Flip-Flop serpentine flow field for electrolyte distribution in electrochemical cells FIELD OF THE INVENTION

The present invention relates to an electrolyte flow field plate with improved flow distribution for application in electrochemical cells such as fuel cells, flow batteries and electrolyzers.

BACKGROUND OF THE INVENTION Electrochemical devices such as fuel cells and flow batteries depend on heterogeneous electrochemical reactions at the anode and the cathode. These reactions are often carried out on planar surfaces over which the electrolyte is circulated. Incorporating grooves with flow channels and ribs on these plates help in achieving uniform distribution of the electro-active species through the reaction zone and thus improves the discharge energy efficiency of these electrochemical devices. The book titled ‘PEM Fuel Cells’ by F. Barbir discloses different types of flow field configurations such as straight, crisscross, multi-channel serpentine, subsequent serpentine, mixed serpentine, interdigitated, screen mesh, porous, fractal, biomimetic, etc., which are employed on a graphite current collector plate. The major limitations of some of these configurations include maldistribution or uneven distribution of reactant fluids, excessive pressure drop leading to parasitic energy losses, inadequate velocity, unwanted bypassing, and channel blockage.

T. Jyothi Latha and S. Jayanti from Indian Institute of Technology Madras (IIT Madras), published an article titled ‘Hydrodynamic analysis of flow fields for redox flow battery applications. S. Kumar and S. Jayanti from IIT Madras published an article titled ‘Effect of flow field on the performance of an all-vanadium redox flow battery’, disclosing single serpentine flow fields in vanadium redox flow batteries exhibiting superior electrochemical performance compared to interdigitated flow fields when used in large cells. However, the pressure drop over the cell is significantly higher in serpentine flow fields.

Splitting the serpentine flow field into a number of sub-serpentine sections or segments is a known approach to overcome high pressure drop. A few flow patterns have been shown in Fig. 1. Fig. la shows a single serpentine flow field in which the flow enters from the top left, travels all along the serpentine flow path over the plate and leaves at the bottom right corner. The length of the flow path, and thus the overall cell pressure drop, can be reduced by splitting the flow path into multi channel serpentine flow field as shown in Fig. lb.

Minkmas V. Williams et al. from The University of Connecticut published an article titled ‘Influence of convection through gas-diffusion layers on limiting current in PEM FCs using a serpentine flow field’ and Nada Zamel et al. from The University of Connecticut published an article titled ‘Effective transport properties for polymer electrolyte membrane fuel cells - with a focus on the gas diffusion layer’, both of which disclose studies related to fuel cell applications. S. Kumar and S. Jayanti from IIT Madras published an article titled ‘Effect of electrode intrusion on pressure drop and electrochemical performance of an all-vanadium redox flow battery’ while R Gundlapalli and S. Jayanti from IIT Madras published an article titled ‘Effect of channel dimensions of serpentine flow fields on the performance of a vanadium redox flow battery’ disclosing studies related to flow battery applications. It is evident from all these studies that there is a significant undercurrent of reactant flow through the electrode area. This is a desirable feature of the serpentine flow field and is to be preserved while modifying the serpentine flow field.

K.B. Shyam Prasad and S. Jayanti from IIT Madras published an article titled ‘Effect of channel- to-channel cross-flow on local flooding in serpentine flow-fields’, which discloses that the local flooding at U-bend regions is due to lower channel-to-channel cross-flow in the electrodes between consecutive serpentine channels. S. Jayanti and S. Kumar from IIT Madras published an article titled ‘High energy efficiency with low-pressure drop configuration for an all-vanadium redox flow battery’ which discloses splitting of the flow path into three sections in a manner that the flow path enhances inter-channel cross-flow even in the U-bend regions of the flow field (Figure lc).

S. Jayanti et al. from IIT Madras published an article titled ‘An improved serpentine flow field with enhanced cross-flow for fuel cell applications’ which discloses detailed computational fluid dynamics (CFD) simulations of the flow through the enhanced cross-flow split (ECFS) serpentine flow field and confirms a reduction in the pressure drop and increased cross-flow in the U-bend regions. However, the arrangement of split sections in this design leads to regions of low through- flow at the electrode and uneven distribution.

Fin Fin et al. from The University of Science and Technology Beijing published an article titled Optimization of a serpentine flow field with variable channel heights and widths for PEM fuel cells’ disclosing a modified serpentine flow field design in which the channel heights vary along each straight flow path to enhance reactant transport and liquid water removal. The optimal design has tapered channels for channels 1, 3 and 4, and increasing heights for channels 2 and 5, with the flow channel widths first increasing and then decreasing.

WO201709767A1 discloses a bipolar plate for a fuel cell, in which the flow channels formed on top surface of the bipolar plate are divided into plurality of groups, such that each flow channel has the same length and forms one group. Each group forms a plurality of subgroups. The length of the flow channels of the groups and the sub groups is substantially the same.

US20070298311A1 discloses a fuel cell separator wherein the ridge area per unit area of the membrane electrode assembly upstream along the fluid path is larger than the ridge area per unit area of the membrane electrode assembly downstream along the fluid path.

US20050042493A1 discloses a fuel cell device, wherein the channel is wider, and the channel area is larger in the upstream part with higher reactivity, so as to improve power generating efficiency. In the downstream part with lower reactivity, the channel is narrower and the channel area is smaller to increase the flow rate and enhance the discharge of carbon dioxide generated. Thus, the power generating efficiency can be improved in the whole cell. The width of a rib acting as a collector may be constant or may gradually taper towards the downstream part. IN301913B discloses a fuel cell with enhanced cross-flow serpentine flow fields, characterized in that a serpentine channel is split into independent serpentine channels with individual inlets from a common inlet manifold, such that a high pressure differential is maintained between the flow channel and the U-bends. This causes a cross-flow of the reactant from the flow channel to the U- bends through the porous diffusion layer, the lay out of the flow field being such that the cross- flow is higher in the oxygen-depleted portion of the adjacent serpentine flow field.

Even though split serpentine with cross-flow helps in reducing the overall pressure drop thereby reducing parasitic losses, lack of specific direction for the flow may lead to cancellation of flow. The coherence between the splits is essential without which one segment of circulation may interfere with another segment, finally leading to inefficient cell performance.

CN101800317A discloses a proton exchange membrane fuel cell bipolar plate with flow field, in which the cross-sectional area of each flow channel gradually decreases from the inlet to the outlet in the running direction of the reaction gas.

US6756149B2 discloses an electrochemical fuel cell with a non-uniform fluid flow design, wherein at least one reactant flow passage is narrower at the inlet than at the outlet, and the cross- sectional area of at least one reactant flow passage is substantially constant from the inlet to the outlet.

US20050042493A1 discloses a fuel cell device, wherein the width of the port may be narrower than the width of the manifold such that the channel has a shape smoothly broadening from the port toward the manifold.

The flow field designs disclosed in the prior art have incorporated channels with gradually increasing or decreasing widths all over the plate (CN101800317A, US6756149B2). These flow field design patterns disclosed in the prior art have gradually varying channels or rib dimensions and are not incorporated into the split serpentine flow field.

There is a need for a flow field design pattern which can improve electrolyte circulation, avoid unwanted bypassing, make the fluid flow in a required order, and regulate the overall pressure drop.

The present invention overcomes the problem of high pressure drop in single serpentine flow field, by proposing a new flow field design with a pattern henceforth called as Flip-Flop pattern. The said flow field pattern is designed in such a manner that the uniformity of cross-flow in the electrode region is preserved while ensuring a short flow path over the cell area.

OBJECT OF THE INVENTION

The principal object of the present invention is to improve electrolyte circulation in electrochemical cells such as redox flow batteries, fuel cells and electrolyzers by preventing unwanted early bypassing of electrolyte through the flow channels and electrode region.

Another object of the present invention is to reduce flow mal-distribution over the electrode region. Yet another object of the present invention is to reduce residence time of electrolyte in the electrode region for quick evacuation of the used electrolyte. Still another object of the present invention is to improve the overall performance of the electrochemical cell.

These and other objects, benefits, and advantages may be obtained by a Flip-Flop serpentine flow field with two or more segments.

SUMMARY OF THE INVENTION

The present invention provides a Flip-Flop serpentine flow field with two or more segments to improve electrolyte circulation in electrochemical cells such as redox flow batteries, fuel cells, and electrolyzers. The same flow path direction is employed in alternate segments of the flow field. The segments are provided with ribs of varying widths at certain locations of the flow field.

BRIEF DESCRIPTION OF THE DRAWINGS

The summary of the present invention, as well as the detailed description, are better understood when read in conjunction with the accompanying drawings that illustrate one or more possible embodiments of the present invention, of which:

Fig. 1 illustrates a schematic electrolyte flow path in single serpentine (a), triple / multiple serpentine (b) and ECFS serpentine (c) flow fields;

Fig. 2 illustrates a schematic arrangement of flow path in the Flip-Flop serpentine flow field; Fig. 3 illustrates a CAD rendering of the Flip-Flop serpentine flow field;

Fig. 4 (a) illustrates a predicted variation of the horizontal (from inlet to the outlet) velocity component in the electrode region in the Flip-Flop and ECFS serpentine flow fields; Fig. 4 (b) illustrates a predicted variation of the horizontal (from inlet to the outlet) velocity component in the electrode region in the Flip-Flop serpentine, ECFS serpentine, multiple serpentine, and single serpentine flow field configurations;

Fig. 5 illustrates a sketch showing varying rib width at select places (marked by ellipses), and Flip- Flop convective circulation in the electrode region;

Fig. 5a, 5b, 5c and 5d illustrate sketches showing the magnified view of varying rib width at select places (marked by ellipses), and Flip-Flop convective circulation in the electrode region; and Fig. 6 illustrates bar graphs showing relative performance for an operating current density of 90 mA/cm ² and flow rate of 0.62 ml/min/cm ² of four serpentine flow fields, namely, the single serpentine, the multiple serpentine, ECFS, and the Flip-Flop serpentine flow field based on measured polarization data of: (a) pressure drop, (b) net energy efficiency (including pump parasitic losses), and (c) discharge energy density.

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises a Flip-Flop serpentine flow field with two or more segments to improve electrolyte circulation in electrochemical cells such as redox flow batteries, fuel cells, and electrolyzers. The same flow path direction is employed in alternate segments of the flow field. The said segments of the flow field comprise ribs of varying widths at certain locations of the flow field.

A comparative assessment was made between the proposed Flip-Flop serpentine flow field design and other flow field configurations, using the predicted velocity profiles obtained from Computational Fluid Dynamics (CFD) simulations. The proposed Flip-Flop serpentine flow field was tested experimentally to evaluate its performance with respect to other flow field configurations.

Fig. 1 illustrates a schematic electrolyte flow path in single serpentine (a), triple / multiple serpentine (b) and ECFS serpentine (c) flow fields while Fig. 2 illustrates a schematic arrangement of the flow path in the Flip-Flop serpentine flow field, and Fig. 3 illustrates a Computer-aided design (CAD) rendering of the Flip-Flop serpentine flow field. The electrolyte fluid enters the flow field from the top left corner through a common inlet manifold ( 1 ) and splits into three inlet streams (2), (3), (4) entering into three different Segments A, B and C respectively, as shown in Fig. 2. Each of these segments contains a short serpentine flow path and straight flow path sections both above and below the return bends in the serpentine flow path. The inlet fluid stream (2) enters Segment A from the top left corner of the segment and it is provided with wider ribs (6) at the entry from straight flow path to serpentine flow path of the segment to prevent the unspent electrolyte inlet fluid stream (2) from entering the common outlet manifold (15) and to avoid unwanted early bypassing of electrolyte to its serpentine channels. Segment A has another set of wider ribs (5) near the common inlet manifold to prevent unwanted bypassing of the electrolyte from the inlet fluid streams (2), (3) to the serpentine channels of Segment A. The inlet fluid stream (2) enters into the serpentine from its bottom straight flow path as indicated by the arrow mark (9), travels all along the segment, comes across the wider ribs (5) and (6), and finally exits the segment at the bottom right corner as indicated by the arrow mark (12). The fluid streams (3) and (4) enter Segments B and C from the top right and top left corners of the segments respectively. Wider ribs (7) are placed at the bottom of the flow field where the Segments B and C part, in order to prevent the entry of unspent electrolyte fluid streams (3) and (4) into the common outlet manifold (15). The directions of the fluid flow into the serpentine segments B and C from their top straight flow paths indicated by the arrow marks (10) and (11) respectively, depart from each other in opposite direction as indicated by the long directional arrows in Segments B and C. The exit fluid streams (13) and (14) leave the flow field through a common outlet manifold (15). The inlet fluid stream (4) in Segment C travels across the wider ribs (8) which prevents early unwanted bypassing of the electrolyte to its serpentine channels before entering its serpentine flow path and leaves the segment to the outlet manifold (15). The exit fluid streams (12), (13), and (14) from all the segments enter a common outlet manifold (15). The relative position of the alternate serpentine flow path segments A, B, and C is same, and the said flow path segments are positioned in such a manner that the direction of the fluid flow in Segment A is opposite to the direction of fluid flow in Segment B. Similarly, the direction of fluid flow in Segment C is in the opposite direction relative to B. In all the three segments, the straight sections of the serpentine flow path are parallel to each other, unlike in the ECFS serpentine pattern shown in Fig. lc. However, the inlet fluid flow stream (2) enters the serpentine Segment A from the top left corner and leaves the segment from the bottom right corner, while in the serpentine Segment B, the inlet fluid stream (3) enters from the top right corner and exits from the bottom left corner. In the serpentine Segment C, the inlet fluid stream (4) adopts the same flow path direction as serpentine Segment A. It enters from the top left corner and exits from the bottom right corner. If there is a fourth serpentine Segment D, the entry and exit paths of corresponding fluid stream will be similar to those of serpentine Segment B and this pattern can be extended to multiple Flip-Flop segments. This arrangement is made in such a manner that a horizontal pressure gradient is created in the flow path, wherein the flow path direction in adjacent segments (for example A & B, and B & C) is opposite, and same in the alternate Segments A and C. The direction of the fluid flow in the segments is marked with long arrows, as shown in Fig. 2. This Flip-Flop pressure gradient induces a corresponding alternating cross-flow in the porous electrode region attached to the graphite flow field. This enables each of the streams to have its own flow direction within its reaction zone and evacuates the streams in an orderly manner with less residence time compared to other serpentine configurations. The flow field design is provided with an arrangement wherein the ribs of varying widths are placed at specific locations as shown in Fig. 5. Fig. 5a, Fig. 5b, Fig. 5c, and Fig. 5d represent a magnified view of Fig. 5. These locations are considered for the said arrangement as they are highly prone to unwanted early bypassing of electrolyte into the respective serpentine channels and/or flow of the unspent electrolyte into the common outlet manifold. This arrangement of ribs with varying widths would reduce the flow mal-distribution over the electrode region and prevent local electrolyte starvation.

According to an embodiment of the present invention, the flow field design is provided with an arrangement, wherein the varying widths or wider ribs are placed only at specified locations near the entry and exit regions of the serpentine segment, thus creating a local resistance, and preventing unwanted early bypassing of the electrolyte.

According to an embodiment of the present invention, the flow field is provided with parallel serpentine channels of approximately equal length except at the regions where wider ribs are provided. According to an embodiment of the present invention, the flow field is designed with a single inlet and single outlet only, such that the Flip-Flop patterned flow division is achieved internally in the flow field. Evaluating flow distribution through CFD simulations

S. Jayanti et al. published articles disclosing predictions of the flow field features through computational fluid dynamics simulations. Using these observations, Figs. 4 (a) and 4 (b) illustrate a predicted variation of the horizontal (from inlet to the outlet) velocity component in the electrode region in the Flip-Flop and other flow field configurations. The figures show the velocity variation in the electrode region for the Flip-Flop serpentine, ECFS serpentine, and other serpentine flow field configurations for the same flow rate of the electrolyte. The figures indicate a much smoother flow profile in the Flip-Flop serpentine with a higher and more uniform convective velocity over a large area of the electrode compared to ECFS serpentine. This shows that the wider ribs employed at specific locations smoothen the local convective peaks. It is evident from the graphs that the convective velocity goes to negative values from the positive values, and reverts back to positive values while moving along Segments A to B, and then to C. Even though the ECFS serpentine flow field exhibits regions of high velocity, high velocities prevailed only in a lesser area with the larger area experiencing significantly low velocities, and such mal-distribution will eventually reduce the electrochemical performance of the cell.

Experimental Performance Evaluation

The discharge energy, discharge capacity, and round-trip energy efficiency values measured for different current density values as well as different serpentine flow field configurations for a fixed flow rate of 0.41 ml/min/cm ² are presented below in Table 1.

Fig. 6 illustrates bar graphs showing relative performance of four serpentine flow fields, namely, the single serpentine, the multiple serpentine, the ECFS serpentine, and the Flip-Flop serpentine flow field, based on measured polarization data of: (a) pressure drop, (b) net energy efficiency, and (c) discharge energy density. R. Gundlapalli and S. Jayanti, published articles titled ‘Effect of channel dimensions of serpentine flow fields on the performance of a vanadium redox flow battery’ and ‘Effect of electrode compression and operating parameters on the performance of large vanadium redox flow battery cells’ which disclose experimental studies with single serpentine flow field to measure pressure drop, round-trip energy efficiency, the discharge energy and capacity. Fig. 6a shows considerably lower cell pressure drop in the serpentine flow fields having either multiple serpentine entries or multiple serpentine segments compared to the single serpentine flow field. The said lower cell pressure drop is due to the presence of shorter fluid travelling paths and low electrode velocities. Fig. 6b shows that the low pressure drop is accompanied by a significant reduction in energy efficiency of the cell in the case of the multiple serpentine and ECFS serpentine flow fields. Unlike this, a low pressure drop accompanying increasing energy efficiency can be observed in the case of Flip-Flop serpentine flow field. Fig. 6c shows highest discharge energy capacity for Flip-Flop serpentine flow field compared to the other flow field configurations.

It is to be understood, however, that the present invention would not be limited by any means to the techniques, and approaches that are not specifically described, and any change and modifications to the techniques and approaches can be made without departing from the spirit and scope described in the present invention.