VO2 as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction

Understanding the Super-Theoretical Capacity Behavior of VO2 in Aqueous Zn Batteries

VO2 material, as a promising intercalation host, is widely investigated not only in aqueous lithium-ion batteries, but also in aqueous zinc-ion batteries (AZIBs) owing to its stable tunnel-like framework and multivalence of vanadium. Different from lithium-ion storage, VO2 can provide higher capacity when storing zinc ions, even exceeding its theoretical capacity (323 mAh g-1), but the specific reason for this unconventional performance in AZIBs is still unclear. The present study proposes a catalytic oxygen evolution reaction (OER) coupled with an interface oxidation mechanism of VO2 during the initial charging to a high voltage. This coupling induces a phase transformation of VO2 into a high oxidation state of V5O12∙6H2O, enabling a nearly two-electron reaction and providing additional zinc storage sites to achieve super-theoretical capacity. Furthermore, it is demonstrated that these vanadium oxide cathodes (V2O3, VO2, and V2O5) will all undergo phase change after the first charge or short cycle. Notably, water molecules participate in the final formation of layered vanadium-based hydrate, highlighting their crucial role as "pillars" for stabilizing the structure. This work significantly enhances the understanding of vanadium-based oxide cathodes.

V2O5-Fe3O4/rGO Ternary Nanocomposite with Dual Applications as a Dye Degradation Photocatalyst and OER Electrocatalyst

The design and synthesis of structured nanomaterials with dual properties have always been highly attractive in various fields, especially in the reduction of environmental pollution as well as the generation of renewable energy. In this study, the synthesized ternary V2O5-Fe3O4/rGO nanocomposite was investigated to evaluate both the photocatalytic and electrocatalytic activities for the removal of methylene blue (MB) dye under UV/visible light radiation and oxygen evolution reaction (OER), respectively. The magnetized V2O5-Fe3O4/rGO nanocomposite is characterized by TEM, FE-SEM (with coupling by elemental mapping), EDS, XRD, FTIR, Raman, PL, DRS, and UV-vis analyses. The obtained results show that the graphene oxide substrate is decorated very well using Fe3O4 and V2O5 nanoparticles and converted to reduced graphene oxide (rGO). Furthermore, the V2O5-Fe3O4/rGO nanocomposite is considered as an active catalyst material to modify the commercial glassy carbon electrode for OER using linear sweep voltammetry (LSV). The photocatalytic activity of this novel nanocomposite revealed 89.2% (kobs = 1.7 × 10-2 min-1) and 76% (kobs = 8.3 × 10-3 min-1) degradation efficiencies of MB dye under UV and visible light irradiation at room temperature, respectively, and the surface area of the V2O5-Fe3O4/rGO nanocomposite was examined to be 705.8 cm2/g by N2 adsorption-desorption isotherms. In addition, electrochemical measurements determined the best OER performance of the ternary nanocomposite with the lowest overpotential (458 mV) and Tafel slope (132 mV dec-1) compared to the rGO substrate, Fe3O4, V2O5 nanoparticles, and binary nanocomposites. This work shows much enhancements in both photocatalytic and electrocatalytic activities due to the synergistic effect of the decorated GO support with V2O5 and Fe3O4 nanoparticles.

Electrocatalytic Properties of Co3O4 Prepared on Carbon Fibers by Thermal Metal-Organic Deposition for the Oxygen Evolution Reaction in Alkaline Water Electrolysis

Cobalt oxide (Co₃O₄) serves as a promising electrocatalyst for oxygen evolution reactions (OER) in water-electrolytic hydrogen production. For more practical applications, advances in dry-deposition processes for the high-throughput fabrication of such Co₃O₄ electrocatalysts are needed. In this work, a thermal metal-organic deposition (MOD) technique is developed to form Co₃O₄ deposits on microscale-diameter carbon fibers constituting a carbon fiber paper (CFP) substrate for high-efficiency OER electrocatalyst applications. The Co₃O₄ electrocatalysts are deposited while uniformly covering the surface of individual carbon fibers in the reaction temperature range from 400 to 800 °C under an ambient Ar atmosphere. It is found that the microstructure of deposits is dependent on the reaction temperature. The Co₃O₄ electrocatalysts prepared at 500 °C and over exhibit values of 355-384 mV in overpotential (η₁₀) required to reach a current density of 10 mA cm^-2 and 70-79 mV dec^-1 in Tafel slope, measured in 1 M KOH aqueous solution. As a result, it is highlighted that the improved crystallinity of the Co₃O₄ electrocatalyst with the increased reaction temperature leads to an enhancement in electrode-level OER activity with the high electrochemically active surface area (ECSA), low charge transfer resistance (R_ct), and low η₁₀, due to the enhanced electrical conductivity. On the other hand, it is found that the inherent catalytic activity of the surface sites of the Co₃O₄, represented by the turnover frequency (TOF), decreases with reaction temperature due to the high-temperature sintering effect. This work provides the groundwork for the high-throughput fabrication and rational design of high-performance electrocatalysts.

Graphitic Carbon Cloth-Based Hybrid Molecular Catalyst: A Non-conventional, Synthetic Strategy of the Drop Casting Method for a Stable and Bifunctional Electrocatalyst for Enhanced Hydrogen and Oxygen Evolution Reactions

Hydrogen energy production through water electrolysis is envisaged as one of the most promising, sustainable, and viable alternate sources to cater to the incessant demands of renewable energy storage. Germane to our effort in this field, we report easily synthesizable and very cost-effective isoperthiocyanic acid (IPA) molecular complexes as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under acidic and alkaline conditions. The Pd(II)IPA, Co(II)IPA, and Ni(II)IPA complexes were synthesized and were evaluated for HER and OER applications. These complexes when embedded onto graphitized carbon cloth (GrCC) exhibited a significant enhancement in the HER activity in contrast to their pristine counterparts. The hybrid electrocatalyst Pd(II)IPA among the three showed an extremely low overpotential of 94.1 mV to achieve a current density of 10 mA cm^-2, while Co(II)IPA and Ni(II)IPA complexes showed overpotentials of 367 and 394 mV, respectively, to achieve a current density of 10 mA cm^-2. These complexes on carbon cloth showed decreased charge transfer resistance compared to that of pristine metal complexes. The enhanced catalytic activity of the complexes on carbon cloth can be attributed to the porous and conducting nature of the graphitized carbon cloth. For OER activity, the Pd(II)IPA complex showed an excellent performance with an overpotential value of 210 mV, while Co(II)IPA and Ni(II)IPA exhibited overpotentials of 400 and 270 mV, respectively, to drive a current density of 10 mA cm^-2 in 0.1 M KOH. This work further widens the scope and application of molecular complexes in combination with an excellent carbon support for renewable energy storage applications.