Stacking of Flowcells for the CO2-to-CO Electrolysis

Abstract

The low-temperature CO2-to-CO electrolysis is one of the most mature CO2 electrolysis processes regarding technical and economical readiness. For this rather young technology Ag is established as a stable and highly selective catalyst. Research focus is therefore on increasing the process´ energy efficiency as well as the space-time yield. Promising results were reported for continuous single flowcells in lab scale size operating with aqueous electrolytes and gas diffusion electrodes (GDE) [1,2].This work specifically addresses the performance optimization of such flowcells under the condition of designing a stackable flowcell architecture. For two different flowcell concepts, short-stacks were designed and tested.Media flow compartments in a catholyte buffered GDE-flowcell operating with a proton exchange membrane were modified on two levels. The feed gas distribution over the GDE was optimized by introducing gas flowfields in the gas chamber, which supports the GDE from the far side of the anode and provides electrical contact. In the catholyte chamber, which separates the membrane from the cathode, an ionically conductive spacer was introduced. This spacer suppressed gas bubble-induced noise during cell voltage measurements and thus enabled a more stable process. The anode was mounted directly to the membrane (Zero-Gap) eliminating the second electrolyte gap (Figure1). In combination, these three modifications provided a supporting structure all the way from the anode flowfield to the cathode flowfield and thus force closure within the cell. Additionally these modifications ensured electric and ionic contact through the entire active cell area and provide mechanical support for the GDE and the membrane. By assembling and successfully testing a two-cell short-stack, it was demonstrated that the applied modifications yielded a stackable flowcell concept. [3]As catholyte buffered flowcells are limited in reaching high energy efficiencies (e.g. 35%) due to the ohmic resistance in the electrolyte gap, a flowcell containing a membrane electrode assembly (MEA) was designed applying the lessons learned previously and stackability as a design requirement. These flowcells operate with anion exchange membranes as polymer electrolytes rendering the catholyte buffer redundant. The electrodes can be mated directly on the membrane which minimized the electrode distance to the thickness of the membrane. The MEA flowcells, therefore, show low cell voltages and hence high energy efficiencies (e.g. 45%). Furthermore, the number of media flows that are to be managed within a single flowcell and hence in a stack is reduced. Aside from this simplification, anolytes with low ionic conductivity (e.g. 10mM KHCO3) can be used, reducing shunt currents within the stack and thus simplifying the electrolyte manifolds.A modular bipolar plate consisting of two plates was designed, which allows a free combination of different coated flowfields for the anode and cathode sides. For instance, the anode plates were coated with an Ir-MMO coating to enhance the OER appearing at the anode and thus to increase the single-cell performance. The bipolar plates were implemented in a three-cell short-stack containing MEA cells (Figure1). The results from electrochemical experiments with the short-stack showed that the results obtained with the single flowcell (200 mA/cm²; 3.2 V; FE(CO) >90%) could be adapted to the three-cell short-stack.