In high temperature polymer electrolyte fuel cells, at high current densities, phosphoric acid (PA) migrates toward the anode and invades catalyst, microporous and gas diffusion layers (GDL). This ...work studies this PA redistribution using synchrotron based operando X-Ray tomographic microscopy (XTM) and electrochemical impedance spectroscopy (EIS) during a current cycling protocol. It is shown that under reformate conditions, during the first 2 minutes after a positive current step, the cell voltage increases due to better wetting of the anode catalyst layer (CL). From 2 to 20 minutes, the cell voltage drops due to increasing mass transport losses in the microporous layer (MPL) and the GDL. At the anode, cracks in MPL and CL, both with widths up to 150 μm, are flooded within 2 minutes after a current density increase. Acid flooding is only observed for MPL cracks that overlap with CL cracks. The CL cracks therefore act as injection points for the flooding of the MPL cracks and the gas diffusion layer. No change in the PA content of any of the cathodic porous components was observed.
Synchrotron based X-ray tomographic microscopy was used to image the redistribution of phosphoric acid in HT-PEFC due to electrolyte migration from cathode to anode. The acid migration rate, ...transference number of the hydrogen phosphate ion and flooding of the anode gas diffusion layer (GDL) was analyzed for MEAs with different membrane acid doping levels (24-36 mgcm−2) and membrane materials (imbibed m-polybenzimidazole (PBI) and polyphosphoric acid (PPA) processed p-PBI). The most influential factors for the acid migration rate are current density and the amount of free acid in the membrane. High doping level of the membrane and current density above 0.4 A cm−2 significantly increase the migration rate. From the migration rates apparent transference numbers for the hydrogen phosphate anions in the order 10−5 to 10−4 are calculated at the high current densities. Besides the membrane properties, also the influence of the microstructure of the porous transport layers was analyzed. Most probably cracks in the catalyst and microporous layers facilitate the migration of acid into the anode GDL.
In this paper, we present an experimental study on the development of gas diffusion layer (GDL) materials for fuel cells with dedicated water removal pathways generated using radiation induced ...grafting of hydrophilic compounds onto the hydrophobic polymer coating. The impact of several material parameters was studied: the carbon substrate type, the coating load, the grafted chemical compound and the pattern design (width and separation of the hydrophilic pathways). The corresponding materials were characterized for their capillary pressure characteristic during water imbibition experiments, in which we also evidenced the differences between injection from a narrow distribution channel in the center of the material (and thus strongly relying on lateral transport) and homogeneous injection from one face of the material. All materials parameters were observed to have a significant influence on the water distribution. In particular, the type of substrate has a dramatic impact, with results ranging from a nearly perfect separation of water between hydrophilic and hydrophobic domains for substrates having a narrow pore size distribution to a fully random imbibition of the material for substrates having a broad pore size distribution.
Synchrotron‐based X‐ray tomographic microscopy is investigated for imaging the local distribution and concentration of phosphoric acid in high‐temperature polymer electrolyte fuel cells. Phosphoric ...acid fills the pores of the macro‐ and microporous fuel cell components. Its concentration in the fuel cell varies over a wide range (40–100 wt% H3PO4). This renders the quantification and concentration determination challenging. The problem is solved by using propagation‐based phase contrast imaging and a referencing method. Fuel cell components with known acid concentrations were used to correlate greyscale values and acid concentrations. Thus calibration curves were established for the gas diffusion layer, catalyst layer and membrane in a non‐operating fuel cell. The non‐destructive imaging methodology was verified by comparing image‐based values for acid content and concentration in the gas diffusion layer with those from chemical analysis.
In-situ infrared spectroelectrochemical measurements have been carried out in a high temperature PEFC setup. The design of the setup allowed to acquire infrared spectra at the catalytic surface of a ...Pt layer on a conventional phosphoric acid doped polymer electrolyte membrane. CO has been used as the probing molecule and the spectroscopic signature of the resulting adsorbate has been recorded under technically relevant conditions.
A novel electrochemical setup that allowed IR spectroelectrochemical measurements at technically relevant conditions for high temperature polymer electrolyte fuel cells. Display omitted
•Novel setup for in-situ IR spectroelectrochemical measurements.•Technically relevant operating conditions for HT PEFCs achieved.•Surface induced effect observed for the IR signal of adsorbed CO.
Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of ...the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.
Management and removal of liquid water is essential to maintain and improve the overall performance of polymer electrolyte fuel cells (PEFC). X-ray tomographic microscopy (XTM) of PEFCs has been ...proven as a valuable tool in order to understand accumulation of liquid water in the gas diffusion layer (GDL), both with in-situ and operando setups. Progress in operando XTM of PEFCs has paved the way for 4D imaging studies of the water distribution in the GDL. In order to capture the water dynamics at high current density operation a further decrease of the scan time towards 0.1 s is aspired. In this work different imaging parameters, filters and their consequences on the contrast-to-noise ratio (CNR) of water versus void and water feature detectability have been studied with an ex-situ XTM experiment at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI).
Channel‐to‐channel cross convection in serpentine flow fields of polymer electrolyte fuel cells (PEFC) can influence the overall cell performance. The effect strongly depends on the gas transport ...properties of the gas diffusion layer (GDL). For the first time measured anisotropic, compression dependent permeability and effective diffusivity of GDLs are used to quantify the influence of cross convection on the local current distribution and performance. A model was developed to examine different channel‐rib geometries and GDL characteristics. The results show that cross convection can significantly increase the current density and consequently the power density of PEFCs. A strong sensitivity to GDL compression, flow velocity and rib width was found. As an optimised case the GDL thickness under the rib was increased resulting in about 20% higher current densities. Precise knowledge of the GDL characteristics and its compression are key to understand channel‐to‐channel cross convection and optimise perfomance.
Synchrotron radiation (SR) based imaging of polymer electrolyte fuel cells (PEFC), both radiography and tomography, is an attractive tool for the visualization of water in the gas diffusion layer as ...it provides temporal and spatial resolutions one order of magnitude superior to neutron imaging. Here we report on the degradation of cell performance and changes in GDL water saturation after SR irradiation of about 43% of a cell's active area. Fast X-ray tomographic microscopy (XTM) scans of 11 s duration are used to compare the GDL saturation before and after a 5 min irradiation period of the imaged section. The cell voltage and the water saturation decreased clearly during and after the exposure. Estimates of the current density of the SR exposed and non exposed cell domains underline the effect of irradiation.
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