Construction And Electrocatalytic Performance Of Iron Triad-Based Nanostructured Catalysts For Water Splitting

Water electrolysis is a promising method for producing green hydrogen.It is currently a research hotspot in the field of hydrogen energy due to its high efficiency and environmental sustainability.The efficiency of water electrolysis depends on the catalysts’performance for the cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.Commercial HER/OER electrocatalysts are mainly based on noble metals such as Pt/Ir/Ru.However,the scarcity and high cost of these catalysts limit their large-scale industrial application.Developing high-efficiency,low-cost non-noble metal-based catalysts for water electrolysis is of great scientific value and engineering significance.To address these issues,this dissertation focuses on low-cost Iron Triad（Fe,Co,Ni）based catalysts and explores their structural design and controllable preparation methods.Three highly efficient Iron Triad-based nano catalysts were designed from the perspectives of optimizing the intrinsic activity and increasing the number of active sites.The catalysts’OER performance,HER performance and overall water splitting performance were studied to reveal the catalytic mechanism.Specific research findings are as follows:1.Self-supported composite catalysts consisting of iron-based alloys nanoparticles and nitrogen-doped vertical graphene（NVG）arrays were prepared on carbon cloth（CC）using a pyrolysis method.The electrocatalytic performance and mechanism of these catalysts were investigated.The results demonstrate that the unique nanostructure of iron-based alloys nanoparticles anchored to NVG has numerous catalytic active sites.Furthermore,the electronic band structure is modified by alloying,and electron transfer is accelerated due to the high conductivity of nitrogen-doped graphene.The structure of vertical graphene nanosheet arrays is conducive to the diffusion of ions and oxygen gas.These advantages enable Ni Fe@NVG/CC to exhibit highly efficient and stable oxygen evolution.According to the tests,Ni Fe@NVG/CC can achieve a current density of 10m A cm^-2 with a low Tafel slope of 36.2 m V dec^-1 at an overpotential of 245 m V.High-resolution transmission electron microscopy（HRTEM）and X-ray photoelectron spectroscopy（XPS）analysis were conducted pre-and post-reaction,combined with density function theory（DFT）calculations,revealed that the catalytic active sites of Ni Fe@NVG/CC were Ni-Fe hydroxyl oxides（Ni Fe OOH）formed by the surface reconstruction of the Ni Fe alloy nanoparticles during the OER process,and nitrogen doping in graphene could lower the reaction energy barrier of the reaction intermediate O*to OOH*,is the essence of the enhanced OER performance.2.A self-supported composite catalyst（Ni Fe-LDH@Co P/CC）with Co P nanowires as core and Ni Fe hydroxide nanosheets as shell on a carbon cloth was synthesized through a hydrothermal method combined with electrodeposition.The electrocatalytic performance and mechanism in both alkaline electrolyte and seawater were investigated.The study demonstrates that the conductive Co P nanowire core provides sufficient electron transfer pathways and promotes electron transfer,while the shell layer,composed of ultrathin layered nanosheets of Ni Fe-LDH,provides ample catalytic sites and facilitates mass transport.The unique core-shell nanostructure of Ni Fe-LDH@Co P exhibits excellent OER catalytic activity and fast reaction kinetics.According to the tests in 1.0 M KOH electrolyte,a current density of 100 m A cm^-2 can be achieved with a small overpotential of 154 m V,and the Tafel slope is as low as 24.9 m V dec^-1.As to the alkaline seawater electrolyte,Ni Fe-LDH@Co P/CC also demonstrates excellent OER performance as well,requiring only 164 m V overpotential to achieve a current density of100 m A cm^-2 with a Tafel slope as low as 33.0 m V dec^-1.Specifically,it shows a superb electrocatalytic and structural stability over 300 h at a high current density of 500 m A cm^-2.Based on results above,the reasons for the excellent OER performance of the catalysts were analyzed:Ni Fe-LDH@Co P underwent a surface reconstruction in OER process and the generated Ni Fe OOH acting as the active sites.Furthermore,in situ-generated phosphate can effectively inhabit the corrosion of chlorine in seawater,explaining the excellent OER activity and long-term stability of Ni Fe-LDH@Co P in alkaline seawater.3.A self-supported HER catalyst based on phosphorus-doped cobalt disulfide on carbon cloth（P-Co S₂-h）was designed and constructed by the strategy of hydrothermal sulphuration combined with low-temperature phosphorization.Its HER performance and electrocatalytic mechanism were then investigated.The phosphorus-doped cobalt disulfide has a heterogeneous composite structure with octahedral nanoparticles decorated on nanotubes.This unique heterostructure not only provides abundant electron transfer pathways and catalytic active sites for HER,but also facilitates mass transport.These advantages enable P-Co S₂-h to exhibit excellent HER performance:in 0.5 M H₂SO₄ electrolyte,a current density of 10 m A cm^-2 can be obtained at 129.4 m V overpotential with a Tafel slope of 85.7 m V dec^-1;in 1.0 M KOH electrolyte,a current density of 10 m A cm^-2 can be obtained at 170.0 m V overpotential with a Tafel slope of10 m V dec^-1.The reasons of the enhanced electrocatalytic performance of P-Co S₂-h were further analyzed through structural and electrochemical characterization combined with DFT calculations:the unique heterostructure is conducive to the enhancement of the electrocatalytic activity.Furthermore,the introduction of phosphorus can activate inert sulfur sites on the surface of P-Co S₂-h,thus greatly promoting its HER performance.4.In order to investigate the performance of Iron Triad-based electrocatalysts for overall water splitting（OWS）,a P-Co S₂-h/CC||Ni Fe-LDH@Co P/CC dual-electrode OWS system was constructed.The overall water splitting cell achieved a current density of 100 m A cm^-2 with a low cell voltage of 1.77 V and maintained this activity for more than 100 h.This demonstrates that Iron Triad-based electrocatalysts can achieve high-performance in OWS,providing a feasible solution for the development of low-cost alkaline OWS.