A Green Synthesis of CoFe2O4 Decorated ZIF-8 Composite for Electrochemical Oxygen Evolution

Low-cost, sustainable hydrogen production requires noble metal-free electrocatalysts for water splitting. In this study, we prepared zeolitic imidazolate frameworks (ZIF) decorated with CoFe₂O₄ spinel nanoparticles as active catalysts for oxygen evolution reaction (OER). The CoFe₂O₄ nanoparticles were synthesized by converting agricultural bio-waste (potato peel extract) into economically valuable electrode materials. The biogenic CoFe₂O₄ composite showed an overpotential of 370 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 283 mV dec^-1, whereas the ZIF@CoFe₂O₄ composite prepared using an in situ hydrothermal method showed an overpotential of 105 mV at 10 mA cm^-2 and a low Tafel slope of 43 mV dec^-1 in a 1 M KOH medium. The results demonstrated an exciting prospect of high-performance noble metal-free electrocatalysts for low-cost, high-efficiency, and sustainable hydrogen production.

Fabrication of thermostable and reusable nanobiocatalyst for dye decolourization by immobilization of lignin peroxidase on graphene oxide functionalized MnFe2O4 superparamagnetic nanoparticles

In view of the potential applications of immobilized enzymes, partially purified Lignin Peroxidase (LiP) from Pseudomonas fluorescens LiP-RL5 was immobilized on Graphene Oxide functionalized MnFe₂O₄ nanoparticles (10 nm, synthesized by sol-gel auto-combustion) to fabricate a new hyperactive and thermostable nanobiocatalyst and thereafter characterized by using standard techniques. Immobilized LiP was quite stable at 50 °C with the half-life of 14 h and showed higher tolerance towards various metal ions and solvents than free LiP. Immobilized LiP retained 50% of enzyme activity even after nine consecutive runs. When tested against various textile dyes, the immobilized LiP was found quite effective with higher dye decolourization efficiency (up to 88%) within 1 h of incubation at 30 °C. The results of this research effort confirmed that the immobilization of LiP and fabrication of nanobiocatalyst increase the efficacy, stability, and reusability of the enzyme which could be efficiently utilized under harsh industrial conditions.

Attuning the Electronic Properties of Two-Dimensional Co-Fe-O for Accelerating Water Electrolysis and Photolysis

Two-dimensional (2D) materials such as layered double hydroxides (LDH) are promising electrocatalysts, especially for water oxidation, owing to their unique physical and electronic properties besides having adequate surface area and availability of unsaturated active metal centers. Herein, we illustrate the high-temperature transformation of bimetallic LDH to semicrystalline 2D metal oxide nanoplates that can maneuver their electronic properties and thereby accelerate the water dissociation reactions. The nanoplates prepared at 300 °C require only 280 ± 13 and 177 ± 7 mV overpotentials for oxygen/hydrogen evolution reactions (OER and HER) to achieve a current density of ±10 mA cm^-2 in 1 M KOH, respectively. In a two-electrode water splitting cell, while this bifunctional catalyst needs 1.69 V to deliver a current density of 10 mA cm^-2, the LDH precursor demands a cell voltage of 1.93 V. However, both the catalysts demonstrate excellent durability for more than 200 h. When the bifunctional metal oxide electrolyzer is connected to perovskite solar cells for unassisted solar-driven water splitting, impressively, such an integrated photovoltaic-electrolyzer can achieve a solar-to-hydrogen (STH) efficiency of 9.3%. The predominantly superior catalytic activity of the nanoplates is due to the abundance of unsaturated oxygen which decreases the free energy of adsorption of the intermediates.

Amorphous CoFe Double Hydroxides Decorated with N-Doped CNTs for Efficient Electrochemical Oxygen Evolution

The development and design of a highly active and affordable nanostructured material as an efficient electrocatalyst for electrochemical oxygen evolution is a pressing necessity to realize industrial production of hydrogen by water electrolysis. Amorphous nanocomposites have recently attracted interest owing to their superior electrocatalytic activity derived from their unique structure. Herein, amorphous CoFe double hydroxides (Am-CFDH) decorated with N-doped carbon nanotubes (NCNTs) is synthesized by a facile and simple one-pot approach under room temperature. Through electrochemical measurement, the bare Am-CFDH nanocomposite already exhibits a comparable oxygen evolution reaction (OER) activity to the commercial IrO₂ catalyst on account of its amorphous nature and the interaction between Co and Fe. The introduced NCNTs can provide better electrical conductivity, more anchoring sites, and functional groups for enhancing the transfer of electrons and reactants, preventing the agglomeration of Am-CFDH to expose more active sites, and improving the synergistic effect between Am-CFDH and NCNTs. Thus, the Am-CFDH/NCNTs hybrid displays favorable durability beyond 20 h and advanced OER activity, owning a small overpotential of 270 mV at 10 mA cm^-2 and a low Tafel slope of 56.88 mV dec^-1 in alkaline medium.

Self-Template Synthesis of Co-Se-S-O Hierarchical Nanotubes as Efficient Electrocatalysts for Oxygen Evolution under Alkaline and Neutral Conditions

We develop a facile self-template synthetic method to construct hierarchical Co-Se-S-O (CoSe _xS_{2- x}@Co(OH)₂) nanotubes on a carbon cloth as a self-standing electrode for electrocatalytic oxygen evolution reaction (OER). In the synthetic process, separate selenization and sulfurization on the Co(OH)F precursor in different solvents have played an important role in constructing CoSe _xS_{2- x} (Co-Se-S) hierarchical nanotubes, which was further transformed into the nanotube-like Co-Se-S-O via an in situ electrochemical oxidation process. The Co-Se-S-O obtained by the Kirkendall effect through two stepwise anion-exchange reactions represents the first quaternary Co-Se-S-O nanotube array, which dramatically enhances its surface area and conductivity. Further, it only requires low overpotentials of 230 and 480 mV to achieve a 10 mA cm^-2 current density. The OER performance of Co-Se-S-O is much more efficient than that of its monochalcogenide counterparts, as well as the commercial benchmark catalyst IrO₂.

Simple preparation of carbon–bimetal oxide nanospinels for high-performance bifunctional oxygen electrocatalysts

Carbon–bimetal oxide nanospinels synthesized by a single step of autocombustion show high activity for oxygen evolution and reduction.