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

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

Development of open-cathode polymer electrolyte fuel cells using printed circuit board flow-field plates: Flow geometry characterisation

Open-cathode air-breathing fuel cells have the advantage of reduced system complexity and simplified operation, as oxygen is taken directly from ambient air without the need for blowers/compressors. In this study, printed circuit boards (PCBs) are used as flow-field plates. The use of PCBs offers the potential for significant cost reduction due to their well-established manufacturing processing and low materials cost. This study investigates the effect of varying the cathode geometry (parallel and circular) and opening ratios (43%, 53% and 63%) on fuel cell performance using polarisation curves, electrochemical impedance spectroscopy (EIS) and thermal imaging. The results obtained indicate that circular openings afford lower Ohmic resistance than parallel flow-field designs, which helps improve contact between the gas diffusion layer and flow-field plate. However, flow-field plates with circular openings suffer from greater mass transport limitation effects. Likewise, greater opening ratios offer better mass transport but increased Ohmic resistance as a result of the reduced area of lands/ribs. The thermal imaging results reveal lower temperature in the middle of the fuel cell due to “bowing” of the printed circuit board flow field plates which reduces the local current density. A trade-off between these factors results in a design with a maximum area specific power density of 250 mW cm⁻².     

Lightweight current collector based on printed-circuit-board technology and its structural effects on the passive air-breathing direct methanol fuel cell

To realize lightweight design of the fuel cell system is a critical issue before it is put into practical use. The printed-circuit-board (PCB) technology can be potentially used for production of current collectors or flow distributors. This study develops prototypes of a single passive air-breathing direct methanol fuel cell (DMFC) and also an 8-cell mono-polar DMFC stack based on PCB current collectors. The effects of diverse structural and operational factors on the cell performance are explored. Results show that the methanol concentration of 6 M promotes a higher cell performance with a peak power density of 18.3 mW cm⁻². The combination of current collectors using a relatively higher anode open ratio and inversely a lower cathode open ratio helps enhance the cell performance. Dynamic tests are also conducted to reveal transient behaviors and its dependence on the operating conditions. To validate the real working status of the DMFC stack, it is coupled with an LED lightening system. The performance of this hybrid system is also reported in this study.     

Performance characteristics of air-breathing anion-exchange membrane direct ethanol fuel cells

An air-breathing direct ethanol fuel cell (DEFC) with an anion-exchange membrane (AEM) and Pt-free electrodes is designed and investigated. Particular attention is paid to studying the performance characteristics of the air-breathing AEM DEFC. Experimental results reveal that this air-breathing AEM DEFC yields a peak power density as high as 38 mW cm⁻² at room temperature, which is comparable to the conventional Pt-based proton exchange membrane direct methanol fuel cells (PEM DMFCs). The overshoot/undershoot behaviors of both the cell voltage and cell temperature are avoided in the air-breathing AEM DEFC due to the use of ethanol-tolerant cathode catalyst. It is also found that the cathode water flooding behavior occurs in this air-breathing AEM DEFC, thus lowering the cell performance.     

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.     

Optimization of graded catalyst layer to enhance uniformity of current density and performance of high temperature-polymer electrolyte membrane fuel cell

The optimal use of catalyst materials is essential to improve the performance, durability and reduce the overall cost of the fuel cell. The present study is related to spatial distributions of current and overpotential for various graded catalyst structures in a high temperature-polymer electrolyte membrane fuel cell (HT-PEMFC). The effect of catalyst gradient across the catalytic layer (CL) thickness and along the channel and their combination on cell performance and catalyst utilization is investigated. The graded catalytic structure comprises two, three, or multiple layers of catalyst distribution. For a total cathode catalyst loading of 0.35 mg/cm², higher loading near the membrane presents improved cell performance and catalyst utilization due to reduced limitations caused by oxygen and ion diffusions. However, non-uniformity in the current distribution is significantly increased. The increase in the catalyst loading along the reactant flow provides a substantially uniform current density but lower cell performance. The synergy of varying catalytic profiles across the CL thickness and along the cathode flow direction is investigated. The results emphasize the importance of a rational design of cathode structure and mathematical functions as a strategic tool for functional grading of a CL towards improved uniform current distribution and catalyst utilization.     

Innovative water management in micro air-breathing polymer electrolyte membrane fuel cells

The role of cathodic cover opening ratio on water management was investigated for micro air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The results demonstrate the possibility to manage water content in micro-PEMFC using cover opening ratio variation. By measuring the internal resistance of a cell in various cover configurations (0.33 Ω cm² to 4.0 Ω cm²), the influence of cover opening ratio on water management was shown. Indeed, for a cell situated in a 10% relative humidity atmosphere and operated at 0.5 V, the addition of a 5% opening ratio cover allowed to reach similar current densities (270 mA cm⁻²) to those recorded for the same potential at 70% relative humidity without cover. Although the starting current density for a cell operated at 60 °C without gas humidification was extremely low (15 mA cm⁻²), the total closure of the cover allowed to maintain the water produced and accumulated by the cell at the cathode, and current density of 800 mA cm⁻² were reached after height minutes of operation. The influence of the opening ratio on back-diffused water was also evaluated and the maximum of back-diffused water was observed for a cell operated with a 5% cover opening ratio and represented 33% of the total water product at 150 mA cm⁻².A new method of anodic water evacuation, which does not increase the cell volume and which does not require any control tool was carried out and experimentally evaluated.     

Air-breathing miniature planar stack using the flexible printed circuit board as a current collector

To maximize power density, the volume of a fuel cell stack should be reduced by miniaturizing the stack components. In this study, thin flexible printed circuit board was utilized as a current collector in order to reduce an air-breathing monopolar stack's volume. Also, the effects of varying the geometry and opening ratios of the ports to the cathode on stack performance were evaluated in order to determine the optimal cathode structure. Use of the thin current collector and cathode port optimization resulted in an output of 3.5 W from an 18 cm³ stack (power density of 350 mW/cm²). The effects of orientation under passive air-breathing operation were determined to be nearly negligible. All data was measured at ambient pressure and temperature, baseline conditions for mobile fuel cell intended for use in consumer electronics.     

Planar air breathing PEMFC with self-humidifying MEA and open cathode geometry design for portable applications

In this paper, planar air breathing PEMFCs without the need for endplates are proposed for low power portable applications. PEMFCs with 3 different cathode designs (parallel slit, circular open and oblique slit) with the same opening ratio and employing self-humidifying MEAs were investigated. Performance and stability tests were conducted in hydrogen dead-end operation under both self-breathing and forced convection condition. It was found that rib geometry and hydraulic diameter have significant impact on oxygen transportation. It was concluded that circular opening design yields the best performance and highest limiting current. This is because this design provides the shortest rib distance and smallest hydraulic diameter. However, fuel cell instability was observed under self-breathing and forced convection condition. This is due to the water accumulation that could not be removed by natural-evaporation at the opening cathode. Overall, our proposed air breathing PEMFC achieves a specific power of 150 W kg⁻¹ and a power density of 347 mW cm⁻³.     

In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells

Air-delivery is typically the largest parasitic loss in PEM fuel cell systems. We develop a passive water management system that minimizes this loss by enabling stable, flood-free performance in parallel channel architectures, at very low air stoichiometries. Our system employs in situ-polymerized wicks which conform to and coat cathode flow field channel walls, thereby spatially defining regions for water and air transport. We first present the fabrication procedure, which incorporates a flow field plate geometry comparable to many state-of-the-art architectures (e.g., stamped metal or injection molded flow fields). We then experimentally compare water management flow field performance versus a control case with no wick integration. At the very low air stoichiometry of 1.15, our system delivers a peak power density of 0.68 W cm⁻². This represents a 62% increase in peak power over the control case. The open channel and manifold geometries are identical for both cases, and we demonstrate near identical inlet-to-outlet cathode pressure drops at all fuel cell operating points. Our water management system therefore achieves significant performance enhancement without introducing additional parasitic losses.     

Current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes

Scale-up is required in the practical application of microfluidic fuel cells. Using an array of electrodes is demonstrated as a promising way. However, the non-uniform current density distribution in array anodes will significantly limit the power output. In this study, current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes is tested under acidic and alkaline conditions. The array anode is divided into four layers according to their distance to cathode. Current density of each layer is recorded individually. The cell performance under alkaline media is better than that under acidic media and various current density distributions are found under different media. When the air-breathing microfluidic fuel cell is operated under acidic media, at current densities lower than 50 mA cm^?3, current densities of two-layer anodes far from the cathode are higher than that of the other layers, while the reverse happens at current densities higher than 50 mA cm^?3. This is mainly due to the enhanced fuel transport caused by CO₂ bubbles and the lower ohmic resistance. Moreover, the generated CO₂ bubbles lead to fluctuation of discharging densities especially at low voltages. However, for the air-breathing microfluidic fuel cell operated under alkaline media, two-layer anodes far from the cathode are main contributor to the total current density at all operation voltages.     

Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates

We present a study of proton exchange membrane (PEM) fuel cells with parallel channel flow fields for the cathode, dry inlet gases, and ambient pressure at the outlets. The study compares the performance of two designs: a standard, non-porous graphite cathode plate design and a porous hydrophilic carbon plate version. The experimental study of the non-porous plate is a control case and highlights the significant challenges of operation with dry gases and non-porous, parallel channel cathodes. These challenges include significant transients in power density and severe performance loss due to flooding and electrolyte dry-out. Our experimental study shows that the porous plate yields significant improvements in performance and robustness of operation. We hypothesize that the porous plate distributes water throughout the cell area by capillary action; including pumping water upstream to normally dry inlet regions. The porous plate reduces membrane resistance and air pressure drop. Further, IR-free polarization curves confirm operation free of flooding. With an air stoichiometric ratio of 1.3, we obtain a maximum power density of 0.40 W cm⁻², which is 3.5 times greater than that achieved with the non-porous plate at the same operating condition.     

Development and performance evaluation of a high temperature proton exchange membrane fuel cell with stamped 304 stainless steel bipolar plates

In this work, a high temperature proton exchange membrane fuel cell (HT-PEMFC) with stamped SS304 bipolar plates is successfully developed. Its performance was evaluated under two types of gaskets at different assembly torques and air stoichiometric ratios. The rates of pressure loss at a torque of 7 N-m with 50 Shore A hardness gaskets was 2.0 × 10^?3 MPa min^?1, which is acceptable. The best performance of the developed HT-PEMFC with stamped SS304 bipolar plates was 228.33 mW cm^?2, which approaches the performance of HT-PEMFCs with graphite bipolar plates. The optimal air stoichiometric ratio for the HT-PEMFC with stamped SS304 bipolar plates was 4.0, which is higher than that for proton exchange membrane fuel cells with CNC milled graphite bipolar plates. This is probably because of the deformation of the flow channels under the assembly compression force, which causes an elevated gas-diffusion drag in the flow channels. After the test, it was observed that some products of corrosion reaction formed on the surface of the SS304 bipolar plate. This phenomenon may lead to a decrease in the operating life of the HT-PEMFC.     

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.     

Air-breathing membraneless laminar flow-based fuel cell with flow-through anode

This paper describes a detailed characterization of laminar flow-based fuel cell (LFFC) with air-breathing cathode for performance (fuel utilization and power density). The effect of flow-over and flow-through anode architectures, as well as operating conditions such as different fuel flow rates and concentrations on the performance of LFFCs was investigated. Formic acid with concentrations of 0.5 M and 1 M in a 0.5 M sulfuric acid solution as supporting electrolyte were exploited with varying flow rates of 20, 50, 100 and 200 μl/min. Because of the improved mass transport to catalytic active sites, the flow-through anode showed improved maximum power density and fuel utilization per single pass compared to flow-over planar anode. Running on 200 μl/min of 1 M formic acid, maximum power densities of 26.5 mW/cm² and 19.4 mW/cm² were obtained for the cells with flow-through and flow-over anodes, respectively. In addition, chronoamperometry experiment at flow rate of 100 μl/min with fuel concentrations of 0.5 M and 1 M revealed average current densities of 34.2 mA/cm² and 52.3 mA/cm² with average fuel utilization of 16.3% and 21.4% respectively for flow-through design. The flow-over design had the corresponding values of 25.1 mA/cm² and 35.5 mA/cm² with fuel utilization of 11.1% and 15.7% for the same fuel concentrations and flow rate.     

Experimental study of a novel piezoelectric proton exchange membrane fuel cell with nozzle and diffuser

The newly designed proton exchange membrane fuel cell with a piezoelectric actuation structure, called a PZT-PEMFC, can force air into an air-breathing PEMFC system. Previous studies indicated the PZT-PEMFC may solve the water-flooding problem and improve cell performance. In this experimental study, a PZT-PEMFC with nozzle and diffuser, PZT-PEMFC-ND, is built to verify the previous theoretical study. This innovative design may direct air flow into the cathode channel through the diffuser and prevent air backflow without valves. The performance test includes an analysis of PZT vibration frequencies, cell operation temperatures, gravity effect, and designs of the nozzle and diffuser. The optimal operating temperature for the PZT-PEMFC-ND is 323 K to avoid the risk of higher temperatures drying out the membrane electrode assembly (MEA). The optimal vibration frequency of the PZT-PEMFC-ND is 180 Hz, which may pump in enough air and solve the water-flooding problem in the cathode channel. This study also concludes that the innovative design of the PZT-PEMFC-ND, may reach the performance of an open cathode stack configuration, 0.18 W cm⁻², without an external air supply device.     

Numerical and experimental investigation on 25 cm2 and 100 cm2 PEMFC with novel sinuous flow field for effective water removal and enhanced performance

Commercial viability of fuel cells is limited as it does not produce the same power density while scaling and stacking, generation and safe storage of hydrogen is another snag. This work addresses water lodging at cathode (a scaling issue) through a novel sinuous flow field both numerically and experimentally, by scaling up of PEMFC from 25 cm² to 100 cm². Conventional serpentine flow field of 25 cm² widely studied in literature is experimented to validate the numerical model in a multiphysics tool. The model developed was applied to sinuous flow field of 25 cm² and the results revealed better water removal and 7.7% higher power density than serpentine flow field due to inter channel diffusion and under rib convection. In order to increase power density further the dwell time at anode has to be increased in sinuous flow field, hence anode side flow field was made serpentine while retaining sinuous flow field at cathode. This combination enhanced the performance the power density by about 14%. This serpentine-sinuous combination was then scaled to 100 cm² and experimented, revealing a lower power drop than serpentine flow field.     

Development of cylindrical PEM fuel cells with semi-cylindrical cathode current collectors

Proton exchange membrane (PEM) fuel cells are attractive because of advantages such as low-temperature operation, no emission of harmful gases and high efficiency. However, the bipolar plates used in the state-of-the-art planar architecture are costly and increase the dead weight of the cell. In addition, the flow channels in the planar fuel cell increase the difficulty in removing the water produced in the cathode during cell operation. Cylindrical PEM fuel cells, on the other hand, do not require bipolar plates and there is no need for precisely machined flow channels. Thus, cylindrical PEM fuel cells are cheap, efficient in water management, and possess higher volumetric and gravimetric power density compared to planar PEM fuel cells. The design of a cylindrical fuel cell is very simple, but the fabrication of the same is fairly complex. In this work, a novel cathode current collector design for cylindrical PEM fuel cell has been developed. The cell performance was limited by low open circuit voltage and high ohmic resistance. The open circuit voltage of the cell is increased from 0.85 V to 0.95 V using an acrylic based adhesive to seal the membrane edges. The contact resistance of the cell is reduced from 75 mOhm to 50 mOhm by increasing the contact pressure on the membrane electrode assembly and it is further reduced to 30 mOhm by gold coating the current collectors. Furthermore, a cumulative 40% increase in peak power has been achieved from the optimization of cathode rib width and hydrogen flow rate. The optimized cell delivered a current density of 400 mA/cm² at 0.6 V and peak power of 2 W, which is appreciable considering the fact that the cell is air-breathing and operated with very minimal subsystems.     

Validation of an externally oil-cooled 1 kWel HT-PEMFC stack operating at various experimental conditions

The performance of 1 kW_el 48-cell HT-PEMFC at various experimental conditions is presented, particularly at several CO concentrations (up to 1.0%). Polarization curves measured at various anode (1.0–2.5) and cathode (1.6–4.0) stoichiometries; stack operating temperatures (120–160 °C) and gas pressures (up to 0.5 bar_g) are reported and analysed. The minimum gas stoichiometries of 1.25 and 2.0 were determined for the anode and cathode, respectively. The highest stack power density of 225 mW cm⁻² was measured at 160 °C and 0.4 A cm⁻². Operation at CO concentrations up to 1% was achieved, although a loss of performance of about 4% was observed for low CO concentrations. The operating temperature enhanced fuel cell performance and tolerance to CO, even when supplied with higher CO concentration in the anode feed gas.     

Effect of chloride impurities on the performance and durability of polybenzimidazole-based high temperature proton exchange membrane fuel cells

The effect of chloride as an air impurity and as a catalyst contaminant on the performance and durability of polybenzimidazole (PBI)-based high temperature proton exchange membrane fuel cell (HT-PEMFC) was studied. The ion chromatographic analysis reveals the existence of chloride contaminations in the Pt/C catalysts. Linear sweep voltammetry was employed to study the redox behavior of platinum in 85% phosphoric acid containing chloride ions, showing increase in oxidation and decrease in reduction current densities during the potential scans at room temperature. The potential scans at high temperatures in 85% phosphoric acid containing chloride ions showed both increase in oxidation and reduction current densities. The fuel cell performance, i.e. the current density at a constant voltage of 0.4 V and 0.5 V was found to be degraded as soon as HCl was introduced in the air humidifier. The performance loss was recovered when switching from the HCl solution back to pure water in the air humidifier. Under an accelerated aging performance test conducted through potential cycling between 0.9 V and 1.2 V, the PBI-based fuel cell initially containing 0.5 NaCl mg cm⁻² on the cathode catalyst layer exhibited a drastic degradation in the performance as compared to the chloride free MEAs. The mechanisms of the chloride effect on the fuel cell performance and durability were further discussed.