This study presents a comparative thermodynamic analysis of various pathways for electrochemical hydrogen production coupled with the anodic oxidation of urea, offering a sustainable alternative to the conventional oxygen evolution reaction. For the first time, the feasibility and efficiency of these processes were evaluated using integrated green chemistry metrics, including atom economy and a newly proposed metric, electricity economy, which quantifies the theoretical minimum electrical energy required for the equilibrium formation of reaction products. The analysis demonstrated that urea-oxidation pathways generally require significantly less energy input than water electrolysis. Among the examined reactions, the oxidation of urea to gaseous nitrogen and carbonate ions was identified as the most efficient, with an electricity economy of –4650.83 J mol–1 and an atom economy of 6.4%. However, practical application is hindered by issues such as low product selectivity and high anodic potentials dictated by the redox thermodynamics of commonly used nickel-based catalysts. These findings underscore the need for next-generation electrocatalysts with enhanced selectivity and lower overpotentials to fully exploit the energetic advantages of urea oxidation for green hydrogen production.

An important task of modern materials science is the development of highly efficient electrocatalysts for green hydrogen production. Specifically, this involves the urea oxidation reaction (UOR), which is an energetically advantageous and attractive alternative to the anodic oxygen evolution reaction, coupled with hydrogen evolution at the cathode. In this work, we present for the first time the use of systems based on a new generation of environmentally friendly room-temperature ionic liquids – deep eutectic solvents (DESs) – for the electrodeposition of electrocatalysts for UOR. The electrochemical performance of electrodeposited nanocomposite Ni–CeO2 electrocatalysts was evaluated in alkaline solution, showing an appreciable reduction in the anodic potential of UOR compared to oxygen evolution, reaching up to approximately 0.2 V at a current density of 0.1 mA cm–2. The obtained results are significant for the development of electrochemical synthesis methods for electrocatalysts used in green renewable energy.

The electrocatalytic behavior of electrodeposited Ni and Ni–Mo alloy coatings in the hydrogen evolution reaction in a 1 M NaOH aqueous solution was investigated by means of the electrochemical impedance spectroscopy method. The electrochemical deposition of electrocatalytic coatings was carried out using electrolytes based on deep eutectic solvents (eutectic mixtures of choline chloride with ethylene glycol or urea). To simulate the recorded Nyquist plots reflecting the electrocatalytic performance of deposited coatings, a modified Armstrong-Henderson equivalent circuit was employed, which accounts for the involvement of adsorbed intermediates in the reaction. The equivalent circuit included three polarization resistances and three constant phase elements, allowing for the consideration of the localization of the electrochemical process on different surface microdomains. It was found that the electrocatalytic activity of nickel coatings deposited from deep eutectic solvents exceeded the activity of nickel fabricated in an aqueous electrolyte. The increase in molybdenum content in the coating was shown to enhance electrocatalytic activity. It was established that the main reasons for improving the electrocatalytic properties of the Ni–Mo alloy coatings are structural-morphological factors (increase in the degree of microheterogeneity of the surface and the development of the surface area available for electrochemical reaction) and the formation of a favorable electronic structure of the metal, leading to the acceleration of the rate-determining Volmer step.

The development of efficient and energy-saving electrochemical processes for "green" hydrogen generation represents one of the most crucial challenges in modern chemistry and technology. An effective approach to realizing such processes involves alkaline water electrolysis with cathodic hydrogen evolution coupled with anodic urea oxidation reaction. As extensively reported in the literature, this configuration allows for a significant reduction in cell voltage. In this work, we present thermodynamic analysis of the electrochemical reaction CO(NH2)(2) + 2OH(-) +/- 6e(-) = 3H(2) + N-2 + CO32-, for the first time accounting for the inevitable carbonate formation due to interaction of carbon dioxide with alkali. Standard changes in enthalpy, entropy, and Gibbs free energy, as well as the electromotive force were calculated over the temperature range of 298-338 K. The influence of pH on the electromotive force value was investigated. It was observed that the process under study is accompanied by heat release and an increase in entropy, becoming thermodynamically spontaneous at pH > 12.66. From a thermodynamic point of view, this suggests exceptionally favorable energy characteristics for the studied process, promoting the development of an energy-efficient "green" hydrogen generation technology. Additionally, we demonstrate that increasing the temperature further enhances the thermodynamic features of this reaction by reducing the energy consumption for its progression.

For the first time, we investigated the process of potentiostatic anodic treatment of the surface of a copper (≈55%)-nickel alloy in a eutectic mixture of urea and choline chloride (reline), which is a typical representative of a new generation of ionic liquids, deep eutectic solvents. The anodic behavior of the alloy in the used solvent was characterized by cyclic voltammetry, and the nature of the electrochemical dissolution reactions of individual components of the alloy corresponding to several anodic current waves registered in voltammograms was determined. It was established that the anodic dissolution of the alloy occurs under conditions of salt surface passivation due to the formation of a layer of poorly soluble products of the electrode reaction. It was shown that under conditions of prolonged (150 min) potentiostatic polarization of the alloy in reline for various values of the electrode potential (in the range from 0.1 to 1.7 V relative to the Ag reference electrode), the chemical composition of the surface remained unchanged (i.e., there was no selective etching of individual components of the alloy), but an evolution of surface morphology patterns was observed, the specific type of which depended on the value of the applied potential. Anodic treatment of the Cu-Ni alloy in the reline solvent at any of the investigated anodic potentials led to an increase in the surface roughness coefficient, and electrochemical polishing did not occur. Analysis of kinetic data related to the hydrogen evolution reaction on the surfaces of reline-treated copper-nickel alloys in a 1 M NaOH aqueous solution showed a significant increase in exchange current density. This indicates enhancement of electrocatalytic activity compared to the untreated surface. The observed effect is likely associated with an increase in the true surface area of the alloy available for electrochemical reaction and an increase in the surface concentration of electrocatalytic sites resulting from the anodic dissolution of the alloy. The obtained results can be used in the development of highly efficient and relatively inexpensive electrocatalysts for hydrogen energy.

This study reports the patterns of electroplating a composite coating containing phases of nanocrystalline metallic nickel and cerium dioxide from an electrolyte based on a new generation of ionic liquids, deep eutectic solvents (DESs). Currently, the use of DESs is an extremely promising direction for the electrochemical deposition of electrocatalytic coatings for electrochemical energy, particularly for "green" hydrogen energy. The electrolyte for deposition contained a liquid eutectic mixture of choline chloride and urea (the so-called "reline", a typical representative of DESs), in which nickel (II) chloride (0.1 mol/dm³) and cerium (III) chloride (0.2–0.4 mol/dm3) were dissolved. According to the results of energy-dispersive X-ray analysis and XRD investigations, the composite coating contains approximately 12–16 at.% cerium, presumably in the form of cerium dioxide. A reaction scheme for the formation of the composite nanocoating, which includes a combination of chemical and electrochemical stages, is proposed. It was found that the incorporation of cerium oxide leads to a significant increase in electrocatalytic activity (compared to nickel coating) in the cathodic hydrogen evolution reaction and anodic oxygen evolution and urea oxidation reactions. It is hypothesized that the observed enhancement in electrocatalytic effect is due to the formation of additional active catalytic sites on the surface containing cerium in different oxidation states (e.g., Ce(+4)/Ce(+3)), which can act as electron carriers in both cathodic and anodic reactions. A significant advantage of this composite is its bifunctionality as an electrocatalyst, meaning it can be used for both cathodic and anodic processes. The obtained results can be used in the development of new high-efficiency processes for the electrochemical synthesis of "green" hydrogen.

This review provides a systematic analysis of the literature data on the electrodeposition of composite coatings using plating baths based on a new generation of room-temperature ionic liquids known as deep eutectic solvents (DESs). Such systems offer several advantages over traditionally used aqueous electrolytes and organic solvent-based electrolytes. The colloidal–chemical properties of suspension and colloidal electrolytes for composite deposition are thoroughly examined. New theories describing the kinetics of the co-deposition of composite layers are characterized. The kinetics and mechanisms of electrochemical deposition processes of composite coatings with metallic matrices are discussed. Case studies regarding the electrodeposition of composite coatings based on electrodeposited copper, silver, zinc, tin, nickel, cobalt, and chromium from DES-assisted electroplating baths are described and systematized. The main prospective directions for further research in the discussed scientific area are highlighted.

The influence of anodic potentiostatic treatment of nickel surface in deep eutectic solvents, ethaline and reline (eutectic mixtures of choline chloride with ethylene glycol and urea, respectively), on the electrocatalytic activity in the electrochemical reactions of oxygen evolution and urea oxidation in an aqueous alkaline medium (1 M NaOH) was investigated for the first time. It was shown that, depending on the chosen treatment potential and the nature of the eutectic solvent used, a significant increase in the rate of the studied processes was observed. Specifically, after anodic treatment of nickel under certain conditions, the polarization of the oxygen evolution reaction at a current density of 0.1 A/cm2 could be reduced by approximately 150–200 mV, and the maximum current density of urea oxidation could be increased by an order of magnitude (from 0.012 A/cm2 to 0.131 A/cm2 at a urea concentration of 0.33 mol/dm3 in alkaline solution). The observed increase in electrocatalytic activity after anodic treatment of nickel in deep eutectic solvents is likely related to changes in surface morphology patterns and the nature and concentration of relevant electroactive sites on the electrode surface. The results obtained in this work can be used for the development of highly efficient electrode materials for green hydrogen energy.

Electrodeposited nickel-based coatings microalloyed with lanthanum (up to approximately 1.75 wt%) were investigated. Electrodeposition was carried out using a deep eutectic solvent containing dissolved anhydrous salts of Ni(II) and Ce(III) as precursors. Electrochemical impedance spectroscopy results revealed that microalloying the chemical composition of coatings with lanthanum leads to a significant enhancement in electrocatalytic activity towards the hydrogen evolution reaction in an alkaline medium, as well as an improvement in corrosion resistance, compared to coatings not doped with lanthanum. These findings may be used in the development of high-performance electrocatalysts for hydrogen energy.

The paper reports the impact of anodic potentiostatic treatment of nickel in two representatives of a new type of eutectic ionic liquids (deep eutectic solvents), ethaline and reline, which are eutectic mixtures of choline chloride with ethylene glycol and urea, respectively. The influence of anodic treatment on surface morphology, roughness coefficients, and electrocatalytic activity towards the hydrogen evolution reaction is characterized. It is demonstrated that the current densities of nickel anodic dissolution in reline are approximately an order of magnitude lower than in ethaline under all other identical conditions. Significant differences in the kinetics of nickel anodic dissolution and passivation during anodic polarization in ethaline and reline have been established, which may be attributed to both a substantial difference in the viscosity of these solvents and differences in the chemical nature and composition of the ions present in them. It is found that anodic treatment in ethaline, at certain potentials, results in electrochemical polishing of the surface, confirmed by a decrease in measured roughness coefficients, while anodic treatment in reline does not allow effective electropolishing and only surface etching (increase in roughness coefficients) is observed. Anodic potentiostatic treatment of nickel in both investigated deep eutectic solvents at specific electrode potential values significantly enhances the electrocatalytic activity of the surface towards the hydrogen evolution reaction in an alkaline environment. This finding can be utilized in the development of electrocatalytic materials for the electrolytic synthesis of green hydrogen.

DESs, as new-generation room temperature ILs, are very promising to perform anodic processing of different metals and alloys. We report Ni-Cu alloy (45 wt.% Ni) anodic treatment in a DES, ethaline, for the first time. It is shown that the anodic dissolution originated the formation of sparingly soluble Ni and Cu chloride salts in the near-electrode layer. Ni-Cu alloy anodic treatment in ethaline resulted in changed patterns of the metallic surface morphology. Depending on the applied electrode E, star like crystallites, with sharply pointed shapes and thorns, may appear on the anodically etched surface, and surface morphology smoothing was observed, in some cases. The Ni-Cu alloy anodic processing in ethaline, which dramatically changed its surface morphology, contributed to a considerable increase in the electrocatalytic activity towards HER in an alkaline medium. Considering favorable environmental aspects of DES use and marked improvement in electrocatalytic properties, this treatment can be further used to develop high-efficient, eco-friendly and relatively inexpensive electrocatalysts methods for water electrolysis within H-2 energy concept.

The similarity theory and dimensional analysis were used to establish the mathematical dependences of the properties of chemical elements on their atomic numbers in the periodic system. Dimensionless quantities of the complex type (similarity criteria) were derived, which include, among other quantities, the first ionization energies and atomic radii in their structure. The values of the proposed dimensionless quantities for eighty-six chemical elements of the first six periods of the periodic system (1H-86Rn) were calculated, and the corresponding criterion dependences were analyzed. The obtained criterion dependences were expanded in Fourier series with a finite number of terms; the characteristic features of the corresponding spectral diagrams were discussed. Analysis of the dependences of the properties of chemical elements on the atomic numbers, carried out within the framework of the mathematical apparatus of the similarity theory and dimensional analysis, led to the conclusion that the fundamental chemical law, known as the periodic law, is better called the law of oscillations and formulated as follows: the dependences of the properties of chemical elements on their atomic number have the character of a superposition of a series of oscillations.

A deep eutectic solvent, ethaline (as a typical representative of new-generation room temperature ionic liquids), was used to anodically treat the surface of copper-nickel alloy (55 wt.% Cu). Anodic treatment in ethaline allows flexibly affecting the patterns of surface morphology: formation of stellated crystallites and surface smoothing (i.e. electropolishing) are observed depending on the applied electrode potential. The measured values of roughness coefficient ( Ra ) well correlate with the changes in surface morphology. Anodic treatment of Cu-Ni alloy in ethaline contributes to a considerable increase in the electrocatalytic activity towards the hydrogen evolution reaction in an alkaline aqueous medium, which can be used to develop new high-efficient and inexpensive electrocatalysts within the framework of the concept of carbon-free hydrogen economy.

This work reports the revisited thermodynamic parameters of urea oxidation reaction CO(NH2) (2)+H2O -> N-2+3H(2)+CO2, which is used in the urea-assisted electrolytic cell for the electrochemical production of hydrogen. It has been repeatedly stated in the literature that the open-circuit voltage for this reaction is equal to 0.37 V, which is significantly less than in the case of hydrogen evolution with a coupled oxygen evolution reaction (1.23 V at 298 K). This feature is considered as a very important advantage of urea-assisted systems. Our calculations showed that the value of 0.37 V is erroneous, and the correct value of the open-circuit voltage is similar to 0.07 V. The specified thermodynamic parameters confirm the excellent prospects of electrochemical hydrogen production with the coupled reaction of urea oxidation and indicate wide potential opportunities for further reduction of energy consumption in the electrolysis process. (c) 2023 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license ( http://creativecommons.org/ licenses/by/4.0/).

An electrolyte based on the deep eutectic solvent reline (a mixture of choline chloride and urea) was used for electrodeposition of coatings containing nickel and cerium. It was shown that a decrease in temperature and an increase in the concentration of cerium(III) ions in the plating bath lead to an increase in the content of cerium in the electrodeposits. Under certain conditions of electrolysis, it was possible to prepare coatings that contain nanocrystalline nickel with the inclusion of up to 40–59 wt.

We investigated the electrochemical modification of metal surfaces by using electrolytes based on a novel type of ionic liquids known as deep eutectic solvents (DESs). The anodic treatment of the Cu–Ni alloy in DESs significantly improves its electrocatalytic properties towards the hydrogen evolution reaction (HER). Modification of the chemical composition of nickel coatings via codeposition from DES-based electrolytes containing Fe(II), Mo(VI), Ce(III), and La(III) salts leads to a significant increase in electrocatalytic activity towards the HER, which can be used in development of hydrogen energy.

A literature review was conducted to examine the current understanding of the kinetics and mechanism of electrochemical reactions occurring during the electrodeposition of chromium coatings from electrolytes based on trivalent chromium compounds. The research in this scientific field is crucial, as it addresses the pressing need for an alternative to chromium plating processes that rely on solutions containing highly toxic and harmful hexavalent chromium compounds. Numerous literature data on the kinetics and mechanism of the stepwise reduction process of Cr(III) complex ions were analyzed. The influence of various additives and surfactants on the reaction kinetics of the stepwise reduction of trivalent chromium ions was considered. Special attention was given to the kinetics of the stepwise discharge of trivalent chromium ions in ionic liquids and deep eutectic solvents.

In this work, nanocrystalline nickel and nickel-molybdenum alloys were electrodeposited from electrolytes based on deep eutectic solvents. Eutectic mixtures of choline chloride with ethylene glycol (ethaline) and urea (reline) were used as typical representatives of deep eutectic solvents. The deposited Ni and Ni-Mo films were evaluated as potential electrocatalytic materials for green hydrogen production via electrolysis of alkaline aqueous solutions. The electrodeposited samples were characterized by XRD, SEM and EDX techniques, and the electrochemical behavior was evaluated by means of linear voltammetry and Tafel analysis. It was shown that the deposition of nickel (without molybdenum) from the electrolytes based on ethaline provides a higher elec-trocatalytic activity of the material with respect to the hydrogen evolution reaction than the material deposited from the reline-based electrolytes. The reline-based plating electrolytes contribute to a greater inclusion of molybdenum in the fabricated Ni-Mo alloys and therefore ensure increased electrocatalytic activity as compared with the ethaline-based electrolytes. The electrocatalytic behavior well correlates with the molybdenum content in the coatings. Ni and Ni-Mo electrodeposits produced from the deep eutectic solvent-mediated plating baths exhibit improved electrocatalytic performance and can be considered as promising catalytic materials for water electrolysis in green hydrogen energy.

We considered the kinetics of electrochemical processes occurring during electrodeposition of coatings from a low-temperature ionic liquid based on a eutectic mixture of choline chloride and ethylene glycol, in which a trivalent chromium salt is dissolved. Irreversible current waves of Cr(III) ions discharge on a glassy carbon electrode in the electrolytes of studied compositions are not described by the "classical" equations of linear and cyclic voltammetry, which is due to both the presence of the migration component of the current and the cathodic passivation of the electrode. It has been established that the introduction of additional water into the electrolyte leads to an increase in the current density of the wave of irreversible discharge of Cr(III) ions on the glassy carbon electrode, which is caused by a significant decrease in the viscosity of the solution. The current efficiency of the chromium deposition reaction decreases when water is introduced into the ionic liquid. The X-ray amorphous coatings electrodeposited from the electrolyte under study, along with chromium, contain carbon and oxygen, the inclusion of which is due to the electrocatalytic properties of the freshly deposited chromium surface.