Wetting properties of phosphoric acid in porous materials of high temperature fuel cells (HT-PEFC), operating at around 160 °C, are important for cell performance and durability, but the underlying ...wetting parameters have been unknown so far. Therefore, the influence of phosphoric acid temperature and concentration on the wetting behavior of porous HT-PEFC materials is investigated. The acid filling of gas diffusion and catalyst layers as function of capillary pressure is monitored with X-ray tomographic microscopy under the well defined conditions of an
ex situ
set-up at temperatures up to 160 °C. For the wetting of gas diffusion layers, with pore sizes in the order of few 10 μm, two opposing trends are shown. With increasing phosphoric acid concentration, less capillary pressure is required, while with increasing temperatures, higher capillary pressures are needed for filling up to a given saturation. The same trends are also found for the contact angle of phosphoric acid on PTFE. A higher contact angle is observed with increasing temperature while increasing the phosphoric acid concentration decreases the contact angle. As both trends are of a similar order of magnitude, the wetting behavior of concentrated (113 wt%) phosphoric acid at 160 °C is astonishingly similar to the wetting behavior of water at room temperature. Another important property for HT-PEFC operation is the filling of cracks in the catalyst layer, which have widths up to 100 μm. For large cracks (>60 μm), a capillary pressure of only 15 mbar was deduced from the measurement, increasing to 30 mbar for cracks between 20 and 60 μm. This, for the first time, allows for assessing the membrane phosphoric acid pressure during fuel cell operation. This can guide the development of improved porous materials for HT-PEFC.
The influence of phosphoric acid temperature and concentration on the wetting behavior of porous high temperature polymer electrolyte fuel cell materials is investigated.
•Methods are developed to predict transport properties of dry GDL in PE Fuel Cells.•Diffusivity and Permeability are reliably predicted based on 3D characteristics.•Predictions based on 3D ...microstructure match well with numerical simulations.•Anisotropy is due to in- and through-plane variation of tortuosity and hydraulic rad.•The methods can be used to predict relative permeability and diffusivity in wet GDL.
New quantitative relationships are established between effective properties (gas diffusivity, permeability and electrical conductivity) for a dry GDL (25 BA) from SGL Carbon with the corresponding microstructure characteristics from 3D analysis. These microstructure characteristics include phase volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. The latter two parameters include information from two different size distribution curves for bulges (continuous PSD) and for bottlenecks (MIP-PSD). X-ray tomographic microscopy is performed for GDL at different compression levels and the micro-macro-relationships are then established for the in-plane and through-plane directions. The predicted properties based on these relationships are compared with numerical transport simulations, which give very similar results and which can be summarized as follows:
Gas diffusivity is higher in the in-plane than in the through-plane direction. Its variation with compression is mainly related to changes of porosity and geodesic tortuosity. Permeability is dominated by variations in hydraulic radius. Through-plane permeability is slightly higher than in-plane. Anisotropy of electrical conductivity is controlled by tortuosity, which is higher for the through-plane direction. A table with new quantitative relationships is provided, which are considered to be more accurate and precise than older descriptions (e.g. Carman-Kozeny, Bruggeman), because they are based on detailed topological information from 3D analysis. Furthermore, when using these relationships as input for macro-homogenous modeling, this enables to simulate microstructure effects of real GDL (SGL 25 BA) more accurately. In future, the same methodology can be used to study micro-macro relationships in wet GDL and to predict relative liquid permeability and relative gas diffusivity.
Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) ...in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples.
Performance and durability of polymer electrolyte fuel cells are closely related to the water management. In gas diffusion layers (GDL), the presence of liquid water is associated with mass transport ...losses. For optimization of the materials, mechanisms and parameters influencing the water saturation have to be understood. Ex-situ water injection and withdrawal experiments, allowing for well-defined boundary conditions, have been performed with three different GDL materials, using X-ray tomographic microscopy to image the liquid water phase on the pore scale of the materials. The liquid saturation in the GDLs has been imaged as function of the capillary pressure. The results reveal that, due to the anisotropic structure of the GDLs, transport of water occurs mainly in the through-plane direction via parallel water paths. When the GDL is coated with a microporous layer (MPL), liquid saturation requires higher capillary pressure to overcome the MPL/GDL mixed region where pore and throat sizes are reduced and the water paths are restricted to the crack regions of the MPL.
Acid migration and loss at high current densities has previously been identified as an important degradation mechanism for phosphoric acid in high temperature polymer electrolyte fuel cells. In this ...process, the structure of the anode catalyst layer plays an important role for the acid migration mechanism. Therefore, the acid retaining capabilities of two significantly different catalyst layer structures were investigated by means of operando X-ray tomographic microscopy.
In a commercial catalyst layer, with cracks with a mean width of 39 μm, ideal crack connectivity and no bottlenecks in the crack structure, phosphoric acid penetrates and traverses the catalyst layer and migrates to the GDL and gas channel. In contrast, an in-house catalyst layer retained phosphoric acid within itself. Although with lower mean crack size of only 20 μm, the different crack connectivity and accessibility, as determined by crack width and simulated mercury intrusion crack size analysis, were identified as main causes for better acid retaining capabilities. A fraction of over 95% of the crack-volume is protected by bottlenecks smaller than 20 μm. The present analysis is a guideline for engineering acid retaining catalyst layers structures for high temperature polymer electrolyte fuel cells.
•Acid pressure build-up in high temperature polymer electrolyte fuel cells•Influence of catalyst layer crack structure on phosphoric acid migration•Mitigation of phosphoric acid migration
A computational fluid dynamics model of a channel (one liquid inlet, one liquid inlet and one two-phase outlet), applicable for PEFC gas channel water transport, is developed. A volume of fluid ...approach is used to study the two-phase flow behavior (interface-resolved) inside the gas channel, including the surface of GDL (gas diffusion layer), which is verified by experimental results from synchrotron based X-ray radiography and tomography imaging. A reasonably good agreement is found between the model and the measurements in terms of droplet dynamics, shape, and size. The channel height strongly influences the droplet transport behavior, with the droplet being attached to the GDL surface, as well as to the wall on the opposite side to the GDL at the same time for the shallowest channel (150 μm). The GDL contact angle influences the droplet size, with an increased GDL contact angle creating smaller droplets.
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•VOF model for a PEFC gas channel was verified by synchrotron measurements.•A smaller GDL surface liquid inlet yields significantly smaller droplets.•An increased contact angle gives smaller droplets.•Big impact on droplet dynamics from channel dimensions.
A real time control strategy for fuel cell hybrid vehicles is proposed. The objective is to reduce the hydrogen consumption by using an efficient power sharing strategy between the fuel cell system ...(FCS) and the energy buffer (EB). The energy buffer (battery or supercapacitor) is charge-sustained (no plug-in capabilities). The real time control strategy is derived from a non-causal optimization algorithm based on optimal control theory. The strategy is validated experimentally with a hardware-in-the-loop (HiL) test bench based on a 600
W fuel cell system.
In high temperature polymer electrolyte fuel cells, phosphoric acid migration induces flooding of the anode gas diffusion layer at high current densities. The present study focuses on determining the ...influence of phosphoric acid flooding of the anode GDL on hydrogen mass transport limitations. Two methods for quantifying the performance losses at high current densities, related to acid migration, are discussed: anodic limiting current density measurements and electrochemical impedance spectroscopy. It is demonstrated that the limiting current measurements, the common method for determining transport resistances, is unable to detect the changes induced by acid migration, due to the transient time required when switching to the required low hydrogen concentrations, while EIS is able to capture the changes induced by acid migration because it is faster and less invasive. For diluted hydrogen, an increase of the transport resistance is measured, however the effect on the cell performance is negligible. The time constants for anode GDL flooding and de-flooding are determined based on the EIS data and found to be 8.1 ± 0.1 min for flooding and about 5.8 ± 0.9 min for de-flooding under the applied conditions.
•Electrochemical impedance spectroscopy can monitor phosphoric acid redistribution.•Changes in low frequency regime of the EIS spectra are observed.•Time constants for PA flooding (8.1 min) and PA de-flooding (5.8 min) were obtained.•First method to monitor phosphoric acid redistribution electrochemically.•No impact on cell performance is observed; likely due to improved kinetics.
•Liquid distribution in GDL is investigated with injection tomography experiments.•Anisotropy of liquid breakthrough is triggered by short-range effect in through-plane.•Quantitative 3D analysis ...reveals microstructure limitations for liquid permeability.•Relative permeability (κrel)-curves are ‘S-shaped' due to distinct microstructure effects.•Macroscopic descriptions for κrel from literature are not capable to capture the ‘S-shape'.
The performance of polymer electrolyte fuel cells (PEFC) strongly depends on a controlled water management within the porous layers. For this purpose we investigate liquid water transport in a commercial gas diffusion layer (SGL 25BA) on the pore scale. X-ray tomography experiments combined with pressure-induced water injection provide 3D images of the liquid water distribution inside the GDL at incremental pressure steps between 0 and 100mbar.
The breakthrough behavior of the liquid phase is highly anisotropic. In through-plane (tp) direction first bubble points appear at the outlet plane already at 5mbar and the ‘breakthrough' then evolves continuously over an extended pressure range up to >30mbar. For in-plane (ip) direction the breakthrough is discontinuous and takes place at 27mbar. Simulations of the intrusion process reveal that the different breakthrough behaviors are mainly triggered by different ip- and tp-transport distances. Short tp-transport distances through the thin gas diffusion layer (ca. 100μm) are responsible for the characteristic continuous tp-breakthrough behavior, which is thus attributed to a so-called short-range effect.
Dedicated methods for 3D-image analysis adapted to fibrous GDL microstructures were presented in part I. With these methods we quantify all microstructure characteristics that are relevant for liquid permeability. These characteristics of pore and liquid phases include size distributions of bulges and bottlenecks, connectivity, effective volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. Quantitative relationships are established between these microstructure characteristics and the liquid permeability, which provide a better understanding of the underlying microstructure limitations for injection and liquid transport.
For the in-plane direction the liquid permeability is limited to roughly a similar extent by tortuosity, constrictivity and effective volume fraction. In contrast, for through-plane direction relatively low volume fractions of the liquid phase put stronger limitations to the liquid permeability than tortuosity, constrictivity and hydraulic radius.
The curves for relative permeability vs. saturation (and vs. capillary pressure, respectively) achieved from 3D-analysis reveal complex but characteristic (reproducible) shapes with concave, linear and convex segments. The shape of these segments can be attributed to distinct microstructure effects. In contrast, the conventional macroscopic descriptions from literature cannot capture these complex shapes and the underlying microstructure effects. Future investigations with different GDL materials are necessary in order to understand whether these complex shapes for the relative permeability represent a general feature of gas diffusion layers or if they are specific to the investigated SGL material.
Phosphoric acid electrolyte evaporation in a polybenzimidazole based high temperature polymer electrolyte fuel cell is analyzed as a function of reactant gas stoichiometry and temperature. Based on ...these results a phosphoric acid vapor pressure curve is derived to predict the fuel cell liftetime with respect to electrolyte inventory. The predicted fuel cell life was validated by means of an accelerated stress test. Additionally, the correlation between electrolyte inventory and fuel cell performance was investigated by recording H2/air and H2/O2 polarization curves during the course of the stress test to gain insight into the relation between acid inventory and the different degradation modes.
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