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The electrochemistry of Sn(II), Sn(IV), and Se(IV) in the choline chloride-glycerol deep eutectic solvent was investigated on glassy carbon at 110 °C by various electrochemical methods including cyclic voltammetry, chronopotentiometry, and chronoamperometry. The Sn(IV)/Sn(II) couple displays very slow electrochemical kinetics, so that the reduction of Sn(IV) essentially proceeds directly to Sn(0) through a 4 e− process, even if the potential of the Sn(IV)/Sn(II) couple is more positive than the Sn(II)/Sn(0) couple. Because of this sequence of potentials, the comproportionation reaction Sn(IV) + Sn(0) → 2Sn(II) takes place when elemental tin is deposited from the Sn(IV) solution. The electrodeposition of tin proceeds through an instantaneous nucleation process with Sn(II) whereas a progressive nucleation mechanism is observed with Sn(IV). In the presence of both Sn(IV) and Se(IV) in solution, voltammetric features associated with the formation of a Sn-Se compound are clearly identified and the suitable potential range where the compound can be deposited without co-deposition of elemental tin is determined, as confirmed by SEM images and EDX analyses. [ABSTRACT FROM AUTHOR]

• Reduction of Au(III) via a 2e−+1e− path on GC but via a direct 3e− path on pt. • Comproportionation/disproportionation equilibrium of the Au(III)-Au(I)-Au(0) system. • Standard potentials of the involved redox couples were estimated. The electrochemistry of the Au(III)-Au(I)-Au(0) system was studied in the choline chloride - oxalic acid dihydrate deep eutectic solvent at 60 °C. At glassy carbon electrodes, the 2 e− waves of the Au(III)/Au(I) and the 1 e− waves of the Au(I)/Au(0) are well separated in the cyclic voltammograms, whereas at platinum electrodes two anodic peaks but only one single 3 e− cathodic peak are observed. Electrochemical quartz crystal microbalance and zero-current chronopotentiometric experiments demonstrate the occurrence of a comproportionation reaction AuCl 4 − + 2 Au (0) + 2 C l − ⇌ 3 AuCl 2 − when gold is electrodeposited from an Au(III) solutions. The standard potentials of the Au(III)/Au(I), Au(I)/Au(0) and Au(III)/Au(0) couples were estimated, and even though E Au (III) / Au (I) 0 < E Au (I) / Au (0) 0 translates into an equilibrium constant of the reaction being unfavourable to the comproportionation (K c,comp < 1), it is shown that the reaction can be quantitatively displaced to the right under non-equilibrium conditions imposed by mass-transport. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electroreduction of furfural in deep eutectic solvents on gold and copper. • Spontaneous reaction of in choline chloride/urea 1:2. • Decreased overpotentials on Au/C and Cu/C nanoparticles. • Formation of furfuryl alcohol was evidenced by in-situ IR. Electroreduction of furfural, a platform chemical that can be obtained from lignocellulosic biomass and agricultural waste, was studied by coupling in-situ FTIR spectroscopy and cyclic voltammetry on gold and copper electrodes in Deep Eutectic Solvent (DES) unconventional medium. Choline-glycerol DES medium was selected by investigating with the help of NMR spectroscopy the stability of furfural in the two commonly used DES: choline chloride/urea 1:2 and glycerol 1:2. Instability of furfural in choline chloride/urea 1:2 and the presence of a specific reaction between urea and furfural was evidenced with the same technique. After that, the electrochemical stability of the glycerol DES itself was monitored by FTIR measurements in the absence and presence of furfural. The product of furfural reduction, furfuryl alcohol, was evidenced. The influence of the electrode morphology on the stability of the DES and the furfural behavior was investigated by synthesizing Au/C and Cu/C nanoparticles. [ABSTRACT FROM AUTHOR]

The electrodeposition of copper-selenium binary compounds on gold electrode was investigated in the choline chloride – urea (ChCl-U) deep eutectic solvent at 110 °C. Cyclic voltammetry and potentiostatic deposition followed by stripping voltammetries were employed to gain thermodynamic insights pointing to the formation of Cu 2 Se, which is supported by X-Ray Photoelectron Spectroscopy. The electrochemical stability domain of the compound is about 900 mV (from ≈−0.81 V to ≈+0.07 V vs. a silver quasi-reference electrode) and is consistent with the thermodynamics of induced co-deposition. The domain is limited at positive potentials by the oxidation to Se(0) and at negative potentials by the reduction to Cu(0). While copper is readily deposited on the gold electrode, the electrodeposition of selenium requires a significant overpotential. As a consequence, copper is kinetically favoured in the co-deposition and is present in its pure form at short deposition times. This is circumvented by using longer deposition times or more negative potentials. Photoelectrochemical measurements show that the as-deposited compound behaves as a p-type semiconductor. [ABSTRACT FROM AUTHOR]