DFT Study Of Fe-X (X=N, S) Doped Carbon Materials As Oxygen Reduction Catalysts

Proton exchange membrane fuel cell(PEMFC)is an emerging energy conversion device.There is a big problem that kinetics of the cathodic oxygen reduction reaction(ORR)is very slow,therefore,the high catalyst loading is required.Currently,ORR catalysts with the best performance and only commercial application are rare and expensive platinum.However,the disadvantages of Pt-based catalysts,such as high price,poor stability and inactivation of poisoning,seriously hinder the commercialization of PEMFC.It is imperative to develop a non-precious metal ORR catalyst which is inexpensive and excellent performance.In this paper,we applied density functional theory(DFT)to study non-precious metal oxygen reduction electrocatalysts.These contents are as follows:(1)We evaluated the reliability of four GGA functionals,including BLYP,PW91,PBE,and RPBE.The selected catalysts are the Pt(111)and FeN4 doped graphene(Fe-N4/G).We analyzed the bond length,bond dissociation energy of isolated ORR species and adsorption energy of ORR species compared with experimental values,and the difference about reaction energies of ORR steps.The results indicated that for Pt(111)catalyst,the PW91 and PBE functionals are proved to be the better functionals to investigate the ORR on its surface,While for Fe-N4/G,the BLYP is demonstrated to be the most suitable functional to study the ORR properties.Therefore,we will use BLYP functional in this paper.(2)We screened the stability of several Fe/S/C catalysts(Fe-Sx/C,x=2-6).The results suggest that only the Fe-S2/C structure could maintain its initial planar structure after the geometry optimization,indicating that it is most likely to have ORR catalytic ability.Further investigations find that the O2 molecule is firstly chemisorbed on the Fe-S2 active site with a stable side-on mode,and then is gradually reduced to H2O following a 4e-OOH dissociation pathway.Both thermodynamic adsorption energies of ORR intermediates and kinetic activation energies of proton transfers indicate that the current Fe-S2/C catalytic site possesses the comparable catalytic activity to that of precious Pt catalysts.Furthermore,compared with Pt(111)surface,it also has very excellent tolerance to some impurities such as sulfur compounds(SO2,H2S),carbon compounds(CO),and nitrogen compounds(NO,NH3).(3)We explored the stability of the Fe-N4 sites in carbon nanotubes(Fe-N4/CNTs).The results indicate that the stability will be enhanced with a larger tube diameter,but reduced with shorter tube length.A tube with too small diameter makes a Fe-N4 site unstable in acid medium.Since Fe-N and C-N bonds must be significantly bent at smaller diameters due to hoop strain.The adsorption energy of the ORR intermediates,especially of the OH group,becomes weaker with the increase of the tube diameter.The OH adsorption energy of Fe-N4/CNTs with the largest tube diameter is close to that on Pt(111)surface,indicating that its catalytic property is similar to Pt.Electronic structure analysis shows that the OH adsorption energy is mainly controlled by the energy levels of Fe 3d orbital.The calculation results uncover that Fe-N4/CNTs with larger tube diameters and shorter lengths will exhibit better ORR activity and stability.In addition,the rate determined step(RDS)of these catalysts is the reaction:OH?H2O.With the largest tube diameter,the relative energy of RDS on Fe-N4/CNTs is closer to that on Pt(111)surface.In summary,the ORR catalytic performance of Fe-N4(8,8)-9.8 is the best.