Dino Klotz

Studies on LiFePO4 as Cathode Material in Li-Ion Batteries

Lithium iron phosphate is a promising cathode material for the use in lithium-ion batteries meeting the demands of good stability during cycling and safe operation due to reduced risk of thermal runaway. However, slow solid state diffusion and poor electrical conductivity reduce power capability. For further improvement, the identification of the rate determining processes is necessary. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the characterization of electrochemical systems. In this contribution a deconvolution of the impedance with the distribution of relaxation times (DRT) is used to obtain a better resolution in frequency domain. Therewith, the cathodic and anodic polarization processes are identified and an impedance model for the cell is proposed. Introduction The demand for high energy density rechargeable batteries for electric and hybrid electrical vehicle systems has rised particular interest in lithium-ion batteries in recent times. Many kinds of material are in consideration as the cathode material to meet the requirements of a satisfying cathode performance (1). In 1997, Lithium iron phosphate (LiFePO4) was proposed by John B. Goodenough’s group for the first time (2). It has been investigated as a promising candidate for cathode material in lithium-ion batteries due to its high theoretical capacity (170 mAh/g) compared with other iron-based compounds (3), its good thermal stability leading to high safety (3;4), and its cycling stability (5). One major drawback of lithium iron phosphate is its poor conductivity (ionic and electronic), which generally causes capacity losses at higher charge–discharge currents (6;7). Recently, many studies on lithium iron phosphate have been published, mainly focused on the understanding and improvement of lithium-ion conduction in the active material (8;9;10). However, there are other loss processes which influence the performance of LiFePO4-cells (11). Their identification and physical interpretation according to literature is not consistent. EIS has proven to be a powerful tool for the characterization of electrochemical systems (3;11;12). In general, physio-chemical processes with different time constants can be distinguished in the EIS spectra. However, the identification of these processes remains ambiguous as they overlap in the frequency domain. ECS Transactions, 28 (30) 3-17 (2010) 10.1149/1.3505456

Evaluation of the Rate Determining Processes for LiFePO4 as Cathode Material in Lithium-Ion-Batteries

Lithium iron phosphate is a promising cathode material for the use in lithium-ion batteries meeting the demands of good stability during cycling and safe operation due to reduced risk of thermal runaway. However, slow solid state diffusion and poor electrical conductivity reduce power capability. For further improvement, all rate determining electrode processes have to be quantified. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the characterization of electrochemical systems. In recent publications (1,2), a physical interpretation for the impedance response of LiFePO4/Li-cells was given and an equivalent circuit model was proposed. In this contribution, the proposed equivalent circuit is used to quantify the predominant electrode processes depending on temperature (0°C…30°C), SoC (10% … 100%) and microstructure characteristics (porosity).

Accurate determination of the charge transfer efficiency of photoanodes for solar water splitting

The oxygen evolution reaction (OER) at the surface of semiconductor photoanodes is critical for photoelectrochemical water splitting. This reaction involves photo-generated holes that oxidize water via charge transfer at the photoanode/electrolyte interface. However, a certain fraction of the holes that reach the surface recombine with electrons from the conduction band, giving rise to the surface recombination loss. The charge transfer efficiency, ηt, defined as the ratio between the flux of holes that contribute to the water oxidation reaction and the total flux of holes that reach the surface, is an important parameter that helps to distinguish between bulk and surface recombination losses. However, accurate determination of ηt by conventional voltammetry measurements is complicated because only the total current is measured and it is difficult to discern between different contributions to the current. Chopped light measurement (CLM) and hole scavenger measurement (HSM) techniques are widely employed to determine ηt, but they often lead to errors resulting from instrumental as well as fundamental limitations. Intensity modulated photocurrent spectroscopy (IMPS) is better suited for accurate determination of ηt because it provides direct information on both the total photocurrent and the surface recombination current. However, careful analysis of IMPS measurements at different light intensities is required to account for nonlinear effects. This work compares the ηt values obtained by these methods using heteroepitaxial thin-film hematite photoanodes as a case study. We show that a wide spread of ηt values is obtained by different analysis methods, and even within the same method different values may be obtained depending on instrumental and experimental conditions such as the light source and light intensity. Statistical analysis of the results obtained for our model hematite photoanode show good correlation between different methods for measurements carried out with the same light source, light intensity and potential. However, there is a considerable spread in the results obtained by different methods. For accurate determination of ηt, we recommend IMPS measurements in operando with a bias light intensity such that the irradiance is as close as possible to the AM1.5 Global solar spectrum.

Electrochemical Studies on Anode Supported Solid Oxide Electrolyzer Cells

Introduction Solid Oxide Fuel Cell (SOFC), usually converting hydrogen as well as hydrocarbon fuels directly into electricity, have the ability to operate as a Solid Oxide Electrolyser Cell (SOEC). In the electrolysis operation mode hydrogen is produced by electrochemical reduction of water. Due to the high operating temperature a reversible operation is possible, enabling a high efficiency. The performance and stability of state of the art SOFC cells in this reverse mode of operation is however still open. In this work experimental and modelling results of the SOFCand SOEC-operation of anode supported cells are presented.

The Spatial Collection Efficiency of Charge Carriers in Photovoltaic and Photoelectrochemical Cells

The spatial collection efficiency portrays the driving forces and loss mechanisms in photovoltaic and photoelectrochemical devices. It is defined as the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent. In stratified planar structures, the spatial collection efficiency can be extracted out of photocurrent action spectra measurements empirically, with few a priori assumptions. Although this method was applied to photovoltaic cells made of well-understood materials, it has never been used to study unconventional materials such as metal-oxide semiconductors that are often employed in photoelectrochemical cells. This perspective shows the opportunities that this method has to offer for investigating new materials and devices with unknown properties. The relative simplicity of the method, and its applicability to operando performance characterization, makes it an important tool for analysis and design of new photovoltaic and photoelectrochemical materials and devices.

Empirical Analysis of the Photoelectrochemical Impedance Response of Hematite Photoanodes for Water Photo-oxidation

Photoelectrochemical impedance spectroscopy (PEIS) is a useful tool for the characterization of photoelectrodes for solar water splitting. However, the analysis of PEIS spectra often involves a priori assumptions that might bias the results. This work puts forward an empirical method that analyzes the distribution of relaxation times (DRT), obtained directly from the measured PEIS spectra of a model hematite photoanode. By following how the DRT evolves as a function of control parameters such as the applied potential and composition of the electrolyte solution, we obtain unbiased insights into the underlying mechanisms that shape the photocurrent. In a subsequent step, we fit the data to a process-oriented equivalent circuit model (ECM) whose makeup is derived from the DRT analysis in the first step. This yields consistent quantitative trends of the dominant polarization processes observed. Our observations reveal a common step for the photo-oxidation reactions of water and H2O2 in alkaline solution.

Different Roles of Fe1–xNixOOH Cocatalyst on Hematite (α-Fe2O3) Photoanodes with Different Dopants

Transparent Fe1–xNixOOH overlayers (∼2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (α-Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the cocatalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1–xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1–xNixOOH cocatalysts can play ...

Beneficial Use of a Virtual Reference Electrode for the Determination of SOC Dependent Half Cell Potentials

A method is presented which obtains the half cell potentials of a sealed commercial cell from a measured OCV-curve. The underlying model is introduced and the physical interpretable parameters are discussed. The method is validated by two experiments simulating different aging mechanisms and future potentials are discussed.

Two-site H 2 O 2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.Understanding fundamental processes that occur using solar-to-fuel conversion materials is crucial for constructing effective renewable energy collection. Here, authors find the hydrogen peroxide light-driven hole-scavenging mechanism over haematite to proceed with two active sites rather than one