Oxygen Evolution Reaction In Alkaline Media Research Articles

Nanoconfined Synthesis of Nitrogen-Rich Metal-Free Mesoporous Carbon Nitride Electrocatalyst for the Oxygen Evolution Reaction

Synthesizing metal-free, low-cost, and durable electrocatalysts that are active for the oxygen evolution reaction (OER) is essential for the development of commercial alkaline water electrolyzers. Herein, we develop a nanoconfined synthesis approach for the fabrication of a metal-free graphitic mesoporous carbon nitride (gMesoCN) electrocatalyst with a high surface area of 406 m2/g and high nitrogen content of 48%. This is achieved by a nanohard-templating approach through simple polymerization of guanidine hydrochloride (GndCl) as a single carbon-nitrogen source inside the organized mesopore channels of a mesoporous SBA15 silica nanotemplate. The produced material is characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM), which confirmed the formation of a well-ordered mesoporous carbon nitride, while analysis of the pore size distribution indicated the formation of uniformly sized pore channels of 4.56 nm. X-ray photoelectron spectroscopy (XPS) indicated that gMesoCN consisted of C and N. The metal-free gMesoCN material showed good electrocatalytic performance for the OER in alkaline medium, where a Tafel slope of 52.4 mV/dec indicated favorable OER kinetics. Significantly, the gMesoCN material demonstrates long-term durability with 98.4% retention of current density after 24 h. The reported gMesoCN material is inexpensive, environmentally friendly, and easy-to-synthesize with the potential for applicability in the field of electrocatalysis.

Nanocomposites Based on Ruthenium Nanoparticles Supported on Cobalt and Nitrogen-Codoped Graphene Nanosheets as Bifunctional Catalysts for Electrochemical Water Splitting.

Rational design and engineering of high-efficiency electrocatalysts toward overall water splitting is crucial for the development of hydrogen energy technology. Herein, a facile procedure is described for the preparation of effective bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), where ruthenium nanoparticles are supported on graphene nanosheets that are codoped with atomic cobalt and nitrogen by controlled pyrolysis of melamine-functionalized graphene oxide and metal ion precursors. The obtained nanocomposites (CoNG/Ru) exhibit a remarkable electrocatalytic activity toward both HER and OER in alkaline media, with a respective overpotential of only -15 and +350 mV to reach the current density of 10 mA cm-2, which is much better than the monometallic counterparts and relevant catalysts in the literature. With CoNG/Ru as bifunctional catalysts for overall water splitting in a two-electrode system, a low potential of 1.58 V is needed to reach the current density of 10 mA cm-2, which is even better than that with commercial Pt/C and RuO2 catalysts. This is ascribed to the synergistic interactions between the metal species by metal-metal charge transfer. These results highlight the significance of exploiting the electronic interactions between metal species in carbon-based nanocomposites to develop bifunctional catalysts for electrochemical energy technologies.

NiSe-Ni3Se2/Multiwalled Carbon Nanotube Composites as Efficient Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Media

The development of effective electrocatalysts for the oxygen evolution reaction (OER) is of great importance to combat energy-related concerns in the environment. Herein, we report a one-step solvothermal method employed for the fabrication of nickel selenide hybrids (NiSe-Ni3Se2) and a series of nickel selenide hybrid/multiwalled carbon nanotube composites (NiSe-Ni3Se2/MWCNT) as electrocatalysts for the OER in alkaline media. The catalytic activities of these electrocatalysts were investigated via several electrochemical characterization techniques, such as linear sweep voltammetry, chronoamperometric studies at constant potential, electrochemical surface area determination, and Tafel slope calculation, under alkaline conditions. Morphological observations demonstrated the agglomeration of non-uniform NiSe-Ni3Se2 microspheres around carbon nanotubes (CNTs), demonstrating the successful synthesis of NiSe-Ni3Se2/MWCNT nanocomposites. Among the tested electrocatalysts, the 20% NiSe-Ni3Se2/MWCNT nanocomposite demonstrated the highest activity, exhibiting an overpotential of 325 mV to achieve a current density of 10 mA.cm−2 in 0.1 mol.dm−3 KOH solution. The NiSe-Ni3Se2/MWCNT nanocomposites showed improved activity toward the OER compared to bare NiSe-Ni3Se2 hybrids and MWCNTs, exhibiting an overpotential of 528, 392, and 434 mV for 10%, 30%, and 50% NiSe-Ni3Se2/MWCNT nanocomposites, respectively. These results compare favorably to the overpotential of noble electrocatalysts, such as RuO2 and IrO2. Our results imply that the addition of MWCNTs increased the activity of NiSe-Ni3Se2 hybrids due to an increased number of catalytic sites, dispersion of NiSe-Ni3Se2 hybrid nanoparticles, and electronic conductivity of the nanocomposites. These nanocomposites also demonstrated better long-term stability compared to NiSe-Ni3Se2 hybrids and MWCNTs. Hence, NiSe-Ni3Se2/MWCNT nanocomposites possess the potential as effective electrocatalysts for the OER in alkaline media.

Oxygen evolution on gold: The effects of alkali-metal cations and iron impurities from alkaline electrolytes

Effects of alkali cations and iron impurities in alkaline electrolytes on the activity of gold surface for the oxygen evolution reaction (OER) are studied. Cyclic voltammograms obtained for bare gold evidenced no activity for OER in the potential window bellow 2.0 VRHE, while for gold electrodes multi-cycled in the hydr(oxide) region, i.e., 1.25–1.50 VRHE, the OER activity is high and present an onset potential of 1.55 VRHE. Linear sweep voltammetry shows that the OER activity depends on the nature of alkali cation in the solution and also on the content of iron incorporated onto gold oxide electrodes. In situ surface-enhanced Raman spectroscopy (SERS) detected a band at 790 cm−1 suggesting the presence of OO− species on Au close to 1.90 VRHE and a broader band at a relatively lower potential (1.55 VRHE) that could be related to Fe-like species OO−, OOH and/or FeO that were adsorbed on gold electrode at a relatively low potential (∼1.55 VRHE), such species are likely to be the reactive centers participating in the electrocatalysis of the OER in alkaline media.

Antiperovskite Nitrides CuNCo3-xVx: Highly Efficient and Durable Electrocatalysts for the Oxygen-Evolution Reaction.

Perovskite oxides have attracted much attention for enabling the oxygen-evolution reaction (OER) over the past decades. Nevertheless, their poor conductivity is still a barrier hindering their use. Herein, we report a catalyst prototype of Co-based antiperovskite nitrides CuNCo3-xVx (0 ≤ x ≤ 1) to be a highly effective OER electrocatalyst. The synthesized CuNCo3-xVx exhibits greatly enhanced activity and stability toward the OER in alkaline medium. The CuNCo2.4V0.6 shows a mere 235 mV of overpotential to reach 10 mA cm-2, which is comparable to that of Ir/C (232 mV). More importantly, the CuNCo2.4V0.6 is more durable than the conventional Ir/C catalyst. The CuNCo2.4V0.6 catalyst enabled a Zn-air battery to exhibit a cycle life of 143 h with a much higher cell efficiency. The V-substituted CuNCo2.4V0.6 provides a higher content of the desirable Co3+ species in the post-OER catalyst, which ensures a high activity over a long-term operation. With these enhanced effects enabled by the compositional flexibility of CuNCo3-xVx antiperovskite nitride, a feasible strategy for optimizing an electrocatalyst with tunable properties is provided.

Front Cover: Oxygen‐Deficient NiFe2O4 Spinel Nanoparticles as an Enhanced Electrocatalyst for the Oxygen Evolution Reaction (ChemNanoMat 10/2019)

Spinel-type NiFe2O4 with oxygen vacancies was prepared via a simple hydrothermal method followed by mild reduction under hydrogen flow and used as a catalyst for the oxygen evolution reaction (OER). The oxygen-deficient spinel-type oxide nanoparticles exhibited excellent electrocatalytic activity and long-term durability for the OER in alkaline medium. The outstanding electrochemical performance was attributed to the improved physicochemical properties by induced oxygen vacancies. More information can be found in the Full Paper by Sung-Hyeon Baeck et al. on page 1296 in Issue 10, 2019 (DOI: 10.1002/cnma.201900231).

Influence of Elemental Compositions and Crystalline Structures on Electrocatalytic Activity of Fe-Based Oxides for Oxygen Evolution Reaction in Alkaline Water Splitting

Current energy and environmental issues, alongside the depletion in fossil fuels, have increased the demand for the development and use of sustainable alternative energy resources. Due to their inexhaustibility, renewable energy resources such as solar and wind energies, are highly promising candidates for the construction of sustainable energy systems. To effectively use these intermittent sources in nature, it is essential that it is converted into chemical fuels that can be efficiently stored and transported. One such example of harnessing natural energy is the electrochemical water splitting of naturally abundant water to produce hydrogen, which is a promising carbon-free chemical fuel with a high energy density that, with some further work, can be stored and transported for use on a large scale. Electrochemical water splitting can be carried out using renewable energy generated electricity, making it a clean and cost-effective process. However, the anodic oxygen evolution reaction (OER) in the water splitting process has a large overpotential. Thus, there is a need to develop highly active and inexpensive electrocatalysts for OER to overcome this barrier. Iron (Fe) is an abundant element in the Earth’s crust that is inexpensive and has low toxicity, making it a potential candidate for widespread use in catalysts. Although simple Fe oxides, such as Fe2O3, have intrinsically low OER activity, the activity can be dramatically enhanced by combining the Fe species with other metals, resulting in optimization of the energy levels of the Fe 3d and O 2p bands and modification of the valence state of the Fe cation. Among the OER catalysts, perovskite-type metal oxides with the general formula ABO3, where the A- and B-sites are occupied by alkaline-earth or rare-earth metals and transition metals, respectively, have attracted great interest due to their prominent OER activities, facile synthesis and environmental friendliness. Although several active Fe-based perovskite-type OER catalysts have been previously reported,[1,2] most of these studies focused on the choice of elements for the A-site metal. While the effects of crystalline structures and elemental compositions of catalysts with Fe and A-site metals have not yet been systematically investigated, the knowledge is crucial to understanding the OER process catalyzed by Fe-based oxides and for the efficient design of promising electroactive materials. In this study, a variety of Fe-based oxides with the general formula AxFeyOz were synthesized, and their electrocatalytic OER activities in alkaline media were systematically investigated to elucidate the effects that the structure and composition of the materials have on the OER activity. Common alkaline-earth metals, Ca, Sr, and Ba, were selected as A-site metals, as shown in Figure 1A. Fe-based oxides were synthesized via a carboxylic acid-aided sol–gel method, which is a versatile synthetic tool to easily control the elemental compositions of the A- and B-site metals and has been previously employed to produce excellent electrocatalysts.[3] The structures of the fabricated oxides were characterized by X-ray diffraction, and the Brunauer–Emmet–Teller specific surface areas of the oxides were also measured. Subsequently, the OER activities of the oxides in alkaline medium were evaluated using a rotating-disk electrode and compared. Figure 1B shows the OER specific activities of the Fe-based oxides and reveals that a Ca-containing oxide was found to possess the highest OER specific activity among the synthesized oxides, exhibiting one of the best performances observed when compared to previously reported Fe-based oxides. These findings not only present an excellent electrocatalyst for OER in alkaline media, but also provide new guidelines for the design of metal oxide-type OER electrocatalysts.

Amorphous Cobalt Iron Borate Grown on Carbon Paper as a Precatalyst for Water Oxidation.

The key to improving water oxidation is to develop efficient and earth-abundant catalysts for the oxygen evolution reaction (OER). Herein, a new amorphous cobalt iron borate supported on 3D carbon paper integrated electrode is reported as a precatalyst for the OER, which was synthesized by using a one-pot hydrothermal method. An optimum OER activity was obtained at 25 % Fe doping by screening the compositions of the Co and Fe. The best synthesized catalyst needs an overpotential of 227 mV to deliver a current density of 10 mA cm-2 and also exhibits a long-term durability of 24 h. Impressively, we find that CoFe oxyhydroxide was formed in situ in the OER process, which serves as the real catalytic active species for OER. Moreover, the direct conversion from CoFe borate to CoFe oxyhydroxide is reported for the first time in metal borate OER catalysts. The discovery in this work, that is, that metal borate as a precursor can efficiently catalyze the OER in alkaline media, significantly widens the family of OER catalysts.

Oxygen Evolution Reaction of Co-Mn-O Electrocatalyst Prepared by Solution Combustion Synthesis

High-performance oxygen evolution reaction (OER) electrocatalysts are needed to produce hydrogen for energy generation through a carbon-free route. In this work, the solution combustion synthesis (SCS) method was employed to synthesize mixed phases of Co- and Mn-based oxides, and the relationships between the crystalline structure and the catalytic properties in the mixed phases were established. The mixed phases of Co- and Mn-based oxides shows promising OER properties, such as acceptable overpotential (450 mV for 10 mA∙cm−2) and Tafel slope (35.8 mV∙dec−1), highlighting the use of the mixed phases of Co- and Mn-based oxides as a new efficient catalysts for water splitting. Electronic structure of the mixed phases of Co- and Mn based oxides is studied in detail to give insight for the origin of high catalytic activities. In addition, excellent long-term stability for OER in alkaline media is achieved for the mixed phase of Co- and Mn based oxides.

An ingenious approach for ZIFs derived N-doped hierarchical porous carbon hybrids with FeCo alloy nanoparticles as efficient bifunctional oxygen electrocatalysts

An ingenious approach is proposed to achieve exceedingly efficient bifunctional oxygen electrocatalysts of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Hierarchical porous structure constituted by nanotubes and nanobulks with high specific surface area and encapsulated FeCo alloy nanoparticles is successfully prepared via the hybrid of Fe(II)-doped ZIF-8 (FeZIF) and cobalt (III) acetylacetonate (Co(acac)3), which is greatly redound to mass transfer and electron transport. Interestingly, Co(acac)3 plays dual-roles during preparation process, which not only generates new metal species but improves porous structure. Moreover, FeZIF acts as a self-sacrificing template and iron source. Owing to the coaction of the unique porous structure, bimetal-N active sites and FeCo alloy, the resultant catalyst carbonized at 850 °C exhibits a prominent ORR activity with a positive half-wave potential of 0.864 V (vs the reversible hydrogen electrode, RHE), and a low operating potential of 1.675 V at a current density of 10 mA cm−2 for OER in alkaline media. The potential gap (∆E) of overall oxygen electrode is 0.81 V, which surpass that of numerous non-noble metal-based catalysts reported to date, indicating a promising bifunctional oxygen electrocatalyst.

Solution blow spun nickel oxide/carbon nanocomposite hollow fibres as an efficient oxygen evolution reaction electrocatalyst

The development of efficient electrocatalysts for slow reaction of the oxygen evolution reaction (OER) is fundamental for viability of the electrochemical water splitting technologies. Here we report for the first time the synthesis of NiO/carbon hollow fibres (NiO-HF) by the Solution Blow Spinning (SBS) technique, and a study of their catalytic activity towards the OER in alkaline medium. The hollow fibres were obtained with ca. 300 nm in diameter consisting of agglomerated NiO nanoparticles with an average size of 50 nm which is close to the tubular wall thickness. The formation mechanism of the hollow structure was discussed. It was revealed that the carbon from polyenic branch of polyvinylpyrrolidone (PVP) resists the firing treatment and acts as an agglomerating agent, thus ensuring a conductive and percolating path between NiO nanoparticles along the fibres. A battery of electrochemical tests of NiO-HF supported by commercial Ni foam reveals excellent electrochemical activity for OER in 1 M KOH, in comparison with reference NiO nanoparticles (NiO-NP, diameter ca. 23 nm). NiO-HF attains an overpotential of 340 mV vs. RHE at a current density of 10 mA cm−2, which is amongst the lowest values reported in the literature for undoped NiO. Chronopotentiometry reveals stable NiO-HF electrodes over 15 h under an electrolysis current of 25 mA cm−2. Microscopic analysis shows that the fibrillar morphology is completely preserved after the electrolysis test. The remarkable performance of the NiO-HF catalyst is ascribed to the enhanced electronic conductivity resulting from the interpenetrating NiO-HF/carbon microstructure.

Study the Activity of a Novel Ni-Based Electrocatalyst for Oxygen Evolution Reaction in Alkaline Media

Electrocatalytic water splitting into hydrogen and oxygen has a great potential to produce hydrogen as a carbon neutral source of energy. However, the sluggish kinetics of oxygen evolution reaction (OER) leads to an inefficient process prohibiting industrial application [1]. Therefore, highly efficient OER electrocatalysts are needed to reduce the energy barrier and enhance the efficiency of water electrolysis cell [2]. IrO2 and RuO2 are the most efficient electrocatalysts for OER, however their commercial industrial applications are limited due to their high cost and low earth abundance. To address this issue, several non-noble metal based OER electrocatalysts including transition metal oxide/hydroxides, perovskite, and molecular electrocatalysts have been developed [3]. Among them, Ni-based materials attract substantial attention owing to their low cost, high corrosion stability, and remarkable catalytic activity in alkaline media. The present work aims to evaluate and compare the electrochemical activity of a novel Ni-based electrocatalyst prepared from two different plating baths toward OER. Electrochemical measurements including cyclic voltammetry (CV), quasi-steady state polarization (QSSP), electrochemical impedance spectroscopy (EIS), and chronopotentiometry (CP) were carried out to study and compare the activity, charge transfer resistance and stability of different electrocatalysts developed in this study toward OER in alkaline media.

Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction

Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is crucial for the development of sustainable energy sources. A novel catalyst which could be a promising candidate for such electrocatalysts is described. Co0.85Se supported on three-dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic performances for OER in alkaline medium. It is found that the onset overpotential is 311 mV on the Co0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co3O4/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV dec−1 and the overpotential at the current density of 10 mA cm−2 is 385 mV on this electrode. The Co0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic electrolyte.

Constructing MoPx@MnPy Heteronanoparticle-Supported Mesoporous N,P-Codoped Graphene for Boosting Oxygen Reduction and Oxygen Evolution Reaction

Developing efficient and cost-effective bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly important for fabricating energy conversion and storage technologies, such as fuel cells, water electrolyzers, and metal–air batteries. Herein, we report a facile and economical route for synthesizing ultrasmall molybdenum phosphide (MoPy) nanocrystal-attached mesoporous manganese phosphides on N,P-codoped graphene nanosheets, which display equivalent ORR (OER) activity to that of Pt/C (RuO2) catalysts. This is manifested by the positive onset potential (0.969 V) and half-wave potential (0.842 V) for ORR, as well as a mere overpotential of 301 mV at a current density of 20 mA·cm–2 and a small Tafel slope of 105 mV·dec–1 for OER in alkaline medium. It also demonstrates remarkable stability in comparison with Pt/C and RuO2 for both ORR and OER, respectively. The excellent performance can be attributed to the mesoporous structure with enhanced multiple t...

Fe-Doping in Double Perovskite PrBaCo2(1-x)Fe2xO6-δ: Insights into Structural and Electronic Effects to Enhance Oxygen Evolution Catalyst Stability

Perovskite oxides have been gaining attention for its capability to be designed as an ideal electrocatalyst for oxygen evolution reaction (OER). Among promising candidates, the layered double perovskite—PrBaCo2O6-δ (PBC)—has been identified as the most active perovskite electrocatalyst for OER in alkaline media. For a single transition metal oxide catalyst, the addition of Fe enhances its electrocatalytic performance towards OER. To understand the role of Fe, herein, Fe is incorporated in PBC in different ratios, which yielded PrBaCo2(1-x)Fe2xCo6-δ (x = 0, 0.2 and 0.5). Fe-doped PBCF’s demonstrate enhanced OER activities and stabilities. Operando X-ray absorption spectroscopy (XAS) revealed that Co is more stable in a lower oxidation state upon Fe incorporation by establishing charge stability. Hence, the degradation of Co is inhibited such that the perovskite structure is prolonged under the OER conditions, which allows it to serve as a platform for the oxy(hydroxide) layer formation. Overall, our findings underline synergetic effects of incorporating Fe into Co-based layered double perovskite in achieving a higher activity and stability during oxygen evolution reaction.

Self-reconstruction mechanism in NiSe2 nanoparticles/carbon fiber paper bifunctional electrocatalysts for water splitting

Developing efficient bifunctional electrocatalysts and gaining fundamental understanding of reaction mechanisms are crucial for practical water splitting. Herein, a bifunctional NiSe2 nanoparticles/carbon fiber paper (NSN/CFP) electrode is fabricated by the pyrolysis of Ni(NO3)2 on CFP, followed by a selenization step. The as-prepared electrocatalysts exhibit superior overall water splitting behavior in 1 M KOH with low overpotentials of 145 mV and 280 mV at current densities of 10 mA cm−2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) respectively, comparable to the performance of 20% Pt/C and RuO2. Detailed compositional and morphological studies reveal that the NiSe2 gradually transforms into an amorphous Ni(OH)2/NiOOH heterojunction during both HER and OER in alkaline medium. Based on these experimental results, an oxidation-induced self-reconstruction mechanism is proposed. Owing to the highly-oxidized Ni(OH)2/NiOOH active species, the self-reconstructed structure enhances the water splitting under fixed potentials for a prolonged time of 96 h with negligible current degradation. This work not only provides a facile route to fabricate efficient and stable electrocatalysts for large-scale water splitting but also reveals an underlying structural evolution mechanism, which guides the rational design of heterogeneous catalysts.

Electrodeposited Ni-Co-Sn alloy as a highly efficient electrocatalyst for water splitting

Water splitting to produce hydrogen and oxygen is considered as a feasible solution to solve the current energy crisis. It is highly desirable to develop inexpensive and efficient electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this paper, nanostructured Ni-Co-Sn alloys were electrodeposited on copper foil and the excellent electrocatalytic performances for both HER and OER in alkaline media were achieved. The optimized Ni-Co-Sn electrode shows a low onset overpotential of −18 mV and a small Tafel slope of 63 mV/dec for the HER, comparable to many state-of-the-art non-precious metal HER catalysts. For the OER, it produces an overpotential of 270 mV (1.50 V vs. RHE) at current density of 10 mA/cm2, which is better than that of the commercial Ir/C catalyst. In addition to high electrocatalytic activities, it exhibits good stability for both HER and OER. This is the first report that Ni-Co-Sn is served as a cost-effective and highly efficient bifunctional catalyst for water splitting and it will be of great practical value.

Cobalt oxyhydroxide with highly porous structures as active and stable phase for efficient water oxidation

Providing the direct experimental evidence for confirming active and stable phase in cobalt-based electrocatalysts for oxygen evolution reaction (OER) in alkaline medium is crucial to design and fabricate high-performance electrocatalysts. Herein, we report the in situ fabrication of pure cobalt oxyhydroxide (CoOOH) phase with highly porous structures during OER testing process, which exhibits outstanding performance for OER with a low overpotential of 245 mV at 10 mA cm−2 and retain stable catalytic activity for 250 h with no appreciable decay. Experimental characterizations revealed that the superior catalytic activity of as-formed CoOOH phase mainly arises from the highly porous structures, which endows the electrocatalyst with high specific surface area (∼200 m2 g−1), numerous accessible channels and abundant active sites. Evidence has been produced showing that the in situ formed pure CoOOH is the active and stable phase in cobalt-based electrocatalysts for OER in alkaline medium. In addition, the synthetic strategy applied here could provide a novel way to fabricate nanoporous structures under mild conditions using electrochemical oxidation method.

Features of inorganic nanocrystals formation in conditions of successive ionic layers deposition in water solutions and the Co(II)Co(III) 2D layered double hydroxide synthesis

The article provides a brief overview of the results concerning synthesis of inorganic nanocrystals including such compounds as Ag, Au, NiO1+x∙nH2O, CoAl and ZnCo layered double hydroxides (LDH) by Successive Ionic Layer Deposition (SILD) method in aqueous solutions. It is shown that by specifying the synthesis conditions, for example, the number of SILD cycles, it is possible to change the size of nanocrystals, and for 2D crystals - their orientation with respect to the substrate. Along with this, in the article for the first time a novel SILD route of Co(II)Co(III) LDH synthesis is presented. It was stated that the nanolayers of this compound consist of randomly oriented 2D nanocrystals aggregates with hexagonal hydrotalcite-like crystal structure, each of which has a thickness of less than 3 nm. The method is based on the sequential processing of the substrate in aqueous solutions of CoCl2 and NaBH4 salts of certain concentrations and intermediate washing excess of these reagents with distilled water. The obtained nanolayers were characterized via SEM, TEM, XRD, FT-IR and XPS methods. Models of chemical processes occurring on the surface during the synthesis were suggested on the basis of the obtained experimental results. The samples were acquired on the nickel foam (NF) surface and were studied as electrodes for pseudocapacitors and as electrocatalysts for oxygen evolution reaction (OER) in alkaline medium. The electrodes of the pseudocapacitor showed high specific capacitance (2260 F/g at the current density of 1 A/g), as well as good cyclic stability (after 500 charge-discharge cycles, the capacitance drop do not exceed 2%). In addition, the obtained material showed a high electrocatalytic activity for OER in alkaline medium, characterized at a current density of 10 mA/cm2 by a relatively low overpotential of 285 mV and Tafel slope of 95 mV/dec.

One-pot synthesis of Fe3O4/Fe/C by microwave sintering as an efficient bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

The Fe3O4/Fe/C catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was prepared using a facile microwave sintering method. The Fe3O4/Fe/C electrode showed excellent electro catalytic ability on both ORR and OER in alkaline medium. The phase was characterised by X-ray diffraction (XRD). The current density of the Fe/C electrode was 147 mA cm−2 at −0.2 V (vs. Hg/HgO) and 97 mA cm−2 at 0.7 V (vs. Hg/HgO) in 6 mol L−1 KOH electrolyte at room temperature. The mechanism of ORR and OER was explored by the AC impedance.