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Metal-Free Organic Sensitizers for Use in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

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Metal-Free Organic Sensitizers for Use in Water-Splitting Dye-Sensitized Photoelectrochemical Cells Correction CHEMISTRY Correction for “Metal-free organic sensitizers for use in water- splitting dye-sensitized photoelectrochemical cells,” by John R. Swierk, Dalvin D. Méndez-Hernádez, Nicholas S. McCool, Paul Liddell, Yuichi Terazono, Ian Pahk, John J. Tomlin, Nolan V. Oster, Thomas A. Moore, Ana L. Moore, Devens Gust, and Thomas E. Mallouk, which appeared in issue 6, February 10, 2015, of Proc Natl Acad Sci USA (112:1681–1686; first published January 12, 2015; 10.1073/pnas.1414901112). The authors note that the author name Dalvin D. Méndez- Hernádez should instead appear as Dalvin D. Méndez-Hernández. The corrected author line appears below. The online version has been corrected. John R. Swierk, Dalvin D. Méndez-Hernández, Nicholas S. McCool, Paul Liddell, Yuichi Terazono, Ian Pahk, John J. Tomlin, Nolan V. Oster, Thomas A. Moore, Ana L. Moore, Devens Gust, and Thomas E. Mallouk www.pnas.org/cgi/doi/10.1073/pnas.1501448112 CORRECTION www.pnas.org PNAS | February 24, 2015 | vol. 112 | no. 8 | E921 Downloaded by guest on September 23, 2021 Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells John R. Swierka, Dalvin D. Méndez-Hernádezb, Nicholas S. McCoola, Paul Liddellb, Yuichi Terazonob, Ian Pahkb, John J. Tomlinb, Nolan V. Osterb, Thomas A. Mooreb, Ana L. Mooreb, Devens Gustb, and Thomas E. Mallouka,c,1 Departments of aChemistry and cBiochemistry and Molecular Biology, Pennsylvania State University, University Park, PA 16802; and bDepartment of Chemistry and Biochemistry and Center for Bio-Inspired Solar Fuel Production, Arizona State University, Tempe, AZ 85287 Edited by Richard Eisenberg, University of Rochester, Rochester, NY, and approved December 12, 2014 (received for review August 4, 2014) Solar fuel generation requires the efficient capture and conversion onto a mesoporous TiO2 electrode. The sensitizer absorbs visible of visible light. In both natural and artificial systems, molecular light, injects an electron into the conduction band of TiO2, and is sensitizers can be tuned to capture, convert, and transfer visible then re-reduced by the water oxidation catalyst, which oxidizes light energy. We demonstrate that a series of metal-free porphyr- water to give molecular oxygen and protons. The photoinjected ins can drive photoelectrochemical water splitting under broad- electron migrates through the TiO2 film to a dark cathode where band and red light (λ > 590 nm) illumination in a dye-sensitized protons are reduced to molecular hydrogen (1). At high light TiO2 solar cell. We report the synthesis, spectral, and electrochem- intensity in the blue part of the visible spectrum, incident photon ical properties of the sensitizers. Despite slow recombination of current efficiencies (IPCEs) up to 14% have been demonstrated photoinjected electrons with oxidized porphyrins, photocurrents with WS-DSPECs (10). are low because of low injection yields and slow electron self- Earth-abundant catalysts and sensitizers will be needed for exchange between oxidized porphyrins. The free-base porphyrins large-scale deployment of artificial photosynthesis. Much effort are stable under conditions of water photoelectrolysis and in some has been devoted to the development of earth-abundant water cases photovoltages in excess of 1 V are observed. oxidation catalysts (11–14), including a recently reported com- pletely organic catalyst (15). Less attention has been paid to water-splitting | photoelectrochemical | metal-free porphyrins | visible light | CHEMISTRY the development of earth-abundant sensitizers—an important artificial photosynthesis problem for WS-DSPECs where absorber-to-catalyst ratios can exceed 1,000:1. Ruthenium polypyridyl sensitizers are most The capture of solar energy and storage as reduced chemical commonly used in WS-DSPECs (10, 16–19), although Moore fuels is a significant challenge for a future renewable energy et al. (20) demonstrated a WS-DSPEC sensitized with a zinc economy. Solar fuels may take the form of H2 or reduced car- porphyrin that produced modest photocurrent (∼30 μA/cm2). bon-containing molecules (CH4,C2H6,CH3OH, etc.). Large- Organic sensitizers (containing C, H, N, and O) are earth scale reduction of water or CO2 requires an abundant electron abundant and offer the possibility of being low cost. The viability donor to provide the reducing equivalents. Natural photosyn- of organic sensitizers has been studied in conventional DSSCs thesis uses water as the electron source, generating oxygen as (21, 22) and organic solar cells (23, 24), but is largely unexplored a byproduct. Most artificial photosynthetic systems also seek to in the context of water splitting. Tachan et al. recently proposed use water as the electron donor, even though the kinetically slow type II sensitization with catechol on TiO2 (25). They observed oxygen evolution step is a performance bottleneck (1). current enhancement, although the Faradaic efficiency of oxygen The thermodynamic requirements for water oxidation are relatively modest. A minimum potential of 1.23 V is required, Significance although practical systems need higher voltages because of cat- alytic overpotentials and series losses in photoelectrolysis cells. This minimum thermodynamic requirement can be satisfied by The capture and conversion of sunlight into a useful chemical lightofallwavelengthsshorterthan1μm. Allowing for rea- fuel (H2,CH4,CH3OH, etc.) is a central goal of the field of ar- sonable overpotentials and series losses, the minimum onset for tificial photosynthesis. Water oxidation to generate O2 and light absorption in a one-photon-per-electron system is near 650 protons stands as the major bottleneck in these processes. nm, and the maximum theoretical power conversion efficiency is Relatively few stable photosensitizers can generate sufficient about 20% (2). Although many molecular sensitizers and solid- oxidizing power to drive water oxidation, and those that do contain rare elements such as ruthenium. In this paper, we state semiconductors absorb in this range, finding stable sensi- show that metal-free organic photosensitizers are capable of tizers with redox or band potentials that span the water oxidation driving photoelectrochemical water oxidation. Significantly, and reduction potentials is a significant challenge. these photosensitizers exhibit comparable activity to that of Light harvesting in natural photosynthesis is accomplished by ruthenium-containing photosensitizers under broadband illu- a hierarchical assembly of accessory pigments that funnel their mination. In addition, we report to our knowledge the first excitation energy to chlorophyll molecules (3). The core of chlo- demonstration of a molecular photosensitizer, outside of nat- rophyll-a is a substituted chlorin (4), a porphyrin ring with a re- ural photosynthesis, that can drive water oxidation utilizing duced exo double bond (5). Because porphyrins are synthetically only red light. more accessible than chlorins and bacteriochlorins, many groups have studied them as light-harvesting molecules in dye-sensitized Author contributions: J.R.S., D.D.M.-H., T.A.M., A.L.M., D.G., and T.E.M. designed research; solar cells (DSSCs) (6, 7). Unlike ruthenium polypyridyl dyes, J.R.S., D.D.M.-H., N.S.M., P.L., Y.T., I.P., J.J.T., and N.V.O. performed research; J.R.S., porphyrins contain only abundant elements and strongly absorb D.D.M.-H., N.S.M., P.L., Y.T., I.P., J.J.T., N.V.O., T.A.M., A.L.M., D.G., and T.E.M. analyzed across most of the visible spectrum (8). Grätzel and coworkers data; and J.R.S. and D.D.M.-H. wrote the paper. recently demonstrated a 12.3% efficient DSSC for electricity The authors declare no conflict of interest. generation, using a push–pull Zn porphyrin (9). This article is a PNAS Direct Submission. In contrast to conventional DSSCs, water-splitting dye-sensi- 1To whom correspondence should be addressed. Email: [email protected]. tized photoelectrochemical cells (WS-DSPECs) use molecular This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. sensitizers and water oxidation catalysts that are coadsorbed 1073/pnas.1414901112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1414901112 PNAS Early Edition | 1of6 generation was not measured and it is unclear how much of the (26). The Soret bands for the sensitizers studied in this work (Fig. 1) enhancement was specific to visible light sensitization. typically had maxima at ∼410 nm, with extinction coefficients in − − In this paper, we demonstrate overall water splitting using metal- excess of 105 M 1·cm 1. As the light source used for photo- free organic sensitizers. We show that a series of free-base por- electrochemical measurements is equipped with a 410-nm long-pass phyrins can drive the photoelectrochemical water-splitting reaction, filter, it is the Q bands of the sensitizers that are of particular im- using only visible light illumination at photocurrents comparable portance in this study. SI Appendix,TableS1gives the absorbance to those of Ru polypyridyl sensitizers. TiO2 electrodes sensitized maxima and molar extinction coefficients for the Q bands of with these porphyrins generate photocurrent corresponding to each sensitizer. water oxidation, even when illuminated with red light (λ > 590 nm). An analysis of the absorbance maxima in SI Appendix, Table Although hole transport and injection yields are poor, back S1 shows that the position of the Q bands is influenced by the electron transfer recombination is much slower than it is with
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