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.     

Passive water management at the cathode of a planar air-breathing proton exchange membrane fuel cell

Water management is a significant challenge in portable polymer electrolyte membrane (PEM) fuel cells and particularly in proton exchange membrane (PEM) fuel cells with air-breathing cathodes. Liquid water condensation and accumulation at the cathode surface is unavoidable in a passive design operated over a wide range of ambient and load conditions. Excessive flooding or dry out of the open cathode can lead to a dramatic reduction of fuel cell power. We report a water management design based on a hydrophilic and electrically conductive wick. A prototype air-breathing fuel cell with the proposed water management design successfully operated under severe flooding conditions, ambient temperature 10 °C and relative humidity of 80%, for up to 6 h with no observable cathode flooding or loss of performance.     

Effect of the cathode open ratios on the water management of a passive vapor-feed direct methanol fuel cell fed with neat methanol

A novel approach has been proposed to improve the water management of a passive direct methanol fuel cell (DMFC) fed with neat methanol without increasing its volume or weight. By adopting perforated covers with different open ratios at the cathode, the water management has been significantly improved in a DMFC fed with neat methanol. An optimized cathode open ratio could ensure both the sufficient supply of oxygen and low water loss. While changing the open ratio of anode vaporizer can adjust the methanol crossover rate in a DMFC. Furthermore, the gas mixing layer, added between the anode vaporizer and the anode current collector to increase the mass transfer resistance, can improve the cell performance, decrease the methanol crossover, and increase the fuel efficiency. For the case of a DMFC fed with neat methanol, an anode vaporizer with the open ratio of 12% and a cathode open ratio of 20% produced the highest peak power density, 22.7 mW cm⁻², and high fuel efficiency, 70.1%, at room temperature of 25 ± 1 °C and ambient humidity of 25-50%.     

Iron phthalocyanine as a cathode catalyst for a direct borohydride fuel cell

A direct borohydride fuel cell (DBFC) is constructed using a cathode based on iron phthalocyanine (FePc) catalyst supported on active carbon (AC), and a AB₅-type hydrogen storage alloy (MmNi_3.55Co_0.75Mn_0.4Al_0.3) was used as the anode catalyst. The electrochemical properties are investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), etc. methods. The electrochemical experiments show that FePc-catalyzed cathode not only exhibits considerable electrocatalytic activity for oxygen reduction in the BH₄⁻ solutions, but also the existence of BH₄⁻ ions has almost no negative influences on the discharge performances of the air-breathing cathode. At the optimum conditions of 6 M KOH + 0.8 M KBH₄ and room temperature, the maximal power density of 92 mW cm⁻² is obtained for this cell with a discharge current density of 175 mA cm⁻² at a cell voltage of 0.53 V. The new type alkaline fuel cell overcomes the problem of the conventional fuel cell in which both noble metal catalysts and expensive ion exchange membrane were used.     

Operational condition analysis for vapor-fed direct methanol fuel cells

This paper investigates the analysis and design of optimal operational conditions for vapor-fed direct methanol fuel cells (DMFCs). Methanol vapor at a temperature of 35 °C is carried with nitrogen gas together with water vapor at 75 °C. In this experimental condition, stoichiometry of 10 is maintained for each fuel gas. The results show that the optimal operational concentration was 25–30 wt.% under methanol vapor feeding at the anode. The peak power was 14 mW cm² in polarization curves. To analyze major losses, the activation losses of the anode and cathode were measured by an in situ reference electrode and a working electrode. The activation loss of the anode is proportional to the water content and the high methanol concentration caused the activation loss of the cathode to increase due to methanol crossover. In the vapor-fed DMFC, the activation loss of the anode is higher than that of the cathode. Also, depending on the variation of the methanol concentration, the IR loss and Faradaic impedance is measured via impedance analysis. The methanol concentration significantly affects the IR loss and kinetics. Although the IR loss was more than the desired value at the optimal condition (25–30 wt.%), it did not significantly affect the cell’s performance. The cell operated at room temperature and ambient pressure that is a typical operation environment of air-breathing fuel cells.     

Design for segmented-in-series solid oxide fuel cell through mathematical modeling

The segmented-in-series solid oxide fuel cell comprising fuel channel, anode, cathode and electrolyte layers has been evaluated by developing a two-dimensional model, in which the equations have been solved numerically through finite element methods. The results indicate that the voltage of each membrane electrode assembly (MEA) exhibits a parabola-like curve and is higher than the appointed voltage of unit cell (0.7 V). From fuel inlet to outlet, the voltage of each MEA deceases due to the decreasing local H₂ concentration. When both the interconnector and electrolyte gap lengths are fixed, the cell module with 5 mm long anode gives the maximal power density for the SS-SOFC. Higher power densities can be achieved through increasing the cathode thickness.     

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.     

A long-term degradation study of power generation characteristics of anode-supported solid oxide fuel cells using LaNi(Fe)O3 electrode

The long-term operation of an anode-supported solid oxide fuel cell was examined to study the degradation factor. The cell was constructed using LaNi_0.6Fe_0.4O₃ (LNF), alumina-doped scandia stabilized zirconia (SASZ), and NiO-SASZ as the cathode, electrolyte, and anode respectively. The cell had Pt current collectors and was operated for 6500 h. The test was carried out at 1073 K with a constant load of 0.4 A cm⁻² and included thermal cycling. The cell voltage degradation rate was below 0.86%/1000 h when the cell was operated for up to 5200 h. Changes in the resistance of the cells during the experiments were analyzed by impedance spectroscopy. The cathode polarization resistance and ohmic resistance increased with time. The elements (Si and B) contained in the water condensed from the cathode exhaust gas were identified using inductively coupled plasma (ICP).     

Performance of a miniaturized silicon reformer-PrOx-fuel cell system

A fuel cell made with silicon is operated with hydrogen supplied by a reformer and a preferential oxidation (PrOx) reactor those are also made with silicon. The performance and durability of the fuel cell is analyzed and tested, then compared with the results obtained with pure hydrogen. Three components of the system are made using silicon technologies and micro electro-mechanical system (MEMS) technology. The commercial Cu-ZnO-Al₂O₃ catalyst for the reformer and the Pt-Al₂O₃ catalyst for the PrOx reactor are coated by means of a fill-and-dry method. A conventional membrane electrode assembly composed of a 0.375 mg cm⁻² PtRu/C catalyst for the anode, a 0.4 mg cm⁻² Pt/C catalyst for the cathode, and a Nafion™ 112 membrane is introduced to the fuel cell. The reformer gives a 27 cm³ min⁻¹ gas production rate with 3177 ppm CO concentration at a 1 cm³ h⁻¹ methanol feed rate and the PrOx reactor shows almost 100% CO conversion under the experimental conditions. Fuel cells operated with this fuel-processing system produce 230 mW cm⁻² at 0.6 V, which is similar to that obtained with pure hydrogen.     

Ni/YSZ anode - Effect of pre-treatments on cell degradation and microstructures

Anode supported (Ni/YSZ-YSZ-LSM/YSZ) solid oxide fuel cells were tested and the degradation over hundreds of hours was monitored and analyzed by impedance spectroscopy. Test conditions were chosen to focus on the Ni/YSZ anode degradation and all tests were operated at 750 °C, a current density of 0.75 A cm⁻². Oxygen was supplied to the cathode and the anode inlet gas mixture had a high p(H₂O)/p(H₂) ratio of 0.4/0.6. Commercially available gasses were applied. The effect of different types of pre-treatments on the Ni/YSZ electrode degradation during subsequent fuel cell testing was investigated. Pre-treatments included operating at OCV (4% and 40% H₂O in H₂) prior to fuel cell testing, cleaning of the inlet H₂ gas at 700 °C and processing the anode half cell via multilayer tape casting. Analyses of impedance spectra showed that the increase in the charge transfer reaction resistance in the Ni/YSZ (R_Ni,TPB) was decreased to ¼ or less for the pre-treated and fuel cell tested cells when compared with a non-pre-treated reference tested cell; all operated at the same fuel cell test conditions. Scanning electron microscopy and image analyses for the non-pre-treated reference tested cell and selected pre-treated cells showed significant differences in the area fractions of percolating nickel both in the active anode and support layer.     

Experimental analysis of spatio-temporal behavior of anodic dead-end mode operated polymer electrolyte fuel cell

During the anodic dead-end mode operation of fuel cells, the inert gases (nitrogen and water) present in the cathode side gas channel permeate to the anode side and accumulate in the anode gas channel. The inert gas accumulation in the anode decreases the fuel cell performance by impeding the access of hydrogen to the catalyst. The performance of fuel cell under potentiostatic dead-end mode operation is shown to have three distinct regions viz. time lag region, transient current region and a steady state current region. A current distribution measurement setup is used to capture the evolution of the current distribution as a function of time and space. Co- and counter-flow operations of dead-end mode confirm the propagation of inert gas from the dead-end of anode channel to the inlet of anode. Experiments with different oxidants, oxygen and air, under dead-end mode confirm that nitrogen which permeates from cathode to anode causes the performance drop of the fuel cell. For different starting current densities of 0.15 A cm⁻², 0.3 A cm⁻² and 0.6 A cm⁻² the inert gas occupies 35%, 45% and 57%, respectively of anode channel volume at the end of 60 min of dead-end mode operation.     

Mixed-reactant, micro-tubular solid oxide fuel cells: An experimental study

Anode-supported, micro-tubular solid oxide fuel cells were prepared and operated, utilizing mixed-reactant (methane and air mixture) supply. The cells were composed of conventional materials, i.e. nickel, yttria-stabilized zirconia (Ni-YSZ) as anode supported material, yttria-stabilized zirconia (YSZ) as electrolyte, and lanthanum strontium manganite (LSM) as cathode material. The cells were operated at various temperatures in between 550 and 800 °C with varying methane/air ratio (1:1-1:4.76). Cell performance was found to be strongly dependent on flow rate and mixing ratio. At 750 °C, the maximum open circuit voltage (OCV) of the cell was 1.05 V at a methane/air ratio of 1:4.76, with a maximum power output of 122 mW cm⁻². The degradation test shows 0.05% performance loss per 24 h, thereafter, fluctuations in current density were observed due to oxidation-reduction cycles over nickel surface. It is therefore concluded that although the methane/air ratio of 1:4.76 gives the best performance but the long-term performance is not guaranteed under such conditions.     

A novel direct ethanol fuel cell with high power density

A new type of direct ethanol fuel cell (DEFC) that is composed of an alkaline anode and an acid cathode separated with a charger conducting membrane is developed. Theoretically it is shown that the voltage of this novel fuel cell is 2.52 V, while, experimentally it has been demonstrated that this fuel cell can yield an open-circuit voltage (OCV) of 1.60 V and a peak power density of 240 mW cm⁻² at 60 °C, which represent the highest performance of DEFCs that has so far been reported in the open literature.     

Silver (Ag) as anode and cathode current collectors in high temperature planar solid oxide fuel cells

Planar electrolyte supported solid oxide fuel cells were operated at 900 °C with humidified H₂ for 200 h using silver mesh and paste for cathode current collection. Continuous potentiostatic tests at 0.7 V appeared to induce migration of Ag towards electrode-electrolyte interphase, while continuous OCV tests caused no mass transport. Similar SOFCs fueled by coal syngas at 850 °C using Ag for both anode and cathode current collection indicated little, if any, Ag migration; providing the possibility of employing Ag for 100 h laboratory scale tests using coal-derived syngas. Use of high temperature steam, carbon dioxide and carbon monoxide did not result in the formation of silver carbonates.     

Polarization measurements of anode-supported solid oxide fuel cells studied by incorporation of a reference electrode

A three-electrode system configuration was applied to an anode-supported solid oxide fuel cell where the anode to cathode surface area ratio was ∼7.9, and Ni/YSZ was used as the anode, LSM as the cathode, Pt as the reference electrode, and thin YSZ film as the electrolyte. The cell was polarized potentiostatically at −0.2, −0.4, −0.6 and −0.8 V versus open circuit voltage (OCV) and the potential change versus a reference electrode were recorded to ascertain the relative electrode polarization contributions. The results of these studies suggested that, while the anode contributions to cell polarization were less significant than that observed for the cathode, they were not negligible. Furthermore, the disparity in the relative electrode polarization contribution was observed to decrease with increasing temperature and polarization. Electrode polarization studies suggested that cathodic overvoltage decreased remarkably with increasing temperature whereas anodic overvoltage increased slightly with increasing temperature. Electrode kinetic parameters were extracted from these polarization experiments and the implications of these parameters to cell performance were discussed. Lastly, electrochemical impedance spectroscopy (EIS) data was presented to further elucidate the relative contributions of the anode and cathode impedances on button cell performance.     

Influences of fuel crossover on cathode performance in a micro borohydride fuel cell

Influences of borohydride crossover on cathode performance were studied in a micro direct borohydride fuel cell (DBFC). The results showed that fuel crossover resulted in decreases in open circuit potentials of cathodes. On the other hand, effects of fuel crossover on cathode overpotentials strongly depended on the cathode material. The Pt/C cathode demonstrated a small potential drop of 0.11 V, while the Ag/C cathode had a much larger potential drop of 0.26 V under the same condition. Fuel crossover was found to be depressed during current operations. It is possible that fuel depletion happened around the anode during operation, resulting in a decrease of fuel concentration gradient across the membrane and thus less crossover. Experiments also showed that a balance in wet-proof property is essential for the cathode in the direct borohydride fuel cell and the stability of cell performance was mainly dependent on wet-proof durability of the cathode.     

Electrochemical performance of a glucose/oxygen microfluidic biofuel cell

A microfluidic glucose/O₂ biofuel cell, delivering electrical power, is developed based on both laminar flow and biological enzyme strategies. The device consists of a Y-shaped microfluidic channel in which fuel and oxidant streams flow laminarly in parallel at gold electrode surfaces without convective mixing. At the anode, the glucose is oxidized by the enzyme glucose oxidase whereas at the cathode, the oxygen is reduced by the enzyme laccase, in the presence of specific redox mediators. Such cell design protects the anode from interfering parasite reaction of O₂ at the anode and works with different streams of oxidant and fuel for optimal operation of the enzymes. The dependence of the flow rate on the current is evaluated in order to determine the optimum flow that would provide little to no mixing while yielding high current densities. The maximum power density delivered by the assembled biofuel cell reaches 110 μW cm⁻² at 0.3 V with 10 mM glucose at 23 °C. This research demonstrates the feasibility of advanced microfabrication techniques to build an efficient microfluidic glucose/O₂ biofuel cell device.     

Analysis of operating parameters considering flow orientation for the performance of a proton exchange membrane fuel cell using the Taguchi method

This study has applied the L₁₈ 2 × 3⁷ orthogonal array of the Taguchi method to determine the optimal combination of six primary operating parameters (flow orientation, temperature of fuel cell, anode and cathode humidification temperatures, anode, and cathode stoichiometric flow ratios) of a PEM fuel cell. The optimal combination factor is co-flow, a cell temperature of 333 K, an anode humidification temperature of 353 K, a cathode humidification temperature of 333 K, a stoichiometric flow ratio for hydrogen of 2, and a stoichiometric flow ratio for oxygen of 3; and the amount of maximum power is 17.61 W. The results for the experiment indicate that flow orientation, temperature of fuel cell, and anode and cathode humidification temperatures are significant factors for affecting the performance. Furthermore, this study simulates the transport phenomenon and electrochemical reactions using a finite-element method at the optimal combination factor from the experimental results of Taguchi method.     

Conceptual design of a novel ammonia-fuelled portable solid oxide fuel cell system

A novel portable electric power generation system, fuelled by ammonia, is introduced and its performance is evaluated. In this system, a solid oxide fuel cell (SOFC) stack that consists of anode-supported planar cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode is used to generate electric power. The small size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. The results predicted through computer simulation of this system confirm that the first-law efficiency of 41.1% with the system operating voltage of 25.6 V is attainable for a 100 W portable system, operated at the cell voltage of 0.73 V and fuel utilization ratio of 80%. In these operating conditions, an ammonia cylinder with a capacity of 0.8 l is sufficient to sustain full-load operation of the portable system for 9 h and 34 min. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required in the SOFC stack, the first- and second-law efficiencies, the system operating voltage, the excess air, the heat transfer from the SOFC stack, and the duration of operation of the portable system with a cylinder of ammonia fuel, are also studied through a detailed sensitivity analysis. Overall, the ammonia-fuelled SOFC system introduced in this paper exhibits an appropriate performance for portable power generation applications.     

Electrochemical characterizations of microtubular solid oxide fuel cells under a long-term testing at intermediate temperature operation

We report the long-term stability of a microtubular solid oxide fuel cell (SOFC) operable at ∼500 °C. The SOFC consists of NiO-Gd doped ceria (GDC) as the anode as well as the tubular support, GDC as an electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-GDC as the cathode. A single tubular cell with a diameter of approximately 1.8 mm and an effective electrode length of approximately 20 mm generated 150 mW cm⁻² and 340 mW cm⁻² at 500 °C and 550 °C, respectively, under the operation conditions of 0.7 V and humidified H₂ fuel flow. The cell exhibited good stability with a degradation rate of 0.25%/100 h under operation conditions of 200 mA and 0.75 V.