www.sciencemag.org/cgi/content/full/338/6103/90/DC1

Supplementary Material for

A Local Proton Source Enhances CO2 Electroreduction to CO by a Molecular Fe Catalyst

Cyrille Costentin, Samuel Drouet, Marc Robert, Jean-Michel Savéant*

*To whom correspondence should be addressed. E-mail: saveant@univ-paris-diderot.fr

Published 5 October 2012, Science 338, 90 (2012)

DOI: 10.1126/science.1224581

This PDF file includes:

Materials and Methods

Supplementary Text

Figs. S1 to S6

References (25–40)

Submitted Manuscript: Confidential 10 May 2012

Supplementary Materials:

1. Experimental Details

Chemicals. Dimethylformamide (Sigma-Aldrich, >99.8 %, over molecular sieves), the supporting electrolyte NBu4PF6 (Fluka,

purriss.). All starting materials were obtained from Sigma-Aldrich, Fluka and Alfa-aesar, used without further purification.

MeOH, CHCl3, CH2Cl2 were distilled from calcium hydride and stored under an argon atmosphere. 1H NMR spectra were

recorded on a Bruker Avance III400 MHz spectrometer and were referenced to the resonances of the solvent used. The mass

spectra were recorded on a Microtof-Q of Bruker Daltonics.

Synthesis of 5, 10, 15, 20-tetrakis(2’,6’-dimethoxyphenyl)-21H,23H-porphyrin [1].

A solution of 2’-6’-dimethoxybenzaldehyde (1g, 6.02 mmol) and pyrrole (0.419 mL, 602 mmol) in chloroform (600 mL) was

degassed by argon for 20 minutes, then BF3.OEt2 (0.228 mL, 0.87 mmol) was added via a syringe. The solution was stirred at

room temperature under inert atmosphere in the dark for 1.5 hours, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (1.02

g, 4.51 mmol) was added to the reaction. The mixture was stirred for an additional 1.5 hours at reflux, cooled to room

temperature, and 1 mL of triethylamine was added to neutralize the excessive acid. Then the solvent was removed, and the

resulting black solid was purified by column chromatography (silica gel, dichloromethane) affording porphyrin 1 as a purple

powder (290 mg, 23%). 1H NMR (400 MHz, CDCl3): s, 8H), 7.60 (t, J = 8 Hz, 4H), 6.89 (d, J = 8 Hz, 8H), 3.41 (s, 24H),

-2.57 (s, 2H). HRESI-MS ([M+H]+) calcd for C52H47N4O8 855.3388, found 855.3358.

Synthesis of 5, 10, 15, 20-tetrakis(2’,6’-dihydroxyphenyl)-21H,23H-porphyrin [2]

To a solution of porphyrin 1 (400 mg, 0.47 mmol) in dry dichloromethane (25 mL) at -20°C was added BBr3 (451 L, 4.68

mmol). The resulting green solution was stirred for 12 hours at room temperature, then placed in ice water, ethyl acetate was

added to the suspension and the mixture was washed with NaHCO3. The organic layer was separated, washed twice with water

and then dried over anhydrous Na2SO4. The resulting solution was evaporated. The residue was purified by column

chromatography (silica gel, 20:1 ethyl acetate/methanol) to yield porphyrin 2 as a purple powder (300 mg, 87%). 1H NMR (400

MHz, MeOD): s, 8H), 7.38 (t, J = 8 Hz, 4H), 6.72 (d, J = 8 Hz, 8H). HRESI-MS ([M+H]+) calcd for

C44H31N4O8 743.2436, found 743.2136.

Synthesis of Chloro iron (III) 5, 10, 15, 20-tetrakis(2’,6’-dihydroxyphenyl)-porphyrin [3]

A solution of compound 2 (200 mg, 0.27 mmol), anhydrous iron (II) bromide (1.04 g, 4.85 mmol) and 2,6-lutidine (78 L, 0.67

mmol) was heated at 50°C and stirred 3 hours under inert atmosphere in dry methanol. After methanol was removed, the resulting

solid was dissolved in ethyl acetate, washed with 1.2 M HCl solution and then washed until pH was neutral. The crude product

was purified by column chromatography (silica gel, 1:1 methanol/ethyl acetate) to give compound 3 as a brown solid (211 mg,

94%). HRESI-MS ([M]+) calcd for C44H28FeN4O8 796.1242, found 796.1252.

Synthesis of Chloro iron (III) 5, 10, 15, 20-tetrakis(2’,6’-dimethoxyphenyl)-porphyrin [4]

A mixture of 2 (90 mg, 0.105 mmol), anhydrous iron (II) bromide (227 mg, 1.053 mmol) and anhydrous dimethylformamide (23

ml) was refluxed under inert conditions for 2 hours, opened to air and brought to dryness under vacuum. The residue was re-

dissolved in dichloromethane, washed with water. The organic layer was stirred with 20% HCl for 75 min, washed with water

and taken to dryness. The residue was purified using column chromatography (silica gel, dichloromethane to 1%

methanol/dichloromethane), re-dissolved in dichloromethane and stirred with 4N HCl for 1h. The organic layer was separated,

washed with water and dried over Na2SO4 and evaporated to furnish 4 as a brown solid (60 mg, 54%). HRESI-MS ([M]+) calcd

for C52H44FeN4O8 908.2469, found 908.2504.

Methods and Instrumentation

Cyclic voltammetry. The working electrode was a 3 mm-diameter glassy carbon (Tokai) disk carefully polished and ultrasonically

rinsed in absolute ethanol before use. For scan rate above 0.1 V/s the working electrode was a 1 mm-diameter glassy carbon rod

obtained by mechanical abrasion of the original 3 mm-diameter rod. A mercury drop hung to a 1 mm diameter gold disk was also

used as working electrode to determine the FeTDHPP standard potential. The counter-electrode was a platinum wire and the

reference electrode an aqueous SCE electrode. All experiments were carried out under argon or carbon dioxide at 21°C, the

double-wall jacketed cell being thermostated by circulation of water. Cyclic voltammograms were obtained by use of a Metrohm

AUTOLAB instrument. Ohmic drop was compensated using the positive feedback compensation implemented in the instrument.

Electrolysis. Electrolyses were performed using a Princeton Applied Research (PARSTAT 2273) potentiostat. The experiments

were carried out in a cell (figure S1) with a carbon crucible as working electrode (S = 20 cm2), the volume of the solution is 10

mL. The reference electrode was an aqueous SCE electrode and the counter electrode a platinum wire in a bridge separated from

the cathodic compartment by a glass frit, containing a 0.4M EtNCO2CH3 + 0.1M NBu4PF6 DMF solution. The electrolysis

solution was purged with CO2 during 20 min prior to electrolysis.

Ohmic drop was minimized as follows: the reference electrode was directly immerged in the solution (without separated bridge)

and put progressively closer to the working electrode until oscillations appear. It is then slightly moved away until the remaining

oscillations are compatible with recording of the catalytic current-potential curve. The appearance of oscillations in this cell

configuration does not require positive feedback compensation as it does with micro-electrodes (25).

The potentiostat is equivalent to a self inductance (25). Oscillations thus appear as soon as the resistance that is not compensated

by the potentiostat comes close to zero as the reference electrode comes closer and closer to the working electrode surface.

Gaz detection. Gas chromatography analyses of gas evolved in the course of electrolysis were performed with a HP 6890 series

equipped with a thermal conductivity detector (TCD). CO and H2 production was quantitatively detected using a carbosieve 5 III

60-80 Mesh column 2 m in length and 1/8 inch in diameter. Temperature was held at 230ºC for the detector and 34 ºC for the

oven. The carrier gas was helium flowing at constant pressure with a flow of 20 mL/min. Injection was performed via a syringe

Fig. S1. Electrolysis cell. WE: carbon crucible working

electrode, CE: platinum grid counter-electrode, RE:

aqueous saturated calomel electrode, EV: expansion

vessel.

(500 µL) previously degazed with CO2. The retention time of CO was 7 min. Calibration curves for H2 and CO were determined

separately by injecting known quantities of pure gas.

2. Determination of 2

0

CO COE according to the solvent and to the acids present

We are first looking for2

0CO CO,S,AHE , the standard

potential for the conversion of CO2 into CO in a

solvent S and in the presence of an acid, AH:

CO2(S) + 2 HA(S) + 2 e- CO(S) + H2O(S) + 2 A-(S)

referred to the aqueous standard hydrogen electrode

(NHE). In practice, the potential measurements were

made against the aqueous standard calomel electrode

(SCE) of an electrode reaction that takes place in the

solvent S, here DMF (potentials referred to aq NHE

are 0.241 V more positive than when referred to the aq.

SCE). We consider the thermodynamical cycle shown

aside (Chart S1). The change of solvent introduces an

interliquid potential ,L SE between the aqueous NHE

and DMF solution of a 0.1 M tetraalkylammonium supporting electrolyte in the equation relating 2

0CO CO,S,AH

E , the standard

potential corresponding to the top reaction to, 2

0CO CO,aq

E , the standard potential corresponding to the bottom reaction:

+22

2 2

2

0 0,,H O,S->aq,H ,S->aq,CO ,aq->g,CO,S->g0 0

CO CO,S,AH ,S CO CO, a,HA,S,CO,aq->g ,CO ,S->

2ln10

ln2 2

tthhL aq

h h g

G GKKRT RTE E E pK

F F K K F

This problem has been previously been addressed in the framework of a concerted dissociative electron transfer to organic halide

(26, 27) and generalized to any redox couple in any solvent (28). ,SLE can be estimated through the following relationship:

+0, , 0, , 0

,S , ,aq->SAg Ag,S Ag Ag,aq

1SHE aq SHE aqL t Ag

E E E GF

the various parameters being obtained as follows:

0, ,

Ag Ag,aq0.799

SHE aqE V

0, ,

Ag Ag,

SHE aq

DMFE 0.778 and

3

0, ,

Ag Ag,CH CN

SHE aqE 0.660 V (29)

+0

, ,aq->DMFt AgG = -0.161 eV and +

3

0

, ,aq->CH CNt AgG = -0.238 eV (30)

leading to ,DMFLE = 0.141 V and 3,CH CNLE = 0.099 V.

+

0

,H ,DMF->aqtG = 0.186 eV (30),

2

0,H O,DMF->aqt

G -0.2 eV (considering that it is about the same as for HO2, itself calculated

quantum mechanically (31). 2

0CO CO,aq

E -0.106 V vs. NHE (11)

2

2

0CO

,CO ,S->g 02CO

h

P PK

C ; [CO2] being the solubility of CO2 in S at

2COP = 1 bar with 0P =1 bar and 0C 1 M.

0CO

,CO,S->g 0COh

P PK

C ; [CO] being the solubility of CO in S at COP = 1 bar with 0P =1 bar and 0C 1 M.

[CO2] (M) [CO] (M) 2,CO ,S->ghK ,CO,S->ghK

H2O 0.038 (32) 0.00096 (32) 29 1040

DMF 0.2 (33) 0.0025 (34) 5 400

CH3CN 0.28 (33) 0.0025 * 3.6 400

* considering that it is the same as in DMF.

leading to: 2

0CO CO,DMF,AH

E -0.259 ,HA,DMFln10

aRT

pKF

V vs. NHE

and 2 3

0CO CO,CH CN,AHE 0.349

3,HA,CH CNln10

aRT

pKF

V vs. NHE

In order to obtain the standard potential2

0CO CO,S

E , the strongest acid present has to be considered. In our experiments (and in

previous studies corresponding to references 15, 23 and 24 (see Table 1)), CO2 in presence of water is the strongest acid present

(phenol, present in the FeTDHPP molecule has a pK = 18.8 in DMF (35))

In other words, the redox reaction to be considered is:

CO2(S) + 2 CO2(S) + 2 H2O (S) + 2 e- CO(S) + H2O(S) + 2 HCO3-(S)

Thus, replacing HA/A- by CO2+H2O / HCO3-, an estimation of

2 2,CO +H O,DMFapK is needed to apply the above equations.

In water:

Thus, for the reaction:

2 2 3

6.37,CO ,aq ,H CO ,aq 10a a hK K K (32)

We then consider the thermodynamic cycle in Chart S2,

leading to :

2

2 2

2

- +32

,CO ,aq->g,CO ,S ,CO ,aq

,CO ,S->g

0 00,HCO ,aq->S,H O,aq->S ,H ,aq->S

log

ln10 ln10 ln10

ha a

h

tt t

KpK pK

K

G GG

RT RT RT

We have already obtained the values of +0

,H ,S->aqtG and

2

0,H O,S->aqtG in DMF and CH3CN. In addition,

-3

0

,HCO ,aq->DMF0.4

tG eV and -

3 3

0

,HCO ,aq->CH CN0.3

tG eV assuming a comparable transfer free energy as other monoanions

(see reference 35).

Finally:

2,CO ,DMFapK 7.37 and thus :2

0CO CO,DMF

E -0.690 V. vs. NHE

2 3,CO ,CH CNapK 17.03 and thus :2 3

0CO CO,CH CNE -0.650 V. vs. NHE

In the case of the experiments reported in reference 22, the strongest acid is HBF4 (pKa = 0.1 in CH3CN and thus assumed to be a

strong acid in DMF with a pKa nil ) thus: :2 4

0CO CO,DMF,HBFE -0.260 V. vs. NHE.

3. Cyclic voltammetry of FeIIITDMPP

-5

0

5

10

15

20

25

0.5 0 -0.5 -1 -1.5 -2

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0.5 0 -0.5 -1 -1.5

( A)i

E vs. NHE

( A)i

E vs. NHE

a b

Fig. S2. Cyclic voltammetry of 1 mM FeIIITDMPP in DMF + 0.1 M n-Bu4NPF6+ 2M H2O, at 0.1 V/s in the absence (a) and

presence (b) of 0.23 M CO2, after normalization toward the FeII/FeI peak current, 0p

i .

4. Standard potentials and standard rate constants of FeI/0TDHPP and FeI/0TDMPP

In all experiments, the FeII/I wave serves as an internal standard. The corresponding standard potential for FeII/ITDHPP is -0.918

V vs. NHE. At low scan rate the FeI/0TDHPP wave is chemically irreversible. Attempts to restore reversibility by raising the scan

rate to obtain the standard potential and the standard rate constant in presence of 2 M H2O failed on glassy carbon, because of the

poor responses on this electrode at high scan rates. High scan rate cyclic voltammograms were thus recorded on a mercury drop

electrode. Simulation using DigiElch software (36) allows the determination of I 00

Fe TDHPPE -1.333 V vs. NHE and

/ SD k 0.029 s1/2 (figure S3).

At low scan rate the FeI/0TDMPP wave is chemically irreversible. Raising the scan rate on a 1 mm-diameter glassy carbon

electrode allows to restore reversibility. Simulation allows to determine I 00

Fe TDMPPE -1.69 V vs. NHE and / SD k 0.043

s1/2 (figure S3b).

-20

-15

-10

-5

0

5

10

15

20

25

30

-1.1 -1.2 -1.3 -1.4 -1.5 -1.6

-1

-0.5

0

0.5

1

1.5

-1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2

( A)i

E vs. NHE

( A)i

E vs. NHE

ab

Fig. S3. Cyclic voltammetry of 1 mM FeITDHPP and FeITDMPP in DMF + 0.1 M n-Bu4NPF6 + 2 M H2O. Full line: experiment;

dashed lines simulation. a: at 70 V/s on a Hg microelectrode. b: 2 V/s on a glassy carbon electrode.

5. The turnover frequency and its relationship with the overpotential in preparative-scale electrolysis

The reaction scheme on which the discussion in the main text was based (Figure 3) is simplified as follows (Scheme S3).

Scheme S3

solutiondiffusion-convection

layerreaction –diffusion

layerelectrode

2e

A

C

A

C

CA

x

dCD

dx

2Q

2P

C CA

b

x

dC dCVD

dx S dt

AA

x

dCD

dx

x= 0

Q 0

cat x

I

F

k D C

AA

x

dCD

dx

A

Fig. S4. Catalysis and diffusion for a homogeneous catalytic reaction under preparative scale steady-state conditions. For

symbols and equations, see text.

In this framework, figure S4 depicts the main features of the kinetics and diffusion processes for a homogeneous catalytic

reaction under preparative scale steady-state conditions, bearing in mind that we consider the case where the concentration of the

substrate in the bulk of the solution is maintained constant (in the present case, electrolysis is carried out under a constant

pressure of CO2, with a quick) equilibration between the gas and the solution phases). x is the coordinate for diffusion, assumed

as being linear. is the thickness of the reaction layer,, the thickness of the diffusion-convection layer, depending on the rate of

agitation of the solution or of the circulation rate for flow cells. S is the electrode surface area, V the volume of the solution. CA

and CC are the concentrations of the substrate and product respectively. AbC and C

bC are the concentrations of the substrate and

product in the bulk respectively. 0AC is the initial concentration of substrate. i being the current flowing through the electrode, I =

i / S is the current density. The equations figured in the various space zones of figure S4 describe the way in which the catalytic

current in the film triggers a flux of A toward the electrode and the production of a flux of C toward the solution resulting in the

consumption of A and production of C in the bulk of the solution (37). The fact that the substrate concentration is maintained

constant in the bulk of the solution does not mean that it is necessarily constant over the diffusion-convection layer and the

reaction layer. In any case, the production of C is thus related to the current density according to:

C CAb

x

dC dCSD IS

dt V dx FV

Cb IS

C tFV

The steady state approximation applied to B allows the introduction of a catalysis rate constant, 0A2catk kC

The Q- concentration profile, Q( )C x is restricted to a thin reaction-diffusion layer adjacent to the electrode surface (its thickness

is of the order of / catD k (38); D being diffusion coefficient of P and Q) within which:

2Q

P Q20cat

d CD k C

dx , with:

QQ 0, 0

xx

dCC

dx

Q 0cat x

Ik D C

F

In heterogeneous catalysis, the turnover number is defined as the number of moles of substrate transformed by one mole of

catalyst present in the film deposited on the surface.

In the homogeneous case, as discussed here, the amount of active catalyst is the number of moles contained within the diffusion

reaction-diffusion layer that develops adjacent to the electrode surface:

C

(P+Q)

molTON

mol

Within this portion of space, the Q-profile, obtained by space integration of the above differential equation, taking the two

boundary conditions into account, may be expressed as:

Q Q 0( ) exp cat

x

kC x C x

D

leading to:

0P

(P Q)

cat

Dmol S C

k ( 0

PC is the total concentration of catalyst)

and:

QC 00

0 0 PP P

C

(P+Q)

bcat x

cat cat

k Cmol VC ITON t t

mol D D CS C F C

k k

and to the turnover frequency:

Q 00

0 PP

cat x

cat

k CITOF

D CF C

k

If electron transfer between the two forms of the catalyst is fast, the Nernst law is obeyed and:

0cat1 exp

catkTOF

FE E

RT

In a more general case, the electron transfer kinetics of the catalyst has to be taken into account. Assuming a Butler-Volmer law

with a 0.5 transfer coefficient (39) :

0 0

P Q0 0exp exp

2

cat cat

S x x

F E E F E EIk C C

F RT RT

with 0P Q P0 0x x

C C C

. Thus:

0 0

0P Q 0

exp 1 exp2

cat cat

S x

F E E F E EIk C C

F RT RT

and

0 0 0P

0 0P P

1 1 1

exp exp2

cat cat cat

cat S

F

I k DC F E E F E Ek DC k C

RT RT

At potentials well in front of the standard potential:

0

exp 1catF E E

RT

(S1)

and:

0 0

0 0P P

1 1

exp exp2

cat cat

cat S

F

I F E E F E Ek DC k C

RT RT

leading to:

0

0P

0

exp

1 exp2

cat

cat

cat

catS

F E Ek DC

RTI

F F E EDk

k RT

(S2)

0

00P

exp

1 exp2

cat

cat

cat

catcatS

F E Ek

RTI

TOFD F E EDF C

kkk RT

Without any simplification the expression is:

0 0

1 exp exp2

cat

cat catcat

S

kTOF

F E E F E Ek D

RT k RT

, i.e., equation (1) in the text.

Introducing the overpotential 0AE E , 0 0 0

Acat catE E E E

and 0 0

A

0log logln10

cat

cat

F E ETOF k

RT

leads, without simplification to equation (3) in the text.

6. Foot-of the-wave analysis of catalytic current-potential responses in cyclic voltammetry

All the equations derived in the preceding section, notably equation (S2), with preparative-scale conditions in mind, are valid in

the framework of cyclic voltammetry as well since we deal with reactions fast enough to be in steady-state "kinetic conditions",

which do no hinge upon the time-constant of the system. In the foot-of the-wave analysis of catalytic current-potential responses

in cyclic voltammetry, condition (S1) applies leading to equation (S2). The foot-of the-wave response is then analyzed so as to

derive the two parameters 22 COcatk k and / SD k from the fitting with equation (S2), taking into account that the current

has been normalized toward the peak current of the FeII/FeI wave:

0 0P0.446p

Fvi FSC D

RT .

The most efficient way of deriving the two parameters is to plot the function 0catFIT E E defined as:

00 2

0 0

0

2 CO 2.24

1 0.446 exp 1 exp2

pcat

cat cat

Sp

i

i k RTFIT E E

FvF E E F E Ei D Fv

k RT RT RTi

(S3)

as a function of 1

01 exp /catF E E RT

using for / SD k the values found in section 4. The results obtained in this

way for FeTDHPP, FeTDMPP and FeTPP + 3M PhOH are shown in figures 4d, f, h in the text. The value of 22 COk is then

derived from the slope of the linear portion of the FIT diagram.

7. Prolonged electrolysis

A solution of 1mM FeTDHPP in DMF + 2 M H2O is electrolyzed at -1.16 V vs. NHE on a 20 cm2 carbon crucible as electrode

over 2h. 43 C are transferred corresponding to an averaged current density of 0.31 mA/cm2 (figure S5). CO is the main product

and detection of gas in the headspace after 1 h and 2h electrolysis leads to a faradaic yield of 94% and 6% of H2. The diffusion

coefficient being equal to D = 5 10-6 cm2/s, it follows from:

00 PA

02

1 expcat

CQ i t FS kC D t

F E E

RT

It follows that: 0A2kC 3 106 s-1, and thus logTOF = 3.5 at a 0.466 V overpotential.

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100 120

Q (C)

t (min) t (min)

I (mA/cm2)

Fig. S5. Left: charge passed during electrolysis. Right: current density over time

8. TOF and overpotential estimation for previously reported catalysts

In the treatment of the data that we could retrieve from previous literature examples, we assume, in the absence of contrary

indication, that electron transfer to the catalyst is fast, obeying the Nernst law. The following equations derived from section 5

were therefore applied.

0P

0cat1 exp

catk DCi

FFSE E

RT

(S4)

0cat1 exp

catkTOF

FE E

RT

(S5)

a. Rhenium catalyst (21)

A solution of 7.5 10-8 mol/cm3 catalyst and CO2 saturated in DMF + 10% H2O is electrolyzed at -1.25 V vs. NHE on a 10 cm2

electrode over 14h. 260 C are transferred and the faradaic yield is 98%, thus Q = 255 C. The standard potential of the catalyst is

approximate to -1.25 V vs. NHE (40) and the diffusion coefficient is assumed to be D = 5 10-6 cm2/s. kcat is derived from the

charge passed (Q = i × t) after application of equation (1), leading to kcat = 1018 s-1 and, by application of equation (2), to the

TOF value listed in Table 1.

b. Palladium catalyst (22)

A cyclic voltammetry study of a CO2 saturated solution containing {m-(triphos)2-[Pd(CH3CN)]2} as catalyst in DMF + HBF4 was

reported leading to kcat = 10 s-1, derived from the plateau of the catalytic wave. The standard potential of the catalyst is -0.76 V

vs. NHE. An electrolysis experiment is reported at -1.7 vs Fc+/Fc, i.e., -1.06 vs. NHE, corresponding to an overpotential of 0.80

V taking into account the estimation of 2 4

0CO CO,DMF,HBFE -0.259 V vs. NHE, established in section 2. The corresponding

TOF value (Table 1) is equal to kcat since electrolysis is performed on the plateau of the catalytic wave.

c. Manganese catalyst (23)

An electrolysis was performed at -1.16 V vs. NHE on a saturated CO2 solution containing a manganese complex in CH3CN + 5

% H2O. The standard potential of the catalyst appears to be close to -1.16 V vs. NHE and its diffusion coefficient DP = 5 10-6

cm2/s. The volume of the solution is not given but assumed to be 25 mL (corresponding, as seems likely to a 1 mM concentration

of the catalyst). The current density is reported to be 0.2 mA/cm2 and the faradaic yield 85%, leading to i/S = 0.17 mA/cm2. It

follows (equation S1) that kcat = 2.5 s-1, giving, by application of equation (S2), the TOF value reported in Table 1.

d. Ruthenium catalysts (24)

An electrolysis was performed at -1.52 V vs. NHE on a saturated CO2

solution containing 1 mM of a polypyridyl ruthenium catalyst I (Chart S3)

in CH3CN. The standard potential of the catalyst is -1.3 V vs. NHE and its

diffusion coefficient DP = 5 10-6 cm2/s. The electrode surface is 0.071 cm2

and the averaged current 125 µA with a faradaic yield of 76% thus: i/S =

1.34 mA/cm2. Since the electrolysis potential is again on the plateau of the

catalytic wave 0P/ cati FS k DC leading to kcat = 38.4 s-1 and to the TOF value reported in Table 1. An electrolysis was

performed at -1.46 V vs. NHE on a saturated CO2 solution containing 1 mM of a polypyridyl ruthenium catalyst II (see Chart S3)

in CH3CN. The standard potential of the catalyst is -1.25 V vs. NHE and its diffusion coefficient D = 5 10-6 cm2/s. The electrode

surface is 0.071 cm2. 1.8 C are transferred over 5h electrolysis with a faradaic yield of 85%. Thus: i/S = 1.2 mA/cm2, and

therefore ( 0P/ cati FS k DC ): kcat = 31 s-1, leading to the TOF value reported in Table 1.

e. Nickel catalyst (15)

An electrolysis was performed at -1.2 V vs. NHE, practically at the standard potential of the catalyst, of a saturated CO2 solution

containing a Ni(cyclam) catalyst (1 mM) in a 1 – 4 water – acetonitrile mixture. Assuming the same value, DP = 5 10-6 cm2/s, as

before diffusion coefficient assumed to be. The current density is reported to be 1.8 mA/cm2 and the faradaic yield 90% this leads

to i/S = 1.6 mA/cm2. In these conditions, kcat = 570 s-1, leading to the TOF value reported in Table 1.

It has to be noted that the current density in comparable conditions is much higher (ten times) on a mercury electrode. It has been

shown that the complex is adsorbed on the mercury electrode surface (14). The exact identity of the active species adsorbed

remains to be identified and the amount of catalyst adsorbed has to be known to evaluate its catalytic properties.

References

(25) J. M. Savéant, Elements of Molecular and Biomolecular Electrochemistry (Wiley-Interscience, New York, 2006), Chap. 1,

pp. 14-20; Chap. 6, pp. 353-361.

Chart S3

References and Notes

1. The standard potential of the CO2/CO2– couple has been estimated to be –1.97 V versus NHE

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10. The last term in Eq. 1 is lacking in the derivations of (9) because electron transfer between the electrode and the catalyst was assumed to be unconditionally fast. In the present case, catalysis is so fast that the catalyst electron transfer kinetics cannot be neglected. Full proof of Eq. 1 is given in the supplementary materials, section 5.

11. A. J. Bard, R. Parsons, J. Jordan, Ed., Standard Potentials in Aqueous Solution (Marcel Dekker, New York, 1995).

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1

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21. J. Hawecker, J. M. Lehn, R. Ziessel, Electrocatalytic reduction of carbon dioxide mediated by Re(bipy)(CO)3Cl (bipy = 2,2′-bipyridine). J. Chem. Soc. Chem. Commun. 6, 328 (1984). doi:10.1039/c39840000328

22. J. W. Raebiger et al., Electrochemical reduction of CO2 to CO catalyzed by a bimetallic palladium complex. Organometallics 25, 3345 (2006). doi:10.1021/om060228g

23. M. Bourrez, F. Molton, S. Chardon-Noblat, A. Deronzier, [Mn(bipyridyl)(CO)3Br]: An abundant metal carbonyl complex as efficient electrocatalyst for CO2 reduction. Angew. Chem. Int. Ed. 50, 9903 (2011). doi:10.1002/anie.201103616

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25. J. M. Savéant, Elements of Molecular and Biomolecular Electrochemistry (Wiley-Interscience, New York, 2006), chap. 1, pp. 14–20; chap. 6, pp. 353–361.

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2

3

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37. J. M. Savéant, Elements of Molecular and Biomolecular Electrochemistry (Wiley-Interscience, New York, 2006), Chap. 2, pp. 132–136.

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39. J. M. Savéant, Elements of Molecular and Biomolecular Electrochemistry (Wiley-Interscience, New York, 2006), Chap. 1.

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