Upon engaging the CE fixture to establish a sealed cell, the final mechanical preparation of the cell involves compression of the GDL interface piston to a force of ca. 100 N using a linear actuator (Firgelli Automation). This piston resides within the GH fixture, where a gasket creates an air-tight seal around its perimeter while providing flexibility in its vertical position so that in the "engaged" (raised) position, the piston presses the catalyst layer atop the GDE against the working electrode contact ring. This positioning also establishes a gas flow path from the perimeter of the piston, laterally through the GDL, to the center bore of the piston. A photograph of the AutoGDE system and the fixtures in engaged and disengaged states are shown in Fig. .