Optimizing temperature treatment of copper hollow fibers for the electrochemical reduction of co₂ to co

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.