Standard electrode potential | EPFL Graph Search

In electrochemistry, standard electrode potential E^\ominus, or E^\ominus_{red}, is a measure of the reducing power of any element or compound. The IUPAC "Gold Book" defines it as: "the value of the standard emf (electromotive force) of a cell in which molecular hydrogen under standard pressure is oxidized to solvated protons at the left-hand electrode". Background The basis for an electrochemical cell, such as the galvanic cell, is always a redox reaction which can be broken down into two half-reactions: oxidation at anode (loss of electron) and reduction at cathode (gain of electron). Electricity is produced due to the difference of electric potential between the individual potentials of the two metal electrodes with respect to the electrolyte. Although the overall potential of a cell can be measured, there is no simple way to accurately measure the electrode/electrolyte potentials in isolation. The electric potential also varies with temperat

Vanadium‐manganese redox flow battery: Study of Mn(III) disproportionation in the presence of other metallic ions.

Vimanshu Chanda, Solène Cindy Gentil, Hubert Girault, Sunny Isaïe Maye, Pekka Eero Peljo, Danick Reynard

The Mn(III)/Mn(II) redox couple with a standard potential of +1.51 V vs . SHE has drawn interest for the design of V/Mn redox flow battery (RFB). However, Mn(III) disproportionation leads to a loss of capacity, an increase of pressure drop and electrode passivation due to the formation of MnO 2 during battery cycling. In this work, we studied the influence of Ti(IV) or/and V(V) on Mn(III) stability in acidic conditions, by formulating 4 different electrolytes at equimolar ratios (Mn, Mn:Ti, Mn:V, Mn:V:Ti). Voltammetric studies have revealed a EC i process for Mn(II) oxidation responsible for the electrode passivation. SEM and XPS analysis demonstrate that the nature and the morphology of the passivating oxides layer strongly rely on the electrolyte composition. Spectroelectrochemistry highlights the stabilization effect of Ti(IV) and V(V) on Mn(III). At a comparable pH, the amount of Mn(III) losses through disproportionation is decreased by a factor of 2.5 in the presence of Ti(IV) or/and V(V). V(V) is an efficient substitute to Ti(IV) in order to stabilize Mn(III) electrolyte for redox flow battery applications.

Wiley2020

Engineering Dynamic Hydrogels For In Vitro Organoid Culture

Delphine Lorraine Perle Blondel

Over the past decade, methods to culture stem cells in three dimensions have opened up a plethora of new opportunities for basic and translational research in the life sciences. In particular, the use of natural extracellular matrix (ECM) surrogates, such as mouse tumour-derived Matrigel, unleashed the possibility to recapitulate complex multicellular behaviours in vitro, culminating in the successful derivation of miniaturized organs, termed organoids. While organoids hold tremendous potential as model systems in basic biology, the translational potential of these systems is hampered by their strict dependency on animal-derived matrices that suffer from batch-to-batch variability, potential immunogenicity, and ethical concerns. Recent efforts in engineering covalently crosslinked synthetic hydrogels have attempted to substitute these ill-defined organoid culture systems. However, although these bio-artificial matrices have shown significant potential for 3D organoid culture, their stability and their inherent elastic nature hamper organoid development. Indeed, the native ECM, just like Matrigel, is a highly viscoelastic and dynamic 3D milieu that can relax in response to tissue-induced stress by breaking and subsequently rearranging its network, thus permitting cellular remodelling without compromising the macroscopic stability of the material over time. Therefore, there is a need to develop the next generation of synthetic organoid culture matrices, exhibiting in vivo-like stress-relaxation properties. This thesis provides a perspective on how chemically defined, bio-inspired hydrogels can be designed as potential replacements for native ECM-based matrices for 3D organoid culture. In contrast to traditional synthetic hydrogels that cannot accommodate the dynamic mechanical changes occurring during organoid development, four types of synthetic matrices were developed that are crosslinked, either fully or partially, through dynamic physical bonds. The introduction of such network dynamics is shown to facilitate key morphogenetic processes, such as tissue budding, that are normally absent in purely elastic networks. A unifying hypothesis emerging from these results is that stress relaxation is a crucial requirement for 3D organoid culture. The mechanisms by which stress relaxation influence ISC fate and organoid development remain to be further studied. Preliminary experiments indicated that the stem cells may sense and respond to these characteristics through differential activation of mechanosensing pathways including the transcriptional co-activator YAP. The dynamic hydrogels developed in this thesis hold great promise as tunable environments for stem cell-based self-organization, enabling morphogenetic processes that may be suppressed in conventional synthetic hydrogels systems predominantly used in 3D cell culture applications.

EPFL2019

Chemical-mechanical polishing of tungsten

Jelena Stojadinovic

The chemical-mechanical polishing process (CMP) is an essential part of the production of integrated circuits. Metal to be polished in the CMP reacts with the oxidants from the aqueous suspension (slurry) and the passive film is formed on the metal surface. Suspended abrasive nano particles from the slurry cause detachment of the formed friable passive films. The CMP represents the tribocorrosion process where material deterioration results from the interaction of wear and corrosion which takes place in tribological contact exposed to an aggressive environment. Due to increasing number of the materials to be polished in the modern integrated circuits it is necessary to gather fundamental understanding of the removal mechanism of each material. The aim of this work is to elucidate the removal mechanisms of tungsten (W), as a typical microelectronic metal (chosen as a model system), under different oxidising conditions (represented by electrochemically imposed potential). Also, we want to evaluate the effect of the slurry composition (chelating agents lactic and phosphoric acid) on material removal. To achieve this electrochemical techniques (chrono amperometry, cyclic voltametry, passivation kinetics measurements under mass transport control), and tribocorrosion techniques (tests on tribometer with electrochemical cell with reciprocating sliding motion of alumina ball) were used. Surface analysis techniques (SEM, XPS, AES) were used to assess worn and unworn surfaces' morphology and chemistry. In addition, electrochemical tests and polishing tests on the CMP machine were performed with technical CMP slurries. The sliding action of the alumina ball on tungsten in the 0.01 M H2SO4 solution was found to cause three distinct wear responses depending on imposed electrode potential. Below the first threshold potential the wear is low due to negligible oxide film formation. In the following wear region, up to the second threshold potential, materials deterioration in the rubbed area proceeds by cyclic mechanical removal of the WO3 passive film followed by repassivation of tungsten. The amount of anodic oxidation corresponds to the overall removed metal volume. Beyond the second threshold potential a compact tribolayer of WO3 (more than 400 nm thick), probably formed by agglomeration of oxide particles detached from the metal surface, forms and covers the wear track. Similar tribocorrosion behavior of tungsten was also observed in the presence of 0.1 M H3PO4. The presence of chelating agents, lactic acid and phosphoric acid, in the 0.01 M H2SO4 solution does not change the volume of material wastage at potentials below the second threshold potential. Beyond this electrode potential, lactic acid promotes WO3 dissolution and thus impedes the third body formation and enhances wear. Electrochemical factors have a strong influence on the tungsten CMP removal rate. It was found that CMP removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A good correlation of tribocorrosion results and CMP practice was observed. Tribocorrosion models can be successfully applied to the tribocorrosion of tungsten and as well can be fairly used for modelling the material removal in CMP.

EPFL2009