E. Slavcheva

Sputtered Iridium Oxide Films as Charge Injection Material for Functional Electrostimulation

The paper reports on the deposition of iridium oxide thin films by dc reactive magnetron sputtering and their characterization by surface analysis methods (X-ray diffraction, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy) and electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic square waves). The sputtering process was investigated by applying the method of generic curves. An optimal combination of sputtering parameters has been established [40 standard cubic centimeters per minute (sccm) argon/8-12 sccm oxygen/40% effective pump power/100 W dc power/cold sputtering] that yields stable microporous amorphous films with a highly extended surface. These films have shown an excellent electrochemical reversibility in physiological saline solution with a safe potential range of -1.2 to 1.3 V vs. Ag/AgCl reference electrode, a charge delivery capacity of 90-95 mC/cm 2 , a constant impedance of 70-80 Ω in the frequency range of 100 kHz down to 0.5-1.0 Hz, and a quick response to bipolar current pulses with a safe charge per phase of 2 mC/cm 2 /ph. The demonstrated remarkable electrochemical characteristics combined with their established long-term mechanical stability and corrosion resistance allow one to recommend the use of sputtered iridium oxide films as an ideal electrode material for neural stimulation.

Screening of new corrosion inhibitors via electrochemical noise analysis

The inhibition efficiency of a series of organic compounds against corrosion of magnesium alloy AZ91 in a 50 wt.-% aqueous solutions of ethylene glycol has been studied using polarisation curves and electrochemical noise (ECN) measurements. The comparative analysis of the results obtained has demonstrated unambiguously that the method of ECN is an effective tool for screening of new corrosion inhibitors which offers some advantages comparing to the common electrochemical techniques. The very fast measuring rate enables to instantaneously register and monitor the rate of the corrosion process and changes in its mechanism in presence of the organic additives. The measurements are performed at freely corroding identical electrodes without perturbation from the external signals, which could lead (as in the polarisation measurements), to changes in the surface structure, the adsorption of the inhibitor etc. The statistical analysis of the noise data (in terms of standard deviations) in the time domain shows that the noise intensity of the system correlates well with the intensity of the corrosion process. The total amount of the charges exchanged during the tests is a quantitative measure for the inhibiting effect of the substances under study, while the spectral analysis of the noise in the frequency domain can be used to characterise the type of the corrosion attack.

Hampering of the Stability of Gold Electrodes by Ferri-/Ferrocyanide Redox Couple Electrolytes during Electrochemical Impedance Spectroscopy

In the past decades, numerous measurements have applied electrochemical impedance spectroscopy (EIS) in an electrode-electrolyte system consisting of gold electrodes and the redox couple potassium ferrocyanide/potassium ferricyanide (HCF). Yet these measurements are often hampered by false positive and negative results. Electrochemical impedance signals often display a nonlinear drift in electrolyte systems containing the HCF redox couple, which can mask the accuracy of the analysis. Thus, this Article aims to elucidate the stability and reliability of this particular electrode-electrolyte system. Here, different gold electrode cleaning treatments were compared with respect to adsorption and roughness of the surface of gold electrodes. They show substantial nonlinear long-term drifts of the charge-transfer resistance RD. In particular, the use of HCF-containing electrolytes causes adsorption and corrosion on the gold electrode surface, resulting in a nonlinear impedance behavior that depends on the incubation period as well as on electrolyte composition. Consequently, it is strongly recommended not to use HCF containing electrolytes in combination with gold electrodes.

ECN‐measurements at copper in artificial tap water – Investigation of anion‐effects

Electrochemical noise analysis was succesfully applied to study anion effects in the first 20 h starting period of copper corrosion in tap water. The influence of chloride, sulfate and bicarbonate as single anions and the interactions between them in three anion solutions were characterized in terms of standard deviation and PSD(I) data as well as interaction coefficients obtained from the evaluation of a set of experiments designed according to a statistical 23-factorial plan. The results let conclude that sulfate has an activating and chloride a passivating effect. Bicarbonate seems to act as a buffering agent. It is obviously the combination of the presence of all three anions in a narrow range of concentration ratios which finally leads to pitting corrosion. The relevance of these results from short term exposure for the long term performance of copper in tap water and the likelihood of pitting corrosion should now be investigated in long term exposure tests. Elektrochemische Rauschmessungen an Kupfer in kunstlichem Trinkwasser – Untersuchung von Anionen-Effekten Die elektrochemische Rauschanalyse wurde erfolgreich zum Studium von Anioneneffekten in den ersten 20 h der Korrosion von Kupfer in Trinkwasser eingesetzt. Der Einflus von Chlorid, Sulfat und Bikarbonat als Einzelionen und deren Wechselwirkung untereinander wurden als Standardabweichungen und PSD(I)-Steigungen und uber Wechselwirkungskoeffizienten charakterisiert, welche durch Auswertung von Experimenten aus einem 23-Faktorenplan berechnet wurden. Aus den Resultaten geht hervor, das Sulfat einen aktivierenden und Chlorid einen passivierenden Einflus besitzt. Bikarbonat scheint als Puffersubstanz wirksam zu werden. Es ist offensichtlich die Kombination aus der Anwesenheit aller drei Anionen in einem engen Bereich von Konzentrationsverhaltnissen, welche letztlich zur Lochkorrosion fuhrt. Die Relevanz dieser Resultate aus Kurzzeitversuchen fur das Langzeitverhalten von Kupfer in Trinkwasser und die Lochkorrosionswahrscheinlichkeit sollte nunmehr durch Langzeitauslagerungsversuche uberpruft werden.

Evaluation of SIROF Microelectrodes for Single Neuron Stimulation in Biohybrid Circuits

Microelectrodes with an upper functional layer of dc sputtered iridium oxide film (SIROF) are intended to be applied in biohybrid circuits for single insect neuron stimulation. In this paper, we report on electrochemical evaluation and of planar disc microelectrodes of varying size (diameter of 10 to 300 mum) and their charge injection properties of SIROF at microelectrode level during fast current pulses. The pulse injection charge (Qinj) limits represent only a fraction of the charge storage capacity (Qcsc) measured by cyclic voltammetry. Electrodes with diameter down to 100 mum and 80 -100 mC .cm-2 oxide charge capacity sustain about 2 mC.cm-2 in pulse mode, while the maximum charge injection per phase for d=10 mum electrodes the respective values were Qcsc = 892.6 mC.cm-2 and Qinj = 9.1 mC.cm-2 .

Sputtered Pd as Hydrogen Storage for a Chip-Integrated Microenergy System

The work presents a research on preparation and physical and electrochemical characterisation of dc magnetron sputtered Pd films envisaged for application as hydrogen storage in a chip-integrated hydrogen microenergy system. The influence of the changes in the sputtering pressure on the surface structure, morphology, and roughness was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AMF). The electrochemical activity towards hydrogen adsorption/desorption and formation of PdH were investigated in 0.5 M H2SO4 using the methods of cyclic voltammetry and galvanostatic polarisation. The changes in the electrical properties of the films as a function of the sputtering pressure and the level of hydrogenation were evaluated before and immediately after the electrochemical charging tests, using a four-probe technique. The research resulted in establishment of optimal sputter regime, ensuring fully reproducible Pd layers with highly developed surface, moderate porosity, and mechanical stability. Selected samples were integrated as hydrogen storage in a newly developed unitized microenergy system and tested in charging (water electrolysis) and discharging (fuel cell) operative mode at ambient conditions demonstrating a stable recycling performance.

The glyoxylate pathway contributes to enhanced extracellular electron transfer in yeast-based biofuel cell

This study provides a new insight into our understanding of yeast response to starvation conditions (sole acetate as carbon source) and applied polarization and offers important information about the role of the glyoxylate cycle in the carbohydrate synthesis and extracellular charge transfer processes in biofuel cells. The biosynthetic capabilities of yeast C. melibiosica 2491 and the up/down-regulation of the glyoxylate cycle are evaluated by modifying the cellular metabolism by feedback inhibition or carbohydrate presence and establishing the malate dehydrogenase activity and carbohydrate content together with the electric charge passed through bioelectrochemical system. 10mM malate leads to a decrease of the produced quantity of electricity with ca. 55%. At the same time, 24-times lower intracellular malate dehydrogenase activity is established. At polarization conditions the glyoxylate pathway is up-regulated and huge amount of malate is intra-converted into oxaloacetate. The yeasts are able to synthesize carbohydrates from acetate and a part of them is used for the electricity generation. It is recognized that the enhanced charge transfer in acetate fed yeast-based biofuel cell is implemented by secreted endogenous mediator and changes in the cellular surface redox activity depending on the addition of carbohydrate in the medium.

Sputtered platinum-iridium as catalyst for hydrogen fuel cells

Co-sputtered platinum-iridium films were deposited and tested as catalysts for an application in hydrogen fuel cells. The morphological and electrochemical investigation of samples with different ratios of both metals revealed that by systematic varying of the components content it is possible to obtain PtIr alloy with optimal composition ensuring high catalytic activity at reduced noble metal loading. Among the tested samples the Pt14Ir86 is the most promising candidate for the oxygen reduction reaction.

Electrochemical Properties and Applications of Sputtered Iridium Oxide Thin Films

Abstract The study reports on deposition of iridium oxide thin films by DC reactive magnetron sputtering. The sputtering process has been investigated applying the method of the generic curves. The composition, surface structure and morphology of the sputtered iridium oxide films (SIROFs) have been characterised using X-ray diffraction, SEM, and AFM analysis. An optimal combination of sputtering parameters has been established (40 seem argon −8/18 seem oxygen – 40% effective pump power), which yields stable microporous amorphous films with highly extended fractal surface. The electrochemical properties of these films have been investigated in neutral and acid aqueous solutions by methods of cyclovoltyammetry (CV), electrochemical impedance spectroscopy (EIS), and steady state polarization curves (PC). The SIROFs have shown an excellent electrochemical reversibility in physiological saline solution and have demonstrated high catalytic activity towards oxygen evolution reaction (OER) in 0.5M H2SO4. A current density of 100 mA/cm2 at potential of 1.6V (vs.SSE) was obtained at catalyst load of only 10μg/cm2.