Electrochemical reduction of carbon dioxide on copper electrodes
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Global warming, climate change and over-dependence on non-renewable fossil fuels demand long-term solutions to reduce CO2 emissions and develop alternative and renewable fuels. The electrochemical reduction of CO2 is part of the ambitious, but certainly not impossible, “carbon neutral cycle”, which incorporates CO2 as the unlimited carbon source for the production of high density fuels, and renewable energy as the driving force behind the process.
The majority of this work focusses on various aspects of electrochemical CO2 reduction on polycrystalline Cu electrodes, although preliminary work was also performed on a number of Au9/TiO2 modified Cu electrodes. Initially, the general behaviour of the electrode potential and CO2 reduction activity over long periods of galvanostatic electrolysis was investigated, along with the effects of current density and electrolyte concentration. Overall, the results obtained are consistent with those in the literature, and cover important observations including the major reduction products on Cu electrodes, their pH and potential dependence, and the widely reported deactivation of CO2 reduction.
Following reports in the literature regarding the deactivation of CO2 reduction, attempts were made to prolong the CO2 reduction activity using periodic cyclic voltammetry and potentiostatic steps throughout extended periods of galvanostatic CO2 reduction. However, contrary to previous literature, it is demonstrated that temporarily interrupting galvanostatic CO2 reduction with short periods at potentials between −0.5 and −0.1 V vs Ag|AgCl suppresses the formation of CH4, CO and C2H4. It is proposed that the suppression is caused by the partial removal or oxidation of adsorbed CO2 reduction intermediates, the absence of which allowed the Cu surface to be more active for the hydrogen evolution reaction. Unexpectedly, when brief potentiostatic steps were conducted at more negative potentials (−1.2 V vs Ag|AgCl), the CO2 reduction selectivity switched from CH4 to CO, and was maintained for at least 2 hours. This change in selectivity is proposed to be caused by an increase in the surface coverage of COads (at the expense of Hads) during the brief −1.2 V steps, which then enabled the Cu cathode to switch between multiple steady-state surface coverages when the cathodic current is re-applied.
The observation of the sensitivity of CO2 reduction on cell hydrodynamics prompted a systematic investigation into the effects of mass transfer on CO2 reduction using a polycrystalline Cu rotating cylinder electrode. When the mass transfer rate increases (by increasing the rotation rate), the current efficiencies toward CO2 reduction products decreased while that for the HER increased. Additionally, the selectivity of CO2 reduction was observed to change, with CO becoming favoured over CH4 with increasing mass transfer rates. These observations are in contrast to the widely reported effects of pH and CO2 concentration, the values of which can be indirectly controlled by varying the rotation rate. Instead, the results are more consistent with the enhanced mass transfer of dissolved CO away from the electrode surface, which decreases the surface coverage of COads, preventing the further reduction of COads to hydrocarbons and changing the selectivity from CH4 to CO. This particular work highlights the importance of cell hydrodynamics, and the need to consider these effects when comparing results between different experimental configurations or designing electrochemical cells and cathodes for industrial applications.
Following the strategy of developing novel electrocatalysts with a level of surface heterogeneity, the catalytic ability of TiO2/Cu and Au9/TiO2/Cu electrodes prepared through spin-coating of commercial TiO2 (P25) and chemically synthesised Au9/TiO2 nanoparticles onto polished Cu substrates were investigated. It was determined that as the TiO2 loading increases, the electrode potential during constant current electrolysis tend to become more positive, pointing toward an enhancement in the electrochemical activity of the electrode. The increase in electrode potential is further observed when Au9 nanoparticles are introduced into the TiO2/Cu electrocatalyst. However, the enhancement in electrochemical activity is found to be largely in favour toward the HER rather than CO2 reduction. Nevertheless, despite the very low overpotentials at the modified Cu electrodes, surprising amounts of CO are still produced with current efficiencies generally comparable to that of the Cu controls and Ti electrodes at similar current densities but at much higher overpotentials. This suggests a form of synergy at the active sites of the Au9/TiO2/Cu interfaces which may have lowered the CO adsorption strength, hence allowing similar amounts of CO to be produced at much lower overpotentials.