Study of Active Center Structures for Fe/N/C Type Oxygen Reduction Catalyst Using Semi-Empirical Molecular Orbital Calculations

Abstract

Polymer electrolyte fuel cells (PEFCs) are one of the most important devices in realizing hydrogen society. Platinum, which is used as the catalyst of PEFCs, is expensive and less abundant. Thus, the development of a non-platinum oxygen reduction catalyst is required for the wide-spread use of PEFCs. Fe/N/C non-platinum oxygen reduction catalysts are promising platinum alternative catalyst candidates. We have reported new metal/nitrogen/carbon-electrocatalysts synthesized by the pyrolysis of precursors consisting of Ketjenblack®, a Zn complex with 4,4’-bipyridine, 1,10-phenanthroline (phen), and iron(II) chloride1)．The density of active sites on the catalysts need to be increased for the improvement of for oxygen reduction reaction (ORR). Although we obtained that pyridine type nitrogen is related to the ORR activity, the chemical structure of the active center is still unknown. The conclusion does not match whether Fe atoms function as activity centers2-3．In this study, the structures of active center for Fe/N/C ORR catalysts were studied using semi-empirical molecular orbital calculations． Assuming that four electrons were injected into the compound, the distance between an O2 molecule and atoms was stepwise shortened with the parameters4 using MOPAC 20165. The result of the calculation was visualized by Jmol6. We used model compounds modeling a part of the edge of a graphite layer (Fig. 1(a)). Fig. 1(b) shows the dependence of ΔH on the distance between O and target atoms during shortening distance. The activation energies of route (i) was the smallest of the four routes. The cleavage of O-O bond occurred at routes (i) and (iv). The supply of further proton via H3O+ is necessary in the route (ii) and (iii). References and Notes Shimbori, H. Shiroishi, R. Ono, S. Kosaka, M. Saito, T. Yoshida, H. Katagiri, K. Miyazawa, Y. Tanaka, Mat. Sci. Eng. B, 228, 190 (2018). Lefèvre, J.P. Dodelet, P. Bertrand, J. Phys. Chem. B , 106, 8705 (2002). Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science, 351, 361 (2016).Typical calculation parameters of MOPAC used in the simulations: PM6-DH2 EF PRECISE PULAY CAMP ITRY=9999 SHIFT=50 SYMMETRY STEP=-0.025 POINT=74 NOINTER GRAPHF EPS=78.5 NSPA=200 (UHF) CHARGE=(2 to -2).MOPAC2016, James J. P. Stewart, Stewart Computational Chemistry, Colorado Springs, CO, USA, HTTP://OpenMOPAC.net (2016).Jmol: an open-source Java viewer for chemical structures in 3D. http://www.jmol.org/ Figure 1