DMFC Operation Research Articles

Characterization and evaluation of Nafion HP JP as proton exchange membrane: transport properties, nanostructure, morphology, and cell performance

In this study, a novel membrane (Nafion HP JP) was studied and investigated intensively for use as a proton exchange membrane for fuel cell. A standard membrane, Nafion NRE212, was also studied under the same conditions for comparison. Fourier transform infrared spectroscopy, thermogravimetric analysis, wide-angle x-ray diffraction, atomic force microscope (AFM), proton conductivity, and mechanical test were used to study the structure and properties of the membranes. In addition, methanol permeability was measured as a function of methanol concentration. Nafion HP JP membrane exhibited a lower methanol permeability over the concentration range studied. Furthermore, Nafion HP JP has Young’s modulus of 88.3 MPa and tensile strength of 13.2 MPa which are higher than those of Nafion NRE212 (39.85 and 9.5 MPa, respectively). AFM data confirmed the higher value of water uptake for Nafion HP JP membrane. Free volume distribution data measured using positron annihilation lifetime (PAL) showed that the novel membrane has a smaller free volume hole size than that of Nafion NRE212. Moreover, the cell performance of the single cell test using the investigated membrane, hydrogen as fuel and oxygen as oxidant, was investigated at different temperatures and humidities. The power density delivered using Nafion HP JP is higher than that from Nafion NRE212 especially at 50 °C and 30% relative humidity. In addition, the membrane durability of both samples was performed for about 220 h and Nafion HP JP has an open circuit voltage reduction rate of 0.136 mV/h that nearly half that for Nafion NRE212 membrane. The optimal concentration for direct methanol fuel cell DMFC operation using both membranes was found to be 2 M which totally agrees with methanol permeability and PAL data. The obtained results reflect that Nafion HP JP shows great potential for fuel cell applications.

A locally resolved investigation on direct methanol fuel cell uneven components fading: Steady state and degradation local analysis

DMFC technology widespread commercialization is still hindered by durability issues. In literature, locally resolved measurements and post-mortem analyses reveal the onset of strong heterogeneous components fading, higher at air outlet region. Local aging mechanisms could be enhanced by DMFC cathode cycling operation in highly uneven presence of water. An innovative PEM macro-Segmented Fuel Cell (mSFC) setup has been developed to investigate DMFC local performance and degradation, coupling electrochemical and ex-situ analyses as TEM and XPS. An MEA based on highly graphitized carbon supported cathode electrode, confirmed in AST to be stable to cycling operation under flooded conditions, has been implemented in DMFC operation. 500 h local degradation test, despite 32% heterogenenous current density distribution, reveals homogeneous ECSA loss, consistent with homogeneous nanoparticles growth from 3.16 to 5.4 nm, which leads to 70% lower degradation rate (31.2 μV h−1) than the reference MEA. Water-related limitations, such as dehydration and flooding, are revealed to increase local performance loss by 25% and 100% respectively at cathode inlet and outlet regions, leading to current redistribution and uneven voltage loss. Hence, local optimization of MEA properties is foreseen to permit further important durability improvements.

Stability of SPEEK/Cloisite®/TAP nanocomposite membrane under Fenton reagent condition for direct methanol fuel cell application

The stability of SPEEK/Cloisite®/triaminopyrimidine (SP/CL/TAP) nanocomposite membrane against radical attack during DMFC operation was elucidated by the Fenton reagent test. The nanocomposite membrane was soaked in the Fenton reagent solution with 0.8–50 ppm iron salts concentration for up to 96 h. The results indicate that presence of Cloisite® inorganic particles can improve the stability of SP/CL/TAP nanocomposite membrane against the radical attack. FT-IR characterization combined with DFT study has shown that C‒O‒C and ‒SO3H bonding with phenylene ring, and hydrogen bonding between SPEEK, Cloisite®, and TAP are vulnerable to the radical attack. Loss of these functional groups has caused structural deformation, deterioration of mechanical strength, and changes of hydrophilicity in the SP/CL/TAP nanocomposite membrane. Additionally, changes in its chemical structure have caused its water uptake, proton conductivity, and methanol barrier properties to drop, up to 2 × higher than the Nafion® 117 membrane. However, the selectivity value of the SP/CL/TAP nanocomposite membrane (27,037 S∙s/cm3) remains higher than the Nafion® 117 membrane (3292 S∙s/cm3) due to its lower methanol permeability value (2.72 × 10−7 cm2/s) as compared to Nafion® 117 membrane (2.95 × 10−6 cm2/s). Based on the correlation graph, the SP/CL/TAP nanocomposite membrane could operate as PEM in the DMFC system up to 9800 h. Based on the results, it can be concluded that the SP/CL/TAP nanocomposite membrane has good stability in DMFC harsh environment and suitable to be employed as PEM for high-performance and long lifespan DMFC system.

Sulfonated poly(styrene-block-(ethylene-ran-butylene)-block-styrene (SSEBS)-zirconium phosphate (ZrP) composite membranes for direct methanol fuel cells

In this work, S40 (sulfonated poly(styrene-block-(ethylene-ran-butylene)-block-styrene) with 40% degree of sulfonation)-ZrP (zirconium phosphate) composite membranes, serving as proton conducting membranes for direct methanol fuel cells, were prepared via a precursor infiltration method. The obtained S40-ZrP membranes exhibited homogeneous distribution of ZrP nanoplatlets within continuous proton conducting sulfonated polystyrene domains of the microphase separated S40 matrix. Employing different solvent of the infiltrating ZrP precursor solution could tailor the loading, the geometry and the crystallinity of ZrP, which considerably affect the transport properties. S40-N3, the composite membrane containing only 3wt% of ZrP using weak basic N-methyl-2-pyrrolidone as the solvent, exhibited a remarkable 16 fold increment in selectivity as compared to the parent S40 membrane due to enhanced proton conductivity and significantly suppressed methanol permeability. S40-N3 also exhibited improved mechanical properties to enable the integrity of the corresponding membrane electrode assembly during DMFC operation, leading to a power density of 66mWcm−2 (1M methanol at 60°C) and a maximum current density of 450mAcm−2 without cathode flooding. The DMFC performance was better than a DMFC single cell using Nafion 117 by 50% under the same setup. Enhancement in proton conductivity at low relative humidity was also observed for S40-N3 due to the strong water retention capability of ZrP, suggesting a good potential for both DMFC and PEMFC.

The Effect of Nafion Content on DMFC Electrode Characteristics

In this work the influence of the ionomer content in the electrodes on DMFC operation is studied. Nafion content in the anode can be used to control permeation of methanol and water, but has an influence on cell voltage as well. An optimum was found at intermediate Nafion content, where permeation was already reduced without a significant influence on cell voltage. Nafion content in the cathode can be used to improve tolerance towards low air stoichiometry and operation with humidified air. While a Nafion-to-carbon ratio around 0.8 is the optimum for operation with dry air, a significantly lower Nafion-to-carbon ratio of 0.4 should be used for operation with humidified air. On the cathode it was shown that the catalyst layer with the lowest Nafion content had the finest nanopores. For higher Nafion-contents the pore-size maximum was shifted to larger pores and the total nanopore volume was reduced.

Current and clamping pressure distribution studies on the scale up issues in direct methanol fuel cells

The focus of the present study is to obtain the performance on a larger area (45cm2) equivalent to that obtained on a smaller area (4cm2) single cell DMFC overcoming performance losses associated with scale-up of single cell DMFCs due to uneven clamping pressure distribution on MEA through the end-plates and non-uniform distribution of reactants to MEA. The current distribution profile along the cathode flow field channel is measured using a segmented current measurement plate to understand the influence of uneven clamping pressure distribution on the MEA. Uniform current distribution in a single cell DMFC with 45cm2 electrode area is achieved using angular ribbed end-plates. Also, in the present study performance of conventional DMFC is compared with that of mixed-reactant (MR), i.e. methanol and air. Performance of DMFC operating on mixed reactant (MeOH+air) at anode and air at cathode is superior compared with DMFC operating on aqueous methanol at anode and air at cathode. A power density of 55mWcm−2 at 0.3V is obtained for a DMFC single cell with an effective area of 45cm2 using angular ribbed end-plates and MR at anode and air at cathode under ambient conditions. Pressure film test is conducted to obtain the clamping pressure distribution and cyclic voltammetry is performed to obtain the electrochemical surface area of small and large area electrodes.

Overview on Vapor Feed Direct Methanol Fuel Cell

Direct methanol fuel cell is the best candidate in generating high energy density and compatible as power source for small mobile device. DMFC operate in two basic manners which are active and passive where methanol can be fed in liquid or vapoor phase. Passive DMFC is simpler than active DMFC since it operates without using any external devices to deliver methanol and air to the fuel cell. Due to methanol crossover problem faced by liquid feed DMFC operated at high methanol concentration, vapor feed DMFC is an alternative way to solve this problem. Vapor feed DMFC proposed by some groups since it can increase the fuel cell performance and reliable for high methanol concentration. Methanol vapor delivered to the anode by heating the liquid methanol or using a pervaporation membrane. However, there are challenges to overcome such as methanol crossover and water management. Mass transfer resistance should be increased to reduce the methanol crossover and Water management also needed in order to block the water removal from the cathode and thus help to push more water backward from the cathode to the anode in vapor feed DMFC operation to achieve high performance.

A Key Factor for the Actual Application of a Vapor Feed Passive DMFC Operated with High Concentration of Methanol

The supply of water to the anode from the cathode through the electrolyte membrane is a critical factor for the operation of vapor feed DMFC at high methanol concentrations. A comparative study in a passive vapor feed DMFC employing a PCP on the anode surface had been carried out with and without a hydrophobic cathode filter. Compact and a noncompact cells were fabricated and used. The noncompact cell could be operated with high methanol concentrations of even 100 wt%, while the compact one could not be operated with 90 wt% methanol. This was related to the deficiency of water at the anode surface in the compact cell design, where heat dissipation was not enhanced. For the supply of water to the anode surface, the employment of a hydrophobic filter on the cathode surface was necessary, especially for the compact cell.

High pressure anode operation of direct methanol fuel cells for carbon dioxide management

Experiments with independent pressurization of the direct methanol fuel cell anode and cathode allow for the observation of DMFC operation with carbon dioxide gas formation suppressed. Results indicate that the limiting current density is strongly related to the applied pressure, and, therefore, to the presence of CO 2 in the liquid phase. An additional experiment where CO 2 is allowed to accumulate in recycled anode fuel solution over a period of time and is then stripped from solution using nitrogen gas indicates that the presence of CO 2 in anode fuel solution at any pressure contributes to significant decreases in power and current density. Because CO 2 bubbles are ubiquitous in direct methanol fuel cells, this finding is key to the optimization of these systems.

Inorganic–organic hybrid polymer electrolyte based on polysiloxane/poly(maleic imide- co-styrene) network

Covalently cross-linked nonfluorinated hydrocarbon ionomers are synthesized by introducing sulfonate groups and a siloxane cross-linker through thermally and chemically stable imide bonding on poly(styrene -co-maleic anhydride). The three-dimensional polysiloxane framework, which does not only act as a robust scaffold but also provide sites for the hydrogen bonding with water, contribute to the increase in bound water degree, higher proton conductivity at lower ion exchange capacity, and greatly decreased methanol permeability. The spherical-shaped ionic clusters produce a comparable proton conductivity (10 −1 S cm −1 above 60 °C) to Nafion-117. The conductivity of the hybrid ionomer does not decrease to gain its selectivity, but instead increased. Methanol permeability is ∼70% lower than that of Nafion-117, but has a higher water uptake and IEC. The membrane with IEC values of 1.1 mequiv. g −1 exhibits a constant conductivity for 200 h in hydrolytic stability test, and produce a power density 20% higher than Nafion-117 in single DMFC operation.

Vertical operation of passive direct methanol fuel cell employing a porous carbon plate

Vertical operation of a passive DMFC employing a porous carbon plate, PCP, with different resistances for fluid flow and bubble point pressure was investigated to clarify the properties required for the PCP for vertical operation. Moreover, the cell performance was investigated under different solution head pressures within 22 mm height and was discussed based on the methanol transport through the PCP. In contrast to the horizontal orientation, the static pressure of the liquid as a function of its height on the vertical axis h along the PCP surface, in the vertical orientation, enhanced the convective methanol flux through the PCP and affected the DMFC performance depending on the properties of the PCP and the methanol concentration used. The effect of the solution head pressure on the DMFC performance in the vertical orientation could be controlled by using PCP with high bubble point pressures. The DMFC could attain stable performance under both horizontal and vertical cell orientations and different solution head pressures even with 100% methanol by using a PCP with the proper resistance for the methanol transport and bubble point pressure. A thin PCP of 0.5 mm thickness with the proper resistance enabled the vertical operation producing a constant power density over 40 mW/cm 2 using 100% methanol at 0.25 V.

Ionomer Membrane and MEA Development for DMFC

Membrane-electrode assemblies (MEAs) have been prepared from different acid-base-blends consisting of different sulfonated arylene main-chain polymers and polybenzimidazole PBI and a microphase-separated arylene main-chain block copolymer consisting of a sulfonated and proton-conducting and a hydrophobic microphase. The MEAs have been prepared using 4 different methods: Method A: The membrane has been prepared first as a free film, and the electrodes have subsequently been coated onto the membrane; Method B: The membrane has been prepared first as a polyester-supported film, and the electrodes have thereafter been coated onto the membrane; Method C: The MEA has been built up from the anode; Method D: The MEA has been built up from the cathode. The MEAs have been tested under different temperatures and different meOH concentrations. Three different polyacid-PBI blend membranes could be identified which showed comparable or even better DMFC performance than Nafion®105: a sulfonated polyethersulfone-PBI blend membrane, a sulfonated polyetherketone-PBI blend membrane, and a partially fluorinated sulfonated polyether-PBI blend membrane. The proton-conducting block co-ionomer membrane initially showed an excellent DMFC performance due to reduced meOH permeability, compared to the polyacid-PBI blend membranes, which however degraded with time of the DMFC operation probably being due to irreversible morphology changes. Among all tested MEAs the MEAs prepared by Method B showed the best DMFC performance. The DMFC performance of the MEAs prepared by Method C and Method D was slightly worse than that of the MEAs made via Method B. The DMFC performance of a MEA from the sulfonated polyetherketone-PBI blend membrane which was built up using Method D improved steadily during 4 weeks of DMFC operation.

Design and Control Method for Compact BOP of Direct Methanol Fuel Cell

We have developed an electro-chemical model of a stack to analyze its performance. The model includes methanol solution at the anode to "cross over" to the cathode through, which reduces the system efficiency and increases fuel consumption. BOP (Balance of plants) system - condenser, cooler, and methanol solution tank - was modeled. Dynamic simulation was performed to evaluate and develop a control algorithm of the fuel cell system for an efficient operation of DMFC. We simulated the effects of the varying total UA (overall heat transfer coefficient), and proposed minimum UA of the condenser and cooler. When the volume of the methanol solution tank decreases below a certain level, ON/OFF based control algorithm leads to an unstable fuel cell operation. By adopting PID based control algorithm, the system has better controllability of methanol concentration and power generation in response to load change under the same methanol solution tank volume.

High DMFC performance output using modified acid–base polymer blend

An acid–base polymer blend membrane based on sulphonated poly(etheretherketone) (SPEEK) and poly(benzimidazole) (PBI) was developed for direct methanol fuel cells. Thermal stability, water uptake, ion exchange capacity, conductivity and fuel cell performance of the membrane were studied and compared to that of nafion. The conductivity of nafion was found to be superior to SPEEK/PBI membrane; however, the thickness of SPEEK/PBI membranes can be reduced considerably. SPEEK/PBI membrane with a thickness of 55μm showed a significant improvement in the DMFC performance as compared to Nafion 117. The maximum power densities obtained with SPEEK/PBI membranes are twice better than Nafion 117 at 60°C. SPEEK/PBI membranes showed excellent stability under DMFC operating conditions up to 60°C and therefore are seen as ideal candidates for portable DMFC applications.

Structural analysis of sonochemically prepared PtRu versus Johnson Matthey PtRu in operating direct methanol fuel cells

Sonochemically prepared PtRu (3 : 1) and Johnson Matthey PtRu (1 : 1) were analyzed by X-ray absorption spectroscopy in operating liquid feed direct methanol fuel cells. The total metal loadings were 4 mg cm(-2) unsupported catalysts at the anode and cathode of the membrane electrode assembly. Ex situ XRD lattice parameter analysis indicates partial segregation of the Ru from the PtRu fcc alloy in both catalysts. A comparison of the in situ DMFC EXAFS to that of the as-received catalyst shows that catalyst restructuring during DMFC operation increases the total metal coordination numbers. A combined analysis of XRD determined grain sizes and lattice parameters, ex situ and in situ EXAFS analysis, and XRF of the as-received catalysts enables determination of the catalyst shell composition. The multi-spectrum analysis shows that the core size increases during DMFC operation by reduction of Pt oxides and incorporation of Pt into the core. This increases the mole fraction of Ru in the catalyst shell structure.

Characteristics of Performance Degradation and CO2 Emissions in Starved Operation of DMFC

The fatal performance decay caused by potential reversal in a single cell of a DMFC stack under methanol starved operation was investigated. As a result, it was made clear that the decay could be attributed to carbon corrosion in anode catalyst which depended on the oxygen which was made by the anode reaction of water electrolysis in starved operation. In this study, water oxidation and carbon corrosion was confirmed by measuring O2 and CO2 emissions in anode off-gas. The active surface area loss of the anode catalyst was also detected by cyclic voltammetry (CV) and the way to minimize MEA performance degradation is proposed.

Characteristics of Performance Degradation and CO2 Emissions in Starved Operation of DMFC

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Understanding the Anodic Dissolution of Ru from Select PtRu Electrocatalysts During DMFC Operating Environment

not Available.

Durability of Membrane-Electrode Interface under DMFC Operating Conditions

In this study, interfacial degradation under DMFC operating conditions was systematically investigated using series of alternative sulfonated poly(sulfone) membranes. In-situ cell high frequency resistance measurement indicated that membrane dimensional stability was the key parameter for interfacial delamination. Based on this result, less water swollen sulfonated poly(sulfone) membranes were prepared by introducing fluorine and other polar groups such as benzonitirle or phenyl phosphine oxide in the backbone of poly (sulfone) copolymer. Long-term fuel cell performance of an interface-optimized poly (sulfone) membrane showed a stable interfacial resistance and an excellent cell performance for 3,000h under continuous DMFC operation at fixed voltage of 0.5 V.

DMFC: Galvanic or electrolytic cell?

We report measurements of local current distribution in a direct methanol fuel cell with single straight channels and segmented electrodes. The experiments directly confirm the existence of a bifunctional regime of DMFC operation: at low air flow rate the segments located close to the inlet of the oxygen channel generate current (galvanic domain), while the other segments consume current to produce hydrogen (electrolytic domain). The cell voltage in this regime in not a single-valued function of current in the external load. The electrolytic domain disappears at a critical air flow rate, which does not depend on current in the cell. We show that a model (A.A. Kulikovsky, Electrochem. Commun. 6 (2004) 1259) explains this critical behavior. Finally, we extend the method of measurement of methanol crossover rate suggested by Ye and Zhao (J. Electrochem. Soc. 152 (2005) A2238) to the case of arbitrary current in the load.