CHARACTERIZATION OF PEM ELECTROCHEMICAL HYDROGEN COMPRESSORS

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1 CARACTERIZATION OF PEM ELECTROCEMICAL YDROGEN COMPRESSORS S.A. GRIGORIEV *, K.A. DJOUS, P. MILLET b, A.A. KALINNIKOV, V.I. POREMBSKIY, V.N. FATEEV, R. BLAC c ydrogen Energy nd Plsm Technology Institute of Russin Reserch Center Kurchtov Institute, Moscow Russi, Kurchtov sq., 1 b Institut de Chimie Moléculire et des Mtériux, UMR CNRS n 8182, Université Pris sud, 15 rue Georges Clémenceu, Orsy Cedex Frnce c Dvid Systems nd Technology S.L., Vlverde del Mjno, Segovi, Spin * S.Grigoriev@hepti.kie.ru ABSTRACT This reserch work concerns the development nd chrcteriztion of n electrochemicl hydrogencompressor (C) bsed on proton exchnge membrne (PEM) technology. In C device, pure hydrogen (or hydrogen-contining mixtures) is supplied to the node, nd pure (99.7% nd more) pressurized (potentilly severl hundred brs) hydrogen is collected t the cthode. Cs cn be used to extrct hydrogen from different gseous mixtures nd wste gses (for exmple, from outlet gses of fuel cells, products of orgnic fuel conversion). Results reported in this communiction concern the role of vrious operting prmeters (wter vpor prtil pressure, current density, operting temperture) on the mximum output pressure which cn be reched with such Cs. A model, which tkes into ccount the effect of hydrogen bck diffusion due to the pressure grdient, hs been developed. Experimentl compression up to 28 brs hs been mesured. 1. INTRODUCTION In hydrogen compressor (C) device, pure hydrogen (or hydrogen-contining mixtures) is supplied to the node, nd pure (99.7% nd more [1]) hydrogen, possibly pressurized [2, 3], is collected t the cthode. The following electrochemicl rections re tking plce: node rection: ē; cthode rection: ē 2 ; overll cell rection: 2 2. Cs cn potentilly be used to extrct hydrogen from different gseous mixtures nd wste gses (for exmple, from outlet gses of fuel cells, products of orgnic fuel conversion) nd to produce purified nd compressed hydrogen. In such systems, the low-pressure hydrogen input cn be pressurized up to severl hundred brs (using the series connection of severl cells) without dditionl energy inputs. Electrochemicl hydrogen compression hs been studied by different reserch groups. Reserch ctivities on hydrogen electro-trnsport in PEM cells strted t RRC Kurchtov Institute c. 20 yers go [4]. Lter, reserch group from Center for Solr Energy nd ydrogen Reserch in Bden-Wuerttemberg (Ulm, Germny) demonstrted the possibility to compress hydrogen using Nfion 117 membrne up to 43 br [2]. The mximum pressure difference chieved by uthors of pper [3] using Nfion 105, 115 nd 117 membrnes ws c. 54 br. They concluded tht using improved cell design, it could be possible to rech pressures bove 100 brs. According to ptent [5], pressures of up to psi (857 br) cn be reched using multi-cell ssemblies. More recently, n Itlin reserch group hs studied the glvnosttic nd tensiosttic mode of hydrogen pump opertion using Nfion 117 membrne [6] t tmospheric pressure. Other studies in Cnd [7, 8] hve been reported on the hydrogen pump, however without considertion on high-pressure opertion. Concerning the purity of hydrogen obtined using hydrogen pump, it ws shown by Lee et l [1], tht 99.72% hydrogen purity cn be obtined using Nfion 115 membrne nd two-stge seprtion process from 2 /N 2 /CO 2 mixture. According to [9], Mitsubishi Corportion hs developed highpressure prototype electrolyser ble to operte t mximum pressure of 350 br. It thus seems possible to rech similr pressure levels with the C, using the sme membrne nd properly supported nodic electrode. owever, it cn be expected tht such high pressures will not be esy to rech in one single step nd tht multi-cell systems connected in series will be required. The purpose of the work reported here is to 1

estimte the mximum output pressure tht cn be reched, to provide dt on the kinetics of the compression process nd to determine the best operting conditions for single-cell Cs. 2.

2 estimte the mximum output pressure tht cn be reched, to provide dt on the kinetics of the compression process nd to determine the best operting conditions for single-cell Cs. 2. EXPERIMENTAL DETAILS The experimentl PEM-C used in this work is pictured in figure 1. This prototype ws designed nd mnufctured t Kurchtov Institute. Briefly, it consists of n electrochemicl cell clmped between n upper nd lower stinless steel (grde 12Х18Н10Т or SS316) cover vessel, both designed for high pressure opertion. On top of the upper cover vessel, mnometer (visul control) nd pressure sensor (remote control) re positionned. Both cover vessels re equipped with electric heters nd electronic temperture regultors for temperture mngement. The electrochemicl cell consists of two (stinless steel SS316) flnges. The solid polymer electrolyte (SPE) is n ion-exchnge membrne (Nfion 117 from DuPont de Nemous, USA), pressed between two fluoroplstic gskets. An nodic current collector mde of hydrophobic porous crbon-pper, nd cthodic current collector mde of porous titnium, re pressed on both sides of the SPE. Ech current collector is surfce-covered with n pproprite ctlytic lyer mde of pltinum nnoprticles (1 mg.cm -2 ) nd 25 wt. % of ion-exchnge resin. Figure 1. Experimentl electrochemicl C. A schemtic digrm of the utomted test-bench used for the mesurements is pictured in figure 2. A PEM wter electrolyser (E) is used to produce the hydrogen which is then used in the C. Gseous oxygen nd hydrogen produced by E re extrcted from the biphsic wter-gs mixtures in seprtors С1 nd С2. Oxygen is rejected to the tmosphere, nd hydrogen is sent to the C through vlve V3. ydrogen in excess is rejected through vlve V4. Power unit CS2 supplies the electric power to the electrochemicl cell of the C. Before opertion, wter tnks re filled up with deionized (18 MΩ.cm) wter (figure 2). Admission nd exhust chmbers of the C re purged with pure nitrogen stored in reservoir V using the pressure regultor PR. Then hydrogen is injected into the C nd voltge is pplied to the electrochemicl cell. ydrogen is oxidized t the node nd decomposed into protons nd electrons. Protons then migrte through the membrne to the cthode where they re reduced into moleculr hydrogen. As the rection proceeds, the pressure in the upper cover vessel grdully increses. The output pressure of the C is monitored by the PC with the help of pressure sensors, until sttionry vlues re reched. 2

3 Figure 2. Experimentl test-bench. E electrolyser; C - hydrogen compressor; C1 - oxygen seprtor; C2 - hydrogen seprtor; CS1 - electrolyser current supply; CS2 - hydrogen compressor current supply; V1 V5 vlves; М1, М2 mnometers; V - vessel with inert gs (nitrogen); PR - pressure reducer; PS - pressure sensor; А mperemeter; V voltmeter. 3. MODELING In first step, the behvior of the C ws modeled to estimte the mximum output pressure nd quntify the role of gs permebility. It turns out tht gs permebility of SPE cn produce significnt limittions in terms of frdic efficiency nd output pressure, minly becuse of the bck-diffusion of moleculr hydrogen from the cthodic comprtment (high pressure chmber) to the nodic comprtment (low pressure chmber). From physicl viewpoint, prsite trnsport of moleculr hydrogen bck to the node through the membrne is cused by two bsic processes: - diffusion of moleculr hydrogen cross the SPE (from cthode to node); - convective trnsfer of hydrogen dissolved in wter linked to protons (electroosmotic flow). In the C, both flows hve opposite directions. Becuse of the symmetry of the cell, modeling is onedimension problem which cn be expressed s: dc Q = + υcг C = const 0 < y < dy (1) y = 0 C = Cg ) y = C = Cgc c ) In trnsfer eqution (1), the different terms hve the following mening : y is the one-dimension spce coordinte nd h m the membrne thickness; C gc nd C g re gs phse concentrtions of hydrogen in cthode nd node comprtment; τ с, τ denote the rtio of sturted wter vpor to full pressure for cthode nd node chmbers respectively; υ c is the convective wter velocity due to the electro-osmotic flow; Г is the enry fctor (proportionlity fctor between concentrtion of hydrogen in liquid nd gs phses (here, Г = mol/mol); D is the hydrogen diffusion coefficient in the SPE t full humidifiction D = exp m / s (n expression pproximting dt provided in Ref. [10]). T Convective velocity is obtined from Eq. (2) : 3

4 I 0 n p υ c = (2) FCl Eqution (1) contins two unknown: the hydrogen concentrtion C nd the full hydrogen flow through the membrne Q. Eqution (1) is first order nd hs two boundry conditions. Using (2), we obtin : C Q I 0n I 0n p Г C ( ) ( ) gc 1 τ c exp y 1 + Cg 1 τ exp D = I 0n exp 1 I 0n Cgc c ) Cg ) exp I 0n = FCl I 0n exp 1 I 0n p Г h m exp D At lrge operting pressure, τ c (cthodic comprtment) is close to zero, nd Q in eqution (3) simplifies into: Cgc Cg ) exp I 0n p Г = Q (4) FCl p Г exp 1 At smll current densities, further simplified expression of Q is: Q [ Cgc Cg ( 1 τ )], (5) From (5), the flow vlue does not depend on the current density. At high current densities, Q cn be pproximtely expressed s follows: I0n Q = [ Cgc Cg )], (6) FC l In tht cse, the vlue of hydrogen flow does not depend on diffusion but only on convective velocity. The totl flow of hydrogen Q Σ through the membrne is obtined by summing both (diffusion nd electroosmosis) contributions, yielding : Cgc Cg ) exp I 2n 0 QΣ = 1 (7) 2F Cl exp 1 In first pproximtion, ssuming tht hydrogen behves like n idel gs, its density cn be obtined the low of idel gses : P = CgRT Pc = CgcRT (8) From eqution (7), it ppers tht for certin vlue of pressure P c, the full hydrogen flow through the membrne is zero. The mximl pressure vlue is obtined for Q Σ =0 nd this cn be written s : C I l 0n I0n ( ) P = + c mx RT exp 1 P Ps exp (9) 2n Mximum output pressures of the C, clculted by tking into considertion hydrogen bck-diffusion, re plotted in figures 3 nd 4, s function of current density nd membrne thickness, for two different operting tempertures (50 nd 90 C). From these results, it ppers tht very high pressures cn be reched with single PEM-C cells, even t reltively low current densities. y (3) 4

5 Figure 3. Clculted mximum output pressure s function of current density nd membrne thickness. T = 50 C nd input pressure = 1 br. Figure 4. Clculted mximum output pressure s function of current density nd membrne thickness. T = 90 C nd input pressure = 1 br. 3. EXPERIMENTAL RESULTS AND DISCUSSION Experimentl results reported in this communiction were obtined to vlidte predictions from the model. As cn be seen from figures 3, 4 nd 5, higher output pressures re obtined t lower tempertures. This is explined by the fct tht membrne gs permebility decreses with temperture. owever, permetion kinetics increses with temperture nd this is positive to reduce the cost of electric power ssocited with 2 compression. The role of vrious experimentl prmeters (minly wter vpor prtil pressure nd current density) ws evluted nd n optimum temperture vlue of 70 C ws determined for both the cell nd the node gs flow humidifiction unit. At tmospheric pressure, cell voltges of 0.16 nd 0.39 V ws mesured for current densities of respectively 0.5 nd 1.3 A/cm 2 (figure 5). No kinetic limittion ws observed up to 1.3 A/cm 2. Figure 5. Experimentl currentvoltge reltionship of C with optimized MEA t T cell = T hum = 60 C (curve 1) nd 69 C (curve 2). Active re = 7 cm 2. Input 2 pressure = 1 br. Output 2 pressure = 1 br. Similr experiments hve been performed up to 28 br (figure 6) with no significnt kinetic differences. owever, it ws found difficult to mintin sttionry performnces over severl hours of opertion, minly due to wter mngement issues. 5

6 Figure 6. Sttionry cell voltges mesured vs. output pressure t constnt current density of 0.18 A/cm 2. Active re = cm CONCLUSIONS The mximum output pressure of PEM hydrogen electrochemicl compressor hs been clculted s function of membrne thickness, current density nd operting temperture. Tking into ccount hydrogen bck-diffusion due to the pressure grdient, numericl nlysis hs shown tht for conventionl PEM cell (membrne thickness of 180 microns) mximum output pressure of 400 br (t 90 C nd 0.5 A/cm 2 ) cn be reched. It is shown tht the most significnt experimentl prmeters to consider for designing nd operting hydrogen compressor re current density, operting temperture nd membrne thickness. Experiments hve shown tht there is no significnt kinetic limittion for current densities up to 1.3 A/cm 2 nd pressure up to 28 br. This experimentl work is currently pursued to rech operting pressures up to 150 brs. ACKNOWLEDGEMENT This work hs been performed within the frmework of the ICTS contrct no nd inititive young scientists project no 115 (2008) of RRC Kurchtov Institute. REFERENCES 1. Lee.K., Choi.Y., Choi K.., Prk J.., Lee T.., 2004, ydrogen seprtion using electrochemicl method, Journl of Power Sources, vol. 132: pp Rohlnd B., Eberle K., Stobel R., Scholt J., Grche J., 1998, Electrochemicl hydrogen compressor, Electrochimic Act, vol. 43: pp Ströbel R., Oszcipok M., Fsil M., Rohlnd B., Jörissen L., Grche J., 2002, The compression of hydrogen in n electrochemicl cell bsed on PE fuel cell design, Journl of Power Sources, vol. 105: pp Morozov A.V., Porembsskii V.I., Rozenkevich M.B., Fteev V.N., 1990, ydrogen electrotrnsport in the solid polymer electrolyte cell, Russin Journl of Physicl Chemistry A, vol. 64, n 11: pp Electrochemicl hydrogen compressor, Ptent number: WO 03/075379; Applicnt: Ntionl Reserch Council of Cnd, Cnd; Inventors: Wong T.Y.., Girrd F. nd Vnderhoek T.P.K.; Publiction dte: 12 September Csti C., Longhi P., Znderighi L., Binchi F., 2008, Some fundmentl spects in electrochemicl hydrogen purifiction/compression, Journl of Power Sources, vol. 180: pp Grdner C.L., Ternn M., 2007, Electrochemicl seprtion of hydrogen from reformte using PEM fuel cell technology, Journl of Power Sources, vol. 171: pp Ibeh B., Grdner C., Ternn M., 2007, Seprtion of hydrogen from hydrogen/methne mixture using PEM fuel cell, Interntionl Journl of ydrogen Energy, vol. 32: pp Mitsubishi genertes 350 br hydrogen without compressor, 2004, Fuel Cells Bulletin, n 6: p Brok K., Ekdunge P., 1997, Oxygen nd hydrogen permetion properties nd wter uptke of Nfion 117 membrne nd recst film for PEM fuel cell, Journl of Applied Electrochemistry, vol. 27: pp

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