Highly fluorinated comb-shaped copolymer as proton exchange membranes (PEMs): Fuel cell performance

The fuel cell performance (DMFC and H₂/air) of highly fluorinated comb-shaped copolymer is reported. The initial performance of membrane electrode assemblies (MEAs) fabricated from comb-shaped copolymer containing a side-chain weight fraction of 22% are compared with those derived from Nafion and sulfonated polysulfone (BPSH-35) under DMFC conditions. The low water uptake of comb copolymer enabled an increase in proton exchange site concentrations in the hydrated polymer, which is a desirable membrane property for DMFC application. The comb-shaped copolymer architecture induces phase separated morphology between the hydrophobic fluoroaromatic backbone and the polysulfonic acid side chains. The initial performance of the MEAs using BPSH-35 and Comb 22 copolymer were comparable and higher than that of the Nafion MEA at all methanol concentrations. For example, the power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 0.5 M methanol was 145 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 136 mW cm⁻². The power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 2.0 M methanol was 144.5 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 143 mW cm⁻².     

Performance and design analysis of tubular-shaped passive direct methanol fuel cells

To achieve the maximum performance from a Direct Methanol Fuel Cell (DMFC), one must not only investigate the materials and configuration of the MEA layers, but also consider alternative cell geometries that produce a higher instantaneous power while occupying the same cell volume. In this work, a two-dimensional, two-phase, non-isothermal model was developed to investigate the steady-state performance and design characteristics of a tubular-shaped, passive DMFC. Under certain geometric conditions, it was found that a tubular DMFC can produce a higher instantaneous Volumetric Power Density than a planar DMFC. Increasing the ambient temperature from 20 to 40 °C increases the peak power density produced by the fuel cell by 11.3 mW cm⁻² with 1 M, 16.3 mW cm⁻² with 2 M, but by only 8.4 mW cm⁻² with 3 M methanol. The poor performance with 3 M methanol at a higher ambient temperature is caused by increased methanol crossover and significant oxygen depletion along the Cathode Transport Layer (CTL). For a 5 cm long tubular DMFC to maintain sufficient Oxygen transport, the thickness of the CTL must be greater than 1 mm for 1 M operation, greater than 5 mm for 2 M operation, and greater than 10 mm for 3 M or higher operation.     

Effect of the porous carbon layer in the cathode gas diffusion media on direct methanol fuel cell performances

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm⁻²) at 0.4 V and 70 °C than commercial GDL (SIGRACET^® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm⁻²). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm³ cm⁻² s⁻¹, while the one for SGL 35BC GDLs was 21.7 cm³ cm⁻² s⁻¹. Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.     

Single passive direct methanol fuel cell supplied with pure methanol

A new single passive direct methanol fuel cell (DMFC) supplied with pure methanol is designed, assembled and tested using a pervaporation membrane (PM) to control the methanol transport. The effect of the PM size on the fuel cell performances and the constant current discharge of the fuel cell with one-fueling are studied. The results show that the fuel cell with PM 9 cm² can yield a maximum power density of about 21 mW cm⁻², and a stable performances at a discharge current of 100 mA can last about 45 h. Compared with DMFC supplied with 3 M methanol solution, the energy density provided by this new DMFC has increased about 6 times.     

Improved fuel use efficiency in microchannel direct methanol fuel cells using a hydrophilic macroporous layer

We demonstrate state-of-the-art room temperature operation of silicon microchannel-based micro-direct methanol fuel cells (μDMFC) having a very high fuel use efficiency of 75.4% operating at an output power density of 9.25 mW cm⁻² for an input fuel (3 M aqueous methanol solution) flow rate as low as 0.55 μL min⁻¹. In addition, an output power density of 12.7 mW cm⁻² has been observed for a fuel flow rate of 2.76 μL min⁻¹. These results were obtained via the insertion of novel hydrophilic macroporous layer between the standard hydrophobic carbon gas diffusion layer (GDL) and the anode catalyst layer of a μDMFC; the hydrophilic macroporous layer acts to improve mass transport, as a wicking layer for the fuel, enhancing fuel supply to the anode at low flow rates. The results were obtained with the fuel being supplied to the anode catalyst layer via a network of microscopic microchannels etched in a silicon wafer.     

Performance of a direct methanol alkaline membrane fuel cell

A study of a direct methanol fuel cell (DMFC) operating with hydroxide ion conducting membranes is reported. Evaluation of the fuel cell was performed using membrane electrode assemblies incorporating carbon-supported platinum/ruthenium anode and platinum cathode catalysts and ADP alkaline membranes. Catalyst loadings used were 1 mg cm⁻² Pt for both anode and cathode. The effect of temperature, oxidant (air or oxygen) and methanol concentration on cell performance is reported. The cell achieved a power density of 16 mW cm⁻², at 60 °C using oxygen. The performance under near ambient conditions with air gave a peak power density of approximately 6 mW cm⁻².     

Polyelectrolyte membranes based on hydrocarbon polymer containing fullerene

In the present study, composite polyelectrolyte membranes were prepared from sulfonated polystyrene and fullerene. The additive effect of the fullerene on the membrane properties – electric resistance, mechanical strength, oxidation resistance, and methanol permeability – were measured. The addition of fullerene improved the oxidation resistance, and reduced the methanol crossover. The mechanical strength of the fullerene-composite membrane, on the other hand, was not improved. The direct methanol fuel cell (DMFC) based on a 1.4 wt% fullerene-composite membrane showed the highest power density of 47 mW cm⁻² at the current density of 200 mA cm⁻² (this value is 60% of the Nafion-based DMFC). The transmission electron microscopy (TEM) observations suggest that the improved dispersity of the fullerene and the reduced number of micropores in the membranes would improve its performance in the fuel cell.     

Vapor feed direct methanol fuel cells with passive thermal-fluids management system

The present paper describes a novel technology that can be used to manage methanol and water in miniature direct methanol fuel cells (DMFCs) without the need for a complex micro-fluidics subsystem. At the core of this new technology is a unique passive fuel delivery system that allows for fuel delivery at an adjustable rate from a reservoir to the anode. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The innovative thermal management mechanism is the key for effective operation of the fuel cell system. The vapor feed DMFC reached a power density of 16.5 mW cm⁻² at current density of 60 mA cm⁻². A series of fuel cell prototypes in the 0.5 W range have been successfully developed. The prototypes have demonstrated long-term stable operation, easy fuel delivery control and are scalable to larger power systems. A two-cell stack has successfully operated for 6 months with negligible degradation.     

Effect of the cathode open ratios on the water management of a passive vapor-feed direct methanol fuel cell fed with neat methanol

A novel approach has been proposed to improve the water management of a passive direct methanol fuel cell (DMFC) fed with neat methanol without increasing its volume or weight. By adopting perforated covers with different open ratios at the cathode, the water management has been significantly improved in a DMFC fed with neat methanol. An optimized cathode open ratio could ensure both the sufficient supply of oxygen and low water loss. While changing the open ratio of anode vaporizer can adjust the methanol crossover rate in a DMFC. Furthermore, the gas mixing layer, added between the anode vaporizer and the anode current collector to increase the mass transfer resistance, can improve the cell performance, decrease the methanol crossover, and increase the fuel efficiency. For the case of a DMFC fed with neat methanol, an anode vaporizer with the open ratio of 12% and a cathode open ratio of 20% produced the highest peak power density, 22.7 mW cm⁻², and high fuel efficiency, 70.1%, at room temperature of 25 ± 1 °C and ambient humidity of 25-50%.     

Performance of an integrated composite membrane electrode assembly in DMFC

We report here the performance of a metal-based integrated composite membrane electrode assembly (IC-MEA) in direct methanol fuel cell (DMFC). The IC-MEA integrates the multi-functions of a conventional MEA, gas diffusion layer (GDL) and current collector. It was fabricated by impregnating Nafion electrolyte into a sandwiched structure containing expanding-Polytetrafluoroethylene (e-PTFE) and porous titanium sheets and subsequently coating with catalyst layer and microporous layer (MPL). While operating with air and 2 M methanol under ambient pressure, the IC-MEA in DMFC can yield a maximum power density of 19 mW cm⁻² at 26 °C, higher than a in-house made Nafion 115 MEA under the same working conditions. The IC-MEAs has been successfully applied to planar multi-cell stacks.     

Structural optimization of the direct methanol fuel cell passively fed with a high-concentration methanol solution

In a high-concentration direct methanol fuel cell (HC-DMFC), the methanol crossover is typically decreased to an acceptable level by two main mechanisms: high methanol transport resistance between the anode reservoir and the membrane electrode assembly (MEA), and high water back flow from the cathode to the anode. Based on the semi-passive HC-DMFC fabricated in this work, the effects of methanol barrier layer (MBL) thickness and electrolyte membrane thickness on cell performance, methanol and water crossover, and fuel efficiency have been studied. The results showed that a thicker MBL could significantly decrease the methanol and water crossover by increasing the mass transport resistance between the anode reservoir and the MEA, while a thinner Nafion^® membrane could also significantly decrease the methanol and water crossover by enhancing the water back flow from the cathode through the electrolyte membrane to the anode. Using Nafion^® 212 as the electrolyte membrane, and a 6.4 mm porous PTFE plate as the MBL, a semi-passive HC-DMFC operating at 70 °C produced the maximum power density of 115.8 mW cm⁻² when 20 M methanol solution was fed as the fuel.     

Use of in situ polymerized phenol-formaldehyde resin to modify a Nafion membrane for the direct methanol fuel cell

Commercial Nafion^®-115 (trademark registered to DuPont) membranes were modified by in situ polymerized phenol formaldehyde resin (PFR) to suppress methanol crossover, and SO₃⁻ groups were introduced to PFR by post-sulfonatation. A series of membranes with different sulfonated phenol formaldehyde resin (sPFR) loadings have been fabricated and investigated. SEM-EDX characterization shows that the PFR was well dispersed throughout the Nafion^® membrane. The composite membranes have a similar or slightly lower proton conductivity compared with a native Nafion^® membrane, but show a significant reduction in methanol crossover (the methanol permeability of sPFR/Nafion^® composite membrane with 2.3 wt.% sPFR loading was 1.5 × 10⁻⁶ cm² s⁻¹, compared with the 2.5 × 10⁻⁶ cm² s⁻¹ for the native Nafion^® membrane). In direct methanol fuel cell (DMFC) evaluation, the membrane electrode assembly (MEA) using a composite membrane with a 2.3 wt.% sPFR loading shows a higher performance than that of a native Nafion^® membrane with 1 M methanol feed, and at higher methanol concentrations (5 M), the composite membrane achieved a 114 mW cm⁻² maximum power density, while the maximum power density of the native Nafion^® was only 78 mW cm⁻².     

Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs

According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm⁻² and maintains a high power density of 30 mW cm⁻² (0.405 V) under constant current operation.     

Polymer electrolytes based on sulfonated polysulfone for direct methanol fuel cells

This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SPSf-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30–40 °C) and high temperatures (100–120 °C). DMFC power densities were about 140 mW cm⁻² at 100 °C with the bare SPSf-60 membrane and 180 mW cm⁻² at 120 °C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm⁻² at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices.     

Self-regulating passive fuel supply for small direct methanol fuel cells operating in all orientations

A microfluidic fuel supply concept for passive and portable direct methanol fuel cells (DMFCs) that operates in all spatial orientations is presented. The concept has been proven by fabricating and testing a passive DMFC prototype. Methanol transport at the anode is propelled by the surface energy of deformed carbon dioxide bubbles, generated as a reaction product during DMFC operation. The experimental study reveals that in any orientation, the proposed pumping mechanism transports at least 3.5 times more methanol to the reactive area of the DMFC than the stoichiometry of the methanol oxidation would require to sustain DMFC operation. Additionally, the flow rates closely follow the applied electric load; hence the pumping mechanism is self-regulating. Oxygen is supplied to the cathode by diffusion and the reaction product water is transported out of the fuel cell along a continuous capillary pressure gradient. Results are presented that demonstrate the continuous passive operation for more than 40 h at ambient temperature with a power output of p = 4 mW cm⁻² in the preferred vertical orientation and of p = 3.2 mW cm⁻² in the least favorable horizontal orientation with the anode facing downwards.     

Investigation of sulfonated polysulfone membranes as electrolyte in a passive-mode direct methanol fuel cell mini-stack

This paper reports on the development of polymer electrolyte membranes (PEMs) based on sulfonated polysulfone for application in a DMFC mini-stack operating at room temperature in passive mode. The sulfonated polysulfone (SPSf) with two degrees of sulfonation (57 and 66%) was synthesized by a well-known sulfonation process. SPSf membranes with different thicknesses were prepared and investigated. These membranes were characterized in terms of methanol/water uptake, proton conductivity, and fuel cell performance in a DMFC single cell and mini-stack operating at room temperature. The study addressed (a) control of the synthesis of sulfonated polysulfone, (b) optimization of the assembling procedure, (c) a short lifetime investigation and (d) a comparison of DMFC performance in active-mode operation vs. passive-mode operation.The best passive DMFC performance was 220 mW (average cell power density of about 19 mW cm⁻²), obtained with a thin SPSf membrane (70 μm) at room temperature, whereas the performance of the same membrane-based DMFC in active mode was 38 mW cm⁻². The conductivity of this membrane, SPSf (IEC = 1.34 mequiv. g⁻¹) was 2.8 × 10⁻² S cm⁻¹. A preliminary short-term test (200 min) showed good stability during chrono-amperometry measurements.     

Passive direct methanol fuel cells for portable electronic devices

Due to the increasing demand for electricity, clean, renewable energy resources must be developed. Thus, the objective of the present study was to develop a passive direct methanol fuel cell (DMFC) for portable electronic devices. The power output of six dual DMFCs connected in series with an active area of 4 cm² was approximately 600 mW, and the power density of the DMFCs was 25 mW cm⁻². The DMFCs were evaluated as a power source for mobile phone chargers and media players. The results indicated that the open circuit voltage of the DMFC was between 6.0 V and 6.5 V, and the voltage under operating conditions was 4.0 V. The fuel cell was tested on a variety of cell phone chargers, media players and PDAs. The cost of energy consumption by the proposed DMFC was estimated to be USD 20 W⁻¹, and the cost of methanol is USD 4 kW h. Alternatively, the local conventional electricity tariff is USD 2 kW h. However, for the large-scale production of electronic devices, the cost of methanol will be significantly lower. Moreover, the electricity tariff is expected to increase due to the constraints of fossil fuel resources and pollution. As a result, DMFCs will become competitive with conventional power sources.     

Methanol permeability and performance of Nafion-zirconium phosphate composite membranes in active and passive direct methanol fuel cells

Methanol crossover through polymer electrolyte membranes represents one of the major problems to be solved in order to improve direct methanol fuel cell (DMFC) performance. With this aim, Nafion/zirconium phosphate (ZrP) composite membranes, with ZrP loading in the range 1-6 wt%, were prepared by casting from mixtures of gels of exfoliated ZrP and Nafion 1100 dispersions in dimethylformamide. These membranes were characterised by methanol permeability, swelling and proton conductivity measurements, as well as by tests in active and passive DMFCs in the temperature range 30-80 °C. Increase in filler loading results in a decrease in both methanol permeability and proton conductivity. As a consequence of the reduced conductivity the power density of active DMFCs decreases with increasing ZrP loading (from 46 to 32 mW cm⁻² at 80 °C). However, due to the lower methanol permeability, the room temperature Faraday efficiency of passive DMFCs, with 20 mA cm⁻² discharge current, nearly doubles when Nafion 1100 is replaced by the composite membrane containing 4 wt% ZrP.     

Chemical modification of Nafion membrane with 3,4-ethylenedioxythiophene for direct methanol fuel cell application

Nafion 117 membranes were modified by in situ chemical polymerization of 3,4-ethylenedioxythiophene using H₂O₂ as oxidant for direct methanol fuel cell application. Methanol permeability and proton conductivity of the poly(3,4-ethylenedioxythiophene)-modified Nafion membranes as a function of temperature were investigated. An Arrhenius-type dependency of methanol permeability and proton conductivity on temperature exists for all the modified membranes. Compared with Nafion 117 membrane at 60 °C, the methanol permeability of these modified membranes is reduced from 30% to 72%, while the proton conductivity is decreased from 4% to 58%, respectively. Because of low methanol permeability and adequate proton conductivity, the DMFC performances of these modified membranes were better than that of Nafion 117 membrane. A maximum power density of 48.4 mW cm⁻² was obtained for the modified membrane, while under same condition Nafion 117 membrane got 37 mW cm⁻².     

Development and performance analysis of a metallic micro-direct methanol fuel cell for high-performance applications

As a promising candidate for conventional micro-power sources, the micro-direct methanol fuel cell (μDMFC) is currently attracting increased attention due to its various advantages and prospective suitability for portable applications. This paper reports the design, fabrication and analysis of a high-performance μDMFC with two metal current collectors. Employing micro-stamping technology, the current collectors are fabricated on 300-μm-thick stainless steel plates. The flow fields for both cathode and anode are uniform in shape and size. Two sheets of stainless steel mesh are added between the membrane electrode assembly (MEA) and current collectors in order to improve cell performance. To avoid electrochemical corrosion, titanium nitride (TiN) layers with thickness of 500 nm are deposited onto the surface of current collectors and stainless steel mesh. The performance of this metallic μDMFC is thoroughly studied by both simulation and experimental methods. The results show that all the parameters investigated, including current collector material, stainless steel mesh, anode feeding mode, methanol concentration, anode flow rate, and operating temperature have significant effects on cell performance. Moreover, the results show that under optimal operating conditions, the metallic μDMFC exhibits promising performance, yielding a maximum power density of 65.66 mW cm⁻² at 40 °C and 115.0 mW cm⁻² at 80 °C.