Electrodes for cell cycling tests are carbon papers (SGL 39AA) baked at 400 °C overnight. Center chambers for three-chamber cells were made by PDMS molding or 3-D printing (see Supporting information: Three-chamber two-membrane cell setup characterization and optimization). The center chamber thickness varies from 0.2 mm to centimeters. Several porous plastic spacers were put in the center chamber window to physically avoid AEM CEM touching each other. For this paper, we were using a 3-D printed center chamber with curved in membrane to achieve even flow with 0.2 mm thickness for cell cycling tests, if not further specified.