Acid-base machines: electrical work from neutralization reactions.

We have developed an electrochemical system that performs electrical work due to changes in alkaline ion and proton activities associated with acidic solution neutralization. This system can be used to treat wastewater, contributing to sustainable growth. The system includes an electrochemical machine that operates between an acidic and a basic reservoir to produce work in cycles comprising four stages: two isothermal ionic insertion/de-insertion steps and two steps involving acid and base injection. On the basis of the mixing free energy associated with the reaction free energy, we have developed the thermodynamic formalism by considering reversible electrochemical processes to determine the maximum work performed by this acid-base machine and the efficiency. Electrochemical methods in the time and frequency domains helped in investigating the kinetics of sodium ions and proton insertion in host matrices consisting of copper hexacyanoferrate and phosphomolybdic acid, respectively, to improve our understanding of the factors underlying dissipation as a function of pH and pNa. The full cell composed of these insertion electrodes was used as a proof of concept. It performed a maximum work of 26.4 kJ per mol of electro-inserted ion from HCl solution neutralization with the addition of NaOH, to simulate acidic wastewater treatment in a profitable and sustainable way.

Energy Harvesting by Nickel Prussian Blue Analogue Electrode in Neutralization and Mixing Entropy Batteries.

Some industries usually reduce the concentration of protons in acidic wastewater by conducting neutralization reactions and/or adding seawater to industrial effluents. This work proposes a novel electrochemical system that can harvest energy originating from entropic changes due to alteration in the concentration of sodium ions along wastewater treatment. Preparation of a self-assembled material from nickel Prussian blue analogue (NPBA) was the first step to obtain such electrochemical system. Investigation into the electrochemical properties of this material helped to evaluate its potential use in neutralization and mixing entropy batteries. Assessment of parameters such as the potentiodynamic profile of the current density as a function of the concentration of protons and sodium ions, charge capacity, and cyclability as well as the reversibility of the sodium ion electroinsertion process aided estimation of the energy storage efficiency of the system. Frequency-domain measurements and models and the proposed charge compensation mechanism provided the rate constants at different dc potentials. After each charge/discharge cycle, the NPBA electrode harvested 12.4 kJ per mol of intercalated sodium ion in aqueous solutions of NaCl at concentrations of 20 mM and 3.0 M. The full electrochemical cell consisted of an NPBA positive electrode and a negative electrode of silver particles dispersed in a polypyrrole electrode. This cell extracted 16.8 kJ per mol of intercalated ion after each charge/discharge cycle. On the basis of these results, the developed electrochemical system should encourage wastewater treatment and help to achieve sustainable growth.

Proton electroinsertion in self-assembled materials for neutralization pseudocapacitors.

We propose novel pseudocapacitors that can store energy related to the partial entropy change associated with proton concentration variations following neutralization reactions. In this situation, it is possible to obtain electrochemical energy after the complete charge/discharge cycle conducted in electrolytic solutions with different proton concentrations. To this end, we prepared modified electrodes from phosphomolybdic acid (PMA), poly(3,4-ethylenedioxythiophene/poly(styrenesulfonate) (PEDOT-PSS), and polyallylamine (PAH) by the layer-by-layer (LbL) method and investigated their electrochemical behavior, aiming to use them in these neutralization pseudocapacitors. We analyzed the potentiodynamic profile of the current density at several scan rates, to evaluate the reversibility of the proton electroinsertion process, which is crucial to maximum energy storage efficiency. On the basis of the proposed reaction mechanism and by using frequency-domain measurements and models, we determined rate constants at different potentials. Our results demonstrated that the conducting polymer affects the self-assembled matrixes, ensuring that energy storage is high (22.5 kJ mol(-1)). The process involved neutralization of a hydrochloric acid solution from pH = 1 to pH = 6, which corresponds to 40% of the neutralization enthalpy.

Effects of self-assembled materials prepared from V2O5 for lithium ion electroinsertion.

Self-assembled materials consisting of V(2)O(5), polyallylamine (PAH) and silver nanoparticles (AgNPs) were obtained by the layer-by-layer (LbL) method, aiming at their application as electrodes for lithium-ion batteries and electrochromic devices. The method employed herein allowed for linear growth of visually homogeneous films composed of V(2)O(5), V(2)O(5)/PAH, and V(2)O(5)/PAH/AgNP with 15 bilayers. According to the Fourier transform infrared spectra, interaction between the oxygen atom of the vanadyl group and the amino group should be responsible for the growth of these films. This interaction also enabled establishment of an electrostatic shield between the lithium ions and the sites with higher negative charge, thereby raising the ionic mobility and consequently increasing the energy storage capacity and reducing the response time. According to the site-saturation model and the electrochemical and spectroelectrochemical results, the presence of PAH in the self-assembled host matrix decreased the number of V(2)O(5) electroactive sites. Thus, AgNPs were stabilized in PAH and inserted into the nanoarchitecture, so as to enhance the specific capacity. This should provide new conducting pathways and connect isolated V(2)O(5) particles in the host matrix. Therefore, new nanoarchitectures for specific interactions were formed spontaneously and chosen as examples in this work, aiming to demonstrate the potentiality of the adopted self-assembled method for enhancing the charge transport rate into the host matrices. The obtained materials displayed suitable properties for use as electrodes in lithium batteries and electrochromic devices.

Spectroelectrochemical properties and lithium ion storage in self-assembled nanocomposites from TiO2.

Layer-by-layer (LbL) nanocomposite films from TiO(2) nanoparticles and tungsten-based oxides (WO(x)H(y)), as well as dip-coating films of TiO(2) nanoparticles, were prepared and investigated by electrochemical techniques under visible light beams, aiming to evaluate the lithium ion storage and chromogenic properties. Atomic force microscopy (AFM) images were obtained for morphological characterization of the surface of the materials, which have similar roughness. Cyclic voltammetry and chronoamperometry measurements indicated high storage capacity of lithium ions in the LbL nanocomposite compared with the dip-coating film, which was attributed to the faster lithium ion diffusion rate within the self-assembled matrix. On the basis of the data obtained from galvanostatic intermittent titration technique (GITT), the values of lithium ion diffusion coefficient (D(Li)) for TiO(2)/WO(x)H(y) were larger compared with those for TiO(2). The rate of the coloration front in the matrices was investigated using a spectroelectrochemical method based on GITT, allowing the determination of the "optical" diffusion coefficient (D(op)) as a function of the amount of lithium ions previously inserted into the matrices. The values of D(Li) and D(op) suggested the existence of phases with distinct contribution to lithium ion diffusion rates and electrochromic efficiency. Moreover, these results aided a better understanding of the temporal change of current density and absorbance during the ionic electro-insertion, which is important for the possible application of these materials in lithium ion batteries and electrohromic devices.

Pt/TiO2/poly(vinyl sulfonic acid) layer-by-layer films for methanol electrocatalytic oxidation.

One major challenge for the widespread application of direct methanol fuel cells (DMFCs) is to decrease the amount of platinum used in the electrodes, which has motivated a search for novel electrodes containing platinum nanoparticles. In this study, platinum nanoparticles were electrodeposited on layer-by-layer (LbL) films from TiO2 and poly(vinyl sulfonic) (PVS), by immersing the films into a H2PtCl6 solution and applying a 100 microA current during different electrodeposition times. Scanning tunnel microscopy (STM) and atomic force microscopy (AFM) images showed increased platinum particle size and electrode roughness for increasing electrodeposition times. The potentiodynamic profile of the electrodes indicated that oxygen-like species in 0.5 mol L(-1) H2SO4 were formed at less positive potentials for the smallest platinum particles. Electrochemical impedance spectroscopy measurements confirmed the high reactivity for the water dissociation and the large amount of oxygen-like species adsorbed on the smallest platinum nanoparticles. This high oxophilicity of the smallest nanoparticles was responsible for the electrocatalytic activity of Pt-TiO2/PVS systems for methanol electrooxidation, according to the Langmuir-Hinshelwood bifunctional mechanism. Significantly, the approach used here combining platinum electrodeposition and LbL matrices allows one to both control the particle size and optimize methanol electrooxidation, being therefore promising for producing membrane-electrode assemblies of DMFCs.

The nature of the interactions between Pt4 cluster and the adsorbates *H, *OH, and H2O.

The nature of the interactions between the platinum cluster Pt4 and the adsorbates (*)H, (*)OH, and H2O, as well as the influence of these adsorbates on the electronic structure of the Pt4 cluster, was investigated by density functional theory (B3LYP, B3PW91, and BP86) together with the effective core potential MWB for the platinum atoms, and 6-311++G(d,p) and aug-cc-pVTZ basis set for the H and O atoms. Identification of the optimal spin multiplicity state and the preferential adsorption sites were also evaluated. Adsorption changes the cluster geometry significantly, but the relaxation effects on the adsorption energy are negligible. The adsorbates bind preferentially atop of the cluster, where high bonding energies were observed for the radical species. Adsorption is followed by a charge transfer from the Pt4 cluster toward radical adsorbates, but this charge transfer occurs in a reversed way when the adsorbate is H2O. In contrast with water, adsorption of the radicals (*)H and (*)OH on platinum causes a remarkable re-distribution of the spin density, characterized by a spin density sharing between the (*)H and (*)OH radicals and the cluster. The covalent character of the cluster-adsorbate interactions, determined by electron density topological analysis, reveals that the Pt4-H interaction is completely covalent, Pt4-OH is partially covalent, and Pt4-H2O is almost noncovalent.

Enhanced Li+ charge storage in naphthalene diimide/vanadium pentoxide intercalates.

N,N'-Bis(4-aminophenyl)-1,4,5,8-naphthalene diimide (NDI-ph) was intercalated into lamellar vanadium pentoxide (V2O5) in different amounts to prepare hybrid intercalates. The presence of the imide supports the material's ability to form lithium salts with the structural stabilization of the oxide matrix. This effect is remarkable in charge/discharge cycles in terms of Li+ uptake and discharge by the lamellar intercalates, as we could double the ion uptake capacity (1.27 Li+ per V2O5 unit vs. 0.66 for pure V2O5), enhance the chemical reversibility and double the specific charge capacity (188 mA h g-1 vs. 98 mA h g-1 for pure V2O5) with very small amounts of this imide. This is the first paper dealing with naphthalene diimide intercalates in vanadium pentoxide xerogel for Li+ storage.

Trapping of charge carriers in colloidal particles of self-assembled films from TiO(2) and poly(vinyl sulfonic acid).

Self-assembled electrodes consisting of TiO(2) nanoparticles and poly(vinyl sulfonic acid) (PVS) were prepared by the layer-by-layer (LbL) technique. The electrostatic interaction between the TiO(2) nanoparticles and PVS allowed the growth of visually uniform multilayers of the composite, with high control of the thickness and nanoarchitecture. The electrochemical and chromogenic properties of these TiO(2)/PVS films were examined in an electrolytic solution of 0.5 M LiClO(4)/propylene carbonate. The presence of two intercalation sites was noted during the positive potential scan, and they were attributed to different mobilities of charge carriers. Several charge/discharge cycles demonstrated the trapping of charge carriers in the TiO(2) sites. The absorbance change associated with the oxidation of the trapping sites was attributed to electronic transitions involving energy states in the gap band formed due to the strong distortion of the TiO(2) host. Using the quadratic logistic equation (QLE), it was possible to analyze the electronic intervalence transfer from Ti(3+) to Ti(4+). Using the parameters obtained from this fitting, the amount of trapping sites in the LbL film was also determined. Electrochemical impedance spectroscopy (EIS) data gave the time constant associated with diffusion and the trapping sites. The diffusion coefficient of lithium ions changed from ca. 4.5 x 10(-13) cm(2) s(-1) to 3.0 x 10(-14) cm(2) s(-1) for all the potential range applied, indicating that PVS did not hinder the ionic transport within the LbL film. Finally, on the basis of the spectroelectrochemical data and scanning electron micrographs, the trapping effects were attributed to the colloidal particles of Li(0.55)TiO(2).

Electrochemical and electrochromic properties of layer-by-layer films from WO(3) and chitosan.

The design of improved materials for electrochromic applications now involves extensive use of novel composites, thus requiring an investigation of the mechanisms responsible for electrochromism in these structures. Using films of WO(3) and chitosan produced with the layer-by-layer (LBL) technique, we demonstrate that characteristics such as the number of electrochemical active sites (K), the molar absorption coefficient (epsilon), and the electrochromic efficiency (eta) can be obtained using the quadratic logistic equation (QLE). The complexation ability between chitosan and WO(3) allowed the growth of visually uniform multilayers of the composite, with the same amount of material adsorbed in each deposition cycle. By fitting the absorbance changes (DeltaA) resulting from the electronic intervalence transfer from W(V) to W(VI) sites in four-bilayer LBL films of WO(3)/chitosan and WO(3)/chitosan with ethanol in the precursor dispersion, K was estimated to be ca. 5.5 x 10(-8) mol cm(-2) and 3.6 x 10(-8) mol cm(-2), respectively. The molar absorption coefficient and electrochromic efficiency vary with the charge injected because of the saturation of W(V) sites and the dissipation and feedback effects implicit in the QLE associated with ion-network interactions, such as the proton trapping effect. The LBL film of WO(3)/chitosan showed a smaller molar absorption coefficient and electrochromic efficiency than that containing ethanol because of a greater proton trapping effect for the LBL film with no ethanol. This enhanced trapping effect was seen as a decrease in the electronic flux involved in intervalence transfer in electrochemical impedance spectroscopy experiments.

Layer-by-layer nanostructured hybrid films of polyaniline and vanadium oxide.

Supramolecular structures of polyaniline (PANI) and vanadium oxide (V2O5) have been assembled via the electrostatic layer-by-layer (ELBL) technique. Strong ionic interactions and H-bonding impart unique features to the ELBL films, which are distinct from cast films obtained with the same materials. The interactions were manifested in UV-vis and Fourier transform infrared spectroscopy data. They are enhanced by the intimate contact between the components, as the films are molecularly thin, with 25 A per PANI/V2O5 bilayer.