Study On The Preparation And Electrocatalytic Performance Of Ferrite Series Compounds

The increasingly serious environmental pollution caused by the burning of fossil fuels has triggered the exploration of the use of sustainable and clean energy.H₂is deemed as the substitute for the traditional fossil fuels.The water splitting using electrocatalysts is a promising strategy to generate H₂gas.Electrochemical water splitting includes HER and OER,and their complex kinetics processes increase the overpotentials beyond the theoretical voltage.Highly efficient and stable catalysts can effectively address these issues.The state-of-the-art noble metal catalysts cannot be applied in the large-scale water electrolysis system due to the high price of noble metal catalysts.Therefore,development of non-precious metal-based catalysts with high catalytic activity has been attracted extensive attentions recently.Non-precious metal-based catalysts that can be used for the industrial production always show the following characteristics:（1）Excellent intrinsic catalytic activity;（2）Good stability;（3）Wide p H adaptability.In this context,this dissertation focuses on the synthesis of nanomaterials,the design of porous structure and the optimization of electronic structure.As a result,a series of ferrite-based self-supported catalysts were rationally constructed and their electrochemical performances were evaluated through material characterizations,electrochemical analysis and density functional theory calculations（DFT）.The details are as follows:（1）NiFe mesoporous nanosheets were fabricated on the surface of SSM via hydrothermal electrodeposition followed by the removal of template.Compared with the traditional electrodeposition,the synthesized catalyst by hydrothermal electrodeposition showed the increased specific surface area,improved crystallinity and enhanced OER properties;mesoporous Ni₃S₂nanosheets were grown on nickel foam（Ni₃S₂/NF）via hydrothermal electrodeposition and in-situ electrochemical dealloying combined with the low-temperature sulfuration.Theη₁₀of Ni₃S₂/NF was as low as 233mV with a Tafel slope of 60.5 mV dec^-1for OER;self-supportive Ni/Co/Fe phosphosulfide mesoporous nanowires（NiCoFe-PS）were constructed based on hydrothermal electrodeposition to further improve the HER and water splitting performances.Compared with NiCoFe-P or NiCoFe-S,NiCoFe-PS exhibited better catalytic activity and stability and showed the small overpotentials of 195 and 98 mV at10 m A cm^-2for OER and HER,respectively.In a two-electrode system,a 1.5 V battery can drive the device with a good stability up to 200 h.（2）To further reduce the production costs and improve stability,the modified SSM was constructed to catalyze the OER performance in alkaline condition.The self-supported mesoporous NiFe O_xnanorods were synthesized through an in situ anodic oxidation where the surfacial Cr atoms and the generated bubbles were used as hard and soft templates.The（Fe/Ni）（P/S）mesoporous nanorods（FNPS）was fabricated by the sulfuration/phosphorization to further improve the conductivity and reduce reaction barriers.In the 1.0 M KOH solution,FNPS presented a lowη₁₀of 195 mV with a Tafel slope of 65.7 mV dec^-1for OER and showed excellent stability beyond 60-day test;the N-doped anodized 316 SSM（NASSM）,as a bifunctional electrocatalyst,showed a potential of 1.61 V at 10 m A cm^-2for overall water-splitting with only a 0.01 V increase after a 100-h stability test.（3）Although the surface-modified SSM-based catalyst showed excellent stability,the acidic conditionsynthesis is not conducive to the fabrication of large-scale catalysts.The self-supported composites of（Ni,Co）₃C mesoporous nanosheets/N-doped carbon with adjustable sizes from 1×1 to 25×25 cm²was synthesized through a facile and rapid electrodeposition followed by carbonization.Theoretical calculations indicated that bimetallic carbide is favorable for HER due to the metallic conductivity,close-to-zero Gibbs free energy change（ΔG_H*）,and downshifted d-band center.Experiments showed thatη₁₀,η₅₀₀and j₀of bimetallic carbide catalyst were 58,384 mV and 0.72 m A cm^-2,respectively,in 0.5 M H₂SO₄solution.The excellent HER properties can be attributed to the ultrathin nanosheet-like structure with the large specific surface area,and electronic structure modulations.（4）Compared with acidic/alkaline water splitting,neutral water electrolysis might reduce the cost due to the low corrosiveness for the electrolyzer.A hierarchical strategy was used to modulate their electronic structure and active sites of S&Co co-doped Cu₃P nanowires（S&Co-Cu₃P NWs）.In the 1.0 M PBS solution,theη₁₀of S&Co-Cu₃P NWs was measured to be 118.1 mV with a Tafel slope of 80.3 mV dec^-1.In addition,theη₁₀of S&Co-Cu₃P NWs in artificial seawater was only 6 mV higher than that in PBS solution,showing the feasibility of direct generating H₂from seawater;a simple,fast and scalable synthesis of Cu@WC core-shell nanowires were synthesized.DFT showed that the lattice mismatch between Cu and WC reduced the bonding energy between H⁺and WC,the electrons at the interface were highly delocalized,and Cu increased the density of the electronic states near the Fermi level,thereby providing the higher WC’s carrier density for HER.The matched work functions between Cu and WC preserving the high level of Pt-like electronic structure of WC.Electrochemical experiments showed that Cu@WC exhibited excellent HER performances in a wide p H range.（5）Gas crossover leads to an explosive hydrogen-oxygen gas mixture in the practicl industrial water electrolysis.This dissertation addressed this issue by replacing OER with urea oxidation reaction（UOR）,which generates inert gas.Nitrogen-doped nickel iron oxyhydroxide（N-NiFe OOH@WRIF）was synthesized on rusty foam iron by wet chemical etching and ammonia/argon plasma treatment.Plasma engineering reconstructed the surface of NiFe OOH/WRIF to a 3D nanosheet-like porous network with abundant oxygen vacancies.N-NiFe OOH@WRIF only needed 1.52 V to reach 500m A cm^-2for UOR.For the overall water-urea electrolysis,it only required 1.58 V to deliver 100 m A cm^-2,which was 0.33 V less than that of the urea-free water splitting,lowering the overall energy consumption by 17.3%.Without OER,thus free of explosive hydrogen-oxygen mixture,the water-urea electrolysis was driven by solar energy,safely generating green hydrogen with the significantly decreased cost.