Exposing how gas transport occurs in porous transport layers and how it affects the performance of proton exchange membrane water electrolysis

Significance 

Recent advancements in the field of renewable energy have led to the development of more efficient hydrogen production technologies. In particular, proton exchange membrane water electrolysis is a technology that has recently caught the attention of industry and academia. Such technology generates hydrogen by electrochemically dissociating water and is innately capable of operating intermittently, making it an ideal technology for coupling with renewable electricity production.

Currently, the design of these systems takes into consideration the choice of cost-effective materials that can drive down the system costs as well as the operating conditions required to efficiently generate hydrogen. Parameters such as pressure, water flow, current density and temperature have a large influence in determining the efficiency of water electrolysis; however, these operating conditions have not been fully explored thus attracting significant attention of researchers.

Recent findings have shown that the two-phase flow of the evolved gases (hydrogen and oxygen) and the inlet water (which is required both as coolant and as reagent) is a crucial phenomenon that needs to be understood to drive the optimization of the design and operating conditions of membrane-based water electrolyzers. Unfortunately, this problem has not yet been fully investigated; therefore, the development and validation of a multiphase mass transport model is highly desirable.

To this note, Julio Garcia-Navarro and Mathias Schulze, together with Professor Andreas Friedrich at the Institute of Engineering Thermodynamics of the German Aerospace Center (DLR) investigated the role of mass transport in proton exchange membrane water electrolyzers. Specifically, they conducted experiments through the porous transport layer of an electrolyzer, which is one of the most critical components of the system regarding mass transport. Their experiments took into consideration the interaction between gas and liquid in microscopic pores and how the pressure drop is affected by water flow. Their research work is currently published in the research journal, ACS Sustainable Chemistry & Engineering.

In brief, the research team cross-examined the measured mass transport losses in proton exchange membrane electrolyzers by developing an energy balance model that takes into account the capillary pressure of the pores of the porous transport layer via the van Genuchten-Mualem capillary pressure model together with the Carman-Kozeny equation. With the developed energy balance equation, they assessed pressure loss associated with the friction between the gas exiting and the water entering the electrolyzer. Furthermore, water was pumped to force the gas over the porous transport layer, simulating the actual conditions during gas evolution in the electrolyzer. This enabled the determination of pressure drops and calculation of energy losses, and their dependence on the water flow. Eventually, the permeability of the gas through the pores was determined through a design energy balance on the system.

From their experiments, the authors ascertained that single-phase permeability was the main mass transport mechanism in proton exchange membrane electrolyzers. Single-phase permeability is a less energy-demanding flow mechanism as compared to two-phase flows in microscopic pores, which is the withstanding mass transport theory found in the literature. This was attributed to the induced shear stress on the gas exerted by the water flow in the boundaries of the microscopic channels in the porous transport layer. In addition, they noted a correlation between the tortuosity of the pores and pressure loss with regard to water flow.

In summary, the authors successfully demonstrated the effects of water flow and porous transport layer on the performance of proton exchange membrane water electrolyzers. In general, the study provided vital information that will advance further studies aimed at improving cell design. For instance, it will enable the optimization of porous transport layers by providing a theoretical framework to understand the effects of material properties such as porosity as well as operating parameters such as water flow.

Exposing how gas transport occurs in porous transport layers and how it affects the performance of proton exchange membrane water electrolysis - Advances in Engineering

Exposing how gas transport occurs in porous transport layers and how it affects the performance of proton exchange membrane water electrolysis - Advances in Engineering Exposing how gas transport occurs in porous transport layers and how it affects the performance of proton exchange membrane water electrolysis - Advances in Engineering

About the author

Dr. K. Andreas Friedrich is a Professor of Mechanical Engineering at University of Stuttgart and the Head of the Electrochemical Energy Technology Department at the German Aerospace Center (DLR) in Stuttgart, Germany.

His research areas are electrochemical energy conversion and storage, in particular polymer electrolyte fuel cells and electrolysis, solid oxide cells, system design and optimization. Dr. Friedrich has authored and coauthored about 200 reviewed papers. He received the Fischer medal (Dechema) in 2009 and the Ertl prize 2014 for his scientific work.

Dr. Friedrich has an initial background in Physics and Physical Chemistry. Starting with early fundamental work on electrochemical interfaces and electrocatalysts the research has become increasingly application oriented. His doctoral study on non-linear optics at electrochemical interfaces was performed at the Fritz-Haber Institute of the Max-Planck-Society from 1987 –1990 under the supervision of Prof. Dr. G. Ertl (Nobel prize laurate in Chemistry).

In 2004 he joined the DLR and University of Stuttgart heading the group at DLR focusing on electrolysis, fuel cells and advanced batteries. Priorities are polymer membrane and solid oxide technology as well as “beyond Li-ion” technology in batteries. The activities of the group have received the f-cell Award in Silver 2016 for electrolysis components, the Clean Tech Media Award 2012 (Aviation) and the f-cell Award in Silver 2008 (DLR with Airbus).

About the author

After graduating cum laude as a Chemical Engineer from the National Autonomous University of Mexico in 2012, Julio Cesar Garcia-Navarro obtained a Master of Science degree from Delft University of Technology in 2014. His thesis presented a non-equilibrium thermodynamics model of a PEM fuel cell, and was awarded with a cum laude distinction.

Between 2015 and 2018 he carried out his PhD research at the Institute of Engineering Thermodynamics of the German Aerospace Center in Stuttgart, Germany under the supervision of Prof. K. Andreas Friedrich, where he studied the multiphase mass transport phenomena in PEM electrolyzers using imaging techniques as well as in-situ characterization, focusing on electrochemical impedance spectroscopy.

Currently he works as a Senior Technology Manager at HyET Hydrogen in Arnhem, the Netherlands, where he is in charge of developing the world’s first PEM electrochemical hydrogen compression stack to perform single-stage compression up to 1000 bar.

About the author

Dr. Mathias Schulze received his PhD in Physics in 1988 from the University in Hannover in the field of surface science. Soon after in 1989 he joined the German Aerospace Center first as postdoc and later he was promoted to staff scientist.

He specialized in the application of surface analytical methods to investigate ageing effects of technical electrodes in fuel research in the department of Electrochemical Energy Technology. He widened his activity to the characterization of polymer electrolyte membrane fuel cells and their components with physical and electrochemical methods and became leader of many national and European projects. In addition, currently he manages the interreg NWE project “fccp” which aims to develop and deploy fuel cell cargo pedelecs in several European cities to reduce the emissions on the last mile transport of goods.

Reference

Garcia-Navarro, J., Schulze, M., & Friedrich, K. A. (2019). Understanding the role of water flow and the porous transport layer on the performance of Proton Exchange Membrane Water Electrolyzers. ACS Sustainable Chemistry & Engineering, 7(1), 1600-1610.

Go To ACS Sustainable Chemistry & Engineering

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