Design,fabrication and performance analysis of a 200 W PEM fuel cell short stack

A PEM fuel cell short stack of 200 W capacity, with an active area of 100 cm² has been designed and fabricated in-house. The status of unit cell performance was 0.55 W cm⁻². Based on the unit cell technology, a short stack has been developed. The proper design of uniform flow distribution, cooling plate and compressed end plate were important to achieve the best performance of the short stack. The performance of four cells stack was analyzed in static and dynamic modes. In the static mode of polarization curve, the stack has peak power density of 0.55 W cm⁻² (220 W) at 0.5 V per cell, when the voltage was scanning from low to high voltage (1.5–3.5 V), and resulted in minimum water flooding inside the stack. In this study a series of dynamic loadings were tested to simulate the vehicle acceleration. The fuel cell performances respond to dynamic loading influenced by the hydrogen/air stoichiometric, back pressure, and dynamic-loading time. It was needed high hydrogen stoichiometric and back pressure to maintain high dynamic performance. In the long-time stable power testing, the stack was difficult to maintain at high performance, due to the water flooding at high output power. An adjusting cathode back-pressure method for purging water was proposed to prevent the water flooding at flow channels and maintain the stable output power at 170 W (0.42 W cm⁻²).     

Planar air-breathing micro-direct methanol fuel cell stacks based on micro-electronic–mechanical-system technology

To meet the demands for high power micro-electronic devices, two silicon-based micro-direct methanol fuel cell (μDMFC) stacks consisting of six individual cells with two different anode flow fields were designed, fabricated and evaluated. Micro-electronic–mechanical-system (MEMS) technology was used to fabricate both flow field plate and fuel distribution plate on the silicon wafer. Experimental results show that either an individual cell or a stack with double serpentine-type flow fields presents better cell performance than those with pin-type flow fields. A μDMFC stack with double serpentine-type flow fields generates a peak output power of ca. 151 mW at a working voltage of 1.5 V, corresponding to an average power density of ca. 17.5 mW cm⁻², which is ca. 20.7% higher than that with pin-type flow fields. The volume and weight of the stacks are only 5.3 cm³ and 10.7 g, respectively. Such small stacks could be used as power sources for micro-electronic devices.     

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.     

Performance of a proton exchange membrane fuel cell stack using conductive amorphous carbon-coated 304 stainless steel bipolar plates

In this study, 304 stainless steel (SS) bipolar plates are fabricated by flexible forming process and an amorphous carbon (a-C) film is coated by closed field unbalanced magnetron sputter ion plating (CFUBMSIP). The interfacial contact resistance (ICR), in-plane conductivity and surface energy of the a-C coated 304SS samples are investigated. The initial performance of the single cell with a-C coated bipolar plates is 923.9 mW cm⁻² at a cell voltage of 0.6 V, and the peak power density is 1150.6 mW cm⁻² at a current density of 2573.2 mA cm⁻². Performance comparison experiments between a-C coated and bare 304SS bipolar plates show that the single cell performance is greatly improved by the a-C coating. Lifetime test of the single cell over 200 h and contamination analysis of the tested membrane electrode assemble (MEA) indicate that the a-C coating has excellent chemical stability. A 100 W-class proton exchange membrane fuel cell (PEMFC) short stack with a-C coated bipolar plates is assembled and shows exciting initial performance. The stack also exhibits uniform voltage distribution, good short-term lifetime performance, and high volumetric power density and specific power. Therefore, a-C coated 304SS bipolar plates may be practically applied for commercialization of PEMFC technology.     

Direct operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell stack with methane

Operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell (SIS-SOFC) stack directly on methane is studied. A cone-shaped solid oxide fuel cell stack is assembled by connecting 11 cone-shaped anode-supported single cells in series. The 11-cell-stack provides a maximum power output of about 8 W (421.4 mW cm⁻² calculated using active cathode area) at 800 °C and 6 W (310.8 mW cm⁻²) at 700 °C, when operated with humidified methane fuel. The maximum volumetric power density of the stack is 0.9 W cm⁻³ at 800 °C. Good stability is observed during 10 periods of thermal cycling test. SEM-EDX measurements are taken for analyzing the microstructures and the coking degrees.     

Scale-up of a high temperature polymer electrolyte membrane fuel cell based on polybenzimidazole

A high temperature PEM fuel cell stack with a total active area 150 cm² has been studied. The PEM technology is based on a polybenzimidazole (PBI) membrane. Cast from a PBI polymer synthesised in our lab, the performance of a three-cell stack was analysed in static and dynamic modes. In static mode, operating at high constant oxygen flow rate (QO₂>1105 ml O₂/min) produces a small decrease on the stack performance. High constant oxygen stoichiometry (λO₂>3) does not produce a decrease on the performance of the stack. There are not differences between operating at constant flow rate of oxygen and constant stoichiometry of oxygen in the stack performance. The effect of operating at high temperature with a pressurization system and operating at higher temperatures are beneficial since the performance of the fuel cell is enhanced. A large shut-down stage produces important performance losses due to the loss of catalyst activity and the loss of membrane conductivity. After 150 h of operation at 0.2 A cm⁻², it is observed a very high voltage drop. The phosphoric acid leached from the stack was also evaluated and did not exceed 2% (w/w). This fact suggests that the main degradation mechanism of a fuel cell stack based on polybenzimidazole is not the electrolyte loss. In dynamic test mode, it was observed a rapid response of power and current output even at the lower step-time (10 s). In the static mode at 125 °C and 1 atm, the stack reached a power density peak of 0.29 W cm⁻² (43.5 W) at 1 V.     

Performance of a direct methanol alkaline membrane fuel cell

A study of a direct methanol fuel cell (DMFC) operating with hydroxide ion conducting membranes is reported. Evaluation of the fuel cell was performed using membrane electrode assemblies incorporating carbon-supported platinum/ruthenium anode and platinum cathode catalysts and ADP alkaline membranes. Catalyst loadings used were 1 mg cm⁻² Pt for both anode and cathode. The effect of temperature, oxidant (air or oxygen) and methanol concentration on cell performance is reported. The cell achieved a power density of 16 mW cm⁻², at 60 °C using oxygen. The performance under near ambient conditions with air gave a peak power density of approximately 6 mW cm⁻².     

Effect of carbon fiber cloth with different structure on the performance of low temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different structure in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the structure of the carbon fiber cloth and fuel cell performance.The paper discusses the relationship between fuel cell performance and structure of the carbon fiber cloth, and also examines the effect of the carbon fiber cloth’s thickness, air permeability, surface resistivity, XRD and elemental analysis. Carbon fiber cloth is carbonized at rates of 190, 220, 250, 280, and 310 °C min⁻¹ respectively, and the resulting carbon fiber cloth is tested in cells. When the test piece area is 25 cm², the test temperature 40 °C, the gasket thickness 0.36 mm, and the carbonization rate 280 °C min⁻¹, a fuel cell using the carbon fiber cloth achieves a current density of 1968 mA cm⁻² and a maximum power density of 633 mW cm⁻² at 0.3 V.     

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.     

Performance of an annular solid-oxide fuel cell micro-stack array operating in single-chamber conditions

An annular micro-stack array consisting of four fuel cells has been fabricated and operated successfully in single-chamber conditions using a nitrogen-diluted oxygen-methane mixture as the operating gas. The single cells consist of a state-of-the-art porous NiO/Y₂O₃-stabilized ZrO₂ (YSZ) anode support, a YSZ electrolyte membrane and a modified La_0.7Sr_0.3MnO₃ (LSM) cathode. The annular configuration of the array is favorable for utilizing the heating effect. The maximum power output of the annular stack decreases with increasingCH₄/O₂ ratio. Its performance increases with increasing CH₄ flow rate and decreases with increasing N₂ flow rate. The power output of the stack is ∼380 mW at CH₄/O₂ = 1 and an N₂ flow rate of 100 sccm and the average maximum power density of each cell is ∼190 mW cm⁻². The average performance of each cell in the annular micro-stack array is higher than that of an additional single cell placed next to the stack.     

Design, fabrication, and characterization of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell with a bridge-shaped microchannel cross-section

We report the fabrication of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell (μLFFC) using standard MEMS and IC-compatible fabrication technologies. The μLFFC operates with acid supported solutions of formic acid and potassium permanganate, as a fuel and oxidant respectively. The micro-fuel cell design features two in-plane anodic and cathodic microchannels connected via a bridge to confine the diffusive liquid-liquid interface away from the electrode areas and to minimize crossover. Palladium high-active-surface-area catalyst was selectively integrated into the anodic microchannel by electrodeposition, whereas no catalyst was required in the cathodic microchannel. A three-dimensional (3D) diffusion-convection model was developed to study the behavior of the diffusion zone and to extract appropriate cell-design parameters and operating conditions. Experimentally, we observed peak power densities as high as 26 mW cm⁻² when operating single cells at a flow rate of 60 μL min⁻¹ at room temperature. The miniature membraneless fuel cell design presented herein offers potential for on-chip power generation, which has long been prohibited by integration complexities associated with the membrane.     

Quaternized-chitosan membranes for possible applications in alkaline fuel cells

Novel crosslinked quaternized-chitosan membranes were fabricated and further investigated for possible applications in alkaline polyelectrolyte fuel cells. Impedance analysis indicated that some hydrated membranes could exhibit a conductivity close to 10⁻² S cm⁻¹. Several membranes were selected and integrated into unit fuel cells for the evaluations on their cell performance, using hydrogen as fuel, air as oxidant and platinum as the electrode catalyst, and a current density of 65 mA cm⁻² was already achieved with a flow rate of hydrogen at 50 mL min⁻¹ and air at 250 mL min⁻¹ at a relatively low running temperature of 50 °C.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

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.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

A miniature passive direct formic acid fuel cell based twin-cell stack with highly stable and reproducible long-term discharge performance

A miniature air-breathing twin-cell stack is designed and evaluated for direct formic acid fuel cell (DFAFC) applications. The stack consists of two face-to-face single cells with one shared fuel reservoir. This particular design has advantages in volume reduction relative to single cells in series connection. The performance, stability and reproducibility of the stack are investigated extensively for practical fuel cell applications. A maximum power density of 44.5 mW cm⁻² is obtained with 5.0 M formic acid solution as the fuel and Pt catalysts in electrodes. It is also found that the stack yields high stability and reproducibility when discharged at a constant current of 20 mA. The output voltage can be maintained at 1.14 V for about 5 h by feeding 3.5 ml 5.0 M formic acid solution and the performance can almost be reproduced when the fresh fuel is injected.     

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.     

Performance of alkaline electrolyte-membrane-based direct ethanol fuel cells

A single alkaline direct ethanol fuel cell (alkaline DEFC) with an anion-exchange membrane and non-platinum (non-Pt) catalysts is designed, fabricated, and tested. Particular attention is paid to investigating the effects of different operating parameters, including the cell operating temperature, concentrations of both ethanol and the added electrolyte (KOH) solution, as well as the mass flow rates of the reactants. The alkaline DEFC yields a maximum power density of 60 mW cm⁻², a limiting current density of about 550 mA cm⁻², and an open-circuit voltage of about 900 mV at 40 °C. The experimental results show that the cell performance is improved on increasing the operating temperature, but there exists an optimum ethanol concentration under which the fuel cell has the best performance. In addition, cell performance increases monotonically with increasing KOH concentration in the region of low current density, while in the region of high current density, there exists an optimum KOH concentration in terms of cell performance. The effect of flow rate of the fuel solution is negligible when the ethanol concentration is higher than 1.0 M, although the cell performance improves on increasing the oxygen flow rate.     

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.     

Cathode contact optimization and performance evaluation of intermediate temperature-operating solid oxide fuel cell stacks based on anode-supported planar cells with LaNi0.6Fe0.4O3 cathode

This paper reports the development of intermediate temperature-operating solid oxide fuel cell stacks using anode-supported planar cells with LaNi_0.6Fe_0.4O₃ (LNF)cathode. We developed metallic separators with radial gas flow channels and an anode seal structure. To achieve good power-generating characteristics, we propose two cathode contact methods. According to a performance evaluation at 800 °C, power density of 0.5 W cm⁻² is obtained at the current density of 1.0 A cm⁻² when operating with a sufficient fuel amount, and power conversion efficiency of over 50% LHV is obtained at the current density of more than 0.2 A cm⁻² when operating at a high fuel utilization rate.