Kinetics and scale-up of electrochemical reduction of aqueous CO2 at Sn Cathodes
Creators: name: Bumroongsakulsawat, Palang
Publisher: name: Imperial College London
Year: 2013

description: Electrochemical reduction of CO2 in aqueous buffer solutions of different pHs at tin electrodes was studied. The partial current densities exhibited the expected two Tafel regions; with increasingly negative electrode potential, the first to be encountered depended on pH strongly, while the second showed only weak dependence. Formate and CO were observed as the main products. In the range of potentials studied, the highest charge yield of 0.67 was achieved at -1.55 V (AgCl|Ag) in 0.5 M NaOH saturated with 1 atm CO2 with a total current density of 36.2 A m-2. pH was also found to affect the formation ratios of CO to formate, the value of which ranged from 1 to 0.15 as pH was increased from 2.9 to 7.8. However, pH was not an ideal variable to adjust the product distributions because lower pH led to a lower charge yield of CO2 reduction due to increasingly competing hydrogen evolution. Two mathematical kinetic models based on slightly different concepts were developed to quantify the dependence of the formation ratios on pH and to predict partial current densities of CO2 reduction at different pHs and electrode potentials. The first model assumed that there could be multiple reactions having different stoichiometric coefficients of proton consumption occurring simultaneously; the greater the coefficient, the stronger was the preference for CO production from the reaction. The other concept was that there were multiple protonation states of the intermediate in CO2 reduction; the higher the protonation state, the higher was the tendency to form CO from the intermediate. Both concepts allowed variations of the product ratios as a function of pH, but the latter model was preferred because more accurate predictions of the partial current densities were achieved and a generalised reaction mechanism could be derived. Electrochemical reduction of CO2 in 0.5 M NaOH saturated with 1 atm CO2 at tin-coated graphite felt electrodes was also studied. Tin was deposited on graphite felt from aqueous solutions of 0.3 M K2[Sn(OH)6], 0.4 M KOH and 0.5 M K3PO4. The effects of electrode potential and electrolyte flow rates on the performances of CO2 reduction at 3D electrodes were explored. The best performance was achieved at -1.62 V (AgCl|Ag) and 99 ml min-1 electrolyte flow rate; the total current density and the charge yield of formate were 971 A m-2 and 0.58, respectively. Accumulation of gas bubbles composed of H2 and CO was found to have detrimental effects on superficial electrolyte conductivities and mass transport of CO2, but can be partially alleviated by increasing the electrolyte flow rate. Two mathematical models were developed to include the effect of flow rates into account in predicting total current densities and formate charge yields from 3D electrodes. The first model assumed that the gas bubbles inside porous electrodes travelled at the same velocities as the electrolyte solutions; this led to highly under-predicted potential drops across the electrode thickness and over-predicted total current densities. On the other hand, the second model allowed slip between bubble flows and electrolyte flows, resulting in more accurate predictions. Bubbles were found to travel 0.0016 times slower than electrolyte flow velocities.