Electrochemical Impedance Spectra Data Research Articles

Validation of and Simulations using a High-Temperature Electrolysis Model

Efforts for increasing the share of renewable power generation are dramatically boosted since electrical power from fossil fuels has been used as weapon [1]. Therefore, efficient conversion systems are of great importance to conserve most of the delivered power by nature. High-temperature electrolysis is such an efficient type of energy conversion system compared to similar technologies [2-4]. To bridge the gap to an industrial scale application, the electrochemical and chemical processes have to be understood in detail which is enlightened by some publications from our group [5-8]. In this contribution, a multiphysics model for three types of electrolysis processes (steam, CO2 and co-electrolysis) is presented and evaluated. Relevant parameters like the exchange current density or the anodic / cathodic transfer coefficient including parameter variations for the inlet gas stream, flow rate of the gas, temperature and applied current are investigated. For the microscale properties in the functional layer, Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is used with the “Slice and View” technique [9,10]. Extracting information like the Triple Phase Boundary Length (TPBL), the fraction of electronic and ionic phase in mixed ionic-electronic conducting (MIEC) materials or the porosity and tortuosity of the (La,Sr)CoO3-δ (LSC) air electrode and the Ni / 8 mol% yttria stabilized zirconia (Ni-8YSZ) fuel electrode help in enhancing the modeling quality of high-temperature SOC model. A conversion from the TPBL (s. Figure 1) to an active area to volume ratio for the computation is needed.Calculated iV-characteristics and experimental curves are compared and serve as a validation for (I,V) data points at higher currents. Depending on the degree of agreement, the (missing) main contributions to loss mechanisms can be identified and initiate a feedback loop for material optimization. Sensitivity analyses are carried out for parameters that are difficult to vary in experiments (e.g. porosity). Porosity and tortuosity parametrize the porous structure for a homogeneously distributed property assumption across the material layers. The ratio between electronic and ionic phase fine-tunes the positioning of iV-characteristics within the potential regime of Ohmic and polarization losses. Calculated electrochemical impedance spectra (EIS) enhance the insight into the theoretical system and may be compared to experimental spectroscopy data via Equivalent Circuit Models (ECM). Calculated data from the model are acquired as in the used experimental setup. As an example, calculated EIS data from steam electrolysis show good agreement with a R-(RQ)4 ECM which as well describes most of the corresponding experimental data in our group. Effects of degradation are envisioned to be incorporated into the model as well.[1] Hosp, G.; Höltschi, R.; Keusch, N.; Schürpf, T.: https://www.nzz.ch/wirtschaft/rohstoffe-als-waffen-die-neusten-entwicklungen-ld.1681268 (28th October 2022, 2.13 pm)[2] Foit, S.R.; Dittrich, L.; Duyster, T.; Vinke, I.; Eichel, R.-A.; de Haart, L.G.J. (2020): Direct Solid Oxide Electrolysis of Carbon Dioxide: Analysis of Performance and Processes. In: Processes 8 (11), 1390[3] Foit, S.R.; Vinke, I.C.; de Haart, L.G.J.; Eichel, R.-A. (2017): Power-to-Syngas: An Enabling Technology for the Transition of the Energy System? In: Angew. Chem. Int. Edit. 56 (20), 5402–5411[4] Tanaka, Y.; Hoerlein, M.P.; Schiller, G. (2016): Numerical simulation of steam electrolysis with a solid oxide cell for proper evaluation of cell performances. In: Int. J. Hydrogen Energ. 41, 752–763[5] Wolf, S.E.; Dittrich, L.; Nohl, M.; Duyster, T.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Boundary Investigation of High-Temperature Co-Electrolysis Towards Direct CO2 Electrolysis. In: J. Electrochem. Soc. 169, 034531[6] Unachukwu, I.D.; Vibhu, V.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Sr Substituted La2-xSrxNi0.8Co0.2O4+ d (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties.In: Energies 15, 2136[7] Mebrahtu, C.+; Nohl, M.+; Dittrich, L.; Foit, S.R.; de Haart, L.G.J.; Eichel, R.-A.; Palkovits, R. (2021): Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO2 and H2O. In: ChemSusChem 14, 2295 – 2302[8] Dittrich, L.; Nohl, M.; Jaekel, E.E.; Foit, S.R.; de Haart, L.G.J. (Bert), Eichel, R.-A. (2019): High-Temperature Co-Electrolysis: A Versatile Method to Sustainably Produce Tailored Syngas Compositions. In: J. Electrochem. Soc. 166 (13), F971 – F975[9] Wilson, J.R.; Kobsiriphat, W.; Mendoza, R.; Chen, H.; Hiller, J.M.; Miller, D.J.; Thornton, K.; Voorhees, P.W.; Adler, S.B.; Barnett, S.A. (2006): Three-dimensional reconstruction of a solid-oxide fuel-cell anode. In: Nat. Mater. 5, 541–544[10] Gostovic, D.; Smith, J.R.; Kundinger, D.P.; Jones, K.S.; Wachsman, E.D. (2007): Three-Dimensional Reconstruction of Porous LSCF Cathodes. In: Electrochem. Solid St. 10 (12), B214–B217Figure 1: Histogram of the TPBL from a 400 image sample set of a FIB-SEM cut of a Ni-8YSZ electrode. Figure 1

Use of reduced-voltage EIS to establish a relation between oxygen concentration and EIS responses of large commercial PEM fuel cell modules

This paper employs equivalent circuit analysis of electrochemical impedance spectra (EIS) signatures of commercial proton exchange membrane fuel cell stacks (PEMFC) to establish a relation between oxygen concentration and EIS response within individual cells of the stack. This paper is part of an ongoing effort to use EIS for detecting and quantifying hydrogen crossover to understand and reduce hydrogen cathode exhaust emissions. Selected results from different sets of experiments performed on Ballard water-cooled and air-cooled PEMFC stacks are presented. A novel reduced-voltage EIS system with the ability to perform impedance measurements over different cell counts is employed to record EIS scans of these stacks. In the first set of experiments, hydrogen was injected into the cathode side of a 30 kW 150+-cell water-cooled module at various rates to emulate the phenomenon of hydrogen crossover at different levels. In the second set of experiments, nitrogen was injected into a target single-cell in an air-cooled stack with over 50 cells (50+-cell) to dilute the cathode available oxygen, and the impedance was measured over various numbers of cells spanning around this target cell. A novel equivalent circuit model with resistors, constant phase elements (CPEs), and a linear finite Warburg parameter was used to study the EIS data via an optimization-based fitting approach in python. Results of these experiments have shown there is a proportional relationship between the Warburg parameter (in the equivalent circuit model used) and hydrogen crossover (at nominal idle and low-current air flows), and that this Warburg parameter increases significantly with reduced cathode air flows. Also, it is concluded that this relationship can be further used, along with decomposition of the EIS scan of a stack into different cell-level scans, to estimate the number of large-leaky cells as well as the total hydrogen crossover rate.

Experimental and Numerical Investigation of the Low-Frequency Inductive Features in Differential PEMFCs: Ionomer Humidification and Platinum Oxide Effects

The low-frequency inductive features in PEMFC are studied by differential measurements and numerical simulation. Systematic parameter variations are conducted and the discrepancies between the local polarization curve slopes and the capacitive loops of electrochemical impedance spectra (EIS) are evaluated to compute the inductive contributions. These contributions, primarily slow platinum oxide kinetics and ionomer humidification, are disentangled and we show that the latter is more relevant at medium to high currents, leaving mainly kinetics contributions of around 35 mV dec−1 at small currents. We demonstrate that the inductivity reaches over 150 mV dec−1 at high load and that it strongly depends on the current density (j) and on the relative gas humidity (RH), whereas temperature (T) and oxygen partial pressure () play a minor role. A new approach for modeling the combination of the oxygen reduction reaction and platinum oxidation leading to inductive loops is presented and integrated into a 1D through-plane model which we parameterize based on our large dataset. We present a comprehensive parameter study with this model. Its current version contains platinum oxide kinetics and electron, proton as well as oxygen transport and yields a good match with both steady-state and EIS data.

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1

Electrochemical Pressure Impedance Spectroscopy for Polymer Electrolyte Membrane Fuel Cells: A Combined Modeling and Experimental Analysis

Electrochemical pressure impedance spectroscopy (EPIS) has recently been developed as a potential diagnosis tool for polymer electrolyte membrane fuel cells (PEMFC). It is based on analyzing the frequency response of the cell voltage with respect to an excitation of the gas-phase pressure. We present here a combined modeling and experimental study of EPIS. A pseudo-two-dimensional PEMFC model was parameterized to a 100 cm2 laboratory cell installed in its test bench, and used to reproduce steady-state cell polarization and electrochemical impedance spectra (EIS). Pressure impedance spectra were obtained both in experiment and simulation by applying a harmonic pressure excitation at the cathode outlet. The model shows good agreement with experimental data for current densities 0.4 A cm−2. Here it allows a further simulative analysis of observed EPIS features, including the magnitude and shape of spectra. Key findings include a strong influence of the humidifier gas volume on EPIS and a substantial increase in oxygen partial pressure oscillations towards the channel outlet at the resonance frequency. At current densities 0.8 A cm−2 the experimental EIS and EPIS data cannot be fully reproduced. This deviation might be associated with the formation and transport of liquid water, which is not included in the model.

High‐Rate and Long‐Life Au Nanorods/LiFePO4 Composite Cathode for Lithium‐Ion Batteries

LiFePO4 is considered a promising cathode material for lithium‐ion batteries for its high theoretical specific capacity and cycling stability. However, low electrical conductivity and Li‐ion diffusion coefficient limit its electrochemical performances. Herein, gold nanorods (Au NRs) with a length of about 40 nm (aspect ratio of 7:1) are successfully prepared by a seed‐mediated growth method. LiFePO4 composite cathodes with 0.1/0.5/1 wt% Au NRs are compared with a pristine LiFePO4 cathode, which shows that the addition of Au NRs can improve the electron and lithium‐ion transport. Specifically, the addition of 1 wt% Au NRs increases the electrical conductivity and lithium‐ion diffusion coefficient to 1.99 S cm−1 and 1.32 × 10−14 cm2 s−1, respectively. The distribution of relaxation times analysis is carried out for the electrochemical impedance spectrum data to distinguish different electrochemical reactions within corresponding frequency regions. Particularly, LiFePO4 with 0.1 wt% Au NRs exhibits a superior rate capability (133.7 mAh g−1 at 2C and 91.9 mAh g−1 at 5C) and stable cycling performance.

Analysis of the DRT as Evaluation Tool for EIS Data Analysis

The Distribution of Relaxation Times (DRT) is an important analysis tool that is capable of giving initial information from Electrochemical Impedance Spectra (EIS) with respect to the number of relaxation processes occurring in the system and their corresponding relaxation frequencies. From this the possible number of circuit elements for the equivalent circuit model in the nonlinear least square fit of the data can be selected. There are several techniques and possibilities to perform the transformation [1–3], nevertheless every possibility has its limitation and/or will remain an ill-posed problem [4]. For our EIS data, the DRT transformation with the Tikhonov regularization is used for further analysis. Hence, we want to understand and more precisely describe the effects of this transformation together with occurring pitfalls on the most commonly implemented circuit elements used to describe EIS data for characterizing Solid Oxide Fuel or Electrolysis Cells (SOFC/SOEC) operating at high temperatures. Therefore, a database with the future possibility of pattern recognition is created for DRT transformations on selected circuit elements like a resistor in series with a finite length diffusion or Warburg-short element (R-Ws) or a resistor in parallel with a constant phase element (RQ). A parameter interval is selected for each occurring element. The origin for these data comes from a series of experiments where quite well fitting Equivalent Circuit Models (ECM) were analyzed with respect to their minimum and maximum values per element. Depending on the coverage of the interval, useful intermediate numbers are selected in order to perform parameter sweeps for every possible combination together with a downstream DRT transformation step. We obtain the behavior of the DRT transformation as a function of the individual circuit element parameters and can evaluate which occurring peaks belong to a circuit element and which ones are artefacts or numerical issues in the transformation. Additionally, the regularization parameter (λ) is taken as a sweep parameter as well to see its influence on the transformation. From both perspectives, we can state a range of optimal numbers of recorded data points per decade of frequency for experiments. We also can give a direction for selecting λ depending on what exactly is to be evaluated with the respective DRT transformation. Additionally, we are able to categorize peak-forming behavior of the transformation since the selected ECMs are known and so, in theory, its transformation result. Knowledge for the DRT behavior is also useful for modeling the electrolysis process where theoretical characteristics (i-V, EIS data) are calculated in order to validate and evaluate the model. Figure caption: Figure 1: The DRT transformation of the model RQ-RQ is converted into the frequency domain with a variation of αP1 from 0.5 to 1.0. The fixed parameters are q1 = 10-1 S, r1 = 0.303 Ω, q2 = 10-2 S, r2 = 0.303 Ω, αP2 = 0.8, λ = 10-7.

Analysis of the DRT as Evaluation Tool for EIS Data Analysis

The Distribution of Relaxation Times (DRT) is an important analytical tool that is capable of giving initial information from Electrochemical Impedance Spectra (EIS) with respect to the number of relaxation processes occurring in the system and their corresponding relaxation frequencies. The DRT transformation with the Tikhonov regularization is used for analysis of EIS data obtained from the characterization Solid Oxide Fuel and Electrolysis Cells (SOFC/SOEC) operating at high temperatures. The effects of this transformation together with occurring pitfalls on the most commonly implemented circuit elements used to describe EIS data was investigated to gain a better understanding. The behavior of the DRT transformation as a function of the individual circuit elements is reported, the regularization parameter λ is taken as a sweep parameter to investigate its influence and optimal ranges for the selection of λ are presented.

A pressurized ammonia-fed planar anode-supported solid oxide fuel cell at 1–5 atm and 750–850°C and its loaded short stability test

Ammonia is a useful energy carrier for solid oxide fuel cell (SOFC) with advantages over hydrogen. Understanding of the performance and stability of ammonia-fed SOFC operated at elevated pressure (p) is an important step towards the development of high-efficiency hybrid SOFC power system with micro gas turbine (MGT). This paper reports cell performance, electrochemical impedance spectra (EIS), and stability measurements of a pressurized ammonia-fed anode-supported SOFC at p = 1–5 atm and T = 750–850°C using a planar (50 × 50 mm2) single-full-cell (400 μm Ni-YSZ anode/3 μm YSZ electrolyte/12 μm LSC-GDC cathode). The full cell together with metallic frames and current collectors are sandwiched by a pair of rib-channel flow distributors (interconnectors) in a high-pressure testing facility. Results show that pressurization and increasing temperature enhance the ammonia-fed SOFC performance significantly having almost the same power densities as those of hydrogen/nitrogen-fed SOFC, as substantiated and explained by EIS data and an equivalence circuit model where the effects of p and T on ohmic, gas diffusion, and gas conversion impedances are shown. Moreover, the loaded short (10 h) stability tests at 700°C and 0.8 V for 1 atm/3 atm cases reveal no/little power degradation, where the microstructures without any crack within the scale of SEM observation and nearly the same element atomic percentages of Ni-YSZ anode surfaces from EDX spectra are found. These results suggest that the pressurized ammonia-fed SOFC is a promising candidate for the hybrid SOFC-MGT power generation.

Identification of Battery State of Health By Combining Machine Learning and Impedance Spectroscopy

The possibility of diagnosing Li-ion battery’s state of health (SoH) and remaining useful life from electrochemical impedance spectra (EIS) using Machine Learning is investigated. The Gaussian Process (GP) with automatic relevance determination is used to identify degradation patterns without relying on any equivalent circuit models. The experiment consists of continuous charge-discharge cycling of commercial Li-ion coin cells. With the GP model trained with the entire spectrum measured on identical cells cycled under the same condition, the cycle number of a testing cell can be predicted by providing the corresponding EIS data. The trained GP model selected one frequency that is most indicative for the battery degradation, indicating the potential that the battery SoH can be diagnosed by measuring impedance at certain frequencies instead of taking a full-range EIS. Moreover, the GP model trained with all sets of data is able to identify the cycle number without knowing temperature or SoC, showing the possibility of developing a universal diagnosis methodology for battery management systems. [1] R. R. Richardson, M. A. Osborne, D. A. Howey, J. Power Sources , Vol. 357, 31 Jul. 2017, pp209-219 [2] M. Galeotti, L. Cina, C. Giammanco, S. Cordiner, A. D. Carlo, Energy , 89 (2015) 678-686 Figure 1

Impedance Spectra and Surface Coverages Simulated Directly from the Electrochemical Reaction Mechanism: A Nonlinear State-Space Approach

Current voltage curves and electrochemical impedance spectra (EIS) are crucial for investigating the performance and the electrochemical limitations of electrochemical cells. Therefore, we developed an approach which allows the direct simulation of such data based on microkinetic modeling. This approach allows us to assess the influence of various input parameters on the EIS and on the current–voltage curves and, hence, the overall performance of electrochemical cells. We develop our approach for the oxygen evolution reaction (OER) taking place at the semiconductor–electrolyte interface. At this interface, the microkinetic equations, that is, electrochemical reactions, for the multiple steps in OER are formulated and the resulting set of equations are modeled in a state-space form. As an input to the state-space model, we use the theoretical reaction rates calculated using density functional theory and Gerischer theory for semiconductors. Then, the electrochemical data are simulated as a function of applied potential. Next to the theory and the model development, a case study on the hematite–electrolyte interface which is a typical interface in photoelectrochemical cells is presented. Current voltage curves and EIS data for the hematite interface are simulated from the electrochemical model. The data are compared to experimental measurements. Apart from the current density and the EIS, the model can simulate the coverage of intermediate species as a function of applied potential which is highly demanded for identifying the limiting processes at the interface, but not available from experimental studies. This approach is generic and can be used for other electrochemical interfaces, such as present in fuel cells, electrolysers, or batteries.

Online Estimation of the Electrochemical Impedance Spectrum and Remaining Useful Life of Lithium-Ion Batteries

An electrochemical impedance spectrum (EIS) is considered to be one of the key indicators to monitor the health status of lithium-ion batteries. Experimental procedures to measure the EIS of a battery are offline and require manual intervention. So, in order to monitor the state of health of a battery in real time, online methods for EIS estimation would be very useful. This paper presents an approach for estimation of the EIS of lithium-ion batteries based on a fractional-order equivalent circuit model (FOECM) which can be implemented online. First, the parameters of the fractional-order model are determined using recursive least-squares technique in conjunction with a fractional-order state variable filter based on current and voltage measurements. The parameters obtained are then used to generate the estimated EIS of the battery under different aging conditions. Thereafter, a regression model is obtained based on the estimated EIS spectrum which can represent the degradation trend of the battery in terms of its internal resistance growth. Finally, the obtained regression model is used in the particle filtering framework to predict the remaining useful life (RUL) of the battery quite satisfactorily as compared to the RUL obtained based on the measured EIS data. Moreover, in order to justify the proposed RUL estimation method based on FOECM, comparative analyses with respect to other FOECM-based regression models and an integer order model have also been carried out.

Characterization of High Temperature PEM Fuel Cells for Electrochemical Hydrogen Pumping

High temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric acid doped polybenzimidazole operate at a temperature range between 140°C and 200°C. Due to their high operating temperature, HT-PEMFCs can tolerate a relatively high concentration of impurities in the anodic fuel stream. This makes HT-PEMFCs suitable for stationary applications where hydrogen is generated on site by steam reforming of hydrocarbons such as methane or oil derivatives. In particular, the preferential oxidation stage (PROX), which is typically required to reduce the CO concentration to ≈10 ppm after the reforming and water gas shift (WGS) reactors, is no longer necessary. Thus, fuel treatment units are simpler and cheaper with HT-PEMFCs [1]. The proven robustness of HT-PEMFCs against fuel impurities has recently attracted interest to operate the HT-PEMFC as a ‘hydrogen pump’ for hydrogen purification applications. In this configuration, a hydrogen rich gas stream is supplied to the cell’s anode compartment and a current is externally applied through the cell. Different from a typical fuel cell, no oxygen is supplied on the cathode side. During this process, hydrogen is electrochemically ‘pumped’ from the anode to the cathode in the form of protons through the electrolyte membrane. The ‘pump effect’ is highly selective and a pure hydrogen stream is obtained at the cathode outlet of the cell (≈10 ppm). Compared with low temperature Nafion based fuel cells, HT-PEMFCs are much more promising for purification of hydrogen streams that contain considerable amounts of impurities, as proven by preliminary studies [2, 3]. In this work, an investigation of the HT-PEMFC operated as an electrochemical hydrogen pump is carried out. The impact of hydrogen dilution and CO poisoning is studied as well as the effects of hydrogen stoichiometry, gas humidification, and electrolyte resistance. The cell is characterized with various techniques including polarization curves, electrochemical impedance spectra (EIS), and distribution of relaxation times (DRT) analysis (Fig. 1). Complementary to the equivalent circuit approach, DRT maps the EIS data to a function that contains the characteristic time scales of the system [4]. These results are important for the assessment of the HT-PEMFC’s potential for hydrogen purification. Although this application is distinctly different from that of a typical fuel cell, the electrochemical process at the anode is actually the same as in a normal HT-PEMFC. Therefore, the study also helps us gain valuable insight into the hydrogen oxidation reaction process in the presence of impurities.

In-Cell Chromium Getters to Mitigate Cathode Poisoning in SOFC Stack

Incorporation of a porous and electrically conducting chromium getter layer consisting of a mixture of complex metal oxide getter and electrically conducting perovskite phases has been examined as a potential cost effective approach for the capture of gaseous chromium species originating from the metallic interconnect alloys conventionally used in SOFC stacks. Screen printed/ brush coated SrxNiyOz and conventional LSM/LSCF containing porous layer, deposited on the cathode surface, was electrically tested in a half cell configuration (3%H2O-Air/Pt//YSZ//LSM/Getter/3% H2O-Air-CrOx) at 850 °C for 100 hours. Cell current at 0.5 V bias was monitored and electrochemical impedance spectra (EIS) in frequency range of 0.5 Hz to 200 kHz were obtained at 1-hour interval with 10 mV sinusoidal amplitude. Post tested cells were analyzed for chromium concentration profile, reaction products chemistry and morphological changes. Our observations indicate that the gaseous chromium predominantly concentrates in the getter layer. The bulk electrode and electrode-electrolyte interface remain free of chromium. The electrical performance remains largely stable as evidenced by I-t and EIS data obtained during the test. Experimental details and results will be presented and discussed.

Quantitative Rapid Analysis Method for Ofloxacin in Raw Milk Based on Molecule-Specific Recognition and Electrochemical Impedance Spectrum

Abstract. A quantitative rapid analysis method for ofloxacin detection in raw milk using molecule-specific recognition and an electrochemical impedance spectrum (EIS) technique was investigated in this study. An association complex (AC) formed by a combination of ofloxacin and sodium tetraphenylboron (ST) was used as the active material for electrochemical analysis. A carbon screen-printed electrode (CSE) was modified with the AC to form an electrochemical active membrane for ofloxacin detection. EIS data of pretreated raw milk samples were measured and analyzed with a non-linear bistable stochastic resonance (SR) model. Trace ofloxacin concentrations were characterized by SR output signal-to-noise ratio (SNR) eigen values. An ofloxacin quantitative analysis model was built based on SNR eigen values. Experimental results demonstrated that the proposed method presented good accuracy, repeatability, and recovery. It is a promising way for ofloxacin detection in raw milk. Keywords: Association complex, Electrochemical impedance spectrum, Ofloxacin, Quantitative analysis, Sodium tetraphenylboron.

Separation and Characterization of Overpotentials in Electrochemical Hydrogen Pump with a Reference Electrode

An electrochemical hydrogen pump (EHP) is a device employed for compressing hydrogen gas. The compression efficiency of the EHP, which operates by an isothermal process in principle, is theoretically higher than that of a conventional mechanical compressor, which operates by an adiabatic process. The structure of an EHP cell is similar to that of polymer electrolyte membrane fuel cell (PEMFC). In the EHP, compressing process is realized by imposing a potential difference to catalyst coated membrane (CCM) and completed by 3 steps: hydrogen oxidation reaction (HOR) at anode, conduction of hydrogen protons through membrane and hydrogen evolution reaction (HER) at cathode. Similar with PEMFC, overpotentials involved in EHP are categorized into two components: ohmic overpotential, mainly attributed to the conduction of protons through the membrane, and non-ohmic overpotential, which involves activation and concentration overpotentials. Due to the fast speed of hydrogen electrode reaction, ohmic overpotential is the main overpotential in the EHP. In order to reduce membrane resistance, bubbler humidifier same as that used in PEMFC system is conventionally utilized for humidifying membrane. However, different with PEMFC there is no water generation reaction occurs in the cathode side of EHP, the water at cathode dragged by hydrogen protons may be not enough for humidifying membrane at low current and high temperature conditions. Moreover, the effects of compression ratio (pressure of hydrogen at cathode) on overpotentials are important but not well discussed yet. In order to solve the problems which mentioned above, we supposed a different humidification method and employed a reference electrode for separating overpotentials. An internal humidification method is employed which is realized by storing liquid water in the cathode end plate shown in Fig.1. The liquid water directly humidifies membrane, leading to higher conductivity of membrane than that using a conventional bubbler humidifier [1]. This method is expected to balance water transport through membrane and keep membrane hydrated at high temperature. Electrochemical impedance spectroscopy (EIS) is introduced to clarify the effects of compression ratio on ohmic overpotential and non-ohmic overpotential. The compression ratio effects on overpotentials evaluated by EIS show following characteristics: For the part of ohmic overpotential, experimental results don’t show any dependence on compression ratio which suggests that water transport in EHP is more like diffusion process rather than convection process. On the other hand, non-ohmic overpotential decreases with increasing of hydrogen pressure of cathode. Separation results with reference electrode clearly shows that decreasing of non-ohmic overpotential occurs at cathode and it is attributed to the increasing of exchange current density of HER by increasing hydrogen pressure. Fitting I-V curves with a Volmer-Heyrovsy-Tafel expression [2] shows that the kinetic of HER and HOR are limited by diffusion process to/from electrode at anode/cathode. This is also confirmed by electrochemical impedance spectra data obtained at low frequency region. The concentration overpotential at anode is attributed to hydrogen diffusion through gas diffusion layer (GDL). In the EHP, in order to decrease hydrogen leakage, compression pressure supplied by end plates is set to 5MPa, which is much higher than common value (1~2MPa) used in PEMFC cell. At such a high compression pressure, GDL is compressed extremely, leading to small value of efficient gas diffusivity of hydrogen. Limiting current of anode calculated by hydrogen gas diffusion through GDL is less than 2A/cm2which agrees with the fitting results generally. The concentration overpotential at cathode is larger than that at anode. Frist, we suspected that water layer built at cathode may work as a mass transfer barrier for hydrogen gas and lead to large mass transfer resistance. However, an additional experiment carried out with bubbler humidification showed even larger mass transfer resistance at cathode. It is clear that mass transfer of hydrogen at cathode is not limited by water layer. It is possible that hydrogen diffusion in catalyst layer leads to the high concentration overpotential. The structure of catalyst layer is assumed to consist of gas pores and aggregates of Pt/C covered by ionomer layers [3]. Hydrogen that produced at reaction sites of cathode need to diffuse through this ionomer layer. With the liquid water humidification, water content of ionomer layer increases [4] which enhances the permeability of hydrogen and decreases mass transfer resistance. Reference: [1] T. A. Zawodzinski Jr., T. E. Springer, and S. Gottesfeld. Journal of The Electrochemical Society, 1993, 140(7) 1981-1985. [2] P.M. Quaino, A.C. Chialvo, Electrochimica Acta, 2007(52) 7396–7403. [3] Firat C, Ajay K. Prasad, Journal of The Electrochemical Society, 2014,161 (6) F803-F813. [4] T. Sakai, E. Torikai. Journal of The Electrochemical Society, 1985, 132(6) 1328-1332. Figure 1

A Simple Pressurized SOFC Test Rig for Measurements of Cell Performance, Impedance, and Various Overvoltages

Nowadays, our modern society is facing many serious environmental and energy shortage problems. Delay of action will jeopardize the ability of future generations to meet their needs. Thus, how to develop highly efficient power generation systems with very low emissions is an urgent need for our modern society. Probably, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, which has the highest efficiency up to 70%, is the best candidate. The development of such hybrid power system helps to the consolidation of the fuel cell industry, the expansion of the gas turbine industry, and the improvement of the environment. As such, the hybrid PSOFC-MGT power system has become an emerging topic in the 21th century with wide interest, as evidenced by a large volume of literature and several demonstration projects. For examples, Siemens demonstrated a 220 kW PSOFC-MGT hybrid system in 2002, the Korea Institute of Energy Research displayed a 30 kW (5 kW PSOFC integrated with 25 kW MGT) hybrid system in 2006, and Mitsubishi Heavy Industries run a 200 kW hybrid PSOFC-MGT combined-cycle power plant in 2011. Indeed, considerable progresses have been made over the past two decades in optimization of geometry and development of new materials and in understanding of mass and heat transports together with electrochemical reactions in porous microstructures. However, there are still technical challenges remaining to be solved step by step with limited but controllable parameters before a stable operation among different components of the hybrid PSOFC-MGT system can be assured and the status of the development can be then elevated from a module level. One of the challenges is the detail electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to design a simple efficient high-pressure full/half button cell testing platform for measurements of the effect of pressurization and increasing temperature on EIS and various overvoltages of PSOFC. The testing platform has several parts, from inside out including a specially thread-designed housing carrier for the full/half button cell, a serpentine heating pipe system for uniform heating of the supplied fuel and air gases, a temperature-controlled furnace, and a large high-pressure vessel together with measuring devices and their associates. Hence, current-voltage curves and AC impedance spectra of any full/half button cells can be measured. In this study, we present power-generating characteristics and EIS data of an anode-supported full button cell over wide ranges of pressure (p = 1~5 atm) and temperature (T = 700~850 oC). We apply constant gas flow rates, 200ml min-1 H2 in anode and 200ml min-1 air in cathode, for all experimental events. Results show that, for any given values of T or p, power densities increase with increasing p or T at any fixed current densities. Such enhancement on power densities is more sensitive to the increase of T than to the increase of p. These results are explained by the corresponding EIS data. The latter shows that both high and low frequency arcs of impedance spectra decrease with increasing p, resulting in a reduction of the total polarization resistance. Specifically, the high frequency arcs decrease rather weakly with increasing p as compared to that of the low frequency arcs. Further, we found that the characteristic frequencies of high and low frequency arcs occur around 100 ~ 1000 Hz and at about 10 Hz, respectively. The former may be attributed to the cathode activation polarization and the latter may be due to the diffusion processes in the anode electrode. Hence, pressurization can simultaneously decrease the cathode activation polarization and the anodic diffusion concentration polarization. In addition to the aforesaid power-generating characteristics and EIS results, this study reports analyses of activation and concentration overvoltages using the Butler-Volmer equation and the related concentration overvoltage equation based on a 1D diffusion model. We found that both activation and concentration overvoltages decrease with increasing p. Two important parameters, the exchange current density and the anodic effective diffusion coefficient, are also calculated by applying the present measured data. Results show that pressurization enhances the exchange current density leading to an increase of the electrochemical reaction rate. Though pressurization reduces slightly the anodic effective diffusion coefficient, it can increase the molar concentration and thus the overall gas-phase diffusion rate in porous electrode. These overvoltage results will be compared with previous numerical and experimental data to enhance our understanding of the effect of pressurization on the polarization mechanisms in SOFCs. Figure 1

The Impact of Pressurization on Anode-Supported and Electrolyte-Supported Planar Solid Oxide Fuel Cells at 750oC ~ 850oC

How to reduce effectively the severe impact of various environmental and energy problems is one of the most important issues in our human society. This continues to drive the development of hybrid power generation systems aiming to achieve much higher efficiencies together with much lower emissions than conventional power systems. Among only a few available hybrid energy conversion systems, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, has the highest efficiency up to 70%. Moreover, the development of such hybrid PSOFC-GT or –MGT power system is not only beneficial for the environment, but also it is important to the consolidation of the fuel cell industry and the expansion of the gas turbine industry. Although several demos of the hybrid PSOFC-MGT power system have been established, many technical challenges existing in the design of such hybrid power system and its control still need to be solved in order to assure a stable operation among different components of the hybrid system. One of the most important technical challenges is the detail experimental information on electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to measure experimentally the effect of pressurization on EIS of PSOFC. In this work, we measure the pressurization influence on power generating characteristics and EIS for both anode-supported and electrolyte-supported cells (ASC and ESC) using a recently-established high-pressure SOFC facility. It is worth of noting that the present study has an important advantage due to the fact that both ASC and ESC use the same single-cell setup, a planar full cell having 40 × 40 mm2 effective reactive area sandwiched by a pair of rib-channel flow distributors (interconnectors) on both anode and cathode, under exactly the same experimental conditions. Specifically, both ASC and ESC apply exactly the same flow rates (Q anode = 0.5 slpm H2 + 0.4 slpm N2 and Q cathode = 0.9 slpm air) at three operating temperatures (T = 750 oC, 800 oC, 850oC), each T under five different pressures (p = 1, 2, 3, 4, 5 atm), having a total of 30 data sets for comparison. Hence, a direct comparison of experimental EIS data and power generating characteristics between ASC and ESC at elevated pressures can be clearly made. To the best knowledge of the authors, the present measured EIS comparison between ASC and ESC is new, and experimental EIS data of pressurized electrolyte-supported planar SOFC are still not available in literatures. These ASC and ESC measurements under the same experimental conditions suggest the following points: (1) At T = 850oC and 0.7 V, power densities of ASC/ESC increase from 309/193 mW cm-2 to 476/250 mW cm-2, as p increases from 1 atm to 5 atm. As T increases from 750oC to 850oC at 0.7 V, power densities of ASC/ESC increase from 175/97 mW cm-2 to 309/193 mW cm-2 at p = 1 atm, while at p = 5 atm, the corresponding increases of power densities are 281/137 mW cm-2 to 476/250 mW cm-2. It is found that ASC is more sensitive to pressurization than ESC, while the latter is more temperature-sensitive than the former. (2) The ohmic resistance is not varied with the loaded current density and it is independent of pressure for both ASC and ESC cases. Furthermore, ESC has higher values of the ohmic impedance, about 16 % larger, than that of ASC. The thicker the electrolyte thickness is, the larger the ohmic impedance is. (3) The polarization impedance decreases noticeably with increasing p for both ASC and ESC cases at either unloaded or loaded conditions. This explains why power densities increase with increasing pfor both ASC and ESC. (4) These measurements reveal that the better cell performance of ASC than that of ESC is not simply due to the larger ohmic overvoltage of ESC as previously predicted by numerical simulations. The variation of the polarization overvoltage between ASC and ESC also plays an important role. We hasten to note that it is not sure whether the present conclusions can be extended to all ASCs and ESCs due to different electrolytes and anodes used in the present ASC and ESC, but these results should be of help in the further development of PSOFC integrating with micro gas turbines for future stationary power generation. Figure 1

A Simple Pressurized SOFC Test Rig for Measurements of Cell Performance, Impedance, and Various Overvoltages

This paper reports a newly designed high-pressure full button cell test platform for measurements of various effects on pressurized solid oxide fuel cells (PSOFC). The testing platform has several parts, from inside out including a specially thread-designed housing carrier for the full button cell, a serpentine heating pipe system for uniform heating of the supplied fuel and air gases, a temperature-controlled furnace, and a large high-pressure vessel together with measuring devices and their associates. Hence, current-voltage curves and AC impedance spectra of any full/half button cells can be conveniently measured. In this study, we present power-generating characteristics and electrochemical impedance spectra (EIS) data of an anode-supported full button cell over a range of pressure (p = 1~5 atm) at a temperature T = 800 oC. Results show that power densities increase with increasing p at any given current densities. These results are explained by the corresponding EIS data. In addition to the aforesaid power-generating characteristics and EIS results, this study also presents analyses of activation and concentration overvoltages using the Butler-Volmer equation and the related concentration overvoltage equation based on a 1D diffusion model. These overvoltage results will be compared with previous numerical and experimental data to enhance our understanding of the effect of pressurization on the polarization mechanisms in SOFCs.

Eliminating voltage decay of lithium-rich li1.14 mn0.54 ni0.14 co0.14 o2 cathodes by controlling the electrochemical process.

A lithium-rich cathode material Li1.14 Mn0.54 Ni0.14 Co0.14 O2 (LNMCO) is prepared by a co-precipitation method. The issue of voltage decay in long-term cycling is largely eliminated by control of the charge-discharge voltage range. The LNMCO material exhibits 9.8 % decay in discharge voltage over 200 cycles between 2.0-4.6 V, during which the working voltage decays significantly, from 3.57 V to 3.22 V. The decay was decelerated by a factor of six by using a voltage window of 2.0-4.4 V, from 3.53 V to 3.47 V. IR and Raman spectra reveal that the transformation of layered structure to spinel is significantly retarded under 2.0-4.4 V cycling conditions. Transmission electron microscopy (TEM) was also applied for examining phase change in an individual particle during cycling, showing that the spinel phase occurs both at 2.0-4.6 V and at 2.0-4.4 V, but is not dominant in the latter. Normalization of Li can remove the additional impact on the voltage decay which is brought by different amounts of Li intercalation. The mechanism of no voltage decay at 2.0-4.4 V cycling is raised and electrochemical impedance spectrum data also support the hypothesis.