Remarkable purification of organic dyes by NiOOH-modified industrial waste residues

Extracting functional materials from industrial waste residues to absorb organic dyes can maximize waste reuse and minimize water pollution. However, the extraordinarily low purification efficiency still limits the practical application of this strategy. Herein, the lamellar NiOOH is in-situ anchored on the industrial waste red mud surface (ARM/NiOOH) as an adsorbent to purify organic dyes in wastewater. ARM/NiOOH adsorbent with high specific surface area and porosity provides considerable active sites for the congo red (CR), thereby significantly enhancing the removal efficiency of CR. Besides, we fit a reasonable adsorption model for ARM/NiOOH adsorbent and investigate its adsorption kinetics. Resultantly, ARM/NiOOH adsorbent can remarkably adsorb 348.0 mg g-1 CR within 5 min, which is 7.91 times that of raw RM. Our work provides a strategy for reusing industrial waste and purifying sewage pollution, which advances wastewater treatment engineering.

Fe-doping and oxygen vacancy achieved by electrochemical activation and precipitation/dissolution equilibrium in NiOOH for oxygen evolution reaction

The poor conductivities and instabilities of accessible nickel oxyhydroxides hinder their use as oxygen evolution reaction (OER) electrocatalysts. Herein, we constructed Fe-NiOOH-OV-600, an Fe-doped nickel oxide hydroxide with abundant oxygen vacancies supported on nickel foam (NF), using a hydrothermal method and an electrochemical activation strategy involving 600 cycles of cyclic voltammetry, assisted by the precipitation/dissolution equilibrium of ferrous sulfide (FeS) in the electrolyte. This two-step method endows the catalyst with abundant Fe-containing active sites while maintaining the ordered structure of nickel oxide hydroxide (NiOOH). Characterization and density functional theory (DFT) calculations revealed that synergy between trace amounts of the Fe dopant and the oxygen vacancies not only promotes the generation of reconstructed active layers but also optimizes the electronic structure and adsorption capacity of the active sites. Consequently, the as-prepared Fe-NiOOH-OV-600 delivered large current densities of 100 and 1000 mA cm-2 for the OER at overpotentials of only 253 and 333 mV in 1 mol/L KOH. Moreover, the catalyst is stable for at least 100 h at 500 mA cm-2. This work provides insight into the design of efficient transition-metal-based electrocatalysts for the OER.

Formation and Stabilization of NiOOH by Introducing α-FeOOH in LDH: Composite Electrocatalyst for Oxygen Evolution and Urea Oxidation Reactions

NiOOH is considered as the most active intermediate during electrochemical oxidation reaction, however, it is hard to directly synthesize due to high oxidation energy. Herein, theoretical calculations predict that α-FeOOH enables a decline in formation energy and an improvement in stabilization of NiOOH in NiFe-based layered double hydroxide (LDH). Inspiringly, a composite composed of α-FeOOH and LDH is well-designed and successfully fabricated in hydrothermal treatment by adding extra Fe³⁺ resource, and stable NiOOH is obtained by the following electro-oxidation method. Benefiting from strong electron-capturing capability of α-FeOOH, it efficiently promotes charge redistribution around the Ni/Fe sites and activates Ni atoms of LDH, verified by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The d-band center is optimized that balances the absorption and desorption energy, and thus Gibbs free energy barrier is lowered dramatically toward oxygen evolution reaction (OER) and urea oxidation reaction (UOR), and finally showing an outstanding overpotential of 195 mV and a potential of 1.35 V at 10 mA cm^-2 , respectively. This study provides a novel strategy to construct highly efficient catalysts via the introduction of a new phase for complex multiple-electron reactions.

Electrochemical Oxidation of HMF via Hydrogen Atom Transfer and Hydride Transfer on NiOOH and the Impact of NiOOH Composition

A great deal of attention has been directed toward studying the electrochemical oxidation of 5-hydroxymethylfurfural (HMF), a molecule that can be obtained from biomass-derived cellulose and hemicellulose, to 2,5-furandicarboxylic acid (FDCA), a molecule that can replace the petroleum-derived terephthalic acid in the production of widely used polymers such as polyethylene terephthalate. NiOOH is one of the best and most well studied electrocatalysts for achieving this transformation; however, the mechanism by which it does so is still poorly understood. This study quantitatively examines how two different dehydrogenation mechanisms on NiOOH impact the oxidation of HMF and its oxidation intermediates on the way to FDCA. The first mechanism is a well-established indirect oxidation mechanism featuring chemical hydrogen atom transfer to Ni³⁺ sites while the second mechanism is a newly discovered potential-dependent (PD) oxidation mechanism involving electrochemically induced hydride transfer to Ni⁴⁺ sites. The composition of NiOOH was also tuned to shift the potential of the Ni(OH)₂ /NiOOH redox couple and to investigate how this affects the rates of indirect and PD oxidation as well as intermediate accumulation during a constant potential electrolysis. The new insights gained by this study will allow for the rational design of more efficient electrochemical dehydrogenation catalysts.

Charge-transfer-induced 2D ferromagnetism and realization of thermo-remnant memory effect in ultrathinß-NiOOH-encapsulated graphene

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagneticβ-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thinβ-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure ofβ-NiOOH/graphene/β-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

A Three-Dimensional Branched TiO2 Photoanode with an Ultrathin Al2O3 Passivation Layer and a NiOOH Cocatalyst toward Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting provides an alternative strategy for clean and renewable hydrogen production; however, the practical application is severely limited by the low solar conversion. Herein, a novel and simple strategy has been developed to construct a 3D branched TiO₂ photoanode with an ultrathin Al₂O₃ passivation layer and NiOOH cocatalyst. The structure and properties of the as-obtained photoanodes are explored by X-ray diffraction, Mott-Schottky, electrochemical impedance spectroscopy, and open circuit voltage measurements. The as-obtained B-TiO₂/Al₂O₃/NiOOH ternary heterojunction with a high-quality contact interface exhibits improved light absorption ability, an enhanced photocurrent density of 1.42 mA/cm² at 1.23 V_RHE, high conversion efficiency (0.44% at 0.80 V_RHE), and excellent stability compared to pristine TiO₂ and alone-Al₂O₃ or NiOOH decorated TiO₂ photoanodes. Therefore, this work could offer a new approach to designing and fabricating high-quality contact interfaces between photoelectrodes and various cocatalysts.

Superior Pseudocapacitive Storage of a Novel Ni3Si2/NiOOH/Graphene Nanostructure for an All-Solid-State Supercapacitor

Recent developments in the synthesis of graphene-based structures focus on continuous improvement of porous nanostructures, doping of thin films, and mechanisms for the construction of three-dimensional architectures. Herein, we synthesize creeper-like Ni₃Si₂/NiOOH/graphene nanostructures via low-pressure all-solid melting-reconstruction chemical vapor deposition. In a carbon-rich atmosphere, high-energy atoms bombard the Ni and Si surface, and reduce the free energy in the thermodynamic equilibrium of solid Ni-Si particles, considerably catalyzing the growth of Ni-Si nanocrystals. By controlling the carbon source content, a Ni₃Si₂ single crystal with high crystallinity and good homogeneity is stably synthesized. Electrochemical measurements indicate that the nanostructures exhibit an ultrahigh specific capacity of 835.3 C g^-1 (1193.28 F g^-1) at 1 A g^-1; when integrated as an all-solid-state supercapacitor, it provides a remarkable energy density as high as 25.9 Wh kg^-1 at 750 W kg^-1, which can be attributed to the free-standing Ni₃Si₂/graphene skeleton providing a large specific area and NiOOH inhibits insulation on the electrode surface in an alkaline solution, thereby accelerating the electron exchange rate. The growth of the high-performance composite nanostructure is simple and controllable, enabling the large-scale production and application of microenergy storage devices.

Towards Long-Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long-term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched-NiOOH/BiVO₄ , leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long-term durability.

Electro-oxidation and characterization of nickel foam electrode for removing boron

The electrocoagulation (EC) using metallic Ni foam as electrodes was studied for the removal of boron from solution. The electrolytic parameters were pH (4-12), current density (0.6-2.5 mA cm^-2), and initial concentration of boron (10-100 mg L^-1). Experimental results revealed that removal efficiency was maximized at pH 8-9, and decreased as the pH increased beyond that range. At particular onset potentials (0.5-0.8 V vs. Hg/HgO), the micro-granular nickel oxide that was created on the surface of the nickel metal substrate depended on pH, as determined by cyclic voltammetry. Most of the crystallites of the precipitates comprised a mixed phase of β-Ni(OH)₂, a theophrastite phase, and NiOOH, as revealed by XRD and SEM analyses. A current density of 1.25 mA cm^-2 was effective in the EC of boron, and increasing the concentration of boric acid from 10 to 100 mg L^-1 did not greatly impair removal efficiency. A kinetic investigation revealed that the reaction followed a pseudo-second order rate model. The optimal conditions under which 99.2% of boron was removed from treated wastewater with 10 mg L^-1-B, leaving less than 0.1 mg L^-1-B in the electrolyte, were pH 8 and 1.25 mA cm^-2 for 120 min.

NiSe@NiOOH Core-Shell Hyacinth-like Nanostructures on Nickel Foam Synthesized by in Situ Electrochemical Oxidation as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

NiSe@NiOOH core-shell hyacinth-like nanostructures supported on nickel foam (NF) have been successfully synthesized by a facile solvothermal selenization and subsequent in situ electrochemical oxidation (ISEO). First, the unique NiSe/NF nanopillar arrays were prepared in N,N-dimethylformamide (DMF) as a precursor template that can provide a large surface area, excellent conductivity, and robust support. Next, amorphous NiOOH covering the surface of NiSe nanopillars was fabricated by ISEO, as confirmed by XPS andEDX spectroscopy. SEM images revealed the hyacinth-like morphology of NiSe@NiOOH/NF with NiOOH as the shell and NiSe as the core. The electrochemical performance of NiSe@NiOOH/NF for the oxygen evolution reaction (OER) was investigated. NiSe@NiOOH/NF demonstrates an obviously enhanced OER activity with much lower overpotential of 332 mV at 50 mA cm(-2) compared to other Ni-based electrocatalysts. The low charge-transfer resistance (Rct), large electrochemical double-layer capacitance (Cdl) of electrochemically active surface areas (ECSAs), and excellent long-term stability of NiSe@NiOOH/NF confirm the enhancement of its electrochemical performance for the OER, which can be ascribed to the large amount of active sites derived from the amorphous NiOOH shell and the good conductivity and stability derived from the NiSe core. In addition, the synergistic effect between the NiSe core and NiOOH shell could serve for a highly efficient OER electrocatalyst.