First-Principles Study Of Electrochemical Reaction On The CoO_x Surface

The oxygen electrode consists of two major electrode reactions: oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In which the catalytic performance for OER and ORR catalysts has become a key factor in the efficient use of the fuel cell, therefore developing efficient electrode catalyst in this area will be an important issue. The noble metal catalysts with good catalytic performance are limited in the large-scale commercial production due to the high price and the lack of resources. Recently, the materials with wide range of sources such as cobalt-based oxides Co3O4, CoO were extensive investigated due to its high activity and stability. However, theoretical aspects related reaction mechanism is unclear. Therefore, the microscopic processe was studied in this paper combined with quantum chemistry software VASP and corrected density functional theory (DFT+U), the main work is as follows:1. The First-Principles calculations were used to investigate the oxygen evolution reaction (OER) on the (100) surface of the spinel Co3O4. We compared the OER activities by the free-energy changes on three different covered surfaces including (i) clean, (ii) 0.5 monolayer O covered, and (iii) 0.5 monolayer OH covered surfaces, and the computed overpotential (r)) for the three surfaces followed the order:O covered (η= 0.45 V)< OH covered (0.93 V)< clean (1.82 V). Furthermore, the Bader analysis also showed that the Co Bader charge of +1.52/+1.50 on the O/OH covered (100) surface was more than that on the clean surface (+1.28). Based the above analysis, the O-containing species covered Co3O4 (100) surface exhibited better OER catalytic performance than that of the clean surface in the OER cycle. Overall, the desorption energy order was obtained from:clean surface> 0.5 monolayer O covered surface ≈0.5 monolayer OH covered surface, indicating the importance of O-containing species covered conditions for the fifth step of the O2 desorption reaction.2. The model of CoO (111) and (100) planes was used to explore the dissociative and associative mechanism and the Gibbs free energy change in ORR cycle. The details are summarized as following:In accordance with the dissociative mechanism of O2 adsorption process, transition state energy barrier (1.41eV) on CoO (111) surface is lower than that on the CoO (100) surface corresponding to the value of 2.20eV, and O2 adsorption process on these two crystal planes are exothermic process, the heat release value of 6.79eV on the (111) surface was more than that on the CoO (100) surface. In addition, through the electronic density analysis of the two crystal plane, the d band center of the central sites Co 12 on the CoO (111) surface are more closer with Fermi level than Co 6 atoms on the CoO (100) surface, indicating the O2 adsorption proceeded more easily on the CoO (111) surface than CoO (100) surface.In summary, it is energetically more favorable for the O-containing covered Co3O4(100) surfaces than the clean surface in the OER cycle, and the catalytic activity for ORR on CoO(111) surface was higher than that on the CoO(100) surface. These results have very important significance for the further exploration of catalytic efficiency on cobalt-based oxides for electrochemical reaction.