Erjun Zhao

A direct four-electron process on Fe–N3 doped graphene for the oxygen reduction reaction: a theoretical perspective

As one of the potential candidates for electrocatalysis, non-precious transition metal and nitrogen co-doped graphene has attracted extensive attention in recent years. A deep understanding of the oxygen reduction reaction (ORR) mechanism including the specific active sites and reaction pathways will contribute to the further enhancement of the catalytic activity. In this study, the reaction mechanism for ORR on Fe–N3 doped graphene (Fe–N3-Gra) is investigated theoretically. Our results show that Fe–N3-Gra is thermodynamically stable. The ORR elementary reactions take place within a small region around the Fe–N3 moiety and its adjacent six C atoms. HOOH does not exist on the catalyst surface, indicating a direct four-electron process for Fe–N3-Gra. The kinetically most favorable pathway is O2 hydrogenation, in which the formation of the second H2O is the rate-determining step with an energy barrier of 0.87 eV. This value is close to 0.80 eV for pure Pt, suggesting that Fe–N3-Gra could be a potential electrocatalyst. Free energy changes at different electrode potentials are also discussed.

THE EFFECT OF HIGH N-DOPED ANATASE TiO2 ON THE BAND GAP NARROWING AND REDSHIFT BY FIRST-PRINCIPLES

Anatase TiO2 supercells were studied by first-principles, in which one was undoped and another three were high N-doping. Partial densities of states, band structure, population and absorption spectrum were calculated. The calculated results indicated that in the condition of TiO2-xNx (x = 0.0625, 0.125, 0.25), the higher the doping concentration is, the shorter will be the lattice parameters parallel to the direction of c-axis. The strength of covalent bond significantly varied. The formation energy increases at first, and then decreases. The doping models become less stable as N-doping concentration increases. Meanwhile, the narrower the band gap is, the more significant will be the redshift, which is in agreement with the experimental results.

Structural optimization and physical properties of TcB3 and MoB3 at high-pressure: First-principles

The thermodynamic, mechanical and dynamic properties of TcB3 and MoB3 are systematically investigated at high-pressure by first-principles within density functional theory (DFT). The calculated formation enthalpies are negative for TcB3 with considered structures under the pressure range from 0 to 100 GPa. Triboride hP4-TcB3 (i.e., TcB3 in hP4-OsB3 type structure) has the lowest formation enthalpy of −1.44 eV under ambient condition. The largest shear modulus of 240 GPa and smallest Poisson’s ratio of 0.20 for oP16-TcB3 are comparable to those of 267 GPa and 0.15 for ReB2. The calculated elastic constants show that MB3 (M=Tc and Mo) are mechanically stable at ambient conditions, except for mP8-MoB3. The estimated high hardness of 33.4 and 33.1 GPa for oP16-TcB3 and hP4-TcB3, respectively, are reported for the first time. The calculated lattice parameters for MoB3 are in good agreement with the previously theoretical and experimental studies. Below 13 GPa, hP16-MoB3 and hR24-MoB3 are thermodynamically more favorable than MoB3 in other structures. A pressure-induced phase transition is predicted at 13 GPa from hP16-MoB3 and hR24-MoB3 to hP4-MoB3. Above 13 GPa, hP4-MoB3 becomes the thermodynamically most stable phase among MoB3 in considered structures. All compounds with considered structures are metallic, and the electronic structures of MB3 are governed by a strong hybridization between M-4d and B-2p states. The strong and directional covalent bonding between M-4d and B-2p as well as the strong interlayer interactions of boron layers are correlated to the high hardness of 38.0 and 38.4 GPa for hP16-MoB3 and hR24-MoB3, respectively.

Theoretical Investigation on the Reaction Pathways of the Oxygen Reduction Reaction on Graphene Codoped with Manganese and Phosphorus as a Potential Nonprecious Metal Catalyst

Nonprecious‐metal‐doped graphene catalysts have been proposed recently as promising candidates to substitute Pt catalysts for the oxygen reduction reaction (ORR) in fuel cells. We codoped Mn and P in divacancy graphene (MnPx, x=1–4) and we studied the stability and the catalytic activity for the ORR. The calculated formation energy indicates that MnP2‐doped divacancy graphene is energetically the most stable. The MnP2 moiety and its adjacent six C atoms are catalytically active sites for the ORR. The kinetically most favorable pathway is the hydrogenation of OOH to form O+H2O, which is a four‐electron process. The rate‐determining step is the second H2O formation, which has an energy barrier of 0.91 eV. The free energy diagrams show that for OOH hydrogenation into O+H2O all of the elementary steps are downhill at potentials of 0.0–0.67 V except for the second H2O formation.

Structural determination and physical properties of 4d transitional metal diborides by first-principles calculations

The structural determination, thermodynamic, mechanical, dynamic and electronic properties of 4d transitional metal diborides MB2 (M = Y–Ag) are systematically investigated by first-principles within the density functional theory (DFT). For each diboride, five structures are considered, i.e. AlB2-, ReB2-, OsB2-, MoB2- and WB2-type structures. The calculated lattice parameters are in good agreement with the previously theoretical and experimental studies. The formation enthalpy increases from YB2 to AgB2 in AlB2-type structure (similar to MoB2- and WB2-type). While the formation enthalpy decreases from YB2 to MoB2, reached minimum value to TcB2, and then increases gradually in ReB2-type structure (similar to OsB2-type), which is consistent with the results of the calculated density of states. The structural stability of these materials relates mainly on electronegative of metals, boron structure and bond characters. Among the considered structures, TcB2–ReB2 (TcB2–ReB2 represents TcB2 in ReB2-type structure, the same hereinafter) has the largest shear modulus (248 GPa), and is the hardest compound. The number of electrons transferred from metals to boron atoms and the calculated densities of states (DOS) indicate that each diboride is a complex mixture of metallic, ionic and covalent characteristics. Trends are discussed.