Impact of the PiperION Anion Exchange Membrane Thickness on the Performance of a CO₂‑to-HCOOH Three-Compartment Electrolyzer

The electrochemical reduction of CO₂ to formic acid (HCOOH) is a sustainable synthetic approach with the potential to substitute for energy-demanding conventional processes. In this framework, the three-compartment electrolyzer presents a crucial technological advancement, facilitating the direct production of diluted HCOOH in the center compartment, which is separated from the anode and cathode by cation and anion exchange membranes (CEM and AEM), respectively. However, the impact of the AEM on both selectivity and energy consumption in the three-compartment electrolyzer remains largely unexplored. Herein, the use of PiperION AEMs, investigated under different thicknesses (13–80 μm), current densities (200–500 mA cm^–2), and center compartment flow rates (50–200 μL min^–1), confirms that the AEM acts as a barrier between the acidic center and the alkaline cathodic compartment. Thicker AEMs provide the optimal alkaline media in the cathode manifested by enhanced catalytic efficiency and selectivity (FE_FA up to 84%). The thinnest membrane (13 μm) yields diminished performance in terms of the faradaic efficiency of HCOOH, whereas the thickest membrane (80 μm) shows high cell voltages and limiting applicable current densities. However, medium thick membranes (22 and 35) present high faradaic efficiencies of HCOOH (FE_FA = 76%) with low specific energy consumptions (Q_FA = 5.9 kWh kg^–1) and increased HCOOH concentrations (c = 2.3 mol L^–1), given their enhanced shielding effects while maintaining moderate cell voltages (U = 3.8 V).