Using X-rays to Look inside a Palladium Membrane During Hydrogen Purification


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.

About the author

Mengyao Yuan is a Postdoctoral Research Scientist at the Carnegie Institution for Science in California, USA. She currently models and analyzes near-zero-emission energy systems for transition to a sustainable energy future.

In her PhD work, she investigated carbon capture and hydrogen purification technologies using both experimental and computational methods. She has experience in a variety of tools and techniques, including synchrotron x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), density functional theory calculations (VASP), chemical process modeling (Aspen Plus), and energy systems optimization (MATLAB, Python). Mengyao obtained her BEng in Chemical Engineering with a minor in Humanities from the Hong Kong University of Science and Technology (HKUST), and her MS in Environmental Engineering and Science and PhD in Engineering Resources Engineering from Stanford University.

About the author

Douglas Van Campen received his Ph.D. in Physics from Lehigh University in 1993. He has head up the End Station Engineering and Development team for SSRL/SLAC National Accelerator Laboratory over the past 7 years. The group develops operando, in-situ and environmental sample systems for use with X-ray measurement techniques.


About the author

Simona Liguori is an Assistant Research Professor at Worcester Polytechnic Institute. She received her Ph.D. in Chemical Engineering from the University of Calabria, Italy, in 2012. She continued her research at the Institute on Membrane Technology—the Italian National Lab—as a post-doctoral researcher for two years. In 2014, Dr. Liguori joined Stanford University as Physical Science Research Associate and in 2016 she was appointed as Research Assistant Professor at Colorado School of Mines. After two years at Mines, she moved to her current position at Worcester Polytechnic Institute.

Dr. Liguori’s research efforts are devoted to developing processes and materials that play pivotal roles in energy conversion with enhanced efficiency and environmental sustainability. In particular, her objective is to create and develop clean energy and cost-effective technologies to be beneficial for society and encourage economic growth, while reducing the environmental impact. To achieve these goals, Dr. Liguori’s research is focused on fundamental studies into membrane and catalyst materials to investigate, develop, design, and optimize catalytic processes and membrane technologies to produce energy and energy vector, while mitigating the CO2 emissions and intensifying the process.

About the author

Michael Toney is head of the Materials Sciences Division and a distinguished staff scientist at the SLAC National Accelerator Laboratory. He is a pioneer in the use of X-ray scattering and spectroscopy for the determination of atomic and mesoscale structure in materials for sustainable energy applications, especially solar energy and energy storage. His present interests are in hybrid metal-halide perovskites solar absorbers, Li-rich cathodes, electrode-electrolyte interfaces in energy storage systems and hydrogen storage and transformation. Toney received his BS from Caltech in 1979 and his PhD from the University of Washington in 1983, both in physics. He spent one year as a postdoc at the Risoe National Lab (now DTU) in Denmark, where he participated in some of the first surface X-ray diffraction experiments.

After his PhD in surface physics and a postdoc conducting some of the first surface X-ray diffraction experiments, he then began working at IBM Almaden Research in materials sciences. He left IBM in 2003 to join SLAC National Accelerator Laboratory and Stanford, where he started programs in sustainable energy materials focusing on solar energy and energy storage.

About the author

Jennifer Wilcox is the James H. Manning Chaired Professor of Chemical Engineering at Worcester Polytechnic Institute. Having grown up in rural Maine, she has a profound respect and appreciation of nature, which permeates her work as she focuses on minimizing negative impacts of humankind on our natural environment.

Wilcox’s research takes aim at the nexus of energy and the environment, developing both mitigation and adaptation strategies to minimize negative climate impacts associated with society’s dependence on fossil fuels. This work carefully examines the role of carbon management and opportunities therein that could assist in preventing 2° C warming by 2100. Carbon management includes a mix of technologies spanning from the direct removal of carbon dioxide from the atmosphere to its capture from industrial, utility-scale and micro-emitter (motor vehicle) exhaust streams, followed by utilization or reliable storage of carbon dioxide on a timescale and magnitude that will have a positive impact on our current climate change crisis.

Funding for her research is primarily sourced through the National Science Foundation, Department of Energy and the private sector. She has served on a number of committees including the National Academy of Sciences and the American Physical Society to assess carbon capture methods and impacts on climate. She is the author of the first textbook on carbon capture, published in March 2012.


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.

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