Selective CO Production in Photoelectrochemical Reduction of CO2

Jan 30, 2017 - Photoelectrochemical reduction of CO2 occurred using cobalt(II) chlorin (CoII(Ch)) as a cathode active material adsorbed on multiwalled...
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Selective CO Production in Photoelectrochemical Reduction of CO2 with a Cobalt Chlorin Complex Adsorbed on Multiwalled Carbon Nanotubes in Water Shoko Aoi,† Kentaro Mase,† Kei Ohkubo,*,‡ Tomoyoshi Suenobu,† and Shunichi Fukuzumi*,§,¶ †

Department of Material and Life Science, Graduate School of Engineering, Osaka University, SENTAN, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Division of Innovative Research for Drug Development, Institute for Academic Initiatives, Osaka University, Suita, Osaka 565-0871, Japan § Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea ¶ Faculty of Science and Technology, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-8502, Japan S Supporting Information *

ABSTRACT: Photoelectrochemical reduction of CO2 occurred using cobalt(II) chlorin (CoII(Ch)) as a cathode active material adsorbed on multiwalled carbon nanotubes as a current collector in combination with a surface-modified BiVO4 photoanode with iron(III) oxide(hydroxide), FeO(OH), to produce CO with 83% Faradaic efficiency at an applied bias voltage of −1.3 V at the CoII(Ch)-modified cathode vs the FeO(OH)/BiVO4/FTO photoanode under visible light irradiation in a CO2-saturated aqueous solution (pH 4.6). The difference in the oxidation potential of the FeO(OH)/BiVO4/FTO electrode under dark and that under light illumination was ∼1.5 V, which was smaller than the band gap of BiVO4 (band gap energy ≈ 2.4 eV), indicating that the FeO(OH)/ BiVO4/FTO photoanode lowered the total bias that enabled simultaneous water oxidation and CO2 reduction.

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We report herein photoelectrochemical reduction of CO2 to CO with high Faradaic efficiency using a cobalt chlorin complex (CoII(Ch)) adsorbed on multiwalled carbon nanotubes (MWCNTs) as a cathode and a surface-modified BiVO4 photoanode with iron(III) oxide(hydroxide) (FeO(OH)) for oxidation of water in a CO2-saturated aqueous solution (pH 4.6). The photoelectrochemical reduction of CO2 was performed in a two-compartment cell composed of an FeO(OH)/BiVO4/ FTO photoanode and a CoII(Ch)-modified cathode. These two electrodes were connected with conducting wire as an external circuit and separated by a Nafion membrane, as shown in Figure 1. BiVO4 was prepared according to the literature and was deposited on a fluorine-doped tin oxide (FTO) glass

hotoelectrochemical reduction of CO2 and H2O to CO and H2 has merited increasing attention because synthesis gas,1−8 which is a fuel gas mixture consisting primarily of CO and H2, can be converted to liquid hydrocarbon fuels by Fischer−Tropsch processes.9−13 CO2 reduction to CO normally competes with proton reduction to H2 as well as CO2 reduction to HCOOH due to their similar reduction potentials.14−17 Nonaqueous media have been usually used for selective production of CO in the photoelectrochemical reduction of CO2.18−21 Thus, selective CO production in the photoelectrochemical reduction of CO2 in water has been a challenging issue. A high Faradaic efficiency of ∼90% for CO production has recently been achieved using an Au coupled ZnTe/ZnO nanowire heterojunction photocathode.22 The CO selectivity relative to H2 was improved by 47.5 fold with Au deposition.22 However, selective CO production in the photoelectrochemical reduction of CO2 in water using non-noble metal cathode materials has yet to be achieved.23,24 © XXXX American Chemical Society

Received: November 24, 2016 Accepted: January 29, 2017 Published: January 30, 2017 532

DOI: 10.1021/acsenergylett.6b00630 ACS Energy Lett. 2017, 2, 532−536

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Table 1. pH Dependence of the Amount of Produced Gas, Faradaic Efficiency for Produced Gas, and Total Electric Charge (Q) in Photoelectrochemical Reduction of CO2 with the CoII(Ch)-Modified Cathode with an Applied Bias Voltage of −1.3 V at the Cathode vs FeO(OH)/BiVO4/FTO Photoanode in a CO2-Saturated Aqueous Solution

Figure 1. (a) Photoelectrochemical cell composed of an FeO(OH)/ BiVO4/FTO photoanode for the photocatalytic oxidation of water to O2 and a CoII(Ch)-modified cathode for the catalytic reduction of CO2 to CO. Cathode and anode compartments are separated by a Nafion membrane. (b) Chemical structure of CoII(Ch).

pH

amount of CO, [μmol] ( f CO %)b

2.0 2.8 3.6 4.6 6.7

0 (0) 0.0448 (14) 0.175 (27) 0.517 (83) 1.24 (59)

amount of H2, [μmol] ( f H2 %) 0.789 0.285 0.476 0.106 0.847

(100) (86) (73) (17) (41)

amount of O2, [μmol] ( f O2 %)

total Q, [μmol]a

0.396 (100) 0.165 (100) 0.327 (100) 0.311 (100) 1.05 (100)

1.60 0.660 1.31 1.25 4.17

a

Q represents the total amount of electrons [μmol] consumed at the CoII(Ch)-modified cathode during photoassisted controlled potential electrolysis for 2 h. f is the Faraday constant. bf CO = (Amount of CO produced [μmol] × 2)/(Total Q [μmol]) × 100.

Figure 2. I−V curves of BiVO4/FTO (black) and FeO(OH)/ BiVO4/FTO (red) electrodes in an Ar-saturated aqueous solution containing 5.0 mM Na2SO4 under simulated 1 sun (AM 1.5G) illumination. I−V curve of an FeO(OH)/BiVO4/FTO electrode in the same solution under dark (black dotted line). Sweep rate: 100 mV s−1.

Figure 4. (a) Time courses of formation of CO (red) and H2 (black) in photoassisted controlled potential electrolysis with the CoII(Ch)-modified cathode at an applied bias voltage of −1.3 V at the cathode vs the FeO(OH)/BiVO4/FTO photoanode in a CO2saturated aqueous solution containing 5.0 mM Na2SO4 (pH 4.6) under simulated 1 sun (AM 1.5G) illumination at 298 K. (b) Time courses of formation of O2 (blue) and CO + H2 (red) in photoassisted controlled potential electrolysis with the CoII(Ch)modified cathode at an applied bias voltage of −1.3 V at the cathode vs the photoanode in a CO2-saturated aqueous solution containing 5.0 mM Na2SO4 (pH 4.6) under simulated 1 sun (AM 1.5G) illumination at 298 K. CO2 was bubbled every 2 h.

Figure 3. Cyclic voltammograms of a CO2-saturated aqueous solution at pH 4.6 (red line) and at pH 6.7 (red dotted line) containing 5.0 mM Na2SO4 recorded at the CoII(Ch)-modified cathode at an applied bias voltage at the cathode vs the FeO(OH)/ BiVO4/FTO photoanode under simulated 1 sun (AM 1.5G) illumination. The black line shows the cyclic voltammogram recorded at the CoII(Ch)-modified cathode at an applied bias voltage at the cathode vs the FeO(OH)/BiVO4/FTO photoanode in an Ar-saturated aqueous solution (pH 6.7) under simulated 1 sun (AM 1.5G) illumination. The black dotted line shows a cyclic voltammogram recorded at the CoII(Ch)-modified cathode at an applied bias voltage at the cathode vs the FeO(OH)/BiVO4/FTO photoanode in a CO2-saturated aqueous solution (pH 4.6) under dark. Sweep rate: 10 mV s−1.

substrate as a photoanode (BiVO4/FTO).25−28 The surface of the BiVO4 layer was modified with an FeO(OH) layer as a water oxidation catalyst to form FeO(OH)/BiVO4/FTO by photodeposition and a subsequent electrodeposition method.29 Preparation details of the FeO(OH)/BiVO4/FTO electrode are 533

DOI: 10.1021/acsenergylett.6b00630 ACS Energy Lett. 2017, 2, 532−536

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various experimental conditions such as pH’s of a CO2saturated aqueous solution and applied bias voltage. Table 1 summarizes the gas amount produced in electrolysis of a CO2saturated aqueous solution in the presence of Na2SO4 at various pH’s and an applied bias voltage (see also Figure S8 and Table S1). The maximum current efficiency for CO production for the initial 2 h was 83% at pH 4.6. When the pH value was lower than 4.6, proton reduction to evolve H2 preferentially occurred rather than CO2 reduction.30 The small Faradaic efficiency for CO production at pH 6.7 (59%) is attributable to slow protoncoupled electron-transfer reduction of CO2 to CO under high pH conditions.32−34 The maximum amount of electrons consumed at the CoII(Ch)-modified cathode during photoassisted controlled potential electrolysis for 2 h was 4.17 μmol at pH 6.735 because neutral conditions in an anode electrolyte solution are favorable for water oxidation on the FeO(OH)/ BiVO4/FTO photoanode.29,36 The amount of O2 produced in the photoelectrochemical oxidation of water is one-half of the amount of the sum of CO and H2 produced in the electrochemical reduction of CO2 and H2O on the cathode (Figure 4b). Faradaic efficiency for CO production was much improved by adsorption of CoII(Ch) on MWCNTs because of a suitable hydrophobic environment for binding of CO2 instead of a proton, where the binding of CO2 to the Co(I) complex is required for the formation of CO.30,37,38 In the electric circuit (two-electrode system: 2E system) in Figure S1a, the potential difference between the FeO(OH)/ BiVO4/FTO photoanode and CoII(Ch)-modified cathode is constantly kept during the controlled potential electrolysis. However, the absolute potentials of the FeO(OH)/BiVO4/ FTO photoanode and CoII(Ch)-modified cathode are not known.39 Thus, the photoassisted controlled potential electrolysis was conducted in a three-electrode system (3E system) with a potentiostat, as shown in Figure S1b. The potential of the CoII(Ch)-modified cathode as a working electrode was variably changed at appropriate fixed potentials. On the other hand, the potential of the FeO(OH)/BiVO4/FTO photoanode as a counter electrode was automatically changed according to the current flowed to the CoII(Ch)-modified cathode; thus, the corresponding potential was precisely and externally measured with a Precision Source/Measure Unit. Experimental details are described in the Supporting Information. When the potential for controlled potential electrolysis (E red) under light illumination was applied at −1.1 V vs NHE, the Faradaic efficiency for CO formation was 88%, with H2 production accounting for the remaining 12% at pH 4.6 under simulated 1 sun illumination (Table S2). The potential of the FeO(OH)/ BiVO4/FTO photoanode (Eox) was 0.32 V vs NHE, and the Faradaic efficiency for O2 produced by oxidation of water was 100%. In addition, also in the case of Ered values higher than −1.1 V vs NHE under light illumination, oxidation of water occurred at low Eox values (1.6 V vs NHE) is necessary for oxidation of water under dark. The potential difference between Eox under dark and that under light illumination was smaller than

Figure 5. Plots of current values versus Eox − Ered for controlled potential electrolysis with the CoII(Ch)-modified cathode at various applied potentials (Ered) vs NHE in a CO2-saturated aqueous solution containing 5.0 mM Na2SO4 (pH 4.6) with the FeO(OH)/ BiVO4/FTO photoanode under dark (black) and simulated 1 sun (AM 1.5G) illumination (red) at 298 K.

explained in the Supporting Information (SI). The purity and crystal structure of the nanoporous BiVO4/FTO and FeO(OH)/BiVO4/FTO were determined with powder X-ray diffraction (Figure S2). The cathode was modified with CoII(Ch) adsorbed on MWCNTs that exhibit both good electrical conductivity and good dispersibility of CoII(Ch) on their surface. Detailed preparation procedures are described in the experimental section in the SI.30 The effect of surface deposition of FeO(OH) over BiVO4/ FTO on the photoelectrochemical properties is shown in Figure 2, where the photocurrent obtained from the FeO(OH)/BiVO4/FTO anode was more positive than that from the bare BiVO4/FTO anode at applied potentials more positive than 0.1 V vs NHE. The photoelectrochemical performance of the FeO(OH)/ BiVO4/FTO photoanode and the CoII(Ch)-modified cathode was investigated in a CO2-saturated aqueous solution, as shown in Figure 3, where photocurrent generation was observed at the bias voltage at the cathode vs the photoanode more negative than −0.8 V and the catalytic current significantly increased in the presence of CO2 (Figures 3 and S3). Photoassisted controlled potential electrolysis of a CO2saturated aqueous solution with 5.0 mM Na2SO4 as an electrolyte was performed using a photoelectrochemical cell, shown in Figure 1. Formation of CO and H2 was quantified by gas chromatography analyses. No formation of other reduced products such as methane was observed under the present experimental conditions. No formation of formic acid was confirmed by the formate dehydrogenase assay (Figure S4 and the experimental section in the SI).30 Figure 4a shows the time courses of formation of CO and H2 in the photoassisted electrolysis of a CO2-saturated aqueous solution in the presence of Na2SO4 (pH 4.6) at an applied bias voltage of −1.3 V at the cathode vs the photoanode. The CO yield is significantly higher than the H2 yield. The existence of CoIICh on the CoII(Ch)modified MWCNTs cathode after the reaction was confirmed by comparison of UV−vis absorption spectra (Figure S5).31 No CO formation was confirmed by the controlled potential electrolysis at an applied bias voltage of −1.3 V at the cathode vs the photoanode under dark (Figure S6). In contrast, a large current was observed when the FeO(OH)/BiVO4/FTO photoanode was illuminated (Figure S7). We also examined 534

DOI: 10.1021/acsenergylett.6b00630 ACS Energy Lett. 2017, 2, 532−536

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ACS Energy Letters the band gap of BiVO4 (band gap energy ≈ 2.4 eV),29 indicating that the FeO(OH)/BiVO4/FTO photoanode lowered the total bias (Eox − Ered) by ∼1.5 V, which enabled simultaneous water oxidation and CO2 reduction (Figure 5). In conclusion, CoII(Ch) adsorbed on MWCNTs acts as an efficient catalyst for selective photoelectrocatalytic reduction of CO2 to CO in H2O (pH 4.6) using a BiVO4 photoanode modified with FeO(OH) with an applied bias voltage of −1.3 V at the cathode vs the photoanode, exhibiting a high Faradaic efficiency of 83%. The present study provides a unique strategy for selective photoelectrocatalytic reduction of CO2 to CO over proton reduction to H2 using an earth-abundant metal cathode.

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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.6b00630. Experimental section: Experimental details, images of photoelectrochemical cells (Figure S1), powder X-ray diffraction patterns (Figure S2), cyclic voltammograms (Figure S3), UV−vis absorption spectra (Figures S4 and S5), time courses of evolution of CO and H2 under various conditions (Figures S6 and S8), time courses of currents (Figures S7 and S10), dependence of the CO production rate on CoII(Ch) concentrations (Figure S9), amount of produced gas (Tables S1 and S2), and potentials of the FeO(OH)/BiVO4/FTO photoanode during controlled potential electrolysis (Table S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.O.). *E-mail: [email protected] (S.F.). ORCID

Shunichi Fukuzumi: 0000-0002-3559-4107 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid (No. 16H02268 to S.F., Nos. 26620154 and 26288037 to K.O., No. 16K05721 to T.S.) and a JSPS fellowship (No. 25•727 to K.M.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and ALCA and SENTAN projects from JST, Japan (to S.F. and T.S). Photoelectrochemical studies especially on the potential difference between the FeO(OH)/BiVO4/ FTO anode and the SCE reference electrode were technically supported by Prof. Ken-ichi Nakayama at Osaka University and were financially supported by Grants-in-Aid (No. 25288113 to K.N.) from MEXT.

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REFERENCES

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ACS Energy Letters (21) Guo, Z.; Cheng, S.; Cometto, C.; Anxolabéhère-Mallart, E.; Ng, S.-M.; Ko, C.-C.; Liu, G.; Chen, L.; Robert, M.; Lau, T.-C. Highly Efficient and Selective Photocatalytic CO2 Reduction by Iron and Cobalt Quaterpyridine Complexes. J. Am. Chem. Soc. 2016, 138, 9413−9416. (22) Jang, Y. J.; Jang, J.-W.; Lee, J.; Kim, J. H.; Kumagai, H.; Lee, J.; Minegishi, T.; Kubota, J.; Domen, K.; Lee, J. S. Selective CO Production by Au Coupled ZnTe/ZnO in the Photoelectrochemical CO2 Reduction System. Energy Environ. Sci. 2015, 8, 3597−3604. (23) For the electrochemical reduction of aqueous CO2 to CO using a non-noble metal electrocatalyst based on a copper−indium (Cu−In) alloy, see: Rasul, S.; Anjum, D. H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanabe, K. A Highly Selective Copper−Indium Bimetallic Electrocatalyst for the Electrochemical Reduction of Aqueous CO2 to CO. Angew. Chem., Int. Ed. 2015, 54, 2146−2150. (24) Photocatalytic reduction of CO2 to CO accompanied by the nonstoichiometric evolution of H2 in excess was recently reported using a CoOx/BiVO4/reduced graphene oxide/CuGaS2 composite, see: Iwase, A.; Yoshino, S.; Takayama, T.; Ng, Y. H.; Amal, R.; Kudo, A. Water Splitting and CO2 Reduction under Visible Light Irradiation Using Z-Scheme Systems Consisting of Metal Sulfides, CoOx-Loaded BiVO4, and a Reduced Graphene Oxide Electron Mediator. J. Am. Chem. Soc. 2016, 138, 10260−10264. (25) Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Nanostructured Bismuth Vanadate-Based Materials for Solar-EnergyDriven Water Oxidation: A Review on Recent Progress. Nanoscale 2014, 6, 14044−14063. (26) Sivula, K. Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis. J. Phys. Chem. Lett. 2013, 4, 1624−1633. (27) Seabold, J. A.; Choi, K.-S. Efficient and Stable Photo-Oxidation of Water by a Bismuth Vanadate Photoanode Coupled with an Iron Oxyhydroxide Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 2186−2192. (28) McDonald, K. J.; Choi, K.-S. A New Electrochemical Synthesis Route for a BiOI Electrode and Its Conversion to a Highly Efficient Porous BiVO4 Photoanode for Solar Water Oxidation. Energy Environ. Sci. 2012, 5, 8553−8557. (29) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (30) Aoi, S.; Mase, K.; Ohkubo, K.; Fukuzumi, S. Selective Electrochemical Reduction of CO2 to CO with a Cobalt Chlorin Complex Adsorbed on Multi-Walled Carbon Nanotubes in Water. Chem. Commun. 2015, 51, 10226−10228. (31) When the CoII(Ch)-modified MWCNTs cathode was immersed in MeCN (2.0 mL) and sonicated, the Co complex loaded on the MWCNTs was dissolved in MeCN. After removal of detached MWCNTs by centrifugation, the amount of loaded Co complex was determined from the UV−vis spectrum of the supernatant. About 70% of the CoIICh remained on the CoII(Ch)-modified MWNCTs cathode after the photocatalytic reaction, as shown in Figure S5. (32) The solubility of CO2 decreases with an increase in pH due to equilibrium between CO2 and HCO3− reported in ref 33. Under the catalytic reaction conditions at pH 4.6, the ratio of CO2 to HCO3− is ∼0.95; thus, the CO2 solubility is virtually unchanged. At pH 6.7 in Table 1, the ratio of CO2 to HCO3− decreased to ∼0.35 to lower the selectivity for CO production. (33) Lide, D. R. Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2004. (34) The ratio of CO to H2 drastically decreased with a positive increase in the bias voltage at the cathode vs the photoanode according to Table S1 due to preference of H2 evolution to CO2 reduction at a more positive bias voltage on the cathode. With a decrease in proton concentration from pH 4.6 to 6.7, cathode reactions would be suppressed and photocurrent might be reduced. In order to compensate the loss of current, the potential of the photoanode would be positively shifted, leading to a positive shift of the cathode

potential. As a result, the ratio of CO to H2 at pH 6.7 in Table 1 is less than that reported in ref 30. (35) In the bulk electrolysis experiment at fixed pH, as the current decreases, the potential of the photoanode automatically is shifted to the positive direction and the cathode potential would be shifted to the positive direction in the same manner. As a result, the total electric charge may not significantly increase by changing pH in comparison with the photocatalytic current observed in CV. (36) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. Efficient Photocatalytic Production of Hydrogen Peroxide from Water and Dioxygen with Bismuth Vanadate and a Cobalt(II) Chlorin Complex. ACS Energy Lett. 2016, 1, 913−919. (37) Aoi, S.; Mase, K.; Ohkubo, K.; Fukuzumi, S. Photocatalytic Reduction of CO2 and H2O to CO and H2 with a Cobalt Chlorin Complex Adsorbed on Multi-Walled Carbon Nanotubes. Catal. Sci. Technol. 2016, 6, 4077−4080. (38) The dependence of the CO production rate on the amount of CoII(Ch) on MWCNTs was examined, as shown in Figure S9, where the initial rate of CO production increased with an increase in the amount of CoII(Ch) but decreased with a further increase in the amount of CoII(Ch). The existence of such an optimized amount of CoII(Ch) for catalytic CO production suggests that there may be an optimized distance between two CoII(Ch) molecules on MWCNTs to interact with CO2 for selective CO2 reduction to CO. However, a detailed mechanism of the catalytic CO2 reduction to CO with CoII(Ch) on MWCNTs has yet to be fully clarified. (39) In a 2E system, the bias voltage between the cathode and photoanode has been kept fixed under dark. Under light illumination, additional bias voltage has been applied, exhibiting a negative shift of the cathode potential. On the other hand, in a 3E system, almost the same current generation was observed under either dark or light illumination because the fixed potential vs NHE reference electrode has been applied to the CoII(Ch)-modified cathode as a working electrode. Thus, the 2E system is preferable to the 3E one in terms of photoresponse.

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DOI: 10.1021/acsenergylett.6b00630 ACS Energy Lett. 2017, 2, 532−536