Gold-supported cerium-doped NiOx catalysts for water oxidation · 2016-05-06 · Gold‐Supported...

SUPPLEMENTARY INFORMATIONARTICLE NUMBER: 16053 | DOI: 10.1038/NENERGY.2016.53

NATURE ENERGY | www.nature.com/natureenergy 1

Supplementary Information

Gold‐Supported Cerium‐Doped NiOx Catalysts for Water Oxidation

Jia Wei Desmond Ng1, Max García‐Melchor2, Michal Bajdich2, Pongkarn Chakthranont1, Charlotte Kirk1, Aleksandra Vojvodic2*, Thomas F. Jaramillo1,2*

1 Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA

2 SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.

* Authors to whom correspondence should be addressed: [email protected], [email protected]

Gold-supported cerium-doped NiOx catalysts for water oxidation

http://dx.doi.org/10.1038/nenergy.2016.53

2 NATURE ENERGY | www.nature.com/natureenergy

SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.53

Supplementary Figures

(a)

Supplementary Figure 1. Physical and chemical characterization of NiCeOx−Au. (a) Ni and Ce elemental mappings and the corresponding SEM image of NiCeOx−Au taken using Auger electron spectroscopy. (b) High resolution XPS spectra of the O 1s region of NiCeOx−Au. The dotted vertical lines indicate the region where O is expected to exist as hydroxide. (c) XRD diffractograms of NiCeOx−Au and bare Au‐coated glassy carbon substrate. The "•" symbol indicates a peak arising from the glassy carbon substrate.

533 532 531 530 529

NiCeOx-Au(b)

Inte

nsity

(a.u

.)

hydroxide

Binding Energy (eV)10 20 30 40 50 60 70 80 90

(c)

Au

(222

)

Inte

nsity

(a.u

.)

Au

(111

)2 (degrees)

Bare Au-GC substrate NiCeOx-Au



SUPPLEMENTARY INFORMATIONDOI: 10.1038/NENERGY.2016.53

Supplementary Figure 2. Optimization of the Ni to Ce ratio of Ni1‐xCexOy−Au. (a) Cyclic voltammograms displaying the OER performance of the Ni1‐xCexOy−Au catalysts. The sweeps are color‐coded according to the rainbow spectrum in order of increasing amounts of Ce. (b) Activity of Ni1‐xCexOy−Au as a function of the percentage of Ni in the deposition solution. The left y‐axis represents the current density at 350 mV overpotential, while the right y‐axis represents the overpotential required to reach 10 mA/cm2.

1.2 1.3 1.4 1.5 1.6 1.7-20

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70(a) NiOx-Au Ni99Ce1Ox-Au Ni97Ce3Ox-Au Ni95Ce5Ox-Au Ni90Ce10Ox-Au Ni80Ce20Ox-Au Ni75Ce25Ox-Au Ni50Ce50Ox-Au CeOx-Au

Potential (V versus RHE)

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Supplementary Figure 3. Electrochemical profile of NiCeOx−Au in two distinct systems. Cyclic voltammograms displaying the electrochemical profile of NiCeOx‐Au in an unpurified system and a Fe‐free system.

1.2 1.3 1.4 1.5 1.6 1.7-20

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70 Unpurified system Fe-free system


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Supplementary Figure 4. Electrochemical profile of NiCeOx−Au, NiCeOx−GC, NiOx−Au and NiOx−GC. (a) Cyclic voltammograms displaying the electrochemical profile of the four catalysts for the first sweep. (b) Cyclic voltammograms of NiCeOx−Au and NiCeOx−GC for both the first and the second sweep.

1.2 1.3 1.4 1.5 1.6 1.7-10

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40(a) NiCeOx-Au NiCeOx-GC NiOx-Au NiOx-GC


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1.2 1.3 1.4 1.5 1.6 1.7-10

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40(b) NiCeOx-Au, 1st cycle NiCeOx-Au, 2nd cycle NiCeOx-GC, 1st cycle NiCeOx-GC, 2nd cycle


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1 M NaOH, 10 mV/s




Supplementary Figure 5. Optimization of the Ni to Fe ratio of Ni1‐xFexOy. (a) Activity of Ni1‐xFexOy−GC as a function of the percentage of Ni in the deposition solution. The left y‐axis represents the current density at 350 mV overpotential, while the right y‐axis represents the overpotential required to reach 10 mA/cm2. (b) Cyclic voltammograms displaying the electrochemical profile of Ni97Fe3Ox−Au and Ni97Fe3Ox−GC.

0 50 60 70 80 90 1000

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(a) (b)

(c) (d)

Supplementary Figure 6. Images of the electrodes used for water electrolysis. SEM images of (a) NiCeOx on Au‐coated Ni foam, (b) NiMo on Ni foam, and (c) bare Ni foam for comparison. (d) Photograph of the three electrodes.




Supplementary Figure 7. Evaluation of the performance of the NiCeOx−Au‐ and NiMo‐coated electrodes versus a bare Ni foam substrate. Chronopotentiometric runs of the individual electrodes at 50 mA in a three‐electrode electrochemical cell setup. The electrolyte was 6 M KOH, while the counter and reference electrodes were a carbon rod and Ag/AgCl respectively.

0 1 2 3 4 5-0.5

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NiCeOx on Au-coated Ni foam NiMo on Ni foam bare Ni foam

Time (minutes)

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l (V

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us R

HE)




Supplementary Figure 8. Photograph of electrolyser setup. The NiCeOx on Au‐coated Ni foam anode and the NiMo on Ni foam cathode was held in separate halves of an H‐cell in a 6 M KOH electrolyte and separated by an anion‐exchange membrane.




Supplementary Figure 9. Geometric structure and magnetic moments for modelled NiCex–Au catalysts. Top view of the most energetically favourable relaxed atomic structure of the finite undoped and Ce‐doped NiOx thin films supported on Au(111) during the different OER reaction steps given by Eqs. (1)‐(4) in the main text. The colour code is as follows: Au (gold), Ni (green), Ce (grey), O (red), H (white).




Supplementary Figure 10. DFT‐calculated Pourbaix diagram. Pourbaix diagram calculated using the (1x4) finite NiOx−Au thin film shown in Supplementary Fig. 13. The nomenclature (H−X, O−X and OH−X) refers to the number of H atoms on the basal plane, and the number of O and OH groups on the edge sites of the finite NiOx−Au film.




Supplementary Figure 11. Optimization of the Au substrate thickness and catalyst loading of NiCeOx−Au. (a) Cyclic voltammograms displaying the OER performance of NiCeOx−Au loaded on glassy carbon disks with different Au thicknesses. (b) Cyclic voltammograms displaying the OER performance of NiCeOx−Au with different electrodeposition times.

1.2 1.3 1.4 1.5 1.6 1.7-10

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50NiCeOx-Au

(a)

100 nm Au 20 nm Au 1 nm Au


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(b)

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Supplementary Figure 12. Stability of NiCeOx−Au. (a) Variation in the potential required to reach 10 mA/cm2 over a period of 2 hours to demonstrate the stability of NiCeOx−Au under OER testing. (b) Cyclic voltammograms displaying the OER activities of NiCeOx−Au before and after the stability test.

0 20 40 60 80 100 1201.40

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NiCeOx-AuChronopotentiometry at 10 mA/cm2 in 1M NaOH

(a)

Time (minutes)

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)

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Supplementary Figure 13. Optimized structure of a (1x4) finite NiOx−Au thin film. Atomistic side‐view of the finite NiOx−Au (1x4) thin film structure used to calculate the resting state of the catalyst surface under relevant OER conditions, i.e. pink region in the Pourbaix diagram in Supplementary Figure 10. The colour code is as follows: Au (gold), Ni (green), O (red).




Supplementary Tables

Supplementary Table 1. Corrections to the Gibbs free energies (in eV’s) for the adsorbates in the gas phase and on the surface. Zero point energies (ZPE), vibrational enthalpies and entropy contributions to the Gibbs free energies calculated for the adsorbates in the gas phase and on the surface at T = 300 K and P = 1 atm. In the case of H2O, we use P = 0.035 bars, as it is the pressure at which vapour water is in equilibrium with liquid water at T = 300 K.

ZPE Hvib TS ZPE + Hvib – TS

H2O 0.560 0.103 0.675 ‐0.012

H2 0.268 0.091 0.408 ‐0.049

O* 0.061 0.033 0.051 0.043

HO* 0.360 0.049 0.081 0.328

HOO* 0.468 0.068 0.116 0.420




Supplementary Discussion

Optimization of the thickness of the gold support and the catalyst loading

A study varying the thickness of the gold film on the GC substrate from 1 to 100 nm was conducted. Electrochemical testing revealed that increasing the amount of gold from 1 to 20 nm results in a significant improvement in activity possibly due to the increased number of metal oxide‐gold interfacial sites; however, an additional 80 nm of gold only leads to a slightly further boost in the OER activity which suggests that only a limited amount of additional gold came into contact with the catalyst layer (Supplementary Fig. 11a).

The amount of catalyst deposited on 100 nm of gold was also varied by modifying the deposition times from 1 to 60 seconds, which translates to 8 to 480 nm of catalyst assuming a linear relation between deposition time and catalyst thickness. We found that the 10‐second deposition sample (80 nm catalyst thickness) delivers the best performance (Supplementary Fig. 11b). This suggests that shorter deposition times result in less active sites to turnover the water molecules to oxygen, while longer deposition times lead to films that are structurally unstable or too thick to benefit from the influence of gold1.

Stability of NiCeOx−Au under OER testing

The stability of NiCeOx−Au was examined using the benchmarking protocol, which consists in a chronopotentiometry run with the catalyst held at 10 mA/cm2 for 2 hours in 1 M NaOH (Supplementary Fig. 12a)2. The observed slow and steady increase in potential might be attributed to a number of reasons: blocking of active sites by the O2 bubbles formed during the reaction, morphological changes of the catalyst leading to reduced access to active sites, and/or loss of catalytic material possibly due to dissolution or collateral loss of catalyst, which might also be induced by the O2 bubbles leaving the electrode surface.

The removal of O2 bubbles on the catalyst surface after the chronopotentiometry run, followed by a typical cyclic voltammetry scan, revealed that the catalyst requires only an additional 39 mV overpotential to reach 10 mA/cm2 (Supplementary Fig. 12b), thus demonstrating the excellent stability of NiCeOx−Au.




Supplementary References

1 Yeo, B. S. & Bell, A. T. In situ Raman study of nickel oxide and gold‐supported nickel oxide catalysts for the electrochemical evolution of oxygen. J. Phys. Chem. C 116, 8394‐8400 (2012).

2 McCrory, C. C. L. et al. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 137, 4347‐4357 (2015).


Gold-supported cerium-doped NiOx catalysts for water oxidation · 2016-05-06 · Gold‐Supported...

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