Enhanced performance and electrocatalytic kinetics of Ni-Mo/graphene nanocatalysts towards alkaline urea oxidation reaction

Heterostructural Coupling of Phase-Modulated NiMoO4 with NiCo2O4 for Enhanced Urea Electro-Oxidation

Developing catalysts with high efficiency and low cost for the urea oxidation reaction (UOR) is attractive but challenging. Herein, relying on the high catalytic activity of Ni, low overpotential of Co, and superior antipoisoning resistance of Mo, a NiMoO4-NiCo2O4 p-p heterojunction is constructed via a hydrothermal strategy followed by calcination. Interestingly, phase transformation of β-NiMoO4 to α-NiMoO4 occurs when a heterojunction is generated. The unique structure of NiMoO4-NiCo2O4 enables faster charge transfer capability, greater active site availability, lower impedance, and reduced activation energy. Thus, a much better catalytic performance for the UOR is triggered when employing NiMoO4-NiCo2O4 as a catalyst. A specific current density of 1306 mA cm-2 mg-1 (at 0.6 V vs Hg/HgO) is achieved for NiMoO4-NiCo2O4, which is much larger than that for NiMoO4 and NiCo2O4. Potential-dependent impedance analyses unveil that Ni3+ should be active sites and both indirect and direct urea oxidation paths should be accelerated on NiMoO4-NiCo2O4. Phase transformation of β-NiMoO4 to α-NiMoO4 is vital. Making the energy bands of NiCo2O4 and NiMoO4 match better, promoting Ni3+ formation, facilitating active sites exposure, and reducing alkalinity to enhance antipoisoning capacity all make sense. This work stresses the importance of crystal phases in developing heterojunctions with high catalytic performance.

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.

Direct photoelectrochemical detection of ethanol in complex biological sample

The development of advanced photoelectrochemical (PEC) technology for the direct detection of ethanol in complex biological sample, has become a hot topic. However, the photo-active nanomaterials, which could generate the photo-induced carriers under illumination, are susceptible to biofouling and interference in complex bio-matrices. In this work, the silica nanochannel-protected TiO2 nanomaterials was reported for the first time that enables the direct sensing of ethanol in real fruits and untreated whole blood. The modification of SNC enhanced the sensitivity of ethanol detection by promoting light absorption, electron-hole separation, and surface reaction rate of photo-active materials. Meanwhile, the biofouling macromolecules and interference signals can be effectively excluded due to the hydrophilic properties, size, and electrostatic exclusion of nanochannels. As a result, without any complex sample pretreatments, the proposed PEC sensor can be directly immersed in complex biological samples for ethanol detection, exhibiting a broad linear range (1.775 μM-20 mM) and a low detection limit (1.2 μM), as well as excellent reproducibility and stability. This work paves a new path for PEC sensors in real biomedical applications.

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.

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.

Superwettable and photothermal all-in-one electrocatalyst for boosting water/urea electrolysis

Developing multifunctional all-in-one electrocatalysts for energy-saving hydrogen generation remains a challenge. In this study, a simple and feasible thermal phosphorization strategy is explored to rationally construct P-doped MoO₂-NiMoO₄ heterostructure on nickel foam (NF). The heterointerfaced P-MoO₂-NiMoO₄/NF can simultaneously realize the integrated all-in-one functionalities, innovatively introducing superwettable surfaces, photothermal conversion capabilities and electrocatalytic functions. The superwettability gives P-MoO₂-NiMoO₄/NF sufficient electrolyte permeation and smooth bubble detachment. The plasmonic MoO₂ with photothermal performance greatly elevates the local surface temperature of in P-MoO₂-NiMoO₄/NF, which is conducive to improve the electrocatalytic efficiency. The favorable in-situ surface reconstruction brings abundant active sites for electrocatalytic reactions. As an advanced multifunctional electrocatalyst, the superwettable and photothermal P-MoO₂-NiMoO₄/NF exhibits significantly improved performances in oxygen evolution reaction (OER) and urea oxidation reaction (UOR). More importantly, the highly efficient and stable overall water-urea electrolysis assisted by photothermal fields can be simply achieved by exposing P-MoO₂-NiMoO₄/NF to near-infrared (NIR) light.

Methanol electro oxidation on Ni-Pt-CrO/CNFs composite: morphology, structural, and electrochemical characterization

In this work, prepared nanoparticle samples of Ni_1-xCr_x with a fixed ratio of platinum (3%) were synthesized and loaded onto carbon nanofibers, which were produced by an electrospinning technique and carbonized at 900 °C for 7 h in an argon atmosphere. A variety of analysis techniques were applied to examine the stoichiometry, structure, surface morphology, and electrochemical activity. The carbonization process produces carbon nanofibers decorated with metal nanoparticles. Typical fibre diameters are 250-520 nm. The fibre morphologies of the treated samples don't exhibit any overt alterations. A study of the samples' methanol electrocatalytic capabilities was conducted. Cyclic voltammetry, chronoamperometry, and electrochemical impedance measurements were used to investigate catalytic performance and electrode stability as a function of electrolyte concentration, scan rate, and reaction time. The electrooxidation reaction's activation energy is increased, and the electrode's stability is increased, when Cr is added to Ni. In sample C3, the maximum current density (JPE) was 170.3 mA/cm² at 0.8 V with an onset potential of 0.352 V. Utilizing our electrocatalysts, the electrooxidation of methanol involves a mix of kinetic and diffusion control limiting reactions. This study has shown how to fabricate a powerful Ni-Pt-Cr-based methanol electrooxidation catalyst using a novel approach.

Molybdenum carbide/Ni nanoparticles-incorporated carbon nanofibers as effective non-precious catalyst for urea electrooxidation reaction

In this study, molybdenum carbide and carbon were investigated as co-catalysts to enhance the nickel electro-activity toward urea oxidation. The proposed electrocatalyst has been formulated in the form of nanofibrous morphology to exploit the advantage of the large axial ratio. Typically, calcination of electropsun polymeric nanofibers composed of poly(vinyl alcohol), molybdenum chloride and nickel acetate under vacuum resulted in producing good morphology molybdenum carbide/Ni NPs-incorporated carbon nanofibers. Investigation on the composition and morphology of the proposed catalyst was achieved by XRD, SEM, XPS, elemental mapping and TEM analyses which concluded formation of molybdenum carbide and nickel nanoparticles embedded in a carbon nanofiber matrix. As an electrocatalyst for urea oxidation, the electrochemical measurements indicated that the proposed composite has a distinct activity when the molybdenum content is optimized. Typically, the nanofibers prepared from electrospun nanofibers containing 25 wt% molybdenum precursor with respect to nickel acetate revealed the best performance. Numerically, using 0.33 M urea in 1.0 M KOH, the obtained current densities were 15.5, 44.9, 52.6, 30.6, 87.9 and 17.6 mA/cm² for nanofibers prepared at 850 °C from electropsun mats containing 0, 5, 10, 15, 25 and 35 molybdenum chloride, respectively. Study the synthesis temperature of the proposed composite indicated that 1000 °C is the optimum calcination temperature. Kinetic studies indicated that electrooxidation reaction of urea does not follow Arrhenius's law.

Stimulation of ethylene glycol electrooxidation on electrodeposited Ni–PbO2–GN nanocomposite in alkaline medium

In this work, a novel system composed of non-precious nickel-based metal oxide/reduced graphene oxide nanocomposite (Ni–PbO2–GN) is used for electrooxidation of ethylene glycol (EG) in 1.0 M NaOH solution and compares its activity with that of Ni, Ni–GN, and Ni–PbO2. The facile electrodeposition technique is used to prepare the catalysts on glassy carbon (GC) substrates. The outcomes of electrochemical measurements show a high performance towards EG oxidation is obtained for Ni-nanocomposite electrodes compared to that of Ni mainly due to their higher surface areas. The excellent electrocatalytic properties of the Ni-nanocomposite could be ascribed to the synergistic contributions of PbO2 and graphene (GN) nano-sheets that help the reduction of Ni grains. A smaller charge transfer resistance value of 34.5 Ω cm2 for EG oxidation reaction at + 360 mV is recorded for GC/Ni–PbO2–GN compared to the other prepared electrodes. Moreover, it exhibits higher kinetic parameters of EG such as diffusion coefficient (D = 3.9 × 10–10 cm2 s−1) and charge transfer rate constant (ks = 32.5 mol−1 cm3 s−1). The overall performance and stability of the prepared catalysts towards EG electrooxidation have been estimated to be in the order of GC/Ni–PbO2–GN > GC/Ni–GN > GC/Ni–PbO2 > GC/Ni.

Graphical abstract

Crystalline-Amorphous Ni3S2-NiMoO4 Heterostructure for Durable Urea Electrolysis-Assisted Hydrogen Production at High Current Density

Developing bifunctional catalysts with good performance at a high current density for the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER) can effectively relieve the severe environmental and energy pressures. Herein, amorphous NiMoO₄ decorated Ni₃S₂ grown on nickel foam (Ni₃S₂-NiMoO₄/NF) is prepared to accelerate UOR and HER. The crystalline-amorphous heterostructure could regulate the interfacial electron structure to reduce the electron density near Ni₃S₂ for optimizing UOR and HER. The decoration of NiMoO₄ enhances its anti-poisoning ability for CO-intermediate species to show good stability at high current densities. Meanwhile, the nano-/microstructure with high hydrophilicity improves mass transfer and the accessibility of electrolyte. Driving high current densities of ±1000 mA cm^-2, it merely needs 1.38 V (UOR) and -263 mV (HER). For urea electrolysis, it can deliver 1000 mA cm^-2 at 1.73 V and stably operate at 500 mA cm^-2 for 120 h. Therefore, this study provides new ideas for durable urea electrolysis-assisted H₂ production.

Hydrothermal synthesis of Mn3O4 nanorods modified indium tin oxide electrode as an efficient nanocatalyst towards direct urea electrooxidation

Control fabrication of metal-oxide nanocatalysts for electrochemical reactions has received considerable research attention. Here, manganese oxide (Mn₃O₄) nanorods modified indium tin oxide (ITO) electrodes were prepared based on the in-situ one-step hydrothermal methods. The nanorods were well characterized using field emission scanning electron microscopy, Fourier transform infrared, and X-ray diffraction spectroscopy. The results showed the formation of pure crystalline Mn₃O₄ nanorods with a length of approximately 1.4 μm and a thickness of approximately 100 ± 30 nm. The Mn₃O₄ nanorod-modified ITO electrodes were used for accelerating urea electrochemical oxidation at room temperature using cyclic and square wave voltammetry techniques. The results indicated that the modified electrode demonstrated excellent electrocatalytic performance toward urea electrooxidation in an alkaline medium over concentrations ranging from 0.2 to 4 mol/L. The modified electrode showed high durability, attaining more than 88% of its baseline performance after 150 cycles; furthermore, the chronoamperometry technique demonstrated high stability. Thus, the Mn₃O₄ nanorod-modified ITO electrode is a promising anode for direct urea fuel cell applications.

Ultrathin Two-Dimensional Bimetal-Organic Framework Nanosheets as High-Performance Electrocatalysts for Benzyl Alcohol Oxidation

Ultrathin two-dimensional metal-organic frameworks (2D MOFs) have the potential to improve the oxidation of benzyl alcohol (BA) with a large surface area and open catalytic active sites. To achieve high-efficiency electrocatalysts for the oxidation of benzyl alcohol, a moderate solvothermal method was evolved to synthesize a series of 2D MOFs on nickel foam (Ni-MOF/NF, NiCo-61-MOF/NF, NiCo-21-MOF/NF). As the electrocatalyst used for the oxidation of benzyl alcohol, NiCo-61-MOF/NF presented a lower overpotential and superior chemical durability than other electrocatalysts; it only required a potential of ∼1.52 V (vs RHE) to reach 338.16 mA cm^-2, with an oxidation efficiency of more than 86%. Besides, after continuous electrocatalysis for 20 000 s at 1.42 V (vs RHE), the current density of NiCo-61-MOF/NF nanosheets was still 38.67 mA cm^-2 with 77.34% retention. This demonstrated that NiCo-61-MOF/NF nanosheet electrocatalysts had great potential for benzyl alcohol oxidation. From both the experimental and theoretical studies, it was discovered that NiCo-61-MOF/NF nanosheets have the highest electrocatalytic activity due to their distinctive ultrathin 2D structure, optimized electron structure, and more accessible active sites. This finding would pave a brand-new thought for the design of electrocatalysts with electrocatalytic activity for benzyl alcohol oxidation (EBO).

NiO‐MnOx/Polyaniline/Graphite Electrodes for Urea Electrocatalysis: Synergetic Effect between Polymorphs of MnOx and NiO

In this work, we are enhancing the catalytic activity of the urea electrooxidation (UEO) process by using the composite of polyaniline (Pani), nickel oxide (NiO), and polymorphs of manganese oxide (MnOx) based on a graphite electrode. The hydro‐gel method was used to prepare catalyst suspension with different ratios from NiO and MnOx. The chemical structures and surface morphology were characterized by employing the IR, XRD, and SEM/EDEX techniques. The catalytic activity of four modified electrocatalysts G/Pani/NiMn1, G/Pani/NiMn2, G/Pani/NiMn3, G/Pani/NiMn4 was investigated using cyclic voltammetry, chronoamperometry, and electrochemical impedance. The kinetic parameters such as diffusion coefficient, Tafel slope, charge transfer coefficient, and surface coverage were calculated to choose the best electrocatalysts toward UEO in alkaline solution. The anodic current of new electrodes achieved about 16 mA.cm−2at potential of 550 mV (vs. Ag/AgCl). Density functional theory studies (DFT) have been carried out to assess the adsorption energy between polyaniline (Pani) and the metal oxides.

Synthesis and Characterization of NiCoPt/CNFs Nanoparticles as an Effective Electrocatalyst for Energy Applications

In this work, three nanoparticle samples, Ni4Co2Pt/CNFs, Ni5CoPt/CNFs and Ni6Pt/CNFs, were designed according to the molar ratio during loading on carbon nanofibers (CNFs) using electrospinning and carbonization at 900 °C for 7 h in an argon atmosphere. The metal loading and carbon ratio were fixed at 20 and 80 wt%, respectively. Various analysis tools were used to investigate the chemical composition, structural, morphological, and electrochemical (EC) properties. For samples with varying Co%, the carbonization process reduces the fiber diameter of the obtained electrospun nanofibers from 200-580 nm to 150-200 nm. The EDX mapping revealed that nickel, platinum, and cobalt were evenly and uniformly incorporated into the carbonized PVANFs. The prepared Ni-Co-Pt/CNFs have a face-centered cubic (FCC) structure with slightly increased crystallite size as the Co% decreased. The electrocatalytic properties of the samples were investigated for ethanol, methanol and urea electrooxidation. Using cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance measurements, the catalytic performance and electrode stability were investigated as a function of electrolyte concentration, scan rate, and reaction time. When Co is added to Ni, the activation energy required for the electrooxidation reaction decreases and the electrode stability increases. In 1.5 M methanol, the Ni5CoPt/CNFs electrode showed the lowest onset potential and the highest current density (30.6 A/g). This current density is reduced to 28.2 and 21.2 A/g for 1.5 M ethanol and 0.33 M urea, respectively. The electrooxidation of ethanol, methanol, and urea using our electrocatalysts is a combination of kinetic/diffusion control limiting reactions. This research provided a unique approach to developing an efficient Ni-Co-Pt-based electrooxidation catalyst for ethanol, methanol and urea.

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.

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.

Ni-Co-B nanoparticle decorated carbon felt by electroless plating as a bi-functional catalyst for urea electrolysis

A free-standing catalyst electrode for urea electrolysis was synthesized by electroless plating of NiCoB alloy onto a flexible carbon felt. The synthesized NiCoB@C catalyst exhibited porous and partially amorphous metallic structure depending on its composition, as analysed by XRD, XPS, and TEM; thus, NiCoB@C catalyst showed a high catalytic activity for urea oxidation reaction as well as hydrogen evolution reaction. The required cell voltage in the electrolysis cell with NiCoB@C as anode and cathode was as low as 1.34 V for the current densities 10 mA cm^-2. Similar performance of the urea electrolysis for H₂ production using 0.33 M urea and a fresh urine in 1 M KOH was observed. The result indicated that NiCoB could be incorporated on to carbon felt by electroless plating, and it could be used as free-standing bifunctional electrodes for urea electrolysis using urea as well as urine.

Effect of Anode Material on Electrochemical Oxidation of Low Molecular Weight Alcohols-A Review

The growing climate crisis inspires one of the greatest challenges of the 21st century-developing novel power sources. One of the concepts that offer clean, non-fossil electricity production is fuel cells, especially when the role of fuel is played by simple organic molecules, such as low molecular weight alcohols. The greatest drawback of this technology is the lack of electrocatalytic materials that would enhance reaction kinetics and good stability under process conditions. Currently, electrodes for direct alcohol fuel cells (DAFCs) are mainly based on platinum, which not only provides a poor reaction rate but also readily deactivates because of poisoning by reaction products. Because of these disadvantages, many researchers have focused on developing novel electrode materials with electrocatalytic properties towards the oxidation of simple alcohols, such as methanol, ethanol, ethylene glycol or propanol. This paper presents the development of electrode materials and addresses future challenges that still need to be overcome before direct alcohol fuel cells can be commercialized.

A highly active Ni-based anode material for urea electrocatalysis by a modified sol-gel method

A highly electroactive Ni-based catalyst for urea oxidation is prepared by a sol-gel method with bubbling of gel mixture. It is observed that the conditions for the gel formation strongly affect the morphology and electrochemical properties of the catalyst materials. As synthesized Ni-catalysts are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The Ni-based catalyst prepared at optimum conditions in the scope of this study exhibits the urea oxidation activity of 570 mA mg^-1 (at 0.54 V). In a single urea/hydrogen peroxide fuel cell test, the Ni-catalyst provides maximum power densities of 19.6 and 36.4 mW cm^-2 at 30 and 70 °C, respectively. Additionally, the cell catalyst shows a stable voltage for 3 days. Thus, this work suggests that a novel Ni-based catalyst derived from a facile method can be used for urea oxidation and as an efficient anode material for urea fuel cells.

Trimetallic NiCoMo/graphene multifunctional electrocatalysts with moderate structural/electronic effects for highly efficient alkaline urea oxidation reaction

Trimetallic NiCoMo/graphene (NCM/G 811) multifunctional electrocatalysts demonstrate remarkable catalytic activity, fast kinetics, a low onset potential and high stability towards alkaline urea oxidation reaction (UOR). Moderate structural/electronic effects among Ni, Co and Mo species are responsible for the outstanding catalytic behavior.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

Metal-organic framework-derived Ni@C and NiO@C as anode catalysts for urea fuel cells

Highly porous self-assembled nanostructured Ni@C and NiO@C were synthesized via calcination of a Ni-based metal–organic framework. The morphology, structure, and composition of as synthesized Ni@C and NiO@C were characterized by SEM, FIB-SEM, TEM, and XRD. The electro-catalytic activity of the Ni@C and NiO@C catalysts towards urea oxidation was investigated using cyclic voltammetry. It was found that the Ni@C had a higher residual carbon content and a higher specific surface area than NiO@C, thus exhibiting an enhanced electrochemical performance for urea oxidation. A direct urea fuel cell with Ni@C as an anode catalyst featured an excellent maximum power density of 13.8 mW cm⁻² with 0.33 M urea solution in 1 M KOH as fuel and humidified air as oxidant at 50 °C, additionally showing excellent stability during continuous 20-h operation. Thus, this work showed that the highly porous carbon-supported Ni catalysts derived from Ni-based metal–organic framework can be used for urea oxidation and as an efficient anode material for urea fuel cells.

Recent progress with electrocatalysts for urea electrolysis in alkaline media for energy-saving hydrogen production

Urea electrolysis is a promising energy-saving avenue for hydrogen production owing to the low cell voltage, wastewater remediation and abundant electrocatalysts.