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. 2020 Jun 19;5(25):15268-15279.
doi: 10.1021/acsomega.0c01303. eCollection 2020 Jun 30.

Single Si-Doped Graphene as a Catalyst in Oxygen Reduction Reactions: An In Silico Study

Anton V Kuzmin  1 , Bagrat A Shainyan  1
Affiliations

Single Si-Doped Graphene as a Catalyst in Oxygen Reduction Reactions: An In Silico Study

Anton V Kuzmin et al. ACS Omega. .

Abstract

Single Si-doped graphene C53H18Si with one carbon atom replaced by a three-coordinate silicon atom is studied by density functional theory (DFT) calculations as a catalyst for the oxygen reduction reactions (ORRs) in both acidic and alkaline media. The active sites for oxygen adsorption were determined from the distribution of the charge density difference analysis. At the equilibrium electrode potential, the most stable intermediate was found to have the structure HO*O*-C53H18Si with both oxygen atoms bound to the support, one of them being incorporated in between Si and C atoms, corresponding to the transfer of one hydrogen atom [H+ + e-]. The 2e ORR mechanism is shown to be very unlikely because the alternative 4e ORR pathway occurring via intermediates with a broken O-O bond is much more exothermic. In addition to the commonly adopted ORR mechanism, new reaction pathways have been discovered and shown to be potentially preferable over the traditional mechanism. The new proposed four-electron ORR route was predicted to proceed spontaneously in acidic media at U < 0.99 V and in alkaline media at U < 0.22.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Model Si-doped graphenes with a four-coordinate (left) and a three-coordinate silicon atom (right).
Figure 2
Figure 2
Optimized structure of planar (1a) and distorted (1b) Si-doped graphene C53H18Si.
Figure 3
Figure 3
Charge density difference plot for the first excited state of Si-doped graphene C53H18Si 1b. Isocontour values are ±0.001. The red color corresponds to charge accumulation, and the blue color corresponds to charge depletion.
Figure 4
Figure 4
B3LYP/6-311+G(d)-optimized structures of the key oxygen-containing intermediates involved in ORR catalytic cycle on Si-doped graphene C53H18Si (denoted *): O2* (2a–c); [O*O*] (2d and 2e), HOO* (3a–e), HO*HO* (4a–c), O* (5a–c, products of H2O elimination from HOO* 3 and 4), and HO* (6).
Figure 5
Figure 5
Proposed mechanism for the ORR on Si-doped graphene C53H18Si 1b. The values under intermediates and arrows (transition states) refer to the free energy ΔG at electrode potential U = 1.23 V in acidic media corresponding to 4[H+ + e–] + O2 = 2H2O equilibrium calculated at B3LYP and wB97XD (in parentheses) levels of theory.
Figure 6
Figure 6
Conventional mechanism of the oxygen reduction reaction (ORR).
Figure 7
Figure 7
Free energy plots for the possible 4e ORR pathways on Si-doped graphene, at U = 1.23 V in acidic media: common route via HOO* (3a) → O* (5a) (A), alternative route via HOO* (3a) → HO*HO* (4c) (B), and route via O–O bond rupture after O2 adsorption and oxygen atom insertion into the graphene sheet (C).
Figure 8
Figure 8
Free energy diagrams for ORR on Si-doped graphene for pathways 1 (top row), 2 (middle row), and 3 (bottom row) at different electrode potentials U in acidic (A, C, E) and alkaline (B, D, F) media calculated at the B3LYP/6-311G(d,p)//B3LYP/6-311+G(d) level of theory.
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