Research On Electrochemical And Mechanical Properties Of Nanoporous Silver

The nanoporous materials prepared by dealloying method are expected to be applied in the fields of filtration,catalysis,sensors and actuators with their high specific surface area,high porosity and small pore characteristics.In this paper,Ag-Zn alloy was used as the mother alloy to prepare nanoporous silver by dealloying method.The preparation process,electrochemical catalytic performance and mechanical properties were systematically investigated.The following experimental results were obtained.The dealloying process of Ag-Zn alloy is controlled by diffusion rate of silver atoms in the alloy surface/electrolyte interface.The corrosion current density decreased,merely about half of that at room temperature,when the electrolyte solution decreases from room temperature?25??to 2?.Critical potential and self-corrosion potential increased significantly.Calculated surface diffusion rate of silver atoms in 0.1M H₂SO₄ is 1.2×10^-12cm²s^-1 at 2?.Pore size decreased by 30%through measurement.The process of coarsening of nanoporous structure was significantly retarded.The pores/ligaments become smaller after 0.2 g SDS was added to 250 mL 0.1 M HNO₃with identical condition.The pore size scale is only 70%of that without the surfactant.The diffusion rate D_s of silver atoms in 0.1 M HNO₃ decreased from 1.65×10^-11 cm²s^-1 to 2.69×10^-12 cm²s^-11 after adding SDS,showing the adsorbed surfactant on the ligaments can significantly increase surface diffusion resistance of the silver atoms and limit their mobility.The massive nanoporous silver with thickness of 5.4 mm is obtained after dealloying for100 hours.Nevertheles,volume shrinkage of 14.2%was detected and obvious surface peelings with a thickness of several tens of microns can be found.Experimental results show that the overall structure of the massive nanoporous silver is inhomogenous,and external morphology is rougher than interior.These also indicate that the dealloying time has an obvious effect on the pore structure,and coarsening phenomenon will be more serious with the extension of the dealloying time in the electrolyte.Both the SAED and the XRD results verify that the as-dealloyed specimen is composed of a single phase of fcc Ag.Moreover,the interplanar spacing was determined from the HRTEM image and is 0.235 nm,which corresponds to the?111?reflection of fcc Ag.It should be noted that some lattice defects can be seen in the ligaments of nanoporous silver,such as stacking faults,lattice distortions and twins.Electrochemical catalytic formaldehyde experiment showed the peak current density of nanoporous silver six times as much of dense silver in 0.9 M KOH,indicating that nanoporous silver has the higher catalytic activity to the formaldehyde.It is needs to be noted that the oxidation potential of formaldehyde on nanoporous electrode is 300 mV lower than that on dense silver electrode,which will increase the output voltage and improve the efficiency of fuel cell in actual application.A good linear relationship between the peak current density and the square root of the scanning rate indicates that formaldehyde electrochemical behavior on the nanoporous silver electrode is controlled by the diffusion process.The peak current density and the corresponding potential both increase with scanning rate increasing.In 1 mM/L H₂PtCl₆+0.5 M H₂SO₄ solution,deposition of platinum on the surface of the NPS electrode by electrochemical deposition resulted in a decrease in the pore size and an increase in ligament size.Compared with the NPS electrode,the activation energy of formaldehyde oxidation on NPS-Pt electrode is low,the reaction speed is fast,and the activity of catalyzing of formaldehyde is improved.After 100 cycles,the peak current density was76%for the NPS electrode,and 92.5%for the NPS-Pt electrode,which improved the electrocatalytic stability.The mechanical properties of nanoporous silver were measured by nanoindentation.It was found that under the same loading rate and the same indentation depth,the hardness and elastic modulus of nanoporous silver decrease sharply with increasing size scale of the ligament and pore.And there is no work hardening phenomenon in the course of the whole press-in process.The hardness and elastic modulus both show the strong surface size effect when the depth is less than 200 nm.With the same loading rate and different indentation depth,the hardness decreases with increasing depth and the elastic modulus increases.