Multi-Factor Approach on How to Enhance the System Stability of CO2 Electrochemical Conversion to C2+ Products from Hours to a Month
Authors: Sahin Baran , Facci Allegrini Vinicius , Kraehling Marc , Wiesner-Fleischer Kerstin , Tawil Angelika , Pastusiak Remigiusz , Magori Erhard , Simon Elfriede , Hinrichsen Olaf 
Published: February 2024

Renewable electricity-powered electrochemical conversion of CO2 into fuels and value-added chemicals has immense potential to defossilize related sectors and provide an alternative storage solution for intermittent renewable energy supply. Studies show that low-temperature CO2 electrochemical conversion can be economically feasible if operated efficiently, particularly when the final price of electrolysis products and market size are attractive [1-2]. Cu is the only catalyst material that can reduce CO2 electrochemically into C2+ products (ethylene, acetate, n-propanol, propionate, propionaldehyde, and allyl alcohol) in significant amounts. Higher current densities (>200 mA/cm2) for the electrochemical reduction reaction of CO2 (CO2RR) on Cu-based gas diffusion electrodes (GDE) are already achieved; however, researchers still report a wide range of challenges, poor reaction stability being arguably the most critical one. To realize its full potential for large-scale industrial implementation, research efforts in both industry and academia focus primarily on selective catalysts, novel electrode designs, and cell design improvements, leaving a research gap for a multi-aspect approach centering on long-term stability.
In our work, based on our own research and state-of-the-art knowledge, we focused on three main factors which play significant roles in the stability of Cu-based CO2RR: Cu surface restructuring, GDE flooding and liquid product accumulation in the electrolyte. These factors were addressed by taking various countermeasures, which mainly included changes in the electrode layer design, material selection in the electrode and cell, and optimization of numerous operation conditions.
First, the Cu surface restructuring factor was considered in the multi-layered GDE design. Carbon-containing layers were used on top of the catalyst layer to stabilize otherwise agglomerating Cu catalysts as the literature [3-4] hypothesized. Secondly, the GDE flooding factor was counteracted with multiple measures: 1) PTFE material was chosen as the gas diffusion layer due to its electrical insulator feature as well as its electrochemical inertness. 2) GDE layers were prepared with polymers binders consisting of hydrophobic backbones with hydrophilic functional groups, controlling the water uptake in the layers. 3) Coexistence of potassium, carbonate, and bicarbonate ions in the catalyst and gas diffusion layer was inhibited by the layer design in which the binders had specific permeance for cations or anions (i.e., ion exchange ionomers), undermining the potassium carbonate and potassium bicarbonate salt formation inside the catalyst and gas diffusion layers. 4) Temperature was optimized to prevent the salt formation in the gas chamber of the cell. Third, liquid product accumulation in the electrolyte was determined to be a destabilizing factor for the GDE and the membrane separator as a result of a prior experimental study [5]. Therefore, the liquid products of CO2RR were prevented from accumulating in the electrolyte by periodical electrolyte refurbishment.
After implementing all the identified countermeasures in Siemens Energy test rigs, we conducted a series of experiments focusing only on the system's stability. Internally designed 10 cm2 flow-cells, which had comparable cell architecture to the cell upscaled to 5000 cm2 scale in previous work [6], were employed combined with Cu-based GDEs as cathode and Ir-based anodes in this study. We have achieved 720 hours (one month) of stable CO2RR at 100 mA/cm2 at around 20% faradaic efficiencies for the aimed product, C2H4. Faradaic efficiency for all C2+ products was up to 50%. Increasing current densities from 100 mA/cm2 to 300 mA/cm2 compromised the stability while enhancing the C2+ selectivity.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101006701.

References

[1] Sisler et al., ACS Energy Lett. 2021, 6, 997.

[2] De Luna et al, Science 2019, 364.

[3] Dinh et al., Science, 2018, 360, 783

[4] Melchionna et al., Energy Environ. Sci., 2021,14, 5816-5833

[5] Sahin et al., [Conference lecture: Investigations on the effects of liquid product accumulation in
the electrolyte on CO2 electrolysis with CuO-based electrodes] Electrochemistry 2022 Conference (Berlin, Germany on 29.09.2022)

[6] Kopernikus project P2X: Exploration, Validation and Implementation of Power-to-X Concepts, Flyer published in August 2022, retrieved from: https://www.kopernikus-projekte.de/lw_resource/datapool/systemfiles/elements/files/EC51478D257959E3E0537E695E869833/live/document/P2X-Flyer_Results_Phase_II.pdf

Figure 1