The development of efficient CO2 capture, utilization, and storage (CCUS) technologies has aroused worldwide concern in recent years. As one of the most promising technological paths, electrocatalytic CO2 reduction can convert greenhouse gas CO2 into various chemicals or fuels. Its practical application requires the conversion of CO2 into value-added products at an industrial-scale current density, high selectivity, and low electrolysis voltage, in order to ensure sufficient production efficiency and reduce the cost of product separation as well as electricity consumption.
The CO2 electrolysis at an industrial-scale current density can be realized by utilizing electrochemical reactors with direct CO2 gas feeding, including flow cells and membrane electrode assembly (MEA) electrolyzers. The flow cell setup has a millimeter-scale gap between anode and cathode (Fig. 1a), resulting in a high electricity waste to overcome the huge internal resistance for ionic transport. In contrast, the MEA electrolyzer is a zero-gap setup (Fig. 1b). It enables a much lower cell voltage for high current density CO2 electrolysis, thereby greatly increasing the energy conversion efficiency.
Fig. 1. Scheme and characteristics of the flow cell (a), MEA electrolyzer with liquid electrolyte (b), and MEA electrolyzer with pure water (c) for CO2 electrolysis.
Although more and more studies start to employ the MEA electrolyzer in recent few years, they still rely on the use of liquid electrolytes, mainly for two reasons: (1) it was usually considered that the alkali cations in liquid electrolytes are essential promoters for electrocatalytic CO2 reduction. Some studies even suggest that the CO2 reduction cannot happen in the absence of alkali cations; (2) it is easier to achieve high utilization of porous electrode inside the MEA, due to the good permeability and high ionic conductivity of liquid electrolytes. However, the flowable liquid electrolytes will also bring problems such as electrolyte leakage and cathode failure (caused by salt precipitation in the gas diffusion electrode). Using immobile polymer electrolytes and operating with pure water (Fig. 1c) can address these problems caused by liquid electrolytes. It represents a more advanced technological path. As similar electrochemical techniques, fuel cells and water electrolyzers have already completed the innovation from liquid electrolytes to polymer electrolytes/pure water. The MEA electrolyzer with pure water is rarely utilized in electrocatalytic CO2 reduction yet, probably due to the absence of alkali cation promoter and strong alkaline environment, which is generally beneficial for CO2 reduction and C2H4 production.
In our article, for the first time, we realize the C2H4 production from CO2/pure water co-electrolysis at industrial-scale current density, by applying a bifunctional alkaline polymer electrolyte (ionomer) in the cathode. It can not only conduct anions and thus enlarge the cathode electrochemical surface area (i.e., make a high utilization of the porous gas diffusion electrode), but also serve as an alkali cation-like promoter to promote CO2 reduction. Our findings open up a new avenue for the development of practical CO2 electrolyzers.
For more details, please see our recent publication in Nature Energy:
Bifunctional ionomers for efficient co-electrolysis of CO2 and pure water towards ethylene production at industrial-scale current densities