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Effects of Cell Temperature and Reactant Humidification on Anion materials Article Effects of Cell Temperature and Reactant Humidification on Anion Exchange Membrane Fuel Cells Van Men Truong 1, Ngoc Bich Duong 1, Chih-Liang Wang 1,* and Hsiharng Yang 1,2,* 1 Graduate Institute of Precision Engineering, National Chung Hsing University, 145 Xingda Road, South District, Taichung City 402, Taiwan 2 Innovation and Development Center of Sustainable Agriculture (IDCSA), National Chung Hsing University, 145 Xingda Road, South District, Taichung City 402, Taiwan * Correspondence: [email protected] (C.-L.W.); [email protected] (H.Y.) Received: 24 May 2019; Accepted: 22 June 2019; Published: 26 June 2019 Abstract: The performance of an anion exchange membrane fuel cell (AEMFC) under various operating conditions, including cell temperature and humidification of inlet gases, was systematically investigated in this study. The experimental results indicate that the power density of an AEMFC is susceptible to the cell temperature and inlet gas humidification. A high performance AEMFC can be achieved by elevating the cell operating temperature along with the optimization of the gas feed dew points at the anode and cathode. As excess inlet gas humidification at the anode is supplied, the flooding is less severe at a higher cell temperature because the water transport in the gas diffusion substrate by evaporation is more effective upon operation at a higher cell temperature. The cell performance is slightly affected when the humidification at the anode is inadequate, owing to dehydration of the membrane, especially at a higher cell temperature. Furthermore, the cell performance in conditions of under-humidification or over-humidification at the cathode is greatly reduced at the different cell temperatures tested due to the dehydration of the anion exchange membrane and the water shortage or oxygen mass transport limitations, respectively, for the oxygen reduction reaction. In addition, back diffusion could partly support the water demand at the cathode once a water concentration gradient between the anode and cathode is formed. These results, in which sophisticated water management was achieved, can provide useful information regarding the development of high-performance AEMFC systems. Keywords: AEMFC; water management; cell temperature; reactant humidification 1. Introduction Anion exchange membrane fuel cells (AEMFCs) utilizing an anion exchange membrane (AEM) as a solid polymer ion-conducting membrane have been considered as a viable alternative to proton exchange membrane fuel cells (PEMFCs) due to their potential to adopt a non-precious catalyst and low-cost electrolyte, mitigating the cost concern of fuel cell technology [1–3]. The AEMFCs, different from the PEMFCs, also provide the practical advantages of lower overpotential electrode reactions [4,5], lower fuel crossover, a less corrosive environment, and the reduction of carbon monoxide (CO) poisoning [6,7], as well as a wider choice of fuels including hydrogen (H2), ammonia (NH3), urea, and nitrogen-based fuels [8–10] for the requirement of the applications. Compared to PEMFCs, the design and development of AEMFCs are focused on the preparation of a high-performance ionic membrane and non-precious catalysts [11–19]. Unfortunately, the performance achieved by AEMFCs has struggled due to the operating conditions, such as the temperature, pressure, and humidification of the reactant gases and the interplay of produced/consumed water in the transport of reactant gases. Materials 2019, 12, 2048; doi:10.3390/ma12132048 www.mdpi.com/journal/materials Materials 2019, 12, x FOR PEER REVIEW 2 of 11 Materials 2019, 12, 2048 2 of 11 the reactant gases and the interplay of produced/consumed water in the transport of reactant gases. Particularly,Particularly, the the AEMFC AEMFC involves involves the the production production and consumptionand consumption of water of atwater the anodeat the and anode cathode, and respectively,cathode, respectively, which makes which it more makes di ffiit cultmore to difficult control theto control water managementthe water management in the AEMFC in the compared AEMFC tocompared the PEMFC. to the PEMFC. AA typicaltypical schematicschematic diagramdiagram ofof AEMFC,AEMFC, composedcomposed ofof anodeanode/membrane/cathode,/membrane/cathode, involvinginvolving thethe conduction medium of OH− within the alkaline membrane is shown in Figure 1. Although the stack conduction medium of OH− within the alkaline membrane is shown in Figure1. Although the stack designdesign ofof AEMFC AEMFC is similaris similar to that to ofthat PEMFC, of PEMFC, the diff erentthe different conduction conduction medium betweenmedium the between membrane the ofmembrane AEMFC andof AEMFC PEMFC and results PEMFC in their results distinct in their redox. distinct redox. FigureFigure 1.1. Schematic diagram of a typical anionanion exchangeexchange membranemembrane fuelfuel cellcell (AEMFC).(AEMFC). OnceOnce the electro-osmotic drag drag moves moves the the water water pr producedoduced at atthe the anode anode toward toward the the cathode cathode in the in thePEMFC, PEMFC, the back-diffusion the back-diffusion force, force, built builtat the at water- the water-richrich cathode, cathode, drives drives the water the waterback to back the water- to the water-deficientdeficient anode, anode, sustaining sustaining membrane membrane hydratio hydrationn and and water water management management during during the the operatingoperating conditions.conditions. In contrast, the water balance in AEMFC didiffersffers fromfrom thatthat inin PEMFC.PEMFC. ExtraExtra consumptionconsumption ofof waterwater inin thethe AEMFCAEMFC isis electrochemicallyelectrochemically requiredrequired forfor thethe oxygenoxygen reductionreduction reactionreaction (ORR)(ORR) atat thethe cathode.cathode. Accordingly,Accordingly, the the gradient gradient of intrinsicof intrinsic water water content content between between the two the electrodes two electrodes in the AEMFC in the isAEMFC more severe is more than severe that than of the that PEMFC, of the PEMFC, easily resulting easily resulting in a dry-out in a situationdry-out situation at the cathode. at the cathode. On the otherOn the hand, other the hand, ionic the conductivity ionic conductivity of the AEM of the also AEM changes also due changes to the due hydration to the levelhydration of the level membrane of the andmembrane thereby and affects thereby the performance affects the pe andrformance stability and of AEMFCs. stability of AEMFCs. TheseThese complicatedcomplicated situationssituations revealreveal thatthat anan in-depthin-depth studystudy ofof waterwater transporttransport insideinside AEMFCsAEMFCs isis urgentlyurgently requiredrequired toto enableenable themthem toto bebe appliedapplied inin devices.devices. Although the water transport in flowflow fieldsfields cancan bebe observedobserved withwith thethe visible cell made of transparenttransparent materials, itit is didifficultfficult toto visualize thethe water distributiondistribution inin thethe gasgas didiffusionffusion layerlayer (GDL)(GDL) andand catalystcatalyst layerlayer (CL)(CL) becausebecause thesethese layerslayers areare formedformed fromfrom opaqueopaque materialsmaterials (e.g.,(e.g., carbon).carbon). The neutron-imagingneutron-imaging technique has been usedused toto observeobserve thethe waterwater distributiondistribution insideinside GDLGDL andand CLCL byby somesome researchers,researchers, howeverhowever thisthis techniquetechnique seemsseems toto require furtherfurther timetime andand spatial spatial resolution resolution [20 [20–22].–22]. Thus, Thus, an an adequate adequate method method for for capturing capturing water water transport transport in GDLin GDL has has not not been been eff ectivelyeffectively established. established. Alternatively,someAlternatively, some numerical numerical modeling modeling and experimentaland experimental works haveworks been have proposed been toproposed investigate to theinvestigate water transport the water properties transport of properties AEMFCs. of For AEMFCs example,. For Huo example, et al. [23 Huo] developed et al. [23] a developed three-dimensional a three- multiphasedimensional model multiphase to investigate model to water investigate transport water in thetransport anode in side the of anode AEMFCs. side of It AEMFCs. can be found It can that be thefound current that the density current and density operating and temperature operating temper haveature a major have influence a major oninfluence the liquid on waterthe liquid transport water attransport the anode at the while anode the stoichiometricwhile the stoichiometric ratio of anode ratio inlet of anode flow hasinlet a flow minor has eff aect minor on water effect transport. on water Thetransport. removal The of removal the water of at the the water anode at can thebe anode enhanced can be through enhanced the through back-diff theusion back-diffusion effect as a thinner effect membraneas a thinner is membrane used (around is used 50 µ(aroundm). Increasing 50 µm). theIncrea porositysing the of porosity GDL at theof GDL anode at linearlythe anode decreases linearly thedecreases amount the of liquidamount water of liquid inside water the GDL inside and the CL GDL because and ofCL a higherbecause porosity of a higher which porosity facilitates which the Materials 2019, 12, 2048 3 of 11 removal of water to the flow channel. Similar work has been carried out by Jiao et al. [24]. Their simulation results showed that the mechanism of water removal, depending on the humidification
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