Phosphoric Acid Doped Polybenzimidazole Membranes Research Articles

Cross-Linked Phosphoric Acid Doped PBI Membranes and Their Performance on Operation As Automotive Range Extender

One of the promising application areas for high temperature proton exchange membrane technology is as a range extender in a battery electric vehicle. Unlike low temperature PEM fuel cells dependent on perfluorosulfonic acid type membranes, HT-PEM fuel cells need no humidification, the conductivity of the membrane having much lower dependence on degree of hydration, and their use therefore eliminates water management issues. The operation conditions of a fuel cell for range extender application differ from those when the PEMFC stack is used for the power train. Thus the stack is subjected less to transients and operates in semi-continuous mode, but with stop/start events corresponding to the normal start up and shut down of vehicle operation. HT-PEM membranes are based upon polybenzimidazole (PBI) polymers that are doped with (generally) phosphoric acid. This acid "doping" may be achieved by immersion of a polymer membrane in acid, or by methods involving the preparation of the PBI polymer in polyphosphoric acid followed by membrane casting and a controlled hydrolysis step, or the dissolution of PBI in a polyphosphoric acid/phosphoric acid followed by membrane casting, in a thermoreversible sol-gel transition. The last two approaches lead to very high acid contents, easily exceeding 20 molecules H3PO4/PBI repeat unit, and the resulting membrane is soft with poor mechanical properties. Polymer cross-linking is a recognized means for improving membrane strength. To date, in the case of PBI, acid doping has been carried out on the cross-linked membrane, resulting in low phosphoric acid uptake. We have developed an approach that allows the covalent cross-linking of PBI by introduction of a molecular cross-linking species at the polymer dissolution stage, following by membrane casting and thermal curing to give the cross-linked membrane. Since this approach provides mechanical stabilization to an acid-swollen membrane containing a high content of acid, it is a means leading to highly proton conducting membranes of satisfactory mechanical properties. A range of synthesis parameters including the amount of cross-linking, temperature for thermal curing etc., have been examined and, most recently, we have provided experimental evidence for cross-linking with the help of X-ray photoelectron and nuclear magnetic resonance spectroscopies. Membrane electrode assemblies (MEAs) have been developed using the novel cross-linked acid doped PBI membranes, and fuel cell test protocols relevant to range extender application, based upon load cycling at 160 °C, have been applied. The results demonstrate that the novel MEAs comprising cross-linked PBI have lower voltage decay on load cycling than is observed using commercial MEAs, reaching >1000 h operation without the voltage dipping below the end-of-life criterion. The research leading to these results has received funding from the European Community’s Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Undertaking under grant agreement ARTEMIS no. 303482.

Imaging Phosphoric Acid Migration in High Temperature Polymer Electrolyte Fuel Cells by X-Ray Tomographic Microscopy

High temperature polymer electrolyte fuel cells (HT-PEFC) use phosphoric acid (PA) as the proton conducting electrolyte, which is imbibed in a polybenzimidazole (PBI) membrane. The low vapour pressure of PA allows for increased operating temperatures of up to 200°C. At the higher operating temperatures, the tolerance to fuel gas impurities increases significantly and operation with CO levels up to 3% and H2S up to 10 ppm can be achieved.1 Phosphoric acid doped PBI membranes are therefore ideal candidates for cost-efficient combined heat and power (CHP) as well as hydrogen separation, purification and pumping applications. Unlike low temperature sulfonic acid based PEFC, the electrolyte is not covalently bound to the polymer backbone of the membrane and migration of anions in the electric field can occur similar to what has been observed in phosphoric acid fuel cells (PAFC).2,3 We have recently demonstrated how X-ray tomographic microscopy (XTM) can be applied for imaging electrolyte migration in HT-PEFC under dynamic operation.4 It was shown, for the first time, that PBI based membrane systems exhibit extensive PA migration from cathode to anode under high current operation. PA flooding of the anode gas diffusion layer (GLD) and flow field channel was observed. In this contribution, the application of synchrotron based XTM for in-operando imaging and quantification of migrated phosphoric acid in HT-PEFC will be presented. The influence of current density, temperature, membrane material (conventional m-PBI film casted5 and sol-gel based6 membranes) and phosphoric acid loading will be discussed. [1] T. J. Schmidt and J. Baurmeister, ECS Trans., 3, 861–869 (2006). [2] H. R. Kunz, ECS Trans., 11, 1447–1460 (2007). [3] H. R. Kunz, Electrochem. Soc. Proc., 99-14, 191–207 (1999). [4] S. H. Eberhardt, M. Toulec, F. Marone, M. Stampanoni, F. N. Büchi, and T. J. Schmidt, J. Electrochem. Soc., 162, F310–F316 (2015). [5] R. Savinell, E. Yeager, D. Tryk, U. Landau, J. Wainright, D. Weng, K. Lux, M. Litt, and C. Rogers, J. Elelectrochemical Soc., 141, L46–L48 (1994). [6] L. Xiao, H. Zhang, T. Jana, E. Scanlon, R. Chen, E.-W. Choe, L. S. Ramanathan, S. Yu, and B. C. Benicewicz, Fuel Cells, 5, 287–295 (2005).

Performance Degradation Tests of Phosphoric Acid Doped Polybenzimidazole Membrane Based High Temperature Polymer Electrolyte Membrane Fuel Cells

Degradation tests of two phosphoric acid (PA) doped polybenzimidazole (PBI) membrane based high temperature polymer electrolyte membrane (HT-PEM) fuel cells were reported in this paper to investigate the effects of start/stop and the presence of methanol in the fuel to the performance degradation. Continuous tests with H2 and simulated reformate which was composed of H2, water steam and methanol as the fuel were performed on both single cells. 12-h-startup/12-h-shutdown dynamic tests were performed on the first single cell with pure dry H2 as the fuel and on the second single cell with simulated reformate as the fuel. Along with the tests electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy (EIS) were employed to study the degradation mechanisms of the fuel cells. Both single cells showed an increase in the performance in the H2 continuous tests, because of a decrease in the oxygen reduction reaction (ORR) kinetic resistance probably due to the redistribution of PA between the membrane and electrodes. EIS measurement of first fuel cell during the start/stop test showed that the mass transfer resistance and ohmic resistance increased which can be attributed to the corrosion of carbon support in the catalyst layer and degradation of the PBI membrane. During the continuous test with simulated reformate as the fuel the ORR kinetic resistance and mass transfer resistance of both single cells increased. The performance of the second single cell experienced a slight decrease during the start/stop test with simulated reformate as the fuel.

Temperature dependence of CO poisoning in high-temperature proton exchange membrane fuel cells with phosphoric acid-doped polybenzimidazole membranes

In this study, we numerically investigate the effects of CO poisoning on the performance of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) with phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes. A CO poisoning model that rigorously accounts for the adsorption/desorption processes of CO and hydrogen on the anode Pt catalysts and for the subsequent electrochemical oxidation is incorporated into a comprehensive three-dimensional HT-PEMFC model developed in our previous study. The numerical simulations are conducted under different operating temperatures and various percentages of CO in the anode feed stream. The experimental CO poisoning data presented in the literature are used for the model validation study. In particular, a major focus is placed on analyzing the reaction order of the hydrogen adsorption kinetics on the Pt catalyst sites; the reaction order is strongly influenced by the fractional CO coverage on the Pt sites and was mainly assumed to be either first or second order in the previous studies. The simulation results generally agree well with the experimental data, successfully capturing the high poisoning loss at the high current density and/or low operating temperature. The detailed numerical analysis further elucidates the key CO poisoning mechanisms and characteristics in HT-PEMFC at various operating temperatures.

Transmission electron microscopy study of silica reinforced polybenzimidazole membranes

The incorporation of silica nanostructures in polymer membranes is beneficial for the chemical, mechanical and the life time stability of high temperature proton exchange membrane fuel cells. Since the properties are closely related to the morphology and chemical composition of the constituents, their detailed investigation is of importance. In this paper transmission electron microscopy is utilized to study cross-linked silica nanoparticles in phosphoric acid doped polybenzimidazole membranes, which are formed by an in-situ sol–gel reaction using varying reaction parameters. The results show that by an additional heating step before membrane casting, several hundred nanometer large, ellipsoidal silica particles are formed in the membrane. Small, elongated nanoparticles with sizes in the range of about 20nm are present in all samples independent of the heat treatment. In-depth energy dispersive X-ray spectroscopy and electron energy loss spectroscopy analysis proved the homogeneous distribution and composition of the amorphous silica nanoparticles and revealed that no enrichment occurs at the interface between nanoparticles and polymer. The ex-situ membrane investigations reveal a higher chemical and mechanical stability for membranes containing the large, ellipsoidal particles. However, the membrane with only smaller silica nanoparticles shows a better degradation behavior than the membrane with large particles.

Gas-phase electrocatalytic reduction of carbon dioxide using electrolytic cell based on phosphoric acid-doped polybenzimidazole membrane

Carbon dioxide transformation to fuels or chemicals provides an attractive approach for its utilization as feedstock and its emission reduction. Herein, we report a gas-phase electrocatalytic reduction of CO2 in an electrolytic cell, constructed using phosphoric acid-doped polybenzimidazole (PBI) membrane, which allowed operation at 170 °C. Pt/C and PtMo/C with variable ratio of Pt/Mo were studied as the cathode catalysts. The results showed that PtMo/C catalysts significantly enhanced CO formation and inhibited CH4 formation compared with Pt/C catalyst. Characterization by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy revealed that most Mo species existed as MoO3 in PtMo/C catalysts and the interaction between Pt and MoOx was likely responsible for the enhanced CO formation rate although these bicomponent catalysts in general had a larger particle size than Pt/C catalyst.

A CO poisoning model for high-temperature proton exchange membrane fuel cells comprising phosphoric acid-doped polybenzimidazole membranes

We present herein a carbon monoxide (CO) poisoning model for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) comprising phosphoric acid-doped polybenzimidazole membranes. In the model, the adsorption/desorption processes of CO and hydrogen on the anode Pt catalysts, and subsequent electrochemical oxidation are rigorously considered. The CO poisoning model is incorporated into a previously developed three-dimensional HT-PEMFC model, and then both numerical simulations and experimental measurements are conducted for a comparative study. The simulation results generally agree well with the experimental data under various current density conditions, highlighting that it is necessary to consider the variable hydrogen adsorption kinetics as a function of CO fractional coverage to achieve better agreement with the experimental data. Furthermore, detailed key contours for hydrogen/CO fractional coverage, anode overpotential, temperature, and current density are provided to derive greater insights into the CO poisoning mechanisms and characteristics in HT-PEMFCs.

One-dimensional modeling and analysis for performance degradation of high temperature proton exchange membrane fuel cell using PA doped PBI membrane

Abstract The cell degradation in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) using phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes is one of the most important challenges to be improved for its early commercialization since the reported lifetime of the HT-PEM is too deficient to meet the commercially required target. The aim of this paper is to numerically investigate the effects of operating conditions on the performance degradation of HT-PEMFC. In order to resolve this problem, a one-dimensional model for the HT-PEM with PA-doped PBI membrane has been developed based on the hybrid modeling method and the model validated with HT-PEMFC polarization curves obtained by ourselves. Using the developed model, activation and ohmic overpotentials at each layer could be analyzed through membrane electrode assembly (MEA). The simulation results demonstrate that the durability of HT-PEMFC is changeable with respect to current density and PA doping level. In addition, the lifetimes have been predicted according to different operating conditions and thus we found whether any conditions are helpful to improve lifetime of HT-PEM or not.

Effect of treatment temperature on the performance of RuO2 anode electrocatalyst for high temperature proton exchange membrane water electrolysers

Ruthenium oxide catalysts were prepared by a sol–gel technique and calcined at different temperatures e.g., 400 °C, 500 °C and 600 °C. The catalysts performance for the oxygen evolution reaction was studied using cyclic voltammetry and their performance in a high temperature proton exchange membrane water electrolyser (PEMWE) examined. Physio-chemical characterization was carried out to study the thermal stability, oxygen-metal bond formation, crystallinity phase and crystallite size, particle size and elemental analysis by TGA, FTIR, XRD, TEM and EDX respectively. The electrolyte used for electrochemical characterisation was 1.0 M H3PO4 and 0.5 M H2SO4. Additionally, the effect of calcination and electrolyte temperature on oxygen evolution reaction of RuO2 catalysts was studied and the apparent activation energy was determined using chronoamperometry. The prepared RuO2 were tested as anode catalyst in PEMWE in the temperature range of 120–150 °C using phosphoric acid doped polybenzimidazole membrane electrolyte. The physio-chemical and electrochemical characterization results indicate that RuO2 calcined at 500 °C gave the best performance with a current density of 0.875 A cm−2 at 1.8 V in a PEMWE operated at 150 °C.

Catalyst Degradation in High Temperature Proton Exchange Membrane Fuel Cells Based on Acid Doped Polybenzimidazole Membranes

AbstractDegradation of carbon supported platinum catalysts is a major failure mode for the long term durability of high temperature proton exchange membrane fuel cells based on phosphoric acid doped polybenzimidazole membranes. With Vulcan carbon black as a reference, thermally treated carbon black and multi‐walled carbon nanotubes were used as supports for electrode catalysts and evaluated in accelerated durability tests under potential cycling at 150 °C. Measurements of open circuit voltage, area specific resistance and hydrogen permeation through the membrane were carried out, indicating little contribution of the membrane degradation to the performance losses during the potential cycling tests. As the major mechanism of the fuel cell performance degradation, the electrochemical active area of the cathodic catalysts showed a steady decrease in the cyclic voltammetric measurements, which was also confirmed by the post TEM and XRD analysis. A strong dependence of the fuel cell performance degradation on the catalyst supports was observed. Graphitization of the carbon blacks improved the stability and catalyst durability though at the expense of a significant decrease in the specific surface area. Multi‐walled carbon nanotubes as catalyst supports showed further significant improvement in the catalyst and fuel cell durability.

Oxidative degradation of acid doped polybenzimidazole membranes and fuel cell durability in the presence of ferrous ions

Phosphoric acid doped polybenzimidazole membranes have been explored as proton exchange membranes for high temperature polymer electrolyte membrane fuel cells. Long-term durability of the membrane is of critical concern and has been evaluated by accelerated degradation tests under Fenton conditions. In this study effects of phosphoric acid and ferrous ions were investigated by measurements of the weight loss, intrinsic viscosity and size exclusion chromatography (SEC) of the polymer membranes. Ferrous ions resulted in, as expected, catalytic formation of peroxide radicals and hence the accelerated polymer degradation in terms of weight loss and molecular weight decrease. The presence of phosphoric acid as an inevitable dopant of the membranes, on the other hand, significantly impeded the membrane degradation by means of metal ion complexing, decreased pH, and acid–base interactions with the amino groups of the polymer. Fuel cell durability tests with contaminations of ferrous ions did show considerable performance degradation, however, primarily due to the catalyst deterioration rather than the membrane degradation.

Performance of the HT-PEM Membrane Electrode Assembly

High Temperature Polymer Electrolyte Membrane (HT-PEM) Fuel Cells provide an attractive alternative to Low Temperature (LT) PEM fuel cells with respect to fuel flexibility, simplified cooling system and balance of plant, higher value waste heat and no need for humidification of reactants. Polybenzimidazole (PBI) membranes doped with phosphoric acid have been demonstrated to be the most successful system to achieve high temperature operation of HT-PEM fuel cells. HT-PEM fuel cells face a number of challenges related to the performance that needs to be addressed. In this work we present the recent PBI membrane and single-cell fuel cell testing results with respect to performance and durability. In addition, we present results on the in-plane and through-plane ionic conductivity of phosphoric acid-doped PBI membrane. Lastly, we show preliminary results on post-mortem cell characterizations which are the key for understanding the breakdown mechanisms and maximizing cell performance and lifetime.

Control and experimental characterization of a methanol reformer for a 350 W high temperature polymer electrolyte membrane fuel cell system

This work presents a control strategy for controlling the methanol reformer temperature of a 350 W high temperature polymer electrolyte membrane fuel cell system, by using a cascade control structure for reliable system operation. The primary states affecting the methanol catalyst bed temperature is the water and methanol mixture fuel flow and the burner fuel/air ratio and combined flow. An experimental setup is presented capable of testing the methanol reformer used in the Serenergy H3 350 Mobile Battery Charger; a high temperature polymer electrolyte membrane (HTPEM) fuel cell system. The experimental system consists of a fuel evaporator utilizing the high temperature waste gas from the cathode air cooled 45 cell HTPEM fuel cell stack. The fuel cells used are BASF P1000 MEAs which use phosphoric acid doped polybenzimidazole membranes. The resulting reformate gas output of the reformer system is shown at different reformer temperatures and fuel flows, using the implemented reformer control strategy. The gas quality of the output reformate gas is of HTPEM grade quality, and sufficient for supporting efficient and reliable HTPEM fuel cell operation with CO concentrations of around 1% at the nominal reformer operating temperatures. As expected increasing temperatures also increase the dry gas CO content of the reformate gas and decreases the methanol slip. The hydrogen content of the gas was measured at around 73% with 25% CO2.

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.

Phosphoric acid doped polybenzimidazole membrane for high temperature PEM fuel cell

AbstractIn the present study, phosphoric acid doped polybenzimidazole (PBI) membranes with potential applications in high temperature PEM fuel cells were investigated. PBI was synthesized by polycondensation of 3,3′‐diaminobenzidine and isophtalic acid in polyphosphoric acid. The formation of PBI was validated by H1‐NMR and elemental analysis. PBI membranes were prepared by solution casting method and immersed in phosphoric acid in order to provide ionic conductivity. The phosphoric acid doped membranes were used to prepare membrane electrode assemblies (MEA) for PEMFC operation. Gas diffusion layer (GDL) spraying method was used to prepare the electrodes. In order to determine optimum electrode structure, the effect of electrode preparation technique on fuel cell performances was studied. Two methods were applied in which the binder differs in the catalyst ink. In the first method, 5 wt % PBI solution was used as the binder. In the second method, polyvinylidene fluoride (PVDF) was used in addition to PBI as the binder in the catalyst ink. The MEA were tested in a single cell operating at 100°C without need for humidification of reactant gases. The observed maximum power output was increased considerably from 0.015 to 0.072 W/cm2 at 150°C when the binder of the catalyst was changed from PBI to PBI and PVDF mixture (PVDF : PBI = 3 : 1). A single cell was operated up to 160°C and the power outputs of 0.032 and 0.063 W/cm2 were obtained at operating temperatures of 125 and 160°C, respectively. The PVDF : PBI ratio was 1 : 3 in the catalyst ink at both temperatures. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Intermediate Temperature Fuel Cell and Oxygen Reduction Studies With Carbon-Supported Platinum Alloy Catalysts in Phosphoric Acid Based Systems

The oxygen reduction activities of platinum and platinum alloy catalysts were evaluated at temperatures up to 150 °C in phosphoric acid solution. The oxygen reduction currents and open circuit potentials were measured using chronoamperometry with double potential steps to eliminate the effects of double layer charging currents and metal deactivationcould be eliminated effectively. Based on the mass activity at 0.7 V versus Ag/AgCl, the commercial PtNi catalyst showed higher performances than commercial Pt catalyst at 150 °C. The commercial PtCo catalyst showed high activities at 90 °C and 120 °C. Intermediate temperature fuel cells based on phosphoric acid doped polybenzimidazole membranes were tested with the alloy cathode catalysts. In the case of Pt–Fe alloy an enhanced performance was achieved in comparison to that with Pt carbon catalysts.

Sub-Tg Macromolecular Motions in Phosphoric Acid Doped Polybenzimidazole Membranes for High Temperature Fuel Cell Applications

Broadband dielectric spectroscopy was used to identify sub-Tg macromolecular motions for undoped and phosphoric acid - doped polybenzimidazole (PBI) films. Loss permittivity vs. frequency spectra showed three secondary relaxation processes labeled beta, gamma and delta in order of decreasing temperature for undoped PBI. As acid content in the membrane increased, peak broadening was observed for all relaxations while the peak positions shift closer to each other. The loss and storage permittivity vs. frequency curves for PBI membranes are displaced upwards with increased acid content which reflects increasingly greater polarizability due to increasingly greater populations of polar phosphoric acid groups. The high storage permittivity values observed for acid doped samples in the low frequency regime could be due to membrane | electrode interfacial polarization relaxation. The Kramers-Krönig integral transform was used to calculate pure loss permittivity relaxation peaks from experimental dc-free storage permittivity curves to eliminate obscuring dc conductivity contribution.

Multinuclear NMR Study of the Effect of Acid Concentration on Ion Transport in Phosphoric Acid Doped Polybenzimidazole Membranes

not Available.

Demonstration of a 20 W class high-temperature polymer electrolyte fuel cell stack with novel fabrication of a membrane electrode assembly

Acid-doped polybenzimidazole (PBI) membrane and polytetrafluoroethylene (PTFE)-based electrodes are used for the membrane electrode assembly (MEA) in high-temperature polymer electrolyte fuel cells (HTPEFCs). To find the optimum PTFE content for the catalyst layer, the PTFE ratio in the electrodes is varied from 25 to 50 wt%. To improve the performance of the electrodes, PBI is added to the catalyst layer. With a weight ratio of PTFE to Pt/C of 45:55 (45 wt% PTFE in the catalyst layer), the fuel cell shows good performance at 150 °C under non-humidified conditions. When 5 wt% PBI is added to the electrodes, performance is further improved (250 mA cm −2 at 0.6 V). Our 20 W class HTPEFC stack is fabricated with a novel MEA. This MEA consists of 8 layers (1 phosphoric acid-doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-diffusion media, 2 gas-sealing gaskets). The sub-gasket mitigates the destruction of a highly acid-doped PBI membrane and provides long-term durability to the fuel cell stack. The stack operates for 1200 h without noticeable cell degradation.

Multinuclear NMR Study of the Effect of Acid Concentration on Ion Transport in Phosphoric Acid Doped Polybenzimidazole Membranes

not Available.