ELECTROCATALYTIC COBALT-VANADIUM COATINGS FOR THE HYDROGEN EVOLUTION REACTION

The study of existing energy-saving materials and obtaining the new ones for reducing the cost of the hydrogen production, is relevant for modern hydrogen energy industry. Such properties can be predicted for materials containing vanadium, molybdenum, tungsten and exhibiting catalytic activity for the hydrogen evolution reaction Aforementioned metals can be co-deposited from aqueous solutions with iron subgroup metal-catalysts through the formation of cluster intermetallic compounds with Me-V bond adsorbed on the cathode surface. The induced co-deposition of cobalt with vanadium from the complex citrate electrolyte was investigated in the current work. As a result of the research, it was found that the uniform microcrystalline light-gray high-quality cobalt-vanadium alloy coating is possible to precipitate from a citrate electrolyte with content of 20 g/dm3 vanadium (in terms of metal) as a citrate complex The process was carried out at a current density of 5–10 A/dm2, at a temperature of 30–40°С, pH = 2,8–3,2. The content of vanadium in the coating is 0,37–0,53 % by weight. The maximum vanadium content in the coating is observed at current densities 8–9 А/dm2. The catalytic activity study of the coating that was obtained using cobalt-vanadium alloy in the reaction of hydrogen reduction at the cathode was performed in solution of 2,5М NaOH + 0,02 M NaCl. By increasing the vanadium content in the coating from 0,37 to 0,53% the hydrogen evolution overvoltage is reduced by 0,5 V. It was found that the overvoltage of the hydrogen ion evolution reaction on cathodes from steel 20 with cobalt-vanadium coating is 0.08–0,1 V lower, and the exchange current is higher than on electrodes made of steel 20, which are used in industrial water-alkali electrolysis. This indicates the electrocatalytic activity of the investigated materials for the hydrogen evolution reaction. Electrodes with coating, obtained by cobalt-vanadium alloy can be recommended as a cathode material for the hydrogen electrochemical production. Hydrogen evolution overvoltage reduction also decrease the energy consumption for this process by 15–20 %.

Electrochemical deposition of cobalt alloy

Electrodeposition of cobalt alloys with refractory metals makes it possible to obtain coatings with a unique combination of physicochemical properties that are unattainable using other deposition methods. For the deposition of high-quality coatings with a cobalt-vanadium alloy, it is proposed to use a citrate electrolyte. Co-V coating was deposited on steel samples from citrate electrolyte at a temperature of 35-40 °C and a current density of 5-12 A/dm2 using soluble cobalt anodes. The vanadium content in the coating deposited at a ligand concentration of 0.3 mol / dm3 is 0.1-0.5 wt%. An increase in the concentration of the ligand to 0.4 mol / dm3 promotes the binding of cobalt into complexes, and, accordingly, the vanadium content in the coating increases to 0.6-1.2 wt.%. Moreover, the tendency to change the percentage of alloying elements with current density remains. Deposition coatings are dense, shiny, without internal stresses and cracks. The proposed compositions of electrolytes and modes of deposition of Co-V coatings with a vanadium content of up to 1.5 wt.% And a current efficiency of 50%. It was found that Co-V coatings are characterized by increased carbon content and are substitutional solid solutions, and the surface morphology of the obtained coatings depends significantly on the current density and changes from fine-crystalline to globular spheroid. The optimal current density for obtaining high-quality coatings with a cobalt alloy in a galvanostatic mode is ік = 10 A / dm2. Management of the storage of galvanic cobalt alloys in a quite wide range of concentrations of alloy-forming components is achieved by varying the electrolysis parameters, which allows the deposition technology to be adapted to the needs of the modern market.

ELECTROCATALYSIS OF THE HYDROGEN EVOLUTION REACTION ON CoRe, CoWRe SUPERALLOYS DEPOSITED FROM CITRATE ELECTROLYTE

The reaction of electroreduction of hydrogen ions on binary CoRe and ternary CoWRe alloys electrodeposited from a citrate electrolyte with different amount of potassium perrhenate (0.01 and 0.05 mol·L-1) depending on the deposition current density (5–40 A·cm-2) has been investigated by the method of stationary voltammetry. The kinetic parameters of the reaction have been calculated, and it is shown that the use of ternary alloys allows one to increase the value of exchange current density by almost an order of magnitude and significantly reduce the overvoltage of hydrogen reduction in comparison with cobalt. It is shown that the best electrocatalysts for the reduction of hydrogen in alkaline solution can be ternary CoWRe alloys with a rhenium content of 15–20 at. %.

Electrochemical deposition of nanocrystalline Ni–W coatings from citrate electrolytes

In this paper, requirements for wear-resistant coatings of Ni–40%W manufactured by electrochemical deposition are determined, and the electrolyte stability is studied. The influence of temperature and current density on the deposition of the Ni–W alloy from a citrate electrolyte was researched, and the optimal deposition mode was found. The maximum operating time of the electrolyte is established by the dependence of the current efficiency for the Ni–W alloy on the electric power transmission. The necessity of using membrane electrolyzers during the deposition of nickel-tungsten coatings is substantiated.

Effect of Electrodeposition Parameters on the Composition and Surface Topography of Nanostructured Coatings by Tungsten with Iron and Cobalt

The electrodeposition of binary and ternary coatings Fe-W and Fe-Co-W from mono ligand citrate electrolyte has been investigated. The Fe-Co-W coatings were formed from electrolytes, which composition differs in the ratio of the concentrations of the alloying components and the ligand content. The investigation results indicate a competitive reduction of iron, cobalt and tungsten, the nature of which depends both on the ratio of electrolyte components, and electrolysis parameters. The effect of both current density amplitude and pulse on off time on quality, composition and surface morphology of the galvanic alloys was determined. Coatings deposited on a direct current with a density of more than 6.5 A/dm2, crack and peel off from the substrate due to the inclusion of Fe (III) compounds containing hydroxide anions. The use of non-stationary electrolysis allows us to extend the working range of current density to 8.0 A/dm2 and form electrolytic coatings of sufficient quality with significant current efficiency and the content of the refractory component. The presence of the Co7W6, Fe7W6, α-Fe, and Fe3C phases detected in the Fe-Co-W deposits reflects the competition between the alloying metals reducing from hetero-nuclear complexes. The surface of binary and ternary coatings is characterized by the presence of spherical agglomerates and is more developed in comparison with steel substrate. The parameters Ra and Rq for electrolytic alloy Fe-W are of 0.1, for Fe-Co-W are 0.3, which exceeds the performance of a polished steel substrate (Ra = 0.007 and Rq = 0.010). These properties prospect such alloys as a multifunctional layer are associated with structural features, surface morphology, and phase composition.

Synthesis and Characterization of Ni–W/Cr2O3 Nanocomposite Coatings Using Electrochemical Deposition Technique

Ni–W/Cr2O3 nanocomposite coatings were synthesized from aqueous sulphate-citrate electrolyte containing Cr2O3 nanoparticles on a steel surface using conventional electrodeposition technique. This study was aimed at investigating the influence of Cr2O3 nanoparticle content on the microstructure, corrosion resistance, and mechanical properties of electrodeposited Ni–W/Cr2O3 nanocomposite coatings. Ni–W binary alloy coatings were deposited and optimized before addition of the nanoparticles to produce high-quality coatings. The microstructure and chemical composition of the Ni–W/Cr2O3 nanocomposite coatings were evaluated using scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), and XRD. Corrosion resistance properties were evaluated using potentiodynamic polarization (Tafel) measurements in 3.5 wt.% NaCl medium. The corrosion resistance and microhardness are significantly higher in Ni–W/Cr2O3 nanocomposite coatings compared to pure Ni–W binary alloy and increase with the increase in content of Cr2O3 nanoparticles in the coatings. Wear resistance is also higher in Ni–W/Cr2O3 nanocomposite coatings.