Searching for electrolytes and electrodes for CO2 reduction below 300 °C

Abstract

Electrochemical CO2 reduction research is driven by the desire to reduce reliance on fossil fuels and lower greenhouse gas emissions. The conversion of CO2 into fuels and chemicals using energy derived from a renewable source, such as wind or solar, could replace the use of fossil fuels. This thesis uses the knowledge derived from earlier investigations on electrolysis techniques as the foundation for an exploratory work to find acceptable materials and fabricate an electrochemical cell able to produce hydrocarbons and alcohols directly from reduction of CO2 and steam. The operating conditions should be between 200 – 300 ⁰C and at elevated pressure in the range of 20 – 30 bar. The temperature range is chosen according to the thermal stability of the hydrocarbons produced by conversion of CO2. The electrochemical performance of the fabricated cells was evaluated using electrochemical impedance spectroscopy and chronoamperometry, while the gas analysis was carried out via gas chromatography. The initial part of the study focused on electrolyte materials in order to identify a promising candidate to be implemented in the full cells. Some proton conducting materials, such as Y-doped BaZrO3-BaCeO3 solid solutions and K-doped BaZr1-xYxO3-δ were evaluated. BaCe0.5Zr0.4Y0.1O3-δ would have been the best electrolyte candidate because of its low resistivity in high pH2O (2 · 10-3 S/cm) even at temperatures as low as 240 °C. However, the instability in the acidic CO2 gas atmosphere hinders practical application for carbon dioxide reduction at high pressure. K-doped BaZr1-xYxO3-δ was successfully synthesized by hydrothermal technique, but the conductivity recorded in high pH2O and at 240 °C was too low (3 · 10-5 S/cm) to be considered as a suitable electrolyte. A literature survey showed that most CO2 reduction studies were performed in aqueous potassium bicarbonate (KHCO3) and with a copper metal catalyst. Therefore, it was decided to investigate the electrocatalytic activity of copper foam in aqueous media at ambient conditions for electrochemical reduction of CO2. The measurements were conducted at Stanford University – Chemical Engineer Department, where it was possible to utilize an experimental setup which ensures high sensitivity for minor products from the CO2 reduction reaction. Seven products were identified with the copper foam electrode tested to -0.98 V vs. RHE. H2, formate and CO were the main products observed and in particular the faradaic efficiency of H2 was ca. 90 %. The highest current density that could be sustained with this setup was about -20 mA/cm2. Therefore, it was decided to develop a new cell that could operate at higher current densities, pressures and temperatures. A foam based CO2 conversion cell with gas diffusion electrodes and a ceramic porous structure in which the liquid electrolyte is immobilized by capillary forces was developed and tested up to 20 bar and to a maximum temperature of 50 °C. Potassium carbonate was selected as aqueous electrolyte and various concentrations of this electrolyte were immobilized in a ceramic porous matrix at both ambient and elevated temperatures and pressures. Copper and silver metal foams were tested as cathode. Nickel metal foam was chosen as anode. When copper was used as electrocatalyst, a high faradaic efficiency for the evolution of H2, i.e. between 92 to 99 % was registered. The other products detected were CO and during one test also methane was identified. The performance of Ag cathode metal foam confirmed its higher selectivity for CO2 reduction to CO. The formation of passive oxide layers and the subsequent degradation of nickel foam electrodes affected the electrochemical performance and the stability of the cells negatively.