An effective layout of polyaniline nanofibers incorporated in membrane-electrode assembly as methanol transport regulator for direct methanol fuel cells

This study reports a novel strategy by using polyaniline nanofibers (PANFs) to modify membrane-electrode assembly (MEA) for improving direct methanol fuel cell (DMFC) performance. First of all, a series of PANFs emeraldine salt was synthesized and characterized. Then, we investigated the effect of PANFs layout in MEA on DMFC performance. Three different placements to incorporate the as-synthesized PANFs in anodes include (1) placing a layer of PANFs between catalyst layer (CL) and proton exchange membrane (PEM), (2) mixing with catalyst slurry and coating onto gas diffusion layer (GDL), and (3) placing a layer of PANFs between CL and GDL. Polarization curves indicate that the third method is superior to the others and is adopted as the incorporation layout thereafter. Both methanol transport resistance and methanol crossover of the PANFs-modified MEA are studied further. The DMFC incorporated with H₂SO₄-doped PANFs obtained after the re-doping process with 2 mol L⁻¹ H₂SO₄ performs a power density as high as 53 mW cm⁻², about 20% higher than that of the pristine one without PANFs incorporation. However, an excessive doping level may result in a higher methanol transport resistance due to PANFs aggregation and thus deteriorate DMFC performance. This study provides a simple and effective way by placing a layer of PANFs between CL and GDL in anode to act as methanol transport regulator and improve DMFC performance consequently.     

Effect of SiO2 additives on the PEM fuel cell electrode performance

The Nafion membrane used as an electrolyte in the Polymer Electrolyte Membrane Fuel Cell (PEMFC) needs hydration to retain the proton conductivity. In PEMFC operation the reactant gas needs to be humidified either externally or internally. To reduce the cost, weight and complexities of the PEMFC system, it is beneficial to operate the PMEFC without humidification of the reactant gases because it eliminates the need for a complex gas-humidification sub-system. In recent years, worldwide R&D efforts have been made to remove the external humidifying unit from the PEMFC system by endowing the membrane electrode assembly (MEA) with self-humidifying ability. Efforts have been made to minimize humidification of the ionic polymer by introducing SiO₂ either in the catalyst layer or on the gas diffusion layer or on the membrane directly. In-house has made two silica powders, 1. Aerogel silica, surface area is 582 m²/g and 2. Silica powder with surface area of 45 m²/g is incorporated in the fuel cell electrode. This improves the hydrophilic and protonation properties of the SiO₂ powders when treated with H₂SO₄. Initial experiments under humidified conditions showed that the Silica powder, which was not treated with H₂SO_4, gave marginally lower performance compared to the H₂SO₄ treated sample. The polarization behaviors of the electrode with and without SiO₂ in the catalyst layer were studied. The PEMFC was also studied under different humidity conditions. The electrodes and the PEMFC were characterized by different electrochemical techniques like cyclic voltammetry and Electrochemical Impedance Spectroscopy (EIS). The results are presented in this paper.     

Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.     

Enhanced low-humidity performance of proton exchange membrane fuel cell by incorporating phosphoric acid-loaded covalent organic framework in anode catalyst layer

Developing self-humidifying membrane electrode assembly (MEA) is of great significance for the practical use of proton exchange membrane fuel cell (PEMFC). In this work, a phosphoric acid (PA)-loaded Schiff base networks (SNW)-type covalent organic framework (COF) is proposed as the anode catalyst layer (CL) additive to enhance the PEMFC performance under low humidity conditions. The unique polymer structure and immobilized PA endow the proposed COF network with not only excellent water retention capacity but also proton transfer ability, thus leading to the superior low humidity performance of the PEMFC. The optimization of the additive content, the effect of relative humidity (RH) and PEMFC operating temperature are investigated by means of electrochemical characterization and single cell test. At a normal operation temperature of 60 °C and 38% RH, the MEA with optimized COF content (10 wt%) showes the maximum power density of 582 mW cm^?2, which is almost 7 times higher than that of the routine MEA (85 mW cm^?2). Furthermore, a preliminary durability test demonstrates the potential of the proposed PEMFC for practice operation under low humidity environment.     

Performance optimization of ultra-low platinum loading membrane electrode assembly prepared by electrostatic spraying

To improve the utilization of platinum and reduce the manufacturing cost of proton exchange membrane fuel cell (PEMFC), the electrostatic spraying was used to prepare the cathode catalyst layer of membrane electrode assembly (MEA) with platinum loading varying from 0.1 to 0.01 mg cm^?2. The performance of fuel cell was tested and analyzed by electrochemical impedance and polarization curve. Our results show that the platinum carbon (Pt/C) particles deposited by electrostatic spraying were well dispersed and the microporous structure of catalyst layer (CL) were relatively uniform. Replacing the CCS type MEA (catalyst coated on gas diffusion layer substrate) with the CCM type MEA (catalyst coated on proton exchange membrane) can reduce its electrochemical impedance and improve the power density of fuel cell. Compared to the Pt/C catalyst with a platinum mass fraction of 60%, a lower platinum-carbon ratio catalyst is more conducive to the uniform dispersion of catalyst particles and efficient utilization of platinum in the preparation of MEA with ultra-low platinum loading. However, their difference in peak power density decreases with the increase of platinum loading. Besides, increasing the back pressure can improve the performance of fuel cell, when the back pressure increased to 0.15 Mpa and the feeding gases were set as H₂/O₂, the peak power density of 0.56 W cm^?2 was obtained by the MEA with cathode platinum loading of 0.01 mg cm^?2, which is corresponding to the cathode platinum utilization of 56 kW·g_Pt^?1_cathode.     

Enhanced low-humidity performance in a proton exchange membrane fuel cell by developing a novel hydrophilic gas diffusion layer

An ultrathin layer of hydrophilic titanium dioxide (TiO₂) is coated on the gas diffusion layer (GDL) to enhance the performance of a proton exchange membrane fuel cell (PEMFC) at low relative humidity (RH) and high cell temperature. Both of the modified and unmodified GDLs are characterized using contact angles, and the cell performance is evaluated at various RHs and cell temperatures. It is found that the modified GDL, which contains a hydrophilic TiO₂ layer between the microporous layer (MPL) and the gas diffusion-backing layer (GDBL), exhibits better self-humidification performance than a conventional GDL without the TiO₂ layer. At 12% RH and 65 °C cell temperature, the current density is 1190 mA cm⁻² at 0.6 V, and it maintains 95.8% of its initial performance after 50 h of continuous testing. The conventional GDL, however, exhibits 55.7% (580 mA cm⁻²) of its initial performance (1040 mA cm⁻²) within 12 h of testing. The coated hydrophilic TiO₂ layer acts as a mini humidifier retaining sufficient moisture for a PEMFC to function at low humidity conditions.     

A new cheap asymmetric aqueous supercapacitor: Activated carbon//NaMnO2

A new cheap asymmetric supercapacitor based on activated carbon (AC) and NaMnO₂ as electrodes and aqueous Na₂SO₄ solution as electrolyte was assembled. It shows an energy density of 19.5 Wh kg⁻¹ at a power density of 130 W kg⁻¹ based on the total mass of the active electrode materials and an excellent cycling behavior. This asymmetric aqueous AC//NaMnO₂ capacitor is promising for practical applications due to its low price, easy preparation of NaMnO₂ and friendliness to environment.     

Enhanced catalytic properties from platinum nanodots covered carbon nanotubes for proton-exchange membrane fuel cells

An efficient fabrication method for carbon nanotube (CNT)-based electrode with a nanosized Pt catalyst is developed for high efficiency proton-exchange membrane fuel cells (PEMFC). The integrated Pt/CNT layer is prepared by in situ growth of a CNT layer on carbon paper and subsequent direct sputter-deposition of the Pt catalyst. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrate that this Pt/CNT layer consists of a highly porous CNT layer covered by well-dispersed Pt nanodots with a narrow size distribution. Compared with conventional gas-diffusion layer assisted electrodes, the CNT-based electrode with a Pt/CNT layer acting as a combined gas-diffusion layer and catalyst layer shows pronounced improvement in polarization tests. A high maximum power density of 595 mW cm⁻² is observed for a low Pt loading of 0.04 mg cm⁻² at the cathode.     

Microwave-synthesized self-supporting CoSe2/carbon fiber felt electrode for ultra–high cycling life flexible supercapacitors

Flexible electrodes are candidate for portable and wearable electronic storage devices. In this work, high-performance flexible self-supporting CoSe₂/carbon fiber felt (CoSe₂/CFF) electrode was prepared via the microwave method without any binder. The CoSe₂/CFF electrodes exhibited superior electrochemical performance (621 F g^?1 at 1 A g^?1) and an ultra-high cycling life (84.7% capacitance retention after 100,000 cycles). When the CoSe₂/CFF was used as the positive electrode for the flexible supercapacitor, the assembled device exhibited an outstanding energy density of 22.43 W h Kg^?1 at a power density of 823.12 W kg^?1. Because of the excellent mechanical stability of the device, it maintained 91.3% of its initial C after bending from 0° to 180°. The CoSe₂/CFF proposed in this work shows electrode promising applicability to a low-cost, small-sized wearable and portable energy storage device.     

Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method

A proton exchange membrane fuel cell (PEMFC) electrode having a modified morphology of conventional Teflon (PTFE) bonded electrodes was studied using the AC impedance method. The electrode differs from other types of electrodes in the presence of a thin catalyst-supporting layer between the gas diffusion backing and the catalyst layer. The thickness and composition of the supporting layer were optimized on the basis of the information from AC impedance measurements. The optimal thickness of the supporting layer and its PTFE content turned out to be approximately 3.5 mg cm⁻² and 30 wt.%, respectively. The catalyst layer was cast on top of the supporting layer, from solution that has the proper ratio of ionomer Nafion and Pt/C catalyst. The optimal amount of the ionomer in the catalyst layer was approximately 0.8 mg cm⁻² when Pt loading was kept at 0.4 mg cm⁻². These values are rationalized in terms of the catalyst active area and the transport of the involved species for the electrode reaction.     

Impact of ultra-low Pt loadings on the performance of anode/cathode in a proton-exchange membrane fuel cell

This study focuses on the elaboration of PEMFC electrodes containing ultra-low platinum (Pt) loadings by direct liquid injection metal organic chemical vapor deposition (DLI-MOCVD). DLI-MOCVD offers a large number of advantages for the elaboration of model PEMFC electrodes. First, by using different metal precursors or elaboration temperature, the size of the Pt nanoparticles and thus the intrinsic catalytic activity can easily be tailored in the nanometer range. In this work, Pt nanoparticles (1-5 nm) with remarkable low degree of agglomeration and uniform distribution were deposited onto the microporous side of a commercial gas-diffusion layer (GDL). Second, reduction of the Pt loading is made possible by varying the Pt deposition time and its influence of the cell performance can be extracted without variation of the thickness of the catalytic layer (in previous studies, a decrease of the catalyst utilization was observed when increasing the Pt loading, i.e. the thickness of the catalytic layer (CL)). The electrocatalytic activity of home-made Pt nanoparticles elaborated by DLI-MOCVD was measured in liquid electrolyte or in complete fuel cell operating on H₂/O₂ or H₂/air and compared vs. that of a commercially available electrode containing 500 μg_Pt cm⁻² (Pt_Ref500). At the cathode, the performance of the electrodes containing 104-226 μg of Pt per cm² of electrode compares favorably with that of the Pt_Ref500 in H₂/O₂ conditions. In H₂/air conditions, additional mass-transport losses are detected in the low-current density region but the high effectiveness of our electrodes improves the performance in the high-current density region. At the anode, the Pt loading can be reduced to 35 μg_Pt cm⁻² without any voltage loss in agreement with previous observations.     

A novel composite oxygen electrode: PrBaCo2O5+δ combined with negative thermal expansion oxide applied to reversible solid oxide cells

The thermal expansion coefficient (TEC) of Co-based layered perovskite electrodes is about twice than that of common electrolytes, and this thermal mismatch between electrode and electrolyte leads to the delamination and cracking of electrode. To solve this problem thoroughly, a strategy of introducing negative thermal expansion (NTE) material is proposed. The average linear thermal expansion coefficient of PrBaCo₂O_5+δ (PBC) decreases significantly from 22.3 × 10^?6 K^?1 to 12.2 × 10^?6 K^?1 when compounds with 50 wt% NdMnO_3-δ (NM). The results of multi physical field coupling calculation show that the introduction of NTE oxide can reduce the residual stress of oxygen electrodes and electrolytes, which decreases the trend of electrodes damaged by thermal stress. The composite electrode (PBC-NM) shows excellent electrocatalytic activities and reversible cycle performance. The peak power densities (PPDs) of the cell with PBC decreases from 1.4 to 0.48 W cm^?2 (65.7% decrease) after 20 thermal cycles, while the one of PBC-NM-based cell decreases from 1.55 to 1.25 W cm^?2 (19.4% decrease). Electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and distribution of relaxation time (DRT) analysis show that the improvement of thermal cycle stability of SOFC can be attributed to the ideal thermal matching between electrolyte and oxygen electrode.     

The influence of graphitization on the thermal conductivity of catalyst layers and temperature gradients in proton exchange membrane fuel cells

As the proton exchange membrane fuel cell (PEMFC) has improved its performance and power density, the efficiency has remained unchanged. With around half the reaction enthalpy released as heat, thermal gradients grow. To improve the understanding of such gradients, PEMFC component thermal conductivity has been increasingly investigated over the last ten years, and the catalyst layer (CL) is one of the components where thermal conductivity values are still scarce. CLs in PEMFC are where the electrochemical reactions occur and most of the heat is released. The thermal conductivity in this region affects the heat distribution significantly within a PEMFC. Thermal conductivities for a graphitized and a non-graphitized CL were measured for compaction pressures in the range of 3 and 23 bar. The graphitized CL has a thermal conductivity of 0.12 ± 0.05 WK^–1m^–1, whilst the non-graphitized CL conductivity is 0.061 ± 0.006 WK^–1m^–1, both at 10 bar compaction pressure. These results suggest that the graphitization of the catalyst material causes a doubling of the thermal conductivity of the CL. This important finding bridges the very few existing studies. Additionally, a 2D thermal model was constructed to represent the impact of the results on the temperature distribution inside a fuel cell.     

Feasibility of ultra-low Pt loading electrodes for high temperature proton exchange membrane fuel cells based in phosphoric acid-doped membrane

Factors as the Pt/C ratio of the catalyst, the binder content of the electrode and the catalyst deposition method were studied within the scope of ultra-low Pt loading electrodes for high temperature proton exchange membrane fuel cells (HT-PEMFCs). The Pt/C ratio of the catalyst allowed to tune the thickness of the catalytic layer and so to minimize the detrimental effect of the phosphoric acid flooding. A membrane electrode assembly (MEA) with 0.05 mg_Ptcm⁻² at anode and 0.1 mg_Ptcm⁻² at cathode (0.150 mg_Ptcm⁻² in total) attained a peak power density of 346 mW cm⁻². It was proven that including a binder in the catalytic layer of ultra-low Pt loading electrodes lowers its performance. Electrospraying-based MEAs with ultra-low Pt loaded electrodes (0.1 mg_Ptcm⁻²) rendered the best (peak power density of 400 mW cm⁻²) compared to conventional methods (spraying or ultrasonic spraying) but with the penalty of a low catalyst deposition rate.     

Membrane electrode assembly with doped polyaniline interlayer for proton exchange membrane fuel cells under low relative humidity conditions

A membrane electrode assembly (MEA) was designed by incorporating an interlayer between the catalyst layer and the gas diffusion layer (GDL) to improve the low relative humidity (RH) performance of proton exchange membrane fuel cells (PEMFCs). On the top of the micro-porous layer of the GDL, a thin layer of doped polyaniline (PANI) was deposited to retain moisture content in order to maintain the electrolyte moist, especially when the fuel cell is working at lower RH conditions, which is typical for automotive applications. The surface morphology and wetting angle characteristics of the GDLs coated with doped PANI samples were examined using FESEM and Goniometer, respectively. The surface modified GDLs fabricated into MEAs were evaluated in single cell PEMFC between 50 and 100% RH conditions using H₂ and O₂ as reactants at ambient pressure. It was observed that the MEA with camphor sulfonic acid doped PANI interlayer showed an excellent fuel cell performance at all RH conditions including that at 50% at 80 °C using H₂ and O_2.     

Optimization of ionomer-free ultra-low loading Pt catalyst for anode/cathode of PEMFC via magnetron sputtering

In this study, thin-film Pt catalysts with ultra-low metal loadings (ranging from 1 to 200 μg cm⁻²) were prepared by magnetron sputtering onto various carbon-based substrates. Performance of these catalysts acting as anode, cathode, or both electrodes in a proton exchange membrane fuel cell (PEMFC) was investigated in H₂/O₂ and H₂/air mode. As base substrates we used standard microporous layers comprising carbon nanoparticles with polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP) supported on a gas diffusion layer. Some substrates were further modified by magnetron sputtering of carbon in N₂ atmosphere (leading to CN_x) followed by simultaneous plasma etching and cerium oxide deposition. The CN_x structure exhibits higher resistance to electrochemical etching as compared to pure carbon as was determined by mass spectrometry analysis of PEMFC exhaust at different cell potentials for both sides of PEMFC. The role of platinum content and membrane thickness was investigated with the above four different combinations of ionomer-free carbon-based substrates. The results were compared with a series of benchmark electrodes made from commercially available state-of-the-art Pt/C catalysts. It was demonstrated that the platinum utilization in PEMFC with magnetron sputtered thin-film Pt electrodes can be up to 2 orders of magnitude higher than with the standard Pt/C catalysts while keeping the similar power efficiency and long-term stability.     

Three-layered absorptive glass mat separator with membrane for application in valve-regulated lead-acid batteries

During charge and discharge of the lead-acid cell equal amounts of H₂SO₄ participate in the reactions at the two types of plates (electrodes). However, the charge and discharge reactions at the positive plates involve also 2 mol of water per every mole of reacted PbO₂. Consequently, a concentration difference appears in the electrolyte between the two electrodes (horizontal stratification), which affects the reversibility of the processes at the two electrodes and thus the cycle life of the battery. The present paper proposes the use of a three-layered absorptive glass mat (AGM) separator, the middle layer playing the role of a membrane that divides (separates) the anodic and cathodic electrolyte spaces, and controls the exchange rates of H₂SO₄, H⁺ ions, O₂ and H₂O flows between the two electrode spaces. To be able to perform this membrane function, the thinner middle AGM layer (0.2 mm) is processed with an appropriate polymeric emulsion to acquire balanced hydrophobic/hydrophilic properties, which sustain constant H₂SO₄ concentration in the two electrode spaces during cycling. Three types of polymeric emulsions have been used for treatment of the membrane: (a) polyvinylpyrollidonestyrene (MPVS), (b) polyvinylpyrrolidone “Luviskol” (MPVP), or (c) polytetrafluorethylene modified with Luviskol (MMAGM). It is established experimentally that the MMAGM membrane maintains equal acid concentration in the anodic and cathodic spaces (no horizontal stratification) during battery cycling and hence ensures longer cycle life performance.     

Design,simulation and experimental characterization of a high-power density fuel cell

This research proposes a novel design of a millimeter-sized air-breathing proton exchange membrane fuel cell (AB-PEMFC). The producing method of the air-breathing PEMFC consisted of the sequential deposition of the elements in layers: 1) endplate of a flat polymer, 2) fuel flow channel with gas diffusion layers (GDL), 3) silicone seal with embedded micro Pt (platinum) wires, 4) membrane electrode assembly (MEA) with high platinum load, 5) GDL with micro Pt wires. Performance tests were done using an AB-PEMFC and the stack of two mono cells under different conditions. The results obtained by the computational fluid dynamics (CFD) allowed analyzing the current collector (CC) position. The performance results show 110 W L^?1 for the cell power density and 2200 W kg^?1 for the specific power density.     

Optimization of gas diffusion electrode for polybenzimidazole-based high temperature proton exchange membrane fuel cell: Evaluation of polymer binders in catalyst layer

Gas diffusion electrodes (GDEs) prepared with various polymer binders in their catalyst layers (CLs) were investigated to optimize the performance of phosphoric acid doped polybenzimidazole (PBI)-based high temperature proton exchange membrane fuel cells (HT-PEMFCs). The properties of these binders in the CLs were evaluated by structure characterization, electrochemical analysis, single cell polarization and durability test. The results showed that polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF) are more attractive as CL binders than conventional PBI or Nafion binder. At ambient pressure and 160 °C, the maximum power density can reach ∼ 0.61 W cm⁻² (PTFE GDE), and the current density at 0.6 V is up to ca. 0.52 A cm⁻² (PVDF GDE), with H₂/air and a platinum loading of 0.5 mg cm⁻² on these electrodes. Also, both GDEs showed good stability for fuel cell operation in a short term durability test.     

Corrosion behavior of SS316L in simulated and accelerated PEMFC environments

The electrochemical behavior and change in the passive film formation of SS316L are investigated under polymer electrolyte membrane fuel cell (PEMFC) simulated (pH from 3 to 6 containing F⁻, SO₄²⁻ and Cl⁻ anions) and accelerated conditions (0.5 M and 1 M H₂SO₄ + 2 ppm HF). Potentiodynamic, potentiostatic, and EIS measurements are performed to investigate the electrochemical behavior of the SS316L specimens in both the anode and cathode PEMFC environments. The chemical composition of the passive film, surface topography of the specimens, and degree of metal ion release is characterized by XPS, SEM, and ICP-OES, respectively. The results reveal that the nature of the passive film depends on the pH value, external medium/environment, as well as applied potential during polarization. The corrosion behavior of SS316L is closely related to the chemical composition and structure of the passive film.