Biswas, Indro

Friedrich, K. Andreas

Garcia Sanchez, Daniel

Ruiu, Tiziana

Sanchez, Juan

Vera, Marcos

Institute of Transport Research:Publications

MODELING THE EFFECT OF THE INLET GASHUMIDIFICATION ON PEMFC BEHAVIOR AND CURRENTDENSITY DISTRIBUTIONJuan Sánchez-Monreal1, Marcos Vera1, Daniel. G. Sánchez2,Tiziana Ruiu2, Indro Biswas2 & K. Andreas Friedrich21Dept. Ingeniería Térmica y de Fluidos, Universidad Carlos III de Madrid, Spain2 DLR Institut für Technische Thermodynamik, Stuttgart, GermanyABSTRACTAn in-house single cell fixture developed at DLR has been tested experimentally under different temperatures and anode/cathode humidification conditions. The cell was equippedwith a segmented board [1-3] to perform current density distribution measurements. Maps of fuel cell responses for cell temperatures of 80oC and 60oC and various anode/cathodehumidifications have been obtained. The experimental results are compared with a global water balance model that takes into account the water mass flow rates carried by the streamsentering/exiting the cell, as well as the water produced at the cathode by the Oxygen Reduction Reaction (ORR).EXPERIMENTAL SETUPThe cell was operated in galvanostatic mode (100A) with a fixed stoichiometry of the inletgases (1.2A/2C). Reference humidity conditions were fixed at RH 50 % on both sides.Experimental ConditionsTemperature 60oC / 80oCHydrogen / Air flow 840 / 3320 ml min−1, 1.5 barCurrent 100 ASurface 142 cm2Membrane Nafion c© XLCatalist Layer 0.3 mgPt cm2 (Ion Power Inc)Gas Diffusion Layer Sigracet 25 BC (SGL Group)SEGMENTED BOARDA segmented printed circuit board (PCB)was used as anode bipolar plate. Theprinted board features 25 integratedtemperature sensors [2, 3]. The plate isfurther divided into a matrix of 10× 9segments that provides local currentdensity measurements. Using this data,deactivated zones of the membrane canbe easily identified. For the flow channelgeometry considered here, each segmenthas a unique channel direction.This configuration also provides good information along the channel direction.EXPERIMENTSSeveral experiments were performed at different humidifications conditions. They are identi-fied with square symbols in the left plots. A color legend was used to indicate the behavior ofthe cell: indicates stable behavior, is used for flooding conditions, for slight performancelosses, and  for drastic deactivations.At 80oC drastic cell deactivations were observed at low cathode humidifications [1] due tothe high drying capacity of the cathode gas stream. Conversely, at 60oC no fully deactivatedconditions were found at low cathode humidification; only local deactivation of the cell wasobserved, leading to a slight drop in the cell performance.Completely dry conditions can be operated at 60oC with slight performance losses. At 80oC nostable condition was found below RH 20 % in cathode side.MODEL RESULTS20203030304040Cathode RHAnodeRH0 25 50 75 1000255075100125150175200 60oC-20 -10-10-5-5-555510101020202020505050000Cathode RHAnodeRH0 25 50 75 1000255075100125150175200 80oCA global water balance model can be derived from a control-volume approachBOW = m˙aW,in − m˙aW,out︸ ︷︷ ︸Anode water removal+ m˙cW,in − m˙cW,out︸ ︷︷ ︸Cathode water removal+IMw2F︸ ︷︷ ︸Productionwhich states that the mass of water that accumulates in the cell per unit time is equal to thewater flow rates provided by the anode and cathode feeding streams, minus those leavingthe cell with the outlet gases, plus the water production rate due to the ORR. The watercontent is calculated using the molar fraction of water χW and the mass flow rate of theother speciesm˙W =χW1− χWMW(∑km˙kMk)k ={H2 AnodeN2,O2 CathodeThe model considers the outlet gases as fully saturated, and takes into account the hydro-gen and oxygen consumption rates for the evaluation of the water flow rates leaving thecell. The global balance of water (BOW ) is negative for drying and positive for hydrationconditions. The figure shows BOW calculated in gr/h for 60oC (left) and 80oC (right).SEGMENTED BOARD RESULTSReference condition: 50 % - 50 % @ 60oCThe current densitydistribution is nearlyhomogeneous. Smalldeactivations in theflow field corners.Dry condition: 0 % - 0 % @ 60oCBOW is still positive.Only small lossesare observed. Thewater produced atthe cathode is enoughto auto-humidify thecell.20 %Anode - 50 %Cathode @ 80oCBOW is negative butsmall. No performan-ce losses are observed.The back-diffusion ofwater from cathode toanode plays an im-portant role.50 %Anode - 20 %Cathode @ 80oCBOW is largely nega-tive. Dramatic voltagedrop. The graphshows the currentdensity distributionjust before completedeactivation, startingat the cathode inlet.CONCLUSIONSA global water balance model can explain the qualitative behavior of the cell in most ca-ses. Unbalanced conditions (those with a big difference in water content between sides)shows the worst agreement. Flooding conditions are not well represented either due theabsence of water evaporation/condensation effects. Further work is still needed to ex-plain cell behavior under these conditions.FUTURE WORKThe water transport through the membrane should be incorporated to the model so as toexplain cell behavior in unbalanced conditions, where water transport plays a key role.Several complex models have been developed to describe water management in PEMFCs[4, 6] but none of them consider the channel evolution and therefore the downstream va-riation of the current density. At least a 1D+1D model (across+along the channel) shouldbe considered in future work.ACKNOWLEDGEMENTSThis work was supported by Project ENE2011-24574 of the Spanish Ministerio de Eco-nomía y Competitividad and by the European Union s Seventh Framework Programme(FP7/2007-20013) for Fuel Cell and Hydrogen Joint Technology Initiative under grantsno 303446, and 325239. Authors would also like to thank H. Sander at DLR for helpfulsuggestions and discussions.REFERENCES[1] D.G. Sánchez, et al. , ECS Transactions 64(3) (2014), pp. 603-617.[2] E. Gulzow, et al., J. Power Sources 86 (2000), pp 352-362.[3] M. Schulze, et al. J. Power Sources 173 (2007), pp 19-27.[4] D. G. Sanchez, P. L. Garcia-Ybarra Int. J. Hydrogen Energ 37 (2012) 7279-7288.[5] D. Gerteisen, T. Heilmann, C.J. Ziegler J. Power Sources 187(1) (2009), pp 165-181.[6] R. Alink, D. Gerteisen Int. J. Hydrogen Energ 39 (2014) 8457-8473.

MODELING THE EFFECT OF THE INLETGAS HUMIDIFICATION ON PEMFC BEHAVIOR AND CURRENT DENSITYDISTRIBUTION

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