Study On The Mechanism Of Percolative Anodization In Porous Titanium Microchannels And Its Application To The Redox Flow Battery Membrane

The innovation of energy storage technology will trigger a deep change in the global energy pattern.Due to the long service life,fast response,decoupling between power and capacity,deep charge and discharge,all vanadium redox flow battery has become the most promising large-scale energy storage technology.Diaphragm material is the core component of redox flow battery.At present,the widely used commercial Nafion polymer diaphragm has the advantages of high proton conductivity and strong chemical stability.However,disadvantages of Nafion polymer diaphragm such as high vanadium ion permeability,poor mechanical stability and high cost limit its application and development.Porous titanium has the characteristics of large specific surface area,high specific strength,good corrosion resistance and low cost.Based on porous titanium,this paper proposed a membrane material by using screening effect of ultra-fine pore size and super wetting coating on the surface of internal three-dimensional channel.In this paper,the growth characteristics of TiO₂ nanotube arrays（TNTAs）on the surface of ultra-fine microchannels were studied,and the principle of porous metal infiltration anodization was established.Moreover,the percolation and barrier characteristics of super infiltrated microchannels to ions were studied,and the action mechanism of TNTAs on ion selectivity was revealed.Finally,the liquid flow battery based on porous metal diaphragm material is explored,the charge and discharge characteristics of static and dynamic battery were studied,and the influence mechanism of porous metal diaphragm material on battery performance was clarified.By studying the growth law and geometric characteristics of nanotubes in three-dimensional microchannels and one-dimensional straight channels,the principle of porous metal penetration anodization was established.The key parameter K value was introduced to clarify the action mechanism of electric field and concentration gradient on osmotic anodic oxidation.When K<0.4,osmotic anodizing requires a low viscosity electrolyte system to realize the full coverage of nanotube coating in microchannels;When K>0.4,the electrolyte system with high viscosity can be used to obtain the super infiltrated microchannel surface fully covered by the nanotube coating.Firstly,the growth characteristics of TNTAs on the surface of porous titanium ultra-fine three-dimensional microchannels were studied by electrochemical anodic oxidation,and the wettability and mass transfer characteristics of microchannels were regulated.By optimizing the viscosity of electrolyte,the average pore diameter is 0.5μm,5μm and 10μm porous titanium three-dimensional microchannel with uniform super hydrophilic TNTAs coating（average nanotube length:1μm,diameter:90 nm）.Secondly,the ion permeation characteristics of super hydrophilic microchannels modified by TiO₂ nanotube arrays were studied,and the anion selectivity mechanism of porous titanium microchannels was revealed.The mechanism of ion selectivity was clarified by adjusting the surface potential,adsorption state and electrochemical state of TNTAs coating.The results showed that the Cu²⁺permeability of super hydrophilic porous titanium microchannel was the lowest,which is 2.26×10^-5 cm² min^-1,the lowest VO²⁺permeability was 1.35×10^-6 cm² min^-1,the highest conductivity was 20.6 m S cm^-1.The ion movement in porous titanium microchannels mainly depends on the adsorption of cations by TNTAs coating,resulting in the change of microchannel surface potential from negative（-40 m V）to positive（40-50 m V）.The concentration gradient of cations in the channel converges to the nanotube array,which increases the diffusion and transmission resistance of cations.Finally,the porous titanium microchannel membrane modified by TNTAs was designed and explored,and the charge and discharge characteristics of static redox flow battery were studied to obtain better stability.When the current density is 5 m A cm^-2,the battery is stably cycled for 55 times,the coulomb efficiency（CE）is about 70%,the voltage efficiency（VE）is about 65%,the energy efficiency（EE）is about 43%,and the capacity attenuation is better than that of Nafion membrane.The performance improvement is mainly due to the porous microchannel diaphragm,which improves the vanadium ion barrier performance and proton conductivity,and reduces the battery polarization.At the same time,the adsorption of ions in electrolyte by TiO₂ nanotube array improves the capacity of the battery.On this basis,a redox flow battery system based on porous titanium super infiltrated microchannel membrane was further designed,and the charge discharge and cycle characteristics of the battery were studied.The results showed that the introduction of PTFE film on the surface of microchannel effectively improved the barrier performance of vanadium ion and prevented battery short circuit.The liquid flow battery based on three-dimensional microchannel diaphragm can cycle 100 times at a current density of 60 m A cm^-2.The average coulomb efficiency is about 92%,the voltage efficiency is about 82%,and the energy efficiency is about 75%.The improvement of vanadium ion barrier performance of super-infiltrated porous titanium microchannels treated with PTFE reduces the polarization of the battery and improves the coulomb efficiency of the battery;However,porous titanium has strong water absorption（>6%）due to its loose and porous characteristics.It is necessary to update the electrolyte to maintain the stability of the battery.This paper provides an experimental and theoretical basis for the surface modification of porous metal microchannels,and provides a new research idea for the development of membrane materials for porous metal flow battery.