Temperature-Dependent Water Transport Mode in Fuel Cell
▶ PEFC Water Transport Mode
Publication: H Xu, et al. Temperature Dependent Water Transport Mechanism in PEFC Gas Diffusion Layers Revealed by subsecond Operando X-ray Tomographic Microscopy [J] 2020, In progress. Co-paper with TOYOTA
Figure 1: Schematic of two proposed water transport mechanisms in GDLs under the rib.
At high current densities operation demanded by automotive applications, the excessive product water pervades the microporous gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs), resulting shortages of gas diffusion pathways hence leading to significant performance losses. Previous experimental and modelling studies on water transport inside GDLs proposed two feasible mechanisms namely capillary-force driven and two-phase transport. Thoroughly understanding of the water transport mechanism is vitally important for the GDL materials design and operational optimizations in order to advance the water management strategy. Subsecond and submicron operando X-ray tomographic microscopy (XTM) was employed to reveal the dominating water transport mechanism inside cathode GDL at different temperatures with increasing water saturation levels driven by current ramp-up process. The results suggest that two-phase water transport dominates the initial increasing of saturation levels at typical automotive PEFC operating temperature of about 80 oC, and facilitates removal of the water accumulates impeding gases delivery to the catalyst layers. Whereas the capillary-force driven transport dominates the contribution to the total saturation from the beginning to stagnating stage at 40 oC.
Figure 2: a) Schematic plot for PEFC components; b) subsecond Operando XTM setup at TOMCAT beamtime with slip ring device used for continuous imaging; c) illustration of region of interest (ROI, yellow line indicated) selection under the rib regarding the field of view (FoV, 806 µm × 806 µm) of the 26-fold high magnification microscope.
Figure 3: Image processing pipeline including image filtering, segmentation and water cluster labelling. The legends of water cluster labelling are: BC (bottom-connected, in red), TC (top-connected, in green), FC (fully-connected, in blue) and NC (not-connected/isolated, in purple).
Figure 5: 3D surface rendering of GDL substrates for a) cell T1 and b) cell T2; local thickness mapping d) and f) of GDL substrates for cell T1 and T2, and its corresponding histogram plots c) and e) .
Figure 6: Liquid water morphologies found in Cell T1 MPL cracks, fiber pores, binder pores and binder solid areas by exploiting high magnification (26x) microscope.
3D rendering presentations of cluster connectivity labelled water saturations with selected time steps (15, 25, 45, 300 s) for 2 cells operated at 40 and 80 degrees.
▶ PEFC Water Saturation Dynamics
Publication: H Xu, et al. Influence of Gas Diffusion Layer Substrates on Operando Liquid Water Saturation in Polymer Electrolyte Fuel Cells [J] 2020, In progress. Co-paper with Swiss Light Source
Figure 6. In-plane view of water volume fraction plots of 3 different GDL substrates at selected current densities (0.25 A/cm2, 0.5 A/cm2, 0.75 A/cm2, j-lim) according to the polarization curve in Figure 3.
Figure 1: (a) Schematic of PEFC components; (b) Operando XTM imaging setup with PEFC placed inside a housing
Figure 2. Image processing and evaluation pipeline for operando fuel cell XTM imaging.
Figure 4. 3D rendering schemes of 3 different GDL substrates. (a) Freudenberg I6; (b) Toray 60; (c) SGL 24BA. And their corresponding continuous pore size distribution (PSD) indicated with Gaussian fitting for peak position as subfigure (d), (e) and (f).
Figure 11. Time series liquid water volume evolution of 3 different GDL substrates from the beginning of experiment (dry state) until 1 h of current jump from 0 to 0.5 A/cm2. And their corresponding time-series liquid saturation profile for analyzed domain under the rib and under the channels.
Figure 13. Through-plane and in-plane relative diffusivity versus current density, saturation and normalized relative diffusivity versus saturation plots of 3 different GDL substrates at all current densities according to the polarization curve in Figure 3. Solid lines represent for the analyzed domain under the rib and dashed lines for the analyzed domain under the channels. Grey dashed lines in figure (c) and (f) represents for Dnorm. rel.= (1-s) λ when λ equals to 1, 2, 3 and 4.
▶ PEFC Water Management
Publication: Nagai Y, Eller J, -Xu H, et al. Improving water management in fuel cells through microporous layer modifications: Fast operando tomographic imaging of liquid water[J]. Journal of Power Sources, 2019, 435: 226809. [Link]
Fig. 8. Schematic representation of water drainage and gas diffusion in the cathode GDL: a) Cell-M1 and b) Cell-M2.
Polymer electrolyte fuel cells (PEFCs) have been actively developed for a wide range of power generation applications. At the high power densities required for automotive applications, sophisticated water management is vital to further improve cell performance. In particular, the gas diffusion layer (GDL), which includes a microporous layer (MPL), plays a crucial role in optimizing both water drainage and gas transport between the catalyst layer and gas channels. The present work studied the effect of cathode MPL porosity on the water distribution in the GDL. The results show that cells in which MPL materials have larger, micron-sized pores exhibit better performance. Advanced 4D operando X-ray imaging (3D structure plus time) was employed to analyse the water content in GDLs, and demonstrated that the superior performance of cells with large MPL pores is due to the efficient formation of water pathways. These pathways are based on water clusters allowing percolation in the through-plane direction from the bottom to the top of the GDL. Such pathways decrease the liquid water level in the entire cathode GDL. Especially, large MPL pores merge numerous small liquid water pathways in the catalyst layer and stabilize them morphologically, thus creating primary pathways for effective water drainage.
Fig. 1. a) Components of the operando cell employed for XTM and b) the XTM cell, complete with head socket and heated gas feed tube, and unheated exhaust gas lines at the cell top. XTM reconstructions of the operando fuel cell: c) horizontal slice through the cell and d) in-plane slice through the cathode GDL; in c) and d) the regions of interest for the evaluation of channel (red dashed line), rib (blue) and repetition unit (green) are indicated. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Materials
Fig. 3. a) Pore size distribution in the MPL region obtained from micro (0.65 μm/voxel) and nano (70 nm/voxel) XTM measurements. GDL-M1(open symbols), GDL-M2 (solid symbols). b) Percolation paths of GDL-M1 and GDL-M2 in the MPL region obtained from micro XTM (0.65 μm/voxel). c) Variations in water pressure during water permeability tests and d) liquid water distributions of GDL-M1 and GDL-M2 during the water injection at 100 kPa, obtained from micro XTM at 0.65 μm/voxel.
Fig. 5. a) Diagram showing the different water cluster types in the cathode GDL domain. Temporal variations in the liquid water saturation in the cathode GDL under constant currentoperation: b) Cell-M1 at 0.5 A cm-2, c) Cell-M2 at 0.5 A/cm-2, d) Cell-M1 at 1.0 A cm-2 and e) Cell-M2 at 1.0 A cm-2. Conditions: cell temperature 40 °C, dry gas velocities 7.6 m s-1 for air and 5.3 m s-1 for H2, relative humidity 105% for air and 100% for H2, gas pressure 100 kPa.
