Electronic Structure Modulation Of Cobalt-Based Electrocatalysts For Applications In Energy Conversion Reactions

Energy depletion and environmental pollution have become urgent global problems,and the development of eco-friendly green energy is crucial to the sustainable development of human society.Compared with traditional fossil fuels,hydrogen energy has a series of advantages such as zero emission,high efficiency and renewable,and plays an indispensable role in achieving carbon peaking and helping carbon neutrality.Hydrogen production by electrocatalytic decomposition of water is considered as an effective way for green hydrogen production,but the slow kinetic process and high thermodynamic energy barrier of anodic oxygen precipitation reaction（OER）seriously limit its industrial application.In contrast,the electro-oxidation reaction using low-value organic small molecules instead of OER,such as the oxidation of small molecules like urea and hydrazine in wastewater,can effectively reduce the potential and energy consumption of hydrogen production from electrolytic water,and also allow wastewater treatment.In addition,electro-oxidation of small molecules such as propanetriol and ethanol is also expected to yield high value-added chemicals.The key to realize energy-saving hydrogen production coupled with pollutant degradation and high-value-added chemical preparation lies in the development of efficient and inexpensive non-precious metal-based catalysts.With high electrical conductivity,easily adjustable electronic structure and excellent corrosion resistance,transition metal cobalt-based materials have broad application prospects in the field of electrocatalysis.By adjusting the electronic structure and surface interface properties of cobalt-based metal compounds,the catalytic performance can be further optimized and their application scope can be expanded.Based on this,this thesis modulates the electrocatalytic performance of cobalt-based catalysts by doping or constructing heterojunctions to construct bifunctional electrocatalytic systems.In this work,the cobalt-based compounds（CoS₂,Co₃O₄）were used as the main research objects,and the nanostructure design and microstructure modulation were used to optimize the surface/interface properties of the cobalt-based compounds by adjusting their electronic and geometric structures to achieve efficient electrocatalytic organic small molecule oxidation coupled with hydrogen and ammonia production performance.The main research of this thesis is as follows:1.Sn doping enhances the electrocatalytic activity of CoS₂ in urea oxidation and hydrogen precipitation reaction under alkaline conditions.The CoS₂ electrodes with different Sn doping amounts（Sn-CoS₂）were prepared by the strategy of hydrothermal and then gas phase sulfidation.Through the modulation of the electronic structure of CoS₂ by Sn atoms,the intrinsic catalytic activity of the Co sites was enhanced,which facilitated the adsorption of intermediates,and Sn-CoS₂ exhibited excellent electrocatalytic activity and stability.In the electrolyte of 1 M KOH+0.5 M urea,the Sn（2）-CoS₂ electrode required only a potential of 1.301 V to reach a current density of 10 mA·cm^-2 and operated stably for at least 108 h.The electrolytes assembled with Sn（2）-CoS₂/CC and Sn（5）-CoS₂/CC reached a current density of 10 mA·cm^-2 current density requires only a low voltage of 1.45 V and can operate stably for at least 95 h.More importantly,this electrolyzer can continuously generate bubbles driven by a commercial dry cell（1.5 V）,demonstrating the great potential of the fabricated electrodes in constructing a bifunctional system for anode urea degradation and cathode hydrogen production.This study provides a promising strategy for achieving energy conversion and pollutant treatment by designing efficient and low-cost electrocatalysts through functional doping.2.Binary synergy of cobalt-copper nanoarrays enhances the bifunctional electrocatalytic activity of anodic GOR and cathodic NO3RR.The cobalt-copper-based heterojunction materials were prepared as precursors by hydrothermal method,and the electronic structure was regulated by changing the cobalt/copper ratio,and the materials were subjected to different post-treatments:at the anodic end,the cobalt-copper precursors were annealed in air to prepare Co₃O₄/CuO heterojunctions(denoted as Co_1-xCu_x-A);at the cathodic end,the cobalt-copper precursors were electrochemically reduced in situ to construct β-Co（OH）2/Cu heterojunction（denoted as Co1-xCux-CH）.The efficient electrocatalytic glycerol oxidation coupled with cathodic nitrate electrolysis for ammonia production performance was achieved using cobalt-copper binary synergy.The synthesized Co_0.3Cu_0.7-A electrode material required only a very low potential of 1.195 V to reach 10 mA·cm^-2 in an electrolyte of 1 M KOH+0.1 M Glycerol,and there was no significant degradation in performance after 513 h of constant current testing.In addition,the ammonia production by nitrate reduction test demonstrated that the Faraday efficiency（FE）of Co_0.3Cu_0.7-CH at-0.6 V vs.RHE reached 85.16%with a yield of 0.25 mmol·h^-1·cm^-2.The results indicated that the cobalt-copper heterojunction in the electrocatalytic The binary synergy promoted the oxidative decomposition of propanetriol and the deoxygenation and hydrogenation of nitrate,respectively.The cobalt-copper heterojunction catalyst exhibited excellent electrocatalytic performance,and the bifunctional system thus constructed could realize the simultaneous production of valuable products formate and ammonia.This work provides research ideas for the construction of highly efficient bifunctional systems for anodic organic oxidation and cathodic nitrate reduction for ammonia production.