Construction And Oxygen Evolution Mechanism Investigation Of Oxyanion Modulated Nickel-Iron Based Catalyst

Electrocatalytic water splitting has received increasing attention due to its ability to transform the intermittent sustainable energy into hydrogen as the chemical fuel.The rational design and development of high-efficiency catalyst for oxygen evolution reaction（OER）is critical for the application of electrolytic water splitting devices.Nickel-iron（Ni Fe）based materials have been regarded as one of the promising OER electrocatalysts owing to their high intrinsic catalytic activity.However,the difficult electrochemical reconstruction and the leaching of metal sites during OER process leads to the limited catalytic activity and poor long-term stability of Ni Fe-based electrocatalyst.Thus,an oxyanion regulation strategy was adopted to improve the OER activity and stability based on the analysis of Ni/Fe sites.A series of oxyanion modulated Ni Fe-based catalysts with high activities and stabilities have been obtained by adjusting the existing forms and categories of oxyanions.Moreover,the internal factors and the reaction mechanism of oxyanions on enhancing the catalytic performances was clarified,which provided a new path for developing efficient Ni Fe-based catalysts and accelerating the practical application of Ni Fe-based catalysts in water splitting.The main content of this thesis is as follows:（1）NiFeO_x（OH）_y catalysts with varied atomic ratio of Ni/Fe were prepared on the surface of Ni foam by electrodeposition for OER to explore the dominate active site during OER processes.The electrochemical performances shows that the Tafel slopes of catalysts with Fe species are much lower than those of catalysts without Fe species,indicating that the Fe site is the main active site for OER.Meanwhile,the Ni site is proved to be the OH^-adsorption site,which synergistically accelerates the OER process.As a result,the NiFeO_x（OH）_y catalyst with the best Ni/Fe interface（atomic ratio of 1:1.18）shows the best OER performances.It only requires a low overpotential of 250 m V to deliver current density of 10 m A cm^-2and exhibits high stability for 50 h at a current density of 50 m A cm^-2.（2）Considering that SO₄^2-is easy to form covalent bond with metal sites on the surface of catalyst,a surficial SO₄^2-modified NiFeO_x（OH）_ypre-catalyst was synthesized by electrochemical anodizing of NiFefoam for OER to reveal the dual effect of SO₄^2-on enhancing the OER activity.The oxidation process of Ni site and electrochemical reconstruction can be promoted by the leaching of SO₄^2-under OER conditions to form active Ni（Fe）OOH species from pre-catalyst.Meanwhile,the residual SO₄^2-adsorbed on the surface of catalyst can stabilize the OOH*intermediate for reducing the reaction energy barrier and thus improving OER performance.As expected,the SO₄^2-modified NiFeO_x（OH）_y pre-catalyst displays remarkable OER performances.The overpotential to achieve a current density of 50 m A cm^-2 is only 234 m V,which is 48 m V lower than that of NiFeO_x（OH）_y without SO₄^2-modification.Furthermore,it possesses high stability over 100 h at the current density of 100 m A cm^-2.（3）Since the stable structure and the high solubility in alkaline media of Mo O₄^2-,a Fe-doped Ni Mo O_x（OH）_y pre-catalyst was obtained via the electrochemical anodizing of Ni Mo foam and electrodeposition for OER to investigate the effect of dynamic behavior of inactive Mo O₄^2-contained in the pre-catalyst on OER activity.The Mo O₄^2-escapes from the pre-catalyst and dissolves into the electrolyte during the OER process,which is conducive to increasing the reconstruction degree of pre-catalyst to generate the Ni（Fe）OOH active phase.Moreover,the re-adsorption of Mo O₄^2-is favor for enhancing the adsorption of OOH*intermediate and optimizing the reaction free energy,thus improving the OER activity.Therefore,Fe-doped Ni Mo O_x（OH）_y pre-catalyst exhibits significantly boosted OER activity with low overpotential of 217 m V at current density of 10 m A cm^-2,which is 55 m V lower than that of catalyst without Mo species.It can also maintain stable oxygen evolution for more than 200 h at a current density of 100 m A cm^-2.（4）Considering that B-O has strong complexation ability with Fe,a BO₃^3-regulation strategy is proposed.BO₃^3-can easily fill the oxygen vacancy in NiFeLDH and is able to improve the valence state and coordination number of Fe site,which narrows the band gap and favors achieving efficient reconstruction of NiFeLDH under OER conditions.Besides,BO₃^3-enhances the adsorption of OH^-and accelerate the catalytic reaction,further improving the activity of OER.As a result,BO₃^3-modulated NiFeLDH shows desirable OER catalytic activity with an ultralow overpotential of 201 m V at current density 10 m A cm^-2,which is40 m V lower than that of the pure NiFeLDH.In addition,the assembled anion exchange membrane water electrolysis system using BO₃^3-modulated NiFeLDH as anode electrode and Pt/C as cathode electrode for water splitting needs a voltage of only 2.0 V to drive a current density of540 m A cm^-2.（5）The planar configuration of NO₃^-makes it easy to enter the lattice of layered materials,therefore a lattice occupied NO₃^-was introduced to alleviate the Fe segregation and improve the OER stability of NiFecatalyst.A stable Fe OOH/Ni₃（NO₃）₂（OH）₄ biphasic interphase catalyst was constructed through hydrothermal and electrodeposition process.The stable NO₃^-in the lattice tends to coordinate with Fe in Fe OOH to stabilize the Fe site,thus improving the structural stability of catalyst.The chemical interaction between NO₃^-and Fe can significantly weaken the dissolution level of Fe and favor the homogeneous redeposition of Fe,thus effectively alleviates Fe segregation.Thus,Fe OOH/Ni₃（NO₃）₂（OH）₄ catalyst exhibits significantly enhanced long-term OER stability with six-fold improvement compared with the Fe OOH/Ni（OH）₂ catalyst without NO₃^-modification.This paper includes 123 figures,23 tables and 260 references...