The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.     

Computational analysis of fuel saving by using porous-end configuration for a PEM fuel cell

In the present study, a novel porous-end PEMFC inspired by the characteristics of open-end and dead-end PEMFCs is proposed for fuel saving. For this purpose, a porous media region with a certain thickness is added to the outlet region of the anode channel of an open-end PEMFC. The effect of porous media thickness at the anode channel on the current density and hydrogen mass flow was numerically analyzed. Results indicate that in comparison to the base model PEMFC, the presence of porous media at the end of the anode channel of porous-end PEMFC leads to an increase in the pressure and a decrease in the velocity magnitude in the anode channel. Results illustrate that the porous-end PEMFC with t = 1 mm thickness can be an adequate choice to gain an optimum design for the porous-end configuration. This conclusion becomes more highlighted when the results give the 66.17% reduction in fuel consumption.     

Experimental study and optimization of flow field structures in proton exchange membrane fuel cell under different anode modes

As one of the critical components in the proton exchange membrane fuel cell (PEMFC), the flow field is crucial to the improvement of cell performance. However, the current research on flow field structure lacks consideration of the influence of different anode modes, which makes the existing flow field structure rules have limitations in the practical application of PEMFC. In this paper, the PEMFC characteristics of parallel flow field, S-shaped flow field, multi-serpentine flow field and single-serpentine flow field at the cathode side are compared experimentally in the dead-end anode (DEA) mode and hydrogen circulation anode (HCA) mode, respectively. Especially, the spatial current density distribution and parasitic power of different flow field structures are measured. The results show that the performance trends of different flow field structures change with the DEA and HCA anode modes. In DEA mode, the PEMFC is prone to flooding, and the flow field with high gas velocity in the channel has better drainage ability, which can obtain higher cell performance. The HCA mode is helpful for the discharge of water in the PEMFC, which effectively alleviates the adverse impact of water accumulation on the overall performance, and the mass transport ability of the flow field structure plays a leading role in the cell performance improvement. In addition, although the high gas flow velocity has better drainage ability in the flow field, it may lead to a decrease in the current density distribution uniformity and PEMFC net output power density. Based on the comprehensive consideration of the experimental results, the multi-serpentine flow field is more suitable for DEA mode, and the S-shaped flow field is more suitable for HCA mode.     

Simultaneous direct visualisation of liquid water in the cathode and anode serpentine flow channels of proton exchange membrane (PEM) fuel cells

Water flooding is detrimental to the performance of the proton exchange membrane fuel cell (PEMFC) and therefore it has to be addressed. To better understand how liquid water affects the fuel cell performance, direct visualisation of liquid water in the flow channels of a transparent PEMFC is performed under different operating conditions. Two high-resolution digital cameras were simultaneously used for recording and capturing the images at the anode and cathode flow channels. A new parameter extracted from the captured images, namely the wetted bend ratio, has been introduced as an indicator of the amount of liquid water present at the flow channel. This parameter, along with another previously used parameter (wetted area ratio), has been used to explain the variation in the fuel cell performance as the operating conditions of flow rates, operating pressure and relative humidity change. The results have shown that, except for hydrogen flow rate, the wetted bend ratio strongly linked to the operating condition of the fuel cell; namely: the wetted bend ratio was found to increase with decreasing air flow rate, increasing operating pressure and increasing relative humidity. Also, the status of liquid water at the anode was found to be similar to that at the cathode for most of the cases and therefore the water dynamics at the anode side can also be used to explain the relationships between the fuel cell performance and the investigated operating conditions.     

Review on water management methods for proton exchange membrane fuel cells

Control of water content of proton exchange membrane fuel cells (PEMFCs) within a reasonable rangeis a question worthy of study. This paper addresses questions of water transport, water fault, and water management methods in a PEMFC. Both an excess (overflow) or lack (dehydration) of water in a fuel cell may affect the performance and the service life. Herein, we describe in detail the effects of water content on the cathode, anode, gas diffusion layer (GDL), catalyst layer (CL) and flow channel. Monitoring the flow and accumulation of water directly in the PEMFC is the most effective approach to determine which of the two scenarios, overflow or dehydration, occurs. The water transport can be effectively investigated in a transparent fuel cell, using neutron scanning, nuclear magnetic resonance, and X-ray irradiation. Regarding the PEMFC water management, this paper reviews some current methods, such as improvement of the flow field structure, changing hydrophilic materials, and optimizing control systems.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.     

Experimental study on enhancing the fuel efficiency of an anodic dead-end mode polymer electrolyte membrane fuel cell by oscillating the hydrogen

This paper investigates how to improve the fuel efficiency of an anodic dead-end mode fuel cell for portable power generation. Generally, a periodic purge process in anodic dead-end operation is required to avoid anode flooding caused by back diffusive water from the cathode. However, during the purge process, small amounts of the hydrogen are discharged with the water, lowering the fuel utilization efficiency. Therefore, hydrogen pulsations are introduced and experimental attempt to minimize the purge frequency is conducted in this study. The experimental results indicate that pulsation reduces partial pressure of the water vapor in the anode channel, increasing the interval between purges by approximately three times, thus improving overall efficiency.     

Three-dimensional multi-phase numerical study for the effect of coolant flow field designs on water and thermal management for the large-scale PEMFCs

The multi-phase numerical study is performed for the large-scale proton exchange membrane fuel cells (PEMFCs) regarding coolant flow field design. In this study, three coolant flow fields were designed to explore the effect of different temperature distributions on the water management of the PEMFCs. The numerical results show that increasing the temperature gradient along the gas flow direction and improving the temperature uniformity perpendicular to the gas flow direction enhances PEMFC performance and makes the liquid water distribution in the gas diffusion layers more reasonable. The co-flow for the cathode gas stream and the coolant flow is beneficial to raise the temperature along the cathode gas flow direction and reduce the risk of flooding near the cathode outlet. Then, it is noted that the coolant flow field design is not necessary to keep the temperature absolutely uniform for the PEMFCs. Although increasing the coolant volume flow rate will reduce the IUT, it dramatically increases the risk of flooding near the cathode outlet. Therefore, the moderate volume flow rate is preferred. Finally, the effect of the coolant manifold on the volume flow rate uniformity in the coolant channels is investigated, and it is found that reducing the number of coolant channels is the best strategy to improve volume flow rate uniformity and thermal management performance.     

Visualization of flooding in a single cell and stacks by using a newly-designed transparent PEMFC

The flow phenomenon in the fuel-cell channels is difficult to understand and predict because of the two-phase flow. Proton exchange membrane fuel cells (PEMFCs) with transparent windows are widely used for visualizing the two-phase flow in the channels. In this paper, the visualization of the two-phase flow in the channels was accomplished under various current-density conditions using a transparent cell. The visualization of the single serpentine flow field clearly reveals that anode flooding is more severe than cathode flooding. The main cause for anode flooding is a low gas-flow rate in the channel because of the absence of the carrier gas. In addition, flooding is more significant under a low current-density condition than under a high current-density condition; under the latter condition, there is significantly more reaction heat that prevents flooding. The flow phenomena in the PEMFC stack were also visualized by electrically connecting three transparent cells in series and supplying fuel to each cell from a manifold. Sudden voltage drops and overshoots were detected, and the voltage fluctuations were found to be strongly related to flooding.     

The critical pressure drop for the purge process in the anode of a fuel cell

Purge operation is an effective way to remove the accumulated liquid water in the anode of proton exchange membrane fuel cells (PEMFCs). This paper studies the phenomenon of the two-phase flow as well as the pressure drop fluctuation inside the flow field of a single cell during the purge process. The flow patterns are identified as intermittent purge and annular purge, and the two purge processes are contrastively analyzed and discussed. The intermittent purge greatly affects the fuel cell performance and thus it is not suitable for the in situ application. The annular purge process requires a higher pressure drop, and the critical pressure drop is calculated from the annular purge model. Furthermore, this value is quantitatively analyzed and validated by experiments. The results show that the annular purge is appropriate for removing liquid water out of the anode in the fuel cell.     

Reticulated porous and parallel channel cathode flow fields in real scale polymer electrolyte membrane fuel cells: A comparative experimental study

This paper focuses on understanding the effect of reticulated porous cathode flow fields in real scale close and open cathode polymer electrolyte membrane fuel cells (PEMFCs) in terms of their thermo-electrical performance. This research contributes to addressing challenges with PEMFCs linked to oxygen supply to the cathode and proper mixing of gasses as well as water removal issues. Parallel channel and porous cathode flow fields in both open and closed cathode PEMFCs of medium scale (active area of 15 × 15 cm²) have been investigated. The porous material consisted of 20 pores per inch with a porosity level of 80%. The cells’ polarisation and impedance characteristics have been analysed. The porous flow field has been found providing better electrical performance in closed cathode PEMFC compared to the open cathode. Improvements in gas diffusion and temperature uniformity were observed with porous flow field; however, water removal has been observed challenging, which need to be addressed before the benefits of using porous flow field are fully realised.     

A method for the ad hoc and real-time determination of the water balance in a PEMFC

A novel method for the ad hoc and real-time determination of the water balance in a proton exchange membrane fuel cell is presented. The method requires the anode side of the fuel cell to be operated in open-ended mode and to use dry, pure hydrogen as is typical for vehicular applications. In that case there is a linear relationship between the anode outlet velocity and the effective drag coefficient of water through the membrane, r_d, provided the stoichiometric flow ratio is sufficiently low (below ξ = 1.2). The anode outlet velocity can then be directly measured e.g. by using hot wire anemometry, and this method provides a voltage signal that can be fed to the board computer of a fuel cell vehicle for PEMFC diagnosis. It is also shown that the nitrogen cross-over from cathode to anode has only a small effect on the anode outlet velocity. In addition to detecting the velocity, the relative humidity may be measured which is shown to be independent of the current density, but measurement techniques suffer from lower accuracy. It is argued that this method can also be applied to quantify fuel cell degradation. Finally, it is fundamentally shown that when operating the fuel cell in steady state mode at a hydrogen stoichiometric flow ratio as low as 1.03, the molar fraction of hydrogen in the gas mixture at the anode outlet is at least 50% which means that at steady state there will be no hydrogen starvation at the anode outlet.     

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.     

Dynamic performance for a kW-grade air-cooled proton exchange membrane fuel cell stack

In this study, a kW-grade air-cooled proton exchange membrane fuel cell (PEMFC) stack with a dead-end anode (DEA) operation is designed and manufactured. The gravity-assisted drainage principle is applied for the stack to design the wettability of gas diffusion layers (GDLs) and the anode channel geometry, which can help the liquid water that diffuses to the anode to drain out of the anode porous electrode and move down the anode channel outlets. As a result, the stack can stably operate in a long purge interval of 268 s and in a short purge time of 2 s. In addition, using this design, only four small-power fans are employed to pump air to the cathode to provide oxygen for the electrochemical reaction and cool the stack. With a constant load current of 30, 45, or 60 A, the stack output voltage is experimentally tested at various cathode air flow rates (CAFRs). The local temperatures (60 measurement points) inside the stack and the pressure differences across anode channels are also monitored to understand heat dissipation and the back diffusion of liquid water. In a wide range of operating conditions, the designed stack possesses superior and stable voltage output characteristics with relatively uniform temperature distributions. The measured maximum output power is 3.83 kW, and the parasitic powers of fans are only 80～112 W.     

An experimental study on the purge characteristics of the cathodic dead-end mode PEMFC for the submarine or aerospace applications and performance improvement with the pulsation effects

In polymer electrolyte membrane fuel cells for the submarine or the aerospace systems, the pure hydrogen and oxygen are generally supplied to both sides of the membrane electrode assembly (MEA). Additionally, the fuel cell becomes a dead-end system which needs no fuel and oxidant supplying devices such as a blower, a fan and a re-circulation unit, therefore, the improvement in efficiency and the simplification of the system are expected. Instead, the purge process for removing the water that is produced in the reaction is necessary for the stable and durable system operation. However, during a purge process, small amount of oxygen can be discharged with the water. Thus, the number of purge cycle should be minimized for saving the oxidant. This paper presents experimental results on the periodic characteristics of the voltage change in the cathodic dead-end mode for various conditions. Moreover, the pulsation effect is introduced and applied to the outlet of the cathode channel for decreasing the purge frequency. The experimental results show that the pulsation effect decreases the voltage decay rate, and makes the interval between two purges about three times longer than that without the pulsation effect, which can make the oxidant efficiency of the dead-end mode operation higher.     

Computational study of water transport in proton exchange membrane fuel cells

A computational fuel cell dynamics framework is used to develop a unified water transport equation for a proton exchange membrane fuel cell (PEMFC). Various modes of water transport, i.e., diffusion, convection and electro-osmotic drag, are incorporated in the unified water transport equation. The water transport model is then applied to elucidate water management in three-dimensional fuel cells with dry-to-low humidified inlet gases after its validation against available experimental data for dry oxidant and fuel streams. An internal circulation of water with the aid of counter-flow design is found to be of vital importance for low-humidity operation, for example, in the portable application of a PEMFC without an external humidifier. The general features of water transport in PEMFCs are discussed to show various water transport regimes of practical interest, such as anode water loss, cathode flooding, and the equilibrium condition of water at the channel outlets, particularly for limiting situations where anode and cathode water profiles acquire an equilibrium state. From the practical point of view, the effects of the flow arrangement, membrane thickness, and inlet gas humidity as important determinants of fuel cell performance are also analyzed to elucidate fuel cell water transport characteristics.     

Transient response of a unit proton-exchange membrane fuel cell under various operating conditions

The transient response of proton-exchange membrane fuel cells (PEMFCs) is an important criterion in their application to automotive systems. Nevertheless, few papers have attempted to study experimentally this dynamic behaviour and its causes. Using a large-effective-area (330 cm²) unit PEMFC and a transparent unit PEMFC (25 cm²), systematic transient response and cathode flooding during load changes are investigated. The cell voltage is acquired according to the current density change under a variety of stoichiometry, temperature and humidity conditions, as well as different flooding intensities. In the case of the transparent fuel cell, the cathode gas channel images are obtained simultaneously with a CCD imaging system. The different levels of undershoot occur at the moment of load change under different cathode stoichiometry, both cathode and anode side humidity and flooding intensity conditions. It is shown that undershoot behaviour consists of two stages with different time delays: one is of the order of 1 s and the other is of the order of 10 s. It takes about 1 s for the product water to come up on to the flow channel surface so that oxygen supply is temporarily blocked, which causes voltage loss in that “undershoot”. The correlation of dynamic behaviour with stoichiometry and cathode flooding is analyzed from the results of these experiments.     

Water transport characteristics of polymer electrolyte membrane fuel cell

This paper describes the performance of a polymer electrolyte membrane fuel cell (PEMFC) system without humidification of the reactants which consumes a lot of parasitic power, increases the weight of the PEMFC system and thus adds complexity. Such PEMFC systems are preferable for portable applications. The results indicate that dry gas operation depends on various factors like reactant flow field design, area of the electrode and equilibration time for the product water. The performance of the fuel cell can be improved by giving some equilibration time for the product water, produced by the electrochemical reactions, to get transported across the membrane to the anode side, thus increasing the conductivity of the membrane. The water transported through the membrane across the cell was investigated by measuring the amount of product water at the anode side which allows humidification for the anode gas and less condensed water in the fluid flow channels of the cathode.     

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.     

不同型式流场的PEMFC内部传质分析

对采用不同型式流场的PEMFC进行建模,并用控制容积法对控制方程进行离散,求解得到PEMFC内部各物理量的分布以及综合水拖带系数、质子交换膜平均电导率等。分析了采用交趾型流场和常规流场时PEMFC的内部传质以及阴极性能、电池性能和膜性能,结果认为采用交趾型流场时,PEMFC阴极性能高于采用常规流场的PEMFC阴极性能,但质子交换膜的平均电导率低于采用常规流场时。在没有液态水产生时常规流场PEMFC性能高于交趾型流场PEMFC。