Monitoring Fluid Transport In Low Temperature Water Electrolyzers And Fuel Cells Advances Green Hydrogen Production

Low temperature water electrolyzers and fuel cells offer a compelling pathway to sustainable, green hydrogen energy, and are crucial for achieving net-zero emissions goals. However, efficient operation of these devices is significantly hampered by the complex interplay between fluid transport and reaction kinetics within their intricate structures. Zehua Dou from Technische Universität Dresden, Laura Tropf from ZBT GmbH, and Tobias Lappan from Helmholtz-Zentrum Dresden-Rossendorf, alongside colleagues, present a comprehensive review of recent advances in measuring fluid transport within these systems. Their work assesses the capabilities and limitations of current imaging techniques, including optical, X-ray, and neutron-based methods, and highlights innovative strategies employing miniaturized sensors and ultrasound technology. By outlining future directions in this interdisciplinary field, the team emphasizes the critical role of next-generation sensing concepts in overcoming fluid transport limitations and accelerating the widespread adoption of green hydrogen technologies.

In-Situ Diagnostics of Fuel Cells and Electrolyzers

This document presents a comprehensive overview of techniques used to characterize and monitor the performance of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and Polymer Electrolyte Membrane Water Electrolyzers (PEMWEs). Several methods provide valuable insights into device operation, categorized by their approach. Imaging and spectroscopy techniques reveal internal structures and processes. Neutron radiography and tomography map water distribution, highlighting flooding and drying, while X-ray computed tomography visualizes three-dimensional structures and identifies degradation mechanisms. Magnetic resonance imaging sensitively maps hydration states, and optical microscopy and spectroscopy analyze components.

Raman and infrared spectroscopy identify chemical species and monitor reactions. Electrochemical techniques, such as electrochemical impedance spectroscopy, characterize cell components and processes, and potentiostatic/galvanostatic cycling assesses durability. Scanning electrochemical microscopy provides localized electrochemical information, mapping activity and identifying defects. Acoustic techniques detect internal processes through sound, excelling at characterizing bubbles within the cell or electrolyzer. Acoustic emission detects transient elastic waves generated by bubble formation, crack propagation, and phase changes, diagnosing flow regimes, hydration states, and degradation.

Ultrasound offers versatile applications, including time-of-flight imaging to map internal structures and flow characteristics, super-resolution microscopy to visualize microstructures and bubble evolution, and ultrasound localization microscopy to reconstruct high-resolution images. Guided waves detect water accumulation in bipolar plates, and sono-electrolysis utilizes ultrasound to enhance electrolysis efficiency. Other techniques, such as magnetometry, detect corrosion, and fiber Bragg grating sensors monitor strain, temperature, and flow. Emerging trends include deep learning and machine learning for complex data analysis, combining multiple techniques for comprehensive understanding, miniaturization for operando measurements, advanced signal processing, and a focus on water management. Overall, this work highlights the growing importance of in-situ and operando diagnostics for accelerating the development and deployment of PEMFCs and PEMWEs, with acoustic techniques emerging as powerful tools for characterizing these complex systems.

Visualizing Fluid Dynamics in Electrochemical Devices

Researchers developed advanced analytical techniques to investigate fluid transport within low temperature water electrolyzers and low temperature hydrogen fuel cells, critical components in green hydrogen production. Recognizing that performance limitations stem from the interplay between reaction kinetics and fluid dynamics, the study pioneered methods to visualize and quantify gas and liquid behavior within these devices. The team employed a range of imaging systems, including optical, X-ray, and neutron-based techniques, to resolve fluid transport across the complex, multiscale structures of operating electrolyzers and fuel cells. To overcome limitations of established methods, scientists integrated miniaturized sensors and ultrasound technologies to achieve operando measurements, enabling high-resolution, scalable measurements at both the device and system levels.

Investigations focused on understanding how gas bubbles and water droplets accumulate within the porous electrodes and flow channels, hindering reactant access and reducing catalyst utilization. Researchers observed bubble and droplet confinement within the gas diffusion layer, adhesion to electrode surfaces, and accumulation in flow channels, directly linking these phenomena to performance limitations. The study demonstrates how inadequate bubble and droplet management increases local mechanical and thermal stresses, accelerating component degradation and limiting device lifespan. To address these challenges, scientists explored strategies to optimize designs and operating schemes, including superaerophobic electrodes to facilitate bubble removal and optimized flow channel geometries. Furthermore, the team investigated pulse electrolysis schemes to enhance bubble removal and improve overall system efficiency. These analytical tools provide quantitative insights into the relationship between fluid transport, device performance, and long-term durability, paving the way for reliable and efficient low temperature hydrogen energy systems at industrial scales.

Flow Regimes Visualized in Electrolyzers and Fuel Cells

Researchers have achieved significant advancements in understanding fluid transport within low temperature water electrolyzers (LTWEs) and low temperature hydrogen fuel cells (LTFCs), critical components in the development of green hydrogen energy. Investigations into two-phase flow regimes within these devices reveal three typical patterns: bubbly/droplet, slug/plug, and annular/film flow, observed in the channels. Optical visualizations demonstrate that increasing reactant flow rates reduces bubble or droplet size and mitigates water flooding in LTFCs and bubble accumulation in LTWEs. Precise flow rate controls enabled the determination of superficial velocity boundaries defining these different flow regimes, expressed using dimensionless Weber numbers, providing a unified relation between flow and operating parameters.

Understanding fluid transport within the porous gas diffusion layers (GDLs) of LTWEs and LTFCs is also crucial, and researchers overcame limitations of light penetration through opaque materials by fabricating transparent 2D and 3D porous models. Using these models, scientists mapped flow regimes into a generalized phase diagram, expressed with respect to dimensionless capillary number and viscosity ratio between the non-wetting and wetting phases. This diagram serves as a valuable tool for elucidating fluid transport regimes and guiding the engineering of GDL structures to mitigate performance losses caused by water flooding or gas trapping. To resolve convective flows, optical velocimetry techniques, including particle image velocimetry (PIV), were employed. PIV utilizes pulsed lasers and high-speed cameras to capture images of moving tracer particles, enabling the calculation of instantaneous velocity vector maps with a spatial resolution of 640μm. This method successfully mapped the velocities of bubbles in narrow vertical flow channels under varying current densities and quantified the velocity fields of humidified water droplets in LTFCs, providing detailed insights into efficient bubble and droplet removal strategies.

Visualizing Fluid Dynamics in Electrochemical Systems

Recent advances in measurement techniques significantly enhance the ability to understand fluid transport within low temperature water electrolyzers and low temperature hydrogen fuel cells. Researchers have successfully applied and refined optical imaging methods, including direct imaging, optical velocimetry, and schlieren imaging, to visualize and quantify key phenomena influencing device performance. These techniques provide detailed information on bubble and droplet dynamics, convection patterns, and fluid velocities, offering insights into optimizing bubble and droplet removal strategies and improving overall efficiency. Schlieren imaging, in particular, stands out as a tracer-free method capable of directly visualizing variations in species concentration by measuring refractive index changes.

This technique has proven effective in investigating hydrogen and oxygen bubble dynamics during water electrolysis and holds promising potential for application in fuel cell systems, based on analogous studies of gas flow in internal combustion engines. While optical methods have yielded valuable results, researchers acknowledge the importance of carefully considering potential limitations, such as the possibility that tracer particles may alter bubble dynamics in certain measurements. Furthermore, addressing cross-sensitivity between temperature and concentration is crucial for accurate interpretation of schlieren imaging data. Future work will likely focus on refining these techniques and developing next-generation sensing concepts to overcome remaining challenges in understanding and controlling fluid transport within these devices, ultimately accelerating the development of efficient electrochemical systems.