Preparation And Electrolytic Overall Water Splitting Properties Of Self-supporting Transition-Metal-Based Nanocomposites

Hydrogen energy,as a green renewable energy source,is extremely likely to become an alternative to traditional fossil energy sources,which has attracted much attention for energy conversion.In recent years,with the rapid development of green power industries such as solar photovoltaic and wind power generation,this has brought new opportunities and challenges to the electrochemical water splitting to produce green hydrogen industry.Electrocatalytic overall water splitting（OWS）is composed of two half-reactions,hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode.The ultimate decomposition voltage of electrolytic water splitting is 1.23 V,which requires to improve its slow kinetics and high reaction energy barrier by the use of highly active HER and OER electrocatalysts.Currently,a lot of research work has been done in developing high-activity and low-cost transition metal-based self-supporting electrocatalysts,but the activity and durability of the catalysts are still unsatisfactory.Therefore,a series of highly active,long-term stable,and low-cost self-supported nano electrocatalysts have been designed and controllably prepared from the perspectives of improving the binding ability to self-supported substrates,regulating the microscopic morphology,interfacial effects,atomic doping,and optimizing gas-liquid transport.The specific research contents are as follows:（1）A self-supporting material with only metal substrates（nickel foam,cobalt foam）providing metal sources without additional metal ions was developed to grow Ni-MOFs/Co-MOFs in situ and further phosphorylated and carbonized to form phosphides（Ni₂P,CoP）and NPC.This self-supporting material grown on the substrate material exhibits excellent catalytic activity,low impedance,and good reaction kinetics.The process strategy is simple and efficient,which does not require the supply of additional metal ions and the organic ligands can be recycled again and again,making it suitable for large-scale industrial production.This provides a new idea for the industrial production of self-supported transition metal-based electrolytic water catalysts and the realization of green electricity to produce green hydrogen.（2）The self-supporting materials of Ni₂P@C/NF with Ni-BTC MOF-derived 3D nanosphere/nanoflower array structures grown in situ on nickel foam substrates were developed.The constructed nanosphere interfaces protrude outward to optimize contact with the electrolyte,while the nanoflowers showing rough nanowires on the surface expose more active sites to promote fast electron transfer as well as abundant void channels to accelerate gas diffusion.Meanwhile,graphitic carbon stabilizes Ni₂P and enhances conductivity.DFT simulations confirm that the synergistic effect of Ni₂P and C facilitates the tuning of electronic structure,accelerates electron transfer ability and promotes catalytic kinetics.This work provides an in-depth study of the nuclei growth to the maturation process,deepens the application of Ostwald Ripening phenomenon in electrolytic water,and also elucidates the mechanism of significant bifunctional activity of synergistic effect between active components,which provides new ideas for rational design based on multiphase interface engineering.（3）The electrode material of Ni₃S₂/NiCo₂S₄ with heterogeneous heterojunction structures was synthesized on nickel foam substrates using a simple strategy of anion and cation exchange.The catalysts exhibit an upward-growing nanorod structure,presenting more active surface sites,and can effectively modulate the local electronic structure by coupling the multidimensional interface with defects in the sulfide.This heterojunction material provides a multi-phase heterojunction interface and exhibits excellent electrocatalytic activity and long-term durability with a current density of 10m A cm^-2 at a voltage of 1.54 V.（4）Two-dimensional lamellar Co-ZIF/NF grown on nickel foam substrates were carbonized and transformed into Co-N-CNT/NF electrode materials with one-dimensional carbon nanotube structures.The composition of cobalt nanoparticles confined in the N-CNT exhibits a synergistic adjustment of the electronic structure;In contrast,the two-dimensional Co-ZIF/NF phosphorylated transformed into nanowires of Ni₂P-CoP-N-CNT,which may be due to the phosphorylation-induced production of phosphine gas that limits the lateral growth of the nanotubes.This exhibits the exposure of more active sites,where the active species of Ni₂P and CoP act synergistically to contribute to the rotational speed transfer of electrons,together contributing to the catalyst performance.This provides a new design idea for the design of one-dimensional nanotube electrode materials.（5）A self-supported hydrogel bimetallic phosphide Ni₂P-CoP film with high activity and long-term stability was developed using the oriented ice template method.This self-supporting carbon film possesses vertically aligned macropores and a pore structure within the carbon wall network,which facilitates smooth ion transfer and gas transport.Meanwhile,Ni₂P and CoP act synergistically as active substances anchored to the carbon film to facilitate the rotational speed transfer of electrons,together contributing to the catalyst performance.A current density of 10 m A cm^-2 can be achieved when electrolyzing water splitting at a voltage of only 1.57 V.This catalyst formed on a hydrogel substrate opens up new directions for the design of new self-supporting electrode materials.（6）An integrated photothermal actuator-sensor system was developed by hydrogel as a flexible self-supporting substrate and graphene as a photothermal response medium.The system possesses a photothermal response actuation capability and achieves sensing and monitoring through surface temperature bending angle-electron current（T-A-I）transient changes.The system solves the problem of difficulty in capturing and monitoring the motion state of actuators in an invisible environment.At the same time,the flexible self-supporting system can adapt to various environments,such as air,water,ice-water mixtures and seawater.What’s more,three demonstrations were innovatively designed-imitating a soccer player and a goalkeeper kicking a penalty kick,and an adhesion-photo thermically driven de-adhesive gripper.A circuit was designed to monitor the motion of the hydrogel actuator during the three demonstrations,which has the advantage of sensing monitoring.This provides theoretical support for the extended application of hydrogel self-supporting actuators.