Controllable Synthesis And Electrocatalytic Performance Of Molybdenum Disulfied-based Nanostructures For Hydrogen Evolution

Ever-increasing global concerns about environmental pollutions and energy crisis have kindled a great desire for alternative clean energy sources and renewable carriers.Molecular hydrogen(H₂),is not only considered as one of the most promising clean fuel sources,but also regarded as an ideal carrier to store energy from renewable energy sources(e.g.,solar and wind)into the chemical bond via electro-chemical hydrogen evolution reaction(HER),which is a half reaction in water splitting.To improve the HER energy conversion efficiency,a leapfrog development is needed to design inexpensive commercially available and non-noble electrocatalyst to the replacement of high-cost and scarce precious-metal catalysts(i.e.,Pt).Up to date,transition metal sulfides,selenides,phosphides,carbides,and nitride have been widely investigated.Among these,MoS₂,in a thermodynamically stable 2H phase(2H MoS₂)normally,has been emerged as a promising candidate for HER under acid media,due to low cost,nontoxicity,and abundance.However,theoretical and experimental studies reveal that only the edges of 2H MoS₂,rather than the large number of basal planes,are active in the acidic HER catalysis.Motivated by these findings,substantial efforts have been made to improve the density of accessible edges for enhancing the HER performances,including the fabrication material consisting precious surface at nanoscale.Following an approach radically different from that mentioned above,efficient acidic HER activity can also be achieved by structural engineering of 1T phase MoS₂(1T MoS₂,1T and 2H are polymorphs of MoS₂).Owing to its metallic conductivity and intrinsic catalytic activity,1T MoS₂ exhibits excellent acidic HER activity.The incorporation of small guest molecules or ions by bottom-up hydrothermal synthesis has recently emerged as a promising new way to engineer 1T MoS₂.However,the mechanism of the associated structural evolution remains elusive and controversial,leading to a lack of effective routes to prepare 1T MoS₂ with controlled structure and morphology,along with high purity and stability.Furthermore,although MoS₂-based materials have been considered as a promising electrocatalyst for acidic HER,corresponding studies have been hampered by the lack of effective routes to design binding O-containing intermediates sites for catalyzing HER or even oxygen evolution reaction(OER)in alkaline media,limiting its potential of being a p H-universal and overall water splitting electrocatalyst to address large-scale industrial requirements.In this dissertation,fitstly,urea is chosen as precursor of small molecules or ions to simultaneously engineer the phase and size of MoS₂ nanosheets,which represent an ideal model system for investigating the structural evolution in these materials,as well as developing a new type of 1T MoS₂ arrays or even 2H MoS₂ arrays.Using reaction intermediate monitoring and theoretical calculations,we show that the oriented growth of 1T MoS₂ is controlled by ammonia-assisted assembly,recrystallization,intercalation,and stabilization processes.A superior HER performance in acidic media is obtained,with an overpotential of only 76 m V required to achieve a stable current density of 10 m A·cm^-2 for 15 h.This excellent performance is attributed to the unique array structure,involving well-dispersed,edge-terminated,and high-purity 1T MoS₂ nanosheets.Secondly,for efficient catalyzing HER over a broad p H value,we develop a novel ammonia ions-guided-nitrogenization-phosphorization strategy toward the preparation of N and P co-doped MoS₂ array anchored on carbon cloth(NP-MoS₂/CC)as a self-supported electrode,relying on the intercalated NH₄⁺in MoS₂ according to urea assisted synthesis.The intercalated NH₄⁺guaranteed in situ homogeneously doping of N and the introduction of N further favor the co-doping of P.In addition,this strategy can be extended to fabricate single P or N doped MoS₂ grown on CC(P-MoS₂/CC and N-MoS₂/CC,respectively)with fixed structure to NP-MoS₂/CC,providing a scalable pathway to systematically investigate the effect of co-doping.Electrochemical measurements demonstrated that the activity and stability of NP-MoS₂/CC were much higher than that of P-MoS₂/CC or N-MoS₂/CC under both acidic and alkaline condition.Impressively,the activity of NP-MoS₂/CC under both sea and river water was superior to those of all previously reported electrocatalysts and outperform commercial Pt/C catalyst.Experimental and theoretical results revealed that the excellent activity and durability originate from the new formation Mo-N-P site.Finally,for efficient catalyzing overall water splitting,the roles of atomic transition-metal(ATM)modification synergy with confined vacancy on bulk sites are identified.1T MoS₂ with a[MoS₆] octahedron,prepared by modified urea assisted synthesis,provides a suitable coordination environment to stabilize ATM at a high concentration(4.6 atom%),as well as to the advantage of high conductivity and bulk H affinity.Theory and experimental results prove that saturated coordination ATM further trigger bulk sites for binding H atom,whereas that of in low-coordination activate O-containing intermediates.The optimized sites for overall water splitting can be tailored by engineering the amount of confined vacancy.Meanwhile,with due consideration for design principles from atomic to macro-scale,all-in-one self-supporting electrodes supported by CC are integrated to drive a better performance than commercial noble metal electrocatalysts.Above all,this study may open new avenues to design various MoS₂-based catalytic materials for H₂ production and provide atomic-scale insights into the relationship between activity and structure.