Post-transition (p-block) metals such as indium, tin, lead, and bismuth have historically been known to electrochemically reduce CO2 to formate in aqueous media with good efficiency; however, the mechanism of this transformation has recently come into question. Recent research in our group seeks both to apply these materials to real electrochemical systems and to understand the mechanisms through which these transformations occur.
A recent project done in collaboration with Liquid Light, Inc., developed a solar-powered, indium-based electrolyzer. Three flow cells, each containing a high surface area indium cathode, a proton-exchange membrane, and a mixed-metal oxide anode, were connected in series electrically and shared catholyte and anolyte flows. This electrolyzer stack was then connected to a commercial SunTech solar panel, lent by local utility company PSE&G, and run outdoors in actual sunlight. Faradaic efficiencies for formate were ~60%, and the energy conversion from light energy to chemical energy was 1.8%, a new record for artificial CO2 photosynthesis at the time.
The electrochemical behavior of post transition metal electrodes has recently been shown to be much more complicated than initially predicted. Specifically, the electrochemical reduction of CO2 on tin and indium electrodes is strongly dependent on the amount of oxide at the electrode/electrolyte interface. Anodized electrodes show an increase in the Faradaic efficiency for the production of formate in comparison to their native counterparts. This is surprising because, under the conditions employed, the electrode surface is thermodynamically expected to be entirely in the metallic state.
Our group has recently set out to determine the role played by surface oxides in the CO2 reduction mechanism on these materials. In addition to standard electrochemical investigations, we have employed a number of ex situ spectroscopic techniques including X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS) to probe these systems.
Our mechanistic studies on p-block metals have focused on the application of in situ spectroelectrochemistry to probe molecules near the electrode interface using vibrational spectroscopy techniques while performing electrochemical experiments. In order to accomplish this, standard attenuated total reflectance infrared (ATR-IR) spectroscopy single crystals are modified by a thin metal film that is employed as the working electrode.
We have shown the presence of a meta-stable oxide at the electrode/electrolyte interface on indium and tin cathodes which plays a significant role in the mechanism of CO2 reduction on these materials. Additionally, a surface-confined carbonate intermediate is observed at these surfaces, indicating that the oxide is necessary for the binding of CO2 prior to the electrochemical reduction steps. This information allowed for the following proposed mechanisms:
Recent work has extended the ATR-IR investigations to lead and bismuth cathodes which, despite producing the same products, have very different electrochemical and spectroscopic behavior. Reduction on these two cathodes does not proceed through the metal-carbonate intermediate as on indium and tin. Despite this, lead still exhibits a meta-stable oxide, while bismuth does not, indicating a wide variety of possible oxide interactions on these materials.