Phosphoric Acid Doped Polybenzimidazole Membranes Research Articles

Improved performance of phosphonated carbon nanotube–polybenzimidazole composite membranes in proton exchange membrane fuel cells

Development of thermally stable polymer electrolyte membranes with higher proton conductivity as well as mechanical stability is a key challenge in commercializing PEM fuel cells operating above 100 °C. Polybenzimidazole membranes are one of the promising candidates in this category although with limited mechanical stability and moderate proton conductivity. Here the incorporation of functionalized MWCNT is shown to increase both these key parameters of the polybenzimidazole membranes. Further, formation of a domain like structure after the incorporation of phosphonated MWCNTs (P-MWCNTs) in phosphoric acid doped polybenzimidazole membranes is demonstrated. The enhanced performance has been attributed to the formation of proton conducting networks that formed along the sidewalls of P-MWCNTs with a domain size of 17 nm as estimated from the small angle X-ray scattering measurements. Membrane electrode assembly (MEA) impedance measurements further reveal that the activation energy of oxygen reduction reaction (ORR) reduced for the composite membranes with enhanced proton conductivity. In addition, the mechanical strength measurements reveal a significant improvement in the yield strength and ultimate strength. Also, the mechanical strength of the composite membrane has been increased significantly as indicated by the improvement in the ultimate strength from 65 MPa to 100 MPa for the pristine and composite membranes, respectively. The optimum loading of P-MWCNTs is found to be 1% as inferred from the polarization measurements carried out using pure hydrogen and oxygen. Thus, this study provides a unique opportunity to tune the properties of polymer electrolytes to prepare application oriented hybrid membranes using CNTs with tailor-made functional groups.

Composite membranes of polybenzimidazole and caesium-salts-of-heteropolyacids for intermediate temperature fuel cells

Inorganic–organic composite electrolyte membranes were fabricated from Cs substituted heteropolyacids (CsHPAs) and polybenzimidazole (PBI) for application in intermediate temperature hydrogen fuel cells. Four caesium salts of heteropolyacid, (CsXH3−XPMo12O40 (CsPOMo), CsXH3−XPW12O40 (CsPOW), CsXH4−XSiMo12O40 (CsSiOMo) and CsXH4−XSiW12O40 (CsSiOW)), were used to form composite membranes with PBI. The membranes were characterized by using SEM, FTIR and XRD. The CsHPA powders were nano-sized as shown in the XRD and SEM data. The CsHPA/PBI composite membranes loaded with H3PO4 had high conductivity, greater than that of a phosphoric acid loaded PBI membrane. The conductivity increased with an increase in the percentage of the powder in the composite. The 30% CsPOMo/PBI/H3PO4 exhibited a conductivity of 0.12 S cm−1 under anhydrous conditions, although its mechanical strength was the poorest, but still promising with a value 40 MPa. The performance of the hydrogen fuel cell with composite membranes was better than that with a phosphoric acid-doped PBI membrane under the same conditions. The CsPOMo gave the best power density of around 0.6 W cm−2 with oxygen at atmospheric pressure.

Three-dimensional non-isothermal modeling of carbon monoxide poisoning in high temperature proton exchange membrane fuel cells with phosphoric acid doped polybenzimidazole membranes

The performance of proton exchange membrane fuel cell (PEMFC) degrades when carbon monoxide ( CO) is present in the supplied fuel, which is referred to as CO poisoning. Even though the high temperature PEMFC (HT-PEMFC) with a typical operating temperature range from 100 °C to 200 °C features higher CO tolerance than the conventional PEMFC operating at lower than 100 °C, the performance degradation of HT-PEMFC is still significant with high CO concentrations (e.g. ⩾0.5% CO by volume at 130 °C) in the supplied fuel. In this study, a CO poisoning model is developed for HT-PEMFCs with phosphoric acid doped polybenzimidazole (PBI) membranes. The present three-dimensional non-isothermal model compares well with published experimental data at various operating temperatures and CO concentrations in the supplied fuel. It is found that the CO adsorption/desorption processes follow Langmuir kinetics in HT-PEMFCs instead of the well-known Temkin kinetics in conventional PEMFCs. The results indicate that a HT-PEMFC can operate with sufficiently good performance at 130 °C or higher with hydrogen gas produced by methanol reforming with selective oxidation process, and at 160 °C or higher even without the selective oxidation process. At high current densities, it is also observed that severe performance degradation due to CO poisoning only occurs if the volume averaged hydrogen coverage is lower than 0.1 in the anode catalyst layer (CL).

A Three‐Dimensional Non‐isothermal Model of High Temperature Proton Exchange Membrane Fuel Cells with Phosphoric Acid Doped Polybenzimidazole Membranes

AbstractHigh temperature proton exchange membrane fuel cells (HT‐PEMFCs) with phosphoric acid doped polybenzimidazole (PBI) membranes have gained tremendous attentions due to its attractive advantages over conventional PEMFCs such as faster electrochemical kinetics, simpler water management, higher carbon monoxide (CO) tolerance and easier cell cooling and waste heat recovery. In this study, a three‐dimensional non‐isothermal model is developed for HT‐PEMFCs with phosphoric acid doped PBI membranes. A good agreement is obtained by comparing the numerical results with the published experimental data. Numerical simulations have been carried out to investigate the effects of operating temperature, phosphoric acid doping level of the PBI membrane, inlet relative humidity (RH), stoichiometry ratios of the feed gases, operating pressure and air/oxygen on the cell performance. Numerical results indicate that increasing both the operating temperature and phosphoric acid doping level are favourable for improving the cell performance. Humidifying the feed gases at room temperature has negligible improvement on the cell performance, and further humidification is needed for a meaningful performance enhancement. Pressurising the cell and using oxygen instead of air all have significant improvements on the cell performance, and increasing the stoichiometry ratios only helps prevent the concentration loss at high current densities.

Thermal properties of phosphoric acid-doped polybenzimidazole membranes in water and methanol–water mixtures

The thermal properties of phosphoric acid-doped poly[2-2′-(m-phenylene)-5-5′ bi-benzimidazole] (PBI) and poly[2,5-benzimidazole] (ABPBI) membranes, ionomeric materials with promising properties to be used as electrolytes in direct methanol and in high temperature polymer electrolyte membrane (PEM) fuel cells, were studied by means of differential scanning calorimetry (DSC) technique in the temperature range from −145 °C to 200 °C. The DSC scans of samples equilibrated in water at different relative humidities (RH) and in liquid water–methanol mixtures were analyzed in relation to glass transition, water crystallization/melting and solvent desorption in different temperature regions. The thermal relaxation observed in the very low temperature region could be ascribed to the glass transition of the H 3PO 4–H 2O mixture confined in the polymeric matrix. After cooling the samples up to −145 °C, frozen water was detected in PBI and ABPBI at different RH, although at 100% RH less amount of water had crystallized than that observed in Nafion membranes under the same conditions. Even more important is the fact that the freezing degree of water is much lower in ABPBI membranes equilibrated in liquid water–methanol mixtures than that observed for PBI and, in a previous study, for Nafion. Thus, apart from other well known properties, acid-doped ABPBI emerges as an excellent ionomer for applications in direct methanol fuel cells working in cold environments.

The effect of electrode parameters on performance of a phosphoric acid-doped PBI membrane fuel cell

A study of the effect of electrode parameters in a phosphoric acid-doped polybenzimidazole (PBI) membrane fuel cell is reported. The influence of phosphoric acid and PBI content are described. High levels of PBI in the catalyst layer did not enhance electrode layer performance for both hydrogen oxidation or oxygen reduction. High levels of doping with phosphoric acid in the anode catalyst layer were beneficial to fuel cell performance. Anode polarisation data confirmed that PBI fuel cells could operate at 175 °C with carbon monoxide contents in the feed gas of 10% in volume. There was an optimum amount of acid doping that gave good performance for oxygen reduction in the cathode layer. The effect of a perfluorinated surfactant improved the oxygen reduction behaviour in the catalyst layer. The carbon in catalyst support and it's interaction with the electrolyte have a large influence on cell performance.

A rapid break-in procedure for PBI fuel cells

Different break-in procedures for phosphoric acid doped polybenzimidazole membrane fuel cells (PBI-FC) have been studied, including voltage cycling and the use of constant current at different temperatures. A rapid break-in procedure consisting of high-temperature constant current break-in and relaxation has been developed. The break-in procedure is considered suitable for contamination and performance studies.

Novel Porous Bipolar Plates for Use with Acid-Doped PBI Fuel Cells at Elevated Temperatures

Light-weight, porous bipolar plates were developed that are ideally suited for use with phosphoric acid-doped polybenzimidazole membrane fuel cells. The porous nature of the bipolar plates allows for a reservoir of acid to remain in intimate contact with the MEA to replenish lost acid. Single-cell and 4-cell stack data show good repeatability, both in flow distribution and performance scans. Over 1000 hour endurance data show performance losses are due to MEA decay, and not bipolar plate corrosion.

A high conductivity Cs 2.5H 0.5PMo 12O 40/polybenzimidazole (PBI)/H 3PO 4 composite membrane for proton-exchange membrane fuel cells operating at high temperature

A high conductivity composite proton-exchange membrane Cs 2.5H 0.5PMo 12O 40 (CsPOM)/polybenzimidazole (PBI) for use in hydrogen proton-exchange fuel cells has been prepared. The CsPOM composite membrane is insoluble in water. The composite membrane doped with H 3PO 4 showed high-proton conductivity (>0.15 S cm −1) and good thermal stability. 31P NMR analysis has suggested the formation of a chemical bond between the CsPOM and PBI in the composite membrane. The performance of the membrane in a high-temperature proton-exchange membrane fuel cell (PEMFC) fueled with hydrogen was better than that with a phosphoric acid-doped PBI membrane under the same conditions and at temperatures greater than 150 °C. The CsPOM/PBI composite would appear to be a promising material for high-temperature PEMFC applications.

Kinetic characterization of a Pt–Ni/C catalyst with a phosphoric acid doped PBI membrane in a proton exchange membrane fuel cell

The performance of a phosphoric acid doped polybenzimidazole (PBI) membrane, with Pt and Pt–Ni/C catalysts on the anode and cathode, respectively, was experimentally determined at 160 °C using neat H 2 and O 2. The resulting current voltage relation was then compared to a performance curve calculated from previously established values for the exchange current density and activation energy of the oxygen reduction reaction (ORR) on Pt–Ni/C. An overall voltage loss >200 mV, regardless of current density, was observed for the MEA relative to the predicted performance, implying about two orders of magnitude decrease in the exchange current density for the ORR. The reduction in exchange current density was attributed to anion (H 2PO 4 −) adsorption.

A MEMS-based reformed methanol fuel cell for portable power

A reformed methanol fuel cell system is described. The use of a microfluidic fuel processor enables component scaling and integration sufficient to achieve power sources in the 2–10 W regime that are competitive in size and energy density in comparison to alternative power sources. While carbon monoxide tolerance of proton conducting membranes has typically limited the performance of reformed methanol fuel cells, phosphoric-acid-doped polybenzimidazole (PBI) membranes have been tested that exhibit no degradation for carbon monoxide >2% mole fraction. Further benefits of the PBI membrane include operating temperature of 150–200 °C, and no need for water to assist protonic conduction. As a result, a chemically and thermally robust fuel cell power source is realized. Results of methanol steam reforming and catalytic combustor heating elements formed in a silicon MEMS platform, and PBI membrane performance with reformate fuel feed will be discussed.

Physicochemical properties of phosphoric acid doped polybenzimidazole membranes for fuel cells

Polybenzimidazole (PBI) membranes have been prepared with different molecular weights. The water and acid swelling, mechanical strength, gas permeability and proton conductivity were studied for the pristine and acid doped PBI membranes. When doped with 5 mol of phosphoric acid per mole repeat unit of the polymer, a level necessary to obtain high enough proton conductivity for fuel cell uses, the polymer membrane exhibits a volume swelling by 118%, resulting in separation of the polymer backbones. The separation in turn reduces the mechanical strength of the membrane especially at high temperatures. Another consequence is the increased H 2 and O 2 permeability through the membrane. In the temperature range from 120 to 180 °C, the hydrogen permeability was found to be 1.6–4.3 × 10 −17 and 1.2–4.0 × 10 −15 mol cm cm −2 s −1 Pa −1 for pristine and acid doped PBI membranes, respectively, while for oxygen it was 5.0–10 × 10 −19 and 3.0–9.4 × 10 −16 mol cm cm −2 s −1 Pa −1, respectively. High molecular weights of the polymers improve the mechanical strength but have little influence on the proton conductivity of the membranes.

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation, Characterization and Fuel Cell Demonstration

AbstractProton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.

The CO Poisoning Effect in PEMFCs Operational at Temperatures up to 200°C

The CO poisoning effect on carbon-supported platinum catalysts (at a loading of 0.5 mg per electrode) in polymer electrolyte membrane fuel cells (PEMFCs) has been investigated in a temperature range from 125 to 200°C with the phosphoric acid-doped polybenzimidazole membranes as electrolyte. The effect is very temperature-dependent and can be sufficiently suppressed at elevated temperature. By defining the CO tolerance as a voltage loss less than 10 mV, it is evaluated that 3% CO in hydrogen can be tolerated at current densities up to at 200°C, while at 125°C 0.1% CO in hydrogen can be tolerated at current densities lower than For comparison, the tolerance is only 0.0025% CO (25 ppm) at 80°C at current densities up to The relative anode activity for hydrogen oxidation was calculated as a function of the CO concentration and temperature. The effect of in hydrogen was also studied. At 175°C, 25% in the fuel stream showed only the dilution effect. © 2003 The Electrochemical Society. All rights reserved.

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities. For this purpose polybenzimidazole (PBI) membranes have been prepared and H3PO4-doped in a doping range from 300 to 1600 mol %. Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient. Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level. At 160 °C a conductivity as high as 0.13 S cm−1 is obtained for membranes of high doping levels. Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties. At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide.