A polymer electrolyte fuel cell life test: 3 years of continuous operation

Implementation of polymer electrolyte fuel cells (PEMFCs) for stationary power applications requires the demonstration of reliable fuel cell stack life. One of the most critical components in the stack and that most likely to ultimately dictate stack life is the membrane electrode assembly (MEA). This publication reports the results of a 26,300 h single cell life test operated with a commercial MEA at conditions relevant to stationary fuel cell applications. In this experiment, the ultimate MEA life was dictated by failure of the membrane. In addition, the performance degradation rate of the cell was determined to be between 4 and 6 μV h⁻¹, at the operating current density of 800 mA cm⁻². AC impedance analysis and DC electrochemical tests (cyclic voltammetry and polarization curves) were performed as diagnostics during and on completion the test, to understand materials changes occurring during the test. Post mortem analyses of the fuel cell components were also performed.     

Performance and endurance of a high temperature PEM fuel cell operated on methanol reformate

This paper analyzes the effects of methanol and water vapor on the performance of a high temperature proton exchange membrane fuel cell (HT-PEMFC) at varying temperatures, ranging from 140 °C to 180 °C. For the study, a H₃PO₄ – doped polybenzimidazole (PBI) – based membrane electrode assembly (MEA) of 45 cm² active surface area from BASF was employed. The study showed overall negligible effects of methanol-water vapor mixture slips on performance, even at relatively low simulated steam methanol reforming conversion of 90%, which corresponds to 3% methanol vapor by volume in the anode gas feed. Temperature on the other hand has significant impact on the performance of an HT-PEMFC. To assess the effects of methanol-water vapor mixture alone, CO₂ and CO are not considered in these tests. The analysis is based on polarization curves and impedance spectra registered for all the test points. After the performance tests, endurance test was performed for 100 h at 90% methanol conversion and an overall degradation rate of −55 μV/h was recorded.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

Performance and efficiency of a DMFC using non-fluorinated composite membranes operating at low/medium temperatures

In order to increase the chemical/thermal stability of the sulfonated poly(ether ether ketone) (sPEEK) polymer for direct methanol fuel cell (DMFC) applications at medium temperatures (up to 130 °C), novel inorganic–organic composite membranes were prepared using sPEEK polymer as organic matrix (sulfonation degree, SD, of 42 and 68%) modified with zirconium phosphate (ZrPh) pretreated with n-propylamine and polybenzimidazole (PBI). The final compositions obtained were: 10.0 wt.% ZrPh and 5.6 wt.% PBI; 20.0 wt.% ZrPh and 11.2 wt.% PBI. These composite membranes were tested in DMFC at several temperatures by evaluating the current–voltage polarization curve, open circuit voltage (OCV) and constant voltage current (CV, 35 mV). The fuel cell ohmic resistance (null phase angle impedance, NPAI) and CO₂ concentration in the cathode outlet were also measured. A method is also proposed to evaluate the fuel cell Faraday and global efficiency considering the CH₃OH, CO₂, H₂O, O₂ and N₂ permeation through the proton exchange membrane (PEM) and parasitic oxidation of the crossover methanol in the cathode. In order to improve the analysis of the composite membrane properties, selected characterization results presented in [V.S. Silva, B. Ruffmann, S. Vetter, A. Mendes, L.M. Madeira, S.P. Nunes, Catal. Today, in press] were also used in the present study. The unmodified sPEEK membrane with SD = 42% (S42) was used as the reference material. In the present study, the composite membrane prepared with sPEEK SD = 68% and inorganic composition of 20.0 wt.% ZrPh and 11.2 wt.% PBI proved to have a good relationship between proton conductivity, aqueous methanol swelling and permeability. DMFC tests results for this membrane showed similar current density output and higher open circuit voltage compared to that of sPEEK with SD = 42%, but with much lower CO₂ concentration in the cathode outlet (thus higher global efficiency) and higher thermal/chemical stability. This membrane was also tested at 130 °C with pure oxygen (cathode inlet) and achieved a maximum power density of 50.1 mW cm⁻² at 250 mA cm⁻².     

Current density dependence on performance degradation of direct methanol fuel cells

A stability test on direct methanol fuel cells (DMFCs) was carried out at current densities of 100, 150, and 200 mA cm⁻². Each test lasted for 145 h in the three cases. X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy were used for analysis of the membrane electrode assemblies (MEAs). The maximum power densities were 93.9, 79.9, and 55.1% of the initial value after operation at 100, 150, and 200 mA cm⁻², respectively. A PtRu black catalyst with an original particle size of 3.3 nm was used for the anode electrode. For the MEAs operated at 100, 150, and 200 mA cm⁻², the PtRu particle sizes increased from the original size to 3.4, 3.9, and 4.2 nm, respectively, while a Pt black catalyst used for the cathode electrode did not change in size. Dissolution of the Ru was observed, and the ratio of (Pt:Ru) changed from (53:47) in the case of the fresh MEA, to (54:46), (56:44), and (73:27) for the MEAs after operation at 100, 150, and 200 mA cm⁻², respectively. The equivalent weight of the Nafion^TM membrane increased from a weight of 1264 g for a fresh membrane, to a weight of 1322, 1500, and 1945 g with the increases in operating current density to 100, 150, and 200 mA cm⁻², respectively.     

Effect of open circuit voltage on performance and degradation of high temperature PBI–H3PO4 fuel cells

The impact of open circuit voltage (OCV) on the performance and degradation of polybenzimidazole–phosphoric acid (PBI–H₃PO₄) fuel cells operated at 180 °C was investigated. The OCV showed an initial quick increase in the first few minutes, followed by a much slower increase, and peaked after about 35 min. It then started an exponential and monotonous decline. Along with the decline of OCV, the performance of the fuel cell also declined. Operating the fuel cell with a load of 0.2 A cm⁻² could temporarily boost the OCV and the fuel cell performance, but it could not recover the lost performance permanently. Electrochemical impedance spectroscopy (EIS) indicated significant loss of catalyst activity and increase in mass transport resistance due to the relatively high potential at OCV. X-ray diffraction (XRD) measurement showed that the cathode Pt crystallite size increased by as much as 430% after a total of 244.5 h of exposure to OCV.     

Proton conducting behavior of a novel polymeric gel membrane based on poly(ethylene oxide)-grafted-poly(methacrylate)

A novel proton conducting polymeric gel membrane that consists of poly(ethylene oxide)-grafted-poly(methacrylate) (PEO-PMA) with poly(ethylene glycol) dimethyl ether (PEGDE) as a plasticizer doped with aqueous phosphoric acid (H₃PO₄) has been prepared and its physicochemical properties were studied in detail. The ionic conductivity was dependent much on the concentration of H₃PO₄, the immersion time, and content of the plasticizer. This type of proton conducting polymeric gels shares not only good mechanical properties but also thermal stability. Maximum conductivities up to 2.6×10⁻² S cm⁻¹ at room temperature (25 °C) and 2.8×10⁻² S cm⁻¹ at 70 °C were obtained for the composition of the polymer matrix to the plasticizer as 35/65 (in mass) after the H₃PO₄ doping from the aqueous solution with 2.93 mol l⁻¹. FT-IR spectra showed that these high proton conductivities are attributed to the presence of excesses free H₃PO₄ in the polymeric gel in addition to the hydrogen-bonded H₃PO₄ to the polymer matrix.     

The effect of the MEA preparation procedure on both ethanol crossover and DEFC performance

In the present work, the changes of Nafion^®-115 membrane porosity in the presence of ethanol aqueous solutions of different concentrations were determined by weighing vacuum-dried and ethanol solution-equilibrated membranes. It was found that membrane porosity increases as ethanol concentration increases. Membrane electrode assemblies (MEAs) have been prepared by following both the conventional and the decal transfer method. The ethanol crossover through these two MEAs was electrochemically quantified by a voltammetric method. A 10 h stability test of direct ethanol fuel cell (DEFC) at a current density of 50 mA cm⁻² was carried out. It was found that the electrode preparation procedure has an obvious effect on ethanol crossover and direct ethanol fuel cell's performance and stability. The single DEFC test results showed that about 15 and 34% of the original peak power density was lost after 10 h of life test for the MEAs prepared by the decal transfer method and the conventional method, respectively. Electrochemical impedance spectrum (EIS) results of the MEAs showed that, in the case of the membrane electrode assembly prepared by the following decal transfer method, the internal cell resistance was almost the same, 0.236 Ω cm² before the life test and 0.239 Ω cm² after 10 h of life test, while the respective values for the membrane electrode assembly by the conventional method are 0.289 and 0.435 Ω cm². It is supposed that the improved cell performance with MEA by the decal transfer method could be resorted to both a better contact between the catalyst layer and the electrolyte membrane and higher catalyst utilization. Furthermore, based on the experimental results, the increased internal cell resistance and the degraded single DEFC performance could be attributed to the delamination of the catalyst layer from the electrolyte membrane.     

Investigating the effects of methanol-water vapor mixture on a PBI-based high temperature PEM fuel cell

This paper investigates the effects of methanol and water vapor on the performance of a high temperature proton exchange membrane fuel cell (HT-PEMFC). A H₃PO₄-doped polybenzimidazole (PBI) membrane electrode assembly (MEA), Celtec P2100 of 45 cm² of active surface area from BASF was employed. A long-term durability test of around 1250 h was performed, in which the concentrations of methanol-water vapor mixture in the anode feed gas were varied. The fuel cell showed a continuous performance decay in the presence of vapor mixtures of methanol and water of 5% and 8% by volume in anode feed. Impedance measurements followed by equivalent circuit fitting revealed that the effects were most significant for intermediate-high frequency resistances, implying that charge transfer losses were the most significant losses. Vapor mixture of 3% in feed, however, when introduced after operation at 8%, showed positive or no effect on the cell's performance in these tests.     

Dependence of high-temperature PEM fuel cell performance on Nafion® content

Operating a proton exchange membrane (PEM) fuel cell at elevated temperatures (above 100 °C) has significant advantages, such as reduced CO poisoning, increased reaction rates, faster heat rejection, easier and more efficient water management and more useful waste heat. Catalyst materials and membrane electrode assembly (MEA) structure must be considered to improve PEM fuel cell performance. As one of the most important electrode design parameters, Nafion^® content was optimized in the high-temperature electrodes in order to achieve high performance. The effect of Nafion^® content on the electrode performance in H₂/air or H₂/O₂ operation was studied under three different operation conditions (cell temperature (°C)/anode (%RH)/cathode (%RH)): 80/100/75, 100/70/70 and 120/35/35, all at atmospheric pressure. Different Nafion^® contents in the cathode catalyst layers, 15–40 wt%, were evaluated. For electrodes with 0.5 mg cm⁻² Pt loading, cell voltages of 0.70, 0.68 and 0.60 V at a current density of 400 mA cm⁻² were obtained at 35 wt% Nafion^® ionomer loading, when the cells were operated at the three test conditions, respectively. Cyclic voltammetry was conducted to evaluate the electrochemical surface area. The experimental polarization curves were analyzed by Tafel slope, catalyst activity and diffusion capability to determine the influence of the Nafion^® loading, mainly associated with the cathode.     

Effects of coal syngas and H2S on the performance of solid oxide fuel cells: Single-cell tests

The performance of single-cell planar solid oxide fuel cells using coal syngas, with and without hydrogen sulfide (H₂S), was studied. A state-of-the-art gas delivery system, data acquisition system, and test stand were designed and assembled for experimentation. All cells were tested at 850 °C with a constant current load of 14.3 A (current density of 0.20 A cm⁻²). The results from using syngas with no H₂S indicated no degradation after 290 h of operation. After immediately injecting CO (and water) in the H₂–N₂ mixture, there was a slight tendency of improving performance (power) and then the behavior remained steady. On the other hand, results for the test with syngas in the presence of H₂S (200–240 ppm) indicated good performance over 570 h (650 h total operation time) with 10–12.5% degradation. The results suggest these cells can be used for extended periods of time for syngas applications, and in the presence of H₂S the cells show no major degradation.     

Zn4Sb3(C7) powders as a potential anode material for lithium-ion batteries

The electrochemical properties of β-Zn₄Sb₃ and Zn₄Sb₃C₇ as new lithium-ion anode materials were investigated. The reversible capacities of the pure Zn₄Sb₃ alloy electrode and 100 h milled Zn₄Sb₃ in the first cycle reached 503 and 566 mA h/g, respectively, but the cycle stability of Zn₄Sb₃ whether milled or not were obviously bad. It was demonstrated that cycle stability of Zn₄Sb₃ could be largely improved by milling after mixing with graphite. It was shown that Zn₄Sb₃C₇ composite has a lithium-ion extraction capacity of 581 mA h/g at the first cycle and 402 mA h/g at 10th cycle.     

Polybenzimidazole and benzyl-methyl-phosphoric acid grafted polybenzimidazole blend crosslinked membrane for proton exchange membrane fuel cells

We synthesize polybenzimidazole (PBI; M_w = 1.65 × 10⁵ g mol⁻¹) and benzyl-methyl-phosphoric acid grafted PBI (PBI-BP; 24 mol% degree of grafting). We demonstrate that blending 20 to −40 wt.% PBI-BP in the PBI membrane enhances the H₃PO₄ doping level, proton conductivity, and mechanical strength. However, the membrane is highly dissolved in an 85 wt.% H₃PO₄ aqueous solution as the PBI-BP content in the blend membrane is larger than 50 wt.%. To prevent PBI-BP from being dissolved out of the blend membrane by the H₃PO₄ aqueous solution, we fabricated a PBI/PBI-BP/epoxy (8/2/1.23 by wt.) crosslinked membrane. The crosslinked membrane demonstrated good fuel cell performance and excellent stability after a 23 on/off (12 h on at 160 °C with a current density of 200 mA cm⁻² and 12 h off at room temperature) fuel cell cycle test with an unhumidified H₂/O₂.     

RuxCrySez electrocatalyst loading and stability effects on the electrochemical performance in a PEMFC

The present paper presents a study of the Ru_xCr_ySe_z chalcogenide electrocatalyst based on physical–chemical characterization through scanning electron (SEM), atomic force (AFM) microscopy and energy dispersion elemental analysis (EDS), thermal stability using differential scanning calorimeter (DSC), electrochemical kinetics towards the oxygen reduction reaction (ORR) in acid media by rotating ring-disk electrode (RRDE) and single and three-stack membrane-electrode assembly (MEA) performance as a function of catalyst loading (10%, 20% and 40% W from 0.2 to 2 mg cm⁻²). Results indicate an electrocatalyst with chemical composition of Ru₆Cr₄Se₅. AFM images showed 80–160 nm nanoparticle agglomerates. Good thermal stability of the cathode Ru₆Cr₄Se₅ was established after 100 h of continuous operation. The electrochemical kinetics study (RRDE) resulted in a electrocatalyst with high activity towards the ORR, preferentially proceeding via 4e⁻ charge transfer pathway towards water formation (i.e., O₂+4H⁺+4e⁻→2H₂O), with a maximum of 2.8% H₂O₂ formation at 25 °C. Finally, MEA tests revealed a maximum power density of 220 mW cm⁻² with a catalyst loading of 20 wt% at 1.6 mg cm⁻².     

Membrane electrode assemblies doped with H3PO4 for high temperature proton exchange membrane fuel cells

H₃PO₄ content plays a critical role in high temperature proton exchange membrane fuel cells (HT-PEMFC), as it is responsible for the majority of the conductivity of the key components under high temperature operation. The conductivities of commercial AB-PBI membranes doped by immersing in 85 wt.% H₃PO₄ for different times and temperatures are investigated. The effect of H₃PO₄ loading in electrodes, including the AB-PBI polymer and a Pt/C catalyst, is also studied. The as-prepared electrodes and membranes are combined to fabricate a membrane electrode assembly for HT-PEMFCs. The results reveal that AB-PBI membranes doped with 85 wt.% H₃PO₄ at 90 °C for 9 h display a maximum conductivity of 33 mS cm⁻¹. This membrane was selected and combined with electrodes including 15 wt.% AB-PBI and 0.75 mg cm⁻² Pt with different H₃PO₄ loadings. A maximum current density of 260 mA cm⁻² was achieved in the as-prepared MEA (with 5 mg cm⁻² H₃PO₄ in electrodes) operating at 0.6 V and 160 °C, using oxygen and hydrogen.     

Composite membrane by incorporating sulfonated graphene oxide in polybenzimidazole for high temperature proton exchange membrane fuel cells

The objective of this work is to examine the polybenzimidazole (PBI)/sulfonated graphene oxide (sGO) membranes as alternative materials for high-temperature proton exchange membrane fuel cell (HT-PEMFC). PBI/sGO composite membranes were characterized by TGA, FTIR, SEM analysis, acid doping&acid leaching tests, mechanical analysis, and proton conductivity measurements. The proton conductivity of composite membranes was considerably enhanced by the existence of sGO filler. The enhancement of these properties is related to the increased content of –SO₃H groups in the PBI/sGO composite membrane, increasing the channel availability required for the proton transport. The PBI/sGO membranes were tested in a single HT-PEMFC to evaluate high-temperature fuel cell performance. Amongst the PBI/sGO composite membranes, the membrane containing 5 wt. % GO (PBI/sGO-2) showed the highest HT-PEMFC performance. The maximum power density of 364 mW/cm² was yielded by PBI/sGO-2 membrane when operating the cell at 160 °C under non humidified conditions. In comparison, a maximum power density of 235 mW/cm² was determined by the PBI membrane under the same operating conditions. To investigate the HT-PEMFC stability, long-term stability tests were performed in comparison with the PBI membrane. After a long-term performance test for 200 h, the HT-PEMFC performance loss was obtained as 9% and 13% for PBI/sGO-2 and PBI membranes, respectively. The improved HT-PEMFC performance of PBI/sGO composite membranes suggests that PBI/sGO composites are feasible candidates for HT-PEMFC applications.     

Performance of a polyaniline(DMcT)/carbon fiber composite as cathode for rechargeable lithium batteries

The redox reaction of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) is slow at room temperature, but it can be accelerated when the electron transfer reaction is coupled with that of polyaniline (Pani). Films of polyaniline were electrosynthesized onto carbon fiber substrates by cyclic voltammetry from a 0.5 mol L⁻¹ H₂SO₄/0.1 mol L⁻¹ aniline aqueous solution; DMcT was incorporated into the films by two different procedures: A – previous adsorption on the carbon fiber substrate, and B – electropolymerization onto a Pani film from a 20 mmol L⁻¹ DMcT solution in acetonitrile. The Pani(DMcT)/carbon fiber composites were tested as cathodes at 0.1 mA cm⁻² in a cell containing lithium as anode in a 0.5 mol L⁻¹ LiClO₄ solution in propylene carbonate, in a dry box under an argon atmosphere at 25 ± 2 °C. Discharge capacity values of 159 mA h g⁻¹ (after 90 cycles) and 39 mA h g⁻¹ (after 50 cycles) were obtained for the composites prepared by procedures A and B, respectively. The high capacity value and the high electrochemical stability during the cycling indicate that there is a synergetic effect of Pani and DMcT in the composites prepared by procedure A.     

Electrocatalysis of oxygen reduction on carbon supported Ru-based catalysts in a polymer electrolyte fuel cell

Powder of nanosized particles of Ru-based (Ru_x, Ru_xSe_y and Ru_xFe_ySe_z) clusters were prepared as catalysts for oxygen reduction in 0.5 M H₂SO₄ and for fuel cells prepared by pyrolysis in organic solvent. These electrocatalysts show a high uniformity of agglomerated nanometric particles. The reaction kinetics were studied using rotating disk electrodes and an enhanced catalytic activity for the powders containing selenium and iron was observed. The Ru-based electrocatalysts were used as the cathode in a single prototype PEM fuel cell, which was prepared by spray deposition of the catalyst on the surface of Nafion^® 117 membranes. The electrochemical performance of each single fuel cell was compared to that of a platinum/platinum conventional membrane electrode assembly (MEA), using hydrogen and oxygen feed streams. A maximum power density of 140 mW cm⁻², at 80 °C with 460 mA cm⁻² was obtained for the Ru_xFe_ySe_z catalysts; approximately 55% lower power density than that obtained with platinum.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Micro-cogeneration application of a high-temperature PEM fuel cell stack operated with polybenzimidazole based membranes

High temperature Proton Exchange Membrane Fuel Cells (HT-PEMFC) have attracted the attention of researchers in recent years due to their advantages such as working with reformed gases, easy heat management and compatibility with micro-cogeneration systems. In this study, it is aimed to designed, manufactured and tested of the HT-PEMFC stack based on Polybenzimidazole/Graphene Oxide (PBI/GO) composite membranes. The micro-cogeneration application of the PBI/GO composite membrane based stack was investigated using a reformat gas mixture containing Hydrogen/Carbon Dioxide/Carbon Monoxide (H₂/CO₂/CO). The prepared HT-PEMFC stack comprises 12 cells with 150 cm² active cell area. Thermo-oil based liquid cooling was used in the HT-PEMFC stack and cooling plates were used to prevent coolant leakage between the cells. As a result of HT-PEMFC performance studies, maximum 546 W and 468 W power were obtained from PBI/GO and PBI membranes based HT-PEMFC stacks respectively. The results demonstrate that introducing GO into the PBI membranes enhances the performance of HT-PEMFC technology and demonstrated the potential of the HT-PEMFC stack for use in micro-cogeneration applications. It is also underlined that the developed PBI/GO composite membranes have the potential as an alternative to commercially available PBI membranes in the future.