Tapping the Potential of High-Valent Mo and W Metal Centers for Dynamic Electronic Structures in Multimetallic FeVO(OH)/Ni(OH)2 for Ultrastable and Efficient Overall Water Splitting.

Rationally designing a noble metal-free electrocatalyst for OER and HER is pivotal for large-scale energy generation via water splitting. A multimetallic electrocatalyst FeVO(OH)/Ni0.86Mo0.07W0.07(OH)2, aimed at tuning the electronic structure, is fabricated and shows considerable improvement in the water-splitting reaction kinetics, aided by low Tafel slope values of 24 mV/dec for OER and 67 mV/dec for HER, respectively. By taking advantage of (e̅-e̅) repulsions at the t2g level, we introduced high-valency Mo and W to provide a viable path for π-electron donation from oxygen 2p orbitals to vacant Mo and W orbitals for a dynamic electronic structure and an interfacial synergistic effect, which optimized the bond lengths for reaction intermediates to facilitate water splitting. The hybrid catalyst FeVO(OH)/NiMoW(OH)2 shows intrinsic activity and durability toward OER and HER tested for 48 h at a current density of 20 mA/cm2 and a cell voltage of 1.65 V @ 20 mA/cm2.

Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO-(VO)3(PO4)2·6H2O for Enhanced Electrochemical Water Splitting.

A strategical approach for morphological transformation and heterojunction formation was utilized to suppress the shortcomings of uni-metal oxide electrocatalysts and enhance their bifunctionality. In situ generation of copper oxide (CuO) over the surface of manganese oxide (Mn2O3) resulted in a morphological transformation from solid spheres to hollow spherical structures due to the ion-exchange diffusion (Kirkendall effect) of Cu ions into Mn2O3 particles. This hollowness resulted in the advancement of the bifunctional electrocatalytic behavior of Mn2O3/CuO (overpotential (η10) of 280 mV for an OER and 310 mV for an HER at a current density of 10 mA/cm2) by virtue of increased exposed surface active sites aiding the adsorption of water molecules on the surface. The increased electrochemical active surface area (ECSA/Cdl = 34 mF/cm2) and reduced charge transfer resistance resulted in the formation of Mn2O3/CuO hollow spheres to achieve an approximately threefold enhancement in the turnover frequency (TOF) compared to the bare Mn2O3. The electrocatalytic efficiency of Mn2O3/CuO was further enhanced by virtue of the faster charge transfer coefficient of two-dimensional (2D) vanadyl phosphate hexahydrate (VOP) sheets deposited over its surface. This boosted the overall water splitting with attained overpotential (η10) values of 190 and 220 mV with Tafel slopes of 60 and 105 mV/decade for an OER and HER, respectively. The morphological transformation and formation of an n-p heterojunction between Mn2O3 and CuO based on their work function (φ) values evaluated from the density functional theory (DFT) calculation and the effect of the VOP overlayer for faster reaction kinetics at the electrolyte interface resulted in an ∼10-fold increment in TOF values compared to the bare counterpart.

Hierarchical FeO(OH)-CoCeV (Oxy)hydroxide as a Water Cleavage Promoter.

The search for a bifunctional electrocatalyst having water cleavage promoting ability along with the operational stability to efficiently generate oxygen and hydrogen could lead to robust systems for applications. These fundamental ideas can be achieved by designing the morphology, tuning the electronic structure, and using dopants in their higher oxidation states. Herein, we have fabricated a binder-free FeO(OH)-CoCeV-layered triple hydroxide (LTH) bifunctional catalyst by a two-step hydrothermal method, in which the nanograin-shaped FeO(OH) coupled with CoCeV-LTH nanoflakes provides more electrocatalytically active sites and enhances the charge-transfer kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The composition-optimized electrocatalyst (FeO(OH)-Co0.5Ce0.05V0.15-LTH) acts as an efficient water cleavage composite by virtue of its favorable oxidation states leading to cyclic redox couples, which yields an overpotential of 53 mV for HER and 227 mV for OER to drive 10 mA/cm2 current density in 1 M KOH with a corresponding Tafel slope of 70 mV/dec for HER and 52 mV/dec for OER. Furthermore, for the overall water splitting reaction, the heterostructure FeO(OH)-Co0.5Ce0.05V0.15-LTH acts as a dual-functional electrocatalyst, which requires a cell voltage of 1.52 V versus RHE to drive 10 mA/cm2 current density.

A Z-Scheme Strategy that Utilizes ZnIn2 S4 and Hierarchical VS2 Microflowers with Improved Charge-Carrier Dynamics for Superior Photoelectrochemical Water Oxidation.

One of the major limiting factors for efficient photoelectrochemical water oxidation is the fast recombination kinetics of photogenerated charge carriers. Herein, we propose a model system that utilizes ZnIn2 S4 and hierarchical VS2 microflowers for efficient charge separation through a Z-scheme pathway, without the need for an electron mediator. An impressive 18-fold increase in photocurrent was observed for ZnIn2 S4 -VS2 compared to ZnIn2 S4 alone. The charge-transfer dynamics in the composite were found to follow a Z-scheme pathway, which resulted in decreased charge recombination and greater accumulation of the surface charge. Furthermore, slow kinetics of the surface reaction in the ZnIn2 S4 -VS2 composite correlated to an increased surface-charge capacitance. This feature of the composite material facilitated partial storage of the photogenerated charge carriers (e- /h+ ) under illumination and dark-current conditions, thus storing and utilizing solar energy more efficiently.