Effect of channel length on interdigitated flow-field PEMFC performance: A computational and experimental study

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Effect of channel length on interdigitated flow-field PEMFC performance: A computational and experimental study Anthony D. Santamaria, Nathanial J. Cooper, Maxwell K. Becton, Jae Wan Park* Department of Mechanical and Aerospace Engineering, University of California, One Shields Ave., Davis, CA 95616-5294, United States

article info

abstract

Article history:

This study examines the effect of increased channel length on the distribution of flow

Received 12 February 2013

gases and cell performance in an interdigitated flow-field PEMFC. A numerical model was

Received in revised form

used to simulate the pressure distribution and gas flow in 5 cm and 25 cm channel length

13 September 2013

interdigitated flow fields that have eight channels with fixed channel and land widths of

Accepted 16 September 2013

1 mm. The results show the distribution of flow under land areas (cross flow) is subject to

Available online 18 October 2013

maldistribution in the longer cell while the shorter cell produces relatively homogenous cross flow along its length. An experimental test cell was designed to run with the same

Keywords:

channel lengths, under similar conditions to those modeled. Inlet pressure data was

PEM fuel cell

recorded to account for parasitic pump losses, used to calculate net system power curves.

Interdigitated flow field

Results show that a shorter channel interdigitated flow field may produce both higher

Parallel flow field

maximum power and limiting current densities compared with longer cells.

Cross flow

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are being developed for use in both stationary and mobile power applications. Several automotive companies continue to view PEMFCs as a candidate technology for vehicle use to meet new state and federal emissions standards [1]. Additionally, growing renewable energy generation capacity, as well as a booming natural gas industry, has made hydrogen production more economically competitive with current fuels [2]. Many challenges lay ahead of PEMFC technology before it can meet the needs of customers on a large scale basis. PEMFC flow-field design remains an important area of focus for both

researchers and designers investigating cell performance. This paper examines the effects of variable channel length, with respect to interdigitated flow-field designs. Such information may be valuable in situations where cell packaging affects cell dimensions. Maintaining a high reactant concentration in, as well as removal of by-product water from, catalyst layer (CL) and gas diffusion layer (GDL) regions of a PEMFC membrane electrode assembly (MEA) is dependent on mass transport conditions. In a parallel flow field, diffusion is the dominant transport mechanism, while interdigitated designs induce convective transport beneath land areas, known as cross flow [3,4]. This basic principle is illustrated in Fig. 1. Cross flow has been

* Corresponding author. Tel.: þ1 530 752 5559; fax: þ1 530 752 4158. E-mail address: [email protected] (J.W. Park). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.081

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Fig. 1 e Diagram of cross flow between adjacent channels. PH represents a higher pressure channel (inlet channel) and PL a lower pressure channel (outlet channel). Arrows designate transport beneath the land area through the GDL.

shown to reduce concentration overpotential significantly, especially at higher current densities, when compared to diffusion based flow. Interdigitated flow fields have been demonstrated to produce higher maximum power, as well as higher limiting current densities, compared to their parallel counterparts. Interdigitated PEMFC flow fields have been investigated since the mid 1990s for their improvements over conventional designs [5e9]. Nguyen’s group observed a higher maximum cell power and reduced overpotential in the concentration region of an interdigitated design PEMFC [10,11]. Additionally, it was demonstrated that the increase in cell power was greater than the increase in parasitic pump power, caused by higher back pressure from porous flow through the GDL [6]. Kazim et al. developed a mathematical model of an interdigitated flow-field PEMFC outlining different cathode operational effects on performance [12,13]. They concluded that increased cathode porosity and increased operating pressure lead to higher maximum power density and higher limiting current density. PEMFC flow-field design can have large implications for water management. Experimental studies by Wang and Liu, characterized interdigitated flow-field performance under a host of operating conditions [14]. They noticed that anode and cathode humidification played a larger role in interdigitated performance compared to a conventional serpentine flow field, potentially due to the different transport mechanisms. Trabold and Owejan et al. completed imaging studies using neutron radiography (NR) to examine liquid water distribution in serpentine and interdigitated designs [15,16]. Spernjak et al. used NR to compare water transport characteristics between parallel, single-serpentine and interdigitated flow-field PEMFC designs [17]. Results showed that interdigitated flow fields were more effective in removing product water from cathode land areas compared to parallel and serpentine patterns, especially at high current densities. Advances in computational methods have lead to a deeper fundamental understanding of cross flow and its effect on PEMFC performance. Grujici et al. completed an extensive study on flow-field optimization that examined the effects of geometry on interdigitated performance [18]. They concluded that reduced channel length and width may lead to higher performance and that optimal cathode design is associated with oxygen transport to the membrane/CL interface. Lum

and McGuirk examined both 2D and 3D modeling results of interdigitated flow, observing that thinner electrodes enhance performance and that the use of a single-phase model was valid because the majority of water in the electrode exists in vapor phase [19]. They also concluded that 3D models more accurately capture oxygen depletion effects down the length of a channel. Kanezaki et al., using a 2D model, investigated the effects of cross flow on PEMFC performance, establishing that strong convection through the electrode leads to higher CL reactant concentration [3]. They further attributed reduced concentration overpotential and enhanced water removal to cross flow. Shi and Wang developed a 3D model, solved using COMSOL Multiphysics, to investigate the effect of GDL compression on cross flow in a serpentine flow-field design [20]. Further optimization studies involving cross flow with regard to serpentine designs followed [21]. This focus on flow fields that induce cross flow has spurred interest in the optimization of flow fields for operation with minimum parasitic pump power, while attaining high cell performance. Kloess et al. modeled bio-inspired flow channels in 3D using COMSOL Multiphysics, identifying a reduced pressure drop due to GDL convection [22]. Bachman et al. developed and tested a PEMFC capable of operating with varying levels of cross flow; separate cathode manifolds and a manually operated valve system regulated the pressure difference between adjacent channels, thus controlling cross flow between them [23]. Recently, Santamaria et al. demonstrated a flow-field switching PEMFC that could be transitioned between parallel and interdigitated flow configurations so as to operate in the most efficient configuration at a given current density [24]. The study presented here aims to characterize the effect of various channel lengths (in this case 5 cm and 25 cm channels having fixed channel and land widths of 1 mm) on a standard interdigitated PEMFC’s performance as well as to identify mechanisms by which flow is affected by operating conditions. Studies have examined such effects in parallel configuration cells; however, no significant work has been completed with interdigitated designs [25]. This is of interest because cross flow is sensitive to channel pressure gradients which can be greatly affected by channel/cell geometry. Maldistribution of gas flow over an active area may affect cell performance in unforeseen ways.

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2.

Model formulation

2.1.

Geometry and assumptions

The Multiphysics package COMSOL was used to simulate the interdigitated flow field at varying lengths and conditions in three dimensions. Fig. 2 is a breakdown of the computational domains modeled. The model focuses on the cathode flow field and GDL layer and does not take into account the electrochemical reaction. As this study is interested in how channel length affects pressure and cross flow distribution, modeling the entire electrochemical process is not necessary because the electrochemical reaction has little effect on bulk cross flow; additionally, the experimental portion of this work characterizes cell performance. Reaction by-product water is a factor; however, no models can currently account for this completely. The model assumes the following: -

No electrochemical reaction Steady state operating conditions Isotropic and homogeneous GDL structure No effects from land area compression No mass transport across the GDL/CL interface Water exists only in a gas phase Water, nitrogen and oxygen are ideal gases Isothermal

2.2.

Governing equations and boundary conditions

Conservation laws were applied to the flow field and GDL domains via: Continuity equation: V$ðruÞ ¼ 0

(1)

Momentum conservation in the channels: 2 rðu$VÞu ¼ VP þ mV2 u  mVðV$uÞ þ b 3

(2)

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where r is the density of the mixture, u is the velocity field, m is the viscosity of the mixture, P the pressure and b accounts for body forces. A compressible formulation of the state equations was used in this simulation for added accuracy. Although the pressures in the simulation are low enough so the gas may be almost considered incompressible, the pressure distribution in cell is the only flow driving force in the simulation, so accuracy in pressure distribution is paramount. To this end, the variation in density due to pressure was accounted for and included in calculations. Momentum conservation in the porous media (i.e. the GDL) was handled using the Brinkman modification to the momentum conservation equations [26]: r u 2m m ~ m ðu$VÞ ¼ VP  VðV$uÞ þ V2 u  u þ b ε ε 3ε ε K

(3)

where K is the effective permeability, m ~ is the effective viscosity (following Brinkman’s work,~ m is set to m) and ε the porosity [26]. Boundary conditions for the model were applied to all walls, the outlets, and the interface between the GDL and the channels. In this simulation the inlet condition was determined by a laminar inlet flow of a specified velocity. This inlet velocity was calculated based on the mass flow of the gas at the maximum power point. The development length in the channel was small compared to the length of the cell. The edges of the channel were constrained to a velocity of zero, to model the wall condition. Due to the low Reynolds number throughout the cell, the outlet condition was also specified to be laminar. The outlet in the experimental cell vented to the atmosphere, so in the simulation, the outlet channels had a boundary condition requiring them to have an outlet absolute pressure of 1 atm. Finally, the solutions for the NaviereStokes equation and the DarcyeBrinkman equation were set to be equivalent across the interface between the channel and the GDL: PBr ¼ PNS

(4)

uBr ¼ uNS

(5)

2.3.

Mesh and solver

The MUMPS direct solver in COMSOL was used to find solutions for equations (1)e(5) based on the parameters specified and limited by the boundary conditions. The mesh in the GDL of the short cell was composed of 73,500 rectangular elements, and each channel had 2940 rectangular mesh elements, for a total of 102,900 rectangular mesh elements in the model of the 5 cm flow field. The mesh in the GDL of the long cell was composed of 147,000 rectangular elements, and each channel had 5880 rectangular mesh elements, for a total of 205,800 rectangular mesh elements in the model of the 25 cm flow field. Increasing the number of mesh points and changing the mesh geometry resulted in pressure drop differences of less than 1%, indicating the solution was mesh independent at this level of fineness.

2.4. Fig. 2 e Subdomain geometries modeled in COMSOL. Arrows designate inlet/outlet flows while X’s designate blocked boundaries on the channel domains.

Parameter selection

The channel and GDL properties listed in Table 1 are based as closely as possible to the actual material properties of components used in the experimental portion; this was to ensure

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Table 1 e Properties for computational models. Variable

Value

Description

Short



T ( C) Pout (Pa) ε k (m2) tGDL (mm) uin (m s1) wc (mm) dc (mm) wl (mm) Lc (cm)

Long

Normal flow

Excess flow

Normal flow

Excess flow

70 0 0.50 4.15e-13 0.18 1 1 1 1 5

70 0 0.50 4.15e-13 0.18 2 1 1 1 5

70 0 0.50 4.15e-13 0.18 5 1 1 1 25

70 0 0.50 4.15e-13 0.18 10 1 1 1 25

comparable results. One condition not accounted for, as listed under assumptions, is the GDL compression beneath land areas. This compression changes local GDL porosity and can effect fluid transport and inlet pressure [27,28]. The consequences of this will be discussed later when the simulated and experimentally measured pressure drops are compared. Channel lengths, Lc, were selected based on a previous experimental study examining channel length effects on parallel design flow fields, which calculated the required pressure drop for water removal [25]. The resulting channel length to width ratios (channel aspect ratios) were 50:1 and 250:1. The simulation inlet velocities, uin, were determined by examining the flow rates of the actual cells at maximum power and specified stoichiometry, then calculating the bulk velocity based on the entrance geometry. Simulations were run at both standard (1.5 anode, 2.0 cathode) and excess (2.0 anode, 4.0 cathode) stoichiometry based velocities [29].

3.

Simulation results and discussion

3.1.

Pressure distribution

The overall pressure distributions of the flow-field channels, simulated at the excess stoichiometric based flow rates, are displayed in Fig. 3. For the long case, clear changes in pressure distribution are present along the length of both the inlet and outlet channels. In the short cell, on the other hand, the pressure distribution variance is far less down the length of the individual inlet and outlet channels. Fig. 4a and b shows the pressure within the center two inlet and outlet channels along the length of both cells. In the long cell inlet channel, pressure starts high and drops off down the length. These parabolic trends are due to head loss within the channel. Longer channels support a greater head drop down their length, and are thus prone to pressure differentials within the channel itself. By contrast, the short cell inlet pressure remains fairly constant down the length of the cell, while the outlet channel remains close to atmosphere. These results match previous interdigitated studies and are very similar to pressure distribution trends which were observed by Koh et al. in their study of z-configuration PEMFC manifolds which have similarities to interdigitated flow-field designs [7,30].

Temperature Outlet pressure (gage) Porosity of GDL Permeability of GDL Thickness of GDL Inlet velocity Channel width Channel depth Land width Channel length

Fig. 4c overlays the deviation in inlet/outlet pressure difference, DPx, at a position x, from the average pressure difference between the inlet/outlet channels DPave, along the normalized length of the cells, x/Lc. The long cell pressure distribution appears parabolic, starting off with a greater than average pressure, dipping well below average around the center of the cell and then climbing to its maximum at the exit. The large pressure difference at the exit is due to the atmospheric boundary condition at the exit of the outlet channels. Even though the inlet channel pressure is still relatively high, it is maintained by the low permeability of the GDL. In the short cell the pressure difference remains more consistent throughout the length. While not obvious at these scales, the short cell pressure deviation data exhibits a similar parabolic shape as the long cell though it is far less extreme. The reduction in maldistribution in the long cell of the lower flow case may be due to decreased velocities in the channels, and so reduced head losses due to flow. This reduction in contribution to pressure differentials down the length of channels may allow for improved pressure equalization between channels as each whole channel is closer to a constant pressure.

3.2.

Cross flow distribution

The pressure difference between channels induces cross flow under land areas. Fig. 5a and b displays cross flow distribution along the length of the cell between the same two center inlet/ outlet channels as the pressure analysis. In the long case, cross flow is distributed unevenly throughout the cell with higher rates at the cell entrance and exits. Significantly reduced cross flow is observed in the center of the cell. Similar flow maldistribution trends were observed by Lebæk et al. while studying long z-configuration fuel cell manifolds [31]. They noticed that cells at the end of the inlet manifold received higher flow rates than cells toward the entrance. Cross flow in the short cell is distributed evenly throughout its length. Both cases show that cross flow rate is correlated very closely with the difference in pressure between channels, which is expected since pressure differences drive cross flow. Fig. 5c plots the deviation in cross flow, qx, at a position x, from the average cross flow rate of a cell, qave, against the normalized distance down the cells, x/Lc. Clear comparisons between

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Fig. 3 e Overview of the channel pressure distribution in the higher flow rate case.

cross flow rate deviation in Fig. 5c and pressure difference deviation trends in Fig. 4c are noticeable. Fig. 6 is a visualization of the cross flow velocity field at several locations along the lengths of the long and short cells. It is observed that the long flow field, despite higher cross flow rates at the inlet (x/Lc ¼ 10%) and outlet (x/Lc ¼ 90%) regions, displays lower cross flow over the bulk of its length than the short cell. Relatively consistent cross flow is present in the short cell throughout its length. The center region of the short cell cross flow rate is higher than that of the long cell (x/ Lc ¼ 50%) by approximately 20%.

3.3.

configurations. The short flow-field cross flow distribution remains consistent even at reduced flow rates, showing very little difference from the excess stoichiometry case. The long cell cross flow distribution, on the other hand, actually improves with reduced flow rates somewhat in the center of the cell while the inlet and outlet displays less extreme cross flow rates.

4.

Experimental setup

4.1.

Cell design

Effect of flow rate

Simulations conducted at both stoichiometry conditions provided some insight into the effect of reduced inlet flow rate on the cross flow distribution. Fig. 7 displays normalized cross flow for high and low flow rate cases, from excess and normal stoichiometry conditions respectively, for both long and short

A PEMFC was designed that could be configured to run under varying channel length configurations through the use of separate manifold exit ports [25]. Manifolds were sized to eliminate any pressure difference between inlet channels or pressure build-up at the outlets. Fig. 8 is a schematic of the flow field along with an image of one of the bipolar plates.

Fig. 4 e (a) Plot of inlet and outlet channel pressure in the long cell. (b) Plot of inlet and outlet pressure in the short cell. (c) Deviation from average pressure difference between the channels in both cells. All data shown is for the excess flow stoichiometry of 2.0 anode and 4.0 cathode. All data is from the middle two channels.

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Fig. 5 e (a) Plot of cross flow between inlet and outlet channels in the long cell. (b) Plot of cross flow between inlet and outlet channels in the short cell. (c) Deviation from average cross flow between the channels in both cells. All data shown is for the excess flow stoichiometry of 2.0 anode and 4.0 cathode. All data is from the middle two channels.

Outlet A was used for the long setup and outlet B was used for the short setup. The flow-field plates were machined out of 6061-T6 aluminum which was then nickel coated to prevent corrosion. The nickel coating produced a contact angle of approximately 80 , making it slightly hydrophilic. The channel geometry details are given in Table 1. To run the cell under interdigitated conditions, silicon plugs were used to seal off inlets and exits of cathode channels as shown in Fig. 8. The 1 mm  1 mm  3 mm plugs sufficiently blocked channel flow and could be repositioned to accommodate the testing

configuration. Rubber plugs were used to fill and block off manifolds that were not in use. This prevented gases from passing between channels at manifold inlet locations. Kapton tape was placed over unused portions of the cathode and anode MEA to prevent electrochemical reactions from occurring in inactive regions of the short configuration flow field. The MEA was comprised an SGL 10BC carbon GDL and a Dupont Nafion XL membrane with 27.5 mm thickness and 0.4 mg cm2 platinum loading on both sides. Silicon gaskets were used to seal the MEA and cell compression was kept

Fig. 6 e Sectional views of cross flow velocity at x/Lc positions of approximately 10%, 50% and 90% of both long and short flow fields.

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was monitored using k-type thermocouples and maintained at 70  C via four cartridge heaters placed directly into the cathode and anode bipolar plates. Inlet pressure was measured using a Miljoco gauge. The cell was oriented sideways to reduce the effects of gravity and all flow hardware was insulated to maintain temperature. The inlet dew point was set to produce 100% humidity outlet conditions for the anode and cathode gas streams. Both interdigitated and parallel configurations would be tested to highlight performance differences. Polarization curves were conducted at both normal and excess stoichiometries of 1.5 anode/2.0 cathode and 2.0 anode/4.0 cathode respectively.

Fig. 7 e Overview of the effect of flow rate on cross flow in both long and short flow fields. Deviation from average cross flow data is for stoichiometries of 2 anode/4 cathode and 1.5 anode/2 cathode. All data is from the middle two channels.

consistent throughout the testing by measuring and maintaining a consistent bipolar plate separation gap.

4.2.

Operating conditions

Performance testing was conducted using an Arbin Instruments Fuel Cell station which measured and controlled cell voltage, current, temperature and feed gas conditions (temperature, humidity, flow rate). Temperature

5.

Experimental results and discussion

5.1.

Polarization and inlet pressure curves

Fig. 9aed presents the polarization data as well as required inlet pressure for each of the cases. Each case was run with three replications in random order to test for consistency and repeatability. The polarization curves depicted are an average of these three runs. As reported in previous studies, the interdigitated designs outperformed the parallel configurations at the same operating parameters with respect to higher limiting current density [12]. The shorter cells show a drastic increase in performance under interdigitated conditions which can be partially attributed to the inherent instability of shorter parallel cells. This topic was addressed and similar low performance of shorter length parallel flow fields were reported by Bachman et al. [25]. The long interdigitated cell, under excess stoichiometry, showed an increase of 20.7% in limiting current density over the parallel long configured cell. The short interdigitated cell, under excess stoichiometry, showed an increase of 114% over the parallel short

Fig. 8 e Overview of experimental test cell and flow configurations that were setup using silicon plugs to block channel flow.

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Fig. 9 e Polarization curve and inlet pressure plots. Polarization data for parallel and interdigitated is labeled on the left axis. Pressure data is assigned to the right axis. Short/parallel inlet pressures were too low to be recorded on these scales.

configuration. Switching to an interdigitated flow-field design may be more beneficial in cells with lower channel aspect ratios. Inlet pressure increases with rising current density, as expected, since fixed stoichiometry control was maintained. The inlet flow rate increases to provide adequate reactants at each current density step. No pressure curves for the parallel short tests were included because the inlet pressure required was too small to register on the gauge. The interdigitated cases required higher inlet pressure compared with the parallel due to flow through the porous GDL. Additionally, the interdigitated design’s inlet velocity was double that of their parallel counterparts because only half the inlet area is used; this may contribute to some pressure increase. The pressure curve for the long cell, interdigitated at normal stiochiometry becomes more erratic at higher current densities. This may be due to water production in the concentration loss region of the polarization curve. The test pressures at approximately max power are compared to the simulation pressures in Table 2. The differences may be accounted for by the presence of GDL compression and liquid water in the experimental testing.

Higher GDL saturation may reduce its effective permeability, further increasing the pressure drop and adding to the discrepancy between the experimental and modeling inlet pressure results.

5.2.

Net power density curves

To compare the true power output of these systems, E_ sys, the required pumping power, E_ pump, must be subtracted from the cell power, E_ cell: E_ sys ¼ E_ cell  E_ pump

(6)

with units of Watts. Because inlet pumping pressure, pinlet, and mass flow rate, ṁAir (kg s1), change with current density, pumping power is determined by Ref. [32]: _ Air ¼ m

  iAxc MO2 4F 0:21

_ Air Cp T m E_ pump ¼ h

(7)

  k1 pinlet k 1 patm

(8)

Table 2 e Comparison of computed and measured cathode pressure drops at max power. Interdigitated flowfield Short Short Long Long

Stoichiometry (anode/ cathode)

Inlet velocity (m s1)

Computed pressure (kPa)

Measured pressure (kPa)

2/4 1.5/2 2/4 1.5/2

2 1 10 5

0.64 0.32 1.30 0.49

3.10 1.38 7.93 4.14

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Fig. 10 e Power density curves comparing interdigitated with parallel configuration performance.

where i is the current density (A m2), A is the electrode area (m2), xc is the cathode stoichiometry, F is Faradays constant (96,485 C mol1) and MO2 is the molar mass of oxygen (kg mol1), Cp is the specific heat of air (J kg1 K1), T is the temperature of the air (K), h is the pump efficiency (w85%), patm is the atmospheric pressure and k is the specific heat ratio. Fig. 10aed displays the system power density of each condition as a function of current density. In each case the interdigitated configuration lead to a higher maximum power, with the exception of the normal stoichiometry long tests (Fig. 10b) where it appears concentration losses were more severe. Given the larger amount of liquid water produced at higher current densities in the long cell, the lower flow rate may have been inadequate for water removal which in turn reduced performance. Evidence of water issues was also mentioned in the pressure discussion (Section 5.1) where performance was observed to be erratic in the concentration region. The resulting limiting current densities and achieved

maximum system power are presented in Table 3. The shorter length cells benefited more from the switch to the interdigitated configuration, as performance rose dramatically compared to the improvements in the long cell cases. The short interdigitated flow field appears to overcome the issues that shorter parallel fields encounter, potentially because of the even distribution of cross flow as suggested by the results presented in Section 3.2. Longer cells may also benefit from interdigitated flow, but are more susceptible to cross flow maldistribution and higher pressure losses. Fig. 11 displays a comparison of power densities at a given stoichiometry (a and b) and at a given length (c and d). The short interdigitated cell clearly produces the highest power density while its parallel counterpart produces the lowest under both normal and excess flow conditions. The performance of the normal flow rate long configuration is very similar to the parallel case; however, the cell does reach a higher limiting current density. Overall, under interdigitated

Table 3 e Overall experimental results. Flow field

Variable

Value

Description

Short

Parallel Interdigitated

2

jL (A cm ) E_ max (mW cm2) jL (A cm2) E_ max (mW cm2)

Long

Normal flow

Excess flow

Normal flow

Excess flow

0.73 236 1.37 403

0.73 278 1.56 436

1.02 313 1.25 301

1.21 368 1.46 387

Limiting current density Maximum system power Limiting current density Maximum system power

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Fig. 11 e (a and b) Power density of cases with the same stoichiometry. (c and d) Power density of cases with the same length.

conditions the shorter channel length configuration leads to better cell performance.

6.

Conclusions

This study examines the effect of channel length on interdigitated PEMFC performance. The computational modeling of the interdigitated flow field shows that, while transport in a shorter cell may be evenly distributed, with increased length, maldistribution of cross flow may occur. Pressure gradients between channels were shown to drive cross flow beneath land areas. In the 5 cm long case, this pressure gradient was stable throughout the length of the cell, while the 25 cm case showed a variation ranging from 20% less to 45% more than its average. Experimental testing showed that the performance of the 5 cm interdigitated flow field outperformed the 25 cm case cell. This may be explained by the homogeneity of cross flow in the short length cell observed in the modeling work and its potential to reduce liquid water beneath land areas. The limiting current densities of the short interdigitated cases were 6.8% and 9.6% greater than the long interdigitated cases limiting current densities for normal and excess flow conditions respectively. Maximum power density of the short interdigitated cases was found to be 33.9% and 12.7% greater than the long maximum power density cases for normal and excess flow conditions respectively. The 25 cm cell was shown to be more susceptible to instability in the concentration

overpotential region especially under the lower normal stoichiometry condition. This was evident in both cell performance and pressure data and may be due to liquid water. Overall the interdigitated design outperformed the parallel flow field except in the standard stoichiometry case where it performed slightly lower, albeit, with a higher limiting current density. These results show that when designing a PEMFC, channel aspect ratio should be taken into consideration when deciding between using interdigitated or parallel flow-field designs. Interdigitated based designs may be more beneficial to lower channel aspect ratio flow fields.

Acknowledgement This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research(KIER) (B3-2413-01).

references

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