Researchers at the Technion, Israel Institute of Technology, have developed a membrane‑free system that enables the simultaneous production of hydrogen and epoxides, which appear as valuable chemical products. At a current density of 100 mA/cm², the efficiency of hydrogen production reached 98%, while the efficiency of the second product was 95%. The researchers expect this breakthrough to improve the economics of “green” hydrogen by producing higher‑value chemicals instead of oxygen as a by‑product.
Currently, “green” hydrogen remains significantly more expensive than hydrogen from fossil fuels. According to data cited by the researchers, in 2024 its cost was between $3.9 and $9.2 per kg, compared to $1–$4.3 per kg for “grey” hydrogen, and it accounted for only 0.3% of global demand. One feature of conventional electrolysis is that hydrogen production is accompanied by oxygen, which is a relatively cheap by‑product that is often simply vented into the atmosphere. The researchers proposed replacing this reaction with the production of epoxides, whose global market is estimated at about $78 billion. These compounds, including ethylene and propylene oxides, are widely used in the manufacture of plastics, polyurethanes, antifreeze, and other chemical products.
Bromide is used as a mediator in the new system. At the anode, bromide ions are converted to bromine, which remains in solution and reacts with the organic feedstock. At the cathode, hydrogen is produced from water. Through several intermediate reactions, the starting organic compound is converted to the epoxide, while the bromide is regenerated and can be reused. The oxidation of bromide requires a lower potential: the standard electrode potential for this reaction is 1.09 V, compared to 1.23 V for oxygen formation and 1.36 V for chlorine production.
Another advantage of bromine is that under the experimental conditions it remains in solution, whereas chlorine, used in some analogous technologies, is produced in gaseous form. Chlorine therefore needs to be reliably separated from the simultaneously produced hydrogen, which is usually done using membranes. These complicate the electrolyzer design, require electrolyte purification, and degrade over time, especially in halogen‑containing environments. In the new system, the anode and cathode are in a single compartment without a separator membrane, and no gaseous bromine was detected in the produced hydrogen.
For the experiment, the researchers used styrene, which was converted to styrene oxide. The electrolyte consisted of equal parts water and acetonitrile with sodium bromide added. Acetonitrile helped dissolve the organic feedstock, which is poorly miscible with water, while also maintaining the bromine compounds in a form suitable for epoxide formation. Without it, the process efficiency dropped sharply: the styrene oxide yield fell from about 83% to 2%, and signs of degradation appeared on the anode surface.
At a current density of 100 mA/cm², the efficiency of generating hydrogen through electric current stood at 98%, and for the anodic reaction – 95%. The selectivity for styrene oxide formation reached 87%, meaning that the vast majority of the organic products obtained were the desired compound, with an overall yield of 83%. At lower loads, the performance was even higher: at 50 mA, the styrene oxide yield reached 89%, selectivity 91%, and faradaic efficiency 98%. The system maintained high performance even at current densities up to 500 mA/cm².
The cell voltage at a current density of 100 mA/cm² was about 2.3 V. For comparison, modern industrial PEM and alkaline electrolyzers typically operate at about 1.8–2 V, but at significantly higher current densities. In a ten‑hour test, the new system maintained nearly unchanged hydrogen production efficiency, and the voltage rose only from 2.43 to 2.54 V. However, the electrolyte was replaced every hour due to the small volume of the laboratory setup, so long‑term continuous operation remains to be verified.
According to the researchers’ calculations, when adapting the technology for the co‑production of hydrogen and ethylene oxide, about 66.5 kWh of electricity would be required per kg of H₂ – 25–33% more than in conventional water electrolyzers. However, this shortfall could be offset by the value of the second product. At the prices used in the study, the total value of the hydrogen and ethylene oxide produced is estimated at about $18.5 per kg of H₂, compared to about $2.4 for the output of conventional electrolysis, where the by‑product oxygen adds little value.
In the future, the system is planned to be adapted for ethylene and propylene representing high‑volume feedstocks for the production of ethylene and propylene oxides. To move toward industrial application, the researchers will need to develop flow reactors, improve gaseous feedstock delivery, reduce energy consumption, and test long‑term equipment performance. In addition, closed‑loop handling of bromine and acetonitrile will need to be organized, as both substances require reliable capture and control.
