Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells

Graphite oxide/polybenzimidazole synthesized by 3, 3′-diaminobenzidine and 5-tert-butyl isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI) composite membranes were prepared for high temperature polymer proton exchange membrane fuel cells (PEMFCs). All membranes were loaded with different content of phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the chemical stability of BuIPBI membrane and support a higher acid content. The proton conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and 0.027 S/cm, respectively, at 140 °C and without humidity.     

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.     

A novel H3PO4/Nafion–PBI composite membrane for enhanced durability of high temperature PEM fuel cells

A novel phosphoric acid doped Nafion–polybenzimidazole (H₃PO₄/Nafion–PBI) composite membrane was prepared and the H₂/O₂ single cell durability was tested at 150 °C without humidification. The durability was improved 55% compared with that of phosphoric acid doped polybenzimidazole (H₃PO₄/PBI). During the durability test, the hydrogen permeability of the membrane and the internal resistance of the single cell were detected using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), respectively. Before and after the durability test, the mechanical strength of the membranes was measured by stress–strain tests. The results of characterization indicated that the enhanced durability of the membrane attributed to the improved mechanical strength, which benefited from the presence of Nafion in the Nafion and PBI matrix. The preliminary results suggested that the novel H₃PO₄/Nafion–PBI composite membrane is a good candidate in high temperature PEMFC for achieving longer cell lifetime.     

Sputtered Pt loadings of membrane electrode assemblies in proton exchange membrane fuel cells

The fabrication of electrodes use in proton exchange membrane fuel cells (PEMFCs) by Pt sputter deposition has great potential to increase Pt utilization and reduce Pt loading without loss of cell performance. A radio frequency (RF) magnetron sputter deposition process (RF power = 100 W and argon pressure = 10^?3 Torr) was adopted to prepare Pt catalyst layers of PEMFC electrodes. The effects of cathode Pt and Nafion loadings on membrane electrode assembly (MEA)/cell performance were investigated using cell polarization, cyclic voltammetry, AC impedance, and microstructure analysis. Among the tested MEAs with various cathode Pt loadings (0.02–0.4 mg cm^?2), the one with 0.1 mg‐Pt cm^?2 (grain size = 3.90 nm, mainly Pt(111)) exhibited the best cell performance (320 and 285 mW cm^?2 at 0.44 and 0.60 V, respectively), which was similar to or better than those of some commercial nonsputtered/sputtered electrodes with the same or higher Pt loadings. The electrode Pt utilization efficiency increased as the Pt loading decreased. A Pt loading of greater than or lower than 0.1 mg cm^?2 yielded a lower electrode electrochemical active surface (EAS) area but a higher charge transfer and diffusion resistance. Nafion impregnation (0.1 to 0.3 mg cm^?2) into the sputtered Pt layer (Pt = 0.1 mg cm^?2) noticeably increased the EAS area, consistent with the decrease of the capacitance of the electrode double layer, but did not improve MEA/cell performance, mainly because of the increase in the kinetic and mass transfer resistances associated with oxygen reduction on the cathode. Copyright © 2011 John Wiley & Sons, Ltd.     

Development of shut-down process for a proton exchange membrane fuel cell

Several different shut-down procedures were carried out to reduce the degradation of membrane electrode assembly (MEA) in a proton exchange membrane fuel cell (PEMFC). The effects of close/open state of outlets of a single cell and application of a dummy load during the shut-down on the degradation of the MEA were investigated. Also, we elucidated the relationship between the thickness of the electrolyte membrane and the degradation of the MEA for different shut-down procedures. When a thin electrolyte membrane was used, the closer of outlets mitigated the degradation during on/off operation. For the thicker electrolyte membrane, the dummy load which eliminates residual hydrogen and oxygen in the electrodes should be applied to lower the degradation.     

Effects of heat treatment time on electrochemical properties and electrode structure of polytetrafluoroethylene-bonded membrane electrode assemblies for polybenzimidazole-based high-temperature proton exchange membrane fuel cells

To improve cell performance, the effects of heat treatment time on the electrochemical properties and electrode structure of PTFE-bonded membrane electrode assemblies for PBI-based high-temperature proton exchange membrane fuel cells are investigated. The cell performance is observed to decrease in the high-current-density region rather than in the low-current-density region with increasing heat treatment time at 350 °C from 1 to 30 min. Microscopic studies reveal remarkable differences in the electrode structure by the agglomeration of dispersed PTFE and adjacent catalyst particles, depending on the heat treatment time. As the heat treatment time increases, only the large pore (secondary pore) volume in the electrode decreases, resulting in increase in mass transport resistance and concentration overpotential in the high-current-density region. Cell performance is not measured without heat treatment because the electrodes are not formed. When the electrodes are heat treated for 1 min at 350 °C, the best cell performance is obtained, 0.67 V at 200 mA cm⁻².     

The development of PTFE/Nafion/TEOS membranes for application in moderate and high temperature proton exchange membrane fuel cells

PTFE/Nafion (PN) and PTFE/Nafion/TEOS (PNS) membranes were fabricated for the application of moderate and high temperature proton exchange membrane fuel cells (PEMFCs), respectively. Membrane electrode assemblies (MEAs) were fabricated by PTFE/Nafion (and PTFE/Nafion/TEOS) membranes with commercially available low and high temperature gas diffusion electrodes (GDEs). The effects of relative humidity, operation temperature, and back pressure on the performance and durability test of the as-prepared MEAs were investigated. Incorporating TEOS into a PNS membrane and adding another layer of carbon onto a GDE would result in low membrane conductivity and low fuel cell performance respectively. However, in this work it is shown that HT-PNS MEAs demonstrate a higher performance than LT-PN MEAs in severe conditions - high temperature (118 °C) and low humidity (25% RH). The TEOS and additional carbon layer function as water retaining agents which are especially important for high temperature and low humidity conditions. The HT-PNS MEA showed good stability in a 50 h fuel cell test at high temperature, moderate relative humidity (50% RH) and back pressure of 14.7 psi.     

Electrochemical carbon corrosion in high temperature proton exchange membrane fuel cells

Electrochemical carbon corrosion occurring in a high temperature proton exchange membrane fuel cell (HT-PEMFC) operating under non-humidification conditions was investigated by measuring CO₂ generation using on-line mass spectrometry and comparing the results with a low-temperature proton exchange membrane fuel cell (LT-PEMFC) operated under fully humidified conditions. The experimental results showed that more CO₂ was measured for the HT-PEMFC, indicating that more electrochemical carbon corrosion occurs in HT-PEMFCs. This observation is attributed to the enhanced kinetics of electrochemical carbon corrosion due to the elevated operating temperature in HT-PEMFCs. Additionally, electrochemical carbon corrosion in HT-PEMFCs showed a strong dependence on water content. Therefore, it is critical to remove the water content in the supply gases to reduce electrochemical carbon corrosion.     

Acid doped polybenzimidazoles based membrane electrode assembly for high temperature proton exchange membrane fuel cell: A review

High temperature proton exchange membrane fuel cells (HT-PEMFCs) are proficient clean energy conversion devices for automotive and stationary applications. HT-PEMFC could mitigate the CO poisoning, humidity and heat management, and sluggish of oxygen reduction reaction (ORR). Acid doped polybenzimidazoles (PBIs)/functionalized PBIs polymer electrolyte membranes are familiar uses for HT-PEMFC because of high proton conductivity with thermo-mechanical stability. Proton conductivity of PBI membranes is greatly promising by acid doping dimension and cell operating temperature. PBI reactive sites (=NH) and acidic anions prominently contribute the proton transfer through the prolonged hydrogen bonding network. Coating and sprayed methods are prominent techniques for fabrication of gas diffusion electrodes (GDEs), although shrinkage and hairline surface cracks observed on GDEs. Multi walled carbon nanotubes (MWCNTs) has been compromising unique characteristics for steady carbon support materials. Moreover, PTFE and PVDF can be used as catalyst binder to reduce corrosion rate. In this review, it has been focused the PBIs membrane, acid doping, GDEs, MEA and durability of MEA.     

An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.     

Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.     

Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells

The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve.     

Low temperature decal transfer method for hydrocarbon membrane based membrane electrode assemblies in polymer electrolyte membrane fuel cells

Decal transfer is an effective membrane electrode assembly (MEA) fabrication method known for its low interfacial resistance and suitability for mass processing. Previously decal transfer for hydrocarbon membranes was performed at temperatures above 200 °C. Here a novel low temperature decal transfer (LTD) method for hydrocarbon membranes is introduced. The new method applies a small amount (2.2 mg cm⁻²) of liquid (1-pentanol) onto the membrane separator before decal transfer to lower the T_g of the membrane and achieves complete decal transfer at 110 °C and 6 MPa. Nafion binder amount in the catalyst layer and catalyst layer annealing temperature is controlled to optimize the fuel cell performance. Compared to conventional decal transfer (CDT), the novel LTD method shows enhancement in energy efficiency, simplicity in the process scheme, and improvement in fuel cell performance.     

A novel self-humidifying membrane electrode assembly with water transfer region for proton exchange membrane fuel cells

A novel self-humidifying membrane electrode assembly (MEA) with the active electrode region surrounded by a unactive “water transfer region (WTR)” was proposed to achieve effective water management and high performance for proton exchange membrane fuel cells (PEMFCs). By this configuration, excess water in the cathode was transferred to anode through Nafion membrane to humidify hydrogen. Polarization curves and power curves of conventional and the self-humidifying MEAs were compared. The self-humidifying MEA showed power density of 85 mW cm⁻² at 0.5 V, which is two times higher than that of a conventional MEA with cathode open. The effects of anode hydrogen flow rates on the performance of the self-humidifying MEA were investigated and its best performance was obtained at a flow rate of 40 ml min⁻¹. Its performance was the best when the environmental temperature was 40 °C. The performance of the self-humidifying MEA was slightly affected by environmental humidity. The area of WTR was optimized, and feasible area ratio of the self-humidifying MEA was 28%.     

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.     

A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique

A novel ultrasonic-spray method for preparing gas diffusion electrodes (GDEs) for proton exchange membrane fuel cell (PEMFC) is described. Platinum (Pt) loaded on Nafion^®-bonded GDEs were prepared by the ultrasonic-spray method on various commercial woven and non-woven gas diffusion layers (GDLs) at several Pt loadings in the range of 0.40-0.05 mg cm⁻². The ultrasonic-sprayed GDEs were tested and compared to commercial and hand-painted GDEs. It was found that the GDEs prepared by the ultrasonic-spray method exhibited better performances compared to those prepared by the hand-painting technique, especially at low Pt loadings. GDEs fabricated by the ultrasonic-spray method with a platinum loading of 0.05 mg cm⁻² exhibited a peak power rating of 10.9 W mg⁻¹ compared to 9.8 W mg⁻¹ for hand-painted GDEs. For all experiments using various GDLs, Sigracet SGL 10BC exhibited the best performance with a peak power of 0.695 W cm⁻².     

Incorporation of H3PO4 into three-dimensional polyacrylamide-graft-starch hydrogel frameworks for robust high-temperature proton exchange membrane fuel cells

To enhance the anhydrous proton conductivities of proton exchange membranes, we report here the incorporation of H₃PO₄ into three-dimensional (3D) framework of polyacrylamide-graft-starch (PAAm-g-starch) hydrogel materials using extraordinary absorption of hydrogels to H₃PO₄ aqueous solution. Intrinsic microporous structure can close to seal H₃PO₄ molecules in the interconnected 3D frameworks of PAAm-g-starch after suffering from dehydration. The hydrogel membranes are thoroughly characterized by morphology observation, thermal stability, swelling kinetics, proton-conducting performances as well as electrochemical behaviors. The results show that the H₃PO₄ loadings and therefore the proton conductivities of the hydrogel membranes are dramatically enhanced by employing PAAm-g-starch matrix. H₃PO₄ loading of 88.68 wt% and an anhydrous proton conductivity as high as 0.046 S cm⁻¹ at 180 °C are recorded. A fuel cell using a thick membrane shows a peak power density of 517 mW cm⁻² at 180 °C by feeding with H₂/O₂ streams. The high H₃PO₄ loading, reasonable proton conductivity in combination with simple preparation, low cost and scalable matrix demonstrates the potential use of PAAm-g-starch hydrogel membranes in high-temperature proton exchange membrane fuel cells.     

A prediction model of degradation rate for membrane electrode assemblies in direct methanol fuel cells

This article proposes a new prediction model to describe the nonlinear performance degradation paths of membrane electrode assemblies (MEAs) in direct methanol fuel cell (DMFC): a bi-exponential model with random coefficients. The bi-exponential model is constructed on a mathematical basis representing second-order kinetics. Performance variation between MEAs is incorporated by random coefficients in the proposed model. A likelihood ratio test is sequentially executed to select random effects in the nonlinear random-coefficients model. Analysis results indicate that the reliability estimation can be substantially improved by using the nonlinear random-coefficients model to incorporate two heterogeneous degradation characteristics of MEA performance during continuous operation of DMFC. Confidence intervals of failure-time distributions are obtained by the parametric bootstrap method.