A team of researchers led by Zhifeng Ren, director of the Texas Center for Superconductivity at the University of Houston, has developed an oxygen-evolving catalyst that takes just minutes to grow at room temperature on commercially available nickel foam.

Paired with a previously reported hydrogen evolution reaction catalyst, it can achieve industrially required current density for overall seawater splitting at low voltage. The work is described in a paper published in the RSC journal Energy & Environmental Science.

Developing energy- and time-saving methods to synthesize active and stable oxygen evolving catalysts is of great significance to hydrogen production from water electrolysis, which however remains a grand challenge. Here we report a one-step approach to grow highly porous S-doped Ni/Fe (oxy)hydroxide catalysts on Ni foam in several minutes under room temperature.

This ultrafast method effectively engineers the surface of Ni foam into a rough S-doped Ni/Fe (oxy)hydroxide layer, which has multiple levels of porosity and good hydrophilic features and exhibits extraordinary oxygen evolution reaction (OER) performance in both alkaline salty water and seawater electrolytes. Specifically, the S-doped Ni/Fe (oxy)hydroxide catalyst requires low overpotentials of 300 and 398 mV to deliver current densities of 100 and 500 mA cm⁻², respectively, when directly used as an OER catalyst in alkaline natural seawater electrolyte.

Using this OER catalyst together with an efficient hydrogen evolution reaction catalyst, we have achieved the commercially demanded current densities of 500 and 1000 mA cm⁻² at low voltages of 1.837 and 1.951 V, respectively, for overall alkaline seawater electrolysis at room temperature with very good durability. This work affords a cost-efficient surface engineering method to steer commercial Ni foam into robust OER catalysts for seawater electrolysis, which has important implications for both the hydrogen economy and environmental remediation.

—Yu et al.

Seawater makes up about 96% of all water on earth, making it a tempting resource to meet the world’s growing need for clean drinking water and carbon-free energy. It is already technically possible both to desalinate seawater and split it to produce hydrogen. However, existing methods require multiple steps performed at high temperatures over a lengthy period of time in order to produce a catalyst—such as NiFe-based (oxy)hydroxide—with the needed efficiency. That requires substantial amounts of energy and drives up the cost.

Zhifeng Ren, corresponding author for the paper, said speedy, low-cost production is critical to commercialization.

Any discovery, any technology development, no matter how good it is, the end cost is going to play the most important role. If the cost is prohibitive, it will not make it to market. In this paper, we found a way to reduce the cost so commercialization will be easier and more acceptable to customers.

—Zhifeng Ren

Ren’s research group and others have previously reported a nickel-iron-(oxy)hydroxide compound as a catalyst to split seawater, but producing the material required a lengthy process conducted at temperatures between 300 - 600 ˚C. The high energy cost made it impractical for commercial use, and the high temperatures degraded the structural and mechanical integrity of the nickel foam, making long-term stability a concern, said Ren.

To address both cost and stability, the researchers discovered a process to use nickel-iron-(oxy)hydroxide on nickel foam, doped with a small amount of sulfur to produce an effective catalyst at room temperature within five minutes. Working at room temperature both reduced the cost and improved mechanical stability, they said.

Ren said one key to the researchers’ approach was the decision to use a chemical reaction to produce the desired material, rather than the energy-consuming traditional focus on a physical transformation.

Resources

• Luo Yu, Libo Wu, Brian McElhenny, Shaowei Song, Dan Luo, Fanghao Zhang, Ying Yu, Shuo Chen and Zhifeng Ren (2020) “Ultrafast room-temperature synthesis of porous S-doped Ni/Fe (oxy)hydroxide electrodes for oxygen evolution catalysis in seawater splitting” Energy Environ. Sci., 13, 3439-3446 doi: 10.1039/D0EE00921K