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.     

Optimization of water and air management systems for a passive direct methanol fuel cell

A multi-phase, multi-component, thermal and transient model is applied to simulate the operation of a passive direct methanol fuel cell and optimize the design. The model takes into consideration the thermal effects and the variation of methanol concentration at the feeding reservoir above the fuel cell. Polarization and constant current cases are numerically simulated and compared with experiments for liquid feed concentration, membrane thickness, water management and air management systems. Parameters considered when determining an optimal design include power density, fuel utilization and energy efficiencies and water balance coefficients. An optimal liquid feed concentration is determined to be 2.0 mol kg^?1, which achieved a maximum power density of 21 mW cm^?2 and a fuel utilization efficiency of 63.0%. An optimal design of a cell uses a thick membrane (Nafion 117) to reduce methanol crossover and two additional cathode GDLs to improve the water balance coefficient and efficiency of the cell. This combination results in a power density of 23.8 mW cm^?2 and a water balance coefficient of ?1.71. An air filter may also be added to improve the efficiency and water balance coefficient of the cell, however, a small loss in power density will also occur. Using an Oil Sorbents air filter the water balance coefficient is increased to ?0.85, the fuel utilization efficiency is improved by 27.35% and the maximum power density decreased to 21.6 mW cm^?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.     

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.     

NiO–YSZ cermets supported low temperature solid oxide fuel cells

Solid oxide fuel cells with thin electrolyte of two types, Sm_0.2Ce_0.8O_1.9 (SDC) (15 μm) single-layer and 8 mol% Yttria stabilized zirconia (YSZ) (5 μm) + SDC (15 μm) bi-layer on NiO–YSZ cermet substrates were fabricated by screen printing and co-firing. A Sm_0.5Sr_0.5CoO₃ cathode was printed, and in situ sintered during a cell performance test. The SDC single-layer electrolyte cell showed high electrochemical performance at low temperature, with a 1180 mW cm⁻² peak power density at 650 °C. The YSZ + SDC bi-layer electrolyte cell generated 340 mW cm⁻² peak power density at 650 °C, and showed good performance at 700–800 °C, with an open circuit voltage close to theoretical value. Many high Zr-content micro-islands were found on the SDC electrolyte surface prior to the cathode preparation. The influence of co-firing temperature and thin film preparation methods on the Zr-islands’ appearance was investigated.     

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.     

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.     

Ammonia fuel cell using doped barium cerate proton conducting solid electrolytes

Proton-conducting solid electrolytes composed of gadolinium-doped barium cerate (BCG) or gadolinium and praseodymium-doped barium cerate (BCGP) were tested in an intermediate-temperature fuel cell in which hydrogen or ammonia was directly fed. At 700 °C, BCG electrolytes with porous platinum electrodes showed essentially no loss in performance in pure hydrogen. Under direct ammonia at 700 °C, power densities were only slightly lower compared to pure hydrogen feed, yielding an optimal value of 25 mW cm⁻² at a current density of 50 mA cm⁻². This marginal difference can be attributed to a lower partial pressure of hydrogen caused by the production of nitrogen when ammonia is decomposed at the anode.A comparative test using BCGP electrolyte showed that the doubly doped barium cerate electrolyte performed better than BCG electrolyte. Overall fuel cell performance characteristics were enhanced by approximately 40% under either hydrogen or ammonia fuels using BCGP electrolyte. At 700 °C using direct ammonia feed, power density reached 35 mW cm⁻² at a current density of approximately 75 mA cm⁻². Minimal loss of performance was noted over approximately 100 h on-stream in alternating hydrogen/ammonia fuels.     

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.     

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.     

Adhesive copper films for an air-breathing polymer electrolyte fuel cell

A design for an air-breathing and passive polymer electrolyte fuel cell is presented. Such a type of fuel cell is in general promising for portable electronics. In the present design, the anode current collector is made of a thin copper foil. The foil is provided with an adhesive and conductive coating, which firstly tightens the hydrogen compartment without mask or clamping pressure, and secondly secures a good electronic contact between the anode backing and the current collector. The cathode comprises a backing, a gold-plated stainless steel mesh and a current collector cut out from a printed circuit board. Three geometries for the cathode current collector were evaluated. Single cells with an active area of 2 cm² yielded a peak power of 250–300 mW cm⁻² with air and pure H₂ in a complete passive mode except for the controlled flow of H₂. The cells’ response was investigated in steady state and transient modes.     

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.     

Hydrogels in methanol fuel cartridge used as a diffusion-rate-controlling agent suppressing the methanol crossover in passively operated flat-pack type DMFCs

Hydrogels are used in methanol fuel cartridges to control fuel diffusion from the fuel reservoir to the anode electrode. This ability to control fuel diffusion rates enables the reduction of excess fuel supply that causes fuel crossover through the proton-exchange membrane. Cell performance was increased drastically from 14 to 21.5 m W cm⁻² with higher methanol concentrations using hydrogel fuel cartridges that fulfill the role of a diffusion-rate-controlling agent. This result implies that the hydrogel retards fuel diffusion, even at higher concentrations and suppresses the methanol crossover.     

Anode-supported solid oxide fuel cell based on dense electrolyte membrane fabricated by filter-coating

A simple and cost-effective technique, filter-coating, has been developed to fabricate dense electrolyte membranes. Eight mole percent yttria-stabilized zirconia (YSZ) electrolyte membrane as thin as 7 μm was prepared by filter-coating on a porous substrate. The thickness of the YSZ film was uniform, and could be readily controlled by the concentration of the YSZ suspension and the rate of the suspension deposition. The YSZ electrolyte film was dense and was well bonded to the Ni-YSZ anode substrate. An anode-supported solid oxide fuel cell (SOFC) with a YSZ electrolyte film and a La_0.85Sr_0.15MnO₃ (LSM) + YSZ cathode was fabricated and its performance was evaluated between 700 and 850 °C with humidified hydrogen as the fuel and ambient air as the oxidant. An open circuit voltage (OCV) of 1.09 V was observed at 800 °C, which was close to the theoretical value, and the maximum power density measured was 1050 mW cm⁻². The results demonstrate that the dense YSZ film fabricated by filter-coating is suitable for application to SOFCs.     

Polymer separator and low fuel concentration to minimize crossover in microfluidic direct methanol fuel cells

Microfluidic direct methanol fuel cell (DMFC) has some key issues, such as fuel crossover and water management, which typically hamper the development of conventional polymer electrolyte‐based fuel cells. Here a method of minimizing fuel crossover in microfluidic DMFC is reported. A polymer separator at the interface of the fuel and electrolyte streams in a single‐channel microfluidic DMFC is used to reduce the cross‐sectional area across where methanol can diffuse. Based on the optimized fuel rate, fuel concentration and pore radius, a maximum power density of 7.4 mW cm^?2 was obtained with the separator using 2 M methanol. This simple design improvement reduces the voltage loss at the cathode and leads to better performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Dual-phase electrolytes for advanced fuel cells

Double-phase electrolyte (DPE) consisting of doped CeO₂/NiAl solid phase and NaOH liquid phase was used for fuel cells utilizing LiNiO₂ anode and Ag cathode at working temperatures over 450 °C. It was shown that the cells can produce a maximum output power of 716.2 mW cm⁻² at 590 °C even though utilized with relatively large thickness of electrolyte, from 0.8 to 1.2 mm. Most measurements of open circuit voltage (OCV) range between 1 and 1.2 V; a significantly higher OCV value of 1.254 V was also obtained. Liquid channel conductive mechanism of NaOH in DPE is proposed; both O²⁻ and H⁺ concur to conduct the current; the doped CeO₂ transports O²⁻ ions, whereas the molten second phase transports H⁺ protons. Moreover, SEM observations and EDS analysis suggest that Na⁺ and OH⁻ also contribute to enhance both OCV and output power of our cells. The addition of NiAl to the doped CeO₂ increases the mechanical strength and the output power of DPE; however the reasons of this latter effect are still to be further investigated. The results show that DPE is a promising electrolyte to manufacture fuel cells with advanced performances.     

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%.     

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.     

Increasing the operation temperature of polymer electrolyte membranes for fuel cells: From nanocomposites to hybrids

Among the possible systems investigated for energy production with low environmental impact, polymeric electrolyte membrane fuel cells (PEMFCs) are very promising as electrochemical power sources for application in portable technology and electric vehicles. For practical applications, operating FCs at temperatures above 100 °C is desired, both for hydrogen and methanol fuelled cells. When hydrogen is used as fuel, an increase of the cell temperature produces enhanced CO tolerance, faster reaction kinetics, easier water management and reduced heat exchanger requirement. The use of methanol instead of hydrogen as a fuel for vehicles has several practical benefits such as easy transport and storage, but the slow oxidation kinetics of methanol needs operating direct methanol fuel cells (DMFCs) at intermediate temperatures. For this reason, new membranes are required. Our strategy to achieve the goal of operating at temperatures above 120 °C is to develop organic/inorganic hybrid membranes. The first approach was the use of nanocomposite class I hybrids where nanocrystalline ceramic oxides were added to Nafion. Nanocomposite membranes showed enhanced characteristics, hence allowing their operation up to 130 °C when the cell was fuelled with hydrogen and up to 145 °C in DMFCs, reaching power densities of 350 mW cm⁻². The second approach was to prepare Class II hybrids via the formation of covalent bonds between totally aromatic polymers and inorganic clusters. The properties of such covalent hybrids can be modulated by modifying the ratio between organic and inorganic groups and the nature of the chemical components allowing to reach high and stable conductivity values up to 6.4 × 10⁻² S cm⁻¹ at 120 °C.     

Fabrication and properties of hybrid membranes based on salts of heteropolyacid,zirconium phosphate and polyvinyl alcohol

The fabrication and properties of a hybrid membrane based on cesium salt of heteropoly acid, zirconium phosphate and polyvinyl alcohol are described. The fabricated membranes were characterized for their intra molecular interaction, thermal stability, surface morphology, water content and surface-charge properties using Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), water uptake and ion-exchange capacity measurements. These membranes showed reduced methanol crossover (for possible application in DMFC) relative to that of Nafion^® 115. At 50% of relative humidity, the protonic conductivity of the hybrid membranes was in the range of 10⁻³ to 10⁻² S cm⁻¹. The feasibility of these hybrid membranes as proton conducting electrolyte in direct methanol fuel cell (DMFC) was investigated and preliminary results are compared with that of Nafion^® 115. A maximum power density of 6 mW cm⁻² with PVA–ZrP–Cs₂STA hybrid membrane was obtained with the cell operated in passive mode at 373 K and atmospheric pressure. Open circuit voltage of the cell operated with hybrid membranes are high compared to that of Nafion^® 115 indicating reduced methanol crossover.