Research Of Ni Based Integrate Electrodes For Water Electrolysis

With the increasingly severe of energy crisis and environmental pollution issues,electrocatalytic water splitting has attracted intense attention due to its environmetal friendly,fully scalable and easy preparation of high purity hydrogen.However,as a multi-steps reaction,water splitting is characterized as a reaction with high energy barrier and sluggish kinetics.Therefore,the development of cost-effective,highly efficient and stable electrocatalysts remains a huge obstocal for the large-scale application of electrocatalytic water electrolysis techneque.Recent years,electro-catalysts based on transitional metal（e.g.Fe,Co,Ni）have attracted great interests,owing to their aboundant reserves and low-costs.Nevertheness,their catalytic performance is still far away from the thermaldynamic theirotical bench mark.An efficient catalyst should possess an optimized electron configuration,which is beneficial to the selective adsorption/desorption of different reaction intermediates.Moreover,as a result of the existence of gas,liquid and solid three-phase boundery,various external factors（e.g.surface area of a catalyst,the design of mass transfer path,loading,etc.）can also influence the improvement of catalytic activity performance.Based on these considerations,to develop low-cost,highly efficient water electrolysis catalyst,works on design of electrode structre and regulating of electron configuration of the catalyst have been conducted in this thesis:（1）To solve the problems encountered under practical applications（e.g.thick catalyst film,high mass transfer resistance,low conductivity,etc.）,a 3-D integrate coral-like FeNi（OH）_x/Ni electrode for overal water splitting is developed with the assistance of mild corrosion of Fe³⁺to Ni.Firstly,coral-like Ni with unique pore system is constructed on the Ni foam by cathode electro-depositon under large current density.Then,catalytic active FeNi（OH）_x species are in-situ decorated on the coral-like Ni back bone through controlling the corrosion process of Fe³⁺to Ni.The as-obtained electrode demonstrate superior activity toward both OER and HER in 1.0 M KOH solution.A symmetric alkline water splitting electrolysor is constructed by taking FeNi（OH）_x/Ni as both the anode and cathode,which illustrates a low cell voltage of 1.52 V at 10 mA cm^-2,even supassing the performance of a dissymmetric electrolysor(V_cell,10=1.55 V)composed of noble metal based catalyst as the anode（RuO₂）and cathode（Pt/C）,respectively.Multiple electrochemical and physical characterizations demonstrate that the superb activity of FeNi（OH）_x/Ni in 1.0 M KOH can be attributed to several advantageous aspects,including more expourse of active sites（the electrochemical active surface area increased for almost 10 times）,more efficient liquid reactant transfer and gas product release and enhanced charge transfer and the formation of high intrinsically active FeNi（OH）_x species（with Fe/Ni ratio of 4:6）during the oxidative etching and hydrolysis process.（2）Currently,carbon-based water electrolysis catalysts are mostly synthesized in powder form,which are traditionally pasted on the conductive substrate together with conductive binder（e.g.Nafion）for practical application.However,such an electrode structure always suffers from insufficient exposure of active sites,inefficient charge transfer and low loading.To solve these problems,a sacrificial template method for modulating of electrode structure is proposed to prepare 3-D integrate FeCo PBA nanosheet arrays on Ni foam.The as-obtained FeCo PBA can be facilely transformed to FeCo/C NS by low temperature hydrogenation treatment for efficient oxygen evolution reaction（OER）and to FeCoP/C NS by low temperature phosphorization treatment for efficient hydrogen evolution reaction（HER）,respectively.Multiple electrochemical measurements and physical characterization demonstrate that the enhancement of activity can be attributed to:1)more active sites are exposed,and mass transfer is promoted,owing to the rational designed architecture of the catalyst film（i.e.regular nanosheet arrays perpendicular to the substrate）;2)more efficient charge transfer can be achieved,attributing to the intimate contact between catalyst film and Ni substrate;A low cell voltage of 1.55 V affords a current density of 10 mA cm^-2 with an electrolysor constructed by FeCo/C NS and FeCoP/C NS,highlighting the potential practical application.（3）To promote the intrinsic activity of metallic Co based HER catalysts by acceleration the activation of H₂O molecule,a strategy concerning the construction of Co/CoMoO₄ interface and introduction of O vacancies at the same time is proposed.Firstly,core@shell Co（CO₃）_0.5（OH）·0.11H₂O@CoMoO₄（CCH NA@CoMoO₄）nanowire arrays on Ni foam are constructed by hydrothermal and chemical bath depostion method.Then,core@shell Co@CoMoO₄ nanowire arrays（Co@CoMoO₄ NA）are obtained by low temprature hydrogenation transfermation of CCH NA@CoMoO₄.Through the construction of Co/CoMoO₄ interface,redistribution of electrons on catalyst surface is confirmed by multiple electrochimcal and physical characterizations,which contributs greatly to the HER activity.In 1.0 M KOH solution,the overpotential at 10 mA cm^-2 is as low as-46 mV for Co@CoMoO₄ NA,surpassing that of Cobalt nanowire arrays(Co NA,η₁₀=-250 mV)and CoMoO₄(η₁₀=-179 mV).Moreover,the performance is quite close to that of commercial Pt/C(η₁₀=-31 mV),highlighting the advantage of the proposed strategy.