Giner Electrochemical Systems (GES) proposes to develop a cathode liquid feed, proton-exchange membrane electrolyzer stack and system capable of producing 3,600 psi oxygen. In preparation for this Phase I effort, we propose to collaborate with Hamilton-Sundstrand Human Space Systems (H-S) to share unique state-of-the-art technologies that provide the best path to meeting program objectives. GES will share their data and expertise with high balanced pressure electrolyzers and H-S will contribute their data and expertise in high differential pressure electrolyzers. Based on this exchange, GES would modify its electrolyzer performance model. In a third task, GES will build two single cell electrolyzers with GES and H-S components. One stack will be dedicated to balanced pressure operation, while a second unit would be dedicated to high differential pressure (oxygen over hydrogen) operation. A full experimental matrix will be conducted on these units in a cathode liquid feed configuration. Balanced pressure operation would be conducted at GES facilities (from atmospheric to 2,000 psi). Differential pressure testing would be conducted in H-S facilities (at pressures between 2,000 and 3,600 psi). Data would then be integrated into the GES analysis code, and be available as a design analysis tool for future phases of the program.