Study Of Oxygen Reduction Catalyzed By MnO₂ In The Light Of Quantum Chemistry

In the future's electric vehicles (EV), fuel cells and metal air batteries are the major candidates of power for EV. Oxygen reduction reaction (ORR) is the common positive electrode reaction for both fuel cells and metal air batteries. However, the low reaction activity of oxygen is one of the major factors barricading the commercialization of fuel cells and metal air batteries. Generally, the over potential of an oxygen electrode is as high as 300~400mV even at a small current density, which is too high to be acceptable in practice. The key to improve the energy capacity of fuel cell and metal-air batteries are to decrease the over potential of oxygen electrode. The high cost of precious metal used in oxygen electrodes is another barrier for wide use of fuel cells and metal air batteries. Manganese dioxide (MnO2) is a promising alternative to precious metal in some cases for its proper catalytic activity and low price.First-principles calculation based on Hartree-Fock and density functional theory, together with bare cluster and embedded point charge cluster have been applied to study the mechanism of ORR on catalyst MnO2 in the present study. HF method and full electron basis set, together with ideal bare cluster Mn7O14 were applied to calculate the single point energy of the crystal planes (110), (111), (001) of MnO2. According to the calculation results of the orbital energy, average net charge and total surface average net charge, we found that (110) plane has a value of the highest HOMO energy, more surface surplus electrons. Accordingly, it has the highest catalytic activity among the three crystal planes of MnO2. The catalytic activity of the three-crystal planes decreases in order of (110), then (111), and finally (001).Real bare cluster Mn7O14 and embedded point charge cluster Mn3O6, Mn7O14 were used to simulate MnO2 doped by metallic ions and with oxygen defects. HF method and B3LYP method of density functional theory was used to calculate the single point energy of the crystals. Full electron basis set and pseudo-basis set were adopted according to the type of cluster models. The theoretical calculations showed that there are two opposite effects in adulteration of MnO2. The positive effect happens, as traces of metallic ion were adulted. In this case, both dopants and oxygen defects can increase the Fermi energy of MnO2 and increase the number of electrons of MnO2 crystalline. This is conducive to the adsorption of oxygen on MnO2 surface and to the electrons transfer from MnO2 to oxygen molecules. The negative effect takes place, as too much of dopants, regardless of transition metals or non-transition metals, or too much oxygen defects are present in MnO2 crystalline. In this case, the HOMO energy of MnO2 was lowered, especially as too much oxygen defects present. It means that it become difficult for electrons transfer from MnO2 crystalline to oxygen. The study also disclosed that the catalysis enhancement of transition metal adulteration to MnO2 is better than that of non-transition metal.The two kinds of oxygen adsorption models, i.e., Griffiths and Pauling adsorption, were investigated by means of HF approach and pseudo-basis set. The released energy during oxygen adsorption on MnO2 in these two adsorption ways is large enough to excite electrons in HOMO of MnO2 transferring to oxygen molecules. It leads to form...