Design,Construction And Performance Study Of High Current Density Electrolytic Seawater Catalyst Based On Nickel Phosphide

Hydrogen（H₂）is an ideal energy carrier with the advantages of high energy density(142 MJ kg^-1),zero pollution,and zero emissions.Compared with the traditional fossil fuel hydrogen production technology,electrolysis of water hydrogen production technology has the advantages of green,pollution-free and high hydrogen production efficiency,and is currently the most promising hydrogen production method.However,almost all electrolytic water hydrogen production systems currently use alkaline pure water as the electrolyte,which undoubtedly exacerbates the shortage of pure water resources.Compared with pure water,seawater,which accounts for 96.5%of the earth’s water resources,can be regarded as an“inexhaustible and inexhaustible”resource.The development of efficient seawater hydrogen production technology is of great significance to the development of my country’s hydrogen energy economy,the development and utilization of marine resources,and the cause of marine national defense.However,the electrolysis of seawater to produce hydrogen still faces many obstacles:on the one hand,the anodic oxygen evolution reaction（OER）involves a four-electron transfer process,the kinetics of which are slow and lead to a high overpotential,which greatly limits the energy conversion efficiency of electrolysis of seawater;On the one hand,there is a high concentration of chloride ions（Cl^-）in seawater,which will trigger the competitive chlorine oxidation reaction（Cl OR）at the anode during high-current electrolysis of seawater,which will accelerate the occurrence of corrosion and seriously affect the stability of the electrode.Aiming at the problem of low efficiency and poor stability of high-current electrolysis seawater catalysts,the key to solving the problem is to improve the activity of the cathodic hydrogen evolution reaction（HER）and anode OER,and to avoid the anodic Cl OR side reaction under alkaline conditions.Among many catalytic materials,transition metal phosphides（such as Fe P,Co P,Ni₂P,etc.）have the advantages of adjustable electronic structure,variable composition and excellent conductivity,and have shown great promise in the field of hydrogen production by electrolysis of seawater.Huge application potential.However,the current transition metal phosphide materials still cannot meet the needs of high-current electrolysis of seawater.In addition,the research on the catalytic mechanism and structural evolution of electrocatalysts needs to be further deepened.Here,this thesis designed and synthesized a series of nickel phosphide-based nanomaterials with high activity and high stability,and studied their performance and mechanism of seawater electrolysis under high current density conditions.The main research contents are as follows:Oxidation reactions of small molecules usually have lower theoretical potentials.Using the hydrazine oxidation reaction（Hz OR）to assist the electrolysis of seawater to produce hydrogen can effectively avoid Cl OR and realize energy-saving hydrogen production.It is our goal to improve the intrinsic activity of the catalyst Hz OR and HER to enhance the performance of electrolytic seawater.（1）Using metal-organic frameworks（MOFs）as precursors,a Co-doped Fe-Ni₂P/MIL-Fe Co Ni heterostructure catalyst（Fe Co-Ni₂P@MIL-Fe Co Ni）.Benefiting from the synergistic effects of heterogeneous engineering and cation doping,the Fe Co-Ni₂P@MIL-Fe Co Ni catalyst exhibits excellent bifunctional electrocatalytic performance for hydrazine oxidation（Hz OR）and hydrogen evolution reaction（HER）in alkaline seawater electrolyte.The overall hydrazine decomposition（OHz S）system of Fe Co-Ni₂P@MIL-Fe Co Ni in alkaline seawater system requires only an ultra-low voltage of 400 m V to reach a current density of 1000 m A cm^-2,and can achieve a current density of more than 500 m A cm^-2It can run stably for 1000 hours under the current density.As a proof of concept,the overall hydrazine splitting（OHz S）with N₂H₄added saves 3.03 k W·h of electricity compared with the overall seawater system（OWS）without N₂H₄for the same production of 1.0 m³hydrogen,indicating that the hydrazine-assisted electrolysis seawater system can achieve energy saving Hydrogen production.Density functional theory（DFT）calculations show that the synergistic effect of Co ion doping and constructing Fe Co-Ni₂P/MIL-Fe Co-Ni heterointerface can reduce the water dissociation energy barrier of Fe Co-Ni₂P@MIL-Fe Co Ni and optimize HER performance.Adsorption hydrogen free energy to facilitate the dehydrogenation kinetics of the Hz OR process.（2）Using MIL-Fe Ni（a MOFs material）as a precursor,a 1D/3D hierarchical structure electrocatalyst（Fe Ni P/NPHC）consisting of N,P co-doped carbon coated Fe P/Fe Ni₂P was successfully designed and synthesized.Benefiting from the unique1D/3D hierarchical structure of the catalyst and the strong coupling effect between Fe P,Fe Ni₂P and N,P co-doped carbon at the three-phase heterointerface,the as-prepared Fe Ni P-NPHC catalyst exhibited excellent performance in alkaline seawater.Excellent Hz OR(E₁₀₀=7 m V)and HER(E₁₀₀=-180 m V)performance.In addition,the direct hydrazine fuel cell（DHz FC）self-assembled by the prepared Fe Ni P-NPHC catalyst can successfully power OHz S,which can simultaneously realize energy-saving hydrogen production,degradation of hydrazine wastewater,and seawater resource utilization,achieving three birds with one stone.DFT theoretical calculations show that the construction of Fe Ni P-NPHC three-phase heterojunction interface can effectively adjust the d-band center and electronic structure,which is beneficial to balance and optimize the Hz OR and HER electrocatalytic performance of the catalyst.The development of efficient,stable,and economical OER and HER electrocatalysts is crucial to realize the application of electrolysis of seawater to hydrogen production,and it is also a key issue we want to solve.（3）Fe-doped P-vacancy-rich Ni₂P catalyst（Fe-Ni₂Pv）was successfully synthesized on nickel foam by molten salt phosphating method.Fe-Ni₂Pv exhibited excellent bifunctional performance for OER and HER in alkaline seawater environment.In addition,Fe-Ni₂Pv exhibited excellent activity,stability,and selectivity in alkaline seawater environment,and was even able to work stably at a current density of 0.2 A cm^-2under industrial conditions（6.0 M KOH,60°C）More than 100 hours,farad efficiency is close to 100%.Theoretical calculations and experimental results show that Fe doping and P vacancies can synergistically improve the performance of the catalyst.Specifically,Fe doping and P vacancies can promote the generation of active species for OER and optimize the free energy of hydrogen adsorption for HER.In addition,we also clarified the real active sites of the catalyst,in which the Ni atom is the active site for OER,the Fe atom is the active site for HER,and the P vacancy can improve the conductivity of the catalyst.