Dr Tsun-Kong Sham – Dr Jiatang Chen – Dr Zou Finfrock – Dr Zhiqiang Wang | X-Rays Shine Light on Fuel Cell Catalysts

Understanding the electronic behaviour of fuel cell catalysts can be difficult using standard experimental techniques, although this knowledge is critical to their fine-tuning and optimisation. Dr Jiatang Chen at the University of Western Ontario works with colleagues to use the cutting-edge valence-to-core X-ray emission spectroscopy method to determine the precise electronic effects of altering the amounts of platinum and nickel in platinum-nickel catalysts used in fuel cells. Their research demonstrates the potential application of this technique to analysing battery materials, catalysts, and even cancer drug molecules.

Creating New Metals

If we examine the range of naturally occurring metals, all these materials have various electronic, structural, chemical, and mechanical properties. If we want to create a material for a particular purpose, sometimes it is necessary to form a combination of metals and other elements, called an alloy. These alloys can boast a range of properties that can be effectively engineered to perform a particular task, with these new characteristics differing widely from those of their individual components.

Promoting Reactions in Fuel Cells

Different alloys are used for different applications, depending on their particular properties. Proton exchange membrane fuel cells (PEMFCs) are devices that produce electrical power through a chemical reaction between a fuel, such as hydrogen, and an oxidant, like oxygen. The fuel is fed to one region of the fuel cell, while the oxidant is fed to another.

Energy States in a Catalyst

Valence electrons in atoms in any spatially confined particle can only take on certain discrete energy values, called energy states. A valence band is formed when atoms overlap in the neighbourhood. The valence states of catalysts (where the valence of an element is the number of chemical bonds that each of its atoms typically forms) essentially determine intermediate species and final products under certain conditions. This is because these states are responsible for the formation as well as the breaking of chemical bonds.

Tuning the band of valence states for Pt, or its valence band, to optimise both reactivity and selectivity is the most effective approach to Pt-based catalyst engineering. However, it can be difficult to understand specific properties of the Pt-Ni alloy valence band using conventional laboratory techniques.

New X-Ray Analysis Method

X-rays are an extremely penetrative type of radiation with a very short wavelength (if we describe light as periodic waves, this wavelength is the distance between the peaks of two adjacent waves). For over a hundred years, X-rays have been used to observe matter without destroying it, and increasingly precise X-ray-based techniques are constantly evolving.

Dr Jiatang Chen (University of Western Ontario), Dr Zou Finfrock (Argonne National Laboratory), Zhiqiang Wang (University of Western Ontario), and Tsun‑Kong Sham (University of Western Ontario) use a novel form of X-ray analysis called valence-to-core X-ray emission spectroscopy (VTC-XES) to selectively probe the electronic makeup of chemical environments. The researchers employed the highly tuneable X-rays produced by a synchrotron (a machine used to accelerate charged particles that give off strong bursts of X-rays) to detect the valence states of specific elements in Pt-Ni alloys.

Traditional valence characterisation methods can only detect the total valence bands of Pt-Ni alloys. As Dr Chen explains, ‘VTC-XES not only detects the valence states but with elemental specificity… This is important because in a compound, all elements contribute to the valence states but not all function equally in a chemical reaction.’

Effects of Strain and Nearest Neighbours

Dr Chen and his colleagues used VTC-XES to examine the changes in Pt valence states in Pt-Ni alloys composed of various amounts of Pt and Ni. This provided them with direct experimental evidence for certain behaviours of Pt valence electrons in the valence band when alloyed with Ni.

This is the first experimental study showing the interplay between compressive strain arising from the smaller size of an Ni atom compared to Pt, which results in shorter Pt–Pt bonds in the compressed alloy, and the effect of Ni as a nearest neighbour of Pt, which reduces the total number of Pt–Pt neighbours.

The researchers discovered broader valence bands than that of pure Pt for alloys containing either a higher ratio of Pt to Ni atoms, or an equal ratio of Pt to Ni. This happened because compressive strain overcompensated the effect of Pt dilution. In contrast, the valence bands of alloys containing a higher ratio of Ni were narrower. The overall trend observed by the researchers is consistent with previous results indicating a shift in the Pt valence band when alloyed with Ni. They believe these changes have a crucial impact on chemical activity when Pt-Ni alloys are used as catalysts.

A Precise and Diverse Technique

The researchers demonstrated that VTC-XES is a powerful method for studying the precise valence band properties of metals and alloys. They continue to fine-tune their technique, which is especially useful for complicated systems such as Pt-based catalysts, and in studies on working fuel cells, where the valence band of an alloy could be altered by variable chemical and physical conditions. The superior capability of VTC-XES allows its application to an exciting and diverse range of chemical compounds that contain transition metals, including battery materials, catalysts, and even cancer drug molecules.