Design Synthesis And Performance Study Of Nickel Based Bimetallic Oxygen Evolution Electrocatalysts

Electrochemical splitting of water into hydrogen and oxygen is one of the most appealing strategies for producing clean fuels to alleviate ever-increasing energy demands and environmental problems.However,the half oxygen evolution reaction(OER)dramatically impedes the overall efficiency of water splitting because of its intrinsically complex four-electron transfer process.Presently,precious-metal-based electrocatalysts RuO₂ and IrO₂ have been proven as state-of-the-art OER catalysts,yet prohibitive cost and relative scarcity limit their applications.To this end,developing cost-effective OER electrocatalysts is urgently desired.Among various promising alternatives,nickel-based bimetallic materials with adjustable electron configuration and low cost have drawn considerable attention.However,to date,their comprehensive performance is still unsatisfact ory and lower than that of the precious-metal RuO₂.On the other hand,the synergistic interactions and preferred active sites between the bimetals are still unclear.More importantly,there is no consensus on how to reasonably design nickel-based bimetallic electrocatalysts.In this thesis,several highly active nickel-based bimetallic electrocatalysts toward oxygen evolution from water were designed by assembling hierarchical hollow nanostructure,constructing heterostructure,and tai loring the d-band centers.In addition,combining DFT simulations with experiments investigated the OER reaction mechanism of the corresponding catalysts.Hierarchical hollow nanopolyhedra assembled by NiCo-LDH nanosheets with adjustable composition and nanostructure are prepared via a facile self-templated strategy.Extensive electrochemical measurements combined with DFT calculation are performed to acquire an in-depth understanding of the OER mechanism for NiCo-LDH.The results show that step 3(formation*OOH from*O)is the potential-determining step for NiCo-LDH.Additionally,Co³⁺hollow sites have the lowest free energy for water oxidation reaction,indicating that the highly active Co³⁺hollow sites serve as the preferred adsorption and active site s.The exposure of more Co³⁺active sites by optimizing the compositions of Niand Co in NiCo-LDH enables the hierarchical NiCo-LDH hollow nanopolyhedra catalysts to exhibit superior OER activity,obtaining a 10 m A cm^-2 current density at an overpotential of314 m V.A novel porous nanoscale NiO/NiCo₂O₄ heterostructure with abundant interfaces is constructed by two-stage calcination of nickel-cobalt bimetallic hydroxide precursors prepared using a microwave-assisted hydrothermal method.The NiO phase stabilized in NiCo₂O₄ matrix is the nanometer scale(ca.13 nm).The porous nanoscale NiO/NiCo₂O₄heterostructure shows a 10 m A cm^-2 current density under the overpotential of 264 m V which outperforms the noble catalyst RuO ₂.It is demonstrated that the highly active Ni³⁺species generated by the oxidation of nanometer scale NiO surface are responsible for enhanced OER.Additionally,DFT calculations certify that the heterostructure interfaces promote the chemisorption of OH intermediates.The generation of Ni³⁺and the construcvtion of heterostructure interface synergistically solve the problems of low intrinsic catalytic activity and fewer active sites of large-size NiO for OER reaction.Guided by the theoretical mechanism study of the intrinsic high electroactivity revealed in the d-band center theory,the Fe-substituted Ni₂P((Ni_xFe_1-x)₂P)nanosheets grown directly on NiFe foam have been designed and prepared.Density functional theory(DFT)calculations reveal that Fe-substitution can intensify the nonuniform distribution of charge and increase the density of states(DOS)near the Fermi level(E_F),thus increasing the E_d energy level of(Ni_xFe_1-x)₂P.UPS valence band spectra and work function(?)maps characterization consistently demonstrate that the E_d energy level of(Ni_xFe_1-x)₂P nanosheets is indeed increased compared with that of Ni₂P.As OER electrocatalysts in alkaline media,the(Ni_xFe_1-x)₂P nanosheets show an overpotential of 166 m V to deliver the current density of 10 m A cm^-2,which is superior to the Ni₂P and most reported transition-metal-based catalysts.On the other hand,the OER overpotentials are strongly related to the E_denergy levels and present a volcano plot relationship.DFT simulations reveal that this trend results from the moderate rise in E_d energy levels,which balances the adsorption and desorption capacities of the oxygen-containing intermediates(*O,*OH,and*OOH).This work supplies valuable insights into the rational design and construction of efficient doped electrocatalysts under the guidance of the d-band center theory.