A two-dimensional nickel/iron metal–organic framework (2D NiFe-MOF), fabricated by researchers at the University of New South Wales in Sydney, Australia, has established a new record for efficient electrocatalytic water splitting. Published recently in Nature Communications (doi:10.1038/ncomms15341), this work expands the capabilities of MOFs to new applications in energy conversion and storage.
Scientists have only recently begun to develop MOFs, which are structures formed by linking metal clusters in a porous network using organic ligands, for applications such as electrocatalysis. “MOFs are generally believed to be poor electrocatalysts,” says Chuan Zhao, whose group published the work. Traditional MOFs are usually insulating, lack diverse pore sizes to facilitate the transfer of materials, and degrade in water. Researchers must engineer MOFs that allow for the movement of charge, reactants, and products if these MOFs are to be used as electrocatalysts.
To create a MOF with these properties, Zhao’s group devised a novel fabrication method that gave them control over many different factors. According to Sheng Chen, a research fellow in the Zhao group, “Our bottom-up growth method enabled us to manipulate the structure and morphology of the MOFs.” Because of its 2D nature, more of the metal sites in the ultrathin NiFe-MOF are exposed and available as sites for catalysis during the electrochemical reaction. The NiFe-MOF designed by Zhao’s group also has many different types of pores, like the intrinsic micropores and macropores between MOF layers (as seen in the Figure). This diversity in pore size allows electrolytes and gas molecules to diffuse through the MOF during catalysis. Thirdly, growing the NiFe-MOF directly on the electrode gives researchers more control over the final MOF-electrode architecture. This is the first demonstration of a 2D MOF being fabricated directly on a substrate. Lastly, this bottom-up approach is much simpler than other methods for creating 2D MOFs. “This synthetic approach is facile, universal, and adaptable for a range of MOFs and substrates,” says Zongping Shao of Curtin University in Perth, Australia, who was not connected with the publication.
(a) Optical and (b) scanning electron microscope image of the 2D nickel/iron metal–organic framework. Courtesy of Nature Publishing Group.
All of these factors combine to give this 2D NiFe-MOF its versatility and high performance. Electrocatalytic water splitting combines an oxygen evolution reaction at the anode with a hydrogen evolution reaction at the cathode. Zhao and Chen’s 2D NiFe-MOF performs both of these reactions efficiently, significantly mitigating the energy losses caused by the slow kinetics of these reactions. Furthermore, an electrochemical cell with the 2D NiFe-MOF as both the cathode and anode showed excellent catalytic activity, producing a current density of 10 mA cm–2 at a voltage of 1.55 V. This activity is higher than that of most bifunctional catalysts, and is close to the activity demonstrated in standard precious-metal-based catalysts that are used as a benchmark for performance.
These results are only in their infancy, but researchers are excited about what this could mean for future MOF applications. “This could open up a new avenue for further tailoring and utilizing MOFs as high-performance electrocatalysts,” Shao says. He would also like to see a more thorough understanding of how the substrate might affect the catalytic activity of the NiFe-MOF. Looking forward, Zhao says his group hopes to “expand MOF applications beyond water splitting” potentially addressing “challenging problems such as electro-reduction of carbon dioxide to generate liquid fuels.”
