Nanoporous CU-based catalytic electrodes for electrochemical conversion of carbon dioxide

Abstract

This research investigates the strategies to enhance copper electrodes for carbon dioxides reduction reactions (CO2RR) for generation of valuable chemical products, including methanol and acetaldehyde. Through modifications of physical characteristics of electroplated copper electrodes, particularly introduction of porosity, and chemical composition, namely formation of copper oxides, the key performance parameters for CO2RR including selectivity, Faradaic efficiency, and energy efficiency are examined. Furthermore, the relationships between copper electrodes’ plating variables (current density, deposition time, and bath solution), porosity and compositions, and corresponding CO2RR performances are constructed. The study has demonstrated that the ratio of CuSO4 and H2SO4 of bath solution, current density, and deposition time influence the obtained porous structure as confirmed by microstructural and physical evaluations (apparent and true porosity, 2D-morphology, surface roughness, BET surface area and BJH pore distribution). With controlling concentration of H2SO4, it was found that more concentration of CuSO4 increase the amount of copper deposit on the substrate creating a porous copper with higher apparent porosity but identical pore size. HCl minimize the size of porous branch causing the highest apparent pore size and porosity. All constant-current porous coppers were mesoporous materials (2-50 nm pore size). The arrangement of copper particle as grape seed on the dendrite structure of porous copper which fabricated in 0.2 M CuSO4 without HCl providing the highest true surface area as 19.56 m2/g. This value is higher than of other porous coppers approximately 5-6 times. The apparent density, electrodeposition efficiency, and true porosity calculation was also confirmed that porous copper with higher concentration of CuSO4 was the densest porous structure. Both bulk and surface chemical composition of porous coppers were slightly different from copper foil. Cu+ and Cu2+ were detected on the surface of copper foil and porous coppers but Cu0 was detected only on the surface of porous copper which fabricated with HCl. Moreover, pulse electrodeposition was another method which can used to fabricate the denser porous structure manifested by the 10-12 times lower value of surface roughness. Furthermore, increase in surface area of catalytic electrode cause increase in active site as a result of increasing rate of reaction as shown by the approximately 7 times higher of current density with product distribution enhancement of porous coppers compared with copper foil. Porous copper with higher concentration of CuSO4 provide the highest product distribution, moreover, H2 increase with higher applied voltage as presented in the result which performed at Nanotec. After passing through CO2RR, particles of copper had more agglomeration with change in apparent pore characteristic and chemical composition. Ethanol is a main product of porous copper electrocatalyst while aldehyde, ethylene and methanol were also detected. In case of thermally-induced copper oxides catalytic electrode, with controlling oxidation time, oxidation temperature cause the different surface composition. Cu+ and Cu2+ were detected on the surface of oxidized copper at 300 and 500 as copper foil whereas Cu0 was detected only on oxidized copper at 800 and 1,000. There were some copper particles on the surface, obviously the oxidized copper at 300 and 500. Only the oxidized copper at 300 was not broken after passing through the oxidation. Copper oxide catalyst provide higher rate of reaction of CO2RR than of copper foil. Acetaldehyde was detected by the oxidized copper catalyst. Furthermore, thermally-induced porous copper oxides which fabricated by combining the porous copper and copper oxide can deliver 9 and 4 times higher rate of reaction than copper foil and porous coppers, respectively. Ethanol is a main product whereas some rare valuable chemicals (i.e., n-propanol, glycolaldehyde, and propionaldehyde). Cu+ and Cu2+ were detected on the surface of thermally-induced porous copper oxides both before and after passing through the CO2RR process indicating that this electrocatalyst have more stability than porous coppers. It summarized that development in structure and chemistry can enhance the performance of electrocatalyst in CO2RR process.