Degradation mitigation effects of pressure swing in proton exchange membrane fuel cells with dead-ended anode

High cost remains to be one of the primary obstacles for the commercialization of proton exchange membrane fuel cells (PEMFCs). To simplify the fuel cell system and reduce cost, dead-ended anode (DEA) is widely used. However, water and nitrogen can accumulate in the dead-ended anode, resulting in cell performance decrease and severe cell degradation. Anode pressure swing supply is a new technology which has been shown to be effective in reducing local water and nitrogen accumulation in the anode channel. In this work, the effects of pressure swing supply on fuel cell degradation have been experimentally studied. Two sets of experiments on the same fuel cell are conducted, one under conventional constant pressure operation and the other under pressure swing operation. Polarization curves show that pressure swing supply can significantly mitigate cell degradation during DEA operations. Electrochemical characterizations are performed to study the mechanisms of mitigations in cell degradation. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that pressure swing supply can significantly reduce electrolyte membrane degradation, but has no significant mitigation effect on the cathode catalyst degradation during DEA operation. Further examinations of the membrane-electrode-assembly (MEA) by scanning electron microscope (SEM) confirm the significant difference in membrane degradations since there is a very large difference in average thickness of the membranes after the degradation tests.     

Effect of air purging and dry operation on durability of PEMFC under freeze/thaw cycles

The effect of air purging and dry operation on durability of polymer electrolyte membrane fuel cell (PEMFC) under repeated freeze/thaw cycles between −20 °C and 60 °C was investigated. The cathode air purging and the operation with dry air feed were highly effective to mitigate freeze damage. The removal of the air purging of the anode compartment did not lead to the degradation of the anode catalyst layer. It is of practical importance, because the air purging of the anode could cause carbon corrosion of the cathode. The performance degradation by the freeze/thaw cycles was associated with the increased charge transfer and mass transfer resistances. After the freeze/thaw cycles, any discernable morphological changes were not observed in the scanning electron microscopic images of the anode, the membrane and the membrane/electrode interface, however, mechanical damage of the poly(tetrafluoroethylene) phases in the cathode diffusion layer was detected.     

Study of the cell reversal process of large area proton exchange membrane fuel cells under fuel starvation

In this research, the fuel starvation phenomena in a single proton exchange membrane fuel cell (PEMFC) are investigated experimentally. The response characteristics of a single cell under the different degrees of fuel starvation are explored. The key parameters (cell voltage, current distribution, cathode and anode potentials, and local interfacial potentials between anode and membrane, etc.) are measured in situ with a specially constructed segmented fuel cell. Experimental results show that during the cell reversal process due to the fuel starvation, the current distribution is extremely uneven, the local high interfacial potential is suffered near the anode outlet, hydrogen and water are oxidized simultaneously in the different regions at the anode, and the carbon corrosion is proved to occur at the anode by analyzing the anode exhaust gas. When the fuel starvation becomes severer, the water electrolysis current gets larger, the local interfacial potential turns higher, and the carbon corrosion near the anode outlet gets more significant. The local interfacial potential near the anode outlet increases from ca. 1.8 to 2.6 V when the hydrogen stoichiometry decreases from 0.91 to 0.55. The producing rate of the carbon dioxide also increases from 18 to 20 ml min⁻¹.     

Effects of CrN/Cr coating layer on durability of metal bipolar plates under a fuel recirculation system of direct methanol fuel cells

Effects of a CrN/Cr coating layer on the durability of metal bipolar plates (stainless steel (STS) 430) are investigated in direct methanol fuel cells (DMFCs) with a fuel recirculation system, since under fuel recirculation the metal bipolar plates can be faced with a tougher corrosion environment. Before the fuel recirculation, the performance losses of the cells consisting of metal bipolar plates are ascribed to cathode degradation, due to a high corrosion level of the cathode side. However, after fuel recirculation, corrosion of the anode metal bipolar plate by pH decrease and overpotential increase brings about severe anode degradation. It is found that damage by corrosion of the cathode metal bipolar plate is limited to degradation of the cathode catalyst, whereas corrosion of the anode metal bipolar plate deteriorates not only the catalysts but also the electrolyte membrane. These durability tests show that the CrN/Cr coatings deposited on the STS 430 improve the corrosion resistance of the metal substrate and lead to low performance degradation.     

Comparison of the performance and degradation mechanism of PEMFC with Pt/C and Pt black catalyst

Proton exchange membrane fuel cell (PEMFC) has been used in supplying power for Unmanned Underwater Vehicle which operate in a closed environment and dead-ended anode and cathode (DEAC) mode is deemed as an effective way to enhance the fuel utilization rate. Catalyst is an important factor that influences the performance and durability of PEMFC, especially in DEAC mode. In this paper, the degradation characteristics of PEMFCs with Pt black and Pt/C catalyst after 100 h operation have been investigated by electrochemical techniques and morphological characterization methods. It's shown that the degradation of Pt black catalyst layer (CL) was more severe than that of Pt/C CL. The difference of performance degradation is due to the dominant decay mechanism of these two catalysts is different. According to SEM, TEM and XPS results, the decay of Pt black catalyst is mainly caused by Pt agglomeration and oxidation, causing a higher ohmic resistance, higher mass transfer resistance and severer degradation of performance. The degradation of Pt/C catalyst is mainly due to the reduction of electrochemical surface area and carbon corrosion because the larger carbon corrosion makes micropores and the thicker supporting structure, resulting in the performance degradation.     

Effect of CO in the anode fuel on the performance of PEM fuel cell cathode

Even trace amounts of CO in the fuel for a proton-exchange membrane fuel cell (PEMFC) could poison not only the anode, which is directly exposed to the fuel, but also the cathode, which is separated from the fuel by a proton-exchange membrane; and the performance decline of the cathode is sometimes more than that of the anode. Adsorption of CO on the cathode catalyst has been detected electrochemically, and this indicates that CO can pass through the membrane to reach the cathode. To reduce such a poisoning effect, fuel cell operation conditions (e.g. level of membrane humidification, gas pressure difference between cathode and anode), membrane and catalyst layer structures, and CO-tolerant cathode catalysts should be further explored.     

Effects of Pt loading in the anode on the durability of a membrane–electrode assembly for polymer electrolyte membrane fuel cells during startup/shutdown cycling

A polymer electrolyte membrane fuel cell (PEMFC) stack of a fuel cell vehicle (FCV) is inevitably exposed to reverse current conditions, which are formed by the oxygen reduction reaction (ORR) induced at the anode with a hydrogen/air boundary during startup/shutdown processes. With an increase in the reverse current, the degradation rate of the cathode that experiences a highly corrosive condition (locally high potential) increases. In this work, the anode Pt loading is decreased from 0.4 to 0.1 mg cm⁻² to decrease the reverse current. The decrease in the anode Pt loading is found to decrease the hydrogen oxidation rates (HOR) during normal operation, but this loading decrease barely affected the cell performance. However, a decrease in the anode Pt loading can significantly decrease the reverse current, leading to a diminished cathode degradation rate during startup/shutdown cycling. It is revealed by slow decreases in the cell performance (i–V curves) and electrochemical active surface area (EAS), and a slow increase in the charge-transfer resistance (R_ct), which can be attributed to corrosion of the carbon support and dissolution/migration/agglomeration of the platinum catalyst.     

Different flow fields,operation modes and designs for proton exchange membrane fuel cells with dead-ended anode

Water and nitrogen can accumulate in the anode channel in proton exchange membrane fuel cells (PEMFCs) with dead-ended anode (DEA) and can affect cell performance significantly. In this paper, the cell performance characteristics in DEA PEMFCs with three different anode flow fields under two operating modes are studied through measuring the cell voltages and local current densities. The effect of the anode exit reservoir is also studied for the three different anode flow fields. The experimental results show that the interdigitated flow field has the most stable cell performance under both constant pressure and pressure swing supply modes. Parallel and serpentine flow fields lead to very non-uniform local current distributions under constant pressure supply mode and experience severe fluctuations and spikes in local current densities under pressure swing supply mode. The results also show that anode pressure swing supply mode can achieve more stable cell performance than anode constant pressure supply mode for parallel and serpentine anode flow fields. The anode exit reservoir can significantly improve cell performance stability for parallel and serpentine flow fields, but has no significant effect on interdigitated flow fields. Besides, the results further show that PEMFCs with DEA can maintain very stable operation with anode serpentine flow field and an anode exit reservoir under pressure swing operation.     

Polarization study of a PEMFC with four reference electrodes at hydrogen starvation conditions

Hydrogen starvation results in an inhomogeneous distribution of gases within fuel cells and stacks. An inhomogeneous reactant distribution significantly changes the current density distribution over the active cell area and can cause high cathode potentials due to the presence of oxygen in the anode electrode domain. High cathode potentials result in corrosion of the carbon support and thus lead to irreversible cell degradation. A PEMFC single cell assembly was utilised for detailed investigations of these phenomena. The cell was operated with low stoichiometries and in dead-end mode. Product gas analyses were utilised for determining the degradation phenomena.     

Durability and characterization studies of polymer electrolyte membrane fuel cell’s coated aluminum bipolar plates and membrane electrode assembly

Coated aluminum bipolar plates demonstrate better mechanical strength, ease of manufacturability, and lower interfacial contact resistance (ICR) than graphite composite plates in polymer electrolyte membrane (PEM) fuel cell applications. In this study, coated aluminum and graphite composite bipolar plates were installed in separate single PEM fuel cells and tested under normal operating conditions and cyclic loading. After 1000 h of operation, samples of both the bipolar plates and the membrane electrode assembly (MEA) were collected from both the cathode and the anode sides of the cell and characterized to examine the stability and integrity of the plate coating and evaluate possible changes of the ionic conductivity of the membrane due any electrochemical reaction with the coating material. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis were performed on the land and valley surfaces of the reactant flow fields at both the anode and the cathode sides of the bipolar plates. The measurements were superimposed on the reference to identify possible zones of anomalies for the purpose of conducting focused studies in these locations. The X-ray diffraction (XRD) analysis of samples scraped from the anode and cathode electrodes of the MEA showed the tendency for catalyst growth that could result in power degradation. Samples of the by-product water produced during the single fuel cell operation were also collected and tested for the existence of chromium, nickel, carbon, iron, sulfur and aluminum using mass spectroscopy techniques. The EDX measurements indicated the possibility of dissociation and dissolution of nickel chrome that was used as the binder for the carbide-based corrosion-resistant coating with the aluminum substrate.     

Modeling hydrogen starvation conditions in proton-exchange membrane fuel cells

In this study, a steady state and isothermal 2D-PEM fuel cell model is presented. By simulation of a single cell along the channel and in through-plane direction, its behaviour under hydrogen starvation due to nitrogen dilution is analysed. Under these conditions, carbon corrosion and water electrolysis are observed on the cathode side. This phenomenon, causing severe cell degradation, is known as reverse current decay mechanism in literature. Butler-Volmer equations are used to model the electrochemical reactions. In addition, we account for permeation of gases through the membrane and for the local water content within the membrane. The results show that the membrane potential locally drops in areas starved from hydrogen. This leads to potential gradients >1.2 V between electrode and membrane on the cathode side resulting in significant carbon corrosion and electrolysis reaction rates. The model enables the analysis of sub-stoichiometric states occurring during anode gas recirculation or load transients.     

Anode bleeding experiments to improve the performance and durability of proton exchange membrane fuel cells

Cost, durability, efficiency and fuel utilization are important issues that remain to be resolved for commercialization of proton exchange membrane fuel cells (PEMFC). Anode flow mode, which includes recirculation, dead-ended and exit bleeding operation, plays an important role in fuel utilization, durability, performance and the overall cost of the fuel cell system. Depending on the flow mode, water and nitrogen accumulation in the anode leads to voltage transients and local fuel starvation, which causes cell potential reversal and carbon corrosion in the cathode catalyst layers. Controlled anode exit bleeding can avoid the accumulation of nitrogen and water and improve fuel utilization. In this study, we present a method to control the bleed rate with high precision in experiments and demonstrate that hydrogen utilization as high as 0.9988 for a 25 cm² single cell and 0.9974 for an 8.17 cm² single cell can be achieved without significant performance loss. In the experiments, anode pressure is kept at 1 bar higher than the cathode pressure to decrease nitrogen crossover from the cathode, decreasing the crossover from the cathode. Moreover, four load cycle profiles are applied to observe the cumulative loss in the electrochemical surface area (ECSA), which are acquired from cyclic voltammetry (CV) analysis. Experiments confirm that the ECSA loss and severe voltage transients are indicative of fuel starvation induced by prolonged dead-ended or low exit-bleed operation modes whereas bleed rates that are larger than the predicted crossover rate are sufficient to operate the fuel cell without voltage transients and detrimental ECSA loss.     

Study on the processes of start-up and shutdown in proton exchange membrane fuel cells

Several different start-up and shutdown processes of a fuel cell were investigated in this paper by measuring cathode, anode and membrane potentials respectively versus reference hydrogen electrode (RHE). The membrane potential was measured by using thin copper wire sandwiched between two membranes. During the formation of hydrogen/air boundary at the anode caused by unprotected start-up and shutdown, the in-plane potential difference between membrane inlet and outlet was measured to be as high as 0.8 V, and the interfacial potential difference between cathode and membrane outlet increased to about 1.6 V, which in turn would cause carbon corrosion. Appropriate start-up and shutdown processes were suggested to avoid the formation of hydrogen/air boundary.     

PEM fuel cell cathode carbon corrosion due to the formation of air/fuel boundary at the anode

The impacts of unprotected start up and shut down on fuel cell performance degradation was investigated using both single cell and dual cell configurations. It was found that the air/fuel boundary developed at the anode side after a fuel cell shut down or during its restart caused extremely quick degradation of the cathode. The thickness, the electrochemical active surface area, and the performance of the cathode catalyst layer were significantly reduced. By using a dual cell configuration, cathode potential as high as two times of open circuit voltage was measured, and the corrosion current flowing externally between the two cells was detected and quantified. Carbon catalyst-support corrosion/oxidation at such a high potential was largely responsible for the accelerated fuel cell performance degradation.     

Effect of iridium oxide as an additive on catalysts with different Pt contents in cell reversal conditions of polymer electrolyte membrane fuel cells

Cell reversal is observed when a current load is applied to the polymer electrolyte membrane fuel cell under fuel starvation conditions. Cell reversal causes severe corrosion (or oxidation) of the carbon support in the anode, which leads to a decrease in overall fuel cell performance. To suppress the corrosion reaction of carbon under cell reversal conditions and to increase the durability of fuel cells, studies on anode additives are being conducted. However, studies on the effect of additives on catalysts with different platinum contents have not been conducted. In this study, 20 wt%, 40 wt%, 60 wt% commercial Pt/C catalyst was applied to the anode, and 50 cycles of cell reversal were performed. Furthermore, the performance change with and without IrO₂ as an additive was observed and its effect was assessed. Changes in the morphologies of the electrodes before and after cell reversal tests were also observed using a transmission electron microscope and a scanning electron microscope. The higher the platinum content of the catalyst, the more resistant to cell reversal. In addition, the addition of IrO₂ to the anode effectively prevents performance degradation due to cell reversal.     

Corrosion of carbon supports at cathode during hydrogen/air replacement at anode studied by visualization of oxygen partial pressures in a PEFC—Start-up/shut-down simulation

During start-up/shut-down processes of a polymer electrolyte fuel cell, platinum particles are lost from the catalyst layer at the cathode due to corrosion of the carbon supports. We simulated the start-up/shut-down cycle by exchanging gases at the anode between hydrogen and air. During the gas exchange, the distribution of oxygen partial pressures at the anode was visualized by our real-time/space visualization system, which clearly showed the location of H₂- and O₂-rich areas along the gas-flow channel from the inlet to the outlet. The gas exchange rate was found to be much slower than that predicted from the simple replacement and to be correlated to the proton transfer derived from carbon corrosion of the cathode catalyst layer. By the visualization results, it was found that the shut-down process gives more serious effect than the start-up process. After the degradation, the oxygen partial pressure at the cathode was visualized during the cell operation. Oxygen was consumed mainly in the middle of the MEA because the MEA was degraded mainly near the inlet and outlet of reactant gases in the cell.     

Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation

The damage caused by cell reversal during proton exchange membrane fuel cell (PEMFC) operation with fuel starvation was investigated by a single cell experiment. The samples from degraded membrane–electrode assemblies (MEAs) were characterized. Chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) clearly showed ruthenium dissolution from the anode catalyst particles. Severe ruthenium loss was observed especially in the fuel outlet region. A reduced carbon monoxide (CO) tolerance was found by CO stripping voltammetry and measurement of deteriorated the fuel cell performance. Surface area loss of the cathode platinum by sintering was also detected by transmission electron microscopy (TEM) analysis and cyclic voltammetry.     

Uncertainty analysis and robust control of fuel delivery systems considering nitrogen crossover phenomenon

In this paper, the fuel delivery subsystem (FDS) with hydrogen recirculation and anode bleeding is applied in proton exchange membrane fuel cell (PEMFC) system, which is utilized to supply hydrogen to the anode of stack and recirculate fuel back to the supply line. As the diffusion of nitrogen from cathode to anode is inevitable in a real PEMFC during long-term operation. To prevent system performance decline due to nitrogen accumulation. Therefore, this paper firstly develops a control-oriented nonlinear dynamic FDS model involving gas diffusion. Additionally, the FDS is very sensitive to operating environment, uncontrolled operation conditions may cause stack degradation. Specifically, a method based on Monte Carlo simulation is proposed to identify the key parameter boundaries. Then the gas distribution in FDS due to nitrogen crossover is analyzed in detail. After this, a hybrid robust methodology based on sliding mode algorithm is also proposed to maintain adequate hydrogen pressure supply, suitable hydrogen and nitrogen content in the system in presence of nitrogen crossover and renewed uncertainties. Finally, the performance of the presented controller is compared with nonlinear PID (NPID) control and nonlinear multi-input-multi-output (NMIMO) control through a hardware-in-the-loop test bench. Experimental results show that the hybrid controller is accurate and suitable for control purpose, the nitrogen content is restricted to the given range and the variation of output voltage is limited to the desired boundaries, the feasibility and effectiveness are validated.     

Degradation mechanisms of proton exchange membrane fuel cell under typical automotive operating conditions

The proton exchange membrane (PEM) fuel cell is an ideal automotive power source with great potential, owing to its high efficiency and zero emissions. However, the durability and life-span limit its large-scale application. Complex automotive operating conditions significantly accelerate fuel cell aging, and result in diverse degradation mechanisms that require comprehensive understanding. This review focuses on three harsh conditions of open-circuit/idling, dynamic load, and startup-shutdown. In-situ and ex-situ accelerated stress tests (ASTs) for the three conditions are summarized in terms of methodology, research objectives, and conditions of application. Reversible decay may arise during ASTs and lead to an over-estimation of the aging state, of which the causes and recovery procedures are emphasized. The degradation mechanisms are elaborated systematically according to parameter characteristics, microstructure, and aging reactions. First, increased gas permeation and a high cathode potential during open-circuit/idling combine to intensify generation of free radicals that cause membrane degradation. Pt degradation and migration are also accelerated, characterized by increased Pt particle growth and precipitation in the membrane. The debate regarding the effect of Pt precipitation on membrane degradation is resolved based on a literature review. Second, dynamic load brings about changes in the thermal/humidity state, altered reactant demand, and potential cycling, which lead to mechanical degradation, gas starvation, and Pt particle growth, respectively. To account for the accelerated particle growth, electrochemical Ostwald ripening and increased Pt dissolution are reviewed. Third, an air/hydrogen boundary appears in the anode under startup-shutdown condition and causes carbon corrosion in the local cathode via the reverse current mechanism. The cathode thereby suffers from severe and non-uniform structural damage and even structural collapse, accompanied by Pt agglomeration and detachment. Meanwhile, difficulties in mass transfer arise because of ionomer redistribution, decreased porosity, and carbon surface hydrophilization. In addition, cold start produces severe damage to component structures. This paper seeks to guide further investigation into improved fuel cell durability via mechanism analysis, condition optimization, control strategy development, structural design of the membrane electrode assembly, and component material development.     

Three-dimensional multiphase modeling of alkaline anion exchange membrane fuel cell

Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.