Solar Production for a Sustainable Energy Future: Highlights of a Symposium on Renewable from Sunlight and by Heli Wang, Deryn Chu, and Eric L. Miller

ynthesis of fuels from sunlight, water 222nd ECS Meeting in Honolulu in October. In this paper, an emphasis is placed on and , without competing Over 130 abstracts were received for this the summary of the presentations for solar Swith food production, is an important symposium. The symposium attendees came fuels production. We will highlight some of route for sustainable development beyond from North America, Europe, and Asia. The the presentations that reflected the research fossil fuels. The magnitude of the challenge research theme of this symposium focused progress in this field. is very significant that requires scientific on the development of materials and devices Metal are the most studied breakthroughs in materials and processing for generation and CO2 conversion material group. While metal for creating economic opportunities.1 to fuels. One approach is to utilize the solar oxides are more stable in aqueous solutions, There are two straightforward conversion energy to produce fuels. Another approach key issues are wide , band-edge pathways for conversion of is to utilize the electrical energy to generate mismatch and their intrinsic low STH to fuels.2 The most important one is the fuels with electrochemical devices. The efficiencies. A combinatorial screening might photoelectrochemical (PEC) hydrogen symposium presentations covered the be a relevant approach to search for metal production via , which following topics in both approaches: oxides with suitable band gap.4 Different combines the electrical generation and tandem cells5-9 have been developed to solve • solar energy materials; into a single system. The other the band edge mismatch. In the keynote • photocatalysts; pathway is replicating plant speech, Akihiko Kudo at Tokyo University • photo electrochemical cells (PECs); process with water and CO . These direct of Science (Japan) highlighted the Z-scheme 2 • biological devices; solar fuel pathways, especially the PEC water type photocatalysts for water splitting with • solar concentrators; splitting approach, have attracted attention improved Rh-doped SrTiO and BiVO . One • solid electrolysis cells (SOECs); 3 4 world-wide due to their renewable fuel the other hand, Lionel Vayssieres at Xi’an • solid oxide fuel cells (SOFCs); and generation and benefit to the environment. In Jiaotong University (China) introduced an • conductor electrolysis cells electrochemical and photoelectrochemical (PCECs) and fuel cells (PCFC). all-oxide quantum confinement approach to techniques, the combine with (continued on next page) catalysts, inhibitors, and an electrochemical flow reactor to convert CO2 to organic fuels. In PEC water splitting, the grand challenge of 10% solar-to-hydrogen (STH) efficiency and 10 year lifetime was defined A as the “holy grail of chemistry.”3 The requirements for a successful PEC water splitting device is displayed in Fig. 1, as shown with a p-type semiconductor case. To achieve efficient PEC water splitting at an illuminated semiconductor (SC),

1. the band gap (Eg) of the SC must be at least 1.6-1.7 eV since the theoretical H2O/H2 difference in equilibrium potentials be- tween water splitting reactions is 1.228 V at 25 °C. The overpotential must be sufficient for water splitting reactions, 1.23 eV but not over 2.2-2.3 eV in order to ab- hv 1.6-1.7 eV sorb the visible ; Counter 2. the band edges of the SC must straddle the water potentials; H2O/O2 3. the SC must meet the need of efficient charge generation/transfer in the bulk and fast reaction kinetics at the inter- face; and 4. the SC must be stable in aqueous solu- tions. It is challenging to develop materials that meet all the requirements of photoelectrodes/ p-type photocatalysts. To facilitate the research in photocatalysts and solar fuel production, Semiconductor a symposium on “ from Sunlight and Electricity” was held at the Fig. 1. Schematic of the PEC water splitting process at illuminated p-type semiconductor.

The Electrochemical Society Interface • Summer 2013 69 Wang, Chu, and Miller (continued from previous page) Naturally, there is no single material efforts are still needed to design and explore that could meet all the requirements of new photocatalysts and PEC materials to solve the issue. The low solar-to-hydrogen photoelectrodes/photocatalysts for water meet the “holy grail” challenge.3 efficiency is related to wide band gap, light splitting; and this has challenged the PEC In CO2 reduction, the thermodynamically absorption, charge mobility, recombination, communities for decades. Multi-junction uphill nature of this reaction, coupled with interfacial kinetics, etc. To overcome this cells, quantum dots, and plasmonic metals the large activation energy associated hurdle, different approaches have been brought new opportunities for research and with the multi- and multi-proton discussed at the symposium, including development of PECs and photocatalysts. reduction, makes the conversion of CO2 a band gap engineering, doping and co- For example, Nianqiang (Nick) Wu, West big challenge.13 Thomas F. Jaramillo from doping, nanostructuring, and catalyst, etc. Virginia University, talked about the Stanford University observed a total of 16 Moreover, Nam-Gyu Park at Sungkyunkwan development of plasmonic nanostructures different CO2 reduction products (C1-C3) University (Korea) presented new solar for enhancing the . Plasmonic from a polycrystalline copper electrode, five energy materials and nanostructures in his nanostructures convert the solar energy to of which were presented at the meeting. They keynote talk. the plasmonic energy stored in the oscillating have expanded their studies to numerous Semiconductor materials such as III-V . Such energy can transfer from the metals such as Pt, Au, Ag, Ni, Zn, and Fe, as metal to the semiconductor via a plasmonic- well as non-metal surfaces. Andrew Bocarsly and I-III-VI2 compounds have suitable band gaps and high efficiencies. The band induced resonant energy transfer (PIRET) from Princeton University presented the edge mismatch was successfully solved by process,11 leading to generation of electron- pyridinium catalyzed electrochemical 10 a tandem cell design. However, the trade- hole pairs in the semiconductor. The PIRET reduction of CO2 to methanol at illuminated off is the photo-corrosion of III-V materials is an unprecedented mechanism of plasmon- p-GaP photocathodes; with Faradaic in PEC hydrogen generation. Silicon enhanced photocatalysis, which provides the efficiencies exceeding 95%. From this materials have oxidation issue in the dark, guidelines for development of plasmonic effort, they proposed that the reduction of thus eliminating continuous application. photosensitizes for solar energy harvesting CO2 is initiated by a mediated charge transfer Naturally the surface modification devices. process, in which the one electron reduction and protection of the high efficient Government support plays a key role of pyridinium leads to the formation of have been discussed in to foster PEC R&D. In his keynote talk, a carbamate intermediate. Protonated the symposium. In the keynote speech, Eric Miller of the Technologies imidazole was also found to be an active John A. Turner, the National Renewable Program at the Office of Energy Efficiency catalyst for the conversion of CO2 to CO and Energy Laboratory (U.S.), addressed the and of DOE (U.S.) formate in an aqueous electrolyte at an iron competition of PEC shared the “big picture” of the roles of pyrite electrode. vs. photovoltaic (PV) electrolysis, and hydrogen and fuel cells in diverse energy Paul Kenis’ group from the University of delivered cost and economic considerations portfolios. He illustrated the promises and Illinois at Urbana-Champaign has developed in selecting PEC materials. challenges of different renewable solar an electrochemical flow reactor for CO2 Krishnan Rajeshwar, University of Texas hydrogen technologies, and addressed reduction, in which the and at Arlington (U.S.), after a short review on the need for national and international are separated by a flowing liquid electrolyte PEC efforts over generations, focused on collaborations for leveraging research to (Fig. 2). Owing to the differences in binding the economic preparation of new families meet these challenges. He presented the energy of intermediates by different particle techno-economic analysis, indicating the size, the reaction rate for CO reduction of metal oxides for water splitting and CO2 2 reduction in his keynote speech. Moreover, critical need for materials systems with increases upon the catalyst particle size characterization and theoretical approaches conversion efficiency exceeding 10% STH decrease from 200 to 5 nm, but then opened new windows for understanding efficiency (in many cases, even exceeding decreases upon the particle size decreasing the challenge and discovering new PEC the 20% mark) to meet the DOE hydrogen further to 1 nm. A thin, uniformly distributed, materials. The combined synthesis- cost threshold.12 This pushes the bar even and agglomerate-free catalyst layer is a key characterization-theory loop will be an higher for the solar-to-hydrogen materials because it eliminates mass transport issues effective way to approach the 10% STH researchers. Significant research progress while avoiding hydrogen evolution on bare efficiency and 10-year life challenge.3 has been made in the past four decades, carbon in the underlying gas diffusion

ig F . 2. (Left) Photo of the electrochemical flow reactor for the electrochemical reduction of CO2.. (Right) Micro-computed 3D tomographic image of the air- brushed cathode for the electrochemical reduction of CO2 to CO that shows the thin uniformly distributed catalyst layer.

70 The Electrochemical Society Interface • Summer 2013 layer. The various optimization steps of the About the Authors References catalyst and the catalyst layer structure have 2 led to current densities as high as 91 mA/cm Heli Wang is a senior scientist at 1. U.S. Department of Energy: New Sci- (with 94% selectivity for CO) while at the National Renewable Energy Laboratory. ence for a Secure and Sustainable En- same time maintaining energy efficiencies He was a postdoctoral researcher at ergy Future, December 2008, http:// that exceed 42%. These advances will help Uppsala University in Sweden before science.energy.gov/~/media/bes/pdf/ electrochemical CO conversion to become 2 joining NREL in 2001. His research reports/files/nsssef_rpt.pdf. an economically feasible process. interests are photoelectrochemical cells 2. J. A. Turner, Science, 285, 687 (1999). In his keynote presentation, Anil Virkar for hydrogen production, charge transfer 3. A. J. Bard and M. A, Fox, Acc. Chem. from the University of Utah highlighted at semiconductor/electrolyte interface, Res., 28, 141 (1995). a reversible solid oxide electrolysis cell corrosion and protection, and PEMFC 4. M. Woodhouse and B. A. Parkinson, (SOEC), which produces fuel in times of components and system contamination. He Chem. Soc. Rev., 38, 197 (2009). available electrical energy and consumes is the author or co-author of one patent and 5. A. J. Nozik, Appl. Phys. Lett. 30, 567 fuel to produce electrical energy in times one book chapter, as well as over 40 journal (1977). of demand, as a storage publications. He may be reached at heli. 6. M. Grätzel, Nature, 414, 338 (2001). tool. In the SOEC configuration, electrical [email protected]. 7. R. Abe, T. Takata, H. Sugihara, and energy is used to electrochemically convert K. Domen, Chem. Commun., 3829 water and air to hydrogen and . Deryn Chu received his PhD in (2005). and/or carbon dioxide from Case Western 8. H. Wang, T. Deutsch, and J. A. Turner, may be incorporated to produce a syngas or Reserve University (CWRU), Cleveland, J. Electrochem. Soc., 155, B99 (2008). hydrocarbon fuel. During this presentation, Ohio in 1989 under the direction of Ernest 9. M. G. Walter, E. L. Warren, J. R. Prof. Virkar addressed the tendency for B. Yeager. Since January 1993, he has been a McKone, S. W. Boettcher, Q. Mi, E. A. more severe degradation in electrolysis research chemist at the U.S. Army Research Santori, and N. S. Lewis, Chem. Rev., mode (fuel generation) by posing the Laboratory (ARL). He is a team leader for 110, 6446 (2010). fundamental question of if a perfectly the fuel cell program where he is engaged 10. O. Khaselev and J. A. Turner: Science, insulating electrolyte is desired, or if rather in electrochemical research related to high 280, 425 (1998). a mixed ionic electronic conductor (MIEC) performance fuel cell and renewable energy 11. S. K. Cushing, J. Li, F. Meng, T. R. might provide some benefit. He highlighted technologies. Dr. Chu has authored or co- Senty, S. Suri, M. Zhi, M. Li, A. D. that the potential for degradation might be authored approximately 90 referred journal Bristow, and N. Q. Wu, J. Am. Chem. mitigated by introducing some level of papers in fuel cell science and technologies Soc., 134, 15033 (2012). electronic conduction in the electrolyte, and he has given a number of invited keynote 12. U.S. Department of Energy Program while achieving a high overall ionic transport presentations at international conferences. Record 11007 (Offices of Fuel Cell number by suitably designing the interfaces. Among his awards are the Best Paper Award Technologies): Hydrogen Threshold Electrochemical reduction of CO nd 2 for the 22 Army Science Conference in Cost Calculation, March 2011, to small organic molecules is a very 2000, the 2003 and 2008 Army Research http://www.hydrogen.energy.gov/ promising approach for generating useful and Development Achievement Award, and pdfs/11007_h2_threshold_costs.pdf. fuel in the future. A series of fundamental the 2004 Richard A. Glenn Award from 13. B. Kumar, M. Llorente, J. Froehlich, research and development programs are the U.S. American Chemical Society, Fuel T. Dang, A. Sathrum, and C. P. ongoing at the United State Department of and Petroleum Division. He also received Kubiak, Ann. Rev. Phys. Chem., 63, Defense. The Defense Advanced Project the Department of the Army “Commader’s 541 (2012). Agency (DARPA), Defense Science Office, Award for Civilian Service” in January 2012 initiated a program “Convert CO , Sunlight, 2 for his outstanding service and contribution and Water to Syngas” in 2008 under the to fuel cell research and development. He “Surface Catalyst for Energy Program.” may be reached at [email protected]. The performers are the University of San Diego and the California Institute Eric L. Miller currently serves as Technology. The University of San Diego the Hydrogen Production Technology also is leading a program to explore and Development Manager with the Hydrogen develop this technology under the Air and Fuel Cell Technologies Program at Force Research Office’s Multi-University the U.S. DOE Office of Energy Efficiency Research Initiative (MURI) program. The and Renewable Energy. His professional U. S. Army also initiated a “CO Reduction 2 career in alternative energy R&D, with an to Fuels” Program in 2013. The U.S. Army emphasis on solar energy and on hydrogen Research Laboratory (ARL) and the U.S. and fuel cell development, has spanned Army Communication Electronic Research more than 20 years, including work with the and Development Center (CERDEC) Oak Ridge National Laboratory, the NASA are collaborating with the University of Lewis (a/k/a Glenn) Research Center, Southern California (USC) and the West Sunpower Inc., and the University of Hawaii Virginia University for this technology. The at Manoa. Dr. Miller is recognized as a world electrochemical and photo-electrochemical leader in the field of photoelectrochemical conversion of CO to small organic molecules 2 (PEC) hydrogen production, specializing as fuels has great potential to become a in semiconductor-based materials, devices, viable and economical technology. and systems for cost-effective PEC solar water-splitting; and currently serves as leader of the PEC task for the International Energy Agency’s Hydrogen Implementing Agreement. His numerous publications in the PEC field include several book chapters. He may be reached at [email protected].

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