co to co - the royal society of chemistry

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ELECTRONIC SUPPLEMENTARY INFORMATION

Nickel(II) macrocycles: highly efficient electrocatalysts for the selective reduction of CO2 to CO

Jacob Schneider,*a Hongfei Jia,b Kazuya Kobiro,c Diane E. Cabelli,a James T. Muckerman,a and Etsuko Fujita*a

a Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA. b Materials Research Department, Toyota Research Institute of North America, Toyota Motor Engineering & Manufacturing North America, Inc., 1555 Woodridge, Ann Arbor, MI 48105, USA. c School of Environmental Science and Engineering, Kochi University of Technology, Tosayamada, Kami-city, Kochi 782-8502, Japan.

E-mail: [email protected], [email protected]

I. Supplementary Information for CV and LSV experiments

II. Supplementary Information for bulk electrolysis experiments

III. Supplementary Information for pulse radiolysis experiments

IV. Supplementary Information for calculations, calibrations, etc.

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2012

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I. SUPPLEMENTARY INFORMATION FOR CV AND LSV EXPERIMENTS

0.0 -0.5 -1.0 -1.5

0

2

4

6

8

10

0.0 -0.5 -1.0 -1.5

0

2

4

6

8

10

[Ni(cyclam)]2+ under Ar

[Ni(cyclam)]2+ under CO2

Background under Ar Background under CO

2

j / m

A c

m-2

Potential vs NHE / V

[Ni(MCC)]2+ under Ar

[Ni(MCC)]2+ under CO2

0.0 -0.5 -1.0 -1.5

0

2

4

6

8

100.0 -0.5 -1.0 -1.5

0

2

4

6

8

10


[Ni(TM)]2+ under Ar

[Ni(TM)]2+ under CO2

j / m

A c

m-2

[Ni(HTIM)]2+ under Ar

[Ni(HTIM)]2+ under CO2

Figure S1. CVs of 1.6 mM solutions of [Ni(L)]2+ in 0.1 M NaClO4, pH 5, purged with Ar (solid lines) and CO2 (dashed lines), scanning at 100 mV s-1, SMDE working electrode (area = 0.0104 cm2). Background scans are shown in the panel with [Ni(cyclam)]2+ (top left). Arrows indicate the direction of the applied potential.


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-0.6 -0.8 -1.0 -1.2 -1.4

0

3

6

9

12

j / m

A c

m2

Potential / V vs NHE

Ar 0.1% CO

2

3% CO2

9.9% CO2

30.3% CO2

100% CO2

Ar blank

0.00 0.05 0.10 0.15 0.200

2

4

6

8

10

j at

-1.1

3 V

/ m

A c

m-2

[CO2]1/2 / M1/2

R2 = 0.99769

Figure S2. CVs of [Ni(MTC)]2+ under different concentrations of CO2 (left panel); 0.1% (0.036 mM), 3% (1.08 mM), 9.9% (3.56 mM), 30.3% (10.9 mM), and 100% CO2 (36 mM). The right panel shows the linear correlation between the current densities at –1.13 V versus the square root of the concentration of CO2. Conditions: ca. 1 mM [Ni] in 0.1 M NaClO4, pH 4-5; SMDE working (area = 0.0104 cm2), Pt wire auxiliary, and Ag/AgCl (satd KCl) reference electrodes; 100 mV s-1 scan rate. Nitrogen is the gas balance for CO2 at different concentrations.


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-1.5 -1.0 -0.5 0.0 0.5-0.7

-0.6

-0.5

-0.4

-0.3-1.5 -1.0 -0.5 0.0 0.5

-0.7

-0.6

-0.5

-0.4

-0.3

-1.5 -1.0 -0.5 0.0 0.5-0.7

-0.6

-0.5

-0.4

-0.3

η / V

slope = - 89

- 0.55 V

- 0.41 V

η / V slope = - 89

- 0.55 V

- 0.41 V

η / V

log j / mA cm-2

slope = - 122

- 0.55 V

- 0.41 V

Figure S3. Plots of the overpotential (η) for CO2 reduction at pH 5 versus the log of the current density (j) for [Ni(MTC)]2+ (top), [Ni(MCC)]2+ (middle) and [Ni(TM)]2+ (bottom). LSVs run in CO2 saturated solutions containing 1 mM [Ni(L)]2+ in 0.1 M NaClO4, 2 mV s-1 scan rate, SMDE working electrode (area = 0.0104 cm2). Here, η is calculated from the thermodynamic potential for CO2 reduction to CO at pH 5 (–0.41 V vs NHE). The horizontal dotted lines correspond to potentials used in bulk electrolysis experiments (see Bulk electrolysis section in manuscript). The vertical dashed lines correspond to the pre-wave due to [Ni(L)]+ adsorption and geometric rearrangement.

Table S1. Selected data from the LSV slow scan analysisa.

L

mA cm-2 at

η = –0.41 V

mA cm-2 at

η = –0.55 V

cyclam 0.014 0.766

HTIM 0.009 0.971

MTC 0.014 0.764

MCC 0.014 0.639

TM 0.007 0.125 a The thermodynamic overpotential (η) for the reduction of CO2 to CO is taken to be –0.41 V vs NHE at pH 5.


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II. SUPPLEMENTARY INFORMATION FOR BULK ELECTROLYSIS EXPERIMENTS

0.00 0.25 0.50 0.75 1.00

0

5

10

15

20

25

- 0.96 V vs NHEpH 5

q /

C

Time / h

blank cyclam HTIM MTC MCC TM

L q / C F.E. (%) CO:H2

cyclam 3.6 ± 0.1 84 ± 4 Trace H2

HTIM 22 ± 3 88 ± 7 No H2

MTC 15 ± 0 88 ± 7 No H2

MCC 3.3 ± 0.1 92 ± 2 No H2

TM 0.87 ± 0.22 42 ± 4 Trace H2

blank 1.4 ± 0.3 64 ± 12 0.4 ± 0.3

Figure S4. Bulk electrolysis at –0.96 V vs NHE (η = –0.55 V) for 50 μM [Ni(L)]2+ in 0.1 M NaClO4, pH 5; CO:H2 is the ratio (mol CO / mol H2), as quantified by GC analysis after 1 h electrolysis.


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0 1 2 3 4 50

5

10

15

Cycle 474% F.E.

Cycle 383% F.E.

Cycle 282% F.E.

Cycle 186% F.E.

Cycle 579% F.E.

[Ni(MTC)]2+ at - 0.96 V vs NHE

q /

C

Time / h

Figure S5. Bulk electrolysis of 50 µM [Ni(MTC)]2+ in 0.1 M NaClO4, pH 5, at –0.96 V vs NHE cycled five times with the same solution; 1 h each cycle. After Cycle 4, the mercury pool electrode was cleaned and the [Ni(MTC)]2+ solution kept overnight in a vial before use in Cycle 5.

0 3 6 9 120

20

40

60

80

- 0.96 V vs NHE

q /

C

Time / h

5 μM HTIM

Figure S6. Bulk electrolysis data for a 5 µM solution of [Ni(HTIM)]2+ in 0.1 M NaClO4, pH 5, electrolysing at –0.96 V for ca. 12 h.


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0.00 0.25 0.50 0.75 1.00

0.0

0.2

0.4

0.6

0.8

- 0.82 V vs NHEpH 5

q /

C

Time / h

cyclam HTIM MTC MCC blank

L q / C F.E.(%) CO:H2

cyclam 0.26 Trace CO and H2

HTIM 0.12 No CO or H2

MTC 0.24 Trace CO and H2

MCC 0.26 Trace CO and H2

TM - - -

blank 0.68 ± 0.01 87 ± 10 No CO

Figure S7. Bulk electrolysis at –0.82 V vs NHE (η = –0.41 V) for 50 μM [Ni(L)]2+ in 0.1 M NaClO4, pH 5; CO:H2 is the ratio (mol CO / mol H2), as quantified by GC analysis after 1 h electrolysis.


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0 10 20 30

0

5

10

15

20

25

- 0.96 V vs NHEpH < 2

q /

C

Time / min

blank cyclam HTIM MTC MCC TM

L q / C F.E.(%) CO:H2

cyclam 4.60 80 0.4

HTIM 23.2 88 0.8

MTC 23.7 82 0.6

MCC 10.5 83 0.1

TM 4.20 79 0.04

blank 2.85 64 No CO

Figure S8. Bulk electrolysis at –0.96 V vs NHE for 5 μM [Ni(L)]2+ in 0.1 M NaClO4, pH < 2; CO:H2 is the ratio (mol CO / mol H2), as quantified by GC analysis after 0.5 h electrolysis.


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0 2 4 6 8 10

8

12

16

8

12

16

μV

Time / min

pH < 2

H2

μV

pH 5

O2

N2

CO

TCD

20

40

60

0 2 4 6 8 10

20

40

60

CO

pA

pH 5

FID

pA

Time / min

pH < 2

Figure S9. Chromatograms (TCD at left, FID at right) corresponding to gaseous product analysis after bulk electrolysis (–0.96 V vs NHE) at pH 5, 50 uM [Ni(MTC)]2+ after 1 h electrolysis (top panels), and at pH < 2, 5 uM [Ni(MTC)]2+ after 0.5 h electrolysis (bottom panels).


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-0.5 -1.0 -1.5

0

5

10

-0.5 -1.0 -1.5

0

10

20

-1.0 -1.2 -1.4

0.0

0.5

electrolyte only, 100 mV s-1

50 mV s-1

100 mV s-1

200 mV s-1

500 mV s-1

pH 5, under CO2

pH 5, under Ar

j / m

A c

m-2



50 mV s-1

100 mV s-1

200 mV s-1

500 mV s-1

-0.5 -1.0 -1.5

0

10

20

-0.5 -1.0 -1.5

0

5

10

j / m

A c

m-2



50 mV s-1

100 mV s-1

200 mV s-1

500 mV s-1

pH 2, under CO2

pH 2, under Ar


50 mV s-1

100 mV s-1

200 mV s-1

500 mV s-1

Figure S10. Scan rate dependence of the current density (j) vs applied potential for 1 mM solutions of [Ni(HTIM)]2+ in 0.1 M NaClO4 under Ar and CO2, pH 5 (left panel) and pH 2 (right panel), with SMDE working electrode (area = 0.0104 cm2); 10 mM phosphate buffer added to pH 2 study, pH adjusted with concentrated HClO4. Notice that when the system is under Ar, the peak current densities are significantly higher and are shifted to more positive potential at pH 2 compared to at pH 5. When the system is under CO2, however, the peak current densities and their respective potentials are comparable at either pH.


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III. SUPPLEMENTARY INFORMATION FOR PULSE RADIOLYSIS EXPERIMENTS

0 100 200 3000

2

4

6

8

10

k = 3.3 x 109 M-1s-1, R2 = 0.99367k = 3.9 x 109 M-1s-1, R2 = 0.99941k = 6.4 x 109 M-1s-1, R2 = 0.99964k = 7.1 x 109 M-1s-1, R2 = 0.99161

HTIM MTC MCC TM

k ob

s (38

0 n

m)

/ s-1 x

105

[Ni] / μM

Figure S11. Determination of the bimolecular rate constants, k, for the reduction of [Ni(L)]2+ (L = HTIM, MTC, MCC, TM) by CO2

•–, measured in 50 mM formate, pH 7, G([Ni(L)]+) = G(eaq

–) + G(H•) + G(CO2•–) = 6.


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300 400 500 600 7000

1

2

3

300 400 500 600 7000

1

2

3

Ar, 100 mM formate, pH 5 CO

2, 100 mM formate, pH 5

εn

orm

alis

ed a

t 38

5 n

m

Wavelength / nm

Ar, 100 mM formate, pH 5 CO

2, 100 mM formate, pH 5

εn

orm

alis

ed a

t 38

0 n

m

Figure S12. Transient spectra of [Ni(TM)]+ (top panel) and [Ni(HTIM)]+ (bottom panel) obtained by pulse radiolysis in 0.1 M formate under Ar and CO2.


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(A)

2400 2800 3200 3600 40000

50

100

150

ε 380

* [C

O2]

/ cm

-1

ε380

/ dm3 mol-1 cm-1

R2 = 0.99989

K = -0.076 +/- 0.001 M

2400 2800 3200 3600 40000

50

100

150

R2 = 0.98917

K = -0.115 +/- 0.008 M

ε 385

* [C

O2]

/ cm

-1

ε385

/ dm3 mol-1 cm-1

(B)

0 2 4 6 8

-40

-20

0

20

40

60

Ar 9.9% CO

2

30.3% CO2

100% CO2

Ab

s x

103

Time / μs

Figure S13. (A) Plots of the extinction coefficient (ε) at ca. 380 nm times [CO2], versus ε at ca. 380 nm, for [Ni(MTC)]+ (blue triangles) and [Ni(HTIM)]+ (green diamonds). The linear fits allow for the determination of the CO2 binding constant, KCO2. The ε at ca. 380 nm for complexes [Ni(MCC)]+ and [Ni(TM)]+ show no dependence on [CO2] (not shown). (B) Kinetic traces for pulse radiolysis experiments with [Ni(HTIM)]+ in solutions saturated with different concentrations of CO2 and monitoring at 380 nm; 150 µM Ni, 100 mM formate, pH 5, 200 ns pulses of electrons (1.13 µM radicals). Notice the fast component under Ar, from eaq

–, disappears when CO2 is present, even at only 9.9%.


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IV. SUPPLEMENTARY INFORMATION FOR CALCULATIONS, CALIBRATIONS, ETC.

N-RRRS-[Ni(cyclam)]2+

or Trans II, < 1%N-RSRS-[Ni(cyclam)]2+

or Trans I, 15%N-RRSS-[Ni(cyclam)]2+

or Trans III, 85%N-RRRR-[Ni(cyclam)]2+

or Trans V, < 1%N-RSSR-[Ni(cyclam)]2+

or Trans IV, < 1%

H

H

N

HNHN N

H

HN

HN

HNN

H

H

NH

N

H

N N

H HN

HN

HN

N

H

H

H

HN

N

NN

H HN

HN

HN

N

H

H

HH

N N

NN

H HN

HN

HN

N

H H

N

H

N

H

N

N

HHN

HN

HN

N

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Ni2+

Figure S14. Conformational isomers of [Ni(cyclam)]2+ present in solution and their relative abundance at equilibrium.

Figure S15. Side view (left) and bottom view from the ‘mercury surface’ looking up (right) of N-RSRS-[Ni(cyclam)(CO2)]

+ adduct adsorbed to ‘mercury surface’ in CPCM of water solvent, where the ‘mercury surface’ is represented by a single Hg atom in the sixth coordination position of Ni. Atom colors: Ni (yellow), C (orange), H (white), N (cyan), O (red), Hg (yellow). Quite similar results were obtained using the B3LYP and the LC-ωPBE functionals with the 6-31+G(d,p) basis for Ni, C, N, O and H, and the SBKJC ECP basis for Hg.


-S15-

0 1 2 3 4 50

250

500

750

1000

Are

a C

O P

eak

(FID

)

mL CO added to cell (10 mL soln)

y = (183 +/- 5) x, R2 = 0.99286

mL CO = Area CO Peak / 183

0.0 0.5 1.0 1.5 2.0 2.50

7

14

21

28

35y = (0.0134 +/- 0.0004) x, R2 = 0.99359

μL H2 = Area H

2 Peak / 0.0134

Are

a H

2 P

eak

(TC

D)

mL H2 added to cell (10 mL soln)

Figure S16. Calibration plots for measuring CO and H2 produced during bulk electrolysis experiments, the gases were quantified by FID and TCD detectors, respectively. Calibration is for 10 mL solution present in the electrolysis cell.

Figure S17. Example calculation for determining the Faradaic efficiency (F.E.) of a typical bulk electrolysis experiment.


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