Observation of Preferential Pathways for Oxygen Removal through Porous Transport Layers of Polymer Electrolyte Water Electrolyzers (original) (raw)
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Journal of The Electrochemical Society, 2021
Developments of the porous transport layers (PTLs) in recent years resulted in significant performance improvements in polymer electrolyte water electrolyzers (PEWEs). One of the milestones of the material design was the integration of a microporous layer (MPL) on sintered titanium PTLs. Utilizing high-resolution neutron imaging, the water and gas distribution in the multi-layered porous transport media (ML-PTL) was probed at various current densities (up to 4 A cm −2) and pressure conditions up to 8 bar, using a series of four materials, differing in MPL morphology. The water and gas distribution measured is greatly affected by the presence of an MPL. While in the bulk of the PTL, the gas accumulation is increased in the presence of an MPL, in the MPL itself more water is retained. The finer the MPL structure, the higher the liquid saturation. It is observed that the two-phase flow in the MPL has minor influence on the performance of the cell even though the gas accumulation at the CL interface is greatly reduced. The improvements, therefore, appear to be related to the CL and MPL interaction on sub-micron scale and microstructure effect on catalyst area utilization.
ACS Sustainable Chemistry & Engineering, 2018
In this article, we have brought a different perspective to the topic of mass transport losses in a Proton Exchange Membrane (PEM) water electrolyzer, particularly to the role of water flow and on the dominant mass transport mechanism in the Porous Transport Layer (PTL). We conducted permeation experiments on a sintered Ti PTL, where we measured the pressure loss of gas that flows through its pores; furthermore, we presented a model based on the van Genuchten-Mualem capillary pressure and the Carman-Kozeny gas permeability, and we report an increase in the pressure loss with respect to the water flow, which we reported as an increase in the apparent tortuosity of the pores in the PTL. From this we conclude that the water flow exerts a shear stress on the gas flowing through the PTL, proportional to its kinetic energy, and that the gas permeation is the dominant transport mechanism within a PTL, in contrast to a one-or two-phase flow, which is more energy demanding. Finally, we propose that further work be carried out, in particular by comparing these results to insitu measurements on an operating PEM electrolyzer.
Heat and Mass Transfer, 2018
In this paper two-dimensional (2D), two-phase numerical model is proposed to investigate the effect of water velocity in the channel on the two-phase flow regime in polymer electrolyte membrane (PEM) electrolyzer porous transport layer (PTL). To simulate the movement of gas-liquid interface finite element method has been used. The model includes a porous media as PTL and a water channel. The water and air is considered as incompressible. The results showed different water velocities although causing different paths of two-phase flow in the PTL have little effect on the type of two-phase flow regimes in the porous media. On the other hand, different water velocities cause different two-phase flows in the channel. For effective removal of airflow in the PTL, the range of water velocities in which the two-phase flow regime in the channel is a bubbly flow is recommended. Therefore, the minimum velocity is necessary for the bubbly flow in the channel.
2020
In the face of increasing demand for renewable energy sources to replace fossil fuels, current technologies face the issue of steady energy supply from source to power distribution. As most current renewable energy methods either rely on intermittent natural sources or are geographically bound, they require an energy storage medium. Polymer Electrolyte Water Electrolyzers are a theoretically ideal solution to this demand, but they require some performance improvements in practice to be economically viable. This study analyses the current distribution in such electrolyzers in the aim of gaining insight to improve their performance. The current distribution diagnostic can capture local regions of low performance and offers valuable insight into mass transport phenomena for cells with different flow fields and diffusion media. Two types of flow fields and four different diffusion media were used. The parallel flow field showed better performance under normal operating conditions. The
International Journal of Hydrogen Energy, 2019
h i g h l i g h t s We photographed O 2 evolution and bubble nucleation in a PEM electrolyzer. Photographs were analyzed in the flow field channel and on the surface of an Ir electrode. Digital bubble counting showed an invariant bubble radius with respect to water flow. We modeled the radii of small bubbles using a bubble force balance model. Model agrees with observations; bubble radii constant in water flow range 0e90 l h-1 .
Journal of Power Sources, 2009
A novel water porosimeter and its use in determining the capillarity of gas diffusion layers are described. It is found that, in accordance with the Washburn equation, the pressure required to force water into the gas diffusion layer depends on the cosine of the contact angle of water with the surface of the pore. Negative pressure is required to withdraw water from the gas diffusion layer, even when the surface is hydrophobic. The negative pressure required is found to be independent of surface contact angle. It is shown that the performance of gas diffusion layers in an operating fuel cell can be qualitatively predicted from the capillary pressure curves obtained. The advantages of the use of water porosimetry over the use of either mercury porosimetry or porosimetry using wetting fluids are discussed.
Applied Microscopy, 2017
In this investigation, synchrotron X-ray imaging was used to investigate the water distribution inside newly developed gas diffusion media in polymer electrolyte membrane fuel cells. In-situ radiography was used to reveal the relationship between the structure of the microporous layer (MPL) and the water flow in a newly developed MPL equipped with randomly arranged holes. A strong influence of these holes on the overall water transport was found. This contribution provides a brief overview to some of our recent activities on this research field.
Energy, 2016
In this study, synchrotron X-ray imaging is used to investigate the water transport inside newly developed GDM (gas diffusion medium) in polymer electrolyte membrane fuel cells. Two different measurement techniques, namely in-situ radiography and quasi-in-situ tomography were combined to reveal the relationship between the structure of the MPL (microporous layer), the operation temperature and the water flow. The newly developed MPL is equipped with randomly arranged holes. It was found that these holes strongly influence the overall water transport in the whole adjacent GDM. The holes act as nuclei for water transport paths through the GDM. In the future, such tailored GDMs could be used to optimize the efficiency and operating conditions of polymer electrolyte membrane fuel cells.
Physical Review E, 2021
Pore structures and gas transport properties in porous separators for polymer electrolyte fuel cells are evaluated both experimentally and through simulations. In the experiments, the gas permeabilities of two porous samples, a conventional sample and one with low electrical resistivity, are measured by a capillary flow porometer, and the pore size distributions are evaluated with mercury porosimetry. Local pore structures are directly observed with micro X-ray computed tomography (CT). In the simulations, the effective diffusion coefficients of oxygen and the air permeability in porous samples are calculated using random walk Monte Carlo simulations and computational fluid dynamics (CFD) simulations, respectively, based on the X-ray CT images. The calculated porosities and air permeabilities of the porous samples are in good agreement with the experimental values. The simulation results also show that the in-plane permeability is twice the through-plane permeability in the conventional sample, whereas it is slightly higher in the low-resistivity sample. The results of this study show that CFD simulation based on micro X-ray CT images makes it possible to evaluate anisotropic gas permeabilities in anisotropic porous media.