report/national/IEA-PVPS_-_Trends_2015_-_MedRes.pdf.Google Scholar or nano-wires consume only little material. Molecular and hybrid cells, such as dye-sensitized, perovskite, and organic polymer solar cells, can be made with cheap and low-tech methods. All these alternative technologies also offer more lightweight and mechanically flexible solar cells than the ones made from silicon. This is an advantage for integration into buildings. They could also lead to new niche applications in, e.g., consumer products. Nonetheless, most of these emerging solar cells still require more research on combining high efficiency with long-term stability and avoiding the use of scarce and toxic elements in the materials. One problem with photovoltaics is the intermittency of solar irradiation. The large yearly and daily variation in electricity production puts a limit on the amount of solar and wind power our current electrical grids can accept. Power production from other sources—hydroelectric, nuclear, and fossil—is used to balance the grid when the sun and winds are weak. When they are strong, however, they produce more than the grid consumes. Some countries, such as Denmark (mainly wind power) and Germany, have already come close to these grid-capacity limits. Grid development is therefore important, and one part of this is batteries for connection to the grid for storing and balancing electricity production and consumption.4Dunn B. Kamath H. Tarascon J.-M. Science. 2011; 334: 928-935Crossref PubMed Scopus (10188) Google Scholar Research on improved batteries for large-scale applications is an important challenge for chemists. However, it is difficult to store large amounts of energy and to store it for a long time. The best option for long-term and large-scale storage of renewable energy is in the form of a fuel. A renewable fuel in liquid form is a high-energy compound like today’s gasoline, but it is produced by renewable energy. The energy density is about two orders of magnitude greater than for the best batteries.4Dunn B. Kamath H. Tarascon J.-M. Science. 2011; 334: 928-935Crossref PubMed Scopus (10188) Google Scholar Globally, 80% of the energy we use today is in the form of fuel, mostly fossil.5International Energy Agency (2015). 2015 Key World Energy Statistics. https://www.iea.org/publications/freepublications/publication/KeyWorld_Statistics_2015.pdf.Google Scholar Only 18% is in the form of electricity, and although this number is increasing, fuels are likely to dominate our energy system for a long time. If we should phase out fossil fuels, we therefore need to develop ways to produce fuels from renewable sources and on a large scale. An attractive possibility is to produce so-called solar fuels. These are renewable fuels that are produced directly from simple substrates with only the sun’s energy as input. For example, water can be split into hydrogen and oxygen. Hydrogen is a fuel that can be stored and later used in, e.g., fuel cells, to produce electricity. Carbon dioxide can be reduced to carbon-based fuels, such as alcohols. When they are burned, they release only the carbon dioxide that was fixed when they were made. Solar fuels can be produced by artificial photosynthesis.6Sun L. Hammarström L. Åkermark B. Styring S. Chem. Soc. Rev. 2001; 30: 36-49Crossref Scopus (459) Google Scholar, 7Lewis N.S. Nocera D.G. Proc. Natl. Acad. Sci. USA. 2006; 103: 15729-15735Crossref PubMed Scopus (6489) Google Scholar The process is in principle similar to natural photosynthesis, where water and carbon dioxide are converted to high-energy compounds. An important difference is that artificial photosynthesis can be much more efficient than its natural counterpart. A typical plant converts less than 1% of the incoming solar energy to biomass,6Sun L. Hammarström L. Åkermark B. Styring S. Chem. Soc. Rev. 2001; 30: 36-49Crossref Scopus (459) Google Scholar and the best crops for biomass production give an efficiency of only up to 2%–3%. In contrast, artificial photosynthesis can theoretically reach over 40% conversion efficiency.8Hanna M.C. Nozik A.J. J. Appl. Phys. 2006; 100: 074510Crossref Scopus (1192) Google Scholar This is similar to the theoretical efficiency of tandem solar cells, but the product is a fuel instead of electricity. Considering the need for large-scale renewable-energy storage and the low efficiency of biomass production, it is interesting to note that the direct production of solar fuels from water and carbon dioxide is the only envisioned renewable-energy technology that does not target electricity or biomass as products.9Sims R.E.H. Schock R.N. Adegbululgbe A. Fenhann J. Konstantinaviciute I. Moomaw W. Nimir H.B. Schlamadinger B. Torres-Martínez J. Turner C. et al.Energy Supply.in: Metz B. Davidson O.R. Bosch P.R. Dave R. Meyer L.A. Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2007: 251-322Google Scholar There are different concepts for how to realize artificial photosynthesis, but the principal similarities are great. First, sunlight is absorbed by a molecule or semiconductor material, and the excitation energy is used to form an electron-hole pair; the hole is a vacancy from where the electron was taken. Before the electron and hole recombine, the electrons are transported to a catalyst where they are used for the fuel-forming reaction. This could be a liquid or gaseous fuel, instead of the sugar or biomass produced by natural photosynthesis. Examples include the reduction of protons to molecular hydrogen (Equation 1) and the reduction of carbon dioxide to methanol (Equation 2). The holes are filled by another catalyst that takes electrons from water and releases oxygen (Equation 3). This allows for the use of water as a cheap and abundant source of electrons. In nature, this reaction is carried out in the large protein complex photosystem II and is catalyzed by a cluster of manganese and calcium ions. In artificial systems, the same process is catalyzed by a molecule, nano-particle, or material that is directly connected, on a microscopic scale, to the units that perform light absorption and electron-hole separation.2 H+ + 2 e− → H2(Equation 1) CO2 + 6 H+ + 6 e− → CH3OH + H2O(Equation 2) 2 H2O → O2 + 4 H+ + 4 e−(Equation 3) The development of artificial photosynthesis during the last 10 years has been tremendous. Water oxidation was long considered a “holy grail” of chemistry, but today a large number of functional catalysts have been discovered and designed, and they are showing better and better performance. For the reduction of protons or carbon dioxide, the focus has shifted from noble metal catalysts to those made of cheaper and more common elements. A number of new lab-scale devices that accomplish overall water splitting to hydrogen and oxygen solely through solar irradiation have been presented.10Ager J.W. Shaner M.R. Walczak K.A. Sharp I.D. Ardo S. Energy Environ. Sci. 2015; 8: 2811-2824Crossref Google Scholar Few of these show any long-time stability, however. Also, most use rather expensive multi-junction semiconductors as light absorbers, and many contain rare or toxic elements that could hinder their large-scale use. Thus, although important and inspiring results have recently emerged, the production of solar fuels by artificial photosynthesis is not yet a technology, and several years of fundamental research remain. Realizing the large-scale production of solar fuels is a multi-disciplinary effort, but it puts chemistry in focus. The most urgent challenges are in the following areas:1.We need to master catalysis of the fuel-forming and water-oxidizing reactions. As is clear from (Equation 1), (Equation 2), (Equation 3), these are multi-electron reactions coupled to the transfer of multiple protons, as well as the formation of new chemical bonds to make the products. These catalytic reactions must be tuned to proceed with low-energy barriers in order to reach high-energy conversion efficiencies of the overall process. Today, we are reasonably good at reducing protons to hydrogens, but we need to find better catalysts both for water oxidation and for efficient and selective reduction of carbon dioxide; the latter includes being able to make the carbon-carbon bonds required for larger fuel molecules. The catalytic reactions also need to be coupled to the light reactions, where absorption of one photon creates only a single electron-hole pair. Thus, complete catalysis needs the accumulated electrons and holes from the absorption of several photons.Currently, the best-performing systems are heterogeneous. Yet, molecular catalysts have great potential for fine tuning electronic and structural properties and could in the future offer more energy-efficient mechanistic pathways and higher product selectivity. Therefore, we are already today seeing emerging hybrid systems where molecules and materials are combined.2.Solar fuel devices have to be stable under several years of operation. This requires that the components are robust or that they can be regenerated by simple methods such as self-assembly. An important precedential case is dye-sensitized solar cells, where an intrinsically very unstable dye on a nano-structured semiconductor film full of defects forms an efficient solar cell that is stable for up to 20 years.11Mathew S. Yella A. Gao P. Humphry-Baker R. Curchod B.F. Ashari-Astani N. Tavernelli I. Rothlisberger U. Nazeeruddin M.K. Grätzel M. Nat. Chem. 2014; 6: 242-247Crossref Scopus (3821) Google Scholar A key to understanding this result is kinetic stabilization, meaning that the productive reactions are much faster than those that would normally degrade the components. Stability is thus an emerging property of the device—not just of the components—and this is something that needs to be carried over to solar fuel devices.3.The ultimate system design is an open question, and the present lab devices have pointed toward several different concepts. Ultimately, the system needs to harvest sunlight and manage the transport of electrons and protons. It needs to harvest the fuel separate from the oxygen. Also, it must be produced by reasonably cheap and not too energy-intensive methods. The materials used must avoid scarce and toxic elements, given that they could become limiting for future upscaling.4.Further optimization of systems for solar fuel production will involve the light-harvesting and antenna function. This includes optimizing the band gap and optical engineering (including plasmonic effects) to maximize the number of photons that can be absorbed. So far, a solar fuel industry does not exist. Therefore, the Solar Fuels Institute (SOFI) was formed to meet the need for global collaboration both between research groups and between academia and industry in a pre-competitive phase of this technology.12Solar Fuels Institute. Solar Energy Innovation. http://www.solar-fuels.org/.Google Scholar SOFI’s intention is to promote collaboration and information exchange to accelerate the transfer from research to a viable and large-scale technology. The challenges and opportunities for solar fuels are great, and we need a long-term and cross-disciplinary commitment of research and development to make this a reality. Reaction: Public Policy Challenges to Scientific Innovation on Solar EnergyBolsen et al.ChemOctober 13, 2016In BriefDr. Toby Bolsen is an associate professor of political science and director of the Zoukis Research Collaborative at Georgia State University. He studies political communication and political behavior. Dr. James N. Druckman is the Payson S. Wild Professor of Political Science and a fellow at the Institute for Policy Research at Northwestern University. He studies political communication and preference formation. He is currently the co-principal investigator of Time-Sharing Experiments in the Social Sciences. Dr. Fay Lomax Cook is a professor of human development and social policy at Northwestern University. She studies the interrelationships between public opinion and public policy, public deliberation, and the dynamics of public and elite support for social programs. She is on leave as assistant director of the National Science Foundation, where she heads the Social, Behavioral, and Economic Sciences Directorate. Full-Text PDF Open Archive