(CO)3Cl and Carbon Nanotubes

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Electrochemical Reduction of CO by a Gas-Diffusion Electrode Composed of fac-Re(diimine)(CO)Cl and Carbon Nanotubes 3

Kei Murata, Hayato Tanaka, and Kazuyuki Ishii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12505 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

Electrochemical Reduction of CO2 by a GasDiffusion

Electrode

Composed

of

fac-

Re(diimine)(CO)3Cl and Carbon Nanotubes Kei Murata, Hayato Tanaka, and Kazuyuki Ishii* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan

ACS Paragon Plus Environment

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ABSTRACT The electrochemical reduction of carbon dioxide (CO2) was investigated using a gas-diffusion electrode containing fac-Re(diimine)(CO)3Cl and carbon nanotubes (CNTs) as the electrocatalyst and its supporting materials, respectively. The catalytic current attributed to CO2 reduction was successfully observed by cyclic voltammetry under CO2 atmosphere. Electrolysis at a constant potential resulted in the selective conversion of CO2 to CO, which was associated with 2-electron reduction coupled with 2-proton transfers. Screenings of the preparation conditions of the gasdiffusion electrode demonstrated that the catalytic performance strongly depended on the catalyst/carrier ratio, amount of loaded electrode components, and applied electrode potentials. These results indicated that the electrical conductivity and the diffusions of protons and CO2 in the catalyst layer were crucial factors for the reaction efficiency. This study will provide basic strategies for the rational design of gas-diffusion electrode–based electrocatalytic systems for reductions of gas substrates.


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INTRODUCTION Carbon dioxide (CO2) reductions have attracted much attention since they can provide constructive solutions to the current greenhouse gas problem and exhaustion of fossil fuels, via the transformation of CO2 into valuable chemicals.1,2 CO2 reductions are often coupled with proton transfers because the intervention of protons significantly lowers the thermodynamic barrier for a single-electron transfer to CO2. One of the promising strategies to realize multielectron, protoncoupled CO2 reductions is the incorporation of transition-metal electrocatalysts, which decreases the overpotentials for the reaction processes.3-6 To date, various types of metal complexes have been applied to homogeneous electrocatalytic systems for CO2 reduction. One of the typical electrocatalysts for CO2 reduction is fac-Re(bpy)(CO)3Cl (bpy = 2,2’-bipyridine), which was first reported by Lehn et al. in the 1980s.7.8 This complex enabled the efficient electrochemical conversion of CO2 to CO, with 98% faradaic efficiency at -1.25 V vs. NHE, in a DMF-water solution. Efforts on modifications of the ligand structure as well as reaction conditions have been successful in enhancing the catalytic performance.9-15 Parallel to such homogeneous systems, heterogeneous electrochemical systems in electrolyte solution have also been actively studied.16 In these systems, CO2 reduction catalysts are combined with carbon materials as the supporting carriers in a working electrode. In the case of molecular catalytic systems, applicable catalysts have been basically limited to metal complexes possessing an extended -conjugated system, such as polycyclic aromatic hydrocarbons17 and porphyrinic compounds.18 As these complexes can easily interact with carbon surfaces, they are usually incorporated in an adsorbed form onto carbon materials. By taking advantage of the combination of cobalt phthalocyanine catalyst and carbon nanotubes (CNTs), highly selective, efficient electrochemical CO2 reduction has been developed recently.19


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In addition to these studies, the electrochemical CO2 reduction by a gas-diffusion electrode has also been under intense investigations.20,21 In such cases, the CO2 reduction undergoes on an electrode composed of a gas-diffusion layer and a reaction layer with catalyst and aqueous electrolyte solution. Therefore, it enables the construction of environmentally friendly, enlarged systems, with easy isolation of the gas product without contamination by solvent vapor. For instance, Sakata and coworkers successfully observed the CO2 reduction current under a high CO2 pressure (30 atm) upon incorporating Ag particles into a gas-diffusion electrode.22 In addition, Fujishima and coworkers combined Ni nanoparticles and activated carbon fibers in a gas-diffusion electrode, and realized the efficient electrocatalytic reduction of CO2.23 Several efficient systems have been recently developed using metal and metal oxide particles as electrocatalysts,24-28 however, relatively few examples have been reported for molecular catalysts.29,30 For instance, Fujishima and coworkers also demonstrated that a gas-diffusion electrode containing cobalt porphyrins and phthalocyanines adsorbed onto activated carbon fibers worked as active catalysts for electrochemical CO2 reduction.29 However, many of these systems required either complex preparation procedures for gas-diffusion electrodes (such as pressing, heating, and long-time mixing) or harsh conditions (such as high CO2 pressure) to perform the reactions. Therefore, an efficient CO2 reduction system using easily preparable gas-diffusion electrodes under mild conditions is highly required. Furthermore, fac-Re(diimine)(CO)3Cl has never been applied to an electrochemical system with a gas-diffusion electrode despite its high catalytic activity in homogeneous systems. These factors motivated us to incorporate rhenium complexes into gasdiffusion electrodes for the construction of efficient CO2 reduction systems.


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Figure 1. fac-Re(bpy)(CO)3Cl (1). In this study, a new electrochemical system for CO2 reduction was constructed by combining an electrocatalyst for CO2 reduction and carbon-based carriers in a gas-diffusion electrode system. The catalyst layer contained fac-Re(diimine)(CO)3Cl (diimine = bpy (1), phen (phen = 1,10phenanthroline) (2), phen-dione (1,10-phenanthroline-5,6-dione) (3); Figure 1) and CNTs as the catalyst and its supporting carriers, respectively. As the supply of both electrons and protons are usually necessitated for the electrocatalytic CO2 reduction, the gas-diffusion electrode was prepared with reference to fuel cells,31 and was composed of a carbon paper, a catalyst layer, and a proton permeable membrane (Nafion). Using the electrochemical system, CO2 was successfully reduced to CO through 2-electron, 2-proton reductions. The catalytic activity was evaluated in terms of the structures of the Re complex and CNTs, catalyst/carrier ratio, amount of loaded electrode components, and electrode potentials. The simple preparation of the gas-diffusion electrode greatly increased the usability of the system. Furthermore, optimization of the preparation conditions of the gas-diffusion electrode figured out the crucial factors affecting the catalytic performance of the electrocatalytic system.


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METHODS fac-Re(bpy)(CO)3Cl (1),32 fac-Re(phen)(CO)3Cl (2),33,34 and fac-Re(phen-dione)(CO)3Cl (3)35 were synthesized according to the literature procedures. N,N-dimethylformamide (DMF), potassium nitrate (KNO3), tetrabutylammonium hexafluorophosphate (TBAPF6), Co(II) phthalocyanine, and Co(II) tetraphenylporphyrin were purchased from Sigma-Aldrich (TBAPF6) and Wako Pure Chemical Industries, Ltd. (others) and used without further purification unless otherwise noted. Aqueous solutions were prepared with purified water purchased from Kyoei Pharmaceutical Co., Ltd. MWCNTs (≥ 97% purity, 10-30 nm diameter, 5-15 m length, > 700 m2/g specific surface area by BET) and SWCNTs (≥ 90% purity, 0.7-1.3 nm diameter, 1 m length) were purchased from Tokyo Chemical Industry Co., Ltd. and Sigma-Aldrich Co., LLC, respectively. Carbon paper (EC-TP1-060T, 0.2 mm thickness) and Nafion (EC-NM-211, 51 m thickness) were purchased from TOYO Corporation. CNTs were pretreated with acids before use to remove impurities and increase the dispersibility in organic solvents.36 The acid treatment is known to generate structural defects with oxygencontaining functional groups in the side walls, as well as functionalize the open ends.37,38 The MWCNTs were ultrasonicated in a mixed acid (3:1 mixture of 96% aq. H2SO4 and 60-61% aq. HNO3) at 70 °C for 24 h. The SWCNTs were ultrasonicated in a piranha solution (9:1 mixture of 96% aq. H2SO4 and 30% aq. H2O2) at room temperature for 30 min. Nafion membrane was also pretreated with acids by boiling in 3% aq. H2O2 for 1 h, and then in 1 M aq. H2SO4 for 1 h. The working electrode was prepared by solution-casting as follows. A calculated amount of complex 1 and CNTs (0.5 g/L) were mixed in DMF (1 mM), and the mixture was ultrasonicated for 2 h. Then, ca. 80 L of the obtained dispersion was casted onto a carbon paper and dried at


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once under reduced pressure. The casting cycle was repeated until a defined amount of the dispersion was fully introduced to construct a catalyst layer. The diameter of the circular thin film was about 8 mm. The incorporation of the catalyst molecules into the CNT networks was confirmed by SEM-EDX analyses (Figure S1).

Figure 2. Schematic representation of electrochemical system with gas-diffusion electrode.

For electrochemical measurements, a three-compartment cell was constructed using the following procedure (Figures 2 and S2a). An electrolyte membrane (Nafion) dipped in water was layered on the catalyst layer on a carbon paper, and then an electrochemical cell possessing a hole at the bottom ( 6 mm, ~28 mm2) was placed on it. The reference (Ag/AgCl) and counter (Pt wire) electrodes were attached to the cell, and the interior space was filled with 0.5 M KNO3 aqueous solution as the electrolyte, which was degassed by N2 bubbling for 30 min. To establish CO2 (or N2) atmosphere in the catalyst layer, gaseous CO2 (or N2) was blown (2 L/min) onto the carbon paper surface from the outside. Using this electrochemical system, electrochemical measurements were performed with a potentiostat-galvanostat (VersaSTAT 3, Princeton Applied Research). The


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electrocatalytic activity for CO2 reduction was evaluated by measuring cyclic voltammograms under both CO2 and N2 atmospheres (sweep range: from 0 to -1.5 V vs. Ag/AgCl, sweep rate: 0.02 V/s). To analyze the products formed in the electrochemical CO2 reduction, controlled potential electrolysis was carried out using the same gas-diffusion electrode in a gas-tight threecompartment cell containing 50 mL of the gas phase, which was substituted by CO2 (Figure S2b). After the electrolysis, a gas sample (0.5 mL) taken from the headspace of the cell was analyzed using gas chromatography (Shimadzu Corporation, GC-2014AT) to determine the product amount. The faradaic efficiency was then calculated according to the equation shown in Scheme S1. For cyclic voltammetry measurements in solution, a three-compartment cell, with a glassy carbon electrode as the working electrode, Ag/Ag+ electrode as the reference electrode, and Pt wire as the counter electrode, was used. DMF was used as the solvent, and TBAPF6 was used as the electrolyte (0.1 M). The dissolved oxygen was removed by N2 bubbling for 30 min before the electrolysis. The cyclic voltammogram was measured with a sweep rate of 0.1 V/s, and the data were corrected based on the ferrocene standard (Fc0/Fc+).


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RESULTS Electrochemical Reduction of CO2 by the Gas-Diffusion Electrode Combining facRe(bpy)(CO)3Cl (1) and SWCNTs.

Using

the

gas-diffusion

electrode

containing

complex 1 (2×10-3 mol/g) and SWCNTs (0.5 mg), cyclic voltammograms were measured under N2 and CO2 atmospheres, as shown in Figure 3a. In the cyclic voltammogram obtained under N2 atmosphere, current density increases were observed in the negative side.39 The distinct increase from around -1.3 V (vs. Ag/AgCl) was mainly attributed to the proton reductions on CNTs, as it appears regardless of the presence or absence of complex 1. No obvious signal originating from complex 1 was observed in the negative side, which is common in heterogeneous electrochemical systems combining a molecular catalyst and CNTs.29 In the cyclic voltammogram obtained under CO2 atmosphere, the current density distinctly increases from around -1.25 V. The catalytic activity for CO2 reduction was evaluated using the current density difference between CO2 and N2 atmospheres, defined as i, to elucidate the electrochemical events taking place due to the intervention of CO2. In this case, the i was 11 mA/cm2 at -1.5 V, which is the maximum value of

i in the applied sweep range (0 to -1.5 V). A moderate increase in the i observed around -0.8 to -1.2 V was attributed to the surface condition changes of SWCNTs due to the adsorption of CO2.40 The current density increase was repeatedly observed with good reproducibility when switching atmospheres between CO2 and N2, as shown in Figure 3b. To investigate the origin of the i observed under CO2 atmosphere, control experiments were performed using the gas-diffusion electrodes without complex 1 or SWCNTs. The cyclic voltammograms obtained using the gasdiffusion electrodes without complex 1 or SWCNTs are represented in Figures 3c and 3d, respectively. When using the electrode without complex 1, although a small increase in i was observed at around -0.8 to -1.2 V, no distinct increase in i was observed at more negative


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potentials; i at -1.5 V was only about 4 mA/cm2. Furthermore, the cyclic voltammogram obtained using the electrode without SWCNTs shows a substantially small non-faradaic current (< 0.01 mA/cm2) with an almost negligible i. These results indicate that both complex 1 and CNTs are essential for the distinct increase in i at around -1.25 V, which was attributed to the

Current density / mA cm-2

-20 -30

under CO2 under N2 -1.5

-1 -0.5 Potential / V (vs. Ag/AgCl)

0

-10 -20 -30



0

cast solution: SWCNT 0.5 g/L, 1 mL

under CO2 under N2

-10 -20 -30

1 2 3 Number of gas-exchange units (CO2-N2)

-2

cast solution: SWCNT 0.5 g/L, N 2 Re(bpy)10-3 M, 1000 L CO 0.5 M KNO 3 aq 2 -1 (c)Vs 00.02

(b)

current density at -1.5V vsAg/AgCl cast solution: SWCNT 0.5 g/L, -3 Re(bpy)10 M, 1000 L (d) 00.5 M KNO aq N2 atmosphere 3 0.02 Vs -1 CO2 atmosphere -2

-10

0

-10 -20

current density / mA cm

0

(a)

Current density / mA cm

-2


-2

electrochemical CO2 reduction.


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0

-0.1 -0.2 -0.3 -1.5

-1

under CO2 under N2

-0.5

potential / V (vs. Ag/AgCl) 0.5 M KNO 3 aq 0.02 Vs -1 under CO2 under N2

-1.5

CO2 atmosphere N2 atmosphere


0.5 M KNO 3 aq 0.02 Vs -1

0

CO2 atmosphere N2 atmosphere

N2 atmosphere 0.5 M KNO aq Figure 3. Cyclic voltammograms obtained using gas-diffusion electrodes containing (a) fac-1 3 0.02 Vs

CO2 atmosphere

Re(bpy)(CO)3Cl (1) (2×10-3 mol/g) and SWCNTs (0.5 mg), (c) SWCNTs (0.5 mg) without complex 1, and (d) complex 1 (1×10-3 mmol) without CNTs. (b) Change in current density observed in the cyclic voltammogram at -1.5 V upon repetitive exchange of the atmosphere (CO2N2).


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Then, chronoamperometries were performed in a closed system under CO2 atmosphere (1 atm) to characterize the CO2 reduction products. CO was successfully detected in the headspace of the cell after the electrolysis at -1.5 V for 5 h. This result indicates that the i observed in the cyclic voltammogram is attributable to the 2-electron reduction of CO2 to afford CO. Other possible products of CO2 reduction (e.g., formic acid) were not detected in the reaction system; this selectivity in CO2 reduction was identical to that of the homogeneous electrocatalytic system using complex 1.7 In addition to CO, H2 was also detected in the cell after the electrolysis, which indicated that proton reduction competed with the CO2 reduction.

Dependency on Ligand Structure of Re Complex.

The diimine ligand on the

Re(I) center plays a crucial role in the electrocatalytic behaviors since redox properties as well as intermolecular interactions of the complex are tunable via the selection of the ligand. In particular, the extension of the -conjugated backbone as well as the introduction of oxygen functional groups could strengthen the interaction with CNTs.15,17 Therefore, fac-Re(phen)(CO)3Cl (2) and facRe(phen-dione)(CO)3Cl (3), which are known to work as electrocatalysts for CO2 reduction in homogeneous systems,12,15 were incorporated into the gas-diffusion electrode instead of complex 1 (Figure S3). In the case of complex 2, the effect on the expansion of the -conjugated system of the diimine ligand was investigated, as the ligand-based reduction potential was almost comparable to that of complex 1 in solution (Figures S4a and S4b). On the other hand, in the case of complex 3, because the ligand-based first reduction potential was significantly shifted to the positive side due to the introduction of carbonyl groups (Figure S4c), the increase in electron acceptability of the diimine ligand was considered in addition to the structural difference.


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Thus, using the gas-diffusion electrode containing complex 2 or 3 (2×10-3 mol/g) and SWCNTs (0.5 mg), cyclic voltammograms were measured as shown in Figures 4a and 4b. In the case of complex 2, the CO2 reduction current was observed as a distinct increase in the i from -1.25 V. The i was 10 mA/cm2 at -1.5 V, which is almost comparable to that of complex 1. In the case of complex 3, although the ligand-based reduction signal additionally appeared at around -0.2 V under N2 atmosphere (0 V under CO2 atmosphere), the CO2 reduction current was also observed with a potential and i almost similar to those of complex 2. The small differences in the electrocatalytic behaviors among complexes 1-3 indicate that both the structural and electronic factors of the diimine ligand have little effect on the electrocatalytic activity in this electrochemical

0


-2

-2

system.


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(a)

-10 -20 -30



0

10

(b) 0 -10 -20 -30

cast solution: SWCNT 0.5 g/L, Re(phen) 1 mM, 1 mL

Figure 4.

CO2 atmosphere 0.5 M KNO Cyclic obtained 3 aq -1 voltammograms N2 atmosphere 0.02 Vs

using


-1 -0.5 0 Potential / V (vs. Ag/AgCl)

0.5

cast solution: SWCNT 0.5 g/L, Re(phen-dione)electrodes 1 mM, 1 mL with gas-diffusion 0.5 M KNO 3 aq 0.02 Vs -1

(a) fac-

N2 atmosphere CO2 atmosphere

Re(phen)(CO)3Cl (2) (2×10-3 mol/g) and SWCNTs (0.5 mg), and (b) fac-Re(phen-dione)(CO)3Cl (3) (2×10-3 mol/g) and SWCNTs (0.5 mg).

Dependency on Electrode Composition.

To improve the electrocatalytic system, the

electrode compositions were optimized in terms of the complex 1/CNT ratio, amount of loaded


12

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electrode components, and fine structure of the CNTs. Figure 5a shows the dependency on the complex 1/CNT ratio for a constant loading of SWCNTs (0.1 mg). In the range of low complex 1/CNT ratios, the CO2 reduction current significantly increases with an increase in the complex 1/CNT ratio, whereas the proton reduction current decreases (Figure S5). However, the CO2 reduction current tends to decrease for further increase in the ratio; the i at -1.5 V was maximum (6 mA/cm2) when the complex 1/CNT ratio was 2×10-3 mol/g. Meanwhile, Figure 5b shows the dependency on the amount of loaded complex 1 and CNTs for a constant complex 1/CNT ratio (2×10-3 mol/g). The amount of loaded electrode components corresponds to the thickness of the layer, as the loading area of the catalyst layer is almost entirely constant. The catalytic currents for both proton and CO2 reductions increases with an increase in the loaded amount in the range of < 0.5 mg (Figure S6). However, the CO2 reduction current tends to decrease and the proton reduction current becomes almost constant for a further increase in the loaded amount; the i at -1.5 V was maximum (11 mA/cm2) when complex 1 (2×10-3 mol/g) and SWCNTs (0.5 mg) were appropriately loaded. Then, the dependency on the CNT fine structures was also investigated by evaluating the electrocatalytic activities of the gas-diffusion electrodes with MWCNTs. When complex 1 (2.0×10-3 mol/g) and MWCNTs (0.5 mg) were loaded, the CO2 and proton reduction currents rose, similar to those of the electrode with SWCNTs; however, the i was significantly smaller (5 mA/cm2 at -1.5 V). For the dependency on the complex 1/CNT ratio and that on the loaded amount (Figures 5a and 5b), the tendencies of the current density changes are almost similar for both SWCNTs and MWCNTs, but the current densities are higher for SWCNTs than for MWCNTs (Figures S7 and S8). These results were mainly attributed to the smaller specific surface area of MWCNTs than that of SWCNTs. As SWCNTs were found to be more suitable supporting


13


materials for this electrochemical system, the optimized composition was determined to be 2.0×103

mol/g and 0.5 mg for complex 1 and SWCNTs, respectively.

8

15

6

SWCNTs MWCNTs

4 2 0

0.1 1 10 Complex 1 / CNT ratio / 10 -3mmol

(b)  i / mA cm-2

(a)  i / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

SWCNTs MWCNTs

10

5

0

0

0.2 0.4 0.6 0.8 1 Loaded amount (SWCNTs) / mg

Figure 5. Current density change observed in the cyclic voltammogram at -1.5 V depending on (a) the concentration of complex 1, for a constant loading of SWCNTs (0.1 mg), and (b) the amount of loaded electrode components, for a constant complex 1/CNT ratio (2×10-3 mol/g).

Evaluation of Reaction Efficiencies.

Based on the optimized composition of the

electrochemical system, chronoamperometries and product analyses were performed under various conditions to evaluate the reaction efficiencies for electrochemical CO2 reduction. The faradaic efficiency for CO formation was calculated to be about 8% after the electrolysis with a constant potential of Ered = -1.5 V for 1 h (Of the differential current obtained under CO2 and N2 atmospheres, ca. 20% of donated electrons contributed to the CO formation). The corresponding faradaic efficiency for H2 formation was about 74%, demonstrating that it was the major side reaction accompanying with the CO2 reduction. Although CO generation gradually slowed down over time, the catalytic activity was maintained even after 5 h of the electrolysis. The degradation


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of the complex, e.g. dissociation of bpy ligand from the Re center, was not evident even after the electrolysis, indicating the high stability of the catalyst in this electrochemical system.41 The control experiments demonstrated that complex 1, CNTs, and CO2 were all essential for CO generation, which corresponds to the observations of the CO2 catalytic currents in the cyclic voltammograms (Figure S9). Then, the effect of the applied potential on the CO production was investigated as shown in Figure 6. CO production was observed upon electrolysis at -1.3 V, whereas it was not observed at -1.1 V. This result corresponds to the fact that the CO2 reduction current arises at -1.25 V in the cyclic voltammogram measured under CO2 atmosphere. Within the applied potential range of -1.3 to -1.7 V, the CO production increased with a shift in the potential to the negative side. The maximum CO production was observed for the electrolysis at -1.7 V, and the production tended to decrease upon application of more negative potentials. The decline in CO production was attributed to an enhancement in the competing H2 generation. Contrary to the CO production, the H2 production increased almost in proportion to the reaction time (Figure S10). 0.4 CO amount / mL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.9 V -1.7 V -1.5 V -1.3 V

0.3 0.2 0.1 0

0

1

2

3

Time / h

Figure 6. CO production during electrolysis at different applied potentials, using the gas-diffusion electrode containing complex 1 (2×10-3 mol/g) and SWCNTs (0.5 mg).


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Applicability of Other Transition-Metal Catalysts.

Page 16 of 31

To

investigate

the

applicability of other CO2 reduction catalysts to the electrochemical system, Co(II) phthalocyanine (CoPc) and Co(II) tetraphenylporphyrin (CoTPP) were incorporated into the gas-diffusion electrode instead of complex 1 (Figure S1). Both these complexes are well-known catalysts for electrochemical CO2 reduction.29,30,42 As shown in Figure 7a, the distinct CO2 reduction current appears at around -1.25 V when CoPc (2×10-3 mol/g) is combined with SWCNTs (0.5 mg); however, the i at -1.5 V is only around half (5 mA/cm2) of that of the electrode with complex 1 prepared under the same conditions. Similarly, as shown in Figure 7b, when CoTPP (2×10-3 mol/g) is combined with SWCNTs (0.5 mg), the i at -1.5 V was low (5 mA/cm2). In the case of the catalyst/carrier hybridization system, the amount of molecular aggregates is known to be crucial for the catalytic performance, as the aggregates reduce the effective surface area and hamper the electron/proton transports in a catalytic site.19 Therefore, the low catalytic activities observed for CoPc and CoTPP were mainly attributed to an increase in molecular aggregates in the catalyst layer because porphyrinic compounds easily form aggregates via - interactions. This electrochemical system provides a versatile platform to incorporate various molecular catalysts for electrochemical CO2 reduction, regardless of the adsorption property of CNTs. However, these results suggest that for each catalyst, optimization of the electrode composition would be necessary for constructing efficient systems.


16

-2

0

(a)

-10 -20 -30

Figure 7.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2

Page 17 of 31



cast solution: SWCNT 0.5 g/L, CoPc 1 mM, 1 mL Cyclic obtained 0.5 voltammograms M KNO 3 aq CO2 atmosphere 0.02 Vs -1 N2 atmosphere

0

using

0

(b)

-10 -20 -30



0

cast solution: SWCNT 0.5 g/L, CoTPP 1 mM, 1 mL CO2containing atmosphere 0.5 M KNOelectrodes 3 aq gas-diffusion 0.02 Vs -1 N2 atmosphere

(a) CoPc

(2×10-3 mol/g) and SWCNTs (0.5 mg), and (b) CoTPP (2×10-3 mol/g) and SWCNTs (0.5 mg).


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DISCUSSION Product Analysis and Reaction Mechanism.

In the electrochemical CO2 reduction, CO

evolved as the sole product transformed from CO2. The control experiments demonstrated that complex 1, protons, and an appropriate potential were essential for CO evolution, and this was attributed to the catalysis of complex 1 accompanying 2-electron and 2-proton transfers (Eq. 1).

Reaction mechanisms of the electrochemical conversion of CO2 to CO by facRe(diimine)(CO)3Cl have been proposed for many homogeneous systems.8,14 Although our electrochemical system is based on a heterogeneous reaction, the CO2 reduction is considered to proceed via a mechanism similar to that proposed for homogeneous systems.43 As shown in Figure 8, complex 1 is initially transformed into the coordinatively unsaturated complex A through oneelectron reduction followed by the elimination of a chloride ion, to initiate the catalytic cycle. In the cycle, complex A is further reduced to give the anion species B. This complex reacts with CO2 and a proton to give the formate complex D via the CO2 coordinating complex C. Then, a oneelectron reduction and a protonation afford the tetracarbonyl complex E, accompanied by the elimination of H2O, and the successive CO release regenerates the active species A to complete the catalytic cycle.


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Figure 8. Proposed reaction mechanism of electrochemical CO2 reduction by fac-Re(bpy)(CO)3Cl (1).

In the case of the homogeneous reaction, the potential limiting step of CO2 reduction is the one-electron reduction of the coordinatively unsaturated complex A. Interestingly, an initial rise in the catalytic current was observed at around -1.25 V (vs. Ag/AgCl) for our electrochemical system, which was shifted a little to the positive side compared to that observed for the homogeneous system (-1.4 V).7 This result suggests that the heterogeneous environment in the gas-diffusion electrode could influence the electronic structures of the intermediate complexes in the catalytic cycle. The absence of a solvent is considered to be one of the major factors for it. In particular, although the coordinatively unsaturated species easily interacts with a solvent molecule on the metal center in a liquid phase, it hardly interacts in a solid phase. This situation could change the coordination environment of the metal center and the redox behavior of the complexes. The other factor is the interaction of the complexes with CNTs. CNTs have been known to interact


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with molecules possessing an extended -conjugated system, such as pyrenes and porphyrinic compounds, via - or CH- interactions.44,45 Although the -conjugated system of the bpy ligand in complex 1 is rather small, the interaction with CNTs is considered to slightly influence the electronic structure of the complexes.

Catalytic Activity Depending on Electrode Composition.

Since

the

electrode

composition determines the number of “active sites” in which all the key substrates, i.e., CO2, electrons, and protons, encounter a catalytic site, it is considered to be crucial for the electrocatalytic performance. Through optimization of the electrode composition, the catalytic performances were found to strongly depend on the complex 1/CNT ratio, amount of loaded electrode components, and CNT fine structures. For example, one of the rational strategies to realize an efficient catalytic system is to increase the amount of complex 1 on CNT surfaces. In fact, in the range of low complex 1/CNT ratios (< 2×10-3 mol/g), the catalytic current reasonably increases with an increase in the complex 1/CNT ratio. However, in the range of high ratios (> 2×10-3 mol), it significantly decreases. Similarly, the catalytic current increases with an increase in the amounts of CNTs in the range of < 0.5 mg, whereas the current decreases in the range of > 0.5 mg. To analyze the experimental results encompassing these phenomena, multiple factors that could contribute to the generation of “active sites” were considered, as follows. Firstly, the complex 1/CNT ratio is discussed in terms of the electrical conductivity, which corresponds to the capability of donating electrons, and catalyst density in the thin layer. Since complex 1 exhibits a substantially low electrical conductivity, whereas CNTs exhibit high conductivity,46 an increase in the complex 1/CNT ratio should reduce the electrical conductivity of the catalyst layer. This was experimentally shown; the proton reduction current, i.e., 2H+ + 2e-


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→ H2, which reflects the capability of donating electrons to protons, monotonically decreases with an increase in the complex 1/CNT ratio in the range of ~10-4 to ~10-2 mol/g under N2 atmosphere. Meanwhile, under CO2 atmosphere, the CO2 reduction current appeared as a result of a trade-off between the electrical conductivity and catalyst density. Therefore, the increase in the CO2 reduction current in the range of low complex 1/CNT ratios (< 2×10-3 mol/g) could be explained by the increase in the catalyst density, whereas the decrease in the current in the range of high complex 1/CNT ratios (> 2×10-3 mol/g) could be interpreted as a decline in the electrical conductivity. When 2×10-3 mmol of complex 1 and 0.1 mg of SWCNTs were loaded (complex 1/CNT ratio = 2×10-2 mol/g), the total surface area of complex 1 in the catalyst layer was calculated to be about 3×10-2 m2, by applying spherical approximation to complex 1 (0.6 nm diameter). This value is almost comparable to or larger than the estimated total surface area of conventional SWCNTs (10-1 m2).47 In other words, in the case of a higher complex 1/CNT ratio, an excess amount of complex 1 cannot contact the CNT surfaces efficiently and function as the active sites. Thus, the electrical conductivity and catalyst density were proven to be the crucial factors for the generation of active sites in the catalyst layer. Secondly, the dependence on the amount of loaded electrode components, which corresponds to the thickness of the thin films, is discussed. Since protons diffuse from the Nafion membrane, whereas CO2 diffuses from the carbon paper electrode at the opposite side, the thickness change should affect the effective diffusion regions of protons and CO2. They were experimentally analyzed as follows. (i) The effective diffusion region of protons: The proton reduction current increases with an increase in the amount of loaded SWCNTs from 0.1 mg, but becomes constant when the SWCNT loading is more than 0.5 mg. This result indicates that the effective diffusion region of protons, which expands toward the carbon paper electrode, is limited due to the low


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proton conductivity in CNTs. (ii) The effective diffusion region of CO2: The CO2 reduction current increases with an increase in the amount of loaded SWCNTs from 0.1 mg, but decreases when the SWCNT loading is more than 0.5 mg. This result indicates that the effective diffusion region of CO2, which expands toward the Nafion membrane, is also limited, and the part overlapping with the effective diffusion region of protons should decrease with an increase in the thickness. Thirdly, the electrode potential, which corresponds to the donation of electrons, is discussed. The maximum CO production was observed for the electrolysis at -1.7 V when shifting the applied potential from -1.1 to -1.9 V, whereas the H2 production monotonically increased with a shift in the potential in a similar manner. At a highly negative potential, protons are promptly consumed by direct reduction to generate H2 before encountering CO2, since the electrons are efficiently supplied even in the vicinity of the Nafion membrane. Hence, the suppression of CO production at -1.9 V was attributed to the decrease in the effective diffusion region of protons. As discussed above, the dependencies on the complex 1/CNT ratio, amount of loaded electrode components, and electrode potential well-highlight the crucial factors for rational design of an efficient electrochemical system. The electrical conductivity as well as effective diffusion regions of protons and CO2 are all dominant factors that determine the number of active sites in the catalyst layer. These findings will provide basic strategies for the rational design of a catalyst/CNT hybrid system for electrochemical CO2 reduction.


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CONCLUSIONS An electrocatalytic CO2 reduction was successfully realized using a combination of facRe(diimine)(CO)3Cl and CNTs in a gas-diffusion electrode. This heterogeneous electrochemical system selectively generated CO through 2-electron, 2-proton reduction of CO2. Through optimization of the preparation conditions for the gas-diffusion electrodes, the electrical conductivity and effective diffusion regions of protons and CO2 were found to be crucial factors for the catalytic activity in electrochemical CO2 reduction. They could be regulated using the catalyst/carrier ratio, amount of loaded electrode components, and applied electrode potentials. Based on the optimized conditions, the faradaic efficiency was determined to be about 8% for the electrolysis at -1.5 V vs. Ag/AgCl (Of the differential current obtained under CO2 and N2 atmospheres, ca. 20% of donated electrons contributed to the CO formation). This system features a simple and easy-to-handle method for preparing the gas-diffusion electrodes, and catalytic reaction under mild conditions. In addition, since this catalytic system did not necessitate a strong interaction between catalysts and supporting materials, a wide variety of molecules could be applied as catalysts regardless of their inherent structures. Therefore, this method will contribute to the construction of convenient, versatile electrochemical systems with molecular catalysts for CO2 reduction.


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ASSOCIATED CONTENT Supporting Information. Figures of materials and electrochemical systems, Cyclic voltammograms in different conditions, and CO / H2 production curves in control experiments (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81-3-5452-6306. ORCID Kazuyuki Ishii: 0000-0002-8676-1008 Kei Murata: 0000-0002-2350-5330 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP17H06375 and JP16H04128.


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On the UV-vis Photophysics of the Re(I) Intercalator and the Redox Reactions with Pulse Radiolysis-Generated Radicals. Dalton Trans. 2007, 0, 2020–2029. (36) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, H. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B. et al. Fullerene Pipes. Science 1998, 280, 1253–1256. (37) Waki, K.; Wong, R. A.; Oktaviano, H. S.; Fujio, T.; Nagai, T.; Kimoto, K.; Yamada, K. NonNitrogen Doped and Non-Metal Oxygen Reduction Electrocatalysts Based on Carbon Nanotubes: Mechanism and Origin of ORR Activity. Energy Environ. Sci. 2014, 7, 1950–1958. (38) Jiang, Y.; Yang, L.; Sun, T.; Zhao, J.; Lyu, Z.; Zhuo, O.; Wang, X.; Wu, Q.; Ma, J.; Hu, Z. Significant Contribution of Intrinsic Carbon Defects to Oxygen Reduction Activity. ACS Catal. 2015, 5, 6707–6712. (39) A small, shouldered-signal that appeared at around -1.2 V was attributed to the electrochemical doping of SWCNTs. For example, see: Kimizuka, O.; Tanaike, O.; Yamashita, J.; Hiraoka, T.; Futaba, D. N.; Hata, K.; Machida, K.; Suematsu, S.; Tamamitsu, K.; Saeki, S. et al. Electrochemical Doping of Pure Single-Walled Carbon Nanotubes Used as Supercapacitor Electrodes. Carbon 2008, 46, 1999–2001. (40) Rahimi, M.; Singh, J. K.; Babu, S. J.; Schneider, J. J.; Müller-Plathe, F. Understanding Carbon Dioxide Adsorption in Carbon Nanotube Arrays: Molecular Simulation and Adsorption Measurements. J. Phys. Chem. C. 2013, 117, 13492–13501. (41) In the 1H NMR spectrum of the complex extracted from the catalyst layer after the electrolysis at -1.5 V for 5 h, no degradation product of complex 1 was observed. (42) For a review, see: Manbeck, G. F.; Fujita, E. A Review of Iron and Cobalt Porphyrins, Phthalocyanines and Related Complexes for Electrochemical and Photochemical Reduction of Carbon Dioxide. J. Porphyrins Phthalocyanines 2015, 19, 45–64.


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(43) Another possibility of the reaction pathway for the CO formation via the dimer species [facRe(diimine)(CO)3]2 was excluded, as the catalyst molecules hardly interact each other in the heterogeneous system, and the dimer formation would be strongly suppressed. (44) Pérez, E. M.; Martín, N. - Interactions in Carbon Nanostructures. Chem. Soc. Rev. 2015, 44, 6425–6433. (45) Kar, T.; Bettinger, H. F.; Scheiner, S.; Roy, A. K. Noncovalent - Stacking and CH--- Interactions of Aromatics on the Surface of Single-Wall Carbon Nanotubes: An MP2 Study. J. Phys. Chem. C 2008, 112, 20070–20075. (46) Pop, E.; Mann, D.; Wang, Q.; Goodson, K.; Dai, H. Thermal Conductance of an Individual Single-Wall Carbon Nanotube above Room Temperature. Nano Lett. 2006, 6, 96–100. (47) The specific surface area of SWCNT was assumed to be 1000 m2/g.


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TOC Graphic


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(CO)3Cl and Carbon Nanotubes

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