Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting

NiFe-layered double hydroxide/CoP2@MnP heterostructures of clustered flower nanowires on MXene-modified nickel foam for overall water-splitting

Exploiting efficient and economical electrocatalysts is indispensable to promoting the sluggish kinetics of overall water-splitting. Herein, we designed a phosphate reaction and two-step hydrothermal method to construct a 3D porous clustered flower-like heterogeneous structure of NiFe-layered double hydroxide (NiFe) and CoP2@MnP (CMP) grown in-situ on MXene-modified nickel foam (NF) substrate (denoted as NiFe/CMP/MX), with favorable kinetics. Density functional theory calculations (DFT) demonstrate that the self-driven transfer of heterojunction charges causes electron redistribution of the catalyst, and optimizes the electron transfer rate of the active site and the d-band center near the Fermi level, thereby reducing the adsorption energy of H and O reaction intermediates (H*, OH*, OOH*). As expected, the combination of CMP and NiFe with naturally conductive MXene forms a strong chemical and electron synergistic effect, which enables the synthesized NiFe/CMP/MX heterogeneous structure exhibits good activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with a low overpotential of 200 mV and 126 mV at 10 mA cm-2, respectively. Furthermore, the overpotential of 1.58 V is enough to drive a current density of 10 mA cm-2 in a two-electrode configuration, which is better than noble metals (RuO2(+)//Pt/C(-)) (1.68 V).

Room Temperature Preparation of Two-Dimensional Black Phosphorus@Metal Organic Framework Heterojunctions and Their Efficient Overall Water-Splitting Electrocatalytic Reactions

Black phosphorus/two-dimensional (2D) metal-organic framework (BP@MOF) heterojunctions were synthesized via templated growth of 2D MOF-Fe/Co nanoplatelets on the surface of exfoliated BP nanosheets at room temperature. Because Fe³⁺ and Co²⁺ ions were absorbed onto the BP surface through coordination with the lone pair electrons of 2D BP, the BP@MOF heterojunction had an intimate interface with strong interactions. Electrochemical oxygen and hydrogen evolution reactions were studied using BP@MOF as the electrocatalyst. High activity of the overall water splitting in 1.0 M KOH was observed under a current density of 10 mA cm^-2. The corresponding overpotentials for HER and OER were as low as 180 and 246 mV, respectively. Meanwhile, the BP@MOF exhibited good environmental stability and long-term electrocatalytic activity for OER and HER, owing to the encapsulation of BP nanosheets by the 2D MOF-Fe/Co. Through this study, a unique hybrid 2D nanomaterial is discovered for the efficient electrolytic splitting of water.

Solid-state synthesis of CdFe2O4 binary catalyst for potential application in renewable hydrogen fuel generation

Clean energy is highly needed at this time when the energy requirements are rapidly increasing. The observed increasing energy requirement are largely due to continued industrialization and global population explosion. The current means of energy source is not sustainable because of several reasons, most importantly, environmental pollution and human health deterioration due to burning of fossil fuels. Therefore, this study develops a new catalyst for hydrogen and oxygen evolution by water splitting as a potential energy vector. The binary metal oxide catalyst CdFe₂O₄ was synthesized by the solventless solid-mechanical alloying method. The as-prepared catalyst was well characterized by several methods including field emission scanning electron microscopy (FESEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared red spectroscopy (FTIR), energy dispersive X-ray spectroscopy (XEDS). The as-prepared catalyst, CdFe₂O₄ was successfully applied for water electrolysis at a moderate overpotential (470 mV). Specifically, the onset potential for the oxygen and hydrogen evolution reactions (OER and HER) were 1.6 V_/RHE and 0.2 V_/RHE respectively (vs. the reversible hydrogen electrode). The electrode potential required to reach 10 mA/cm^-2 for OER (in alkaline medium) and HER (in acidic medium) was 1.70 V_/RHE (corresponding to overpotential η = 0.47 and − 0.30 V_/RHE (η = − 0.30 V) respectively. Similarly, the developed OER and HER catalyst displayed high current and potential stability for a period of 12 h. This approach is seen as the right track of making water electrolysis for hydrogen energy feasible through provision of low-energy requirement for the electrolytic process. Therefore, CdFe₂O₄ is a potential water splitting catalyst for hydrogen evolution which is a clean fuel and an antidote for world dependence on fossil fuel for energy generation.

Role of Interfacial Defects in Photoelectrochemical Properties of BiVO4 Coated on ZnO Nanodendrites: X-ray Spectroscopic and Microscopic Investigation

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO₄ (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

Hierarchically porous FeNi3@FeNi layered double hydroxide nanostructures: one-step fast electrodeposition and highly efficient electrocatalytic performances for overall water splitting

FeNi-layered double hydroxide (LDH) is thought to be an excellent electrocatalyst for oxygen evolution reaction (OER) but it always shows extremely poor electrocatalytic activity toward hydrogen evolution reaction (HER) in alkaline media. Hence, it is significant to improve its HER activity to make it a bifunctional electrocatalyst for the decomposition of water. Here, a simple galvanostatic electrodeposition method was designed for the successful construction of the bifunctional FeNi₃@FeNi LDH electrocatalyst. The as-prepared catalyst displayed excellent electrocatalytic activity for HER/OER in 1.0 M KOH. To drive the current density of 10 mA cm^-2 for HER/OER, an overpotential of 106/199 mV was needed, respectively. In a two-electrode system with the FeNi₃@FeNi LDH/NF as the anode and the cathode simultaneously, the overpotential hardly changed after continuously working for 168 h at 10 mA cm^-2. Compared with other FeNi-based catalysts, the present catalyst possessed close or better electrocatalytic activity.