Forward-Bias 3D-Junction Bipolar Membranes for Electrochemical CO2 Reduction to CO

Abstract

The ongoing, rapid increase in atmospheric CO2 concentration has led to a growing interest in the electrochemical reduction of CO2 to value-added products like CO. To attain high current densities, the latter reaction is often performed using an anion exchange membrane(AEM) electrolyte that is well known to operate through the transport of (bi)carbonate ions from cathode to anode. This can in turn result in a CO2 pumping effect that decreases the device’s net CO2-consumption, and that can be prevented by using a bipolar membrane in a so-called forward-bias configuration (i.e., with the anion vs. cation exchange layers (AEL, CEL) contacting the cathode vs. anode electrodes, respectively). However, the recombination of protons and (bi)carbonate and concomitant production of H2O and CO2 at the AEL-CEL interface in this operative mode can also lead to the delamination of the membrane and cell failure.In order to prevent such a delamination, this study proposes the introduction of a three-dimensional (3D) AEL-CEL junction, utilizing Nafion® as the cation exchange layer and PiperION as the anion exchange one. Two different methods were applied to fabricate the 3D junction bipolar membrane. The first method involves the fabrication of a 3D interlocking layer using polystyrene (PS) beads that are sprayed (along with Nafion ionomer) onto a Nafion membrane, and removed using toluene. This results in an inverse-opal structure atop the Nafion membrane whose pores can be filled with anion exchange ionomer (AEI). Complementarily, the second method involves the introduction of a layer, consisting of a mixture of AEI and cation exchange ionomer (CEI) between the AEM and CEM. Additionally, a non-conductive oxide material is added to this ionomer layer to increase the contact area of AEI and CEI. Finally, the electrochemical CO2 reduction performance and durability of the resulting, 3D junction bipolar membranes are evaluated and compared to those of an equivalent, 2D junction bipolar membrane, in all cases in a forward bias configuration.