Current Collector Plate Research Articles

Diffusion induced stress in layered Li-ion battery electrode plates

In this paper, structural configuration of layered Li-ion battery electrode plates is evaluated by analytically formulating the diffusion induced stress. Both symmetric electrode and asymmetric bilayer electrode are discussed. The thickness ratio and the modulus ratio of current collector to active plate are analytically identified to be the important influence parameters on the stress. Applying a material with smaller elastic modulus for current collector could reduce the peak stresses in both current collector and active plate. Increasing the thickness of current collector would reduce the stress in itself while promote the stress in active plate. Therefore, from mechanical viewpoint on designing an electrode, the material for current collector should be as soft and flexible as possible. And the thickness of current collector should be in an appropriate range. Basically, it should be as small as possible on the precondition that the mechanical strength is satisfied. Finally, effects of three charging conditions, i.e. uniform, galvanostatic, and potentiostatic, on the diffusion induced stress is discussed. It is found the maximum stresses for three cases are linear to the total amount of intercalated lithium ions. Based on the stresses, an optimized charging operation, i.e. first galvanostatic followed by potentiostatic, is suggested.

Calculation of reversible electrode heats in the proton exchange membrane fuel cell from calorimetric measurements

A calorimeter was used to measure the heat production in polymer electrolyte membrane (PEM) fuel cells operated on hydrogen and oxygen at 50 °C and 1 bar. Two cells were examined, one using a 35 μm thick Nafion membrane and a catalyst loading of 0.6/0.4 mg Pt cm −2, for the cathode and anode layer, respectively, the other using a 180 μm thick Nafion membrane and loading of 0.4/0.4 mg Pt cm −2. The cells investigated thus had different membranes and catalyst layers, but identical porous transport layers and micro-porous layers. The calorimeter is unique in that it provides the heat fluxes out of the cell, separately for the anode and the cathode sides. The corresponding cell potential differences, ohmic cell resistance and current densities are also reported. The heat fluxes through the current collector plates were decomposed by a thermal model to give the contributions from the ohmic and the Tafel heats to the total heat fluxes. Thus, the contributions from the reversible heat (the Peltier heats) to the current collectors were determined. The analysis suggests that the Peltier heat of the anode of these fuel cell materials is small, and that it is the cathode reaction which generates the main fraction of the total heat in a PEM fuel cell. The entropy change of the anode reaction appears to be close to zero, while the corresponding value for the cathode is near −80 J K −1 mol −1.

Systematic characterization of a PBI/H3PO4 sol–gel membrane—Modeling and simulation

This work presents a three-dimensional, steady-state, non-isothermal model of a high-temperature polymer-electrolyte-membrane fuel cell (HTPEMFC) using a phosphoric acid-doped polybenzimidazole (PBI/H3PO4) sol–gel membrane. The model accounts for the gold-plated copper current collector plates, the bipolar plates, all gas flow channels (flow-field), the gas diffusion layers, the reaction layers, and the membrane. Electrochemical reactions are modeled using an agglomerate approach and include the gas diffusivity and the gas solubility. The conductivity of the membrane is modeled using the Arrhenius equation to describe the temperature dependence. Finite elements are used to discretize all computational subdomains, and a commercially available code is used to solve the problem. The predicted values are compared to typical operating conditions, and a good agreement is found. The current density, the solid- and fluid-(gas)-phase temperatures and other quantities are analyzed throughout the computational subdomains. It was observed that the Arrhenius approach is valid in a certain temperature range and may overpredict the PBI/H3PO4 sol–gel membrane conductivity at higher solid-phase temperatures. Moreover, it is shown how the fluid-(gas)-phase temperature influences the solid-phase temperature and the current density distribution. Concrete values are deduced from the simulations and discussed according to experimental test.

Fabrication and characterization of micro PEM fuel cells using pyrolyzed carbon current collector plates

A novel fabrication technique for micro proton exchange membrane fuel cells (μPEMFCs) based on carbon-MEMS (C-MEMS) was optimized to yield higher performance cells. Polymer manufacturing is relatively easy compared to directly patterning graphite as is typically done to make fuel cell bipolar plates. In a C-MEMS approach, fuel cell bipolar plates are fabricated by first patterning polymer Cirlex ® sheets. By subsequently pyrolyzing the machined polymer sheets at high temperature in an inert atmosphere, carbon bipolar plates with intricate groove structures to distribute the reactants are obtained. Using an improved assembly technique such as polishing the carbonized plates to minimize the contact resistance between gas diffusion layers (GDL) and bipolar plates, better pyrolysis temperature control and a better end plate design, a μPEMFC with a 0.64 cm 2 active surface was fabricated using the newly developed bipolar plates. At 1 atm and 25 °C a maximum power density of ∼76 mW cm −2 was obtained, and at 2 atm and 25 °C ∼85 mW cm −2 was achieved. These data are comparable with data reported in the literature for μPEMFCs and are a dramatic improvement over earlier results reported for the same C-MEMS based fuel cell. Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry were carried out to characterize steady-state and transient characteristics of the novel C-MEMS fuel cell.

Fabrication and Characterization of a Thin μ-PEMFC with Microfabricated Grooves on Electroformed Current Collector Plate

We proposed a design and fabrication technique for a micro proton exchange membrane fuel cell (μ-PEMFC). Thin current collector plates had microgrooves for fuel supply. Microgrooves with a line and space of 100-500 μm and depth of 112 μm were fabricated by using microfabrication technologies. Maximum power density increased with increasing line and space of the fuel channel, and a high power density of 109 mW/cm 2 was achieved at the line and space of 100 μm. Ohmic resistance decreased with increasing line and space of the fuel channel. A low ohmic resistance of 0.26 Ω cm 2 was determined. These results were comparable to the conventional PEMFC, and preliminary test results showed that our μ-PEMFC is as useful as a miniature PEMFC.

Characterization of a direct methanol fuel cell using Hilbert curve fractal current collectors

The current collector or bi-polar plate is a key component in direct methanol fuel cells (DMFCs). Current collector geometric designs have significant influence on cell performance. This paper presents a continuous type fractal geometry using the Hilbert curve applied to current collector design in a direct methanol fuel cell. The Hilbert curve fractal geometry current collector is named HFCC (Hilbert curve fractal current collector). This research designs the current collector using a first, second and third order open carved HFCC shape. The cell performances of the different current collector geometries were measured and compared. Two important factors, the free open ratio and total perimeter length of the open carved design are discussed. The results show that both the larger free open ratio and longer carved open perimeter length present higher performance.

Titanium substrate based micro-PEMFC operating under ambient conditions

A novel design and fabrication technique of micro-PEMFC based on titanium substrate is described. The titanium substrate was fabricated with microflow channels using microfabrication techniques, followed by surface treating and coating with a microporous layer (MPL). It served as a combined functional component as gas diffusion layer, current collector and flow plate. A catalyst coated membrane (CCM), fabricated by directly coating catalyst layer onto Nafion 112 membrane, was integrated with the titanium substrates to assemble the micro-PEMFC. The titanium substrate based micro-PEMFC showed some unique characteristics and acceptable performance while operating under ambient conditions.

Formation of a Crack-Free and Debonding-Resistant Boron-Doped Diamond Thin Film on Titanium Using a Dual-Coating Strategy

Producing continuous, conductive diamond film on titanium is critical to using this metal for electrochemical electrodes, bipolar current collector plates in fuel cells, and prosthetic implants. A common problem with coating polycrystalline diamond on titanium is the stress-cracking that results from the difference of a factor of 10 in the thermal expansion coefficient for the two materials. A dual coating strategy was evaluated for its effectiveness at forming stress-free thin films of boron-doped diamond on titanium. Using microwave plasma-assisted chemical vapor deposition, a layer of boron-doped nanocrystalline diamond was first deposited on titanium, followed by deposition of a layer of boron-doped microcrystalline diamond.

Local current measurement in a solid oxide fuel cell repeat element

A planar solid oxide fuel cell repeating unit, 50 cm 2 in total active electrode size, consisting of an anode supported electrolyte cell bearing two 7 mm holes for fuel and air injection, and contacted to two dense metal current collector plates via gas distribution layers, was constructed with the aim of measuring local current densities rather than the integral current over the full area. The cathode side was entirely segmented (i.e. cathode layer, gas distribution layer, metal current collector plate) into eight galvanically separated parts of ca. 6.5 cm 2 each, with own current and potential leads. The element was characterised at 750–800 °C and different H 2 fuel flows, by total and local current–voltage recording as well as by local electrochemical impedance measurement. The segment that incorporates the fuel injection hole for the whole cell always outperforms all other segments, the corner segments furthest away from the fuel injection perform least. Differences in local potential can be higher than 200 mV. Polarizing one segment individually and recording the change in potential of the other segments reveals the different contributions of convection and diffusion on the flow field. Contrarily to small ideal single cells, total performance of such larger sized, stackable cells is decisively governed by the distribution fields and their weakest zones.

Direct methanol fuel cell with non-equipotential electrodes

Our model of a direct methanol fuel cell with straight channels is extended to take into account finite conductivity of current collector plates and local contact resistances. Co-flow conditions are assumed and the two cases are considered: (i) when load resistor is connected at the inlet of the feed channels and (ii) when it is connected at the outlet. In both cases voltage loss in the plates induces growth of current production close to the point of load connection. The increase in local current j is accompanied by the growth of local polarization voltages η a and η c of the anode and the cathode sides, respectively. Moving the point of load connection one can change the distribution of j , η a and η c along the channel. Local “spot” of contact resistance decreases local current and polarization voltages; in the rest part of the cell these values increase.

Multidimensional Modelling of Polymer Electrolyte Fuel Cells under a Current Density Boundary Condition

AbstractA three‐dimensional numerical model of the polymer electrolyte fuel cell (PEFC) is applied to study current distribution and cell performance under a current density boundary condition. Since the electronic resistance in the along‐channel direction in the current collector plate is much larger than in the other two directions, i.e., 50 mΩ cm2 vs. 0.5 mΩ cm2, it significantly affects current flow, and current and cell voltage distributions in a PEFC. An identical polarization curve results with two different boundary conditions, constant cell voltage and constant current density, however, the current density profiles in the along‐channel direction differ significantly; it is much flatter for the constant current boundary condition. Increasing the electronic conductivity of the bipolar plate diminishes the difference in the current density distribution under the two boundary conditions. The results also point out that an experimental validation of a PEFC model based on the polarization curve alone is insufficient, and that detailed current density distribution data in the along‐channel direction is essential.

Transport efficiency of vacuum arc plasma in a curved magnetic filter

We describe two methods for increasing the transmission efficiency of vacuum arc plasma through curved magnetic filters. In the first method the substrate is connected to the anode or biased to a negative voltage. In the second method a metal grid is placed between the substrate and the exit of the magnetic filter, and biased to a positive voltage whereas the substrate is biased negatively. The ion saturation current and electron saturation current of the plasma between the filter exit and the substrate were measured using a current collector plate and a Langmuir probe, respectively, and the ion density estimated. For the experimental conditions of the work described here, the measured ion flux (ion saturation current) near the duct exit was increased by up to about 80% (from 140 to 250 mA), and the measured ion density was increased by up to about 40% (from 3.7×1011 to 5.2×1011cm−3). These results can be explained by the ambipolar influence of enhanced electron flow on the accompanying plasma ion component, leading also to enhanced ion flow.

Effect of cathode structure on planar free-breathing PEMFC

Abstract The effect of cathode structure on the performance of planar free-breathing fuel cell is studied. Three different types of gas diffusion backings and current collector plates on the cathode were used. The gas diffusion backings used were thick carbon sheet, titanium sinter, and carbon paper. Difference between current collector plates was the size of the openings between current collecting ribs. Results showed that with thicker gas diffusion backings the structure of the current collector had only some effect on the cell performance but with thin carbon paper the effect was significant. Very high power densities for free-breathing fuel cell were achieved with thin gas diffusion backing with the record of approximately 360 mW cm −2 . However, the cell was also vulnerable for flooding as there was liquid water observed on the cathode surface. Despite the liquid water saturation it seems that this kind of cell design may be suitable especially for portable applications.

Hydrogen Supply Using Borohydride and Prototyping of a Miniature Fuel Cell by Sand Blasting

This paper describes two works on a miniature fuel cell for portable application: (1) Demonstration of the conceptual prototype of a borohydride fuel cartridge, and (2) Prototyping of a 4-cell-stacked miniature polymer electrolyte fuel cell (PEFC) by sand blasting. The borohydride fuel cartridge consists of aqueous solution of sodium borohydride (NaBH4), a catalyst for hydrolysis reaction of the fuel (BH4- + 2H2O → 4H2 + BO2-), and a hydrogen separation membrane of porous PTFE (polytetrafluoroethylene). The hydrogen separation membrane seals the fuel and reactant, securing users from their leakage. We confirmed the feasibility of this fuel cartridge by experiments. The miniature PEFC consists of two glass-based current collector plates, and a MEA (membrane electrode assembly) with cell interconnections and a gas seal. Four 7×7 mm single cells are placed in 27.5 mm square area, and electrically connected in series. The current collector plate was fabricated by photolithography and sand blasting, which are preferable for mass-production. The miniature PEFC was tested using hydrogen as fuel, and an output of 0.5 mW/cm2 was obtained from a single cell.