In a recent Nature Communications paper, a group of researchers led by Dr. Ning Yan of the Van ‘t Hoff Institute for Molecular Sciences at the University of Amsterdam presented a new type of membrane-free electrolyzer that can split water into hydrogen and oxygen at high current density using only earth-abundant catalysts. Their new electrolyzer concept, developed together with researchers from Wuhan University and Wuhan University of Technology, offers advantages over electrolyzers that are currently being developed for large-scale hydrogen production.

 

 

While electrolyzers have been known for over 200 years, the technology is still facing major challenges. For instance, the conventional alkaline electrolysis is more suitable to operate at low current density and low pressure, while the emerging proton exchange membrane (PEM) electrolyzer requires the use of scarce noble metal catalysts and extensive water purification.

The new electrolyzer comprises of two identical and separate compartments with a sandwich-like architecture. Through this sandwich flow two solutions: a hydrogen-rich catholyte and an oxygen-rich anolyte. During operation, the anolyte and catholyte cycle back and forth so that the roles of each compartment are continuously reversed. As a result, the novel electrolyzer delivers hydrogen gas of over 99% purity.

"The closely packed sandwich structure results in a short travel distance of ions, making the ohmic resistance of our membrane-free cell comparable with that of a PEM electrolyzer," said Yan. "Together with the separation of the two reaction chambers, this opens opportunities for the cell to work at high current densities that are comparable with those of PEMs. Moreover, our electrolyzer design is very robust, and works equally well both in deionized water and in regular tap water."

Cyclic operation

To enable continuous performance, the electrolyzer is operated in a cyclic way where the electrode catalyst is bifunctionally active. Tests have revealed that it performs equally well in both the water reduction reaction and water oxidation reaction. An important advantage here is that no noble metals are needed. Instead, the cell uses a modified version of the nitrogen-doped catalysts that have been developed earlier by Yan and Prof. Gadi Rothenberg for fuel cell and supercapacitor applications. These highly porous and structured materials have now been used by PhD student Jasper Biemolt as supports for iron-cobalt alloys and their phosphide derivatives.

Rothenberg explains that using earth-abundant materials holds the key to real-life applications: "To compete effectively in the market, the cost of green hydrogen should be below 2 euro per kilogram. This means that commercial large-scale production of hydrogen needs to find alternative solutions. By designing electrolyzers with new configurations and using catalysts based on abundant elements, we create the possibility for real-life implementation".

Yan and Rothenberg are aware that scaling up this cell technology requires much more future work. The joint collaboration will continue to tackle various fundamental and application questions such as a techno-economic analysis and the dynamic behavior of the working and auxiliary electrodes in the tap water electrolyte.