Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Adoption of a sintered stainless steel fiber felt was evaluated as gas diffusion backing in air-breathing direct methanol fuel cell (DMFC). By using a sintered stainless steel fiber felt as an anodic gas diffusion backing, the peak power density of an air-breathing DMFC is 24 mW cm⁻², which is better than that of common carbon paper. A 30-h-life test indicates that the degraded performance of the air-breathing DMFC is primarily due to the water flooding of the cathode. Twelve unit cells with each has 6 cm² of active area are connected in series to supply the power to a mobile phone assisted by a constant voltage diode. The maximum power density of 26 mW cm⁻² was achieved in the stack, which is higher than that in single cell. The results show that the sintered stainless steel felt is a promising solution to gas diffusion backing in the air-breathing DMFC, especially in the anodic side because of its high electronical conductivity and hydrophilicity.     

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.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

Influence of cathode opening size and wetting properties of diffusion layers on the performance of air-breathing PEMFCs

Air-breathing PEMFCs consist of an open cathodic side to allow an entirely passive supply of oxygen by diffusion. Furthermore, a large fraction of the produced water is removed by evaporation from the open cathode. Gas diffusion layers (GDLs) and the opening size of the cathode have a crucial influence on the performance of an air-breathing PEMFC. In order to assure an unobstructed supply of oxygen the water has to be removed efficiently and condensation in the GDL has to be avoided. On the other hand good humidification of the membrane has to be achieved to obtain high protonic conductivity.In this paper the influence of varying cathodic opening sizes (33%, 50% and 80% opening ratios) and of GDLs with different wetting properties are analysed. GDLs with hydrophobic and hydrophilic properties are prepared by coating of untreated GDLs (Toray^® carbon paper TGP-H-120, thickness of 350 μm). The air-breathing PEMFC test samples are realised using printed circuit board (PCB) technology.The cell samples were characterised over the entire potential range (0–0.95 V) by extensive measurements of the current density, the temperature and the cell impedance at 1 kHz. Additionally, measurements of the water balance were carried out at distinct operation points.The best cell performance was achieved with the largest opening ratio (80%) and an untreated GDL. At the maximum power point, this cell sample achieved a power density of 100 mW cm⁻² at a moderate cell temperature of 43 °C. Furthermore, it could be shown that GDLs with hydrophilic or intense hydrophobic properties do not improve the performance of an air-breathing PEMFC.Based on the extensive characterisations, two design rules for air-breathing PEMFCs could be formulated.Firstly, it is crucial to maximise the cathode opening as far as an appropriate compression pressure of the cell assembly and therewith low contact resistance can be assured. Secondly, it is advantageous to use an untreated, slightly hydrophobic GDL.     

Fabrication of electrocatalyst layers for direct methanol fuel cells

To optimize the performance of the membrane electrode assembly (MEA), a manufacturing process for electrocatalyst layers is systematically studied by controlling physical parameters such as electrocatalyst loadings at each electrode, electrocatalyst compositions, and layer thickness. The MEA is evaluated in an air-breathing direct methanol fuel cell (DMFC) with various methanol concentrations. The investigation focuses on finding the best compromise between electrocatalyst loadings and utilization of methanol concentration. Surprisingly, the power density is influenced more by the Pt loading than by the Pt–Ru loading, and can be increased further by using a methanol concentration above 3 wt.% for a certain level of electrocatalyst loading. Current–voltage characteristics indicate that increasing Pt and Pt–Ru loadings at each electrode can reduce the activation overpotentials, but the respective variation of current density with cell voltage differs in the voltage range (0.3–0.8 V). Although MEA performance can be improved by increasing the Pt (and Pt–Ru) concentration, a penalty is paid due to the tendency towards increased nanoparticle aggregation. The MEAs are also applied to a small pack of air-breathing DMFCs to assess their operability in mobile phones.     

Development of an air-breathing direct methanol fuel cell with the cathode shutter current collectors

An air-breathing direct methanol fuel cell with a novel cathode shutter current collector is fabricated to develop the power sources for consumer electronic devices. Compared with the conventional circular cathode current collector, the shutter one improves the oxygen consumption and mass transport. The anode and cathode current collectors are made of stainless steel using thermal stamping die process. Moreover, an encapsulation method using the tailor-made clamps is designed to assemble the current collectors and MEA for distributing the stress of the edges and inside uniformly. It is observed that the maximum power density of the air-breathing DMFC operating with 1 M methanol solution achieves 19.7 mW/cm² at room temperature. Based on the individual DMFCs, the air-breathing stack consisting of 36 DMFC units is achieved and applied to power a notebook computer.     

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.     

A new direct methanol fuel cell with a zigzag-folded membrane electrode assembly

We propose a new direct methanol fuel cell with a zigzag-folded membrane electrode assembly. This fuel cell is formed by a membrane, which is made up of anode and cathode electrodes on a zigzag-folded sheet, separated by insulation film and current collectors. Individual anodes, cathodes and membranes form a unit cell, which is connected to the adjacent unit cell. The fuel cell can achieve high output voltage through easy in-series connection. Since it is not necessary to connect electrodes, as in the manner of conventional bipolar plates, there is no increase in fabrication cost and no degradation in reliability. The fuel feeds for the anode and cathode are achieved through methanol and air feeds on each electrode, which do not require electricity to run a pump or blower. The experimental cells were formed with an active area of 16 cm × 2 cm on membrane-folded cells. Filter papers with slits were inserted between anodes to improve their methanol supply. A power density of 3 mW cm⁻² was obtained at a methanol concentration of 2 M at ambient temperature. The cell power was affected by the slit area on cathode.     

All-solid-state supercapacitor using a Nafion® polymer membrane and its hybridization with a direct methanol fuel cell

An all-solid-state supercapacitor is fabricated and optimized using a Nafion^® membrane and an ionomer. The device shows good capacitance (ca. 200 F g⁻¹) as demonstrated by cyclic voltammograms (CVs) and charge–discharge curves. The supercapacitor exhibits a relatively stable capacitance during l0,000 cycles of operation. A hybrid system comprising a direct methanol fuel cell (DMFC) and an all-solid-state supercapacitor has been designed and tested. It is confirmed that the power discharged by the supercapacitor is transferred effectively to the DMFC. The power of the hybrid is immediately improved by 30% compared with that of a DMFC alone operating at 25 °C. The possibilities of using this system for high energy and high instantaneous power devices and integrated fabrication processes are discussed.     

Cathode structure optimization for air-breathing DMFC by application of pore-forming agents

The cathode catalyst layer in an air-breathing, direct methanol fuel cell (DMFC) has been improved by addition of pore-forming materials, such as ammonium carbonate and ammonium hydrocarbonate. These construct an effective electrode pore structure. During fabrication of the membrane electrode assembly (MEA), these pore-formers decompose, increase the BET surface area of the electrodes, and form additional porosity. Ammonium carbonate predominantly produces macropores, while ammonium hydrocarbonate is effective for mesopores. This results in an increase in electrochemical active area and catalyst utilization. The higher open-circuit voltage with MEAs prepared with pore-forming material indicates improved air transfer through the cathode. The power density is increased by 30–40% with the employment of the pore-forming materials.     

Performance of direct methanol fuel cell using carbon nanotube-supported Pt–Ru anode catalyst with controlled composition

The performance of a single-cell direct methanol fuel cell (DMFC) using carbon nanotube-supported Pt–Ru (Pt–Ru/CNT) as an anode catalyst has been investigated. In this study, the Pt–Ru/CNT electrocatalyst was successfully synthesized using a modified polyol approach with a controlled composition very close to 20 wt.%Pt–10 wt.%Ru, and the anode was prepared by coating Pt–Ru/CNT electrocatalyst on a wet-proof carbon cloth substrate with a metal loading of about 4 mg cm⁻². A commercial gas diffusion electrode (GDE) with a platinum black loading of 4 mg cm⁻² obtained from E-TEK was employed as the cathode. The membrane electrode assembly (MEA) was fabricated using Nafion^® 117 membrane and the single-cell DMFC was assembled with graphite endplates as current collectors. Experiments were carried out at moderate low temperatures using 1 M CH₃OH aqueous solution and pure oxygen as reactants. Excellent cell performance was observed. The tested cell significantly outperformed a comparison cell using a commercial anode coated with carbon-supported Pt–Ru (Pt–Ru/C) electrocatalyst of similar composition and loading. High conductivity of carbon nanotube, good catalyst morphology and suitable catalyst composition of the prepared Pt–Ru/CNT electrocatalyst are considered to be some of the key factors leading to enhanced cell performance.     

Development of a miniaturized polymer electrolyte membrane fuel cell with silicon separators

The fabrication process and electrochemical characterization of a miniaturized PEM fuel cell with silicon separators were investigated. Silicon separators were fabricated with silicon fabrication technologies such as by photolithography, anisotropic wet etching, anodic bonding and physical vapor deposition (PVD). A 400 μm × 230 μm flow channel was made with KOH wet etching on the front side of a silicon separator, and then a 550 nm gold current collector and 350 nm TiN_x thin film heater were respectively formed on the front side and the opposite side by PVD. Two separators were assembled with the membrane electrode assembly (MEA) having a 4 cm² active area for the single cell. With pure hydrogen and oxygen under atmospheric pressure without humidification, the performance of the single fuel cell was measured. A single cell operation led to generation of 203 mW cm⁻² at 0.6 V at room temperature, which corresponded to 360 mW cm⁻³ in terms of volumetric fuel cell power density, with 20 ccm of gas flow rate of hydrogen and oxygen at the inlet.     

Behavioral pattern of a monopolar passive direct methanol fuel cell stack

A passive, air-breathing, monopolar, liquid feed direct methanol fuel cell (DMFC) stack consisting of six unit cells with no external pump, fan or auxiliary devices to feed the reactants has been designed and fabricated for its possible employment as a portable power source. The configurations of the stack of monopolar passive feed DMFCs are different from those of bipolar active feed DMFCs and therefore its operational characteristics completely vary from the active ones. Our present investigation primarily focuses on understanding the unique behavioral patterns of monopolar stack under the influence of certain operating conditions, such as temperature, methanol concentration and reactants feeding methods. With passive reactants supply, the temperature of the stack and open circuit voltage (OCV) undergo changes over time due to a decrease in concentration of methanol in the reservoir as the reaction proceeds. Variations in performance and temperature of the stack are mainly influenced by the concentration of methanol. Continuous operation of the passive stack is influenced by the supply of methanol rather than air supply or water accumulation at the cathode. The monopolar stack made up of six unit cells exhibits a total power of 1000 mW (37 mW cm⁻²) with 4 M methanol under ambient conditions.     

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.     

Development of a small DMFC bipolar plate stack for portable applications

The direct methanol fuel cell (DMFC) is regarded as a promising candidate in portable electronic power applications. Bipolar plate stacks were systematically studied by controlling the operating conditions, and by adjusting the stack structure design parameters, to develop more commercial DMFCs. The findings indicate that the peak power of the stack is influenced more strongly by the flow rate of air than by that of the methanol solution. Notably, the stack performance remains constant even as the channel depth is decreased from 1.0 to 0.6 mm, without loss of the performance in each cell. Furthermore, the specific power density of the stack was increased greatly from ∼60 to ∼100 W l⁻¹ for stacks of 10 and 18 cells, respectively. The current status of the work indicates that the power output of an 18-cell short stack reaches 33 W in air at 70 °C. The outer dimensions of this 18-cell short stack are only 80 mm × 80 mm × 51 mm, which are suitable for practical applications in 10–20 W DMFC portable systems.     

Predicting the transient response of a serpentine flow-field PEMFC: II: Normal to minimal fuel and AIR

The three-dimensional (3D) transient model presented in part I is used to study the overshoot and undershoot behavior observed in a PEMFC during operation with fixed normal stoichiometic flow rates of hydrogen and air for a 1.0 V s⁻¹ change in the load. In contrast to the behavior with excess flow shown in part I, the predictions show second-order responses for both decreases and increases in the load. That is, there is current overshoot when the load cell is decreased from 0.7 V to 0.5 V and there is current undershoot when the cell voltage is increased from 0.5 V to 0.7 V. The simulation of a 10 cm² reactive area with a serpentine flow path is used to explain this behavior in terms of the reacting gas concentrations, the flow through the gas diffusion media, the movement of water through the MEA by electro-osmotic and back diffusion forces, and the variation in the distributions of current density. The operating conditions correspond to 101 kPa, 70 °C cell temperature, anode and cathode dew-points and stoichiometries of 65 °C and 57 °C and 1.45 and 2.42 at an initial operating voltage of 0.7 V and current density of 0.33 A cm⁻². The fixed flow rates correspond to stoichiometries of 1.05 and 1.73 at 0.5 V for the 0.46 A cm⁻² predicted current density. The predictions illustrate regions where the MEA may alternate between wet and dry conditions and this may be useful to explain stability and durability of the MEA during transient operation.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.     

Methanol crossover in direct methanol fuel cells: a link between power and energy density

Direct methanol fuel cell performance curves were obtained as a function of three parameters, (1) temperature, (2) fuel flow-rate and (3) concentration. Methanol crossover was measured by gas chromatography as a function of these three parameters at 100 mA/cm² in the single-pass fuel delivery mode. The data was used to model a continuous loop mode where pure methanol is injected into a loop that circulates through the flow-field and recovers water from the cathode. The modeled loop composition is identical to the fuel stream used in the single-pass experiments (dilute aqueous methanol). The model results, presented in three-dimensional surfaces, elucidate the impact of parameter variations on the energy and power density of the direct methanol fuel cell (DMFC) and the link between those two figures of merit. In addition, a reasonable estimate of the contribution of mass transport effects due to the carbon fabric current collectors is made along with in situ CO stripping experiments on membrane electrode assembly (MEA) anode surfaces. The analysis shows that, at present, serious compromises are required if reasonable energy and power densities are to be simultaneously maintained in DMFCs using Nafion™ 117 as an electrolyte.