Performance characteristics of a novel tubular-shaped passive direct methanol fuel cell

The present work consists of a tubular-shaped direct methanol fuel cell (DMFC) that is operated completely passively with methanol solution stored in a central fuel reservoir. The benefit of a tubular-shaped DMFC over a planar-shaped DMFC is the higher instantaneous volumetric power energy density (power/volume) associated with the larger active area provided by the tubular geometry. Membrane electrode assemblies (MEAs) with identical compositions were installed in both tubular and planar-shaped, passive DMFCs and tested with 1, 2, and 3 M methanol solutions at room temperature. The peak power density for the tubular DMFC was 19.0 mW cm⁻² and 24.5 mW cm⁻² while the peak power density for the planar DMFC was 20.0 mW cm⁻² and 23.0 mW cm⁻² with Nafion^® 212 and 115 MEAs, respectively. Even though the performance of the fuel cell improved with each increase in methanol concentration, the fuel and energy efficiencies decreased for both the tubular and planar geometries due to increased methanol crossover. The tubular DMFC experienced higher methanol crossover potentially due to a higher static fluid pressure in the anode fuel reservoir (AFR) caused by the vertical orientation of the tubular fuel reservoir. The performance of the tubular DMFC in this work represents an 870% improvement in power density from the previous best, passive, tubular DMFC found in the literature.     

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.     

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.     

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.     

A Nafion-silica cathode electrolyte for durable elevated-temperature direct methanol fuel cells

A novel catalyst layer assisted by a Nafion-silica electrolyte for elevated-temperature direct methanol fuel cells is fabricated through a self-assembly process. The catalyst layer demonstrates good water retention abilities and structural stability during the fuel cell operation. After a dehydration period of 30 min under 25% relative humidity at 100 °C, the proton conductivity of the novel catalyst layer is maintained at ∼0.014 S cm⁻¹, and the single cell assembled with the novel catalyst layer achieves a maximum power density of 108 mW cm⁻². Moreover, a stability operation test conducted under 20 ppm CO and a current density of 100 mA cm⁻² demonstrates the structural stability and water retention abilities of the catalyst layer. The cell voltage of a fuel cell featuring the novel catalyst layer decreases from 0.45 to 0.38 V at a slight degradation rate of 0.6 mV min⁻¹.     

Sequential flow membraneless microfluidic fuel cell with porous electrodes

A novel convective flow membraneless microfluidic fuel cell with porous disk electrodes is described. In this fuel cell design, the fuel flows radially outward through a thin disk shaped anode and across a gap to a ring shaped cathode. An oxidant is introduced into the gap between anode and cathode and advects radially outward to the cathode. This fuel cell differs from previous membraneless designs in that the fuel and the oxidant flow in series, rather than in parallel, enabling independent control over the fuel and oxidant flow rate and the electrode areas. The cell uses formic acid as a fuel and potassium permanganate as the oxidant, both contained in a sulfuric acid electrolyte. The flow velocity field is examined using microscale particle image velocimetry and shown to be nearly axisymmetric and steady. The results show that increasing the electrolyte concentration reduces the cell Ohmic resistance, resulting in larger maximum currents and peak power densities. Increasing the flow rate delays the onset of mass transport and reduces Ohmic losses resulting in larger maximum currents and peak power densities. An average open circuit potential of 1.2 V is obtained with maximum current and power densities of 5.35 mA cm⁻² and 2.8 mW cm⁻², respectively (cell electrode area of 4.3 cm²). At a flow rate of 100 μL min⁻¹ a fuel utilization of 58% is obtained.     

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%.     

Planar air-breathing micro-direct methanol fuel cell stacks based on micro-electronic–mechanical-system technology

To meet the demands for high power micro-electronic devices, two silicon-based micro-direct methanol fuel cell (μDMFC) stacks consisting of six individual cells with two different anode flow fields were designed, fabricated and evaluated. Micro-electronic–mechanical-system (MEMS) technology was used to fabricate both flow field plate and fuel distribution plate on the silicon wafer. Experimental results show that either an individual cell or a stack with double serpentine-type flow fields presents better cell performance than those with pin-type flow fields. A μDMFC stack with double serpentine-type flow fields generates a peak output power of ca. 151 mW at a working voltage of 1.5 V, corresponding to an average power density of ca. 17.5 mW cm⁻², which is ca. 20.7% higher than that with pin-type flow fields. The volume and weight of the stacks are only 5.3 cm³ and 10.7 g, respectively. Such small stacks could be used as power sources for micro-electronic devices.     

Three-dimensional anode engineering for the direct methanol fuel cell

Catalyzed graphite felt three-dimensional anodes were investigated in direct methanol fuel cells (DMFCs) operated with sulfuric acid supporting electrolyte. With a conventional serpentine channel flow field the preferred anode thickness was 100 μm, while a novel flow-by anode showed the best performance with a thickness of 200-300 μm. The effects of altering the methanol concentration, anolyte flow rate and operating temperature on the fuel cell superficial power density were studied by full (2³ + 1) factorial experiments on a cell with anode area of 5 cm² and excess oxidant O₂ at 200 kPa(abs). For operation in the flow-by mode with 2 M methanol at 2 cm³ min⁻¹ and 353 K the peak power density was 2380 W m⁻² with a PtRuMo anode catalyst, while a PtRu catalyst yielded 2240 W m⁻² under the same conditions.     

A self-pumping and self-breathing micro direct methanol fuel cell with polymer bipolar plates

A passive micro direct methanol fuel cell (DMFC) for reducing volume and parasitic power is designed and fabricated using several integrated technologies. New bipolar plates with tapered channels at the anode and a pillar array at the cathode are first applied to a passive micro-DMFC. The substrate of the bipolar plates made of acrylonitrile butadiene styrene (ABS) is hot embossed with two molds, fabricated by UV-LIGA and micro machining. To make the bipolar plates conductive and hydrophilic, a nickel layer is electroplated on the ABS plates, and three PDDA/PSS bi-layers are self-assembled onto the nickel layer. The bipolar plates are produced using hot embossing, a low cost, highly accurate batch process. A single cell is assembled to verify the self-pumping function, and it can generate a peak power density of 7.4 mW cm⁻² with a 3 M methanol solution. The fuel cell is verified to work in three different orientations. When the fuel cell is placed horizontally, the self-pumping rate is about 0.1-0.15 mL h⁻¹. And the fuel cell can work through self-pumping for 5 h under this condition.     

New anodic diffusive layer for passive micro-direct methanol fuel cell

The addition of carbon nanotubes (CNTs) into anodic micro-porous layer (MPL) of the membrane electrode assembly significantly improves the performance of the passive micro-direct methanol fuel cells (DMFCs). The maximum power density of ca. 32.2 mW cm⁻² at a temperature of ca. 25 °C and under air-breathing mode is achieved with pure CNTs as anode MPL material. Impedance analysis and cyclic voltammetric measurements of the anodes indicate that the increased performance of the passive DMFC with the addition of CNTs into anodic MPLs could be attributed to the decrease in charge transfer resistance of the anode reaction and to the improvement in catalyst utilization. Scanning electron microscopy measurements show the network formation within the MPL due to the one-dimensional structure of CNTs, which could be beneficial to the methanol mass transfer and to the improvement in catalyst utilization. Furthermore, the durability test of a passive DMFC after 300 h operation demonstrates that the passive DMFC with CNTs as anode MPL materials exhibits very good stability.     

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.     

High performance membrane-electrode assembly based on a surface-modified membrane

A surface-modified membrane is prepared using a sputtering technique that deposits gold directly on a Nafion^® 115 membrane surface that is roughened with silicon carbide paper. The surface-modified membranes are characterized by means of a scanning electron microscope (SEM), differential scanning calorimetry (DSC), and water contact-angle analysis. A single direct methanol fuel cell (DMFC) with a surface-modified membrane exhibits enhanced performance (160 mW cm⁻²), while a bare Nafion^® 115 cell yields 113 mW cm⁻² at 0.4 V and an operating temperature of 70 °C. From FE-SEM images and CO_ad stripping voltammograms, it is also found that the gold layer is composed of clusters of porous nodule-like particles, which indicates that an anode with nodule-like gold leads to the preferential oxidation of carbon monoxide. These results suggest that the topology of gold in the interfacial area and its electrocatalytic nature may be the critical factors that affect DMFC performance.     

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.     

Monolithic micro-direct methanol fuel cell in polydimethylsiloxane with microfluidic channel-integrated Nafion strip

We demonstrate a monolithic polymer electrolyte membrane fuel cell by integrating a narrow (200 μm) Nafion strip in a molded polydimethylsiloxane (PDMS) structure. We propose two designs, based on two 200 μm-wide and two 80 μm-wide parallel microfluidic channels, sandwiching the Nafion strip, respectively. Clamping the PDMS/Nafion assembly with a glass chip that has catalyst-covered Au electrodes, results in a leak-tight fuel cell with stable electrical output. Using 1 M CH₃OH in 0.5 M H₂SO₄ solution as fuel in the anodic channel, we compare the performance of (I) O₂-saturated 0.5 M H₂SO₄ and (II) 0.01 M H₂O₂ in 0.5 M H₂SO₄ oxidant solutions in the cathodic channel. For the 200 μm channel width, the fuel cell has a maximum power density of 0.5 mW cm⁻² and 1.5 mW cm⁻² at room temperature, for oxidants I and II, respectively, with fuel and oxidant flow rates in the 50-160 μL min⁻¹ range. A maximum power density of 3.0 mW cm⁻² is obtained, using oxidant II for the chip with 80 μm-wide channel, due to an improved design that reduces oxidant and fuel depletion effects near the electrodes.     

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.     

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.     

Performance evaluation of passive direct methanol fuel cell with methanol vapour supplied through a flow channel

This work examines the effect of fuel delivery configuration on the performance of a passive air-breathing direct methanol fuel cell (DMFC). The performance of a single cell is evaluated while the methanol vapour is supplied through a flow channel from a methanol reservoir connected to the anode. The oxygen is supplied from the ambient air to the cathode via natural convection. The fuel cell employs parallel channel configurations or open chamber configurations for methanol vapour feeding. The opening ratio of the flow channel and the flow channel configuration is changed. The opening ratio is defined as that between the area of the inlet port and the area of the outlet port. The chamber configuration is preferred for optimum fuel feeding. The best performance of the fuel cell is obtained when the opening ratio is 0.8 in the chamber configuration. Under these conditions, the peak power is 10.2 mW cm⁻² at room temperature and ambient pressure. Consequently, passive DMFCs using methanol vapour require sufficient methanol vapour feeding through the flow channel at the anode for best performance. The mediocre performance of a passive DMFC with a channel configuration is attributed to the low differential pressure and insufficient supply of methanol vapour.     

A flexible direct methanol micro-fuel cell based on a metalized, photosensitive polymer film

This paper presents and investigates a concept for a flexible direct methanol micro-fuel cell (FDMMFC) based on the microstructuring of a Cr/Au metalized, thin polymer film of photosensitive SU-8. The inscribed microchannels in the electrodes are 200 μm × 200 μm in crosssection and spanning an active fuel cell area of 10 mm × 10 mm with a Pt-black catalyst on the cathode side of the membrane electrode assembly (MEA) and a Pt-Ru alloy catalyst on the anode side. Subsequently, the paper focuses on a thorough electrical characterization of the FDMMFC, under the employment of a variable resistor simulating an electrical load as well as a classical galvanostatical measurement technique. The fuel cell is also tested while operating in a bent, non-flat configuration. An extensive parameter study revealed an optimal and long-term stable operating condition for the fuel cell employing for both electrodes a serpentine flow field and a volume flow rate of 0.14 ml min⁻¹ of a 1 M methanol solution at the anode side with a gas volume flow rate of 8 ml min⁻¹ of humidified O₂ at the cathode side yielding a power density of 19.0 mW cm² at 75 mA cm² at a temperature of 60 °C.Furthermore, a flow-visualization of the two-phase flow occurring at the anode side has been performed by utilizing fluorescence microscopy. The strong influence of the two-phase flow on the performance of a fuel cell at high current densities becomes apparent in correlating the observed flow patterns with the corresponding current density of the polarization curve. The paper also investigates the functionality of the present FDMMFC under different bent conditions. The tests showed an insignificant drop of the electrical performance under bending due to an inhomogeneous contact resistance.     

Design, fabrication, and characterization of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell with a bridge-shaped microchannel cross-section

We report the fabrication of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell (μLFFC) using standard MEMS and IC-compatible fabrication technologies. The μLFFC operates with acid supported solutions of formic acid and potassium permanganate, as a fuel and oxidant respectively. The micro-fuel cell design features two in-plane anodic and cathodic microchannels connected via a bridge to confine the diffusive liquid-liquid interface away from the electrode areas and to minimize crossover. Palladium high-active-surface-area catalyst was selectively integrated into the anodic microchannel by electrodeposition, whereas no catalyst was required in the cathodic microchannel. A three-dimensional (3D) diffusion-convection model was developed to study the behavior of the diffusion zone and to extract appropriate cell-design parameters and operating conditions. Experimentally, we observed peak power densities as high as 26 mW cm⁻² when operating single cells at a flow rate of 60 μL min⁻¹ at room temperature. The miniature membraneless fuel cell design presented herein offers potential for on-chip power generation, which has long been prohibited by integration complexities associated with the membrane.