A team from Johns Hopkins University, Purdue University and the University of California at Irvine has developed a new strategy to tailor and to optimize the reactivity of metal catalysts in applications such as fuel cells. The application of their method could help cut down on expensive metals needed in fuel cell electrodes.
In a paper in the journal Science, the researchers report that, based on their approach, freestanding palladium nanosheets (three to five monolayers thick) form with an internal compressive strain of 1 to 2% and can be much more active for both the oxygen and hydrogen evolution reactions under alkaline conditions compared with nanoparticles.
Tuning surface strain is a powerful strategy for tailoring the reactivity of metal catalysts. Traditionally, surface strain is imposed by external stress from a heterogeneous substrate, but the effect is often obscured by interfacial reconstructions and nanocatalyst geometries. Here, we report on a strategy to resolve these problems by exploiting intrinsic surface stresses in two-dimensional transition metal nanosheets.
Density functional theory calculations indicate that attractive interactions between surface atoms lead to tensile surface stresses that exert a pressure on the order of 105 atmospheres on the surface atoms and impart up to 10% compressive strain, with the exact magnitude inversely proportional to the nanosheet thickness. Atomic-level control of thickness thus enables generation and fine-tuning of intrinsic strain to optimize catalytic reactivity, which was confirmed experimentally on Pd(110) nanosheets for the oxygen reduction and hydrogen evolution reactions, with activity enhancements that were more than an order of magnitude greater than those of their nanoparticle counterparts.—Wang et al.
The researchers tested their theory on palladium, a metal very similar to platinum.
We’re essentially using force to tune the properties of thin metal sheets that make up electrocatalysts, which are part of the electrodes of fuel cells. The ultimate goal is to test this method on a variety of metals.—Jeffrey Greeley, professor of chemical engineering at Purdue
Researchers in the past have tried using outside forces to expand or compress an electrocatalyst’s surface, but doing so risked making the electrocatalyst less stable.
Greeley’s group predicted through computer simulations that the inherent force on the surface of a palladium electrocatalyst could be manipulated for the best possible properties.
According to the simulations, an electrocatalyst five layers thick, each layer as thin as an atom, would be enough to optimize performance.
Don’t fight forces, use them. This is kind of like how some structures in architecture don’t need external beams or columns because tensional and compressive forces are distributed and balanced.—,Zhenhua Zeng, co-first and co-corresponding author
Experiments in Chao Wang’s lab at Johns Hopkins confirmed the simulation predictions, finding that the method can increase catalyst activity by 10 to 50 times, using 90% less of the metal than what is currently used in fuel cell electrodes.
This is because the surface force on the atomically thin electrodes tunes the strain, or distance between atoms, of the metal sheets, altering their catalytic properties.
By tuning the material’s thickness, we were able to create more strain. This means you have more freedom to accelerate the reaction you want on the material’s surface.—Chao Wang
The study was supported by multiple entities, including the US Department of Energy, National Energy Research Scientific Computing Center and the National Science Foundation.
Lei Wang, Zhenhua Zeng, Wenpei Gao, Tristan Maxson, David Raciti, Michael Giroux, Xiaoqing Pan, Chao Wang, Jeffrey Greeley (2019) “Tunable intrinsic strain in two-dimensional transition metal electrocatalysts” Science Vol. 363, Issue 6429, pp. 870-874 doi: 10.1126/science.aat8051