Yolk-shell nanostructural Ni2P/C composites as the high performance electrocatalysts toward urea oxidation

Synergistic Inclusion of Reaction Activator and Reaction Accelerator to Ni-MOF Toward Extra-Ordinary Performance of Urea Oxidation Reaction

Recently electrochemical urea oxidation reaction (UOR) has emerged as the technology of demand for commercialization of urea-based energy conversion. However, the nascent idea is limited by the energy burden of threshold voltage and the sluggish reaction kinetics involving a six-electron transfer mechanism. Herein, for the first time, the engineering of electrocatalysts are proposed with simultaneous inclusion of UOR activator and UOR accelerator. Nitrogen-doped carbon-decorated Ni-based Metal Organic Framework (MOF) has been synthesized as the base catalyst material. MoO2 and rGO with varied loading have been attached to the MOF to get the desired MoO2/Ni-MOF/rGO heterostructure incorporating defects and crystal strain within the materials. Investigations reveal that the invoked lattice strain and atomic defects promote plenteous Ni3+ active sites. The optimized sample demonstrates extraordinary performance of UOR having the potential value as low as 1.32 V versus RHE to reach the current density of 10 mA cm-2 and the tafel slope is only 31 mV dec-1 reflecting very fast reaction kinetics. Here MoO2 plays the role of UOR activator whereas optimized loading of rGO proliferates the reaction speed. This work, experimentally and theoretically, presents a new insight to enhance electrocatalytic urea oxidation reaction opening an avenue of urea-based energy-harvesting technology.

Nanoarchitectonics for structural tailoring of yolk-shell architectures for electrochemical applications

Developing electrochemical energy storage and conversion systems, such as capacitors, batteries, and fuel cells is crucial to address rapidly growing global energy demands and environmental concerns for a sustainable society. Significant efforts have been devoted to the structural design and engineering of various electrode materials to improve economic applicability and electrochemical performance. The yolk-shell structures represent a special kind of core-shell morphologies, which show great application potential in energy storage, controlled delivery, adsorption, nanoreactors, sensing, and catalysis. Their controllable void spaces may facilitate the exposure of more active sites for redox reactions and enhance selective adsorption. Based on different nanoarchitectonic designs and fabrication techniques, the yolk-shell structures with controllable structural nanofeatures and the homo- or hetero-compositions provide multiple synergistic effects to promote reactions on the electrode/electrolyte interfaces. This review is focused on the key structural features of yolk-shell architectures, highlighting the recent advancements in their fabrication with adjustable space and mono- or multi-metallic composites. The effects of tailorable structure and functionality of yolk-shell nanostructures on various electrochemical processes are also summarized.

Phosphide-Based Electrocatalysts for Urea Electrolysis: Recent Trends and Progress

Electrolysis is a trend in producing hydrogen as a fuel for renewable energy development, and urea electrolysis is considered as one of the advanced electrolysis processes, where efficient materials still need to be explored. Notably, urea electrolysis came into existence to counter-part the electrode reactions in water electrolysis, which has hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Among those reactions, OER is sluggish and limits water splitting. Hence, urea electrolysis emerged with urea oxidation reaction (UOR) and HER as their reactions to tackle the water electrolysis. Among the explored materials, noble-metal catalysts are efficient, but their cost and scarcity limit the scaling-up of the Urea electrolysis. Hence, current challenges must be addressed, and novel efficient electrocatalysts are to be implemented to commercialize urea electrolysis technology. Phosphides, as an efficient UOR electrocatalyst, have gained huge attention due to their exceptional lattice structure geometry. The phosphide group benefits the water molecule adsorption and water dissociation, and facilitates the oxyhydrate of the metal site. This review summarizes recent trends in phosphide-based electrocatalysts for urea electrolysis, discusses synthesis strategies and crystal structure relationship with catalytic activity, and presents the challenges of phosphide electrocatalysts in urea electrolysis.

A universal approach to dual-metal-atom catalytic sites confined in carbon dots for various target reactions

Here, a molecular-design and carbon dot-confinement coupling strategy through the pyrolysis of bimetallic complex of diethylenetriamine pentaacetic acid under low-temperature is proposed as a universal approach to dual-metal-atom sites in carbon dots (DMASs-CDs). CDs as the "carbon islands" could block the migration of DMASs across "islands" to achieve dynamic stability. More than twenty DMASs-CDs with specific compositions of DMASs (pairwise combinations among Fe, Co, Ni, Mn, Zn, Cu, and Mo) have been synthesized successfully. Thereafter, high intrinsic activity is observed for the probe reaction of urea oxidation on NiMn-CDs. In situ and ex situ spectroscopic characterization and first-principle calculations unveil that the synergistic effect in NiMn-DMASs could stretch the urea molecule and weaken the N-H bond, endowing NiMn-CDs with a low energy barrier for urea dehydrogenation. Moreover, DMASs-CDs for various target electrochemical reactions, including but not limited to urea oxidation, are realized by optimizing the specific DMAS combination in CDs.

Spherical Ni/NiO nanoparticles decorated on nanoporous carbon (NNC) as an active electrode material for urea and water oxidation reactions

Herein, we report a chemical method for scalable synthesis of spherical Ni/NiO nanoparticle-decorated nanoporous carbon (NNC) based electrocatalytic system using a simple and easy chemical method with ultra-high activity towards urea electrooxidation. Morphological analysis by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) confirms the formation of Ni/NiO NPs on highly nanoporous carbon with an average size of ∼50 nm. X-ray diffraction (XRD) confirms NNC with a face-centred cubic (FCC) crystal structure. Ni/NiO NPs intercalated with nanoporous carbon exhibited the best electrocatalytic performance towards urea oxidation with an ultra-low onset potential of ∼0.33 V vs. SCE, and faster electrokinetic mechanism confirmed from Tafel slope (∼45 mV dec-1), EIS Rct (∼6.98 Ω), and long term durability for 7 h at 10 mA cm-2 with high CO poisoning tolerance. This work affords noble metal-free electrocatalysts for novel appliances and remarkable potential for urea determination, hydrogen generation, real-time water remediation, and energy conversion.

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.

Metal-Organic Frameworks Offering Tunable Binary Active Sites toward Highly Efficient Urea Oxidation Electrolysis

Electrocatalytic urea oxidation reaction (UOR) is regarded as an effective yet challenging approach for the degradation of urea in wastewater into harmless N2 and CO2. To overcome the sluggish kinetics, catalytically active sites should be rationally designed to maneuver the multiple key steps of intermediate adsorption and desorption. Herein, we demonstrate that metal-organic frameworks (MOFs) can provide an ideal platform for tailoring binary active sites to facilitate the rate-determining steps, achieving remarkable electrocatalytic activity toward UOR. Specifically, the MOF (namely, NiMn0.14-BDC) based on Ni/Mn sites and terephthalic acid (BDC) ligands exhibits a low voltage of 1.317 V to deliver a current density of 10 mA cm-2. As a result, a high turnover frequency (TOF) of 0.15 s-1 is achieved at a voltage of 1.4 V, which enables a urea degradation rate of 81.87% in 0.33 M urea solution. The combination of experimental characterization with theoretical calculation reveals that the Ni and Mn sites play synergistic roles in maneuvering the evolution of urea molecules and key reaction intermediates during the UOR, while the binary Ni/Mn sites in MOF offer the tunability for electronic structure and d-band center impacting on the intermediate evolution. This work provides important insights into active site design by leveraging MOF platform and represents a solid step toward highly efficient UOR with MOF-based electrocatalysts.