Synthesis and Characterisation of Cobalt Ferrite Coatings for Oxygen Evolution Reaction

Large-scale sonochemical fabrication of a Co3O4-CoFe2O4@MWCNT bifunctional electrocatalyst for enhanced OER/HER performances

Herein, we have prepared a mixed-phase Co3O4-CoFe2O4@MWCNT nanocomposite through a cheap, large-scale, and facile ultrasonication route followed by annealing. The structural, morphological, and functional group analyses of the synthesized catalysts were performed by employing various characterization approaches such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The resultant samples were tested for bifunctional electrocatalytic activity through various electrochemical techniques: cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The prepared Co3O4-CoFe2O4@MWCNT nanocomposite achieved a very high current density of 100 mA cm-2 at a lower (290 mV and 342 mV) overpotential (vs. RHE) and a smaller (166 mV dec-1 and 138 mV dec-1) Tafel slope in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, compared to Co3O4-CoFe2O4. The excellent electrochemical activity of the as-prepared electrocatalyst was attributed to the uniform incorporation of Co3O4-CoFe2O4 over MWCNTs which provides high redox active sites, a greater surface area, better conductivity, and faster charge mobility. Furthermore, the enhanced electrochemical active surface, low charge-transfer resistance (Rct), and higher exchange current density (J0) of the Co3O4-CoFe2O4@MWCNT ternary composite are attributed to its superior behavior as a bifunctional electrocatalyst. Conclusively, this study demonstrates a novel and large-scale synthesis approach for bifunctional electrocatalysts with a high aspect ratio and abundance of active sites for high-potential energy applications.

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.

Improving the Oxygen Evolution Reaction on Fe3O4(001) with Single-Atom Catalysts

Doping magnetite surfaces with transition-metal atoms is a promising strategy to improve the catalytic performance toward the oxygen evolution reaction (OER), which governs the overall efficiency of water electrolysis and hydrogen production. In this work, we investigated the Fe₃O₄(001) surface as a support material for single-atom catalysts of the OER. First, we prepared and optimized models of inexpensive and abundant transition-metal atoms, such as Ti, Co, Ni, and Cu, trapped in various configurations on the Fe₃O₄(001) surface. Then, we studied their structural, electronic, and magnetic properties through HSE06 hybrid functional calculations. As a further step, we investigated the performance of these model electrocatalysts toward the OER, considering different possible mechanisms, in comparison with the pristine magnetite surface, on the basis of the computational hydrogen electrode model developed by Nørskov and co-workers. Cobalt-doped systems were found to be the most promising electrocatalytic systems among those considered in this work. Overpotential values (∼0.35 V) were in the range of those experimentally reported for mixed Co/Fe oxide (0.2-0.5 V).

Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe₂ O₄ thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm^-2 in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.

Mechanisms of the Oxygen Evolution Reaction on NiFe2O4 and CoFe2O4 Inverse-Spinel Oxides

Spinel ferrites, especially Nickel ferrite, NiFe₂O₄, and Cobalt ferrite, CoFe₂O₄, are efficient and promising anode catalyst materials in the field of electrochemical water splitting. Using density functional theory, we extensively investigate and quantitatively model the mechanism and energetics of the oxygen evolution reaction (OER) on the (001) facets of their inverse-spinel structure, thought as the most abundant orientations under reaction conditions. We catalogue a wide set of intermediates and mechanistic pathways, including the lattice oxygen mechanism (LOM) and adsorbate evolution mechanism (AEM), along with critical (rate-determining) O–O bond formation barriers and transition-state structures. In the case of NiFe₂O₄, we predict a Fe-site-assisted LOM pathway as the preferred OER mechanism, with a barrier (ΔG^⧧) of 0.84 eV at U = 1.63 V versus SHE and a turnover frequency (TOF) of 0.26 s^–1 at 0.40 V overpotential. In the case of CoFe₂O₄, we find that a Fe-site-assisted LOM pathway (ΔG^⧧ = 0.79 eV at U = 1.63 V vs SHE, TOF = 1.81 s^–1 at 0.40 V overpotential) and a Co-site-assisted AEM pathway (ΔG^⧧ = 0.79 eV at bias > U = 1.34 V vs SHE, TOF = 1.81 s^–1 at bias >1.34 V) could both play a role, suggesting a coexistence of active sites, in keeping with experimental observations. The computationally predicted turnover frequencies exhibit a fair agreement with experimentally reported data and suggest CoFe₂O₄ as a more promising OER catalyst than NiFe₂O₄ in the pristine case, especially for the Co-site-assisted OER pathway, and may offer a basis for further progress and optimization.