Ink Formulation, Binder, Catalyst & Metal Loading - How Can the Properties of Gas Diffusion Electrodes be Influenced to Control CO2 Reduction?

Abstract

Exceptionally hot summers, floods and other extreme weather events of recent years underline the importance of combating climate change. Electrochemical CO2 reduction (CO2RR), which can be applied directly at CO2 point sources such as cement plants is a promising way to convert the harmful greenhouse gas into valuable compounds (e.g., alcohols). The integration of a selective catalyst into a gas diffusion electrode (GDE) allows CO2 reduction at industrially relevant current densities above 200 mA cm-2.[1] Combinations of copper and zinc, or copper and silver, are especially suitable for CO2 reduction to multi-carbon products. Whilst copper is the only metal capable of catalyzing C-C bond formation, zinc and silver are known for their CO selectivity in CO2RR. The selectivity of the reaction is shifted towards the formation of multi-carbon products such as ethylene or alcohols by the enrichment of the important intermediate CO at the catalyst surface due to the activity of Zn or Ag.[2] Here we present the preparation of bimetallic catalysts by impregnating carbon black with copper and zinc, followed by the incorporation of the catalysts into GDEs and testing of their electrocatalytic activity in CO2RR. The ink formulation for preparing a catalyst layer by spray-coating was optimized using, among other compounds, the pore-forming agent methylcellulose, the non-ionic surfactant Triton, and PTFE as binding material, to provide a stable CO2RR with the highest possible yield of C2+ products. In addition, the impact of the post-treatment steps pressing and sintering on the CO2RR performance of the electrodes was assessed.[3] Using the optimized formulation and post-treatment to produce the GDE, the influence on the product distribution was evaluated by varying (1) the PTFE powder (2) the metal loading of the catalyst (Cu and Zn in relation to the carbon black content), (3) the catalyst loading of the GDE, and (4) the current densities during CO2RR.PTFE acts as both a binder and a hydrophobizing agent of the electrode to ensure stable electrolysis on the produced GDE in a flow cell. The variation of PTFE powders shows that mechanical and electrochemical stability of the electrodes as well as the product distribution during CO2RR depend on the PTFE powder used. Although hydrophobicity and surface morphology are similar prior to CO2RR, shifts within the product distributions and differences in stability are evident during electrolysis depending on the PTFE powder chosen. Catalyst loading affects the supply and removal of reactants and products and therefore significantly influences the product distribution, which also showed a strong dependence on metal loading. In general, a trend toward increased formic acid formation was observed with increasing metal loading and film thickness. An optimum for C2+ product formation was found at 17.5% metal loading with Faraday efficiencies of 17% for ethanol and 31% for ethylene at current densities of 200 mA cm-2. SEM-EDX images show increased carbonate formation within the electrode with increasing metal loading as well as with increasing current densities. The increase in metal loading was found to be accompanied by metal agglomeration in the catalyst and is further enhanced by the GDE fabrication process.The work provides insight into the interaction of catalyst, electrode composition, and CO2RR parameters and reveals key tuning screws for a selective and stable reaction in liquid electrolysis cells. [1] Gawel, A.; Jaster, T.; Siegmund, D.; Holzmann, J.; Lohmann, H.; Klemm, E.; Apfel, U.-P. Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects. iScience 2022, 25, 104011. [2] Jaster, T.; Gawel, A.; Siegmund, D.; Holzmann, J.; Lohmann, H.; Klemm, E.; Apfel, U.-P. Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions. iScience 2022, 25, 104010. [3] Jaster, T.; Albers, S.; Leonhard, A.; Kräenbring, M.-A.; Lohmann, H.; Zeidler-Fandrich, B.; Özcan, F.; Segets, D.; Apfel, U.-P. Enhancement of CO2RR product formation on Cu-ZnO-based electrodes by varying ink formulation and post treatment methods. J. Phys. Energy. 2023, 5.