Three-dimensional ordered mesoporous Co3O4 enhanced by Pd for oxygen evolution reaction

Sustainable and energy-saving hydrogen production via binder-free and in situ electrodeposited Ni-Mn-S nanowires on Ni-Cu 3-D substrates

Electrochemical water splitting, with its oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), is undoubtedly the most eco-friendly and sustainable method to produce hydrogen. However, water splitting still requires improvement due to the high energy consumption caused by the slow kinetics and large thermodynamic potential requirements of OER. Urea-water electrolysis has become increasingly appealing compared to water-splitting because of the remarkable decline in the cell potential in the hydrogen production process and less energy consumption; it also offers a favorable opportunity to efficiently treat wastewater containing a significant amount of urea. In this work, Ni-Mn-S/Ni-Cu nano-micro array electrocatalysts were synthesized by a two-step and binder-free electrochemical deposition technique and investigated as an effective electrode for the HER and urea oxidation reaction (UOR). According to the electrochemical results, the optimized electrode (Ni-Mn-S/Ni-Cu/10) showed excellent electrocatalytic activity for the HER (64 mV overpotential to achieve the current density of 10 mA cm-2 and Tafel slope of 81 mV dec-1) in alkaline solution. When Ni-Mn-S/Ni-Cu/10 is employed as a UOR anode in an alkaline solution containing urea, it achieves a current density of 10 mA cm-2 at 1.247 V vs. RHE. In addition, when the optimized sample was utilized as a bi-functional electrode for overall urea-water electrolysis (HER-UOR), the cell voltage reached 1.302 V at 10 mA cm-2 (which is 141 mV less than that for HER-OER). The electrocatalytic stability results unequivocally revealed small changes in voltage during a 24 h test and showed good durability. This non-noble metal electrocatalyst, prepared by the electrodeposition synthesis method, is a promising solution to implement low-cost hydrogen production and wastewater treatment.

Developing porous electrocatalysts to minimize overpotential for the oxygen evolution reaction

The development of electrocatalysts for the oxygen evolution reaction (OER) is one of the most critical issues for improving the efficiency of electrochemical water-splitting, which can produce green hydrogen energy without CO2 emissions. This review outlines the advances in the precise design of inorganic- and organic-based porous electrocatalysts, which are designed by various strategies, to catalyze the OER in the electrolytic cycle for efficient water-splitting. For developing high-performance electrocatalysts with low overpotentials, it is important to design a chemical composition that optimizes binding energy for an intermediate in the OER and allows the easy access of reactants to active sites depending on the porosity of electrocatalysts. Porous structures give us the positive opportunity to increase the accessible surface of active sites and effective diffusion of targeting molecules, which is potentially one of the guidelines for developing active electrocatalysts. Further modification of the frameworks is also powerful for tailoring the function of pore surfaces and the environment of inner spaces. Designable organic molecules can also be embedded inside inorganic- and organic-based frameworks. According to chemical composition (inorganic and organic), nanostructure (crystalline and amorphous) and additional modification (metal doping and organic design) of porous electrocatalysts, the current status of resultant OER performance is surveyed with some problems that should be solved for improving the OER activity. The remarkable progress in OER electrocatalysts is also introduced for demonstrating the bifunctional hydrogen evolution reaction (HER) and for utilizing seawater.

KIT-5-Assisted Synthesis of Mesoporous SnO2 for High-Performance Humidity Sensors with a Swift Response/Recovery Speed

Developing highly efficient semiconductor metal oxide (SMOX) sensors capable of accurate and fast responses to environmental humidity is still a challenging task. In addition to a not so pronounced sensitivity to relative humidity change, most of the SMOXs cannot meet the criteria of real-time humidity sensing due to their long response/recovery time. The way to tackle this problem is to control adsorption/desorption processes, i.e., water-vapor molecular dynamics, over the sensor's active layer through the powder and pore morphology design. With this in mind, a KIT-5-mediated synthesis was used to achieve mesoporous tin (IV) oxide replica (SnO₂-R) with controlled pore size and ordering through template inversion and compared with a sol-gel synthesized powder (SnO₂-SG). Unlike SnO₂-SG, SnO₂-R possessed a high specific surface area and quite an open pore structure, similar to the KIT-5, as observed by TEM, BET and SWAXS analyses. According to TEM, SnO₂-R consisted of fine-grained globular particles and some percent of exaggerated, grown twinned crystals. The distinctive morphology of the SnO₂-R-based sensor, with its specific pore structure and an increased number of oxygen-related defects associated with the powder preparation process and detected at the sensor surface by XPS analysis, contributed to excellent humidity sensing performances at room temperature, comprised of a low hysteresis error (3.7%), sensitivity of 406.8 kΩ/RH% and swift response/recovery speed (4 s/6 s).

Enhanced Electrocatalytic Water Oxidation of Ultrathin Porous Co3O4 Nanosheets by Physically Mixing with Au Nanoparticles

Ultrathin porous Co₃O₄ nanosheets are synthesized successfully, the thickness of which is about three unit-cell dimensions. The enhanced oxygen evolution reaction (OER) performance and electronic interaction between Co₃O₄ and Au is firstly reported in Co₃O₄ ultrathin porous nanosheets by physically mixing with Au nanoparticles. With the loading of the Au nanoparticles, the current density of ultrathin porous Co₃O₄ nanosheets is enhanced from 9.97 to 14.76 mA cm^-2 at an overpotential of 0.5 V, and the overpotential required for 10 mA cm^-2 decreases from 0.51 to 0.46 V, smaller than that of commercial IrO₂ (0.54 V). Furthermore, a smaller Tafel slope and excellent durability are also obtained. Raman spectra, XPS measurement, and X-ray absorption near edge structure spectra (XANES) show that the enhanced OER ascribed to a higher Co²⁺/Co³⁺ ratio and quicker charge transfer due to the electronic interaction between Au and ultrathin Co₃O₄ nanosheets with low-coordinated surface, and Co²⁺ ions are beneficial for the formation of CoOOH active sites.

Pd-Supported Co3O4/C Catalysts as Promising Electrocatalytic Materials for Oxygen Reduction Reaction

This paper describes the activity of PdCo3O4/C obtained by wet impregnation towards the oxygen reduction reaction (ORR). For this purpose, the Co3O4/C substrate was synthesized using the microwave irradiation heating method with further annealing of the substrate at 400 °C for 3 h (Co3O4/C-T). Then, the initial Co3O4/C substrate was impregnated with palladium chloride (Pd-Cl2-Co3O4/C), and then part of the obtained Pd-Cl2-Co3O4/C catalyst was annealed at 400 °C for 3 h (PdOCo3O4/C). The electrocatalytic activity of the prepared catalysts was investigated for the oxygen reduction reaction in alkaline media and compared with the commercial Pt/C (Tanaka wt. 46.6% Pt) catalyst. It was found that the annealed PdOCo3O4/C catalyst showed the largest ORR current density value of −11.27 mA cm−2 compared with Pd-Cl2-Co3O4/C (−7.39 mA cm−2) and commercial Pt/C (−5.25 mA cm−2).

Pt-Co3O4 Superstructures by One-Pot Reduction/Precipitation in Bicontinuous Microemulsion for Electrocatalytic Oxygen Evolution Reaction

Bicontinuous microemulsions (BCME) were used to synthesize hierarchical superstructures (HSs) of Pt-Co3O4 by reduction/precipitation. BCMEs possess water and oil nanochannels, and therefore, both hydrophilic and lipophilic precursors can be used. Thus, PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi were prepared (where Aq and Oi stand for the precursor present in aqueous or oily phase, respectively). The characterization of the Pt-Co3O4-HS confirmed the formation of metallic Pt and Co3O4 whose composition and morphology are controlled by the initial pH and precursor combination, determining the presence of the reducing/precipitant species in the reaction media. The electrocatalytic activity of the Pt-Co3O4-HSs for oxygen evolution reaction (OER) was investigated using linear sweep voltammetry in 0.1 M KOH and compared with Pt-HS. The lowest onset overpotentials for Pt-Co3O4-Hs were achieved with PtOi-CoOi (1.46 V vs. RHE), while the lowest overpotential at a current density of 10 mA cm−2 (η10) was obtained for the PtAq-CoAq (381 mV). Tafel slopes were 102, 89, 157 and 92 mV dec−1, for PtAq-CoAq, PtAq-CoOi, PtOi-CoAq and PtOi-CoOi, respectively. The Pt-Co3O4-HSs showed a better performance than Pt-HS. Our work shows that the properties and performance of metal–metal oxide HSs obtained in BCMEs depend on the phases in which the precursors are present.

Mesoporous Nanocast Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions

Catalyzed oxygen evolution and oxygen reduction reactions (OER and ORR, respectively) are of particular significance in many energy conversion and storage processes. During the last decade, they emerged as potential routes to sustain the ever-growing needs of the future clean energy market. Unfortunately, the state-of-the-art OER and ORR electrocatalysts, which are based on noble metals, are noticeably limited by a generally high activity towards one type of reaction only, high costs and relatively low abundance. Therefore, the development of (bi)functional low-cost non-noble metal or metal-free electrocatalysts is expected to increase the practical energy density and drastically reduce the production costs. Owing to their pore properties and high surface areas, mesoporous materials show high activity towards electrochemical reactions. Among all synthesis methods available for the synthesis of non-noble mesoporous metal oxides, the hard-templating (or nanocasting) approach is one of the most attractive in terms of achieving variable morphology and porosity of the materials. In this review, we thus focus on the recent advances in the design, synthesis, characterization and efficiency of non-noble metal OER and ORR electrocatalysts obtained via the nanocasting route. Critical aspects of these materials and perspectives for future developments are also discussed.

The chemically reduced CuO–Co3O4 composite as a highly efficient electrocatalyst for oxygen evolution reaction in alkaline media

The fabrication of efficient, alkaline-stable and nonprecious electrocatalysts for the oxygen evolution reaction is highly needed; however, it is a challenging task.

Shape-controlled synthesis of Co3O4 for enhanced electrocatalysis of the oxygen evolution reaction

One-dimensional Co₃O₄ nanorods, two-dimensional nanosheets and three-dimensional nanocubes were synthesized; the effect of the morphology on their electrocatalytic activities was studied.

Metal–organic-framework derived Co–Pd bond is preferred over Fe–Pd for reductive upgrading of furfural to tetrahydrofurfuryl alcohol

The metal–organic-framework-derived Co–Pd bond can more efficiently catalyze the reductive upgrading of furfural to tetrahydrofurfuryl alcohol production as compared to the Fe–Pd bond.

A core@shell hollow heterostructure of Co3O4 and Co3S4: an efficient oxygen evolution catalyst

A hollow core@shell nanostructure of Co₃O₄ and Co₃S₄ helps to enhance the electrocatalytic activity for the OER.

Oxygen Evolution Reaction Kinetic Barriers on Nitrogen-Doped Carbon Nanotubes

We investigate kinetic barriers for the oxygen evolution reaction (OER) on singly and doubly nitrogen-doped single-walled carbon nanotubes (NCNTs) using the climbing image nudged elastic band method with solvent effects represented by a 45-water-molecule droplet. The studied sites were chosen based on a previous study of the same systems utilizing a thermodynamic model which ignored both solvent effects and kinetic barriers. According to that model, the two studied sites, one on a singly nitrogen-doped CNT and the other on a doubly doped CNT, were approximately equally suitable for OER. For the four-step OER process, however, our reaction barrier calculations showed a clear difference in the rate-determining *OOH formation step between the two systems, with barrier heights differing by more than 0.4 eV. Thus, the simple thermodynamic model may alone be insufficient for identifying optimal OER sites. Of the remaining three reaction steps, the two H₂O forming ones were found to be barrierless in all cases. We also performed solvent-free barrier calculations on NCNTs and undoped CNTs. Substantial differences were observed in the energies of the intermediates when the solvent was present. In general, the observed low activation energy barriers for these reactions corroborate both experimental and theoretical findings of the utility of NCNTs for OER catalysis.

Graphene and activated carbon-wrapped and Co3O4-intercalated 3D sandwich nanostructure hybrid for high-performance supercapacitance

The graphene/Co₃O₄/activated carbon (GCA) 1 : 9 capacitor shows the best electrochemical properties with Co₃O₄ particles that were homogeneously dispersed between the graphene/Co₃O₄ (GC) layers.

Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts

Finding alternative routes to access and store energy has become a major issue recently. Transition metal oxides have shown promising behaviour as catalysts and supercapacitors. Recently, liquid exfoliation of bulk metal oxides appears to be an effective route which provides access to two-dimensional (2D) nano-flakes, the size of which can be easily selected. These 2D materials exhibit excellent electrochemical charge storage and catalytic activity for the oxygen evolution reaction. In this study, various sized selected cobalt hydroxide nano-flake materials are fabricated by this time efficient and highly reproducible process. Subsquently, the electrochemical properties of the standard size Co(OH)₂ nanoflakes were investigated. The oxide modified electrodes were prepared by spraying the metal oxide flake suspension onto a porous conductive support electrode foam, either glassy carbon or nickel. The cobalt hydroxide/nickel foam system was found to have an overpotential value at 10 mA cm^-2 in 1 M NaOH as low as 280 mV and an associated redox capacitance exhibiting numerical values up to 1500 F g^-1, thereby making it a viable dual use electrode.