Control Synthesis Of Porous Metals For The Electrochemical Energy Application

Constructing renewable energy structure is a long-term strategic objective of China and all the other world,and developing efficient techniques,such as electrochemical energy catalysis and storage,for the conversion and storage of renewable energy resources is critical for this objective.Porous metal materials have been regarded as an ideal and desirable electrode structure for electrochemical energy catalysis and storage applications.To date,some methods have been developed for the fabrication of porous metal materials,but these methods generally have limitations of step complexity,material waste,and enviroment harmful,and the resultant porous metal materials commonly have pore structures with a narrow size range.Accordingly,it is generally required to combine two or more methods together for the synthesis of hierarchically porous metal materials.Moreover,some soft and low-melting-point metals with high catalytic activity can not be easily fabricated into 3D porous structures using the conventional methods.This thesis mainly focuses on the control synthesis of porous metal materials with the simple and efficient techniques.Aiming to obtain hierarchically porous metal materials with widely adjustable pore sizes for the application in electrocatalytic water splitting and electrochemical supercapacitors,and create 3D porous structurs in the soft and low-melting-point metals with good activity for the application of electrocatalytic CO2 reduction reaction.The main contents are as follows:A two step gaseous thermal treatment processe has been exploited to transform solid metal into porous metal foams with widely adjustable pore size,including(?)thermal oxidation in air atmosphere and followed(?)thermal treatment in a gaseous ammonia atmosphere.Taking Cu as an example,the thermal oxidation produced CuO is firstly nitridized into Cu3N at a relatively low temperature,and then the formed Cu3N is denitridized into Cu and N2 at relatively high temperature during the thermal treatment in an ammonia atmosphere.The intermediate Cu3N acts as a blowing agent to uniformly produce pores throughout the matrix by the decomposition of itself into Cu and N2.The release of N2 provides the main driving force for the pore-construction.The obtained porous Cu materials can be derived from any shape Cu matter like foils,tubes and foams,and the pore size can be widely adjustable from nanometers to tens micrometers.A simple technique for the synthesis of hierarchically porous metal materials has been developed for the energy catalysis and storate application by cycling the two step gaseous thermal treatment processe.The Cu foam with macropores was firstly produced by the oxidation-nitridation-denitridation of Cu foil.Then a second oxidation-nitridation-denitridation process was carried out to produce mesopores on the walls of the macropores.And the like,more cyclying times can be applied.Finally,hierarchical porous structures cam be obtained.The resultant hierarchical porous Ni foams can give a specific surface area being one magnitude larger than that of commercial Ni foams,and the remarkably enhanced performance of both electrochemical water splitting as HER/OER electrodes and electrochemical energy storage as the host substrate of capacitive material MnO2.The two-electrode water splitting system exhibits an impressive activity,and a bias of?1.65 V(overpotential of?420 mV)is required to produce a current density of 10 mA/cm2.An areal capacitance of 2.04 F cm-2 has been achieved for the porous-Ni-foam/MnO2 hybrid electrode at 0.5 mA cm-2,which is on the top list of the reported values.3D porous metal Bi particles has been rationally prepared by electrochemically reducing BiVO4 particles at the cathode,exhibiting outstantding electrocatalytic activity of CO2RR for formic acid production.Inspired by the dealloying method,we adoped an electrochemical reduction method to withdraw the V and O elements from BiVO4 particals,and thus pores are formed along with this process.Meanwhile,Bi5+ions in BiVO4 will be left and reduced to Bi metal.Finally,3D porous metal Bi particles are produced,overcoming the challenge on the fabrication of 3D porous structues in soft and low-melting-point metals.The resultant 3D porous Bi particles,having interconnected pore structure and high surface roughness,can effectively convert CO2 into formic acid with high selectivity(Faraday efficiency>90%),good durability and wide operating potential window(?450 mV),better than many other CO2RR electrocatalysts for formic acid products.