Researchers at KTH Royal Institute of Technology (Stockholm, Sweden; www.kth.se) have developed a way to decouple the production of hydrogen and oxygen in water electrolysis, thereby reducing the explosion potential of mixing the two gases. The method, described in a recent issue of Science Advances, combines the electrocatalytic reactions of an electrolyzer with a capacitive-storage mechanism.
In a conventional alkaline electrolyzer, the cathode and anode are separated by an ion-permeable membrane. When an electric current is applied, water reacts at the cathode by forming H2 and OH– ions, which diffuse through the barrier to the anode to produce O2. But the barrier causes resistance, and if the electric charge fluctuates, the risk of an explosive mix between O2 and H2 is heightened.
To avoid this, one of the electrodes is replaced with a super capacitive electrode made from carbon. When the electrode is negatively charged and producing H2, the super capacitor stores energy-rich OH– ions. When the direction of current is reversed, the super capacitor releases the absorbed OH–, and O2 is produced at the now-positive electrode. “One electrode does the evolution of both oxygen and hydrogen,” explains Joydeep Dutta, professor of applied physics at KTH. “It’s a lot like a rechargeable battery producing H2 — alternately charging and discharging. It’s all about completing the circuit.”
According to the Science Advances article, an energy efficiency of 69% lower heating value (48 kWh/kg) at 10 mA/cm2 (5 cm × 5 cm cell) was achieved using a cobalt-iron phosphide bifunctional catalyst with 99% Faradaic efficiency at 100 mA/cm2. The researchers also report that laboratory tests showed no apparent electrode degradation as a result of long-term tests, which is important for commercial applications.
Dutta and Esteban Toledo, a doctoral student at KTH who co-authored the paper, have patented the system. With support from KTH Innovation, a company, Caplyzer AB (Stockholm, Sweden; www.caplyzer.com), was already formed in 2021 to scale up the technology.