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

Authors: 
Jain, D., The Ohio State University
Mamtani, K., The Ohio State University
Co, A., The Ohio State University
Ozkan, U. S., The Ohio State University
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 ‘CNx’ [1], to be active, selective and stable catalysts for ORR in acidic medium. Our current focus is to further evaluate the applicability of CNx 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 CNx compared to state-of-the-art Pt and Ir-based catalysts. Analysis of bifunctional electrocatalytic activity demonstrates much better bifunctional characteristics of CNx 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, CNx 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 CNx 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 CNx. 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 CNx 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.