New electrocatalysts make hydrogen production less precious

Clean hydrogen energy offers a good alternative to fossil fuels and is critical for achieving carbon neutrality. Researchers around the world are looking for ways to enhance the efficiency and lower the cost of hydrogen production, particularly by improving the catalysts involved.

A research team from the City University of Hong Kong (CityU) has now developed a new, ultra-stable hydrogen evolution reaction (HER) electrocatalyst, which is based on two-dimensional mineral gel nanosheets and does not contain any precious metals. The catalyst, reported in a paper in Nature Communications, can be produced in large scale and can help achieve a lower hydrogen price in the future.

The electrochemical hydrogen evolution reaction (HER) is a widely used hydrogen-generation method. But commercial HER electrocatalysts are made from precious metals, which are expensive.

Single-atom catalysts have shown promise for catalytic HER applications because of their high activity, maximized atomic efficiency and minimized catalyst usage. Unfortunately, the conventional fabrication process for single-atom catalysts is complicated. It generally involves introducing single metal atoms to a substrate precursor followed by thermal treatment, usually at temperatures above 700?, which requires a lot of energy and time.

The research team co-led by materials scientists at CityU has developed an innovative, cost-effective and energy-efficient way to produce a highly efficient HER single-atom electrocatalyst that uses precious-metal-free mineral hydrogel nanosheets as a precursor.

“Compared with other common single-atom substrate precursors, such as porous frameworks and carbon, we found that mineral hydrogels have great advantages for mass production of electrocatalysts due to the easy availability of the raw materials, simple and environmental-friendly synthetic procedure, and mild reaction conditions,” said Jian Lu, a professor in the Department of Mechanical Engineering (MNE) and the Department of Materials Science and Engineering (MSE) at CityU, who led the research.

The team prepares its electrocatalyst precursor using a simple method. First, solutions of polyoxometalate acid (PMo) and ferric ions (Fe³⁺) are mixed at room temperature, resulting in novel two-dimensional iron–phosphomolybdic-acid nanosheets. After excess water is removed by centrifugation, the nanosheets turn into a mineral hydrogel free of any organic molecules. This process is much more convenient and economical than previously reported processes for producing single-atom substrate precursors, which typically require high temperatures and pressures, and long assembly times.

Further phosphating treatment (at 500?) of this mineral gel precursor leads to the formation of a single-iron-atom dispersed heterogeneous nanosheet catalyst (Fe/SAs@Mo-based-HNSs), avoiding the time-consuming process of loading single atoms on the substrate.

The research team found that this new catalyst exhibits excellent electrocatalytic activity and long-term durability for the HER, manifesting an overpotential of only 38.5mV at 10mA cm⁻². It also exhibits ultra-stability without performance deterioration over 600 hours at a current density of up to 200mA cm⁻².

“This is one of the best performances achieved by non-noble-metal HER electrocatalysts,” said Lu. “The unique idea of using mineral gel to synthesize monatomic dispersed heterogeneous catalysts provides an important theoretical basis and direction for the next step of scalable production of cheap and efficient catalysts, which can help contribute to lowering the cost of hydrogen production in the long run.”

To tackle the high cost of commercial platinum-based electrocatalysts, the team led by Lu also recently made another breakthrough. Using rational nanostructured alloy design, they developed a second low-cost, high-performance HER electrocatalyst, which they report in a paper in Science Advances.

Lu’s team has been conducting in-depth research on alloy nanostructures that simultaneously possess both crystalline and amorphous phases. They discovered that the local chemical inhomogeneity, short-range order and severe lattice distortion in the nanocrystalline phase are desirable for catalysis, while the amorphous phase can offer abundant active sites with lower energy barrier for the HER. Therefore, they devoted their research efforts to designing and constructing dual-phased alloys as excellent electrocatalysts for hydrogen production.

They proposed a new alloy and nanostructure design strategy based on thermodynamics. First, they predicted the composition range of the ‘crystal-amorphous’ dual-phase formation, according to the amorphous forming ability (GFA). Then, using the facile method of magnetron co-sputtering, they successfully prepared an aluminium-based alloy catalyst with a ‘crystalline-amorphous’ dual-phased nanostructure.

Thanks to this nanostructure, the new catalyst showed better electrocatalytic performance in alkaline solution than a commercial platinum-based electrocatalyst, with an overpotential of just 28.8mV at 10mA cm^-2.

“In this novel aluminium-based alloy catalyst, we use ruthenium, which is cheaper than platinum, as the noble metal component. So it can be less costly than the commercial platinum-based electrocatalysts,” said Lu. “And apart from hydrogen evolution, the nano-dual-phase electrocatalysis mechanism can be applied to other catalytic systems. The ‘crystal-glass’ nanostructure design offers a new approach to develop next-generation catalysts.”

This story is adapted from material from the City University of Hong Kong, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.