GRADUATE SCHOOL OF SCIENCES & ENGINEERING
CHEMICAL AND BIOLOGICAL ENGINEERING
MS THESIS DEFENSE BY SEÇİL ÜNSAL
Title: Supercritical Fluid Assisted Preparation of Co Promoted N-Doped Carbon Aerogel Electrocatalysts for Oxygen Reduction Reaction and Investigation of the Nature of Active Sites
Speaker: Seçil Ünsal
Time: July 03, 2018, 16.00
Rumeli Feneri Yolu
Thesis Committee Members:
Prof Dr. Can Erkey (Advisor, Koç University)
Assoc. Prof. Dr. Uğur Ünal (Koç University)
Asst. Prof Dr. Zeynep Ülker Demir (Altınbaş University, Istanbul)
Oxygen Reduction Reaction (ORR) plays an important role in various devices that are under development for energy conversion and storage such as fuel cells and metal-air batteries. Although carbon supported Pt and its alloys are considered as the best ORR catalysts up to now, high cost and limited supply of noble metals severely hinders the widespread commercialization of these devices. Thus, development of Pt-free carbon based nanomaterials for ORR has been attracting increased attention. Among others, transition metal containing (usually Co, Fe) N-doped carbon materials are the most promising candidates. In this study, it is reported a new highly active family of non-noble Co promoted N-doped Carbon Aerogel catalysts (N-Co-CA) prepared by a supercritical CO2 assisted technique combined with NH3 treatment. The catalyst displayed similar activity as a commercial Pt/C catalyst with a superior methanol tolerance. The active sites are created during the complex transformation of an organic aerogel to a carbon aerogel in the presence of cobalt by ammonia as the source of nitrogen and pyrolysis medium.
In the first part, the effect of various parameters on structural and electrocatalytic properties were investigated. The effect of pyrolysis temperature was examined in the range of 700 oC to 1000 oC, and the mass activity for the ORR had a maximum at 800 oC. The TEM images revealed the existence of Co nanoparticles covered by carbon shells in the samples pyrolyzed at 800 to 1000°C. The effect of Co content on ORR activity was also investigated by varying the Co loading between 8.4 wt% and 23.7 wt%. An optimum in Co loading associated with the ORR activity was detected although the changes in the ORR activity of the samples having different Co contents are not very pronounced. Additionally, the roles of the Co and N doping in enhancing the activities of the N-Co-CA catalysts were studied. N-doped Co-promoted carbon aerogel pyrolyzed at 800 oC under NH3 flow ( 9.06 mA/mgcat) exhibited nearly double the mass activity of N-doped carbon aerogel pyrolyzed at the same temperature (5.21 mA/mgcat) at 0.8 V vs a reversible hydrogen electrode (RHE). The former catalyst exhibited the 4e– reaction pathway with a superior methanol tolerance as compared to commercial 20 wt% Pt/C catalyst for ORR in alkaline media. Moreover, the effect of N source on ORR activity were investigated. In this set of samples, urea was added to the initial solution as N source in different ratios, and TEM images displayed the presence of donut-shape Co nanoparticles. The samples prepared with NH3 treatments displayed greater mass activities compared to the samples prepared with urea. The higher activities of the samples prepared with post NH3 treatment attributed to possible catalytic role of Co to form C-N active sites structures for ORR during NH3 treatment.
In the second part, the nature of the active sites for the ORR in N-Co-CA catalyst was investigated by selectively adding or removing the spectator species via base/acid/H2 treatments. Based on rotating disc electrode (RDE) and X-ray photoelectron spectroscopy (XPS) measurements, a strong correlation was found between the relative distribution of graphitic-N species and the mass activity. RDE and XPS data also suggested that the formation of graphitic-N sites is promoted by the presence of Co species. The results reveal that the introduction of Co during the synthesis is crucial although it is not solely responsible for the ORR activity.