Single-Site, Heterogeneous Electrocatalytic Reduction of CO2 in Water as the Solvent Ying Wang,‡ Seth L. Marquard,‡ Degao Wang,‡ Christopher Dares,‡,† and Thomas J. Meyer*,‡ ‡
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States Department of Chemistry and Biochemistry, Florida International University, 11200 Eighth Street, Miami, Florida 31199, United States
†
S Supporting Information *
ABSTRACT: Creating stable surface-bound molecular catalysts for CO2 reduction in aqueous solutions for either electrochemical or photoelectrochemical reduction is a continuing challenge. We report here the preparation and characterization of thin oxide/carbon nanotube electrodes on fluorine-doped tin oxide (FTO) electrodes. The electrodes were prepared by atomic layer deposition (ALD) of ∼15 nm of TiO2 followed by a layer of carbon nanotubes and a thin (2 nm) overlayer of TiO2 for surface stabilization and binding. The phosphonatederivatized version of the known aqueous solution catalyst for H2/CO reduction, [RuII(tpy-Ph−CH2−PO3H2)(Mebim-py)(H2O)](PF6)2 (tpy-Ph−CH2−PO3H2 = (4([2, 2′:6′,2″-terpyridin]-4′-yl)benzyl)phosphonic acid; Mebim-py = 3-methyl-1pyridyl-benzimidazol-2-ylidene), was added to the surface and stabilized by addition of a thin overlayer of TiO2 by ALD. In the derivatized electrodes, the catalyst maintains its reactivity toward CO2 reduction in the short-term, giving mixtures of H2/CO that vary from 1.5 at −0.96 V to 5.6 at −1.16 V vs NHE in 0.5 M NaHCO3 with a turnover number of 308 after 15 min of electrolysis. oxide.3,13,17,18 On the basis of data from Jaramillo et al.,19 reduction of indium tin oxide (ITO) on fluorine-doped tin oxide (FTO) electrodes occurs after −0.62 V vs RHE and at −0.72 V vs RHE in 0.1 M NaAc solutions, respectively. In earlier experiments, we demonstrated aqueous syngas generation by a version of the Ru(II) polypyridyl carbene complex shown in Scheme 1.1,9,10 In order to extend the solution chemistry to oxide electrode surfaces, we extended the coordination chemistry to the phosphonate derivative shown in Scheme 1.20,21 In the modified complex, and in related Ru(II) polypyridyl complexes, there is a systematic basis for bonding to oxide surfaces based on phosphonate−ester surface bonding.22 Here, we introduce a new attachment strategy. It is based on protection of oxide surfaces by thin overlayer films of TiO2 but with the addition of a reactive outer layer of carbon nanotubes. To enable surface phosphonate bonding to the nanomodified electrodes, they were further derivatized by atomic layer deposition (ALD) to give a thin, external layer of TiO2 for surface attachment of Ru(II) polypyridyl carbene catalyst 1. In the final electrode structure, the catalyst was attached to the outer surface of the electrode and stabilized by ALD deposition with a thin layer of TiO2. In the final electrode, the addition of
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lobal warming and control of CO2 are playing increasingly important roles in energy research.1−3 Significant progress has been made on CO2 reduction to formate,4 carbon monoxide,1,5,6 and hydrocarbons,7,8 including synthesis gas (syngas, H2 + CO) mixtures for Fischer−Tropsch conversion to liquid fuels. Progress has also been made in this area in molecular homogeneous catalysis in nonaqueous solvents with catalysts reported that react with CO2 with high selectivity and efficiency.1,2,5,9−16 Examples include Ni(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane)11 from Kubiak and co-workers and derivatives of Re(bpy)(CO)3Cl. In photoelectrochemical schemes for CO2 reduction based on molecular catalysis, there are additional challenges to address. One is transferring the reactivity properties of the molecular catalyst to the surfaces of semiconducting electrodes. The second is controlling the reactivity of the catalyst under conditions suitable for both water oxidation and CO2 reduction in aqueous environments. A common surface binding strategy in water uses surface-bound phosphonate derivatives but, for CO2 reduction, with significant limitations from the thermodynamics of relevant CO2 couples and the reactivity of the solvent. One-electron reduction of CO2 by the CO2/CO2•− couple occurs at E0 = −1.9 V vs NHE for CO2 to CO2•−, and that for the CO2/CO−H2O couple at pH 7 occurs at E0 = −0.53 V.3 Given the potentials for the half reactions, use of oxide semiconductor electrodes is inhibited by reduction of the © XXXX American Chemical Society
Received: March 13, 2017 Accepted: May 15, 2017 Published: May 15, 2017 1395
DOI: 10.1021/acsenergylett.7b00226 ACS Energy Lett. 2017, 2, 1395−1399
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ACS Energy Letters
significantly decreases electrode resistance, greatly enhancing interfacial electron transfer. Following addition of the nanotubes, an additional layer of TiO2 was added by ALD to enable catalyst loading by phosphonate−surface bond formation to the oxide. The thickness of this layer also controls the extent of catalyst loading with a plateau reached at ∼2 nm with a loading of 1.13 ± 0.1 nmol/cm2 (Figure S3). After electrode fabrication, the catalyst was added by surface loading from 2 mM solutions in methanol overnight. It was further stabilized on the surface by addition of an ALD overlayer of TiO2 to a thickness of ∼0.5 nm. As shown earlier, addition of the final overlayer has the effect of “burying” the phosphonate link with the complex surface-stabilized toward hydrolytic loss with stable electrode performance even at elevated pH values.21,25 Cyclic voltammograms of the resulting surfaces, FTO|TiO2/CNT/TiO2|−1−TiO2, Figure S4, are typical for the surface-bound electrode with E1/2 = 0.94 V vs NHE for the Ru(III/II) wave in CO2-saturated 0.5 M NaHCO3 solutions. Figure 1a−d shows SEM images of the electrode surface (a,b) and cross section (c,d) before (a,c) and after (b,d) bulk electrolysis in CO2-saturated 0.5 M NaHCO3 at −1.06 V for 6700 s. As the figure shows, there is no morphological change before and after bulk electrolysis experiments. The crosssectional images in Figure 1c,d provide an image of a condensed layer of ∼15 nm TiO2 particles that tightly cover the FTO grains. The TiO2 layer prevents the FTO electrode from contacting the solution, avoiding reduction. The function of this layer is observable visually in Figure S1c. It shows that the electrode surface with this layer remains in its initial state while the unprotected layer becomes black. There is also no evidence for Sn-based surface redox couples during the electrochemical cycling process with the added TiO2 overlayer (Figure S1d). Figure 1e shows an XPS spectrum of the designed electrode with catalyst before (red) and after (black) bulk electrolysis at −1.06 V vs NHE for 6700s in 0.5 M NaHCO3. Due to the thickness of the layer on FTO electrodes, the SnO2 signal is not observable by XPS. Characteristic TiO2 2p absorptions at 458.5 and 464.2 eV are observed both before and after bulk electrolysis, suggesting that this is a stable electrode design for CO2 reduction.26,27 Electrolysis. Figure 2 shows cyclic voltammograms of the asprepared electrode with (solid line) and without (dashed line) the Ru(II) polypyridyl carbene catalyst in CO2 (black) and N2 (red) saturated 0.5 M NaHCO3 solutions at 20 mV s−1 at room
Scheme 1. Structure of the Ru(II) Polypyridyl Carbene Complex (1), [RuII(tpy-Ph−CH2−PO3H2)(Mebimpy)(H2O)](PF6)2a
a tpy-Ph−CH2−PO3H2 = (4-([2,2′:6′,2″-terpyridin]-4′-yl)benzyl) phosphonic acid; Mebim-py = 3-methyl-1-pyridyl-benzimidazol-2ylidene.
the carbon nanotube layer played an important role in decreasing the electron transfer resistance. Electrodes. In preparation of the FTO electrodes, electroreduction of the tin oxide surface results in a dark metallic film (Figure S1a) on the surface after bulk electrolysis for 1 h at −1.36 V vs NHE. An X-ray photoelectron spectrum (XPS) (Figure S1b) confirmed the formation of Sn/SnO after bulk electrolysis. As shown in Scheme 2, a first layer of TiO2, ∼15 nm, was added to the FTO electrode surface by ALD. The use of ALD is convenient because it provides a condensed and uniform metal oxide layer on the electrode surface with precise control of layer thickness.23 The initial layer was sufficiently thick that it “isolated” the FTO electrode surface from contacting with external electrolyte solutions, which avoids reduction of the electrode. At the electrode interface, electrons transfer through subsequent layers through the “leaky” TiO2 surface of ALD deposited on the electrode23 with access to the conduction band of the electrode.14 Following the initial layer of TiO2, 1 mg/mL of suspended carbon nanotubes in ethanol was added to the TiO2−FTO electrodes by spin-coating. Prior to depositing, the nanotubes were treated with concentrated nitric acid to increase hydrophilicity.24 Use of spin-coating allows for stable attachment of the carbon nanotubes to the electrode surface without addition of additives such as nafion and its influence on electrochemical behavior. As shown by the results of impedance measurements in Figure S2, addition of the overlayer
Scheme 2. Fabrication of the Surface-Stabilized Ru(II) Polypyridyl Carbene Complex on TiO2−Carbon Nanotube−TiO2 Electrodes
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terpyridine (tpy), with both equivalents involved in the reduction. In subsequent steps, a competition exists between capture of the reduced intermediate by CO2 to give a metallocarboxylate intermediate or with protons, by an added acid, to give an intermediate hydride. Without CO2, twicereduced [RuII(tpy−)(Mebim-py−)(H2O)]0 is protonated and further reduced to give H2.1,9,10 With added CO2, as shown in Figure 2, there is a decrease in current compared to N2 degassed solutions because of the buildup of the CO2 intermediate. The CO2 intermediate is reduced further with loss of water to give a coordinated CO intermediate, which, following loss of CO, returns to the initial catalyst. The background in Figure 2 arises from reaction of the electrode (dashed line) due to filling and emptying of the conduction band of the surface layer of TiO2.14 The electrode is conductive after reaching its conduction band potential. At this potential, it facilitates electrochemical reduction of water; see below. On the basis of the results in Figure 2, in electrolysis experiments, potentials more negative than −1.3 V vs NHE were avoided to minimize background reaction at the electrode. As for the solution catalyst, H2/CO ratios vary with potential, with H2 favored at more negative potentials. The H2/CO ratio was 1.9 at −1.06 V vs NHE. It increased to 5.6 at −1.16 V vs NHE for the solution catalyst in 0.5 M NaHCO3 (Table 1). The value observed at −1.06 V is higher than the value 0.5:1 @ −1.2 V vs NHE2 because of a contribution from competitive reduction of water at the electrode. Table 1. Ratio of H2/CO in 15 min of Bulk Electrolysis Figure 1. Scanning electron microscopy (SEM) images of the surface (a,b) and the cross section (c,d) for the as-designed electrodes before (a,c) and after (b,d) bulk electrolysis; (e) X-ray photoelectron spectroscopy (XPS) spectrum of the as-prepared electrodes before (red) and after (black) bulk electrolysis. Conditions: 0.5 M NaHCO3, −1.06 V vs NHE for 6700 s.
E vs NHE/V
cat/electrode
bare electrode
−0.96 −1.06 −1.16
1.51 1.94 5.64
38.11
In order to explore water/CO2 reduction in further detail, we carried out extended electrolyses at −1.06 V vs NHE. With the catalyzed electrode, after the first 900 s of electrolysis, the persite turnover number (TON) for CO generation at the 0.5 cm2 electrode was 308, and it was 597 for H2, at an overall a rate of 0.34 s−1 for CO/second. The TON for CO is comparable with that reported by Robert et. al of 443 on a iron triphenyl porphyrin supported carbon electrode.5 H2 continues to form past the initial electrolysis period with a 70% faradaic efficiency for H2/CO after 6700 s but with decreasing amounts of CO in the final product mixture (shown in Figure S5). We are exploring the loss in reactivity of the catalyst and its conversion into the H2 evolving catalyst on the surface. Following electrolysis cycles, in CV scans, an electrochemical wave for the original Ru−H form of the catalyst shifts from ∼−0.05 to 0.1 V (Figure S6).1,28 Decomposition is also clear from high-resolution XPS spectra. As shown in Figure S7, there is clear evidence in XPS spectra for the disappearance of a Ru feature at low binding energy (280.8 eV) at the end of the electrolysis cycles. There is also evidence for partial loss of the catalyst from the surface by peak current comparisons. We describe here an important addition to the aqueous electrochemical reduction of CO2 and possibly other substrates at oxide electrodes. A method has been developed for surface binding and stabilization of an important solution catalyst, 1, for CO2 reduction on FTO electrodes. Following surface binding and stabilization, the catalyst was shown to maintain its reactivity toward reduction of CO2, giving potential-dependent
Figure 2. Cyclic voltammograms of FTO|TiO2/CNT/TiO2 electrodes with catalyst (solid line) and without catalyst (dashed line) in CO2-saturated (black line) and N2-saturated (red line) aqueous solutions in 0.5 M NaHCO3 at a scan rate of 20 mV s−1 at room temperature and pH = 7.2. The current density was calculated based on the electrode surface area (0.5 cm2).
temperature. The mechanism of CO2 reduction by the catalyst under the same conditions in solution has been reported (shown in Scheme S1).1,9,10 On the reductive side of the CV, the first two electrons are transferred to ligands, 3-methyl-1pyridyl-benzimidazol-2-ylodene (MeBim-py) or 2,2′:6′,2″1397
DOI: 10.1021/acsenergylett.7b00226 ACS Energy Lett. 2017, 2, 1395−1399
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(6) Zhu, W.; Zhang, Y.-J.; Zhang, H.; Lv, H.; Li, Q.; Michalsky, R.; Peterson, A. A.; Sun, S. Active and Selective Conversion of CO2 to CO on Ultrathin Au Nanowires. J. Am. Chem. Soc. 2014, 136, 16132− 16135. (7) Zhang, S.; Kang, P.; Bakir, M.; Lapides, A. M.; Dares, C. J.; Meyer, T. J. Polymer-supported CuPd nanoalloy as a synergistic catalyst for electrocatalytic reduction of carbon dioxide to methane. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 15809−15814. (8) Hori, Y.; Murata, A.; Takahashi, R. Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2309− 2326. (9) Chen, Z.; Chen, C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes. Chem. Commun. 2011, 47, 12607−12609. (10) Chen, Z.; Concepcion, J. J.; Brennaman, M. K.; Kang, P.; Norris, M. R.; Hoertz, P. G.; Meyer, T. J. Splitting CO2 into CO and O2 by a single catalyst. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15606−15611. (11) Froehlich, J. D.; Kubiak, C. P. Homogeneous CO2 Reduction by Ni(cyclam) at a Glassy Carbon Electrode. Inorg. Chem. 2012, 51, 3932−3934. (12) Kumar, B.; Smieja, J. M.; Sasayama, A. F.; Kubiak, C. P. Tunable, light-assisted co-generation of CO and H2 from CO2 and H2O by Re(bipy-tbu) (CO)3Cl and p-Si in non-aqueous medium. Chem. Commun. 2012, 48, 272−274. (13) Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073−4082. (14) Rosser, T. E.; Windle, C. D.; Reisner, E. Electrocatalytic and Solar-Driven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2. Angew. Chem., Int. Ed. 2016, 55, 7388−7392. (15) Chapovetsky, A.; Do, T. H.; Haiges, R.; Takase, M. K.; Marinescu, S. C. Proton-Assisted Reduction of CO2 by Cobalt Aminopyridine Macrocycles. J. Am. Chem. Soc. 2016, 138, 5765−5768. (16) Sheng, H.; Frei, H. Direct Observation by Rapid-Scan FT-IR Spectroscopy of Two-Electron-Reduced Intermediate of Tetraaza Catalyst [CoIIN4H(MeCN)]2+ Converting CO2 to CO. J. Am. Chem. Soc. 2016, 138, 9959−9967. (17) Hansen, H. A.; Shi, C.; Lausche, A. C.; Peterson, A. A.; Norskov, J. K. Bifunctional alloys for the electroreduction of CO2 and CO. Phys. Chem. Chem. Phys. 2016, 18, 9194−9201. (18) Costentin, C.; Robert, M.; Saveant, J.-M. Catalysis of the electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42, 2423−2436. (19) Benck, J. D.; Pinaud, B. A.; Gorlin, Y.; Jaramillo, T. F. Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte. PLoS One 2014, 9, e107942. (20) Vannucci, A. K.; Alibabaei, L.; Losego, M. D.; Concepcion, J. J.; Kalanyan, B.; Parsons, G. N.; Meyer, T. J. Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 20918−20922. (21) Alibabaei, L.; Sherman, B. D.; Norris, M. R.; Brennaman, M. K.; Meyer, T. J. Visible photoelectrochemical water splitting into H2 and O2 in a dye-sensitized photoelectrosynthesis cell. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5899−5902. (22) Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa, E.; Bignozzi, C. A.; Qu, P.; Meyer, G. J. Phosphonate-Based Bipyridine Dyes for Stable Photovoltaic Devices. Inorg. Chem. 2001, 40, 6073−6079. (23) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science 2014, 344, 1005−1009.
mixtures of CO and H2. A ratio of 2:1 for H2/CO syngas production was found at −1.06 V vs NHE with TONs of 308 for CO and 597 for H2 within a 15 min electrolysis period but with long-term catalysis inhibited by decomposition and partial loss of the catalyst from the surface.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00226. Experimental details, synthetic procedures, cyclic voltammograms, a scheme of reaction mechanism for CO2RR, and XPS data and loading determinations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thomas J. Meyer: 0000-0002-7006-2608 Author Contributions
Y.W. and T.J.M. designed the project. Y.W. performed the research. S.L.M. synthesized the complex. D.W. performed the ALD. Y.W., S.L.M., C.D., and T.J.M. analyzed the data. Y.W., S.L.M., and T.J.M. wrote the paper. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported solely by the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Science under Award Number DE-SC0001011. We thank Dr. Carrie Donley and Dr. Amar Kumbhar for assistance with XPS and SEM measurements. This work made use of instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).
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REFERENCES
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