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.     

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.     

Methanol conditioning for improved performance of formic acid fuel cells

This paper considers the effect of methanol pretreatment on the performance of a direct formic acid fuel cell (DFAFC). We find that conditioning of the cell in methanol results in a substantial increase in current. The current at 60 °C increases from 95 to 320 mA/cm² at 0.3 V. The maximum power density increases from 33 to 119 mW/cm². The cell resistance decreases from 0.37 to 0.32 Ω cm². CO stripping experiments show that the catalyst is not being greatly affected by these changes. Our interpretation of the data is that the anode layer of membrane electrolyte assembly (MEA) undergoes some change during the methanol conditioning. The change improves the performance.     

Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray

In order to develop high performance intermediate temperature (<800 °C) solid oxide fuel cells (SOFCs) with a lower fabrication cost, a pressurized spray process of ceramic suspensions has been established to prepare both dense yttria-stabilized zirconia (YSZ) electrolyte membranes and transition anode layers on NiO + YSZ anode supports. A single cell with 10 μm thick YSZ electrolyte on a porous anode support and ∼20 μm thick cathode layer showed peak power densities of only 212 mW cm⁻² at 700 °C and 407 mW cm⁻² for 800 °C. While a cell with 10 μm thick YSZ electrolyte and a transition layer on the porous anode support using a ultra-fine NiO + YSZ powder showed peak power densities of 346 and 837 mW cm⁻² at 700 and 800 °C, respectively. The dramatic improvement of cell performance was attributed to the much improved anode microstructure that was confirmed by both scanning electron microscopes (SEM) observation and impedance spectroscopy. The results have demonstrated that a pressurized spray coating is a suitable technique to fabricate high performance SOFCs and at lower cost.     

Performance characteristics of a vapor feed passive miniature direct methanol fuel cell

In this paper, a new vapor feed fuel delivery system for a passive direct methanol fuel cell (DMFC) is developed and tested. Anode hydrophilic layers, electrical heating power and carbon dioxide release are examined to find their effects on the power density, efficiency and average temperatures of the cell. The hydrophilic layers act as a buffer layer between the vapor chamber and the anode gas diffusion layer (GDL). This layer allows water and methanol to mix, as well as distribute uniformly across the anode surface. Measurement of several parameters such as current, voltage, power, internal resistance, vapor chamber pressure, relative humidity and carbon dioxide concentration are taken. A maximum power density of 33 mW cm^?2 is achieved as well as 120 h of continuous operation at a constant current of 50 mA cm^?2 using the vapor feed system. The fuel utilization efficiency during the 120 h test is 34.8% and the energy efficiency is 8.2%.     

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.     

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.     

Electrooxidation of ascorbic acid on polyaniline and its implications to fuel cells

l-Ascorbic acid (AA) has been shown to undergo oxidation on polyaniline (PANI) without a platinum-group catalyst. A direct ascorbic acid fuel cell (DAAFC) has been assembled by employing an anode coated with PANI catalyst. From the experimental studies using cyclic voltammetry, amperometry and IR spectroscopy, it has been concluded that PANI facilitates the oxidation of AA. It has been possible to achieve a maximum power density of 4.3 mW cm⁻² at a load current density of 15 mA cm⁻² at 70 °C. As both AA and PANI are inexpensive and environmental-friendly, the present findings are expected to be useful for the development of cost-effective DAAFCs for several low power applications.     

Characterization of a high performing passive direct formic acid fuel cell

Small fuel cells are considered likely replacements for batteries in portable power applications. In this paper, the performance of a passive air breathing direct formic acid fuel cell (DFAFC) at room temperature is reported. The passive fuel cell, with a palladium anode catalyst, produces an excellent cell performance at 30 °C. It produced a high open cell potential of 0.9 V with ambient air. It produced current densities of 139 and 336 mA cm⁻² at 0.72 and 0.53 V, respectively. Its maximum power density was 177 mW cm⁻² at 0.53 V. Our passive air breathing fuel cell runs successfully with formic acid concentration up to 10 and 12 M with little degradation in performance. In this paper, its constant voltage test at 0.72 V is also demonstrated using 10 M formic acid. Additionally, a reference electrode was used to determine distinct anode and cathode electrode performances for our passive air breathing DFAFC.     

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.     

New borohydride fuel cell with multiwalled carbon nanotubes as anode: A step towards increasing the power output

A borohydride fuel cell has been constructed using a platinized multiwalled carbon nanotube (MWCNT) anode and an air cathode having an anionic exchange membrane separating the anode and cathode. The MWCNT was functionalized with carboxylic acid under nitric acid reflux. Platinum metal was subsequently incorporated into it by galvanostatic deposition. The platinized functionalized MWCNT was characterized by thermogravimetric analysis, Fourier transform infrared spectrum, scanning electron microscope and X-ray diffraction. The fuel cell produced a voltage of 0.95 V at low currents and a maximum power density of 44 mW cm⁻² at room temperature in 10% sodium borohydride in a 4 M sodium hydroxide medium. Another borohydride fuel cell under identical conditions using carbon as the anode produced a cell voltage of 0.90 V and power density of about 20 mW cm⁻². The improved performance of the MWCNT is attributed to the higher effective surface area and catalytic activity.     

Performance of an air-breathing direct methanol fuel cell

An air-breathing direct methanol fuel cell (DMFC) is attractive for portable-power applications. There are, however, several barriers that must be overcome before DMFCs reach commercially viability. This study shows that the cell power density is strongly affected by the fabrication conditions of the membrane electrode assembly (MEA) and by the technique used for assembly of the cell components. The results indicate that reducing the pressure and the thickness of catalyst layer in the MEA fabrication process can significantly improve power density. The production of water at the cathode, especially at a high power density, is shown to have a strong impact on the operation of an air-breathing DMFC since water blocks the feeding of air to the cathode. The power density (≧20 mW cm⁻²) of an air-breathing DMFC is found to drop to nearly half of its initial value after 30–40 min of operation in a short-term stability test. This appears to be one of the major limitations for potable electronic applications. Despite the many practical difficulties associated with an air-breathing DMFC, an attempt is also made to highlight the importance of the component assembly technique using a small cell pack with four integrated unit cells.     

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.     

Operational characteristics of a 50 W DMFC stack

The characteristics of a 50 W direct methanol fuel cell (DMFC) stack were investigated under various operating conditions in order to understand the behavior of the stack. The operating variables included the methanol concentration, the flow rate and the flow direction of the reactants (methanol and air) in the stack. The temperature of the stack was autonomously increased in proportion to the magnitude of the electric load, but it decreased with an increase in the flow rates of the reactants. Although the operation of the stack was initiated at room temperature, under a certain condition the internal temperature of the stack was higher than 80 °C. A uniform distribution of the reactants to all the cells was a key factor in determining the performance of the stack. With the supply of 2 M methanol, a maximum power of the stack was found to be 54 W (85 mW cm⁻²) in air and 98 W (154 mW cm⁻²) in oxygen. Further, the system with counter-flow reactants produced a power output that was 20% higher than that of co-flow system. A post-load behavior of the stack was also studied by varying the electric load at various operating conditions.     

Nanoporous separator and low fuel concentration to minimize crossover in direct methanol laminar flow fuel cells

Laminar flow fuel cells (LFFCs) overcome some key issues – most notably fuel crossover and water management – that typically hamper conventional polymer electrolyte-based fuel cells. Here we report two methods to further minimize fuel crossover in LFFCs: (i) reducing the cross-sectional area between the fuel and electrolyte streams, and (ii) reducing the driving force of fuel crossover, i.e. the fuel concentration gradient. First, we integrated a nanoporous tracketch separator at the interface of the fuel and electrolyte streams in a single-channel LFFC to dramatically reduce the cross-sectional area across which methanol can diffuse. Maximum power densities of 48 and 70 mW cm^?2 were obtained without and with a separator, respectively, when using 1 M methanol. This simple design improvement reduces losses at the cathode leading to better performance and enables thinner cells, which is attractive in portable applications. Second, we demonstrated a multichannel cell that utilizes low methanol concentrations (<300 mM) to reduce the driving force for methanol diffusion to the cathode. Using 125 mM methanol as the fuel, a maximum power density of 90 mW cm^?2 was obtained. This multichannel cell further simplifies the LFFC design (one stream only) and its operation, thereby extending its potential for commercial application.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

Direct formic acid fuel cells

The performance of formic acid fuel oxidation on a solid PEM fuel cell at 60 °C is reported. We find that formic acid is an excellent fuel for a fuel cell. A model cell, using a proprietary anode catalyst produced currents up to 134 mA/cm² and power outputs up to 48.8 mW/cm². Open circuit potentials (OCPs) are about 0.72 V. The fuel cell runs successfully over formic acid concentrations between 5 and 20 M with little crossover or degradation in performance. The anodic polarization potential of formic acid is approximately 0.1 V lower than that for methanol on a standard Pt/Ru catalyst. These results show that formic acid fuel cells are attractive alternatives for small portable fuel cell applications.     

Progress in the development of a high-power,direct ethylene glycol fuel cell (DEGFC)

We recently reported on a high-power nanoporous proton-conducting membrane (NP-PCM)-based direct methanol fuel cell (DMFC) operated with triflic acid. However, accompanying the advantages of methanol as a fuel, such as low cost and ease of handling and storage, are several pronounced disadvantages: toxicity, high flammability, low boiling point (65 °C) and the strong tendency to pass through the polymer-exchange membrane (high crossover). The focus of this work is the development of a high-power direct ethylene glycol fuel cell (DEGFC) based on the NP-PCM. Ethylene glycol (EG) has a theoretical capacity 17% higher than that of methanol in terms of Ah ml⁻¹ (4.8 and 4, respectively); this is especially important for portable electronic devices. It is also a safer (bp 198 °C) fuel for direct-oxidation fuel cell (DOFC) applications. Maximum power densities of 320 mW cm⁻² (at 0.32 V) at 130 °C have been achieved in the DEGFC fed with 0.72 M ethylene glycol in 1.7 M triflic acid at 3 atm at the anode and with dry air at 3.7 atm at the cathode. The cell platinum loading was 4 mg Pt cm⁻² on each electrode. The overpotentials at the cathodes and at the anodes of the DEGFC and DMFC were measured, compared and discussed.     

Methoxy methane (dimethyl ether) as an alternative fuel for direct fuel cells

The electrooxidation of methoxy methane (dimethyl ether) was studied at different Pt-based electrocatalysts in a standard three-electrode electrochemical cell. It was shown that alloying platinum with ruthenium or tin leads to shift the onset of the oxidation wave towards lower potentials. On the other hand, the maximum current density achieved was lower with a bimetallic catalyst compared to that obtained with a Pt catalyst. The direct oxidation of dimethoxy methane in a fuel cell was carried out with Pt/C, PtRu/C and PtSn/C catalysts. When Pt/C catalyst is used in the anode, it was shown that the pressure of the fuel and the temperature of the cell played important roles to enhance the fuel cell electrical performance. An increase of the pressure from 1 to 3 bar leads to multiply by two times the maximum achieved power density. An increase of the temperature from 90 to 110 °C has the same effect. When PtRu/C catalyst is used in the anode, it was shown that the electrical performance of the cell was only a little bit enhanced. The maximum power density only increased from 50 to 60 mW cm⁻² at 110 °C using a Pt/C anode and a Pt_0.8Ru_0.2/C anode, respectively. But, the maximum power density is achieved at lower current densities, i.e. higher cell voltages. The addition of ruthenium to platinum has other effect: it introduces a large potential drop at relatively low current densities. With the Pt_0.5Ru_0.5/C anode, it has not been possible to applied current densities higher than 20 mA cm⁻² under fuel cell operating conditions, whereas 250 and almost 400 mA cm⁻² were achieved with Pt_0.8Ru_0.2/C and Pt/C anodes. The Pt_0.9Sn_0.1/C anode leads to higher power densities at low current densities and to the same maximum power density as the Pt/C anode.     

Improved photographic production of Pt nano-particles for fuel cell electrodes

An improved photographic Pt printing process has been developed, which is called the print-out process (POP). No developer is required in this process and the deposition efficiency was significantly improved by more than 6 times on carbon paper (CP) and 22 times on carbon-black-coated carbon paper (CB/CP) over the previously reported develop-out process (DOP) [1]. The Pt particle size can be easily controlled by varying the moisture content in the substrate and was reduced to 5 nm on blank CP by adding a stabilizing agent, ethylene glycol (EG), to the photo-emulsion. Due to the hydrophobic nature of CB/CP, both Nafion ionomer solution and ethylene glycol (EG) were mixed with the emulsion during the printing. SEM revealed that on this substrate Pt was distributed as ∼25 nm clusters consisting of 5 nm particles on the carbon-black. The mass specific catalytic activity for methanol oxidation of Pt printed on CB/CP by POP was increased five times compared to that of Pt printed by the previous DOP. The performance of the POP Pt in a H₂ PEM single fuel cell (5 cm²) was also evaluated. A peak power density of 288 mW cm⁻² was achieved with an anode POP Pt catalyst loading of 0.16 mg cm⁻² at 70 °C and 0.9 mg cm⁻² JM Pt at the cathode. Compared to the DOP Pt catalyst at about the same loading, peak power density was improved more than four times by using the POP Pt.