Proton exchange membrane water electrolyzers, or PEMWE systems, split water into hydrogen and oxygen using an acidic polymer membrane sandwiched between two electrodes. They are prized for their high efficiency and rapid dynamic response, which makes them natural partners for intermittent renewable electricity. But achieving the target lifetimes of more than 80,000 hours remains a critical challenge. Despite intensive research into degradation, the transferability and comparability of insights across different systems remain limited, and the field has struggled to generate reproducible data that meaningfully compare one laboratory’s results with another’s.

The core difficulty is manufacturing variability. Even when two catalyst coated membranes nominally contain the same membrane material, differences in production—solvent content, catalyst layer structure, interfacial contact quality, and microscopic defects such as pinholes—can produce markedly different operating behavior. Because manufacturers rarely disclose their production processes, these influences are difficult to assess directly. Harmonized testing protocols and interlaboratory round-robin initiatives have tried to close the gap, but they cannot fully account for variations baked into nominally identical materials before a cell is ever assembled.

The Graz team, led by Özge Kiziltan with colleagues including Viktor Hacker and Merit Bodner, focused on a metric that is almost trivially easy to measure: the open circuit voltage, or OCV, recorded for 120 seconds immediately after the conditioning step. In fuel cells, OCV is a well-established indicator of membrane health, because a degraded, gas-permeable membrane produces mixed potentials at the electrodes that drag the measured voltage down. In water electrolyzers, by contrast, only a handful of studies have examined OCV behavior, even though deviations from the ideal Nernst potential are known to be linked mainly to gas crossover through the membrane, with secondary contributions from internal leakages, catalyst oxidation states, and hydration differences.

The researchers tested two commercially available catalyst coated membranes that both used the same Nafion 115 membrane material, one from Quintech and one from IonPower’s Hydrion line. Each sample underwent a 12-hour hydration period followed by electrochemical activation, after which the team recorded the 120-second OCV response, polarization curves, and electrochemical impedance spectra. The cells were 4 square centimeter single-cell fixtures operated at 80 degrees Celsius with ultrapure water circulated through the anode at 40 milliliters per minute. To probe robustness, the experiments were run in two independent configurations using different flow plates—one titanium, one platinized titanium—and two different activation protocols ending at either 2 or 3 amperes per square centimeter.

The results were striking. One group of membranes, designated CCM.A, exhibited a rapid decline in open circuit voltage, plunging to a minimum voltage of roughly 0.32 volts within ten seconds of current interruption. The other group, CCM.B, showed a far more gradual response, with the voltage continuing to decrease throughout the entire 120-second hold and stabilizing near 1.21 volts. In a direct comparison between two samples measured with identical flow plates and identical activation protocols, the difference in minimum voltage reached 0.357 volts—an enormous gap for devices built around the same membrane material. Notably, switching the flow plate material barely changed the OCV decay of CCM.A, with the coefficient of variation rising only from 4.9 to 5.1 percent, suggesting the effect is intrinsic to the membrane assembly rather than an artifact of the cell hardware.

The physics behind the signal is subtle. Open circuit conditions are not purely passive; capacitive relaxation, changes in electrode oxidation state, and gas crossover all overlap within the measurement window. When current stops flowing, the overpotentials built up during operation relax quickly, which explains an initial voltage drop. But fast relaxation alone cannot account for the extremely low minimum voltages observed for CCM.A. Instead, the researchers argue, parasitic electrochemical reactions driven by gas crossover—hydrogen oxidation or oxygen reduction at the opposing electrode—establish mixed potentials that collapse the measured cell voltage. Prior work cited in the study shows that even crossover fluxes as small as about one nanomole per square centimeter per second can shift electrode potentials by several millivolts per second under open circuit conditions, so tiny leaks can dominate the response.

Crucially, the OCV signature predicted real operational trouble. During a 100-hour constant current hold, the sample with the unstable OCV response showed a pronounced but fully reversible voltage transient: interruptions for characterization triggered a voltage increase peaking around six hours after returning to constant current, which then recovered over roughly twenty hours. The reversible nature and long duration of this effect point to crossover-induced mixed-potential phenomena rather than irreversible catalyst or membrane degradation. The membrane with the stable OCV response, operated at an even lower current density, showed no comparable reversible voltage loss at all. Reapplying current gradually restores the catalyst oxidation states, which explains why the elevated overpotentials fade with time.

The team also examined whether conventional membrane degradation metrics could explain the OCV differences, and here the answer was a clear no. Using combustion ion chromatography to measure total fluorine and ion-selective electrodes to measure fluoride ions in the effluent water, they found that only the more stable membrane group produced quantifiable concentrations, while samples from the rapidly decaying group fell below detection limits—likely because the large volume of circulating anode water dilutes emitted fluorine species below what instruments can detect. Total fluorine values exceeded fluoride values by about 9.9 percent on average, confirming the presence of non-ionic fluorinated fragments that conventional fluoride analysis misses. Most importantly, no clear correlation emerged between these degradation metrics and OCV behavior, underscoring that fluoride emission and total fluorine are poorly suited as universal early-stage indicators, particularly given detection limits and sampling dilution.

The study’s conclusions are deliberately two-sided. On one hand, the early OCV response proved repeatable within a controlled material set and offers a practical, observation-based screening tool: two minutes of measurement, requiring no specialized instrumentation, can flag membranes susceptible to transient instability before more obvious degradation indicators appear and before operational safety is compromised. On the other hand, the researchers caution that OCV behavior is not a global metric. It reflects the combined influence of gas crossover, electrode-state effects, and conditioning history—the final activation current density demonstrably shaped the response—and nominally identical materials from different manufacturers produced irreconcilable differences. The authors call for reproducibility-aware diagnostic frameworks, comprehensive reporting of material properties, and healthy skepticism when comparing metrics across systems based solely on nominal material equivalence. For a hydrogen economy racing toward durability targets measured in decades of operation, the message is that sometimes the most informative diagnostic is also the simplest one: watch what the voltage does when you let go.

Subject of Research: Open circuit voltage diagnostics for proton exchange membrane water electrolysis

Article Title: Initial open circuit voltage response as a practical diagnostic approach in proton exchange membrane water electrolysis

Article References: Kiziltan, Ö., Preihs, M. A., Raab, A., Feldmann, J., Hacker, V., & Bodner, M. (2026). Initial open circuit voltage response as a practical diagnostic approach in proton exchange membrane water electrolysis. Discover Electrochemistry, 3(1), Article 58. https://doi.org/10.1007/s44373-026-00147-w

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00147-w

Keywords: proton exchange membrane water electrolysis, open circuit voltage, green hydrogen, gas crossover, catalyst coated membrane, membrane degradation, fluoride emission, total fluorine, electrochemical diagnostics, Nafion 115, durability testing, mixed potential