Experimental dynamic load cycling and current density measurements of different inlet/outlet configurations of a parallel-serpentine PEMFC

Ene gy 283 (2023) 128455 A ailable online 20 July 2023 0360-5442/© 2023 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by- nc-nd/4.0/). Expe imen al dynamic load cycling and cu en densi y measu emen s o di e en inle /ou le con igu a ions o a pa allel-se pen ine PEMFC Ch is ian Su´ a ez a , b , * , Bal asa Toha ias a , Ma ía Sal a Agui e a , A em Chesalkin c , Felipe Rosa a , Al edo I anzo a , b a The mal Enginee ing G oup, Ene gy Enginee ing Depa men , School o Enginee ing, Uni e si y o Se ille, Spain b AICIA, Andalusian Associa ion o Resea ch & Indus ial Coope a ion, Spain Camino de los Descub imien os s/n, 41092, Se ille, Spain c ENET Cen e – Resea ch Cen e o Ene gy Uni s o U iliza ion o Non T adi ional Ene gy Sou ces, VSB Technical Uni e si y o Os a a, Czech Republic ARTICLE INFO Handling Edi o : A. Olabi Keywo ds: Polyme elec oly e memb ane uel cell Dynamic load cycling Cu en densi y mapping ABSTRACT P o on-exchange memb ane uel cells (PEMFCs) in he anspo sec o equi e speci ic design and du abili y, and s able and eliable pe o mance unde a ying cycling loads. In his s udy, expe imen al dynamic load cycling (DLC) es s and cu en densi y mapping (CDM) measu emen s o local cu en densi ies and empe a u es we e pe o med o di e en inle /ou le con igu a ions o eac an s in a pa allel-se pen ine PEMFC. Resul s we e analyzed in e ms o he pola iza ion and powe cu es and he DLC es s, indica ing ha he In e se Hyd ogen Flow con igu a ion pe o med bes . Howe e , he di e ences wi h espec o he o he inle /ou le con igu a ions (No mal Flow, In e se Ai Flow and In e se Flow) we e no signi ican wi h maximum ela i e ol age and powe densi ies di e ences below 5% in he pola iza ion and powe cu es. Also esul s o he expe imen ed inle /ou le con igu a ions du ing he DLCs we e simila , wi h maximum di e ences in e ms o ene gy du ing he cycle below 10% compa ing he bes con igu a ion (In e se Hyd ogen Flow) wi h he wo s . CDM mea- su emen s showed an in e se bell-shaped dis ibu ion wi h highe cu en densi y alues in he ex e nal pa o he bipola pla e and lowe alues in he cen al pa and a highly homogeneous empe a u e dis ibu ions in all con igu a ions. 1. In oduc ion Cu en ly, he e is a high demand o p opulsion sys ems suppo ed by enewable ene gy sou ces in he anspo sec o , one o hem being PEMFCs. Due o hei ze o-emission p ope ies, high e iciency and low ope a ing empe a u e, uel cells a e a good al e na i e, and hey ha e become a po en ial subs i u e o con en ional combus ion engines [1]. The ope a ing condi ions o PEMFCs in he anspo sec o equi e speci ic design and du abili y, and s able and eliable pe o mance unde changeable and cycling loads. A numbe o s udies ha e ocused on he pe o mance o du abili y du ing PEMFCs ope a ion a di e en loads, wi hou accen on he PEMFC speci ic u ban cycle dynamic load [2–5]. The deg ada ion mechanisms associa ed wi h he s a -up and shu -down o PEMFCs o speci ic au omo i e applica ions equi es u u e design in es iga ions on he way o imp o e PEMFCs pe o mance and s able ope a ion in u ban mode [6,7]. The PMFCs deg ada ion and pe o mance imp o emen mus be i s s udied a memb ane elec ode assembly (MEA) le el be o e any scale-up o s ack and sys em le els, while s a e-o - he-a MEAs a e some imes obse ed o be su e ing om insu icien eliabili y and s abili y unde ce ain condi ions [8–10]. One o he key componen s in PEMFCs a e he bipola pla es (BPs) as hey ep esen a signi ican pa o he olume, weigh and cos [11,12]. The ma e ial and design o he channels in he BPs ha e o ensu e an app op ia e wa e managemen wi hin he cell and imp o e he pe - o mance o he cell a high cu en densi ies [13]. Such ma e ials could include g aphi e, coa ed me allic o composi e pla es [14,15]. Al e na- i ely, BPs imp o emen could be done ia di e en low ield designs such as pa allel, se pen ine, pa allel-se pen ine and as well bioinspi ed designs [16,17]. E ec o BPs ma e ial and low ield design on PEMFCs Abb e ia ions: BP, bipola pla e; CDM, cu en densi y mapping; DLC, dynamic load cycling; FC, uel cell; GDL, gas di usion laye ; IAF, in e se ai low; IF, in e se low; IHF, in e se hyd ogen low; JRC, join esea ch cen e; NEDC, new Eu opean d i ing cycle; NF, no mal low; OCV, open ci cui ol age; PEM, p o on-exchange memb ane; PEMFC, p o on-exchange memb ane uel cell; PID, p opo ional in eg al de i a i e. * Co esponding au ho . The mal Enginee ing G oup, Ene gy Enginee ing Depa men , School o Enginee ing, Uni e si y o Se ille, Spain. E-mail add ess: [email p o ec ed] (C. Su´ a ez). Con en s lis s a ailable a ScienceDi ec Ene gy jou nal homepage: www.else ie .com/loca e/ene gy h ps://doi.o g/10.1016/j.ene gy.2023.128455 Recei ed 26 Oc obe 2022; Recei ed in e ised o m 27 June 2023; Accep ed 15 July 2023 Ene gy 283 (2023) 128455 2 pe o mance ha e been s udied in a signi ican numbe o esea ches, and he s a e o he a in his ield has been summa ized in he e iew wo ks by He mann A. e al. [18], Zhang S. e al. [19], Wang Y. e al. [20], Wang J [21]. and Po s mann S. e al. [22]. Analyzing he local dis ibu ions o bo h cu en densi y and em- pe a u e ields du ing a iable load ope a ion is impo an o a be e unde s anding o PEMFC beha iou , e iciency and possible poin s o u u e imp o emen s in he design o he BP. P e ious wo ks ha e s udied he design o BP channels by measu ing he pola iza ion cu e [23]. A numbe o s udies also analyzed he local phenomena om senso s ha measu e he cu en densi y and empe a u e dis ibu ion wi hin he cell [24–29]. In esea ches [24–27], hese dis ibu ions we e analyzed along he eac an gas low di ec ion and also ac oss la e al di ec ions h ough land and channel a eas, concluding ha local mea- su emen s o cu en densi ies and empe a u es could p o ide c i ical in o ma ion o op imize low ield design and uel cell pe o mance. Di e en local condi ions may exis in PEMFCs ha lead o an inho- mogeneous dis ibu ion o cu en and hea . Cu en densi y dis ibu ion and he FC pe o mance homogenei y demons a ed high sensi i i y o small humidi y changes and d y-ou phenomena in PEMFC s acks [28] and ha inc easing he ca hode s oichiome y can e ec i ely imp o e he uni o mi y o cu en densi y dis ibu ion, and mi iga e he local oxygen s a a ion, especially a high FC loads [29]. A numbe o p e ious s udies on cold s a and dynamic FC loading [10,30–34] show he impo ance o he u u e in es iga ion o PEMFC pe o mance and du abili y imp o emen . Cold s a is a c i ical con- di ion in FCs au omo i e applica ion whe e FCs ope a ion ela ed o he dynamically changeable load. The s udy o Jiao K [30]. in es iga es he cold s a cha ac e is ics o a PEMFC h ough he simul aneous mea- su emen s o cu en and empe a u e dis ibu ions. Fo bo h he ailed and he success ul cold s a p ocesses, he highes empe a u e is ini ially nea he inle egion o he cell, and is also in he middle egion a e he o e all peak cu en densi y is eached. The PEMFC li e ime p edic ion h ough load and po en ial cycling accele a ed deg ada ion was desc ibed by Choi S.R. e al. [31]. Han J. e al. [32] ound ha e en a e y low cu en densi ies (0.52 A/cm 2 ) o he ull dynamic cycling load condi ions, signi ican FC pe o mance deg ada ion has been obse ed, especially by ol age deg ada ion a high empe a u es. Wang Y. e al. [33] in es iga ed ha inc ease in he cu en densi y leads o anode d ying due o elec o- osmo ic esis ance, while i akes se e al seconds o wa e back-di usion and anode humidi ied gas o ewe he anode side o he polyme memb ane. Mayu M. e al. [34] modelled a PEMFC pe o - mance and du abili y o a i ual FC ca . The wo k demons a ed FC du abili y analysis due o memb ane deg ada ion unde highly dy- namic load le els changes o he New Eu opean D i ing Cycle (NEDC) and iden i ied c i ical cell loading egions imposed by he NEDC FC ope a ion. F om he abo e li e a u e e iew, i is clea ha p e ious ela ed s udies ha e ocused on he analysis o local dis ibu ions o cu en densi ies and empe a u es o di e en low channel’s designs and ope a ing condi ions, as well as on he pe o mance and du abili y du ing dynamic load cycling es s. Howe e , as a as we a e conce ned, no p e ious esea ch has in es iga ed he e olu ion o local dis ibu ions o cu en densi ies and empe a u es du ing he PEMFC u ban cycle ope a ion. In he p esen wo k, his no el y is p esen ed, and he expe imen al me hodology ollowed o examine he PEMFC pe o - mance unde ope a ing and s ess condi ions o au omo i e applica- ions desc ibed by he Eu opean Commission in he JRC Science Policy Repo [35]. Cu en densi y and empe a u e dis ibu ion we e expe - imen ally measu ed o ou di e en inle /ou le con igu a ions o a pa allel-se pen ine c oss- low PEMFC, and pe o mance esul s we e analyzed and compa ed. 2. Ma e ials and expe imen al p ocedu e The ma e ials and he expe imen al p ocedu e ollowed in his esea ch a e summa ized in his sec ion. 2.1. Ma e ials The expe imen al wo k was pe o med wi h a PEM uel cell es s a ion, dedica ed o expe imen al es ing o PEM single cells and sho s acks up o 500 W. The es en i onmen shown in Fig. 1 (a) ep esen s a ypical uel cell es bench, wi h a eac an gas handling uni including humidi ie s, a hea ing and cooling sys em con olled by a PID mecha- nism using ilm hea e s and ai ans, and an elec onic load. The cu en densi y mapping senso shown in Fig. 1 (b) was used o measu e he cu en densi y and empe a u e dis ibu ions inside he uel cell. Wi h a ela i ely small hickness o 0.7 mm, he CDM senso was inse ed be ween he bipola ca hode pla e and he ca hode cu en collec o . The senso was manu ac u ed ad hoc o he Elec oChem uel cell, hus ha ing he same ac i e a ea, and p o ided a uni o mly dis ibu ed sequence o measu emen s in an 18 ×18 ma ix o cu en densi y and a 9 ×9 ma ix o empe a u e du ing he expe imen s. The pa allel-se pen ine low ield design o an app oxima ely 50 cm 2 (wid h: 69.8 mm, heigh : 69.9 mm) ac i e a ea Elec oChem Inc. PEM uel cell, wi h 9.5 mm hick g aphi e bipola pla es o bo h anode and ca hode, was expe imen ally in es iga ed in his wo k. The low ield consis ed o i e pa allel se pen ine channels connec ed by mani olds a he end o each pass, see Fig. 2. The eac an s low om he inle loca ed a one co ne o he bipola pla e and a e di ec ed h ough he pa allel se pen ine channels o he ou le loca ed a he opposi e co ne . The channel geome y had a wid h o 0.71 mm, a heigh o 1.1 mm, and a ib wid h o 0.86 mm. The collec o channels we e 1.5 mm wid h and 1.7 mm heigh . Su ace oughness measu emen s using high-p ecision 3D op ical p o ile s based on con ocal and in e e ome y showed a alue o Ra =0.227 μ m o he g aphi e bipola pla es. The anode and ca hode a angemen s and a schema ic diag am o he pa allel-se pen ine channels o he bipola pla es a e shown in Fig. 2. While he anode pa allel-se pen ine channels we e a anged ho izon- ally as depic ed in Fig. 2 (a), he ca hode bipola pla e channels we e a anged e ically as shown in Fig. 2 (b). This e ical a angemen was p e e ed in he ca hode pla e o acili a e wa e emo al by g a i y and o a oid he ho izon al con igu a ion o he ca hode, which is mo e p one o wa e looding. Bipola pla es we e igh ened wi h end pla es o a alue o 5 Nm using a o que w ench, bol s and nu s o p e en leakage and o uni o m cu en collec ion, wi h he anode posi ioned o e he ca hode in he di ec ion indica ed by he a ow in Fig. 2 (a) and (b). A c oss low in which he anode ( ed colo ) and ca hode (blue colo ) low channels a e pe pendicula , as depic ed in Fig. 2 (c). To cla i y he loca ion o e- ac an s inle and ou le and he way anode and ca hode low channels a e posi ioned, a schema ic diag am o he eac an s low di ec ions o he anode, he ca hode and hei in e sec ions a e shown in Fig. 2 (d), Fig. 2 (e) and Fig. 2 ( ) espec i ely. In hese diag ams, only he cen al channel o he i e pa allel channels in each pass o he se pen ine is ep esen ed wi h black colo o be e isualize he eac an s low di ec ions. 2.2. Expe imen al p ocedu e Expe imen al measu emen s we e pe o med using a PEM uel cell es s a ion o ob ain he pola iza ion and powe cu es o each inle / ou le con igu a ion. The powe equipmen included a p og ammable di ec cu en load om Adap i e Powe Sys ems, wi h an accu acies o 0.025%, 0.1%, and 0.125% o ol age, cu en and powe measu e- men s, espec i ely. Fuel cell dynamic load cycle (FC-DLC) es s we e also pe o med, o simula e he ol age and cu en densi y du ing a eal d i ing cycle. Du ing all expe imen s, cu en densi y and empe a u e C. Su´ a ez e al. Ene gy 283 (2023) 128455 3 dis ibu ion da a we e collec ed using he CDM senso p o ided by S++ Simula ion Se ices [36] wi h a da a acquisi ion a e o 1 s and a eso- lu ion o ol age, empe a u e and cu en a iables o 1.5 mV, 0.1 ◦C and 0.001% o he measu ed cu en , espec i ely. In o de o analyze only he e ec o he inle /ou le eac an s con- igu a ions on uel cell pe o mance, uel cell ope a ing condi ions we e kep ixed a ypical ope a ing alues h oughou all es s, as summa- ized in Table 1. Cell empe a u e and ela i e p essu e we e se a 65 ◦C and 0.5 ba espec i ely. In e media e anode and ca hode ela i e hu- midi ies o 60% we e conside ed du ing he expe imen s, o a oid po- en ial p oblems o memb ane dehyd a ion a e y low ela i e humidi ies o wa e looding a e y high ela i e humidi ies. Anode and ca hode s oichiome ic ac o s we e se a 1.3 and 2.5 espec i ely, wi h hyd ogen and ai inle mass low alues o 0.457 NL/min and 2.140 NL/ min o a ep esen a i e ope a ing condi ion o 1 A/cm 2 cu en densi y. Fig. 1. Fuel cell es s a ion (a) and CDM senso (b). Fig. 2. Bipola pla e channels low: (a) Elec oChem anode, (b) Elec oChem ca hode, (c) Resul ing c oss- low dis ibu ion, (d) Schema ic anode low di ec ions, (e) Schema ic ca hode low di ec ions and ( ) Schema ic c oss- low dis ibu ion. C. Su´ a ez e al. Ene gy 283 (2023) 128455 4 2.2.1. Inle /ou le low- ield con igu a ions Fou low con igu a ions we e analyzed by changing he ela i e inle s and ou le s posi ions o he eac an s, as shown in Fig. 3: No mal Flow (NF), In e se Ai Flow (IAF), In e se Hyd ogen Flow (IHF) and In e se Flow (IF). In he NF con igu a ion, he eac an s inle s and ou le s we e connec ed acco ding o he o iginal Elec oChem uel cell design. The o he h ee con igu a ions we e chosen o in es iga e he po en ial o imp o ing uel cell pe o mance by using o he sui able inle /ou le posi ions ha did no in ol e any addi ional ma e ials o ope a ion cos s. In he IAF con igu a ion, he ai inle and ou le we e e e sed while he hyd ogen low was main ained in i s no mal con igu a ion. Simila ly, in he IHF con igu a ion, he hyd ogen inle and ou le we e in e ed while he ai low was main ained in i s no mal con igu a ion. Finally, in he IF con igu a ion, bo h he ai and hyd ogen lows, we e in e ed. Conside ing he geome y o he pa allel se pen ine low ield design and he inle mass low alues o a ep esen a i e ope a ing condi ion o a 1 A/cm 2 cu en densi y, lamina low wi h ela i ely low alues o p essu e d op and eloci ies we e ob ained wi hin he ou con igu a- ions conside ed. An exploded iew d awing wi h he desc ip ion o he p ima y componen s and comple e assembly a e shown in Fig. 4 (a) and (b) espec i ely, whe e i can be obse ed ha he CDM senso was posi- ioned on he ca hode side, be ween he bipola pla e and he cu en collec o . Expe imen al se up o he NF con igu a ion is also shown in Fig. 4 (c), whe e he posi ions o he inle /ou le can be obse ed. Insula ed ai (black colo ) and hyd ogen (g ey colo ) inle connec o s, eed he uel cell in he uppe igh co ne while non- eac an gases a e ex ac ed h ough he ai (blue colo ) and hyd ogen ( ed colo ) ou le pipes. 2.2.2. Pola iza ion and powe cu es Pola iza ion and powe cu es we e ob ained acco ding o he Join Resea ch Cen e (JRC) p o ocol o he di e en p oposed con igu a- ions [35]. Each es s a ed wi h p econdi ioning o he cell by se ing he ope a ing condi ions o he speci ied ope a ing condi ion alues. To each hese condi ions, he cell cu en densi y was g adually inc eased in s eps o 0.1 A/cm 2 un il a ol age o 0.5 V was eached. This cu en densi y was kep cons an 3600 s o s abilize he condi ions. The cell was hen placed unde OCV (Open Ci cui Vol age) condi ions o a pe iod o 90 s. The measu emen o hese cu es was pe o med in gal anos a ic mode, s a ing om he OCV ol age and using ixed cu en densi y s eps as indica ed in he JRC p o ocol [35]. The pola iza ion cu e was di ided in o wo cu es, he i s om he cu en densi y o 0 A/cm 2 (OCV) o maximum cu en densi y, and he second om he maximum cu en densi y o OCV. This es was epea ed h ee imes o analyze he ep oducibili y o he esul s and o ob ain a be e eliabili y o he da a. The da a acquisi ion a e o he es bench is 1 s, so he a e age o he las 30 alues was used o pos -p ocess o he da a. A e p ocessing he da a, he pola iza ion and powe cu es we e plo ed o each eac an s low con igu a ion. Simul aneously wi h he acquisi ion o he pola iza ion Table 1 Fuel cell e e ence ope a ing condi ions. Ope a ing condi ion Value Cell empe a u e (◦C) 65 Cell ela i e p essu e (ba ) 0.5 Anode and ca hode ela i e humidi y (%) 60 Anode s oichiome ic ac o (−) 1.3 Ca hode s oichiome ic ac o (−) 2.5 Fig. 3. Inle /ou le con igu a ions. C. Su´ a ez e al. Ene gy 283 (2023) 128455 5 and powe cu es, he elec o- he mal mapping ins umen was used o measu e and collec he cu en densi y and empe a u e da a a an acquisi ion a e o 1 s. 2.2.3. FC-DLC es The objec i e o he FC-DLC es in his wo k was o subjec he uel cell o epe i i e load cycles and measu e he e ec o he di e en pe- iods o accele a ion, b aking and cons an speed had on he cu en densi y and empe a u e ields wi hin he uel cell. Du ing he es s, he cu en densi y is a ied acco ding o he es ablished dynamic load cy- cles depic ed in Fig. 5 and he ol age is ob ained. The cu en densi y a io was de ined as a pe cen age o he maximum cu en densi y alue. The cycle consis ed o ou epe i ions o 195 s each a a low speed, such as ha used on u ban oads (u ban d i ing cycle zone), ollowed by a pe iod o 400 s simula ing a highway condi ion and i s co esponding accele a ion (ex a u ban d i ing cycle zone). The comple e cycle would co espond o a dis ance o app oxima ely 11 km d i en o 20 min. 3. Resul s and discussion In his sec ion, he main esul s o he expe imen al esea ch a e p esen ed and discussed. 3.1. Pola iza ion cu es and CDM measu emen s In a i s se o expe imen s, pola iza ion and powe cu es, including CDM measu emen s, we e pe o med o he ou di e en inle /ou le con igu a ions (NF, IAF, IHF and IF). The pola iza ion cu es esul s we e also used o de ine he 100% cu en load o each con igu a ion in he subsequen DLC es s. 3.1.1. Pola iza ion and powe densi y cu es Pola iza ion and powe cu es we e ob ained ollowing he expe i- men al p ocedu e desc ibed in sec ion 2.2.2, and a compa ison o he esul s o he di e en con igu a ions was depic ed in Fig. 6. Ascending ( om open ci cui ol age o maximum cu en ) and descending ( om maximum cu en o open ci cui ol age) cu es we e included o each con igu a ion. The expe imen al esul s indica ed ha he hys e esis phenomenon was p esen in all he s udied con igu a ions, as he o wa d cu e (inc easing cu en ) was below he backwa d cu e (dec easing cu - en ), sugges ing ha he e was a ce ain deg ee o d ying in he cell. The pola iza ion cu e in each case also showed he h ee pola iza- ion egions (ac i a ion, ohmic, and mass anspo ). Simila ol age and powe densi ies we e ob ained in he egion o ac i a ion losses o densi y cu en alues below 0.2 A/cm 2 . Fo example, a a cu en densi y o 0.1 A/cm 2 , he di e ence in ol age be ween he bes con igu a ion NF and he wo s con igu a ion IHF was only abou 0.02 V. The ou con igu a ions also pe o med simila ly in his egion in e ms o powe densi ies, wi h negligible di e ences. Howe e , in he ohmic losses egion, whe e he cell po en ial dec eases app oxima ely linea ly wi h he cu en , he IHF con igu a ion pe o med be e , ob aining he highes ol age wi h highe alues o app oxima ely 0.02 V compa ed o he wo s IF and NF con igu a ions a a densi y cu en alue o 0.7 A/cm 2 . Fo he same densi y cu en alue highe ela i e powe densi ies o 3–4% we e ob ained o he IHF con igu a ion. The IHF con igu a ion pe o med e en be e in he egion o mass anspo losses, whe e he cell po en ial d op de ia es om he linea ela ionship wi h cu en densi y due o a mo e p onounced concen a ion pola i- za ion, wi h highe ol age alues in he ange o 0.02–0.03 V compa ed o he o he con igu a ion NF. The same beha io was obse ed when compa ing he powe cu es, in which he IHF con igu a ion achie ed highe powe densi ies compa ed o he o he con igu a ions. Howe e , hese di e ences we e small, wi h ela i e powe densi y di e ences in he ange o 3–5% a a cu en densi y o 0.9 A/cm 2 . Rep esen a i e alues o he a e age cu en densi ies in he pola - iza ion cu es, ob ained as he a e age be ween he ascending and descending cu es, we e la e used in he FC-DLC es s o de ine he espec i e 100% cu en load alues. In pa icula , o a ep esen a i e ol age alue o 0.6 V, he expe imen al cu en densi y o IHF con ig- u a ion was 0.898 A/cm 2 (44.90 A), a 14.4% highe han he wo s case o he NF con igu a ion wi h 0.785 A/cm 2 (39.25 A). The IF and IAF con igu a ions achie ed simila a e age cu en densi y a 0.6 V o 0.810 A/cm 2 (40.5 A) and 0.820 A/cm 2 (40.1 A) espec i ely. Al hough he IHF con igu a ion pe o med bes , he maximum cu en densi y ach- ie ed was he lowes (0.90 A/cm 2 ), compa ed o he 1.04 A/cm 2 ach- ie ed by he IAF con igu a ion. Fig. 4. Expe imen al se up o No mal Flow con igu a ion (NF). Fig. 5. Fuel cell dynamic load cycle [35]. C. Su´ a ez e al. Ene gy 283 (2023) 128455 6 The main esul s desc ibed abo e o he pola iza ion and powe cu es a e summa ized in Table 2, which shows he a e age cu en densi y a 0.6 V, he maximum cu en densi ies achie ed and he maximum powe densi ies a 0.9 A/cm 2 a e shown o he ou con ig- u a ions s udied. 3.1.2. CDM measu emen s Du ing he expe imen al acquisi ion o he pola iza ion cu es o each con igu a ion, CDM measu emen s we e pe o med wi h a da a acquisi ion a e o 1 s. Figs. 7 and 8 show a compa ison o he CDM esul s o a cu en densi y o 0.4 A/cm 2 (20 A) as a ep esen a i e illus a ion o he cu en densi y and empe a u e dis ibu ions ob ained. Rega ding he cu en densi y, simila a ia ions we e ob ained o he di e en con igu a ions, obse ing an in e se bell-shaped dis ibu- ion wi h highe alues in he ou e pa o he bipola pla e and lowe alues in he cen al pa ega dless o he inle /ou le posi ions. Due o he c oss- low con igu a ion o he anode and ca hode channels, he highe cu en densi ies we e ob ained in he collec o channels, whe e he in e cep ion o he anode and ca hode channels is much highe compa ed o he pa allel-se pen ine channels a e sing he cen al pa o he bipola pla e. In addi ion, he la ge wid h and heigh dimensions o he collec o channels allowed highe eac an mass low a es in hese zones, esul ing in highe local cu en densi ies. Ve y homogeneous dis ibu ions o local empe a u es we e obse ed o he di e en con igu a ions. The empe a u es measu ed wi h he CDM senso we e 2-3 ◦C highe han he cell empe a u e ope a ing condi ion o 65 ◦C, due o he ac ha he cell empe a u e he mocouple was placed a a mo e dis an posi ion om he GDL, whe e hea gene a ion occu s du ing he exo he mic eac ion. Simila dis ibu ions we e ob ained o he o he cu en densi ies s udied (0.2 A/cm 2 , 0.7 A/cm 2 and 0.9 A/cm 2 ). To quan i y he deg ee o homogenei y o cu en densi y and em- pe a u e, he ollowing a iables we e de ined o cu en densi y (I d ) and empe a u e (T): a e age (a g), s anda d de ia ion (s d) and maximum ela i e de ia ion ( el, max): Id,a g(A/cm2)=∑ i=18×18 i=1 Id(i) 18 ×18 (1) Id,s d(A/cm2)= 1 18 ×18 ∑ i=18×18 i=1(Id(i) − Id,a g)2 √ √ √ √(2) Id, el,max(%) = max[abs (Id(i) − Id,a g) Id,a g ⋅ 100](3) Ta g(◦C) = ∑ i=9×9 i=1 T(i) 9×9(4) Ts d(◦C) =  1 9×9∑ i=9×9 i=1(T(i) − Ta g)2 √ √ √ √(5) T el,max(%) = max[abs (T(i) − Ta g) Ta g ⋅ 100](6) No e ha while he CDM senso p o ides 324 local cu en densi y measu emen s e enly dis ibu ed in a ma ix o 18 ×18 loca ions, he senso p o ides only 81 empe a u e measu emen s dis ibu ed in a ma ix o 9 ×9 loca ions. The esul s o he CDM s a is ic o cu en densi y and empe a u e a e summa ized in Table 3 and Table 4, espec i ely. Fo cu en den- si y, i was ound ha o each con igu a ion an app oxima ely linea inc ease o he s anda d de ia ion wi h cu en densi y. Almos cons an he e ogenei ies we e obse ed o he di e en in ensi ies, wi h lowe alues o 88.7% o he maximum ela i e de ia ions om he mean o he IF con igu a ion compa ed o he o he s, which emained in he Fig. 6. Pola iza ion and powe cu es o he di e en con igu a ions No mal Flow (NF), In e se Ai Flow (IAF), In e se Hyd ogen Flow (IHF) and In e se Flow (IF). C. Su´ a ez e al. Ene gy 283 (2023) 128455 7 ange o 90.5–91.0%. In e ms o empe a u es, an app oxima ely linea dec ease o a e age empe a u es wi h cu en densi y was obse ed in all con ig- u a ions. The cooling e ec due o he inc ease in eac an s mass low a a lowe ope a ing empe a u e o 65 ◦C had a g ea e e ec on he a e age cell empe a u e han he inc ease in hea gene a ion a highe cu en densi y alues. An app oxima ely linea inc ease o he em- pe a u e s anda d de ia ion wi h cu en densi y was also obse ed o each con igu a ion. The maximum ela i e de ia ions om he a e age empe a u e we e e y low, less han 1.5% e en o he highe case (IHF con igu a ion). 3.2. FC-DLC es s and CDM measu emen s In a second se o expe imen s, DLC es s we e pe o med, including CDM measu emen s. FC-DLC es s we e pe o med o he ou p oposed con igu a ions, epea edly subjec ing he cell o he same load cycle, including as a ia ions in load, p olonged OCV exposu e as well as pe iods o s eady-s a e ope a ion as desc ibed in sec ion 2.2.2. Du ing he DLC es s, CDM measu emen s o cu en densi y and empe a u e we e eco ded simul aneously e e y second. 3.2.1. FC-DLC es s Th ee load cycles we e epea ed sequen ially o each con igu a ion, aking in o accoun a s abiliza ion pe iod o 1 h be o e each cycle. The 100% cu en densi y e e ences we e de ined acco ding o he alues p e iously ob ained in Table 2: 0.96, 1.04, 0.90 and 1.00 A/cm 2 o he NF, IAF, IHF and IF con igu a ions espec i ely. Simila ol age esul s we e ob ained in each epe i ion, wi h ela i e di e ences o less han 1% wi h espec o he a e age o he h ee epe i ions. As a ep esen a i e example, Fig. 9 shows he h ee epe i- ions o he DLC es we e shown o he IHF con igu a ion. Small ol age a ia ions we e obse ed du ing he s abiliza ion pe iod p io o each DLC cycle a 0.678 A/cm 2 . A compa ison o he FC-DLC es s o he di e en con igu a ions is shown in Fig. 10 in e ms o ol age. Fo simplici y, he a e age o he h ee epe i ions in each expe imen a e shown, s a ing om =0 s. The ou p oposed con igu a ions we e able o comple e he DLC es s wi h a co ec cell esponse beha iou . In he ange o he u ban d i ing cycle, wo di e en cha ac e is ic zones we e obse ed. In a i s zone, du ing he i s 60 s, a ansien s abiliza ion beha io was obse ed o all con igu a ions, wi h an app oxima ely linea inc ease in cu en densi y and an app oxima ely linea dec ease in ol age. This ini ial beha io was due o he ini ial cu en densi y a ios de ined in he DLC es , in which he cu en Fig. 7. CDM measu emen s o cu en densi y a 0.4 A/cm 2 (20 A). Table 2 Summa y o pola iza ion and powe cu e esul s o he ou con igu a ions. Con igu a ion I a g (A/cm 2 ) I max (A/cm 2 ) P max (W/cm 2 ) NF 0.785 0.960 0.520 IAF 0.810 1.040 0.517 IHF 0.898 0.900 0.535 IF 0.820 1.000 0.520 C. Su´ a ez e al. Ene gy 283 (2023) 128455 8 densi y a io was se o ze o du ing he i s 15 s o he es , hen apidly inc eased om 0 o 12.5% and main ained a his le el o 13 s, and hen, he cu en densi y a io was apidly dec eased o 5% o 33 s. A e wa ds, in a second zone, he ansien e olu ion o he cu en densi y and ol age s ick mo e signi ican ly o he DLC cu en densi y a io inpu . Wi hin he u ban d i ing cycle ange, wi h low cu en densi y alues below 0.26 A/cm 2 , he NF con igu a ion p esen ed highe ol ages compa ed o he o he con igu a ions, e en hough he IHF con igu a ion had a highe cu en densi y alue. Fo example, a = 190 s he IHF con igu a ion showed a ol age o 0.733 V a a cu en densi y o 0.256 A/cm 2 , while a he same ime he NF con igu a ion eached a highe ol age o 0.753 V wi h a lowe cu en densi y o 0.229 A/cm 2 . Simila beha io bu mo e p onounced di e ences we e obse ed in he ex a u ban cycle, wi h cu en densi y alues up o 0.898 A/cm 2 . Fo example, a =1150 s, he IHF con igu a ion showed a ol age o 0.550 V wi h a cu en densi y o 0.874 A/cm 2 , and he NF con igu a ion showed a highe ol age o 0.582 V wi h a lowe cu en densi y o 0.781 A/cm 2 . No wi hs anding he abo e, as depic ed in Fig. 11, he IHF con igu- a ion p esen ed a highe powe densi y cu e, because he p oduc o cu en densi y and ol age was highe o his con igu a ion compa ed o he o he s o bo h u ban and ex a-u ban d i ing cycle egions. In pa icula , he o al ene gy densi y du ing he cycle o he IHF con ig- u a ion a 210.6 J/cm 2 was in he ange o 8.6–9.1% highe compa ed o he es o con igu a ions wi h ene gy densi ies in he ange o 193–194 J/cm 2 . Fig. 8. CDM measu emen s o empe a u e a 0.4 A/cm 2 (20 A). Table 3 S a is ical cu en densi y esul s o each con igu a ion. Cu en densi y alues o 0.2 A/cm 2 , 0.4 A/cm 2 , 0.7 A/cm 2 and 0.9 A/cm 2 , sepa a ed by commas o each pa ame e . Con igu a ion I d,a g (A/cm 2 ) I d,s d (A/cm 2 ) I d, el,max (%) NF 0.031, 0.062, 0.108, 0.139 0.012, 0.024, 0.042, 0.054 90.6, 90.6, 90.6, 90.5 IAF 0.031, 0.062, 0.108, 0.139 0.012, 0.023, 0.041, 0.053 90.7, 90.6, 90.9, 90.7 IHF 0.031, 0.062, 0.110, 0.139 0.012, 0.023, 0.042, 0.053 91.0, 90.8, 90.9, 90.9 IF 0.031, 0.062, 0.108, 0.139 0.0111, 0.022, 0.039, 0.050 88.7, 88.7, 88.7, 88.7 Table 4 S a is ical empe a u e esul s o each con igu a ion. Tempe a u e alues o 0.2 A/cm 2 , 0.4 A/cm 2 , 0.7 A/cm 2 and 0.9 A/cm 2 , sepa a ed by commas o each pa ame e . Con igu a ion T a g (◦C) T s d (◦C) T el,max (%) NF 68.2, 67.9, 67.9, 67.4 0.114, 0.146, 0.266, 0.387 0.4, 0.4, 0.7, 1.2 IAF 68.2, 68.2, 67.8, 67.7 0.120, 0.157, 0.303, 0.336 0.5, 0.6, 1.1, 1.2 IHF 68.5, 67.9, 67.6, 66.9 0.101, 0.162, 0.288, 0.451 0.4, 0.5, 0.9, 1.5 IF 68.2, 68.5, 67.3, 66.9 0.118, 0.178, 0.356, 0.445 0.5, 0.7, 1.2, 1.5 C. Su´ a ez e al. Ene gy 283 (2023) 128455 9 3.2.2. CDM measu emen s CDM measu emen s we e also pe o med du ing he DLC es s wi h a da a acquisi ion a e o 1 s. As an illus a i e example, he esul s o he IHF con igu a ion can be isualized in ideo o ma as supplemen a y da a in Appendix A, showing bo h cu en densi y and empe a u e dis ibu ions a e e y second o he DLC es . To analyze he esul s ob ained, Fig. 12 shows pa ial esul s o ep esen a i e cu en densi ies o 0.2, 0.4, 0.7 and 0.9 A/cm 2 o he same IHF con igu a ion. A o al o i e measu emen s we e included in he analysis: wo measu emen s a 0.2 A/cm 2 cu en densi y o compa e he esul s du ing an inc easing load a ia ion (poin 1) and a dec easing load a ia ion (poin 2), and one o each o he emaining cu en densi ies (poin s 3, 4 and 5). The scale in Fig. 12 has been ixed o he 5 poin s in o compa e he esul s. The s a is ical esul s o cu en densi y and empe a u e du ing DLC es a e summa ized in Table 5 and Table 6 espec i ely, o he ep e- sen a i e poin s 1–5. Wi h espec o he cu en densi y dis ibu ion, simila o he p e- ious CDM measu emen s o he pola iza ion cu e, he in e se bell- shaped dis ibu ion was obse ed in he i e poin s analyzed. No sig- ni ican di e ences in he CDM esul s we e ound when compa ing he ascending (poin 1) and descending (poin 2) load a ia ions. Al hough he in e se bell-shaped dis ibu ion was isually mo e p onounced a highe cu en densi ies (poin s 4 and 5), he maximum ela i e de ia- ion om he cu en densi y a e age was app oxima ely cons an a all poin s, in he ange o 78.9–79.6%. These pe cen ages ob ained we e abou 10% lowe compa ed o he mo e he e ogeneous dis ibu ions ob ained du ing he pola iza ion cu es. Also, lowe s anda d de ia ions we e ound du ing he DLC es s, especially in he high cu en densi y cases, indica ing ha he apid changes in ascending and descending cu en densi ies caused less he e ogenei y in he local cu en densi y dis ibu ions. Rega ding he CDM measu emen s o empe a u e, again e y ho- mogeneous dis ibu ions we e obse ed, consis en wi h he p e ious CDM expe imen s o ob aining he pola iza ion cu e. No signi ican di e ences we e ound when compa ing ascending (poin 1) and descending (poin 2) load a ia ions. Howe e , highe a e age em- pe a u es wi h a maximum di e ence o 1.4 ◦C we e ound wi hin he cell du ing he DLC es s in compa ed o he pola iza ion cu es. 4. Conclusions and esea ch limi a ions In his wo k, he pe o mance o ou di e en inle /ou le con igu- a ions (No mal Flow, In e se Oxygen Flow, In e se Hyd ogen Flow and Fig. 9. DLC es s esul s o he IHF con igu a ion. Fig. 10. DLC es s cu en densi y and ol age esul s o he di e en inle /ou le con igu a ions. C. Su´ a ez e al.