Exchange Membrane Fuel Cell Gas Research Articles

A multivariable sliding mode predictive control method for the air management system of automotive fuel cells

The proton exchange membrane fuel cell gas control has been one key point in fuel cell management systems. The complexity and coupling of the air management system make it difficult to achieve precise air intake adjustment. In this paper, an accurate joint control method for the air flow and pressure regulation is proposed. The nonlinear mathematical model of the air management system is developed to describe the output characteristic and state change. Based on this, the feedback linearization method is proposed to obtain the direct correspondence between control variables and controlled variables. In addition, to solve the problem that the controlled variables cannot be measured directly, an extended state observer is applied to estimate the stack cathode pressure. The sliding mode predictive control method is proposed to control the oxygen excess ratio and cathode pressure simultaneously. The relative order of the system is used to design the sliding mode surface, and the corresponding predictive model is proposed. The results obtained by simulation experiments show that pressure and mass flow have little effect on each other through decoupling. The proposed algorithm has been verified to have high precision, fast response, and robustness through comparative experiments.

Effect of three-dimensional geometric structure and surface wettability on two-phase flow behavior in a proton exchange membrane fuel cell gas flow channel: modeling and simulation

Improving water management capability is crucial for enhancing the performance and extending the lifespan of a proton exchange membrane fuel cell (PEMFC). To achieve this goal, this study investigates the coupling effects of geometric features induced by actual gas flow channel (GFC) manufacturing processes and the surface wettability on the dynamic droplet change by simulating 32 cases using the volume of fluid (VOF) method. In addition, the simulated dynamic characteristics of the droplets, such as the centroid position and droplet height, have been verified by comparing them with experimental observations. The reliable results of the simulation show that varied draft angles contribute to different local vortices in the flow channels, arousing two drainage mechanisms—spontaneous and overflow drainage. Besides, the compared results of velocity contours suggest that the curvature effect along the water movement direction could strengthen the speed-up effect around a water droplet, thereby facilitating its detachment. By contrast, a cross-section with filleted corners could cause velocity loss and poor drainage efficiency. A geometric structure with a draft angle of 30° and a bend radius of 1 mm of wavy GFC is beneficial to shorten the water removal time and minimize gas diffusion layer (GDL) water coverage. The hydrophobic treatment of the GDL and channel wall meets both criteria of rapidly reducing liquid water coverage on both the GDL surface and the entire channel. This study's findings are expected to guide the target of both geometric manufacturing and surface treatment processes.

Influence of Three-Dimensional Geometric Structure and Surface Wettability on Two-Phase Flow Behavior in a Proton Exchange Membrane Fuel Cell Gas Flow Channel: Modeling and Simulation

Influence of Three-Dimensional Geometric Structure and Surface Wettability on Two-Phase Flow Behavior in a Proton Exchange Membrane Fuel Cell Gas Flow Channel: Modeling and Simulation

Influence of Three-Dimensional Geometric Structure and Surface Wettability on Two-Phase Flow Behavior in a Proton Exchange Membrane Fuel Cell Gas Flow Channel: Modeling and Simulation

Influence of Three-Dimensional Geometric Structure and Surface Wettability on Two-Phase Flow Behavior in a Proton Exchange Membrane Fuel Cell Gas Flow Channel: Modeling and Simulation

Mechanism analysis of the effect of different gas manifold positions on proton exchange membrane fuel cell cold start performance

The balanced distribution of water and heat inside the cell during the startup step is the key to the successful cold start of proton exchange membrane fuel cell. The cold start experiments prove that due to the influence on the internal water and heat distribution, different gas manifold positions have a significant effect on cold start performance. When cold start is at −10°C, the generated water in the cathode side mostly exists in the form of supercooled water. This allows all −10°C cases successfully reach the temperature above the freezing point. When the H2 and air inlet manifolds are arranged on the different side, sudden current rise occurs at the beginning of the startup step. When cold start is at −15°C, the generated water freezes quickly, which causes the performance degradation in local areas. Two cases with the same side H2 and air inlet manifolds show the better cold start performance at −15°C. Combining all the results, during cold start, the proton exchange membrane fuel cell gas manifold position that the H2 and air inlets at the bottom and outlets at the top is recommended.

Study of the anisotropic permeability of proton exchange membrane fuel cell gas diffusion layer by lattice Boltzmann method

In proton exchange membrane fuel cells (PEMFCs), the gas diffusion layers (GDLs) are located between the flow channel and the catalytic layer, which not only provides a transport channel for the reaction gas and the output water, but also its fibrous skeleton is the key structure for the electron load transfer, and its transport performance is an important factor that affects the performance of the cell. In this paper, GDLs are stochastically reconstructed and the Lattice Boltzmann method (LBM) is adopted to study permeability. The changes of through-plane permeability and in-plane permeability are studied by varying the porosity, fiber diameter, and pressure drop. It is found that the porosity plays a dominant role in the variation of permeability, and the through-plane permeability at a porosity of 0.85 is 24.78 times greater than that at 0.5. The influence of fiber diameter can not be ignored. When the diameter is larger than 8 μm, the influence on permeability is more obvious, and the permeability is no longer sensitive to changes in diameter after a certain degree of porosity has been reached. The pressure drop only changes the magnitude of the average cross-sectional velocity of the gas diffusion layer, and the effect on the permeability is negligible. The in-plane permeability is larger than the through-plane permeability. In the scope of the paper research, the ratio of the two values ranges from 1.3 to 1.7.

Modeling of liquid water transport in a proton exchange membrane fuel cell gas flow channel with dynamic wettability

Liquid water transport in the gas flow channel is significantly important for the water removal and management in proton exchange membrane fuel cells. Previous numerical studies consider a single and constant static contact angle for the liquid water transport on the channel surface, which is insufficient to account for the dynamic wettability behavior of the flow. In this study, a dynamic wettability model is developed that incorporates the sliding angle and dynamic contact angles for the simulation of water transport in the flow channel. It is found that both the sliding and dynamic contact angles have significant impact on the characteristics of the water transport and dynamics in the flow channel. Water spreading on the channel surface is elliptic, and its minor and major axes oscillate out of phase with the droplet height. The pressure loss for the 2-phase flow in the channel is directly related to this oscillation and deformation of the droplet shape. Flow channel surface with a small sliding angle facilitates the water transport and removal and reduces the associated pressure loss in the channel. The conventional static wettability model would overpredict droplet deformation and breakup as well as the pressure loss in the channel.

Water emergence from the land region and water–sidewall interactions in Proton Exchange Membrane Fuel Cell gas channels with microgrooves

Liquid water produced in a Proton Exchange Membrane Fuel Cell (PEMFC) can adversely affect the fuel cell performance in two ways: (a) reduction in surface area available for reactant transport at the channel–gas diffusion layer (GDL) interface, and (b) increase in two-phase pressure drop in channels leading to flow maldistribution and increased pumping power. Further, the channels blocked by water reduce reactant availability at reaction sites. Most of the earlier water transport studies were focused on water droplet formation on the gas diffusion layer (GDL) in the channel and its removal from the gas flow without considering the sidewall interactions. In an actual fuel cell, water under the land emerges in the channel and fills the corner, drawing in additional water from the GDL surface. The present work explores water droplet–sidewall interactions and the transport of water from the corner region. Transverse micro-grooves are introduced on the sidewalls and their effect on water removal from the corner region, flow patterns, area coverage ratio and pressure drop are investigated. The micro-grooves are also seen to introduce a wetting regime that facilitates removal of water at the channel exit without causing blockage at the manifold region.

Compact and flexible hydrocarbon polymer sensor for sensing humidity in confined spaces

Hydrophilic polymers generally respond reversibly to changes in humidity and hence can behave as humidity sensors. A hydrocarbon based polymer, Sulfonated Poly (ether ether ketone) (SPEEK) has been investigated as humidity sensor. Sensors of size 2 mm × 4 mm with inter-digitated gold electrodes were fabricated using photolithography technique. The resistance change was noted at varying relative humidity in a humidity chamber. The polymeric membrane was characterized using Fourier Transform Infrared Spectroscopy for the functional groups and with a Dynamic Mechanical Analysis for flexibility. The sensor showed a fast response time of approximately 20 s. Low hysteresis, good cyclability and repeatability were observed for the sensor. As an application to test in confined spaces the sensor was placed inside narrow proton exchange membrane fuel cell gas flow fields for humidity mapping of the cathode plates while operating the cell. For a fuel cell operating at 50 °C with dry and humidified fuel gas (dry oxidant) the sensor placed at the centre of the cathode shows an RH of 30–40% and 60–90% respectively at different current densities.

Contact line characteristics of liquid–gas interfaces over grooved surfaces

Surface wetting is an important phenomenon in many industrial processes including micro- and nanofluidics. The wetting characteristics depend on the surface tension forces at the three-phase contact line and can be altered by introducing patterned groove structures. This study investigates the effect of the grooves on the transition in the wetting behavior between the Cassie to Wenzel regimes. The experiments demonstrate that the wettability on a patterned surface depends on the spacing factor (S = channel depth/channel width). The spacing factor influences the contact angle, contact angle hysteresis, and the transition characteristics between the Cassie and Wenzel states. It was noted that under certain conditions (S > 1) the droplet behaved as a Cassie droplet, while exhibiting Wenzel wetting the rest of the time on the silicon microchannels tested. This criterion was used to design the groove structures on the sidewall of the proton exchange membrane fuel cell gas channel to remove the water effectively. The water coming from the land region into the gas channel is pulled by the grooves to the top wall where the airflow aided in its removal. Also, the contact angles measured on the surfaces were compared with the classical models that use wetted area, and the contact line model that uses the three-phase contact line length. It was found in our experiments that the contact line model predicts the contact angle on the patterned groove surfaces more accurately than the classical models.

Water droplet dynamic behavior during removal from a proton exchange membrane fuel cell gas diffusion layer by Lattice-Boltzmann method

A major challenge in the application of proton exchange membrane fuel cells (PEMFCs) is water management, with the flooding of electrodes as the main issue. The Lattice-Boltzmann method (LBM) is a relatively new technique that is superior in modeling the dynamic interface of multiphase fluid flow in complex microstructures such as non-homogeneous and anisotropic porous media of PEMFC electrodes. In this study, the dynamic behavior of a water droplet during removal from gas diffusion layer (GDL) of a PEMFC electrode with interdigitated flow field is simulated using LBM. The effects of GDL wettability and its spanwise and transverse gradients on the removal process are investigated. The results demonstrate great influence of wettability and its spanwise and transverse gradients on the dynamic behavior of droplets during the removal process. Although increasing the hydrophobicity of GDL results in better droplet removal, its increase beyond a critical value does not show a significant effect.

Effect of channel materials and trapezoidal corner angles on emerging droplet behavior in Proton Exchange Membrane Fuel Cell gas channels

Ex situ experiments were conducted to study water droplet dynamics in Proton Exchange Membrane Fuel Cell (PEMFC) gas channels with different trapezoid channel open angles and GDL materials under controlled air velocities (0.48–7.23 m s−1). High speed videos revealed that the droplets interacted with the channel walls at lower air velocities (0.48–4 m s−1) and led to channel corner filling depending on the channel open angle. However, at higher air velocities (4.1–7.23 m s−1), the droplets did not contact the channel walls and slid off from the GDL surface. For higher air velocities, a correlation was obtained for the minimum velocity required to remove the droplet from the channel without interacting with the channel walls. For lower air velocities, the minimum pressure drop required for removing the droplet from the gas channel for both corner filling and non-filling scenarios was obtained. The study clearly establishes the important role played by the channel sidewalls and the channel angle on the water droplet transport characteristics.

Assessing porosity of proton exchange membrane fuel cell gas diffusion layers by scanning electron microscope image analysis

A gas diffusion layer (GDL) in a proton exchange membrane fuel cell may consist of several, materials of different porosities, with each material serving a specific set of functions. For example, samples analyzed in this work consisted of a macro porous carbon paper substrate treated with a, hydrophobic wet proofing material in differing amounts, which was then coupled to a micro porous, layer. The porosities of four such GDLs were determined by using 2D scanning electron microscope (SEM) images to mathematically model the volumes filled by each solid in the 3D structures. Results, were then compared with mercury intrusion porosimetry (MIP) measurements to verify the accuracy, of the method. It was found that the use of SEM not only allowed for detailed porosity analysis of, separate porous materials within the GDL, but also porosity for the entire GDL could be calculated for, the seemingly complex structures with reasonable accuracy. With some basic geometric assumptions, and use of the superposition principle, the calculated results were accurate to less than a 2% absolute, difference of the porosity measured by MIP for each of the four samples analyzed.

Numerical determination of the effective thermal conductivity of fibrous materials. Application to proton exchange membrane fuel cell gas diffusion layers

The knowledge of physical properties, such as the thermal conductivity, plays an important role in the management of the heat transfer in fibrous materials like PEFC gas diffusion layers. Measurement of thermal conductivity by experimental means is not easy (due to the anisotropy and the high porosity), therefore the availability of experimental data is rather limited. In this paper, the numerical determination of the effective thermal conductivity of fibrous materials is investigated using a three-dimensional approach. Two different fiber geometries were studied with randomly generated fiber structures with overlapping and non-overlapping fibers. The corresponding anisotropic thermal conductivities are computed through the solution of the energy transport equation. The results were validated through a comparison with existing experimental data and the influence of different parameters such as fiber orientation, fiber diameter and binding material were investigated.

The effects of wetproofing on the capillary properties of proton exchange membrane fuel cell gas diffusion layers

In the analysis of proton exchange membrane (PEM) fuel cell components, the capillary pressure vs. saturation ( P C ( S L )) curve is an increasingly popular tool for understanding the interaction of liquid water with the porous gas diffusion layer (GDL) material. In this study, hysteretic water/air P C ( S L ) measurements were combined with mercury intrusion porosimetry (MIP) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging to quantify the effects of fluoropolymer loading on GDL samples. Commercially wetproofed carbon fiber papers with 0–40 wt.% Teflon loading were investigated. MIP showed a slight reduction in characteristic pore radii and a significant loss of pore volume at the highest Teflon loading. Water/air P C ( S L ) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Teflon loading, but negligible additional wetproofing at loadings from 10 to 40 wt.%. ToF-SIMS imaging, a technique that is sensitive to monolayer surfaces coverages, found that GDL materials with 5 wt.% Teflon loading displayed nearly complete fluoropolymer coverage on the carbon substrate, confirming P C ( S L ) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for P C ( S L ) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130° and 133° in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66° and 70° for the same materials.

Aging Studies of Pt/Glassy Carbon Model Electrocatalysts

A model Pt catalyst nanoarray on planar glassy carbon (GC) was synthesized using a diblock copolymer synthesis route and subjected to a potential cycle aging procedure. Preliminary experimental data show initial and degradation behavior similar to that of proton exchange membrane fuel cell gas diffusion electrodes. Commonalities include similar cyclic voltammetry and CO stripping voltammetry features, Pt active surface area temporal changes, and Pt particle size effect on CO oxidation peak potential. Furthermore, atomic force microscopy provides evidence for both Pt dissolution and Pt nanoparticle migration/coalescence as aging mechanisms. Experimental data obtained with Pt model catalysts also revealed that Pt dissolution may be the rate-determining step under certain operating conditions. Thus, the beneficial use of Pt/GC model electrocatalysts as surrogates for commercial gas diffusion electrodes is validated.

Electrically Conductive Polymer-based Blends for Proton Exchange Membrane Fuel Cell Gas Diffusion Layers

Two electrically conductive polymer-based blends were carefully formulated, using twin-screw extrusion process, in order to develop porous gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Two low-viscosity polymers, polyamide 11 (PA11) and Polyethylene terephtalate (PET), were selected as the matrix phase. Blend electrical conductivity was ensured by using a mixture of high specific surface area carbon black and synthetic flake graphite. Final GDL porosity was generated by melt blending the PA11 or the PET matrix with polystyrene (PS) followed by PS extraction with tetrahydrofuran, THF, solvent. Blends processability and final GDL conductivity and porosity were compared with those recently obtained by Yakisir et al. (1, 2). PA-11 was found to be the most suitable polymer for this study since it led to best GDL porosity and through-plane conductivity.

Characterization of Ionic Conductivity Profiles within Proton Exchange Membrane Fuel Cell Gas Diffusion Electrodes by Impedance Spectroscopy

Characterization of Ionic Conductivity Profiles within Proton Exchange Membrane Fuel Cell Gas Diffusion Electrodes by Impedance Spectroscopy