Using In Situ Environmental TEM to Visualize Hydrogen Absorption in Palladium Thin Films

Hydrogen (H2), as a chemical fuel, is an environmentally friendly energy source and is often considered the fuel of the future. It is currently used in internal combustion engines and in systems such as fuel cells to generate electricity. In both cases, the chemical reaction product is only water and heat.

Although this clean energy source looks promising, there are two main barriers to its widespread adoption. The first barrier is created the fuel itself, which is often produced from water through a chemical splitting reaction. While a range of reaction methods have been shown, these take considerable amount of energy and resources.

Storage is the second barrier to widespread commercialization of H2. H2 is a light molecule and can seep out of holding tanks and pipes over time, making long term storage problematic. Globally, research and engineering groups are exploring a number of potential strategies and materials for hydrogen storage. Scientists are studying hydrogen storage in certain metals, particularly palladium (Pd), which is a promising material.

It a well-known fact that palladium can effectively absorb hydrogen at moderate pressures and temperatures. Hydrogen occupies interstitial sites in the Pd fcc lattice. The Pd lattice slightly expands at low absorbed concentrations of hydrogen. However, above a concentration threshold of about 2%, the lattice further expands to accommodate the increased amount of hydrogen.

The Pd alpha phase and beta phase, called palladium hydride, are used to describe the Pd/H structure under low and high concentrations, respectively. When transitioning into the beta phase, the expansion can be identified through diffraction and by measuring slight changes in the lattice spacing. This is a reversible process and when Pd returns to the alpha phase, the lattice contraction can also be measured.

X-ray diffraction and other bulk in situ measurements are capable of detecting these changes in the lattice spacing of Pd, but these techniques cannot provide direct visual data at the atomic and nano scale.

Subtle changes in the microstructure, such as particle coalescence in nanoparticles and grain growth in thin films, which may affect the material’s storage behavior, cannot be directly visualized. Therefore, it can be difficult to analyze material changes at the nano-scale due to heat and hydrogen pressure.

TEM is one of the best ways to directly image materials at small length scales, and provides a way analyze samples using diffraction and element identification with EELS and EDS, which is particularly important when working with materials such as Pd alloys. However, high vacuum is required to operate the TEM, and is typically not amiable to high pressures without making significant modifications to the instrument.

With the launch of the Protochips Atmosphere™ Gas E-cell system, researchers can now expose the samples to temperatures and gas environments that imitate real-world conditions, including the conditions needed to create palladium hydride. Researchers can view atomic scale processes in real time using Atmosphere with new and existing TEMs, so more relevant data can be acquired from experiments with minimal additional effort.

Experiment

At the Materials Science department of the University of Manchester, researchers carried out in situ hydrogen absorption experiments on a thin film of Pd. The Pd sample was directly deposited onto a temperature-controlled support known as a thermal E-chip.

The E-chip features a thin, ceramic heating membrane, which is actively controlled with the Atmosphere software to automatically adjust the temperature under different pressures (up to 1 atm) and gas species. The Pd was deposited on the heating membrane by sputtering through a shadow mask with a thin slit directly above the membrane to localize the deposition in the observation region.

A second window E-chip with a SiN membrane is located on top of the thermal E-chip in the TEM holder, producing a thin gas cavity sealed with small o-rings maintaining the high vacuum of the TEM column.

During the experiment, the sample was imaged under 1 atm (760 Torr) of H2 at temperatures from 200 °C to 300 °C. At the University of Manchester’s Materials Performance Centre, an FEI Titan, Cs probe corrected was used and operated in STEM mode at 200 kV.

Discussion

Using electron diffraction in the TEM, the lattice expansion resulting from the formation of the beta phase can be detected. For this experiment, a diffraction pattern was taken keeping the Atmopshere cell at 20 Torr of H2 at 200 °C, and this was followed by taking a second pattern during exposure to 1 atm of H2 at 300 °C.

The two patterns were then compared. It is to be noted that this sample is a thin polycrystalline palladium film, and as a result, formed diffraction rings due to multiple orientations of small crystalline grains in the sample.

Figure 1 shows a one dimensional line scan demonstrating the intensity of the diffraction rings on each pattern. The lattice expansion is instantly apparent, and using this graph, spacings can easily be extracted, which are displayed in Table 1.

Figure 1. One dimensional line scans of before and after powder diffraction patterns. After exposure to hydrogen at elevated temperatures (black line), the lattice expands to accommodate it.

Table 1. Lattice spacings measured from the diffraction pattern showing the difference in inter planar spacing before and after hydrogen absorption

Reflection Before H absorption
Lattice spacing (nm)
Lattice Parameter
(pure Pd - 0.389 nm)
After H absorption
Lattice Spacing (nm)
Lattice Parameter
(beta phase - 0.4025)
111 0.223 0.386 0.228 0.395
002 0.195 0.39 0.199 0.398
202 0.137 0.387 0.14 0.396
113 0.117 0.388 0.119 0.394

0.389 nm is the lattice parameter for metallic Pd, and the measurements taken from each reflection in the diffraction pattern closely match this value within a small margin of error. 0.4025 nm is the lattice parameter for palladium hydride, and the measured values taken during hydrogen exposure, also closely match this value.

High-resolution images of the Pd lattice can be taken to correlate physical changes in the microstructure. Figure 2 shows one such image taken under 1 atm of H2 at 300 °C.

Figure 2. High resolution HAADF STEM image of a Pd grain during exposure to 1 atm of H2 at 300 °C. Scale bar is 2 nm.

Applications

Pd and Pd alloys can provide an effective solution to the hydrogen storage issues faced in a number of important applications. However, more research is needed to fully understand the behavior of materials under specific environmental conditions. The TEM can play a key role in characterizing the behavior of Pd and Pd alloys during systematic exposure to H2 at certain pressures and temperature.

With the addition of Protochips Atmosphere Gas E-cell system in the TEM, exposing samples to specific gas species and applying accurate temperatures across a wide pressure range have now become a routine process. Atmosphere maintains the analysis and resolution capabilities of the most powerful TEMs available on the market today.

Atmosphere can be used with most modern TEMs, as it is a holder-based system, and can be added to new and existing instruments without any need for special modifications.

Reference:

Images and data courtesy Prof Sarah Haigh and Eric Prestat, University of Manchester; Tadahiro Yokosawa, Karlsruhe Institute of Technology

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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