Preparation Of Self-supported Transition Metal Catalysts For Electrochemical Water Splitting

Driven by renewable resources such as solar-or wind-derived electricity,water splitting technique provides an appealing way to produce environment-friendly and recyclable hydrogen.Generally,the benchmark electrocatalysts for electrochemical water splitting are still precious metal.For example,platinum（Pt）is regarded as the best electrocatalysts for HER,and iridium dioxide（IrO₂）for OER.However,the precious metals with poor stability are expensive and rare.A large amount of research has been devoted to the development of transition metals,especially self-supporting transition metal catalysts to replace precious metal catalysts.In particular,advances in nanotechnology and materials chemistry have greatly contributed to the emergence of a series of nanostructured materials for advanced electrocatalysis.The transition metal catalysts directly growing on the conductive substrate,not only reduces the cost of the synthesis technology but also promotes the electron transfer between the substrate and the active sites,preventing the electrocatalysts from falling off during the bubble production process,thus improving the electrocatalytic activity and stability of the electrocatalysts.In particular,bifunctional electrocatalysts provide a potential direction for the exploration of new electrocatalytic materials.At present,recent researches were focused on recent research concerning how to effectively modify the self-supported transition metal electrocatalysts to realize the unification of catalytic performance and structure composition.According to the above-mentioned research backgrounds,the self-supported transition metal electrocatalysts can be modified by forming the heterogeneous structure,introducing dopant into the lattice,and changing the intrinsic structure.The main contents and results of this thesis are summarized as the following:1.We first successfully prepared the different self-supported Cu-S compound nanoarrays with controllable morphology on Cu foam by a wet-chemical method at room temperature.Moreover,there is a corresponding relationship between different compounds and the corresponding performance of HER.Then the Cu₂S/NiCo₂O₄ heterostructure was successfully prepared by a hydrothermal method using the self-supported Cu₂S with the best performance as the precursor.The innovation of this research is that the synergetic effect between two electrocatalysts in the heterostructure greatly enhanced the electrocatalytic activity.The HER activity of Cu₂S/NiCo₂O₄ heterostructure far exceeds the pure Cu₂S with the good durability for 12 hours in acidic solution.The Cu₂S/NiCo₂O₄ heterostructure provides a new possibility for the commercialization of the self-supported transition metal electrocatalysts in the catalytic field.2.We developed a self-supported ternary nanoporous sulfur-doped copper oxide(Cu₂O_xS_1-x)by applying electrochemical cyclic-potential on the Cu₂S/Cu electrode.In the electrochemical activation process,the scale of Cu₂S nanorods with the diameter of 500 nm was transformed into loose Cu₂O_xS_1-x nanoporous with the diameter of 30 nm.This nanoporous Cu₂O_xS_1-x structure exhibits exceptional hydrogen evolution reaction（HER）properties,with a low overpotential（40 m V）at the current density of 10 m A cm-2,small Tafel slope（68 m V dec-1）in 0.5 M H₂SO₄ electrolyte.The activity and durability of Cu₂O_xS_1-x far exceeds pure Cu₂S and pure Cu2 O.The mechanism was verified by using the density functional theory（DFT）calculations with the constructed structure models.These results indicate the successful preparation of a self-supported ternary nanoporous Cu₂O_xS_1-x catalyst that approaches the performance of the benchmark Pt/C catalyst.Moreover,the Cu₂O_xS_1-x shows excellent electrocatalytic activity for OER,HER,and overall water splitting as a bifunctional catalyst in 1.0 M KOH electrolyte.This work provides a novel strategy of introducing dopants into the crystal lattice for developing the nanoporous transition metal oxides（TMO）-based overall water splitting catalysts.3.We propose a novel strategy of utilizing a Fe OOH substrate to induce the epitaxial growth of layered transition cobalt oxyhydroxides,during which Fe was in situ incorporated to form Fe_0.35Co_0.65 OOH.The as-prepared oxyhydroxides exhibit enhanced electrocatalytic performance in the OER with an overpotential of 240 m V at 10 m A cm-2 in 1.0 M KOH.The activity of Fe_0.35Co_0.65 OOH far exceeds that of α-Co（OH）2 nanosheets,which were obtained on pure carbon cloth without the Fe OOH interface.However,MOOH is normally obtained by chemical oxidation or electrochemical oxidation of M（OH）2 that was first prepared by base precipitation methods.Moreover,Fe_0.35Co_0.65 OOH shows excellent enhanced supercapacitor performance.The specific capacitance of the Fe_0.35Co_0.65 OOH nanosheets in a 1.0 M KOH electrolyte reaches 1144 F g-1 at 2 A g-1,which is a great improvement compared with the value of 807 F g-1 for α-Co（OH）2.Our work shows obvious advantages over the complicated traditional methods of using the Co（OH）2 precursor for oxidization.This study provides a new strategy for designing high-performance layered transition metal oxyhydroxide electrocatalysts for efficient energy conversion and storage.