Synthesis Of Lamellar Transition Metal Nitrides(Phosphides)for Electrocatalytic Water Splitting

Based on the current situation of energy exhaustion and environmental pollution in the world,renewable energy sources have received widespread attention.As an ideal clean chemical fuel with superb gravimetric energy density and energy conversion efficiency,hydrogen energy is expected to be an excellent candidate for the traditional fossil fuels.At present,heavy oil and methane steam catalytic reforming,coal gasification and other methods are still the primary way to produce hydrogen,which undoubtedly accelerates the use of fossil fuels and environmental deterioration.In contrast,electrocatalytic water splitting can convert low-voltage electricity generated by solar and wind energy into hydrogen fuel,providing a promising and sustainable way for large-scale hydrogen production.Designing low-cost,high-efficiency electrocatalysts to replace the most efficient noble metal catalysts is the key to achieving this goal.Transition metal nitrides（phosphides）have a noble metal-like electronic structure and high stability,making them a potential non-platinum catalytic material in the field of electrocatalytic hydrogen production.Two-dimensional nanomaterials have become one of the most promising materials to replace noble metal electrocatalysts due to their unique physical and chemical properties and electronic properties.Therefore,this paper has developed a variety of effective methods for preparing two-dimensional transition metal nitrides（phosphides）with controllable structure and morphology,and obtained a series of electrocatalysts with excellent performance.The main research contents are as follows:The advantages of Single-atom catalysts（SACs）with high atomic utilization and high activity for many catalytic reactions have opened a new era of heterogeneous catalysis research,but the controllable synthesis of SACs with high stability remains challenging.In this part,we show a robust strategy toward the fabrication of highly efficient and stable SACs based on the competitive occupancy of two metal species on graphene oxide（GO）.The abundant Ti species predominantly occupy more groups of GO,thereby leaving a tiny number of groups in the gaps of the Ti species to combine with Co²⁺.The experimental and characterization results show that the competitive relationship between Co²⁺and Ti species can regulate the amount of Co single atoms on the graphite oxide.Moreover,due to the competition and spacer action of Ti species,a stable Co-N₄ single atom embedded in the two-dimensional TiN/rGO sheet is formed after nitriding the precursor.The prepared Co-N₄/Ti N-r GO catalyst not only has good electrocatalytic hydrogen production activity and stability,but also exhibits outstanding catalytic performance for the fast conversion of highly concentrated aromatic nitro compounds into amino compounds and excellent recyclability.The reaction mechanism is also proposed based on theoretical calculations.The strategy can be used to design Ni（Fe,Cr,Cu）-based SACs.The construction of heterojunction is an effective way to improve the performance of dual-function catalysts.The chemical properties of Vanadium（V）is similar to those of molybdenum and tungsten,but V has a more abundant valence state and a lower price.In this part,a hierarchical VN-Co _XN/NF heterojunction fixed on nickel foam was designed and prepared.Firstly,the triangular Co-MOF sheets were synthesized on the nickel foam substrate as the growth framework,then vanadium oxide nanosheets were deposited to synthesize V-Co precursors.Then,a hierarchical VN/Co _XN heterojunction with uniform growth of VN sheets outside Co _XN triangulated flakes was obtained by nitriding treatment.The formation of hierarchical structure not only made the catalyst have larger surface area,but also improved the catalytic activity of the catalyst with abundant porous structure and a large number of heterogeneous interfaces.At the same time,the synergistic effect between VN and Co _XN further promoted the catalytic performance,which made the fractional VN-Co _XN/CC heterojunction catalyst exhibit excellent electrocatalytic hydrogen evolution and oxygen evolution performance.The prepared hierarchical VN-Co _XN/NF heterostructures are bifunctional electrocatalysts with excellent activity for oxygen evolution and hydrogen evolution reactions.In alkaline electrolyte,the current density of 10 m A cm^-2 can be achieved at a low overpotential.At the same time,the catalyst showed excellent stability.We present a new strategy for the synthesis of 2D porous Mo P/Mo₂N heterojunction nanosheets based on the pyrolysis of 2D[PMo₁₂O₄₀]^3--melamine(PMo₁₂-MA)nanosheet precursor from a polyethylene glycol（PEG）-mediated assembly route.First,the theoretical calculation shows that the PEG-4000 can interact with MA through hydrogen bonds,thus inducing the arrangement of MA along 2D direction.Therefore,with the assistance of PEG-4000,the coordination process of POMs with MA can be modulated.There is a strong amino-hydroxyl interaction between PMo₁₂ cluster and MA,so after the introduction of PMo₁₂ cluster into the system,PEG is gradually replaced by PMo₁₂,thus inducing the growth of PMo₁₂ and MA along the two-dimensional direction of coordination to form a two-dimensional thin layer precursor.In the controllable phosphating process of the two-dimensional PMo₁₂-MA nanosheet precursor to form two-dimensional porous Mo P/Mo₂N Heterojunction nanosheets.The two-dimensional porous heterojunction material has excellent HER performance over a broad p H range.Especially in alkaline and neutral medium,the performance under high current density is better than commercial Pt/C catalysts.In combination with theoretical calculation and a series of experiments,this paper analyzes the two-dimensional porous Mo P/Mo₂N heterostructure nano piece of excellent electrical performance reasons of catalytic activity of hydrogen evolution,mainly including two-dimensional nanosheet structure provides abundant active sites,the heterojunction engineering can create active interface,leading to rearrangement of electrons for favorable water dissociation kinetics.Moreover,the rich pores offers a larger surface and abundance of exposed edges.The adjustment of the electronic structure of the catalyst is conducive to improving the catalytic activity of the catalyst.Firstly,thin Ni（OH）₂ nanosheets are prepared on the self-supporting carbon fiber cloth by hydrothermal method,and the post-transition metal Mo oxide was introduced onto the Ni（OH）₂ nanosheets by a coupling agent..The small size Mo N-modified two-dimensional porous Ni₃N nanosheets（Mo N-Ni₃N/CC）with excellent electrocatalytic performance was obtained after high temperature nitridation.The modification of small-sized Mo N can not only regulate the electronic structure of Ni₃N,but also make it form a porous structure,exposing more active sites,thereby improving the HER activity of the catalyst.In 1 M KOH and 1 M PBS electrolyte,the Mo N-Ni₃N/CC nanosheet catalyst needs 23 and 63 mV overpotential to provide the current density of 10 m A cm^-2,respectively.Its performance of electrocatalytic hydrogen evolution at high current density is better than that of commercial Pt/C catalysts.In addition,density functional theory（DFT）calculations further revealed that the Mo N modified Ni₃N promoted the water dissociation and the adsorption of H*,which was conducive to the realization of efficient HER activity.