(544hd) Nitrogen-Doped Carbon Nanostructures As Bifunctional Catalysts for Unitized Regenerative PEM Fuel Cells

Unitized regenerative polymer electrolyte membrane (PEM) fuel cells have a promising future in sustainable power generation applications, especially when combined with renewable sources like solar and wind energy. Successful operation of these electrochemical devices requires catalysts with good bifunctional activity towards oxygen reduction (ORR) and oxygen evolution (OER) reactions that take place during operation of the device in the fuel cell mode and electrolyzer mode, respectively. State-of-the-art catalyst for ORR is Pt and OER is Ir (or Ru). These precious metal-based catalysts are not only expensive, but also exhibit poor bifunctional characteristics. Nitrogen-doped carbon nanostructures (CN_x) have been shown to be active towards ORR in acidic media¹. In this study, we have evaluated the bifunctional characteristics of CN_x, while further investigating the nature of ORR and OER active sites in these catalysts.

Electrocatalytic activity measurements demonstrate much better bifunctional ORR and OER characteristics of CN_x compared to state-of-the-art Pt and Ir-based materials². CN_x catalysts are also resistant to poisoning in the presence of CO, H₂S and CN^-, indicating the absence of a metal-centered active site³. We have recently identified that phosphate anions (H₂PO₄^-) can be used as poisoning probes for investigating nature of active sites in CN_x catalysts⁴. Ex-situ and in-situ electrocatalytic measurements reveal a decrease in ORR activity after exposure to phosphate anions. X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy shed light on the poisoning phenomenon in CN_x and help identify two possible active sites. Pyridinic nitrogen content in CN_x catalysts is varied by changing the pyrolysis temperature. ORR and OER activity, and therefore, the bifunctional characteristics of CN_x catalysts are found to vary with varying pyridinic nitrogen site density in the CN_x samples².

References

1. Matter, P. H.; Zhang, L.; Ozkan, U. S., The role of nanostructure in nitrogen-containing carbon catalysts for the oxygen reduction reaction. Journal of Catalysis 2006, 239, 83-96.
2. Mamtani, K.; Jain, D.; Dogu, D.; Gustin, V.; Gunduz, S.; Co, A. C.; Ozkan, U. S., Insights into oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) active sites for nitrogen-doped carbon nanostructures (CNx) in acidic media. Appl. Catal. B-Environ. 2018, 220, 88-97.
3. Mamtani, K.; Ozkan, U. S., Heteroatom-doped carbon nanostructures as oxygen reduction reaction catalysts in acidic media: An overview". Catal. Lett. 2015, 145, 436-450.
4. Mamtani, K.; Jain, D.; Zemlyanov, D.; Celik, G.; Luthman, J.; Renkes, G.; Co, A. C.; Ozkan, U. S., Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CN x) in Acidic Media Using Phosphate Anion. ACS Catalysis 2016, 6 (10), 7249-7259.