The function of hydrophobic cathodic backing layers for high energy passive direct methanol fuel cell

Homemade wet-proofing carbon papers with back-flow effect were applied as backing layers in the cathode of passive air-breathing direct methanol fuel cell (DMFC) fed by pure methanol. With the increase of polytetrafluoroethylene (PTFE) content, the carbon papers exhibited different water transport resistance and generated different back-flow effects. Moreover, PTFE-treated carbon papers were observed by scanning electronic microscope (SEM) to investigate the function of the cross-linked microstructure. Maximum energy density (438 Wh L⁻¹) of the improved pure methanol DMFC was obtained by using carbon paper with 40 wt.% PTFE content as the cathodic backing layer. This value was 6 times larger than that of the conventional DMFC fed by 2 M methanol solution.     

Review on Li-air batteries—Opportunities, limitations and perspective

Li-air batteries are potentially viable ultrahigh energy density chemical power sources, which could potentially offer specific energies up to ∼3000 Wh kg⁻¹ being rechargeable. The modern state of art and the challenges in the field of Li-air batteries are considered. Although their implementation holds the greatest promise in a number of applications ranging from portable electronics to electric vehicles, there are also impressive challenges in development of cathode materials and electrolyte systems of these batteries.     

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.     

Development of a passive direct methanol fuel cell (DMFC) twin-stack for long-term operation

Direct methanol fuel cell (DMFC), with benefits such as high energy efficiency, quick start capability and instantaneous refueling, is a promising power source to meet the ever-increasing power demand for portable electronic products. In this paper, a novel CO₂-driven fuel-feed device was produced and equipped in a passive 8-cell DMFC twin-stack for long-term operation. It was shown that this fuel-feed device was capable of supplying methanol solution continuously in response to the change in discharging current of the stack. Stainless steel sheet was photochemically etched as current collectors based on MEMS techniques. Series interconnections between two neighbor cells were realized in banded configuration which avoided the external connection. TiN-plated mesh was placed between current collector and membrane electrode assembly (MEA), which was used to lessen the internal resistance of the stack. A peak power density of 16.9 mW cm⁻² was achieved with 4 M methanol at ambient temperature and passive operation. The stack equipped with the fuel feed device successfully powered a sensor node for 39 h with the consumption of 80 ml of 4 M methanol.     

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.     

Carbide-based fuel system for undersea vehicles

In underwater applications such as unmanned undersea vehicle (UUV) propulsion, mass and volume constraints often dictate system energy density and specific energy, which are targeted to exceed 300 Wh L⁻¹ and 300 Wh kg⁻¹, respectively, in order to compete with state-of-the-art battery technologies. To address this need, a novel carbide-based fuel system (CFS) intended for use with a solid oxide fuel cell (SOFC) is under development that is capable of achieving these energy metrics as well as sequestering carbon dioxide. The proposed CFS uses calcium carbide and calcium hydride that react with water to generate acetylene and hydrogen as the fuel and calcium hydroxide as a carbon dioxide scrubber. The acetylene is hydrogenated to ethane and then reformed to syngas (carbon monoxide and hydrogen) before being utilized by the SOFC. Carbon dioxide effluent from the SOFC is reacted with the calcium hydroxide to produce a storable solid, calcium carbonate, thus eliminating gas evolution from the UUV. A system configuration is proposed and discussion follows concerning energy storage metrics, operational parameters and preliminary safety analysis.     

An asymmetric anthraquinone-modified carbon/ruthenium oxide supercapacitor

An asymmetric supercapacitor with improved energy and power density, relative to a symmetric Ru oxide device, has been constructed with anthraquinone-modified carbon fabric (Spectracarb 2225) as the negative electrode and Ru oxide as the positive electrode. The performance of the supercapacitor was characterized by cyclic voltammetry and constant current discharging. Use of the anthraquinone-modified electrode extends the negative potential limit that can be used, relative to Ru oxide, and allows higher cell voltages to be used. The maximum energy density obtained was 26.7 Wh kg⁻¹ and an energy density of 12.7 Wh kg⁻¹ was obtained at a 0.8 A cm⁻² discharge rate and average power density of 17.3 kW kg⁻¹. The C-AQ/Ru oxide supercapacitor requires 64% less Ru relative to a symmetric Ru oxide supercapacitor.     

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.     

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.     

High energy density capacitor using coal tar pitch derived nanoporous carbon/MnO2 electrodes in aqueous electrolytes

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg⁻¹, power densities of 11 kW kg⁻¹ and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g⁻¹) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.     

Passive direct methanol fuel cells for portable electronic devices

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

Water and air management systems for a passive direct methanol fuel cell

In this paper water and air management systems were developed for a miniature, passive direct methanol fuel cell (DMFC). The membrane thickness, water management system, air management system and gas diffusion electrodes (GDE) were examined to find their effects on the water balance coefficient, fuel utilization efficiency, energy efficiency and power density. Two membranes were used, Nafion^® 112 and Nafion^® 117. Nafion^® 117 cells had greater water balance coefficients, higher fuel utilization efficiency and greater energy efficiency. A passive water management system which utilizes additional cathode gas diffusion layers (GDL) and a passive air management system which makes use of air filters was developed and tested. Water management was improved with the addition of two additional cathode GDLs. The water balance coefficients were increased from −1.930 to 1.021 for a cell using a 3.0 mol kg⁻¹ solution at a current density of 33 mA cm⁻². The addition of an air filter further increased the water balance coefficient to 1.131. Maximum power density was improved from 20 mW cm⁻² to 25 mW cm⁻² for 3.0 mol kg⁻¹ solutions by upgrading from second to third generation GDEs, obtained from E-TEK. There was no significant difference in water management found between second and third generation GDEs. A fuel utilization efficiency of 63% and energy efficiency of 16% was achieved for a 3.0 mol kg⁻¹ solution with a current density of 66 mA cm⁻² for third generation GDEs.     

Electrochemical micro-capacitors of patterned electrodes loaded with manganese oxide and carbon nanotubes

MnO₂ and carbon nanotubes (CNT) composite electrodes have been built on the interdigital stack layers of Fe-Al/SiO₂ and Fe-Al/Au/Ti/SiO₂ for the electrochemical micro-capacitors, using photolithography and thin-film technologies. The electrode properties and the performance of micro-cells are measured and analyzed with cyclic voltammetry (CV), impedance spectroscopy, and galvanostatic charge/discharge test in 0.1 M Na₂SO₄ electrolyte. The vertically aligned CNT, grown on Fe-Al/SiO₂, is more suitable for supporting the pseudocapacitive MnO₂ than the random CNT on Fe-Al/Au/Ti/SiO₂, but ohmic resistance of the former electrode is higher. We have prepared three cells on each stack layer with different electrode materials. The Ragone plot shows systematic variations in power and energy performance, reflecting their differences in electrode structure and polarization loss. The asymmetric cell of a pseudocapacitive positive electrode, loaded with MnO₂ and CNT, exhibits a small IR drop and a high specific energy during discharge. Built on Fe-Al/SiO₂, this asymmetric cell discharges at specific power 0.96 kW kg⁻¹ with specific energy 10.3 Wh kg⁻¹; while on Fe-Al/Au/Ti/SiO₂, the asymmetric cell discharges at power 1.16 kW kg⁻¹ with energy 5.71 Wh kg⁻¹.     

Development and characteristics of a 400 W-class direct methanol fuel cell stack

In this study, a 400 W-class direct methanol fuel cell (DMFC) stack is developed for large size portable applications and its operating behaviors under the various conditions are monitored. The DMFC stack comprising of 42-cells is assembled with graphite bipolar plates and membrane–electrode assemblies (MEAs) having an active area of 138 cm² per each. The stack is operated by varying the concentrations of methanol, stoichiometry (λ), and the electric load. In addition, other associated factors, such as voltage and temperature distributions along the individual unit cells, pressure drops inside the stack, voltage behaviors in response to the dynamic change of the electric load and the pHs of the effluent solutions from the outlets of both electrodes, are also studied in a detailed manner. The stack produces a power of 400 W under an operating condition of feeding 0.8 M methanol and 34 l/min air at 1 atm, and uniform distributions of temperature and voltage prevail in all the 42 unit cells. A long-term operation coupled with performance restoration processes shows that a typical single cell used in this stack is able to run with a good stability for more than 500 h without any substantial degradation in the performance.     

A small mono-polar direct methanol fuel cell stack with passive operation

A passive direct methanol fuel cell (DMFC) stack that consists of six unit cells was designed, fabricated, and tested. The stack was tested with different methanol concentrations under ambient conditions. It was found that the stack performance increased when the methanol concentration inside the fuel tank was increased from 2.0 to 6.0 M. The improved performance is primarily due to the increased cell temperature as a result of the exothermic reaction between the permeated methanol and oxygen on the cathode. Moreover, the increased cell temperature enhanced the water evaporation rate on the air-breathing cathode, which significantly reduced water flooding on the cathode and further improved the stack performance. This passive DMFC stack, providing 350 mW at 1.8 V, was successfully applied to power a seagull display kit. The seagull display kit can continuously run for about 4 h on a single charge of 25 cm³ 4.0-M methanol solution.     

Design,fabrication and testing of a PMMA-based passive single-cell and a multi-cell stack micro-DMFC

A passive, air-breathing polymethyl methacrylate (PMMA) based single-cell and a multi-cell stack micro-direct methanol fuel cell (DMFC) with 1.0 cm² active area with a novel cathode plate structure and assembly layer are designed, fabricated and tested. The fuel cell is completely passive with no auxiliary device such as pump or fan. Oxygen is taken from the surrounding air, and the methanol solution is stored in a built-in reservoir. The performance of the single cell is tested with different methanol concentrations ranging from 1.0 M to 5.0 M, and the optimum performance is achieved by using methanol at a concentration of 4.0 M. A stack with 6 cells is fabricated and tested with the optimum methanol concentration of 4.0 M, and power levels produced by different catalyst loadings on the anode are compared. Besides, this study also considers the cost analysis of micro-DMFC. The combination of a catalyst loading of 3.0 mg cm⁻² Pt/Ru on the anode and 2.0 mg cm⁻² Pt on the cathode yield the highest power of 12.05 mW at 1.08 V and 11.2 mA. The total cost for the micro-DMFC in this study is only about USD 2 mW⁻¹.     

Recent advances in NiMH battery technology

Nickel-metal hydride (NiMH) is a commercially important rechargeable battery technology for both consumer and industrial applications due to design flexibility, excellent energy and power, environmental acceptability and cost. [1] From the initial product introduction in 1991 of cylindrical cells having an energy of 54 Wh kg⁻¹, today's small consumer cells have a specific energy over 100 Wh kg⁻¹. Numerous licensed manufacturers produce a myriad of NiMH products ranging from 30 mAh button cells to a wide variety of consumer cylindrical products, prismatic cells up to 250 Ah for electric buses and 6 Ah multicell modules for hybrid electric vehicles. Power has increased from under 200 to 1200 W kg⁻¹ commercially and up to 2000 W kg⁻¹ at a development level [2].     

A miniature direct formic acid fuel cell battery

This work describes miniature formic acid fuel cell batteries, which are built based on a Nafion^® membrane and thin metal foils. The intrinsic advantages of formic acid fuel allow for a very simple design of the fuel cell, and the volume of the complete system, including fuel reservoir, can be as small as 11 mm³. This work examines the effect of membrane thicknesses and fuel concentrations on the cell performance. The optimized cell performance is obtained with N117 membrane and 12 M fuel. Peak power density of the optimized cell is 112 mW cm⁻². Life tests are conducted at various conditions using 6 μL of fuel. An energy density of 70 Wh L⁻¹ with 40% fuel utilization rate is observed when 12 M formic acid is used at 0.5 V.     

Electrochemical profile of an asymmetric supercapacitor using carbon-coated LiTi2(PO4)3 and active carbon electrodes

In this work, we reported an asymmetric supercapacitor in which active carbon (AC) was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode in 1 M Li₂SO₄ aqueous electrolyte. The LiTi₂(PO₄)₃/AC hybrid supercapacitor showed a sloping voltage profile from 0.3 to 1.5 V, at an average voltage near 0.9 V, and delivered a capacity of 30 mAh g⁻¹ and an energy density of 27 Wh kg⁻¹ based on the total weight of the active electrode materials. It exhibited a desirable profile and maintained over 85% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1000 W kg⁻¹, it had a specific energy 15 Wh kg⁻¹ compared with 24 Wh kg⁻¹ at the power density about 200 W kg⁻¹.     

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

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