Quantum Chemical Study Of Electrochemical Catalysis On Controllable Oxygen Reduction Mechanisms

Platinum is still the most widely used and promising cathodic electrocatalyst for oxygen reduction reaction(ORR) in the proton exchange membrane fuel cells(PEMFC). However, as a noble metal, Pt is not only very rare and extremely expensive, but it is also apt to be poisoned and lost in the presence of the intermediates of ORR and the impure gases in the antioxidant. Therefore, reducing the high cost, improving the catalytic activity and enhancing the stability of electrocatalyts are important ways to solve problems of the widespread commercialization of fuel cells. In this work, the first-principles calculation is applied to study the nature chemistry reason behind the improved ORR activity, providing a theoretical basis for preparing more effective catalysts in the experiment. The work mainly includes the following two aspects.Firstly, origin of the enhanced catalytic activity of Pt M/Pd(M=Au, Os) is investigated by the density functional theory(DFT). Au is a chemically inert metal; while Os is quite active to react with oxygen. Though Au and Os are in the two extremes in chemical properties, unexpectedly, both of them can enhance the ORR activity of Pt based alloys. In the work, we use density function theory to elucidate the mechanisms of enhanced activity in Pt M/Pd with doped atoms changing from chemically inert Au to active Os. We find doped chemically inert metal(Au) and active metal(Os) can benefit different ORR steps, respectively. Thus, regardless of the inert or the active, either of them can speed up the overall ORR. The calculations show that Pt Au/Pd and Pt Os/Pd have heterogeneous electronic structures, leading surface Pt atoms to play a different role in ORR. For Pt Au/Pd, only the Pt atoms away from Au can make the O2 adsorption and dissociation, and the doped Au is beneficial to the OH desorption. For Pt Os/Pd, however, all atoms benefit the O2 adsorption and dissociation. On Pt Os/Pd, although Os atom has strongest interaction with OH, the adsorption energy of OH is weak on Pt atoms away from Os and average EOH on top-layer Pt atoms is close to that on Pt. In addition, the easy diffusion of OH and repulsion between OH species alleviate the Pt-OHad interaction of Pt Os/Pd, which facilitates the OH desorption.Secondly, relationship between the electronic structure and the activity of Pt catalyst is investigated. Considerable experiments have demonstrated that single atom, sub-nanometer and nanometer catalysts not only improve the utilization rate of precious metal, but they also have good electrocatalytic activity. However, the changes of geometry structure and electronic structure are unclear with the change of particle size. And the relationship between the electronic structure of catalysts and catalytic property also need to be investigated further. But when the particles become smaller, the surface free energy is bigger and the catalyst is more unstable. Therefore, an appropriate support is essential to study the relationship between the particle size and catalytic property accuratly. Our team has demonstated that Ti3C2 has the special structure, ellcellent chemical stability and conductivity after removing Al layer of Ti3 Al C2 with HF acid treatment. In this work, theory calculation is applied to study the relationship between the particle size and catalytic property from the following two sides. One is the interaction between the Pt catalysts and Ti3C2 support; the other is the relationship of the electronic structure and the catalytic performance. The calculations show that the bind energy of single layer Pt atoms on the Ti3C2 support is bigger than that of two layers, regardless of Pt cluster or Pt overlayers catalyst. And the strong interaction between Pt catalysts and Ti3C2 support originate from the large overlap between the orbital of Pt-d and the orbital of Ti-d. The d band center of single layer Pt atoms shifts down and two layer Pt atoms shifts up. The adsorption energy of intermediate O on the single layer catalysts is weaker, which is beneficial to enhance the ORR activity.