Electrocatalysis in deep eutectic solvents: from fundamental properties to applications

Electrocatalysis stands out as a promising avenue for synthesizing high-value products with minimal environmental footprint, aligning with the imperative for sustainable energy solutions. Deep eutectic solvents (DESs), renowned for their eco-friendly, safe, and cost-effective nature, present myriad advantages, including extensive opportunities for material innovation and utilization as reaction media in electrocatalysis. This review initiates with an exposition on the distinctive features of DESs, progressing to explore their applications as solvents in electrocatalyst synthesis and electrocatalysis. Additionally, it offers an insightful analysis of the challenges and prospects inherent in electrocatalysis within DESs. By delving into these aspects comprehensively, this review aims to furnish a nuanced understanding of DESs, thus broadening their horizons in the realm of electrocatalysis and facilitating their expanded application.

Electrodeposition of Ni-Mo alloy coatings from choline chloride and propylene glycol deep eutectic solvent plating bath

Ni–Mo alloy coatings were deposited on a copper base material from a non-aqueous plating bath based on a deep eutectic solvent (DES) of choline chloride and propylene glycol in a 1:2 molar ratio containing 0.2 mol dm⁻³ NiCl₂ · 6H₂O and 0.01 mol dm⁻³ (NH₄)₆Mo₇O₂₄·4H₂O. Uniform and adherent Ni–Mo deposits with a nodular morphology were obtained at all the deposition potentials investigated (from − 0.5 to − 0.9 V vs. Ag). By shifting the potential from − 0.5 to − 0.9 V, the deposition current density increased from − 0.4 to − 1.5 mA cm⁻² and the overall surface roughness increased. It was also accompanied by an increase in the Mo content from ~ 7 to ~ 13 wt% in the potential range from − 0.5 to − 0.7 V. A further change in the potential from − 0.8 to − 0.9 V caused a decrease in the Mo content to ~ 10 wt% and a deterioration in the quality of the coating. For the most uniform coating, deposited at − 0.6 V and having a thickness of ca. 660 nm, the crystallite size did not exceed 10 nm. With the content of Ni (89 at.%) and Mo (11 at.%), the selected area electron diffraction (SAED) analysis allowed us to identify the cubic phase Ni_3.64Mo_0.36. The corrosion resistance of Ni–Mo coatings in 0.05 mol dm⁻³ NaCl solution generally increased during exposure of 18 h, as evidenced by ever higher polarization resistance. Finally, regardless of the applied deposition potential, low corrosion currents (in the range of 0.1–0.3 μA cm⁻²) have been measured for the coatings. EIS revealed that charge transfer resistances were the highest (57–67 kΩ cm²) for coatings deposited at − 0.5 V, − 0.6 V and − 0.7 V. Further increase in the deposition potential in the negative direction was unfavorable.

The analysis of nickel-obsidian composite formation on mild steel surface through electrocodeposition process

Composite, the engineering material consisting of two different macroscopic phases, can be synthesized through electro co-deposition reaction by adding particles into the solution of the electroplating process. The obsidian powder as dispersed particles and the nickel-metal as the matrix are the materials that can be synthesized using that technique. The hardness of obsidian is in the range of 650-900 HV, whereas nickel is 380 HV. We have investigated the formation mechanism of the coated composite layer made via electrocodeposition reaction. The electrocodeposition of the obsidian particle was controlled by kinetic, electrophoretic, and gravity forces that act on the particles. The resultant of these forces determines the direction of particles. The experimental result shows that increasing particle concentration from 0 to 25 g/L increased the hardness of the coated layer from 380 to 504 HV. The kinetic force can keep the particles suspended in solution at this concentration range. However, a further increase in particle concentration to 50 g/L decreases the kinetic energy of particles significantly, causing the particles to fall without depositing at the cathode and reducing the hardness down to 430 HV. Meanwhile, increasing current density increase the rapidness of the electro co-deposition reaction. At the increasing current density at a range of 3.3-10 Ampere/dm², the hardness of the coated layer is at the range of 420-540 HV. Coated layer thickness decreases slightly as particle concentration increases but increases significantly as current density increases. The highest hardness of the coated layer, 540 HV, was obtained at a particle concentration of 25 g/L and a current density of 10 A/dm².

High-Performance Ni-SiC Coatings Fabricated by Flash Heating

In this research, a novel flash heating coating application technique was utilized to create Ni-SiC coatings on carbon steel substrates with SiC contents much higher than is achievable using certain conventional coating techniques. Hardness profiles showed that the coatings improved the substrate by as much as 121%, without affecting the substrate. Tribotests showed that the wear performance was improved by as much as 4.7× in terms of the wear rate (mm3/N·m) for the same coating when using an Al2O3 counterpart. Pure SiC coatings as a reference were also fabricated. However, the SiC coatings experienced elemental diffusion of Fe from the carbon steel substrate into the coating during fabrication. This occurred due to the increased heat input required for pure SiC to fuse to the substrate compared to the Ni-SiC coatings and resulted in decreased tribological performance. Diffusion of Fe into the coating weakened the coating’s hardness and reduced the resistance to wear. It was concluded that ceramic–metallic composite coatings can successfully be fabricated utilizing this novel flash heating technique to improve the wear resistance of ceramic counterparts.

Nucleation and growth mechanisms of an electrodeposited Ni-Se-Cu coating on nickel foam

Electrocatalysts for water splitting have been widely explored among recent years. In this study, nickel-selenium-copper (Ni-Se-Cu) coating was synthesized on nickel foam through potentiostatic electrodeposition. The electrochemical kinetics and nucleation mechanisms of the deposition were investigated, and the diffusion coefficient D from different deposition potentials and temperatures was calculated. Results reveal that the electrodeposition of Ni-Se-Cu follows an instantaneous nucleation and diffusion-controlled three-dimensional (3D) growth mechanism. Deposition potential and bath temperature slightly effect the nucleation mechanism of electrodeposition. The apparent activation energy E_a of the hydrogen evolution reaction (HER) in 1.0 M KOH electrolyte of Ni-Se-Cu is 21.1 kJ·mol^-1, which is lower than that of Ni-Se (37.7 kJ·mol^-1). The majority phase formed by nickel and selenium is Ni₃Se₂, and a Ni(Cu) solid solution forms after the incorporation of Cu atoms into a Ni lattice.

Mechanical and Tribological Properties of Co-Electrodeposited Particulate-Reinforced Metal Matrix Composites: A Critical Review with Interfacial Aspects

Particulate-reinforced metal matrix composites (PRMMCs) with excellent tribo-mechanical properties are important engineering materials and have attracted constant scientific interest over the years. Among the various fabrication methods used, co-electrodeposition (CED) is valued due to its efficiency, accuracy, and affordability. However, the way this easy-to-perform process is carried out is inconsistent, with researchers using different methods for volume fraction measurement and tribo-mechanical testing, as well as failing to carry out proper interface characterization. The main contribution of this work lies in its determination of the gaps in the tribo-mechanical research of CED PRMMCs. For mechanical properties, hardness is described with respect to measurement methods, models, and experiments concerning CED PRMMCs. The tribology of such composites is described, taking into account the reinforcement volume fraction, size, and composite fabrication route (direct/pulsed current). Interfacial aspects are discussed using experimental direct strength measurements. Each part includes a critical overview, and future prospects are anticipated. This review paper provides an overview of the tribo-mechanical parameters of Ni-based co-electrodeposited particulate-reinforced metal matrix composite coatings with an interfacial viewpoint and a focus on hardness, wear, and friction behavior.

Potentiostatic electrodeposition of Ni-Se-Cu on nickel foam as an electrocatalyst for hydrogen evolution reaction

Development of cost-effective and efficient earth-abundant catalysts for hydrogen evolution reaction (HER) is a great challenge. In this study, by one-step potentiostatic electrodeposition, the Ni-Se-Cu electrocatalyst on nickel foam was fabricated as a binder-free HER electrocatalyst. As compared with Ni-Se electrocatalysts, such fabricated Ni-Se-Cu electrocatalyst exhibited prominent electrocatalytic activity to the HER in alkaline electrolyte. This Ni-Se-Cu electrocatalyst exhibits a small overpotential of 136 mV to achieve a current density of 10 mA·cm^-2 and high electrochemical stability. The remarkable HER properties of Ni-Se-Cu electrocatalyst mainly originate from high electronic conductivity induced by Cu-doping. This work shows a cheap and simple avenue to develop high efficient non-noble electrochemical electrocatalysts for HER.

Nanolubrication in deep eutectic solvents

We report surface force balance measurements of the normal surface force and friction between two mica surfaces separated by a nanofilm of the deep eutectic solvent ethaline. Ethaline, a 1 : 2 mixture of choline chloride and ethylene glycol, was studied under dry conditions, under ambient conditions and with added water, revealing surface structural layers and quantised frictional response highly sensitive to water content, including regions of super-lubric behaviour under dry conditions and with added water. We also report exceptionally long-ranged electrostatic repulsion far in excess of that predicted by Debye-Hückel theory for a system with such high electrolyte content, consistent with previously reported observations of "underscreening" in ionic liquid and concentrated aqueous electrolyte systems [Smith et al., J. Phys. Chem. Lett., 2016, 7(12), 2157].

Influence of Silica Particle Size on the Corrosion Behavior of Electroplated Silica-Ni Hybrid Layer

Silica-Ni hybrid layers were electroplated on a Q235 steel plate in a Watt nickel plating solution with varying sizes of silica, and a pure nickel layer was prepared as a blank sample. The surface morphology, composition, and crystal structure of the electroplated layers were examined using a field emission scanning electron microscope (FESEM), an energy-dispersive spectrometer (EDS), and a X-ray diffractor (XRD). The corrosion resistance of the samples was evaluated by the experiments of potentiodynamic polarization, electrochemical impedance spectroscopy, and salt spray test in NaCl solutions. The results showed that the smaller the particle size of silicon, the higher the content of silicon in the layer was, and the better the corrosion resistance of the layer was. However, when the particle size is 10 nm, agglomeration would happen, which leads to a decrease in the corrosion resistance performance, 50 nm silica-Ni hybrid layer showed the optimal corrosion resistance with the highest silica content.

Ni/cerium Molybdenum Oxide Hydrate Microflakes Composite Coatings Electrodeposited From Choline Chloride: Ethylene Glycol Deep Eutectic Solvent

Cerium molybdenum oxide hydrate microflakes are codeposited with nickel from a deep eutectic solvent-based bath. During seven days of exposure in 0.05 M NaCl solution, the corrosion resistance of composite coating (Ni/CeMoOxide) is slightly reduced, due to the existence of some microcracks caused by large microflakes. Multielemental analysis of the solution, in which coatings are exposed and the qualitative changes in the surface chemistry (XPS) show selective etching molybdenum from microflakes. The amount of various molybdenum species within the surface of coating nearly completely disappear, due to the corrosion process. Significant amounts of Ce³⁺ compounds are removed, however the corrosion process is less selective towards the cerium, and the overall cerium chemistry remains unchanged. Initially, blank Ni coatings are covered by NiO and Ni(OH)₂ in an atomic ratio of 1:2. After exposure, the amount of Ni(OH)₂ increases in relation to NiO (ratio 1:3). For the composite coating, the atomic ratios of both forms of nickel vary from 1:0.8 to 1:1.3. Despite achieving lower corrosion resistance of the composite coating, the applied concept of using micro-flakes, whose skeleton is a system of Ce(III) species and active form are molybdate ions, may be interesting for applications in materials with potential self-healing properties.

Potential applications of deep eutectic solvents in natural gas sweetening for CO2 capture

Novel solvents named deep eutectic solvents (DESs) have been intensively investigated in recent years. Their non-toxicity, biodegradability, low volatility, easy preparation and low cost make them promising green solvents for several industrial processes. This article provides a status review of the possible applications of DESs in natural gas (NG) sweetening by carbon dioxide (CO

Studies on the electrodeposition and characterization of PTFE polymer inclusion in Ni–W–BN nanocomposite coatings for industrial applications

A Ni–W–BN–PTFE nanocomposite coating with excellent corrosion and friction resistance alongside hardness and a smooth surface was developed.

Enhanced corrosion performance of Zn coating by incorporating graphene oxide electrodeposited from deep eutectic solvent

The corrosion resistance of Zn–GO is much better than that of pure Zn.