Catalyst degradation diagnostics of proton exchange membrane fuel cells using electrochemical impedance spectroscopy

A previously validated equivalent circuit model, in which two resonant circuits were inserted to represent the processes in the catalyst layers, is applied to fit the electrochemical impedance spectroscopy results of a single proton exchange membrane fuel cell exposed to accelerated stress test targeting catalyst degradation. The simulation results of the applied equivalent circuit model show very good agreement with the experimental data. The applied model is able to extract contributions of each of the model elements to the cell degradation. The obtained results indicate that the cathode catalyst layer resonant loop parameters, together with the cathode charge transfer resistance and cathode double-layer capacitance, change the most during the accelerated stress test. If each of the elements of the cathode resonant loop can be associated with physical processes inside the catalyst layer, the model may be used to give more insight into the degradation effects on functioning of the catalyst layer. From the conducted electrochemical impedance spectroscopy analysis, it seems that the low-frequency intercept in Nyquist plot shows the most significant change with degradation, so it may be used directly as a sufficient indicator of fuel cell performance degradation due to catalyst layer degradation.     

基于电化学阻抗法的质子交换膜燃料电池“三步活化法”机理研究

活化能够有效地发挥质子交换膜燃料电池膜电极的性能,"三步活化法"是其中一种比较理想的方法。为了研究"三步活化法"活化质子交换膜燃料电池的机理,利用电化学阻抗谱测试"三步活化法"过程中的膜电极阻抗,并建立等效电路模型拟合所得实验数据。结果表明,"三步活化法"可以有效降低欧姆阻抗、阳极法拉第阻抗、阴极法拉第阻抗以及阴极传质阻抗,这表明"三步活化法"有利于电子、质子、气体与水的传输通道的形成。     

Analysis of the electrochemical behaviour of polymer electrolyte fuel cells using simple impedance models

The analysis of the electrochemical behaviour of polymer electrolyte fuel cells (PEFC) both in time and frequency domain requires appropriate impedance models. Simple impedance models with lumped parameters as resistances and capacitances or Warburg impedances do have limitations: often the validity is limited to a certain frequency range, effects at very low or very high frequencies can not be described properly.     

On the electrochemical impedance spectroscopy of direct methanol fuel cell

The direct methanol fuel cell (DMFC) was operated under a variety of current densities to monitor the electrochemical impedance spectroscopy (EIS) for understanding its reaction mechanism. Based on the EIS analysis, the impedance of the cell reaction is divided into three components, two of them are current dependent and the remainder is current independent. Through detailed exploration of the impedance components, the high-frequency impedance was attributed to interfacial behavior, the medium-frequency impedance to electrochemical reactions, and the low-frequency impedance to the adsorption/relaxation of CO. Based on EIS analysis, a qualitative model is proposed to delineate the reaction mechanisms of DMFC, which is confirmed quantitatively by one set of equivalent circuit elements. The experimental data are satisfactorily consistent with the results simulated from the proposed model.     

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.     

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.     

Dynamic modeling of proton exchange membrane fuel cell using non-integer derivatives

The modeling of proton exchange membrane fuel cells (PEMFC) may work as a powerful tool in the development and widespread testing of alternative energy sources in the next decade. In order to obtain a suitable PEMFC model, which can be used in the analysis of fuel cell-based power generation systems, it is necessary to define the values of a specific group of modeling parameters. In this paper, the authors propose a dynamic model of PEMFC, the originality of which lays on the use of non-integer derivatives to model diffusion phenomena. This model has the advantage of having least number of parameters while being valid on a wide frequency range and allows simulating an accurate dynamic response of the PEMFC.

In this model, the fuel cell is represented by an equivalent circuit, whose components are identified with the experimental technique of electrochemical impedance spectroscopy (EIS). This identification process is applied to a commercially available air-breathing PEMFC and its relevance is validated by comparing model simulations and laboratory experiments. Finally, the dynamic response derived from this fractional model is studied and validated experimentally.     

Using electrochemical impedance to determine airflow rates

This paper proposes the use of electrochemical impedance spectroscopy (EIS) to estimate the cathode flow rate in a fuel cell system. Through experimental testing of an eight-cell, hydrogen-fueled polymer electrolyte stack, it shows that the ac impedance measurements are highly sensitive to the airflow rates at varying current densities. The ac impedance magnitude at 0.1 Hz allows the distinction of airflow rates (stoichiometry of 1.5–3.0) at current densities as low as 0.1 A/cm². Using experimental data and regression analysis, a simple algebraic equation that estimates the airflow rate using impedance measurements at a frequency of 0.1 Hz is developed. The derivation of this equation is based on the operating cell voltage equation that accounts for all the irreversibilities.     

Analytic parameter identification of proton exchange membrane fuel cell catalyst layer using electrochemical impedance spectroscopy

A finite transmission line is proposed for proton exchange membrane fuel cell reaction layer, when the faradic current is absent due to purging of Inert gas at the back of cathode and anode. Also a finite transmission line is presented when a charge transfer accrued among catalyst and electrolyte interface. The electrochemical impedances of finite transmission lines are computed using MATLAB software. Relative to the orders and types of the evaluated impedances, some relations to determine and identify the parameters of the proposed models are derived. In first model, it is shown that the electrical elements of transmission line can be extracted explicitly from the Nyquist and Bode diagrams whereas for the second one, some of the parameters cannot directly be investigated. However, using a numerical procedure, some valuable results about parameter variations are obtained.     

Study of crossover and depletion effects in laminar flow-based fuel cells using electrochemical impedance spectroscopy

The impedance characteristics of the laminar flow-based fuel cells (LFFCs), which are also called microfluidic fuel cells (MFFCs), are measured to study the crossover and depletion phenomena. To study the effect of the crossover and depletion separately, the impedances are measured at different flow rates simulating different crossover conditions; while the effect of depletion is controlled by keeping the current density constant through changing the fuel concentration at different flow rates. Then the depletion effect is studied by measuring the impedance characteristics of the cell at various fuel concentrations. It is shown that the crossover affects the moderate frequency ranges in the measured Nyquist plot; whereas depletion impacts the low frequency domain. Finally, the impedance measurements at various potentials suggest that increasing the current density increases the effect of crossover while decreases the effect of depletion. This could be the result of carbon dioxide production in the anode.     

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.     

Impedance characterization of high temperature proton exchange membrane fuel cell stack under the influence of carbon monoxide and methanol vapor

This work presents a comprehensive mapping of electrochemical impedance measurements under the influence of CO and methanol vapor contamination of the anode gas in a high temperature proton exchange membrane fuel cell, at varying load current. Electrical equivalent circuit model parameters based on experimental evaluation of electrochemical impedance spectroscopy measurements were used to quantify the changes caused by different contamination levels. The changes are generally in good agreement with what is found in the literature. It is shown that an increased level of CO contamination resulted in an increase in the high frequency and intermediate frequency impedances. When adding CO and methanol to the anode gas, the low frequency part of the impedance spectrum is especially affected at high load currents, which is clearly seen as a result of the high load current resolution used in this work. The negative effects of methanol vapor are found to be more pronounced on the series resistance. When CO and methanol vapor are both present in anode gas, the entire frequency spectrum and thereby all the equivalent circuit model parameters are affected. It is also shown that the trends of contamination effects are similar for all the test cases, namely, CO alone, methanol alone and a mix of the two, suggesting that effects of methanol may include oxidation into CO on the catalyst layer.     

Development and performance evaluation of a high temperature proton exchange membrane fuel cell with stamped 304 stainless steel bipolar plates

In this work, a high temperature proton exchange membrane fuel cell (HT-PEMFC) with stamped SS304 bipolar plates is successfully developed. Its performance was evaluated under two types of gaskets at different assembly torques and air stoichiometric ratios. The rates of pressure loss at a torque of 7 N-m with 50 Shore A hardness gaskets was 2.0 × 10^?3 MPa min^?1, which is acceptable. The best performance of the developed HT-PEMFC with stamped SS304 bipolar plates was 228.33 mW cm^?2, which approaches the performance of HT-PEMFCs with graphite bipolar plates. The optimal air stoichiometric ratio for the HT-PEMFC with stamped SS304 bipolar plates was 4.0, which is higher than that for proton exchange membrane fuel cells with CNC milled graphite bipolar plates. This is probably because of the deformation of the flow channels under the assembly compression force, which causes an elevated gas-diffusion drag in the flow channels. After the test, it was observed that some products of corrosion reaction formed on the surface of the SS304 bipolar plate. This phenomenon may lead to a decrease in the operating life of the HT-PEMFC.     

Study of temperature, air dew point temperature and reactant flow effects on proton exchange membrane fuel cell performances using electrochemical spectroscopy and voltammetry techniques

A single PEMFC has been operated by varying the assembly temperature, the air dew point temperature and the anode/cathode stoichiometry rates with the aim to identify the parameters and combinations of factors affecting the cell performance. Some of the experiments were conducted with low humidified reactants (relative humidity of 12%). The FC characterizations tests have been conducted using in situ electrochemical methods based on load current and cell voltage signal analysis, namely: polarization curves, EIS measurements, cyclic and linear sweep voltammetries (CV and LSV). The impacts of the parameters on the global FC performances were observed using the polarization curves whereas EIS, CV and LSV test results were used to discriminate the different voltage loss sources. The test results suggest that some parameter sets allow maximal output voltages but can also induce material degradation. For instance, higher FC temperature and air flow values can induce significant electrical efficiency benefits, notably by increasing the reversible potential and the reaction kinetics. However, raising the cell temperature can also gradually dry the FC and increase the risk of membrane failure. LSV has also shown that elevated FC temperature and relative humidity can also accelerate the electrolyte degradation (i.e. slightly higher fuel crossover rate) and reduce the lifetime consequently.     

Electrochemical impedance investigation of proton exchange membrane fuel cells experienced subzero temperature

Polarization losses of the fuel cells with different residual water amount frozen at subzero temperature were investigated by electrochemical impedance spectroscopy (EIS) taking into account the ohmic resistance, charge transfer process, and oxygen mass transport. The potential-dependent impedance before and after eight freeze/thaw cycles suggested that the ohmic resistance did not change, while the change of the charge transfer resistance greatly depended on the residual water amount. Among the four cells, the mass transport resistance of the cell with the largest water amount increased significantly even at the small current density region. According to the thin film-flooded agglomerate model, the interfacial charge transfer process and oxygen mass transport within the agglomerate and through the ionomer thin film in the catalyst layer both contributed to the high frequency impedance arc. From the analysis of the Tafel slopes, the mechanism of the oxygen reduction reaction (ORR) was the same after the cells experienced subzero temperature. The agglomerate diffusion changed a little in all cells and the thin film diffusion effect was obvious for the cell with the largest residual water amount. These results indicated that the slower oxygen diffusion within the catalyst layer (CL) was the main contributor for the evident performance loss after eight freeze/thaw cycles.     

Investigation of the effect of cathode stoichiometry of proton exchange membrane fuel cell using localized electrochemical impedance spectroscopy based on print circuit board

Proton Exchange Membrane Fuel Cell can have a large active area, and the working condition in different areas can be entirely different. Localized electrochemical impedance spectroscopy can directly observe the proton exchange membrane fuel cell internal reaction conditions. In this work, localized electrochemical impedance spectroscopy test system based on print circuit board is implemented in a 50 cm² multi-channel serpentine flow fields. The localized electrochemical impedance spectroscopy performances of different segments with different cathode stoichiometry (1.8, 2.3 and 2.8) at different current density (100  mA cm⁻², 500  mA cm⁻² and 900 mA cm⁻²) are studied. The result demonstrates that the fuel cell may suffer from local drying and flooding at the same time. To make full use of the potential of a fuel cell, a suitable cathode stoichiometry should be identified to control the drying of the inlet and the flooding of the outlet at the same time. It is shown that a cathode stoichiometry of 2.3 is close to the optimum cathode stoichiometry to keep the fuel cell in good consistency without gas waste. Besides, a current density distribution measurement is performed to verify the conclusions of this work.     

Investigation of solar cell degradation using electrochemical impedance spectroscopy

The performance of solar cells reduces annually due to various unavoidable phenomena of thermal cycling, damp heat, UV exposure, and mechanical stress, etc. Generally, I-V characteristic is used to check the performance of the solar cell, but the minor stress conditions mentioned above are difficult to characterize by I-V measurement. Impedance spectroscopy is a widely used method in fuel cell and a battery that can be used to detect minor degradation in the solar cell by analyzing the change in equivalent circuit parameters. In this work, commercially available polycrystalline silicon solar cell is investigated under the condition of hotspot, mechanical stress, and disconnection of interconnection ribbon and then characterized by impedance measurement, Fourier transform (Bode plot) as well as I-V characteristic. The results show noteworthy decrease in parallel resistance (Rp) which is clearly visible in Nyquist plot in compare to the I-V characteristic. The Rp decreases in EIS from 283.60 to 234.80 Ω for mechanical stress test, from 273.0 to 187.10 Ω for hotspot and from 352.80 to 345.20 Ω for disconnection of interconnection ribbon test. The results confirm potential application of impedance measurement for solar cell characterization due to noteworthy change in equivalent circuit parameters after test conditions.     

Evaluation of polymer electrolyte membrane fuel cells by electrochemical impedance spectroscopy under different operation conditions and corrosion

Electrochemical impedance spectroscopy (EIS) was employed for in situ diagnosis for polymer electrolyte membrane fuel cells during operation. First, EIS was measured as a function of operation parameters such as applied current density, gas flow rates and gas humidification temperature. The resistance that correlated with conductivity of the membrane and the contact resistance between bipolar plate and gas diffusion layer (GDL) was set as R_m in the assumed equivalent circuit. The charge transfer resistances were considered for cathode (R_ct(C)). The value of R_ct(C) was sensitive to the parameters that affected cell voltage. Additionally, the diffusion resistance (R_d) was ascribed to the effect of oxygen supply and drainage of generated water. Second, the influence of corrosion of type 430 stainless steel bipolar plates was evaluated by EIS method during operation. Corrosion of the stainless steel bipolar plates resulted in an increase in the value of R_d.