Micro-reference Electrode Research Articles

The timescale identification and quantification of complicated kinetic processes in lithium-ion batteries based on micro-reference electrodes

The lack of reliable methods for acquiring accurate and stable electrochemical impedance spectroscopy (EIS) data at both the cathode and anode makes overlapping kinetic processes in commercial lithium-ion batteries (LIBs) with enhanced capacity and size indistinguishable. Micro-reference electrodes offer an elegant solution for obtaining accurate and stable EIS data for both the cathode and anode. The proposed selection model ensures that the mean absolute percentage error between the EIS of the combined cathode and anode and the battery remains below 3.5 % under varying temperatures and states of charge (SOCs). Different SOCs, temperatures, and pressures are utilized on the timescale to identify and quantify the kinetic processes of the cathode and anode. The results indicate that under varying temperature conditions, the process of Li+ traversing the solid electrolyte interphase dominates the total impedance of the battery, contributing over 75 %. At −10 °C, polarization resistance from this process exceeds a 2500 % increase compared to 20 °C. Moreover, the variations in the kinetic processes of LIB cathodes and anodes with temperature are revealed on the timescale, with potential reasons for the overlap of kinetic processes provided. This study offers new insights into LIB electrode kinetics and lays a theoretical groundwork for enhancing battery design and performance.

Impedance spectroscopy using microscopic reference electrodes to analyze different rate-determining steps in aqueous dye-sensitized solar cells using nitroxide radicals as redox mediators

For new components in dye-sensitized solar cells (DSSCs), identification and quantification of rate-limiting steps is needed to evaluate their applicability. This is particularly important if fundamental changes are studied. In this work, the use of a micro-reference electrode in DSSCs is proposed to increase the significance of electrochemical impedance spectroscopy. The recombination of charge carriers at the photoanode and the regeneration of redox mediators at the counter electrode, which typically occur on similar timescales, could be studied separately but simultaneously in cells under operating conditions. This is particularly useful in the study of water-based DSSCs. Here, cells with 2,2,6,6-Tetramethylpiperidinoxyl (TEMPO) or 4-Hydroxy-TEMPO (OH-TEMPO) as redox mediators using additives like 1-Methylbenzimidazole (MBI) are discussed. It was revealed that the charge transfer resistance (RCE) for the reduction of the oxidized redox mediator at the counter electrode (CE) limits the fill factor (FF) of such DSSCs. TEMPO/MBI electrolytes yielded low RCE and high FF, whereas OH-TEMPO in otherwise identical cells resulted in large RCE, low FF, and low conversion efficiencies. This indicates that the interface between the CE and the electrolyte significantly influences the DSSC performance of these cells and strongly depends on the electrolyte composition. Important optimization strategies could be discussed based on the present results.

Online adaptive anode potential-controlled fast charging of lithium-ion cells using a validated electrochemical model-based virtual reference electrode

Optimal utilization of the safe operating area of lithium-ion battery cells is fundamental to achieving peak performance. Pushing the currents close to the intrinsic electrochemical cell limits during fast charging of battery electric vehicles reduces charging times noticeably and improves customer satisfaction. With electrochemical model-based state observers embedded in the battery management system, predicting the critical anode potential is feasible. This allows the charging process to be adjusted accordingly for effective prevention of lithium plating. However, validating the model, especially the internal potentials, is particularly challenging, and the real-time capability is often doubted. This work addresses the parameterization and experimental validation of a full-order electrochemical model utilizing cells instrumented with micro-reference electrodes. Special emphasis is put on validating the estimate of the anode surface potential, which enables the model to act as a virtual reference electrode. For the first time, it is shown how this can be efficiently implemented on a next-generation automotive microcontroller, in combination with a charging current controller, for online adaptive anode potential-controlled fast charging. Execution times well below 4 ms prove real-time capability, and reference electrodes validate good agreement between model estimate and measurements in closed-loop charging current control.

Reliable unencapsulated Ag/Ag2S micro-reference electrodes for battery research and other applications in organic media

Reliable electrochemical measurements depend on the availability of robust reference electrodes (RE) with well-defined potentials. While many reliable REs are known, they are not applicable in certain demanding media such as ionic liquids, nor in small confined spaces. Here, we describe the fabrication of a simple yet robust Ag/Ag2S micro-reference electrode (μ-RE) where a micron-thick Ag2S layer is formed by isothermal reaction with sulfur vapor. Scanning electron microscopy, X-ray photoemission spectroscopy, and spectroscopic ellipsometry characterization reveals that the optimal morphology corresponds to a slightly porous Ag2S film. We demonstrate that the Ag/Ag2S μ-RE can be operated in and cycled through a wide variety of polar organic solvents, including common protic solvents (EtOH), aprotic solvents (ACN, DMSO, NMP, DMF) and ionic liquids (EMIM-TFSI, BMP-TFSI), with short equilibration time (tens of seconds) and little drift (<20 mV), without requiring encapsulation, protective liquid junctions, nor special conditioning. A redox potential of 0.54 ± 0.02 V was obtained for ferrocene in acetonitrile, which places this RE at 0.08 V vs Ag/AgCl. We have also successfully embedded the electrode inside the CR2032 coin cell to perform cyclic voltammetry of battery materials. These results underpin the suitability of this simple micro-reference electrode for a wide variety of electrochemical measurements in demanding and/or miniaturized environments.

Dynamic Electrochemical Impedance Spectroscopy for Characterization of the State of Health of Graphite Electrodes

Graphitic carbon is the most used material as anode in lithium-ion batteries since the last 30 years. To make batteries cost-effective for mobility or stationary applications the optimization of every aspect is crucial. Advanced techniques can be implied to undercover or demonstrate the main chemical processes at the graphite electrode such as the Electrochemical Impedance Spectroscopy (EIS). EIS is a powerful technique able to reveal and separate resistive and capacitive properties of an interface in a non-destructive way. It has been applied for the characterization of graphite in order to quantify the charge transfer resistance on the intercalation process, to study the mechanism of formation of solid-electrolyte-interface (SEI), and reveal the lithium plating process [1]. There are high requirements for the collection of impedance spectra on a broad range of frequencies (typically from kHz to tens of mHz) at different electrode potentials regarding measurement time and the condition of a system in equilibrium. To overcome these requirements a technique has been developed called Dynamic Electrochemical Impedance Spectroscopy (DEIS). For DEIS a multi-frequency signal, containing all frequencies to probe in a form of summed sine waves, is superimposed to the galvanostatic current. Using this method, it is possible to measure the time-varying impedance while the system is drifting through non-equilibrium states, e.g. during charge/discharge cycling.In this study we propose DEIS measurements performed on a freshly assembled graphite/lithium half-cell cycled at current rates of C/10 and C/5 and 1C (for aging). Figure 1 A shows the working electrode voltage profile of the first 16 cycles. The chosen geometry for the cell is a three-electrode pouch cell with 2x2cm square electrodes and an insulated copper wire with lithium plated at the tip as micro-reference electrode (see Figure 1 B).The novelty of the proposed method is the fully automatized procedure to acquire the dynamic impedance on a wide frequency band (1kHz-10mHz) continuously over an arbitrary number of cycles to investigate the degradation of the electrode. The experimental set-up is shown as a scheme in Figure 1 C [2]. On base of the recorded current and voltage signals, we computed the time-varying impedance using the Dynamic Multi-Frequencies Analysis (DMFA). This method allows to calculate the impedance at the low frequency band by filtering of the sine components in the Fourier domain; the voltage drift is calculate in the same way [3]. An example of the computed time-varying impedance and the respective potential profile for the first cycle is reported in Figure 1 D.

Investigating the Change in the Microporous Structure of the Graphite Electrode during Formation

Till today, the graphite-based electrode is the most widely used negative electrode in commercially available lithium-ion batteries (LIBs).[1] While extended research has been conducted in the investigation of the graphite electrode’s properties, especially the formation of the solid-electrolyte interphase (SEI) within the first cycles of LIBs remains a field of ongoing research.[2] The SEI forms during the first cycle(s) on the anode surface at potentials below the stability window of the electrolyte components, due to the reductive decomposition of the respective solvents, additives, and conductive salts.[2,3] The initially formed SEI has an influence both on the long-term cycling stability of the cell, depending on how good the passivation, i.e., electronic insulation, of the graphite surface is,[4] and on the rate performance of the anode, depending on the thickness of the SEI, its chemical composition, and its lithium-ion conductivity, referred to as SEI resistance.[5] Additionally, with a thickness of 10-20 nm when employing additives such as vinylene carbonate (VC), the SEI takes up parts of the void volume of the pristine electrode and may change the pores and the electrolyte conduction pathways within the electrode (see Figure 1).[6] In this study, we investigated the change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 3:7 w:w with 2 wt% VC, Gotion, USA) during formation at 45 °C. The electrode’s microstructure is often connected to the macroscopic properties using the McMullin number N M in the form of:[7] N M = τ / ε = R Ion ·A · κ / d, (1)correlating the porosity ε, electrodes thickness d, ionic pore resistance R Ion, area of the electrode A, conductivity of the electrolyte κ, and the tortuosity τ (see Figure 1).Here, we analyzed graphite electrodes with varying initial porosities from 20 to 50 %, employing electrochemical impedance spectroscopy (EIS) in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.[8,9] Measuring EIS at temperatures of -5 °C allowed the reliable determination of the ionic resistance before and after formation. Due to the formed SEI covering the graphite particles and partially filling the void of the porous electrode, we discovered an increase of the ionic pore resistance throughout the whole range of initial porosities.Next, we determined a porosity dependent increase in thickness of the electrodes from electrodes cycled at the same conditions in coin cells that were subsequently harvested from the cells and analyzed for thickness changes. Additionally, mercury intrusion porosimetry (MIP) was used to determine the porosity of formed graphite electrodes, revealing a decrease in porosity after formation for all samples. The combination of these findings - using Equation 1 as described above with a constant electrode area A and assuming a constant electrolyte conductivity κ - allowed us to evaluate the change in the tortuosity of the graphite electrodes, giving insights into the overall changes in the graphite anode properties due to SEI formation.

Dynamic Change in Throwing Power for the Cu-to-Carbon Steel Galvanic Couple in the Presence of Bentonite

In galvanic corrosion, the maximum distance from the periphery of the cathode to the anode is often defined as the throwing power. It dictates the surface area of the cathode involved in the galvanic corrosion and is also related to the extent of the accelerated anodic reaction. Therefore, its estimation is of great interest and importance in engineering applications, such as fastener and panel assembly [1], metal coatings [2], buried metallic structures [3], etc. Throwing power may be estimated experimentally by measuring the solution potential variation in close proximity to the electrode surface (Es) or through modelling, such as finite element analysis (FEA), provided the boundary conditions (polarization characteristics) are correctly determined. In this work, we investigated the throwing power for the Cu-to-carbon steel galvanic couple in the presence of bentonite clay, a scenario that could arise in the vicinity of a through-coating defect in the Canadian-designed nuclear waste container. Using a homemade micro-reference electrode array coupled to a Multichannel Microelectrode Analyzer (MMA), Es was measured simultaneously in 10 locations as a function of time in 0.1 M NaCl solution with and without bentonite clay, Fig. 1. Transient potential peaks were found when bentonite clay was added to the solution and are attributed to the increased galvanic activities driven by the reduction of air-formed Cu oxides.We further investigated this phenomenon using FEA. Boundary conditions were established experimentally under circumstances reflecting the actual surface states, i.e., with or without Cu oxides. Simulations were conducted considering secondary current distribution, using the appropriate boundary conditions. The obtained results match well with the experimental measurements, including the peak and the stable Es. Additionally, the dynamic change of the throwing power due to the evolution of the surface state was investigated through simulation and discussed. Reference [1] R.S. Marshall, A. Goff, C. Sprinkle, A. Britos, R.G. Kelly, Estimating the Throwing Power of SS316 when Coupled with AA7075 Through Finite Element Modeling, Corrosion, 76 (2020) 476-484.[2] J.R. Scully, F. Presuel-Moreno, M. Goldman, R.G. Kelly, N. Tailleart, User-Selectable Barrier, Sacrificial Anode, and Active Corrosion Inhibiting Properties of Al-Co-Ce Alloys for Coating Applications, Corrosion, 64 (2008) 210-229.[3] L. Liu, J. Li, M. Peng, W. Li, B. Lei, G. Meng, Macro-galvanic corrosion of tower grounding device consisting of graphite and Zn-coated steel in a simulated soil environment, Engineering Failure Analysis, 135 (2022) 106136. Figure 1

Lifting the Lids on Reference Electrodes in PEMWE: Previous Approaches and Current Application

The authors of this contribution have undertaken a comprehensive review of reference electrodes (REs) used in PEM Water Electrolysis (PEMWE) research. The importance of REs cannot be understated as they are instrumental in determining the potential within the system and in separating the two electrodes. Examining the electrodes' characteristics yields insights into their individual performance and can e.g. help to assess new catalyst layer designs, their interplay with the adjacent porous transport layer or understand the complex and multi-faceted mechanisms of degradation1.Previous research on REs has generated a significant amount of information and detailed data for each reference electrode setup. However, this information is currently distributed across various publications. As a result, the present review compresses this information into a single, comprehensive resource. The review covers the classification of various reference electrodes, their advantages and disadvantages, and the process of implementing them. The purpose of the work is to provide guidance to researchers in the field, enabling them to effectively tailor the reference electrode to meet the specific demands and contexts of their research.Furthermore, two different RE approaches have been selected for experimental studies. The first one is a salt bridge reference electrode (SBRE), which probes to the proton potential of the catalyst layer at the interface to the porous transport layer (PTL). It has been used for analysis of half-cell kinetics. At current densities beyond 0.1 A cm-2, a trade-off between the needed ionic connectivity of the PTL and additional losses limited the application2. A systematic study is carried out to improve the PTL impregnation with ionomer resulting in almost no performance decline. The improved setup is used to study the effect of temperature on the half-cell kinetics in a range from 80 to 30 °C. Corresponding polarization curves are presented in Figure 1, with UA-REa including the kinetic overpotential from oxygen evolution reaction and electric losses at the anode, UC-REa containing all remaining losses from the protonic losses in the anode ionomer towards the cathode bipolar plate.The second type is a platinum wire micro-reference electrode based on the concept of a dynamic hydrogen electrode3. The wire is in contact with the membrane outside the active CL area in an isolated compartment of humidified hydrogen. A total of 119 individual tests are carried out to assess the RE reproducibility. The results of the half-cell potential with HFR measurements, and kinetic analysis lead to a discussion on the prevalence of misalignment in solid state systems and the benefits of using EIS to diagnose it. Finally, the robustness and the limitations of the method are discussed through a set of variations of influencing parameters.References(1) Kuhnert, E.; Hacker, V.; Bodner, M. A Review of Accelerated Stress Tests for Enhancing MEA Durability in PEM Water Electrolysis Cells. International Journal of Energy Research 2023, 2023, 1–23. DOI: 10.1155/2023/3183108.(2) Bühre, L. V.; Bullerdiek, S.; Trinke, P.; Bensmann, B.; Deutsch, A.-L.; Behrens, P.; Hanke-Rauschenbach, R. Application and Analysis of a Salt Bridge Reference Electrode Setup for PEM Water Electrolysis: Towards an Extended Voltage Loss Break Down. J. Electrochem. Soc. 2022. DOI: 10.1149/1945-7111/ac9ee1.(3) Bühre, L. V.; McLeod, A. J.; Trinke, P.; Bensmann, B.; Herrera, O. E., Mérida, W.; Hanke-Rauschenbach, R in preparation Figure 1

Corrosion Electrochemistry at Micro/Nano-Scale

A fundamental understanding of corrosion mechanisms is the key to developing suitable corrosion protection approaches for metallic structures and new corrosion resistant alloys for emerging technologies. However, corrosion testing methods such as mass loss and conventional electrochemical testing average over large areas and thus often mask the underlying mechanisms, due to either low sensitivity, low resolution, and/or a lack of element-resolved information. Recent advances in electron microscopy have led to development of liquid-cell transmission electron microscopy (LC-TEM) that allows in-situ monitoring of morphological and even compositional evolution in materials at high spatial resolution during interactions of surfaces with aqueous environments. However, electrochemical testing in such restricted volume with a very thin (~ 500 nm) electrolyte layer places stringent demands not only for a reference electrode, but also for a comprehensive understanding of the factors affecting electrochemical kinetics at the nano-scale1.In order to solve the need for a micro-reference electrode, we recently proposed the concept of metallic Luggin-Haber probe2. The metallic Luggin-Haber probe is a metal wire that serves as a bridge between the test solution and the reference electrode that was in a physically-distinct container. In standard-scale electrochemical cells, it was found that the metal bridge (MB) provides a means of making potential measurements that are identical to those made using a direct reference electrode or using a salt bridge.The MB is shown to be compatible with a wide range of electrochemical techniques including cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. The present work explores the compatibility of the MB for electrochemistry in LC-TEM using Pt as working electrode. The work includes the effect of radiolysis of electrolyte, electrolyte flow rate, pH and potential scan rate on electrochemical kinetics when using LC-TEM, in a detailed comparison with results obtained using a standard-scale electrochemical cell. Present work is a discrete effort towards developing advanced methodologies for electrochemical measurement in situations involving restricted electrolyte volume including thin electrolyte layers, droplets, localized corrosion sites, batteries and fuel cells. References A. Kosari, H. Zandbergen, F. Tichelaar, P. Visser, H. Terryn, and A. Mol, Corrosion, 76, 4–17 (2020).S. Choudhary, K. Marusak, T. Eldred, and R. G. Kelly, J. Electrochem. Soc., 169, 111505 (2022).

The Implications of Particle Morphology on the Capacity Retention, Side Reactions, and Impedance Build-Up of Nickel-Rich NCMs upon Cycling in Full-Cells: Poly- vs. Single-Crystalline NCM851005

The rising interest in single-crystalline NCMs (LiMO2, M = Ni, Co, Mn) has generated numerous publications which feature promising results in terms of cycle-life improvement when compared to the conventional polycrystalline analogues. To elucidate the effect of the two morphologies on the capacity retention and the internal resistance, this study aims to discriminate the effect of different degradation phenomena of polycrystalline and single-crystalline NCM851005 (LiNi0.85Co0.10Mn0.05O2) in coin full-cells cycled against graphite anodes. The impact of the particle morphology is analyzed over the course of more than 200 charge/discharge cycles for two temperatures of 25 and 45 °C, applying 4.1 or 4.4 V as upper cutoff voltages. The morphology-dependent surface area changes, resulting mainly from the tendency of polycrystalline NCMs towards particle cracking upon calendering, charging, and extended cycling, are quantified via krypton-gas physisorption, and the consequences of particle cracking regarding the amount of gas evolution, transition-metal dissolution, loss of lithium inventory, and resistance build-up are evaluated. In particular, the pronounced cathode impedance build-up of polycrystalline NCMs, investigated by electrochemical impedance spectroscopy using a micro-reference electrode in full-cells, exposes the impact of particle cracking and the induced electronic resistances within a secondary agglomerate on the rate capability.

Adaptation of a PEMFC Reference Electrode to PEMWE: Possibilities and Limitations

This work presents a platinum wire micro-reference electrode suitable for recording in-operando individual electrochemical impedance spectra of both the anode and the cathode in a proton exchange membrane water electrolyzer. The setup enables the protonic membrane potential to be accessed outside the active area. The reference electrode allows for the analysis of kinetic parameters for anode and cathode reactions separately. However, the exact location of the measured membrane potential is unknown due to a potentially asymmetric potential distribution, which makes the calculation of exact kinetic parameters non-trivial. Qualitative analyses of the potential distribution behavior over temperature, cell compression, and current density are shown.

Electrochemical Multiplexed N-Terminal Natriuretic Peptide and Cortisol Detection in Human Artificial Saliva: Heart Failure Biomedical Application

The early detection at low concentration, by non-invasive methods, of cardiac biomarkers in physiological fluids has attracted the interest of researchers over the last decade. This enables early diagnosis and prediction of the first signs of heart failure (HF). In this respect, the analysis of human saliva remains the most suitable medium for this non-invasive approach, as it contains a highly interesting biological matrix for general health and disease monitoring. In this work, we developed a highly sensitive multiplexed immunosensor for direct simultaneous detection of both N-terminal Natriuretic Peptide (NT-proBNP) and Cortisol in human artificial saliva (AS). The developed biosensor platform based on silicon nitride substrate was composed from four gold working microelectrodes (WEs) and an integrated counter and reference microelectrode. Gold WEs were biofunctionalized through carboxyl diazonium (4-APA) to immobilize both anti-NT-proBNP and anti-Cortisol antibodies for simultaneous detection. The electroaddressing of the 4-APA onto the gold WE surfaces was realized with cyclic voltammetry (CV), while the interaction between antibodies and antigens in PBS was monitored using electrochemical impedance spectroscopy (EIS). The antigen detection in human AS was realized with EIS combined with the standard addition method. The immunosensor was highly sensitive and selective toward the corresponding biomarkers in both PBS and artificial human saliva as well as in the presence of other potential interfering biomarkers such as tumor necrosis factor alpha (TNF-α) and interleukin-10 (IL-10). The limit of detection (LOD) was at 0.2 pg/mL for NT-proBNP within the range of 0.03 to 0.9 pg/mL, while the LOD for Cortisol was 0.06 ng/mL within the range of 0.02 to 0.6 ng/mL for Cortisol in artificial saliva. The developed immunosensor is very promising for significant detection in physiological media, and time reducing as it allows the simultaneous detection of various biomarkers.

A Micro-Reference Electrode for Electrode-Resolved Impedance and Potential Measurements in All-Solid-State Battery Pouch Cells and Its Application to the Study of Indium-Lithium Anodes

Impedance measurements are a powerful tool to investigate interfaces in lithium-ion batteries (LIBs). In order to deconvolute the anode and cathode contributions to the cell impedance, a reference electrode (RE) is required. However, there are only very few reports on the use of a three-electrode setup with an RE for all-solid-state batteries (ASSBs), which is due to the complexity of integrating an RE with a suitable geometry into the typical ASSB test cells that are based on a compressed electrolyte pellet. In this study, we present a straightforward approach to implement a micro-reference electrode (μ-RE) for electrode-resolved impedance and potential measurements into ASSB pouch cells. The μ-RE consists of an insulated ∼64 μm diameter gold wire that is sandwiched between two Li6PS5Cl/polymer separator sheets and activated by in situ electrochemical lithiation. Using this μ-RE, we investigate the electrode potential and the accessibility of cyclable lithium at the separator interface of indium-lithium anodes, which are prepared by stacking lithium and indium foils with a molar excess of indium. We compare two different cell assembly configurations, with the separator faced by either (i) the formerly In-side or (ii) the formerly Li-side, showing that only the latter case provides a reservoir of cyclable lithium.

Development of an EGFET microsensor with 3D structure for high-specificity cardiac troponin I detection

In this study, we used microelectromechanical systems (MEMS) technology to design a three-dimensional microsensor, which is based on an extended gate field-effect transistor (EGFET), to detect cardiac troponin (cTnI). A planar micro-reference electrode was integrated onto the same chip. All fabrication processes were compatible with integrated circuit and MEMS technologies, and the dimension of our cTnI microsensor was 1.0 cm × 0.8 cm. From measurements, microsensor sensitivity was 11.408 V (ng ml−1)−1 and sensing linearity was 0.961 in the 0–0.05 ng ml−1 range. In the 0.05–1 ng ml−1 range, sensitivity was 0.887 V (ng ml−1)−1 and linearity 0.933. The detection limit was 0.0063 ng ml−1, and the response time was approximately 300 s. Bridge-type source read-out, temperature correction and temperature compensation circuits were also successfully designed and were used to reduce the influence of temperature on sensing signals.

Implications of the Thermal Stability of FEC-Based Electrolytes for Li-Ion Batteries

Fluoroethylene-carbonate (FEC) is a common co-solvent for high-voltage cathodes and for silicon-based anodes in lithium-ion batteries. However, FEC has a limited thermal stability when used with LiPF6 as conductive salt, and its decomposition can trigger detrimental side reactions. Here, we will examine the reaction mechanism of FEC with LiPF6, confirming that vinylene-carbonate (VC) and HF are produced at elevated temperatures. By full-cell cycling at 45 °C in a T-cell setup with a micro-reference electrode (μ-RE), we can show by electrochemical impedance spectroscopy that these side reactions not only lead to an impedance increase of the anode and the cathode, but also trigger transition metal dissolution. By comparison of FEC and ethylene-carbonate (EC) as cyclic carbonate, we demonstrate that FEC has no advantage at high-voltage operation compared to EC, when employing cathode materials or cathode potentials for which no lattice oxygen is evolved. Finally, we use multi-layer pouch-cells to analyze the gassing of an EC- and FEC-based electrolyte upon extended charge/discharge cycling at 45 °C, showing that the latter leads to cell bulging upon extended charge/discharge cycling at 45 °C due to the oxidation of the VC formed by the thermal decomposition of FEC above ≈4.4 − 4.5 V vs. Li+/Li.

Split aptamers immobilized array microelectrodes for detection of chlorpyrifos pesticide using electrochemical impedance spectroscopy

Pesticide contamination in foods and its high persistence in environments are major serious issues globally. Chlorpyrifos (CPS), a broad-spectrum organophosphate insecticide, is commonly used as an active ingredient for pest control in agricultures and industries, and eventually it is found in high prevalence of contamination in food chains. Unfortunately, CPS exerts several adverse effects, especially as a neurotoxic agent, that cause severe health implications on humans and animals. To prevent the overexposure of pesticide residues in contaminated foods and environments, a label-free impedimetric aptamer-based biosensor for CPS determination was developed. The split aptamers for CPS were designed from its parent sequence to encounter nonspecific binding, and were immobilized onto the array microelectrodes consisting of four gold (Au) working microelectrodes, a platinum (Pt) counter microelectrode, and two Ag/AgCl reference microelectrodes. The aptasensor showed a good linear EIS response with CPS concentration ranging from 0.01 to 1 μg/mL with a detection limit (LOD) of 0.01 μg/mL. Moreover, the aptamer-immobilized Au multi-microelectrodes provided excellent selectivity to CPS over high level of other interferences, yet it also revealed good reproducibility. The sensitivity of CPS was successfully determined with acceptable recoveries in the range of 75–128% in spiked coriander, orange juice, and tap water. Therefore, the split aptamer-functionalized microchip could be a potential biosensor for monitoring CPS contamination in fruits, vegetables, and environments to improve human health and well-being.

Monitoring the Electrochemical Capacitance By in Situ Impedance Spectroscopy As Indicator for Particle Cracking of (Nickel-Rich) Cathode Active Materials: Development of a Simplified Measurement Setup

Due to their high specific capacity, the market share of nickel-rich layered lithium nickel cobalt manganese oxides (NCMs, LiNixCoyMnzO2, x+y+z = 1) as cathode active materials (CAMs) for lithium-ion batteries is constantly growing. With higher nickel content, however, the increased capacity is often accompanied by shorter cycle life due to undesired side reactions on the CAM/electrolyte interface. As the optimization of the electrochemical performance of lithium-ion batteries by the adjustment of the composition of the cathode active materials has come to a limit, the focus has shifted to the modification of the morphological aspects. One way to minimize side reactions is to reduce the specific surface area of the CAM through a greater NCM crystallite size or through a customized particle morphology, both exhibiting less particle cracking upon charge/discharge cycling [1]. However, new methodologies for the quantification of aspects such as particle size, particle cracking, and surface area change are needed.Recently, we have developed a novel in situ analytical method which is able to quantify the increase of the CAM surface area upon extended charge/discharge cycling using electrochemical impedance spectroscopy (EIS) [2]. There, we make use of the direct correlation between the capacitance and the surface area of the electrode, whereby an increase of the capacitance indicates cracking of CAM particles, caused by the repeated volume change of the NCM material upon (de)lithiation and/or oxygen release at high state of charge [3]. In these works, the direct relationship between capacitance and NCM particle surface area was validated by ex situ krypton physisorption measurements.Unfortunately, this impedance-based method relies on a sophisticated experimental setup, using a micro-reference electrode (i.e., a gold-wire µ-RE) and a partially pre-lithiated lithium titanate (LTO) counter electrode. Therefore, in this study, we deduce a stepwise simplification of the capacitance measurements from the setup with a µ-RE and a pre-lithiated LTO counter electrode to a conventional coin half-cell setup, which is commonly used in industry for quick and routine material benchmarking. Additionally, it will be shown that the capacitance does not have to be extracted from a full impedance spectrum provided by an impedance analyzer, but that it can be obtained solely from a low-frequency single-point impedance measurement performed at a battery cycler. The working principle of this approach is validated using four different cell and potentiostat / battery cycler configurations over several charge/discharge cycles [4].The here demonstrated setup provides a simple method suitable for conventional coin half-cells to identify surface area changes of cathode active materials, being easily implemented in standard cycling procedures. Reference s [1] W. Li, E. M. Erickson, and A. Manthiram, Nature Energy 5 26 (2020).[2] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J. Electrochem. Soc. 167 100511 (2020).[3] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J . Electrochem . Soc . 168 120501 (2021).[4] S. Oswald, F. Riewald, H. A. Gasteiger, manuscript in preparation. Acknowledgements This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.

A Micro-Reference Electrode for Impedance and Potential Measurements in All-Solid-State Battery Pouch Cells

Electrochemical impedance spectroscopy (EIS) is a powerful and versatile tool to investigate interfaces in batteries. In order to disentangle the anode and cathode contributions from the full-cell impedance, a reference electrode (RE) is required. In the field of batteries based on liquid electrolytes, the concept of a RE has become a widespread tool for the EIS analysis of small-scale cells [1]. However, there are only very few reports on the use of a three-electrode setup with a reference electrode for all-solid-state batteries (ASSBs) [2,3], which is due to the complexity of integrating a RE with a suitable geometry in the typical ASSB test cells that are based on a compressed electrolyte pellet (further on referred to as bulk-type ASSB cells), since for artifact-free single-electrode impedance spectra, the RE should be placed between the electrodes and should be thin compared to the thickness of the pellet. In contrast to the widely used bulk-type ASSB cells, a recently available alternative construction is offered by the use of free-standing separator sheets based on a solid electrolyte / polymer binder composite (further on referred to as sheet-type ASSB cells),[4,5] in which case a micro-RE can be placed between two separator sheets, in analogy to the micro-RE concept used with batteries based on liquid electrolytes [1].In this study, we use sheet-type separators based on a composite consisting of Li6PS5Cl (LPSCl) solid electrolyte and a hydrogenated nitrile butadiene rubber (HNBR) binder to build ASSB pouch cells that include a gold wire micro-RE (µ-GWRE). We show that upon in-situ lithiation of the µ-GWRE a stable reference potential is obtained and that artifact-free single-electrode impedance spectra can be obtained, analogous to what we had found previously for a µ-GWRE in a lithium ion battery with liquid electrolyte.[1] Figure 1 shows both half-cell impedance spectra of an InLi | separator sheet | Li cell. The sum of both half-cell impedances (blue) is identical to the full-cell impedance (green) and now impedance loops or other common artefacts are observed for the InLi (black) and the Li (red) electrodes, indicating the viability of this setup to determine single-electrode impedances. Since the InLi electrode is commonly used as counter electrode (CE) for ASSB testing cells, we will also use this setup to investigate the potential stability of InLi alloys and their impedance evolution upon lithiation and delithiation. Acknowledgements: This work was carried out as part of the research project “Industrialisierbarkeit Festkörperelektrolytzellen”, funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy.

Deep Eutectic Solvents Enhance Stability of Ag/AgCl Solid State Miniaturised Reference Electrode

A new class of solid-state miniaturised reference electrodes with a deep eutectic solvent as an alternate enhancement electrode system is described. A simple and accurate stable electrochemical sensor was prepared by developing a conventional reference electrode using an Ag/AgCl planar micro-reference electrode covered with a PVC polymer. A conductive deep eutectic solvent (DES), ethaline, was added in small quantities and mixed with an internal electrolyte to maintain the Cl− ion concentration in the constructed electrode. The fabricated microelectrode showed good stability, reproducibility, and long-term stability against varying concentrations of different ions. The potential response of the fabricated microelectrode was studied under varying concentrations of Cl− ions in the presence of 0.1 to 1.0% DES in a concentrated electrolyte system (20 mM Na2SO4). The stability of the fabricated microelectrode was addressed against Br− and Cl− ions using different inorganic salts, and the potential measurements were found to be insensitive toward all responsive ions. The stability response of the fabricated microelectrode against Cl− ions was optimised in the presence of 1.0% DES. The experimental data showed good agreement with the potential change of the fabricated electrode in the presence of the supporting DES electrolyte. The liquid junction-free PVC solid-state miniaturised reference electrode demonstrated a constant potentiometric measurement over a long period of time. The concentrated supporting DES electrolyte solution (20 mM) exhibited better stability values and was a more suitable fabricated microelectrode than other additive concentrations. The long-term stability of the developed microelectrode displayed a good lifetime and high stability of around 60 days.

Novel Method for Monitoring the Electrochemical Capacitance by In Situ Impedance Spectroscopy as Indicator for Particle Cracking of Nickel-Rich NCMs: Part II. Effect of Oxygen Release Dependent on Particle Morphology

Nickel-rich NCMs (LiMO2, with M = Ni, Co, and Mn) are increasingly commercialized as cathode active materials for lithium-ion batteries due to their high specific capacity. However, the available capacity is limited due to their structural instability at high state of charge, causing the formation of a resistive surface layer upon release of lattice oxygen, observed at different upper cutoff potentials depending on the NCM composition. To understand the impact of this instability, the correlation of oxygen release, capacity fading, and particle cracking was investigated as a function of state of charge for three nickel-rich NCMs, differing either in composition (i.e., in transition metal ratio) or in morphology (i.e., in primary crystallite size). First, the onset of the release of lattice oxygen was identified by on-line electrochemical mass spectrometry (OEMS). In electrochemical cycling experiments, the NCM capacitance was tracked in situ by impedance spectroscopy (EIS) using a micro-reference electrode while the upper cutoff potential was increased every third cycle stepwise from 3.9 V to 5.0 V. Hereby, the effect of the degree of delithiation on the discharge capacity and on the particle integrity (tracked via its surface area) was examined, both for poly- and single-crystalline NCMs.