Researchers at the University of Houston (UH), with colleagues from Central China Normal University and the Chinese University of Hong Kong, Hong Kong SAR, have developed a two-electrode catalyst that relies on one compound to produce hydrogen and oxygen efficiently from both seawater and freshwater. A paper on their work is published in the RSC journal Energy & Environmental Science.
Previous attempts at such bi-functional catalysts to split water into hydrogen and oxygen have generally resulted in poor performance in one of the two functions. Using two separate catalysts works but increases the catalysts’ manufacturing cost.
The researchers used a nickel/molybdenum/nitrogen compound, tweaked with a small amount of iron and grown on nickel foam to produce hydrogen efficiently and then, through a process of electrochemical reconstruction sparked by cycling voltage, converted to a compound that produced a similarly powerful oxygen evolution reaction.
Electrochemical reconstruction is a powerful tool for generating highly active oxygen evolution reaction (OER) catalysts. Utilizing electrochemical reconstruction to fabricate an OER active catalyst based on a hydrogen evolution reaction (HER) catalyst enables the development of a bifunctional catalyst that possesses state-of-the-art HER and OER activity simultaneously.
Here we successfully synthesized Fe-doped Ni&Ni0.2Mo0.8N (Fe0.01-Ni&Ni0.2Mo0.8N) on Ni foam and, after rapid electrochemical reconstruction, the Fe-doped Ni&Ni0.2Mo0.8N compound was reconstructed into NiO co-doped with Fe and Mo (Fe0.01&Mo-NiO). The Fe0.01-Ni&Ni0.2Mo0.8N and Fe0.01&Mo-NiO compounds were found to exhibit state-of-the-art HER and OER performance, respectively.
Alkaline fresh water/seawater electrolysis was then systematically studied using the two-electrode electrolyzer Fe0.01-Ni&Ni0.2Mo0.8N‖Fe0.01&Mo-NiO. Due to the excellent seawater HER and OER activity of the individual catalysts, the electrolyzer exhibited record-high performance for seawater electrolysis, achieving a current density of 688 mA cm−2 at 1.7 V. Further experiments under quasi-industrial conditions (6 M KOH & seawater, 60 °C) showed that the electrolyzer delivers a current density of 1000 mA cm−2 at the extremely low voltage of 1.562 V, which is only 1.49% higher than that required for fresh water under the same conditions.
Stability testing of the electrolyzer showed that it exhibits good durability over 80 h under the harsh industrial conditions. Therefore, the findings in this research promote the development of bifunctional catalysts and address the small performance differences between alkaline fresh water and seawater electrolysis under industrial conditions.—Ning et al.
The researchers said using a single compound for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER)—albeit slightly changed through the reconstruction process—not only makes water splitting more affordable, it also simplifies the engineering challenges.
Most materials are best suited for either HER or OER, but both reactions are required to complete the chemical reaction and produce hydrogen from water. Zhifeng Ren, director of the Texas Center for Superconductivity at UH and a corresponding author for the paper, said the new catalyst not only allows for efficient operations with a single catalyst but also works equally well in seawater and freshwater. This is on par with the best existing catalysts ever reported, he said.
Using alkaline seawater and operating under quasi-industrial conditions, the catalyst delivered a current density of 1,000 milliamps/centimeter squared using just 1.56 volts in seawater, remaining stable for 80 hours of testing.
The catalyst’s strong performance in seawater could solve a problem: most available catalysts work best in freshwater. Splitting seawater is more complicated, in part because of corrosion associated with the salt and other minerals. Ren, who is also M.D. Anderson Chair Professor of Physics at UH, said the new catalyst also generates pure oxygen, avoiding the potential byproduct of corrosive chlorine gas produced by some catalysts.
Supplies of freshwater are increasingly limited by drought and population growth. Seawater, in contrast, is abundant.
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
Shuo Chen, associate professor of physics at UH and co-corresponding author on the paper, said the catalyst’s reported strong current density at a relatively low voltage lowers the energy cost of producing hydrogen. But that’s just one way the catalyst addresses affordability, said Chen, who is also a principal investigator with TcSUH Texas Center for Superconductivity at the University of Houston).
By using one material—the iron-tweaked nickel/molybdenum/nitrogen compound—for the HER and then using cycling voltage to trigger an electrochemical reconstruction to produce a slightly different material, an iron-oxide/molybdenum/nickel oxide, for the OER, researchers eliminate the need for a second catalyst while also simplifying engineering requirements, 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
In addition to Ren and Chen, researchers on the paper include Minghui Ning, Fanghao Zhang, Libo Wu, Xinxin Xing, Dezhi Wang, Shaowei Song and Jiming Bao, all with UH; Qiancheng Zhou of Central China Normal University; and Luo Yu of the Chinese University of Hong Kong.
Minghui Ning, Fanghao Zhang, Libo Wu, Xinxin Xing, Dezhi Wang, Shaowei Song, Qiancheng Zhou, Luo Yu, Jiming Bao, Shuo Chen and Zhifeng Ren (2022) “Boosting efficient alkaline fresh water and seawater electrolysis via electrochemical reconstruction” Energy Environ. Sci. doi: 10.1039/D2EE01094A