Increasingly stringent regulations on carbon emissions highly favor the adoption of low-carbon technologies, such wind and solar power generation, energy storage, and hydrogen-integrated systems. In particular, broader deployment of hydrogen could reduce carbon dioxide emissions in mobile and distributed applications, such as in automobiles, where carbon emissions are often more challenging to remove. However, several challenges still hinder full deployment of hydrogen as a clean energy carrier. For instance, high purification levels required for fuel cell applications are energy-consuming to reach and often achieved in large, stationary facilities. Palladium (Pd)-based membranes are highly selective to hydrogen gas, compact, and can be operated flexibly. Thus, Pd-based membranes could potentially facilitate on-demand hydrogen gas production to reduce emissions even from small and distributed applications.
In fact, Pd-based membranes have the distinct ability to achieve hydrogen purities as high as 99.999% or higher. Although hydrogen transport in Pd, Pd alloys, and Pd-based membranes has long been studied using advanced techniques such as x-ray diffraction (XRD), operando (“in operation”) XRD characterization of Pd-based membranes during hydrogen purification remains a largely unexplored area. Research in this area could contribute to the understanding needed for long-term performance improvement and system scale-up.
To this note, Dr. Mengyao Yuan at Stanford University, together with Dr. Kyoungjin Lee at Applied Materials in California, Dr. Douglas Van Campen and Dr. Michael Toney at the SLAC National Accelerator Laboratory, and Dr. Simona Liguori and Professor Jennifer Wilcox at the Worcester Polytechnic Institute, developed an operando XRD experiment to directly observe Pd-based membranes during hydrogen gas permeation at typical operating conditions. The research team intended to develop the technique for extensive applications in metallic-membrane research. This research work is published in the journal Industrial & Engineering Chemistry Research.
The researchers investigated hydrogen gas permeation in a dense Pd membrane at typical operating conditions, where they were able to detect structural changes of the Pd lattice during hydrogen permeation in real time. The authors simultaneously collected hydrogen flux and XRD measurements and calculated important hydrogen transport properties, including permeability, solubility, and diffusivity. These estimates are in reasonable agreement with existing literature. In addition, the authors were able to show that the Pd lattice remained in the α phase throughout the operating conditions investigated (280–440 °C, 1–3 bar pressure difference). While this work is a proof-of-concept study, the team noted that the operando XRD method could be used to characterize real-time structural changes and measure hydrogen transport properties in other Pd-based membranes during hydrogen permeation at wider temperature and pressure conditions.
In summary, the study presented a novel operando XRD experiment to study Pd-based membranes for hydrogen purification. It is considered to be the first to perform time-resolved operando XRD measurements of Pd-based membranes at typical operating temperatures and pressures. Altogether, the methodology and findings reported here may have broader implications for similar metal−hydrogen systems, such as Pd-alloy membranes, hydrogen-storing metal hydrides, and Pd-based fuel-cell catalysts.
Mengyao Yuan, Kyoungjin Lee, Douglas G. Van Campen, Simona Liguori, Michael F. Toney, Jennifer Wilcox. Hydrogen Purification in Palladium-Based Membranes: An Operando X-ray Diffraction Study. Industrial & Engineering Chemistry Research 2019, volume 58, page 926−934.Go To Industrial & Engineering Chemistry Research