Researchers at Ruhr-Universität Bochum (RUB) and Max-Planck-Institut für Eisenforschung in Germany have discovered an 5-element alloy that is noble metal-free and as active as platinum. They published their paper in the journal Advanced Energy Materials.

The oxygen reduction reaction (ORR), as a key reaction in electrocatalysis, attracts considerable attention due to its complex four-electron/four-proton transfer, which causes sluggish kinetics. Pt-based materials are the state-of-the-art electrocatalysts for ORR due to their outstanding catalytic activity, selectivity, and long-term stability under operating conditions. However, their scarcity and, thus, high cost limit their applications. Accordingly, research on non-noble metal catalysts increased substantially over the past decades. … new materials capable of replacing Pt-based catalysts for ORR are yet to be found.

One approach in exploring novel materials is to increase their chemical complexity. Multinary metal alloys require a more challenging synthesis pathway, but enable a virtually unlimited amount of different compositions.

… we investigated nanoparticles of a noble metal–free Cr–Mn–Fe–Co–Ni multinary element alloy that shows the high-entropy effect promoting single solid solution formation. We report for the first time that such materials can compete with Pt in their ORR activity. Based on a new method for nanoparticle-based investigation of intrinsic electrocatalytic activity, we show that this outstanding activity is indeed an inherent material property.

—Löffler et al.

The catalytic properties of non-noble elements and their alloys are usually rather poor. To the researchers’ surprise, one alloy made up of five almost equally balanced components offer much better properties. This is because of the so-called high entropy effect. It causes multinary alloys to maintain a simple crystal structure.

Through the interaction of different neighboring elements, new active centers are formed that present entirely new properties and are therefore no longer bound to the limited properties of the individual elements. Our research has demonstrated that this alloy might be relevant for catalysis.

—Tobias Löffler, lead author

Searching for an alternative to platinum, researchers at the RUB Chair of Materials for Microtechnology headed by Professor Alfred Ludwig deployed a special method to generate an alloy nanoparticle library of five source elements. Their atoms blend in plasma and form nanoparticles in a substrate of ionic liquid. The liquid is placed in small cavities on a carrier.

If the nanoparticles are located in the vicinity of the respective atom source, the percentage of atoms from that source is higher in the respective particle. In the center of the carrier, all five elements are present in more or less equal quantities. The combinatorial process enables the precise control of the composition of the alloy nanoparticles anywhere in the material library, said Ludwig.

Headed by Professor Christina Scheu, the research team at the Max-Planck-Institut für Eisenforschung analyzed the thus generated nanoparticles using transmission electron microscopy. RUB chemists determined their catalytic activity and compared it with that of platinum nanoparticles.

In the process, they identified a system made of up five elements in which the high entropy effect result in catalytic activity for an oxygen reduction that is similar to that of platinum. By optimizing the composition further, they successfully improved the overall activity.

The researchers are hoping to adapt the properties for any required reactions by taking advantage of the almost infinite number of possible combinations of the elements and modifications of their composition. The research team has already applied for a patent.

However, as the interplay of the elements is not fully understood, the researchers cannot develop any specific catalysts as yet.

These results demonstrate the potential of multinary alloys toward tailoring of catalytic properties by a high-entropy-induced formation of a single solid solution phase with homogeneous distribution of all constituents leading to a high number of novel active sites resulting from this mixture. However, suitable selection of element combination and composition is required as this affects number and nature of active sites while overcoming limitations of single elements.

This was confirmed experimentally by variation of the Mn content in the quinary alloy, yielding even higher intrinsic catalytic activity for the ORR. The basically unlimited number of multinary alloys should allow finding new material combinations with potentially superior properties. For achieving discoveries in this multidimensional search space, the proposed method is considered to be efficient and will open up a new promising pathway which is not restricted to the discovery of ORR catalysts alone.

—Löffler et al.

The project was funded by the Federal Ministry of Education and Research (NEMEZU, FKZ 03SF0497B and Mangan (FKZ 03EK3548)) and by the German Research Foundation (LU1175/23-1, SCHE634/21-1, Exploring Multinary Nanoparticles by Combinatorial Sputtering into Ionic Liquids and Advanced Transmission Electron Microscopy) as well as under the umbrella of the Transregio Collaborative Research Centre 247 and the Cluster of Excellence Ruhr explores Solvation, short Resolv (EXC1069).

Resources

• Tobias Löffler, Hajo Meyer, Alan Savan, Patrick Wilde, Alba Garzón Manjón, Yen Ting Chen, Edgar Ventosa, Christina Scheu, Alfred Ludwig, Wolfgang Schuhmann (2018) “Discovery of a multinary noble metal free oxygen reduction catalyst,” Advanced Energy Materials doi: 10.1002/aenm.201802269.