Low-Energy Electrocatalytic CO2 Reduction in Water over Mn

Apr 10, 2018 - ... provided an environment to stabilize CO2 adjacent to the Mn-complex and significantly lowered the overpotential for CO2 reduction i...
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Low-energy electrocatalytic CO2 reduction in water over Mn-complex catalyst electrode aided by a nanocarbon support and K+ cations Shunsuke Sato, Kenichiro Saita, Keita Sekizawa, Satoshi Maeda, and Takeshi Morikawa ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01068 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Low-energy electrocatalytic CO2 reduction in water over Mn-complex catalyst electrode aided by a nanocarbon support and K+ cations

Shunsuke Sato†*, Kenichiro Saita‡, Keita Sekizawa†, Satoshi Maeda‡, Takeshi Morikawa†

†

Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan.

‡

Department of Chemistry, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo

060-0810, Japan *Correspondence to: [email protected]

Abstract Electrocatalytic CO2 reduction over a Mn-complex catalyst in an aqueous solution was achieved at very low energy with a combination of multi-walled carbon nanotubes (MWCNTs) and K+ cations. Although the bare Mn-complex did not function as a catalyst in an aqueous solution, the combined Mn-complex/MWCNT cathode promoted electrocatalytic CO2 reduction, at an overpotential of 100 mV where neither the bare MWCNTs nor bare Mn-complex were catalytically active. The Mn-complex/MWCNT produced CO at a constant rate for 48 h with a current density of greater than 2.0 mA cm-2 at -0.39 V (vs. RHE). The MWCNTs with electron accumulation properties, together with surface adsorbed K+ ions, provided an environment to stabilize CO2 adjacent to the Mn-complex and significantly lowered the overpotential for CO2 reduction in an aqueous solution, and these results were consistent with density functional theory (DFT) calculations. Experiments clarified that the synergetic effect of the MWCNTs and K+ ions was also ACS Paragon Plus Environment

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applicable to Co and Re complexes that were almost inert with regard to CO2 reduction in an aqueous solution.

Keywords: Mn complex • electrocatalyst • CO2 reduction• carbon nanotube • heterogeneous catalyst

Introduction The development of a photocatalytic system that can produce useful organic chemicals by the reduction of carbon dioxide (CO2) using solar energy is an important research area because of climate change and fossil fuel shortages. However, direct solar reduction of CO2 to organic chemicals using electrons and protons extracted from water molecules, a process similar to photosynthesis in plants, is difficult. Although direct fuel generation is the ultimate goal, the production of basic components, such as carbon monoxide (CO), methanol, and ethane, from CO2 with high energy efficiency is considered a realistic approach. Electrocatalytic CO2 reduction is a feasible approach that can be accomplished using metal catalysts and molecular catalysts (metal complexes) under certain electrical biases. These electrocatalysts require a large electrical potential to achieve catalytic CO2 reduction because the first step in CO2 conversion is the formation of CO2- radical anions during single-electron reduction.1 Therefore, H2 is likely to be preferentially generated by water splitting in an aqueous solution. Currently employed electrocatalysts facilitate proton-coupled multi-electron reactions (e.g., CO2 + 2H+ + 2e- →CO + H2O, -0.11 V vs. RHE), which require lower potentials than those for single-electron reactions.2 Metals such as Au and Ag act as electrocatalysts for selective CO2 reduction in a NaHCO3 solution, but the overpotentials required are generally high (>600 mV).3 In contrast, metal complex catalysts (MCs) such as Re, Fe, and Mn complexes ACS Paragon Plus Environment

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promote electrocatalytic CO2 conversion to CO or formic acid with high product selectivity. CO2 reduction reactions over such MCs have generally been conducted in an organic solvent to avoid H2 generation or to prevent decomposition of the complexes under an electrical bias in the presence of water.4-7 Use of an organic solvent is essential because it provides basic science information such as correlation of ligand species and CO2 reduction activities in addition to a high turnover frequency. On the other hand, reaction in water, which competes with preferential H2 generation, is also desirable, and recently CO2 reduction in aqueous media using Cu, Ni, Co, Mn, and Fe complexes at biases ranging from -0.46 to -1.38 V vs. RHE has been reported.8-18 CO2 reduction at a low overpotential is also important with respect to achieving a high reaction rate and for future industrialization of production processes. An oxide-derived Au catalyst and a porous Cu metal catalyst were both reported to promote CO2 reduction to CO with high selectivity at a low overpotential of around 200 mV in an aqueous carbonate solution.19,20

Ionic liquid facilitated the conversion of CO2 to CO

over a metallic Ag catalyst at an overpotential of about 200 mV.21 In contrast, MCs generally required higher overpotentials for CO2 reduction in aqueous solution, except for a Ru complex.22,23 In order to decrease the overpotential, we need to decrease lowest unoccupied molecular orbital (LUMO) potential of MCs. However, catalytic ability for CO2 reduction is also decreased by lowering LUMO potential of the MCs.24,25 The LUMO potential and the CO2 reduction ability are in a trade-of relation. Demand also exists for the development of electrocatalysts that include abundant and thus low-cost elements. The first notable example of electrocatalytic CO2 reduction to CO using a Mn complex was reported by Deronzier and Chardon-Noblat’s group,6 which was followed by many related studies by other research groups17,18,26-32 because Mn is an abundant element and less expensive than Co or Ni.33 Carbon monoxide is ACS Paragon Plus Environment

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an important chemical because of its use as a feedstock in the production of chemicals ranging from acetic acid to polycarbonates, its use in syngas (mixture of CO and H2), and its role in the production of aldehydes and alcohols. It is more beneficial if the overpotential for CO2 reduction (ca. 700 mV) of these noble Mn complexes could be reduced or be operated in water-based solvent together with advanced stability under irradiation. The present report describes CO2 reduction over MCs in water at very low overpotentials. A new Mn complex electrocatalyst, [Mn{4,4’-di(1H-pyrrolyl-3-propyl carbonate)-2,2’-bipyridine}(CO)3MeCN]+(PF6)([Mn-MeCN]), was developed and loaded onto conductive MWCNTs ([Mn-MeCN]/MWCNT). We carried out the electrocatalytic CO2 reduction using [Mn-MeCN]/MWCNT in aqueous solution. Although bare [Mn-MeCN] did not promote the CO2 reduction reaction, [Mn-MeCN]/MWCNT selectively catalyzed CO2 reduction at a very low overpotential (ca. 100 mV) even in an aqueous solution. This overpotential is significantly lower than those reported for previous Mn-complex catalysts and those for noble metal catalysts. The electrocatalytic CO2 reduction reaction was deactivated without MWCNT and K+ cations condition. Therefore, the CO2 reduction activity in aqueous solution was highly improved by the presence of K+ ions and the nanocarbon support. This effect can be applied for other catalysts such as Co and Re complexes for CO2 reduction.

Results and discussion 1. [Mn-MeCN]/MWCNT electrode for CO2 reduction The synthesis of the [Mn-MeCN] electrocatalyst was accomplished through ligand replacement, as described previously6 (see Figure S1), and the [Mn-MeCN]/MWCNT combined catalyst electrode was ACS Paragon Plus Environment

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created by drop-casting on a stainless steel (SUS) substrate (details in Supporting Information and Scheme S1). The [Mn-MeCN]/MWCNT electrode catalyzed CO2 molecules to form CO under an electrical bias in 0.1 M K2B4O7 + 0.2 M K2SO4 (pH = 6.9, saturated with CO2). Among various solvents such as KHCO3, a mixed solution of K2B4O7 and K2SO4 was found to be the best electrolyte for the [Mn-MeCN]/MWCNT electrode. Figure 1a shows the electrocatalytic formation of CO over [Mn-MeCN]/MWCNT at -0.39 V (vs. RHE) using a flow reactor,22 in which a reaction current density of -2.6 to -2.0 mA cm-2 was obtained. The main product was CO with a Faradaic efficiency (FE) of 84±4 % and the electrocatalytic reaction continued for at least 24 h. H2 (FE = 16±4%) was also produced over the Mn catalyst. Another measurement with repeated evacuation confirmed that the electrocatalytic reaction continued without deactivation of the Mn catalyst for 48 h (Figure S2). Figure 1b shows linear sweep voltammetry results (LSV) for the [Mn-MeCN]/MWCNT electrode before and after electrocatalytic reaction for 24 h. Almost no deterioration of the catalytic current occurred after electrolysis for 24 h, as determined by Fourier transform infrared (FT-IR) spectroscopy analysis of the [Mn-MeCN] catalyst after the reaction (Figure S3). The turnover number (TON) for CO reached 722 after 24 h in the aqueous solution, which is comparable to that for previous Mn complex systems measured in organic solvents such as MeCN/H2O (5%) and MeCN in a solution containing Mg2+ ions (TON = 34 after 18 h and TON = 36 after 6 h).6,28 The [Mn-MeCN]/MWCNT electrode operated at a high reaction current of greater than -5 mA cm-2 at -0.49 V (vs. RHE) (Table 1 and Figures 2 and S4). The MWCNT support enabled greater loading of the [Mn-MeCN] catalyst within the same area of the SUS substrate while maintaining the CO2 reduction activity of [Mn-MeCN], which is speculated to be due to the increased specific surface area of the MWCNT as we reported recently.23 The results for electrochemical CO2 reduction conducted under various electrical biases over cathodes of ACS Paragon Plus Environment

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[Mn-MeCN]/MWCNT, MWCNT/SUS, and [Mn-MeCN]/SUS are shown in Table 1. Neither the MWCNT/SUS electrode under CO2 nor the [Mn-MeCN]/MWCNT cathode under an Ar atmosphere acted as a CO2 reduction catalyst, and they produced only H2. The current density for MWCNT/SUS was very low (Figure S5) because the bare MWCNTs possessed negligible activity for either CO2 reduction or H2 production at these low potentials (from -0.1 to -0.6 V vs. RHE), which is consistent with a previous report.34 In contrast, the [Mn-MeCN]/MWCNT electrode catalyzed selective CO2 reduction at -0.21 V (vs. RHE; overpotential of ca. 100 mV for CO production), which indicates that [Mn-MeCN]/MWCNT possessed the low overpotential compared to systems using a N-doped CNT catalyst or an Earth-abundant element catalyst, even without the need for additional aids such as ionic liquids.21,34-36 A reaction current of 0.23 mA cm-2 and an FE value of 80.6% were found at -0.21 V (vs. RHE) over [Mn-MeCN]/MWCNT, accompanied by the generation of 7.1 µmol of CO after electrolysis for 2 h. At -0.39 V, the reaction current increased to 2.42 mA cm-2, which generated 75.5 µmol of CO with an FE of 87.3% after electrolysis for 2 h. By comparison, for [Mn-MeCN]/SUS measured at -0.39 V, the reaction current of 0.46 mA cm-2 was one-sixth of that for [Mn-MeCN]/MWCNT, and no CO was generated, but only H2 with an FE of 97.6%. Comparison with previous Mn complexes clarified that the [Mn-MeCN]/MWCNT electrode results in a very low overpotential, even in water (Table S1 and S2). Previous reports on Fe-, Ni-, and Co-complex electrocatalysts were also compared with the present [Mn-MeCN]/MWCNT electrode (Table S3). The catalytic ability for CO2 reduction is generally known to be lost when the lowest unoccupied molecular orbital (LUMO) potential of the MC becomes too low.24,25 Therefore, design of MCs with an electron-withdrawing group in the diamine ligand has been considered impractical. Bare [Mn-MeCN] did not function as a CO2 reduction electrocatalyst when using a MeCN/H2O (5%) solution (Table 2), because ACS Paragon Plus Environment

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the LUMO potential of [Mn-MeCN] was approximately Ep = -0.84 V (vs. Ag/AgCl), which was 0.4 V lower than that for the conventional [Mn(bpy)(CO)3Br]([Mn(bpy)]) catalyst (Figure S6). This could be the reason why the bare [Mn-MeCN]/SUS electrode and [Mn-MeCN] in an organic solvent could not reduce CO2. Therefore, we cannot calculate turnover frequency for the [Mn-MeCN] catalyst, because measurement in a non-aqueous medium is necessary.11 In contrast, [Mn-MeCN]/MWCNT catalyzed selective CO2 reduction at a very low overpotential, even in water without the aid of an ionic liquid, which indicates that the MWCNT support is crucial for the enhanced activity of the [Mn-MeCN]/MWCNT system.

2. Effect of carbon support on the metal complex 2-1. Possibility of modified electronic interaction of MWCNTs and metal complex The bare [Mn-MeCN]/SUS cathode in the absence of MWCNTs was inactive for CO2 reduction, but produced H2 instead. There are two possibilities that led to the improved activity with the MWCNT support; electronic interaction between the MC and MWCNTs, and an improvement in the environment for CO2 reduction in the aqueous solution. Attempts were made to elucidate the mechanism for the effect of carbon on CO2 reduction using Mn complexes. First, the electronic state of the Mn complex was examined using attenuated total reflectance infrared (ATR-IR) spectroscopy. A peak shift in the CO stretching mode for [Mn-MeCN]/MWCNT was observed (Figures S3); however, the shift did not originate from the electronic interaction between the Mn complex and MWCNTs, but from polymerization of the [Mn-MeCN] during catalyst formation (Figure S7). This result indicates that no strong electronic interaction occurred between the [Mn-MeCN] and MWCNTs. Therefore, the MWCNTs may provide a preferable environment for CO2 reduction in an aqueous solution, which will be discussed later. The selectivity for CO2 reduction over ACS Paragon Plus Environment

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[Mn-MeCN]/MWCNT was dependent on the surface hydrophobicity of the MWCNTs, and hydrophilic treatment of the MWCNTs decreased the FE for CO production but increased the FE for H2 production (Table S4).

2-2. Effect of MWCNT electrode thickness The activity of [Mn-MeCN]/MWCNT for CO2 reduction was found to be dependent on the thickness of the MWCNT film (Table 3). When the MWCNT solution was deposited only once or spin-coated only once (thickness ca. 20–30 µm), the CO2 reduction rate for the electrode was very small (very low total reaction charge with poor CO selectivity of one-fifth or less) compared to the sample with 10-times coating (thickness ca. 200–300 µm). Although the H2 production rate on SUS can be suppressed by a single-drop or spin-coating (total charge decreased by one order of magnitude, as in Table 3), the [Mn-MeCN] catalyst should be dispersed deep in the MWCNT scaffold to enhance CO selectivity. A similar phenomenon has been reported for CO2 selectivity in carbon fiber cloth electrodes dispersed with a Ru-complex catalyst,22 and for formate and CO generation over Ag and Au metal catalysts in carbon fiber.37 In the case of metal catalysts incorporated in thick carbon fiber films, the current density and selectivity for CO2 reduction were greater than those for metal catalysts deposited on thin carbon fiber films.38 Furthermore, it was found that hydrogen production over a Pt metal catalyst was suppressed by surrounding the catalyst with carbon fiber.37 Although the current density for CO production over Ag and Au catalysts was increased by the surrounding carbon fiber, the current density for hydrogen production was reduced. These results suggest that hydrophobic carbon such as MWCNTs suppresses the approach of H+ to the catalyst, which results in decreased H2 generation. We have also found that hydrophilic pre-treatment of MWCNTs increased the FE ACS Paragon Plus Environment

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for H2 production (Table 3); when a small amount of CNTs or hydrophilic carbon materials were used, H2 production at the catalysts occurred readily. The environment adjacent to the MC molecules with hydrophobic MWCNTs thus plays a crucial role for highly selective CO2 reduction in these aqueous solutions. Electrocatalytic CO2 reduction using MCs combined with CNTs has been reported recently.9,10,17,18 However, the amount of CNTs was small, because CNTs were used only to increase the surface area or enhance the conductivity of a Nafion support. In contrast, the present approach with a larger amount of MWCNTs significantly improved the selectivity toward CO2 reduction, even in an aqueous solution without the aid of an ionic liquid.

2-3 Effect of K+ cations on CO2 reduction [Mn-MeCN]/MWCNT exhibited highly selective CO2 reduction in a K2B4O7 + K2SO4 mixed solution. In contrast, CO2 selectivity was much lower in a (NH4)2B4O7 + (NH4)2SO4 mixed solution, and H2 was mainly produced, even in the case of a thick hydrophobic MWCNT film (Table 1 and Figure S8). These results indicated that K+ cations played a very important role in CO2 reduction at very low overpotential over the [Mn-MeCN]/MWCNT electrode. The electrocatalytic activities of homogeneous MC systems can be improved by the addition of a Lewis acid such as Mg2+ in organic solvents.29,38 In the present system, experiments verified that bare [Mn-MeCN] yielded minimal amounts of CO2 reduction products under conditions (MeCN + 5% H2O) that were the same as those reported previously, even in the presence of K+ cations (Table 2). Therefore, the effect of the Lewis acid is negligible. On the other hand, the electrode without MWCNTs ([Mn-MeCN]/SUS) did not catalyze CO2 reduction (Table 1), even in the presence of K+ cations, which indicated that the presence of ACS Paragon Plus Environment

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both MWCNTs and K+ cations is decisive for a very high CO2 reduction rate at a low overpotential.

2-4. Adsorption of K+ cations on the MWCNT surfaces MWCNTs operate as electric double-layer capacitors;39 therefore, the properties of the present MWCNT films were determined using cyclic voltammetry (Figure S9). The results suggested that the MWCNT surfaces are covered with H+ and K+ cations during electrocatalytic CO2 reduction in the K2B4O7 + K2SO4 mixed solution. It was clearly demonstrated that electric double-layer capacitors can repeatedly charge and discharge by the formation and release of electrical double layers through the accumulation of electrolyte ions at the electrode/electrolyte interface.40,41 Adsorbed cations were reported to have an effect on the electric double-layer capacitor properties of MWCNTs, in that they stored more electrons.42 Alkali metal ions such as Na+ and K+ in zeolites exhibit strong CO2 adsorption ability at room temperature;43,44 therefore, the concentration of CO2 molecules on the MWCNT surfaces is likely to be improved by adsorbed K+ cations that accumulate on [Mn-MeCN]/MWCNT under negatively biased conditions. High CO2 concentration conditions improve the CO2 reduction activity of metal complex catalysts.45 The results in Sections 2-1, 2-2 and 2-3 strongly suggest that the CO2 reduction activity of the [Mn-MeCN]/MWCNT electrode is significantly improved by the cooperative effect of K+ and MWCNTs, and the effect does not occur if either of these is lacking.

2-5. Effect of K+ cations on the overpotential As described above, the cooperative effect of K+ and MWCNTs at the [Mn-MeCN]/MWCNT electrode appears to promote electrocatalytic CO2 reduction with a high current density and CO2 selectivity in an ACS Paragon Plus Environment

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aqueous solution. Thus, the mechanism for the low overpotential for CO2 reduction should be discussed. We have shown that CO2 reduction using the [Mn-MeCN]/MWCNT electrode occurs at -0.21 V vs. RHE (Table 1), which is almost the same potential as the LUMO for [Mn-MeCN]. This indicates that the CO2 reduction reaction can proceed with a one-electron-reduced (OER) species of [Mn-MeCN] and thermal energy at room temperature. However, it has been reported that the OER species of metal complex catalysts with electron-withdrawing groups do not readily react with CO2 molecules (the rate constant is very low) due to their very low LUMO potentials.24 We have confirmed that [Mn-MeCN] cannot act as a CO2 reduction catalyst without the MWCNT support and K+ cations. Furthermore, no catalytic current was observed in MeCN with [Mn-MeCN]. These results indicate that there is a one of the bottlenecks for CO2 reduction between the OER species of [Mn-MeCN] and CO2. To clarify the reason for the low overpotential, DFT calculations for the OER species of a Mn complex ([Mn(bpy)]-) with K+ were performed (Figure 3) because the reaction between the OER species of [Mn-MeCN] and CO2 is one of the bottlenecks in the CO2 reduction reaction. Based on previous DFT calculations for [Mn(bpy)]- for CO2 reduction, CO2 binding to the Mn complex is recognized to be endergonic.27,30 In contrast, DFT calculations indicated that CO2 binding to the Mn complex became exergonic in the presence of K+. Furthermore, the activation energy for the Mn complex upon CO2 addition was lowered by the presence of adjacent K+ ions. Therefore, we consider that the lower activation energy due to the presence of K+ cations is one of the reasons for the low overpotential for CO2 reduction. However, the electrode without MWCNTs ([Mn-MeCN]/SUS) produced only hydrogen, even in the presence of K+. Although the factors described in Sections 2-2, 2-3 and 2-4 are effective, we consider that the adsorption of K+ cations on the MWCNT surfaces may also be crucial for achieving a low overpotential. A recent report on a needle-like Au catalyst demonstrated that K+ cations can improve ACS Paragon Plus Environment

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electrochemical CO2 reduction. High electric fields at the nanostructured Au edge concentrated electrolyte K+ cations, which led to a high local concentration of CO2 close to the active CO2 reduction reaction surface of Au, lowering the applied overpotential, which was speculated to be due to the stabilization of CO2-derived molecules.46 In the present [Mn-MeCN]/MWCNT electrode, MWCNTs could concentrate K+ cations similarly, which would enrich CO2 adjacent to the Mn-complex catalyst. A schematic illustration of the proposed carbon-K+ synergetic effect for CO2 reduction at lower energy is shown in Figure 4.

2-6. Application of the synergetic effect of carbon-K+ on Co- and Re-complex catalysts The carbon-K+ synergetic effect to lower overpotential is also applicable to CO2 reduction over other metal complexes because it provides a energetically favorable environment for CO2 reduction adjacent to the active sites of the metal complex catalysts in aqueous solution. To demonstrate the universality of this effect, MWCNTs and K+ were applied to a Co-complex catalyst. Similar phenomena were observed for a Co tetraphenylporphyrin ([Co(TPP)]) catalyst. Bare [Co(TPP)] did not act as a CO2 reduction catalyst in the absence of the carbon support;47 however, [Co(TPP)]/MWCNT catalyzed selective CO2 reduction at a low overpotential of -0.39 V vs. RHE in an aqueous solution containing K+ (Table S5 and Figures S10 and S11). A larger CO2 reduction current of 3.12 mA cm-2 was generated at -0.59 v vs. RHE. Conversely, the [Co(TPP)]/MWCNT electrode was deactivated in a NH4+ solution that did not contain K+ (Figure S12). The lowered overpotential due to the carbon-K+ synergetic effect was also realized with a Re-complex system. [Re{4,4’-di(1H-pyrrolyl-3-propyl carbonate)-2,2’-bipyridine}(CO)3(Cl)] ([Re-Cl]) cannot act as a CO2 reduction photocatalyst, because the LUMO potential is too low with the electron withdrawing group (Figure S13). Therefore, [Re-Cl] also has a bottleneck in the reaction between the OER species and CO2.24 ACS Paragon Plus Environment

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However, when introduced with a carbon support and K+, [Re-Cl] catalyzed selective CO2 reduction in an aqueous solution (Figure S14), although the CO2 reduction reaction was deactivated without K+ (Figure S15). Thus, the carbon-K+ synergetic effect to lower the overpotential for CO2 reduction could be extended to many other MCs.

3. Isotope tracer experiment To verify the carbon source for the generation of CO, an isotope tracer analysis with 13CO2 was performed for [Mn-MeCN]/MWCNT (Figure S16). CO (m/z = 29) was the main product, which confirmed that the CO detected in these electrocatalytic reactions over [Mn-MeCN]/MWCNT did not originate from the MWCNTs, but from CO2 molecules dissolved in the aqueous solution.

Conclusions A new method was developed for improving the CO2 reduction activity of MCs, and for achieve both high CO2 selectivity and low reaction overpotential in aqueous solution. Electrocatalytic CO2 reduction over MCs was performed at much lower energy with the aid of a MWCNT support combined with alkali metal cations (K+), where the nanoscale carbon provided a significant improvement in the CO2 reduction rate in an aqueous solution. This effect was confirmed for [Co(TPP)], [Re-Cl], and [Mn-MeCN] molecular catalysts. The [Mn-MeCN]/MWCNT electrocatalyst, which contained non-noble Mn as a central metal, was the focus of the present study. Although the bare [Mn-MeCN] cathode did not catalyze CO2 reduction in an aqueous solution, the [Mn-MeCN]/MWCNT electrode generated CO with high efficiency. The potential necessary for electrochemical CO2 reduction to CO over Mn-MeCN was decreased to -0.21 V (vs. RHE) by ACS Paragon Plus Environment

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combination with the MWCNT support, due to lowering of the CO2 reduction energy by K+ adsorbed on the MWCNTs. The [Mn-MeCN]/MWCNT electrode operated even at -5 mA cm-2 (at -0.49 V vs. RHE) to convert CO2 to CO. The significantly lowered overpotential by the synergetic effect of the MWCNT support and K+ cations could be applicable to many electrocatalysts and semiconductor/MC hybrid photosystems22,23,48-50 to produce organic chemicals from CO2 at low input energy. The lowered overpotential could also contribute to the formation of simple monolithic photodevices conjugated with H2O oxidation to reduce CO2 in a one-compartment reactor under solar irradiation.

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Figure 1. Electrocatalytic activity of [Mn-MeCN]/MWCNT electrode. a) Long-term bulk electrolysis at -0.39 V (vs. RHE) in a solution of CO2-saturated 0.1 M K2B4O7 + 0.2 M K2SO4, indicating the volume of carbon monoxide (black) and hydrogen (blue) produced using the [Mn-MeCN]/MWCNT electrocatalyst. b) LSV for the [Mn-MeCN]/MWCNT electrocatalyst in CO2-saturated 0.1 M K2B4O7 + 0.2 M K2SO4 solution, before (black) electrolysis and after (red) 24 h bulk electrolysis (counter electrode, reference electrode, and electrolyte solution were replaced).

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Figure 2. Chronoamperograms for [Mn-MeCN]-catalyzed CO2 reduction over [Mn-MeCN]/MWCNT electrode for 2 h with several bias voltages [-0.21 V (black), -0.29 V (red), -0.39 V (blue), and -0.49 V (green)].

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Figure 3. Results of DFT calculations and summarized experimental results. Free energy changes are shown for catalytic cycle reaction steps from dissociation of the monodentate ligand complex to CO2 addition for the Mn-complex catalyst.

Figure 4. Schematic illustration of [Mn-MeCN]/MWCNT electrode during electrocatalytic CO2 reduction.

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Table 1. Summary of electrocatalytic CO2 reduction using Mn-complex-based electrodes at several applied potentials, from electrochemical CO2 reduction over Mn complex-based electrodes at -0.21 to -0.49 V (vs. RHE) in 0.1 M K2B4O7 + 0.2 M K2SO4 purged with CO2 for 2 h. (n.d. = not determined) Applied Catalyst

potential / V (vs. RHE)

Charge

[CO] /

FE (CO)

FE (H2)

/C

µmol

/%

/%

Current density / mA cm-2

[Mn-MeCN]/MWCNT*

-0.21

1.7

7.1

80.6

17.6

0.23

[Mn-MeCN]/MWCNT

*

-0.29

5.1

22.0

83.3

13.9

0.75

[Mn-MeCN]/MWCNT

*

-0.39

16.7

75.5

87.3

12.6

2.42

[Mn-MeCN]/MWCNT

*

-0.49

35.1

148.5

81.6

19.7

5.02

**

-0.49

6.4

0.8

2.4

94.4

0.82

***

-0.39

9.2

n.d.

-

93.8

1.26

[Mn-MeCN]/SUS

-0.39

3.1

n.d.

-

97.6

0.46

MWCNT/SUS

-0.39

0.3

n.d.

-

98.6

0.03

[Mn-MeCN]/MWCNT [Mn-MeCN]/MWCNT

*Chronoamperograms for [Mn-MeCN]/MWCNT electrode shown in Figure 2. ** Electrochemical CO2 reduction over [Mn-MeCN]/MWCNT electrode at -0.49 V (vs. RHE) in 0.1 M (NH4)2B4O7 + 0.2 M (NH4)2SO4 purged with CO2 in for 2 h. The pH of the 0.1 M (NH4)2B4O7 + 0.2 M (NH4)2SO4 solution saturated with CO2 was approximately 6.6. *** Electrochemical reaction was conducted under Ar.

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Table 2. Summary of electrocatalytic CO2 reduction using Mn complexes in organic solvents. Mn complex catalyst [Mn-MeCN]* [Mn-MeCN]* [Mn-Br]* [Mn(bpy)]* [Mn(bpy)]*

Applied potential

Condition MeCN + 5% H2O with 0.1 M TBAP MeCN + 5% H2O with 0.1 M TBAP + 0.1 M K(Otf) MeCN + 5% H2O with 0.1 M TBAP MeCN + 5% H2O with 0.1 M TBAP MeCN + 5% H2O with 0.1 M TBAP

FE (CO) Current density /

vs. Ag/AgCl

/%

mA cm-2

-1.0

n.d.

0.05

-1.0

19.7

0.07

-1.0

n.d.

0.04

-1.0

n.d.

~0.01

-1.5

94.1

0.39

* Concentration of Mn complexes was 0.5 mM in MeCN/5% H2O solution including 0.1 M tetrabutylammonium perchlorate (TBAP), according to previously reported method.6 MWCNTs on a SUS electrode, Ag/AgNO3 (0.1 M), and platinum wire were used as working, reference, and counter electrodes, respectively. Electrolysis time for all experiments was 2 h. (n.d. = not determined, Otf = Triflate)

Table 3. Electrochemical CO2 reduction over Mn complex electrodes with carbon supports. The electrodes were measured at -0.39 V (vs. RHE) in an aqueous solution with electrolytes purged with CO2.

Charge

FE (CO)

FE (H2)

MWCNT coating

/C

/%

/%

procedure

[Mn-MeCN]/MWCNT*

16.7

87.3

12.6

[Mn-MeCN]/MWCNT*

4.4

7.9

80.5

[Mn-MeCN]/MWCNT*

1.5

15.4

75.1

SUS*

0.1

n.d.

84.6

SUS*

4.5

n.d.

98.2

Catalyst

drop-cast 10 times drop-cast once spin-coated 3 times spin-coated 3 times -

*Electrochemical CO2 reduction conducted at -0.39 V (vs. RHE) in CO2-saturated 0.1 M K2B4O7 +0.2 M K2SO4 for 2 h. (n.d. = not determined)

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Supporting information Materials, General Procedures, and Methods are described in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

Acknowledgements This study was supported in part by the Precursory Research for Embryonic Science and Technology (PRESTO) and Core Research for Evolutional Science and Technology (CREST) programs. The authors would like to thank, T. Arai, H. Uchiyama, T. Ikuno, Y. Takatani, M. Yamamoto and A. Ohshima for experimental support.

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