Cobalt Oxide Electrode Doped with Iridium Oxide as Highly Efficient Water Oxidation Electrode

Utilizing the oxygen-atom trapping effect of Co3O4 with oxygen vacancies to promote chlorite activation for water decontamination

Heterogeneous high-valent cobalt-oxo [≡Co(IV)=O] is a widely focused reactive species in oxidant activation; however, the relationship between the catalyst interfacial defects and ≡Co(IV)=O formation remains poorly understood. Herein, photoexcited oxygen vacancies (OVs) were introduced into Co3O4 (OV-Co3O4) by a UV-induced modification method to facilitate chlorite (ClO2-) activation. Density functional theory calculations indicate that OVs result in low-coordinated Co atom, which can directionally anchor chlorite under the oxygen-atom trapping effect. Chlorite first undergoes homolytic O-Cl cleavage and transfers the dissociated O atom to the low-coordinated Co atom to form reactive ≡Co(IV)=O with a higher spin state. The reactive ≡Co(IV)=O rapidly extracts one electron from ClO2- to form chlorine dioxide (ClO2), accompanied by the Co atom returning a lower spin state. As a result of the oxygen-atom trapping effect, the OV-Co3O4/chlorite system achieved a 3.5 times higher efficiency of sulfamethoxazole degradation (~0.1331 min-1) than the pristine Co3O4/chlorite system. Besides, the refiled OVs can be easily restored by re-exposure to UV light, indicating the sustainability of the oxygen atom trap. The OV-Co3O4 was further fabricated on a polyacrylonitrile membrane for back-end water purification, achieving continuous flow degradation of pollutants with low cobalt leakage. This work presents an enhancement strategy for constructing OV as an oxygen-atom trapping site in heterogeneous advanced oxidation processes and provides insight into modulating the formation of ≡Co(IV)=O via defect engineering.

Nano-spinel cobalt decorated sulphur doped graphene: an efficient and durable electrocatalyst for oxygen evolution reaction and non-enzymatic sensing of H2O2

We report the synthesis of a nano-spinel cobalt decorated sulphur doped reduced graphene oxide (Co@S–rGO) composite exhibiting excellent electrocatalytic performance and electrochemical stability toward oxygen evolution reaction in an alkaline medium.

Assembly of a Highly Active Iridium-Based Oxide Oxygen Evolution Reaction Catalyst by Using Metal-Organic Framework Self-Dissolution

The proton exchange membrane (PEM) electrolyzer for hydrogen production has multiple advantages but is greatly restricted by expensive iridium and sluggish oxygen evolution reaction (OER) kinetics. The most promising way to reduce the precious metal loading is to design and develop highly active Ir-based catalysts. In this study, a versatile approach is reported to prepare a hybrid in the form of a catalyst-support structure (Fe-IrO_x@α-Fe₂O₃, abbreviated Ir@Fe-MF) by utilizing the self-dissolving properties of Fe-MIL-101 under aqueous conditions. The formation of this hybrid is mainly due to the Ir⁴⁺ and released Fe³⁺ ions coprecipitated to assemble into Fe-IrO_x nanoparticles, and the Fe³⁺ released from the inward collapse of the metal-organic framework (MOF) spontaneously forms α-Fe₂O₃. The prepared Ir@Fe-MF-2 hybrid exhibits enhanced catalytic activity toward OER with a lower onset potential and Tafel slop, and only 260 mV overpotential is required to drive the current density to reach 10 mA cm^-2. The performed characterizations clearly indicate that the IrO₆ coordination structure is changed significantly by Fe incorporated into the IrO₂ lattice. The performed X-ray adsorption spectra (XAS) provides evidence that Ir 5d orbital degeneracy is eliminated because of multiple orbitals being semi-occupied in the presence of Fe, which is mainly responsible for the enhancement of OER activity. These findings open an opportunity for the design and preparation of more efficient OER catalysts of transition metal oxides by utilization of the MOF materials. It should be highlighted that a long-term stability of this catalyst run at a high current density in acidic conditions still faces great challenges.

Preparation of Yolk-Shell-Structured Cox Fe1-x P with Enhanced OER Performance

The design and development of low-cost, highly efficient, and stable electrocatalysts to take the place of noble-metal catalysts for the oxygen evolution reaction (OER) remain a significant challenge. Herein, the synthesis of yolk-shell-structured binary transition metal phosphide Co_x Fe_1-x P with different Co/Fe ratios by phosphidation of a cobalt ferrite precursor is reported. The as-synthesized Co_x Fe_1-x P catalysts were used for the OER. All yolk-shell Co_x Fe_1-x P catalysts with different Co/Fe ratios showed much better performance than the corresponding solid catalyst. The formation of Co oxides on the catalyst surface during OER and the optimal Co/Fe ratio were found to be critical to their activity. Among the as-prepared Co_x Fe_1-x P catalysts, that with a Co/Fe ratio of 0.47/0.53 (Co_0.47 Fe_0.53 P) exhibited the best performance. Co_0.47 Fe_0.53 P has an overpotential of 277 mV at a current density of 10 mA cm^-2 , a Tafel slope of 37 mV dec^-1 , and superior stability in alkaline medium. The outstanding performance is partly ascribed to the transfer of valence electrons from Co to P and Fe. The Co_0.47 Fe_0.53 P matrix with excellent conductivity and Fe phosphate that is stable on the surface of the catalyst are also helpful for the OER performance. In addition, the yolk-shell structure of Co_0.47 Fe_0.53 P increases the contact area between electrolyte and catalyst. These characteristics of Co_0.47 Fe_0.53 P greatly improve its OER performance. This optimized binary transition metal phosphide provides a new approach for the design of nonprecious-metal electrocatalysts.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

High-Performance Transition Metal Phosphide Alloy Catalyst for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is a pivotal process in many energy conversion and storage techniques, such as water splitting, regenerative fuel cells, and rechargeable metal-air batteries. The synthesis of stable, efficient, non-noble metal-based electrocatalysts for OER has been a long-standing challenge. In this work, a facile and scalable method to synthesize hollow and conductive iron-cobalt phosphide (Fe-Co-P) alloy nanostructures using an Fe-Co metal organic complex as a precursor is described. The Fe-Co-P alloy exhibits excellent OER activity with a specific current density of 10 mA/cm² being achieved at an overpotential as low as 252 mV. The current density at 1.5 V (vs reversible hydrogen electrode) of the Fe-Co-P catalyst is 30.7 mA/cm², which is more than 3 orders of magnitude greater than that obtained with state-of-the-art Fe-Co oxide catalysts. Our mechanistic experiments and theoretical analysis suggest that the electrochemical-induced high-valent iron stabilizes the cobalt in a low-valent state, leading to the simultaneous enhancement of activity and stability of the OER catalyst.

Hierarchical nanosheet-based Bi2MoO6 microboxes for efficient photocatalytic performance

Hierarchical nanosheet-based Bi₂MoO₆ microboxes are synthesized by a facile template-assisted strategy.

Nanotubular Iridium-Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis

Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as Ir_xCo_1-xO_y, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of Ir_xCo_1-xO_y (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that Ir_xCo_1-xO_y polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO₂ and Co₃O₄, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of Ir_xCo_1-xO_y nanotubes was mainly ascribed to the inclination of Co₃O₄ to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO₂ by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained Ir_xCo_1-xO_y nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse Ir_xCo_1-xO_y, Ir_0.46Co_0.54O_y nanotubes showed the best OER activity (the least-positive onset potential, greatest current density, and low Tafel slope), which was even better than that of commercial Ir/C. The Ir_0.46Co_0.54O_y nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts.

CoO x nanoparticle anchored on sulfonated-graphite as efficient water oxidation catalyst

Ultrasmall CoO_x nanoparticles on sulfonated graphite exhibit highly efficient water oxidation activity and can be used for electrochemical and solar water oxidation.

Development of efficient, robust and earth-abundant water oxidation catalysts (WOCs) is extremely desirable for water splitting by electrolysis or photocatalysis. Herein, we report cobalt oxide nanoparticles anchored on the surface of sulfonated graphite (denoted as “CoO_x@G-Ph-SN”) to exhibit unexpectedly efficient water oxidation activity with a turnover frequency (TOF) of 1.2 s^–1; two or three orders of magnitude higher than most cobalt-based oxide WOCs reported so far. The CoO_x@G-Ph-SN nanocomposite can be easily prepared by a soft hydrothermal route to have an average CoO_x size below 2 nm. Additionally, the loading of CoO_x@G-Ph-SN catalyst on the surface of a BiVO₄ or Fe₂O₃ photoanode can boost remarkably the photoanode currents for robust photocatalytic water oxidation under visible light irradiation. Its excellent activity and photochemical stability for water oxidation suggest that this ultrasmall cobalt-based composite is a promising candidate for solar fuel production.

Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis, Morphological Stability, and Electrocatalytic Application

Inspired by graphene, ultrathin two-dimensional nanomaterials with atomic thickness have attracted more and more attention because of their unique physicochemical properties and electronic structure. In this work, the atomically thick ultrathin Rh₂O₃ nanosheet nanoassemblies (Rh₂O₃-NSNSs) were obtained by oxidizing the atomically thick ultrathin Rh nanosheet nanoassemblies with HClO. For the first time, Rh-based nanostructures were used as the oxygen evolution reaction (OER) electrocatalyst in an alkaline medium. Surprisingly, the as-prepared Rh₂O₃-NSNSs displayed extremely improved catalytic activity and durability for the OER compared with those of the commercial Ir/C catalyst and most recently reported Ir-based electrocatalysts. The result indicated Rh-based nanostructures that have great promise to become a potential candidate for efficient OER electrocatalyst because of the similarity of Rh and Ir prices. These experimental results demonstrated the reasonable morphological control of Rh₂O₃ nanostructures could significantly improve their catalytic activity and durability during heterogeneous catalysis.

Nanosized manganese oxide/holmium oxide: a new composite for water oxidation

Ho₂O₃ as a support for nanosized Mn oxide was used for the synthesis of a new water-oxidizing catalyst.

An iron-based thin film as a highly efficient catalyst for electrochemical water oxidation in a carbonate electrolyte

An iron-based thin film electrodeposited from a CO₂ saturated bicarbonate solution showed remarkable activity with a Tafel slope as low as 34 mV dec⁻¹ in a HCO₃⁻/CO₃²⁻ electrolyte.