A two-sided electrocatalyst developed at Rice University splits water into hydrogen on one side and oxygen on the other. The hydrogen side seen in electron microscope images features platinum particles (the dark dots at right) evenly dispersed in laser-induced graphene (left). Courtesy of the Tour Group. Click to enlarge.

By serial connecting of the electrodes with a power source in an O-ring setup, H₂ and O₂ are simultaneously generated on either side of the plastic sheet at a current density of 10 mA/cm² at 1.66 V and can thereby be selectively captured.

The team said that said their demonstration provides a promising route to simple, efficient and complete water splitting, and that the inexpensive material may be a practical component in generating the hydrogen for use in future fuel cells.

… the sluggish kinetics of hydrogen and oxygen evolution reactions (HER and OER, respectively) in water splitting requires catalysts to increase the efficiencies. Apart from the noble metal materials such as Pt, RuO₂ and IrO₂, numerous noble-metal-free catalysts have been developed. For example, transition-metal carbides, phosphides and sulfides have shown favorable performance in HER. The oxides/hydroxides of cobalt, nickel, manganese and iron also have proven to be efficient catalysts for OER. However, for the scalable production of H₂ and O₂, the catalytic electrodes should be easily fabricated to minimize their overall cost. It is therefore desirable to couple the HER and OER processes into a single simple package.

Laser-induced graphene (LIG) is a 3D porous graphene material fused to a flexible substrate that is prepared by a one-step laser scribing process on commercial polyimide … While LIG has been used for direct patterning applications in microsupercapacitors and related devices, in this study we exploit the LIG process to fabricate efficient catalytic electrodes for splitting water to H₂ and O₂ starting from a single sheet of plastic.

—Zhang et al.

LIG is produced by treating the surface of a sheet of polyimide, an inexpensive plastic, with a laser. Rather than a flat sheet of hexagonal carbon atoms, LIG is a foam of graphene sheets with one edge attached to the underlying surface and chemically active edges exposed to the air.

Illustration of the integration of catalytic LIG electrodes as a full water electrolyzer. (a) Schematic drawing of an integrated LIG device. (b) LSV curve of the electrolyzer in 1 M KOH with 95% iR compensation. (c) A photograph of the electrolyzer working at 50 mA/cm2. Bubbles are the generated H2 (left side) and O2 (right side) as depicted. Credit: ACS, Zhang et al.Click to enlarge.

LIG itself is inert, so turning it into a water splitter involves a few more steps. First, the lab impregnated the side of the plastic destined to pull hydrogen from water with platinum particles; then the lab used a laser to heat the surface and make LIG. The Rice material uses only a quarter of the platinum found in commercial catalysts, said Jibo Zhang, a Rice graduate student and lead author of the paper.

The other side, for oxygen evolution, was first turned into LIG and then enhanced with nickel and iron through electrochemical deposition. Both sides showed low onset potentials (the voltage needed to start a reaction) and strong performance over 1,000 cycles.

The lab came up with another variation: making the polyimide into an LIG catalyst with cobalt and phosphorus that could replace either the platinum or nickel-iron sides to produce hydrogen or oxygen. While the low-cost material benefits by eliminating expensive noble metals, it sacrifices some efficiency in hydrogen generation, Tour said.

When configured with cobalt-phosphorus for hydrogen evolution and nickel-iron for oxygen, the catalyst delivered a current density of 10 milliamps per square centimeter at 1.66 volts. It could be increased to 400 milliamps per square centimeter at 1.9 volts without degrading the material. The current density governs the rate of the chemical reaction.

Tour said enhanced LIG offers water-splitting performance that’s comparable and often better than many current systems, with an advantage in its inherent separator between oxygen and hydrogen products. He noted it may find great value as a way to chemically store energy from remote solar or wind power plants that would otherwise be lost in transmission.

The material might also serve as the basis for efficient electrocatalysis platforms for carbon dioxide or oxygen reduction, he said.

Co-authors are graduate students Chenhao Zhang, Huilong Fei, Yilun Li and Junwei Sha. Sha is also a student at Tianjin University and the Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, China. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

The research was supported by the Air Force Office of Scientific Research, the National Science Foundation-funded and Rice-based Nanotechnology-Enabled Water Treatment Engineering Research Center and the Chinese Scholarship Council.

Resources

• Jibo Zhang, Chenhao Zhang, Junwei Sha, Huilong Fei, Yilun Li, and James M. Tour (2017) “Efficient Water Splitting Electrodes Based on Laser-Induced Graphene” ACS Applied Materials & Interfaces doi: 10.1021/acsami.7b06727