Increasing the service life of polymerelectrolyte fuel cells with a nanodispersedionomer17 February 2020, by Thamarasee Jeewandara

Schematics of Nafion ionomers on the catalyst surfaces.(A) Distribution of conventional ionomers synthesized byemulsion polymerization. (B) Distribution of thelaboratory-made ionomers synthesized by supercriticalfluid (SCF) process. Enlarged conceptual diagramshowing the distribution of both conventional andprepared ionomer on the Pt/C catalyst surface. The SCFprocess contributes to the formation of nanodispersedNafion ionomer, leading to improved electrochemicalperformance and durability. Credit: Science Advances,doi: 10.1126/sciadv.aaw0870

Protons (subatomic particles) can be transferredfrom the anode to the cathode through the ionomermembrane in polymer electrolyte fuel cells (PEFC).Scientists can extend proton pathways byimpregnating the ionomer (type of polymer) into theelectrodes to achieve improved proton transferefficiency. Since the impregnated ionomer canmechanically bind catalysts within the electrode,they are known as a binder. In a new report on Science Advances, Chi-Yeong Ahn and a researchteam introduced a simple approach to use asupercritical fluid and prepare a homogenousnanoscale dispersion of binder material in aqueousalcohol. The preparation showed high dispersioncharacter, crystallinity and proton conductivity forhigh performance and durable applications in aPEFC cathode electrode.

Polymer electrolyte fuel cells (PEFCs) are electrochemical devices that can efficiently convertchemical energy of the fuel directly into electricalenergy. The PEFCs are largely influenced by keycomponents including polymer electrolytemembranes, catalysts and perfluorinated sulfonicacid (PFSA) ionomers. Redox reactions that occurin a PEFC mainly occur at the electrode interfaceknown as the triple phase boundary (TPB) at whichthe reactant gases (H2 at the anode and O2 at thecathode) can come into contact with platinum (Pt)catalyst particles, on electron-conducting carbonmaterials (hence the triple phase). In the presentstudy, Ahn et al. described an ionomer dispersionwith an average colloidal particle size much smallerthan commercially available dispersions by treatinga Nafion 117 membrane with aliphatic alcoholunder supercritical conditions. The Nafionmembrane, a brand name for a perfluorinatedsulfonic acid (PFSA) membrane introduced by E. I.du Pont de Nemours and Company in the 1960s,can separate the anode and cathode compartmentin proton exchange membrane fuel cells and in water electrolysers.

Surface morphologies and pore distributions of MEAswith conventional D521 ionomer and ND ionomer. SEM

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results of (A to C and G) MEA with conventional D521ionomer and (D to F and H) MEA with ND ionomer. (I)MIP results of MEA with conventional D521 ionomer(blue) and ND ionomer (red). The inset is an enlargedgraph showing the pore distribution near 0.1 to 10 ?m.Credit: Science Advances, doi: 10.1126/sciadv.aaw0870

Supercritical fluids (SCFs) are widely used inindustry and research to synthesize specialmedicine, polymers and nanomaterials, withadditional applications to prepare materials forelectrochemical studies. However, researchersremain to explore the efficacy of superacidicperfluorinated sulfonic acid (PFSA) ionomers aselectrode binders. To accomplish this, Ahn et al.first obtained laboratory-made ionomer dispersionby treating a commercially available Nafionmembrane in an aqueous medium of isopropylalcohol (IPA) in the supercritical fluid state (SCF).Then using dynamic light scattering analysis, theresearchers observed ionomer particle distributionswith sizes smaller than 100 nm and named thelaboratory-made dispersion a 'nanodispersion'(ND). The ND underwent a phase transition from anaqueous dispersion to a solid, for its use as acathode binder. Using X-ray diffraction (XRD)analysis they obtained the crystallinities of ND andshowed them as semi-crystalline chains packeduniformly with improved regularity, compared to Nafion D521 used in other PEFC systems. Theimproved proton conductivity of the ND impliedlower resistance; predicting a high-level of performance of the membrane-electrode assembly(MEA) for single-cell operation.

Ahn et al. characterized (tested) the ionomerdispersion made with SCF, using scanning electronmicroscopy to observe the topography and mercuryintrusion porosimetry (MIP) to measure porosities.They observed a relatively uniform surface of theND on the MEA (membrane-electrode assembly)surface; the ND ionomer was well dispersed on thePt/C catalyst in the ink slurry to prepare the MEA tobegin with. Based on the morphologies and poresize distribution of the catalyst, the ND had betterionomer dispersibility for fuel use within themembrane-electrode assembly.

Physical properties of solidified-state ionomers. (A)Particle size distribution pattern by DLS. Most D521particles are in the ~100-nm range, but the laboratory-made dispersion has a large number of particles in thenanoscale region. (B) Viscosity behaviors of D521 andND. Because of the relatively small ionomer particlesizes, ND shows four times the viscosity of D521. (C andD) XRD patterns of solid-state D521 and ND. The sharpXRD peak of ND ionomer indicates that semi-crystallineND ionomer chains are relatively uniformly packed withimproved regularity. This feature is quantitativelyanalyzed by deconvoluting each XRD peak as individualamorphous (green line) and crystalline (blue line) peakswith Gaussian equations. (E) SAXS spectra of solid-stateD521 and ND. The narrow width of the SAXS peak of NDindicates the relatively small average size of itshydrophilic domains. TEM images of (F) D521 and (G)ND to compare the size difference of hydrophilic domains(dark regions). (H) Proton conductivities of D521 and NDmembrane coupons obtained in deionized water as afunction of temperatures. Each coupon was thermallytreated at 140°C for 1 hour. a.u., arbitrary units. Credit:Science Advances, doi: 10.1126/sciadv.aaw0870

The binder content for electrode formulation wasimportant as one of the components thatdetermined the triple phase boundary (TPB). Thescientists tuned the ionomer ratio in the electrodeswhenever one of the electrode components werechanged. To understand the performance ofionomers, they detected the electrochemicalperformances of MEAs using cathodes with 30percent weight of D521 (MEA-0) vs. 10, 20, and 30percent weight ND (MEA-10, MEA-20 andMEA-30). The MEA performances increased withthe ionomer content. They determined anappropriate amount of ionomer for MEA fabricationand decided on MEA-20, which exhibited thehighest performance in an atmosphere of oxygen.

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When they measured electrochemical performanceof MEAs in air, the fuel cell performance decreaseddue to the presence of inert nitrogen and reducedconcentrations of oxygen.

Polarization curves and EIS results of MEAs withconventional D521 ionomer and ND ionomer before andafter AST. Fuel cell performances of MEAs before andafter AST. (A) MEA with conventional D521 ionomer. (B)MEA with ND ionomer in air. EIS results for MEAs beforeand after AST. (C) MEA with conventional D521 ionomer.(D) MEA with ND ionomer in air. Credit: ScienceAdvances, doi: 10.1126/sciadv.aaw0870

To understand single-cell performance andelectrochemical durability, the team selected twosamples (MEA-0 and MEA-20) and conducted the accelerated stress test (AST). They performed ASTusing a load-cycling method, which caused severedegradation of the cathode electrode. The degreeof electrochemical degradation depended on thetype of ionomer used in the cathode. For example,MEA-20 (ND ionomer) maintained itselectrochemical performance at 3.33 percent in thepresence of oxygen and its electrochemicaldurability increased about six times more than thatof MEA-0, relative to the current density.

Even after the accelerated stress tests (AST), thecurrent density of MEA-20 was higher than theinitial current density of MEA-0. The degradation ofcatalyst was therefore serious in MEA-0 but barelynoticeable in MEA-20. Ahn et al. credited the high

molecular weight and improved crystallinity of theconstituent ionomer to justify the improvedelectrochemical tolerance of the ND electrode,which helped prevent catalyst degradation. Theyperformed transmission electron microscopy (TEM)to confirm physical changes in the electrode afterAST and noted less catalyst degradation in the NDelectrode. The extremely enhanced electrochemicaldurability was due to improved mechanical strengthbased on the high molecular weight and improvedcrystalline character of the ND, which was moredifficult to be washed away during PEFC function.

TEM images and particle distribution of Pt/C catalystsbefore and after AST. (A, B and C) initial Pt/C beforeAST. (D, E, and F) Pt/C with conventional ionomer afterAST. (G, H and I) Pt/C with conventional ionomer afterAST. Credit: Science Advances, doi:10.1126/sciadv.aaw0870

In this way, Chi-Yeong Ahn and colleaguesdemonstrated the preparation and characterizationof ND ionomer containing an average particle sizesmaller than D521 ionomer (which had an identicalchemical architecture and equivalent weight). Theyconfirmed the electrochemical efficacy of ND as acathode binder material and observed unique

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morphologies for ND ionomer obtained from theSCF (Supercritical fluid) process. Thesemorphologies corresponded to improved protonconductivity and single cell performances—resultingfrom an effective proton transport pathway. Thehigher crystalline content and molecular weight ofND improved the mechanical strength andenhanced the MEA lifetime by a factor of six at acurrent density of 0.6 V. The results showedimproved performance and durability of PEFCelectrodes. The research team expect the electrodeto further improve performance and durability uponapplying the newly formed ionomer with a high-performance catalyst within a polymer electrolytefuel cell.

More information: Chi-Yeong Ahn et al.Enhancement of service life of polymer electrolytefuel cells through application of nanodispersedionomer, Science Advances (2020). DOI:10.1126/sciadv.aaw0870

Zhi Wei Seh et al. Combining theory andexperiment in electrocatalysis: Insights intomaterials design, Science (2017). DOI:10.1126/science.aad4998

B. C.-Y. Lu et al. Solubility enhancement insupercritical solvents, Pure and Applied Chemistry(2007). DOI: 10.1351/pac199062122277

© 2020 Science X NetworkAPA citation: Increasing the service life of polymer electrolyte fuel cells with a nanodispersed ionomer(2020, February 17) retrieved 8 September 2021 from https://phys.org/news/2020-02-life-polymer-electrolyte-fuel-cells.html

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