The production of hydrogen by water splitting via electrolysis relies on two reactions—the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). OER and HER suffer from sluggish reactions kinetics due to high overpotentials (a measure of kinetic energy barriers). Thus, each requires highly effective catalysts—that generally use noble metals—to minimize the overpotentials for OER qnd HER towards efficient hydrogen and oxygen production.

The reliance on such metals for catalysts results in unfavorable costs, and thus has spurred wide-spread investigation into the use of non-noble-metal-based electrocatalysts.

Now, a team from RIKEN in Japan and the Dalian Institute of Chemical Physics in China investigating OER catalysts now reports that incorporating Mn into the spinel lattice of Co₃O₄ can extend the catalyst lifetime in acid by two orders of magnitude while maintaining the activity. A paper on their work is published in Nature Catalysis.

The activation barrier of the obtained spinel Co₂MnO₄ is comparable to that of state-of-the-art iridium oxides, most probably due to the ideal binding energies of the oxygen evolution reaction intermediates, as shown using density functional theory calculations. The calculations also show that the thermodynamic landscape of Co₂MnO₄ suppresses dissolution, which results in a lifetime of over 2 months (1,500 hours) at 200 mA cm⁻²_geo at pH 1.

As the lifetimes of other 3d metal oxygen evolution catalysts are in the order of days and weeks, despite current densities being lower by an order of magnitude, our results are an important step towards the realization of noble-metal-free water electrolysers.

—Li et al.

(Left) The mixed cobalt manganese oxide, Co₂MnO₄. (Right) a frame from a video showing hydrogen being produced through electrolysis at the current density of 1000 milliamperes per square centimeter.

In their search for a better catalyst, the researchers looked at mixed cobalt and manganese oxides. Cobalt oxides can be active for the required reaction, but corrode very quickly in the acidic environment. Manganese oxides are more stable, but are not nearly active enough. By combining them, the researchers hoped to take advantage of their complimentary properties. They also had to consider the high current density needed for practical application outside the laboratory.

For industrial scale hydrogen production, we needed to set our study’s target current density to about 10 to 100 times higher than what has been used in past experiments. The high currents led to a number of problems such as physical decomposition of the catalyst.

—co-first author Shuang Kong

Eventually, the team overcame these issues by trial and error, and discovered an active and stable catalyst by inserting manganese into the spinel lattice of Co₃O₄, producing the mixed cobalt manganese oxide Co₂MnO₄.

We have achieved what has eluded scientists for decades. Hydrogen production using a highly active and stable catalyst made from abundant metals. In the long run, we believe that this is a huge step towards creating a sustainable hydrogen economy. Like other renewable technologies such as solar cells and wind power, we expect the cost of green hydrogen technology to plummet in the near future as more advances are made.

—co-first author Ailong Li

The next step in lab will be to find ways to extend the lifetime of the new catalyst and increase its activity levels even more.

Resources

• Li, A., Kong, S., Guo, C. et al. (2022) “Enhancing the stability of cobalt spinel oxide towards sustainable oxygen evolution in acid.” Nat Catal doi: 10.1038/s41929-021-00732-9

• Wang, S., Lu, A. & Zhong, CJ. (2021) “Hydrogen production from water electrolysis: role of catalysts.” Nano Convergence 8, 4 doi: 10.1186/s40580-021-00254-x (Open Access)