Higher Potential Differences Research Articles

(Invited) Highly Stable, Crosslinked Polymer Membranes for Non-Aqueous Flow Batteries

Long-term energy storage is a crucial stepping stone for the introduction of renewable sources in the energy mix due to their intermittent nature. Non-aqueous Flow Batteries (NFBs) are expected to operate at higher potential differences in comparison to conventional aqueous systems, thus yielding an improved power density. However, among the several issues of NFBs, it is pivotal to develop ion-conducting separators stable in organic solvents.Here we present a new family of ionomers which proved promising as candidates to implement this crucial function. The ionomers are founded on the strong ionic interactions between the functionalities of a polyamide-based material (P) and a perfluorosulfonic acid macromolecule (N). The latter interactions establish crosslinks between the two starting macromolecules. Consequently, the polymeric structure of the final ionomer is reorganized, improving the mechanical properties and curtailing the swelling upon immersion in organic solvents.The membranes obtained from two of the proposed ionomers, labelled “P-N0.77” and “P-N0.89”, show a swelling < 7% after 1000 hours of immersion both in acetonitrile (ACN) and in a 1:1 vol./vol. mixture of ethylene carbonate and propylene carbonate (EC:PC). Upon exchanging the membranes with tetraethylammonium (TEA+) cations, a common benchmark mobile specie for NFBs, it is shown that: (i) P-N0.77 exhibits a conductivity of 0.4∙10-3 S∙cm-1 and 0.9∙10-3 S∙cm-1 at 25°C in 0.1 M TEA+ in ACN and 0.1 M TEA+ in EC:PC, respectively; and (ii) P-N0.89 shows a conductivity of 1.5∙10-3 S∙cm-1 and 0.5∙10-3 S∙cm-1 in the same conditions.These results suggest that the proposed materials are suitable for application in NFBs, opening the door to the long-term operation of these devices.

Structure of cholinium glycinate biocompatible ionic liquid at graphite electrode interface.

We use constant potential molecular dynamics simulations to investigate the interfacial structure of the cholinium glycinate biocompatible ionic liquid (bio-IL) sandwiched between graphite electrodes with varying potential differences. Through number density profiles, we observe that the cation and anion densities oscillate up to ∼1.5 nm from the nearest electrode. The range of these oscillations does not change significantly with increasing electrode potential. However, the amplitudes of the cation (anion) density oscillations show a notable increase with increasing potential at the negative (positive) electrode. At higher potential differences, the bulkier N(CH3)3CH2 group of cholinium cations ([Ch]+) overcomes the steric barrier and comes closer to the negative electrode as compared to oxygen atom (O[Ch]+ ). We observe an increase in the interaction between O[Ch]+ and the positive electrode with a decrease in the distance between them on increasing the potential difference. We also observe hydrogen bonding between the hydroxyl group of [Ch]+ cations and oxygens of glycinate anions through the simulated tangential radial distribution function. Orientational order parameter analysis shows that the cation (anion) prefers to align parallel to the negative (positive) electrode at higher applied potential differences. Charge density profiles show a positive charge density peak near the positive electrode at all the potential differences because of the presence of partially positive charged hydrogen atoms of cations and anions. The differential capacitance (Cd) of the bio-IL shows two constant regimes, one for each electrode. The magnitude of these Cd values clearly suggests potential application of such bio-ILs as promising battery electrolytes.

A Pulse Voltage Application in Electrochemical Reduction of Solid CaWO4 Powder

Scheelite is the most abundant mineral of tungsten, but wolframite is used more than scheelite due to easier dissolution in alkaline solutions used in present processing methods. An alternative method based on electrochemical reduction of CaWO4 by direct current applications in molten salt solutions was recently reported. Pulse voltage and constant voltage reduction mechanisms were compared and an optimization of reduction kinetics was achieved in this study. The results have shown that faster reduction rates could be achieved during pulse voltage applications compared to constant voltage applications, when average value of pulse voltage application was the same as its constant counterpart. Furthermore, analysis of charge, energy and theoretical reduction graphs showed that reduction of calcium tungstate occurs at higher potential differences than 2.2 V, between calcium tungstate and graphite.

A Pulse Voltage Application in Electrochemical Reduction of Solid CaWO4 Powder

Among more than thirty known tungsten containing minerals, only scheelite (CaWO4) and wolframite ((Fe, Mn)WO4) are industrially important [1]. Scheelite is the most abundant mineral of tungsten, but wolframite is used more than scheelite due to easier dissolution in alkaline solutions used in present processing methods [2]. An alternative method based on electrochemical reduction of CaWO4by direct current applications in molten salt solutions was recently reported [3-5]. In this study, pulse voltage and constant voltage reduction mechanisms were compared and an optimization of reduction kinetics was achieved. Studies have shown that faster reduction rates could be achieved during pulse voltage applications compared to constant voltage applications, when average voltage value of pulse voltage application was the same as its constant counterpart. Furthermore, analysis of charge, energy and theoretical reduction graphs showed that reduction of calcium tungstate occurs at higher potential differences than 2.2 V, between calcium tungstate and graphite.

Corrosion behaviour of different tempers of AA7075 aluminium alloy

AA7075 is susceptible to localised corrosion like pitting, intergranular and exfoliation corrosion. Its susceptibility is strongly affected by heat treatments. In order to study the initiation of localised corrosion, solution heat treated, peak-strength (T6) and overaged (T76) tempers were characterised using complementary techniques like SEM-EDS, TEM, scanning confocal laser microscopy, scanning Kelvin probe force microscopy and micro-capillary cell. The localised attack initiated at intermetallics, which caused local dissolution of the matrix. At a later stage intergranular corrosion and exfoliation corrosion took place. The SKPFM investigation proved that the initiation of the attack at the intermetallics was caused by strong potential differences between intermetallics and matrix. The investigation with the micro-capillary cell showed that higher potential differences were associated with more negative local breakdown potentials for areas containing a few intermetallics. Besides, the susceptibility to intergranular corrosion was related to the precipitation of MgZn 2 particles at the grain boundaries. It is concluded that the corrosion behaviour of AA7075 is dependent on microstructure changes caused by heat treatment.