Preparation Of Transition Metal Nitride Based Catalysts And Their Catalytic Performance Towards Oxygen Reduction Reaction

Proton exchange membrane fuel cells(PEMFCs) have been recognized as a kind of advanced energy technologies, due to their advantages such as high energy conversion efficiency, environmental benign, start quickly, not rely on the fossil fuels etc. However, the commercialization of fuel cell technologies is still hampered by the several obstructions, one of them is the high cost, mainly caused by the utilization of precious platinum as catalyst. To address these issues, non-platinum and low-platinum catalysts have been widely explored in the past decade and great achievements have been obtained. As one of the most important low-platinum catalyst, core–shell structured catalyst is considered as one of the most promising catalyst due to it could minimize the Pt usage. Actually, the core-shell structured catalyst is regarded to be the most likely applied practically in the near future. However, the core elements are still costly metals of current core-shell catalysts, such as Pd, Ir, Au and Ru. In addition, carbon black is still the most widely used support for the catalyst; yet corrosion of the carbon support by electrochemical oxidation under fuel cell operating conditions causes the precious metals to aggregate and separate from the carbon support, with consequent performance degradation have been extensively reported. Thus, new strategies to develop efficient ORR catalysts remain greatly needed.By a method called as complexation-nitridation method suggested by our group previously, titanium nitride and titanium based binary transition metal nitride with a particle size of less than 10 nm were successfully prepared, it is interesting that the nitrides exhibited good oxygen reduction activity in alkaline media, and the performance can be further enhanced by doping second transition metal elements. Then the transition metal nitride nanoparticles were used as the core, and ultra-thin platinum shell composited of 2-3 atomic layers were deposited on the nitride surface via a pulsed electrodeposition technology. The platinum mass activity of the catalyst could be 4-6 times higher than that of commercial Pt/C catalyst, and comparable completely to that of the typical core-shell catalysts with the noble metals as the core. Furthermore, in order to improve the dispersion and reduce the particle size of the nitride nanoparticles, we try to use nitrided carbon nanotubes as the support of nitride nanoparticles, then we prepared Pt coated catalyst by depositing Pt layer on the nitride nanoparticles. The structure of the nitride based core-shell catalysts and the interactions between the nitride core and the Pt shell were also investigated in this thesis.(1) Genarally, transition metal nitrides were synthesized via nitridation of the transition metal oxide in nitrogen or ammonia gas atmosphere with a high temperature, 1000℃ or higher, resulting in the large particle(micron scale) and poor activity. To solve this problem, a method was suggested to prepare nitride nanoparticles, in which the complex of transition metals, but not oxide, was used as precursor, and the nitridation was undergone at a very low temperature(600-700℃). With this new method, the nitride nanoparticles with a particle size less than 10 nm was successfully prepared. It is interesting that the nitride prepared with this new method, not coated with Pt, exhibited good oxygen reduction activity in alkaline media. Furthermore, we found that the performance of the nitrides could be enhanced significantly by doping with second transition elecment, such as Fe, Co and Ni etc, In 0.1 M KOH solution, the performance of Ni doped TiN catalyst was almost comparable to that of commercial JM Pt/C; the diffusion current density reached 5.3 mAcm–2, and the halfway potential was only 71 mV less than that of commercial JM Pt/C. It should be noted that the nitride catalyst showed high stability and only a slight drop in its current density after durability testing in acidic conditions.(2) On the above works, an ultra-thin Pt layer was deposited successfully on the nitride nanoparticles with a pulse deposition method, and core-shell structured Ti N@Pt and TiMN@Pt(M=Fe, Co, Ni, Mo etc.) were synthesized. It was demonstrated that the catalyst exhibited outstanding ORR activity and stability. The TiN@Pt and TiNiN@Pt catalysts exhibited 2.8 and 4.1 times in mass activity, respectively, compared with the commercial Pt/C catalyst. The Pt mass activity of our nitride-based core–shell catalyst was completely comparable to activities reported for most core–shell catalysts with precious metal cores. In addition, it also showed excellent stability/durability, experiencing only a slight performance loss after 10,000 potential cycles(9%), while Pt/C dropped 55% of its initial value, suggesting the catalyst’s outstanding performance.(3) In the synthesis process of transition metal nitride nanoparticles, we found that the nanoparticles tended to aggregate and cannot be well dispersed. To solve this problem, we used nitrided carbon nanotubes(NCNTs) as a support for nitride nanoparticles, and then the ultra-thin Pt layer was deposited on the nitride nanoparticles via a pulse deposition method. Interestingly, Pt atoms tend to deposite on the nitride nanoparticles instead of the carbon nanotubes with the optimized experimental conditions. Well-dispersed nitride nanoparticles with a particle size of around 5 nm compared with ca. 10 nm without the CNTs were prepared. After Pt deposition, a novel nitride-based low-Pt catalyst, with outstanding ORR performance and high stability was successfully prepared. The Pt mass activity and specific activity of TiN@Pt/NCNTs at 0.9 V were 3.6 and 2.9 times higher than that of the Pt/C catalyst, and the Pt mass activity of the catalyst is 40% higher than that of the catalyst without CNTs. We also found that the addition of a small amount of Cu significantly enhanced the catalyst’s performance, and we identified that the addition of Cu caused a negative shift in the Pt 4f binding energy, resulting in the catalyst’s improved ORR performance. Most importantly, our catalyst also exhibited outstanding stability/durability after 10000 CV cycles, indicating that the introduction of NCNTs did not have a negative impact on the advantages of our original nitride-based core–shell catalyst.(4) Inspired by the promotion of the nitrided carbon nanotubes, we attempted to prepare transition metal nitride nanotubes and Pt layer coated nitride nanotubes. Firstly, we synthesized the metal oxide nanotubes with controllable morphology through solvothermal method, followed by nitriding the oxide nanotubes in the ammonia flow, then, we deposited Pt layer on the surface of the nitride nanotubes to prepare a Pt coated catalyst. The experimental results showed that the synthesized titanium nitride nanotubes exhibited good oxygen reduction activity in both alkaline and acidic media, and the ORR activity is superior to that of the nitride nanoparticles with the half-wave potential shift of 38 mV. The limiting current density reached 4.7 mA cm–2 compared with 4 mAcm–2 for nitride nanoparticles. Furthermore, Pt coated nitride nanotubes catalysts were prepared by depositing Pt layer on the surface of the TiN nanotubes, it is found that the ORR activity of the catalyst used nitride nanotubes as Pt support is almost 50% higher than that used the nitride particles as support. The performance, structure and forming mechanism remain to be further in-depth study.