Carbon dioxide, as a greenhouse gas, is one of the most significant contributors to climate changes on the planet. The CO2 electrocatalytic reduction to value-added products is the key to the achieving of practical renewable energy conversion and storage, as well as green chemical production based on the CO2 and H2O. Sn- and Bi-based electrocatalysts are considered being the most promising for the electrolytic reduction of CO2 to formic acid, which has a wide range of applications, a pure hydrogen carrier, in particular. Due to the low solubility of carbon dioxide in aqueous solutions, the most promising high-rate and selective electrodes for its reduction are gas-diffusion electrodes, which make it possible to overcome restrictions on the mass transfer of the substrate (CO2) to a highly developed three-phase contact surface. In this review, we have systematized the most significant practical results obtained over the 2019–2021 period by various research groups in the electrocatalytic reduction of carbon dioxide to formic acid in aqueous electrolyte solutions at gas-diffusion electrodes based on Sn and Bi and their compounds.

Electrocatalytic oxidation of aromatic pollutants (aniline, Methyl Orange, Eriochrome blue SE) is studied on lead dioxide, boron doped diamond, and ruthenium- and titanium-oxide-based anodes (DSA, dimensionally stable anode). The catalytic properties of the tested materials are studied using cyclic voltammetry and galvanostatic electrolysis. The activity of electrodes toward the electrochemical conversion of organics is shown to increase in the sequence of DSA < lead dioxide < boron doped diamond. The oxidation rate decreases in the order of Eriochrome blue SE > Methyl Orange > aniline for all electrodes. The oxidation process of the compounds corresponds to the pseudo-first-order reaction kinetics. The apparent rate constant grows at an increase in the applied current density and decrease in the initial pollutant concentration. The formation of both •OH and $${\text{SO}}_{4}^{{2\centerdot {\kern 1pt} - }}$$ radicals is confirmed by the free radical quenching studies; their contribution to the Eriochrome blue SE dye destruction process is evaluated.

The efficiency of new brands of technical carbon CH210 and C40 in the electrogeneration of Н2О2 from О2 in gas-diffusion electrodes (GDE) on their basis is studied in acidic solutions of the 0.5 М K2SO4 : 0.1 M H2SO4 (3 : 1) electrolyte. The textural characteristics of original materials and their mixtures with polytetrafluoroethylene used as the hydrophobizer are determined by low-temperature nitrogen adsorption as well as their electric double layer capacitance. For a comparison, similar studies are also carried out with technical carbon Vulcan XC-72. The rate constants for hydrogen peroxide decomposition on these materials in acidic solutions are determined. The data on the kinetics of hydrogen peroxide accumulation at the overall current density of 1500 А/m2 are as follows: the 6 h electrolysis produces the solution with Н2О2 concentration exceeding 2.5 М with the current efficiency higher than 70.0%. It is concluded that in this case, the surface area is not the determining factor. The technical carbons СН210 and С40 are comparable as regards their efficiency with the well-known technical carbon Vulcan XC-72 and can be used as electrocatalysts in GDE for electrosynthesis of Н2О2 from О2.

The results and prospects of the in situ (in the cell volume) and ex situ (outside the cell) use of aqueous solutions of hydrogen peroxide electrogenerated from oxygen in gas-diffusion electrodes (GDE) of carbon black are discussed. It is shown that using GDE based on technological A-437E carbon (acetylene black) and mesostructured carbon CMK-3 allows the Н2О2 solution with the concentration higher than 3 M to be obtained. It is found that electrosynthesized hydrogen peroxide may be used in situ with the high efficiency both in the indirect electrosynthesis of important organic and inorganic target products and in the destruction of organic and inorganic pollutants present in waste waters of different origin. Under the ex situ conditions, it is possible to synthesize the more concentrated solutions of Н2О2, organic peroxoacids, and inorganic peroxosolvates and also to carry out mineralization of exometabolites in autonomous life-support systems. These results may be helpful in selecting the most appropriate versions of using hydrogen peroxide solutions electrogenerated from oxygen for solving particular problems.

The efficiency of new brands of technical carbon CH210 and C40 in the electrogeneration of H(2)O(2)from O(2)in gas-diffusion electrodes (GDE) on their basis is studied in acidic solutions of the 0.5 \M K2SO4: 0.1 M H2SO4(3 : 1) electrolyte. The textural characteristics of original materials and their mixtures with polytetrafluoroethylene used as the hydrophobizer are determined by low-temperature nitrogen adsorption as well as their electric double layer capacitance. For a comparison, similar studies are also carried out with technical carbon Vulcan XC-72. The rate constants for hydrogen peroxide decomposition on these materials in acidic solutions are determined. The data on the kinetics of hydrogen peroxide accumulation at the overall current density of 1500 A/m(2)are as follows: the 6 h electrolysis produces the solution with H(2)O(2)concentration exceeding 2.5 \M with the current efficiency higher than 70.0%. It is concluded that in this case, the surface area is not the determining factor. The technical carbons CH210 and C40 are comparable as regards their efficiency with the well-known technical carbon Vulcan XC-72 and can be used as electrocatalysts in GDE for electrosynthesis of H(2)O(2)from O-2.

The results on how polytetrafluoroethylene (PTFE) affects the structural and electrochemical characteristics of porous composite materials based on furnace black СН600, acetylene black А437E, and mesostructured carbon СМК-3 are analyzed. Carbon materials differ by their preparation method, texture, wettability with respect to aqueous electrolyte, and specific surface area. The characteristics of the texture of original carbon materials and their mixtures with PTFE (5–70 wt %) are determined by the low-temperature adsorption of nitrogen. The composition materials are used as the electrode material in the working layer of gas-diffusion electrodes (GDE). The effect of PTFE on the volume and size of pores, the surface area of carbon materials, the volume of pores filled with electrolyte, the electric double layer capacitance, and the parameters of Н2О2 electrogeneration from О2 in aqueous sulfuric acid solutions at the current density of 150 mA/cm2 is described. It is shown that the effect of the PTFE concentration in composite materials on their characteristics depends on the properties of carbon materials. All carbon materials listed can be used in GDE for the reduction of О2 to Н2О2. For GDE with the optimal ratio of PTFE to the carbon material, the 5 h electrolysis produces the Н2О2 solution with the concentration of 1.9–2.4 М at 65–87% current efficiency.

We study the effect of the treatment by an electric current of two-layer gas diffusion electrodes made of porous (66–68 vol %) composite material based on A-437E acetylene black and polytetrafluoroethylene. Polarization is carried out cyclically in the range 0.0 ± 2.0 V by anodic and cathodic currents, respectively, in 1 M H2SO4 and 0.5 M KOH with the addition of tetrabutylammonium bromide (TBAB). In both cases, the increase in the charge passed leads to an increase in the volume of electrolyte pores and the electrical capacitance of the electrodes. Under anodic polarization, the increase in the wettability and capacitance of the electrodes is larger than under the cathodic process; however, in the latter case, the carbon surface does not undergo oxidation. As the charge passed and the TBAB concentration increases, the ohmic loss grows. The possible causes of the observed phenomena are considered.

Electrochemical oxidation for degradation of industrial dye Methyl Orange in aqueous sulfate solutions with various electrocatalytic materials: boron-doped diamond electrode and electrode based on titanium and ruthenium oxides. The influence exerted by the main working parameters of electrolysis (current density, concentration of Methyl Orange, pH) on the discoloration efficiency and on the chemical oxygen demand (COD) was examined. It was shown that an increase in the current density and a decrease in the pollutant concentration improve the process efficiency. However, this leads to an increase in the specific electric energy consumption per unit mass of COD. It was found that the boron-doped diamond electrode is a more efficient electrocatalytic material, compared with electrode based on titanium and ruthenium oxides. At low concentrations of Methyl Orange (<50 mg L–1), there exists the possibility in principle of using the electrode based on titanium and ruthenium oxides not only for discoloration, but also for making lower the COD level.

The effect of (0.05 M) tetraalkylammonium salt additions in aqueous 0.5 M KOH on the rate of impregnation of carbon black electrodes with a polytetrafluoroethylene binder (5–20 wt % PTFE) at hydrogen evolution potentials was studied. It was shown that tetraalkylammonium salts facilitate the fast filling of electrodes with electrolyte, and their effect increases with the molecular mass of the cation. Tetramethylammonium bromide showed the weakest effect. In solutions with tetrabutylammonium bromide, the electrodes were completely flooded with 5 wt % PTFE within 15 min. Diethyldibenzylammonium bromide had a similar effect. The influence of the PTFE concentration in the electrodes on their capacity was studied. The specific capacity of acetylene black in acid and alkaline aqueous solutions was evaluated from the electrode surface area determined by low-temperature nitrogen adsorption (BET).

Mesostructured carbon CMK-3 (Carbon Mesostructured by KAIST) synthesized by the template method is studied as the electrocatalyst for electrosynthesis of Н2О2 from О2 in a gas-diffusion electrode (GDE) in alkaline and acidic solutions. The texture characteristics of the original material and its mixture with hydrophobizer (polytetrafluoroethylene) are studied by the method of low-temperature nitrogen adsorption. The rate constants for hydrogen peroxide decomposition on these materials in alkaline and acidic solutions are calculated. Kinetic parameters of oxygen reduction in alkaline and acidic solutions are determined as well as the capacitance of gas-diffusion electrodes based on mesocarbon. The selectivity of the electrocatalyst is estimated by finding the current fracture γ consumed in oxygen reduction to hydrogen peroxide. Data on the kinetics of hydrogen peroxide accumulation during electrosynthesis of Н2О2 from О2 are obtained. The acidic solution of hydrogen peroxide with the concentration more than 3 M is obtained with the current efficiency higher than 80%.

Furnace black СН600 was studied as a catalyst for electrosynthesis of Н 2 О 2 from О 2 in a gas diffusion electrode in alkaline and acidic solutions. The texture properties of the starting black СН600 and gas diffusion electrodes (GDEs) based on it were determined by the low-temperature nitrogen adsorption (LTNA) method. The rate constants of Н 2 О 2 decomposition on black and its mixtures with fluoroplast-4D in acidic and alkaline solutions of electrolyte were calculated. The process selectivity γ (the current fraction spent on the two-electron reduction of oxygen), kinetic parameters of oxygen reduction, and double-layer capacity of the СН600-based GDE were determined. Data on the kinetics of hydrogen peroxide accumulation during electrosynthesis from oxygen in СН600-based GDE in acidic and alkaline solutions were obtained.

The paper studied the effect of texture characteristics of gas diffusion electrodes (GDE) based on A 437-E acetylene black for electrosynthesis of H2O2 from O-2 in acid solution. Texture characteristics of the initial material (A437-E) and its mixtures with a hydrophobisator fluoroplastic-4D (F-4D) were determined by the low-temperature nitrogen adsorption (LTNA) method. Rate constants for hydrogen peroxide decomposition (K-chem) over these materials in acid solution were calculated. The dependence of electrochemical activity of GDE on the quantitative of F-4D content in the working layer was studied and process selectivity (gamma) was determined. The effect of the working layer of GDE has a substantial impact on the development of highly developed surface of 3-phase contact and removal efficiency of the target product of electrode pore volume. These conditions were met in electrodes with a low content of a hydrophobisator at comparatively high hydrophilic porosity. Minor microporosity of composites of working layers ensured low losses of the target product due to its heterogeneous decomposition and contributed to its efficient removal from the electrode. The findings on accumulation kinetics of hydrogen peroxide during electrosynthesis from O2 in acid solution at a current density of 150 mA/cm(2) in GDE with different F-4D contents were acquired. Acid solution of 2.43. hydrogen peroxide with a current efficiency (CE) of 76 % and process selectivity of 0.69 was obtained in GDE using 5 mass % of F-4D for 5.5 h of preparative electrosynthesis.

The effect exerted by treatment with cyclic anodic current in 1 M H2SO4 in the interval 0.0–2.0 V on electrodes made of a porous (55 vol %) composite material consisting of A-437E acetylene black and polytetrafluoroethylene (60 wt %) was studied. The cyclic volt–ampere curves were recorded in 3 M KOH and 1 M H2SO4 to determine the double layer capacity. The anodic treatment leads to an increase in the volume of pores filled with the electrolyte and in the electrical capacity of the electrode due both to an increase in the area of the surface wetted with the electrolyte and to the pseudocapacity caused by oxidation of the carbon black surface.