High Polymer Content 2,5‐Pyridine‐Polybenzimidazole Copolymer Membranes with Improved Compressive Properties

Three series of polybenzimidazole (PBI) random copolymers (2,5‐pyridine‐r‐meta‐PBI, 2,5‐pyridine‐r‐para‐PBI, and 2,5‐pyridine‐r‐2OH‐PBI) were synthesized and cast into phosphoric acid (PA) doped membranes using the PolyPhosphoric Acid (PPA) Process. Copolymer composition was adjusted using co‐monomers that impart high and low solubility characteristics to simultaneously control overall copolymer solubility and gel membrane stability. Measured under a static compressive force at 180 °C, copolymer membranes generally exhibited decreased creep compliance with increasing polymer content. Within each series of copolymer membranes, increasing polymer contents proportionally reduced the phosphoric acid/polymer repeat unit (PA/PRU) ratios and their respective proton conductivities. Some copolymer membranes exhibited comparable fuel cell performances (up to 0.66 V at 0.2 A cm⁻² following break‐in) to para‐PBI (0.68 V at 0.2 A cm⁻²) and equal to 3,5‐pyridine‐based high solids membranes. Furthermore, 2,5‐pyridine copolymer membranes maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm⁻² for over 8600 h under steady‐state operation conditions. Phosphoric acid loss was monitored during long‐term studies and demonstrated acid losses as low as 5.55 ng cm⁻² h⁻¹. The high‐temperature creep resistance and long‐term operational stabilities of the 2,5‐pyridine copolymer membranes suggest that they are excellent candidates for use in extended lifetime electrochemical applications.     

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

Proton 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.     

Synthesis and Characterization of a New Fluorine‐Containing Polybenzimidazole (PBI) for Proton‐Conducting Membranes in Fuel Cells

A new type of fluorine‐containing polybenzimidazole, namely poly(2,2′‐(2,2′‐bis(trifluoromethyl)‐4,4′‐biphenylene)‐5,5′‐bibenzimidazole) (BTBP‐PBI), was developed as a candidate for proton‐conducting membranes in fuel cells. Polymerization conditions were experimentally investigated to achieve high molecular weight polymers with an inherent viscosity (IV) up to 1.60 dl g^–1. The introduction of the highly twisted 2,2′‐disubstituted biphenyl moiety into the polymer backbone suppressed the polymer chain packing efficiency and improved polymer solubility in certain polar organic solvents. The polymer also exhibited excellent thermal and oxidative stability. Phosphoric acid (PA)‐doped BTBP‐PBI membranes were prepared by the conventional acid imbibing procedure and their corresponding properties such as mechanical properties and proton conductivity were carefully studied. The maximum membrane proton conductivity was approximately 0.02 S cm^–1 at 180 °C with a PA doping level of 7.08 PA/RU. The fuel cell performance of BTBP‐PBI membranes was also evaluated in membrane electrode assemblies (MEA) in single cells at elevated temperatures. The testing results showed reliable performance at 180 °C and confirmed the material as a candidate for high‐temperature polymer electrolyte membrane fuel cell (PEMFC) applications.     

Preparation and Characterization of Semi‐IPN Fluorine Containing Polybenzimidazole/Nafion Composite Membrane for Fuel Cells

A facile way to prepare semi‐interpenetrating polymer network (semi‐IPN) membrane which adopted 1,3‐benzenedisulfonyl azide (1,3‐BDSA) to crosslink with fluorine containing polybenzimidazole (Aliphatic‐16F‐PBI) in the Aliphatic‐16F‐PBI/Nafion composite membranes was proposed. By means of Fourier transformed infrared (FTIR) spectra analysis, the possible crosslinking reaction mechanism was investigated. Results suggested that 1,3‐BDSA molecule loses a nitrogen and forms nitrene upon heating. Then this nitrene reacts with C–H bond of Aliphatic‐16F‐PBI. Scanning electron microscope (SEM) images showed that the compatibility of PBI and Nafion improved while hexadecafluoro‐octyl groups were implanted into Aliphatic‐16F‐PBI molecule. The properties of Aliphatic‐16F‐PBI/Nafion composite membranes for fuel cell applications were determined through tests of gel fraction, thermogravimetry (TG), dimensional stability, mechanical property and proton conductivity. The gel fraction could reach 27.9% when 7.4% 1,3‐BDSA was added into the composite membranes. The proton conductivity of the semi‐IPN Aliphatic‐16F‐PBI/Nafion composite membranes could reach 0.69 × 10^–2 S cm^–1 at 120 °C at 100% relative humidity. Such high crosslink degree resulted in the improvement of the tensile strength, dimensional stability and chemical oxidative stability of semi‐IPN Aliphatic‐16F‐PBI/Nafion composite membranes. Nonetheless, it had little effect on the thermal stability.     

Improving of Micro Porous Layer based on Advanced Carbon Materials for High Temperature Proton Exchange Membrane Fuel Cell Electrodes

This work reports the study of four different carbon materials for their application as carbon material in microporous layers for high temperature proton exchange membrane fuel cells electrodes. The microporous layers were prepared with carbon black (a commercial one, Vulcan XC72), two different carbon nanofibers, CNF, (Ribbon and Platelet structure) and carbon nanospheres, all of them prepared in our lab. The microporous layers were characterized by XRD. The hydrophobicity, electrical conductivity, and permeability to different gases were also evaluated. The stability is an important issue to be overcome in the field of proton exchange membrane fuel cells. Thus, accelerated thermal and electrochemical degradation tests in phosphoric acid media were carried out to evaluate the stability of the different advanced materials tested under the same conditions. From all the performed essays, the carbon nanospheres were the best nano‐carbon materials because of the lower degradation degree shown by the microporous layer prepared with them and the good conductivity and permeability achieved, whereas CNF with a Platelet structure showed a low electrochemical stability due to their greater edge plane exposure which favors their corrosion.     

Preparation of Highly Active 40 wt.% Pt on Multiwalled Carbon Nanotube by Improved Impregnation Method for Fuel Cell Applications

40 wt.% Pt catalyst supported on multiwalled carbon nanotube (MWCNT) was successfully synthesized by using improved aqueous impregnation method. Catalysts were characterized by HR‐TEM, XRD, and X‐Ray photoelectron spectroscopy. Electrocatalytic performance of the catalyst materials was investigated by electrochemical half cell test measurements. According to the results of electrochemical measurements, synthesized Pt/MWCNT catalyst presented high electrochemical activity which is mostly due to high utilization of catalyst particles and good physical properties of MWCNT supporting material. It was revealed that, improved aqueous impregnation method has a satisfactory efficiency for production of Pt/MWCNT catalyst.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

Applying Shape‐Controlled Pt Nano‐dendrites Supported on Carbon for Membrane‐Electrode Assembly in a Proton Exchange Membrane Fuel Cell

Platinum nano‐dendrites of various sizes were applied as cathode catalysts in a proton exchange membrane fuel cell (PEMFC). The membrane electrode assembly (MEA) fabricated with Pt nano‐dendrites showed very high ORR activity at high potential ranges due to lower activation overpotential. But it showed the cell performance only comparable to commercial Pt/C due to Ohmic and mass transfer resistance at high current density ranges. These results demonstrate that both high intrinsic activity of the catalysts and the formation of three‐phase interface with efficient proton, electron, and mass transfer should be considered together when shape‐controlled metal nanoparticles are used as cathode catalysts in a PEMFC.     

Properties of Sulfonated Poly(Arylene Ether Sulfone)/Functionalized Carbon Nanotube Composite Membrane for High Temperature PEMFCs

Composite membranes are prepared using sulfonated poly (arylene ether sulfone) (SPAES) copolymers and the incorporation of functionalized multiwall carbon nanotubes (CNTs) for high temperature (120 °C) proton exchange membrane fuel cells (PEMFCs). The CNT is functionalized with sulfonated groups that are expected to support the improvement of water absorption and mechanical properties. The SPAES copolymers are synthesized with sulfonation degree (DS) = 0.5 and the sulfonated CNT (s‐CNT) is dispersed into the SPAES copolymers in varying ratios to fabricate the composite membranes. In this study, the proton conductivity, water uptake, and single cell test of the composite membrane are investigated for verifying the effects of the enhancement at high temperature and low humidity. The composite membrane containing 0.2 wt.% s‐CNT increases proton conductivity approximately 45% at 120 °C and 50% relative humidity and enhances the tensile strength by about 1.3 times compared to the pristine membrane. However, the proton conductivity and water absorption shows a decline when more than 0.2 wt.% s‐CNT is added in the composite membrane, due to the aggregation of the s‐CNT, which serves as a proton barrier. For the single cell test, the developed composite membrane with 0.2 wt.% s‐CNT exhibits a notable performance for high temperature PEMFC.     

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

A porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N₂, O₂, CO and under mixed reactant O₂/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.     

A Non‐isothermal Model of a Laboratory Intermediate Temperature Fuel Cell Using PBI Doped Phosphoric Acid Membranes

A two‐dimensional non‐isothermal model developed for a single intermediate temperature fuel cell with a phosphoric acid (PA) doped PBI membrane is developed. The model of the experimental cell incorporates the external heaters, and the body of the fuel cell. The catalyst layers were treated as spherical catalyst particles agglomerates with porous inter‐agglomerate space. The inter‐agglomerate space is filled with a mixture of electrolyte (hot PA) and PTFE. All the major transport phenomena are taken into account except the crossover of species through the membrane. This model was used to study the influence of two different geometries (along the channel direction and cross the channel direction) on performance. It became clear, through the performance analyses, that the predictions obtained by along the channel geometry did not represent the general performance trend, and therefore this geometry is not appropriate for fuel cell simulations. Results also indicate that the catalyst layer was not efficiently used, which leads to large temperature differences through the MEA.     

Amphiphilic Biopolyester‐Carbon Nanotube Anode Enhances Electrochemical Activities of Microbial Fuel Cell

Hydrophobic bacterial polyhydroxyalkanoates were rendered amphiphilic by grafting with poly(ethylene glycol) methacrylate, followed by compositing with carbon nanotubes. The polymer graft composite as an anode material encouraged superior biofilm surface growth; thus enhancing electrochemical activities in microbial fuel cells and resulting in higher current and power densities. The internal resistance of the cell was greatly reduced due to improved electron transfer from the biofilm to the anode.     

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

High 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.     

Effect of Anode off‐gas Recycling on Reforming of Natural Gas for Solid Oxide Fuel Cell Systems

The effect of anode off‐gas recycling (AOGR) on the characteristic performance of a natural gas reformer equipped with a precious metal catalyst is investigated experimentally. The reformer is operated both with synthetic AOGR gas and in steam reforming (SR) conditions. The characteristic performance in SR and AOGR mode are compared with equilibrium, and it is found that equilibrium is more readily achieved in AOGR mode. The reformer is used for extended periods of time (100–1,000 h) in conditions where carbon formation is thermodynamically possible to measure any changes in characteristic performance. No significant change in the performance is observed due to carbon formation or catalyst deactivation. The reformer could be successfully implemented in a 10 kW SOFC system with an anode off‐gas recycling loop.     

An Algorithm for On‐line Measurement of the Internal Resistance of Proton Exchange Membrane Fuel Cell

The internal resistance of proton exchange membrane fuel cell (PEMFC) system is difficult to measure on‐line due to its variation with time. The traditional electrochemical impedance spectroscopy (EIS) and its variants such as high frequency resistance (HFR) can be used to measure the resistance when the system is in steady state, but they fail in automotive applications where a change in speed or inclination modifications could lead to a sharp fluctuation in demand on power. In order to resolve this problem, a novel algorithm is proposed in this paper to estimate the resistance based on the alternating current (AC) impedance spectroscopy technique by adding an extra term to eliminate the errors caused by voltage variation or when the system is under unsteady state. Numerical simulations show that the proposed algorithm can not only accurately track the variation of the internal resistance, but is also robust against the noises caused by uncertainty and measurements.     

Evaluating the Effectiveness of Using Flexography Printing for Manufacturing Catalyst‐Coated Membranes for Fuel Cells

This study is an evaluation of the effectiveness of the flexography printing process for manufacturing catalyst‐coated membranes (CCMs) for use in proton exchange membrane fuel cells (PEMFCs). Flexography is a maskless and continuous process that is used in large‐scale production with water‐based inks to reduce the cost of production of PEMFC components. Unfortunately, water has undesirable effects on the Nafion® membrane: water wets the membrane surface poorly and causes the membrane to bulge outwards significantly. Membrane printability was improved by pre‐treating membrane samples by water immersion for short periods (<2 min). This pre‐treatment was used to control the membrane deformation before printing to limit the impact of the ink transfer. Water and ink drop deposition experiments were performed to estimate the liquid‐air‐Nafion® apparent contact angle and the locally induced membrane deformation. Despite the short immersion times used in the tests, the immersion pre‐treatment appeared to induce structural modifications that enhanced both the membrane wettability and the dimensional stability. Flexography printability tests were performed on these treated membranes and showed that the dimensional instability of the Nafion® membrane was the critical parameter for limiting the ink transfer. The immersion pre‐treatment improved the printability of the Nafion® membranes, which were used to fabricate cathodes that were tested in an operational fuel cell.     

Synthesis of Truncated‐Octahedral Pt–Pd Nanocrystals Supported on Carbon Black as a Highly Efficient Catalyst for Methanol Oxidation

A novel PtPd/C nanocrystals catalyst with truncated‐octahedral structure has been successfully prepared by ethylene glycol reduction to induce anisotropic growth in an isotropic medium by adding a small amount of Fe³⁺ species. Its structure, composition, and morphology are characterized by X‐ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy‐energy dispersive spectroscopy elemental maps, respectively. The electrochemical measurements demonstrate that the highly dispersed and uniform PtPd/C nanocrystals have excellent poisoning tolerance, significantly higher electrocatalytic activity and durability for the methanol oxidation, as compared to solid solution PtPd/C and Pt/C catalysts. This may be ascribed to the stepped atoms and dangling bonds, which served as active sites for breaking chemical bonds during oxidation–reduction reaction; the high density of preferred crystal planes of (111) facets greatly enhanced the oxidation of poisonous residues during reaction.     

Characterization of Solution‐Processed Double‐Walled Carbon Nanotube/Poly(vinylidene fluoride) Nanocomposites

Dispersion of CNTs in polymers can yield impressive property enhancements at low volume fractions, thus maintaining the inherent processability of the polymer. In particular, they can improve the electromechanical response of piezoelectric polymers by lowering the actuation voltage and increasing strain and stress response. In this work, piezoelectric PVDF and DWNTs are solution‐cast into films. SEM of fracture surfaces confirms good dispersion, and electrical conductivity measurements reveal a low percolation threshold (0.23 vol.‐%). The effect of CNTs on storage modulus, T_c, T_m and T_g of PVDF is studied. Electromechanical strain is observed at low actuation voltages, possibly due to enhanced local electric field in the presence of DWNTs.

     

Effect of Nickel Addition on Ceria‐Supported Platinum Catalysts for Medium‐Temperature Shift Reaction in Fuel Processors

Medium‐temperature shift reaction (MTS, 280–340 °C) has received much attention for use in fuel processors. In this study, bifunctional Pt‐Ni/CeO₂ catalysts were prepared by different Pt (0.1–0.5 %) and Ni (5–20 %) loadings, and investigated for MTS reaction. X‐ray diffraction, N₂ adsorption and temperature‐programmed reduction tests were used to characterize the prepared samples. The results showed that Pt‐Ni bimetallic catalysts have higher CO conversion in comparison to Pt/CeO₂ monometallic catalyst. Furthermore, the sequential synthesis method of Pt and Ni impregnation was preferred to the simultaneous one, which is due to the better Pt dispersion on catalytic surface. Steam to carbon ratio variations study showed the maximum CO conversion to be in the range of 4.5.