Conclusions

In summary, nitrogen doped carbon nanotube arrays with total N content of 5 at.% were synthesized by liquid CVD method. The NCNT array acts as highly active, selective and stable catalysts for electrocatalytic reduction of CO2 to CO. Compared to

noble metals Au and Ag, these NCNTs exhibit lower overpotentials to achieve the similar selectivity towards the production of CO. The maximum FE of CO reaches around 80% at a low overpotential of -0.26 V. As a contrast, pristine CNTs show poor activity and selectivity towards electroreduction of CO2. The superior activity for NCNTs is mainly

attributed to a low free energy barrier for the potential-limiting step to form adsorbed COOH. Moreover, the suitable binding energy of the key intermediates enables strong COOH adsorption and feasible CO desorption that contributes to the high selectivity towards CO formation. The economical NCNTs are proposed as successful alternatives to expensive noble metal electrocatalysts for commercial CO2 reduction application, with

References

1. Chen Y, Li CW, Kanan MW. Aqueous CO2 Reduction at Very Low Overpotential

on Oxide-Derived Au Nanoparticles. J Am Chem Soc134, 19969-19972 (2012). 2. Li CW, Kanan MW. CO2 Reduction at Low Overpotential on Cu Electrodes

Resulting from the Reduction of Thick Cu2O Films. J Am Chem Soc134, 7231-

7234 (2012).

3. Wu JJ, Risalvato FG, Ke FS, Pellechia PJ, Zhou XD. Electrochemical Reduction of Carbon Dioxide I. Effects of the Electrolyte on the Selectivity and Activity with Sn Electrode. Journal of the Electrochemical Society159, F353-F359 (2012).

4. Zhang S, Kang P, Meyer TJ. Nanostructured Tin Catalysts for Selective Electrochemical Reduction of Carbon Dioxide to Formate. Journal of the American Chemical Society136, 1734-1737 (2014).

5. Rosen BA, et al. Ionic Liquid–Mediated Selective Conversion of CO2 to CO at

Low Overpotentials. Science334, 643-644 (2011).

6. Chen Z, Kang P, Zhang M-T, Stoner BR, Meyer TJ. Cu(ii)/Cu(0) electrocatalyzed CO2 and H2O splitting. Energy & Environmental Science6, 813-817 (2013).

7. Jermann B, Augustynski J. Long-term activation of the copper cathode in the course of CO2 reduction. Electrochimica Acta39, 1891-1896 (1994).

8. Voiry D, et al. Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution

Reaction. Nano Lett13, 6222-6227 (2013).

9. DeWulf DW, Jin T, Bard AJ. Electrochemical and Surface Studies of Carbon Dioxide Reduction to Methane and Ethylene at Copper Electrodes in Aqueous Solutions. Journal of The Electrochemical Society136, 1686-1691 (1989). 10. Fan MY, Bai ZY, Zhang Q, Ma CY, Zhou XD, Qiao JL. Aqueous CO2 reduction

on morphology controlled CuxO nanocatalysts at low overpotential. Rsc Advances

4, 44583-44591 (2014).

11. Kedzierzawski P, Augustynski J. Poisoning and Activation of the Gold Cathode During Electroreduction of CO2. Journal of the Electrochemical Society141,

L58-L60 (1994).

12. Delacourt C, Ridgway PL, Kerr JB, Newman J. Design of an electrochemical cell making syngas (CO+H2) from CO2 and H2O reduction at room temperature.

13. Li H, Oloman C. The electro-reduction of carbon dioxide in a continuous reactor.

Journal of Applied Electrochemistry35, 955-965 (2005).

14. Koleli F, Atilan T, Palamut N, Gizir AM, Aydin R, Hamann CH. Electrochemical reduction of CO2 at Pb- and Sn-electrodes in a fixed-bed reactor in aqueous

K2CO3 and KHCO3 media. Journal of Applied Electrochemistry33, 447-450

(2003).

15. Gong K, Du F, Xia Z, Durstock M, Dai L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science323, 760-764 (2009).

16. Li Y, et al. An oxygen reduction electrocatalyst based on carbon nanotube- graphene complexes. Nat Nano7, 394-400 (2012).

17. Kang P, Zhang S, Meyer TJ, Brookhart M. Rapid Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by an Iridium Pincer Catalyst Immobilized on Carbon Nanotube Electrodes. Angewandte Chemie126, 8853- 8857 (2014).

18. Zhao H, Zhang Y, Zhao B, Chang Y, Li Z. Electrochemical Reduction of Carbon Dioxide in an MFC–MEC System with a Layer-by-Layer Self-Assembly Carbon Nanotube/Cobalt Phthalocyanine Modified Electrode. Environmental Science & Technology46, 5198-5204 (2012).

19. Zhang S, et al. Polyethylenimine-Enhanced Electrocatalytic Reduction of CO2 to

Formate at Nitrogen-Doped Carbon Nanomaterials. J Am Chem Soc136, 7845- 7848 (2014).

20. Yadav R, Dobal P, Shripathi T, Katiyar R, Srivastava O. Effect of Growth Temperature on Bamboo-shaped Carbon-Nitrogen (C-N) Nanotubes Synthesized Using Ferrocene Acetonitrile Precursor. Nanoscale Research Letters4, 197 - 203 (2008).

21. Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Carbon nanotube filters. Nat Mater3, 610-614 (2004).

22. Kresse G, Hafner J. \textit{Ab initio} molecular dynamics for liquid metals.

Physical Review B47, 558-561 (1993).

23. Kresse G, Furthmüller J. Efficient iterative schemes for \textit{ab initio} total- energy calculations using a plane-wave basis set. Physical Review B54, 11169- 11186 (1996).

24. Perdew JP, Burke K, Ernzerhof M. Generalized Gradient Approximation Made Simple. Physical Review Letters77, 3865-3868 (1996).

25. Nørskov JK, et al. Origin of the Overpotential for Oxygen Reduction at a Fuel- Cell Cathode. The Journal of Physical Chemistry B108, 17886-17892 (2004). 26. Peterson AA, Abild-Pedersen F, Studt F, Rossmeisl J, Norskov JK. How copper

catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy & Environmental Science3, 1311-1315 (2010).

27. Dresselhaus MS, Jorio A, Hofmann M, Dresselhaus G, Saito R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett10, 751-758 (2010).

28. Wu J, Risalvato FG, Sharma PP, Pellechia PJ, Ke F-S, Zhou X-D.

Electrochemical Reduction of Carbon Dioxide II. Design, Assembly, and

Performance of Low Temperature Full Electrochemical Cells. J Electrochem Soc

160, F953-F957 (2013).

29. Kumar B, et al. Renewable and metal-free carbon nanofibre catalysts for carbon dioxide reduction. Nat Commun4, (2013).

30. Li CW, Ciston J, Kanan MW. Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature508, 504-507 (2014).

31. Azuma M, Hashimoto K, Hiramoto M, Watanabe M, Sakata T. Electrochemical Reduction of Carbon Dioxide on Various Metal Electrodes in Low‐Temperature Aqueous  KHCO3 Media. J Electrochem Soc137, 1772-1778 (1990).

32. Hori Y, et al. "Deactivation of copper electrode" in electrochemical reduction of CO2. Electrochimica Acta50, 5354-5369 (2005).

33. Kresse G, Furthmüller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B54, 11169-11186 (1996). 34. Hansen HA, Varley JB, Peterson AA, Nørskov JK. Understanding Trends in the

Electrocatalytic Activity of Metals and Enzymes for CO2 Reduction to CO. J

Phys Chem Lett4, 388-392 (2013).

35. Peterson AA, Nørskov JK. Activity Descriptors for CO2 Electroreduction to

Methane on Transition-Metal Catalysts. J Phys Chem Lett3, 251-258 (2012).