Cathode Channel Research Articles

Effect of liquid water in flow channel on proton exchange membrane fuel cell: Focusing on flow pattern

The liquid water in channel is a critical factor affecting the oxygen transport and fuel cell performance. However, the research considering the real gas–liquid interface is insufficient and the effect of flow pattern on mass transfer is still not clear. In this research, the volume of fluid model is added into the fuel cell model to track the two-phase flow in cathode channel (CCH) and a two-dimensional multi-phase fuel cell model is established and validated. The result shows that the liquid water in CCH could change the gas flow path by forming the variable cross-section gas channel. Then it changes the oxygen concentration and distribution, and finally affects the fuel cells performance. The flow pattern is founded to be the critical factor affecting the performance. The initial film flow leads to the long-time large-area water coverage on the cathode gas diffusion layer surface, and the serious local oxygen starvation and current density degradation. When the water content is 10.0%, the average and minimum current density decreases by 4.23% and 7.49% respectively. Besides, the initial slug flow has server deformation and fragmentation, which leads to the output performance fluctuation. Besides, the initial droplet flow has small effect on the performance, and even strengthen the performance by enhancing the local oxygen transportation. Based on this research, regulating the flow pattern is the key means to improve the influence of two-phase flow on fuel cell performance.

Transient temperature heterogeneous distribution analysis in three-dimensional passive µDMFC

In this paper, the transient temperature heterogeneous distribution analysis in three-dimensional passive micro Direct Methanol Fuel Cell (µDMFC) is presented in detail. The metal-based µDMFC with the effective area of 1.0 cm2 is fabricated to verify the correctness of the model. By coupling the mass transport, momentum transport, electron-proton conduction and heat transfer mechanisms, the models of the whole cell and a single flow channel are established, and the non-uniform temperature changes in different stages of the cell are analyzed. Simulation results manifest that the temperature of collector and channel is in heterogeneous distribution state during the process of the fuel cell from starting work to reaching a stable trend. Due to the different fluids in the cathode and anode flow channels, the inside of flow channels also presents different heating trends. In addition, the temperature-increasing rate of each part of the cell after starting work is also simulated. The results can be used as a reference for the inner heterogeneous transfer and startup phase optimization of fuel cells.

Numerical simulation of water and heat transport in the cathode channel of a PEM fuel cell

Cathode channel of a PEM fuel cell is the critical domain for the transport of water and heat. In this study, a mathematical model of water and heat transport in the cathode channel is established by considering two-phase flow of water and air as well as the phase change between water and vapor. The transport process of the species of air is governed by the convection-diffusion equation. The VOSET (coupled volume-of-fluid and level set method) method is used to track the interface between air and water, and the phase equilibrium method of water and vapor is employed to calculate the mass transfer rate on the two-phase interface. The present model is validated against the results in the literature, then applied to investigate the characteristics of two-phase flow and heat transfer in the cathode channel. The results indicate that in the inlet section, water droplets experience three evolution stages: the growing stage, the coalescence stage and the generation stage of dispersed water drops. However, in the middle and outlet sections of the channel, there are only two stages: the growth of water droplets, and the formation of a water film. The mass transfer rate of phase change in the inlet section of the channel varies over time, exhibiting an initial increase, a decrease followed, and a stabilization finally, with the maximum and stable values of 1.78 × 10−4 kg/s and 1.52 × 10−4 kg/s for Part 1, respectively. In the middle and outlet sections, the mass transfer rate increase firstly and then keeps stable gradually. Furthermore, regarding the distribution of the temperature and vapor mass fraction in the channel, near the upper surface of the channel, the temperature and vapor mass fraction first change slightly (x < 0.03 m) and then rapidly decrease with fluctuations (x > 0.03 m). In the middle of the channel, the temperature and vapor mass fraction slowly decrease with fluctuation.

Experimental study of key operating parameters effects on the characteristics of proton exchange membrane fuel cell with anode recirculation

Water management is essential to ensure high performance and stable operation of the proton exchange membrane fuel cell (PEMFC). However, little is known about water management in the hydrogen circulation anode (HCA) mode which can achieve high hydrogen utilization. In this work, the effects of hydrogen circulation rate, a key parameter of HCA mode, on water management and cell performance are experimentally investigated. In particular, transparent fuel cell and printed circuit board segmented fuel cell technologies are used to study the water distribution and the local current density distribution, respectively. Furthermore, the electrochemical properties and PEMFC stability at different hydrogen circulation rates are discussed in detail. The experimental results indicate that increasing the hydrogen circulation rate reduces liquid water in the cathode and anode flow channels, which can effectively alleviate the adverse effects of flooding on overall performance and cell stability. However, subsequent drying of the membrane and catalyst layer results in an increase in ohmic resistance and a decline in cell performance, especially at low current densities. The current density distribution shows that severe local flooding is likely to occur in the hydrogen and air outlet regions at a low hydrogen circulation rate. For a high hydrogen circulation rate, membrane drying will occur in some areas, especially near the hydrogen inlet. To achieve uniform current density distribution and high fuel cell performance, the hydrogen circulation rate should be increased with the load current.

A novel channel design and heat and mass transfer analysis of fuel cells

In this study, a novel partially separated-partially coupled cathode channel (PSPC) of proton exchange membrane fuel cell (PEMFC) is developed on the basis of the PEMFC with independent cooling channel, in order to accelerate the removal of liquid water and maintain the overall water balance. The novel cathode channel is numerically investigated by a 3D PEMFC model. The research shows that the PEMFC with PSPC can accelerate the water removal at the cathodic outlet side and improve the uniformity of water and heat distribution as the length of partition (“L”) declines. When the L = 25 mm, PEMFC with PSPC can have a 13.5% higher current density than PEMFC with independent cooling channels. According to the analysis of local mass transfer for PSPC, the local water axial diffusion transport at coupled channel section evidently improves to induce water lateral diffusion transport of the separated channel section. This water transport mechanism of PSPC can also be adjusted by the cooling channel height, but the water balance should be considered. The PEMFC without partition easily leads to severe water imbalance. Thus, the current density of PEMFC with L = 25 mm is higher 15.9% than that without partitions under 1.4 mm total channel height of cathode.

Measurement of water concentration along the straight channel of proton exchange membrane fuel cell

Given that the ionic conductivity of a proton exchange membrane fuel cell is significantly influenced by water concentration in the channel, an elaborate water management strategy is crucial to ensure durability and performance of these types of fuel cells. In this study, the vapour concentration of a gas mixture was observed in the through-plane direction in a bipolar plate. The stainless steel bipolar plate used here has straight channels containing installed relative humidity/temperature micro-sensors such that the vapour flow behaviour can be observed. The vapour concentration in the anode channel is increased by the back diffusion of water vapour from the cathode. The vapour concentration at the cathode channel increases with the water produced during the electrochemical reaction. The distribution of the dew point temperature at the anode/cathode channel shows that water transport to the anode increases the vapour concentration of anode gases.

Influence of hydrodynamic parameters on the critical current density at water electrolysis: Mass flux, channel inclination and inlet void fraction

Hydrogen production rate increases as current density increases in a water electrolysis. At a certain high current density, the cell potential abruptly increases due to the hydrogen film formed at the cathode, which is known as the critical current density (CCD). Hence, it is imperative to increase the CCD for effective hydrogen production. However, the investigations regarding hydrodynamic parameters, which affect the CCD have hardly been performed. This work investigated the influence of the hydrodynamic parameters such as mass flux, inclination of cathode channel and inlet void fraction on the CCD in water electrolysis. The increase in the mass flux increased the CCD regardless of the channel inclination and the inlet void fraction due to the enhanced hydrogen bubble elimination near the surface, which retards hydrogen film formation. The influence of the inlet void fraction showed different trend according to the channel inclination. Monotonically decreasing trend was measured in the vertical channel due to reduced flow rate near the surface as the void fraction increased. Meanwhile, a peak was measured in the inclined channels. The inlet voids at bubbly flow regime dispersed the hydrogen bubbles from the cathode, while those at the slug flow regime aided and enhanced the hydrogen film formation at the cathode. It is concluded that the inlet void fraction either enhances or impairs the CCD depending on the flow regime. The authors expect that this work would shed light on the roles of hydrodynamic parameters for efficient hydrogen production.

Effect of cathode pulsating flow on mass transport and performance of solid oxide fuel cell

In the context of attempts to improve the performance of solid oxide fuel cell (SOFC), we investigate whether -and to which degree - pulsating flow in the cathode channel affects the performance of the fuel cell. In this study, a two-dimensional Lattice Boltzmann, non-isothermal model of planar SOFC is developed for simulating multicomponent gas transport in the channels and porous electrodes and an electrochemical model is established to study the SOFC performance. After model validation, our study focused on the effects of pulsating air flow on the distribution of temperature and oxygen and the cell performance. The results prove that a pulsating flow enhances the convective species transport to the reaction sites and consequently increases cell performance. The effects of pulsation frequency and amplitude on cell performance are also investigated. The effect of pulsation frequency is marginally small, while the pulsation amplitude increase improves the cell performance.

Modeling and numerical study of SO2-depolarized electrolysis for hydrogen production in the hybrid sulfur cycle

The SO2-depolarized electrolysis (SDE) is one of the main processes in the hybrid sulfur cycle for hydrogen production, the microscopic characteristics of which are still not clear. In this work, a numerical analysis of SDE was conducted. The steady-state, laminar and incompressible 2D model of the electrolytic cell (EC) for carrying out SDE was first developed and verified. Physical field characterization indicated that the velocity was almost uniformly distributed in the flow channel, but higher pressure drops and pumping power consumption existed in the anode channel than in the cathode channel. Meanwhile, the continuous conversion of SO2 to H2SO4 at anode and generation of H2 at cathode along the channel length were determined. Further exploring the impacts of three main operating parameters implied that an appropriate higher electrolytic potential, lower inlet flow rate (not less than 0.005 m/s) and operating temperature were favorable for improving the SDE. After comprehensive consideration of SDE performance and energy consumption, a feasible operating condition of 0.8 V, 303 K, and 0.005 m/s was proposed for SDE process. The developed model and microscopic characterization of SDE will favor the design and optimization of EC for SDE process.

Non-Dimensional Analysis of Diffusion Characteristics in Polymer Electrolyte Membrane Fuel Cells with Mismatched Anodic and Cathodic Flow Channels

Polymer electrolyte membrane fuel cells were analyzed to investigate changes in the structure of the flow field and operating conditions. The cell performance, which was controlled by adjusting the width of the cathodic channel, improved as the backpressure increases. With the anodic and cathodic flow channels mismatched, the maximum power densities at 3.0 bar for a narrow cathodic channel were 1115 and 1024 mW/cm2, and those for a wide cathodic channel were 959 and 868 mW/cm2, respectively. The diffusion characteristics were investigated using the non-dimensional numbers Re (Reynolds), Sc (Schmidt), and Sh (Sherwood) to confirm the improvement of mass transport. The narrower the channel or the higher the operating pressure, the larger Re was and the smaller Sc and Sh became. In particular, the wider the anodic channel, the larger the value of Sh.

Numerical study of optimized three-dimension novel Key-shaped flow field design for proton exchange membrane fuel cell

Key-shaped three-dimension (3D) flow field channel is designed to improve the performance and mass transfer of proton exchange membrane fuel cell (PEMFC). This study comprehensively analyses the impacts on the performance and mass transfer of the flow channel from multiple dimensions such as the size, shape, and placement of the blocks. In comparison with the conventional straight single flow field channel, the new channel with rectangular blocks can effectively improve performance by 30%. Semi-elliptical and quarter-elliptical blocks are designed to make forced convection and increase the diffusion area of oxygen. The results indicate that the flow velocity in the Z-axis direction can be increased to 0.08–0.2 m/s due to the narrow space formed by variable cross-sections. In conclusion, the Key-shaped design has a potential to improve the performance of mass transfer in the cathode channel, providing a new strategy for the development of flow field design in PEMFC field.

Numerical study on the influence of waveform channel and related design parameters on the cold start of proton exchange membrane fuel cell

In order to enable the proton exchange membrane fuel cell to be used under outdoor conditions, research on the cold start of the fuel cell is essential. In this paper, numerical simulation is used to study the influence of fuel cell cathode channel geometry and porosity on the fuel cell cold start process. By analyzing the internal parameters of the fuel cell at different times, including current density, ice volume fraction, temperature and water content, the optimal fuel cell design was obtained. From a comprehensive comparison, although the current density and temperature rise of the ordinary wave channel fuel cell with higher porosity increase slowly at the beginning of the cold start, the decline process and icing process of it are also relatively slow. Therefore, a fuel cell with an ordinary wave channel and a higher porosity may perform better at cold start. This research contributes to the application and commercialization of fuel cells in a wider range of practical conditions.

Experimental study on non-uniform arrangement of 3D printed structure for cathodic flow channel in PEMFC

Enhancing mass transfer capability of flow channel is important subject to improve fuel cell performance. In this study, an experimental study about non-uniform arrangement of metallic structures in cathodic flow channel on polymer electrolyte membrane fuel cell is conducted. Metallic 3D printer is used to manufacture 3D mesh with complex geometry. Channel width and bottom-rear channel depth are selected to change the geometry of structure. Assuming that accelerating flow velocity and reinforcing flow direction to gas diffusion layer can improve fuel cell performance, eight basic arrangements are inserted into cathodic carbon bipolar plate to measure fuel cell performance. With I–V curve, the unit cells with non-uniform arrangements of width and tapered structure show performance enhancement of 12.8% in maximum power density, compared with conventional parallel flow channel. According to electrochemical impedance spectroscopy results, performance enhancement by non-uniform arrangement is mainly occur in high current density operation due to low mass transfer resistance.

Analytical Impedance of Oxygen Transport in the Channel and Gas Diffusion Layer of a PEM Fuel Cell

Analytical model for impedance of oxygen transport in the gas–diffusion layer (GDL) and cathode channel of a PEM fuel cell is developed. The model is based on transient oxygen mass conservation equations coupled to the proton current conservation equation in the catalyst layer. Analytical formula for the “GDL+channel” impedance is derived assuming fast oxygen and proton transport in the cathode catalyst layer (CCL) In the Nyquist plot, the transport impedance consists of two arcs describing oxygen transport in the air channel (low–frequency arc) and in the GDL. The characteristic frequency of GDL arc depends on the CCL thickness: large CCL thickness strongly lowers this frequency. At small CCL thickness, the high–frequency feature on the arc shape forms. This effect is important for identification of peaks in distribution of relaxation times spectra of low–Pt PEMFCs.

A Feasiblity Study of the Placement of a Heated Turbulence Grid in Front of an Air-Cooled Fuel Cell Stack in Subzero Conditions

Within telecom back-up applications, the air-cooled fuel cell is a promising alternative to diesel generators as a backup powersystem. Additionally, these fuel cell systems are required to operate in a subzero environment with temperaturesas low as -40C. In this study, a numerical analysis of a heated turbulence-inducing grid to preheat the air before the cathode channel, has been conducted. The turbulence model used is the comprehensive Reynolds Stress Model. Four different grid dimensions, namely grid with 1mm in depth and six 1 mm2 holes, grid with 2mm in depth and six 1 mm2 holes, grid with 1mm in depth and twenty-four 0.17 mm2 holes, grid with 2mm in depth and twenty-four 0.17 mm2 holes have been considered. The outside temperature has been varied from -20C and -10C and heated up to 5C before air flow enters the cathode. A three-dimensional, steady state numerical analysis has been conducted on these grid dimensions using the finite volume method. First results indicate very high grid surface temperatures. Additionally, it can be concluded that the grid surfacetemperature decreases as the turbulence grid thickens. Figure 1

A Computational Analysis of Heat Transfer in an Air-Cooled Proton Exchange Membrane Fuel Cell for Unsteady Flow Conditions Using a Reynolds Stress Turbulence Model

Within telecom applications, the air-cooled fuel cell, as a backup power system, has recently been gaining popularity. These fuel cells are cost effective, simple, and additionally they do not require a secondary coolant loop. However, since airflow is used as coolant, the heat generated by the fuel cell limits the current density and thus the power density. A turbulence inducing grid is placed in front of the cathode channel to make the airflow turbulent and hence increase the convective cooling effect from the air. The turbulence inducing grid is placed 2.5mm from the cathode channel since previous studies have shown this distance to be most effective.In this study, a three dimensional CFD analysis on the effect of a transient boundary conditions on the cooling effect of the turbulence grid has been conducted. Special emphasis has been on using the comprehensive Reynolds Stress Model as turbulence model. Additional analysis has been made on the cooling effect of pulsating flow with a period of 10 seconds excluding the turbulence inducing grid. At an electrode heat flux of 2600 W/m2, the results indicate that the convective cooling effect increases when using transient boundary conditions compared to steady state boundary conditions. Furthermore, results show that the start-up time is seven minutes, and the time-averaged peak temperature in the transient case is two degrees Celsius below the peak temperature of the steady state case. The CFD analysis show an additional cooling effect due to accelerated-decelerated flow conditions. Figure 1

Measurement Technique of Oxygen Concentration in Narrow Channels of PEFCs Based on Transmission Laser Absorption Spectroscopy

Oxygen transport at cathodes of polymer electrolyte fuel cells (PEFCs) is one of the key factors for determining their power generation performance. To maintain their stable operations, it’s necessary to establish a technique for real-time monitoring of oxygen concentration inside cathode channels of operational cells. In the previous works, the measurements of oxygen distribution within gas channels of operating PEFCs have been conducted using gas chromatograph (GC) [1], oxygen-quenching of fluorescence and phosphorescence [2], and tunable diode laser absorption spectroscopy (TDLAS) [3]. GC techniques provide better accuracy for gas detection in flow channels, however, it takes a long time (at least a few minutes) to collect a gas sample and identify a species. On the other hand, TDLAS enables the non-invasive in-situ monitoring of chemical species inside cells with a high time resolution of 100 milliseconds. Fujii et al. [3] applied the single-ended TDLAS sensor to measure the oxygen concentration in a PEFC channel. In their experiments, since the laser beam is injected perpendicular to the flow direction, the optical path length is extremely short and the sensitivity is insufficient. The detection sensitivity of oxygen is relatively low due to the weak infrared absorption. Therefore, the optical path needs to be lengthened to measure its concentration with high accuracy. In this study, to solve the above-mentioned problem, we developed the fiber-optic transmission TDLAS system and applied it to the quantitative measurement of oxygen concentration in the narrow channel of a simulated cell.Fig. 1 shows the experimental apparatus for measuring the oxygen concentration in a simulated flow cell using the fiber-optic transmission TDLAS system. TDLAS is one of the high sensitive absorption spectroscopy techniques which measure the absorbance of a chemical species and identify its concentration using infrared diode laser. The triangular modulated beam emitted from a DFB laser diode (wavelength: 764 nm) is directly injected into the straight narrow channel (width and depth: 1.0 or 1.5 mm, length: 50 mm) through the single-mode optical fiber. To collimate the laser beam at the channel inlet, a GRIN lens is attached at the top of the pitch fiber. The injected beam passes one or two channels along the flow direction. In the case of passing two channels, the beam transmitted through the first channel is turned 180 degrees by a triangular prism at the bend section (length: 5 mm). The returned beam is passed through the second channel and detected at a photodiode. The output signal is transmitted to a lock-in amplifier, and then the second harmonic (2f) absorption spectrum can be obtained by phase-sensitive detection. In the experiments, the mixed gas of oxygen and nitrogen was supplied to the channel at 100 ml/min. The oxygen concentration in the channels was adjusted to 0 to 100% by controlling each gas flow rate. This measurement was performed at 25 oC and 1 atm.Fig. 2 presents the 2f absorption signals of oxygen in the narrow channel of the simulated flow cell. The incident laser beam passed two straight channels (channel width and depth: 1.5 mm, optical path: 105 mm). The oxygen mole fraction was varied from 0 to 100%. Although the TDLAS measurements were performed inside the narrow channels, the optical fringe noise was hardly observed and the 2f spectra were clearly obtained. It was also noted that the peak-valley height of 2f spectrum increased with an increase in oxygen mole fraction. The oxygen concentration in fuel cell channels can be quantitatively estimated from only the 2f spectrum data using the fiber-optic transmission TDLAS technique.

In-situ measurement of humidity distribution and its effect on the performance of a proton exchange membrane fuel cell

Understanding the internal water distribution in proton exchange membrane fuel cell (PEMFC) is critical to the development of high-performance PEMFCs. In this study, the cathode flow field is divided into 9 areas, and the relative humidity under different conditions is measured in-situ by using microsensors embedded in the cathode flow field plate. Polarization curves are measured and electrochemical impedance spectroscopy is carried out under different conditions. The results show that the relative humidity has a significant effect on the internal resistance of PEMFCs. When the inlet gas is not humidified, too high of a temperature will lead to low relative humidity and membrane dehydration. Increasing pressure and cathode humidification can alleviate this problem. However, the relative humidity of the outlet area is close to 100% when the cathode back pressure is 100 kPa, and a too high relative humidity will result in flooding in the cathode channel. Moreover, the degree of corrosion of the carbon support in the catalyst is closely related to the water concentration in the cathode, and the water management downstream of the flow field is particularly important.

Finger Gate Vacuum Channel Field Emission Transistors: Performance and Sensitivity Analysis

In the current research, for the first time, Nano-scale finger gate vacuum channel field emission transistor (FGVFET) scaling down and its limitations considering electrical characteristics are studied. The FGVFET with different cathode electrode materials, channel dimensions, and finger widths is considered and the impacts of these structural and physical changes on the key indicators of the device are assessed to achieve a comprehensive design guideline. The sensitivity analysis reveals that the channel height modification by changing gate-anode oxide and cathode–gate oxide thicknesses can be assumed as the most significant design factor in the determination of ON–OFF-state currents. The results indicate that the gate leakage current as a salient parameter in device performance is dependent on the cathode material, channel depth, and height. Furthermore, investigating cutoff frequency as the prominent factor in high-speed applications indicates that cathode–gate oxide thickness may fundamentally modify this factor.

Performance of proton exchange membrane fuel cells with honeycomb-like flow channel design

A honeycomb-like flow channel was proposed and investigated for the performance of proton exchange membrane fuel cells (PEMFCs). The effects of various thicknesses and porosities of the gas diffusion layer (GDL) on the honeycomb-like flow channel were studied. Compared with parallel and serpentine flow channels, the honeycomb-like flow channel exhibited the lowest oxygen non-uniformity value of 0.59 at 0.4 V, and the pressure drop was 6.9 times lower than that of the serpentine flow channel. The current density was 8034.9 A/m2, which was 14.0% and 10.4% higher than that of the parallel and serpentine flow channels. For a porosity of 0.4, the decrease in GDL thickness from 0.58 to 0.38 mm for the honeycomb-like flow channel facilitated oxygen diffusion, and the current density increased from 7717.2 to 8034.9 A/m2; the oxygen mass fraction gradually increased at the cathode channel but decreased at the center of the honeycomb-like structure. At a thickness of 0.38 mm, the porosity increased from 0.2 to 0.6, leading to a decrease in the oxygen non-uniformity value from 0.89 to 0.42. For a porosity of 0.6, the current density was 8787 A/m2, which was 60% and 9.4% greater when compared with the porosities of 0.2 and 0.4.