Performance analysis and impedance spectral signatures of high temperature PBI–phosphoric acid gel membrane fuel cells

Polarization curves, i.e., dc performance, and impedance spectral signatures of polybenzimidazole (PBI)–phosphoric acid (H₃PO₄) membrane fuel cells are obtained in the temperature range of 160–180 °C, in an effort to investigate the effect of temperature, anode humidification, various cathode stoichs, and use of oxygen versus air. Thus, in situ electrochemical impedance spectroscopy (EIS) was used to obtain various resistances, ohmic as well as charge-transfer resistances, under these conditions. The results obtained show that PBI–H₃PO₄ gel membrane fuel cells exhibit very good performance in the temperature range of 160–180 °C with an ohmic resistance similar to Nafion. Mass transfer limitations were determined by comparing performance polarization curves with air and oxygen along with EIS. Further EIS was also used to obtain signatures during fuel starvation, and electrical shorting across the cell.     

Estimation of kinetic and electric parameters of liquid feed passive DMFCs

The study of polarization curves is an important part for evaluating the performance of passive direct methanol fuel cells (DMFCs). This study must be essentially followed by other experiments to gain knowledge of important kinetic and electrical parameters such as exchange current density, charge transfer coefficients, ohmic resistance, etc. This experimentation include in-situ characterization techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and current interrupt (CI) technique which are expensive to carry out. Hence, this study develops a semi-analytical model for estimating exchange current density, charge transfer coefficients and ohmic resistance directly from the polarization curves. In addition, a simple “3/4-Point method” is introduced, utilizes the polarization curves for the estimation of kinetic and electric parameters. The triad (proposed model, 3/4-Point method and polarization/EIS experiment) are compared and exhibited similar trends, thereby validating it as a tool for estimation of a large class of parameters. The model is comprehensive as it is valid throughout the polarization curves, thereby avoiding the need for multiple strategies in different regions of the polarization curves to calculate various parameters. The model can serve to know the values of these parameters by-hand before other experimentations which involves high cost and sophisticated experimental conditions.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part II. Impedance of the positive plate revisited

In the second part of this publication series, dedicated to the pulse charge of the lead-acid battery, a special attention is paid to the impedance spectrum of the positive plate as a source for estimation of the electrostatic capacitance of the double layer (C_dl) on the surface of the positive active mass. The impedance spectra were measured at open circuit for different states of charge (SoC) in H₂SO₄ with specific gravity 1.24 and 1.28 g ml⁻¹. A substantial difference was observed in the impedance spectra of partially charged and partially discharged positive plates keeping the same value of the SOC. The impedance data were subjected to inductance error correction, followed by differential impedance analysis (DIA). Considering the results from DIA, the recently published equivalent circuits of the positive plate in charged and in discharged state and the gel-crystal model of the lead dioxide, we proposed a model of the positive plate in partial state of charge (PSoC). The analysis of the obtained experimental results using this model and DIA show that the double layer capacitance is not frequency distributed. The influence of the state of charge and state of health on the model parameters is discussed. One of the most interesting results is the dependence of C_dl on SOC—it features a hysteresis at which the values of C_dl during the charge are 5–6 times higher than the corresponding ones during the discharge. This result was discussed in terms of changes in the double layer structure considering the gel-crystal model of the lead dioxide. During the discharge in H₂SO₄ with specific gravity 1.28 g ml⁻¹ a passivation process was detected as a high frequency pseudo-inductive loop in the Nyquist plots in PSoC. The passivation time constant is higher at 50–60% SOC and decreases to zero in the end of the discharge. During the charge in both electrolytes, pseudo-inductive time constant was observed too. It was attributed to the phenomena of the dehydration of Pb(OH)₄, an intermediate in the reaction scheme of the PbSO₄ oxidation. The state of health influences mostly the ohmic resistance R_Ω, the charge transfer resistance R_ct and the parameters of the constant phase element accounting the diffusion in the pores (CPE_diff), when the plate is well charged.     

High pressure polymer electrolyte water electrolysis: Test bench development and electrochemical analysis

A test bench for a polymer electrolyte water electrolysis (PEWE) cell for high pressure operation of up to 100 bar in differential and balanced pressure mode is described. Important aspects referring to the design, safety and operability of the test bench and the design of a small scale electrolysis cell are described. The electrolyzer cell comprises a special compression mechanism which allows accommodating porous transport layers of different thickness and setting of the compression pressure independent of the clamping pressure. In order to analyze the electrochemical results with respect to the overpotentials, a power source with integrated high frequency resistance (HFR) as well as electrochemical impedance spectroscopy (EIS) measurement capabilities is implemented. The versatility of the test environment is demonstrated by comparing the DC, HFR and EIS data as a function of operating pressure, temperature (up to 70 °C) and current density (up to 4 A/cm²). With respect to pressurized operation of PEWE cells, only the differential pressure mode (hydrogen pressurized) shows the expected isothermal compression behavior, for balanced pressure operation a different characteristic is observed.     

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.     

Characterisation tools development for PEM electrolysers

Electrochemical impedance spectroscopy (EIS), current interrupt (CI) and current mapping (CM) were investigated as in-situ characterisation tools for PEM electrolysers. A 25 cm² cell with titanium anode and carbon cathode plates were utilised in this study. A commercial MEA consisting of 1 mg IrO₂/cm² on the anode and 0.3 mg Pt/cm² on the cathode was used. The electrocatalyst was deposited on Nafion^® membranes. The electrochemical losses in a PEM electrolyser namely: activation, ohmic and mass transfer losses were identified using EIS and CI and both the advantages and disadvantages of the methods were discussed. The current distribution over the membrane electrode assembly (MEA) at different current densities was measured using the current mapping method. It is also shown that under the given experimental conditions the current density decreases along the serpentine flow field.     

AC Impedance of Li(Ni1/3Co1/3Mn1/3)O2/graphite cell as UPS

以Li（Ni_1/3Co_1/3Mn_1/3）O₂/graphite动力电池为研究对象,在模拟备用电源工况下对动力电池进行交流阻抗测试。通过建立等效电路来研究欧姆阻抗R_s、电荷传递阻抗R_ct和扩散阻抗CPE_W随不同搁置时间、荷电状态（state of charge,SOC）的变化规律,研究Li（Ni_1/3Co_1/3Mn_1/3）O₂/graphite动力电池在备用电源工况下,容量和阻抗的变化趋势。结果表明：随着搁置时间的增加,电池容量衰减1.7%左右。随着搁置时间的增加,不同SOC下的欧姆阻抗R_s具有相同的变化趋势,电荷传递阻抗明显增加。随着SOC的降低,由双电层产生的电荷传递阻抗在逐渐增加。在SOC=0%时,扩散阻抗随搁置时间的增加而增加,在SOC=100%、50%的扩散阻抗有细微的增加。容量衰退和阻抗结果显示出Li（Ni_1/3Co_1/3Mn_1/3）O₂/graphite动力电池可以很好地在备用电源工况上使用。     

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.     

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.     

Principle and application of electrochemical impedance spectroscopy method

电化学阻抗测定技术是一种研究电极反应动力学和电化学体系中物质传递与电荷转移的有效方法,通过电化学阻抗数据所提供的信息,能够分析电极过程的特征,包括动力学极化,欧姆极化和浓差极化,为电化学过程设计,电极材料开发和电极结构研究提供基本依据.本文在介绍电化学阻抗谱技术原理的基础上,分别以液流电池,空气扩散电极过程为对象,阐述电化学阻抗谱中电荷转移,物质传递等过程以及多孔电极本身的电荷传递电阻等,并综述阻抗技术在设计电池结构,优化电极材料等方面的应用实例.     

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.     

Development of a method to estimate the lifespan of proton exchange membrane fuel cell using electrochemical impedance spectroscopy

This paper proposes a new method for estimating the state and lifespan of fuel cells in operation by fuel cell equivalent impedance modeling by electrochemical impedance spectroscopy (EIS) and observing degradation. The performance change of fuel cells takes place in the form of changes in each parameter value comprising an equivalent AC impedance circuit; monitoring such changes allows for the prediction of the state and lifespan of a fuel cell. In the experiments, the AC impedance of high-temperature proton exchange membrane (PEM) fuel cells was measured at constant time intervals during their continuous operation for over 2200 h. The expression for the lifespan of a fuel cell was deduced by curve fitting the changes in each parameter to a polynomial. Electric double layer capacitance and charge transfer resistance, which show the reduction reaction of the cathode, were used as major parameters for judging the degradation; a method of using time constants is proposed to more accurately estimate the degree of degradation. In addition, an algorithm that can evaluate the soundness and lifespan of a fuel cell is proposed; it compares the measured time constant of the fuel cell being tested with that of average lifespan fuel cell.     

Coupling RTD and EIS modelling to characterize operating non-uniformities on PEM cathodes

Large PEM cells will be used in future proton exchange membrane fuel cell (PEMFC) power plants and appropriate tools are therefore be needed to study their behaviour. One approach to understanding single cell behaviour involves using mathematical models. The numerous techniques used in this work to describe PEM electrode behaviour require different scientific disciplines: chemical engineering and electrochemistry. This study proposes combining residence time distribution (RTD) and electrochemical impedance spectroscopy (EIS). The investigation focuses on cathodic DC and AC responses where over-voltage is critical. Results demonstrate that although gas distribution does not cause additional loops on impedance diagrams, it is strongly related to both the shape and amplitude of these diagrams. The simulations have drawn attention to operating conditions that can threaten the life of the PEM cell: under these setting points EIS method is not sufficient to detect this risk.     

Application of electrochemical impedance spectroscopy in bio-fuel cell characterization: A review

Fuel cell is an efficient energy conversion device converting chemical energy directly into electrical energy. It is a fact that, to boost the fuel cell performance, resistance (charge transfer resistance, mass transfer resistance and electrolyte resistance) should be decreased. For this, many techniques have been used for cell testing such as: cyclic voltammetry, current interruption measurement, chronoamperometry, chronopotentiometry, polarization curve and electrochemical impedance spectroscopy (EIS). Among these techniques, EIS is a well-established, non-intrusive, non-destructive, semi-quantitative, and an efficient technique for identification of each circuit element. In this review article, application of electrochemical impedance spectroscopy in identification of individual components of total resistance and their dependence on different factors in biofuel cell along with some recent advancement in this technique have been discussed.     

集胞藻PCC-6803催化的光合微生物燃料电池性能研究

以集胞藻PCC-6803(Synechocystis PCC-6803)为阳极催化剂搭建直接利用太阳能的双室H-型光合微生物燃料电池(PMFC),通过极化曲线法、交流阻抗法、循环伏安法等电化学方法,开展电极面积比、质子交换膜、内阻等因素对光合微生物燃料电池产电的影响研究。试验结果显示:在PMFC运转过程中,其输出功率稳定,且达到的最大功率密度为72.3 mW/m2;阴阳极面积大小对PMFC产电性能没有显著影响,说明双室光合微生物燃料电池中,质子交换膜传递质子的速率较慢,限制了PMFC发电效能的提高。PMFC启动后,随着生物膜的增长,其欧姆内阻、极化内阻、总内阻都呈现下降的趋势,且欧姆内阻下降的速率小于极化内阻,从而使欧姆内阻占总内阻的比率变大,进一步说明质子交换膜传递质子的速率是限制PMFC发电的关键因素。     

Anion behaviour in a polyacetylene cathode for a secondary lithium battery

The impedance of polyacetylene has been measured in electrolyte solutions of LiClO₄, LiPF₆ or LiBF₄ in propylene carbonate (PC). In the high frequency range, an impedance corresponding to a charge-transfer reaction has been observed in each electrolyte. The charge-transfer resistance has been estimated from the impedance measurement and has the lowest value in PC-LiClO₄.     

Two dimensional modeling study of PBI/H3PO4 high temperature PEMFCs based on electrochemical methods

A two dimensional model was constructed for a high temperature PBI/H₃PO₄ fuel cell, the model was focused on the cathode side since it is the most important component of the overall polarization loss in unit cell. Linear sweep voltammetry (LSV) technique was used to supply the model with the most important kinetic parameter—cathodic exchange current density. The cathode polarization was calculated by the resulting two dimensional model and the ohmic polarization was estimated by electrochemical impedance spectroscopy (EIS), the overall polarization curve was simulated according to the above procedure; three unit cells with different catalysts were assembled and their steady state polarization curves were simulated based on the mathematical model. Good agreement between the simulated and experimental results was achieved.     

Effect of operating conditions on current density distribution and high frequency resistance in a segmented PEM fuel cell

Understanding losses in polymer electrolyte membrane fuel cells, in the form of ohmic and mass transport, is of great importance to their commercialization. In this study, we use a spatially resolved cell consisting of 49 segments to measure the local current density distribution and high frequency resistance (HFR). A parametric study is used to investigate the effects of cell voltage, inlet relative humidity and flow rate and configuration using a three-channel serpentine flow field. We found that as the cell voltage decreased, the current density increased, while the HFR decreased. However, at a low cell voltage of 200 mV, we found the HFR to be higher than that at 500 mV. This increase is attributed to the increased electro-osmotic drag. This trend is independent of the flow configuration. Further, we found that the effect of the inlet relative humidity on the HFR highly depends on the flow configurations. Finally, a sharp decrease in the current density at some specific bend segments was observed, which correlates with lower OCV values and higher HFR values at this position.     

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.     

Electrochemical parameter identification—An efficient method for fuel cell impedance characterisation

Electrochemical parameter identification (EPI) is a novel, application-oriented characterisation method for fuel cell impedances. EPI strictly works in the time domain, with a model of the fuel cell impedance and measurements of the excitation and the response in the time domain. This approach reduces the measurement time considerably in comparison to frequency domain measurements for electrochemical impedance spectroscopy. The use of a superimposed signal as system excitation leads to less interference of the fuel cell operation than a current interrupt. Short measurement time and little interference enable an online application of the method during the operation of the fuel cell. A simple discrete-time model describing the dynamic electrical behaviour of the fuel cell is depicted as an equivalent circuit which consists of a voltage source and the impedance as internal resistance. The model parameters are identified by a hybrid optimisation algorithm using the sampled excitation and response signals. A comparison of measured impedance spectra at various operation conditions with impedance models identified by EPI shows very good agreement over a wide frequency range and emphasizes the reliability of EPI.