Salt-Enhanced Photocatalytic Hydrogen Production from ...

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Salt-Enhanced Photocatalytic Hydrogen Production from Water with Carbon Nitride Nanorod Photocatalysts: Cation and pH Dependence

Xiaobo Lia†, Stuart A. Bartletta, James M. Hookb, Ivan Sergeyevc, Edwin B. Clatworthya, Anthony F. Mastersa, Thomas Maschmeyera a Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, 2006, Australia b Mark Wainwright Analytical Centre, and School of Chemistry, The University of New South Wales, Sydney, 2052, Australia c Bruker Biospin Corporation, 15 Fortune Drive, Billerica, MA, USA, 01821

Experimental details:

All chemicals used were reagent grade and were used as received.

The polymeric carbon nitride precursor was thermally polymerized in a tube furnace (Carbolite, GHA 12/300). UV-vis spectra were measured on a Varian Cary 5 UV/vis spectrophotometer. The instrument that measures the zeta potential is the Malvern Zetasizer Nano ZS. The instrument for photoluminescence is the Cary Eclipse Fluorescence Spectrometer. Surface area and porosity measurements were carried out on a Micromeritics Accelerated Surface Area and Porosimetry System 2020 instrument. Each sample was degassed at 120 ºC, and nitrogen adsorption/desorption isotherms were collected at -196 ºC. The BET surface areas were calculated from the adsorption branch of the isotherms by using the Micromeritics software. Powder XRD patterns were collected using a PANalytical X’Pert PRO MPD X-ray diffractometer using Cu Kα (λ= 1.5419 Å) radiation. Elemental Microanalysis was performed on Elemental Analyser, Model PE2400 CHNS/O (PerkinElmer, Shelton, CT, USA) with PC based data system, PE Datamanager 2400 for WindowsTM and a PerkinElmer AD-6 Ultra Micro Balance. Dynamic light scattering characterization was collected on Malvern Instruments High Performance Particle Sizer.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

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Synthesis of CNK

A two-step polymerization strategy was applied here. Initial precursor of CNK , melamine, which was loaded in an alumina crucible covered with a lid, was heated to 390 ºC for 24 h with a ramp rate of 2.2 ºC /min in a quartz tube with flowing N2 gas at a flow rate of 100 ml/min. The obtained white solid is boiled in water with 10 vol% acetic acid overnight. After thoroughly washing with water, the white solid was dried and kept in a 110 ºC oven for a second step polymerization. 1.5 g of the obtained white solid from the previous step was mixed and ground with 3.375 g LiCl and 4.125 g KCl, then, the mixture was loaded into a lid-covered crucible and heated to 400 ºC in a flowing N2 atmosphere at a flow rate of 100 ml/min with a ramping rate of 6.3 ºC and kept at this temperature for an additional 6 h. Then, the temperature was further increased to 600 ºC with a ramping rate of 6.7 K and kept for another 12 h. The mixture was cooled to room temperature under flowing N2. The solid obtained was washed in boiling water and collected following centrifugation at least three times. After drying the solid in at 110 ºC in an oven overnight, a yellow solid was obtained.

Synthesis of conventional polymeric carbon nitrides

Synthesis of CN

The carbon nitride precursor, melamine, was loaded on an alumina boat with a lid and heated to 600 ºC from room temperature with a ramp rate of 2.2 ºC /min in a quartz tube (2.5 cm in diameter) with flowing N2 gas at a flow rate of 100 ml/min and then held for 4 hr.

Synthesis of CNP

PCN_P was synthesized according to the literature.[1] Thus, cyanamide (12 g) was dissolved in a 40 % dispersion of 12 nm SiO2 particles (Ludox HS40, Aldrich) in water (15 g ) with stirring at 60 ºC overnight. The resulting transparent mixture was then heated to and held at 550 ºC over 4 h in N2 with a ramp rate of 2.2 ºC /min. The resulting brown-yellow powder was treated with aqueous 4 M NH4HF2 solution for 36 hr to remove the silica template. The powder was then centrifuged, washed with distilled water and finally, dried at 80 ºC in an oven.

Synthesis of potassium melonate and cyamelurate

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Potassium melonate was prepared according to a literature preparation.[2] Potassium cyamelurate was prepared according to a literature preparation.[3]

Photocatalytic hydrogen-evolution experiments

The quartz reactor was irradiated with a 350 W mercury arc lamp (Oriel). The spectrum of the output light was adjusted by using a water filter and a cut-off filter (Newport). The photocatalysis was performed in a continuous-flow reactor system.[2] Within the inner compartment of a double-walled quartz tube (50 mL), polymeric carbon nitride (5 mg) was suspended in an aqueous solution of 10 vol% triethanolamine (TEOA) (20 mL) containing variable salts. Platinum metal (~3 %wt) was photo-deposited by addition of H2PtCl6 at the beginning. The suspension was de-aerated by purging with argon for 1 hr before photo-irradiation. During the photocatalytic experiment, the suspension was continuously agitated by using a magnetic stirrer and kept cooled by running coolant water at 20 ºC through the outer compartment. Argon was continuously purged through the suspension at a controlled flow rate (30 mL/min) to carry any hydrogen formed into a GC (Shimadzu GC-2014; sample loop 1 mL) equipped with a discharge ionization detector (Vici pulsed discharge detector D-4-I-SH14-R) for quantification of the hydrogen gas.

Solid State NMR Spectroscopy

13C and 15N NMR MAS spectra of all carbon nitrides were acquired on a Bruker Avance III 300 MHz NMR spectrometer (7 T) at 75 and 30 MHz, respectively (UNSW). Samples were packed into 4 mm zirconia rotors for use in 4 mm H-X CPMAS probe on a 300 MHz system. All spectra were acquired at ambient probe temperature.

13C CPMAS spectra of carbon nitrides were typically acquired with the following parameters: MAS: 6.5 kHz, on the 300, relaxation delay 2-120 s; 2 ms contact pulse, ramped from 50 to 100% on the 1H channel; 1H 90° pulse of 2.55 μs (98 kHz) decoupling with SPINAL64; 256 scans were usually acquired for satisfactory signal to noise; total time from 12 min to 8.5 hr to acquire. 1-D 15N CPMAS spectra were acquired on the same samples and were typically acquired with the following parameters: MAS: 4 kHz, on a 300 system; relaxation delay 2-120 s; 2-8 ms contact pulse, ramped from 50 to 100% on the 1H channel; 1H 90° pulse of 4 - 4.5 μs

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(55 - 62.5 kHz) decoupling with SPINAL64; 2800-7000 scans were usually acquired for satisfactory signal to noise; total time from 3 - 8.5 hr to acquire. The TOSS (Total Suppression of Spinning Side Bands) pulse sequence was also employed with 13C and 15N data acquisition to distinguish the isotropic peaks from the side bands, and to suppress baseline artifacts.

13C SEDPMAS (Spin Echo Direct Polarisation) spectra of carbon nitrides were typically acquired with the following parameters: MAS: 12 kHz, on the 300, relaxation delay 5-1200 s; 13C 90° excitation pulse of 4.5 μs; echo time 77 μs; 1H 90° pulse of 4 μs (98 kHz) for decoupling with SPINAL64; up to 1288 scans were usually acquired for satisfactory signal to noise; total time from 12 min to 85 hr to acquire. All spectra were acquired at ambient probe temperature. Chemical shift referencing was achieved externally with 1H to solid DSS; with 13C to glycine carbonyl at 176 ppm; with 15N to [NH4]2 SO4, 24 ppm on the NH3 scale."

DNP MAS NMR

DNP measurements were performed on a 400 MHz (9.4 T) Bruker AVANCE-III HD DNP system equipped with a 263 GHz gyrotron and a 3.2 mm 3-channel HXY MAS DNP probe tuned to 1H-13C-15N. MAS rates of 8 kHz, controlled by a Bruker MAS2 unit, were used for all samples. Sapphire 3.2 mm rotors with zirconia drive caps were used for cryogenic spinning; active sample volume was approximately 28 μL. Typical LTMAS temperatures were 98 K for variable temperature (VT) gas, 105 K for bearing gas, and 104 K for drive gas.

Samples of CNK (ca. 30 mg) were finely ground repeatedly in an agate mortar and pestle to achieve uniform particle size. 30 µL of 30 mM AMUPol solution in 30% glycerol-d8, 70% D2O were added to wet the solid mass, followed by vigorous grinding to homogenize the mixture. The mortar was allowed to let stand at ambient temperature for approximately 10 minutes to give the solution time to soak into the particles. A standard powder funnel was used to transfer the impregnated solid into sapphire 3.2 mm DNP rotors. A fully packed rotor contained circa 40 mg of radical-solution-impregnated PCN.

Prior to NMR experiments on the carbon nitrides, gyrotron parameters were calibrated to yield a smooth power curve/enhancement profile, signal enhancement was checked at the experimental conditions using a U−13C,15N proline sample with 10 mM of AMUPol

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in water/glycerol mixture (glycerol-d8/D2O/H2O, 60/30/10), yielding an enhancement factor of 230. All spectra were acquired with a recycle delay of 3 s. CP experiments were performed with a 10% tangential ramp, a 1H 90° pulse of 2.5 μs (100 kHz), and with proton decoupling field strength of 100 kHz. The 13C and 15N NMR spectra are indirectly referenced to 4,4-dimethyl-4-silapentane-1-sulfonic acid and ammonium chloride, respectively.

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10 20 30 40

2

CNK

CN

CNP

Figure S1. XRD patterns of conventional bulk polymeric carbon nitride (CN), porous polymeric carbon nitride (CNP) and CNK.

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0 100 200 300 400 500 600 700 800 9000

10

20

30

40

50

60

70

80

90

100

W

eig

ht lo

ss (

%)

Temperature (C)

16.7 % loss

50 100 150 200 250 300 350

-2

0

2

4

He

at f

low

T (°C)

.

Figure S2. Thermogravimetric (top) and DSC (bottom) analysis of CNK in nitrogen.

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Figure S3. 75 MHz solid state 13C solid state MAS NMR spectra of CNK acquired with CP at 6.5 kHz MAS (A) and DP direct polarisation with high power decoupling at 12 kHz MAS (B).

Figure S4. 30 MHz solid state 15N CPMAS NMR spectrum of CNK at 4kHz MAS.

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400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d F

(R

)

Wavelength (nm)

CNK CN CNP

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.40.0

0.5

1.0

1.5

2.0

(hvF

(R))

1/2

h(eV)

2.71 eV

Figure S5. Solid state UV-vis diffuse reflectance spectra of CNK and conventional PCN materials (CN and CNP) (a), and Tauc-plot of CNK from the absorbance spectrum (b).

10

0 1 2 3 40

2

4

6

8

CNK CNP CN

H2 e

volu

tion

rat

e (

mol/h)

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

50

60

CNK CNK + Rb

2SO

4

CNK + K2SO

4

CNK + Na2SO

4

CNK + Li2SO

4

H2 e

volu

tion

ra

te (

mol/h)

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50

10

20

30

40

50

60

70

80

CNK CNK + RbF CNK + KF CNK + NaF CNK + LiF

H2 e

volu

tion

ra

te (

mol/h)

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

50

60

CNK CNK + BaCl

2

CNK + CaCl2

CNK + MgCl2

H2 e

volu

tion

ra

te (

mol/h)

Time (h)

Figure S6. Photocatalytic hydrogen evolution rates of CNK, CNP and CN in the absence of salts in the reaction solution (a), and CNK in the presence of different salts in the reaction solution (b, c and d).

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

2

4

6

8

10

12

CNP CNP + RbF CNP + kF CNP + NaF CNP + LiF

H2 e

volu

tion

rate

(

mol/h)

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5

2.0

CNM CNM + RbF CNM + kF CNM + NaF CNM + LiF

H2 e

volu

tion

rate

(

mol/h)

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

3.5

4.0

4.5

5.0

5.5

6.0

CNNS CNNS + K

SO

4

Hyd

roge

n e

volu

tion

rate

(m

ol/h

)

Time (h)

Figure S7. Photocatalytic hydrogen evolution rates of carbon nitrides in the presence of salts in the reaction solution, CNP (a), CNM (b), and CNNS (c).

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

10

20

30

40

50

60

70

80

0 g KF 0.04 g KF 0.16 g KF 0.33 g KF 0.66 g KF

H2 e

volu

tion

ra

te (

mol/h)

Time (h)

0.0 0.1 0.2 0.3 0.4 0.5 0.60

10

20

30

40

50

60

70

Ave

rag

e P

HE

R (m

ol/h

)

KF (mol/L)

Figure S8. Photocatalytic hydrogen evolution of CNK in the presence of different amounts of potassium salt, KF

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350 400 450 500 550 600 650

0

10

20

30

40

F (

R)

Wavelength (nm)/Cut off filter (nm)

0

20

40

60

80

100

120

140

H2 e

volu

tion

rate

(

mol/h)

Figure S9. Action spectrum of CNK with different long-pass cut off filters.

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0 10 20 30 40 50 60 7020

30

40

50

60

70

H

2 evo

lutio

n r

ate

(

mol/h)

Time (h)

Figure S10. Long-term stability test of CNK in the presence of K+ cations. Reaction conditions: 5 mg catalysts, 3 wt% photodeposited Pt, 20 mL H2O with 10 vol% triethanolamine (TEOA) and irradiation at λ > 420 nm. The cation concentration is 0.287 M K (as the KF salt).

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300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

Wavelength (nm)

Abs

orb

ance

(a

.u)

With the addition of increased salt concentration

Li2SO

4

300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5Na

2SO

4

Wavelength (nm)

Abs

orba

nce

(a

.u)

With the addition of increased salt concentration

300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

With the addition of increased salt concentration

Wavelength (nm)

A

bso

rban

ce (

a.u

)

K2SO

4

300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

With the addition of increased salt concentration

Wavelength (nm)

Abs

orb

ance

(a

.u)

Rb2SO

4

Figure S11. UV-vis spectra of CNK with increasing salt concentration from 48 M to 0.275 M.

16

300 400 500 600 700 800-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Abs

orba

nce

(a.u

)

Wavelength (nm)

+ MgCl2

+ CaCl2

+ BaCl2

CNK

Figure S12. The UV-vis spectra of CNK in 10 wt% TEOA solution (photocatalytic reaction solution) in the presence of different chloride salts (Mg2+, Ca2+, and Ba2+).

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orb

ance

(a

.u)

Wavelength (nm)

NH4Cl

N-C4 Cl N-C2 Cl N-C1 Cl CNK

Figure S13. The UV-vis spectra of CNK in 10 wt% TEOA solution (photocatalytic reaction solution) in the presence of NH4

+ and quaternary ammonium cations ( N-C1: tetramethylammonium; N-C2: tetraethylammonium; N-C4: tetrabutylammonium).

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300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

Wavelength (nm)

CNK

CNK with 0.026 M K2SO

4 from 0 to 60 min

Figure S 14 .The 60 min UV-vis time course of CNK solution in the presence of 0.026 M K2SO4.

0.00 0.05 0.10 0.15 0.20 0.25 0.30-10

-15

-20

-25

-30

-35

-40

Ze

ta p

ote

ntia

l (m

V)

K2SO

4 concentration (M)

Figure S 15. K2SO4 concentration dependent zeta potential of CNK.

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300 400 500 600 700 800

0.00

0.05

0.10

0.15

0.20

0.25

0.30

A

bso

rban

ce (

a.u

)

Wavelength (nm)

CNNS + Li

2SO

4

+ Na2SO

4

+ K2SO

4

+ Rb2SO

4

Figure S 16. The UV-vis spectra of CNNS in 10 wt% TEOA solution (photocatalytic reaction solution) in the presence of alkali chloride salts. The concentration of each cation is 0.275 M. Here, CNNS was chosen as it could be dispersed in 10 wt% TEOA solution for the solution UV-vis experiment.

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0.19 M K+2 M K

+

PL

inte

nsi

ty (

a.u)

0 M K+

PL

inte

nsi

ty (

a.u

)

0.19 M Rb+2 M Rb+

0 M Rb+

PL

inte

nsity

(a.

u)

0.19 M Li+2 M Li+

0 M Li+

PL

inte

nsity

(a.

u)

0.19 M Na+

2 M Na+

0 M Na+

Figure S17. PL spectra of CNK in the photocatalytic reaction solution with the addition of varying concentrations of salts.

20

400 450 500 550 600 650

PL

Inte

nsi

ty (

a.u

)

K salts with with SO4

2-, NO3

-, Cl- and F-

Li salts with SO4

2-, NO3

-, Cl-

Wavelength (nm)

Na salts with SO4

2-, NO3

-, Cl- and F-

Figure S18. Anion effect on the PL spectroscopy.

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0.00 0.05 0.10 0.15 0.20 0.25 0.3060

80

100

120

140

160

Li2SO

4

Li2SO

4 + K

2SO

4

PL

inte

nsity

at 4

48 n

m (

a.u)

Cation concentration (mM)

Addition of K2SO

4

0.00 0.05 0.10 0.15 0.20 0.25 0.3060

80

100

120

140

160

Na2SO

4

Na2SO

4 + K

2SO

4

PL

inte

nsity

at 4

48 n

m (

a.u)

Cation concentration (mM)

Addition of K2SO

4

0.00 0.05 0.10 0.15 0.20 0.25 0.3060

80

100

120

140

160

Rb2SO

4

Rb2SO

4 + K

2SO

4

PL

inte

nsi

ty a

t 44

8 n

m (

a.u

)

Cation concentration (mM)

Addition of K2SO

4

Figure S19 . The effect of potassium cations on the PL spectroscopy of CNK in the presence of Rb+ cations, and Rb+ and K+ cations

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Figure S20. TEM images of CNK after photocatalytic hydrogen evolution with or without the addition of salts (scale bars 0.1μm and 100 nm for K2SO4).

K2SO

4 Na

2SO

4

Li2SO

4

23

Figure S21. TEM images of CNK after photocatalytic hydrogen evolution with or without the addition of salts (scale bars 100 nm).

BaCl2 CaCl

2

MgCl2

24

Figure S22. XRD patterns of CNK after photocatalytic hydrogen evolution with or without the addition of salts.

0 10 20 30 40 50 60 70

CNK after reaction with Rb2SO

4

CNK after reaction with K2SO

4

CNK after reaction with Na2SO

4

CNK after reaction with Li2SO

4

CNK after reaction

Inte

nsi

ty (

a.u

)

2 theta (o)

CNK before reaction

25

Figure S23. TEM image of CNK after long-term stability test in the presence of 0.287 M K (KF).

26

300 400 500 600 700

0.0

0.1

0.2

0.3

0.4

0.5

A

bso

rban

ce (

a.u)

Wavelength (nm)

CNK

pH from 8.9 to 12.76

Figure S24. The effect of different pH on the UV-vis absorption of CNK in the photocatalytic hydrogen evolution reaction solution. The pH is adjusted with potassium phosphate salts, while fixing the potassium concentration at 0.275 M.

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400 450 500 550 600 650

PL

inte

nsi

ty (

a.u

)

Emission wavelength (nm)

pH increases from 8.9 to 12.76

Figure S25. PL spectroscopy of CNK in the photocatalytic solution with adjusted pH.

Figure S26. Slurry-state XRD of CNK in water with addition of potassium phosphate salts.

6 9 12 15 18 21 24 27 30

Inte

nsity

(a

.u)

2 theta (o)

CNK CNK in water CNK in water + KH

2PO

4

CNK in water + K2HPO

4

CNK in water + K3PO

4

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0 2 4 6 8 10 12 14 16 18

6

8

10

12

H2 e

volu

tion

ra

te (

mol/h)

Time (h)

a

0 10 20 30 40 50

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

H2 e

volu

tion

rate

(

mol/h)

Time (h)

b

0 2 4 6 8 10 12 14

3.5

4.0

4.5

5.0

5.5

6.0

H2 e

volu

tion

rate

(

mol/h)

Time (h)

c

Figure S27. The effect of adding potassium phosphate salt to the reaction solution on the photocatalytic hydrogen evolution rate with conventional polymeric carbon nitrides, (a) CNP; (b) CN and (c) CNNS.

29

Figure S28. Comparison of DNP enhanced 1D 13C CP NMR spectra (a) and 1D 15N CP NMR (b) between CNK and other polymeric carbon nitrides, CNP and CN.

30

Figure S29. The DNP-enhanced 2D 13C-15N correlation spectrum (double-CP) of CNK at natural isotopic abundance.

31

1000 1500 2000 2500 3000 3500 4000

0.0

0.1

0.2

0.3

0.4

Abs

orba

nce

(a.u

)

Wavenumber (cm-1)

Figure S30. IR-ATF of CNK.

1000 1500 2000 2500 3000 3500 4000

0.0

0.1

0.2

0.3

0.4

Abs

orb

ance

(a

.u)

Wavenumber (cm-1)

CNK Potassium melonate

Figure S31. IR-ATR of CNK and potassium melonate. The height ratios of the 2180 cm-1 signal (nitrile group) to the ~ 803 cm-1 absorption (from heptazine ring vibration) are 0.0375 and 0.73, respectively for CNK and potassium melonate. Each heptazine ring has 3 nitrile groups in potassium melonate, this gives 0.15 nitrile group for one heptazine on CNK.

N

N

NN

N

N N

N

N N

C

N

CNC N

K+

K+K

+

Potassium melonate

32

800 1200 1600 2000 2400 2800 3200 3600

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ize

d a

bso

rba

nce

(a

.u)

Wavenumbers (cm-1)

CNK CNK Without salt CNK with K

2SO

4

CNK with Na2SO

4

CNK with Li2SO

4

Figure S32. IR-ATR spectra of CNK after photocatalytic hydrogen evolution.

33

Supporting information K EXAFS part

X-ray Absorption Spectra

The Extended X-ray absorption fine structure (EXAFS) at the potassium K edge were measured at the Taiwan National Synchrotron Radiation Research Centre on beamline BL16A1. The X-ray data were collected with Si(111) monochromator crystal in Fluorescence mode at 10 K in vacuum. The raw data were analysed using the IFEFFIT 1.2.11 software package.

Figure S33. Potassium K Edge EXAFS data analysis with k2 weighting. Displaying KCl (Blue) and best fit (Red dash). Left – Fourier transform EXAFS; Right - inverse FT plot (R-range 1.3 – 5.0 Å).

‐0.5

‐0.25

0

0.25

0.5

0.75

0 1 2 3 4 5

FT (χ)

R (Å)‐1

‐0.5

0

0.5

1

0 1 2 3 4 5 6 7 8 9 10

Re χ(R) (Å

‐2)

k space (Å‐1)

34

Figure S34. The normalised EXAFS spectra. Displaying CNK (Red); potassium melonate (blue) and potassium cyamelurate (green).

Table S1 Potassium K edge EXAFS fitting analysis of KCl data. (Error in brackets- indicate variable)

Path (with Coordination) Distance (Å) Debye-Waller factor 6 K – Cl 3.10(3) 0.019(3) 12 K – K 4.42(2) 0.014(2) E0 = 0(1); R-Factor = 0.010, S02 = 0.7; k-range = 1.7 – 8.2; R-range = 1.3 – 5; k-weight = 3.

35

Table S2 Potassium K edge EXAFS fitting analysis of CNK data. (Error in brackets- indicates variable)

Path (with Coordination) Distance (Å) Debye-Waller factor 1 K - N 2.56(4) 0.003(6) 5 K – N 2.83(2) 0.010(2) 2 K – K 4.34(3) 0.016(4) 2 K – K 5.09(3) 0.006(3) E0 = -6(1); R-Factor = 0.006, S02 = 0.85; k-range = 1.7 – 7.5; R-range = 1.6 – 5; k-weight = 2,3.

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