Investigations of the Electrochemical Reduction of Carbon Dioxide
- Author(s): Clark, Ezra Lee
- Advisor(s): Bell, Alexis T
- et al.
The electrochemical reduction of CO2 offers a potential means for producing carbon-neutral fuels and chemicals. Cu is the most effective contemporary electrocatalyst for reducing CO2 to products such as methane, ethene, and ethanol. Unfortunately, the current efficiency of the process is limited by competition with the relatively facile H2 evolution reaction. Thus, there is considerable interest in identifying ways to modify Cu that will suppress the evolution of H2 and enhance the selectivity to desired products. Accomplishing this goal has proven difficult, partially because objective evaluation of CO2 reduction electrocatalysts has been convoluted by a lack of standardized methods for measuring and reporting activity data. Furthermore, the electrochemical reduction of CO2 is sensitive to electrolyte polarization, which is characterized by the formation of gradients in both pH and the concentration of CO2 near the cathode surface. Since the intrinsic kinetics of CO2 reduction depend on the composition of the local reaction environment it is desirable to measure the concentration of reaction-relevant species in the immediate vicinity of the cathode. However, meeting this objective has proven difficult since conventional analytical methods only sample species from the bulk electrolyte.
In the first study, we identify extraneous variables that influence the measured activity of CO2 reduction electrocatalysts and propose procedures to improve the accuracy and precision of reported data. We demonstrate that when these proposed procedures are followed that the activity of Ag and Cu electrocatalysts prepared and tested in different laboratories exhibit little variation. We advocate that standardizing the experimental methods for measuring the activity of CO2 reduction electrocatalysts will greatly facilitate the search for electrocatalysts with superior activity and selectivity.
In the second study, we investigate the impact of surface atomic structure on the CO evolution activity of Ag by conducting CO2 reduction over Ag(111), Ag(100), and Ag(110) thin films. We directly observe the surface atomic structure of these Ag thin films under electrochemical conditions by conducting in-situ electrochemical scanning tunneling microscopy, which enables atom resolved images to be acquired under an applied potential. We find that the CO2 reduction activity of the corrugated Ag(110) surface is roughly an order of magnitude higher than either Ag(111) or Ag(100). We have determined that these activity trends are caused by variations in the local electric field strength, which stabilizes the polarizable intermediates of CO2 reduction. The strength of these local electric fields is enhanced over undercoordinated surface atoms due to their elevated surface charge density when polarized to a given potential.
In the third study, we report a novel differential electrochemical mass spectrometer (DEMS) cell design that enables the partial current densities of volatile CO2 reduction products to be quantified in real time. The capabilities of the novel DEMS cell are demonstrated by conducting CO2 reduction over polycrystalline Cu. The reaction products are quantified in real time as a function of the applied potential during linear sweep voltammetry, demonstrating that the technique can determine the product distribution produced over a given electrocatalyst as a function of the applied potential on the timescale of roughly one hour.
In the fourth study, we utilize DEMS to measure the concentration of CO2 and reaction products in the immediate vicinity of the cathode surface. This capability is achieved by coating the electrocatalyst directly onto the pervaporation membrane used to transfer volatile species into the mass spectrometer, thereby enabling these species to be sampled directly from the electrode-electrolyte interface. Using this approach, we observe the depletion of CO2 within the local reaction environment due to reaction with hydroxide anions evolved from the cathode, providing insights into the detrimental effects of concentration polarization on the performance of CO2 reduction electrocatalysts. Furthermore, we observe an abundance of aldehydes relative to alcohols within the local reaction environment over Cu, supporting their hypothesized role as intermediate reaction products in the mechanism of alcohol formation.
In the fifth study, we investigate CO2 reduction over CuAg bimetallic electrodes and surface alloys, which we have found to be more selective for the formation of multi-carbon products than pure Cu. This selectivity enhancement is a result of the selective suppression of H2 evolution, which occurs due to compressive strain induced by the incorporation of relatively large Ag atoms into the Cu surface. Furthermore, we report that these bimetallic electrocatalysts exhibit an unusually high selectivity for the formation of multi-carbon carbonyl-containing products, which we hypothesize to be the consequence of a reduced coverage of adsorbed H and the reduced oxophilicity of the compressively strained Cu.
Overall, these studies demonstrate that the activity and selectivity of CO2 reduction electrocatalysts can be tuned by modifying both the surface atomic structure and elemental composition of the electrocatalyst. Furthermore, they demonstrate that the composition of the electrolyte near the cathode surface varies substantially from the bulk and contains a substantial concentration of transient reaction products that are typically reduced further.