Collecting hydrogen from nanogardens

According to research by chemist Ning Yan and his team at the Van ‘t Hoff Institute for Molecular Sciences at the University of Amsterdam, along with co-workers at the University of Amsterdam, the Cobalt Phosphate Nanostructure is a catalyst highly effective for electrolysis of water. Wuhan University School of Physics and Technology, China. In a document presented on the cover of the Materials Chemistry Journal A, describe how relatively simple electrochemical deposition methods produce grass, leaf and flower-like nanostructures that carry the promise of efficient hydrogen generation.

To prepare nanostructures, top-down approaches, such as lithography, have been common for a long time. This has proven quite useful in semiconductor manufacturing, but for more dedicated applications it is time consuming and not particularly cost effective. As an alternative, many researchers explore bottom-up synthesis of nanostructures, for example, based on the self-assembly of nanoscale molecules or building blocks. However, achieving geometry control often requires expensive additives and surfactants, making large-scale material preparation quite challenging.

As an alternative, Assistant Professor Ning Yan, along with his Ph.D. Students Jasper Biemolt and Pieter Laan of the Van ‘t Hoff Institute for Molecular Sciences at the University of Amsterdam have explored a relatively simple method of electrodeposition of cobalt hydroxide. In cooperation with researchers from the Faculty of Physics and Technology, Wuhan University, China, they have been able to design and prepare a variety of nano-architectures that resemble various elements in a garden: soil, sprouts, herbs, flowers, and leaves.

The researchers report that they have mastered the system in such a way that they can grow any of these structures at will.

In addition to this, they were able to make the catalytically active nanostructures by a simple phosphorylation procedure. The resulting cobalt phosphide nanostructures show bifunctional catalytic activity in the electrolytic division of water, improving hydrogen and oxygen generation reactions.

Hierarchical nanostructures through controlled synthesis

Ning Yan and his coworkers cultivated their nanogardens on a cloth made of carbon fibers about 10 microns in diameter, an electrode material common in the fuel cell and electrolyzer industries. Gardening began with the deposition of a layer of “soil” by hydrothermally encapsulating the fibers with a dense layer of cobalt hydroxide. This layer increased the structural stability of the nanostructures. By varying ion concentration and temperature, they were able to induce the “bud” of grass-like characteristics that are strongly “rooted” in the soil.

These herbs have an average length of 1.5 mm and a thickness of around 100 nm. To add flowers and leaves to herbaceous characteristics, the researchers applied an electrodeposition method. In a dilute solution, electrodeposition proceeds predominantly from the tip of the grass stem, where the small radius of curvature results in a higher density of space charge. In more concentrated solutions, electrodeposition comes primarily from the bottom of the stems. This results in the deposit of “leafy” characteristics, which are in fact interlocking dendritic deposit structures.

After converting cobalt hydroxide nanostructures to cobalt phosphide using phosphorylation, the researchers evaluated their catalytic activity in an environment that adequately represented the conditions relevant to industry. As it turned out, catalyst performance in an acidic environment is one of the best non-precious metal catalysts for hydrogen evolution. Furthermore, under acidic, as well as alkaline and neutral conditions, flowery nanofeatures resulted in significantly higher rotational frequencies than leafy characteristics, particularly at higher overpotentials when hydrogen evolution is influenced by the limitations of mass transport. The researchers attribute this to the geometry of the nanofeatures where the flowers allow for a smoother release of hydrogen. However, the different reaction environments at the top and bottom positions of the nanostructures complement each other, resulting in optimal overall performance.

Finally, in electrolysis experiments on the division of water, the researchers demonstrated that their nanogardens not only catalyze the evolution reaction of hydrogen but also the evolution of oxygen. This bifunctional activity was shown using a symmetrical two-electrode configuration with completely identical nanogardens at the anode and cathode. The team will further investigate the use of electrons to control the growth of nanocrystals in a synthesis of “electrified” materials that promises a sustainable future.