The Good, the Neutral, and the Positive: Buffer Identity ...The buffer concentration, pH adjusted to pH 7.0, is as indicated in figure with the atmosphere denoted in parentheses. CVs

The Good, the Neutral, and the Positive: Buffer Identity Impacts CO2 Reduction Activity by Nickel(II) Cyclam

Camille R. Schneider, Luke C. Lewis, and Hannah S. Shafaat

Electronic Supporting Information Table of Contents

Figure S1. Midpoint potentials as a function of buffer concentration ................................................. S2 Figure S2. Eonset determination for each buffer. ................................................................................... S3 Figure S3: Gas chromatography calibration curves ........................................................................... S4 Figure S4. Cyclic voltammetry under an inert atmosphere and carbon dioxide atmospheres ............ S5 Figure S5. Square scheme used to derive the binding constant, KA .................................................... S6 Figure S6. CVs of [Ni(cyclam)]2+ in increasing concentrations of sodium bicarbonate .................... S7 Figure S7. CVs of [Ni(cyclam)]2+ in increasing concentrations of HEPES ........................................ S8 Figure S8. CVs of [Ni(cyclam)]2+ in increasing concentrations of PIPES ......................................... S9 Figure S9. CVs of [Ni(cyclam)]2+ in increasing concentrations of MOPS ....................................... S10 Figure S10. Baseline corrected and normalized data under an inert atmosphere .............................. S11 Figure S11. CVs of [Ni(cyclam)]2+ in water at varying scan rates .................................................. S12 Figure S12. CVs of [Ni(cyclam)]2+ in buffer at varying scan rates .................................................. S13 Figure S13. Midpoint potential as a function of scan rate ................................................................. S14 Figure S14. Splitting in anodic and cathodic peak potentials as a function of scan rate .................. S15 Figure S15. CVs of [Ni(cyclam)]2+ in anionic buffers at varying scan rates .................................... S16 Figure S16. CVs of [Ni(cyclam)]2+ in Good’s buffers at varying scan rates .................................... S17 Figure S17. CVs of [Ni(cyclam)]2+ in cationic buffers at varying scan rates ................................... S18 Figure S18. Eonset as a function of scan rate ...................................................................................... S19 Figure S19. CVs of [Ni(cyclam)]2+ with increasing [CO2] in phosphate, HEPES, and TEOA ........ S20 Figure S20. CO2 binding curves to determine K1,CO2 ........................................................................ S21 Table S1. Fit values for K1,CO2 and K2, CO2 in Figure S20 .................................................................. S22 Table S2. Electrocatalytic properties of [Ni(cyclam)]2+ in anionic buffers ...................................... S23 Table S3. Electrocatalytic properties of [Ni(cyclam)]2+ in Good’s buffers ...................................... S24 Table S4. Electrocatalytic properties of [Ni(cyclam)]2+ in cationic buffers ...................................... S25 Table S5. Photoassay product formation ........................................................................................... S26 Figure S21. Product distribution by [Ni(cyclam)]2+ in sodium bicarbonate ..................................... S27 Figure S22. Product distribution by [Ni(cyclam)]2+ in HEPES ......................................................... S28 Figure S23. Product distribution by [Ni(cyclam)]2+ in imidazole ..................................................... S29 Figure S24. Product distribution by [Ni(cyclam)]2+ in MOPS .......................................................... S30 Figure S25. Product distribution by [Ni(cyclam)]2+ in phosphate ..................................................... S31 Figure S26. Product distribution by [Ni(cyclam)]2+ in PIPES .......................................................... S32 Figure S27. Product distribution by [Ni(cyclam)]2+ in TEOA .......................................................... S33 Figure S28. Product distribution by [Ni(cyclam)]2+ in Tris .............................................................. S34 Figure S29. icat vs. pKa at the onset potential - 100 mV .................................................................... S35 Figure S30. icat vs. pKa at a constant overpotential ........................................................................... S36

Electronic Supplementary Material (ESI) for Dalton Transactions.This journal is © The Royal Society of Chemistry 2019

Supplementary Material

S2

Figure S1. Midpoint potentials (E1/2) for [NiIII/II(cyclam)] as a function of buffer concentration (0, 10, 50, and 100 mM). Specific buffer used in each experiment is noted in the panel. All samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and indicated concentration of buffer at a final pH of 7.0. CVs were conducted under an inert atmosphere at a scan rate of 10 mV/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials are reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values. The KN for each buffer was determined using Equation 4 of the main text. KIII

represents preferential binding to the NiIII state of [Ni(cyclam)], while KII represents preferential binding to the NiII state. Best-fits to Equation 4 were determined using Igor Pro 8.

0.80

0.75

0.70E1/

2(N

iIII/I

I) (V

vs.

NH

E)

100806040200Concentration (mM)

KIII = 4440.3 ± 8330.835

0.830

0.825

0.820

0.815

E1/

2(N

iIII/I

I) (V

vs.

NH

E)


KII =1.1593 ± 1.44

0.845

0.840

0.835

0.830

0.825

0.820

0.815

0.810E 1

/2(N

iIII/I

I) (V

vs.

NH

E)100806040200

Concentration (mM)

KII = 0.09507 ± 0.756 0.82

0.80

0.78

0.76

0.74

0.72

0.70

E 1/2

(NiII

I/II)

(V v

s. N

HE)


KIII = 212.46 ± 53.3

0.830

0.825

0.820

0.815

0.810

E 1/2

(NiII

I/II)

(V v

s. N

HE)


KIII = 5.5878 ± 0.2610.84

0.83

0.82

0.81

0.80

E 1/2

(NiII

I/II)

(V v

s. N

HE)


KIII = 10.136 ± 0.681

0.845

0.840

0.835

0.830

0.825

0.820

0.815

0.810

E1/

2(N

iIII/I

I) (V

vs.

NH

E)


KIII = 1.6593 ± 0.09880.86

0.85

0.84

0.83

0.82

0.81

E1/

2(N

iIII/I

I) (V

vs.

NH

E)


KII = 22.01 ± 2.48

Bicarbonate HEPES Imidazole MOPS

Phosphate PIPES TEOA Tris

A B C D

E F G H

S3

Figure S2. Eonset determination for CO2 reduction by [Ni(cyclam)] in each buffer. Specific buffer used for each experiment is noted in the panel. The maximum of the first derivative of the cathodic scan was used to determine the onset potential of catalysis for each buffer which is indicated as a dotted line in each panel. All reactions contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0. CVs were conducted under a CO2-saturating atmosphere at a scan rate of 1 V/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials are reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values. Data were analyzed using Igor Pro 8.

-80

-60

-40

-20

0C

urre

nt (µ

A)

-1.5 -1.4 -1.3 -1.2 -1.1Potential (V vs. NHE)

∂i/∂E

-80

-60

-40

-20

0

Cur

rent

(µA

)

-1.5 -1.4 -1.3 -1.2 -1.1 -1.0Potential (V vs. NHE)

∂i/∂E

-80

-60

-40

-20

0

Cur

rent

(µA

)


∂i/∂E

-50

-40

-30

-20

-10

0

Cur

rent

(µA

)


∂i/∂E

-60

-50

-40

-30

-20

-10

0

Cur

rent

(µA

)


∂i/∂E

-140

-120

-100

-80

-60

-40

-20

0

Cur

rent

(µA

)

-1.5 -1.4 -1.3 -1.2Potential (V vs. NHE)

∂i/∂E

-60

-40

-20

0C

urre

nt (µ

A)


∂i/∂E

-30

-20

-10

0

Cur

rent

(µA

)


∂i/∂E

Eonset EonsetEonset Eonset

EonsetEonset

EonsetEonset

A B C D

E F G H


Phosphate PIPES TEOA Tris


S4

Figure S3. Gas chromatography calibration curves and equations for (A) carbon monoxide and (B) hydrogen.

1000

800

600

400

200

Hyd

roge

n TC

D p

eak

area

3.02.52.01.51.00.5nmole hydrogen

H2 peak area = 351.1 * nmole H2

250

200

150

100

50

CO

FID

pea

k ar

ea (1

03 )

3.02.52.01.51.00.5nmole CO

CO peak area = 88416 * nmole COA B

S5

Figure S4. Cyclic voltammograms at negative potentials of [Ni(cyclam)]2+ under an inert Ar atmosphere (grey) compared to a CO2-saturated atmosphere (colored and labeled as indicated in panel). All experiments contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0. CVs were conducted at a scan rate of 100 mV/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

Tris


Phosphate PIPES-600

-500

-400

-300

-200

-100

0

Cur

rent

(µA

)


-500

-400

-300

-200

-100

0

Cur

rent

(µA

)


-800

-600

-400

-200

0

Cur

rent

(µA

)-1.4 -1.2 -1.0

Potential (V vs. NHE)

-400

-300

-200

-100

0

Cur

rent

(µA

)


-40

-30

-20

-10

0

Cur

rent

(µA

)


-500

-400

-300

-200

-100

0

Cur

rent

(µA

)


-800

-600

-400

-200

0

Cur

rent

(µA

)

-1.4 -1.2 -1.0Potential (V vs. NHE)

-400

-300

-200

-100

0

Cur

rent

(µA

)-1.5 -1.4 -1.3 -1.2 -1.1


A B C D

E F G H

TEOA


S6

Figure S5. Square scheme used to derive the binding constant KN. X denotes the buffer.

[NiIII(cyclam)]3+

[NiIII(cyclam)X](3-n)+ [NiII(cyclam)X](2-n)+

[NiIII(cyclam)]2+ E˚(NiIII/II)

E˚(Ni(3-n)/(2-n))

KIII+ Xn- KII+ Xn-

S7

Figure S6. Cyclic voltammograms of [Ni(cyclam)]2+ in increasing concentrations of sodium bicarbonate buffer. All measurements were conducted at a scan rate of 25 mV/s and contained 100 µM [Ni(cyclam)]2+. The buffer concentration, pH adjusted to pH 7.0, is as indicated in figure with the atmosphere denoted in parentheses. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

-60

-40

-20

0

Cur

rent

(µA

)


0 mM (N2)10 mM (N2)50 mM (N2)

100 mM (N2)100 mM (CO2)


S8

Figure S7. Cyclic voltammograms of [Ni(cyclam)]2+ in increasing concentrations of HEPES buffer under an inert atmosphere. All samples were collected at a scan rate of 10 mV/s and contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in HEPES buffer, pH 7.0, at the concentrations indicated in the figure. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

20

15

10

5

0

Raw

cur

rent

(µA

)

1.00.80.60.40.2Potential (V vs. NHE)

600

400

200

0

-200

Raw

cur

rent

(nA

)

1.11.00.90.80.70.6Potential (V vs. NHE)

10 mM50 mM100 mM

S9

Figure S8. Cyclic voltammograms of [Ni(cyclam)]2+ in increasing concentrations of PIPES buffer under an inert atmosphere. All samples were collected at a scan rate of 10 mV/s and contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in PIPES buffer, pH 7.0, at the concentrations indicated in the figure. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Raw

cur

rent

(µA

)


10 mM50 mM100 mM


S10

Figure S9. Cyclic voltammograms of [Ni(cyclam)]2+ in increasing concentrations of MOPS buffer under an inert atmosphere. All samples were collected at a scan rate of 10 mV/s and contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in MOPS buffer, pH 7.0, at the concentrations indicated in the figure. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

3

2

1

0

Raw

cur

rent

(µA)


10 mM50 mM100 mM

S11

Figure S10. Baseline corrected and normalized data under an inert atmosphere. All reactions contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0. Specific buffer used is indicated in the figure. CVs were conducted at a scan rate of 10 mV/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values. CVs were baseline-corrected in QSOAS and normalized using Igor Pro 8.

1.0

0.5

0.0

-0.5

-1.0

Nor

mal

ized

cur

rent

1.000.900.800.70Potential (V vs. NHE)

HEPESMOPSPIPESH2O

ImidazoleTEOATrisH2O

BicarbonatePhosphateH2O

1.0

0.5

0.0

-0.5

-1.0

Nor

mal

ized

cur

rent

1.00.90.80.70.60.5Potential (V vs. NHE)

1.0

0.5

0.0

-0.5

-1.0

Nor

mal

ized

cur

rent

1.000.900.800.70Potential (V vs. NHE)

A B C


S12

Figure S11. Cyclic voltammograms of [Ni(cyclam)]2+ in water under an inert atmosphere at varying scan rates. Samples were collected at the following scan rates as depicted by increased line thickness: 10 mV/s, 25 mV/s, 50 mV/s, 100 mV/s, 250 mV/s, 500 mV/s, and 1 V/s. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in pH-adjusted, unbuffered water, pH 7.0. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

8

6

4

2

0

-2

Cur

rent

(µA

)


S13

Figure S12. Cyclic voltammograms of [Ni(cyclam)]2+ in 100 mM of the indicated buffer under an inert atmosphere at varying scan rates. Samples were collected at the scan rates listed in the figure. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in 100 mM buffer, pH 7.0. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

15

10

5

0

-5

Cur

rent

(µA

)1.00.80.60.40.2


15

10

5

0

-5

Cur

rent

(µA

)


10

8

6

4

2

0

-2

-4

Cur

rent

(µA

)


10

8

6

4

2

0

-2

Cur

rent

(µA

)


10

5

0

-5C

urre

nt (µ

A)


10

8

6

4

2

0

-2

-4

Cur

rent

(µA)


10

5

0

-5

Cur

rent

(µA

)


8

6

4

2

0

-2

Cur

rent

(µA

)


10 mV/s25 mV/s50 mV/s100 mV/s250 mV/s500 mV/s1 V/s



E G H

TrisTEOAPhosphate






A

Bicarbonate

PIPES

Imidazole MOPSHEPES

B C D


S14

Figure S13. Midpoint [NiIII/II(cyclam)] potentials (E1/2) as a function of scan rate for each buffer. Cyclic voltammetry was conducted under an inert atmosphere for the buffers indicated in the figure. All reactions contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer where appropriate at a final pH of 7.0. CVs were conducted at the following scan rates: 10, 25, 50, 100, 250, 500 and 1000 mV/s. A three-electrode set up was employed with a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimental determined values.

0.85

0.80

0.75

0.70

0.65

E 1/2

(NiII

I/II)

(V v

s. N

HE)

102 4 6 8

1002 4 6 8

1000Scan rate (mV/s)

0.85

0.80

0.75

0.70

0.65

E 1/2

(NiII

I/II)

(V v

s. N

HE)

102 4 6 8

1002 4 6 8


0.85

0.80

0.75

0.70

0.65

E 1/2

(NiII

I/II)

(V v

s. N

HE)

102 4 6 8

1002 4 6 8


Phosphate

HEPESMOPSPIPESH2O


H2O

Bicarbonate

S15

Figure S14. Splitting between [NiIII/II(cyclam)] anodic and cathodic peak potentials (DEP) as a function of scan rate in each buffer (as indicated). Cyclic voltammetry was conducted under an inert atmosphere. All samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0. CVs were conducted at the following scan rates: 10, 25, 50, 100, 250, 500 and 1000 mV/s. A three-electrode set up was employed using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

0.25

0.20

0.15

0.10

0.05

ΔE P

(NiII

I/II)

(V)

102 4 6 8

1002 4 6 8


0.25

0.20

0.15

0.10

0.05

ΔE P

(NiII

I/II)

(V)

102 4 6 8

1002 4 6 8


0.25

0.20

0.15

0.10

0.05

ΔE P

(NiII

I/II)

(V)

102 4 6 8

1002 4 6 8


BicarbonatePhosphateH2O

HEPESMOPSPIPESH2O



S16

Figure S15. (A) and (B) Cyclic voltammograms of [Ni(cyclam)]2+ and (C) catalytic current as a function of scan rate in 100 mM of the indicated anionic buffer under a CO2-saturating atmosphere at varying scan rates. Samples were collected at the scan rates listed in each panel. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in 100 mM buffer, pH 7.0. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

16

14

12

10

8

i c (µ

A)

1.00.80.60.40.2Scan rate (V/s)

Bicarbonate-60

-50

-40

-30

-20

-10

0

10C

urre

nt (µ

A)


A25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/s-200

-150

-100

-50

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sPhosphate

B C

BicarbonatePhosphate

S17

Figure S16. (A), (B), (C) Cyclic voltammograms of [Ni(cyclam)]2+ and (D) catalytic current as a function of scan rate in 100 mM of the indicated Good’s buffer under a CO2-saturating atmosphere at varying scan rates. Samples were collected at the scan rates listed in each panel. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in 100 mM buffer, pH 7.0. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

22

20

18

16

14

12

10

8

i c (µ

A)

1.00.80.60.40.2Scan rate (V/s)

-80

-60

-40

-20

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sHEPES -150

-100

-50

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sMOPS

HEPESMOPSPIPES

-120

-100

-80

-60

-40

-20

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sPIPES

D

A B

C


S18

Figure S17. (A), (B), (C) Cyclic voltammograms of [Ni(cyclam)]2+ and (D) catalytic current as a function of scan rate in 100 mM of the indicated anionic buffer under a CO2-saturating atmosphere at varying scan rates. Samples were collected at the scan rates listed in each panel. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in 100 mM buffer, pH 7.0. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

20

15

10

i c (µ

A)

1.00.80.60.40.2Scan rate (V/s)

-140

-120

-100

-80

-60

-40

-20

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sTris

-350

-300

-250

-200

-150

-100

-50

0

Cur

rent

(µA)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sImidazole

-300

-200

-100

0

Cur

rent

(µA

)


25 mV/s50 mV/s

100 mV/s250 mV/s500 mV/s

1000 mV/sTEOA

ImidazoleTEOATris

A B

DC

S19

Figure S18. Eonset as a function of scan rate. All samples contained 100 µM [Ni(cyclam)]2+ and 100 mM KCl in 100 mM buffer, pH 7.0. CVs were conducted under a CO2-saturating atmosphere using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Reduction potentials were reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

-1.25

-1.20

-1.15

-1.10

Eon

set (

V v

s. N

HE

)

3 4 5 6 7100

2 3 4 5 6 71000

Scan rate (mV/s)

BicarbonateHEPES

ImidazoleMOPS

PhosphatePIPESTEOATris


S20

Figure S19. Representative cyclic voltammograms (n = 50 mV/s) of [Ni(cyclam)]2+ in (A) phosphate, (B) HEPES, and (C) TEOA buffers with varying concentrations of CO2 as indicated. Samples contained 150 µM [Ni(cyclam)]2+ and 100 mM KCl in 1 M buffer, pH 7.2. CVs were conducted using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode. Potentials are reported against the normal hydrogen electrode by the addition of +198 mV to the experimentally determined values.

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Nor

mal

ized

Cur

rent


250uM 20mM 36mM

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Nor

mal

ized

Cur

rent

-1.4 -1.2 -1.0

Potential (V vs.NHE)

250uM 20mM 36mM

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Nor

mal

ized

Cur

rent


250uM 20mM 36mM

A B CPhosphate TEOAHEPES

250 μM20 mM36 mM

250 μM20 mM36 mM

250 μM20 mM36 mM

S21

Figure S20. Best-fit traces for CO2 binding affinity to [Ni(cyclam)]+. Potentials (reported against NHE) were obtained from cyclic voltammograms measured at different CO2 concentrations (see Figure S19 for representative examples). Samples contained 150 µM [Ni(cyclam)]2+ in 1 M buffer with 100 mM KCl, maintained at a pH of 7.2. The data were fit to Eqn 6 with fixed K2,CO2 values of (A) K2,CO2 =1; (B) K2,CO2 = 10; (C) K2,CO2 = 100; and (D) K2,CO2 = 1000 to obtain the best-fit values for K1,CO2 and EN2 given in Table S1.

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)


-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)


-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)


-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)


A B

C D

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)

4 6 80.001

2 4 6 80.01

2

Log(Concentration)

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pote

ntia

l (V

vs. N

HE)

4 6 80.001

2 4 6 80.01

2

Log(Concentration)

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)

4 6 80.001

2 4 6 80.01

2

Log(Concentration)

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.12

-1.10

-1.08

Pot

entia

l (V

vs.

NH

E)

4 6 80.001

2 4 6 80.01

2

Log(Concentration)

HEPESPhosphate

TEOA

HEPESPhosphate

TEOA

HEPESPhosphate

TEOA

HEPESPhosphate

TEOA


S22

Table S1. c2 values for the K1,CO2 and EN2 values presented in Figure S20. Fits selected for main text figure highlighted.

Buffer Fit Panel K1,CO2 (M-1) K2,CO2 (M-1) EN2 (V vs. NHE) c2

Phosphate A 200000 1 -1.511 0.001024

Phosphate B 300000 10 -1.518 0.000796

Phosphate C 10000000 100 -1.594 0.001638

Phosphate D 600000 1000 -1.484 0.008970

TEOA A 100000 1 -1.501 0.001895

TEOA B 300000 10 -1.521 0.001615

TEOA C 300000 100 -1.506 0.001968

TEOA D 500000 1000 -1.485 0.008263

HEPES A 40 1 -1.320 0.001456

HEPES B 60 10 -1.321 0.001399

HEPES C 400 100 -1.328 0.001078

HEPES D 200000 1000 -1.441 0.000376

S23

Table S2. Cyclic voltammetry data for the catalytic redox couple of [Ni(cyclam)]2+ under CO2-saturating conditions. Data was collected at the indicated scan rate and all samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl and 100 mM of the indicated anionic buffer, pH 7.0.

Scan rate

(mV/s)

Bicarbonate Eonset

Bicarbonate ic (𝝁𝑨)

Bicarbonate TOF (s-1)

Phosphate Eonset

Phosphate ic (𝝁𝑨)

Phosphate TOF (s-1)

25 -1.20 ± 0.01 -8 ± 2 6 -1.17 ± 0.02 -16 ± 10 25

50 -1.20 ± 0.01 -9 ± 3 8 -1.20 ± 0.01 -12 ± 4 14

100 -1.20 ± 0.01 -15 ± 6 22 -1.20 ± 0.01 -13 ± 3 16

250 -1.20 ± 0.01 -15 ± 5 22 -1.21 ± 0.01 -13 ± 4 16

500 -1.21 ± 0.01 -15 ± 3 22 -1.22 ± 0.02 -12.3 ± 0.4 15

1000 -1.21 ± 0.01 -17 ± 4 28 -1.22 ± 0.02 -13.8 ± 0.1 18


S24

Table S3. Cyclic voltammetry data for the catalytic redox couple of [Ni(cyclam)]2+ under CO2-saturating conditions. Data was collected at the indicated scan rate and all samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl and 100 mM of the indicated Good’s buffer, pH 7.0.

Scan rate (mV/s)

HEPES Eonset

HEPES ic (𝝁𝑨)

HEPES TOF (s-1)

MOPS Eonset

MOPS ic (𝝁𝑨)

MOPS TOF (s-1)

PIPES Eonset

PIPES ic (𝝁𝑨)

PIPES TOF (s-1)

25 -1.20 ± 0.05 -14 ± 9 19 -1.13 ±

0.02 -7 ± 3 5 -1.16 ± 0.04

-10 ± 10 10

50 -1.22 ± 0.03 -12 ± 8 14 -1.19 ±

0.04 -8 ± 2 6 -1.22 ± 0.01

-10 ± 10 10

100 -1.21 ± 0.03 -13 ± 9 16 -1.21 ±

0.01 -10 ± 3 10 -1.21 ± 0.01 -12 ± 7 14

250 -1.22 ± 0.03 -15 ± 6 22 -1.22 ±

0.03 -12 ± 4 14 -1.21 ± 0.02 -15 ± 8 22

500 -1.24 ± 0.03

-20 ± 20 38 -1.21 ±

0.02 -10.1 ±

0.1 10 -1.21 ± 0.01 -19 ± 9 35

1000 -1.26 ± 0.04

-20 ± 10 38 -1.22 ±

0.03 -12.2 ±

0.2 14 -1.22 ± 0.01 -23 ± 9 51

S25

Table S4. Cyclic voltammetry data for the catalytic redox couple of [Ni(cyclam)]2+ under CO2-saturating conditions. Data was collected at the indicated scan rate and all samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl and 100 mM of the indicated cationic buffer, pH 7.0.

Scan rate

(mV/s)

Imidazole Eonset

Imidazole ic (𝝁𝑨)

Imidazole TOF (s-1)

TEOA Eonset

TEAO ic (𝝁𝑨)

TEAO TOF (s-1)

Tris Eonset

Tris ic

(𝝁𝑨)

Tris TOF (s-1)

25 -1.23 ± 0.04 -20 ± 6 38 -1.10

± 0.02 -20 ±

20 31 -1.16

± 0.01

-6 ± 4 3

50 -1.29 ± 0.04 -25 ± 2 60 -1.14

± 0.05 -14 ±

9 19 -1.22

± 0.03

-10 ± 4 10

100 -1.30 ± 0.03 -24 ± 5 55 -1.22

± 0.01 -17 ±

5 28 -1.21

± 0.02

-12 ± 5 14

250 -1.29 ± 0.04 -20 ± 3 38 -1.22

± 0.01 -18 ±

8 31 -1.21

± 0.03

-10 ± 3 10

500 -1.27 ± 0.03 -17 ± 3 28 -1.23

± 0.02 -17 ±

5 28 -1.23

± 0.04

-10 ± 3 10

1000 -1.29 ± 0.04 -21 ± 4 38 -1.25

± 0.01 -21 ±

5 42 -1.23

± 0.05

-11 ± 4 12


S26

Table S5. Product formation by [Ni(cyclam)]2+ following 2.5 hours of irradiation. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under a saturating carbon dioxide atmosphere at 4 °C. Values reported are the average and standard deviation of at least three independent trials. All values were corrected for baseline activity by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ and 100 mM ascorbate in 1 M buffer control assays.

a ND = not detected b Buffers reported to be 100% selective produced no hydrogen above the control samples.

Buffer CO (nmole) CO TON H2 (nmole) H2 TON % CO selective

Bicarbonate 310 ± 20 10.2 ± 0.7 7 ± 4 0.2 ± 0.8 98 ± 5

HEPES 47 ± 17 1.6 ± 0.6 23 ± 17 0.8 ± 0.5 67 ± 9

Imidazole 2700 ± 600 90 ± 20 NDa ND 100b

MOPS 18 ± 4 0.6 ± 0.1 ND ND 100

Phosphate 390 ± 60 13 ± 2 1300 ± 400 40 ± 10 23 ± 4

PIPES 44 ± 9 1.5 ± 0.3 130 ± 70 4 ± 2 30 ± 10

TEOA 30 ± 9 1.0 ± 0.3 ND ND 100

Tris 250 ± 98 8 ± 3 ND ND 100

S27

Figure S21. Photoassay product distribution by [Ni(cyclam)]2+ in sodium bicarbonate buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under a saturating carbon dioxide atmosphere. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M bicarbonate.

300

250

200

150

100

50

0

CO

pro

duce

d (n

mol

e)

12080400Time (minutes)

10

8

6

4

2

0

CO

TON

6

5

4

3

2

1

0

H2 p

rodu

ced

(nm

ole)


0.20

0.15

0.10

0.05

0.00

H2 TO

N

100

95

90

85

80

% C

O s

elec

tive

140120100806040Time (minutes)

A B C


S28

Figure S22. Photoassay product distribution by [Ni(cyclam)]2+ in HEPES buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under a saturating carbon dioxide atmosphere. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M HEPES.

50

40

30

20

10

0

CO

pro

duce

d (n

mol

e)


1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

CO

TON

20

15

10

5

0

H2 p

rodu

ced

(nm

ole)


0.6

0.4

0.2

0.0

H2 TO

N

100

80

60

40

20

0

% C

O s

elec

tive

140120100806040Time (minutes)

A B C

S29

Figure S23. Photoassay product distribution by [Ni(cyclam)]2+ in imidazole buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under a saturating carbon dioxide atmosphere. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M imidazole.

2500

2000

1500

1000

500

0

CO

pro

duce

d (n

mol

e)


80

60

40

20

0

CO

TON

20

15

10

5

0

H2 p

rodu

ced

(nm

ole)


0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0H

2 TON

100

98

96

94

92

90

% C

O s

elec

tive

140120100806040Time (minutes)

A B C


S30

Figure S24. Photoassay product distribution by [Ni(cyclam)]2+ in MOPS buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under saturating carbon dioxide atmospheres. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M MOPS.

20

15

10

5

0

CO

pro

duce

d (n

mol

e)


0.6

0.4

0.2

0.0

CO

TON

1.0

0.8

0.6

0.4

0.2

0.0

H2 p

rodu

ced

(nm

ole)


1.0

0.8

0.6

0.4

0.2

0.0

H2 TO

N

100

98

96

94

92

90

% C

O s

elec

tive

140120100806040Time (minutes)

A B C

S31

Figure S25. Photoassay product distribution by [Ni(cyclam)]2+ in phosphate buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under saturating carbon dioxide atmospheres. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M phosphate.

400

300

200

100

0

CO

pro

duce

d (n

mol

e)


12

10

8

6

4

2

0

CO

TON

1400

1200

1000

800

600

400

200

0

H2 p

rodu

ced

(nm

ole)


40

30

20

10

0

H2 TO

N

60

50

40

30

% C

O s

elec

tive

140120100806040Time (minutes)

A B C


S32

Figure S26. Photoassay product distribution by [Ni(cyclam)]2+ in PIPES buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under saturating carbon dioxide atmospheres. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M PIPES.

40

30

20

10

0

CO

pro

duce

d (n

mol

e)


1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

CO

TON

120

100

80

60

40

20

0

H2

prod

uced

(nm

ole)


4

3

2

1

0H

2 TON

32

30

28

26

24

% C

O s

elec

tive

140120100806040Time (minutes)

A B C

S33

Figure S27. Photoassay product distribution by [Ni(cyclam)]2+ in TEOA buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under saturating carbon dioxide atmospheres. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M TEOA.

30

25

20

15

10

5

0

CO

pro

duce

d (n

mol

e)


1.0

0.8

0.6

0.4

0.2

0.0

CO

TON

1.0

0.8

0.6

0.4

0.2

0.0

H2 p

rodu

ced

(nm

ole)


1.0

0.8

0.6

0.4

0.2

0.0

H2 TO

N

100

98

96

94

92

90

% C

O s

elec

tive

140120100806040Time (minutes)

A B C


S34

Figure S28. Photoassay product distribution by [Ni(cyclam)]2+ in Tris buffer. (A) CO production; (B) H2 production; (C) Selectivity as indicated by % CO selective, as described in Section 2.5 using Equation 8. Samples contained 10 µM [Ni(cyclam)]2+, 1 mM [Ru(bpy)3]2+, and 100 mM ascorbate in 1 M buffer, pH 7.0. Assays were conducted under saturating carbon dioxide atmospheres. Samples were irradiated using 447.5 nm LEDs at 4 °C. All values were corrected for baseline levels by subtracting any CO or H2 produced by the corresponding control of 1 mM [Ru(bpy)3]2+ with 100 mM ascorbate in 1 M Tris.

250

200

150

100

50

0

CO

pro

duce

d (n

mol

e)


8

6

4

2

0

CO

TON

1.0

0.8

0.6

0.4

0.2

0.0

H2 p

rodu

ced

(nm

ole)


1.0

0.8

0.6

0.4

0.2

0.0

H2 TO

N

100

98

96

94

92

90

% C

O s

elec

tive

140120100806040Time (minutes)

A B C

S35

Figure S29. Current as a function of pKa. Current was determined as the current at the onset potential determined for each buffer - 100 mV. All samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0 under a saturating CO2 atmosphere. CVs were conducted at a scan rate of 1 V/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode.

-40

-35

-30

-25

-20

i Eon

set +

100

mV (µ

A)

8.07.67.26.86.4pKa

BicarbonateHEPESImidazoleMOPS

PhosphatePIPESTEOA

Tris

i E(on

set)

–10

0 m

V (µ

A)

pKa


S36

Figure S30. Current as a function of pKa. Current was determined as the current at a constant overpotential of 800 mV compared to the CO2/CO reduction potential of – 520 mV vs. NHE. All samples contained 100 µM [Ni(cyclam)]2+, 100 mM KCl, and 100 mM buffer at a final pH of 7.0 under a saturating CO2 atmosphere. CVs were conducted at a scan rate of 1 V/s using a glassy carbon working electrode, Ag/AgCl reference electrode, and a Pt wire counter electrode.

-40

-35

-30

-25

-20

i η=80

0 m

V (µ

A)

8.07.67.26.86.4pKa

BicarbonateHEPESImidazoleMOPS

PhosphatePIPESTEOATris

pKa

The Good, the Neutral, and the Positive: Buffer Identity ...The buffer concentration, pH adjusted to pH 7.0, is as indicated in figure with the atmosphere denoted in parentheses. CVs

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