Finding and understanding low cost catalysts: it all comes down to the iron

Chromium enhances the current density which is directly related to catalytic activity.

Chromium enhances the current density which is directly related to catalytic activity. © HZB/RUB

A team has investigated more than one hundred iron-nickel catalysts containing various admixtures of chromium. At BESSY II, they also analysed the configurations of the electrons in the individual elements. The team showed that an increasing proportion of chromium primarily influences the energy levels of the iron electrons, which are important for the catalytic effect. The results of this high-throughput study will assist the knowledge-based search for better specific catalysts.

Catalysts can accelerate chemical reactions, while remaining unchanged. They are necessary for many reactions – including if you want to split water into hydrogen and oxygen using sunlight to chemically store solar energy. And yet, the best catalysts for this reaction are still made of platinum or other rare elements. These are too expensive for broad usage. Research teams are therefore searching everywhere for alternatives.

But how do you actually go about finding good new catalysts? And why are some materials better catalysts than others, though they only differ slightly in their compositions? The Young Investigator Group named Operando Characterization of Solar Fuel Materials headed by Prof. Kathrin Aziz-Lange together with partners from the Ruhr-Universität-Bochum has dedicated itself to this problem.

They selected a material system for this of iron, nickel, and oxygen that is currently a particularly promising candidate for catalysts. Then they admixed chromium as a tertiary metal that can further increase the efficiency. They did not produce just one material, though, but instead an entire material library in which the composition of the three metals varies continuously over a total of more than one hundred samples. In this way, the team was able to empirically determine how the introduction of chromium in the catalyst altered the reaction speed and which features of the materials facilitate higher speeds.

They determined the catalytic performance for all of the samples (s. illustration). In parallel to this, they investigated the energy levels of the electrons for the individual elements that are related to the catalytic activity using spectroscopic methods at the BESSY II synchrotron source in Berlin. “We recorded more than 500 spectra of the material library”, explains Christoph Schwanke who carried out these measurements as part of his doctoral studies.

The results of this high-throughput study show that the configuration of the electrons around the nickel and chromium hardly changes with rising chromium content. However, the rising proportion of chromium alters the energy level of the iron electrons. “We were able to observe that the performance of the catalysts correlates with a specific configuration of iron electrons”, explains Prof. Kathrin Aziz-Lange. “This is important information, not just for deeper understanding of catalytic materials, but for targeted synthesis of good catalysts as well.”

The study gives clues for the systematic and knowledge-based development of new catalysts. Combinatoric materials research thus permits a great many materials to be investigated in a short time and to identify candidates that are especially promising.

To the publication: Correlating Oxygen Evolution Catalysts Activity and Electronic Structure by a High-Throughput Investigation of Ni1-y-zFeyCrzOx 

Christoph Schwanke, Helge Sören Stein, Lifei Xi, Kirill Sliozberg, Wolfgang Schuhmann, Alfred Ludwig and Kathrin M. Lange

Scientific Reports 7, (2017) doi: 10.1038/srep44192

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