First-principles Study On The Mechanisms For The Oxygen Reduction Reaction Activity And Resistance To Sulfur Poisoning Of TiC And HfC-based Electrod Materials Of Fuel Cells

The development of new materials that can solve challenging problems in the areas of clean energy production,conversion and storage is of paramount importance in the quest to find an alternative to environmentally unfriendly fossil-fuel use.The proton exchange membrane fuel cell is considered to be an attractive alternative to conventional power generators,due to its high efficiency in energy production and low degree of pollution emission.Although the traditional Pt/C electrocatalyst for PEMFC is active,it also encounters some issues when regarding the particle dissolution,high cost and low stability in the electrochemical environment.To this end,cheaper and more effective electrocatalysts are needed for the further development of fuel cell technology.Transition metal carbide has been the subject of many investigations in surface science and the fields of catalysis,which has been proposed as optional candidate choices to replace of the more expensive Pt-group metal catalysts due to its lower cost and equal or even better catalytic properties.First-principles methods are employed to investigate the interaction of H₂S?SH and atomic S with the TiC?001?and try to reveal how to avoid or remove the adsorbed sulfur from the dissociation of H₂S,which may shed light on the sulfur tolerance mechanism of the TiC?001?system in rather low concentration of H₂S.Then,the adsorption and dissociation of O2 on the Ptn/TiC?001?with different coverages of Pt as a simple descriptor of oxygen reduction reaction activity are studied in detail.Finally,the M4?M=Au,Pd,Pt?clusters supported on HfC?001?is designed,on which the adsorption and dissociation of O₂ are studied.Our findings provide a guidance for designing high efficient and low cost oxygen reduction reaction catalysts.The main work performed and results reached are as follows:?1?The adsorption and dissociation reactions of H₂S on TiC?001?are investigated using first principles density functional theory calculations.The geometric and electronic structures of the adsorbed S-based species?including H₂S,SH and S?on TiC?001?are analyzed in detail.It is found that the H₂S is bound weakly,while SH and atomic S are bound strongly on the TiC?001?surface.The transition state calculations show that the formation of SH from H₂S?H₂S?SH+H?is very easy,while the presence of a co-adsorbed H will inhibit the further dissociation of SH?SH+H?S+H+H?.In contrast,the hydrogenation of the adsorbed SH is rather easy?SH+H?H₂S?.Therefore,the dissociative SH can be removed via the hydrogen-ation reaction.It is concluded that H₂S is difficult to dissociate completely to form atomic S and poison the TiC surface.The results will further provide understanding on the mechanism of the sulfur tolerance of the TiC anode of proton exchange membrane fuel cell.?2?The adsorption and dissociation of O₂ on the Pt modified TiC?001?surfaces with different Pt coverages of 1/4,1/2,3/4 and 1 ML are comparatively investigated using first principles density functional theory calculations.The geometric and electronic structures are analyzed in detail.The strong interaction of Pt atoms with the TiC?001?is beneficial to improving the stability and activity of Pt catalyst.Compared with Pt?111?,the MLPt/TiC?001??3×3?have a positive impact on promoting the scission of the O-O bond leading to a dissociation barrier comparable to that on Pt?111?and weakening the adsorption of atomic O?the dissociation product of O₂?,which shed meaningful light on the important role of TiC?001?as support to improve the efficiency of Pt for oxygen reduction reaction.?3?The adsorption and dissociation of O2 on the M4?M=Au,Pd,Pt?clusters supported on HfC?001??Hafnium Carbide?are investigated using first principles density functional theory calculations.The geometric and electronic structures are analyzed in detail.It is found that the dissociation barriers of O₂ on Au₄/HfC?001??0.26 eV?,Pd₄/HfC?001??0.49 eV?and Pt4/HfC?001??0.09 eV?are much smaller than those on the clean surfaces of HfC?001??1.60eV?,Au?111??1.37 eV?,Pd?111??1.0 and 0.91 eV?and Pt?111??0.27-0.7 eV?,respectively.The low dissociation barriers imply that the Pt₄/HfC?001?exhibits the highest catalytic activity for O₂ dissociation,and the Au₄/HfC?001?and Pd₄/HfC?001?may also be possible substitutes with lower cost for the current Pt/C catalyst for O₂ dissociation.The present study is conductive to designing new efficient noble metal catalyst using HfC support for efficiently promoting O₂ dissociation.