Synthesis of Silver Nanocatalysts Using Microwave Irradiation in Ionic Liquid for Reduction of Carbon Dioxide to CO By Solar-Driven Electrochemical
Authors: Lee Jeon Ryang , Park Jae Cheol , Han Eun Mi , KIM Tae-Won 
Published: December 2020

Over the last few decades electrochemical CO2 reduction has attracted attention as a promising technology for greenhouse gas reduction. The Carbon monoxide product from CO2 reduction is of interest as a component of syngas for methanol synthesis. Silver is a reactive electrocatalyst toward carbon dioxide reduction to Carbon monoxide among single metals (Au, Zn etc.). In order to carry out CO2 reduction reaction effectively, it is require to nano-sized electro catalyst for wide active area. However, there is aggregation issue of nano-sized particle in liquid phase reduction method from metal precursor. It can be solved simply using of ionic liquid as a solvent and capping agent to suppress aggregation of nano-particle. In addition, Microwave-assisted fabrication has much potential due to their distinctive characteristics such as rapid heating ability, efficient and uniform internal heating, and relatively short process time.

Here we describe its use as a single electrocatalyst made in Microwave method with Ionic liquid for reduction of CO2 to CO. We have simply synthesized Ag nanoparticles with an ultra-fine size (~20nm) by microwave irradiation process using Ag precursors dissolve in Ionic liquid. Silver carbonate (Ag2CO3, purchased from Sigma Aldrich) as a precursor was dissolved in Ionic liquid, Butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4-, purchased from C-TRI). The 5mL mixture solutions with a concentration of 10, 20 and 30mM were prepared by stirring under atmosphere at room temperature. Each mixture solution was heated by microwave irradiation system (CEM Co.,Ltd Discover) at the power of 10~30W within just 10min.

Characterization of synthesized silver nanoparticles was analyzed by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to confirm the morphology, chemical state, and crystal structure of the samples. XRD analysis showed (111), (200), (220), (311), and (222) diffractions of Ag metal with a face-centered cubic structure, confirming that the silver nanoparticles could be synthesized. TEM images showed that the nanoparticles have ultra-fine sized silver, less than 20nm without aggregation. Based on CO2 reduction reaction (CO2RR) response, the elctrocatalystic performance of synthesized silver was evaluated. In order to compare the performance, the same mass of commercial Ag and carbon black were used in three electrode system with 0.5M KHCO3 aqueous solution saturated with CO2. The catalytic potential reduced 0.374eV compared to carbon black.

In order to confirm the efficiency of Solar-driven electrochemical carbon dioxide reduction, synthsized silver nano catalyst of 9mg/cm2 was loaded at the cathode(6.25cm2) in electrochemical reactor which is a closed two-component separated by anion exchange membrane (Sustainion® X37-50, Dioxide Materials). The anode was IrOx (purchased from Dioxide materials) sprayed on Carbon paper (9cm2). A Si solar cell of series connection solar panels supplied electric power to electrochemical reactor. We achieved the Faradaic Efficiency of 92% to Carbon monoxide conversion by synthesized silver catalyst. We obtained 8.81% to Power conversion efficiency employing series connected Si photovoltaics for CO2 conversion to CO.