Iron oxide promoted nickel/nickel oxide rough nanorods for efficient urea assisted water splitting

Hierarchically Structured Graphene Aerogel Supported Nickel-Cobalt Oxide Nanowires as an Efficient Electrocatalyst for Oxygen Evolution Reaction

The rational design of a heterostructure electrocatalyst is an attractive strategy to produce hydrogen energy by electrochemical water splitting. Herein, we have constructed hierarchically structured architectures by immobilizing nickel-cobalt oxide nanowires on/beneath the surface of reduced graphene aerogels (NiCoO2/rGAs) through solvent-thermal and activation treatments. The morphological structure of NiCoO2/rGAs was characterized by microscopic analysis, and the porous structure not only accelerates the electrolyte ion diffusion but also prevents the agglomeration of NiCoO2 nanowires, which is favorable to expose the large surface area and active sites. As further confirmed by the spectroscopic analysis, the tuned surface chemical state can boost the catalytic active sites to show the improved oxygen evolution reaction performance in alkaline electrolytes. Due to the synergistic effect of morphology and composition effect, NiCoO2/rGAs show the overpotential of 258 mV at the current density of 10 mA cm-2. Meanwhile, the small values of the Tafel slope and charge transfer resistance imply that NiCoO2/rGAs own fast kinetic behavior during the OER test. The overlap of CV curves at the initial and 1001st cycles and almost no change in current density after the chronoamperometric (CA) test for 10 h confirm that NiCoO2/rGAs own exceptional catalytic stability in a 1 M KOH electrolyte. This work provides a promising way to fabricate the hierarchically structured nanomaterials as efficient electrocatalysts for hydrogen production.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Ni2P Nanoparticle-Inserted Porous Layered NiO Hetero-Structured Nanosheets as a Durable Catalyst for the Electro-Oxidation of Urea

The electro-oxidation of urea (EOU) is a remarkable but challenging sustainable technology, which largely needs a reduced electro-chemical potential, that demonstrates the ability to remove a notable harmful material from wastewater and/or transform the excretory product of humans into treasure. In this work, an Ni₂P-nanoparticle-integrated porous nickel oxide (NiO) hetero-structured nanosheet (Ni₂P@NiO/NiF) catalyst was synthesized through in situ acid etching and a gas-phase phosphating process. The as-synthesized Ni₂P@NiO/NiF catalyst sample was then used to enhance the electro-oxidation reaction of urea with a higher urea oxidation response (50 mA cm^-2 at 1.31 V vs. RHE) and low onset oxidation potential (1.31 V). The enhanced activity of the Ni₂P@NiO/NiF catalyst was mainly attributed to effective electron transport after Ni₂P nanoparticle insertion through a substantial improvement in active sites due to a larger electrochemical surface area, and a faster diffusion of ions occurred via the interactive sites at the interface of Ni₂P and NiO; thus, the structural reliability was retained, which was further evidenced by the low charge transfer resistance. Further, the Ni₂P nanoparticle insertion process into the NiO hetero-structured nanosheets effectively enabled a synergetic effect when compared to the counter of the Ni₂P/NiF and NiO/NiF catalysts. Finally, we demonstrate that the as-synthesized Ni₂P@NiO/NiF catalyst could be a promising electrode for the EOU in urea-rich wastewater and human urine samples for environmental safety management. Overall, the Ni₂P@NiO/NiF catalyst electrode combines the advantages of the Ni₂P catalyst, NiO nanosheet network, and NiF current collector for enhanced EOU performance, which is highly valuable in catalyst development for environmental safety applications.

Systematic Engineering on Ni-Based Nanocatalysts Effectively Promote Hydrogen Evolution Reaction

Designing a synthesis of ultra-small Ni-based nanomaterials with high intrinsic activity and stability in alkaline hydrogen evolution reaction (HER) is a major challenge. Herein, a series of noble metal doped ultra-small size (4 nm) M-Ni/NiO nanoparticles supported on CNT are rationally designed by a solvent-free microwave reduction method that is fast (60 s), simple, includes no surfactants, extensive (>1 g), and has high yield (82.7%). The Ir-Ni/NiO@CNT has superior performance with a low overpotential of 24.6 mV at 10 mA cm^-2 . In addition, the turnover frequency (TOF) value up to 2.51 s^-1 and the exchange current density reaches 4.34 mA cm^-2 , indicating that the catalyst has better intrinsic catalytic activity. It is further proved by density functional theory (DFT) that the NiO surface is conducive to the adsorption of OH* in the Volmer step while the Ni is inclined to adsorb H*, which synergistically promotes the water-splitting reaction, thereby increasing the catalytic rate of HER. It is believed that this work will provide valuable contributions and inspirations toward the large-scale production of high-performance Ni-based electrocatalysts for HER.

Recent progress in direct urea fuel cell

Direct urea fuel cell (DUFC) has attracted many researchers’ attention due to the use of wastewater, for example urine, which contains urea for the fuel. The main factor to improve the electrochemical oxidation performance of urea and further enhance the performances of DUFC is the use of a good anode catalyst. Non-noble metal catalyst, such as nickel, is reported to have a good catalytic activity in alkaline medium towards urea electro-oxidation. Besides optimizing the anode catalyst, the use of supporting electrode which has a large surface area as well as the use of H2O2 as an oxidant to replace O2 could help to improve the performances. The recent progress in anode catalysts for DUFC is overviewed in this article. In addition, the advantages and disadvantages as well as the factors that could help to escalate the performance of DUFC are discussed together with the challenges and future perspectives.

Pure and Ce-doped spinel CuFe2O4 photocatalysts for efficient rhodamine B degradation

Wastewater management is becoming a serious issue worldwide. To enhance the reuse of wastewater, one has to remove toxic pollutants present in it. High amount of dye is present in wastewater, and to remove these dyes is the large scope of this research. Herein, we report production of pure and Ce-doped copper ferrite via hydrothermal route. The synthesized nanoparticles were collected and analyzed by basic characterization techniques. The bandgap energy calculated for pure, 1% Ce, and 2% Ce-doped CuFe₂O₄ was found to be 2.77, 2.57, and 2.36eV, respectively. Reduction in bandgap was attributed to the doping element. The shape and size of pure and Ce-doped products were investigated using a scanning electron microscope. Agglomeration was observed in the pure copper ferrite sample. In the Ce-doped sample, agglomeration was clearly reduced and the 2% Ce-doped CuFe₂O₄ sample showed growth of small nanoparticles. They showed complete growth and were arranged in a uniform manner without agglomeration. The surface area of the 2% Ce-CuFe₂O₄ sample was found to be 65.89 m²/g with 7.02 nm pore diameter. The photocatalytic activity of the prepared material was observed for rhodamine B degradation. The pure and catalyst-added dye was exposed under visible light. The samples were tested for UV. The efficiency obtained for pure dye solution, pristine CuFe₂O₄-added, and 1% Ce and 2% Ce-doped CuFe₂O₄-added dye solutions were 48%, 50%, 66%, and 88% within 2 h of irradiation. The 2% Ce-doped CuFe₂O₄ sample showed excellent photocatalytic activity as the bandgap and morphology were enhanced by doping an appropriate ratio of Ce ions.

Ni2P nanoflakes for the high-performing urea oxidation reaction: linking active sites to a UOR mechanism

Urea electrolysis is regarded as an effective method for addressing both energy and environment issues. Herein, we successfully synthesized Ni2P nanoflakes for catalyzing the urea oxidation reaction (UOR). Due to the higher electrical conductivity as well as the prevailing tendency in triggering the UOR via a direct electro-oxidation mechanism, Ni2P nanoflakes exhibit comparable UOR activity (1.33 V vs. RHE for onset-potential, and 95.47 mA·cm-2 at 1.6 V vs. RHE) to the most active state-of-the-art catalysts, rendering them an effective alternative to precious metals such as Pt and Rh. The accelerated proton-coupled electron transfer (PCET) process caused by PO43- facilitates the in situ generation of NiOOH; thus, the UOR process is initiated at a lower onset-potential on Ni2P nanoflakes than on β-Ni(OH)2 nanoflakes. The in situ generated NiOOH instead of the Ni2P phase in Ni2P nanoflakes functions as an active site during the UOR process, while both NiOOH and the Ni2P phase serve as active sites in the OER process. This work provides insights into the understanding of the UOR mechanism and opens a new avenue to design low-cost Ni-based phosphide UOR catalysts.