One of the most abundant resources on earth that offers promise as a source of not only hydrogen (preferred as a clean energy source) but also drinking water in arid climates is seawater.
Although water-splitting technologies with the potential to produce hydrogen from freshwater have turned more effective, seawater has constantly posed challenges.
Scientists at the University of Houston have described a major advancement with a new oxygen evolution reaction catalyst that, when blended with a hydrogen evolution reaction catalyst, realized current densities adequate to support industrial requirements, while necessitating comparatively low voltage to initiate the electrolysis of seawater.
According to the researchers, the device, developed using low-cost non-noble metal nitrides, has the ability to overcome a number of obstacles that have hampered earlier efforts to synthesize hydrogen or safe drinking water from seawater at low cost. The study has been reported in Nature Communications.
Zhifeng Ren, director of the Texas Center for Superconductivity at UH and a corresponding author for the paper, stated that a major challenge has been the lack of a catalyst with the potential to effectively split seawater to synthesize hydrogen without also letting loose ions of chlorine, sodium, calcium, and other components of seawater.
If these ions are released, they can settle on the catalyst and make it inactive. Specifically problematic are the chlorine ions, partially because chlorine needs only slightly higher voltage to free than that required for free hydrogen.
The scientists performed tests of the catalysts on seawater obtained from Galveston Bay off the Texas coast. According to Ren, M.D. Anderson Chair Professor of physics at UH, it would also work with wastewater, offering another source to extract hydrogen from water that is otherwise unusable without expensive treatment.
Most people use clean freshwater to produce hydrogen by water splitting. But the availability of clean freshwater is limited.
Zhifeng Ren, M.D. Anderson Chair Professor of Physics, University of Houston
In order to overcome the obstacles, the scientists used transition metal-nitride to formulate and produce a 3D core-shell oxygen evolution reaction catalyst, with nanoparticles formed of a nickel-iron-nitride compound and nickel-molybdenum-nitride nanorods on porous nickel foam.
According to Luo Yu, the first author of the paper, who is a postdoctoral researcher at UH and also affiliated with Central China Normal University, the new oxygen evolution reaction catalyst was combined with a formerly described hydrogen evolution reaction catalyst of nickel-molybdenum-nitride nanorods.
The catalysts were incorporated into a two-electrode alkaline electrolyzer, which can be activated by waste heat by an AA battery or through a thermoelectric device.
Cell voltages needed to generate a current density of 100 milliamperes per square centimeter (a measure of current density, or mA cm−2) ranged from 1.564 to 1.581 V.
Yu stated that the voltage is considerable because while at least 1.23 V is needed to synthesize hydrogen, chlorine is synthesized at a voltage of 1.73 V. This implies that the device had to have the potential to generate useful levels of current density with a voltage between the two levels.
Apart from Ren and Yu, the other researchers on the paper are Qing Zhu, Shaowei Song, Brian McElhennyy, Dezhi Wang, Chunzheng Wu, Zhaojun Qin, Jiming Bao, and Shuo Chen, all of UH; and Ying Yu of Central China Normal University.