The Impact of Relative Humidity on Electrode Ionomer Deposition for Hydroxide Exchange Membrane Electrolyzers
Authors: Polsky Abigayle 
Published: December 2024

Hydrogen is a multifaceted tool that is prospected to be a key alternative energy source in the fight against climate change as it is energy dense, can be used directly as a fuel source in hydrogen fuel cells, and its storage & energy capacity are seasonally/temperature independent. Grey hydrogen is the predominant H2 manufacturing process—producing over 10x more CO2, eq than H2. Only about 5% of H2 is made by green hydrogen, or electrolysis, which produces up to 10x less CO2,eq than its counterpart1. The Hydroxide Exchange Membrane Electrolyzer (HEMEL) is a contemporary green hydrogen device. Other green hydrogen devices exist, such as the alkaline electrolyzer—which is inefficient due to its high resistance liquid electrolyte—and the proton exchange membrane electrolyzer—which is costly and requires the use of platinum group metal catalysts. The HEMEL uses a highly efficient solid electrolyte but does not depend on the use of rare metal catalysts, instead allowing abundant metals, such as nickel, to be utilized.
HEMELs function with a coupled set of reactions—the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)—that, together, produce hydrogen and oxygen gas through electrolysis. At the cathode, H2O splits at HER active sites to form H2 and OH-. The H2 bubbles out of the system and is collected. The OH- is shuttled across an anion-selective membrane made of poly(aryl piperidinium) ionomer. After crossing the membrane, the OH- meets FeNiOOH-F active sites on the OER anode side to satisfy the full-cell reaction.
Industrial-scale manufacturing is the ultimate goal of HEMELs. Since the technology is maturing, production research must be closely studied to prepare for the transition from lab-scale research to large-scale processing. This study will focus on the ionomer deposition of the anode and how the relative humidity (RH) of that environment impacts HEMEL performance. The catalyst-covered anode is dip-coated in an ionomer solution and let to dry at room temperature and varying RHs, between 9% and 92%. Nickel felt and nickel foam anode porous transport layers are tested as well as three ionomer types. Through full-cell polarization curves, electrochemical impedance spectroscopy, and scanning electron microscopy, RH at the time of anode-ionomer dip-coating is found to be a substantial factor in HEMEL performance. Up to a 30% increase in HEMEL performance could be attributed to lowering the RH of the environment that the anode is prepared in. Through analysis, it is suggested that the higher RH environments do not allow the ionomer to fully dry and form a strong, interlocked resin film. Thus, when the ionomer comes into contact with water flowing through the HEMEL, it swells substantially and obstructs the substrate’s pores. In turn, this hinders gas, water, and hydroxide transport—leading to high mass transport overpotentials.
[1] Decarbonized Hydrogen in the US Power and Industrial Sectors: Identifying and Incentivizing Opportunities to Lower Emissions. Resources for the Future https://www.rff.org/publications/reports/decarbonizing-hydrogen-us-power-and-industrial-sectors/ (2020).

Figure 1