Electrochemical Reduction Of Carbon Dioxide On The Threedimensional Ag And TiO₂ Nanotube Electrodes

The continuing overconsumption of traditional fossil fuels will eventually lead to an energy crisis and their combustion raises atmospheric levels of CO₂.Electricity can be provided via renewable sources such as solar,tide and wind energy,without giving rise to any CO₂ emissions.The use of electricity as a medium for the electrochemical reduction of CO₂ to value added fuels or chemical products is an attractive process for addressing both energy and environmental issues.Nonetheless,activation of CO₂ by breaking the very stable C=O bond is a major challenge,thermodynamically requiring 220–330 k J mol^-1 of energy.Therefore,an excellent catalyst is needed to overcome the high activation energy barrier.In this paper,an Ag electrode was surface-modified by ozonation and subsequent electroreduction.Numerous aspects of the modified and precursor Ag electrodes such as chemical composition,surface structure,roughness factor,as well as electrochemical activity and stability towards CO₂ reduction are investigated using X-ray diffraction（XRD）,field-emission scanning electron microscope（FESEM）,potential steps determination,potentiostatic electrolysis and potentiostatic pulse experiments.Experimental results show that the modified electrode was more active and selective than its smooth-surfaced counterpart in reducing CO₂ to CO.Surface modification facilitated the formation of a three-dimensional structure on the Ag surface,thereby enlarging the number of reactive species available for CO₂ electroreduction.Moreover,the modified Ag electrode retained durable activity using potentiostatic pulse electrolysis.The main carbon-containing substances produced were large amounts of CO accompanied with relatively low quantities of formic acid,methanol,methane and formaldehyde.After 5 h of electroreduction at-1.1 V（vs.reversible hygrogen electrode（RHE））,the product amounts for CO,formic acid,methanol,formaldehyde and methane were 1620 μmol,71.8 μmol,11.9 μmol,0.65 μmol and 2.70 μmol,respectively.A maximum Faradaic effeciency（FE）of 88.87% and 79.14%,corresponding to partial current densities（PCD）of 5.73 m A cm^-2 and 3.51 m A cm^-2,was obtained for CO formation at a cathode potential of-1.1 Vvs.RHE over a potential range from -0.5 to -1.6 Vvs.RHE for both r-Ag and p-Ag,respectively.On the basis of Tafel plots and the addition of known products,it is proposed that the electroreduction of CO₂ on the Ag electrode proceeds via a mechanism involving generation of CO and HCOOH.The Ag electrode deactivation was caused by CHOads and/or COHads absorbed species on the Ag surface.A series of highly ordered TiO₂ nanotubes（TiO₂NTs）electrodes were prepared via potentiostatic anodization of Ti foil followed by calcining in air.XRD,FESEM,XPS,potential steps determination were used to characterize the electrodes.The electrochemical reduction properties of CO₂ on these TiO₂ NTs electrodes were investigated by cyclic voltammetry,potentiostatic electrolysis in aqueous solution,respectively.It was found that methanol was the major product in electrochemical CO₂ reduction.Formic acid,formaldehyde,methane and CO were formed as minor products.Compared with the electrodes sintered at 550 ℃ and 650 ℃,The optimal TiO₂ NTs electrode was found to be that calcined at 450 ℃（TiO₂NTs-450）.After 3 h of reaction,the methanol yield was 15.3 μmol,corresponding to a FE and PCD of 85.8% and 0.2 m A cm^-2 at -0.56 V vs.RHE,respectively.The three valence titanium in TiO₂ was demonstrated to serve as efficient site for adsorption of CO₂ and facilitate the stablilization of adsorbed ·CO₂^- radical.As a consequence,the reduction process of CO₂ on TiO₂ NTs electrodes involved a fast first electron and proton transfer followed by a slow second proton transfer.The observations presented herein are potentially significant in the design of useful electrodes and procedures for generation of valuable carbon-containing products using CO₂ as the carbon source.