Polymer Electrolyte Layer Research Articles

Analysis of Oxygen Transport Resistance of Nafion Thin Film on Pt Electrode

To clarify the rate determining process for the oxygen transport in a catalyst layer of polymer electrolyte fuel cells, the oxygen transport resistance of a Nafion thin film was precisely evaluated using a microelectrode technique for cast Nafion films with 30-150 nm thickness and data analysis on the relationship between the Nafion thickness on the Pt electrode and the diffusion-limited current density on the electrode. While the oxygen transport resistivity inside the Nafion thin film was roughly equal to that of Nafion thick membrane, the interfacial resistance at the Pt/Nafion or Nafion/gas interface was estimated to be equivalent to 60-80 nm thickness of Nafion film and to account for the dominant part of the total oxygen transport loss in a real PEFC electrode, especially with a low platinum loading.

Composition Dependence of the Pore Structure and Water Transport of Composite Catalyst Layers for Polymer Electrolyte Fuel Cells

The material distribution, porous features, and ionomer behavior in the catalyst layer (20wt% platinum on Vulcan carbon impregnated with Nafion) of polymer electrolyte fuel cells were investigated by microscopy, nitrogen adsorption, and water uptake as a function of ionomer loading or relative humidity. As revealed by microscopy, thin ionomer coverage along the carbon surface is observed when ionomer loading is low, while for higher loading, the pore spaces among the carbon agglomerates were mostly flooded. Even at a low ionomer to carbon ratio, the micro-porosity is closed off and the BET surface area decreases by more than 50%. At 100% relative humidity, the water content in the ionomer within the I/C = 0.5 catalyst layer reaches only 4 water molecules per sulfonate, presumably due to the inhibition of swelling by the carbon filler. The resulting structures and properties in the catalyst layer induce a tortuous pathway for water or proton transport in the ionomer phase, particularly at low water content. A distinct feature is observed in the catalyst layers using differential scanning calorimetry (DSC). DSC measurements on the sub-components lead us to conclude that the feature reflects interactions between the ionomer and the carbon or the Pt particles.

Numerical Analysis of Finite Element Method for a Transient Two-phase Transport Model of Polymer Electrolyte Fuel Cell

In this paper, we study a 2D transient two-phase transport model for water species in the cathode gas diffusion layer of hydrogen polymer electrolyte fuel cell (PEFC), the reformulation of water concentration equation is described by using Kirchhoff transformation, and its numerical efficiency is demonstrated by successfully dealing with the discontinuous and degenerate water diffusivity. The semi-discrete and fully discrete finite element approximations with Crank-Nicolson scheme are developed for the present model and the optimal error estimate in H1 norm and the sub-optimal error estimate in L2 norm are established for both finite element schemes.

Dielectric relaxations and conduction mechanisms in polyether–clay composite polymer electrolytes under high carbon dioxide pressure

The composite material P(EO/EM)-Sa consisting of synthetic saponite (Sa) dispersed in poly[ethylene oxide-co-2-(2-methoxyethoxy)ethyl glycidyl ether] (P(EO/EM)) is studied by "in situ" measurements using broadband electrical spectroscopy (BES) under pressurized CO2 to characterize the dynamic behavior of conductivity and the dielectric relaxations of the ion host polymer matrix. It is revealed that there are three dielectric relaxation processes associated with: (I) the dipolar motions in the short oxyethylene side chains of P(EO/EM) (β); and (II) the segmental motion of the main chains comprising the polyether components (αfast, αslow). αslow is attributed to the slow α-relaxation of P(EO/EM) macromolecules, which is hindered by the strong coordination interactions with the ions. Two conduction processes are observed, σDC and σID, which are attributed, respectively, to the bulk conductivity and the interdomain conductivity. The temperature dependence of conductivity and relaxation processes reveals that αfast and αslow are strongly correlated with σDC and σID. The "in situ" BES measurements under pressurized CO2 indicate a fast decrease in σDC at the initial CO2 treatment time resulting from the decrease in the concentration of polyether-M(n+) complexes, which is driven by the CO2 permeation. The relaxation frequency (fR) of αslow at the initial CO2 treatment time increases and shows a steep rise with time with the same behavior of the αfast mode. It is demonstrated that the interactions between polyether chains of P(EO/EM) and cations in the polymer electrolyte layers embedded in Sa are probably weakened by the low permittivity of CO2 (ε = 1.08). Thus, the formation of ion pairs in the polymer electrolyte domains of P(EO/EM)-Sa occurs, with a corresponding reduction in the concentration of ion carriers.

Model for investigation of oxygen transport limitation in a polymer electrolyte fuel cell

A model for oxygen transport in the catalyst layer of polymer electrolyte fuel cells was developed. The model includes oxygen transport resistance for dissolution from the gas phase into the ionomer, which is typically assumed to be negligible in conventional models. The dissolution kinetics were experimentally assessed by measuring the diffusion-limited current density, id, in a planar Pt electrode covered with thin ionomer films as a function of film thickness. The extrapolation of id−1 to zero thickness reflects the dissolution resistance. The effect of dissolution resistance is added to a conventional agglomerate model of the catalyst layer as an additional cause of transport loss. The agglomerate size was determined so that the model predicted the same diffusion-limited current density as the experimental results. The agglomerate size was close to the particle size observed in scanning electron micrographs of the cross-section of the catalyst layer prepared by focused ion beam milling. In contrast, the conventional agglomerate model must adopt an agglomerate size that is larger by one order-of-magnitude than that experimentally observed. As a result, it is reasonable to attribute the hindrance of oxygen transport in the catalyst layer to the slow oxygen dissolution at the gas–ionomer interface, rather than to the presence of large agglomerates.

Mobility enhancement and highly efficient gating of monolayer MoS2 transistors with polymer electrolyte

We report electrical characterization of monolayer molybdenum disulfide (MoS2) devices using a thin layer of polymer electrolyte (PE) consisting of poly(ethylene oxide) (PEO) and lithium perchlorate (LiClO4) as both a contact-barrier reducer and channel mobility booster. We find that bare MoS2 devices (without PE) fabricated on Si/SiO2 have low channel mobility and large contact resistance, both of which severely limit the field-effect mobility of the devices. A thin layer of PEO/LiClO4 deposited on top of the devices not only substantially reduces the contact resistance but also boost the channel mobility, leading up to three-orders-of-magnitude enhancement of the field-effect mobility of the device. When the PE is used as a gate medium, the MoS2 field-effect transistors exhibit excellent device characteristics such as a near ideal subthreshold swing and an on/off ratio of 106 as a result of the strong gate-channel coupling.

Structure, dielectric and optical properties of p-type (PVA/CuI) nanocomposite polymer electrolyte for photovoltaic cells

A novel PVA/CuI nanocomposite polymer electrolyte layer synthesized via the reduction of CuCl2 by NaI in an aqueous PVA solution. The as-prepared films were characterized by X-ray diffraction, scanning electron microscope, as well as impedance spectroscopy. The obtained results indicated the formation of hexagonal CuI nano particles of ≈55nm sizes embedded in the PVA matrix. In addition, the study of dielectric parameters and conductivity of PVA/CuI nanocomposite in wide range of temperature and frequency are given and discussed. The frequency dependence of ac-conductivity suggests power law with an exponent 0.026<s<0.73 which predicts hopping of charge carriers. The bulk conductivity showed activation with temperature, significant values of activation energy are deduced and discussed. An average value of the energy gap width, 2.05eV obtained using optical absorption in UV–visible spectra for PVA/CuI nanocomposite polymer electrolyte.

Towards Ultra-Fast X-ray Tomographic Microscopy of Liquid Water in PEFC

Synchrotron radiation X-ray tomographic microscopy (SRXTM) allows the simultaneous in-situ visualization of the water and carbonaceous structures in the gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFC) on the pore scale. Recently, the TOMCAT beamline of the Swiss Light Source (SLS) was upgraded with a new CMOS camera, enabling ultra fast in-situ XTM investigations of PEFC with temporal resolution of about 8 s. This acquisition speed limits irradiation of the cell, preventing radiation damage and subsequent measurement bias. In this way, PEFC can be imaged several times before deterioration is arising. The stability of the 3D liquid water distribution in porous GDL structures was studied with a temporal resolution of 10 minutes and liquid water fraction data is presented.

A Representative Volume Element Approach for Pore-Scale Modeling of Fuel Cell Materials

The objective of this work is to utilize the recent advances in microstructure quantification to select small volumes (referred to as representative volume elements, or "RVEs") for use in pore scale modeling, which accurately reflect the overall microstructure and transport properties of large fuel cell material datasets. The micro-porous layer (MPL) of polymer electrolyte fuel cells is chosen for demonstration. Focused ion beam scanning electron microscopy is utilized to obtain a 3-D structural dataset for the selected MPL sample. Using n-point statistics, RVEs are selected from the full dataset to reflect the ensemble averaged statistics of the full dataset to within acceptable tolerance. Metric comparisons between RVEs and the full dataset indicate that the selected RVEs offer a very good representation of the full dataset, albeit in a volume that is significantly smaller in spatial extent, therefore providing a computationally efficient and reliable model domain for pore-scale modeling efforts.

Ionic Transport Across Interfaces of Solid Glass and Polymer Electrolytes for Lithium Ion Batteries

A study of lithium cation transport across solid-solid electrolyte interfaces to identify critical resistances in nanostructured solid electrolytes is reported. Bilayers of glass and polymer thin film electrolytes were fabricated and characterized for this study. The glass electrolyte was lithium phosphorous oxynitride (Lipon), and two polymer electrolytes were studied: poly(methyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate) and poly(styrene-co-poly(ethylene glycol) methyl ether methacrylate). Both copolymers contained LiClO4 salt. In bilayers where polymer electrolyte layers are fabricated on top of Lipon, the interfacial resistance dominates transport. At 25 °C, the interfacial resistance is at least three times greater than the sum of the Lipon and polymer electrolyte resistances. By reversing the structure and fabricating Lipon on top of the polymer electrolytes, the interfacial resistance is eliminated. Experiments to elucidate the origin of the interfacial resistance in the polymer-on-Lipon bilayers reveal that the solvent mixtures used to fabricate the polymer layers do not degrade the Lipon layer. The importance of the polymer electrolytes’ mechanical properties is also discussed.

Polymer-electrolyte gated graphene transistors for analog and digital phase detection

We present an alternating current (ac) circuit based on a misoriented bilayer graphene device for analog and digital phase detection. We exploit the ambipolar nature of the transfer characteristics of a misoriented bilayer graphene transistor. The transistor action here is realized using an electrochemical gate integrated into a solid polymer electrolyte layer. This unique combination provides a voltage gain close to unity under ambient conditions, which is one order of magnitude higher than that attainable in back-gated devices. The achieved gain provides sufficient sensitivity to detect phase differences between pairs of analog or digital signals.

Nano-morphology of a polymer electrolyte fuel cell catalyst layer—imaging, reconstruction and analysis

The oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) of polymer electrolyte fuel cells (PEFC) is one of the major causes of performance loss during operation. In addition, the CCL is the most expensive component due to the use of a Pt catalyst. Apart from the ORR itself, the species transport to and from the reactive sites determines the performance of the PEFC. The effective transport properties of the species in the CCL depend on its nanostructure. Therefore a three-dimensional reconstruction of the CCL is required. A series of two-dimensional images was obtained from focused ion beam — scanning electron microscope (FIB-SEM) imaging and a segmentation method for the two-dimensional images has been developed. The pore size distribution (PSD) was calculated for the three-dimensional geometry. The influence of the alignment and the anisotropic pixel size on the PSD has been investigated. Pores were found in the range between 5 nm and 205 nm. Evaluation of the Knudsen number showed that gas transport in the CCL is governed by the transition flow regime. The liquid water transport can be described within continuum hydrodynamics by including suitable slip flow boundary conditions.

Electrochemical study of double-walled carbon nanotube electrode/block polyether-lithium bis(trifluorosulphonyl)imide salt polymer electrolyte interface

Electrodes of double-walled carbon nanotubes functionalized with ∼5 mass% of carboxyl groups (DWCNT-COOH) were prepared as an entangled and porous mat structure to facilitate the infiltration of polymer electrolytes. A system composed by a block copolymer of polyethylene-b-poly(ethylene oxide) or PE-b-PEO with lithium bis(trifluorosulphonyl)imide salt was optimized with respect to salt concentration. The 25 mass% of polymer electrolyte-DWCNT-COOH composite showed typical electronic conductivity of carbon nanotube-based composites (i.e., conductivities of 10 −2 S cm −1 were obtained). The Nyquist plots for the nanocomposite led to the assignment of separate semi-circles to the electronic transfer through the nanotube and at the polymer electrolyte–nanotube interface. A complete cell with two electrode nanocomposites and a polymer electrolyte layer was tested in a wide range of cell potentials and temperatures. The operation of this configuration at 100 °C and a 4 V window was demonstrated.

S053013 PEFC触媒層に対する局所的酸素供給と出力向上の関係([S05301]次世代電池のナノ・マイクロフルイディクス(1))

In the present study, a numerical method is suggested to solve the diffusion equations including the chemical reaction between oxygen and protons in the cathode catalyst layer of polymer electrolyte fuel cells (PEFCs). The concentration distributions of oxygen and the activated one, which depend on oxygen supply from a cathode pore and on output current density, are quantitatively investigated. The theoretical result indicates that local oxygen supply from the pores can increase the concentration of activated oxygen in the cathode catalyst layer and that power generation efficiency may be improved under the condition of suitable reaction fields of oxygen and protons.

Conformal Coating of Thin Polymer Electrolyte Layer on Nanostructured Electrode Materials for Three-Dimensional Battery Applications

Various three-dimensional (3D) battery architectures have been proposed to address effective power delivery in micro/nanoscale devices and for increasing the stored energy per electrode footprint area. One step toward obtaining 3D configurations in batteries is the formation of core-shell nanowires that combines electrode and electrolyte materials. One of the major challenges however in creating such architectures has been the coating of conformal thin nanolayers of polymer electrolytes around nanostructured electrodes. Here we show conformal coatings of 25-30 nm poly(methyl methacralate) electrolyte layers around individual Ni-Sn nanowires used as anodes for Li ion battery. This configuration shows high discharge capacity and excellent capacity retention even at high rates over extended cycling, allowing for scalable increase in areal capacity with electrode thickness. Our results demonstrate conformal nanoscale anode-electrolyte architectures for an efficient Li ion battery system.

Ultrathin solid polymer electrolyte PEI/Pebax2533/AgBF 4 composite membrane for propylene/propane separation

In this paper, a new solid polymer electrolyte PEI/Pebax2533/AgBF 4 composite membrane (PEI: polyetherimide; Pebax2533: Polynylon12/tetramethylene oxide block copolymer, PA12-PTMO) is prepared with complexing method. When the external surface of Pebax2533 layer is immersed into AgBF 4 solution, silver salt transfers into Pebax2533 layer according to the complexation with ether oxygens in Pebax2533 backbone, and an ultrathin solid polymer electrolyte layer is formed. When AgBF 4 concentration is less than the threshold silver concentration C *(4.5 mol/L), because of low silver salt content in the solid polymer electrolyte layer, no carrier-facilitated transport phenomenon occurs. But for PEI/Pebax2533/AgBF 4 composite membrane prepared with AgBF 4 concentration above C *(4.5 mol/L), the percolation channels for carrier-facilitated transport are formed and the preferential adsorbed propylene molecules can transfer across the membrane easily, which result in an increase of propylene permeance and a sharp decrease of propane permeance. According to TEM observation, an ultrathin solid polymer electrolyte layer of 100–200 nm thickness exists in the external surface of the Pebax2533 layer of PEI/Pebax2533/AgBF 4 composite membrane.

미세 X선 단층촬영 기법의 개발과 적용

The objective of this study is to elucidate the feasibility of synchrotron X-ray micro CT as a non-destructive imaging method to visualize the three-dimensional morphological structures of biological and non-biological samples. The experiments were conducted in 7B2 X-ray micro CT beamline in Pohang Accelerate Laboratory (PAL). A rotational 3-axis stage was specially designed for <TEX>$0^{\circ}-180^{\circ}$</TEX> scanning of test samples. Preliminary tests were performed for opaque samples including a mosquito head, a plant seed and gas diffusion layer (GDL) of polymer electrolyte fuel cell to verify the feasibility of the X-ray micro CT. It visualized clearly the internal structure of all the test samples, supporting its usefulness.

Performance equations for a polymer electrolyte membrane fuel cell with unsaturated cathode feed

A mathematical formulation for the cathode of a membrane electrode assembly of a polymer electrolyte membrane fuel cell is proposed, in which the effect of unsaturated vapor feed in the cathode is considered. This mechanistic model formulates the water saturation front within the gas diffusion layer with an explicit analytical expression as a function of operating conditions. The multi-phase flows of gaseous species and liquid water are correlated with the established capillary pressure equilibrium in the medium. In addition, less than fully hydrated water contents in the polymer electrolyte and catalyst layers are considered, and are integrated with the relevant liquid and vapor transfers in the gas diffusion layer. The developed performance equations take into account the influences of all pertinent material properties on cell performance using first principles. The mathematical approach is logical and concise in terms of revealing the underlying physical significance in comparison with many other empirical data fitting models.

Polyimide gel polymer electrolyte-nanoencapsulated LiCoO 2 cathode materials for high-voltage Li-ion batteries

We demonstrate a novel and facile approach to surface modification of high-voltage charged LiCoO 2, which is based on encapsulating LiCoO 2 by a polyimide (PI) gel polymer electrolyte layer. The PI is introduced onto the LiCoO 2 by thermally curing 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) polyamic acid. The PI nanoencapsulating layer features the high surface coverage, nanometer thickness, and facile ion transport. These unique characteristics are expected to enable the PI coating layer to effectively suppress the undesirable interfacial reaction of the LiCoO 2 with liquid electrolyte, which plays a key role in noticeably improving the 4.4 V cycle performance and mitigating the vigorous exothermic reaction between the charged LiCoO 2 and liquid electrolyte.

Electrochemical Fluorescence Switching from Anthracene Polymer Films

Electrochemical fluorescence switching of electroactive fluorescent anthracene polymer films was studied using an electro active quencher. The fluorescence of a methylene anthracene polymer (PMAn) was intact under redox reaction in the absence of an electroactive quencher. Interestingly the fluorescence of a methylene bridged anthracene polymer (PMAn) film could be modulated by adjusting the redox state of the PMAn and redox reaction of the quencher. The electrochemical fluorescence switching from the PMAn film with a quencher was reversible and the cyclability was longer than 1,000 cycles with an ON/OFF ratio of 3.86 under the applied potential step of +2 V for 4 second. A multi state fluorescent switch was fabricated with a double layered two electrode cell that was consisting of the PMAn layer doped with a quencher and a solid polymer electrolyte layer (SPE).