[1] B. Sreenivasulu, G. Vasu, V. Dharma Rao, and S. V. Naidu, Effect of Back Pressure and Flow Geometry on PEM Fuel Cell Performance - An Experimental Study, Int. J. Appl. Sci. Eng., 2013. 11, p.1-11.

Straight flow field - dezavantaje

An example is shown in Fig. 4 with the flow channel crosssectionalshape. When air is used as the oxidant, it is foundthat low and unstable cell voltages occur after extendedFig. 5. Variation of configuration in straight or parallel flow fielddesign [19].periods of operation, because of cathode gas flow distributionand 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 bottomand the sides of the channels. The water droplets alsotend to coalesce and form larger droplets. A force, which increaseswith 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.Another problem associated with this design is that thestraight and parallel channels in the BPPs tend to be relativelyshort and have no directional changes. As a consequence,the reactant gas has a very small pressure dropalong these channels, and the pressure drop in the stack distributionmanifold and piping system, which is normal tothe BPPs, tends to be large in comparison. This inadequatepressure loss distribution results in non-uniform flow distributionof 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 aligned in parallel to allow for the concurrent and countercurrentflow 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 machiningand 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).

Serpentine flow field - avantajeIn an attempt to tackle the problems with straight channels,Spurrier et al. [22] and Granata and Woodle [23] describeda modified serpentine gas flow field across the platesurface, as shown schematically in Fig. 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 manner,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 formation of stagnant areas due to the cross channel flowby the spaced slots as shown in Fig. 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 typicallyfollowed a serpentine path. A schematic diagram isshown in Fig. 12. Such a single serpentine flow field forcesthe reactant flow to traverse the entire active area of thecorresponding electrode thereby eliminating areas of stagnantflow. However, this channel layout results in a relativelylong 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 reactionmay promote flooding of the single serpentine, especiallyat 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 parasiticpo wer required to pressurize the air,which can be as much as over 30% of the stack power output.This design, shown schematically in Fig. 13, ensuresadequate water removal by the gas flow through the channel,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 pressuredrop through each of the serpentines remains relativelyhigh due to the relatively long flow path of each serpentinechannel, thus the reactant concentration changes significantlyfrom the flow inlet region to the exit region for eachactive cell.Although reactant pressure losses through the flow distributionfields increase the parasitic load and the degree ofdifficulty for hydrogen recirculation, they are actually helpfulfor the removal of product water in vapour form. Assumingideal gas behaviour, the total reactant gas pressure PT= Pvap + Pgas, where Pvap and Pgas are the partial pressureof 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 relatedas follows:NvapNgas= PvapPgas= PvapPT 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 serpentinechannels and the electrodes after the cathode gasstream has been saturated. However, if the reactant gas temperatureis increased along the flow direction from the inletto the outlet of the fuel cell, the capacity of the gas streamto absorb water also increases.[2]

[1] X. Li, I. Sabir, Reviewof bipolar plates inPEMfuel cells: Flow-field designs, International Journal of Hydrogen Energy 30 (2005) 359 371.[2] Peiyun Yia, Linfa Penga,, Lizhong Fengb, Pin Ganb, Xinmin Lai, Performance of a proton exchange membrane fuel cell stack using conductive amorphous carbon-coated 304 stainless steel bipolar plates, Journal of Power Sources 195 (2010) 70617066.

Graphite Bipolar PlatesGraphite is the commonly used material for fuel cell bipolar plates due to itshigh conductivity, corrosion resistance and chemical compatibility. The production ofhigh density graphite plates is a complex process that involves high-temperaturetreatment which can cause defects in the material such as porosity and cracks. Thematerial has then to be treated with certain resins to reduce its porosity, which causes adecrease in its electrical conductivity.The flow channels in the graphite plates are usually made by machining withdifferent configurations, which increases the cost depending on the complexity of thetopology of the channels. Furthermore, another factor in increasing the cost of thegraphite plates is the fact that they are fragile and prone to damage duringmanufacturing and handling. This compels the designer to select a material of largerthickness so that it can withstand machining stresses and tightening torque in the fuelcell. This also reduces the power density of the fuel cell in terms of kW/m3. In the firstdesign attempt for this research, graphite was chosen as the material for bipolar plates,figure (4-3).

Figure 4-3 A machined graphite plate for use as a bipolar plate, the main four holes at the cornersof the flow field are for the inlet and outlet of gases, the large side holes are for the cooling fluid, thesmall side holes are for guide pins.

SS 316 BIPOLAR PLATES

The end plates made of SS 316 were machined with an intricatemodified serpentine flow field design as shown in Fig. 1.The active area of the flow field was 25 cm2.

[7] A. Kumar, R.G. Reddy, in: D. Chandra, R.G. Baustia (Eds.), Fundamentalof Advanced Materials for Energy Conversion, TMS, Warrendale, 2002, pp. 4153.

Fig. 1. Fabricated SS 316 (1618wt% Cr and 1214wt% Ni) end plate with modified serpentine flow fielddesign.[X] TRIPLE SERPENTINE FLOW FIELD

[X] Biswa R. Padhy, Ramana G. Reddy, Performance of DMFC with SS 316 bipolar/end plates, Journal of Power Sources 153 (2006) 125129.

The main problem with corrosion is not only the distortion of the material,but because the dissociated metal will react with the catalyst and may block the activesites in the catalyst, it can also contaminate the membrane and reduce its protonicconductivity.316 stainless steel has been receiving increasing attention as a replacement for nonporous graphite in bipolar plates.Wang and Northwood [25] investigated the influence of oxygen- and hydrogen-containing environments on thecorrosion behavior of such bipolar plates. Intergranular and pitting corrosion in both the oxygen- and hydrogen containing environments were reported with a greater corrosion resistance shown by 316L SS bipolar plates in simulated cathodes. This has been attributed to the cathodic protection ability of the oxygen-containing environments.[25] Y. Wang and D. O. Northwood, Effects of O2 and H2 on the corrosion of SS316L metallic bipolar plate materials insimulated anode and cathode environments of PEM fuel cells, Electrochimica Acta, vol. 52, no. 24, pp. 67936798,2007.Analiza de flow channels in comsol Multiphysics: Pressure lines graphics were plotted for eachprofile studied. For one predetermined channel length, y =3.50mm, six pressure lines were plotted at the GDL interior and one at the channel outlet (0.05mm from the upper surface of the channel), in order to evaluate the pressure gradient there. The pressure gradient behavior of the studied models provides homogeneity of thereactants gas flux data through diffusion layer. A homogeneous pressure gradient means highergas availability for the fuel cell reactions.2.2. Metallic Bipolar Plates2.2.1. Introduction. In recent years, metallic bipolar plateshave been attracting the attention of the research communitybecause of their desirable characteristics, such as highelectrical conductivity, formability and manufacturability,gas impermeability, and superior mechanical properties [46,47]. Metal plates offer higher strength, toughness and shockresistance than graphite plates, and their unique mechanicalproperties allow for fabrication of thinner plates. Althoughmetals offer many advantages, they are, however, moresusceptible to corrosion, which can adversely affect theirperformance and durability [43, 46]. It is important tonote that corrosion can take place both at the anode andthe cathode of an operating PEMFC. At the anode, theprotective metal oxide layer can be reduced as a result of thepresence of a reducing environment, leading to unwantedhydride formation and dissolution of the metal in water.This problem is amplified by the addition of water vapourto the incoming fuel stream. This can potentially increasethe risk of polymer electrolyte membrane contamination andcan adversely impact the activity of the catalyst layer. At thecathode, the existing oxidizing environment can substantiallyincrease the corrosion rate of metallic bipolar plates, leadingto performance losses and even premature failure of thewhole stack.Surface modification techniques have been proposedand corrosion-resistant coatings have been developed tominimize these effects [40, 41]. Metals also are much denserthan graphite or composites, hence a substantial reductionin their thickness is necessary [43]. Stainless steel, aluminumalloys, titanium alloys, nickel alloys, copper alloys, andmetal-based composites have been used to fabricate plates.Research has mainly been focused on iron-based alloys, suchas stainless steels, because of their low cost; however, morerecently, considerable efforts have been expended to usenoble metals, aluminum, and titanium as the material ofchoice for fabricating bipolar plates.

PERFORATED STAINLESS STEEL PLATE GAS FLOW DISTRIBUTOR PEM FUEL CELL

Figure 5-7 PEM fuel cell based on meshed SS316 electrode plates under testing. The LabJack U12DAQ system is shown. Gas supply lines are marked with red ribbon for Hydrogen and greenribbon for air[Y][Y] Mustafa, M.Y., Design and manufacturing of a proton exchange membranefuel cell, in Mechanical Engineering. 2009, Coventry University: Coventry. ,PhD Thesis, p. 240.