Local Current Density Variations Research Articles

Effects of cathode gas channel design on air-cooled proton exchange membrane fuel cell performance

In the study, the impact of cathode gas channel (CGC) structure on the operation of air-cooled PEMFCs is investigated by a multiphase nonisothermal model. First, the influences of tapered channel configuration on the distribution of key variable distributions are studied. The distributions of these variables are found to be coupled with the reactive transport process within the catalyst layer (CL). The variation of local current density is found to be positively correlated with the dissolved water content. Although the tapered channel design can improve the current density, the weight of the bipolar plates (BPs) will be inevitably increased. Finally, the trapezoidal channel design is proposed aiming at improving the performance without increase of fuel cell weight. Several performance indexes are employed to comprehensively evaluate the different CGC designs, and the results demonstrate that the inverted trapezoidal CGC design, with which good thermal and water management are achieved, presents the best performance among the different CGC designs studied, leading to 18.9 % increase of the mass power density and 18.6 % increase of volume power density compared with the base case design.

Heterogeneous current and concentration distributions in a redox flow battery: Influence on the cell voltage

In order to analyze the heterogeneity of the local behavior of an organic redox flow battery, a dynamic two-dimensional model was built at the engineering scale, combining mass and charges transfer, and electrochemical. It is able to simulate correctly the local behavior of a real segmented organic battery and allows to bring additional information about the effect of the internal heterogeneities on the performances. During charge-discharge cycling, the link between the local current density variation and the Faradaic source of charge and reactant depletion was established, and the effect of the concentration distribution during polarization curves was also analyzed. The influence of some parameters of the model shown some unexpected results on the current density profile observed at the negative collector plate. Analysis tools such as the standard deviations of the local current density and of the redox species concentration were developed in order to quantify their internal distribution giving quantitative criteria for redox flow battery performances. Although ignoring transport phenomena at the fiber level, the model reveals the influence of the concentration distribution on the cell behavior and mass-transport limitations at the macroscopic level.

Morphological Heterogeneities of Stripped and Plated Lithium Metal Analyzed By X-Ray Tomography

Lithium (Li) metal is the most promising anode battery material to be implemented in stationary and electric vehicle applications [1]. For years, its use and subsequent industrialization was hampered because of the inhomogeneous Li electrodeposits onto Li metal leading to dendrite growth [2]. The use of solid polymer electrolyte is a solution to mitigate dendrite growth but the Li redox processes, stripping and plating, remain heterogeneous. Blue Solutions compagny is already commercializing lithium metal batteries using a solid polymer electrolyte to prevent dendrite growth. A precise characterization of the behaviour of these heterogeneities during cycling is essential to move towards an optimized anode. During our research, we have developed a characterization method based on X-ray tomography applied to model Li symmetric cells toquantify and spatially probe the Li stripping/plating process. Ante- and post-mortem cells are recut in order to allow a 1µm voxel sizein a conventional laboratory scanner. The reconstructed cell volume is post-processed to numerically re-flatten the Li electrodes, allowingus a subsequent precise measurement of the electrode and electrolyte thicknesses and revealing local interface modifications. This in-depthanalysis brings information on the location of heterogeneities and the impact on the electrode microstructure both at the electrode grains andgrain boundaries. This simple methodology permits to finely retrieve and then surface-map the local current density at both electrodes basedon the local thickness change during the redox process (see Figure 1a : 2D map of the cathode local current density variations overlaped withgrain boundaries pattern (image analysis of a cell after moving 22µm of lithium)). Thanks to quantitative data that can be extracted from the2D maps (see Figure 1b: anode and cathode quantitative distribution of local current density variations), we show that the plating process(reduction) induces more pronounced heterogeneous deposits compared to the stripping (oxidation) one. The existence of cross-talkingbetween the electrodes is also highlighted. [1] Wu Xu et al., Energy Environ. Sci. 7, no 2 (2014): 513-37[2] Xin-Bing Cheng et al., Chemical Reviews 117, n°15 (2017): 10403-73 Figure 1

Investigation of performance heterogeneity of PEMFC stack based on 1+1D and flow distribution models

Flow distributions among proton exchange membrane fuel cell (PEMFC) stacks remain an important issue owing to the great influences on stack performances. To simultaneously consider flow distributions as well as reactions, phase changes, and transport processes inside every fuel cell, a comprehensive stack model is developed based on the integration of a 1+1 dimensional multiphase stack sub-model and a flow distribution sub-model. Frictional and local pressure drops due to diverging, converging, and bending configurations are calculated. After rigorous model validation, differences between the uniform flow assumption and the authentically non-uniform distribution are quantitatively investigated, including the distribution of voltage, mass flow rate, temperature, reactant concentration, and other parameters. Results show that the uniform assumption not only overestimates the stack output performance but also underestimates the cell voltage variations. Besides, the uniform assumption may lead to higher predictions of the overall stack temperature and lower predictions of temperature variations among different fuel cells. Even though the total amount of air seems abundant, it is still possible for some fuel cells to suffer from the local reactant starvation. Therefore, a higher cathode stoichiometry is preferable since it increases the inlet mass flow rate for middle cells. Increasing the inlet pressure contributes negligibly to the uniformity of reactant distribution, but it improves the stack performance. A larger manifold cross-sectional area leads to more uniform reactant distributions among the stack and less local current density variations inside single cells.

Communication—Implications of Local Current Density Variations on Lithium Plating Affected by Cathode Particle Size

Consistent performance of Li metal electrode (LME) relies on well-distributed current across the electrode surface. Lithium plating and dendrite growth are challenging issues for LME performance in high-energy rechargeable Li battery (RLB) development. The morphology of Li plating is affected by local current density variations on LME. A three-dimensional RLB cell model is used to study current density variations induced by cathode particle size. Smaller cathode particles show lower variances in the current density distribution throughout the separator. The results suggest particle size could affect uniformity of Li plating, propensity for dendrite formation and ultimately cycle life of RLB.

A Study of the Influence of Gas Channel Parameters on HT-PEM Fuel Cell Performance Using FEM Analysis

Proton Exchange Membrane Fuel Cells (PEMFC) are highly efficient power generators, achieving up to 50–60% conversion efficiency, even in sizes of a few kilowatts. Comsol Multiphysics, a commercial solver based on the Finite Element Method (FEM) was used for developing a three dimensional model of a high temperature PEMFC that can deal with both anode and cathode flow field for examining the micro flow channel with electrochemical reaction. Cathode gas flow velocity influence on the cell performance was investigated at first. Polarization curves for three different channel widths (0.8, 1.6 and 2.4 mm) and three different channel depths (1, 2 and 3 mm) were computed at a cathode inlet flow velocity of 0.06 m/s. Oxygen molar concentration at cathode catalyst layer-GDL channel interface and local current density variation along the cell length were also studied for specific gas channel geometries.

Simulating the Effect of Gas Channel Geometry on PEM Fuel Cell Performance by Finite Element Method

Proton exchange membrane fuel cells (PEMFC) become in the last years an attractive alternative clean energy source and the main researches are oriented in order to understand how different cell parameters affects the performance of the fuel cell in real operating conditions and subsequently reduce the cost involved in prototype development. Comsol Multiphysics, a commercial solver based on the Finite Element Method (FEM) was used for developing a three dimensional model of PEMFC working at a high temperature of 150 ̊C. Cathode gas flow velocity influence on the cell performance was investigated at first. The effect of channel width and channel height on the current density at a cell voltage of 0.5V was established, at various cathode gas flow velocities between 0.04 and 0.42 m/s. Local current density variation for cell geometries with three different channel widths (0.8, 1.2 and 1.6mm) at three different channel heights (1, 2 and 3mm) were computed at a cathode inlet flow velocity of 0.1 m/s and cell voltage of 0.3V. Oxygen and water molar concentrations at cathode catalyst layer-GDL channel interface were also studied for specific gas channel geometries.

Dynamic characteristics and mitigations of hydrogen starvations in proton exchange membrane fuel cells during start-ups

Hydrogen starvation during a start-up process in proton exchange membrane (PEM) fuel cells could result in drastic local current density variations, reverse cell voltage and irreversible cell damages. In this work, variations of local current densities and temperatures are measured in situ under both potentiostatic and galvanostatic modes. Experimental results show that when the cell starts up under potentiostatic mode with hydrogen starvation, current density undershoots occur in the downstream; while under the galvanostatic mode, local current density in the downstream almost drops to zero, but the current density near the outlet remains almost constant. The phenomenon of near constant current density near the outlet leads to a novel approach to alleviate hydrogen starvations - a hydrogen reservoir is added at the anode outlet. Experimental results show that the exit hydrogen reservoir can significantly reduce the zero current region and alleviate hydrogen starvations. A non-dimensional current-density variation coefficient is proposed to measure the magnitude of local current density changes during starvations. Experimental results show that the exit hydrogen reservoir can significantly reduce the current-density variations coefficient over the entire flow channel, indicating that adding an exit reservoir is an effective approach in mitigating hydrogen starvations.

Measurement of local current density of all-vanadium redox flow batteries

This article presents a preliminary study of the measurement of local current density in all-vanadium redox flow batteries. Two batteries are designed and manufactured in this study, and the experimental results are compared. In the first cell, the current collector is divided into 25 segments, and the flow field plate is not segmented, whereas in the other cell, the flow field plate is segmented. The effects of the electrolyte flow rate on the battery efficiencies and the local current density variation are investigated. The experimental results show that the current density near the outlet significantly decreases when the discharge capacity approaches zero. In addition, the battery has a larger discharge depth at a higher electrolyte flow rate.

Local current density and water management in PEMFCs

We have used computational fluid dynamics analysis to investigate the local current density distribution at the membrane-gas diffusion layer (GDL) interface at average current densities ranging from 0.1 to 2.4 A/cm 2. A three-dimensional, non-isothermal model was used with a single straight channel geometry. Both anode and cathode humidification were included in the model. In addition, phase transportation was included in the model to predict the distributions of water vapor and liquid water and the related water management for systems operating at different current densities. The dependency of local current density on total water and thermal management of the fuel cell and its other related linkage with physical parameters were investigated. The simulation results showed that at low average current density, the local current density does not vary along the width but gradually decreases along the cell length. However, the opposite trend starts to emerge as the average current density is increased. The anode water activity was found to play a significant role in determining the membrane conductivity and the local current density variation in the cell. Moreover, at high average current density, the local current density in the downstream end of the channel is dominated by the cathode water rather than the membrane conductivity. Specifically, the cathode water accumulates in the shoulder area and congests the pores of the GDL, thereby blocking the passage of oxygen to the reacting area. The resulting scarcity of oxygen in the shoulder area causes a dramatic reduction in the local current density in this region. Simulations using different cathode stoichiometric rates showed that increasing the cathode stoichiometric rate led to better oxygen transportation to the GDL at the shoulder area, and hence improved to smooth the local current density distribution. The model was validated by comparison with the polarization curve ( I– V characteristics) in the literature.

Effects of Operating Parameters on the Current Density Distribution in Proton Exchange Membrane Fuel Cells

During the operation of a proton exchange membrane (PEM) fuel cell, significant variation of the local current density could exist across the cell causing sharp temperature and stress gradients in certain points, and affecting the water management, all of which severely impact the cell performance and reliability. The variation of local current density is a critical issue in the performance of PEM fuel cell, and is influenced by the operating conditions. This article presents a model-assisted parametric design with the objective of determining the operating conditions which maximize the fuel cell performance while maintaining a level of uniformity in the current density distribution. A comprehensive two-dimensional model is adopted to simulate the species transport and electrochemical phenomena in a PEM fuel cell. Numerical simulations are performed for over a wide range of operating conditions to analyze the effects of various operating parameters on the variation of local current density of the fuel cell, and to develop design windows which serve as guideline in the design for maximum power density, minimum reactant stoichiometry, and uniform current density distribution.