Study Of Anion-regulated Transition Metal-based Electrocatalysts For Water Splitting

The energy crisis has forced people to shift their attention to the development and utilization of new clean energy.Hydrogen energy has a high energy density,and its combustion product is water,which is clean and pollution-free,so it is the best alternative to fossil fuels.Combined with various industrial hydrogen production methods,water splitting is undoubtedly the most efficient and feasible scheme.However,the four electron transfer step was involved in the process of water splitting,the reaction kinetics is slow.Therefore,it is the key to develop low-cost,efficient and stable electrocatalysts for industrial hydrogen production by water splitting.In this thesis,we designed and synthesized a series of transition metal-based compounds which were controlled by anions as catalysts for water splitting.We further optimized the electrocatalytic performance of transition metal-based compounds by means of heteroatom doping,morphology control and multi-metal coordination,so that the electrocatalytic performance was comparable to that of noble metal-based electrocatalysts.The main research contents are as follows:（1）The Fe-Co-S/NF bifunctional electrocatalyst with nanoflower-like structure was synthesized on the three-dimensional porous nickel foam through one-step hydrothermal and one-step high-temperature sulfuration operations and the material display high-efficiency electrocatalytic performance.As a catalyst for the hydrogen evolution reaction,Fe-Co-S/NF can drive a current density of 10 m A cm^-2 at overpotential of 143 m V with a Tafel slope of80.2 m V dec^-1.As a catalyst for oxygen evolution reaction,it exhibits good OER performance,low Tafel slope(82.6 m V dec^-1)and only an overpotential of 117 m V is required to drive a current density of 50 m A cm^-2.In addition,the assembled Fe-Co-S/NF//Fe-Co-S/NF electrolyzer can drive a current density of 50 m A cm^-2 with only a cell voltage of 1.64 V,which is one of the most advanced electrolyzers available.This work indicates that the introduction of S,P and Se could effectively improve electrical conductivity of the material and enhance the catalytic activity of the material.（2）In this work,the Co-Ni-M@Ce O₂/NF（M=O,S,P and Se）electrodes was firstly in situ grown on three-dimensional（3D）conductive nickel Foam（NF）support through selective sulfuration,phosphorization and selenylation of the Co-Ni-O@Ce O₂ under a N₂ atmosphere.The Co-Ni-S@Ce O₂/NF display superior oxygen evolution reaction performance with requiring overpotential of 170 m V@20 m A cm^-2 and Co-Ni-P@Ce O₂/NF display excellent hydrogen evolution reaction activity with requiring an overpotential 120 m V@10 m A cm^-2 in an alkaline medium.What’s more,an electrode pairing of Co-Ni-S@Ce O₂/NF//Co-Ni-P@Ce O₂/NF was assembled for overall water splitting using the Co-Ni-S@Ce O₂/NF material as anodic catalyst together with the Co-Ni-P@Ce O₂/NF material as efficient cathodic catalyst.The assembled alkaline electrolyzer required a relatively small cell voltage of 1.60 V to obtain a reference current density of 10 m A cm^-2,which is one of the best electrocatalytic activities reported so far.The Co-Ni-S@Ce O₂/NF//Co-Ni-P@Ce O₂/NF also displayed relatively satisfactory durability and the current density had no significant attenuation during a 10 h electrocatalytic measurement in 1.0 M KOH.This selective sulfuration,phosphorization and selenylation strategy is effective in developing the M（M=O,S,P and Se）hybrid/oxide interface for overall water splitting applications.（3）In this work,a self-grown nanosheet array electrocatalyst on nickel foam with a high structural stability is firstly rationally designed through suitable anionic doping.The combined experimental and theoretical calculations reveal that F-P-Co₃O₄/NF material optimizes the adsorption energy of hydrogen/water through electron coupling,and its nanosheet structure provides abundant active sites,accelerating the mass and electron transfer in the reaction process.It’s worth noting that the as-developed F-P-Co₃O₄/NF materials exhibit outstanding catalytic activity for overpotentials of 192 and 110 m V at a reference current density of 10 m A cm^-2 for oxygen evolution reaction and hydrogen evolution reaction in 1 M KOH,respectively.More notably,an assembled F-P-Co₃O₄/NF//F-P-Co₃O₄/NF alkaline electrolytic cell requires only an ultra-low cell voltage of 1.53 V to achieve a reference current density of 10 m A cm^-2,which is one of the best activities reported so far.Theoretical calculations show that the superior activity of the F-P-Co₃O₄/NF catalyst is attributed to the optimal electron configuration and the lower Gibbs free energy of hydrogen adsorption due to co-doping of P and F.