Construction And Catalytic Performance Of Iron Group Elements For Oxygen Evolution Reaction

Global warming and the depletion of fossil energy sources are important issues that need to be addressed in order to achieve sustainable development.At present,the infrastructure of energy facilities is dominated by fossil fuels.The use of these non-renewable energy not only faces a crisis of energy shortage,but also directly leads to environmental pollution.This is why there is a need to develop renewable energy sources such as solar,wind and geothermal energy.However,these sources of renewable energy are intermittent and unevenly distributed,mostly in isolated areas that cannot be connected to the distribution network,which makes energy storage essential.Energy storage is the weakest link in the energy sector,but it is a key element in the growth of renewable energy.Hydrogen production from electrolysis of water is a promising method of energy storage that converts electricity generated from renewable energy into hydrogen for storage.Hydrogen is a clean energy that does not cause environmental pollution and is an ideal energy carrier due to its high calorific value of combustion and its flexible and convenient storage and transportation methods.Water splitting consists of two half-reactions:the cathodic hydrogen evolution reaction（HER）and the anodic oxygen evolution reaction（OER）,of which the OER is a four-electron transfers reaction with a high reaction energy barrier and requires the development of a cheap and efficient catalyst to reduce the activation energy of the reaction.Ru O₂ and Ir O₂ are widely recognized as the best OER catalysts,but their applications are limited by reserves and price.As a result,the target was turned to inexpensive transition metal-based materials.In this thesis,the relevant research results on OER are first summarized,then based on the iron group elements（FeCo Ni）catalysts,the problems of the currently available materials are modified,the changes of the materials before and after OER are studied,and the final active substances of the alkaline OER catalysts are verified,the main research contents are described as follows:（1）Construction of ultrathin two-dimensional Co2Ni-MOF@Ti₃C₂T_xnanosheets for efficient oxygen evolution.Metal organic frameworks（MOFs）and their derivatives are promising catalysts with ultra-low mass density and large specific surface area,but the poor electrical conductivity and high bulk-phase mass transfer resistance of three-dimensional（3D）MOFs make them less active as catalysts directly.Based on the above problems,we prepared an ultrathin two-dimensional（2D）Co2Ni-MOF from the bottom up by a one-step ultrasonic method and then compounded it with highly conductive 2D Ti₃C₂T_x nanosheets by electrostatic adsorption.This method not only shortens the transport path of MOFs and improves the bulk-phase mass transfer rate of MOFs,but also avoids the stacking of 2D MOF and compensates for the poor electrical conductivity of MOFs,resulting in a better OER activity.In addition,Raman and XPS tests were carried out on the catalysts before and after the OER tests,and the results showed that the catalysts were reconfigured during the OER process,changing from Co2Ni-MOF to Co OOH and Ni OOH.This simple and feasible synthesis method provides a viable idea for optimising the OER performance of MOFs,which is expected to be extended to other applications.（2）Preparation of Ni³⁺-rich Ni₃S₄/FeS nanoflower-like heterojunctions for efficient oxygen evolution.It is well known that Ni³⁺is an efficient catalytic active site for OER,but current synthetic methods are difficult to obtain catalysts with high Ni³⁺ratios.To this end,we optimized the deposition conditions using anodic electrodeposition,guided by the potential-p H diagram,to obtain Ni₃S₄ with a high Ni³⁺ratio,and the material exhibited good OER performance.Moreover,the in situ grown catalysts do not require the use of binder,expose the active sites to a greater extent,and can operate stably at high current densities for a long time.The effect of Fedoping on Ni₃S₄ was then explored in depth.XPS tests showed that the introduction of Fenot only further increased the Ni³⁺ratio but also modulated the electron cloud density of Ni 2p,which together resulted in an outstanding OER catalyst for the composite.Furthermore,both in situ Raman and XPS of the Ni₃S₄/FeS after OER testing demonstrate the transition of the material from sulphide to hydroxyl oxide.This simple and efficient anodic electrodeposition method is universal and instructive for the synthesis of materials rich in high-valent metal ions and is expected to be widely used in other applications.