Designing efficient new catalysts is no easy task, especially when nanoparticles are the active structures. In catalysts that contain more than one element, for example, researchers not only need to take into account all the possible elemental combinations, they must also add a number of other variables, such as particle size, shape and surface structure, as well as the degree of alloying or phase segregation. This ultimately leads to an overwhelmingly large number of potential candidates.

To address this challenge, techniques to make poly-elemental particles and control their alloying or phase segregation state combined with screening methods to reduce their overall number need to be developed. Such techniques require combinatorial approaches coupled with theory calculations.

Combining SPBCL and DFT

A team led by Chad Mirkin, Chris Wolverton and Yijin Kang of Northwestern University in the US has now used an up-and-coming nanoparticle synthetic tool called scanning probe block copolymer lithography (SPBCL) combined with density-functional theory (DFT) calculations to explore three-component particles consisting of different combinations of platinum, gold, copper and nickel.

In their experiments, the researchers chose to study the hydrogen evolution reaction (HER) because it is crucial for commercially producing hydrogen in fuel cells. In an acidic electrolyte, the catalyst’s hydrogen binding energy (HBE) is the most important descriptor for the HER. According to the so-called Sabatier principle, the HBE of a HER catalyst should neither be too strong nor too weak – that is, the surfaces of the metals making up the catalysts should neither be too strongly nor too weakly absorbing.

Reducing the HBE

Although Pt is the best-known single-element catalyst for the HER, it could be further improved if its HBE was reduced. Electronically tuning its d-band structure by alloying it with another element, or indeed other elements, is a good way of doing this.

To find out which combinations of Pt, Au, Cu and Ni were best, Mirkin and colleagues studied the PtAu-M tri-metallic system (where M=Ni or Cu). They first used DFT calculations to calculate the HBEs of the different structures. They then synthesized these target structures using SPBCL and evaluated their catalytic properties. One of the advantages of SPBCL is that researchers can control the growth and composition of individual nanoparticles patterned on a surface, which allows them to produce particles that have uniform stoichiometry and phase.

Seven times more active

Thanks to these experiments, described in PNAS, the researchers identified PtAuCu as having the optimal calculated HBE and thus the highest measured HER activity – seven times more active than state-of the-art commercial platinum, says Mirkin.

“In addition to providing a new way to catalyze the HER, the paper highlights a novel approach for making and discovering new particle catalysts for almost any industrially important process,” says Wolverton.

“To find best-in-class materials that drive any application of interest, we need to identify ways to reduce the number of possibilities that will be studied and increase the rate at which they can be explored,” adds Kang. “This combination of theory and nanoscale particle synthesis begins to take on that challenge,” says Mirkin.