The electrochemical behavior of Co(II) reduction and Co nucleation/growth process on glassy carbon (GC) electrode in 1-ethyl-3methylimidazolium bisulfate ([EMIM]HSO 4 ) ionic liquid (IL) and ethylene glycol (EG) system is investigated. Cyclic voltammetry (CV) measurements indicate that Co(II) reduction occurs by a one-step process, Co(II) to Co(0), and it is an irreversible reaction. The diffusion coefficient of Co(II) is 2.24 × 10 −6 cm 2 s −1 at 323 K in the system. Chronoamperometry measurements show that the growth/nucleation process of Co on GC electrode in [EMIM]HSO 4 -EG is a three-dimensional (3D) progressive nucleation at lower overpotentials and instantaneous nucleation at higher overpotentials under diffusion controlled growth process. These effects of electrodepositing potential, current density and temperature on Co coating thickness are also investigated. The Co coatings are observed by energy dispersive spectrometer (EDS), scanning electron microscope (SEM) and X-ray diffractometer (XRD). SEM micrographs confirm that the Co coatings are relatively loose with a fibrous surface morphology. XRD pattern of the prepared coating reveals the characteristic peak of crystalline Co with a preferred orientation direction and the average size of Co grains is 11 nm. The nanocrystalline Co coating exhibits an excellent catalytic activity and stability for hydrogen evolution reaction (HER) in alkaline medium.

The codeposition behavior of Co(II) and Ni(II) in 1-ethyl-3-methylimidazolium bisulfate ([EMIM]HSO 4 ) ionic liquid (IL) and ethylene glycol (EG) system is recorded using cyclic voltammetry (CV). The result indicates that the mechanism of Co-Ni codeposition is an anomalous codeposition, and both the inhibition for Ni(II) reduction and the anomalous codeposition phenomena can be improved by the increase of Co(II) concentration in the system. These Co-Ni deposits are characterized using EDS, XRD and SEM. SEM micrographs indicate that the surface morphology of Co-Ni deposits can be changed by the increase of Co content, and these deposits with many additional elongated acerose crystals, which are germinated on the spherical "nodules" surface, can be prepared. XRD pattern reveals the feature peak of crystalline Co 1.2 Ni 2.8 with a preferred orientation direction and the average size of grains is calculated to be about 14.6 nm. Moreover, the electrocatalytic activity for hydrogen evolution reaction (HER) on the prepared Co-Ni catalyst surface is investigated using linear sweep voltammetry (LSV) and electrochemical impedance spectra (EIS). These results indicate that the obtained catalyst presents an excellent electrocatalytic activity and durability for HER with an overpotential of 139 mV at 10 mA cm −2 in 1 M KOH solution.

Summary This work describes a two‐step method of in situ electrodeposition and following oxidation to prepare carbon paper (CP) modified with tin dioxide (SnO2) particle as bifunctional electrode for vanadium redox flow battery. Electrodeposition technique was employed to load metal tin particle on CP, which was subsequently oxidized to tin dioxide at high temperature. CPs modified by SnO2 with different content (CP/SnO2‐1, CP/SnO2‐2, and CP/SnO2‐3) were obtained by controlling electrodeposition time. CP/SnO2 presents an increase in electrochemical performance including faster charge and mass transfer compared with pristine CP. Among all samples, CP/SnO2‐2 with proper decorated SnO2 exhibits superior electrocatalytic performances for V3+/V2+ and VO2+/VO2+ reactions. Raised performances can be attributed to that SnO2 nanoparticles provide more active sites leading to rapid electrochemical kinetic of vanadium redox reactions. In addition, mass transfer can be accelerated due to the excellent hydrophilicity of SnO2. The cell using CP/SnO2‐2 as bifunctional electrode exhibits better stability and higher capacity retention during 50‐cycle charge‐discharge test. The cell for CP/SnO2‐2 shows higher energy and voltage efficiency, suggesting that introduction of SnO2 can decrease electrochemical polarization. At 150 mA cm−2, energy efficiency of the cell increases by 7.8% through using CP/SnO2‐2.

Based on cyclic voltammetry (CV) and chronoamperometry (CA) experiments, the effects of deposition potential and temperature on Co(II) reduction and electrocrystallization in ethylene glycol (EG) are reported. CV observations show that Co(II) reduction reaction is irreversible, and both the equilibrium potential and nucleation overpotential, ηnucleation, move to more positive values as the temperature is improved. The diffusion coefficient of Co(II) is 2.24 × 10-6 cm2 s-1 at 323 K and the activation energy for bulk diffusion (Ebd) of Co(II) is 22.03 ± 0.8 kJ mol-1. CA observations show that the Co electrocrystallization mechanism is a 3D nucleation and growth of a new phase under diffusion control. Its mechanism is an instantaneous regime at 323 K at various applied potentials, however, the mechanism changes from a progressive nucleation to an instantaneous nucleation at -1.10 V as the temperature increases. The cobalt nuclei are spherical microstructure, and these innumerable spherical crystal particles can align compactly to form many wirelike crystals bundles. XRD observation confirms the characteristic peak of crystalline Co with a preferred orientation direction and the average size of Co grains is 19 nm. It is also found that the Co coating thickness is affect by the potential and temperature.

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