Electroreduction of Carbon Dioxide to Formate by Homogeneous Ir

Oct 24, 2018 - Formate was selectively produced by use of GC as working electrode, although there still remained a problem of the relatively high appl...
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Electroreduction of Carbon Dioxide to Formate by Homogeneous Ir Catalysts in Water Ryoichi Kanega, Naoya Onishi, Lin Wang, and Yuichiro Himeda ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02525 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Catalysis

Electroreduction of Carbon Dioxide to Formate by Homogeneous Ir Catalysts in Water Ryoichi Kanega,*,† Naoya Onishi,† Lin Wang,† and Yuichiro Himeda*,† †Research

Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan KEYWORDS: electroreduction, carbon dioxide, formate, iridium, electrocatalyst, hydride specie ABSTRACT: Electroreduction of CO2 to formate is one of the approaches converting electrical energy into chemical energy and is expected to be a method storing unevenly distributed renewable sources. However, high performance of the electroreduction system including catalysts is indispensable for establishing the energy storage system. Furthermore, it is desirable to convert CO2/formate more efficiently than hydrogen production by water electrolysis. In this paper, we demonstrate the electroreduction systems of CO2 to formate by the systematic combination of iridium (Ir) catalysts and various electrodes in aqueous solutions without any organic additives. It was found that the combination of the Pt black electrode and the Ir catalyst having a picolinamide type ligand gave faradic efficiency of formate (95 %) and current density (7.2 mA cm-2) with -0.49 V vs SHE at pH 8.3. Moreover, we achieved that electroreduction of CO2 proceeded efficiently even with lower potential (-0.44 V vs SHE) than the theoretical potential of water electrolysis (pH 8.3). The electrochemical measurement, the electro-kinetic analysis and the 1H NMR spectroscopy suggested that Ir hydride (Ir−H) as key intermediates for electroreduction of CO2 were formed via not reaction with hydrogen produced by electrolysis, but twoelectron reduction of [Cp*IrIII(OH2)(L)]n+ (n = 1 or 2). The selective and efficient electroreduction system is attributed to easy generation and strong CO2 reduction ability of the Ir-H intermediates in water.

electroreduction systems including catalyst are strongly desired.

1. INTRODUCTION Toward expanding utilization of renewable sources, developing technologies of conversion of electrical energy into chemical energy, such as hydrogen which is expected a next generation energy, has been extensively studied today. However, conventional methods to transport and store hydrogen are deficient in terms of scale, cost and energy efficiency. Moreover, the alkaline water electrolysis system shows high overpotential of hydrogen production (theoretical potential: 0 V vs the reversible hydrogen electrode (RHE)). On the other hand, electroreduction of CO2 to formic acid (formate) has been attracted much attention not only as a hydrogen carrier but also as CO2 utilization. Formic acid as the hydrogen carrier has hydrogen content of 4.3 % and is capable of easily generating hydrogen.1 In addition, it was recently found that high-pressure gasses of H2 and CO2 (up to 153 MPa) were produced from formic acid as hydrogen storage without using a mechanical compressor.2 The potential for the CO2/formate (eq. 1) is approximately 0 V vs RHE (-0.25 V vs the standard hydrogen electrode (SHE) at pH 8.3; all potentials are given with respect to this reference),3 which means that the possibility of CO2/formate conversion as a storage of electrical energy with lower potential than hydrogen evolution under basic conditions (eq. 2). If this electroreduction system can be realized, it becomes an energy storage method more efficient than alkaline water electrolysis. However, high overpotential on the conventional electroreduction of CO2 still remains as unsolved issues. Therefore, more efficient CO2

CO2 + H2O + 2e− → HCO2− + OH−

(1)

2H2O + 2e− → H2 + 2OH−

(2)

A number of studies about electroreduction of CO2 with homogeneous4 and heterogeneous5 catalysts have been reported (Table 1). According to the previous studies on the heterogeneous catalysts, the activation step of CO2 into CO2•− by one electron transfer was a bottleneck in electroreduction of CO2, which caused a high overpotential.6 Kanan’s group and Wrighton’s group have reported a catalytic system not via CO2•− by one electron transfer using Pd catalysts.3,7 Although a low applied potential of -0.53 V (overpotential: 0.28 V) and a high current density (4.0 mA cm-2) were achieved, Pd NPs/C showed low durability because of being poisoned by the generated CO as by-products.3 On the contrary, using homogeneous catalysts for electroreduction of CO2, metal hydride species are proposed as intermediates.8 The path via metal-hydride could avoid one electron transfer to CO2 which is the cause of the increased overpotential. Several homogeneous catalysts with a variety of metals and ligands for electroreduction of CO2 to formate have been reported.9 However, most of the electroreduction systems required organic solvents or additives which cause the high applied potential. Recently, Berbe’s group have reported

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electroreduction of CO2 to formate without CO formation using [Fe4N(CO)12],10 which is the only example conducted in an aqueous solution without any organic additives by using homogeneous catalysts. However, this reaction also required a high applied potential of -0.96 V (overpotential: 0.44 V). Table 1. Electroreduction of CO2 to Formate with Hydride Intermediate Mechanism. Catalysts [IrH(AN)2(PCP)]+b [IrH3(PNP)]c [CpCo(P2N2)I]d [Fe4N(CO)12]−e Pd NPs/XC72f [Cp*Ir(L)]n+(5)f aSHE

at

Potential FEformate Current Ref. /V vs SHEa /% /mA cm-2 -1.41 93 0.6 9f -0.81 97 0.5 9g -1.33 98g 2.8 9i -0.96 96 3.8 10 -0.53 99 4.0 3 -0.49 95 7.2 This work

pH

8.3, bElectrolyte: H2O/CH3CN/NaHCO3, dElectrolyte: H2O/CH3CN/nBu4NPF6, n e H2O/DMF/ Bu4NBF4, Electrolyte: H2O/phosphate, fElectrolyte: H O/KHCO , gProduct: HCO H, AN: CH CN, 2 3 2 3 XC72: Vulcan XC72 carbon black.

cElectrolyte:

We have developed highly efficient iridium (Ir) catalysts such as 1-5 having N,N-bidentate ligands for hydrogenation of CO2 to formate and dehydrogenation of formic acid in water without any organic additives (Scheme 1).11 Based on our ligand-design concepts, such as the electron-donating effect to Ir and/or pendant base effects, we have developed the catalysts 1-4 for CO2 hydrogenation, which can generate formate even at ambient conditions (25 °C, atmospheric pressure) in water.11a-c,11e Very recently, we have reported the most effective catalyst 5 having a deprotonated picolinamide ligand, which shows strong electron-donating ability.11f These studies mean that formation of metal-hydride intermediates as active species for hydrogenation of CO2 by these catalysts smoothly proceed under ambient conditions. Scheme 1. Ir catalysts for hydrogenation of CO2 to generate formate under ambient conditions. CO2 + H2 + OH

Catalyst 25 C, H2/CO2 = 1/1 (0.1 MPa)

HCO2 + H2O

Catalyst OH SO4

Ir N

N

H2O OH

TOF: 7 h-1 ref. 11a

Cl

HO

1

4

N

Ir

HO

Cl

OH

TOF: 70 h-1 ref. 11c

OH Ir

TOF: 106 h-1 ref. 11e

N

TOF: 27 h-1 ref. 11b

Ir NH

N

3

N N

Ir

OH N

2

OH SO4

HO

H2O

HO

N

Ir H2O

SO4

HO

N

H2O

N N

O

5 TOF: 167 h-1 ref.11f

HSO4

Cl2

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Therefore, by using catalysts with high hydrogenation activity in water, it can be expected that not only electrochemical metal-hydride formation but also reduction of CO2 with metal-hydride intermediates will progress effectively as compared with conventional electroreduction systems. In this paper, we describe the electroreduction systems of CO2 to formate by systematic combination of catalysts 1-5 and various electrodes in water without any organic additives. We also demonstrate more efficient storing the electric energy system by electroreduction of CO2 using Ir catalyst with a picolinamide type ligand and a Pt black electrode than hydrogen evolution under basic conditions. Furthermore, we discussed the reaction mechanism by using experimental approach, electro-kinetic analysis and 1NMR spectroscopy.

2. EXPELIMENTAL Electroreduction of CO2. Electroreduction of CO2 was carried out in two compartment electrolysis cell with a glass filter (G4) as the separator and a three-electrode system using a potentiogalvanostat (IVIUM VERTEX). The working electrode and counter electrode was Pt black prepared by the following procedure. H2PtCl6 (100 mg) and (CH3COO)2Pb (1.0 mg) were dissolved in H2O (10 mL). In the solution, electrolysis was conducted under -10 mA galvanostatic conditions using Pt wire (0.5 cm2) as a working electrode for 10 min. Also, the working electrodes were glassy carbon (GC, BAS) with 2 cm2, Cu wire (Nilaco) with 0.5 cm2, Ni wire (Nilaco) with 0.5 cm2 and Pt wire (Tanaka Kikinzoku) with 0.5 cm2. Cathode compartment contained catalysts (4 μmol) and KHCO3 aq. solution (2.0 M, pH 8.3, 15 mL). Then, the solution was bubbled with CO2 for 20 min. Anode compartment contained KHCO3 aq. solution (2.0 M, pH 8.3, 15 mL), and then the solution was bubbled with N2 for 20 min. After a 20 min. potentiostatic electrolysis or galvanostatic electrolysis carried out with vigorous stirring. The solution and the gas were sampled appropriately and measured by HPLC and TCD-GC respectively. The All potentials were measured against an Ag/AgCl reference electrode (BAS, 3.0 M NaCl) and converted to the SHE reference scale using eq. 3.12 E (vs SHE) = E (vs Ag/AgCl) + 0.21 V (3) Faradic efficiency (FE) was calculated using eq. 4 and 5, where F is the Faraday constant. FEformate = F × 2 formate/total Q = F × 2 formate/(I × t) (4) FEH2 = F × 2 H2/total Q = F × 2 H2/(I × t)

(5)

Electrochemical measurements. Electrochemical measurements were carried out in one compartment electrolysis cell and a three-electrode system using a potentio-galvanostat (BAS Model 624D). A Pt wire

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ACS Catalysis and Ag/AgCl electrode were used as counter and reference electrodes, respectively. The working electrode was Pt electrode (2 mm2). Linear sweep voltammetry with a scan rate of 100 mV s-1 was conducted in CO2 saturated 2.0 M KHCO3 solution (10 ml, pH: 8.3). All potentials were measured against an Ag/AgCl reference electrode (BAS, 3.0 M NaCl) and converted to the SHE reference scale using eq. 2. Catalytic hydrogenation of CO2 at atmospheric pressure conditions with 5. The catalyst 5 (0.2 μmol) was added to a KHCO3 aq. or KHCO3/KCl solution (2.0 M/0.02 M, 20 mL) in a Schleck flask (30 mL) with an attached condenser and then the solution was degassed carefully by the freeze-pump-thaw method five times. Under a 0.1 MPa H2/CO2 (1/1) gas mixture at 25 °C, the catalytic hydrogenation of CO2 was carried out with vigorous stirring (1500 rpm) under H2/CO2 (20 mL/min) for 1 h. The solution was sampled appropriately (0.1 mL) and measured by HPLC. Detection of Ir−H species by 1H NMR spectroscopy. Electrolysis of 3 was carried out in two compartment electrolysis cell with a glass filter (G4) as the separator and a three electrode system using a potentio-galvanostat (IVIUM VERTEX). The working electrode was Pt black. A platinum wire was used as the counter electrode. Cathode compartment contained 3 (24.5 μmol) and D2O (10 mL)/KOD (0.5 mL) solution. Then, the solution was bubbled with N2 for 20 min. Anode compartment contained D2O (10 mL)/KOD (0.1 mL) solution. And then the solution was bubbled with N2 for 20 min. After a 20 min. galvanostatic electrolysis carried out at -2.0 mA with stirring. The solution was sampled appropriately and analyzed by 1H NMR.

3. RESULTS AND DISCUSSION Electrochemical properties of 1-5 were investigated by linear sweep voltammetry (LSV) using a glassy carbon (GC) electrode in water (2.0 M KHCO3 as supporting electrolyte) under Ar and CO2 (Figure S1). Figure S1a shows LSV sweeping the potential from 0 V to the negative side without the Ir complex. When LSV was measured in the presence of 1-5 ([Cp*IrIII(OH2)L]n+; n = 1 or 2), the cathodic current was observed in Ar (Figure S1b-f). These cathodic currents are expected to be attributed to the reduction of [Cp*IrIII(OH2)L]n+ (n = 1 or 2). When LSV was measured in CO2 using 1-5, a cathodic current increased slightly in all cases. The cathodic current shifted to the positive side due to pH change for dissolution of CO2. Subsequently, potentiostatic electrolysis was carried out with 1-5 using the GC as a working electrode at -0.94 V referenced to LSV (Table 2). By use of 1 having 4,4′dihydroxy-2,2′-bipyridine, the faradic efficiency of formate (FEformate) and hydrogen (FEH2) of 71% and 28%, respectively, were obtained. Byproducts such as CO and CH4 were not observed. However, the current density was only 0.5 mA cm-2. The FEformate for 2-5 showed somewhat higher selectivity (FEformate: 86-96%) than 1. In addition,

the current density of 4 and 5 significantly increased to 3.2 and 4.6 mA cm-2, respectively. Interestingly, the order of these catalytic activities in electroreduction of CO2 corresponds to that of catalytic activities in the hydrogenation of CO2. Formate was selectively produced by use of GC as working electrode, although there still remained a problem of the relatively high applied potential (-0.94 V). In order to reduce the applied potential, the effect of the working electrodes on the electroreduction of CO2 using 5 was examined by a galvanostatic electrolysis (Table 3, Figure S2). Cu and Ni as working electrode produced selectively formate at the positive potential (-0.87 V and 0.82 V) compared with the GC (-0.95 V). Surprisingly, formate was produced selectively even using Pt, which is considered to be predominant for hydrogen generation in these conditions. These results indicate that the Pt electrode by using 5 was effective for formate production. Furthermore, we examined the Pt black which showed lower hydrogen overpotential than the Pt. Table 2. Electroreduction of CO2 to formate by 1-5 at 0.94 V using GC electrode. Cat. 1 2 3 4 5

FEaformate/% 71 86 89 91 96

TON 3 5 7 27 41

FEaH2/% 28 15 4 8 3

Current b/mA cm-2 0.5 0.7 0.8 3.2 4.6

Reaction conditions: 2 M KHCO3 solution (15 mL) with catalyst (0.27 mM) in cathode compartment using and 2 M KHCO3 solution (15 mL) in anode compartment. aFE: Faradic efficiency. bAverage value for 1 h. Beyond expectation, by using the Pt black electrode, the electroreduction progressed selectively at much lower cathode potential of -0.45 V (over potential: 0.20 V). This result shows that hydrogen can be stored in CO2 with lower potential than hydrogen production by water electrolysis (-0.49 V at pH 8.3). The relationship between hydrogen overpotential and cathode potential of each electrode was shown in Figure S3. The electrode with lower hydrogen overpotential tended to have lower cathode potential for electroreduction of CO2. Table 3. Electroreduction of CO2 to formate by 5 using various working electrodes under galvanostatic electrolysis (4.6 mA cm-2). WEa GC Cu Ni Pt Pt black

FEbformate/% 95 93 94 95 95

FEbH2/% 6 7 5 7 6

Cathode c/V -0.95 -0.87 -0.82 -0.64 -0.45

Reaction conditions: 2 M KHCO3 solution (15 mL) with 5 (0.27 mM) in cathode compartment using and 2 M KHCO3 solution (15 mL) in anode compartment. aWE: Working electrode. bFE: Faradic efficiency. cAverage value of potential for 1 h.

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ACS Catalysis The effect of the catalysts 1-5 on the electroreduction of CO2 was re-examined using the Pt black electrode in detail. Figure 1 shows the effect of applied potential (-0.39 to 0.64 V) on FEformate and Table S1 and Figure S4-S8 summarize the result of applied potential (-0.49 V). Without the Ir catalyst, hydrogen as a solo product was observed at all potentials. In addition, [Cp*Ir(bpy)OH2]2+ (bpy: 2,2′-bipyridine) having poor activity for CO2 hydrogenation gave hydrogen at FEH2 of > 95%. When using Ir catalysts 1-4, formate and hydrogen were produced. In contrast to using the GC electrode, the FEforamte was 20-90% at -0.49 V with the Pt black electrode and the difference in selectivity of each catalyst appeared remarkably. Further examination, a high FEformate was obtained from 5 even at low applied potential of -0.39 V. Moreover, high FEformate of > 95% was maintained up to 0.49 V (overpotential: 0.24 V). It should be noted that CO2 can be converted with lower potential than hydrogen production by water electrolysis because of redox potential of hydrogen production at pH 8.3 being -0.49 V. In addition, current density of up to 60 mA cm-2 was obtained at overpotential of only 0.39 V and the FEformate was 63% (Figure S9), which is considered to have potential as an electrocatalyst. Thus, only when the catalyst 5 having high CO2 hydrogenation ability is used, formate was produced selectively even by use of the Pt black electrode having high hydrogen evolution ability.

suggest that 5 functions in a solution rather than being adsorbed to the electrode. Figure 2 shows the time course of the FEformate and current density at applied potential of 0.5 V. The catalytic activity of 5 was maintained stably for 12 h and the FEformate was maintained with > 90%. Finally, we investigated the reaction mechanism of electroreduction of CO2 using Pt type electrode. The Tafel plot of 1 exhibits slopes of 58 mV dec−1 from 0.39 to 0.49 V and 615 mV dec−1 at larger overpotential (Figure S10 and Table S3). This value (58 mV dec−1) was consistent with a mechanism for electrochemically generated metal-hydride intermediates reducing CO2 (59 mV dec−1).9i The 615 mV dec−1 slope at larger overpotential, where jformate became nearly independent of potential, is likely the result of the bubbles (H2) accumulated on the working electrode reduced the electrode area. Likewise, due to the Tafel plots of 2-5 exhibited slopes of 54 mV dec−1 from 0.39 to 0.49 V, it was expected that electroreduction proceeded via electrochemically generated metal-hydride species. In addition, at larger overpotential, 2-5 gave slopes of 205280 mV dec−1. As mentioned above, these values could be attributed to the adsorption of bubbles (H2) to the working electrode surface. Additionally, in order to detect the metal-hydride (Ir−H) species, galvanostatic electrolysis was carried out in KOD/D2O using 3. We detected the Ir−H (: –10.83 ppm) derived from the electrochemical reaction of 3 by 1H NMR spectroscopy (Figure S11). -14

100

current densitiy / mA cm-2

80 60 40 20

-10

80

-8

60

-6 40 -4 20

-2 0 -0.35

-0.40

-0.45 -0.50 -0.55 -0.60 applied potential / V vs SHE

-0.65

0

FE of formate / %

100

-12

FE of formate / %

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

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0

120

240

360

480

600

720

0

time / min

Figure 1. Effect of applied potential on FEformate by blank (black square), [Cp*Ir(bpy)OH2]2+ (hollow circle), 1 (red circle), 2 (orange triangle), 3 (green triangle), 4 (blue square), 5 (purple square) in 2.0 M KHCO3 aq. using Pt black electrode. Effect on concentration of 5 is summarized in Table S2. In case of the low concentration of 5 (Entry1, 1.4 × 10-2 mM), although the FEformate was as low as 25.0%, a high TON of 78 was obtained. The FEformate increased with the concentration of 5 and reached 98% (Entry 5, 54.0×10-2 mM). In addition, it found that the current density depended on the concentration of 5. In addition, after electrolysis in Entry 4, electroreduction of CO2 was carried out without 5 with the electrode used in Entry 4, but only formation of H2 was observed (Entry 6). These results

Figure 2. Time course of current density (red circle) and FEformate (blue square) at -0.5 V in 2.0 M KHCO3 aq. with 0.27 mM of 5 using Pt black electrode. Moreover, when CO2 was bubbled into the solution containing Ir−H species formed electrochemically, formate was stoichiometrically produced and the peak of Ir−H eliminated. These Tafel slopes and 1H NMR spectroscopy suggested that the electroreduction of CO2 progressed via the Ir−H species. [Cp*IrIII(OH2)(L)]n+ + H2 → [Cp*IrIIIH(L)]n+ + H+ (n = 1 or 2)

(6)

[Cp*IrIII(OH2)(L)]n+ + 2e− →[Cp*IrI(L)](n-2)+ + H2O

(7)

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ACS Catalysis [Cp*IrI(L)](n-2)+ + H2O → [Cp*IrIIIH(L)]n+ + OH− (8) However, these results do not include information about the production path of the hydride species. Two possibilities are considered for the generation path of hydride species: via hydrogen heterolysis (eq. 2 and 6); or reduction of [Cp*IrIII(H2O)(L)]n+ (eq. 7 and 8). We investigated how hydride species were formed. Some reports including our previous study indicated that the formation of catalytically active species, such as metalhydride species from hydrogen (eq. 6), could be inhibited by the presence of halogen.13 Then, the effect of Cl as halogen in hydrogenation and electroreduction of CO2 using 5 was examined (Table S4). The hydrogenation ability (TOF) of 5 under ambient conditions (H2/CO2 = 1/1 (0.1 MPa), 25 °C) was drastically decreased from 164 h-1 to 7 h-1 by addition of KCl as Cl source. These results imply the possibility that the strong coordination of Cl− to the Ir center inhibits the generation of the Ir−H species. On the other hand, in electroreduction of CO2, the inhibitory effect of Cl− was slight compared with the hydrogenation of CO2. This means that the mechanism of Ir−H generation in electroreduction of CO2 is different with hydrogenation of CO2. It is expected that the Cl− ligand was eliminated from [Cp*IrIIICl(L)](n-1)+ by two-electron reduction and reacted with H2O to produce Ir−H species (eq. 9 and 10). In addition, to detect Ir−H species, cyclic voltammetry under Ar and CO2 using GC or Pt disc electrodes was conducted. The cathodic current derived from the reduction of 5 was observed, but anodic current derived from oxidation of Ir−H species was not observed (Figure S13 and S14). These results indicated that Ir−H species would react with H2O or CO2 quickly to regenerate IrIII species.

[Cp*IrI(L)](n-3)+ + H2O → [Cp*IrIIIH(L)](n-1)+ + OH−

(10)

Thus, we proposed the mechanism of electroreduction of CO2 in case of 5 as follows. [Cp*IrI(L)]− (n = 1) was generated by two electron reduction of [Cp*IrIII(OH2)(L)]+ (eq. 7). From experiment results, using Pt disc electrode, a cathodic current observed under Ar at LSV attributed to two-electron reduction of 5 rather than hydrogen evolution (Figure S12f, solid line). On the other hand, in the case of 1-4, it is difficult to separate current originated by H2 evolution from two-electron reduction of Ir complexes (Figure S11b-e, solid line). After that, [Cp*IrIIIH(L)]+ which produced by reaction of [Cp*IrI(L)]− with H2O (eq. 8) reduced CO2 to formate (eq. 11). The cathodic current observed in LSV under CO2 (Figure S12f dash line) was derived that [Cp*IrIII(L)]+ is regenerated by reaction of [Cp*IrIIIH(L)]+ with CO2.

4. CONCLUSION

(11)

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental section including general methods, electroreduction of CO2 data and electrochemical measurement, and NMR spectra.

AUTHOR INFORMATION

[Cp*IrIIICl(L)](n-1)+ + 2e− → [Cp*IrI(L)](n-3)+ + Cl− (9)

[Cp*IrIIIH(L)]n+ + CO2 + H2O → [Cp*IrIII(OH2)(L)]n+ + HCO2−

In summary, we demonstrated the systematic study of various combinations of Ir complexes and electrodes for the electroreduction of CO2 to formate in water. It was found that the highly effective catalyst for CO2 hydrogenation improved the electroreduction of CO2 in water. Combination the Ir catalyst bearing picolinamide ligand with the electrode material having lower hydrogen overpotential (i.e., Pt black electrode) led to remarkable decrease of the applied potential. From the electrochemical measurement, electro-kinetic analysis, and reaction mechanism analysis, we revealed that the Ir-H species were generated by two-electron reduction. As a result, the combination of 5 with picolinamide ligands and the Pt black working electrode gave high current density (7.2 mA cm-2) and high FEformate (95 %) at -0.49 V. These results represent the lowest overpotential and the highest current density among the previous reports using homogeneous catalysts for electroreduction of CO2 to formate in water. Moreover, the electroreduction system was able to selectively convert CO2 even at -0.44 V and store electricity more effectively saving energy than alkaline water electrolysis. Further development of our reduction protocol, including analysis of electrode surface reaction, has the potential to lead to more efficient technologies storing electrical energy.

Corresponding Author *Ryoichi Kanega: [email protected] *Yuichiro Himeda: [email protected] ORCID Ryoichi Kanega: 0000-0003-0790-956X Naoya Onishi: 0000-0003-4501-1742 Lin Wang: 0000-0001-5644-8865 Yuichiro Himeda: 0000-0002-9869-5554

ACKNOWLEDGMENT This research was supported by the International Joint Research Program for Innovative Energy Technology of the Ministry of Economy, Trade, and Industry (METI).

REFERENCES (1) (a) Wang, W. H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E., CO2 hydrogenation to formate and methanol as an alternative to photo- and electrochemical CO2 reduction. Chem. Rev. 2015, 115, 12936-12973; (b) Mellone, I.; Gorgas, N.; Bertini, F.; Peruzzini, M.; Kirchner, K.; Gonsalvi, L., Selective Formic Acid Dehydrogenation Catalyzed by Fe-PNP Pincer Complexes Based on the 2,6-

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Diaminopyridine Scaffold. Organometallics 2016, 35, 33443349; (c) Eppinger, J. r.; Huang, K.-W., Formic acid as a hydrogen energy carrier. ACS Energy Lett. 2016, 2, 188195; (d) Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.; Beller, M.; Laurenczy, G. b., Homogeneous catalysis for sustainable hydrogen storage in formic acid and alcohols. Chem. Rev. 2017, 118, 372-433. (2) (a) Fellay, C.; Dyson, P. J.; Laurenczy, G., A viable hydrogen-storage system based on selective formic acid decomposition with a ruthenium catalyst. Angew. Chem., Int. Ed. 2008, 47, 3966-3968; (b) Iguchi, M.; Himeda, Y.; Manaka, Y.; Matsuoka, K.; Kawanami, H., Simple continuous high-pressure hydrogen production and separation system from formic acid under mild temperatures. ChemCatChem 2016, 8, 886-890; (c) Iguchi, M.; Himeda, Y.; Manaka, Y.; Kawanami, H., Development of an iridium-based catalyst for high-pressure evolution of hydrogen from formic acid. ChemSusChem 2016, 9, 2749-2753; (d) Iguchi, M.; Chatterjee, M.; Onishi, N.; Himeda, Y.; Kawanami, H., Sequential hydrogen production system from formic acid and H2/CO2 separation under high-pressure conditions. Sustainable Energy & Fuels 2018, 2, 1719-1725. (3) Min, X. Q.; Kanan, M. W., Pd-catalyzed electrohydrogenation of carbon dioxide to formate: High mass activity at low overpotential and identification of the deactivation pathway. J. Am. Chem. Soc. 2015, 137, 47014708. (4) (a) Taheri, A.; Berben, L. A., Making C-H bonds with CO2: production of formate by molecular electrocatalysts. Chem. Commun. 2016, 52, 1768-1777; (b) Min, S. X.; Rasul, S.; Li, H. F.; Grills, D. C.; Takanabe, K.; Li, L. J.; Huang, K. W., Electrocatalytic Reduction of Carbon Dioxide with a WellDefined PN3-Ru Pincer Complex. ChemPlusChem 2016, 81, 166-171; (c) Nakada, A.; Ishitani, O., Selective electrocatalysis of a water-soluble rhenium (i) complex for CO2 reduction using water as an electron donor. ACS Catal. 2017, 8, 354-363; (d) Sato, S.; Saita, K.; Sekizawa, K.; Maeda, S.; Morikawa, T., Low-energy electrocatalytic CO2 reduction in water over Mn-complex catalyst electrode aided by a nanocarbon support and K+ cations. ACS Catal. 2018, 8, 4452-4458; (e) Francke, R.; Schille, B.; Roemelt, M., Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts. Chem. Rev. 2018, 118, 4631-4701. (5) (a) Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O., Electrocatalytic process of co selectivity in electrochemical reduction of CO2 at metal-electrodes in aquous media. Electrochimica Acta 1994, 39, 1833-1839; (b) Jitaru, M.; Lowy, D. A.; Toma, M.; Toma, B. C.; Oniciu, L., Electrochemical reduction of carbon dioxide on flat metallic cathodes. J. Appl. Electrochem. 1997, 27, 875-889; (c) Lee, S.; Lee, J., Electrode build-up of reducible metal composites toward achievable electrochemical conversion of carbon dioxide. ChemSusChem 2016, 9, 333-344; (d) Bullock, R. M.; Das, A. K.; Appel, A. M., Surface immobilization of molecular electrocatalysts for energy conversion. Chem. Eur. J. 2017, 23, 7626-7641; (e) Gao, S.; Lin, Y.; Jiao, X. C.; Sun, Y. F.; Luo, Q. Q.; Zhang, W. H.; Li, D. Q.; Yang, J. L.; Xie, Y., Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature

2016, 529, 68-71. (6) Schwarz, H. A.; Dodson, R. W., Reduction potentials of CO2- and the alcohol radicals. J. Phys. Chem. 1989, 93, 409414. (7) Stalder, C. J.; Chao, S.; Wrighton, M. S., Electrochemical reduction of aqueous bicarbonate to formate with high current efficiency near the thermodynamic potential at chemically derivatized electrodes. J. Am. Chem. Soc. 1984, 106, 3673-3675. (8) (a) Bolinger, C. M.; Story, N.; Sullivan, B. P.; Meyer, T. J., Electrocatalytic reduction of carbon dioxide by 2,2'bipyridine complexes of rhodium and iridium. Inorg. Chem. 1988, 27, 4582-4587; (b) Hawecker, J.; Lehn, J. M.; Ziessel, R., Electrocatalytic reduction of carbon-dioxide mediated by Re(bipy)(CO)3Cl(bipy=2,2'-bipyridine). J. Chem. Soc. Chem. Commun. 1984, 328-330. (9) (a) Caix, C.; ChardonNoblat, S.; Deronzier, A., Electrocatalytic reduction of CO2 into formate with (η5Me5C5)M(L)Cl+ complexes (L=2,2'-bipyridine ligands; M=Rh(III) and Ir(III)). J. Electroanal. Chem. 1997, 434, 163170; (b) Collin, J. P.; Jouaiti, A.; Sauvage, J. P., Electrocatalytic properties of Ni(cyclam)2+ and Ni2(biscyclam)4+ with respect to CO2 and H2O reduction. Inorg. Chem. 1988, 27, 1986-1990; (c) Sypaseuth, F. D.; Matlachowski, C.; Weber, M.; Schwalbe, M.; Tzschucke, C. C., Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands. Chem. Eur. J. 2015, 21, 6564-6571; (d) Machan, C. W.; Sampson, M. D.; Kubiak, C. P., A molecular ruthenium electrocatalyst for the reduction of carbon dioxide to CO and formate. J. Am. Chem. Soc. 2015, 137, 8564-8571; (e) Kang, P.; Cheng, C.; Chen, Z.; Schauer, C. K.; Meyer, T. J.; Brookhart, M., Selective electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride pincer complexes. J. Am. Chem. Soc. 2012, 134, 5500-5503; (f) Kang, P.; Meyer, T. J.; Brookhart, M., Selective electrocatalytic reduction of carbon dioxide to formate by a water-soluble iridium pincer catalyst. Chem. Sci. 2013, 4, 3497-3502; (g) Ahn, S. T.; Bielinski, E. A.; Lane, E. M.; Chen, Y. Q.; Bernskoetter, W. H.; Hazari, N.; Palmore, G. T. R., Enhanced CO2 electroreduction efficiency through secondary coordination effects on a pincer iridium catalyst. Chem. Commun. 2015, 51, 5947-5950; (h) Pun, S.N.; Chung, W.-H.; Lam, K.-M.; Guo, P.; Chan, P.-H.; Wong, K.Y.; Che, C.-M.; Chen, T.-Y.; Peng, S.-M., Iron (I) complexes of 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline(H2dophen) as electrocatalysts for carbon dioxide reduction. X-Ray crystal structures of [Fe(dophen)Cl]2·2HCON(CH3)2 and [Fe(dophen)(N-MeIm)2]ClO4 (N-MeIm=1methylimidazole). J. Chem. Soc., Dalton Trans. 2002, 575583; (i) Roy, S.; Sharma, B.; Pécaut, J.; Simon, P.; Fontecave, M.; Tran, P. D.; Derat, E.; Artero, V., Molecular cobalt complexes with pendant amines for selective electrocatalytic reduction of carbon dioxide to formic acid. J. Am. Chem. Soc 2017, 139, 3685-3696. (10) Taheri, A.; Thompson, E. J.; Fettinger, J. C.; Berben, L. A., An iron electrocatalyst for selective reduction of CO2 to formate in water: including thermochemical insights. ACS Catal. 2015, 5, 7140-7151.

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ACS Catalysis (11) (a) Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Kasuga, K., Simultaneous tuning of activity and water solubility of complex catalysts by acid-base equilibrium of ligands for conversion of carbon dioxide. Organometallics 2007, 26, 702-712; (b) Wang, W. H.; Hull, J. F.; Muckerman, J. T.; Fujita, E.; Himeda, Y., Second-coordination-sphere and electronic effects enhance iridium(III)-catalyzed homogeneous hydrogenation of carbon dioxide in water near ambient temperature and pressure. Energy Environ. Sci. 2012, 5, 7923-7926; (c) Hull, J. F.; Himeda, Y.; Wang, W. H.; Hashiguchi, B.; Periana, R.; Szalda, D. J.; Muckerman, J. T.; Fujita, E., Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nat. Chem. 2012, 4, 383-388; (d) Xu, S. A.; Onishi, N.; Tsurusaki, A.; Manaka, Y.; Wang, W. H.; Muckerman, J. T.; Fujita, E.; Himeda, Y., Efficient Cp*Ir catalysts with imidazoline ligands for CO2 hydrogenation. Eur. J. Inorg. Chem. 2015, 5591-5594; (e) Wang, L.; Onishi, N.; Murata, K.; Hirose, T.; Muckerman, J. T.; Fujita, E.; Himeda, Y., Efficient hydrogen storage and production using a catalyst with an imidazoline-based, proton-responsive ligand. ChemSusChem 2017, 10, 10711075; (f) Kanega, R.; Onishi, N.; Szalda, D. J.; Ertem, M. Z.; Muckerman, J. T.; Fujita, E.; Himeda, Y., CO2 hydrogenation catalysts with deprotonated picolinamide ligands. ACS Catal. 2017, 7, 6426-6429. (12) Smith, T.; Stevenson, K., Handbook of Electrochemistry, ; Zoski, CG, Ed. Elsevier: Amsterdam, 2007; p. 75. (13) (a) Kaim, W.; Reinhardt, R.; Sieger, M., Chemical and Electrochemical Generation of Hydride-Forming Catalytic Intermediates (bpy)M(CnRn): M= Rh, Ir (n=5); M= Ru, Os (n=6). Coordinatively Unsaturated Ground State Models of MLCT Excited States? Inorg. Chem. 1994, 33, 4453-4459; (b) Sasayama, A.; Moore, C.; Kubiak, C., Electronic effects on the catalytic disproportionation of formic acid to methanol by [Cp*IrIII(R-bpy)Cl]Cl complexes. Dalton Trans. 2016, 45, 2436-2439; (c) Tsurusaki, A.; Murata, K.; Onishi, N.; Sordakis, K.; Laurenczy, G.; Himeda, Y., Investigation of Hydrogenation of Formic Acid to Methanol using H2 or Formic Acid as a Hydrogen Source. ACS Catal. 2017, 7, 1123-1131.

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