Theoretical investigation of the ORR on boron–silicon nanotubes (B–SiNTs) as acceptable catalysts in fuel cells

Here, the potential of boron doped silicon nanotubes (7, 0) as ORR catalysts is examined. Acceptable paths for the ORR on studied catalysts are examined through DFT. The optimum mechanism of the ORR on the surface of B2–SiNT (7, 0) is shown. The ORR on the surface of B2–SiNTs (7, 0) can continue through LH and ER mechanisms. The calculated beginning voltage for the ORR on B2–SiNTs (7, 0) is 0.37 V and it is smaller than the beginning voltage (0.45 V) for platinum-based catalysts. In the acidic solution the beginning voltage for the oxygen reduction process can be evaluated to be 0.97 V, which corresponds to 0.37 V as a minimum overvoltage for the ORR. The B2–SiNTs (7, 0) are suggested as an ORR catalyst in acidic environments.


Introduction
Fuel cells as energy machines are important due to their low contamination and great efficiency. The ORR rate in electrodes of cells is slow, therefore the ORR can be evaluated as a significant reason to increase the full cell efficiency. [1][2][3][4] Platinumcompounds have been used as catalysis in the ORR but platinum-compounds have low ability to endure CO. [5][6][7][8][9] The potential of various compounds was investigated to nd and propose effective catalysts for the ORR. Nanostructures and doped nanostructures with high ability for CO endurance can be used as suitable replacements for platinum-compounds. [10][11][12][13][14][15] The B-nanostructures are acceptable catalysts for the ORR in alkaline conditions and mechanisms of action of B-doped nanostructures in acidic position are not clear. [16][17][18][19][20][21][22][23] The nanostructures due to their electrical conductivity and thermal conductivity can be used to product the transistors and non-volatile memory devices. [24][25][26][27][28] The electrical conductivity of doped carbon/silicon nanotubes indicated that the adoption of carbon/silicon nanotubes (with various atoms such as B, N, O and some metals) increased their electrical conductivity, signicantly. These ndings improved the application of carbon/silicon nanotubes in nano-electronic devices and novel catalyst to ORR. [29][30][31][32][33][34][35][36][37][38] Results demonstrated that the adoption of carbon/silicon nanotubes increased their electrical conductivity and enhanced the ORR efficiency. [39][40][41][42][43][44][45][46][47][48] Wang et al. 49 demonstrated that boron-doped graphene nanoribbons are suitable catalyst to ORR catalyst. Xiao et al. 50 proved that the layered silicon-carbon nano sheets represented the high activity in ORR without CO poisoning. Xia and Zhang et al. 51,52 investigated the mechanisms of ORR of fuel cells in acidic environment on graphene cathodes. Stevenson et al. 53 proved in ORR the O 2 in a 2-electron path is reduced to form OOH on carbon nanotubes. Hu and Xiong et al. 54,55 conrmed that nitrogen and boron-doped nanostructures as ORR catalysts have low price, great durability and excellent potential. Zhao and Wei et al. 56,57 conrmed the doping of carbon nanotubes have vital roles on performance of ORR. Ferrighi et al. 58 demonstrated that boron atoms of nanosheets increase the reactions of oxygen with graphene.
In current study, ORR on B-doped silicon nanotube (7, 0) as acceptable catalysts is examined to nd possible mechanisms to ORR on B 2 -SiNT (7, 0) and to suggest high activity nanocatalysts to ORR.

Molecule adsorptions on nanotube
In this section, the B adoption of SiNT (7, 0) were investigated and then interactions of B-SiNT (7, 0) structures with O 2 , OOH, OH, H 2 O and CO molecules were investigated. The one Si atom of the SiNT (7, 0) was replaced with one B atom and the B-SiNT (7, 0) was produced (Fig. 1). Also the two Si atoms of the SiNT (7, 0) in two difference positions were replaced with two B atoms and B-B-SiNT (7, 0) and B 2 -SiNT (7, 0) structures were produced ( Fig. 1). Structures of SiNT, B-SiNT, B-B-SiNT, B 2 -SiNT, O 2 , OH, H 2 O, H 2 O 2 and CO are presented in Fig. 1. The adoption energy (E doped ), adoption free Gibbs energy (G doped ) and bond lengths of B-Si of B-SiNT (7, 0), B-B-SiNT (7, 0) and B 2 -SiNT (7, 0) were reported in Fig. 1. In the B-SiNT (7, 0) the B atom was connected with three neighboring silicon atoms and the E doped and G doped were À2.18 and À2.10 eV and average of bonds of B-Si in B-SiNT (7, 0) is 1.95Å. In the B-B-SiNT (7, 0) the B atoms are connected with four neighboring silicon atoms and the E doped and G doped are À2.23 and À2.14 eV and average of bonds of B-Si in B-B-SiNT (7, 0) is 1.93Å. In the B 2 -SiNT (7, 0) the B atoms are connected with six neighboring silicon atoms and the E doped and G doped values are À2.28 and À2.17 eV and average of bonds of B-Si in B 2 -SiNT (7, 0) are 1.92Å.
In process of ORR the CO can occupy the positions of catalysts and the performance of ORR is reduced and efficiency of catalyst decreases sharply. Previous works showed that reactions between CO molecule and surface of platinum nano-catalyst was powerful (DE ad is À1.90 eV) and CO poisoning was happen. 20,66 The average of DE ad and DG ad of CO on SiNT, B-SiNT, B-B-SiNT and B 2 -SiNT surfaces are À0.11 and À0.08 eV. The average of q and E HLG values for adsorption of CO on SiNT (7, 0), B-SiNT (7, 0), B-B-SiNT (7, 0) and B 2 -SiNT (7, 0) surface is 0.13|e| and 1.70 eV. The CO molecule can be adsorbed on SiNT (7, 0), B-SiNT (7, 0), B-B-SiNT (7, 0) and B 2 -SiNT (7, 0) surfaces as physical adsorption processes. It can be concluded that B 2 -SiNT (7, 0) as acceptable catalyst can be endurance to CO poisoning and it can solve the major problem of platinum nano-catalysts.

Conclusions
Performances of boron-silicon nanotube (7, 0) as novel catalyst to ORR are investigated. The ORR on surface of B 2 -SiNT can be continued through LH and ER mechanisms. The ratedetermining stage (DE a ¼ 0.35 eV) for ORR on B 2 -SiNT (7, 0) surface is creation of B 2 -SiNT (7, 0)-*OH structure. The calculated beginning voltage to ORR on surface of the B 2 -SiNT (7, 0) is 0.37 V. In the acidic solution the beginning voltage to oxygen reduction process can be evaluated to 0.97 V. Results indicated that the B 2 -SiNT (7, 0) is suggested as catalyst to ORR with suitable efficiency.

Conflicts of interest
There are no conicts to declare.