Fabrication Of Highly Efficient Transition Metal Ni/Co-Based Catalysts For Electrocatalytic Performance

Energy is the basis for the development of the world economy and the survival of mankind.However,the two major problems facing mankind at this stage are the depletion of fossil fuel-based energy and the serious environmental pollution caused by its use.The development of renewable and clean energy is imminent and crunch time.Anyway,the core of future international competition is still energy,and the countries that master clean and renewable energy technologies will occupy a leading position in international competition and become the center of economic development.Energy issues are not only scientific issues,economic issues,but also strategic issues.In recent years,the research and development of clean and renewable energy has gradually expanded and become a system.Among the many clean and renewable energy sources,hydrogen energy is considered to be the best alternative to fossil fuels due to its large combustion calorific value,non-polluting,and wide-ranging sources.The technology of water electrolysis is currently the most potential green and pollution-free way to produce clean and renewable energy hydrogen energy.The electrocatalytic water splitting process is divided into two half-reactions:the cathode hydrogen evolution reaction(HER)and the anode oxygen evolution reaction(OER).These two half-reactions are very important in the process of hydrogen production by water electrolysis,which determine the output.Under ideal conditions,the theoretical external voltage required for hydrogen production by water electrolysis is 1.23 V.However,under actual conditions,the potential barrier required in the process of hydrogen production by water electrolysis is high,the energy consumption is large,and the voltage is much greater than the theoretical value,so development high-efficiency catalysts is required to reduce the energy barrier in catalytic reactions,to improve energy conversion efficiency and to reduce the costs for hydrogen production.At present,the most effective HER and OER catalysts are precious metal Pt-based catalysts and Ir and Ru-based catalysts.However,due to the scarcity and high cost of precious metals,the large-scale use of precious metal catalysts is restricted,which in turn restricts development of the electrocatalytic water splitting hydrogen production technology.Therefore,the development of efficient,stable,and inexpensive transition metal-based catalysts which can replace precious metal catalysts,reduce overpotential,reduce energy consumption,and improve energy conversion efficiency has become the most critical step for large-scale industrial production and application of hydrogen production by water electrolysis.The ideal catalyst should have the following four obvious characteristics:(1)High catalytic activity,low over-potential,with catalytic activity comparable to or even higher than that of noble metal catalysts;(2)Long-term operating stability.The longer the time,the better,and it is best to keep the catalytic activity for decades without deactivation and still maintain the initial state of catalytic activity;(3)Catalytic materials are cheap,easily available,and environmentally friendly,which can meet the needs of large-scale promotion;(4)The catalyst synthesis process is simple,low-consumption and easy to expand,and can be produced in a large-scale.At present,most catalyst materials cannot meet the above requirements at the same time.To obtain an ideal catalyst,researchers need to spend time and energy to explore different components,different structures,different morphologies,and different catalytic properties in the vast ocean of materials.Summing up experience,discovering rules,and finally designing and developing an ideal catalyst,lay a solid foundation for the popularization of hydrogen production by water electrolysis.The design of the catalyst in this paper mainly focuses on:(1)Nanostructured array of catalyst materials,development of self-supporting nanostructured array catalyst electrodes,to maximize the exposure of catalytic active sites,increase the utilization rate of catalyst active sites,and then improve the catalytic activity and stability of the catalyst to the hilt;(2)Use doping,heterogeneous introduction and other means to optimize the electronic structure of the catalyst material,thereby improving the intrinsic activity of the catalyst material.Through the combination of different optimizing strategies,a series of non-noble metal Ni/Co-based electrocatalysts with high catalytic activity and good stability were finally designed and synthesized,and their catalytic activity performance was further analyzed through comparative studies,and the electronic structure and catalytic performance of the catalyst materials were further analyzed.The internal structure-activity relationship is summarized.The specific research contents of this paper are as follows:(1)Transition metal phosphide has a unique valence electron configuration and a diverse crystal structure,which is considered as a potential excellent HER catalyst material.It has been reported that transition metal phosphide CoP is used as a dual-functional catalyst for the overall water splitting,but its catalytic activity is not good.Therefore,the aim of this paper is to control the morphology of the CoP catalyst,incorporate heteroatom N into CoP,and prepare self-supporting electrode,thereby increasing the number of exposed catalytic active sites and modulating the electronic structure of the metal phosphide,optimizing the dual-functional catalytic activity of CoP,and finally obtaining the catalysts with high catalytic activity and good stability.Based on the above ideas,we used the self-sacrificing MOF(Metal-Organic Framework)template method to synthesize a regular octahedral CoP nanostructured catalyst,and cleverly introduced a more electronegative N atom doping to prepare a highly efficient 3D self-supporting N-CoO@CoP arrays catalyst.In an alkaline electrolyte,at a current density of100 m A cm^-2,the low overpotentials of OER and HER are 332 m V and 201 m V.When the N-CoO@CoP catalyst is used for the two electrodes at the same time,the voltage of the cathode and anode of the device is 1.79 V at a current density of 100 m A cm^-2.The high catalytic activity of N-CoO@CoP catalyst is achieved by promoting the mass transfer and charge transfer process in the reaction process,optimizing the electronic structure of the catalyst and exposing more catalytic active sites,which improves the catalytic activity and stability of the catalyst.This work verifies the feasibility of using the self-sacrificial MOF template to adjust the nano-morphology of the catalyst material,and proves that N-CoO@CoP is a highly efficient difunctional electrocatalysts that can be used to improve the energy utilization efficiency of the overall water splitting.It provides new ideas for the synthesis of related catalytic materials.(2)We creatively improved the structure of the conductive substrate.By chemical etching the three-dimensional porous substrate of copper foam,a uniform CuO nano-array was generated on the surface,and a CuO/Cu hybrid foam was prepared.The advantages of hybrid foam are:increasing the geometric surface area of??the substrate,providing more attachment points for the catalyst material,collecting and transferring electrons in a larger area,and realizing the most efficient electron transportation.Based on the optimal control of the composition,morphology and structure of the catalyst material,we used a simple and environmentally friendly electrodeposition method to synthesize Ni-Co-S ultra-thin porous nanosheets on CuO nanowires,and prepared 3D hierarchical self-supporting Ni-Co-S@CuO/Cu nanosheet array catalyst.The alkaline catalytic oxygen production test showed the catalytic activity of Ni-Co-S@CuO/Cu material,and the overpotential was 240 and 309 m V at the current density of 10 and 100 mA cm^-2.Its catalytic activity is not only related to its own 3D hierarchical self-supporting structure,but also related to the edge activity effect of metal sulfides.The ultra-thin and porous Ni-Co-S nanosheets can expose more active sites and further improve Catalyst catalytic activity.The above results prove that Ni-Co-S@CuO/Cu is an efficient basic OER catalyst material,and its simple and scalable synthesis method meets the needs of large-scale industrial applications.(3)During the formation of transition metal nitrides,the insertion of N atoms will change the lattice and the distance between metal atoms,so that the metal d-band distance will also change,and the change of d-band leads to the reconstruction of state density at the Fermi level,researchers believe that the redistribution of the density of states makes metal nitrides have catalytic activity similar to noble metals,which can improve their catalytic performance.Based on this,we introduced heterogeneous interfaces through heterogeneous introduction,and used the oxygen-rich promotor Ce O₂ to couple with transition metal nitride Co₄N,and successfully synthesized self-supporting Co₄N-Ce O₂/NF porous nanowires catalysts on a conductive substrate.By anchoring Ce O₂in Co₄N,the prepared catalyst has superhydrophilic and superaerophobic properties,which promotes the adsorption/dissociation of water,thereby significantly improving the HER performance of the catalyst.In 1.0 M KOH electrolyte,when the current density is 10and 100 m A cm^-2,they exhibit ultralow overpotentials of 52 and 149 m V,respectively.The catalytic activity is equivalent to that of Pt-based noble metals,and it has better stability.The above results confirm that Co₄N-Ce O₂/NF is a high-performance HER catalyst.Its performance is far superior to most other HER catalysts reported.It is comparable to precious metal Pt-based catalysts and provides an effective strategy to improve the catalytic activityof other related catalysts.(4)Metal sulfides generally have good electrical conductivity and structural stability,and are a type of catalyst material that has developed relatively quickly in recent years.Some conductive metal sulfides have been reported to have excellent catalytic oxygen production activity,and their development potential in the field of catalysis should not be underestimated.Based on the research on the promotor Ce O₂rich in oxygen vacancies,we added cerium salt during the synthesis of metal sulfides.By adjusting the concentration of the reactants,a one-step hydrothermal method was used to synthesize self-supporting Ce-Ni₃S₂/NiS@NF nanobud array catalyst electrode on the conductive substrate nickel foam.Through structural characterization,we found that the existence of Ce species directly determines the crystal structure phase of the catalyst.Ce hinders the synthesis of pure Ni₃S₂,and then generates multiphase Ni₃S₂/NiS catalysts with rich interfaces.Furthermore,the coupling of Ce species can adjust the electronic structure of the catalyst,thereby reducing the energy barrier of the catalytic reaction,and helping to enhance the OER activity of the Ce-Ni₃S₂/NiS@NF electrocatalyst.The superhydrophilic and superaerophobic properties of Ce-Ni₃S₂/NiS@NF electrode further promote the mass transfer process and increase the utilization of catalytic active sites.Therefore,the Ce-Ni₃S₂/NiS@NF electrode has excellent alkaline OER activity,and can achieve low overpotentials of 236 and 350m V at current densities of 10 and 100 m A cm^-2,and its Faraday efficiency can reach 99%.These results confirm the rationality of the Ce-Ni₃S₂/NiS@NF catalyst electrode design,which is superior to many other related OER catalysts reported,and provides a new idea for the simple preparation and activity improvement of related catalysts.