(282g) Active Sites in Nitrogen-Doped Carbon Nanostructures for Oxygen Reduction and Oxygen Evolution Reactions

Regenerative polymer electrolyte membrane (PEM) fuel cell technology has a promising future for sustainable power generation. In regenerative PEM fuel cells, oxygen reduction (ORR) and oxygen evolution (OER) are two important reactions that take place in the fuel cell mode and electrolyzer mode, respectively. State-of-the-art catalysts used for these reactions – Pt for ORR and Ir (or Ru) for OER – are expensive and found in limited reserves. Also, both these catalysts are active only for one of the two reactions, thus necessitating the development of cheaper materials that can serve as efficient bifunctional catalysts for both ORR and OER in regenerative PEM fuel cells. We have reported nitrogen doped carbon nanostructures, denoted as ‘CN_x’ [1], to be active, selective and stable catalysts for ORR in acidic medium. Our current focus is to further evaluate the applicability of CN_x as bifunctional ORR and OER catalysts, and also understand the nature of active sites in these materials.

Electrocatalytic activity measurements using rotating ring disk electrode (RRDE) show significant ORR and OER activity of CN_x compared to state-of-the-art Pt and Ir-based catalysts. Analysis of bifunctional electrocatalytic activity demonstrates much better bifunctional characteristics of CN_x compared to Ir/C and Pt/C [2].

Use of poisoning probes is a useful methodology to systematically examine the nature of active sites in catalytic materials. However, due to the absence of a metal centered active site, CN_x materials are not susceptible to poisoning by CO, H₂S or CN^- [3, 4]. We have recently identified phosphate anions (H₂PO₄^-) as probe molecules for poisoning CN_x catalysts in an acidic ORR environment [5]. In-situ RRDE measurements show a linear decrease in ORR activity with increasing H₂PO₄^- anion concentration. Transmission IR spectroscopy and Raman spectroscopy reveal the presence of H₂PO₄^- species on H₃PO₄–soaked CN_x. A linear decrease in ORR kinetic current with a decrease in pyridinic nitrogen content obtained using X-ray photoelectron spectroscopy (XPS) is observed. This poisoning phenomenon is consistent with two possible active site models: (i) pyridinic N as the active site which is rendered inactive by protonation [6, 7], and (ii) carbon sites adjacent to pyridinic N as the active site (i.e. pyridinic N as a marker for the active site) wherein poisoning is caused by a site-blocking effect due to adsorption of H₂PO₄^- species on carbon. OER activity has also been found to increase with increasing pyridinic nitrogen site density in CN_x samples.

References

[1] P.H. Matter, U.S. Ozkan, Non-metal catalysts for dioxygen reduction in an acidic electrolyte, Catal. Lett., 109 (2006) 115-123.

[2] K. Mamtani, D. Jain, D. Dogu, A. C. Co, U. S. Ozkan, Insights into Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) Active Sites for Nitrogen-doped Carbon Nanostructures (CNx) in Acidic Media, Applied Catalysis B: Environmental, Under Review, (2017).

[3] D. von Deak, D. Singh, J.C. King, U.S. Ozkan, Use of carbon monoxide and cyanide to probe the active sites on nitrogen-doped carbon catalysts for oxygen reduction, Appl. Catal. B-Environ., 113-114 (2012) 126-133.

[4] D. von Deak, D. Singh, E.J. Biddinger, J.C. King, B. Bayram, J.T. Miller, U.S. Ozkan, Investigation of sulfur poisoning of CNx oxygen reduction catalysts for PEM fuel cells, J. Catal., 285 (2012) 145-151.

[5] K. Mamtani, D. Jain, D. Zemlyanov, G. Celik, J. Luthman, G. Renkes, A.C. Co, U.S. Ozkan, Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CN x) in Acidic Media Using Phosphate Anion, ACS Catalysis, 6 (2016) 7249-7259.

[6] G. Liu, X. Li, P. Ganesan, B.N. Popov, Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells, Electrochim. Acta, 55 (2010) 2853-2858.

[7] X. Li, G. Liu, B.N. Popov, Activity and stability of non-precious metal catalysts for oxygen reduction in acid and alkaline electrolytes, J. Power Sources, 195 (2010) 6373-6378.