university of waterloo flow field designs report

International Journal of Hydrogen Energy 30 (2005) 359–371

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Reviewof bipolar plates inPEM fuel cells: Flow-field designs

Xianguo Li∗, Imran SabirDepartment of Mechanical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1

Received 7 April 2004; received in revised form 11 August 2004; accepted 2 September 2004Available online 23 November 2004

Abstract

The polymer electrolyte membrane (PEM) fuel cell is a promising candidate as zero-emission power source for transportand stationary cogeneration applications due to its high efficiency, low-temperature operation, high power density, fast start-up, and system robustness. Bipolar plate is a vital component of PEM fuel cells, which supplies fuel and oxidant to reactivesites, removes reaction products, collects produced current and provides mechanical support for the cells in the stack. Bipolarplates constitute more than 60% of the weight and 30% of the total cost in a fuel cell stack. For this reason, the weight,volume and cost of the fuel cell stack can be reduced significantly by improving layout configuration of flow field and use oflightweight materials. Different combinations of materials, flow-field layouts and fabrication techniques have been developedfor these plates to achieve aforementioned functions efficiently, with the aim of obtaining high performance and economicadvantages. The present paper presents a comprehensive review of the flow-field layouts developed by different companiesand research groups and the pros and cons associated with these designs.� 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords:PEM fuel cell; Bipolar plate; Flow field; Flow channels

1. Introduction

The polymer electrolyte membrane fuel cell (PEMFC)is one of the most widely researched fuel cell technolo-gies because it offers several advantages for transport anda number of other applications. Its low-temperature oper-ation, high power density, fast start-up, system robustness,and low emissions have ensured that the majority of motormanufacturers are actively pursuing PEMFC research anddevelopment. Already in Europe with demonstration busesand passenger vehicles in California, for example, a firstmarket introduction of fuel cell vehicles will be seen in thenear future. However, there are still technical barriers to beovercome before fuel cell vehicles reach a significant mar-ket penetration[1–3].

∗ Corresponding author. Tel.: +1 5198884567/X 6843;fax: +15198886197.

E-mail address:[email protected](X. Li).

0360-3199/$30.00� 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2004.09.019

During the past two decades, the research and develop-ment of the PEMFC with a Nafion� membrane as elec-trolyte have received much attention. Much research hasfocussed on single cells of PEMFC or their components,such as novel membrane electrolytes, catalysts and structure,electrochemical reaction mechanisms and kinetics, as wellas electrode materials and preparation. This has resulted ina number of alternatives to Nafion� membrane technologywith low load, high utilization catalyst yielding significantlyhigher power density than was available few years back. Im-provements in cell design and manufacturing have furtherincreased power, while reducing manufacturing costs, whichis essential if the fuel cell is to compete with the internalcombustion engine.

For a given membrane/electrode assembly (MEA), thepower density of a fuel cell stack can be significantly in-creased by reducing the profile of the bipolar plates. A keyprerequisite for many power applications is the productionof compact and lightweight PEMFC stacks, which may be

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360 X. Li, I. Sabir / International Journal of Hydrogen Energy 30 (2005) 359–371

achieved with appropriate selection of materials. Bipolarplate design as a whole, and flow channel layout configura-tion, in particular, are potential areas of research for makingthis alternative clean power source compatible to its coun-terparts[4–6]. Challenges remain, however, that include re-ducing the weight, volume and cost of the fuel cell stackwith the present goal being to develop a 50 kW stack systemweighing less than 133 l and a cost of $35/kW (DOE). Thedominance of the bipolar plates (BPPs) in a PEMFC stacksis illustrated inFig. 1. The objective of this review article isto present a comprehensive overview of the state-of-the-arttechnology for the BPPs in PEMFCs and the current andfuture direction of the R&D activities that aim at the reduc-tion in the weight, volume and cost of BPPs with improvedperformance and lifetime.

2. Biopolar plates

As discussed earlier, BPPs account for the bulk of thestack, hence it is desirable to produce plates with the small-est possible dimensions permissible (<3mm in thickness)[US Department of Energy (DOE), 8].With the bipolar platearrangement for current collection each of the MEAs (mem-brane electrode assembly) is interspersed between two fluid-impermeable, electrically conductive plates, commonly re-ferred to as the anode and the cathode plates, respectively.When the reactant flow channels are formed on the anodeand cathode plates, the plates are normally referred to asfluid flow-field plates. When the flow channels are formedon both side of the same plate, one side serves as the an-ode plate and the other side as the cathode plate to the ad-jacent cell, and the plate is called bipolar (separator) plate,as shown inFig. 2. In this stack design, cooling is accom-plished with accommodating separate cooling plates after afew cells in series. It is more often that one of the reactantsflows on one side of such a plate, while a cooling fluid flowson the other side of the same plate in order to remove thewaste heat generated in the cell, and these plates collectivelyhave to keep the fuel, oxidant and cooling fluid apart, pre-venting them from mixing with each other, otherwise safetyconcerns and hazardous situations may arise.

2.1. Functions

BPPs being one of the most important components inPEMFC stacks must perform a number of functions wellsimultaneously in order to achieve good stack performanceand lifetime. BPPs supply the reactant gases through theflow channels to the electrodes and serve the purpose ofelectronically connecting one cell to another in the elec-trochemical cell stack. These plates also provide structuralsupport for the thin and mechanically weak MEAs andmeans to facilitate water management within the cell. Inthe absence of dedicated cooling plates, the BPPs also fa-cilitate heat management. Plate topologies and materials

Fig. 1. Mass distribution in a 33 kW PEMFC stacks[7].

facilitate these functions. Topologies can include straight,serpentine, or interdigitated flow fields, internal manifold-ing, internal humidification, and integrated cooling. There-fore, optimal design must be sought for the BPPs becausethe above functions have conflicting requirements on theBPP design.

X. Li, I. Sabir / International Journal of Hydrogen Energy 30 (2005) 359–371 361

Fig. 2. Stack components of fuel cell[9].

2.2. Requirements

The essential requirements for BPPs, in respect to physic-ochemical characteristics, are uniform distribution of thereactant gases over the respective active electrode surfaceto minimize the concentration overpotential; high values ofelectronic conductivity for current collection; high mechan-ical strength for stack integrity; impermeability to reactantgases for safe operation; resistance to corrosion in severecell environment for long lifetime; cheap materials, easy andautomated fabrication for low cost.

2.3. Channel cross-section

The fluid flow channels are typically rectangular in cross-section, even though other configurations such as trape-zoidal, triangular, semi-circular, etc. have been explored[10]. The flow channel dimensions range from a fraction of1 about 2mm in width and depth as a low limit for a rea-sonable fluid pressure loss due to friction losses. The mostcommon methods of fabricating fluid flow channels on theBPPs require the engraving or milling of flow channels intothe surface of the BPPs. After molding the plates at hightemperature and pressure, the gas distribution channels aremachined, generally in a parallel flow configuration.

Simulation results for values of channel depth, width andland width have also been reported close to 1.5, 1.5, and0.5mm, respectively. Decreasing land width will increasehydrogen concentration at the anode, and triangular andhemispherical cross-sections have land width close to zero[11]. In practice these design suggestions have limitationstoo, for example triangular or near zero land width designmight crush the MEA at high pressure contact areas thathampers the current collection.

3. Flow-field layout design

One of the main obstacles to large-scale commercializa-tion of fuel cells is the gas flow fields and BPPs, includ-ing the development of low-cost lightweight constructionmaterials, optimal design and fabrication methods and their

Fig. 3. Pin-type flow-field[16,17].

impact on PEMFC performance (i.e., energy efficiency andpower density)[12]. As much as 50% increase in the outputpower density has been reported[13,14] just by appropri-ate distribution of gas flow fields alone. In spite of all theindustrial R&D efforts, the time-effective design and opti-mization of the gas flow fields and BPPs remain one of theimportant issues for the cost reduction and performance im-provement of PEM fuel cells.As to the geometrical configurations of the gas flow fields,

a variety of different designs are known and the conventionaldesigns typically comprise either pin, straight or serpentinedesigns of flow-field channels, as summarized in[15]. Fuelcell developers have used alternative designs, such as

1. pin-type flow field,2. series-parallel flow field,3. serpentine flow field,4. integrated flow fields,5. interdigitated flow field,6. flow-field designs made from metal sheets.

3.1. Pin-type flow field

Examples of the pin-type flow fields are illustrated byReiser and Sawyer[16] and Reiser[17] , and an example isshown inFig. 3. The flow-field network is formed by manypins arranged in a regular pattern, and these pins can be inany shape, although cubical and circular pins are most oftenused in practice. Normally, both the cathode and the anodeflow-field plates have an array of regularly spaced cubical


Flow-Channel

Flow Channel

Rib or Channel Support

Inlet

Outlet

Fig. 4. Straight and parallel flow field and flow channelcross-section[18].

or circular pins protruding from the plates, and the reactantgases flow across the plates through the intervening groovesformed by the pins. The actual fluid flow thus goes througha network of series and parallel flow paths. As a result, pin-design flow fields result in low reactant pressure drop. How-ever, reactants flowing through such flow fields tend to fol-low the path of least resistance across the flow field, whichmay lead to channelling and the formation of stagnant areas,thus uneven reactant distribution, inadequate product waterremoval and poor fuel cell performance. Further, relativestable recirculation zones may arise behind each pin sincethe reactant flow is very slow in such a small flow chan-nels, and the Reynolds number for the reactant flow remainssmall, particularly for the fuel stream Reynolds numbersmay range from a few tens to low hundreds. Reactant con-centration may be depleted in the stable recirculation zonesas well, decreasing the cell and stack performance. Theseissues may become particularly problematic with flow fieldshaving certain geometric shapes.

3.2. Straight flow field

Pollegri and Spaziante[18] showed a straight flow-fielddesign, which is further exemplified by General Electric andHamilton Standard LANL No. 9-X53-D6272-1 (1980). Inthis design, the gas flow-field plate includes a number ofseparate parallel flow channels connected to the gas inlet andexhaust headers, which are parallel to the edges of the plate.An example is shown inFig. 4with the flow channel cross-sectional shape. When air is used as the oxidant, it is foundthat low and unstable cell voltages occur after extended

Fig. 5. Variation of configuration in straight or parallel flow fielddesign[19].

periods of operation, because of cathode gas flow distribu-tion and cell water management. As the fuel cell operatedcontinuously, the water formed at the cathode accumulatesin the flow channels adjacent to the cathode, the channelsbecome wet, and the water thus tends to cling to the bot-tom and the sides of the channels. The water droplets alsotend to coalesce and form larger droplets. A force, which in-creases with the size and number of the droplets, is requiredto move the droplets through the channel and out of thecell. Since the number and size of the water droplets in theparallel channels are likely different, the reactant gas thenflows preferentially through the least obstructed channels.Water thus tends to collect in the channels in which little orno gas is passing. Accordingly, stagnant areas tend to format various areas throughout the plate. Hence, the poor cellperformance arises from the inadequate water drainage andpoor gas flow distribution on the cathode side. This problemis similar to the one that occurs in the pin-type flow field, asdiscussed earlier. Few more examples of straight or parallelflow-field design are shown inFigs. 5–7.Another problem associated with this design is that the

straight and parallel channels in the BPPs tend to be rel-atively short and have no directional changes. As a con-sequence, the reactant gas has a very small pressure dropalong these channels, and the pressure drop in the stack dis-tribution manifold and piping system, which is normal tothe BPPs, tends to be large in comparison. This inadequatepressure loss distribution results in non-uniform flow distri-bution of reactant gases among various active cells in thestack, usually the first few cells near the manifold inlet havemore flow than those towards the end portion of the inlet


Fig. 6. Straight or parallel flow field[19].

Fig. 7. Example of straight or parallel flow field design[19].

manifold. One possible solution is to artificially place somerestrictions at the inlet and the exit of these parallel flowchannels to increase the pressure drop in the channels andhence improve the flow distribution among the active cells.However, this complicates the design and fabrication, thusthe cost. A further problem with this design is the possi-ble and often-arisen non-uniform distribution of a compres-sive load carried across the fuel cells within the stack whenthe flow channels on the anode and the cathode plates are

aligned in parallel to allow for the concurrent and counter-current flow arrangements for the fuel and oxidant stream.The contact area, as defined by the overlap of the ribs on theanode and the cathode plates, depends on the manufacturingtolerances affecting the width of the ribs, the smoothness ofthe rib surface, the exact location of the ribs, rib-edge ma-chining and assembly alignments (plate to plate), etc. Thevariation in the contact areas of the ribs results in variationin the local stress and the associated cell strain. A minimumlocal stress is necessary to maintain minimum electrical (aswell as thermal) contact resistance, whereas a significantlyhigh local stress may lead to the damage and prematurefailure of cell components. To ensure uniform compressionload across the cell, it is necessary to have even distributionof both parallel and perpendicular contact areas (i.e., crossflow and co-flow arrangements).

3.2.1. Modified versionsJohnson et al.[20] proposed a design with pressure gra-

dients within the channels, such that the resistance to reac-tant flow differs along the length of the adjacent channelsas shown inFigs. 8and9. Prior to flow field designs withlinear, parallel, uniform channels, there may be no signifi-cant local pressure differential between adjacent channels.However, by appropriately varying flow resistance in thechannels, pressure differentials may desirably be increasedin such flow fields. This improved version of straight or par-allel flow channels performs better than the one mentionedbefore, even though the pressure drop may not be as muchas that achieved by serpentine layout.

The flow-field layout by Yang et al.[21] introduced re-strictions between the inlet of the manifold and inlet of thestraight channels which in turn contribute towards pressuredifferential without using long, tortuous channels as shownin Fig. 10. Essentially the flow restriction is accomplishedthrough the use of fuel inlet tubes, which pass through theframe member and connect the fuel inlet manifold to thefuel channel inlets and similarly on the oxidant side. Thecross-sectional or flow areas of inlet tubes are such that theycreate a flow restriction and, therefore, a pressure differen-tial required to drive reaction products out of the reactivearea.

3.3. Serpentine flow field

In an attempt to tackle the problems with straight chan-nels, Spurrier et al.[22] and Granata and Woodle[23] de-scribed a modified serpentine gas flow field across the platesurface, as shown schematically inFig. 11. The channelsare generally linear and arranged parallel to one another, butskewed to the edge of the plate, while the spaced slots allowcross-channel flow of the reactant gas in a staggered man-ner, which creates a multiple of mini-serpentine flow pathstransverse to the longitudinal gas flow along the channels.Thus, adjacent pairs of the channels are interconnected bythe spaced slots. The flow channels on the anode and cathode


Fig. 8. Modified straight flow channels design[20].

Fig. 9. Straight flow channel design for pressure variation alongthe length[20].

Fig. 10. Straight channels flow field with restrictions[21].

Fig. 11. Serpentine flow field by Spurrier et al.[22].

plates are skewed in opposite directions in such a mannerthat exact co-flow arrangement is avoided, and some crossflow and some nearly co-flow configuration are achieved.So, it is claimed that this design can improve reactant flowdistribution across the electrode surface of the fuel cells,and produce a uniform distribution of stack compressionloading on each fuel cell within the stack. In reality, thisdesign may incur high reactant pressure loss with potential


Fig. 12. Single serpentine flow channel design[24].

formation of stagnant areas due to the cross channel flowby the spaced slots as shown inFig. 11b.

To resolve the problem of water flooding resulting fromthe inadequate water removal from the cells, Watkins et al.[14] proposed using a continuous fluid-flow channel thathad an inlet at one end and an outlet at the other, and typ-ically followed a serpentine path. A schematic diagram isshown inFig. 12. Such a single serpentine flow field forcesthe reactant flow to traverse the entire active area of thecorresponding electrode thereby eliminating areas of stag-nant flow. However, this channel layout results in a rela-tively long reactant flow path, hence a substantial pressuredrop and significant concentration gradients from the flowinlet to outlet. In addition, the use of a single channel tocollect all the liquid water produced from the electrode re-action may promote flooding of the single serpentine, es-pecially at high current densities. Hence, for higher currentdensity operation, especially when air is used as the oxidantor with very large gas flow field plates, Watkins et al.[13]pointed out that several continuous separate flow channelsmight be used in order to limit the pressure drop and thusminimize the parasitic power required to pressurize the air,which can be as much as over 30% of the stack power out-put. This design, shown schematically inFig. 13, ensuresadequate water removal by the gas flow through the chan-nel, and no stagnant area formation at the cathode surfacedue to water accumulation. Watkins et al.[13] reported thatunder the same experimental conditions, the output powerfrom the cell could be increased by almost 50% with thisnew type of flow-field plates. Although multiple serpentineflow-field designs of this type reduce the reactant pressuredrop relative to single serpentine designs, the reactant pres-sure drop through each of the serpentines remains relativelyhigh due to the relatively long flow path of each serpen-tine channel, thus the reactant concentration changes signif-icantly from the flow inlet region to the exit region for eachactive cell.

Fig. 13. Multiple serpentine flow channels[25].

Although reactant pressure losses through the flow dis-tribution fields increase the parasitic load and the degree ofdifficulty for hydrogen recirculation, they are actually help-ful for the removal of product water in vapour form. Assum-ing ideal gas behaviour, the total reactant gas pressurePT= Pvap + Pgas, wherePvap andPgas are the partial pres-sure of the water vapour and reactant gas in the reactant gasstream, respectively. Then the molar flow rate of the watervapour and the reactant (either hydrogen or oxygen) is re-lated as follows:

Nvap

Ngas= Pvap

Pgas= Pvap

PT − Pvap. (1)

Hence, the total pressure loss along a flow channel willincrease the amount of water vapour that can be carried andtaken away by a given amount of the reactant gas flow ifthe relative humidity is maintained. This approach can beused to enhance water removal by both oxidant and fuelstreams. In fact, a sufficient pressure loss in the anode flowchannels can even draw water through the membrane fromthe cathode side, and remove the excess water by the anodestream, so that the fuel cell performance at high currentoperations can be improved significantly, as demonstratedby Voss and Chow[19].

Eq. (1) also indicates that an increase in the water vapourpartial pressure can enhance the ability of the reactant gasstream to remove water, and the water vapour pressure islimited by the saturation pressure determined by the gasstream temperature. Hence, liquid water can flood the ser-pentine channels and the electrodes after the cathode gasstream has been saturated. However, if the reactant gas tem-perature is increased along the flow direction from the inletto the outlet of the fuel cell, the capacity of the gas streamto absorb water also increases. Fletcher et al.[26] describeda stack arrangement where the coolant flow is substantiallyparallel to the reactant flow, such that the coolest regionof each cooling layer coincides with the inlet region of theadjacent reactant layer where the gas stream has the low-est temperature and water content, and the warmest region


Fig. 14. Flow-field design by Cavalca et al.,[27].

of each cooling layer coincides with the outlet region ofthe adjacent reactant layer where the reactant gas streamhas the highest temperature and water content. Thus, thetemperature increase along the cooling path is also usedto increase the cathode stream temperature, enhancing thecathode stream’s capability of absorbing and removing thereaction product water in the vapour form. However, thiscreates an undesirable non-uniform temperature distributionthroughout the cell. Due to the small pressure drop betweenthe inlet and outlet ports, there are no water droplet block-ages in the bipolar plates with the serpentine flow geometry,as used in Ballard PEM Fuel Cells.Another design by Cavalca et al.[27] exhibits distri-

bution of reactants more uniformly with higher averagereactant concentrations and also pressure drop is low andprevents formation of stagnant flow areas. This type isshown in Fig. 14. Here, flow field is divided in severalsections with separate inlet and outlet. Each flow sector hasparallel flow channels, which are further sub-divided intofew sets of channels connected in series. This design givescombined advantages from pin, straight and serpentinedesign.

3.3.1. Modified versionsSerpentine channels are designed to allow some limited

gas movement between adjacent legs of the same chan-

Fig. 15. Serially linked serpentine flow field[28].

nel via the diffusion layer so as to expose the MEA con-fronting the land of consecutive legs of channel. In this re-gard, gas can flow from an upstream (high pressure) legof the channel to a downstream leg of the same channel(low pressure) through the diffusion layer under the land.However, in long channel legs excessive pressure drop canoccur between adjacent legs or between the ends of thelegs. Such excessive pressure drop can in turn result in thegaseous reactant, short circuiting excessively between adja-cent legs, rather than flowing through the full length of thechannel.

The layout design suggested by Rock[28] takes care ofthe above problem by subdividing the channels into a plu-rality of serially arranged segments or stages as shown inFig. 15. Each of these segments has its own serpentine con-figuration whose legs are relatively short; as a result, verylittle pressure drop exists between adjacent legs and betweenthe ends of the legs. Another noticeable point here is thatthe end of the medial leg closest to the inlet leg of the onesegment (high pressure) is spaced farther from the bridgingsection than the end of the medial leg closest to the exit legof the same segment to reduce gas bypass into the bridgingsection from the one segment.


Fig. 16. Reactant gas flow field and cooling fluid flow field builton the same plate surface[29,30].

3.4. Integrated flow field

Chow et al.[29,30] released a BPP design, which pos-sesses both reactant gas flow field and cooling flow field onthe same plate surfaces, as shown inFig. 16. The gas flowfield directly faces the electrochemically active area of theadjacent MEA, while the cooling flow field surrounds thegas flow field. This integrated reactant and coolant flow-fieldplate design eliminates the need for a separate cooling layerin a stack, thus significantly improves the stack power den-sity [29,30]. In the same spirit, Ernst and Mittleman[31]described a fluid flow-field plate assembly, which is dividedinto a multiple of fluid flow sub-plates, as illustrated inFig.17. Each sub-plate is electrically insulated from all othersub-plates of the same plate assembly, and has its own reac-tant flow field adjacent to the electrochemically active areaof the nearby MEA. A cooling flow field may be positionedin-between and around each of the gas flow sub-plates[31].However, these designs cannot maintain a uniform temper-ature distribution over the entire fuel cell surface.

3.5. Interdigitated flow field

For all the above designs of flow fields, the flow channelsare fabricated on the flow distribution plates (or BPPs), orto the lesser degree, on the porous electrode backing layers,and they provide continuous flow passages, from the stackinlet manifold to the exit manifold, while traversing throughthe electrode surface of the active areas of the cell. In thisconfiguration, as schematically shown inFig. 18, the domi-nant reactant flow is in the direction parallel to the electrodesurface, and the reactant flow to the catalyst layer, required

Fig. 17. Example of integrated flow-field design[31].

Fig. 18. Conventional flow-field design mechanism[32].

for electrochemical reaction and electric power generation,is predominantly by molecular diffusion through the elec-trode backing layer. Not only molecular diffusion is a slowprocess, easily leading to the occurrence of large concen-tration gradients across the backing layer and mass transferlimitation phenomenon for the cell operation, but it is also


Fig. 19. Interdigitated flow-field design[32].

difficult to remove liquid water which exists in the porousregion of the backing layer. This difficulty is compoundedby the fact that typical flow in the flow channels is laminardue to the small gas velocity and the small flow channeldimensions. Therefore, interdigitated flow fields have beenexplored to provide convection velocity normal to the elec-trode surface for better mass transfer, and convection flow inthe porous backing layer for enhanced water removal capa-bility [33]. An interdigitated flow field, as shown inFig. 19,consists of dead-ended flow channels built on the flow dis-tribution plates. The flow channels are not continuous fromthe stack inlet manifold to the exit manifold, as the reactantflow is forced under pressure to go through the porous elec-trode backing layer to reach the flow channels connected tothe stack exit manifold, thus developing the convection ve-locity towards the catalyst layer and convection flow in thebacking layer itself. Such flow-field design can remove wa-ter effectively from the electrode structure, preventing waterflooding phenomenon and providing enhanced performanceat high current density operation. However, a large pressureloss occurs for the reactant gas flow, especially the oxidantair stream. The parasitic power required for air compressionmay limit the application of this flow-field design to smallerstack sizes.

The interdigitated flow field is attractive since the reactantgases are forced to flow into the active layer of the elec-trodes, where the forced convection (instead of diffusion)avoids flooding and gas diffusion limitations, thereby ex-

Fig. 20. Improved mass transfer channels (Variable channel crosssection and/or Interdigitated concept)[35].

tending the linear region of the cell potential versus currentdensity plot. When a fuel cell accumulates too much waterat high current density, about one-third of the electrode sur-face area is not utilized[34]. To overcome mass transportlimitations in porous electrodes, the diffusive mass transfermechanism is changed into a forced convective mass transferwhich causes limiting current density and maximum powerdensity to increase significantly. This design outperformsconventional flow-field design, especially on cathode side athigh current densities.

3.5.1. Modified versionsAs gases flow in the channels, the reactants are trans-

ferred into the gas diffusion layer, and thus the concentra-tion of reactant in the flow channels is reduced along theflow direction. This reduction in concentration can result innon-uniform reaction across the fuel cell active area. Gurauet al.[35] proposed a design for improved mass transfer us-ing interdigitated flow-field design concept (Fig. 20). In this


Fig. 21. Gas block mechanism for water removal in interdigitatedflow field [36].

Fig. 22. Biomimetic flow-field concept for bipolar plates[37].

layout the outlet channel volume is less than the inlet chan-nel volume, whereby the rate at which the fluids flow to theoutlet channel is increased causing improved fluid removal.Different approaches have been mentioned to achieve sucha phenomenon, such as outlet channel depth can be less thanthat of the inlet channel or may vary the width, etc. Thechannel width may also be varied so that the land width re-duce along the channel from the inlet to the dead end, e.g.the inlet channel can increase in width towards its terminusand outlet channel can increase in width away from its ter-minus. This will result in a closer positioning of the inlet andthe outlet channel in the direction of flow of the fluids. This

Fig. 23. Configurations for flow fields made by metal sheets[38] .

Fig. 24. Bipolar plate flow-field design from metal sheets[38,39].

reduced land width permits greater mass flow rates as theconcentration of the reactants is decreased. This advantagecan be coupled with improved water removal by decreasingthe outlet channel volume as well as by reducing the depthof it relative to inlet channel or reducing the outlet channellength relative to the inlet channel.

For improved water removal, Issacci and Rehg[36] con-sidered the gas block mechanism for cathode and anode


Fig. 25. Stamped flow-field design on sheet metal[40].

sides in a fuel cell. In this design, they used one or moreporous gas block media at points adjacent to the flow field,as illustrated inFig. 21, that have pore size such that wateris sipped off to the outside of the flow field by capillary flowand the cathode gas is blocked from flowing through themedium. On the other surface of the plate is channel in fluidcommunication with each porous gas block medium[36].

The patented ‘Biomimetic’ BPP technology developed byMFC (Morgan fuel cell) drew its inspiration from the naturalworld [37]. It mimics the structure as seen in animal lungsand plant tissues to allow the gases to flow through the platein a far more efficient way than has never been achievedbefore. The Biomimetic plates also have the added advan-tage of being produced using MFC’s patented ElectroEtchsystem, which allows them to be manufactured at a fractionof the time and cost of conventional methods. Looking athow animal lungs and plant leaves ‘breathe’, a structure con-sisting of large distribution channels feeding progressivelyto smaller capillaries is the most efficient way to distributereactants, as shown inFig. 22. This structure reduces thepressure drop found in the industry-standard serpentine de-sign of flow field and ensures a more even delivery of gasacross the BPP, so that more power can be extracted fromthe fuel cell. Initial results are very promising, with testsalready confirming a 16% increase in peak power[37].

3.6. Flow channels from metal sheets

Flow through porous carbon has also been proposed forimproved water management; a better method may be theuse of flow through porous metallic meshes (with high re-

sistance to corrosion) to improve gas distribution on the cellplane. Proposals have been made to fabricate BPPs frommetals such as titanium, chromium, stainless steel, niobium,etc. [38–41]. Figs. 23and24 illustrate possible configura-tions for the reactant and cooling flow fields. The plates com-prise corrosion-resistant thin metal sheets brazed together toprovide a cooling flow field between the sheets and reactantgas flow fields on the two outside surfaces of the sheets.Such a BPP design eliminates the need for a separate cool-ing plate, decreases material usage for stack constructionand reduces the weight and volume of the stack.

Rock [40] proposed a stamped BPP for PEM fuel cellsfrom a single metallic sheet. The plate has a serpentineflow field formed on one side and an interdigitated flowfield formed on the opposite side such that a single platemember is usable as an anode and cathode side flow fieldsfor adjacent fuel cells (Fig.25).

4. Conclusions

Bipolar plate is one of the key components in PEM fuelcell stacks, and it performs a number of essential func-tions in stack operation, such as reactants supply to the cellactive area, current collection, mechanical support to theMEA, water management, heat management and maintain-ing the reactants separate. In practice, PEM fuel cell stackdesign often boils down to bipolar plate design, which inturn is basically the design of flow channels formed on thetwo surfaces of the bipolar plates, because the requirementson carrying out the bipolar plate functions optimally are


often met by the appropriate design of flow channels. A va-riety of flow channel configurations and layouts have beenproposed in different designs, including pins, straight chan-nels, serpentine channels, integrated channels, interdigitatedchannels and channels formed from sheet metals. These dif-ferent flow field designs have pros and cons associated withthem which in turn make them suitable for different appli-cations. Improvements in the design of bipolar plates canhelp achieve the set goals of cost and performance for thecommercialization of PEM fuel cell.

Acknowledgements

The financial support of the Centre for Automotive Mate-rials and Manufacturing (CAMM) and the NSERC is greatlyacknowledged.

References

[1] Costamagna P, Srinivasan S. Quantum jumps in the PEMFCscience and technology from 1960s to the year 2000, Part II.Engineering, technology development and application aspect.J Power Sources 2001;102:253–69.

[2] Gamburzev S, Appleby AJ. Recent progress in performanceimprovement of PEM fuel cell. J Power Sources 2002;107:5–12.

[3] Arita M. Technical issues of fuel cell systems for automotiveapplications. Yokosuka, Japan: Nissan Motor Co. Ltd.; 2002.

[4] Chalk Steven G, Miller James F, Wagner Fred W. Challengesfor fuel cells in transport applications. J Power Sources2000;86:40–51.

[5] Barbir F, Gomez T. Efficiency and economics of PEM fuelcell. Int J Hydrogen Energ 1997;22(10/11):1027–37.

[6] Cacciola G, Antonucci V, Freni S. Technology update and newstrategies on fuel cells. J Power Sources 2001;100:67–79.

[7] Woodman AS, Anderson EB, Jayne KD, Kimble MC,Development of corrosion-resistant coatings for fuel cellbipolar plates. AESF SUR/FIN ’99 Proceedings, vol. 6,p. 21–4.

[8] http://www.eere.doe.gov/vehiclesandfuels.html(USA Depart-ment of Energy: Office of transportation technologies, pageretrieved on May, 2003).

[9] http://www.Ballard.com(Last retrieved on March, 2004).[10] Hontanon E. et al. Optimisation of flow field in PEM fuel cell

using CFD techniques. J Power Sources 2000;86:363–8.[11] Kumar A, Reddy Ramana G. Effect of channel dimensions

and shape in the flow field distributor on the performance ofPEM fuel cells. J Power Sources 2003;113:11–8.

[12] Gamburzev S, Boyer C, Appleby AJ. Proceedings of the fuelcell seminar, 1998. p. 556–9.

[13] Watkins DS, Dircks KW, Epp DG. US Patent No. 5,108,849,1992.

[14] Watkins DS, Dircks KW, Epp DG. US Patent No. 4,988,583,1991.

[15] Li X. PEM Fuel Cells. Lecture notes, University of Waterloo,August 20, 2002.

[16] Reiser CA, Sawyer RD. Solid polymer electrolyte fuel cellstack water management system. US Patent No. 4,769,297,1988.

[17] Reiser CA. Water and heat management in solid polymer fuelcell stack. US Patent No. 4,826,742, 1989.

[18] Pollegri A, Spaziante PM. US Patent No. 4,197,178, 1980.[19] Voss HH, Chow CY. Coolant flow field plate for

electrochemical fuel cells. US Patent No. 5,230,966, 1993.[20] Johnson MC, Wilkinson DP, Kenna J, Vanderleeden OR,

Zimmerman J, Tabatabaian M. US Patent No. 6,586,128,2003.

[21] Yang Jefferson YS, et al., US Patent No. 6,635,378, 2003.[22] Spurrier FR, Pierce BE,Wright MK. US Patent No. 4,631,239,

1986.[23] Granata Jr SJ, Woodle BM. US Patent No. 4,684,582, 1987.[24] Wilkinson DP, Lamont GJ, Voss HH, Schwab C. Embossed

fluid flow field plate for electrochemical fuel cells. US PatentNo. 5,521,018, 1996.

[25] Marvin Russel H, Carlstrom CM. US Patent No. 6,500,580,2002.

[26] Fletcher NJ, Chow CY, Pow EG, Wozniczka HH, Voss GH,Wilkinson DP. Canadian Patent No. 2,192,170, 1996.

[27] Cavalca C, Homeyer ST, Walsworth E. US Patent No.5,686,199, 1997.

[28] Rock Jeffrey Allan. US Patent No. 6,309,773, 2001.[29] Chow CY, Wozniczka B, Chan JKK. Integrated reactant and

coolant fluid flow field layer for a fuel cell with membraneelectrode assembly. Canadian Patent No. 2,274,974,1999.

[30] Chow CY, et al. US Patent No. 5,804,326, 1998.[31] Ernst WD, Mittleman G. US Patent No. 5,945,232, 1999.[32] Kazim A, Liu HT, Forges P. Modelling of performance of

PEM fuel cell with conventional and interdigitated flow fields.J Appl Electrochem 1999;29:1409–16.

[33] Wood DL, Yi JS, Nguyen TV. Effect of direct liquid waterinjection and interdigitated flow field on the performanceof proton exchange membrane fuel cells. Electrochim Acta1998;43(24):3795–809.

[34] Yi JS, Nguyen TV. Electrochem Soc Proc 1995;23:66–75.[35] Gurau V, et al. US Patent No. 6,551,736, 2003.[36] Issacci F, Rehg TJ. US Patent No. 6,686,084, 2004.[37] Chapman A, Mellor I. Development of biomimetic flow field

plates for PEM fuel cells. Eighth grove fuel cell symposium,September 2003, London, UK.

[38] Vitale NG. US Patent No. 5,981,098, 1999.[39] Neutzler JK. US Patent No. 5,776,624, 1998.[40] Rock JA. US Patent No. 6,503,653, 2003.[41] Marianowski LG. US Patent No. 6,261,710, 2001.

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http://www.Ballard.com

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