Molybdenum Electrodes Research Articles

Toward an in Situ Electrochemical Sensor Measuring Phosphate Concentration in Seawater

Dissolved orthophosphate (phosphate) is one of the major nutrient elements in natural waters (lakes, rivers, oceans) easily uptaken by primary producers. Monitoring its concentration in seawater will allow to better understand the biogeochemical cycles and the ocean role in climate evolution. Phosphate is an electroinactive specie that is previously complexed in situ at acidic pH with molybdates. In order to avoid silicate interference a ratio protons over molybdates equal to 70 is required. The reagents are formed in situ by oxidation of 2 different molybdenum electrodes. The first oxidation allows to form H+ that cross a proton exchange membrane to obtain the pH conditions needed in the complexation cell. Then, the second molybdenum electrode is oxidized to reach the ratio (H+/MoO4 2-) = 70 and form the phosphomolybdic complex. The counter electrode is isolated behind a Nafion® membrane to limit the reduction of H+ into H2. Because phosphate concentration in seawater is quite low, i.e. few nanomolar in the surface ocean, up to 5 µmol.L-1 at deepest waters, electrochemical detection method requires high sensitivity. Therefore, cyclic voltammetry cannot be used neither chronoamperometry as a rotating electrode is needed to control the convection and it is not suitable for an in situintegrated sensor. Square Wave Voltammetry (SVW) is proposed to detect phosphomolybdic complex in seawater as it offers an optimal combination of potential modulation (potential ramp combined with short term potential pulses) and a specific current sampling reducing the effect of capacitive current. The mechanism of reduction of phosphomolybdic complex is still not fully elucidated. Depending on the pH and the phosphate concentration, different types of complex are formed, and polymer structures have also been observed [1]. Therefore, the reduction of Mo(VI) into a mix of Mo(V)/Mo(IV) observed on the square wave voltammograms showed different behaviours depending on experimental conditions. Indeed, at 250 Hz frequency, a saturation of the signal is quickly observed when the phosphate concentration increased (> 1 µmol.L-1) compared to 2.5 Hz frequency that allowing to obtain a linear behaviour on the whole concentration range (0-6 µmol.L-1) [2]. All the experimental parameters such as molybdenum oxidation charges, time of complexation and SWV parameters, have been optimized using a laboratory prototype in order to detect the smallest phosphate concentration possible, as fast as possible. Those parameters are currently on adaptation for the in situ version of the sensor as the design of the electrochemical cells has a strong influence on the diffusion time (homogenisation of the solution) and so on the detection signal. The electronics of the sensor is also in development and a comparison of its performance compared with the commercial Metrohm® potentiostat will be presented. The in situsensor will then be deployed in the ocean for inter-comparison tests. [1] Jonca et al.,Int. J. Electrochem. Sci. 7 (2012) 7325-7348 [2] Barus/Romanytsia et al., Talanta 160 (2016) 417-424

Molten Salt Electrorefining of Indium Metal from Indium Tin Alloy

Indium is an essential element for the fabrication of transparent conductive oxide (TCO) materials. However, it is very rare element and is obtained as a byproduct of zinc metal refining. Moreover, its recycling rate is less than 1%. This provokes the need to recycle indium from its end products. Indium and tin (Sn) are especially well suited for molten salt electrolysis because of their low melting points. In this paper, reduction processes on molybdenum electrode at 500oC in molten LiCl–KCl eutectic (55.7-44.3 mol %) containing varying amount of InCl3 was studied by Cyclic Voltammetry (CV) and Chronopotentiometry (CP). In-Sn alloy was used as raw material. Thermodynamic calculations showed poor stability of InCl3 with metal based crucibles. So, Al2O3 crucible was utilized for the experiments. Concentration of InCl3 was changed from 1 wt% to 3 wt% and it was found that the reduction of indium ions take place by a two-step electrochemical reaction: In3+/In+ and In+/Ino at the potentials of 0.78 and 0.24 V versus Ag/AgCl. Number of transferred electrons and diffusion coefficient of In+3 ion species in molten salt with varying InCl3 content was estimated with the help of CV measurements. However, at 500oC, InCl3 shows chemical reactivity with Sn metal according to the following reaction: InCl3 + 1.5Sn → 1.5SnCl2 + In. This problem was overcome by shifting the molten chloride system to molten fluoride system (LiF-KF-InF3) as fluoride based salts shows high chemical stability with metals. As a result of electrorefining, In-Sn alloy was recycled into pure indium with low levels of impurities.

Autonomous Silicate Electrochemical Sensor in Seawater

Long term monitoring of dissolved inorganic nutrient concentrations such as silicate is the key to improve our understanding of biogeochemical cycles in the ocean. Electrochemical methods are proposed to measure in situ silicate concentration as they allow to develop miniaturized, reagentless sensor. We report here on the development and the optimization of the in situversion of our silicate sensor. Silicate is a non-electroactive specie and its determination is performed by measuring its corresponding silicomolybdic complex formed in situ at acidic pH after the oxidation of a molybdenum electrode while the counter electrode is isolated behind a Nafion® membrane [1]. The mechanical design of the sensor has been adapted from the laboratory prototype containing two connected cells: one for the complexation and one for the detection on a gold electrode (Æ=2mm) by cyclic voltammetry. All the electrochemical and chemical parameters, such as the Mo oxidation, the complexation time, the silicomolybdic complex detection as well as the complex transfer from one cell to the other and the cleaning protocol have been optimized to detect the smallest silicate concentration possible with the shortest time. Reproducible calibrations performed with this sensor provided linear regressions between 2.5 µmol L-1 and 70 µmol L-1 and a limit of detection of 1 µmol L-1. The 1st version of the sensor has been validated so far down to 50 meters using a pressure column and deployed in the Kiel fjord (Baltic Sea, Germany) down to 25 meters. The electronics (including datalogger) was also validated in situ as well as the solenoid pump placed into an equi-pressure compartment filled with dielectric oil. Some mechanical improvements have been done and the 2ndversion of the silicate sensor is currently in fabrication. This new version will be deployed off Chile (South of Coquimbo) in April 2017 and results obtained will be discussed. [1] Lacombe et al., Talanta 77 (2008) 744-750.

Electrochemical studies on the redox behavior of zirconium in the LiF-NaF eutectic melt

In the present paper, a detailed study of the redox behavior of zirconium in the eutectic LiF-NaF system was carried out on an inert molybdenum electrode at 750 °C. Several transient electrochemical methods were used such as cyclic voltammetry, square wave voltammetry, chronopotentiometry, and open circuit voltammetry. The reduction of Zr (IV) was found to follow a two-step mechanism of Zr (IV)/Zr (II) and Zr (II)/Zr at the potentials of about −1.10 and −1.50 V versus Pt, respectively. The theoretical evaluations of the number of transferred electrons according to both cyclic voltammetry and square wave voltammetry further confirmed the Zr reduction mechanism. The estimations of Zr (IV) diffusion coefficient in the LiF-NaF eutectic melt at 750 °C through cyclic voltammetry and chronopotentiometry are in fair agreement, as to be approximately 1.13E-5 and 2.42E-5 cm2/s, respectively.

Optimisation of rear reflectance in ultra-thin CIGS solar cells towards >20% efficiency

In order to decrease their cost and the use of rare metal elements, thin film solar cell thicknesses are continuously reduced at the expense of their efficiency, due to a lack of absorption for long wavelengths. Optimisation of cells rear reflectance (Rb) thus becomes meaningful to provide non-absorbed light a second chance to be harvested by the active cell layer. In this sense, we present a way to keep the rear reflectance in advanced Cu(In, Ga) Se2 (CIGS) cell as high as possible while keeping in mind the progress already done regarding the rear passivation techniques. We show that introducing a stack of thin Al2O3 and aluminium between the CIGS layer and the rear molybdenum electrode increases Rb up to 92% in the long wavelength 800–1100nm range. Several other stacks, using MgF2, SiO2 or TiO2, are optimised in order to investigate the best trade-off between passivation, material consumption and performances, resulting in Rb ranging from 42% (moderate case) to 99% in the best case. Those CIGS rear interface reflectance optimisations were performed by using a standard transfer matrix method (TMM) in the long wavelength range. Seven interesting stacks are then analysed for solar cell performances using SCAPS simulation software to understand the impact of rear reflectance on short circuit current density (Jsc) and eventually on the cell efficiency (η), for ultra-thin CIGS absorber thicknesses (<1μm). Based on these results, we propose Rb optimisation to achieve Jsc>40mA/cm2 and η>20% with a 500nm-thick CIGS absorber film using CIGS-Al2O3-Mo stack, where the Al2O3 thickness can be chosen in between 104 and 139nm. This way, we can ensure good rear reflectance (Rb=65%) and reduced interface recombination while being industrially feasible with present technologies.

Electrodeposition of thin Cu2ZnSnS4 films

The electrochemical behavior of copper(II), zinc(II), and thiosulfate (S2O3 2-) ions on the molybdenum electrode in individual 0.2 М sodium sulfate solutions (рН 6.7) and with addition of either 0.1 М tartaric acid (рН 4.6) or 0.1 М citric acid (рН 4.7) is studied. A one-step electrochemical method is developed for the deposition of thin Cu2ZnSnS4 films, which is carried out on the molybdenum electrode at a constant potential in sodium sulfate solutions containing tartaric acid. The effect of the concentration of electrolyte components on the chemical composition of Cu2ZnSnS4 films is determined. The phase composition is confirmed by the Raman spectroscopy data. The surface morphology of synthesized films is studied by means of scanning-electron and atomic-force microscopes. The photoelectrochemical characteristics of Cu2ZnSnS4 films are determined. Samples of these coatings on the Mo electrode are found to be highly photosensitive.

Stable and efficient operation of a large-bore copper vapor laser with funnel-shaped, grooved copper electrodes

Using an appropriate design of electrodes and adjustment of the thyratron decoupling circuit as a high-repetition-rate and high-voltage switch, very stable operation of a copper vapor laser at high pressures was obtained. This was achieved by canceling the intense filamentation in the laser plasma at the higher pressures. The transverse grooves on the inner surface of the funnel-shaped copper electrodes permit operation of the laser up to 100 torr. This design reduces the cathode-fall voltage, and as a result reduces the thermal loading in the cathode-fall region. The optimum pressure was 80 torr. At this condition the output power was more than that observed with expensive molybdenum electrodes in a similar laser system.

Electrochemical Behavior of Dy2O3 on Molybdenum and Liquid Tin Electrodes in Molten CaCl2

The electrochemical behavior of dysprosium oxide at a solid molybdenum and a liquid tin electrodes have been investigated by cyclic voltammetry and square wave voltammetry in molten CaCl2 at 1173 K. It was investigated that deposition potentials of calcium and dysprosium at liquid tin electrode are more positive than those at a solid molybdenum electrode. Meanwhile, our results proved that the dysprosium oxide can be reduced at both molybdenum and liquid tin electrodes. There are two steps for Dy2O3 reducing on Mo electrode while only one reduction step can be seen on liquid tin electrode. The Dy-Sn alloys were obtained through long time electrolyzing on a liquid tin cathode. The reduction mechanism for the reducing process was also investigated in this paper.

Electrode and Redox Potentials of Molybdenum and Stability of Molybdenum Chloro-Species in Alkali Chloride Melts

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides. Experiments were performed in individual salts (LiCl, NaCl, KCl, RbCl and CsCl) and in several binary and ternary eutectic or low-melting mixtures between 633 and 1173 K (depending on the melting point of the solvent salt). Formal standard electrode potentials E*Mo/Mo(III) and redox potentials E*Mo(III)/Mo(IV) in respect to Cl−/Cl2 couple and Gibbs free energy change of formation of molybdenum(III) chloride in alkali chloride melts were calculated. Electronic absorption spectra of Mo(III) ions were recorded, and spectroscopic parameters of MoCl63− complex ions determined. High temperature spectroscopy measurements were used to study the stability of Mo(III) chloro-species in fused chlorides and the reaction rates of Mo(III) ions disproportionation were also determined. Diffusion coefficients of molybdenum ions in LiCl-KCl-CsCl and NaCl-CsCl eutectic based melts were determined from the results of electrochemical measurements.

Plasma Assisted Generation of Micro- and Nanoparticles

In this research, the peculiarities of micro- and nanoparticles generation are considered. Two techniques of micro- and nanoparticles' formation using electric arc and underwater discharge plasma sources are proposed. Molybdenum oxide crystals were deposited on side surface of the bottom electrode (anode) of the free-burning discharge between metallic molybdenum electrodes. Friable layer of MoO&lt;sub&gt;3&lt;/sub&gt;, which consists of variously oriented transparent prisms and platelets (up to few hundreds of $mu;m in size), was formed by vapor deposition around the electrode. In the second technique, plasma of the underwater electric spark discharges between metal granules was used to obtain stable colloidal solutions with nanoparticles of 20-100 nm sizes.

Preparation and optimization of a molybdenum electrode for CIGS solar cells

Molybdenum (Mo) films were deposited by radio frequency (RF), direct current (DC) and mixed magnetron sputtering, respectively. With changing the deposition parameters including deposition pressure and power, the films show different surface morphology and crystallinity. Lower resistivity of the films is obtained in the DC mode and better reflectivity of the films is obtained in the RF mode. It is shown that the crystallinity increases when the deposition pressure decreases. The crystallinity and the grain size both increase as the deposition power increasing. The lowest resistivity of the single Mo film is 34×10-6 Ω·cm when the deposition pressure is 0.1 Pa and the deposition power is 300 W in the DC mode. In order to obtain lower resistivity, better adhesion and better reflectivity, bilayer films and tri-layer films were both deposited in different mode. They all show good adhesion and low resistivity. The Mo films deposited in mixed mode show better reflectivity. It is demonstrated that the resistivity of about 65×10-6 Ω·cm is achieved in DC/RF mode and the resistivity of about 61×10-6 Ω·cm is achieved in RF/DC/RF mode. And the tri-layer films achieved in RF/DC/RF mode have better reflectivity than bilayer films achieved in DC/RF mode. The tri-layer films achieved in RF/DC/RF mode is appropriate for using as the electrode of CIGS solar cells.

Electrochemical and Thermodynamic Properties of Lanthanum in a Chloride Melt – Liquid Metal System

Separation of spent nuclear fuel (SNF) components in a “molten salt – liquid metal” system is one of prospective directions in developing pyrochemical technology of SNF reprocessing. Modeling separation processes requires basic thermodynamic data for the elements present in SNF, both in the salts and metallic phases. In particular, values of the electrode potentials in the salt melt and activity coefficients in the metallic alloy are required. Present work was aimed at determining electrochemical and thermodynamic properties of lanthanum in alkali chloride melts (based on a low melting ternary LiCl–KCl–CsCl eutectic mixture) and gallium containing alloys. Lanthanum electrode potentials were measured in the melts containing 2.5–5.0 wt. % La. The following galvanic cell was employed: Cl2(C) │ LiCl–KCl–CsCl ║(LiCl–KCl–CsCl)–LaCl3│ La(Mo). Potential values were obtained using the potential-time transient curves recorded after a short time cathodic polarization of a molybdenum electrode. Molybdenum was selected as the working electrode material because it does not form alloys with lanthanum. Chlorine electrode was used as the reference. Typical potentiograms are presented in the Figure. Average potential value on the plateau of the curve was taken as lanthanum electrode potential. From the analysis of the experimental results the following temperature dependence of lanthanum electrode potential in the ternary lithium-potassium-cesium chloride eutectic melt was obtained: E * La(III)/La = –3.46 + 5.1·10-4 · T, V (668–976 K). Gibbs free energy change of lanthanum trichloride formation in LiCl–KCl–CsCl eutectic calculated from the results of electrochemical measurements is described by the following temperature dependence: ΔG*LaCl3 = –1001 + 0.148 · T, kJ/mol. Low melting gallium-aluminum (1.6 wt. % Al) and gallium-zinc (3.64 wt. % Zn) eutectic alloys were considered as liquid metal media. Activity coefficients of lanthanum, required for calculating separation factors, can be obtained from the difference of activity and solubility at a given temperature. Activity of lanthanum was determined employing electromotive force (EMF) measurements using the following galvanic cell: La(Ga–Me) │ (LiCl–KCl–CsCl)–LaCl3│ La(In), where Me = Al or Zn, and La(Ga–Me) and La(In) are saturated two-phase alloys. Supercooled liquid lanthanum was taken as the standard state for calculating thermodynamic values. Temperature dependencies of lanthanum activity in the studied alloys are adequately approximated by the following equations: lga La(Ga–Al) = –16068 · T -1 + 6.139, (573–1074 K), lga La(Ga–Zn) = –15346 · T -1 + 5.638, (573–1073 K). Solubility of lanthanum in the studied alloys was determined using direct physical measurements (sedimentation or filtration of saturated two-phase alloys) or EMF measurements of dilute homogeneous La(Ga–Me) alloys. Results obtained using different techniques showed a very good agreement. Lanthanum solubility in Ga–Zn eutectic at 389–1034 K can be described by the following linear equation: lgX La(Ga–Zn) = –2466 · T -1+ 1.018. Similar dependence for Ga–Al eutectic based alloys has an inflection around 480 K and can be described by two equations: lgX La(Ga–Al) = –4529 · T -1 + 3.487, (480–936 K), lgX La(Ga–Al) = –481 · T -1 + 4.947, (299–480 K). Temperature dependencies of lanthanum activity coefficients were obtained by subtracting solubility from activity, and the following equations were derived: lgγLa(Ga–Al) = –13602 · T -1 + 2.652, (573–936 K), lgγLa(Ga–Zn) = –12880 · T -1 + 4.619, (573–1034 K). The obtained data allow calculating separation factors of lanthanum and other elements present in SNF in the system “LiCl–KCl–CsCl eutectic melt – Ga–Al or Ga–Zn eutectic alloy”. Figure 1

In situ TEM study on structural and electric conduction properties of a multiwall carbon nanotube connected to a Mo electrode

Structural change of a multiwall carbon nanotube (MWNT) contacted with a molybdenum (Mo) electrode during Joule heating and its electrical conducting properties were investigated by in situ transmission electron microscopy. We found that inner shells of the MWNT were retracted (slide and disappeared) to the Mo electrode, and the total electrical conductance decreased accordingly. This result shows that inner shells being retracted to the Mo electrode contribute to the electric conduction in the MWNT. Utilizing this inner‐shell retraction phenomenon, inter‐shell conductance per unit length in a MWNT is derived to be ca. 13–19 mS/µm. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Toward an in situ phosphate sensor in seawater using Square Wave Voltammetry

A Square Wave Voltammetry electrochemical method is proposed to measure phosphate in seawater as pulse techniques offer a higher sensitivity as compared to classical cyclic voltammetry. Chronoamperometry cannot be either adapted for an in situ sensor since this method requires to have controlled convection which will be impossible in a miniaturised sensor. Tests and validation of Square Wave Voltammetry parameters have been performed using an open cell and for the first time with a small volume (<400µL) laboratory prototypes. Two designs of prototypes have been compared. Using high frequency (f=250Hz) allows to obtain a linear behaviour between 0.1 and 1µmolL−1 with a very low limit of detection of 0.05 µmolL−1 after 60min of complexation waiting time. In order to obtain a linear regression for a larger concentration range i.e. 0.25–4µmolL−1, a lower frequency of 2.5Hz is needed. A limit of detection of 0.1µmolL−1 is obtained in this case after 30min of complexation waiting time for the peak measured at E=0.12V. Changing the position of the molybdenum electrode for the complexation step and moving the detection into another electrochemical cell allow to decrease the reaction time down to 5min.

Flexible Molybdenum Electrodes towards Designing Affinity Based Protein Biosensors.

Molybdenum electrode based flexible biosensor on porous polyamide substrates has been fabricated and tested for its functionality as a protein affinity based biosensor. The biosensor performance was evaluated using a key cardiac biomarker; cardiac Troponin-I (cTnI). Molybdenum is a transition metal and demonstrates electrochemical behavior upon interaction with an electrolyte. We have leveraged this property of molybdenum for designing an affinity based biosensor using electrochemical impedance spectroscopy. We have evaluated the feasibility of detection of cTnI in phosphate-buffered saline (PBS) and human serum (HS) by measuring impedance changes over a frequency window from 100 mHz to 1 MHz. Increasing changes to the measured impedance was correlated to the increased dose of cTnI molecules binding to the cTnI antibody functionalized molybdenum surface. We achieved cTnI detection limit of 10 pg/mL in PBS and 1 ng/mL in HS medium. The use of flexible substrates for designing the biosensor demonstrates promise for integration with a large-scale batch manufacturing process.

Study of the electrochemical behavior of U(III) ions on liquid Cd electrode and preparation of the U–Cd intermetallic compound in fused 3LiCl–2KCl eutectic

The uranium (III) ions behaviour in fused 3LiCl–2KCl eutectic versus the Cl−/Cl2 reference electrode in the temperature range of 723–823 K on the liquid cadmium electrode by transient electrochemical techniques on the tungsten or molybdenum electrodes was studied. The mechanism of electrochemical reduction on cadmium cathode and the influence of temperature, cathode current density and the duration of electrolysis were studied. The activity coefficients and the base thermodynamic properties of uranium in fused U–Cd/3LiCl–2KCl system were calculated.

A novel ethanol biosensor on pulsed deposited MnOx-MoOx electrode decorated with Pt nanoparticles

Transition metal oxides provide low cost alternatives to noble metals due to their unique electrocatalytic and electrochromic properties and display strong interactions with noble metal nanoparticles. Present study describes the use of mixed molybdenum and manganese oxide film electrode enriched with platinum nanoparticles to enhance the catalytic performance in the oxygen reduction reaction (ORR). The mixed oxide film, denominated as GCE/MnOx-MoOx/Pt, was co-deposited by pulsed potential deposition technique and then, it was used as an immobilization matrix for the intact bacterial cells. The resulted whole cell biosensor has served as a sensing platform for ethanol detection by monitoring of oxygen consumption as a result of the bacterial metabolism in the presence of the substrate. Following the optimization experiments, GCE/MnOx-MoOx/Pt/Gluconobacter oxydans biofilm was applied to investigate the analytical characteristics and response time to ethanol as well as operational stability and substrate specificity for other carbon sources. The linear range was found as 0.075–5.0mM with a response time of 63s and the developed method was applied to the ethanol determination in alcoholic drinks. Satisfactory recovery figures were obtained in comparison to the standard method.

Electrochemical behavior of Th(IV) and its electrodeposition from ThF4-LiCl-KCl melt

The electrochemical behavior of Th(IV) ion on molybdenum (Mo) electrode was studied by cyclic voltammetry (CV) and square wave voltammetry (SWV) technologies at 773 K in ThF4-LiCl-KCl melt. The reduction of Th(IV) to metal Th at −1.67V (vs. Ag/AgCl) is a four-electron exchange process, which is quasi-reversible and diffusion-controlled. The diffusion coefficient (D) and activation energy of diffusion process for Th(IV) were determined to be 3.77×10−5cm2s−1 and 59.2kJmol−1. The pulse potential electrolysis of ThF4-LiCl-KCl melt revealed that 86.8% of Th(IV) can be separated from the melt based on the inductively coupled plasma atomic emission spectrometer (ICP-AES) results.

外径70 mm のコンフラットフランジにマウントした簡易金属蒸着源の製作

We have constructed a simple metal evaporator mounted on a conflat flange with an outer diameter of 70 mm. It consists of a tungsten filament, an alumina tube, two molybdenum electrodes, tantalum foils, and electric feedthroughs. Evaporation rates are measured as a function of electric current. We used it to prepare an Au thin film and a Si (111)-√3 ×√3 R30°-Au surface.

Electrochemical Behavior of Fe (III) Ion in CaO-MgO-SiO2-Al2O3-NaF-Fe2O3 Melts at 1673 K

Studies were conducted to investigate the cathodic behavior of Fe (III) ion in a CaO-MgO-Al2O3-SiO2-NaF-Fe2O3 melt at 1673 K. The kinetic parameters for the charge transfer steps and the reduction mechanism of Fe (III) were investigated by cyclic voltammetry and square wave voltammetry. The results show that two steps are involved during the electrochemical reduction of Fe (III): Fe3+ + e → Fe2+, Fe2+ + 2e → Fe. Meanwhile, galvanostatic electrolysis was performed on a molybdenum electrode, which further presents the feasibility of electro-depositing iron in the molten CaO-MgO-Al2O3-SiO2-NaF-Fe2O3 system.