Study On Preparation And Application Of Transition-Metal-Based Composite Catalyst In Electrocatalysis

Energy is an important material basis for the survival and development of human society.The depletion of traditional fossil fuel and the serious environmental pollution have forced global efforts to explore environmentally friendly,efficient,and renewable energy.Hydrogen energy,with the high gravimetric energy density,combustion heat value and zero carbon content,is considered as an energy carrier with great application prospects.Recently,in view of various methods of producing hydrogen,electrochemical water splitting has been widely recognized as the most practical and economical method.The overall water-splitting（OWS）process involves two sluggish half-cell reactions,i.e.,the cathodic hydrogen evolution reaction（HER）and the anodic oxygen evolution reaction（OER）.Among them,the OER process involves a complex four-electron transfer process,with slower kinetic reaction and energy conversion efficiency,which greatly hinders the efficiency of hydrogen production.Therefore,the synthesis of high efficiency,low cost and high stability of the catalyst is crucial for electrolysis of water to produce hydrogen on a large scale.As is well known,the precious metal-based catalysts have excellent electrocatalytic properties.However,their large-scale application is restricted by the extreme scarcity and high cost.Due to the special electronic structure（d-orbitals partially filled）and the ability to"receive/give"electrons of transition metals,transition-metal-based catalysts have been widely used in electrocatalytic water splitting for hydrogen production.At the same time,substituting the sluggish water oxidation with small molecule electrooxidation,such as methanol,urea and hydrazine,can effectively reduce the overall potential in the process of water electrolysis and realize efficient hydrogen production.In this paper,a series of economical and efficient transition-metal-based catalysts were prepared on metal foam substrates（Co foam,Ni foam）via hydrothermal synthesis method and low temperature phosphating method.The details are as follows:1.Ni Fe Prussian blue analogue（PBA）was fabricated on Co foam in situ via a facile hydrothermal synthesis.After chemical etching and electrochemical activation,Ni Fe PBA was entirely transformed into amorphous superhydrophilic structure,which exhibited a remarkable OER performance at large current density.To drive the high current density of400 and 800 m A·cm^-2,only ultralow overpetentials of 280 and 309 m V were required,respectively,far exceeding many recently reported OER catalysts.The superior performance can be attributed to:（1）in-situ growth on metal foam substrate can improve structural stability and provide faster charge transfer as well as oxygen bubble release;（2）chemical etching allows to expose more surface active sites;（3）electrochemical activation-induced amorphous surface possesses larger BET surface area,more high-valent oxidation states and higher intrinsic OER activity;（4）superhydrophilic surface structure is conducive to the adsorption of water molecules.These advantages make a-Ni HCF a promising candidate in the field of electrocatalytic water splitting.2.A three-dimensional（3D）self-supported micro-strip-like Co₃O₄assembled from tiny nano-cubes electrocatalyst,grown in situ on commercial Co foam（denoted as Co₃O₄/Co）was fabricated through a facile one-step hydrothermal synthesis method.By TEM,XPS and other characterization methods,it is found that the surface of the material is exposed to abundant C_O²⁺,C_O³⁺and O defects.Meanwhile,the 3D metal foam substrate can provide more active sites,which synergistically makes the catalyst possess good catalytic activity and stability.In1.0 M KOH solution with 0.3 M hydrazine,only-32 m V was required to deliver a current density of 200 m A·cm^-2and the Tafel slope is 53.43 m V·dec^-1.Remarkably,an ultra-small cell voltageof only 1 V is required todrive 764 m A·cm^-2in a two-electrode electrolytic system,which is superior to the noble-metal catalysts system and thereported noble-metal-free electrocatalysts.3.Based on Co₃O₄/Co,a novel bifunctional Co_xP@Co₃O₄nanocomposite with grass-like and block-like structure was fabricated via low-temperature-phosphorization method.Compared with the Co₃O₄/Co,the heterogeneous Co_xP@Co₃O₄,composed of a mixture of Co P,Co₂P and Co₃O₄,possessed superb electrochemical catalytic activity for both the HER and Hz OR in 1.0 M KOH and 0.3 M hydrazine medium.Low overpotentials of 106,and 129m V were required to deliver current density of 10,and 200 m A·cm^-2.Meanwhile,potentials of-100,and-83 m V,are needed to drive 10,and 200 m A·cm^-2,which exceeds these of almost recently reported catalysts.Meanwhile,a low cell voltage of 0.26 V was achieved to deliver a current density of 100 m A·cm^-2when the Co_xP@Co₃O₄behaved as both cathode and anode simultaneously.The excellent performance can be attributed to the fact that the synergistic effect between the presence of multi-phase of Co_xP/Co₃O₄and 3D porous Co foam substrate makes as-synthesized catalyst possess large specific surface area and fast charge/mass transport.Density functional theory（DFT）calculations unravel that phosphorization strategy can not only regulate the electronic structure of the pristine Co₃O₄,enhancing the electronic conductivity,but also can optimize the adsorption/desorption strength of H*and alter the free energy change of the dehydrogenation kinetics of the NH₂NH₂*.