This electrolyzer is a research-grade electrochemical setup designed for advanced electrocatalysis studies under controlled pressure, inspired by the NREL High-Pressure Liquid Testing (HPLT) electrolyzer concept and extended to enable true three-electrode measurements on both the anode and cathode sides. The detailed specification of the electrolyzer developed by National Renewable Energy Laboratory (NREL) can be found on its product page. On the present page, however, there is a description of the extension of functionality to internally pressure-balanced reference electrodes. Unlike conventional two-electrode electrolyzer test cells, this system integrates two independent reference electrodes (REs), one dedicated to the anode and one to the cathode, allowing accurate, potential-resolved analysis of electrode kinetics, overpotentials, and degradation mechanisms. Each reference electrode is a classical glass-bodied electrode with a ceramic frit, mounted in a pressure-rated PEEK cup. The reference electrodes are equipped with a pressure vent, ensuring that the internal pressure of the reference electrode equilibrates with the operating pressure of the electrolyzer. This design eliminates pressure-induced drift and mechanical stress on the reference electrode junction.

The reference electrodes are housed in dedicated PEEK chambers that are installed directly into precision-drilled ports in the flow fields. An ionic connection between the flow field and the reference electrode chamber is established via a Polyethylene (PE) frit, creating a double-junction reference electrode configuration: 1st junction between the flow field and the PE frit; 2nd junction between the electrolyte-filled PEEK bottom compartment and the RE's ceramic frit. This arrangement significantly reduces contamination of the reference electrode, improves long-term stability, and allows operation in aggressive electrochemical environments. Electrical contact to the reference electrodes is implemented in a manner that fully preserves the pressure integrity of the PEEK chamber. No direct pressure leaks or mechanically stressed feedthroughs are introduced, making the system suitable for high-pressure electrolysis experiments. The setup is compatible with both PEM (Proton Exchange Membrane) and AEM (Anion Exchange Membrane) electrolyzers.

Application note The pressure vent is made by installing a thin steel tube (such as those used in medical needles) into one of the openings intended for filling the electrode. This requires the electrode to be a refillable version. Depending on the membrane chemistry and electrolyte environment, different reference electrodes can be used. Our product range includes 30 mm-long Ag/AgCl electrodes for PEM and 30 mm-long mercury-based electrodes (Hg/HgO and Hg/Hg₂Cl₂) for AEM, which, after modification, can be used in this setup. The modification consists of drilling a hole at the top of the glass body. Before shipping the order, we will ask you which reference electrodes are optimal for your experiment. The PEEK RE adaper (bottom compartment must be filled to two-thirds of its height with an appropriate electrolyte to: a) establish a stable ionic connection between the reference electrode and the working electrode (anode or cathode), b) protect the reference electrode from direct exposure to the bulk electrolyte, and c) enable operation with lower electrolyte concentrations, which is particularly critical for AEM systems. If a reference electrode is not used in the experiment, the bottom electrolyte compartment of the adapter must be fully filled. The PEEK reference electrode adapters are fully modular and easily disassembled, allowing rapid replacement of reference electrodes without disturbing the main electrolyzer assembly. This design supports routine maintenance, electrolyte exchange, and experimental flexibility. The main applications of the electrolyzer with internally pressure-balanced REs are: a) electrocatalyst activity and stability studies, b) accurate separation of anode and cathode overpotentials, c) high-pressure water electrolysis research, d) PEM and AEM electrolyzer development and e) mechanistic studies requiring true three-electrode control