Single Catalyst Can Split Water at Low Voltage

In recent years, hydrogen has gained significant attention as a potential clean energy source since it combusts without generating climate-damaging emissions. Nevertheless, conventional hydrogen production techniques have a considerable carbon footprint, and cleaner techniques are costly and technically complex.

Scientists are now reporting major progress, a two-electrode catalyst that depends on a single compound to efficiently generate hydrogen and oxygen from freshwater and seawater.

Earlier attempts at these bi-functional catalysts to separate water into hydrogen and oxygen have primarily resulted in an ineffective performance in one of the two functions. Using two individual catalysts is effective but escalates the manufacturing cost of the catalysts.

In a new study recently published in Energy & Environmental Science, scientists from the University of Houston (HU), Central China Normal University, and the Chinese University of Hong Kong have reported using a nickel/molybdenum/nitrogen compound, adjusted with a small quantity of iron and grown on nickel foam, to efficiently create hydrogen. Then, via a process of electrochemical reconstruction driven by cycling voltage, it is transformed into a compound that creates a similarly robust oxygen evolution reaction.

The scientists explained that employing a single compound for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) — albeit marginally altered through the reconstruction process — not only renders water splitting more economical but also streamlines the engineering issues.

Most materials are ideally matched for either OER or HER, but both reactions are necessary to realize the chemical reaction and generate hydrogen from water. Zhifeng Ren, director of the UH Texas Center for Superconductivity and the study’s corresponding author, stated that the new catalyst enables smooth operations with a single catalyst and functions equally well in freshwater and seawater.

Compared with existing catalysts, this is on par with the best ever reported.

Zhifeng Ren, Study Corresponding Author and Director, Texas Center for Superconductivity, UH

Utilizing alkaline seawater and working under quasi-industrial situations, the catalyst provided a current density of 1,000 milliamps/centimeter squared using merely 1.56 volts in seawater and remained stable during an 80-hour testing period.

The excellent performance of the catalyst in seawater could resolve a key issue: that most available catalysts function best in freshwater. Separating seawater is more complex, partly due to corrosion related to salt and other minerals.

Ren, who is also M.D. Anderson Chair Professor of Physics at UH, explained that the new catalyst also produces pure oxygen, preventing the creation of the possible byproduct of corrosive chlorine gas that is created by specific catalysts.

Freshwater supplies are increasingly limited by population growth and drought; seawater, by contrast, is plentiful.

Normally, even if a catalyst works for salty water, it requires a higher energy consumption. In this case, requiring almost the same energy consumption as freshwater is very good news.

Zhifeng Ren, Study Corresponding Author and Director of Texas Center for Superconductivity, University of Houston

Shuo Chen, UH associate professor of physics and the study’s co-corresponding author, said that the catalyst’s reported strong current density at a comparatively low voltage decreases the energy cost of hydrogen production. This is only one way in which the catalyst resolves the issue of affordability, stated Chen, who is also a principal investigator with TcSUH.

By employing one material — the iron-tweaked nickel/molybdenum/nitrogen compound — for the HER and then using cycling voltage to stimulate an electrochemical reconstruction to create a slightly different material, an iron-oxide/molybdenum/nickel oxide, for the OER, scientists remove the necessity for a second catalyst while also streamlining engineering necessities, Chen said.

If you are making a device with two different materials on two electrodes, you have to figure out how the electric charge can flow through each electrode and design the structure to fit that. In this case, the material is not exactly the same, because one (electrode) undergoes electrochemical reconstruction, but it is a very similar material, so the engineering is easier.

Shuo Chen, Study Co-Corresponding Author and Associate Professor, Physics, University of Houston

Besides Ren and Chen, scientists involved in the study include Minghui Ning, Fanghao Zhang, Libo Wu, Xinxin Xing, Dezhi Wang, Shaowei Song, and Jiming Bao, all with UH; Luo Yu of the Chinese University of Hong Kong; and Qiancheng Zhou of Central China Normal University.

Journal Reference:

Ning, M., et al. (2022) Boosting efficient alkaline fresh water and seawater electrolysis via electrochemical reconstruction. Energy & Environmental Science. doi.org/10.1039/D2EE01094A.