Design,Synthesis and Electrocatalytic Water Splitting Properties of Metal Sulfides and Nitrogen Group Compound Catalysts

Author:Liu Huan

Supervisor:zhen liang


Degree Year:2019





Hydrogen energy as the clean and effcient energy is imperative for the development of energy in the future.Hydrogen evolution reaction(HER)plays a crucial part in hydrogen energy economy.Water electrolysis,is considered as one of the most promising ways to realize large-scaled hydrogen production.However,high energy consumption and economic issue of noble-metal restrict its commercial applications.Especially,large overpotential and sluggish kinetics are big obstacles of water splitting.Electrocatalysts can significantly reduce energy consumption and accelerate reaction kinetics of water splitting.There is an urgent need to develop the nonprecious metal alternative catalysts.Due to the transition metals are widespread in nature and theirs outer orbital electronic structures are similar to noble metals.Transition metal-based compounds are promising nonprecious alternative electrocatalysts for hydrogen evolution.Electrocatalytic water splitting is a three-phase interfacial reaction between catalyst,electrolyte and bubble.Therefore,the surface structures of catalysts play a decisive role in their catalytic activity of water electrolysis.Based on this,this paper aims to develop and regulation the surface active sites of transition metal-based sulfides,nitrides and phosphides,and improve their catalytic activity towards water splitting by the design of microstructure,surface reconstruction,structural defects and interfacial interactions,then study the mechanism of improving electrocatalytic activity.Co9S8 hierarchical hollow microplates were fabricated via sulfurization and subsequent high temperature treatment of Co-EG microplates.The unique hierarchical structures of Co9S8 were assembled by the tiny two-dimensional nanosheets on the surface of hollow microplates.It was found that the diffusion rate of S2-,Co2+ions and the layer structures of Co-EG were the key to the construction of Co9S8 hierarchical hollow microplates.Benefiting from the advantages of hollow structures and nanosheet units,Co9S8 hollow microplates with extensive catalytic active sites and fast charge transfer rate exhibited remarkable catalytic activity towards oxygen evolution reaction(OER)with low overpotential of 278 mV to reach a current density of 10 mA?cm-2,a low Tafel slope of 53 mV?dec-1 and satisfied stability.At the same time,Co and S were also the active sites of electrocatalysis and involved in oxygen evolution reaction.It was found that the S was replaced by O and Co9S8 was converted to CoO after oxygen evolution at 1.58 V vs.RHE for 20 h.In order to further reduce the overpotential of Co9S8 towards oxygen evolution,the nanosheets assembled hierarchical structure with structural advantages were extended to composite with rGO.A sequential synthesis method was developed for in-situ growth of ultrathin Co9S8 nanosheets(3-4 nm)vertically aligned on N and S co-doped reduced graphene oxide(Co9S8/N,S-rGO)as novel and effcient electrocatalysts for water splitting.Co9S8/N,S-rGO presented remarkable electrocatalytic activity towards oxygen evolution with a low overpotential(266 mV to achieve current density of 10 mA?cm-2)and small Tafel slope of 75.5 mV?dec-1 in alkaline medium.Moreover,Co9S8/N,S-rGO also exhibited good catalytic activity towards hydrogen evolution with low overpotential of 332.4 mV to reach the current density of 10 mA?cm-2.This engineering of architectures based on two-dimensional nanosheets could provide abundant catalytic active sites and minimize charge and proton transport distances to accelerate the oxygen evolution.Due to the sophistication of electrolytic hydrogen evolution in alkaline than acid media,there are no ideal non-noble metal catalysts for large scale application.A new method was proposed for the construction of efficient MoP|S nanoparticles catalysts by surface reconstruction.The surface of MoP|S nanoparticles would undergo oxidation in the ambient air and form a layer of MoP-Ox species.During the electrocatalysis process,the MoP-Ox species could dissolve in electrolyte,however S still kept the reduction state in MoP|S nanoparticles.Thus the surface of MoP|S nanoparticles was reconstructed and owned new catalytic active centers and large exposed active sites.Hence,the MoP|S nanoparticles after surface reconstruction(MoP|S-r)exhibited the enhanced hydrogen evolution properties with low overpotential of 85 mV to drive current density of 10 mA?cm-2 and small Tafel slope of 43.0 mV?dec-1 in alkaline media(1 M KOH).It was indicated that surface reconstruction could tune the surface atomic composition(Mo-S riched),increase the electrochemical active areas and fast charge transfer process,which boosted to evolution of hydrogen.This exploration of the real catalytic active centers of catalysts during hydrogen evolution reaction was challenging and significant.Understanding how the original electrocatalytic activity limited and developing specific design catalysts with enhanced activity are significant but important exemplification for future hydrogen energy application.Tungsten-based catalysts exhibited relative low activity owing to their strong adsorption free energy of H*.In our research,the number of structural defects and interfacial interactions were tuned to facilitate the kinetics of hydrogen evolution.Hence,cobalt hydroxides/cubic phase WN nanoparticles anchored on multiwall carbon nanotubes(Co(OH)2/c-WN1-x)with strengthening interfacial interactions and regulating nitrogen vacancies were exploited as the high-efficient alkaline hydrogen evolution electrocatalysts.Co(OH)2/c-WN1-x-x exhibited remarkable activity with an overpotential of 78 mV at 10 mA?cm-2 and a low Tafel slope of 43 mV?dec-11 in 1 M KOH.This activity was better than those of most tungsten-based alkaline hydrogen evolution electrocatalysts.The experiments and theoretical calculation revealed that the crucial roles of Co(OH)2/c-WN1-x for facilitating the kinetics of water adsorption and dissociation,in which the Co(OH)2 accelerated the adsorption of H2O and breakage of HO-H bonds in Volmer step and c-WN1-x provided the active sites for adsorption of hydrogen intermediates and generation of hydrogen.