Boosting The Catalytic Performance Of Transition-Metal Sulfide Catalysts Toward The Oxygen Reduction Reaction Via Tungsten Based Compounds Modulating

The sluggish kinetic of oxygen reduction reaction（ORR）,which is as an important electrode reaction of metal-air batteries and low temperature fuel cells,is a key factor in determining the cell performance.Transition metal sulfides have attracted much attention due to their abundant raw materials,low cost,and platinum-like catalytic performance in alkaline solution,but they are prone to be formed into（oxy）hydroxides via the surface reconstruction during the catalytic process,leading to the structural instability and poor durability.Acoording to the Sabatier principles,we have proposed a facile strategy for preparing tungsten nitride or sulfide and transition metal sulfide materials,which boosting the intrinsic catalytic activity and stability.The details are described as follows:（1）Preparation of tungsten nitride/iron sulfide heterostructures for electrocatalytic oxygen reduction reactionIn chapter 3,we reported a ion migration strategy for successfully preparing tungsten nitride@iron sulfide/carbon nanoboxes(WN/Fe_1-xS)heterostructure.WN layer is introduced to decrease the electronic density around iron active active,and weaken the adsorption of oxygen intermediates,resulting in boosting the intrinsic ORR activity.It is illustrated by the positive shift of the ORR half-wave potential and the increase of the ORR mass activity at 0.90 V vs.RHE from Fe_1-xS@CNS to Fe_1-xS/WN@CNB catalyst(0.84 V vs.RHE versus 0.88 V vs.RHE and 20.99 A g^-1 versus 99.29 A g^-1).Fe_1-xS/WN@CNB catalyst also exhibited excellent durability with only 14 m V negative shift for the E_1/2 after10,000 accelerated durability test（ADT）cycles,while Fe_1-xS@CNS has 61 m V negative shift for the E_1/2 after 5,000 ADT cycles.The outstanding durability performance of Fe_1-xS@WN/CNB catalyst is mainly ascribed to the self-sacrifice of electron-rich and low-valence-state tungsten species during the catalytic process for preserving the valence state of iron active species.（2）Preparation of tungsten sulfide/cobalt sulfide heterostructures for electrocatalytic oxygen reduction reactionAlthough iron active species can significantly accelerate the ORR kinetics,they also can promote the the Fenton reactions to generate highly active free radicals,which likely lead to the degradation of the catalysts,ionomers,and polymer membrane and deteriorate the durability performance.Alternatively,cobalt active species with mitigated Fenton reactions are in recent years pursued for replacing iron active species,whereas cobalt active species showed a weak binding with oxygen molecule,leading to poor intrinsic ORR activity.In chapter 4,we reported a facile hydrolysis and sulfidation approach to prepare cobalt sulfide/tungsten sulfide（Co S_x/WS₂）heterostructures,which are embedded into carbon nanocubes.Tungsten sulfide tempts the electron accumulation around cobalt active centers for strengthening the adsorption of oxygen molecules on them,resulting in improving the ORR performance,accompanied with the positive shift of the half-wave potential from 0.857 V vs.RHE for the Co S_x@CNC to from 0.863 V vs.RHE for the Co S_x/WS₂@CNC.The Co S_x/WS₂@CNC also exhibited excellent durability with no significant shift for the E_1/2 after 30,000 cycles,while Co S_x@CNC showed a 13 m V negative shift after 10,000 cycle.Its superior durability is found to be ascribed by inhibiting the surface transformation of cobalt sulfide into（oxy）hydroxides via the formation of tungsten sulfide protective layer.Besides that,the required overpotential of OER was 306m V for the Co S_x/WS₂@CNC to achieve a current density of 10 m A cm^-2,which was 191mV lower than that for the CoS_x@CNC（497 mV）.