Introduction of surface defects in NiO with effective removal of adsorbed catalyst poisons for improved electrochemical urea oxidation

Multifunctional iron-cobalt heterostructure (FeCoHS) electrocatalysts: accelerating sustainable hydrogen generation through efficient water electrolysis and urea oxidation

The urgent need to address escalating environmental pollution and energy management challenges has underscored the importance of developing efficient, cost-effective, and multifunctional electrocatalysts. To address these issues, we developed an eco-friendly, cost-effective, and multifunctional electrocatalyst via a solvothermal synthesis approach. Due to the merits of the ideal synthesis procedure, the FeCoHS@NF electrocatalyst exhibited multifunctional activities, like OER, HER, OWS, UOR, OUS, and overall alkaline seawater splitting, with required potentials of 1.48, 0.130, 1.59, 1.23, 1.40, and 1.54 V @ 10 mA cm-2, respectively. Moreover, electrolysers required only 1.40 V at 10 mA cm-2 for energy-saving urea-assisted hydrogen production, which was 190 mV lower than that of the alkaline water electrolyser. The alkaline sewage and seawater purification setup combined with the FeCoHS@NF electrolyzer led to a novel approach of producing pure green hydrogen and water. The ultrastability of the FeCoHS@NF electrocatalyst for industrial applications was confirmed using chronopotentiometry at 10 and 100 mA cm-2 over 110 h for OER, HER, UOR, and overall water splitting. The production of hydrogen using the FeCoHS@NF electrocatalyst in alkaline sewage water and seawater offers multiple benefits, including generation of renewable hydrogen energy, purification of wastewater, reduction of environmental pollutants, and low cost and low electricity consumption of the electrolyser system.

Ball-milled Ni@Mo2C/C nanocomposites as efficient electrocatalysts for urea oxidation

Urea oxidation reaction (UOR) has been identified as a promising method for hydrogen production and the remediation of urea-rich wastewater by electrochemical techniques. In the present work, Ni/C and Ni@Mo2C(x)/C electrocatalysts (x = 0.1, 0.2, 0.4, and 0.6 mol fraction of Mo2C in Ni@Mo2C) are prepared by ball milling method followed by annealing at 800 °C for 2 h under nitrogen atmosphere. Electrooxidation of urea is carried out using these electrocatalysts in an alkaline solution. Among them, the Ni@Mo2C(0.4)/C catalyst shows a maximum current density of 96.5 mA cm-2 at 1.7 V (versus RHE) in 1 M KOH and 0.33 M urea electrolyte. The Ni@Mo2C(0.4)/C catalyst exhibits better catalytic activity, low overpotential, and charge transfer resistance with extremely low Tafel slope compared to other compositions for UOR. The synergistic electronic effect between Ni and Mo2C components is responsible for generating active sites and facilitating the catalytic activity of UOR. The Ni@Mo2C(x)/C electrocatalysts are promising for treating urea-rich wastewaters and for use as a substitute for suppressing OER in water-splitting reactions.

Mechanistic Insights into the Stabilization of In Situ Formed γ-NiOOH Species on Ni60Nb40 Nanoglass for Effective Urea Electro-Oxidation

The formation of NiOOH on the catalyst surface is widely considered to be the active species in electrochemical urea oxidation reactions (UOR). Though in situ-formed NiOOH species are reported to be more active than the synthesized ones, the mechanistic study of the actual active species remains a daunting task due to the possibility of different phases and instability of surface-formed NiOOH. Herein, mechanistic UOR aspects of electrochemically activated metallic Ni60Nb40 Nanoglass showing stability toward the γ-NiOOH phase are reported, probed via in situ Raman spectroscopy, supported by electron microscopy analysis and X-ray photoelectron spectroscopy in contrast with the β-NiOOH formation favored on Ni foil. Detailed mechanistic study further reveals that γ-NiOOH predominantly follows a direct UOR mechanism while β-NiOOH favors indirect UOR from time-dependent Raman study, and electrochemical impedance spectroscopy (EIS) analysis. The Nanoglass has shown outstanding UOR performance with a low Tafel slope of 16 mV dec-1 and stability for prolonged electrolysis (≈38 mA cm-2 for 70 h) that can be attributed to the nanostructured glassy interfaces facilitating more γ-NiOOH species formation and stabilization on the surface. The present study opens up a new direction for the development of inexpensive Ni-based UOR catalysts and sheds light on the UOR mechanism.

Barium-induced lattice expansion of Ni(OH)2: enhancing catalytic urea oxidation activity for energy-saving H2 production

Constructing an environmentally friendly and efficient electrocatalyst holds important and profound significance for energy-efficient hydrogen production. Replacing the oxygen evolution reaction with a lower potential urea oxidation reaction (UOR) may save energy in water electrolysis to produce hydrogen. The UOR is characterized by its high energy barrier, which results in slow reaction kinetics. In this study, we introduced Ba(OH)2 into Ni(OH)2 to form uniform nanosheets. Due to the introduction of Ba2+, the lattice expansion of Ni(OH)2 was triggered, leading to significant improvement in UOR activity. The catalyst achieved a current density of 100 mA cm-2 at only 1.316 V and exhibited remarkable stability over time. Density functional theory (DFT) calculations demonstrate that the Ba-Ni(OH)2 site significantly reduces the energy barrier for urea adsorption, intermediate steps, and desorption. This work provides a novel and environmentally friendly strategy for constructing energy-efficient and highly efficient catalysts through the doping of alkaline earth metals.

Engineering advanced noble-metal-free electrocatalysts for energy-saving hydrogen production from alkaline water via urea electrolysis

When the anodic oxygen evolution reaction (OER) of water splitting is replaced by the urea oxidation reaction (UOR), the electrolyzer can fulfill hydrogen generation in an energy-economic manner for urea electrolysis as well as sewage purification. However, owing to the sluggish kinetics from a six-electron process for UOR, it is in great demand to design and fabricate high-performance and affordable electrocatalysts. Over the past years, numerous non-precious materials (especially nickel-involved samples) have offered huge potential as catalysts for urea electrolysis under alkaline conditions, even in comparison with frequently used noble-metal ones. In this review, recent efforts and progress in these high-efficiency noble-metal-free electrocatalysts are comprehensively summarized. The fundamentals and principles of UOR are first described, followed by highlighting UOR mechanism progress, and then some discussion about density functional theory (DFT) calculations and operando investigations is given to disclose the real reaction mechanism. Afterward, aiming to improve or optimize UOR electrocatalytic properties, various noble-metal-free catalytic materials are introduced in detail and classified into different classes, highlighting the underlying activity-structure relationships. Furthermore, new design trends are also discussed, including targetedly designing nanostructured materials, manipulating anodic products, combining theory and in situ experiments, and constructing bifunctional catalysts. Ultimately, we point out the outlook and explore the possible future opportunities by analyzing the remaining challenges in this booming field.

Large scale uniform Ni-P plated carbon fiber for boosting urea electro-oxidation and electro-detection

Seeking an excellent electrocatalyst is the trickiest issue for the application of urea electro-oxidation and electro-detection. Phosphorus-doped nickel plating on carbon fibers (Ni-P/CF) is synthesized by simple electroless plating. SEM results exhibit that the Ni-P densely and uniformly grows onto the surface of carbon fibers (CF), forming carbon fibers-like nanoarchitectures. Benefiting from the carbon fibers-like nano architectures with abundant exposed active sites on the surface of CF, electron transfer can be synchronously facilitated, and Ni-P/CF displays superior urea electrooxidation (UOR) performance with potentials of 1.40 V to reach 100 mA cm-2. Impressively, it can maintain at 20 mA cm-2 for 48 h without evident activity attenuation, demonstrating robust durability. Cycle stability shows that the voltage has only increased by 10 mV at 300 mA cm-2 from the 10th to 20000th cycles. Most importantly, Ni-P/CF at a length of 100 cm with good reproducibility was successfully synthesized, denoting great potential for large-scale industrial production. Therefore, this work not only affords cost-effective tactics for urea-rich wastewater degradation but also can achieve practical medical applications.

Recent advances in the pre-oxidation process in electrocatalytic urea oxidation reactions

The electrocatalytic urea oxidation reaction (UOR) has attracted substantial research interests over the past few years owing to its critical role in coupled electrochemical systems for energy conversion, for example, coupling with the hydrogen evolution reaction (HER) to realize urea-assisted hydrogen production and assembling direct urea fuel cells (DUFC) by coupling with the oxygen reduction reaction (ORR). The UOR process has been proved to be a two-step process which involves an electrochemical pre-oxidation reaction of the metal sites and a subsequent chemical oxidation of the urea molecules on the as-formed high-valence metal sites. Hence, designing advanced (pre-)catalysts with a boosted pre-oxidation reaction is of great importance in improving the UOR performance and thus accelerating the coupled reactions. In this feature article, we discuss the significant role of the pre-oxidation process during the urea electro-oxidation reaction, and summarize detailed strategies and recent advances in promoting the pre-oxidation reaction, including the modulation of the crystallinity, active phase engineering, defect engineering, elemental incorporation and constructing hierarchical nanostructures. We anticipate that this feature article will offer helpful guidance for the design and optimization of advanced (pre-)catalysts for UOR and related energy conversion applications.

MIL-101 supported CeOx-modified NiPt nanoparticles as a highly efficient catalyst toward complete dehydrogenation of hydrazine borane

High hydrogen content and excellent stability at room temperature promote hydrazine borane (HB, N2H4BH3) as a promising chemical hydrogen storage material. However, the development of high efficient and selective catalysts...