Jerónimo Agrisuelas

Electronic Perspective on the Electrochemistry of Prussian Blue Films

The derivative of the voltabsommetric scans, together with previous nano-electrogravimetric and X-ray diffraction results, allow different electrochemical processes to be distinguished during the Prussian blue (PB) voltammetric scan. Potassium, proton, and hydrated proton counterions involved in PB electrochemistry are related here to the electrochemical reactions of specific Fe sites. Potassium counterions show two different sites for their insertion: one located in the crystalline framework and another in ferrocyanide vacancies. From the monitoring of electroactive Fe sites, the covalent-exchange model is suggested as one of the first approaches to explain the origin of the PB magnetic ordering observed at room temperature during voltammetric scanning.

Insights on the Mechanism of Insoluble-to-Soluble Prussian Blue Transformation

The electrochemical transformation of the soluble form of Prussian blue (PB) material from the insoluble form was monitored using electrochemical, gravimetric, acoustic, and spectroscopic techniques simultaneously. The described combination of in situ techniques represents an innovative tool for measurement in electrochemistry, which provides complementary information on the electrochemical systems. The insoluble-to-soluble PB transformation process takes place during the successive voltammetric cycles between the mixed valence form (PB) and the fully reduced form [Everitts salt (ES)]. One of the processes that takes place is the exit of free Fe(CN) 4- 6 ions occluded in the vacancies of the insoluble PB crystalline framework during the fresh PB electrodeposit process. In potassium salt solutions, the exit of each ferrocyanide ion is compensated by the entrance of two potassium ions and six hydroxyl anions. This species exchange increases the manifestation of viscoelastic phenomena of the rigid skeleton from the insoluble PB form to the soluble one, which could facilitate the appearance of an internal magnetic field at room temperature during the soluble PB ⇄ ES voltammetric cycle.

Oscillatory Changes of the Heterogeneous Reactive Layer Detected with the Motional Resistance during the Galvanostatic Deposition of Copper in Sulfuric Solution

Metallic copper was galvanostatically deposited on quartz|gold resonant electrodes by applying a constant current in a 0.5 M CuSO4/0.1 M H2SO4 aqueous solution. Galvanostatic copper deposition is one of the best methodologies to calibrate the electrochemical quartz crystal microbalances (EQCM), a gravimetric sensor to evaluate changes in mass during the electrochemical reactions through the Sauerbrey equation. The simultaneous measurement of mass, current density, and motional resistance by an EQCM with motional resistance monitoring allows us to characterize the processes occurring on the electrode surface and at the interfacial regions with unprecedented detail. During the galvanostatic copper deposition, Cu(H2O)4(OH)2 is accumulated close to the copper surface, generating a passive layer. This passive layer can act as Cu(2+) reservoir for the Cu(2+) → Cu process since the copper deposition is not affected. The analysis of motional resistance evolution in different experimental conditions reveals that the passive layer is formed by the reaction of oxidizing agents generated at the counter electrode with the metallic copper surface. The simplistic Cu(2+) → Cu process is completed with a more detailed mechanism, which includes the passive layer formation/dissolution and the transport of species from the counter electrode surface (Pt) to the working electrode surface. The results further support the calibration procedure of EQCM by the galvanostatic deposition of copper in sulfuric solutions. However, we suggest applying high current densities, separating the counter electrode and quartz|gold resonant electrode about 0.5 cm, and keeping oxygen in solution for the EQCM calibration. Moreover, the better interval time to calculate the Sauerbreys constant from charge and resonant frequency data is between 150 and 300 s.

Motional Resistance Evaluation of the Quartz Crystal Microbalance to Study the Formation of a Passive Layer in the Interfacial Region of a Copper|Diluted Sulfuric Solution

A hyphenated technique based on vis–NIR spectroscopy and electrochemical quartz crystal microbalance with motional resistance monitoring was employed to investigate the dissolution of copper in acid media. Changes in motional resistance, current, mass, and absorbance during copper dissolution allow the evolution of the interfacial region of copper|diluted sulfuric solution to be understood. In particular, motional resistance is presented in this work as a useful tool to observe the evolution of the passive layer at the interface. During the forced copper electrodissolution in sulfuric solution, SO4(2–) favors the formation of soluble [Cu(H2O)6]2+. On the contrary, OH– involves the formation of Cu(H2O)4(OH)2, which precipitates on the electrode surface. The high viscosity and density of Cu(H2O)4(OH)2 formed on surface causes an increase in motional resistance independently of resonance frequency changes. During the copper corrosion in a more natural acidic environment, the results of electrochemical impedance spectra at open circuit potential indicate that corrosion is controlled by the diffusion of copper to the solution at short experimental times. However, copper diffusion is hindered by the formation of a passive layer on the electrode surface at long experimental times. During the copper corrosion, motional resistance shows an oscillatory response because of an oscillatory formation/dissolution of the passive later. Vis–NIR spectroscopy and electrochemical quartz crystal microbalance with motional resistance monitoring give new perspectives for reaching a deep understanding of metal corrosion processes and, in a future, other interfacial processes such as the catalysis or adsorption of (bio)molecules.

Electrochemical Stabilization of Prussian Blue Films in NH4Cl Aqueous Medium

Derivative voltabsommetric scans in near-UV/visible-NIR electromagnetic range together with nano-electrogravimetric measurements during the stabilization process of Prussian Blue in NH4Cl aqueous solution yields information about the localization of the into the crystalline structure of this material. It is proposed that the ammonium ion is placed into the vacancies of the framework that affect to the motional resistance. The simultaneous measure of current, mass, motional resistance, and near-UV-vi-N IR spectra allows to explain the differences observed between the Prussian Blue films placed in a solution of NH4Cl salt regarding the behaviour showed in KCl aqueous solution.

A Comparative Study of Poly(Azure A) Film-Modified Disposable Electrodes for Electrocatalytic Oxidation of H2O2: Effect of Doping Anion

In the present paper, poly(azure A) (PAA) films were electrosynthetized in the presence of different doping anions on disposable screen-printed carbon electrodes (SPCEs). The anions used included inorganic monoatomic (chloride and fluoride), inorganic polyatomic (nitrate and sulfate) and organic polyatomic (dodecyl sulfate, DS) species. The coated electrodes thus obtained were characterized by electrochemical techniques and SEM. They showed improved electrocatalytic activities towards hydrogen peroxide oxidation compared to that of a bare SPCE. In particular, the insertion of DS anions inside PAA films provided a special sensitivity to the electrocatalysis of H2O2, which endowed these electrodes with promising analytical features for H2O2 quantification. We obtained a wide linear response for H2O2 within a range of 5 µM to 3 mM and a limit of detection of 1.43 ± 0.10 µM (signal-to-noise ratio of 3). Furthermore, sensitivity was 72.4 ± 0.49 nA·µM−1∙cm−2 at a relatively low electrocatalytic oxidation overpotential of 0.5 V vs. Ag. The applicability of this boosted system was tested by the analysis of H2O2 in commercial samples of a hair lightener and an antiseptic and was corroborated by spectrophotometric methods.

Poly(neutral red) on passivated nickel films. New insights through EQCM measurements

Poly(Neutral Red) (PNR) has been electrogenerated on a passivated Ni surface. PNR was chosen due to the fact that its electroactivity region overlaps with the Ni dissolution/deposition process. Therefore, both electrochemical processes can compete and by this way, there are evidences about the formation of a Ni(OH)2/PNR composite. It was investigated by classical EQCM and the instantaneous mass/charge ratio (F(dm/dQ)) analysis shed light on the active/passive transition and nickel trans-passive dissolution mechanisms.

The role of NH 4 + cations on the electrochemistry of Prussian Blue studied by electrochemical, mass, and color impedance spectroscopy

The role of the ammonium cation on the reversible electrochemistry of Prussian Blue thin films was analyzed through in situ combination of three different impedance techniques. Electrochemical impedance spectroscopy provides information on the electron transfer. Mass impedance spectroscopy allows the exchange of free water, ammonium, and proton ions to be elucidated. Color impedance spectroscopy provides the kinetics of electrochromic changes of Prussian Blue structure (main backbone structure, ferrocyanide vacancies, and trapped structures). The simultaneous measurement of the three impedances gives detailed information on the mechanism of the whole process, specifically one faster and two slower, which have been identified during the electrochemical reactions of Prussian Blue films in NH4Cl acidic solutions.

An Electronic Perspective On The Electrochemical Changeover In Prussian Blue-Like Materials

directions. The Fe(III)(NC) sites are ascribed to high spin metallic atoms ) 2 / 5 ( = S whereas the Fe(II)(CN) sites are ascribed to low spin atoms ) 0 ( = S where 1/4 of the low spin Fe(II) sites are missed [3]. Herein, it is important to emphasize that the oxidation state of the Fe(III)(NC) sites is modulated during the voltammetric scan between the PB (0.60 V) and Everitt’s Salt (ES, –0.20 V vs Ag/AgCl/KCl sat electrode) forms, where the electrochemical changeover commented above is observed [2].

Usefulness of the Instantaneous Mass-charge Ratio for Elucidating the Ions Role in the Stabilization and Dissolution Processes in Prussian Blue Films

A similar procedure was used for the analysis of the mechanism of reduction of Prussian Blue films to the Everitt’s Salt form and to follow the partial dissolution of iron species during the oxidation to the Prussian Yellow form. The possibility of covering Prussian Blue films by Nafion membranes which make difficult the transport of anions through it allows to discern the role of anions in the solubilization process of Prussian Blue films.