Surface Structure Design And Performance Of The Transition-metal-based Electrocatalysts

The large-scale utilization of renewable energy requires efficient energy storage and conversion devices.Electrolysis of water and electrochemical synthesis of ammonia are green and promising energy storage and conversion technologies,but the high cost and low activity and selectivity of the electrocatalysts limits their development.Thus,developing cheap and highly efficient electrocatalysts is the key to the development of electrolysis of water and electrochemical synthesis of ammonia.At present,the precious-metalbased electrocatalysts such as Pt and RuO₂ show the highest performance,but the scarcity limits their applications.Transition metal-based electrocatalysts are cost-effective and earth-abundant but show inferior performance to the precious-metal-based ones.Their electrocatalytic performance can be enhanced through surface structure design,which is expected to approach or even surpass precious-metal-based electrocatalysts.This thesis focuses on nickel-based,cobalt-based materials and UiO-66 metal-organic frameworks(MOFs)as electrocatalysts.The different strategies such as engineering surface lattice strain,surface and interface defects,surface heteroatoms,surface valence and surface metals and substituents have been used to design the surface structure and improve the electrocatalytic activity and selectivity of the electrocatalysts.Moreover,by combining density functional theory(DFT)calculation,synthesis,characterization and test of electrocatalysts,the structure-performance relationship of the transition-metal-based electrocatalysts has been explored,which will provide theoretical guidance and foundation for practice for developing cheap and efficient transition-metalbased electrocatalysts.The detailed contents are as follows:(1)The surface lattice strain of electrocatalysts has been engineered based on the lattice parameter difference between Ni and NiO.DFT calculations found that the Ni(111)supported NiO(111)shows expanded lattice compared to the unsupported Ni(111),leading to the upshift of d band center of the surface Ni atoms.While the NiO(100)supported Ni(100)shows compressed lattice compared to the unsupported Ni(111),leading to the downshift of d band center of the surface Ni atoms,which is beneficial to lower the free energy barrier of the hydrogen evolution reaction(IER)on the catalyst surface.According to the theoretical model calculated by DFT,the surface-oxidized nickel(so-Ni)and surface-reduced nickel oxide(sr-NiO)catalysts were designed and synthesized.The surface lattices of the synthesized so-Ni and sr-NiO have expanded and compressed respectively,leading to enhanced HER activity.Among them,srNiO shows the highest HER activity with an overpotential of 164mV to deliver the current density of 10mAcm^-2.(2)The nickel oxide is reduced by a safe and cheap reducing agent(i.e.,cellulose)and used as electrocatalyst for oxygen evolution reaction(OER).The defects on the surface and interface of the obtained electrocatalysts are engineered by changing the temperature and duration of the hydrothermal reaction.The cellulose-partially-reduced NiO(CL-prNiO)was obtained by hydrothermal reaction at 250? for 6 hours.The CL-prNiO shows increased electrochemically active surface area and charge transfer efficiency,which is conducive to in-situ generate a large amount of Ni³⁺ at the potential prior to the OER onset.The in-situ generated Ni³⁺ species can activate the lattice oxygen in the unreduced NiO of CL-prNiO and lower the free energy barrier of the OER,leading to the boosted OER activity.The CL-prNiO delivers the current density of 10mACm^-2 with an overpotential of 288 mV,which is comparable to the commercial RuO₂ and outperforms the synthesized Ni and NiO.(3)The nickel foam supported amorphous NiO was synthesized by electrodeposition.The Fe and Se heteroatoms were introduced to engineer the surface structure of the obtained electrocatalyst.The introduced Fe and Se heteroatoms provide more active sites for HER and OER.Moreover,the morphology of the deposited layer has been changed after the introduction of surface Fe and Se heteroatoms,leading to the formation of a hollow porous structure,which is beneficial for the exposure of active sites and mass transfer.The obtained a-FeSeNiO@NF exhibits high bifunctional activity for HER and OER,which can deliver the current density of 100mAcm^-2 with the overpotentials of 192 and 222mV for HER and OER,respectively.Moreover,the a-FeSeNiO@NF can split alkaline water with a total voltage of 1.66V at 100mAcm^-2 and remain stable over 50 hours of operation.(4)The physical mixture of a cobalt-based MOFs and carbon black was pyrolyzed in nitrogen to prepare the nitrogen-doped carbon supported Co-CoOx composite electrocatalysts.By controlling the ratio of MOF(Co)and carbon black,the valence state of Co and the distribution of N species on the surface of the obtained electrocatalysts has been engineered,leading to enhanced bifunctional activity for the OER and oxygen reduction reaction(ORR).The MOF(Co)/C(3:1)-500 shows an overpotential of 400mV at 10mAcm^-2 for OER and a half-wave potential of 0.80V for ORR.Moreover,its performance in zinc-air battery exceeds the precious metal-based 20wt% Pt/C.(5)Defective UiO-66 based MOFs are used as electrocatalysts for HER and nitrogen reduction reaction(NRR).Different metals(Ti,Zr,Hf)and ligand substituents(-NH₂,-OH,-CH₃,-Br,-Cl,-NO₂)were used to engineer the surface structure of the electrocatalysts.DFT calculation was used to investigate the effect of the types of metals and ligand substituents on the structure and energy of the intermediates for HER and NRR.Regulating the metal and ligand substituents will change the d band structure of the surface metal sites of the electrocatalysts,thereby changing the adsorption characteristics of reaction intermediates such as H and NNH on the surface of the electrocatalysts.After screening,it was found that the defective Hf-UiO-66-NO₂ shows high theoretical HER activity,while the Zr-UiO-66-NH₂ can significantly inhibit HER and the formation of hydrazine.Thus,Zr-UiO-66-NH₂ shows high theoretical NRR selectivity.The synthesized Zr-UiO-66-NH₂ shows the highest Faraday efficiency of 85% for electrochemical nitrogen fixation,outperforming most of the recently reported electrocatalysts.