Capacitance Of Electrode Research Articles

Simple and affordable synthesis of a binder-free nickel-zinc sulfide electrode for high-performance supercapacitors

Due to the increasing demand for renewable energy solutions, aqueous energy systems have emerged as promising and sustainable technologies. Nanostructured transition metal sulfides, renowned for their exceptional properties, are prime candidates for electrochemical supercapacitors. In this study, we present the synthesis of nickel-zinc sulfide (Ni-Zn-S) as a binder-free supercapacitor material using the affordable successive ionic layer adsorption and reaction simple technique. The active nanomaterial was directly deposited onto a 3D Ni foam current collector substrate, negating the need for a binder. The synergistic combination of Ni and Zn enhances the electrode capacitance. Various electrochemical techniques were used to evaluate the loaded active material as a binder-free supercapacitor electrode in a 6 M KOH electrolyte. Results indicate pseudocapacitive performance, achieving a high specific capacitance of 400F/g at 0.7 A/g. This material retains 97 % of its capacity, even after 2000 cycles, showcasing exceptional long-term cycling stability. These findings underscore the potential of the Ni-Zn-S electrode for delivering low-cost, high-performance supercapacitors.

Prelithiation of electrodes in lithium-ion capacitors: A review

Lithium-ion capacitors (LICs) are one of the modern state-of-the-art hybrid capacitors, comprising a high potential window and imparting a higher energy density than supercapacitors (SCs). These LICs encompass elevated power density and a longer life span than lithium-ion batteries (LIBs). Preparation of high-performance electrode materials with electrochemically active microstructure and prelithiation are two efficient approaches to fabricate highly efficient LICs. But it comes across as a real dilemma of low initial Columbic efficiency if only microstructure is considered as an efficient way to enhance the performance. Nevertheless, prelithiation plays a crucial role in the manufacturing of LICs, improving the initial Coulombic efficiency and enlarging the voltage window. This paper reviews the recent lithiation approaches for lithium-ion capacitors by providing their methods and discussing their results concerning their energy and power density.

Machine Learning-Assisted Survey on Charge Storage of MXenes in Aqueous Electrolytes.

Pseudocapacitance is capable of both high power and energy densities owing to its fast chemical adsorption with substantial charge transfer. 2D transition-metal carbides/nitrides (MXenes) are an emerging class of pseudocapacitive electrode materials. However, the factors that dominate the physical and chemical properties of MXenes are intercorrelated with each other, giving rise to challenges in the quantitative assessment of their discriminating importance. In this perspective, literature data on the specific capacitance of MXene electrodes in aqueous electrolytes is comprehensively surveyed and analyzed using machine-learning techniques. The specific capacitance of MXene electrodes shows strong dependency on their interlayer spacing, where confined H2O in the interlayer space should play a key role in the charge storage mechanism. The electrochemical behavior of MXene electrodes is overviewed based on atomistic insights obtained from data-driven approaches.

Urea–acetamide-based deep eutectic compound as novel, eco-friendly additives in stable and efficient dye-sensitized solar cells: A performance and electrochemical study

Deep eutectic solvents of urea and acetamide at molar ratios of 1:1 (UA11) and 1:2 (UA12) were investigated as new, eco-friendly, and effective electrolyte additive candidates to replace the commonly used 4‑tert-butylpyridine additive in dye-sensitized solar cells (DSCs). Both UA11 and UA12 resulted in a high open circuit potential (VOC) of about 745 mV, which was very competitive to a VOC of 754 mV from DSCs using 4‑tert-butylpyridine (4-TBP). After 2240 h of storage, performance dropped 8% in devices using UA11 compared to about 25% of devices using 4-TBP as an additive. Our time-series data analysis revealed the correlation between the short circuit current (JSC) drop and the increase of counter-electrode resistance. In addition, by examining the VOC correlation between working electrode resistance and working electrode capacitance in four electrolyte groups (NAd, UA11, UA12, 4-TBP), we could speculate the effects of the DES urea-acetamide as well as 4-TBP. 4-TBP could improve the VOC by adsorption to the bare TiO2 surface and diminish the recombination reaction. On the other hand, 4-TBP could interact with iodine species in the electrolyte solution to minimize the current leakage from the TiO2 surface to the iodine species. In the case of UA11 and UA12, the additive seemed to interact with the iodine species only.

Novel Binder with Cross-Linking Reconfiguration Functionality for Silicon Anodes of Lithium-Ion Batteries.

Silicon is expected to be used as a high theoretical capacity anode material in lithium-ion batteries with high energy densities. However, the huge volume change incurred when silicon de-embeds lithium ions, leading to destruction of the electrode structure and a rapid reduction in battery capacity. Although binders play a key role in maintaining the stability of the electrode structure, commonly used binders cannot withstand the large volume expansion of the silicon. To alleviate this problem, we propose a PGC cross-linking reconfiguration binder based on poly(acrylic acid) (PAA), gelatin (GN), and β-cyclodextrin (β-CD). Within PGC, PAA supports the main chain and provides a large number of carboxyl groups (-COOH), GN provides rich carboxyl and amide groups that can form a cross-linking network with PAA, and β-CD offers rich hydroxyl groups and a cone-shaped hollow ring structure that can alleviate stress accumulation in the polymer chain by forming a new dynamic cross-linking coordination conformation during stretching. In the half cell, the silicon negative prepared by the PGC binder exhibited a high specific capacity and capacity maintenance ratio, and the specific capacity of the silicon negative electrode prepared by the PGC binder is still 1809 mAh g-1 and the capacity maintenance ratio is 73.76% following 200 cycles at 2 A g-1 current density, indicating that PGC sufficiently maintains the silicon negative structure during the battery cycle. The PGC binder has a simple preparation method and good capacity retention ability, making it a potential reference for the further development of silicon negative electrodes.

Non-destructive electrode potential and open-circuit voltage aging estimation for lithium-ion batteries

In this publication we extend a state-of-the-art electrode open circuit potential model for blend electrodes and inhomogeneous lithiation. We introduce a bi-level optimization algorithm to estimate the open parameters of the electrode model using measurements conducted on the full-cell level with state-of-the-art testing equipment. As input for the optimization algorithm, we use data from a pseudo open-circuit voltage aging study to estimate the electrode potential curves, the electrode capacities, and the capacity of cyclable lithium over lifetime without opening the cell. We validate our method against measurements on commercial lithium-ion cells with the electrode materials LFP-C, NMC-SiC and NCA-SiC, and compare the estimated electrode pseudo open-circuit potential curves to measurements conducted on harvested half-cells. We conclude that the presented non-destructive method of half-cell open-circuit potential modeling offers an alternative to complex cell disassembly and electrode material harvesting for degradation mode analysis.

Enhancing electrochemical capacitor performance of N-doped tannin-derived carbons by hydrothermal treatment in ammonia

This study demonstrates that ammonia concentration when doping carbons using hydrothermal treatment has crucial impact on textural properties. The latter translates into an improvement of the electrochemical performance of carbon materials, when used into electrochemical capacitors, especially at high-rate performance. Nitrogen-doped carbons were synthesized by subjecting tannin to hydrothermal carbonization in ammonia solutions of varying concentrations. After carbonization and CO2 activation, the as-produced activated carbons (ACs) were tested as electrodes for electrochemical capacitors in symmetric configuration using 1 M H2SO4 as aqueous electrolyte. Interestingly, ammonia concentration during the hydrothermal synthesis step did not significantly affect the final N content (ca. 3 and 4 at. %) nor the nitrogen and oxygen functionalities on the surface. However, the use of ammonia had crucial impact on the textural properties developed by CO2 activation, and therefore on the electrochemical performance of the ACs. The best-performing N-doped AC showed specific electrode capacitance values, based on carbon material, of 212 F g−1 at 0.5 A g−1 and outstanding capacitance retention of ca. 71 % at 40 A g−1. It also showed high cycling stability, with capacitance retention of ca. 96 % after 30,000 cycles. Furthermore, this AC outperformed similar reported materials, achieving a specific energy of 4.6 W h kg−1 at 12.1 kW kg−1.

Electrophysiological Recording from a "Model" Cell.

The muscle cell or neuron membrane is functionally equivalent to a resistor-capacitor (RC) circuit with the membrane resistance and capacitor in parallel. Once inserted inside the membrane, an electrode introduces a serial resistance and small capacitance to the RC circuit. Through a narrow opening at its tip (∼0.1-μm), current can pass through the electrode, into the cell, and back to the outside (ground) across the membrane to complete the circuit. This arrangement enables a voltage difference between the outside and inside of the cell membrane to be recorded. To determine cell membrane properties, a current can be injected into the cell through the electrode. One complication with this approach, however, is that the voltage difference measured with the electrode includes the voltage drop across the cell membrane and that across the electrode. Furthermore, a small amount of current is drawn by the electrode capacitor, thereby slowing the current flow across the membrane. Fortunately, most amplifiers are equipped with bridge balance and capacitance compensation functions so that the effects of the electrode on cell membrane properties can be canceled out or minimized. This protocol describes the basics of setting up and conducting electrophysiological experiments using a model cell. For the novice, a model cell provides a way to learn the operation of electrophysiology equipment and software without the anxiety of damaging living cells. This protocol also illustrates passive membrane properties such as the input resistance, capacitance, and time constant.

The Progress of Polymer Composites Protecting Safe Li Metal Batteries: Solid-/Quasi-Solid Electrolytes and Electrolyte Additives.

The impressive theoretical capacity and low electrode potential render Li metal anodes the most promising candidate for next-generation Li-based batteries. However, uncontrolled growth of Li dendrites and associated parasitic reactions have impeded their cycling stability and raised safety concerns regarding future commercialization. The uncontrolled growth of Li dendrites and associated parasitic reactions, however, pose challenges to the cycling stability and safety concerns for future commercialization. To tackle these challenges and enhance safety, a range of polymers have demonstrated promising potential owing to their distinctive electrochemical, physical, and mechanical properties. This review provides a comprehensive discussion on the utilization of polymers in rechargeable Li-metal batteries, encompassing solid polymer electrolytes, quasi-solid electrolytes, and electrolyte polymer additives. Furthermore, it conducts an analysis of the benefits and challenges associated with employing polymers in various applications. Lastly, this review puts forward future development directions and proposes potential strategies for integrating polymers into Li metal anodes.

Constructing ionic highway in capacitive deionization via integrated electrodes based on bifunctional ionomer

In this work, bifunctional ionomeric capacitive deionization (BICDI) electrode are constructed by using bifunctional ionomer as electrode binder and ion exchange material for the enhancement of desalination performance. Compared with traditional membrane capacitive deionization (MCDI) electrodes, the BICDI electrodes were fabricated by using the poly(arylene alkylene) based ionomer to substitute the PVDF binder and ion exchange membrane (IEM). The BICDI electrode based on bifunctional ionomer exhibits higher hydrophilicity, special ion exchange capability, as well as excellent electrochemical properties in comparsion with that of MCDI electrode counterpart. The built-in ionic groups in poly(arylene alkylene) based ionomers could create interconnected ionic highway between different electrode particles, which encourages effective ion transport. The desalination results validated that the salt adsorption capacity (SAC) of carbon-based BICDI electrodes reached 19.2 mg g−1, which is 2.2 times that of MCDI electrodes during adsorption-desorption cycles. Meanwhile, BICDI electrode possesses exceptional charge efficiency (96.6 %), which is much higher than that of MCDI (76.8 %). In the case of BICDI device, the ionomer utilization rate increased by a factor of 88 compared to that of ion exchange membrane in MCDI device. Especially, the economic cost of the integrated BICDI electrodes module is only $35 m−2, which is much cheaper than that in conventional MCDI system ($520 m−2). It is reasonable to believe that the integrated BICDI electrode incorporated with charged bifunctional ionomer proposed in this work provide novel insights and strategies for the high-performance and large-scale CDI applications.

Nano-alumina islands enabling superior capacitive lithium-ion storage of pitaya configuration

Currently, high-temperature thermal failure hinders the development of lithium-ion batteries (LIBs). Interfacial side reaction of electrolyte and electrode is the main cause of thermal failure at high temperature. Explicitly, suppressing interfacial side reactions at high temperatures can help the research of capacitive energy storage electrodes. While, thermo-electrochemical inert alumina can be facilitated to solve this problem. In this paper, pitaya configuration of nano-alumina islands uniformly dispersed in carbon matrix can be achieved from NOTT-300(Al). Nano-alumina contributes to the structural stability of porous carbon during carbonization and lithium deintercalation. And nano-alumina islands are effective in suppressing interfacial side reactions at high temperatures. Thus, NC-3 exhibits competitive specific capacity (717.1 mAh/g) and cycling stability (99.15% after 500 cycles). NC-3's capacity was confirmed to be mainly attributed to capacitive energy storage mechanism. This study provides a new research paradigm for nano-alumina to improve the high-temperature thermal stability management of LIBs.

TiH2-based anode: In situ formation of solid electrolyte for high volumetric energy density with minimal content of conducting agent

Herein, we report the fabrication of an electrode composite with an ultra-high active material content (90 wt%). This composite was possible because of the utilization of TiH2 as an anode active material that forms a solid electrolyte (LiH) in situ and functions as an electronic conductor, enabling a substantial reduction of the amount of conductor additive in the electrodes. The electrochemical performance of the TiH2-based electrodes with a high active material content was comparable to that of electrode composites that include a solid electrolyte because of sufficient Li+-ion conduction pathways provided by the LiH formed in situ. To further increase the capacity of the TiH2-based electrode, MgH2 (theoretical capacity: 2036 mAh/g) was mixed with TiH2 to function as an active material. TiH2, which is known to exhibit high electron conductivity and a high density, is expected to be used as a partial substitute for low-density carbon additives to increase the volumetric energy density of batteries and their active material content. Hence, the xMgH2-(1 − x)TiH2-AB (AB: acetylene black) electrode composite exhibited a high active material content (90 wt%) and improved volumetric energy density (1687 Wh/L, based on the anode) compared with a MgH2-AB (70:30 wt%) electrode composite (1157 Wh/L), even when the AB content of the xMgH2-(1 − x)TiH2-AB composite was 10 wt%. In addition, a MgH2-TiH2-VGCF (VGCF: vapor-grown carbon fiber) based electrode composite exhibited the highest volumetric and gravimetric energy density (2154 Wh/L, 1888 Wh/kg) among the investigated composites, facilitated by sufficient electron pathways provided by VGCF.

Highly active and robust A-site deficient La0.77Sr0.2Co0.2Fe0.8O3-δ fiber base electrode for intermediate temperature solid oxide cells

The combination of solid oxide electrolysis cells (SOECs) and solid oxide fuel cells (SOFCs) is expected to solve the problems of energy conversion and storage. However, the insufficient catalytic capacity and poor durability of the oxygen electrodes in solid oxide cells (SOCs) at intermediate temperatures pose a huge challenge for SOCs applications. In this work, A-site deficient La0.77Sr0.2Co0.2Fe0.8O3-δ nanorod materials coated with Ce0.8Gd0.2O1.9 (GDC) are prepared to suppress cation surface segregation and enhance charge transfer kinetics by reducing internal elastic force of perovskite and meanwhile applying external compressive stress with an oxygen-ion conductor. As a result, the composite cathode with a GDC to La0.77Sr0.2Co0.2Fe0.8O3-δ weight ratio of 0.59 (La0.77Sr0.2Co0.2Fe0.8O3-δ@GDC0.59) exhibits excellent electrochemical performance and long-term stability when operating in both SOFCs and SOECs modes. XPS results show that the La0.77Sr0.2Co0.2Fe0.8O3-δ@GDC0.59 oxygen electrode exhibits no significant Sr/Fe surface segregation after operating in SOFCs and SOECs modes for 200 h. Density functional theory calculation and physiochemical characterization confirm that Sr segregation phenomenon is well inhibited through the novel A-site deficient structural design of perovskite materials, which eliminates the major residual internal elastic force of La0.8Sr0.2Co0.2Fe0.8O3-δ crystal, and GDC coating further relieves the lattice transformation of La0.77Sr0.2Co0.2Fe0.8O3-δ upon the additional introduction of A-site deficiency and oxygen vacancy. This well-orchestrated composite cathode design provides a new perspective in stabilizing perovskite crystalline structure toward high-performance SOCs with extended operating life.

Energy-dense aqueous Zn//MnO2 rechargeable battery constructed on dual three-dimensional conductive skeletons

Enhancing the energy density of aqueous rechargeable zinc-based batteries is critical for boosting their application in a variety of practical scenarios. Herein we demonstrate a strategy to improve specific energies of Zn//MnO2 batteries in mild electrolyte via utilizing three-dimensional (3D) current collectors for both the anode and cathode. By electrodepositing limited amounts of Zn on surface-alloyed Ag mesh and coupling it with MnO2 cathode electrodeposited on carbon cloth with no binder, the resultant batteries exhibit specific energy up to 253 and 196 Wh/kg (based on total mass of active materials) at negative-to-positive electrode capacity (N/P) ratios of 1:1 and 2:1. By elevating the working temperature to 35 °C, the specific energy and life span of the battery with N/P = 2 can be further increased to 216 Wh/kg and >225 cycles, respectively. The key to achieving a high specific energy relies primarily on the great reversibility of the Zn anode, which is conferred by the 3D Ag mesh.

3D porous structure Fe3O4@SnO2/MXene composites with enhanced electrochemical performance for lithium ion battery anode

The low theoretical capacity of the commercial graphite electrodes used in lithium-ion batteries (LIBs) necessitates the development of novel anode materials with higher rate performance, reversible specific capacities, long cycle lives, and low costs. In this work, three-dimensional porous structure Fe3O4 nanosphere with extremely thin SnO2 coatings are prepared by a method of getting twice the result with half the effort. And it was uniformly dispersed in an MXene flexible conductive matrix to fabricate a 3D porous heterogeneous Fe3O4@SnO2/MXene composite to act as an anode material for LIBs. The 3D porous heterojunction structure integrates the advantages of Fe3O4@SnO2 and MXene, achieving rapid electron and ion transport at the interface and the effective release of structural stress. The introduction of MXene markedly enhances the conductibility of the materials, limits the expansion of Fe3O4 during the Li+ insertion/removal process, and inhibits its aggregation. The Fe3O4@SnO2/MXene anode exhibits excellent electrochemical performance (1609 mAh g−1 over 200 cycles at 100 mA g−1 and 626.1 mAh g−1 over 900 cycles at 1000 mA g−1). These findings demonstrate that the Fe3O4@SnO2/MXene composite is a promising anode material for LIBs.

Controlling Electrodeposition in Nonplanar High Areal Capacity Battery Anodes via Charge Transport and Chemical Modulation.

Controlling the morphological evolution of electrochemical crystal growth in battery anodes is of fundamental and practical importance, particularly towards realizing practical, high-energy batteries based on metal electrodes. Such batteries require highly reversible plating/stripping reactions at the anode to achieve a long cycle life. While conformal electrodeposition and electrode reversibility have been demonstrated in numerous proof-of-concept experiments featuring moderate to low areal capacity (≤3 mA h/cm2) electrodes, achieving high levels of reversibility is progressively challenging at the higher capacities (e.g., 10 mA h/cm2), required in applications. Nonplanar, "3D" electrodes composed of electrically conductive, porous substrates are conventionally thought to overcome trade-offs between reversibility and capacity because they hypothetically "host" the electrodeposits in an electronically conducting framework, providing redundant pathways for electron flow. Here, we challenge this hypothesis and instead show that a nonplanar substrate with moderate electrical conductivity (ideally, with an electrical conductance similar to the ionic conductance of the electrolyte) and composed of a passivated cathode-facing surface efficiently regulates electro-crystallization. In contrast, an architecture with a high intrinsic electrical conductivity or with a high electrical conductivity coating on the front surface results in dominantly out-of-plane growth, making the 3D architecture in effect function as a 2D substrate. Using Zn as an example, we demonstrate that interconnected carbon fiber substrates coated by SiO2 on the front and Cu on the back successfully ushers electroplated Zn metal into the 3D framework at a macroscopic length scale, maximizing use of the interior space of the framework. The effective integration of electrodeposits into the 3D framework also enables unprecedented plating/stripping reversibility >99.5% at high current density (e.g., 10 mA/cm2) and high areal capacities (e.g., 10 mA h/cm2). Used in full-cell Zn||NaV3O8 batteries with stringent N/P ratios of 3:1, the substrates are also shown to enhance cycle life.

Magnetic Force Dilatometry of Silicon-NMC622 Lithium-Ion Coin Cells: The Effects of Binder, Capacity Ratio, and Electrolyte Selection

Operando cell expansion measurements on Si-NMC622 coin cells using a magnetic dilatometer were performed to understand the effects of electrode binder content, electrode formulation, negative-to-positive electrode capacity ratio (N/P ratio), and electrolyte selection on reversible and irreversible cell expansions. Our experiments reveal a complex relationship between cell properties, imparted by the selected cell parameters, and cell expansion. Reversible cell expansions scaled with cell discharge capacity and electrode mechanical properties, while irreversible cell expansions were sensitive to capacity fade, silicon utilization, and electrolyte decomposition mechanisms. Additionally, volumetric cell energy densities were calculated using the measured capacities and irreversible expansions over the life of the cells. We show that judicious selection of cell parameters can improve volumetric energy density after 200 charge/discharge cycles by approximately two-fold. Our work provides valuable insight, at an early stage of cell development, towards minimizing the effects of cell expansion on battery cell, pack, and module designs.

Bioelectronic Direct Current Stimulation at the Transition Between Reversible and Irreversible Charge Transfer.

Many biological processes rely on endogenous electric fields (EFs), including tissue regeneration, cell development, wound healing, and cancer metastasis. Mimicking these biological EFs by applying external direct current stimulation (DCS) is therefore the key to many new therapeutic strategies. During DCS, the charge transfer from electrode to tissue relies on a combination of reversible and irreversible electrochemical processes, which may generate toxic or bio-altering substances, including metal ions and reactive oxygen species (ROS). Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrodes are emerging as suitable candidates for DCS to improve biocompatibility compared to metals. This work addresses whether PEDOT electrodes can be tailored to favor reversible biocompatible charge transfer. To this end, different PEDOT formulations and their respective back electrodes are studied using cyclic voltammetry, chronopotentiometry, and direct measurements of H2O2 and O2. This combination of electrochemical methods sheds light on the time dynamics of reversible and irreversible charge transfer and the relationship between capacitance and ROS generation. The results presented here show that although all electrode materials investigated generate ROS, the onset of ROS can be delayed by increasing the electrode's capacitance via PEDOT coating, which has implications for future bioelectronic devices that allow longer reversibly driven pulse durations during DCS.

Electrochemical hydrogen storage in carbon nitride electrode using modified reversible polymer electrolyte membrane fuel cell

The human population is now through a process of evolution, whereby there is a gradual shift towards the adoption and utilization of alternative energy sources and technologies for energy storage. In this transition process non-availability of renewable energy sources round the clock is one of the key hurdles. It is possible to meet continuous energy demands by storing excess renewable energy as hydrogen. The presented experimental investigation focuses on a modified reversible fuel cell (MRFC) with a solid carbon-based substance (carbon nitride) integrated into it as a host for electrochemical hydrogen storage. The method for electrode fabrication from carbon nitride powder and PTFE binder and a lab-scaled MRFC is disclosed. The developed MRFC with an integrated hydrogen storage electrode is put through electrochemical testing, and the results are presented. Galvanostatic charging and discharging is done, and the results are examined to ascertain the electrode's electrochemical hydrogen storage capacity. The electrochemical hydrogen storage capacity of the porous carbon nitride electrode is found to be 1.15 wt%, which is comparable to intermetallic metal hydride-based hydrogen storage mediums viz. AB2 and AB5. The obtained results showed the feasibility of a MRFC with an integrated carbon nitride-based hydrogen storage electrode, making it the first device of its kind. Besides, porous carbon nitride is proved to be a potential candidate for reversible electrochemical hydrogen adsorption. This will contribute to the clean energy sector and surely a step forward toward hydrogen economy.

Enhancing the stability of all-solid-state astringent taste sensor through annealed solid contact and slow-drying lipid/polymeric membrane

The stability of the all-solid-state electrode (ASSE) is influenced by its fabrication procedure, which has been insufficiently investigated. In this study, we report that by applying direct annealing of solid contact (polypyrrole-carbon black [PPy-CB]), and slow-drying the membrane during fabrication, electrode capacitance is improved which determines the stability of the ASSE-based astringent taste sensor. The deposition of the PPy-CB by direct annealing accelerated the sensor fabrication time and improved the capacitance. The slow-drying method led to a thinner and denser membrane, which consequently reduced bulk and charge-transfer resistance and improved the electrode capacitance. In synergy with the annealed PPy-CB, the proposed sensor exhibited an areal capacitance of 9090 μF/cm2. In addition, the sensor maintained its linear response to the standard astringent substance (tannic acid), selectivity against various interfering taste substances, and retained repeatable measurements for at least 27 d.