Supplementary Information Ambient ammonia

Supplementary Information

Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at low overpotential

Wang et al.

1

Intensity 410

408

406

404

402

400

398

396

394

392

390

Binding energy (eV)

Supplementary Figure 1. High-resolution XPS spectrum of the Pd/C catalyst in the N 1s region. There are no distinguishable peaks in the region, indicating that no N species were observed within the detection limit of the XPS (~0.1 atomic percent).

2

Supplementary Figure 2. Photograph of the electrochemical cell setup used for the N2RR electrolysis.

3

a

b

1.5

1.0

0.05 M H2SO4

0.20 0.15

0.1 M PBS

Current (mA)

Current (mA)

0.10

0.5

0.275 V 0.0

0.05

0.598 V

0.00 -0.05

-0.5

-0.10

-1.0 -0.15

-0.29

-0.28

-0.27

-0.26

-0.62

-0.25

E (V vs. Ag/AgCl/sat. KCl)

c

-0.60

-0.59

-0.58

-0.57

-0.56


0.3 0.2 0.1

Current (mA)

-0.61

0.1 M NaOH 0.938 V

0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.98 -0.97 -0.96 -0.95 -0.94 -0.93 -0.92 -0.91 -0.90 -0.89


Supplementary Figure 3. Calibration of reference electrodes. The Ag/AgCl/saturated KCl reference electrode was calibrated with respect to RHE in a 0.05 M H2SO4, b 0.1 M PBS, and c 0.1 M NaOH, respectively. Based on the calibrations, we have: in 0.05 M H2SO4, E (vs. RHE) = E (vs. Ag/AgCl) + 0.275 V; in 0.1 M PBS, E (vs. RHE) = E (vs. Ag/AgCl) + 0.598 V; in 0.1 M NaOH, E (vs. RHE) = E (vs. Ag/AgCl) + 0.938 V.

4

1.0

Absorbance

0.8

b

0 mM 0.01 mM 0.02 mM 0.03 mM 0.05 mM 0.1 mM

Absorbance @ 653 nm

a

0.6

0.4

y = 7.929x + 0.135 R2 = 0.998

0.8

0.6

0.4

0.2

0.2

0.0 500

1.0

0.0

550

600

650

700

750

0.00

800

0.02

0.04

Wavelength (nm) 1.0

Absorbance

0.8

1.0

0.6

0.4

0.6

600

650

700

750

0.08

0.10

0.6

0.4

0.00

800

0.02

0.04

0.06

cNH (mM) 3

f

0 mM 0.01 mM 0.02 mM 0.03 mM 0.05 mM 0.1 mM

0.4

0.2

0.0 500

0.10

0.8

0.0

550

Absorbance @ 653 nm

Absorbance

0.8

0.08

y = 8.531x + 0.128 R2 = 0.999

Wavelength (nm)

e

0.10

0.2

0.2

0.0 500

0.08

3

d

0 mM 0.01 mM 0.02 mM 0.03 mM 0.05 mM 0.1 mM

Absorbance @ 653 nm

c

0.06

cNH (mM)

1.0

y = 6.923x + 0.117 R2 = 0.999

0.8

0.6

0.4

0.2

0.0

550

600

650

700

750

800

0.00

0.02

0.04

Wavelength (nm)

0.06

cNH (mM) 3

Supplementary Figure 4. NH3 quantification using indophenol blue method. The UV-Vis absorption spectra and corresponding calibration curves for the colorimetric NH3 assay using the indophenol blue method in different background solutions: a, b 0.05 M H2SO4, c, d 0.1 M PBS, and e, f 0.1 M NaOH. The error bars correspond to the standard deviations of multiple measurements.

5

Absorbance

a

1.5

0 M 2 M 4 M 6 M 8 M

1.0

0.5

0.0 420

430

440

450

460

470

480

490

500

Wavelength (nm)

b

0.7

y = 0.0554x + 0.1754 R2 = 0.998

Absorbance @ 458 nm

0.6

0.5

0.4

0.3

0.2

0.1 0

2

4

6

8

CN H (M) 2

4

Supplementary Figure 5. N2H4 quantification. a The UV-Vis absorption spectra and b corresponding calibration curves for the colorimetric N2H4 assay in 0.1 M PBS. The error bars correspond to the standard deviations of multiple measurements.

6

Current density (mA cm2)

a

0

-5

-10

-15

0.05 M H2SO4 0.1 M PBS 0.1 M NaOH

-20 -0.3

-0.2

-0.1

0.0

Potential (V vs. RHE)

b

40

0.05 M H2SO4 0.1 M PBS 0.1 M NaOH

Z'' (ohm)

30

20

10

0

0

10

20

30

40

50

Z' (ohm)

Supplementary Figure 6. Electrochemical measurements of carbon-paper-supported Pd/C catalyst. a Linear sweep voltammograms and b electrochemical impedance spectra of the carbon-paper-supported Pd/C catalyst (Pd/C loading = 1 mg cm−2, with 30 wt % Pd) measured in three Ar-saturated electrolytes.

7

a 0.2 V

N2

Counts (a.u.)

0.1 V H2

O2

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Retention time (min)

b

100

H2 selectivity (%)

80

60

40

20

0 -0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10


Supplementary Figure 7. Quantifying H2 production at different potentials. a Gas chromatography (GC) spectra of the gas from the headspace of the cell for the N2RR on the Pd/C catalyst in N2-saturated 0.1 M PBS at various potentials, and b the calculated H2 selectivity accordingly. The error bars correspond to standard deviations of measurements over three separately prepared samples under the same conditions. In the GC spectra, trace amount of residue oxygen was detected after long time N2 flushing. H2 signal was detected since 0 V and the H2 selectivity increased rapidly at more negative potentials. Combing the data with the obtained NH3 selectivity, the unaccounted value may be attributed to the capacitance of the carbon support as well as dynamic hydrogen adsorption and absorption on Pd.

8

15

N2

14

N2

Ar

15

NH+4 Standard

14

NH+4 Standard

7.3

7.2

7.1

7.0

6.9

Chemical shift (ppm)

Supplementary Figure 8.

15N

isotope labeling experiment. 1H nuclear magnetic resonance (NMR)

spectra were obtained for the post-electrolysis 0.1 M PBS electrolytes with 15N2, 14N2, or Ar as the feeding gas. The chemical shifts in the spectra were calibrated using dimethyl sulphoxide (DMSO) as an internal standard. In the 1H NMR spectra, a doublet coupling for 15NH4+ and a triplet coupling for 14NH4+ were distinguished for the 15N2- and 14N2-saturated electrolytes after electrolysis, confirming that the NH3 was produced from the feeding gas. No apparent signals were observed when the electrolyte was bubbled with Ar, indicating a negligible amount of background NH3 from the catalyst, the electrolyte or the environment, which is consistent with the control experiment using the indophenol blue method in Fig. 2f.

9

a

0.0


-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0

5.0

3

6

9

12

15

Time (h) 2.5

4.5 4.0

2.0

3.5 3.0

1.5

2.5 2.0

1.0

1.5 1.0

0.5

Faradaic efficiency (%)

5.5

3

b NH3 yield rate (gNH mg1 h1) Pd

0

0.5 0.0

0.0 0

3

6

9

12

15

Time (h)

Supplementary Figure 9. Durability measurement. a Stability test of the Pd/C catalyst in N2-saturated 0.1 M PBS at −0.05 V vs. RHE under consecutive recycling electrolysis. b Corresponding NH3 yield rates and Faradaic efficiencies calculated after each cycle. TEM images of the Pd/C catalyst c before and d after the stability test. The scale bars are 50 nm in c and 100 nm in d.

10

c

d

Au/C PDF#65-2870

(111)

Intensity

Intensity

(111)

Pt/C PDF#65-2868

(200)

(200) (311)

(220)

(220)

(311) (222)

Carbon 20

30

40

50

60

70

80

(222)

Carbon 90

20

30

40

50

2 (O)

f

e

Au 4f7/2

Pt 4f5/2

C 1s

92

90

88

86

84

82

Intensity

Intensity

Intensity

94

70

80

90

Pt 4f7/2

Intensity

Au 4f5/2

60

2 (O)

80

Binding energy (eV)

C 1s

80

78

76

74

72

70

68

Binding energy (eV)

O 1s Pt 4p

O 1s

Pt 4d Pt 4f

Au 4f

900

800

700

600

500

400

300

Binding energy (eV)

200

100

0

900

800

700

600

500

400

300

200

100

0

Binding energy (eV)

Supplementary Figure 10. Physical characterizations of Au/C and Pt/C catalysts. a, b TEM images, c, d XRD patterns, and e, f XPS spectra of the a, c, e Au/C and b, d, f Pt/C catalysts prepared via the polyol reduction method.

11


0

-1

-2

-3

Au/C Pd/C Pt/C

-4

-5

-6 -0.3

-0.2

-0.1

0.0


Supplementary Figure 11. Linear sweep voltammograms of the glassy-carbon-supported Au/C, Pd/C, and Pt/C catalysts measured in Ar-saturated 0.1 M PBS at a scan rate of 5 mV s−1.

12

Supplementary Figure 12. Differential adsorption free energy diagram of *H. Differential adsorption free energy diagram of *H was calculated for different *H coverages on the (211) surfaces of a Au, b Pt, and c -PdH.

13

Supplementary Figure 13. Comparison of free energy change for surface hydrogenation of N2 on PdH(211) with (solid line) and without (dashed line) hydride transfer.

14

Supplementary Table 1. Comparison of the N2RR performance of the Pd/C catalyst with other catalysts reported to date under ambient conditions (room temperature and atmospheric pressure).

Catalyst

Electrolyte

Potential V (vs. RHE) 0.1

Pd/C

NH3 yield rate 4.4 μg mg−1Pd h−1

Faradaic efficiency

Reference

8.2%

0.1 M PBS

This work −1

−0.05

4.9 μg mg

Pd

h

−1

2.4%

Au nanorod

0.1 M KOH

−0.2

1.648 μg cm−2 h−1

3.88%

Adv. Mater. 29, 1604799 (2017).

Au cluster/TiO2

0.1 M HCl

−0.2

21.4 μg mg−1 h−1

8.11%

Adv. Mater. 29, 1606550 (2017).

a-Au/CeO-RGO

0.1 M HCl

−0.2

8.3 μg mg−1 h−1

10.1%

Adv. Mater. 29, 1700001 (2017).

PEBCD/C

0.5 M Li2SO4

−0.5

1.58 μg cm−2 h−1

2.85%

Ru

2 M KOH

0.25 μg cm−2 h−1

0.92%

11.4 μmol m−2 s−1

0.52%

0.22 μg cm−2 h−1