| The modern society is suffering from two dominating challenges of energy crisis and environmental pollution,owing to the over-exploitation and use of carbon-based fossil fuels,and the development of clean and sustainable alternative energy sources is crucial.With higher energy density than all fossil energy sources and zero carbon emissions,hydrogen occupies an unparalleled position in the energy transition.As a hub between electrical energy and hydrogen energy,hydrogen production from electrolytic water including hydrogen precipitation reaction(HER)and oxygen precipitation reaction(OER),is one of the key technologies to cleanly prepare highpurity hydrogen nowadays,which can combine the surplus renewable electricity generated from solar and wind energy with low-cost water resources,thus forming a closed loop of renewable energy.Alkaline water electrolysis is the most mature commercial hydrogen production scheme,but the high cost of hydrogen production limits its large-scale application,so it is highly urgent to develop efficient and cheap electrocatalysts.In addition to the selection of metallic elements with high earth abundance as research materials,developing a variety of strategies to modulate the number and intrinsic activity of catalytic reaction centers,and electrical conductivity is indispensable to reduce reaction energy consumption,improve electrolysis efficiency,and reduce production costs.The purpose of this paper is to design alkaline water electrolysis catalysts with high intrinsic activity and stability,and take into account the simplification of the synthesis process,combined with a variety of material characterization and analysis techniques,in-depth analysis of the intrinsic link between electronic structure and performance of catalysts,to provide data and theoretical support for the development of efficient water electrolysis catalysts,and contribute new development ideas for future industrial applications.Therefore,the main achievements of this paper are as follows:1.To address the high price and lack of HER performance of commercial Pt-based catalysts in alkaline medium,we synthesized sulfur-doped ruthenium(Ru-S)ultrasmall nanoparticles by a facile and scalable one-step solvothermal method,which can be used as efficient and inexpensive HER electrocatalysts in alkaline medium.The preparation method not only saves energy,but also avoids agglomeration of nanoparticles at high temperatures without calcination step.Achieving a current density of 10 mA cm-2 in 1 M KOH electrolyte,the Ru-S catalyst exhibited an overpotential of only 10 mV and a Tafel slope as low as 53 mV dec-1,which are superior to those of the studied Pt-based reference materials.In addition,Ru-S catalysts exhibit greater exchange current density and longer stability than standard materials.The reasons are as follows:Sulfur doping can significantly reduce the size of ruthenium particles and increase the number of effective active sites by increasing the specific surface area of the material.Furthermore,the Ru-S catalyst can form Ru-Sδ--K+-(H2O)x network structure to strengthen the adsorption of H2O molecules and induce the dissociation process more easily.Density functional theory(DFT)calculation proves that S can optimize the electronic structure of metal Ru,resulting in a moderate adsorption and dissociation energy of H2O molecules on the surface of Ru-S catalysts,as well as an enhanced hydrogen atom capture and recombination ability.The effect of sulfur doping on Ru catalyst can be further extended to other chalcogens,such as Se and Te.This study provides a simple strategy for the synthesis of sulfur-modified ruthenium catalysts and even other sulfur-containing materials with promising applications for industrial production of green hydrogen energy in the future.2.In response to the problems of expensive precious metals and transition metal catalysts with OER activity far from meeting the needs of large-scale industrial promotion in alkaline water electrolyzers,we have developed a novel S-doped ironbased metal organic frameworks(Sx-Fe-BTB)with a posy-bouquet shape via a facile and easily scalable one-pot solvothermal method for in situ growth on the surface of nickel foam(NF).The three-dimensional integrated electrode with a composite hierarchical structure can be used as an efficient,cost-efficient and durable OER electrocatalyst in alkaline media.Achieving a current density of cm-2 in 1 M KOH electrolyte,the optimized S0.05-Fe-BTB/NF electrode possessed an OER overpotential of only 231 mV and a Tafel slope as low as 41 mV dec-1.Meanwhile,the S0.05-FeBTB/NF electrode can work continuously for more than 50 h,showing good stability.The main reasons for the excellent electrocatalytic activity of the S0.05-Fe-BTB/NF electrode are as follows:(1)the integrated electrode structure facilitates the reduction of interfacial resistance,exposing more catalytic active sites and improving the stability.(2)The porous morphology of metal organic frameworks(MOFs)materials can not only make full use of the active sites,but also promote the generation and escape of oxygen bubbles,and the introduction of S in Fe-BTB can optimize the surface composition and electronic structure of the catalyst,increase the quality and number of active sites,and accelerate the charge transfer and mass transfer processes.(3)FeOOH evolved from the original MOFs structure,inherited some previous structural advantages,and eventually became a species with higher OER activity in the subsequent electrolysis process.Therefore,this work not only opens up a path for the preparation of novel MOFs electrodes with abundant active sites and excellent OER catalytic performance by doping non-metallic elements,but also provides a direction to explore the conversion process of MOFs and investigate their reactive species.3.The common transition metal catalysts have many disadvantages,such as low activity,poor durability and single functionality,thus degrading the catalytic efficiency and increasing the cost of hydrogen production,which is far from meeting the needs of commercial alkaline electrolyzers.To address this problem,we propose a facile and effective strategy to successfully grow amorphous Ni(OH)2/NiFe2O4/Ni3S2 heterostructures on the surface of 3D porous nickel foam(simplified as Ni0.8-Fe0.2S0.25/NF)by one-step controllable solvothermal treatment,which achieving the simultaneous regulation of the active site and electronic structure.The obtained Ni0.8Fe0.2-S0.25/NF self-supported electrode can be used as a bifunctional electrocatalyst with high activity and stability.When the current density is 10 mA cm-2,the overpotentials of OER and HER of Ni0.8-Fe0.2-S0.25/NF are 247 mV and 128 mV in 1 M KOH solution,respectively.While the Ni0.8-Fe0.2-S0.25/NF electrode is assembled as cathode and anode directly in the alkaline electrolytic cell,the current density of 10 mA cm-2 can be achieved with only 1.59 V voltage and long-term stability.We attribute the excellent activity of Ni0,8-Fe0.2-S0.25/NF electrode to the synergistic effect of multiple interfaces of Am-Ni(OH)2/NiFe2O4/Ni3S2 multi-interface,optimizing the reaction barrier of absorption/desorption or dissociation of electrolytic water reactants and reaction intermediates on the surface of electrocatalysts,thereby exhibiting faster HER and OER kinetic processes than a single component.This facile approach can be extended to the construction of heterogeneous interfaces of other transition metal catalytic materials and design novel high-performance bifunctional electrocatalysts for electrolytic water,which has a great potential to replace commercial precious metal materials. |
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