Oxygen Evolution Catalyzed by Nickel-Iron Oxide Nanocrystals with a Nonequilibrium Phase

Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Intercalation-induced partial exfoliation of NiFe LDHs with abundant active edge sites for highly enhanced oxygen evolution reaction

Edge sites and interlayer space of NiFe layered double hydroxides (LDHs) play an important role in water oxidation. However, the combined effect of interlayer expansion and partial exfoliation on the catalytic activity is yet to be investigated. Herein, scalable synthesis of partially exfoliated citrate-intercalated NiFe LDHs with tunable interlayer space have been achieved. The effect of citrate concentration on the phase, morphology, surface elemental composition, electronic states of surface metals, and electrochemical properties are comprehensively studied. The unique structure results in improved intrinsic catalytic activity and abundant active edge sites for oxygen evolution reaction. The optimal NiFe LDHs show an overpotential of 225 mV at 10 mA cm^-2, which is much smaller than that (∼305 mV) of the single-layer NiFe LDH nanosheets reported in the literature. The high catalytic activity can be mainly attributed to the combined effect between the enlarged interlayer space and the partial exfoliation/nanosheet thickness. That is, the interlayer space is related to the reaction kinetics/mechanism, while the degree of exfoliation affects the magnitude of the current density at a certain potential.

Dendrimer-Ni-Based Material: Toward an Efficient Ni-Fe Layered Double Hydroxide for Oxygen-Evolution Reaction

Ni/Fe oxides are among the most widely used catalysts for water splitting. This paper outlines a new approach to synthesize Ni-Fe layered double hydroxides (Ni-Fe LDHs) for oxygen-evolution reaction (OER). Herein, we show that a dendrimer with carboxylate surface groups (generation 3.5) could react with Ni(II) ions to form a precatalyst for OER. During electrochemical OER, this precatalyst converted to Ni-Fe LDH, which is an efficient catalyst toward OER in the presence of Fe(III) ions. The catalyst was characterized by a number of methods and applied for OER using fluorine-doped tin oxide (FTO), Au, Pt, Ni foam, and glassy carbon electrodes. The catalyst shows a current density of 100 mA/cm² on the surface of the Ni foam, using only 297 mV overpotential and with the Tafel slope of 60.8 mV/decade. A current density of 50 mA/cm² on the surface of Au or Pt requires 333 and 317 mV overpotentials, respectively. The slopes of the Tafel plots for the catalyst on Au, GC, and Pt are 52.5, 47.1, and 37.4 mV/decade, respectively. The dendrimer resulted in a large dispersibility and an increase in active sites of Ni-Fe LDH, as well as the formation of Ni-Fe LDH.

Nickel selenide from single-molecule electrodeposition for efficient electrocatalytic overall water splitting

The prepared electrode (NiSe-TMEDA/CC) from single-molecule electrodeposition is functional for both the OER and HER and shows a superior performance for overall water splitting with a low cell voltage in alkaline medium.

Micro–nanoarchitectures of electrodeposited Ni–ITO nanocomposites on copper foil as electrocatalysts for the oxygen evolution reaction

A simple conventional three electrodes system was used for the preparation of Ni and Ni–ITO nanocomposites as an electrocatalyst for oxygen evolution reaction.

Controlling the 3-D morphology of Ni-Fe-based nanocatalysts for the oxygen evolution reaction

Controlling the 3-D morphology of nanocatalysts is one of the underexplored but important approaches for improving the sluggish kinetics of the oxygen evolution reaction (OER) in water electrolysis. This work reports a scalable, oil-based method based on thermal decomposition of organometallic complexes to yield highly uniform Ni-Fe-based nanocatalysts with a well-defined morphology (i.e. Ni-Fe core-shell, Ni/Fe alloy, and Fe-Ni core-shell). Transmission electron microscopy reveals their morphology and composition to be NiOx-FeOx/NiOx core-mixed shell, NiOx/FeOx alloy, and FeOx-NiOx core-shell. X-ray techniques resolve the electronic structures of the bulk and are supported by electron energy loss spectroscopy analysis of individual nanoparticles. These results suggest that the crystal structure of Ni is most likely to contain α-Ni(OH)2 and that the chemical environment of Fe is variable, depending on the morphology of the nanoparticle. The Ni diffusion from the amorphous Ni-based core to the iron oxide shell makes the NiOx-NiOx/FeOx core-mixed shell structure the most active and the most stable nanocatalyst, which outperforms the comparison NiOx/FeOx alloy nanoparticles expected to be active for the OER. This study suggests that the chemical environment of the mixed NiOx/FeOx alloy composition is important to achieve high electrocatalytic activity for the OER and that the 3-D morphology plays a key role in the optimization of the electrocatalytic activity and stability of the nanocatalyst for the OER.

Synthesis of a Highly Efficient Oxygen-Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co_0.7 Fe_0.3 )₂ B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm^-2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO₂ and RuO₂ . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)₂ are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co_1-x Fe_x )₂ B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.

Tailored transition metal-doped nickel phosphide nanoparticles for the electrochemical oxygen evolution reaction (OER)

Foreign transition metals are doped into the nickel phosphide nanoparticles through a simple wet-chemical process to show a high level of doping without significant distortion of crystal structure and morphology, which endows excellent activity in the electrochemical oxygen evolution reaction.

Nickel oxide–polypyrrole nanocomposite electrode materials for electrocatalytic water oxidation

Electrochemically prepared nickel oxide nanoparticles entrapped into a polymer matrix as efficient material for O₂ evolution.

Surface Electrochemical Modification of a Nickel Substrate to Prepare a NiFe-based Electrode for Water Oxidation

The slow kinetics of water oxidation greatly jeopardizes the efficiency of water electrolysis for H₂ production. Developing highly active water oxidation electrodes with affordable fabrication costs is thus of great importance. Herein, a Ni^II Fe^III surface species on Ni metal substrate was generated by electrochemical modification of Ni in a ferrous solution by a fast, simple, and cost-effective procedure. In the prepared Ni^II Fe^III catalyst film, Fe^III was incorporated uniformly through controlled oxidation of Fe^II cations on the electrode surface. The catalytically active Ni^II originated from the Ni foam substrate, which ensured the close contact between the catalyst and the support toward improved charge-transfer efficiency. The as-prepared electrode exhibited high activity and long-term stability for electrocatalytic water oxidation. The overpotentials required to reach water oxidation current densities of 50, 100, and 500 mA cm^-2 are 276, 290, and 329 mV, respectively.

Solar-Driven H2 O2 Generation From H2 O and O2 Using Earth-Abundant Mixed-Metal Oxide@Carbon Nitride Photocatalysts

Light-driven generation of H2 O2 only from water and molecular oxygen could be an ideal pathway for clean production of solar fuels. In this work, a mixed metal oxide/graphitic-C3 N4 (MMO@C3 N4 ) composite was synthesized as a dual-functional photocatalyst for both water oxidation and oxygen reduction to generate H2 O2 . The MMO was derived from a NiFe-layered double hydroxide (LDH) precursor for obtaining a high dispersion of metal oxides on the surface of the C3 N4 matrix. The C3 N4 is in the graphitic phase and the main crystalline phase in MMO is cubic NiO. The XPS analyses revealed the doping of Fe(3+) in the dominant NiO phase and the existence of surface defects in the C3 N4 matrix. The formation and decomposition kinetics of H2 O2 on the MMO@C3 N4 and the control samples, including bare MMO, C3 N4 matrix, Ni- or Fe-loaded C3 N4 and a simple mixture of MMO and C3 N4 , were investigated. The MMO@C3 N4 composite produced 63 μmol L(-1) of H2 O2 in 90 min in acidic solution (pH 3) and exhibited a significantly higher rate of production for H2 O2 relative to the control samples. The positive shift of the valence band in the composite and the enhanced water oxidation catalysis by incorporating the MMO improved the light-induced hole collection relative to the bare C3 N4 and resulted in the enhanced H2 O2 formation. The positively shifted conduction band in the composite also improved the selectivity of the two-electron reduction of molecular oxygen to H2 O2 .

General Strategy for the Synthesis of Transition Metal Phosphide Films for Electrocatalytic Hydrogen and Oxygen Evolution

Transition metal phosphides recently have been identified as promising Earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Here, we present a general and scalable strategy for the synthesis of transition metal phosphide electrodes based on the reaction of commercially available metal foils (Fe, Co, Ni, Cu, and NiFe) with various organophosphine reagents. The resulting phosphide electrodes were found to exhibit excellent electrocatalytic HER and OER performance. The most active electrodes required overpotentials of only -128 mV for the HER in acid (Ni2P), -183 mV for the HER in base (Ni2P), and 277 mV for the OER in base (NiFeP) to produce operationally relevant current densities of 10 mA cm(-2). Such HER and OER performance compares favorably with samples prepared using significantly more elaborate and costly procedures. Furthermore, we demonstrate that the approach can also be utilized to obtain highly active and conformal metal phosphide coatings on photocathode materials, such as highly doped Si, that are relevant to solar fuels production.

Nanostructured hybrid NiFeOOH/CNT electrocatalysts for oxygen evolution reaction with low overpotential

The OER activity of nanostructured Ni-based LDH can be improved by doping with transition metals, which can be further enhanced by hybridizing with CNT.

Nanosorbcats of methylene blue on novel Fe2O3 nanorods for photocatalytic water oxidation

MB@pfa@Fe₂O₃ nanorods are efficient recyclable nanosorbcats for LED light driven dioxygen generation using water as oxygen source.