In situ detection of anode flooding of a PEM fuel cell

This paper proposes an early detection scheme of anode flooding in a PEM fuel cell. Through experimental testing of an eight-cell hydrogen-fueled polymer electrolyte stack it is shown that anode flooding can be detected prior to a rapid voltage decline. The proposed detection scheme requires no additional costly instrumentation and uses the existing voltage scan cards.     

A gas management strategy for anode recirculation in a proton exchange membrane fuel cell

The anode configuration and gas management strategy are two of factors that affect the energy efficiency of a proton exchange membrane fuel cell. In order to improve the hydrogen utilization, unused hydrogen can be recirculated to the inlet using a pump. However, impurities diffusing from the cathode to the anode may cause the dilution of hydrogen in the anode. As a result, a gas management strategy is required for the anode recirculation configuration. In this preliminary study, a novel configuration for anode recirculation and a gas management strategy are proposed and verified by experiments. Two valves are installed in the recirculation line. The anode is operated in four modes (dead-end, recirculation, compression, and purge), and the real-time local current density (LCD) is monitored for gas management purposes. The results show that the LCD distribution is uniform during the recirculation mode and nonuniform during the dead-end and compression modes. With this configuration and gas management strategy, the cycle duration is increased by a factor of 6.5.     

Investigation of anode flow field for direct dimethyl ether fuel cell

In this work, we presented the novel anode flow field for direct dimethyl ether fuel cell (DDFC). The anode flow field of the DDFC consisted of hydrophilic, hydrophobic, and diffusion region (with a ratio of region areas of 3:6:1). The maximum power density of the DDFC with novel anode flow field was 67 mW cm⁻², which was higher than that of the cell fed with conventional flow field (60 mW cm⁻²). The electrochemical impedance spectra analyses revealed that the mass transfer resistance of novel anode flow field was lower than that of conventional flow field. The constant current operation curves showed that the performance decay ratio of the novel anode flow field was lower than that of conventional one. It indicated that the novel flow field benefited the long-term operation of DDFC.     

Two-phase flow pressure drop hysteresis in parallel channels of a proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in a non-operational PEM fuel cell to understand the effect of stoichiometry, GDL characteristics, operating range, and initial conditions (dry vs. flooded) for flow conditions typical of an operating fuel cell. This hysteresis is noted when the air and water flow rates are increased and then decreased along the same path, exhibiting different pressure drops. When starting from dry conditions, the descending pressure drop tended to be higher than the ascending pressure drop at lower simulated current densities. The hysteresis effect was noted for stoichiometries of 1-4 and was eliminated at a stoichiometry of 5. It was found that the hysteresis was greater when water breakthrough occurred at higher simulated current densities, which is a function of GDL properties. The operating range had to reach a critical simulated current density (800 mA cm⁻² in this case) between the ascending and descending approach to create a pressure drop hysteresis zone. The descending step size does not change the size of the hysteresis effect, but a larger step size leads to lower fluctuations in the pressure drop signal. An initially flooded condition also showed hysteresis, but the ascending approach tended to have a higher pressure drop than the descending approach.     

Two-phase flow pressure drop hysteresis in an operating proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in an operating PEM fuel cell. The variables studied include air stoichiometry (1.5, 2, 3, 4), temperature (50, 75, 90 °C), and the inclusion of a microporous layer. The cathode channel pressure drops can differ in PEM fuel cells when the current density is increased along a path and then decreased along the same path (pressure drop hysteresis). Generally, the descending pressure drop is greater than the ascending pressure drop at low current densities (<200 mA cm⁻²), and the effect is worse at low stoichiometries and low temperatures. The results show that the hysteresis occurs with or without the inclusion of a microporous layer. Initial results show a modified Lockhart-Martinelli approach seems to be able to predict the two-phase flow pressure drop during the ascending path. The results compare well with photographs taken from the cathode flow field channel of a visualization cell.     

Hydrogen pressure drop characteristics in a fuel cell stack

Research on hydrogen pressure characteristics was performed for a fuel cell stack to supply a rule of hydrogen pressure drop for flooding diagnostic systems. Some experiments on the hydrogen pressure drop in various operating pressure, temperature, flowrate and stack current conditions were carried out, and hydrodynamic calculation was managed to compare with the experiment results. Results show that the hydrogen pressure drop is strongly affected by liquid water content in the flow channel of fuel cells, and it is not in normal relation with flowrate when the stoichiometric ratio is inconstant. The total pressure drop can be calculated by a frictional pressure loss formula accurately, relating with mixture viscosity, stack temperature, operating pressure, stoichiometric ratio and stack current. The pressure drop characteristics will be useful for predicting liquid water flooding in fuel cell stacks before flow channels have been jammed as a diagnostic tool in electric control systems.     

Water management studies in PEM fuel cells,Part I: Fuel cell design and in situ water distributions

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.     

An investigation into the effect of anode purging on the fuel cell performance

The anode purge is a crucial process for the fuel cell long time operation because when the hydrogen is supplied in a circulation mode, any impurities present in hydrogen will gradually accumulate which lead to output voltage loss. A mathematical model is proposed for the purge process based on some operational purge parameters. The governing equations are solved and the effect of purge process on the stack working parameters is analyzed. Purge operational parameters are determined in such a way that the minimum pressure fluctuations in the anode compartment and a compromise between the minimum voltage loss and minimum hydrogen waste are achieved. A semi-stable condition is introduced and indicated that the behavior of voltage loss and hydrogen waste at this condition with respect to purge stop time (duration which the purge valve is closed) is semi-logarithmic.     

Hydrogen purge and reactant feeding strategies in self-humidified PEM fuel cell systems

A 6 kW proton exchange membrane fuel cell system, operating in self-humidified conditions, was characterized in two anode operative modes: dead-end and flow through with exhaust recirculation. The anode sub-system was specifically designed in order to adjust the level of recycled anodic stream. The role of anode purge frequency, anode recirculation level and air stoichiometric ratio was analysed in the power range 1–5 kW. The aim of this study was to define management strategies to assure efficient and reliable cell performance during steady-state, warm-up and load variation phases. The results evidenced the combined effect of hydrogen purge, air flow rate impulse, and recycled anodic stream on individual cell performance recovery when unstable working conditions were detected during system start-up and load variations.     

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.     

Analysis of the water and thermal management in proton exchange membrane fuel cell systems

Water and thermal management is essential to the performance of proton exchange membrane (PEM) fuel cell system. The key components in water and thermal management system, namely the fuel cell stack, radiator, condenser and membrane humidifier are all modeled analytically in this paper. Combined with a steady-state, one-dimensional, isothermal fuel cell model, a simple channel-groove pressure drop model is included in the stack analysis. Two compact heat exchangers, radiator and condenser are sized and rated to maintain the heat and material balance. The influence of non-condensable gas is also considered in the calculation of the condenser. Based on the proposed methodology, the effects of two important operating parameters, namely the air stoichiometric ratio and the cathode outlet pressure, and three kinds of anode humidification, namely recycling humidification, membrane humidification and recycling combining membrane humidification are analyzed. The methodology in this article is helpful to the design of water and thermal management system in fuel cell systems.     

Development of a polymer electrolyte fuel cell dead-ended anode purge strategy for use with a nitrogen-containing hydrogen gas supply

The effect of nitrogen content within the hydrogen fuel supplied to a polymer electrolyte fuel cell (PEFC) operating in dead-ended anode mode is examined, with a view to using an ammonia decomposition product gas mix (containing 75H₂:25N₂) as the hydrogen-containing fuel. The impact of this impure hydrogen stream, supplied to the anode, was evaluated in terms of mean cell voltage and in relation to actual operating conditions (purge interval, dead-ended interval and fuel cell load). Design of Experiments (DoE) methodology, using multi-linear models, assessed hydrogen utilisation in terms of stack efficiency and identified an effective and viable dead-ended anode purge strategy for this nitrogen-containing hydrogen fuel.     

Development of the novel control algorithm for the small proton exchange membrane fuel cell stack without external humidification

Small PEM (proton exchange membrane) fuel cell systems do not require humidification and have great commercialization possibilities. However, methods for controlling small PEM fuel cell stacks have not been clearly established. In this paper, a control method for small PEM fuel cell systems using a dual closed loop with a static feed-forward structure is defined and realized using a microcontroller. The fundamental elements that need to be controlled in fuel cell systems include the supply of air and hydrogen, water management inside the stack, and heat management of the stack. For small PEM fuel cell stacks operated without a separate humidifier, fans are essential for air supply, heat management, and water management of the stack. A purge valve discharges surplus water from the stack. The proposed method controls the fan using a dual closed loop with a static feed-forward structure, thereby improving system efficiency and operation stability. The validity of the proposed method is confirmed by experiments using a 150-W PEM fuel cell stack. We expect the proposed algorithm to be widely used for controlling small PEM fuel cell stacks.     

Analysis of solid oxide fuel cell system concepts with anode recycling

The main drivers for anode recirculation are the increased fuel efficiency and the independence of the external water supply for the fuel pre-reforming process. Different system flow-schemes have been defined and a set of parameters have been elaborated as basis for various calculations. Taking into account the combinations of layout, cell type, fuel utilization, fuel, and recycling ratio the total number of cases modeled was about 220. All calculated SOFC systems are on a high level of electrical net efficiency in the range of 50–66%. The electrical and thermal efficiencies are mainly influenced by the fuel utilization. The layout itself, the choice of fuel gas or the type of cell have minor effects on the system efficiency.     

Effectiveness of anode in a solid oxide fuel cell with hydrogen/oxygen mixed gases

A porous Ni/YSZ cermet in mixed hydrogen and oxygen was investigated for its ability to decrease oxygen activity as the anode of a single chamber SOFC. A cell with a dense 300 μm YSZ electrolyte was operated in a double chamber configuration. The Ni–YSZ anode was exposed to a mixture of hydrogen and oxygen of varying compositions while the cathode was exposed to oxygen. Double chamber tests with mixed gas on the anode revealed voltage oscillations linked to lowered power generation and increased resistance. Resistance measurements of the anode during operation revealed a Ni/NiO redox cycle causing the voltage oscillations. The results of these tests, and future tests of similar format, could be useful in the development of single chamber SOFC using hydrogen as fuel.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 1 – Experimental method and serpentine flow field results

Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm² PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180° bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.     

Carbon anode in direct carbon fuel cell

Direct carbon fuel cell (DCFC) is a kind of high temperature fuel cell using carbon materials directly as anode. Electrochemical reactivity and surface property of carbon were taken into account in this paper. Four representative carbon samples were selected. The most suitable ratio of the ternary eutectic mixture Li₂CO₃–K₂CO₃–Al₂O₃ was determined at 1.05:1.2:1(mass ration). Conceptual analysis for electrochemical reactivity of carbon anode shows the importance of (1) reactive characteristics including lattice disorder, edge-carbon ratio and the number of short alkyl side chain of carbon material, which builds the prime foundation of the anodic half-cell reaction; (2) surface wetting ability, which assures the efficient contact of anode surface with electrolyte. It indicates that anode reaction rate and DCFC output can be notably improved if carbon are pre-dispersed into electrolyte before acting as anode, due to the straightway shift from cathode to anode for CO₃²⁻ provided by electrolyte soaked in carbon material.     

Investigation of water droplet kinetics and optimization of channel geometry for PEM fuel cell cathodes

The kinetics and transport mechanisms of water droplets in model flow field channels of a low temperature polymer electrolyte fuel cell were investigated. The pressure drop at different air flows was measured for different channel geometries in a graphite plate as employed for fuel cell bipolar plates. The minimum air flow required for the movement of a water droplet in the flow channel was identified. From the experimental findings, recommendations for the development of flow fields with high condensate removal capabilities combined with low pressure differences were drawn to allow for an efficient operation of PEM fuel cells.     

Hydrogen from aluminium in a flow reactor for fuel cell applications

Aluminium appears to be a promising material for on-board hydrogen generation in fuel cell applications given the comparatively large amount of hydrogen produced per gram of aluminium in a safe system. A microfuel processor with aluminium and water as reactants is developed in a flow reactor for application in portable power sources. Two types of reactor are used. One reactor permits the direct feeding of liquid water in channels containing aluminium pellets, whereas the other utilizes the heat produced from the reaction to vapourize liquid water before entry into the reactor. Two additives, namely, calcium oxide (CaO) and sodium hydroxide (NaOH), are used to enhance the reaction rate. A maximum conversion of 78.6% with respect to aluminium is achieved when the water entering in the reactor is vapourized partially. In the case of liquid water entering the reactor, the conversion is 74.4%.