ERA-Chemistry: Novel Pt-poor catalysts for the electrocatalytic O2 reduction based on modified, nanostructured metal oxides
Final Report Abstract
The main scientific findings of this project can be summarized as follows: First, we succeeded in producing composite materials consisting of a thin titania layer covering CNTs without the need of a surface functionalization and applied them as support material for Pt nanoparticle ORR catalysts, which solved the problem of high ohmic resistances of the titania without sacrificing on the ORR activity. This way we were also able to reduce the Pt dissolution in degradation tests, thus improving its stability. However, the titania layer activated new ORR degradation pathways. For the mechanism we tentatively suggest that similar to the well-known ‘strong metal-support interactions’ the repeated oxidation and reduction of the catalysts results in an overgrowth of Pt particles by a partly reduced TiOx layer, blocking the access of O2 to the Pt surface. Following this approach, we developed nitrided carbon spheres covered with a layer of titanium or tantalum (oxy)nitride and confirmed the ORR activity of these noble metal-free materials. We demonstrated that the nitriding temperature has a major influence on their ORR activity and selectivity, where the former is highest at 850°C for the titania (oxy)nitride-based catalyst and 1000°C for the tantalum (oxy)nitride-based catalysts. The ORR activity correlates mainly with the porosity of the metal (oxy)nitride shell, thus with the access to the nitrided carbon core, which indicates that in addition to the specific (oxy)nitride phases also the nitrided carbon core plays a major role in the ORR activity and selectivity of these materials. However, the stability of these composite materials could not compete with the stability of the commercial Pt/C due to structural changes of the composite during the degradation simulation. Next. Based on the above work, we explored the potential of nitrided carbon spheres on the ORR. We could show that depending on the nitriding temperatures these spheres are highly active for the ORR, exceeding the activity of the metal (oxy)nitride covered nitrided carbon spheres. By combining experimental and theoretical results we established the correlation between the nitriding temperature / nitrogen content and the ORR activity. Low amounts of graphitic nitrogen (~1 wt.%) were found to be beneficial for the ORR activity of nitrided carbon, whereas higher amounts (>7 wt.%) resulted in an increase of the ORR overpotential. Furthermore, based on DFT calculations we could identify carbon atoms at pore edges in nitrided graphenic carbon areas as the ORR active sites. Thus, the increased amounts of micropores and graphenic areas with increasing nitriding temperature result in a maximum ORR activity for samples nitrided at 1000°C, despite the fact that this catalyst showed the lowest amount of nitrogen on the surface. Based on isotope labelling experiments, evaluating kinetic isotope effects, the ORR pathway was found to be strongly pH dependent. The first electron transfer was identified as rate determining step. It is independent of the first proton transfer at high pH and coupled with a proton transfer at low pH. This led to a comprehensive insight on the correlations between structure, pH and ORR activity, which can be used for the optimization of noble metal-free ORR catalysts in future work. Last but not least, a vivid collaboration with our cooperation partners at the University of Salzburg (Prof. Hüsing) and the NIMS group (Dr. K. Sakaushi and Dr. A. Lyalin) has been established, which will continue and extend also over this project.
Publications
- Spherical core-shell titanium (oxy)nitride@nitrided carbon composites as catalysts for the oxygen reduction reaction: Synthesis and electrocatalytic performance. ChemElectroChem 2016, 3, 1641 – 1654
M. Wassner, M. Eckardt, C. Gebauer, N. Hüsing, and R. J. Behm
(See online at https://doi.org/10.1002/celc.201600246) - Synthesis and electrocatalytic performance of spherical core-shell tantalum (oxy)nitride@nitrided carbon composites in the oxygen reduction reaction. Electrochim. Acta 2017, 227, 367 – 381
M. Wassner, M. Eckardt, C. Gebauer, G. R. Bourret, N. Hüsing, and R. J. Behm
(See online at https://doi.org/10.1016/j.electacta.2016.12.145) - Microscopic Electrode Processes in the Four-Electron Oxygen Reduction on Highly-Active Carbon-based Electrocatalysts. ACS Catal. 2018, 8, 8162 – 8176
K. Sakaushi, M. Eckardt, L. Andrey, T. Taketsugu, R. J. Behm and K. Uosaki
(See online at https://doi.org/10.1021/acscatal.8b01953) - Oxygen reduction reaction activity and long-term stability of platinum nanoparticles supported on titania and titania-carbon nanotube composites. J. Power Sources 2018, 400, 580 – 591
M. Eckardt, C. Gebauer, Z. Jusys, M. Wassner, N. Hüsing, and R. J. Behm
(See online at https://doi.org/10.1016/j.jpowsour.2018.08.036) - The role of nitrogen-doping and the effect of the pH on the oxygen reduction reaction on highly active nitrided carbon sphere catalysts. Electrochim. Acta 2019, 299, 736 – 748
M. Eckardt, K. Sakaushi, A. Lyalin, M. Wassner, N. Hüsing, T. Taketsugu and R. J. Behm
(See online at https://doi.org/10.1016/j.electacta.2019.01.046)
