Surface/interfacial Chemical Regulation And Property Investigation Of Nano-cobalt Based Electrocatalysts

Converting intermittent renewable energy into storable chemical fuels is a key to realize a sustainable energy in the future.Water electrolysis is an effective way to achieve clean hydrogen energy production.Designing and developing electrocatalysts with abundant active sites and robust structure is of great significance for the sustainable development of hydrogen energy.The dissertation focuses on the nano-cobalt based materials with promising application in water electrolysis as the research object.Based on the activity of cobalt sites,including electronic structure,coordination environment and the amounts of active sites,explores various surface/interface regulation strategies to optimized the catalytic activity of nano-cobalt based electrode.To guarantee highly exposure active sites of nano-cobalt based electrode,optimizing the reaction energy barrier,and balancing the circulation rate.In this dissertation,by the following strategies,including interface construction,anions doping and synergy of multiple constraints to adjust the activities of nano-cobalt materials,and obtain a series of highly efficient catalysts.Moreover,we researched the catalytic mechanism of modified-nano-cobalt based materials,providing insight into how to optimize the activities by surface/interface regulation strategies.The main research contents are as follows:（1）Mn₃O₄/Co P porous nanorod was successfully prepared using Mn O₂/Co₃O₄ as precursor by selective phosphating and in-situ reduction strategy.Various characterization methods were conducted to investigate the phase composition,morphological structure,and electrocatalytic properties of the composites.Electrochemical measurements indicate as-prepared Mn₃O₄/Co P electrode possess superior activity and durability toward water splitting in comparison to Mn₃O₄ and Co P electrode.It is demonstrated that porous nanorods morphology coupled with interpenetrated interfaces Mn-O-Co-P could contribute to the charge redistribution between mix-valence Mn_x²⁺Mn_3-x³⁺O₄ with Co P,optimizing the surface electronic states of Co P-sites,thus,enhancing the overall catalytic performance.（2）Based on phase transition relaxation,we proposed a non-metal modulation,Co P porous nanosheet arrays were successfully prepared using[200]crystal cobalt metal-organic framework ultrathin nanosheet arrays as precursors by s successively oxidation/phosphating reaction method.With the carefully controlled phosphating degree,the tuned P/O atomic ratio in-plane would result in the changing Co³⁺/Co²⁺couples.Optimal O-Co P-40/NF electrode just requires a small overpotential of 67 and 202 m V to deliver a current density of 10 m A cm^-2 for HER and OER,surpassing majority of reported Co-based phosphide.Based on theoretical and experimental results,the electron-rich P-3p and O-2p orbitals could co-modulate the electronic environment of Co sites,which boosted water dissociation in the HER process and balanced the adsorption/desorption of intermediates in the OER pathway,resulting in an optimal overall water splitting.（3）The spinel Co₃O₄ with octahedral(Co_Oh³⁺)sites was selected as the research object,we successfully prepared ultrathin cobalt oxide nanosheet arrays doped with tungsten（W）and iron（Fe）were by in-situ etching assisted electrochemical anodic oxidation（Fe WCo O_x/CC）.In situ etching can directionally introduce WO₄^2-into the interlayer of LDH precursor,on the other hand,anodic oxidation can accurately replace Co O₆with Fe O₆and achieve short-range ordered lattice arrangement,ensuring the layered structure and constructing high double-center and high exposure catalyst.Typically,the Fe WCo O_x/CC electrode exhibits low overpotential of 271 m V to reach the current density of 100 m A cm^-2for OER,which is better than commercial Pt/C catalysts.The results shows that Co_Td²⁺caused by defects can realize the rapid reconstruction of irreversible Co_Oh³⁺in the oxidation cycle reaction,and high-valance W can reverse assist the original thermodynamic process of Co³⁺/Co⁴⁺to transition to Co²⁺/Co³⁺/Co⁴⁺,thus,balancing the energy barrier in the oxidation cycle and make the oxidation reaction run quickly and smoothly.Fe fixed as a high activity and stable OER sites,and finally realize the synergistic effect company with high-valance W to enhance the OER performance of cobalt oxide.Combined with the characterization analysis before and after OER test,it showed that Co_Td²⁺caused by defects can realize the rapid reconstruction of irreversible Co_Oh³⁺in the oxidation cycle reaction,and high-valance W can reverse assist the original thermodynamic process of Co³⁺/Co⁴⁺to transition to Co²⁺/Co³⁺/Co⁴⁺,thus,balancing the energy barrier in the oxidation cycle and make the oxidation reaction run quickly and smoothly.Fe fixed as a high activity and stable OER sites,and finally realize the synergistic effect company with high-valance W to enhance the OER performance of cobalt oxide.（4）In the work,Ru-doped Co_0.85Se/CC porous nanorods arrays was prepared by selenic-acid-assisted etching strategy.The phase composition,morphological structure and electrocatalytic properties of the Ru-Co_0.85Se/CC were studied by various characterization method.the Fe WCo O_x/CC electrode exhibits low overpotential of 288,189 and 213m V to reach the current density of 100 m A cm^-2for OER,UOR and HER,respectively.Especially,when Ru-Co_0.85Se/CC was employed as the self-supported electrode for H₂ production in coupled HER||UOR system,only a small cell voltage of 1.49 V was required to deliver ahigh current density of 10 m A cm^-2,Finally,the electrode after stability test was characterized,indicating that the electrochemical process would induce the rapid surface reconstruction of the material to form Ru-Co_0.85Se@Ru/Se-Co OOH material,and its activity remained relatively stable.This paper includes 112 figures,11 tables and 301 references...