In the electrochemical reduction of CO 2 , copper electrodes are well known to be active and selective for a variety of products, depending on process conditions. However, the effect of feed composition on performance has not been extensively investigated, especially with respect to the conversion of CO 2 to CO. We now show for copper electrodes in a porous tubular configuration (Hollow Fibre Electrodes, HFEs) at a relatively low working potential (−1.1 V vs Ag/AgCl), that an increasing concentration of CO in the feed results in a decreasing CO 2 conversion rate to CO. Contrary, it is observed that the concomitant hydrogen production rate does not depend on the concentration of CO in the feed. These observations are in good agreement with thermodynamic predictions applying the equation for the Gibbs energy of reaction. On the basis of this conclusion, we anticipate that mass transfer limitations are minimized by the tubular morphology and flow-through mode of operation. Most importantly, this study shows the necessity of a low CO concentration in the feed, to obtain a high CO 2 conversion rate.
Copper hollow fibers were prepared via dry-wet spinning of a polymer solution of N-methylpyrrolidone, Polyetherimide, Polyvinyl Pyrolidone, and copper particles of sizes in the range of 1–2 µm. To remove template molecules and to sinter the copper particles, the time of calcination was varied in a range of 1–4 h at 600 °C. This calcination temperature was determined based on Thermal Gravimetric Analysis (TGA), showing completion of hydrocarbon removal at this temperature. Furthermore, the temperature of the subsequent treatment of the fibers in a flow of 4% H2 (in Ar) was varied in the range of 200 °C to 400 °C, at a fixed time of 1 h. Temperature programmed reduction experiments (TPR) were used to analyze the hydrogen treatment. The Faradaic Efficiency (FE) towards CO in electrochemical reduction of CO2 was determined at −0.45 V vs. RHE (Reversible Hydrogen Electrode), using a 0.3 M KHCO3 electrolyte. A calcination time of 3 h at 600 °C and a hydrogen treatment temperature of 280 °C were found to induce the highest FE to CO of 73% at these constant electrochemical conditions. Optimizing oxidation properties is discussed to likely affect porosity, favoring the CO2 gas distribution over the length of the fiber, and hence the CO2 reduction efficiency. Treatment in H2 in the range of 250 to 300 °C is proposed to affect the content of residual (subsurface) oxygen in Cu, which leads to favorable properties on the nanoscale.
An introduction to electrochemical CO 2 conversion and copper hollow fibre electrodes 1.1 Adding to the carbon cycle Being one of the main resources for plants and trees, carbon dioxide (CO2) acts as a building block for life on planet earth. [1] The supply of CO2 to the earth's atmosphere occurs through combustion of carbon containing species in the presence of air. Take the human body as an example, where carbon containing foods are taken in and combusted together with oxygen to yield energy and CO2. This CO2 is emitted to the atmosphere through breathing. In the many years that planet earth exists, a delicate cycle between CO2 supply and demand has evolved.In man-made combustion processes, carbon containing species are also combusted in the presence of air. These processes provide for energy, amongst others, and heavily rely on fossil-based fuels. [2] Utilization of these fossil-based carbon sources adds carbon to the atmosphere that was long stored in the earth's crust. The carbon cycle evolved to process a certain amount of CO2 each year, and the emission of additional fossilderived CO2 results in accumulation of CO2 in the atmosphere. [1]
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