Surface characterization of Co3O4 electrodes prepared by the sol-gel method

Modulation of the Electronic Properties of Co3O4 through Bi Octahedral Doping for Enhanced Activity in the Oxygen Evolution Reaction

Developing a highly active and stable electrocatalyst for the oxygen evolution reaction (OER) is essential for efficient hydrogen production through anion exchange membrane water electrolysis powered by renewable electricity. Recently, there has been a renewed interest in designing electrocatalysts based on their work function optimization. The insights into the materials' electronic properties gained from developing other heterogeneous catalysts, such as those used for N2O decomposition, can be thus leveraged to enhance the performance of the OER electrocatalysts. Knowing that Bi enhances the catalytic activity of Co3O4 in N2O decomposition, where the surface electronic properties play a crucial role, we hypothesized that it might also improve the electroactivity of the OER electroactivity. Therefore, we synthesized Bi-doped Co3O4 with different bismuth contents and studied the sample with a complementary set of physicochemical, electrochemical, and computational techniques. We found that promoting Co3O4 with atomically dispersed bismuth enhances its OER electrocatalytic properties by reducing the energy of the potential-determining step and improving electron charge transfer properties. Bismuth atoms enter octahedral sites in Co3O4, creating Bi active centers and enhancing the activity of vicinal Co sites in the OER. The Bi and modified Co centers are characterized by increased binding energy of the intermediate state of the metal-oxygen intermediate and increased density of states at the Fermi level. The former reduces the overpotential required for the OER, whereas the latter improves the reaction kinetics by decreasing the charge transfer resistance.

Selective Catalytic Electro-Oxidation of Water with Cobalt Oxide in Ion Impermeable Reduced Graphene Oxide Porous Electrodes

The direct electrolysis of seawater is greatly inhibited by the oxidation of Cl^- to free chlorine, an undesirable, corrosive byproduct. To suppress the parasitic interference of Cl^- and any other ion, we developed a freestanding, electrically conducting, 3D macroporous reduced graphene oxide (rGO) scaffold with cobalt oxide particles selectively deposited on the internal walls of its closed pores (with an average diameter of ∼180 μm). The pore walls act as membranes composed of stacked rGO flakes; the nanochannels between rGO layers (size <1 nm) are permeable to water and gases while preventing the diffusion of dissolved ions such as Cl^-. Due to this, the catalytic particles are selectively accessible to water molecules but not to ions, allowing electrolysis to occur without chlorine evolution. The electrodes developed exhibit a stable generation of O₂ from simulated seawater at pH 14, reaching a specific current density of up to 25 A g^-1 during continuous electrolysis with 89-98% Faradaic efficiency, while chlorine generation is below 6 ppm h^-1, the sensitivity limit of the detection method employed. The strategy here proposed can be generalized to build electrodes that are inherently selective thanks to their architecture, with catalytically active particles loaded into closed pores with selective ion transport properties.

New Limits for Stability of Supercapacitor Electrode Material Based on Graphene Derivative

Supercapacitors offer a promising alternative to batteries, especially due to their excellent power density and fast charging rate capability. However, the cycling stability and material synthesis reproducibility need to be significantly improved to enhance the reliability and durability of supercapacitors in practical applications. Graphene acid (GA) is a conductive graphene derivative dispersible in water that can be prepared on a large scale from fluorographene. Here, we report a synthesis protocol with high reproducibility for preparing GA. The charging/discharging rate stability and cycling stability of GA were tested in a two-electrode cell with a sulfuric acid electrolyte. The rate stability test revealed that GA could be repeatedly measured at current densities ranging from 1 to 20 A g-1 without any capacitance loss. The cycling stability experiment showed that even after 60,000 cycles, the material kept 95.3% of its specific capacitance at a high current density of 3 A g-1. The findings suggested that covalent graphene derivatives are lightweight electrode materials suitable for developing supercapacitors with extremely high durability.

Low-Temperature Thermally Annealed Niobium Oxide Thin Films as a Minimally Color Changing Ion Storage Layer in Solution-Processed Polymer Electrochromic Devices

The limited availability of solution-processable ion storage materials, both inorganic and organic, hinders the adoption of roll-to-roll manufacturing for polymer electrochromic devices (ECDs). The n-type transition metal oxides are known for their ion storage properties. However, the fabrication methods of their amorphous metal oxide thin films typically involve sputtering, thermal deposition, electrical deposition, or sol-gel deposition followed by high-temperature thermal annealing (>300 °C), thus making them incompatible for low-cost roll-to-roll manufacturing on flexible substrates. In this study, we report the synthesis of amorphous niobium oxide(a-Nb₂O₅) thin films from sol-gel precursors through the combination of photoactivation and low-temperature thermal annealing (150 °C). Coupled with p-type electrochromic polymers (ECPs), solution-processed a-Nb₂O₅ thin films were evaluated as a minimally color changing counter electrode (MCC-CE) material for electrochromic devices. We found that ultraviolet ozone (UVO) treated and 150 °C thermally annealed (UVO-150 °C) a-Nb₂O₅ thin films show excellent electrochemical properties and cycling stability. Notably, a-Nb₂O₅/ECP-magenta ECD has a high optical contrast of ∼70% and a fast switching time (bleaching and coloring time of 1.6 and 0.5 s for reaching 95% of optical contrast). In addition, the ECD demonstrates a high coloration efficiency of ∼849.5 mC cm^-2 and a long cycling stability without a noticeable decay up to 3000 cycles.

Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode

Co₃O₄ film coated on Ti substrate is prepared using sol-gel method and applied as cathode material for electrochemical denitrification in this research. Preparation conditions including precursor coating times and calcination temperature are optimized based on NO₃^--N removal, NO₂^--N generation, NH₄⁺-N generation and total nitrogen (TN) removal efficiencies. The influences of electrolysis parameters such as current density and NO₃^--N initial concentration are also investigated. In comparison with other common researched cathodes (Ti, Cu and Fe₂O₃/Ti), Co₃O₄/Ti exhibits better NO₃^--N removal and NH₄⁺-N generation efficiencies. In order to remove NO₃^--N completely from water, Cl^- is added to help further oxidize NH₄⁺-N to N₂. TN removal after 3 h treatment increases from 65% to 80%, 90% and 96% with the increase of Cl^- from 0 mg L^-1 to 500, 1000 and 1500 mg L^-1, respectively. The mechanisms of NO₃^--N reduction on cathode and NH₄⁺-N oxidation on anode in the absence and presence of Cl^- are investigated in a double-cell reactor. Actual textile wastewater containing both NO₃^- and Cl^- is also treated and the Co₃O₄/Ti cathode exhibits excellent stability and reliability. It is interesting to find out that FeCl₂-H₂O₂ Fenton pretreatment is needed to remove extra COD and provide more Cl^- to help oxidize NH₄⁺-N to N₂ at the same time.

One-Step Electrodeposition of Nanocrystalline ZnxCo3-xO4 Films with High Activity and Stability for Electrocatalytic Oxygen Evolution

The development of highly active, environmentally friendly, and long-term stable oxygen evolving catalysts at low costs is critical for efficient and scalable H₂ production from water splitting. Here, we report a new and facile one-step electrodeposition of nanocrystalline spinel-type Zn_xCo_3-xO₄ films from an alkaline Zn²⁺-Co²⁺-tartrate solution. The electrodeposited Zn_xCo_3-xO₄ electrode could be directly used as the anode for the water electrolysis without any post treatment. The Zn_xCo_3-xO₄ film shows a low and stable overpotential of ∼0.33 V at 10 mA cm^-2 (and ∼0.35 V at 20 mA cm^-2) for over 10 h and a Tafel slope of ∼39 mV dec^-1 toward the oxygen evolution reaction (OER) in 1 M NaOH, comparable to the best performance of the nonprecious OER catalysts reported for alkaline media. The enhanced OER activity of Zn_xCo_3-xO₄ compared to Co₃O₄ could be attributed to the surface structural modification and higher density of the accessible active Co³⁺ sites induced by the incorporation of Zn²⁺. The electrodeposition method in this paper could also be used to synthesize other binary and ternary metal oxide based catalytic electrodes for reactions such as the OER and oxygen reduction reaction (ORR).

Codoping-Induced, Rhombus-Shaped Co3O4 Nanosheets as an Active Electrode Material for Oxygen Evolution

Nanostructured Co3O4 doped with Zn(2+), Ni(2+), and both were directly grown on an ITO substrate by an easily available hydrothermal method. The doped Co3O4 showed a unique structural morphology evolution upon controlling the doping elements and the doping ratio of the cations. For the codoped samples, the novel rhombus-shaped Co3O4 nanosheets doped with Zn(2+) and Ni(2+) (concentration ratio of 1:2) exhibited the optimal electrocatalytic performance. The sample showed a current density of 165 mA cm(-2) at 1.75 V, approximately 1.6 and 4 times higher than that of samples doped with Zn(2+) and Ni(2+) at a concentration ratio of 1:1 and 1:3. The unique architecture and its corresponding modified physical properties, such as high active-site density created by codoping, large structural porosity, and high roughness, are together responsible to its superior performance. For codoped Co3O4 nanostructures, Zn(2+) facilitates the creation of Co cations in their high oxidation state as active centers, while Ni(2+) contributed to the new active sites with lower activation energy. The synergistic effect of Zn(2+) and Ni(2+) doping can explain the improved physicochemical properties of codoped Co3O4 nanostructures.

Characterisation of Nanostructured Co3O4 Synthesised by the Thermal Decomposition of an Inorganic Precursor

Nanocrystalline Co3O4 has been synthesised using an inorganic precursor via thermal decomposition. The prepared inorganic precursor Co(cinnamate)2(N2H4)2 was characterised by hydrazine and metal analyses, infrared spectral analysis, and thermogravimetric analysis. Using appropriate annealing conditions, cobalt oxide nanoparticles of average size ~11 nm were synthesised by thermal treatment of the precursor. The nanoparticles’ size and structure were characterised using X-ray diffraction, high-resolution transmission electron microscopy, selected-area electron diffraction, and scanning electron microscopy techniques.

Reactive ballistic deposition of alpha-Fe2O3 thin films for photoelectrochemical water oxidation

We report the preparation of alpha-Fe2O3 electrodes using a technique known as reactive ballistic deposition in which iron metal is evaporatively deposited in an oxygen ambient for photoelectrochemical (PEC) water oxidation. By manipulating synthesis parameters such as deposition angle, film thickness, and annealing temperature, we find that it is possible to optimize the structural and morphological properties of such films in order to improve their PEC efficiency. Incident photon to current conversion efficiencies (IPCE) are used to calculate an AM1.5 photocurrent of 0.55 mA/cm(2) for optimized films with an IPCE reaching 10% at 420 nm in 1 M KOH at +0.5 V versus Ag/AgCl. We also note that the commonly observed low photoactivity of extremely thin hematite films on fluorine-doped tin oxide substrates may be improved by modification of annealing conditions in some cases.

Oxygen evolution on Ti/Co3O4-coated electrodes in alkaline solution

The electrochemical performances of Co3O4 nanopowders, obtained by the sol-gel method, were investigated and compared with those of commercial Co3O4 powders, for oxygen evolution reaction in alkaline solution. The active oxide powder was mixed with teflon and assembled on Ti substrate to form thin catalyst film. Cyclic voltammetry, polarization curves, and electrochemical impedance spectroscopy were used to assess the mechanism of oxygen evolution reaction, chemical structure, and morphology of the catalyst.

Cobalt oxide/tetraruthenated cobalt-porphyrin composite for hydrogen peroxide amperometric sensors

A new composite material constituted by mu-{5,10,15,20-tetra(4-pyridyl)porphyrinato cobalt(iii)}-tetrakis-{chloro-bis-(2,2'-bipyridine)ruthenium(ii)} complex (or CoTRP) and cobalt oxide, exhibiting high stability and sensitivity for the quantification of hydrogen peroxide, was obtained by the electrochemical polymerization of the tetraruthenated cobalt porphyrin in alkaline medium. The optimized experimental conditions for the preparation of the modified glassy carbon electrodes and for analysis of H2O2 were carefully determined. Fast sequential analysis (120 determinations h(-1)) in a wide linear dynamic range (5.0 x 10(-7) mol L(-1) to 2.0 x 10(-3) mol L(-1)), with high sensitivity and low detection limit (2.0 x 10(-7) mol L(-1)), was achieved by using these electrodes and the batch injection analysis (BIA) technique. Such characteristics allied to a good stability were explored for the specific determination of hydrogen peroxide in six commercial cosmetics and pharmaceutical product samples, giving results in excellent agreement with those obtained by the spectrophotometric method.