Organic Hydrogen Sensors for Potential Use in Safety-Critical Environments

Accurate monitoring of the hydrogen concentration is critical for optimizing fuel cell performance, minimizing purge losses, and reducing long-term degradation. Conventional hydrogen sensors often rely on catalytic materials and face limitations such as the need of oxygen purging when operated in fuel cell environments. Here, we report the discovery of a novel hydrogen-sensing mechanism based on organic molecules, without the use of catalytic metals. The sensor is based on a typical vertical stack geometry, containing $\mathrm{Alq_3}$ as active organic material. Upon exposure to hydrogen, the device shows an increase in resistivity, yielding a reliable sensor signal that varies linearly with hydrogen concentration, temperature, and humidity, and exhibits a relative response of up to 3.5 % at 100 %vol hydrogen. By exposing the sensor to an external magnetic field, the rise and fall times of the sensor response were found to be tunable. This novel organic sensor demonstrates sensitivity across a wide range of hydrogen concentrations under fuel cell-relevant conditions and beyond. This new class of hydrogen sensors with high miniaturization potential and cost efficiency paves the way for real-time hydrogen monitoring and advanced control strategies in fuel cells, the chemical industry, or energy storage applications.

💡 Research Summary

The authors present a novel hydrogen‑sensing platform that relies solely on organic materials, eliminating the need for catalytic metals or oxygen‑purging steps that limit conventional sensors in fuel‑cell environments. The device adopts a vertical stack architecture reminiscent of organic light‑emitting diodes, but with a micro‑structured top electrode (50 µm bar width) that directly exposes the active organic layer to the gas phase. The active layer consists of tris(8‑hydroxyquinolinato)aluminium (Alq₃), a small‑molecule semiconductor with a HOMO of ~5.48 eV and a LUMO of ~2.40 eV, sandwiched between a hole‑transport layer (PEDOT:PSS/MoO₃) and a thin LiF/Al electron‑injection electrode. The whole stack is encapsulated in a 0.16 mm epoxy resin that is permeable to hydrogen, providing protection against moisture and contaminants while allowing selective gas diffusion.

Electrical measurements were performed under a range of bias voltages (5–8 V). A bias of 7 V was identified as optimal, delivering a stable current‑voltage regime with a power consumption of ~28 mW. Upon exposure to hydrogen, the device exhibits an increase in resistance (decrease in current), which the authors quantify as a relative change ΔI/I₀. The response is linear over the full concentration range tested, from a few percent up to pure H₂ (100 % vol). At 100 % vol H₂ the relative change reaches 3.5 %, and the sensor retains linearity with respect to temperature (up to 80 °C) and relative humidity (0–100 % r.h.). This demonstrates that the sensor can operate reliably under the harsh, oxygen‑deficient conditions typical of closed‑loop fuel‑cell recirculation.

A key discovery is that the rise and fall times of the sensor can be tuned by applying an external magnetic field, suggesting that spin‑orbit interactions influence charge transport dynamics in the organic layer. This adds a new degree of freedom for tailoring sensor speed without altering the material stack.

To elucidate the sensing mechanism, the authors compare Alq₃‑based devices with those employing the metal‑free donor‑acceptor molecule 4CzIPN. Both materials respond to hydrogen, indicating that a metal centre in Alq₃ is not essential for detection. However, 4CzIPN devices show a larger initial signal but suffer rapid degradation, implying that molecular rigidity and intermolecular packing—properties more favorable in Alq₃—are critical for long‑term stability.

Further experiments dissect the contributions of individual layers. A “Stack PE” device containing only the PEDOT:PSS/MoO₃ hole‑transport layer exhibits a decrease in resistance upon hydrogen exposure, opposite to the increase observed in the full “Stack PE+Alq₃” device. Moreover, the Stack PE shows rise/fall times roughly five times longer than the full stack. These observations point to an interface‑controlled mechanism: hydrogen adsorption modifies the charge‑injection barrier at the Alq₃/electrode interface, leading to the observed resistance increase. Control measurements on the ITO, Al, and LiF electrodes reveal negligible hydrogen sensitivity, confirming that the bulk of the sensing response originates from the organic layers rather than the metallic contacts.

Cross‑sensitivity tests with ethanol, toluene, and ambient air show minimal response, underscoring the selectivity of the organic stack. The sensor also functions without relying on electroluminescence; even after the photoluminescent component decays, the electrical response to hydrogen persists, opening the door to non‑optical device configurations such as organic field‑effect transistors.

In summary, the work demonstrates that an Alq₃‑based organic stack can serve as a robust, linear, and tunable hydrogen sensor suitable for safety‑critical applications. Its advantages include low fabrication cost, compatibility with flexible substrates, miniaturization potential, and operation in pure hydrogen without oxygen regeneration. These attributes make it a promising candidate for integration into fuel‑cell systems, chemical‑process monitoring, and energy‑storage safety platforms, where real‑time hydrogen detection is essential.