Editorial pubs.acs.org/JPCL
Solar Cells versus Solar Fuels: Two Different Outcomes
T
Scheme 1. Outline of the Somewhat Divergent Paths Taken over the Past Four Decades by Photovoltiac and Photocatalyic Systems
he recent thrust toward utilizing nanomaterials for light energy conversion has brought noticeable success in the field of thin film photovoltaics (PV). In fact, thin film solar cells have already demonstrated themselves to be a viable commercial technology. Furthermore, the recent success of achieving more than 20% power conversion efficiency with organic−inorganic lead halide perovskites (http://www.nrel. gov/ncpv/images/efficiency_chart.jpg) has led the speculation that these devices may provide the transformative leap needed to drive widespread adaptation of thin film PV. Although the photovoltaic efficiency of thin film solar cells is steadily increasing, the price of these panels continues to drop at the same time, bringing PV technology closer to grid power.1 Continued reduction in the panel, installation, and balance of systems costs of PV could soon transform an imagined ideal into reality; solar energy may be able to directly compete with, and even outcompete, energy from fossil fuels. Semiconductor-assisted hydrogen production and reduction of CO2 are being projected as attractive approaches for storing solar energy in the form of fuels. Despite the successes achieved in recent years in thin film photovoltaics, the story is markedly different for solar fuel generation by semiconductor assisted photocatalysis (Scheme 1). By integrating two perovskite solar cells with a relatively modest solar-to-electric power conversion efficiency of 15.7% in series with an electrolyzer, Luo and coworkers were able to split water with an overall efficiency of over 12%.2 However, designing a cost-effective stand-alone photocatalytic reactor with efficiency in a similar range remains an ambitious goal. The first photocatalytic water splitting was introduced by Honda and Fujishima in 1972.3 Yet, despite over four decades of photocatalytic research, a practically compatible technology does not seem to be a near term reality. Progress is lagging in terms of material deployment as well as truly scalable devices. It is now generally accepted that a single light absorber system will likely not overcome all the barriers in photocatalytic water splitting. Given the 1.23 V thermodynamic barrier for water splitting and the overpotential required for the water oxidation at the photoanode and/or reduction of H+ at the photocathode, one would require a semiconductor with bandgap energy of 1.5 to 2.0 eV to drive the photocatalytic process efficiently with visible light.4 To date, no single semiconductor system has been identified that can continuously produce hydrogen without the aid of an external bias or use of sacrificial electron donor. New strategies to utilize multijunction tandem devices and photocatalysts assembled with a Z-scheme show promise in bringing about transformative change in the field of photocatalysis;5 however, it is important that the scientific community realize the limitations inherent in implementing proton coupled electron transfer processes in photocatalytic systems. Nevertheless, given the current thrust in funded research for solar fuel production, one can feel optimistic that continued advances could make it as effective as PV technology in the near future. The two Perspectives in this issue focus on the importance of metal oxide−semiconductor interfaces in quantum dot © 2015 American Chemical Society
sensitized solar cells and photocatalysis as a means to synthesize useful chemicals. Tian and Cao discuss how the large surface area of TiO2 and ZnO nanostructures, which facilitates QD loading, also provides an easy pathway for charge recombination and abundant surface defects, limiting overall power conversion efficiency. New approaches for surface modification of the metal oxide nanostructure and recent advances in improving the photoconversion efficiency are discussed. Augugliaro et al. present photocatalytic approaches for the conversion of noxious chemicals into high value chemical products. Although a majority of the published work in this area relates to the photocatalytic remediation of undesired compounds, the controlled photocatalytic conversion into desirable chemical products is also of importance. Dimethyl carbonate is also being considered as an alternative green organic solvent to enhance the solubility of organic compounds, thus overcoming the limitation posed by the low water solubility. The recovery of products is still a challenge for advancing this technology toward industrial applications.
Prashant V. Kamat*
University of Notre Dame, Notre Dame, Indiana 46556, United States
Jeffrey A. Christians
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States Published: May 21, 2015 1917
DOI: 10.1021/acs.jpclett.5b00897 J. Phys. Chem. Lett. 2015, 6, 1917−1918
The Journal of Physical Chemistry Letters
■
Editorial
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Views expressed in this Editorial are those of the authors and not necessarily the views of the ACS.
■
REFERENCES
(1) Jean, J.; Brown, P. R.; Jaffe, R. L.; Buonassisi, T.; Bulovic, V. Pathways for Solar Photovoltaics. Energy Environ. Sci. 2015, 8, 1200− 1219. (2) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Gratzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, 345, 1593−1596. (3) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (4) Sivula, K. Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis. J. Phys. Chem. Lett. 2013, 4, 1624−1633. (5) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473.
1918
DOI: 10.1021/acs.jpclett.5b00897 J. Phys. Chem. Lett. 2015, 6, 1917−1918
