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Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel

Abstract

Electroreduction of CO2 into useful fuels, especially if driven by renewable energy, represents a potentially ‘clean’ strategy for replacing fossil feedstocks and dealing with increasing CO2 emissions and their adverse effects on climate1,2,3,4. The critical bottleneck lies in activating CO2 into the CO2•− radical anion or other intermediates that can be converted further, as the activation usually requires impractically high overpotentials. Recently, electrocatalysts based on oxide-derived metal nanostructures have been shown5,6,7,8 to enable CO2 reduction at low overpotentials. However, it remains unclear how the electrocatalytic activity of these metals is influenced by their native oxides, mainly because microstructural features such as interfaces and defects9 influence CO2 reduction activity yet are difficult to control. To evaluate the role of the two different catalytic sites, here we fabricate two kinds of four-atom-thick layers: pure cobalt metal, and co-existing domains of cobalt metal and cobalt oxide. Cobalt mainly produces formate (HCOO) during CO2 electroreduction; we find that surface cobalt atoms of the atomically thin layers have higher intrinsic activity and selectivity towards formate production, at lower overpotentials, than do surface cobalt atoms on bulk samples. Partial oxidation of the atomic layers further increases their intrinsic activity, allowing us to realize stable current densities of about 10 milliamperes per square centimetre over 40 hours, with approximately 90 per cent formate selectivity at an overpotential of only 0.24 volts, which outperforms previously reported metal or metal oxide electrodes evaluated under comparable conditions1,2,6,7,10. The correct morphology and oxidation state can thus transform a material from one considered nearly non-catalytic for the CO2 electroreduction reaction into an active catalyst. These findings point to new opportunities for manipulating and improving the CO2 electroreduction properties of metal systems, especially once the influence of both the atomic-scale structure and the presence of oxide are mechanistically better understood.

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Figure 1: Synthetic scheme and characterizations of Co 4-atom-thick layers with and without surface oxide.
Figure 2: Characterizations for the partially oxidized Co 4-atom-thick layers obtained at 220 °C for 3 h.
Figure 3: Electroreduction of CO2 to formate.
Figure 4: Comparison of CO2 adsorption amount and ECSA-corrected Tafel plots.

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Acknowledgements

This work was financially supported by the National Basic Research Program of China (grant number 2015CB932302), the National Nature Science Foundation (grant numbers 21422107, 21331005, 91422303, 21201157 and 11321503), the Program for New Century Excellent Talents in University (grant number NCET-13-0546), the Youth Innovation Promotion Association of CAS (grant number CX2340000100), the Chinese Academy of Science (grant number XDB01020300), the Fundamental Research Funds for the Central Universities (grant number WK2340000063) and the Scientific Research Grant of the Hefei Science Center of CAS (grant numbers 2015HSC-UE006 and 2015HSC-UP015).

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Authors

Contributions

Y.X., Y.S. and S.G. conceived the idea and co-wrote the paper. Y.S., S.G., Y.L., X.J. and D.L. carried out the sample synthesis, characterization and CO2 reduction meansurement. Y.S., S.G., Q.L., W.Z. and J.Y. discussed the catalytic process. All the authors contributed to the overall scientific interpretation and edited the manuscript.

Corresponding authors

Correspondence to Yongfu Sun or Yi Xie.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Characterizations for the comparable products.

a, b, SEM image (a) and XRD pattern (b) for Co(OH)2 sheets. c, d, TEM image (c) and XRD pattern (d) for large and irregular Co particles. In the case where only n-butylamine was present, the reaction produced two-dimensional Co(OH)2 sheets (a, b), whereas the reaction yielded large and irregular Co particles when only dimethylformamide was used (c, d). These results indicated that n-butylamine favoured the formation of a sheet-like morphology, while dimethylformamide was beneficial in reducing the cobalt ions with high oxidation states. JCDPS, the Joint Committee on Powder Diffraction Standards.

Extended Data Figure 2 Characterizations for the intermediate products.

TEM images for the obtained products at 220 °C for 0.5 h (a) and 2 h (b).

Extended Data Figure 3 Supplementary characterizations for the partially oxidized Co 4-atom-thick layers.

a, TEM image. b, XRD pattern. c, High-resolution TEM image, in which the majority of these two-dimensional sheets corresponds to the [001]-oriented hexagonal Co, while the other structural domains denoted by red squares correspond to the cubic Co3O4.

Extended Data Figure 4 Characterizations for the Co 4-atom-thick layers.

a, XRD pattern. b, Atomic force microscope image. c, The corresponding height profile. Data are shown for the products obtained at 220 °C for 48 h.

Extended Data Figure 5 Characterizations for partially oxidized bulk Co and bulk Co particles.

a, XRD patterns. b, Micro-Raman spectra. c, SEM image for partially oxidized bulk Co particles. d, SEM image for bulk Co particles.

Extended Data Figure 6 NMR spectra and formate yield.

a, Representative NMR spectra of the electrolyte after CO2 reduction electrolysis at −0.85 V versus SCE for the partially oxidized Co 4-atom-thick layers. DMSO is used as an internal standard for quantification of HCOO. b, Formate yield at the corresponding potentials with the highest Faradaic efficiencies for the partially oxidized Co 4-atom-thick layers, pure Co 4-atom-thick layers, partially oxidized bulk Co and bulk Co. Independently prepared electrodes evaluated under identical conditions in b exhibited a variability of <10% for the formate yield. c, d, 13C spectra (c) and 1H-NMR spectra (d) of the electrolyte after 8 h 13CO2 reduction electrolysis at −0.85 V versus SCE for the partially oxidized Co 4-atom-thick layers.

Extended Data Figure 7 Comparison for Co 4-atom-thick layers in the absence or presence of cobalt oxide with different concentrations.

a, XRD patterns. b, Raman spectra. c, Linear sweep voltammetric curves. Data are for a CO2-saturated 0.1 M Na2SO4 aqueous solution for the partially oxidized Co 4-atom-thick layers obtained at 220 °C for 3 h, partially oxidized Co 4-atom-thick layers obtained at 220 °C for 24 h (the synthesis process is the same as that for fabricating the partially oxidized Co 4-atom-thick layers obtained at 220 °C for 3 h except that the synthetic time is increased from 3 h to 24 h; note that the increased reaction time results in the decreased amount of cobalt oxide in the Co 4-atom-thick layers), and pure Co 4-atom-thick layers obtained at 220 °C for 48 h.

Extended Data Figure 8 Characterizations for the partially oxidized Co 4-atom-thick layers after the 40-h test.

a, TEM image for the partially oxidized Co 4-atom-thick layers after the 40-h CO2 reduction test. b, c, XRD patterns (b) and Raman spectra (c) for the partially oxidized Co 4-atom-thick layers before and after the 40-h CO2 reduction test. The samples for the above characterizations were collected as follows: the working electrodes after 40 h of electrolysis were sonicated in ethanol for about 3 min and then the samples were collected by centrifuging the mixture, washed with cyclohexane and absolute ethanol (1:4) many times, and then dried in vacuum. The above process was performed on approximately 50 similar working electrodes and all the samples collected were used to conduct the above characterizations.

Extended Data Figure 9 XRD patterns and Raman spectra before and after 40-h electrolysis at −0.85 V versus SCE for Co 4-atom-thick layers and bulk Co.

a, b, XRD patterns (a) and Raman spectra (b) for Co 4-atom-thick layers. c, d, XRD patterns (c) and Raman spectra (d) for bulk Co.

Extended Data Figure 10 XRD patterns and Raman spectra before and after repeating linear sweep voltammetry measurement scanning from −0.35 V versus SCE to different potentials (versus SCE) about 300 times.

a, b, XRD patterns (a) and Raman spectra (b) for Co 4-atom-thick layers. c, d, XRD patterns (c) and Raman spectra (d) for bulk Co.

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Gao, S., Lin, Y., Jiao, X. et al. Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature 529, 68–71 (2016). https://doi.org/10.1038/nature16455

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