Urea Oxidation Reaction Research Articles

Coupling interface constructions of flower-like structure MoSe2–NiWSe2 for efficient oxygen evolution reaction and urea oxidation reaction

Because of its low thermodynamic voltage, the electrocatalytic urea oxidation reaction (UOR) is an effective way to replace the sluggish oxygen evolution reaction (OER) and address the issue of urea-rich water pollution. Herein, we have synthesized MoSe2–NiWSe2 with spherical nanoflower structure consisting of MoSe2 nanosheets and NiWSe2 nanosheets crisscrossed longitudinally and transversely. The results show that the prepared MoSe2–NiWSe2 has a significant competitive advantage in the UOR and OER. With 10 mA cm−2, a mere 200 mV overpotential was required, indicating high OER catalytic activity. MoSe2–NiWSe2-driven UOR might operate at a comparatively low potential of 1.36 V (10 mA cm−2). The overpotential did not significantly alter after 40 h of continuous operation, suggesting it possesses high UOR stability. A built-in electric field that aids in controlling the adsorption and oriented distribution of urea molecules and thus promotes the oxidative properties of water and urea forms at the interface of MoSe2 and NiWSe2, where the difference in electronegativity between the elements Mo and Ni induces self-driven charge transfer and interfacial coupling effects. This work offers a method for creating catalysts that produce more oxygen and clean up urea-rich water contamination.

Optimized crystalline/amorphous NiFeS@NiMoP on nickel foam as dual-functional electrocatalysts for urea-water splitting

Urea electrolysis is a potential choice for generating hydrogen (H2) to fuel several purified energy technologies. In practice, the strategy for designing a transition metal sulfides (TMSs) phosphides (TMPs) electrocatalyst coupled with interfacial engineering applied into hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) is a extremely desirable and meaningful challenge. Herein, crystalline/amorphous heterostructure denoted as NiFeS@NiMoP/NF was made. Electrochemical tests show that：NiFeS@NiMoP/NF was active for both the HER and UOR process, which may due to the synergistic effect of Ni-based sulfides together with phosphides markely facilitated electron transfer and reaction kinetics and crystalline/amorphous structure provided ample unsaturated coordination configurations to enhance the adsorption of reactants. Impressively, NiFeS@NiMoP/NF displays comparatively low overpotential with value of 56.4 mV for HER and potential of 1.32 V for UOR to deliver the current density of 10 mA cm−2, and drives the overall splitting of urea-water electrolysis at 1.40 V in 1.0 M KOH +0.5 M urea at 10 mA cm−2. This approach may shed light on the designing and developing crystalline/amorphous bifunctional materials for energy conversion and application.

Morphology engineering and electronic structure remodeling of manganese-incorporated VN for boosting urea-assisted energy-saving hydrogen production

Urea-assisted water electrolysis offers a promising route to reduce energy consumption for hydrogen production and meanwhile treat urea-rich wastewater. Herein, we devised a shear force-involved polyoxometalate-organic supramolecular self-assembly strategy to fabricate 3D hierarchical porous nanoribbon assembly Mn-VN cardoons. A bimetallic polyoxovanadate (POV) with the inherent structural feature of Mn surrounded by [VO6] octahedrons was introduced to trigger precise Mn incorporation in VN lattice, thereby achieving simultaneous morphology engineering and electronic structure modulation. The lattice contraction of VN caused by Mn incorporation drives electron redistribution. The unique hierarchical architecture with modulated electronic structure that provides more exposed active sites, facilitates mass and charge transfer, and optimizes the associated adsorption behavior. Mn-VN exhibits excellent activity with low overpotentials of 86 mV and 1.346 V at 10 mA/cm2 for hydrogen evolution reaction (HER) and urea oxidation reaction (UOR), respectively. Accordingly, in the two-electrode urea-assisted water electrolyzer utilizing Mn-VN as a bifunctional catalyst, hydrogen production can occur at low voltage (1.456 V@10 mA/cm2), which has the advantages of energy saving and competitive durability over traditional water electrolysis. This work provides a simple and mild route to construct nanostructures and modulate electronic structure for designing high-efficiency electrocatalysts.

Superhydrophilic/superaerophobic CoP/CoMoO4 multi-level hierarchitecture electrocatalyst for urea-assisted hydrogen evolution reaction in alkaline media

Utilizing the thermodynamically favorable urea oxidation reaction instead of the anodic oxygen precipitation reaction is an alternative pathway for the energy-saving hydrogen production. Therefore, it is significant to explore advanced electrocatalysts for both HER and UOR. In this work, a dendritic heteroarchitectures of 2D CoMoO4 nanosheets deposited on 1D CoP nanoneedles (CoP/CoMoO4-CC) was fabricated as bifunctional electrocatalyst. 1D CoP nanostructure with fast charge transport pathways and 2D CoMoO4 nanostructure with large specific surface area and short paths for electron/mass transport. The unique morphology endows the superhydrophilic and superaerophobic properties, allowing for the rapid contact with the reactants and rapid removal of surface-generated gases. As a result, the CoP/CoMoO4-CC shows efficient bifunctional activity. This work offers a new avenue to rationally design bifunctional electrocatalysts for large-scale practical hydrogen production.

CoFe(OH)x-Co3O4/NF as highly efficient electrocatalyst for oxygen evolution and urea oxidation reactions at high current densities

The advancement of an affordable and highly effective catalyst for the oxygen evolution reaction (OER) is essential to enhance the performance of overall water splitting (OWS). Here, we report a highly efficient nickel foam (NF)-supported CoFe(OH)x-Co3O4/NF heterostructure catalyst for OER. The CoFe(OH)x-Co3O4/NF heterostructure catalyst contain amorphous phase CoFe(OH)x and crystalline Co3O4, and the synergistic interaction between the two different phases reduces the charge transfer resistance, accelerates the kinetic processes of electrocatalysis and makes the catalysts exhibit excellent catalytic performance. In 1.0 M KOH solution, an overpotential (η) of merely 262 mV is needed to attain a current density of 100 mA cm−2. Even at high current densities demanded for commercial applications, the overpotentials remain very low (η500 = 291 mV, η1000 = 319 mV). The catalytic activity of CoFe(OH)x-Co3O4/NF has exceeded that of most non-noble metal OER catalysts documented in the literature. Furthermore, the CoFe(OH)x-Co3O4/NF catalyst demonstrates superior catalytic stability to OER, with only a 23 mV potential increase after 24 h of stability testing at a high current density of 500 mA cm−2. Owing to the heterostructure, the CoFe(OH)x-Co3O4/NF catalyst also exhibits excellent catalytic performance for the urea oxidation reaction (UOR), and the use of UOR instead of OER at the anode of OWS significantly reduces the cell voltage, thereby reducing the electrolytic energy consumption. The findings in this work offer precious insights into the design of OWS anode catalysts and their important applications in industrial water electrolysis. These insights are crucial for advancing the development of highly efficient and sustainable water electrolysis processes, which is essential for the production of green hydrogen and other valuable chemicals.

Fe-Doped Ni2P/NiSe2 Composite Catalysts for Urea Oxidation Reaction (UOR) for Energy-Saving Hydrogen Production by UOR-Assisted Water Splitting.

The development of efficient urea oxidation reaction (UOR) catalysts helps UOR replace the oxygen evolution reaction (OER) in hydrogen production from water electrolysis. Here, we prepared Fe-doped Ni2P/NiSe2 composite catalyst (Fe-Ni2P/NiSe2-12) by using phosphating-selenizating and acid etching to increase the intrinsic activity and active areas. Spectral characterization and theoretical calculations demonstrated that electrons flowed through the Ni-P-Fe-interface-Ni-Se-Fe, thus conferring high UOR activity to Fe-Ni2P/NiSe2-12, which only needed 1.39 V vs RHE to produce the current density of 100 mA cm-2. Remarkably, this potential was 164 mV lower than that required for the OER under the same conditions. Furthermore, EIS demonstrated that UOR driven by the Fe-Ni2P/NiSe2-12 exhibited faster interfacial reactions, charge transfer, and current response compared to OER. Consequently, the Fe-Ni2P/NiSe2-12 catalyst can effectively prevent competition with OER and NSOR, making it suitable for efficient hydrogen production in UOR-assisted water electrolysis. Notably, when water electrolysis is operated at a current density of 40 mA cm-2, this UOR-assisted system can achieve a decrease of 140 mV in the potential compared to traditional water electrolysis. This study presents a novel strategy for UOR-assisted water splitting for energy-saving hydrogen production.

Recent progress of two-dimensional metal-base catalysts in urea oxidation reaction

Recent progress of two-dimensional metal-base catalysts in urea oxidation reaction

Surface reconstruction and synergistic electrocatalysis of perovskite K-Ni-Zn-F for efficient urea-based energy conversion

Urea oxidation reaction (UOR) is hindered by poor catalytic activity, sluggish kinetics, and complex catalytic mechanism of electrocatalysts. Herein, we develop a novel perovskite K-Ni-Zn-F catalyst (KNZF-31) towards efficient UOR, showing a low potential of 1.348 V vs. RHE at 10 mA cm−2. The designed direct urea-hydrogen peroxide fuel cell (DUHPFC) provides superior open circuit voltages and power densities (0.99 V, 11.41 mW cm−2). Ex situ technology verifies amorphous electro-active/inert NiOOH/ZnO derived from surface conversion of crystalline KNZF-31, which synergistically drive the UOR. Theoretical calculation indicates that Zn-doping in KNiF3 promotes the OH− adsorption and charge transfer of Ni2+-site and formation of surface NiOOH electroactive species, which optimizes subsequent urea adsorption and charge transfer of Ni3+-site and desorption of CO2 on NiOOH/ZnO hetero-interface. This study reveals catalytic mechanism and activity origin of KNZF-31 catalyst towards UOR, offering a bright prospect for energy-sustainable developments.

Bioinspired binary-site catalysts for novel urea-assisted Zn-air battery: A transfer station between renewable energy and hydrogen

The slow reaction kinetics of oxygen electrode is the bottleneck restricting the development of rechargeable Zn-air batteries (ZABs). In order to further improve the energy conversion efficiency of ZABs, a novel urea-assisted ZAB system was proposed by replacing oxygen evolution reaction (OER) with urea oxidation reaction (UOR) with lower theoretical thermodynamic potential. Inspired by nature’s ingenious structure-function, a “leaf-branch” CoNi@NCNTs-LDH/CC binary-site electrocatalyst was designed for the above system, where spatially separated branches and leaves serve as the main active sites for ORR and UOR, respectively. A build-in electric field from CoNi-LDH to CoNi@NCNTs is formed at the interface of the composite, which plays a command effect, effectively triggering the accumulation of O2 and urea reactants around the CoNi and LDH sites, respectively, and thus improving the efficiency of both ORR and UOR reactions. Based on that, a conceptual urea-assisted rechargeable ZAB is demonstrated to have significantly decreased charging voltage, higher energy conversion efficiency (74.6 %) compared with the conventional ZAB, as well as high urea elimination rates. Considering the urea-assisted ZAB with high energy conversion efficiency and wastewater treatment versatility as an energy transfer station applied in a high value-added agricultural system, uninterrupted hydrogen production and wastewater treatment of agricultural wastewater can be simultaneously achieved using sustainable energy sources.

In situ fabrication of transition metal phosphide tree-like nanostructure for energy-saving hydrogen production coupled via urea oxidation reaction

Substituting the Oxygen Evolution Reaction (OER) with the Urea Oxidation Reaction (UOR) which is a thermodynamically more favorable option, has paved the way for the production of energy-saving hydrogen as well as the purification of urea-reach waste water. In this study, a nanostructured, porous and active Ni-Cu-Mn-P electrode was created using the dynamic hydrogen bubble template (DHBT) process in different conditions. The fabricated electrodes were utilized as bi-functional electrodes for both the HER and UOR processes. The electrochemical results displayed that the Ni-Cu-Mn-P (which synthesized at current density of 2 A.cm−2) requires only 84 mV overpotential to achieve a current density of 10 mA.cm−2, in HER process, and the necessary potential value for the UOR is only 1.30 V vs RHE. Also, the favorable electrocatalytic stability for industrial applications was also confirmed. The Ni-Cu-Mn-P electrode also demonstrated satisfactory performance in the overall urea electrolysis process and the cell voltage was 1.40 V to generate 10 mA.cm−2. The excellent performance of the electrode was due to the hierarchical porous nanostructure with high active surface area and also the synergistic effect between the elements. This study introduces an effective catalyst design for energy-saving hydrogen production.

Modulation of energy barrier of reaction steps over S-doped Ni(OH)2/Cu composites to achieve high-performance urea electrolysis catalysts

Ni(OH)2 has been found a cost-effective and promising electrocatalyst for urea oxidation reaction (UOR). However, HER activity is challenging due to its weak adsorption of active H atoms, poor water dissociation, and low conductivity. In this study, heteroatomic S-doped Ni(OH)2/Cu composite has been synthesized using the one-pot electrodeposition process. The incorporation of S atoms significantly enhanced the electron transfer rate and adsorption of active H atoms, which promoted the UOR and HER process. Similarly, the metallic Cu with S-doped Ni(OH)2 also exhibited the synergistic effect and promoted water dissociation, adsorption of active H atoms. As expected, S-doped Ni(OH)2/Cu composites exhibit remarkable urea electrolysis activity with a small cell voltage of 1.35 V to approach the current density of 10 mA cm−2. Results indicate that the adopted strategy has been found effective and the prepared composite is a good bifunctional urea electrolysis catalyst.

Synergistic etching of nickel foam by Fe3+ and Cl− ions to synthesize nickel-iron-layered double hydroxide nanolayers with abundant oxygen vacancies for superior urea oxidation

Urea electrolysis is an appealing topic for hydrogen production due to its ability to extract hydrogen at a lower potential. However, it is plagued by sluggish kinetics and noble-metal catalyst requirements. Herein, we developed nickel-iron-layered double hydroxide (NiFe-LDH) nanolayers with abundant oxygen vacancies (OV) via synergistically etching nickel foam with Fe3+ and Cl- ions, enabling the efficient conversion of urea into H2 and N2. The synthesized OV-NiFe-LDH exhibits a lower potential (1.30 vs. reversible hydrogen electrode, RHE) for achieving 10 mA cm−2 in the urea oxidation reaction (UOR), surpassing most recently reported Ni-based electrodes. OV provides favorable conductivity and a large surface area, which results in a 4.1-fold in electron transport and a 5.1-fold increase in catalyst reactive sites. Density Functional Theory (DFT) calculations indicate that OV can lower the adsorption energy of urea, and enhance the bonding strength of *CONHNH, giving rise to improved UOR. This study provides a viable path toward economical and efficient production of high-purity hydrogen.

Elemental Charge Engineering in Cobalt and Cobalt–Phosphide Interface for Enhanced Oxygen Evolution and Urea Oxidation Reactions

Elemental Charge Engineering in Cobalt and Cobalt–Phosphide Interface for Enhanced Oxygen Evolution and Urea Oxidation Reactions

Nanoneedles like FeP engineered on Ni-Foam as an effective catalyst towards overall alkaline freshwater, urea, and seawater splitting

The slow kinetics of electrochemical urea and water oxidation processes at electrode surfaces, which can be recognised as promising resources for pollution-free hydrogen energy production, motivate the scientific community to design and fabricate low-cost, high-efficiency electrocatalysts. Because the performance of electrocatalysts is dependent on their structural, morphological, and electronic properties, herein, the tuning of these properties of Fe3O4@Ni-Foam via phosphorylation and sulfurization, yielding iron phosphide (FeP@Ni-Foam) and iron sulphide (FeS@Ni-Foam), with nanoneedles (along with microstructures) and nanoflower-like morphologies, respectively, is investigated. Among all the prepared samples, FeP is found to be the most effective electrocatalyst for the oxygen evolution process (OER), the urea oxidation reaction (UOR), and seawater electrolysis. The overpotentials observed for OER, UOR, and seawater splitting are significantly reduced when using FeP as compared to other materials, with values of 207 mV, 133 mV, and 287.1 mV, respectively, at a current density of 10 mA/cm2. The enhanced catalytic activity of FeP over FeS and Fe3O4 is attributed to morphological changes, improved electronic conductivity, and exceptional endurance. The theoretical studies reveal that FeP has a better density of states over the fermi level than FeS and Fe3O4, lowering the energy barriers for the OER and UOR processes and demonstrating significant catalytic activity towards these processes. This work demonstrates that phosphorylation and sulfurization treatments can alter morphologies and electrical characteristics, hence improving the catalytic activity of the materials.

Enhancing hydrogen production capability from urine-containing sewage through optimization of urea oxidation pathways

The electrochemical urea oxidation reaction (UOR) represents a promising route to sustainable hydrogen production and reuse of urea-containing sewage. However, the efficiency of UOR is hindered by the dehydrogenation of intermediate *CONH2NH and the conversion of toxic intermediate the *CO. Herein, we report a robust strategy to elevate UOR performance by introducing iron (Fe) atoms into the Ni3S2@NiSe2 heterojunctions (denoted Fe-Ni3S2@NiSe2). The Fe-Ni3S2@NiSe2 exhibits remarkable selectivity and electrocatalytic activity towards UOR, attributed to its reconstruction into Fe-NiOOH species during UOR process, as confirmed by in-situ Raman technology. Utilizing Fe-Ni3S2@NiSe2 as both the cathode and anode in a single-chamber electrolytic cell, the hydrogen production rate reaches 588.4 μmol h−1 in simulated urea-containing sewage and 432.1 μmol h−1 in actual human urine, respectively. Notably, in both scenarios, no oxygen product is detected, and the hydrogen production efficiency surpasses that of traditional water splitting by 5.8-fold and 4.3-fold, respectively. In-situ infrared spectroscopy study reveals that the UOR process involves the cleavage of C-N bond and the generation of CO2. Density functional theory calculations further signifies that the incorporation of Fe facilitates the dehydrogenation of *CONH2NH intermediates, strengthens the d-p hybridization, and weakens O-H bonds, thereby resulting in reduced energy barriers for UOR. Our strategy holds promise for efficient hydrogen production from sewage via UOR, offering potential implications for wastewater treatment and clean energy generation.

One-step constructing advanced N-doped carbon@metal nitride as ultra-stable electrocatalysts via urea plasma under room temperature

Highly active transition metal nitrides are desirable for electrocatalytic reactions, but their long-term stability is still unsatisfactory and thus limiting commercial applications. Herein, for the first time, we report a unique and universal room-temperature urea plasma method for controllable synthesis of N-doped carbon coated metal (Fe, Co, Ni, etc.) nitrides arrays electrocatalysts. The preformed metal oxides arrays can be successfully converted into metal nitrides arrays with preserved nanostructures and a thin layer of N-doped carbon (NC) via one-step urea plasma. Typically, as a representative case, NC@CoN nanowire arrays are illustrated and corresponding formation mechanism by plasma is proposed. Notably, the designed NC@CoN catalysts deliver excellent electrocatalytic activity and long-term stability both in oxygen evolution reaction (OER) and urea oxidation reaction (UOR). For OER, a low overpotential (264 mV at 10 mA/cm2) and high stability (>50 h at 20 mA/cm2) are acquired. For UOR, a current density of 100 mA/cm2 is achieved at only 1.39 V and maintain over 100 h. Theoretical calculations reveal that the synergetic coupling effect of CoN and NC can significantly facilitate the charge-transfer process, optimize adsorbed intermediates binding strength and further greatly decrease the energy barrier. This strategy provides a novel method for fabrication of NC@ metal nitrides as highly active and stable catalysts.

Hollow NiMo-based nitride heterojunction with super-hydrophilic/aerophobic surface for efficient urea-assisted hydrogen production

Hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are key reactions of the water-cycling associated catalytic process/device. The design of catalysts with a super-hydrophilic/aerophobic structure and optimized electron distribution holds great promise. Here, we have designed a three-dimensional (3D) hollow Ni/NiMoN hierarchical structure with arrayed-sheet surface based on a one-pot hydrothermal route for efficient urea-assisted HER based on a simple hydrothermal process. The Ni/NiMoN catalyst exhibits super-hydrophilic/aerophobic properties with a small droplet contact angle of 6.07° and an underwater bubble contact angle of 155.7°, thus facilitating an escape of bubbles from the electrodes. Density functional theory calculations and X-ray photoelectron spectroscopy results indicate the optimized electronic structure at the interface of Ni and NiMoN, which can promote the adsorption/desorption of reactants and intermediates. The virtues combining with a large specific surface area endow Ni/NiMoN with efficient catalytic activity of low potentials of 25 mV for HER and 1.33 V for UOR at 10 mA cm−2. The coupled HER and UOR system demonstrates a low cell voltage of 1.42 V at 10 mA cm−2, which is approximately 209 mV lower than water electrolysis.

Highly-dispersed 2D NiFeP/CoP heterojunction trifunctional catalyst for efficient electrolysis of water and urea

The electrocatalytic production of “green hydrogen”, such as through the electrolysis of water or urea has been vigorously advocated to alleviate the energy crisis. However, their electrode reactions including oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrogen evolution reaction (HER) all suffer from sluggish kinetics, which urgently need catalysts to accelerate the processes. Herein, we design and prepare an OER/UOR/HER trifunctional catalyst by transforming the homemade CoO nanorod into a two-dimensional (2D) ultrathin heterojunction nickel–iron-cobalt hybrid phosphides nanosheet (NiFeP/CoP) via a hydrothermal-phosphorization method. Consequently, a strong electronic interaction was found among the Ni2P/FeP4/CoP heterogeneous interfaces, which regulates the electronic structure. Besides the high mass transfer property of 2D nanosheet, Ni2P/FeP4/CoP displays improved OER/UOR/HER performance. At 10 mA cm−2, the OER overpotential reaches 274 mV in 1.0 M KOH, and the potential of UOR is only 1.389 V in 1.0 M KOH and 0.33 M urea. More strikingly, the two-electrode systems for electrolysis water and urea-assisted electrolysis water assembled by NiFeP/CoP could maintain long-term stability for 35 h and 12 h, respectively. This work may help to pave the way for upcoming research horizons of multifunctional electrocatalysts.

Vertical Cross-Alignments of 2D Semiconductors with Steered Internal Electric Field for Urea Electrooxidation via Balancing Intermediates Adsorption.

Single-component electrocatalysts generally lead to unbalanced adsorption of OH- and urea during urea oxidation reaction (UOR), thus obtaining low activity and selectivity especially when oxygen evolution reaction (OER) competes at high potentials (>1.5 V). Herein, a cross-alignment strategy of in situ vertically growing Ni(OH)2 nanosheets on 2D semiconductor g-C3N4 is reported to form a hetero-structured electrocatalyst. Various spectroscopy measurements including in situ experiments indicate the existence of enhanced internal electric field at the interfaces of vertical Ni(OH)2 and g-C3N4 nanosheets, favorable for balancing adsorption of reaction intermediates. This heterojunction electrocatalyst shows high-selectivity UOR compared to pure Ni(OH)2, even at high potentials (>1.5 V) and large current density. The computational results show the vertical heterojunction could steer the internal electric field to increase the adsorption of urea, thus efficiently avoiding poisoning of strongly adsorbed OH- on active sites. A membrane electrode assembly (MEA)-based electrolyzer with the heterojunction anode could operate at an industrial-level current density of 200 mA cm-2. This work paves an avenue for designing high-performance electrocatalysts by vertical cross-alignments of active components.

Tailoring competitive adsorption sites of hydroxide ion to enhance urea oxidation-assisted hydrogen production

Alloys with bimetallic electron modulation effect are promising catalysts for the electrooxidation of urea. However, the side reaction oxygen evolution reaction (OER) originating from the competitive adsorption of OH− and urea severely limited the urea oxidation reaction (UOR) activity on the alloy catalysts. This work successfully constructs the defect-rich NiCo alloy with lattice strain (PMo-NiCo/NF) by rapid pyrolysis and co-doping. By taking advantage of the compressive strain, the d-band center of NiCo is shifted downward, inhibiting OH− from adsorbing on the NiCo site and avoiding the detrimental OER. Meanwhile, the oxygenophilic P/Mo tailored specific adsorption sites to adsorb OH− preferentially, which further released the NiCo sites to ensure the enriched adsorption of urea, thus improving the UOR efficiency. As a result, PMo-NiCo/NF only requires 1.27 V and −57 mV to drive a current density of ±10 mA cm−2 for UOR and hydrogen evolution reaction (HER), respectively. With the guidance of this work, reactant competing adsorption sites could be tailored for effective electrocatalytic performance.