AC impedance characteristics of a vehicle PEM fuel cell stack under strengthened road vibrating conditions

Electrochemical impedance spectroscopy is used in this paper to investigate the performance of the fuel cell stack and single cells under long-term vibrating conditions on strengthened roads. During strengthened road vibration test, the electrochemical impedance spectra of fuel cell stack and several cells in the stack are measured nine times at regular intervals. Parameters of a Randles-like equivalent circuit are fitted to the experimental data. The classical Randles cell is extended by changing the standard plane capacitor into a constant phase element so that the quality of fit is improved. The results of the electrochemical impedance analysis indicate that the ohmic resistance of the fuel cell stack is nearly linear with the vibration time and reaches a growth of 0.035695% per hour. While the charge transfer resistance of the fuel cell stack during strengthened vibration test ascends after it falls down firstly, and finally tends to be stable. The influence of cell position on the AC impedance is also studied, and the results of which show that the cell position has a significant impact on the ohmic resistance.     

Research on electrochemical impedance spectroscope behavior of fuel cell stack under different reactant relative humidity

The reactant relative humidity (RH) is a key parameter to keep a suitable water content in the large active area fuel cell. Electrochemical impedance spectroscopy (EIS) is one of the effective methods to identify the water state within the membrane. In this work, the EIS behavior of fuel cell stack under different reactant RH and current density is investigated. Both the whole stack and individual cell impedances are experimentally measured. The internal reactions of the stack and individual cells with different current densities and different reactants RH can be distinguished by impedances. Based on the experiment results, the low frequency impedance has a greater variation than high frequency impedance when the reactant RH changes. And the impedance is more sensitive to the reactant RH under low current density. With the current density increases, the internal self-humidification can be realized to obtain a good performance for large area fuel cells.     

Assessing cell polarity reversal degradation phenomena in PEM fuel cells by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base technique. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data was made using an a priori equivalent circuit describing the transfer function of the system in question – equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT). Results and interpretation of cell polarity reversal due to hydrogen starvation were complemented with ex-situ MEA cross section analysis, using scanning electron microscopy. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation by fluoride depletion are discussed.     

Experimental analysis of a degraded open-cathode PEM fuel cell stack

The well-known challenges to overcome in PEM fuel cell research are their relatively low durability and the high costs for the platinum catalysts. This work focuses on degradation mechanisms that are present in open-cathode PEM fuel cell systems and their links to the decaying fuel cell performance. Therefore a degraded, open-cathode, 20 cell, PEM fuel cell stack was analyzed by means of in-situ and ex-situ techniques. Voltage transients during external perturbations, such as changing temperature, humidity and stoichiometry show that degradation affects individual cells quite differently towards the end of life of the stack. Cells located close to the endplates of the stack show the biggest performance decay. Electrochemical impedance spectroscopy (EIS) data present non-reversible catalyst layer degradation but negligible membrane degradation of several cells. Post-mortem, ex-situ experiments, such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show a significant active area loss of the first cells within the stack due to Pt dissolution, oxidation and agglomeration. Scanning electron microscope (SEM) images of the degraded cells in comparison with the normally working cells in the stack show severe carbon corrosion of the cathode catalyst layers.     

Electrochemical characterization of a CFY-stack with planar electrolyte-supported solid oxide cells in rSOC operation

In this study, a ten-layer SOC-stack, consisting of large planar electrolyte supported cells (ESC) and CFY interconnects, was operated in both fuel cell and electrolysis modes considering steam-, co-, and CO₂-electrolysis. Different gas mixtures containing H₂, H₂O, CO, CO₂, and N₂ were applied for this purpose, and an extensive analyses were performed at two temperatures, 800°C and 850°C. Under these operating conditions, independent Electrochemical Impedance Spectroscopy (EIS) measurements were performed on the stack as a whole, and several cells within the stack concurrently. In order to analyze the losses that occurred during operation, measured impedance values were analyzed, and an electrical equivalent circuit (EEC) for ESCs was applied to the measured impedance spectra. Finally, the electrochemical impedance measurements were performed at both OCV and elevated current densities.     

AC impedance diagnosis of a 500 W PEM fuel cell stack: Part II: Individual cell impedance

AC impedance or electrochemical impedance spectroscopy (EIS) has been demonstrated to be a powerful technique for characterizing and evaluating fuel cells. In this work, as an extension of our previous study on the stack impedance of a 500 W PEM fuel cell, we report the AC impedance studies on individual cells of the same fuel cell stack. The EIS of the stack with an active area of 280 cm² was measured at currents from 10 to 210 A in steps of 20 A using the combination of a FuelCon test station, a TDI loadbank and a Solartron 1260 Frequency Response Analyzer. Measurement of the individual cell EIS was carried out with the help of a rotary switch unit made in our lab. Two methods (floating mode and grounded mode) were utilized for measuring the impedance spectroscopy of the individual cells. The results show that both methods are applicable to individual cells. The results also indicate a good agreement between the total Ohmic loss in the stack and the combined Ohmic losses of the individual cells.     

Estimation of membrane hydration status for standby proton exchange membrane fuel cell systems by complex impedance measurement: Constant temperature stack characterization

Fuel cell-based backup units are characterized by long standby periods but they must be ready to start at any instant in the shortest possible time. In the case of low temperature proton exchange membrane fuel cells, the estimation of the hydration status of the fuel cell's membrane during standby is important for determining the cell's ability to perform a fast and safe startup. In this article, non-conventional electrochemical impedance spectroscopy (EIS) is suggested as a method to estimate the membrane's hydration status. The proposed technique differs from standard EIS in that the current through the fuel cell cannot contain a DC component, since hydrogen is absent. A 56-cell fuel cell stack has been symmetrically fed with air, whose temperature and relative humidity were controlled, and its complex impedance was measured at different frequencies and for different values of relative humidity at constant temperature. Power regression models were applied to the data, and the relationships between complex impedance and relative humidity were found. The results showed that the proposed technique is a viable way for estimating the membrane hydration status of a fuel cell stack during standby. Moreover, the most suitable frequency values at which the measurements should be performed are given.     

Study of PEM fuel cell performance by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a suitable and powerful diagnostic testing method for fuel cells because it is non-destructive and provides useful information about fuel cell performance and its components. This paper presents the diagnostic testing results of a 120 W single cell and a 480 W PEM fuel cell short stack by electrochemical impedance spectroscopy. The effects of clamping torque, non-uniform assembly pressure and operating temperature on the single cell impedance spectrum were studied. Optimal clamping torque of the single cell was determined by inspection of variations of high frequency and mass transport resistances with the clamping torque. The results of the electrochemical impedance analysis show that the non-uniform assembly pressure can deteriorate the fuel cell performance by increasing the ohmic resistance and the mass transport limitation. Break-in procedure of the short stack was monitored and it is indicated that the ohmic resistance as well as the charge transfer resistance decrease to specified values as the break-in process proceeds. The effect of output current on the impedance plots of the short stack was also investigated.     

Analysis of dc link oscillations in a hybrid fuel cell powertrain brought by in situ converter based electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a valuable tool for characterizing fuel cells. It was previously used for offline testing and only applied to single cells or short stacks with low-power equipment. Recently, there has been a trend on bringing it to high-power fuel cell stacks (FCSs) for in situ operation, especially in vehicular applications, with the required EIS ac perturbations generated by the power processing converter. However, the effects of adding such an extra function to a converter in the powertrain have not been investigated previously. Aiming at filling this gap, this paper focuses on the oscillations brought by the converter based EIS operation and their transfer in a typical hybrid fuel cell powertrain. The presented results indicate that it is favorable to operate EIS during motor is idling. The oscillation on the dc link reaches its maximum at the resonant frequency of the energy storage converter, while its magnitude is influenced by various factors, i.e., system parameters and operating conditions. Moreover, it is found that the worst-case condition of oscillation can be determined by a healthy stack at the beginning of its life.     

A study of the effect of compression on the performance of polymer electrolyte fuel cells using electrochemical impedance spectroscopy and dimensional change analysis

Compression plays an important role in the performance of polymer electrolyte fuel cells (PEFCs). In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) with dimension change. The stress/strain characteristics of the MEA are observed to be dominated by the gas diffusion layer (GDL), with the GDL exhibiting a degree of plasticity. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance and mass transfer losses.     

Electrochemical characterization of a polybenzimidazole-based high temperature proton exchange membrane unit cell

This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBI-based HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters, such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC’s are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is an important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has an exponential-like dependency on the air stoichiometry. Based on the present results and results found in recent publications it is still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, the transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally, this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell.     

PEM stack test and analysis in a power system at operational load via ac impedance

This paper presents a method for collecting ac impedance data from a commercial PEFC power system at operational loads. The PEM fuel cell stack in the power system, including 47 MEAs, was operated using room air and pure hydrogen (>99.99%). For a stack test in the power system, the power source for the embedded controller board was simultaneously switched from the fuel cell stack to a similar external power source after the system reached a steady temperature. The PEM fuel cells in the stack were tested by collecting the ac impedance data at different current levels. By using ac impedance, a single fuel cell, a group of fuel cells, and a complete stack were then tested without the embedded control devices for ohmic, activation, and mass transport losses. The ohmic resistance for each cell component in the stack was obtained as 117 mΩ cm² at operational loads from 2.5 A to 35 A. The membrane thickness was further estimated as ca. 51–89 μm. Resistances from ohmic conduction, anode/cathode activation, and mass transport were measured and discussed using the Nyquist plots from the ac impedance spectra.     

Characterization of direct formic acid fuel cells by Impedance Studies: In comparison of direct methanol fuel cells

We characterized direct liquid fuel cells by electrochemical impedance spectroscopy (EIS) combined with reversible hydrogen electrode (RHE) under fuel cell operating conditions. EIS has been successfully implemented as an in-situ diagnostic tool using an impedance setup with RHE, capable of singling out individual contributions to the overall polarization of fuel cells and separating the anode and cathode contributions. While a direct methanol fuel cell (DMFC) anode was subject to substantial poisoning by reaction intermediates due to better accessibility of methanol to catalyst surface regardless of anode diffusion media, a direct formic acid fuel cell (DFAFC) anode suffered from significant mass transfer limitation depending on the anode diffusion media property and formic acid concentration. The high frequency resistance of a DFAFC cathode increased linearly with an increase of formic acid concentration by membrane dehydration effect. Interestingly, on both the DMFC and DFAFC cathodes, decrease in the mixed charge transfer resistance with an increase of fuel crossover was observed together with a drop in the cathode potential.     

Impedance study of membrane dehydration and compression in proton exchange membrane fuel cells

Electrochemical impedance spectroscopy (EIS) is used to measure drying and rehydration in proton exchange membrane fuel cells running under load. The hysteresis between forward and backward acquisition of polarization curves is shown to be largely due to changes in the membrane resistance. Drying tests are carried out with hydrogen and simulated reformate (hydrogen and carbon dioxide), and quasi-periodic drying and rehydration conditions are studied. The membrane hydration state is clearly linked to the high-frequency arc in the impedance spectrum, which increases in size for dry conditions indicating an increase in membrane resistance. Changes in impedance spectra as external compression is applied to the cell assembly show that EIS can separate membrane and interfacial effects, and that changes in membrane resistance dominate. Reasons for the presence of a capacitance in parallel with the membrane resistance are discussed.     

Electrochemical impedance spectroscopy study of methanol oxidation on nanoparticulate PtRu direct methanol fuel cell anodes: Kinetics and performance evaluation

Electrochemical impedance spectroscopy (EIS) along with cyclic voltammetry (CV) has been applied as a tool for the mechanistic investigation of methanol oxidation on nanoparticulate PtRu fuel cell anodes of a commercially available state of the art membrane electrode assembly (MEA). The spectra could be fitted to a circuit derived analytically for multi-step single adsorbed intermediate reactions. The analysis has indicated that methanol adsorption and surface blocking occur below the onset and the surface is ‘poisoned’ to the highest degree just before the onset, implying that the removal of residues before the onset, if any, is slower compared to the formation. The onset potential is marked by a sudden change in the mechanism as the impedance becomes pseudoinductive. It has also been demonstrated that EIS can be applied for analyzing and singling out different contributions behind electrode performance for methanol oxidation reaction under fuel cell operating condition.     

Corrosion resistance of functionally graded TiN/Ti coatings for proton exchange membrane fuel cells

Bipolar Plates (BPP) are important components of proton exchange membrane fuel cell (PEMFC) stacks. In the development of innovative fuel cell designs, it is advantageous to use aluminum for these applications, however, this material lacks the necessary corrosion resistance. Since the performance of PEMFC stacks depends on BPP properties, in particular, corrosion resistance, depositing titanium nitride (TiN) thin films onto aluminum substrates may improve their efficiency and durability. The present work focuses on improving corrosion resistance and hydrophobicity of TiN/Ti by using N graded films deposited onto aluminum substrates (AA-1100) by grid-assisted magnetron sputtering (GAMS). Electrochemical impedance spectroscopy (EIS) and potentiodynamic and potentiostatic polarization are used to investigate the performance of the substrate/film system at room temperature and 70 °C, thus simulating a prototypic PEMFC electrolyte environment. Electrochemical test results showed that graded TiN films improved corrosion resistance when compared with both the homogeneous films and the AA1100 uncoated substrate. Furthermore, contact angle results reveal improved hydrophobicity for both homogeneous and graded TiN coatings when compared with the AA1100 substrate.     

Steady state and dynamic performance of proton exchange membrane fuel cells (PEMFCs) under various operating conditions and load changes

The steady-state performance and transient response for H₂/air polymer electrolyte membrane (PEM) fuel cells are investigated in both single fuel cell and stack configurations under a variety of loading cycles and operating conditions. Detailed experimental parameters are controlled and measured under widely varying operating conditions. In addition to polarization curves, feed gas flow rates, temperatures, pressure drop, and relative humidity are measured. Performance of fuel cells was studied using steady-state polarization curves, transient I–V response and electrochemical impedance spectroscopy (EIS) techniques. Different feed gas humidity, operating temperature, feed gas stoichiometry, air pressure, fuel cell size and gas flow patterns were found to affect both the steady state and dynamic response of the fuel cells. It was found that the humidity of cathode inlet gas had a significant effect on fuel cell performance. The experimental results showed that a decrease in the cathode humidity has a detrimental effect on fuel cell steady state and dynamic performance. Temperature was also found to have a significant effect on the fuel cell performance through its effect on membrane conductivity and water transport in the gas diffusion layer (GDL) and catalyst layer. The polarization curves of the fuel cell at different operating temperatures showed that fuel cell performance was improved with increasing temperature from 65 to 75 °C. The air stoichiometric flow rate also influenced the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane and water flooding in cathode side. The fuel cell steady state and dynamic performance also improved as the operating pressure was increased from 1 to 4 atm. Based on the experimental results, both the steady state and dynamic response of the fuel cells (stack) were analyzed. These experimental data will provide a baseline for validation of fuel cell models.     

Electrical test methods for on-line fuel cell ohmic resistance measurement

The principles and trade-offs of four electrical test methods suitable for on-line measurement of the ohmic resistance (R_Ω) of fuel cells is presented: current interrupt, AC resistance, high frequency resistance (HFR), and electrochemical impedance spectroscopy (EIS). The internal resistance of a proton exchange membrane (PEM) fuel cell determined with the current interrupt, HFR and EIS techniques is compared. The influence of the AC amplitude and frequency of the HFR measurement on the observed ohmic resistance is examined, as is the ohmic resistance extracted from the EIS data by modeling the spectra with a transmission line model for porous electrodes. The ohmic resistance of a H₂/O₂ PEM fuel cell determined via the three methods was within 10–30% of each other. The current interrupt technique consistently produced measured cell resistances that exceeded those of the other two techniques. For the HFR technique, the frequency at which the measurement was conducted influenced the measured resistance (i.e., higher frequency providing smaller R_Ω), whereas the AC amplitude did not effect the observed value. The difference in measured ohmic resistance between these techniques exceeds that reasonably accounted for by measurement error. The source of the discrepancy between current interrupt and impedance-based methods is attributed to the difference in the response of a non-uniformly polarized electrode, such as a porous electrode with non-negligible ohmic resistance, to a large perturbation (current interrupt event) as compared to a small perturbation (impedance measurement).     

AC impedance characteristics of a 2 kW PEM fuel cell stack under different operating conditions and load changes

This paper mainly presents the AC impedance characteristics of a 2 kW PEMFC stack under different operating conditions and load changes. The AC impedances of the fuel cell stack are examined by a fuel cell impedance meter. Air stoichiometry, air humidity, and operation temperature are shown to have significant effects on the AC impedance of stack. When air stoichiometry decreases, the mass transfer resistance of stack increases obviously, but the influences on other resistances are very slight. The air humidity and operation temperature mainly influence the charge transfer resistance of stack. The influences of load changes on the AC impedance of stack are also investigated, and the results of which show that it is quite necessary to adjust the humidity of reactant gas according to the fuel cell load changes during fuel cell running. The AC impedance diagnosis of stack can provide some useful information for the running of fuel cell stack.     

AC impedance technique in PEM fuel cell diagnosis—A review

Because the AC impedance technique, also known as electrochemical impedance spectroscopy (EIS), is being utilized by more and more researchers in proton exchange membrane (PEM) fuel cell studies, the technique has developed into a primary tool in such research. In this paper the recent work on PEM fuel cells using the AC impedance technique is reviewed. Both in situ and ex situ impedance measurements are discussed, with primary focus on the in situ measurements. Within the domain of in situ studies, various methods for measuring the impedance of a PEM fuel cell are examined, and typical impedance spectra in several common scenarios are presented. Representative applications of the AC impedance technique in PEM fuel cell research are also discussed. Finally, the necessity of a time domain rapid AC impedance technique is briefly discussed.