Researchers have devised a reliable means of measuring the catalytic activity of tiny crystals that speed up important chemical reactions [Zhang et al., Materials Today (2019) https://doi.org/10.1016/j.mattod.2019.02.014]. In fact, the approach of Jing Zhang at Rice University and colleagues at the University of Cincinnati, Oak Ridge National Laboratory, and Tsinghua University can determine which facet of a catalytic crystal is most active.
The team chose the hydrogen evolution reaction (HER) as their model system, partly because it is a fundamentally important reaction in physical chemistry and partly because identifying alternative catalysts to platinum (Pt) could open up hydrogen as a clean fuel.
“HER is a clean way to obtain hydrogen using electricity,” explains Zhang. “In combination with solar panels, people could [generate hydrogen] at home to fuel their cars.”
Because platinum is rare and expensive, lower cost and comparably active catalysts are needed for large-scale application of HER in transport and other clean energy sectors.
Instead of taking a trial and error approach to finding new catalysts, the researchers devised a way to fast-screen potential non-noble metal two-dimensional catalysts such as MoS2. First, they deposited tiny flakes of candidate catalysts onto an e-beam lithographically patterned SiO2/Si substrate, which is then coated with a layer of insulating polymer. The process leaves only one facet of the micro-sized single crystals available for analysis via local electrochemical measurements.
“We can then measure the catalytic activity of this facet,” says Zhang. “If we compare the facet activities of multiple single crystals, we can select the one with highest facet activity for long-term tests.”
The ability to measure the facet-dependent activity of micro-sized single catalyst crystals is novel, points out Zhang, and enables systematic screening of potential candidate materials.
“We went through various types of transition-metal sulfides experimentally and, for the first time, precisely measured their efficiency in promoting hydrogen generation,” points out Zhang.
The team discovered that the transition metal sulfide NbS2 behaves in a very similar way to Pt in long-term tests. More specifically, it is the (001) facet of the two-dimensional material that exhibits the best HER catalytic activity.
“Our approach starts from identifying the intrinsic activity of catalyst, followed by observation of its practical-scale performance,” Zhang told Materials Today. “The screening approach can be easily implemented for many other electrochemical reactions, as long as there is a proper ‘model’ catalyst that can be used to explain more complex scenarios.”
The researchers believe that their screening approach could be easily adapted for use in lab-scale electrochemical experiments and, potentially, industrial processes. The team is now looking to expand the approach to more complex reactions including N2 and CO2 reduction.