Synthesis And Property Of Nano Morphology Of SnO₂ And Compounds And Its Application For CO₂ Electroreduction

In recent years, CO2 conversion using electrochemical catalysis approaches has attracted great attention for its several advantages:(1) the process is controllable by electrode potentials and reaction temperature;(2) the supporting electrolytes can be fully recycled so that the overall chemical consumption can be minimized to simply water or waste water;(3) the electrochemical reaction systems are compact, modular, on-demand, and easy for scale-up applications. However,challenges remain, such as the slow kinetics of CO2 electroreduction, even when electrocatalysts and high electrodereduction potential are applied; the low energy efficiency of the process, due to the parasitic or decomposition reaction of the solvent at high reduction potential. Thus, developing the cathode materials with high catalytic activity, high product selectivity is very critical to the developing of CO2 electroreduction.This work reports hierarchical tin oxide microsphere catalysts synthesized by a facile hydrothermal self-assembled process and BP-Cosalen composite nanoparticle catalyst. The electrochemical properties of the catalyst for CO2 reduction were investigated thoroughly by cyclic voltammetry(CV), linear sweep voltammetry(LSV), constant potential electrolysis(i-t Curve). At the same time, the electrolysis potential and electrolysis time and stability of the catalysts of the electroreduction have also been investigated. XRD, SEM, TEM, XPS were used to characterize the structure of SnO2 microsphere and BP-Cosalen composite nanoparticle catalyst. The specific content of this paper are summarized as follows:(1) Tin oxide microsphere is synthesized by a facile hydrothermal self-assembled process,the main synthesize process is using SnCl4 as the precursors, then form SnO2 nanopaticles with carbon of black color by the hydrothermal method, at last use the method of air oxidation to removal of carbon to form hierarchical tin oxide microsphere(HMS-SnO2). Electrochemical measurements demonstrate that these catalysts can have both outstanding catalytic activity and selectivity toward CO2 electroreduction, and the ethanol/water ratio have a significant effect on the catalyst performance。By controling different ratios of ethanol to distilled water, the catalytic activity of SnO2 microsphere catalysts were siginificantly change. When the ratio of material is 86% ethanol and 14% distilled water, forming hierarchical SnO2 microsphere with the best catalytic activity of CO2 electroreduction. High efficient faradaic efficiency of 62% is achieved toward formate formation at-1.7 V vs. SHE, with onset potential of-0.49 V vs. SHE and the maximum current density reaches 21 mA cm-2 at-1.25 V vs. SHE. The results showed that the SnO2 exhibits a large hierarchical microsphere structure composed of nanoparticles in size of 20-25 nm indiameter. When the ethanol content was further increased, the microsphere structures begin to collapse, the catalytic activity was decreased sharply. During electrolysis experiment, KHCO3 electrolyte was found to show some contribution on formateformation on these micro-structured tin oxide catalysts, with a faradaic efficiency of 2.4% and the concentration of 13.2 mg L-1. A perfect durability was observed for the prepared HMS-SnO2 catalysts with almost unchanged current density over 12 h of continuous electrolysis operation. The atomic percentages of Sn 3d5/2 and Sn 3d3/2 was almost unchanged as determined by the XPS spectra analysis. The superior performanceis credited to the morphology- and size-controlled hierarchical structure, which may provide more active sites to accelerate the slow kinetics of CO2 reduction.(2) From SEM characterization, the SnO2 nanoparticle shows a 3-dimensioned hierarchical structure, and was a mixed composition of nanoparticles and nanospheres aggregate, show a diameter of 500 nm-1 ?m with a high porous structure. It is found that the catalytic activity and formate selectivity is strongly depending on the electrolyte concentration. The results show that the catalytic activity increased with the electrolyte concentration from 0.1 M to 0.5 M, where the best catalytic activity was obtained. Meanwhile, the highest faradaic efficiency obtained in this research was 56% in 0.5 M KHCO3. This is attributed to the HCO3― directly involved in the reaction, the formate formation was controlled by mass transfer and charge transfer when its concentration is in a low or high degree. During the reaction of formation of formate, the electrolyte pH value was found to show some contributions on CO2 reduction. Our results suggests that the electrolyte of pH 8.3 is the optimal electrolyte condition for electroreduction of CO2, under which CO2 reduction in neutral partial alkaline environment could offer the possibility of maintaining the oxide stability. The catalyst stability test after 28 h electrolysis indicates a degradation mechanism of the faradaic efficiency toward formate production on a SnO2-50/GDE electrode. The decreasing of faradaic efficiency is a result of deposition of the trace fluoride ions onto the surface of the SnO2-50/GDE electrode, which was determined by the XPS spectra analysis. The presence of fluoride hindered the reduction of CO2 into formate on the surfaces of SnO2-50/GDE electrode.(3) Gas diffusion electrodes modified with Cosalen and conductive carbon black BP(BP-Cosalen/GDE) was successfully applied to CO2 electroreduction. The adsorption of Cosalen onto BP was characterized by scanning electron microscopy(SEM) and ultraviolet and visible spectroscopy(UV-vis). The electrochemical properties of the modified electrodes, ie., BP-Cosalen/GDE with different content of BP, for CO2 reduction were investigated thoroughly by cyclic voltammetry(CV), linear sweep voltammetry(LSV), CO2 electrolysis and ion chromatography analysis. The results show that the BP-Cosalen/GDE60 electrode exhibited the least negative cathode over potential at-0.65 V vs. SHE, a more positive potential than both Cosalen/GDE and bare GDE. BP-Cosalen/GDE60 electrode shows the highest activity for CO2 reduction reaction, wherever increase or decrease the amount of BP do not benefit to CO2 reduction. The synergistic electrocatalysis of Cosalen and BP jointly promoted the reduction reaction of CO2. The improved catalytic activity indicates that the apparent surface of the modified electrode becomes larger due to the presence of BP-Cosalen composite, which may provide more active phases for the electrochemical reduction of CO2. Furthermore, the presence of reduction products is confirmed to be formate through ion chromatography analysis. With 0.5 M KHCO3 as the electrolyte, the faradaic efficiency for producing formate on BP-Cosalen/GDE60 electrode reached a maximum value of 27% at-0.9 V and the corresponding concentration of formate reached 0.44 mM(mmol/L).