Statistical Analysis of the Measurement Noise in Dynamic Impedance Spectra.

Different strategies can be used to acquire dynamic impedance spectra during a cyclic voltammetry experiment. The spectra are then analyzed by fitting them with a model using a weighted non-linear least-squares minimization algorithm. The choice of the weighting factors is not trivial and influences the value of the extracted parameters. At variance with the classic electrochemical impedance spectroscopy, dynamic impedance measurements are performed under non-stationary conditions, making them typically more prone to errors arising from the voltage and current analog-to-digital conversion. Under the assumption that the noise in the voltage and current signals have a constant variance along the measurement and that it is uncorrelated between distinct samples, we calculate an expression for the expected variance of the error of the resulting immittances, which considers the specific procedure used to extract the spectra under the time-varying nature of the measurements. The calculated variance of the error is then used as a rigorous way to evaluate the weighting factors of the least-squares minimization, assuming that the fitted model is ideally exact and that there are no systematic errors. By exploring two classical electrochemical systems and fitting the measured spectra with a transfer function measurement model, namely with the Padé approximants, we show that the variance evaluated with our method captures the frequency dependence of the resulting residuals and can be used for reliably performing the complex non-linear least-squares fitting procedure.

Thermally Regenerable Redox Flow Battery.

The efficient production of energy from low-temperature heat sources (below 100 °C) would open the doors to the exploitation of a huge amount of heat sources such as solar, geothermal, and industrial waste heat. Thermal regenerable redox-flow batteries (TRBs) are flow batteries that store energy in concentration cells that can be recharged by distillation at temperature <100 °C, exploiting low-temperature heat sources. Using a single membrane cell setup and a suitable redox couple (LiBr/Br2 ), a TRB has been developed that is able to store a maximum volumetric energy of 25.5 Wh dm-3 , which can be delivered at a power density of 8 W m-2 . After discharging 30 % of the volumetric energy, a total heat-to-electrical energy conversion efficiency of 4 % is calculated, the highest value reported so far in harvesting of low-temperature heat.

Electrochemical Methods for Lithium Recovery: A Comprehensive and Critical Review.

Due to the ubiquitous presence of lithium-ion batteries in portable applications, and their implementation in the transportation and large-scale energy sectors, the future cost and availability of lithium is currently under debate. Lithium demand is expected to grow in the near future, up to 900 ktons per year in 2025. Lithium utilization would depend on a strong increase in production. However, the currently most extended lithium extraction method, the lime-soda evaporation process, requires a period of time in the range of 1-2 years and depends on weather conditions. The actual global production of lithium by this technology will soon be far exceeded by market demand. Alternative production methods have recently attracted great attention. Among them, electrochemical lithium recovery, based on electrochemical ion-pumping technology, offers higher capacity production, it does not require the use of chemicals for the regeneration of the materials, reduces the consumption of water and the production of chemical wastes, and allows the production rate to be controlled, attending to the market demand. Here, this technology is analyzed with a special focus on the methodology, materials employed, and reactor designs. The state-of-the-art is reevaluated from a critical perspective and the viability of the different proposed methodologies analyzed.

Cylindrical flowing-junction cell for the investigation of fluctuations and pattern-formation in miscible fluids.

We describe a flowing-junction cell with cylindrical symmetry suitable to investigate fluctuations and pattern formation at the diffusing interface between two miscible phases of a liquid mixture. The continuous outflow of the remixed fluid through a thin slit located at the midheight of the sample allows the preparation of an initially sharp interface. The system can be used in both gravity-stable and unstable conditions. In the stable case, the denser liquid is on the bottom of the cell and mass diffusion is the only active process for remixing the two liquids. Once the flow is stopped, one can investigate nonequilibrium fluctuations during free-diffusion in a binary mixture or double diffusive instabilities in multicomponent mixtures. Two horizontal transparent windows allow vertical mapping of the fluid flow by using shadowgraphy. In the unstable condition, with the denser fluid on top, stopping the radial flow at the interface gives rise to a Rayleigh-Taylor instability, which drives the denser liquid toward the bottom of the cell. The fact that the cell can maintain the system in the unstable condition shows that it is suitable to perform experiments under microgravity conditions. With respect to other free-diffusion cells, the proposed configuration has the advantage that the interface is extremely stable and flat, and that the experiments can be repeated by just flowing the cell with fresh liquids.

Asymmetric time-cross-correlation of nonequilibrium concentration fluctuations in a ternary liquid mixture.

Equilibrium phenomena are characterized by time symmetry. Thermodynamic fluctuations are also time-symmetric at equilibrium. Conversely, diffusion of a solute in a liquid in the presence of a gradient is a nonequilibrium phenomenon, which gives rise to long-range fluctuations with amplitude much larger than the equilibrium one for small enough wave number. In the case of diffusion in binary mixtures such fluctuations are time-symmetric, notwithstanding the fact that they are generated by a nonequilibrium condition. In this paper, we investigate diffusion of two solutes in a ternary liquid mixture by means of fluctuating hydrodynamics theory. We show that the time-cross-correlation function of the concentrations is not time-symmetric, hence showing that time symmetry is violated for such nonequilibrium fluctuations. We discuss the feasibility of experiments aimed at the detection of the asymmetry of the cross-correlation function of nonequilibrium concentration fluctuations in ternary mixtures, as envisaged in the Giant Fluctuations (NEUF-DIX) microgravity project of the European Space Agency.

Space-Charge Effects at the Li7La3Zr2O12/Poly(ethylene oxide) Interface.

Composites consisting of garnet-type Li7La3Zr2O12 (LLZO) ceramic particles dispersed in a solid polymer electrolyte based on poly(ethylene oxide) (PEO) have recently been investigated as a possible electrolyte material in all solid state Li ion batteries. The interface between the two materials, that is, LLZO/PEO, is of special interest for the transport of lithium ions in the composite. For obtaining the desired high ionic conductivity, Li+ ions have to pass easily across this interface. However, previous research found that the interface is highly resistive. Here, we further investigate the interface between Al-substituted LLZO and PEO-LiClO4 electrolytes in the frame of a theoretical description, which is based on space-charge layers. By theoretical calculations supported by experiments, we find that the interface is highly resistive. From the results, we have deduced that the highest contribution to this resistance comes from a high activation energy and not from electrostatic repulsion of lithium.

Dynamic Impedance Spectroscopy of Nickel Hexacyanoferrate Thin Films.

Dynamic multi-frequency analysis (DMFA) is capable of acquiring high-quality frequency response of electrochemical systems under non-stationary conditions in a broad range of frequencies. In this work, we used DMFA to study the kinetics of (de-)intercalation of univalent cations (Na+ and K+) in thin films of nickel hexacyanoferrate (NiHCF) during cyclic voltammetry. For this system, the classic stationary electrochemical impedance spectroscopy fails due to the instability of the oxidized form of NiHCF. We are showing that such spectra can be fitted with a physical model described by a simple two-step intercalation mechanism: an adsorption step followed by an insertion step. The extracted kinetic parameters are depending on the state of charge as well on the nature of the inserted cation.

Intercalation into a Prussian Blue Derivative from Solutions Containing Two Species of Cations.

Reversible mixed-ion intercalation in nonselective host structures has promising applications in desalination, mixed-ion batteries, wastewater treatment, and lithium recovery through electrochemical ion pumping. One class of host compound that possesses many of the requirements needed for such applications (cost effectiveness, fast ion kinetics, and stability in an aqueous medium) includes the Prussian blue derivatives. Herein, the fundamental process of intercalation of multiple cations is studied at the thermodynamic level by means of galvanostatic cycling. Nickel hexacyanoferrate is focused upon because of its stability and low potential for electrochemical process relative to other hexacyanoferrates. Various cations can be intercalated; large cations (K+ and NH4+ ) are intercalated at higher potentials than those of smaller cations (Na+ ). When mixtures of cations are present in solution, the potential profile is not qualitatively altered with respect to single-salt solutions, but the potential of (de-)intercalation is shifted; a simple thermodynamic model is introducted that is able to predict the potential and distribution at which intercalation takes place.

Nonequilibrium fluctuations during diffusion in liquid layers.

Theoretical analysis and experiments have provided compelling evidence of the presence of long-range nonequilibrium concentration fluctuations during diffusion processes in fluids. In this paper, we investigate the dependence of the features of the fluctuations from the dimensionality of the system. In three-dimensional fluids the amplitude of nonequilibrium fluctuations can become several orders of magnitude larger than that of equilibrium fluctuations. Notwithstanding that, the amplitude of nonequilibrium fluctuations remains small with respect to the concentration difference driving the diffusion process. By extending the theory to two-dimensional systems, such as liquid monolayers and bilayers, we show that the amplitude of the fluctuations becomes much stronger than in three-dimensional systems. We investigate the properties of the fronts of diffusion and show that they have a self-affine structure characterized by a Hurst exponent H=1. We discuss the implications of these results for diffusion in liquid crystals and in cellular membranes of living organisms.

Salinity Gradients for Sustainable Energy: Primer, Progress, and Prospects.

Combining two solutions of different composition releases the Gibbs free energy of mixing. By using engineered processes to control the mixing, chemical energy stored in salinity gradients can be harnessed for useful work. In this critical review, we present an overview of the current progress in salinity gradient power generation, discuss the prospects and challenges of the foremost technologies - pressure retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix) and provide perspectives on the outlook of salinity gradient power generation. Momentous strides have been made in technical development of salinity gradient technologies and field demonstrations with natural and anthropogenic salinity gradients (for example, seawater-river water and desalination brine-wastewater, respectively), but fouling persists to be a pivotal operational challenge that can significantly ebb away cost-competitiveness. Natural hypersaline sources (e.g., hypersaline lakes and salt domes) can achieve greater concentration difference and, thus, offer opportunities to overcome some of the limitations inherent to seawater-river water. Technological advances needed to fully exploit the larger salinity gradients are identified. While seawater desalination brine is a seemingly attractive high salinity anthropogenic stream that is otherwise wasted, actual feasibility hinges on the appropriate pairing with a suitable low salinity stream. Engineered solutions are foulant-free and can be thermally regenerative for application in low-temperature heat utilization. Alternatively, PRO, RED, and CapMix can be coupled with their analog separation process (reverse osmosis, electrodialysis, and capacitive deionization, respectively) in salinity gradient flow batteries for energy storage in chemical potential of the engineered solutions. Rigorous techno-economic assessments can more clearly identify the prospects of low-grade heat conversion and large-scale energy storage. While research attention is squarely focused on efficiency and power improvements, efforts to mitigate fouling and lower membrane and electrode cost will be equally important to reduce levelized cost of salinity gradient energy production and, thus, boost PRO, RED, and CapMix power generation to be competitive with other renewable technologies. Cognizance of the recent key developments and technical progress on the different technological fronts can help steer the strategic advancement of salinity gradient as a sustainable energy source.

Correlations and scaling properties of nonequilibrium fluctuations in liquid mixtures.

Diffusion in liquids is accompanied by nonequilibrium concentration fluctuations spanning all the length scales comprised between the microscopic scale a and the macroscopic size of the system, L. Up to now, theoretical and experimental investigations of nonequilibrium fluctuations have focused mostly on determining their mean-square amplitude as a function of the wave vector. In this work, we investigate the local properties of nonequilibrium fluctuations arising during a stationary diffusion process occurring in a binary liquid mixture in the presence of a uniform concentration gradient, ∇c_{0}. We characterize the fluctuations by evaluating statistical features of the system, including the mean-square amplitude of fluctuations and the corrugation of the isoconcentration surfaces; we show that they depend on a single mesoscopic length scale l=sqrt[aL] representing the geometric average between the microscopic and macroscopic length scales. We find that the amplitude of the fluctuations is very small in practical cases and vanishes when the macroscopic length scale increases. The isoconcentration surfaces, or fronts of diffusion, have a self-affine structure with corrugation exponent H=1/2. Ideally, the local fractal dimension of the fronts of diffusion would be D_{l}=d-H, where d is the dimensionality of the space, while the global fractal dimension would be D_{g}=d-1. The transition between the local and global regimes occurs at a crossover length scale of the order of the microscopic length scale a. Therefore, notwithstanding the fact that the fronts of diffusion are corrugated, they appear flat at all the length scales probed by experiments, and they do not exhibit a fractal structure.

Fluorimetric detection of the earliest events in amyloid ß oligomerization and its inhibition by pharmacologically active liposomes.

BACKGROUND: Amyloid ß (Aß) peptide aggregation is the main molecular mechanism underlying the development of Alzheimer's disease, the most widespread form of senile dementia worldwide. Increasing evidence suggests that the key factor leading to impaired neuronal function is accumulation of water-soluble Aß oligomers rather than formation of the senile plaques created by the deposition of large fibrillary aggregates of Aß. However, several questions remain about the preliminary steps and the progression of Aß oligomerization. METHODS: We show that the initial stages of the aggregation of fluorescently labeled Aß can be determined with a high degree of precision and at physiological (i.e., nanomolar) concentrations by using either steady-state fluorimetry or time-correlated single-photon counting. RESULTS: We study the dependence of the oligomerization extent and rate on the Aß concentration. We determine the chemical binding affinity of fluorescently labeled Aß for liposomes that have been recently shown to be pharmacologically active in vivo, reducing the Aß burden within the brain. We also probe their capacity to hinder the Aß oligomerization process in vitro. CONCLUSIONS: We introduced a fluorescence assay allowing investigation of the earliest steps of Aß oligomerization, the peptide involved in Alzheimer's disease. The assay proved to be sensitive even at Aß concentrations as low as those physiologically observed in the cerebrospinal fluid. GENERAL SIGNIFICANCE: This work represents an extensive and quantitative study on the initial events of Aß oligomerization at physiological concentration. It may enhance our comprehension of the molecular mechanisms leading to Alzheimer's disease, thus paving the way to novel therapeutic strategies.

Violation of the mass-action law in dilute chemical systems.

The mass-action law, which predicts the rates of chemical reactions, is widely used for modeling the kinetics of the chemical reactions and their stationary states, also for complex chemical reaction networks. However, violations of the mass-action equations have been reported in various cases: in confined systems with a small number of molecules, in non-ideally-stirred systems, when the reactions are limited by the diffusion, at high concentrations of reactants, or in chemical reaction networks with marginally stable mass-action equations. In this paper, I describe a new mechanism, leading to the violation of the mass-action equations, that takes place at a low concentration of at least one of the reactants; in this limit, the reaction rates can be easily inferred from the chemical reaction network. I propose that this mechanism underlies the replication stability of the hypercycles, a class of chemical reaction networks hypothetically connected with abiogenesis. I provide two simple examples of chemical reaction networks in which the mechanism leading to the violation of the mass-action law is present. I study the two chemical reaction networks by means of a simulation performed with a cellular automaton model. The results have a general validity and represent a limitation of the validity of the mass-action law, which has been overlooked up to now in the studies about the chemical reaction networks.

Looking for chemical reaction networks exhibiting a drift along a manifold of marginally stable states.

I recently reported some examples of mass-action equations that have a continuous manifold of marginally stable stationary states [Brogioli, D., 2010. Marginally stable chemical systems as precursors of life. Phys. Rev. Lett. 105, 058102; Brogioli, D., 2011. Marginal stability in chemical systems and its relevance in the origin of life. Phys. Rev. E 84, 031931]. The corresponding chemical reaction networks show nonclassical effects, i.e. a violation of the mass-action equations, under the effect of the concentration fluctuations: the chemical system drifts along the marginally stable states. I proposed that this effect is potentially involved in abiogenesis. In the present paper, I analyze the mathematical properties of mass-action equations of marginally stable chemical reaction networks. The marginal stability implies that the mass-action equations obey some conservation law; I show that the mathematical properties of the conserved quantity characterize the motion along the marginally stable stationary state manifold, i.e. they allow to predict if the fluctuations give rise to a random walk or a drift under the effect of concentration fluctuations. Moreover, I show that the presence of the drift along the manifold of marginally stable stationary-states is a critical property, i.e. at least one of the reaction constants must be fine tuned in order to obtain the drift.

Magnetic tweezers measurements of the nanomechanical stability of DNA against denaturation at various conditions of pH and ionic strength.

The opening of DNA double strands is extremely relevant to several biological functions, such as replication and transcription or binding of specific proteins. Such opening phenomenon is particularly sensitive to the aqueous solvent conditions in which the DNA molecule is dispersed, as it can be observed by considering the classical dependence of DNA melting temperature on pH and salt concentration. In the present work, we report a single-molecule study of the stability of DNA against denaturation when subjected to changes in solvent. We investigated the appearance of DNA instability under specific external applied force and imposed twist values, which was revealed by an increase in the temporal fluctuations in the DNA extension. These fluctuations occur in the presence of a continuous interval of equilibrium states, ranging from a plectonemic state to a state characterized by denaturation bubbles. In particular, we observe the fluctuations only around a characteristic force value. Moreover, this characteristic force is demonstrated to be notably sensitive to variations in the pH and ionic strength. Finally, an extension of a theoretical model of plectoneme formation is used to estimate the average denaturation energy, which is found to be linearly correlated to the melting temperature of the DNA double strands.

Ions Transport and Adsorption Mechanisms in Porous Electrodes During Capacitive-Mixing Double Layer Expansion (CDLE).

A model of the electro-diffusion of ions in porous electrodes is applied to analyze the dynamics of capacitive-mixing extraction of energy from salinity gradients with carbon porous electrodes. The complex time-evolution of the cell voltage observed in experiments is satisfactorily described. The asymmetry on the duration of the solution-change steps performed in open circuit is found to be due to the nonlinear voltage-concentration relationship of the electric double layers and to a current that redistributes the counterions along the depth of the electrode leading to nonuniform charge and salt adsorption. The validated model is an essential tool for the design and optimization of renewable energy extraction by this technique.

Marginal stability in chemical systems and its relevance in the origin of life.

Concentration fluctuations are always present in solutions; it has been noticed that, in chemical systems, they can lead to deviations from what is expected from mass-action equations. I recently described the class of the "marginally stable" chemical systems; namely, a system that have an infinity of stationary states forming a continuous curve, and I showed that they present such deviations, which appear as a drift along the stationary-state curve [Phys. Rev. Lett. 105, 058102 (2010)]. Here I describe various marginally stable chemical reaction networks, including replicating molecules, and I present numerical calculations based on reaction-diffusion master equations, showing that the thermodynamic fluctuations induce a drift. This drift can be interpreted in terms of evolution toward a more efficiently replicating system and is analogous to a Darwinian evolution. The concentration fluctuations observed during the drift are scale invariant. Relevance of this phenomenon to the origin of life is discussed. I propose that marginal stability is the mathematical property defining chemical reaction networks potentially involved in the origin of life.

Quantitative Fourier analysis of schlieren masks: the transition from shadowgraph to schlieren.

In a schlieren setup, a lens system forms an image of the refractive index fluctuations of a transparent sample onto a matrix detector while an intensity mask is positioned in the Fourier plane of a collecting lens to perform the required spatial filtering. In the absence of the mask, the resulting technique is that of a shadowgraph. The two methods provide different information about the refractive index of transparent fluids and can be used both for visualization purposes and scattering measurements. Here, we describe the effect of the intensity mask on the technique transfer function, i.e., its ability to detect different spatial frequencies and show how the special cases of shadowgraph, schlieren, and the transition between the two can be derived. We also present experimental data that agree well with our predictions.

Atomic force microscopy study of DNA conformation in the presence of drugs.

Binding of ligands to DNA gives rise to several relevant biological and biomedical effects. Here, through the use of atomic force microscopy (AFM), we studied the consequences of drug binding on the morphology of single DNA molecules. In particular, we quantitatively analyzed the effects of three different DNA-binding molecules (doxorubicin, ethidium bromide, and netropsin) that exert various pharmacologic and therapeutic effects. The results of this study show the consequences of intercalation and groove molecular binding on DNA conformation. These single-molecule measurements demonstrate morphological features that reflect the specific modes of drug-DNA interaction. This experimental approach may have implications in the design of therapeutically effective agents.

Marginally stable chemical systems as precursors of life.

Current research on the origin of life aims at finding the simplest entity that can undergo spontaneous Darwinian evolution toward increasing replication efficiency. Here I consider some of the models of self-replicating molecular systems, and I show that they exhibit a distinct feature, namely, an infinity of stationary states forming a continuous curve; i.e., they are only marginally stable. I show that, in marginally stable chemical systems, thermodynamic fluctuations induce a drift directed toward increasing replication efficiency. This drift represents a form of evolution, taking place slowly, cooperatively, in macroscopic volumes of water.