Fabrication Of Porous Ag Electrode And Its Electrocatalytic Property Of CO₂ Reduction

Electrochemical reduction of CO₂ is an effective approach to resolving the green-house effect as well as realizing the utilization of CO₂. Silver is a promising electrocatalyst for selective reduction of CO₂ to CO, while it still has the problems of low current density and rapid loss of activity. Aiming to develop an electrode with large active surface area and high stability, we prepared a three-dimentional porous Ag electrode with nanostructured pore walls and studied its catalytic performance in electrochemical reduction of CO₂ to CO.In the electrode preparation process, influnces of the electrolyte composition, current density and deposition time to the electrode morphology and its CO₂ electroreduction performance were studied. And finally the optimized preparation conditions were established. Techniques of SEM, EDS and XRD were employed to characterize the morphology and composition of the electrode. The results show that the porous Ag deposit has an average pore size of 20?m and thickness of 30?m. And its pore walls were fabricated by nanostructured Ag with particle size of 40¹00nm. The porous Ag was polycrystalline Ag with no impurities. CV test revealed that the electrochemically active surface area of the porous Ag electrode was about 120 times of that of a smooth Ag electrode, and its partial current density for CO₂ reduction was about 3.5 times of that of its smooth counterpart.Influences of electrolysis potential, electrolyte concentration and temperature to the CO₂ reduction performance of porous Ag electrode was investigated when it was used in the electrolysis process. Constant potential electrolysis of porous Ag electrode at-1.7V vs.SCE and 25? in 0.5M KHCO3 solution for 1 hour was conducted and achieved a current density of 14 mA/cm2 with a steady CO efficiency of higher than 90% during the 1 hour's electrolysis process. However, under the same condition, the current density of a smooth Ag plate electrode was only 7m A/cm2 and the CO efficiency decreased from 74% to 58%. The deactivation investigation results revealed that the black deposits on the surface of Ag plate electrode after electrolysis which contains carbon and oxygen may be the major factors that leading to the deactivation of the electrode, while the porous Ag electrode exhibited good catalytic stability owing to its unique Ag nanostructures which may have improved the electrode's resistance to deactivation.Tafel test revealed that the rate determining step of CO₂ reduction on the porous Ag electrode and the Ag plate electrode was the same one, which was the process of one initial electron transferring to CO₂ to form a surface adsorbed CO₂?-. With the application of rotating disc electrode technique to minimize the diffusion control in system, the current density of the porous Ag electrode was increased to 21mA/cm2. In the stability investigation of the porous Ag electrode, it exhibited good catalytic stability during a 4 hours' electrolysis while current density decreased gradually with the repeated use of the electrode, which was confirmed by the SEM images that it was oweing to the falling off of some Ag particles on the surface of the electrode.