Exfoliation Of Two-Dimensional Transition Metal Based Materials And Its Application For Electrochemical Ammonia Synthesis

The primary route for ammonia synthesis is the Haber-Bosch method,a process that poses consequences such as high levels of emissions and energy consumption.Therefore,there is an urgency in developing efficient and environmentally friendly ways to reduce nitrogen into ammonia.Among the widely reported studies on nitrogen fixation,the electrochemical nitrogen reduction reaction(NRR)is able to proceed under more ambient conditions,and thus,it is widely considered a promising approach for electrochemical ammonia synthesis.Currently,noble-metal catalysts possess both high selectivity and efficiency for NRR,however,they suffer from scarcity and high price,which limits their large-scale application for industry.As a result,much attention has been paid on designing and developing non-noble-metal NRR electrocatalysts.Recently,two-dimensional(2D)transition metal-based material have gained increasing attention due to their outstanding electrochemical performance and unique layered structure that can result in strong anisotropy.We developed novel exfoliated metallic NbS2 nanosheets possessing a thickness of～12 nm with a lateral dimension of up to～300 nm,produced via a liquid exfoliation treatment of bulk NbS2 in N-Methyl-2-pyrrolidone(NMP)solution.The exfoliated NbS2 nanosheets exhibited a high electrocatalytic activity for NRR at mild conditions with a high NH3 yield rate of 37.58 μg mg-1 Cat h-1 and a Faradaic efficiency(FE)of 10.12%at-0.5 V vs reversible hydrogen electrode(RHE)in 0.1 M HCl,which showed a competitive NRR performance.Experimental results revealed that the Nb4+ species of exfoliated NbS2 was acted as the active sites for NRR.Meanwhile,we report a porous exfoliated FePS3 nanosheet with rich sulfur vacancy(Vs-FePS3NSs)synthesized by electrochemical exfoliation and hydrogenation treatment.Owing to the rich sulfur vacancy and unique 2D morphology with a thickness of～15 nm and lateral dimension of～500 nm,Vs-FePS3 NSs show an excellent activity in electrochemical NRR with a high Faradaic efficiency of 12.36%at-0.20 V and an ammonia yield rate of 3.88 μg h-1 cm-2 at-0.25 V versus reversible hydrogen electrode in acid,ranking top in Fe-based NRR electrocatalysts.Experimental observations reveal the Fe species as real active sites and S vacancies for hydrogenation of N2 toward a synergistic effect for enhanced NRR.In addition,exfoliated NbS2 nanosheets and Vs-FePS3 NSs separately serve as the cathode of a Zn-N2 battery to integrated energy storage with N2 fixation.When the exfoliated NbS2 nanosheets are assembled into a Zn-N2 battery and serve as the cathode,an energy density of 714 mAh g-1 with a power density of 0.31 mW cm-2 is achieved.And when the Vs-FePS3 NSs was assembled into a Zn-N2 battery and serve as the cathode,an energy density of 833 mAh g-1 with a power density of 2.6 mW cm-2 is achieved.