Fabrication Of Mo-based Electrocatalysts And Their Performance In Ammonia Synthesis

As the most promising chemicals and carbon-free energy carriers,ammonia(NH3)has a global annual output of～160 million tons.At present,NH3 mainly produced by the traditional Haber-Bosch process,which had harsh reaction conditions and caused serious pollution.Under the increasing pressure of the fossil energy crisis and global climate change,researches on the nitrogen reduction reaction(NRR)in a sustainable and environmentally friendly manner under ambient conditions are flourishing and thriving.The electrocatalytic NRR to synthesis NH3,with the advantages of mild reaction,clean energy and no greenhouse gas emission has became research hostpot.In order to achieve high catalytic activity,inexpensive and efficient electrocatalysts must be reasonably designed and constructed to realize the adsorption and activation of nitrogen,inhibit the occurrence of HER,reduce NRR reaction overpotential and improve the selectivity of NH3.In order to achieve the preliminary exploration of the Mo-based electrocatalyst in electrocatalytic NRR field from the basic research to practical application,in this thesis,highly-efficiency Mo-based electrocatalysts were prepared to investigate the problems involving catalysts,electrolyte and pollution interference in NRR reactions systematically and deeply.The main contents of this thesis are as follows:1.Using chitosan-derived N-doped porous carbon(NPC@500)as an NRR electrocatalyst.The experiment results showed that the doped pyrrole-N and pyridine-N in NPC@500 were unstable and would be disaggregated and hydrogenated to form NH3,which would interfere with the intrinsic electrocatalytic NRR performance of NPC@500.To solve these problem,metal-nitrogen coordination were proposed as an effective and universal strategy for anchoring doped N.Specifically,single-atom catalysts with Mo(Co)atom and N atom coordination can not only firmly anchored the exposed active N atoms in NPC@500,but also provide a new active site for the electrocatalytic NRR.This research indicated the controversial the application of NPC in the electrocatalytic NH3 synthesis,and proposed a universal solution to stabilize N atoms on the surface of carbon materials,while can further improve the electrocatalytic NRR performance.2.The inhibition of HER in electrocatalytic ammonia synthesis system can be achieved by selecting appropriate electrolyte and regulated the interaction between electrolyte and catalyst,which also can significantly improve the performance of NRR.In this chapter,Li2SO4 solution was selected as electrolyte and MoS2/BCCF as catalyst.The results found that a NH3 yield rate of 43.4 μg h-1 mg-1 MoS2 with a faradaic efficiency(FE)of 9.81%can be achieved in presence of strong Li-S interactions,more than 8 and 18 times by the same electrocatalyst in the absence of Li-S interactions.The theoretical calculation revealed that the Li+ in the electrolyte can strongly interact with S-edge sites of MoS2 during NRR process.The strong Li-S interaction not only can significantly change the adsorption of H*at S-edge site,effectively suppress the Heyrovsky/Tafel process involved in HER.Meanwhile,the strong Li-S interactions can also lead to the positively charged Mo edge site with dramatically increased N2 adsorption energy and reduced NRR activation energy.This work provides a new research idea for promoting the development of MoS2 based catalysts in the field of electrocatalytic NRR.3.The development of high stability and activity electrocatalysts was a great challenge for electrochemical NRR.By adjusting the size of the catalyst,with highly dispersed MoS2 nanodots anchored on reduced graphene oxide(MoS2 NDs/RGO)were prepared in this work.Because the catalysts exposed more active sites,the MoS2 NDs/RGO showed a remarkable NRR performance(faradaic efficiency of 27.93%,NH3 yield rate of 16.41 μg h-1 mgcat.-1)under ambition conditions,which superior to that of MoS2 NS/RGO(NH3 yield of 6.5 μg h-1 mgcat.-1).In addition,The excellent NRR performance of the catalysts were also attributed to the construction of C-S-C bridging bonds in MoS2 NDs/RGO hybrid,which can significantly facilitate the reaction kinetics by boosting electron transfer,thus leading to the improvement of NRR activity.The work in this chapter provides reference and experimental basis for the development of Mo-based heterogeneous electrocatalysts in NRR field.4.By constructing heterojunction,leading to the redistribution of charge in the interfacial,which is beneficial to the adsorption and activation of N2,reduce the activation energy of NRR reaction,and thus promote the performance of the catalyst.In this work,by a high-temperature molten salt strategy,MoB2 nanosheets was introduced in-situ on the surface MoS2 nanosheets to obtain MoB2/MoS2 heterojunction catalyst.The results showed that the electrocatalyst attained an enhanced Faradic efficiency of 15.48%and an excellent ammonia yield rate of 63.98μg h-1 mgcat.-1 in 0.1 M Na2SO4,which is not only better than that of pure MoS2 and much higher than the NRR performance of most MoS2 based catalysts.In this work,two-dimensional transition metal borides(TMBs)were used for electrocatalysis of NRR reaction for the first time,providing a new design thought to promote the development of Mo-based electrocatalyst for nitrogen fixation.