Preparation Of Transition Metal Nitride Based Nanomaterials And Their Applications For Electrochemical Water Splitting

Transition metal compounds have attracted increasing interest in the photo and electrochemistry and energy fields,for example,metal-ion batteries,supercapacitors,solar,cells,sensors,and photo/electro-catalysts because of the multi-valence states,special electronic structure,and physical properties.Introduction of nitrogen atoms to transition metals increases the d-electron density and contraction of the d-band makes the electronic structure of TMNs resembling that of noble metals such as Pd and Pt up to the Fermi level.Besides the metallic conductivity,TMNs are often considered as“interstitial alloys”by integrating nitrogen atoms into the interstitial sites of the parent metals.Therefore,compared to the parent metals,TMNs have high corrosion resistance boding well for electrocatalytic water splitting in acidic and alkaline solutions.Moreover,coupling TMNs with carbon materials is found to create a synergistic effect between the components which can modulate the electron density as well as the distribution of the electronical potentials of the namocomposite to balance the hydrogen surface adsorption/desorption behavior,thus enhancing the electrocatalytic properties.In the introduction section,the principles of electrocatalytic water splitting,methods for the preparation of TMNs electrocatalyst,structure and physical properties of TMNs and TMNs based non-precious metal catalysts for electrochemical water splitting were summarized.Through a simple hydrothermal method,we synthesized a series of TMN-based precursors.Then,after ammonia pyrolysis,a series of TMNs-based nanomaterials with high electrocatalytic activity were successfully prepared.Meanwhile,these TMNs based nanomaterial respectively show excellent catalysis activities for OER or HER.At last,this dissertation consists of the following four aspects:（1）We prepared successful synthesis of Co₂N_0.67 with 3D architecture of hierarchical meso-macroporous nanoflowers（NFWs）stacked by the nanoplates（NPTs）with polystyrene spheres（PSs）as the template.Co₂N_0.67 NFWs is one of best earth-abundant elements based electrocatalysts toward OER to date.Importantly,being different from the previous observation that the cobalt nitrides with higher nitrogen content possess lower OER electrocatalytic abilities,we found Co₂N_0.67 exhibits higher OER activity than Co_5.47N.The density functional theory（DFT）calculations reveal that during the OER,the oxidation step from O*to OOH*on Co₂N_0.67.67 and Co_5.47N is considered as the rate-determining step,making OER to be more thermodynamically favorable on Co₂N_0.67 than that on Co_5.47N.（2）We synthesize a series of cobalt nitride(Co₂N_0.67)-carbon composites with different textural properties.Meanwhile,these nitrides-based composites have been investigated as catalysts in OER.Experimental results demonstrate that the specific morphologies of different carbon matrices play the crucial roles in determining the overall OER catalytic efficacies of corresponding composites.Specially,the hollow CMFs carbon matrices not only enhance the N content in CoN NPs,can also promote the catalytic ability of active centers.Benefitting from the unique open hollow architecture,abundant out-of-plane pores and vast exposed three-phase boundaries dispersed along both inner and outer curve surfaces of Co₂N_0.67-CMFs,the most notable feature of optimal Co₂N_0.67-CMFs can provide much better transport pathways that can promote O2 bubbles fast release from active site surfaces.Therefore,Co₂N_0.67-CMFs composites exhibit excellent OER catalytic activity and long-term stability.（3）We synthesize a series of secondary metals doped Co5.47N nanosheets[donated as CoM（3:1）-N（M=Fe,Ni,Mn,and Zn;3:1 is the molar ratio of Co to M）].X-ray photoelectron spectroscopy（XPS）analysis results indicated that doping Fe,Ni or Mn into the crystal textures of Co_5.47N would lead to the increment for the binding energies（BEs）values of Co.The positive shift of BEs value for Co decrease by the order of CoFe（3:1）-N（¹.2 eV）>CoNi（3:1）-N（⁰.4 eV）>CoMn（3:1）-N（⁰.2 eV）.Benefitting from Lewis acid effect of Fe³⁺promotes the formation of Co⁴⁺active sites,the enhanced electron conduction capability of Fe doped CoFe（3:1）-N and the well-designed rose-like 3D porous architecture,Fe-doped CoFe（3:1）-N electrocatalyst possesses superior OER performance and long-term stability.（4）We design and synthesize a novel three-dimensional（3D）hierarchically meso-macroporous and hollow tube-like architecture of Ni3N nanosheets/carbon microfibers/Ni3N nanosheets（Ni3N/CMFs/Ni3N）assembled by the dispersion of porous Ni3N nanosheets（NSs）over the inner and outer walls of hollow and porous carbon fibers（CMFs）.Benefitting from excellent electrical conductivity and a high density of utilizable active sites,Ni₃N/CMFs/Ni₃N revealed superior OER and HER catalytic activities compared with Ni₃N supported by graphene（Gr）,carbon nanotubes（CNTs）and macroporous carbons（MPCs）,respectively.Because of the advantages of the porous Ni3N@NC NSs and 3D hierarchically meso-macroporous and hollow tube-like architectures,the resulting Ni3N/CMFs/Ni3N exhibited extremely high catalytic activities toward OER and HER.In addition,due to the remarkable structural and chemical stabilities of Ni3N/CMFs/Ni3N,especially the unique protection of graphite carbon shells（GCSs）on the Ni3N nanoparticles（NPs）in Ni3N/CMFs/Ni3N,the Ni3N/CMFs/Ni3N-based water electrolysis cell also displayed excellent stability.