Guest Commentary pubs.acs.org/JPCL
Toward Economically Feasible Direct Solar-to-Fuel Energy Conversion
T
PEC device produced at a hypothetical (very low-end) cost of $80 m−2 with a solar-to-hydrogen efficiency of 10% and a lifetime of 10 years, would produce hydrogen only as low as $7 kg−1,4 which does not put it at a large advantage over PV + electrolysis. There is, however, a bright side: if appropriate materials can be developed that can afford high conversion efficiency while also exhibiting intrinsic stability, and the ability to be processed into thin-film photoelectrodes using radically less expensive techniques (e.g., roll-to-roll solution-based processing), or better yet directly dispersed as micro- or submicro-particle photocatalysts (PC) in an aqueous phase, the price of the produced H2 could reasonably drop to the targeted $1−2 kg−1 range. The development of systems that meet these criteria is now becoming the primary focus of the field, and further advances require new physical insights into promising materials with respect to light absorption, charge transport/ transfer, stability, and (photo)catalytic activity. Three Perspectives in this issue shed some light on these issues for promising materials aimed at highly efficient and inexpensive solar hydrogen production. First, in the Perspective by Richard Hennig and co-workers, the possibly of using 2D semiconducting materials is discussed (http://pubs.acs.org/ doi/abs/10.1021/jz502646d). Two-dimensional semiconducting materials, exemplified by the transition metal dichalcogenides, offer intrinsic advantages as photocatalysts, given their high surface-area-to-volume ratio and tunable semiconducting properties. In addition, materials like WS2 and WSe2 have been recently shown as promising stable materials for this application,5,6 and facile routes for their dispersion and solution-processing have recently been advanced,7,8 giving further hope that this class of materials could enable economically viable solar energy conversion. In their Perspective, the authors summarize the latest advances in using 2D semiconducting materials for solar hydrogen production and further describe a computational strategy for directing their improvement. They finally forecast that advances in computational methods will offer accurate descriptions of the interaction between the 2D materials and the electrolyte affording predictive capabilities over the resulting electronic effects including catalysis, stability, and corrosion. Indeed, given the myriad of possible new 2D materials, performing accurate prioritization with computational methods will undoubtedly speed up the progress in the development of 2D semiconductors for solar hydrogen production. In the second Perspective by Narayan Pradhan and coworkers, (http://pubs.acs.org/doi/abs/10.1021/acs.jpclett. 5b00113) the authors address the issue of photogenerated charge carrier production and separation in nanostructures for solar-to-chemical energy conversion. Indeed, when employing nanostructured semiconductors in PEC or PC applications, the increased proportion of surface states raises the probability of
o enable a future energy economy based primarily on renewable sources, the conversion of solar energy into a vector suitable for storage, transportation, and consumption at a scale commensurate with the global demand is required. Storing solar energy chemically in the form of covalent bonds, that is, producing solar fuels via photosynthesis, is especially attractive for this purpose, given that the infrastructure for the use of liquid and gaseous hydrocarbon fuels is already in place. Because the ability to rely solely on natural (biological) photosynthesis is challenging and uncertain due to low product yields, developing systems capable of artificial photosynthesis at high conversion efficiency has become a clear and important objective.1 In particular, H2produced via solar H2O splittinghas emerged as a promising artificial photosynthetic target given the ubiquitous availability of water and the possibility to further convert H2 to liquid hydrocarbon fuels using established chemistry (e.g., reverse water gas shift and Fischer−Tropsch processes). Using conventional photovoltaic (PV) technology (e.g., p−n junction silicon) coupled with standard water electrolyzers is an obvious method for the production of solar hydrogen. This PV + electrolysis approach is seemingly becoming more and more attractive as the price of electricity from PV installations nears parity with the grid. A reasonable estimate for the cost of the hydrogen produced using this approach is around $10 kg−1 H2 for the combination of silicon PV and a PEM electrolyzer.2 However, this is considerably higher than the price of H2 obtained via the steam reforming of fossil methane (ca. $1 kg−1). So although PV-produced electricity is now at a point to realistically compete with electricity produced from fossil fuels, similar economic motivation to widely adopt current PV + electrolysis technology for artificial photosynthesis does not seem to exist yet; it might not until fossil fuels are in short supply. Because of this, the development of a fundamentally less expensive way to afford solar water splitting is of high interest. For decades, promise has been held that a photoelectrochemical (PEC) device, with a direct semiconductor− water junction, could offer a more straightforward, and thus less expensive, route to solar H2 production. In recent years, there has been a surge of interest in developing PEC electrodes. Device demonstrations with solar-to-hydrogen conversion efficiency comparable to PV + electrolysis (ca. 10% under standard conditions) have typically employed semiconductors known for PV energy conversion with added complexity to protect the semiconductor from the harsh conditions of being immersed in the aqueous electrolyte and to promote the catalysis of the water oxidation and reduction reactions. Although the development of these “PV-biased PEC cells”3 is encouraging for the field, it seems unlikely that adding complexity to traditional PV device architectures can be a viable approach to achieve solar water splitting at a cost significantly less than or even equivalent to PV + electrolysis. Indeed, a recent techno-economic analysis suggests that for a © 2015 American Chemical Society
Published: March 19, 2015 975
DOI: 10.1021/acs.jpclett.5b00406 J. Phys. Chem. Lett. 2015, 6, 975−976
Guest Commentary
The Journal of Physical Chemistry Letters Notes
recombination. Using semiconductor−metal heterostructures can aid in charge separation and even offers improved light harvesting thanks to plasmonic effects. Plasmonics have indeed been identified as a promising approach to enhance light harvesting in wide band gap semiconductors that exhibit good stability for PEC water splitting.9 A current challenge in the field, which is addressed in their Perspective, is the preparation of structures with high-quality heterojunctions. By correlating recent advances with the crystallographic orientation of the heterojunction, the authors present new insights into the importance of epitaxial interfaces on good electronic communication between the materials in the heterostructure. Although high solar-to-hydrogen conversion efficiency has not been yet demonstrated with this approach, the ability to control the nanoparticle morphology and heterojunction properties in promising systems for solar energy conversion (e.g., Cu2ZnSnS4/Au) suggest a promising future for photocatalytic application. Finally the Perspective by Peter Vesborg, Brian Seger, and Ib Chorkendorff addresses recent developments in catalysts for the hydrogen evolution reaction (HER) (http://pubs.acs.org/ doi/abs/10.1021/acs.jpclett.5b00306). For both PEC and PC solar water splitting, enhanced activity can be brought by including a (nonphotoactive) material to promote the water reduction and oxidation reactions. The development of acidstable HER catalysts while avoiding the use of expensive precious metals like platinum is an important goal for economical solar hydrogen production by PC, PEC, and PV + electrolysis approaches alike. In their Perspective, the authors make a case for using grid-scale electrolysis straightaway in local markets to smooth out production fluctuations stemming from a substantial use of renewable electricity (e.g., from the large installed capacity of wind turbines in Denmark). Moreover, they compare the latest HER catalysts with respect to their overpotential required for a given current density over ranges applicable to PEC, PEC using solar concentration, and PV + electrolysis. The authors point out that, remarkably, entirely new families of catalysts have emerged in the last 5−7 years due to the intense interest in the field. There are now promising alternatives to the platinum-group metals to enable economically feasible solar-to-hydrogen conversion. The identification of a suitably stable and inexpensive catalyst for the corresponding water oxidation reaction remains, however, a significant challenge. Overall, because it is likely that economic considerations will be the ultimate driving force of the adoption of any solar fuel production technology, it is encouraging to see research focused on approaches that are restricted to using inexpensive, widely available materials together with suitable processing/ application techniques. With so many promising materials already identified (and innumerable more on the cusp of being discovered), it is reasonable that economically viable systems will be soon available for practical application.
The author declares no competing financial interest.
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REFERENCES
(1) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−2320. (2) Kelly, N.; Gibson, T.; Ouwerkerk, D. A Solar-Powered, HighEfficiency Hydrogen Fueling System Using High-Pressure Electrolysis of Water: Design and Initial Results. Int. J. Hydrogen Energy 2008, 33, 2747−2764. (3) Nielander, A. C.; Shaner, M. R.; Papadantonakis, K. M.; Francis, S. A.; Lewis, N. S. A Taxonomy for Solar Fuels Generators. Energy Environ. Sci. 2015, 8, 16−25. (4) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilitiesfor Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983−2002. (5) McKone, J. R.; Pieterick, A. P.; Gray, H. B.; Lewis, N. S. Hydrogen Evolution from Pt/Ru-Coated p-Type WSe2 Photocathodes. J. Am. Chem. Soc. 2012, 135, 223−231. (6) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121−14127. (7) Yu, X.; Prévot, M. S.; Sivula, K. Multiflake Thin Film Electronic Devices of Solution Processed 2D MoS2 Enabled by Sonopolymer Assisted Exfoliation and Surface Modification. Chem. Mater. 2014, 26, 5892−5899. (8) O’Neill, A.; Khan, U.; Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 2012, 24, 2414−2421. (9) Warren, S. C.; Thimsen, E. Plasmonic Solar Water Splitting. Energy Environ. Sci. 2012, 5, 5133−5146.
Kevin Sivula*
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Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
AUTHOR INFORMATION
Corresponding Author
*E-mail: kevin.sivula@epfl.ch. 976
DOI: 10.1021/acs.jpclett.5b00406 J. Phys. Chem. Lett. 2015, 6, 975−976
