Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation.

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2 ), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2 -based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

An antioxidation strategy based on ultra-small nanobubbles without exogenous antioxidants.

Antioxidation is in demand in living systems, as the excessive reactive oxygen species (ROS) in organisms lead to a variety of diseases. The conventional antioxidation strategies are mostly based on the introduction of exogenous antioxidants. However, antioxidants usually have shortcomings of poor stability, non-sustainability, and potential toxicity. Here, we proposed a novel antioxidation strategy based on ultra-small nanobubbles (NBs), in which the gas-liquid interface was employed to enrich and scavenge ROS. It was found that the ultra-small NBs (~ 10 nm) exhibited a strong inhibition on oxidization of extensive substrates by hydroxyl radicals, while the normal NBs (~ 100 nm) worked only for some substrates. Since the gas-water interface of the ultra-small NBs is non-expendable, its antioxidation would be sustainable and its effect be cumulative, which is different to that using reactive nanobubbles to eliminate free radicals as the gases are consumptive and the reaction is unsustainable. Therefore, our antioxidation strategy based on ultra-small NB would provide a new solution for antioxidation in bioscience as well as other fields such as materials, chemical industry, food industry, etc.

Turning Electrocatalytic Activity Sites for the Oxygen Evolution Reaction on Brownmillerite to Oxyhydroxide.

One of the major challenges that hinder the practical application of water electrolysis lies in the design of advanced electrocatalysts toward the anodic oxygen evolution reaction (OER). In this work, a pure Co-based precatalyst of CoOOH/brownmillerite derived from the surface activation of brownmillerite by a surface acid etching method exhibits high activity and stable electrical properties toward the OER. Different from oxyhydroxide derived from in situ surface reconstruction during the electrochemical process, the growth of highly crystalline CoOOH from the brownmillerite surface enables rational control over the surface/bulk structure as well as the concentration of active sites, and this structure can be well maintained and serve as highly active sites. The catalyst shows a low overpotential of 320 mV to obtain 10 mA cm-2 and high stability in an alkaline electrolyte for the OER, which is comparable to the majority of Co-based electrocatalysts. Moreover, the appropriate interfacial interaction of the composite catalysts greatly contributes to the hydroxide insertion to improve water oxidation ability. This work proposes an effective strategy to develop high-performance metal oxide-based materials for the OER.

Ni-Doped CuO Nanoarrays Activate Urea Adsorption and Stabilizes Reaction Intermediates to Achieve High-Performance Urea Oxidation Catalysts.

Urea oxidation reaction (UOR) with a low equilibrium potential offers a promising route to replace the oxygen evolution reaction for energy-saving hydrogen generation. However, the overpotential of the UOR is still high due to the complicated 6e- transfer process and adsorption/desorption of intermediate products. Herein, utilizing a cation exchange strategy, Ni-doped CuO nanoarrays grown on 3D Cu foam are synthesized. Notably, Ni-CuO NAs/CF requires a low potential of 1.366 V versus a reversible hydrogen electrode to drive a current density of 100 mA cm-2 , outperforming various benchmark electrocatalysts and maintaining robust stability in alkaline media. Theoretical and experimental studies reveal that Ni as the driving force center can effectively enhance the urea adsorption and stabilize CO*/NH* intermediates toward the UOR. These findings suggest a new direction for constructing nanostructures and modulating electronic structures, ultimately developing promising Cu-based electrode catalysts.

Atomic controllable anchoring of uranium into zirconate pyrochlore with ultrahigh loading capacity.

Efficient immobilization of actinide wastes is challenging in the nuclear energy industry. Here, we reported that 100% substitution of Zr4+ by U6+ in a La2Zr2O7 matrix has been achieved for the first time by the molten salt (MS) method. Importantly, we observed that uranium can be precisely anchored into Zr or La sites of the La2Zr2O7 matrix, as confirmed by X-ray diffraction, Raman, and X-ray absorption spectra. This work will guide the construction of site-controlled and high-capacity actinide-immobilized pyrochlore materials and could be extended to other perovskite materials.

Molten Salt Synthesis of High-Performance, Nanostructured La0.6Sr0.4FeO3-δ Oxygen Electrode of a Reversible Solid Oxide Cell.

Nanoscale perovskite oxides with enhanced electrocatalytic activities have been widely used as oxygen electrodes of reversible solid oxide cells (RSOC). Here, La0.6Sr0.4FeO3-δ (LSF) nanoscale powder is synthesized via a novel molten salt method using chlorides as the reaction medium and fired at 850 °C for 5 h after removing the additives. A direct assembly method is employed to fabricate the LSF electrode without a pre-sintering process at high temperature. The microstructure characterization ensures that the direct assembly process will not damage the porosity of LSF. When operating as a solid oxide fuel cell (SOFC), the LSF cell exhibits a peak power density of 1.36, 1.07 and 0.7 W/cm2 at 800, 750 and 700 °C, respectively, while in solid oxide electrolysis cell (SOEC) mode, the electrolysis current density reaches 1.52, 0.98 and 0.53 A/cm2 under an electrolysis voltage of 1.3 V, respectively. Thus, it indicates that the molten salt routine is a promising method for the synthesis of highly active perovskite LSF powders for directly assembled oxygen electrodes of RSOC.

The synthesis of 3D graphene/Au composites via γ-ray irradiation and their use for catalytic reduction of 4-nitrophenol.

Graphene oxide (GO) and gold ions (Au3+) can be simultaneously reduced and self-assembled into a three-dimensional (3D) graphene/Au composite (GA/Au) porous structure at room temperature via one-step γ-ray irradiation. The microstructure of GA/Au composites were observed under different magnifications and the pores were observed to be uniform 3D porous structure. In addition, Au nanoparticles were homogeneously attached to graphene sheets and had a typical diameter of 6 nm. These GA/Au composites were analyzed and characterized by x-ray diffraction analysis, x-ray photoelectron spectroscopy, and thermal gravity analysis. Due to synergistic catalysis between graphene and Au nanoparticles, GA/Au composites catalyzed 4-nitrophenol with excellent catalytic performance, even at concentrations up to 6.48 × 10-3 M. When the concentration of 4-nitrophenol was 2.16 × 10-3 M and 4.22 × 10-3 M, the first-order kinetic constants were 2.00 and 1.43 min-1, respectively.

Investigation of the local structure of molten ThF4-LiF and ThF4-LiF-BeF2 mixtures by high-temperature X-ray absorption spectroscopy and molecular-dynamics simulation.

The microscopic structures of ThF4-LiF and ThF4-LiF-BeF2 molten salts have been systematically investigated by in situ high-temperature X-ray absorption fine-structure (XAFS) spectroscopy combined with molecular-dynamics (MD) simulations. The results reveal that the local structure of thorium ions was much more disordered in the molten state of the ThF4-LiF-BeF2 salt than that in ThF4-LiF, implying that the Th and F ions were exchanged more frequently in the presence of Be ions. The structures of medium-range-ordered coordination shells (such as Th-F2nd and Th-Th) have been emphasized by experimental and theoretical XAFS analysis, and they play a significant role in transport properties. Using MD simulations, the bonding properties in the molten ThF4-LiF and ThF4-LiF-BeF2 mixtures were evaluated, confirming the above conclusion. This research is, to the best of our knowledge, the first systematic study on the ThF4-LiF-BeF2 molten salt via quantitative in situ XAFS analysis and MD simulations.