Regenerative polymer electrolyte membrane (PEM) fuel cell technology is promising for sustainable power generation in the future. The two reactions taking place on the oxygen electrode in a regenerative PEM fuel cell are oxygen reduction (ORR) in the fuel cell mode and oxygen evolution (OER) in the electrolyzer mode. State-of-the-art catalysts for these reactions are precious metal based – Pt for ORR and Ir (or Ru) for OER. These are extremely expensive and are active only for one of the two reactions, either ORR or OER, but not both. This necessitates the development of cheaper alternatives with good bifunctional characteristics as catalysts for both ORR and OER in regenerative PEM fuel cells. Bifunctional characteristics of nitrogen-doped carbon nanostructures (CN_x) that have been shown to demonstrate high ORR activity in acidic medium [1], are explored in this work.

ORR electrochemical activity measurements performed using a rotating disk electrode (RDE) show significant ORR activity of CN_x catalysts which is lower than that of Pt/C but higher than that shown by Ir/C. Pt/C and Ir/C exhibit the lowest and highest OER activity, respectively, while OER over-potential of CN_x is found to be similar to state-of-the-art Ir/C catalyst (1.62 V and 1.59 V vs. RHE). Analysis of bifunctional activity in terms of total ORR and OER over-potential, demonstrates much better bifunctional characteristics of CN_x compared to Ir/C and Pt/C catalysts. Operando mass spectrometry experiments are performed to confirm the absence of carbon corrosion [2].

Use of poisoning probes is a common methodology to investigate the nature of active sites in catalytic materials. However, CN_x catalysts cannot be poisoned using probe molecules like H₂S, CO and CN^- indicating the absence of a metal-centered active site [3]. Our recent work has identified, for the first time, that phosphate anions can be used as poisoning probes for CN_x [4]. Poisoning phenomenon associated with phosphate anions reveals the presence of two possible active sites: (i) pyridinic N itself which is rendered inactive by protonation, and (ii) carbon next to pyridinic N (i.e. pyridinic N as an active site marker) where poisoning is caused by a site-blocking effect caused by H₂PO₄^- adsorption on carbon. As the H₂PO₄^- concentration increases, there is a decrease in pyridinic N content of CN_x which correlates with the decrease in ORR activity.

Based on the above observations, distribution of pyridinic N species in CN_x is changed by changing the pyrolysis conditions during carbon growth. XPS performed on CN_x samples pyrolyzed at different temperatures reveals an increase in pyridinic nitrogen content with increasing temperature, with C:N ratio remaining the same in all samples. Laser Raman Spectroscopy shows a slight increase in the degree of graphitization with increasing pyrolysis temperature. Electrochemical activity measurements of these samples show higher ORR and OER activity at higher pyrolysis temperatures. Specific ORR kinetic current (i_k) and OER current at 1.63 V correlate very well with increasing pyridinic nitrogen content in the samples [2]. ORR activity and hence bifunctional characteristics are also found to improve by incorporation of chloride species in CN_x samples.

The insights gained in this study will help in the rational design of efficient bifunctional catalysts for PEM regenerative fuel cells.

References

[1] P.H. Matter, L. Zhang, U.S. Ozkan, J. Catal., 239 (2006) 83-96.

[2] K. Mamtani, D. Jain, D. Dogu, V. Gustin, S. Gunduz, A.C. Co, U.S. Ozkan, Appl. Catal. B-Environ., 220 (2018) 88-97.

[3] K. Mamtani, U.S. Ozkan, Catal. Lett., 145 (2015) 436-450.

[4] K. Mamtani, D. Jain, D. Zemlyanov, G. Celik, J. Luthman, G. Renkes, A.C. Co, U.S. Ozkan, ACS Catalysis, 6 (2016) 7249-7259.