DFT Calculation Of The Deactivation Mechanism Of TM-N-C For Oxygen Reduction Reaction

The polymer electrolyte membrane fuel cells（PEMFCs）,which directly convert chemical energy stored in hydrogen fuel into electricity with water as the only by-product,are one of promising technologies for clean and sustainable power generation.To reducing the cost of catalysts at the cathode of PEMFCs,extensive researchs on non-precious metal catalysts have been carried out and made a great progress in recent years.Transition-metal and nitrogen-co-doped carbon（TM-N-C）catalysts exhibit reasonably good activity towards ORR,and become the most promising ORR catalyst to replace precious metals.However,the TM-N-C catalysts still face a huge challenge resulting from a rapid initial performance loss in the first few hours in stability testing,which heavily hinders their further application.Thus,uncovering the deactivation mechanisms of TM-N-C catalysts for ORR is important to accelerate the commercialization of PEMFCs.Four mechanisms have been proposed for the deactivation of TM-N-C catalysts for ORR:（1）the attack by H₂O₂（and/or free radicals）,（2）a demetalation of the catalytic sites,（3）the protonation of the active sites involving either N_pyridinicor N species neighboring the active site,followed by anion adsorption,（4）micropore flooding.However,the detailed deactivation mechanisms of TM-N-C catalysts remain unclear.Here,we focused on TM-N₄-C catalysts to explore their ORR activity and stability by using density functional theory（DFT）calculation.These results of calculation would provide some useful guidance to design TM-N₄-C catalysts with high activity and stability.（1）To identify the deactivation mechanism of Fe-N₄-C catalysts,we calculated the protonation of N and oxidation mechanism of Fe and C site on Fe-N₄-C by using density functional theory calculation,probed the competition between ORR and Fe OR,and unveiled the effect of Fe OR on the ORR activity.Firstly,the thermodynamic and kinetic calculations show that the protonation of N is a nonspontaneous process.Secondly,the oxidation mechanism of Fe and C site on Fe-N₄-C surface suggest that,C site can be oxidized partially forming surface oxidation intermediates rather than be oxidized completely.For Fe site,its partial and complete oxidation process are easier than C site oxidation,and the partial oxidation of C would accelerate the oxidation of Fe site.Thirdly,in the Indirect-Fe OR process,the protonation of N and OH adsorption on Fe site would promote the dissociation of Fe-N bond,and eventually leads to the complete oxidation of Fe site.The competition relation between ORR and Fe OR finds that*Fe OH,which is tend to desorb OH or adsorb OH,is the key to determing the ORR activity and anti-oxiation ability of Fe.when the free energy of*Fe OH desorption OH（ORR）equals to the free energy of OH adsorption（Fe OR）（△G₂=△G₁）,the applied electrode potential can indicte the anti-oxidation ability catalyst.The larger the applied electrode potential,the higher the anti-oxidation ability.Finally,ORR activity before and after the oxidation of Fe and C sites indicates that Fe OR is one of the major fortors for the rapid loss of initial performance of Fe-N₄-C in stability testing.（2）Based on above reaserch,we further investigated the ORR activity and anti-oxidation ability of a series of TM–N₄-C composites,including 3d（Cr–Ni）,4d（Mo–Ag）and 5d（Os–Au）transition metals by using the density functional theory.Firstly,binding energy and cohesive energy show that TM-N₄-C catalysts all have thermodynamic stability.Secondly,the apllied electrode potential when the free energy of OH desorption equals to the free energy of OH adsorption on investigated TM-N₄-C catalysts show that the anti-oxidation ability of TM-N₄-C catalysts decrease in the order:Cu>Ni>Pd>Zn>Pt>Co>Sc>Rh>Fe>Ir>Mn>Cr>V>Ti.Thirdly,the compassion of ORR activity and anti-oxidation ability find that Co,Ni,Cu,Zn,Pd and Pt based TM-N₄-C catalysts exhibit high ORR catalytic activity and anti-oxidation ability.Finally,the oxidation mechanism of Co,Cu,and Zn based TM-N₄-C catalysts with high anti-oxidation ability and ORR activity indicate that the competition reaction step of ORR and TMOR on different TM-N₄-C catalysts are different,and it depends on the properties of TM.Moreover,Cu-N₄-C and Zn-N₄-C catalysts have high anti-oxidation ability,and metal active sites can be oxidized hardly during ORR potential range.