Dead-ended Anode Research Articles

A purge strategy for proton exchange membrane fuel cells under varying-load operations

When a proton exchange membrane fuel cell is operated in a dead-ended anode mode, its performance gradually drops due to impurity accumulation in the anode channel. The performance can be recovered by purging the impurities from the anode channel. Developing a purge strategy for a fuel cell is challenging under varying-load operations because the measured or monitored signals vary with the load demand. To capture the local performance throughout the active area during dead-end operation, the local current density distribution was measured using a specially designed single cell. First, the single cell was operated at three different current density levels for the observation of the variation of the local current density. The ratio of the normalized local current density to the load current density can be employed as a characteristic value for the purge strategy. The purge strategy with different threshold values was applied to a single cell operated under varying-load conditions and was discussed. Finally, a procedure for implementing the purge strategy was presented in detail.

Anode purge strategy optimization of the polymer electrode membrane fuel cell system under the dead-end anode operation

Dead-ended anode (DEA) mode is commonly applied in fuel cell vehicles for the hydrogen purge at the anode side, to reduce fuel waste and enhance fuel cell efficiency. Anode purge is necessary and is definitely important with respect to removing liquid water and accumulated nitrogen in the gas diffusion layer and the flow field of the DEA-mode fuel cell. In this paper, the effect of different purge strategies on the stack performance and system efficiency is investigated experimentally using fast data acquisition and advanced tools, such as the fast cell voltage measurement (CVM) system and the mass spectrum. From the fast data acquisition, the voltage stability, liquid water and nitrogen concentration measurement in the anode exhaust are compared and analyzed under different purge strategy designs and using different purge valves. The results show that under the optimal purge strategy, the DEA fuel cell stack can achieve the desired stability and system efficiency based on the analysis of the cell voltage and purge volume. Moreover, the diameter of the purge valve has a great impact on the voltage stability because a diameter change will result in a different pressure drop and purge volume when the purge valve is open.

Mitigation studies of carbon corrosion by optimizing the opening size of the cathode outlet in a proton exchange membrane fuel cell with dead-ended anode

Water management is the key issue for the cathode in a proton exchange membrane fuel cell, nearly all the humidified and generated water is flow out of the fuel cell through the outlet of the cathode. Cathode flooding can lead to dramatic performance decay and irrecoverable material degradation in fuel cell. Therefore, optimization design at the cathode outlet is a significant technical challenge for the performance and lifetime enhancement for fuel cell. To address this problem, this study optimized the opening size at cathode outlet of the fuel cell that operates under dead-ended anode mode. The designed cells were continuously operated for 100h under dead-ended anode mode and the effect of opening size in cathode outlet on cell performance has been investigated. It was found that, with the increase in opening size at cathode outlet, the flooding electrode and the consequent carbon corrosion in catalyst layer can be substantially suppressed.

Nitrogen Blanketing and Hydrogen Starvation in Dead-Ended-Anode Polymer Electrolyte Fuel Cells Revealed by Hydro-Electro-Thermal Analysis

Dead-ended anode operation has a number of practical advantages that simplify system complexity and lower cost for polymer electrolyte fuel cells. However, dead-ended mode leads to performance loss over time which can only be reversed by performing intermittent purge events. This work applies a combined hydro-electro-thermal analysis to an air-cooled open-cathode fuel cell, presenting experimental functional maps of water distribution, current density and temperature. This approach has allowed the identification of a ‘nitrogen blanketing’ effect due to nitrogen cross-over from the cathode and a ‘bypass’ effect where a peripheral gap between the gasket and the GDL offers a hydrogen flow ‘short circuit’ to the border of the electrode. A consequence of high local current density at the margin of the electrode, and resulting high temperatures, may impact the lifetime of the cell in dead-end mode.

(Invited) Modeling and Control of Pemfcs with Dead-Ended Anodes

Nitrogen, oxygen and water crossover and their accumulation in the anode channels of low temperature Proton Exchange Membrane Fuel Cells present system level challenges for the design and operation of devices with high hydrogen utilization. The low flow rates enable operation under self-humidified anode conditions further reducing the system weight, cost, and complexity which is especially important for small portable applications. However, accumulated inert gasses and liquid water need to be purged before severe starvation conditions lead to carbon corrosion, inevitably wasting some Hydrogen. Frequent purging wastes fuel and dries the membrane leading to physical degradation and membrane delamination observed in post mortem Scanning Electron Microscopy images. Model based control methods provide an automated way to schedule the anode purges and study the tradeoff between degradation and fuel efficiency over the life of a fuel cell stack [1]. In addition the models can be used for design purposes such as optimizing the anode channel dimensions to balance the tradeoff between power density and interval between purges. The transport of gas species in the anode and cathode channels can be predicted by partial differential equations, specifically the 1-D the multi-component Stefan-Maxwell equation along the length of the channel. The distribution of membrane hydration level and temperature must also be considered as they impact the crossover rate. The model accurately predict the nitrogen accumulation and resulting recoverable drop in terminal voltage over time between purges when operating with a Dead Ended Anode (DEA). The model has been validated by simultaneous Neutron Imaging (NI) and Gas Chromatography (GC) sampling from within the anode channel [4]. Neutron imaging provides a unique measurement of the liquid water distribution within the cell and can be performed on relevant materials and channel designs without the need for modified hardware with different thermal properties such as transparent plates for optical imaging. The two dimensional images provide information about both the membrane electrode assembly water content, and channel liquid water [5]. This allows us to validate model predictions of the two phase (liquid water) transition point along the length of the channels. Dead Ended Anode (DEA) operation represents the extreme case of operation with high hydrogen utilization. There are several practical advantages to studying this limiting behavior of the system. Foremost is the direct measurement of membrane permeation rates since the only sources of nitrogen and oxygen in the anode channel is crossover. The introduction of oxygen into the anode gas stream from membrane crossover is the primary mechanism for cathode carbon corrosion. The oxygen present in a hydrogen depleted region creates a voltage reversal and hence large cathode potential in the adjacent region of the cell similar to the one observed during startup conditions when the air is pushed out of the channels by hydrogen. A quaternary system for the anode channels using the Stefan-Maxwell framework was augmented with a model of carbon corrosion [3]. The resulting model of cathode active site loss predicts the slow non-recoverable voltage drop over 1000 hours of operation. The modeled corrosion rate matched the decrease in cathode catalyst layer thickness observed in post mortem SEM images [2]. Finally the augmented model has been used to study the optimum purge scheduling considering both corrosion and fuel efficiency [1]. The physical channel dimensions provide a rough estimate of feasible target purge duration (volume) and interval between purges. The minimum purge duration pushes the nitrogen blanketed region to the end of the channel, fully recovering the terminal voltage, whereas the maximum duration completely clears the channel (anything beyond this needlessly wastes fuel). The analysis and methodologies presented herein can be used for the design and control of fuel cell systems operating with high hydrogen utilization to prolong stack life and achieve high efficiency. [1] J. Chen, J. Siegel, A. Stefanopoulou, and J. Waldecker, IJHE, 38, (2013). [2] T. Matsuura, J. Chen, J. Siegel, and A. Stefanopoulou, IJHE, 38 (2013). [3] J. Chen, J. B. Siegel, T. Matsuura, and A. G. Stefanopoulou, J. Electrochem. Soc., 158, 9, (2011). [4] J. B. Siegel, S. V. Bohac, A. G. Stefanopoulou, and S. Yesilyurt, J. Electrochem. Soc., 157, 7 (2010). [5] J. B. Siegel, D. A. McKay, A. G. Stefanopoulou, D. S. Hussey, and D. L. Jacobson, J. Electrochem. Soc. 155, 11 (2008). Figure 1

Carbon corrosion and performance degradation mechanism in a proton exchange membrane fuel cell with dead-ended anode and cathode

This paper presents the work on a PEMFC (proton exchange membrane fuel cell) with DEAC (dead-ended anode and cathode) to achieve high hydrogen and oxygen utilization. Gas purging is an effective way to remove the excess water and maintain normal operation of fuel cell while the voltage drops to the set value chosen arbitrarily due to accumulation of generated water. However, serious carbon corrosion has been observed in MEA (membrane electrode assembly) under this purging model. Voltage degradation process during one purging cycle can be divided into three stages: dehydration of membrane induced ohmic loss, quasi-equilibrium state, and flooding induced concentration polarization. To improve the lifespan of PEMFC, gas purging should be conducted before water flooding. The operation time till the quasi-equilibrium state is suggested as the purging duration in the subsequent research. The results show that with the introduction of time regulator purging strategy, the decay rate of the carbon corrosion is greatly reduced. Moreover, it has also been found that the cell performance and purge cycle duration increase with increasing operating pressure, whereas they decrease with the increase in current density.

A lumped fluidic model of an anode chamber for fault tolerant strategy design

Water management in Proton Exchange Membrane (PEM) fuel cell systems is a critical point. This article will widely explain the importance of anode exhaust gas management for fuel cell and the issues of liquid water accumulation in the anodic compartment of a dead-ended anode fuel cell system. A 0D model of the water behavior in the anode channels is introduced and an identification procedure of the water diffusion coefficient and a tunable parameter, using the least-square criteria, has been developed. The presented results show good correlation, allowing to extend the model to hydrogen and to validate it with experiments. This model will be useful to elaborate a fault tolerant strategy to mitigate the impact of liquid water accumulation into the anode chamber.

Investigation of the purging effect on a dead-end anode PEM fuel cell-powered vehicle during segments of a European driving cycle

The dynamic performance of the PEM fuel cell is one of the key factors for successful operation of a fuel cell-powered vehicle. Maintaining fast time response while keeping stable and high stack performance is of importance, especially during acceleration and deceleration. In this paper, we evaluate the transient response of a PEM fuel cell stack with a dead-end anode during segments of a legislated European driving cycle together with the effect of purging factors. The PEM fuel cell stack comprises of 24 cells with a 300cm2 active catalyst area and operates at a low hydrogen and air pressure. Humidified air is supplied to the cathode side and the dry hydrogen is fed to the anode. The liquid coolant is circulated to the stack and the radiator to maintain the thermal envelope throughout the stack. The stack performance deterioration over time is prevented by utilizing the purging, which removes the accumulated water and impurities. The effect of purging period, purging duration, coolant flow rate and cathode stoichiometry are examined with regard to the fuel cell’s transient performance during the driving cycle. The results show that a low purging duration may avoid the undesired deceleration at a high current, and a high purging period may sustain a better performance over time. Moreover, the coolant flow rate is found to be an important parameter, which affects the stack temperature–time response of the cooling control and the stack performance, especially at high operating currents.

Model-based fuel pressure regulation algorithm for a hydrogen-injected PEM fuel cell engine

In Polymer Electrolyte Membrane Fuel Cells (PEMFCs) with Dead-Ended Anode (DEA) configuration, precise control of hydrogen pressure in anode is beneficial for better performance and durability of fuel cell systems. In most studies, mechanical pressure regulator is implemented to modulate the hydrogen pressure. But when load changes or purge operation activates, hydrogen pressure downstream the regulator varies with hydrogen flow fluctuation. In this study, a hydrogen injection system is developed utilizing gaseous fuel injectors. Based on a fuel cell simulation test bench, the performance of the hydrogen injection system is evaluated. A physics-based nonlinear model of the simulation test bench is presented in this paper. Experimental results validate the model. The model is further linearized and integrated in a Robust Predictive Controller (RPC), a Model-Based Controller (MBC), and an H∞ controller, respectively, in order to compare the control effects on the hydrogen pressure, especially the mitigation of the pressure swing during the purge operation. A comparative study is carried out under different scenarios, i.e. target pressure step response, hydrogen consumption mass flow step response, and periodical pulse response during the purge operation. According to the results, MBC is chosen for injection control, since the system demonstrates preferable performance in comparison with other controllers. Finally, pressure fluctuation data of both simulation test bench with hydrogen injection and conventional mechanical pressure regulator is compared to elucidate the effectiveness.

Operation characteristics and carbon corrosion of PEMFC (Proton exchange membrane fuel cell) with dead-ended anode for high hydrogen utilization

PEMFC (Proton exchange membrane fuel cell) operated under dead-ended anode mode is an effective and simple device for high hydrogen utilization. Herein, the operation characteristics of PEMFC with dead-ended anode are systematically investigated under various operating temperature, anode inlet pressure, and cathode stoichiometry. In each operating parameter, the solenoid valve located at the anode outlet is closed all the time and the hydrogen utilization reaches 100%. It has been found that fuel cell with dead-ended anode can operate continuously for 26 h with slight decrease in output voltage under 600 mA cm−2 at 65 °C, 80% of RH (relative humidity) with hydrogen inlet pressure of 0.4 bar gauge and oxygen stoichiometry of 4.0. After accumulated 150 h operation under dead-ended anode mode, the degradation of according MEA (membrane electrode assembly) has been analyzed using CV (cyclic voltammetry) and cross-sectional morphology. The results show that the accumulation of water induced corrosion of catalyst layer is responsible for the decrease in ECSA (electrochemical surface area) of catalyst layer and the according degradation of cell performance. Moreover, the degradation of catalyst layer became much more serious at the vicinity of cathode outlet than the other regions.

Impact of Water Management on Local Potential Evolutions during PEM Fuel Cell Operation with Dead-Ended Anode

Operating a PEMFC with a dead-ended anode may lead to local fuel-starvation because of water and possibly nitrogen accumulation in the anode compartment. In previous works, we used a segmented linear cell with reference electrodes to monitor simultaneously the local potentials and current densities during dead-ended anode operation. The results indicated that water transport as well as nitrogen crossover through the membrane were most probably the two key factors governing fuel starvation. In this work, we evaluated with more details the contributions of nitrogen crossover and water transport to hydrogen starvation by using different cooling temperature on the cathode and anode sides. The results show that with a colder anode side, fuel-starvation was faster. In the opposite case of a hotter anode plate, water accumulation in the anode compartment was limited, suggesting that nitrogen crossover through the membrane was probably the main reason for hydrogen starvation in this case while water it is water in regular operation (i.e., with identical cooling temperature on both sides).

Development of a Pure Oxygen Fuel Cell System for an Autonomous Underwater Vehicle with Focus on Fuel Starvation due to Inert Gas Accumulation on the Anode

In this paper a LT-PEM fuel cell stack is used in an autonomous underwater vehicle (AUV) as a range-extender to increase its mission duration by continuously supplying the AUV with electricity. A battery buffers the difference between generated power from the fuel cell and consumed power of the AUV motor and payload. For fuel cell operation in an AUV different challenges arise linked to the surrounding sea water. These conditions lead to the design of a closed fuel cell system which will be outlined. Additionally measurements concerning a purge free dead-ended anode operation of the fuel cell stack will be presented.

Impact of Water Management on Local Potential Evolutions during PEM Fuel Cell Operation with Dead-Ended Anode

The operation of proton exchange membrane (PEM) with a dead-ended anode may lead to local fuel starvation due to the excessive accumulation of liquid water and possibly nitrogen (due to crossover through the membrane) in the anode compartment. Such fuel-starvation events are localized and may remain undetected in terms of overall cell performance. However, these events can lead to significant rise of the anode (and thus cathode) local potentials and accelerate carbon corrosion and catalyst degradation, which can be particularly damaging to the cell performance [1-3]. In previous works we used a segmented linear cell with reference electrodes to monitor simultaneously the local anode and cathode potentials (i.e. the difference between the metal and electrolyte potential) and current densities during operation while the anode outlet remaining closed between several hundreds and a few thousands of seconds [4]. Apart from the addition of the reference electrodes, this cell is similar to that described in [5]. Such dead-ended anode operation sequences were repeated during 216 hours, which led to non-uniform ElectroChemical Surface Area (ECSA) losses, strong carbon corrosion rates (estimated by measuring CO2 concentration at the cathode exhaust) and performance degradation along the cell area. The damage was more pronounced in the regions suffering the longest from fuel starvation: i.e., close to the anode outlet. In a first step, several parameters like membrane thickness, air relative humidity and average current density were varied and the results indicated that water transport from the cathode to the anode as well as nitrogen crossover through the membrane [6] were most probably the two key factors governing fuel starvation. Then, we evaluated with more details the contributions of nitrogen crossover and water transport to hydrogen starvation. Based on the recent works of Thomas et al. [7, 8], the cell was operated by using three thermal configurations with different cooling temperature on the cathode and anode sides (while the cell average temperature was kept constant) in order to drive water toward the colder side: Tanode = Tcathode = 65°C;Tanode = 62.5°C < Tcathode = 67.5°C;Tanode = 67.5°C > Tcathode = 62.5°C; Figure 1 shows the cathode potential evolutions for the six first cycles of dead-end/purge for each thermal configurations (i), (ii) and (iii). For the second configuration (ii), i.e. Ta < Tc, water accumulation in the anode side was favored. As a consequence, the average closing time was 35% faster than for configuration (i). For the third configuration (iii), Ta > Tc, the average cycle duration was seven times greater than that of configuration (i) and ten times greater than that of configuration (ii). For the third configuration, water accumulation in the anode compartment was limited – in practice, no liquid water can be observed at the anode outlet following the purge –. Nitrogen crossover through the membrane was probably the main reason for hydrogen starvation. In order to assess the impact of the thermal configuration on carbon corrosion and performance decrease, the three aging protocols (for each thermal configuration) were repeated but by setting a fixed time span between two successive purges (and by this way the total number of purges performed in 216 hours). The results allowed us to state that the configuration (iii), Tanode>Tcathode, was less damaging than the two other configurations. Eventually, such results should help optimizing purge protocols of fuel cell stack operated with a dead-end mode. Figure 1

In situ analysis of voltage degradation in a polymer electrolyte membrane fuel cell with a dead-ended anode

In this paper, the impact of dead-ended anode (DEA) operation on the liquid water management of polymer electrolyte membrane fuel cells (PEMFCs) was investigated using X-ray synchrotron radiography. The fuel cell voltage and concurrent through-plane liquid water saturations in anode and cathode gas diffusion layers (GDLs) were measured during DEA and open anode operation. It was observed that DEA operation led to significant increases in liquid water content in both anode and cathode GDLs, along with fuel cell power output degradation. A strong correlation was observed between the buildup of the liquid water in the anode and the cathode, and as a result, the liquid water transport in the cathode was also significantly impacted by this DEA operation.

Influence of gas channel depth in self-humidified miniature PEM fuel cells with dead-ended anode

For fuel cells to become a viable power source in low power applications, significant gains must be made in maximizing fuel cell specific power density and system specific energy density. To reduce complexity, these miniature PEM fuel cells are passively cooled with a dead-ended anode. Additionally, dry reactant gases can be supplied, resulting in self-humidified membranes. Targeting reductions in mass, this work experimentally investigates the influence of flow field channel depth on cell performance. Over the range of 0.457–1.57 mm, the best performance was found at cathode channel depths of 0.813 mm with no statistically significant difference associated with anode channel depth using static polarization curves following anode purges. As a result, an along-the-channel model of anode nitrogen accumulation identifies the optimal anode channel depth that minimizes the system specific energy density given a driving cycle. The anode volumetric leak rate plays a significant role in the nitrogen frontal evolution and resulting optimal anode channel depth. If deployed in a multi-cell stack with the recommended channel depths, the PEMFC system would be capable of achieving a specific power density of 45 mW/g and a specific energy density of 680 W h/kg, nearing the specific power density and exceeding the specific energy density of off-the-shelf lithium-ion batteries (approximately 80–100 mW/g and 300 W h/kg).

Dynamic cell performance of kW-grade proton exchange membrane fuel cell stack with dead-ended anode

This paper examines the dynamic cell performance of a kW-grade proton exchange membrane fuel cell stack with anode dead-ended mode fuel supply. A self-made kW-grade 40 cells stack with reaction area of 112.85cm2 has been used in the experiment. A single-chip (DSPIC30F4011) is utilized for establishing a control circuit to monitor the voltage and current with constant-current loading. The stack temperature is controlled at a low-level temperature rise. To enhance the hydrogen utilization and reduce the water flooding in the fuel cell stack, the dead-ended anode operation is accomplished by controlling the open or close of the anode outlet solenoid valve. As the loading is heavy, the anode outlet solenoid valve is purged frequently to force the water to flow out. While a light load, the anode outlet solenoid valve is shut down for a period time for hydrogen saving. The solenoid valve is controlled to be opened, referred as purge interval, reaching the discharge amount for 1000C, 1500C, and 2000C as parameter, respectively. The open period of solenoid valve, referred as purge duration, is set as 1s, 3s, and 5s for this study. Experimental results indicate an optimal purge interval and duration for water management and cell performance of the fuel cell stack.

High Potential Excursions during PEM Fuel Cell Operation with Dead-Ended Anode

Operating a proton exchange membrane (PEM) fuel cell with a dead-ended anode may lead to local fuel starvation due to the excessive accumulation of liquid water and possibly nitrogen (because of membrane crossover) in the anode compartment. In this paper, we present experimental results obtained with a segmented, linear cell with reference electrodes along the gas channels, used to record local anode and cathode potentials. By simultaneously monitoring the local potentials and current densities during operation, we assessed the impact of fuel starvation on local fuel cell performance during aging protocols consisting of repeated dead-ended anode operation sequences (with anode outlet closed longer than in real use conditions). During the aging protocols, we observed strong local cathode potential excursions close to the anode outlet. The cathode showed non-uniform ElectroChemical Surface Area (ECSA) losses and performance degradation along the cell area. The damage was more severe in the regions suffering the longest from fuel starvation. Similar experiments performed in different operating conditions and with different membrane thickness showed that water management impacts significantly the cathode potential variations and thus the MEA degradation. Most of the MEA degradation is attributed to local cathode potential excursions above 1.2 V although potential cycling between 0.5 V and 0.7 V also had an impact in the regions well supplied with hydrogen (hydrogen purges were triggered when the fuel cell voltage dropped from about 0.7 V to 0.5 V). According to our results, localized and transient hydrogen starvation events may be difficult to detect by considering only the overall fuel cell performance.

Control by state observer of PEMFC anodic purges in dead-end operating mode

A Proton Exchange Membrane Fuel Cell (PEMFC) needs an active system to control all the ancillaries and ensure optimal operating conditions, especially in a fuel cell vehicle. For the fuel cell system architecture, dead-end anode is the cheapest architecture for the hydrogen line and also the one that leads to important reversible and irreversible degradations if not appropriately managed. To address the cost and durability issues on fuel cell vehicles, this study proposes a state observer which aims at estimating online the nitrogen saturation in the anode side in order to trigger the purge at a given criterion. This observer is based on a simple set of equations extracted from a detailed 2D-meshed model. Nitrogen buildup is evaluated in simulation with different road profiles with less than 5% error. Moreover the fuel cell system efficiency increases compared to the other existing purge strategies, so as the mileage. The observer can also be used to develop other optimal strategies to minimize the irreversible degradations of the fuel cell.

Effect of Controlled Anode Flow Release on Dead-Ended Anode Proton Exchange Membrane Fuel Cells

Dead-ended anode operation is commonly used in polymer electrolyte fuel cells, but suffers from voltage reduction with time. This study shows how a controlled anode bleed can mitigate against voltage loss and fuel dilution. Performance analysis, along with off-gas analysis, is used to characterize cell performance during dead-ended mode and an optimum bleed rate is identified to minimize voltage drop for this particular 5-cell stack.

Transient analysis of gas transport in anode channel of a polymer electrolyte membrane fuel cell with dead-ended anode under pressure swing operation

Further cost reduction is a critical issue for commercialization of fuel-cell electric vehicles (FCEVs) based on polymer electrolyte fuel cells (PEFCs). The cost of the fuel-cell system is driven by the multiple parts required to maximize stack performance and maintain durability and robustness. The fuel-cell system of the FCEV must be simplified while maintaining functionality. The dead-ended anode is considered as a means of simplification in this study. Generally, if hydrogen is supplied under constant pressure during dead-ended operation, stable power generation is impossible because of accumulation of liquid water produced by power generation and of nitrogen via leakage from the cathode through the membrane. Herein, pressure oscillation is applied to address this issue. Empirical and CFD data are employed to elucidate the mechanism of stable power generation using the pressure swing supply. Simultaneous and time-continuous measurements of the current distribution and gas concentration distribution are also conducted. The results demonstrate that the nitrogen concentration in the anode channel under pressure constant operation differs from that under pressure swing supply conditions. The transient two-dimensional CFD results indicate that oscillatory flow is generated by pressure swing supply, which periodically sweeps out nitrogen from the active area, resulting in stable power generation.