Urea Electro-oxidation Research Articles

Improved mass transport imparts stability and durability in nickel catalysts for urea oxidation

In this issue of Chem Catalysis , J. Zhang et al. report on chromium-doped nickel hydroxides electrocatalysts that exceed the performances of current state-of-the-art materials in the electro-oxidation of urea in alkaline media and provide a microscopic understanding of their exceptional stability and durability in industrially relevant conditions. In this issue of Chem Catalysis , J. Zhang et al. report on chromium-doped nickel hydroxides electrocatalysts that exceed the performances of current state-of-the-art materials in the electro-oxidation of urea in alkaline media and provide a microscopic understanding of their exceptional stability and durability in industrially relevant conditions.

Ni-WC nanoparticles/carbon aerogel electrocatalytic electrode for methanol and urea electrooxidation

Direct methanol fuel cells and direct urea fuel cells are significant for mitigating the energy crisis and protecting the environment. However, as the common critical anodes for the electrooxidation of methanol and urea, most Ni-based catalysts are severely poisoned by the CO group, resulting in a decrease in current densities and stability of fuel cells. Herein, efficient bacterial cellulose (BC)-derived carbon aerogel loaded with Ni-based catalyst and WC nanoparticles co-catalyst was developed for the electrooxidation of methanol and urea. As a result, the catalytic electrode exhibits high electrocatalytic activity for methanol electrooxidation (105.7 mA cm−2 at 0.8 V) and urea electrooxidation (90.4 mA cm−2 at 0.8 V). The synergistic effect between Ni and WC can significantly weaken the surface adsorption of CO on the electrode to obtain excellent and durable electrochemical performance. For methanol oxidation reaction and urea oxidation reaction, the retentions of the peak current density after 2000 cycles in the original electrolytes are 48.94% and 74.79%, respectively. Moreover, applying the new electrolyte, the peak current density can return to 88.42% and 84.4% of the original value.

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.

Construction of hierarchical nanostructures and NiO nanosheets@nanorods for efficient urea electrooxidation

Hierarchical NiO nanosheets@nanorods have been rationally designed and constructed for efficient urea electrooxidation in an alkaline solution. The critical synthetic strategy, engaging the one-step anion-competitive reaction, precisely integrates two nickel-based materials into a heterostructure with Ni(OH)2 nanosheets and NiC2O4 nanorods. Benefiting from the hierarchically porous structure and high specific surface area, the NiO NNs can improve the escape efficiency of gas in electrochemical reactions and maintain sustainability. Furthermore, this distinctive structure can expose highly dispersed active sites for enhancing urea molecules' adsorption, surface-dependent redox reactions, and electrical conductivities. As a result, these hierarchical NiO nanosheets@nanorods exhibit superior activity with a low overpotential of 156 mV at 10 mA/cm2, and a slight Tafel slope of 40.7 mV/dec, and high stability with almost no decay of 12,000 s for urea electrooxidation. This work promotes the application of well-designed hierarchical structure in electrooxidizing urea and provides a possibility for highly efficient electrolysis of alkaline urea wastewater.

Insights into the synergistic effect between nickel and molybdenum for catalyzing urea electrooxidation

AbstractThe efficiency of many clean technologies, such as urea fuel cells, hydrogen production from urea‐containing water splitting, and sewage treatment, and so forth, is restricted due to the sluggish kinetics of the urea oxygen reaction (UOR). Mo‐doped Ni‐based catalysts are excellent for UOR because of the synergistic effect between Ni and Mo. However, such applications have indeed been hampered by a lack of mechanistic comprehension. Here, we fabricate Mo‐Ni(OH)2 nanosheets for efficient UOR electrocatalysis with activity at least twice higher than Ni(OH)2 nanosheets. Various in situ characterizations including Raman, differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy reveals that the doping of Mo lowers the onset potential required for Ni2+/Ni3+ conversion and promotes the coupling of the N–N bond inside the molecule and the fracture of the double bond between C and O in the urea molecule. In addition, an entirely flexible direct urea fuel cell with the Mo‐Ni(OH)2 electrocatalyst delivers a maximum output power density of 0.78 mW cm−2 and complete flexibility for the first time. Our research significantly promotes the mechanistic understanding of UOR electrocatalysis and its applications.

Metal organic framework-assisted in-situ synthesis of β-NiMnOOH nanosheets with abundant NiOOH active sites for efficient electro-oxidation of urea

NiOOH has been considered as the active center for urea oxidation reaction (UOR), but it remains challenging to synthesize high-performance NiOOH-based catalysts. Herein, we realize the synthesis of a high-performance NiOOH-based catalyst through in-situ transformation from the NiMn-based metal–organic framework to NiMnOOH. X-ray photoelectron spectroscopy characterization shows that the Ni3+/Ni2+ ratio in the NiMnOOH is 3.9 times as big as that in the Ni(OH)2, and in-situ Raman characterization further consolidates the presence of the NiOOH species in the NiMnOOH and as well unveils the faciliated Ni2+/Ni3+ redox reaction. The abundant NiOOH species, the markedly facilitated Ni2+/Ni3+ redox reaction and the Ni-Mn synergy contribute to the high intrinsic activity of the NiMnOOH towards UOR. The NiMnOOH exhibits an impressively low onset potential of 1.305 V vs reversible hydrogen electrode (RHE) and requires only a small potential of 1.34 V vs RHE to deliver a current density of 100 mA cm−2 in 1.0 M KOH + 0.33 M urea. In addition, the NiMnOOH catalyst possesses good long-term working stability.

Two-dimensional nickel–cobalt bimetallic hydroxides towards urea electrooxidation

The atom-thin metal-based two-dimensional (2D) nanomaterials have exhibited wide application in electrocatalysis because of their particular geometrical and electronic structures. Herein, we report a versatile and simple strategy to synthesize ultrathin nanosheets of nickel–cobalt bimetallic hydroxides (named as NiCo BMHs) through NaBH4 boosted cyanogel hydrolysis strategy. The hydrogen (H2) bubbles from the NaBH4 decomposition can effectively separate the 2D NiCo BMHs and avoid their self-stacking during the synthesis. As a result, NiCo BMHs with rich nanoholes possess only about 1.5 nm thickness, which results in a high surface area and rich defective atoms. The high surface area, rich edge active atoms, and synergistic effect between Ni2+ and Co2+/Co3+ species greatly improve the electrocatalytic activity of NiCo BMHs towards urea oxidation reaction (UOR). The potential of UOR at NiCo BMHs is only 1.33 V at 10 mA cm−2, much lower than that at various reported Ni-based nanomaterials, confirming the high electroactivity of NiCo BMHs.

Green synthesis of NiO/Fe3O4@chitosan composite catalyst based on graphite for urea electro-oxidation

In this work, we modified the NiO-based graphite electrode by a green synthesis nanoparticles composite of magnetite@chitosan for urea electrooxidation (UEO) in 0.5 M KOH solution. Green magnetite (Fe3O4) nanoparticles were prepared with green tea extract that acts as a reducing agent. The new composite catalyst Fe3O4@Chitosan performs a dual function to improve the catalytic properties of UEO. The magnetite Fe3O4 nanoparticles increase the active sites by accelerating the electron transfer between Fe3+/Fe2+ions, while chitosan enhances urea adsorption on the electrode surface. Three graphite electrodes were prepared with different NiO and Fe3O4@Chitosan catalysts ratios under optimized conditions and denoted by Ni0.85Fe0.15, Ni0.75Fe0.25, and Ni0.65Fe0.35. The excellent catalytic parameters as maximum current density (35.4 mA cm−2), Tafel slope (69 mVdec−1), diffusion coefficient (5.759 × 10−6 cm2 s−1), and onset potential (446 mV vs. Ag/AgCl) were achieved by Ni0.75Fe0.25 electrocatalyst. The modified NiO/Fe3O4@chitosan electrocatalyst was characterized by XRD and SEM/EDEX-elemental mapping techniques characterized its morphology and chemical structures. Moreover, cyclic voltammetry, double-step Choronoamerometery, and electrochemical impedance were employed to detect the electrodes' catalytic performance. The EIS fitting parameters deduced from R(RQ)& (RQ) equivalent circuits were used to explain how the Ni/Fe ratios' can affect the charge transfer process of UOR at the prepared electrocatalysts surfaces under the applied potentials.

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.

Sprouts-like Fe(OH)2 hetero-nanostructures assembly on selenium layered nickel foam (NiF–Se) as an efficient and durable catalyst for electro-oxidation of urea

In this study, we develop selenium (Se)-iron hydroxide (NiF–Se@Fe(OH)2) hetero-nanostructured catalyst system for fuel cell, and environmental-relevant urea-electro-oxidation reaction. For the rational engineering, the Se layers are initially deposited on the Ni foam substrate; then we grow Fe(OH)2 hetero-nanostructures with various morphologies by introducing different ratios of Fe precursor sample. The Fe(OH)2 ball-like nanorods to rock-like nanosheets are synthesized on the Se layered Ni foam surface by a simple hydrothermal process. The microscopic characterizations, and spectral analysis reveal the formation of Se integrated Fe(OH)2 hetero-nanostructures such as ball-like nanorods, sprouts-like nanowire network, nanoflowers and rock-like nanosheets through effective Ni–O bond and Fe–Se bond for its typical synergism. The Se induces an interesting morphology transformation from crystalline nanorods to rock-like nanosheets structures that lead to the potential constituents for catalyst electrode that effectively merge the qualities such as high conductivity, large specific surface area, and larger catalytic active sites for electro-oxidation reaction of urea. Among them, NiF–Se@Fe(OH)2 (8 mmol Fe(NO3)2) sprouts-like hetero-nanostructured network displays higher catalytic activity toward oxidation of urea (146.7 mA) with onset potential of 0.11 V vs. Ag/AgCl in 1 M NaOH + 0.1 M urea. Furthermore, the sprouts-like NiF–Se@Fe(OH)2 nanowired network shows superior activity than the other aspect ratio's, excellent long-term stability, and reproducibility.

(Digital Presentation) Paper-Based Sensor for Monitoring Urea Oxidation Using Hierarchical Nickel Cobalt Oxide

Urea has attracted attention because of its various potential applications such as hydrogen production, fuel cells, fertilizers, and electrochemical sensors. [1] Their long-term usages can lead to soil acidification and eutrophication, disturbing the ecosystem.[2] As an end-product of human metabolism, urea is a crucial biomarker that can access various human disorders such as kidney and renal function. Thus, the rapid sensing of the urea level in urine can play an important role in diagnostic areas, especially point-of-care testing devices. Most electrochemical biosensors rely on the enzymatic method. However, the utilization of the enzyme for the sensors appears to be limited due to complex processes of enzyme immobilization, high cost, short shelf life from the denaturation of the enzyme. [3] To overcome the limitations from using enzyme, non-enzymatic catalyst, especially nickel oxide, has been attracted with the advantage of good stability re-usability, high sensitivity, simplicity, low cost, and an excellent catalytic activity on detection of urea by the formation of the redox couple of Ni(II) and Ni(III).[4]Despite many efforts to achieve a higher catalytic effect with bimetallic oxides with Co[5], Mo[6], and Mn[7], the improvement of electrochemical response of oxidation of urea is still challenging due to the low exposure of active sites. Therefore, the formation of hollow structured hierarchical catalysts can be considered to improve the sensitivity of urea detection besides exploration of highly performing compositions. Such a hierarchical structure will provide structural stability and facile transport channels for electrolytes by exploiting its inner and outer surface as active sites. For a flexible and disposable sensor platform, the paper has merit due to a porous cellulose matrix. The paper naturally allows a liquid sample to infiltrate the paper matrix by capillary force. Furthermore, the capillary force-driven transport can be utilized in the catalyst loading process to distribute the catalyst and conductive network uniformly within the paper, which made the fabrication process simpler and enhanced performance.In this study, the combination of the hierarchical structure of nickel oxide and the paper matrix has demonstrated an increase in the sensitivity toward electrochemical sensing of urea. A filter paper and CNTs were used for the porous matrix and the conductive network, respectively. The hierarchical nickel cobalt oxide was synthesized with a one-pot hydrothermal method with the variation of Ni:Co atomic ratio, and then applied to the paper substrate. The structure and morphology of paper-based electrodes were characterized by XRD, SEM, and EDS, and the electrochemical response was measured by a potentiostat. A detailed description of the fabrication of paper-based sensors and the effect of hierarchical structure and bimetallic composition will be presented.[1] B. K. Boggs, R. L. King, and G. G. Botte, “Urea electrolysis: direct hydrogen production from urine,” Chem. Commun., no. 32, pp. 4859–4861, Aug. 2009, doi: 10.1039/B905974A.[2] L. Liu, H. Mo, S. Wei, and D. Raftery, “Quantitative analysis of urea in human urine and serum by 1 H nuclear magnetic resonance,” Analyst, vol. 137, no. 3, pp. 595–600, 2012, doi: 10.1039/C2AN15780B.[3] K. Kim et al., “Fabrication of a Urea Biosensor for Real-Time Dynamic Fluid Measurement,” Sensors, vol. 18, no. 8, Art. no. 8, Aug. 2018, doi: 10.3390/s18082607.[4] Nie, Huagui, et al. "Non-enzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes." Biosensors and Bioelectronics 30.1 (2011): 28-34.[5] Ding, Rui, et al. "Facile synthesis of mesoporous spinel NiCo2O4 nanostructures as highly efficient electrocatalysts for urea electro-oxidation." Nanoscale 6.3 (2014): 1369-1376.[6] Liang, Yanhui, et al. "Enhanced electrooxidation of urea using NiMoO4· xH2O nanosheet arrays on Ni foam as anode." Electrochimica Acta 153 (2015): 456-460.[7] Periyasamy, Sivakumar, et al. "Exceptionally active and stable spinel nickel manganese oxide electrocatalysts for urea oxidation reaction." ACS applied materials & interfaces 8.19 (2016): 12176-12185.

Sulfur-doped NiCo carbonate hydroxide with surface sulfate groups for highly enhanced electro-oxidation of urea

NiCo-based hydroxides are promising catalysts for urea oxidation reaction. To further enhance their catalytic performance, the combined effect of nonmetal heteroatom doping and surface functional groups on their catalytic behaviors deserves to be evaluated. Herein, as a proof of concept, both the sulfur-doped NiCo(OH)2CO3 with surface sulfate groups (SS-NiCo) and the surface-sulfated NiCo(OH)2CO3 (S-NiCo) are synthesized by hydrothermal method. The effect of sulfur doping and surface sulfate treatment on the morphology, crystal structure, electronic structure and electrochemical properties is comprehensively studied. It is found that both the incorporation of sulfur into the crystal structure and the introduction of surface sulfate groups can tune the electronic structure and thus promotes the intrinsic activity of the NiCo(OH)2CO3. Additionally, the surface sulfate groups can effectively reduce the series resistance and thus enhance the charge transfer/reaction kinetics. The optimized SS-NiCo catalyst delivers a current density of 100 mA cm−2 at 1.34 V vs RHE, which is much smaller than those of the S-NiCo (1.38 V) and the pristine NiCo(OH)2CO3 (1.43 V). In addition, the SS-NiCo catalyst shows good performance in the urea-assisted water electrolysis for hydrogen production.

A review of Ni based powder catalyst for urea oxidation in assisting water splitting reaction

Water splitting has been regarded as a sustainable and environmentally-friendly technique to realize green hydrogen generation, while more energy is consumed due to the high overpotentials required for the anode oxygen evolution reaction. Urea electrooxidation, an ideal substitute, is thus received increasing attention in assisting water-splitting reactions. Note that highly efficient catalysts are still required to drive urea oxidation, and the facile generation of high valence state species is significant in the reaction based on the electrochemical-chemical mechanisms. The high cost and rareness make the noble metal catalysts impossible for further consideration in large-scale application. Ni-based catalysts are very promising due to their cheap price, facile structure tuning, good compatibility, and easy active phase formation. In the light of the significant advances made recently, herein, we reviewed the recent advances of Ni-based powder catalysts for urea oxidation in assisting water-splitting reaction. The fundamental of urea oxidation is firstly presented to clarify the mechanism of urea-assisted water splitting, and then the prevailing evaluation indicators are briefly expressed based on the electrochemical measurements. The catalyst design principle including synergistic effect, electronic effect, defect construction and surface reconstruction as well as the main fabrication approaches are presented and the advances of various Ni-based powder catalysts for urea assisted water splitting are summarized and discussed. The problems and challenges are also concluded for the Ni-based powder catalysts fabrication, the performance evaluation, and their application. Considering the key influencing factors for catalytic process and their application, attention should be given to structure−property relationship deciphering, novel Ni-based powder catalysts development and their construction in the real device; specifically, the effort should be directed to the Ni-based powder catalyst with multi-functions to simultaneously promote the fundamental steps and high anti-corrosion ability by revealing the local structure reconstruction as well as the integration in the practical application. We believe the current summarization will be instructive and helpful for the Ni-based powder catalysts development and understanding their catalytic action for urea-assisted hydrogen generation via water splitting technique. Advances and challenges of Ni based powder catalyst were reviewed for urea oxidation in assisting water-splitting reaction.

Deciphering the active origin for urea oxidation reaction over nitrogen penetrated nickel nanoparticles embedded in carbon nanotubes

Urea electrooxidation with favorable thermodynamic potential is highly anticipated but suffering from sluggish kinetics. Deciphering the activity origin and achieving rational structure design are pivotal for developing highly efficient electrocatalyst for urea oxidation reaction (UOR). Herein, nitrogen penetrated nickel nanoparticles confined in carbon nanotubes (Ni-NCNT) is successfully achieved to drive UOR. Active origin of Ni-NCNT is decoded to be the in-situ generated Ni2+δO(OH)ads according to comprehensive analysis. The electrophilic Ni2+δ and protophilic OHads could targeted capture O and H atoms from urea, respectively, achieving molecule activation and accelerating the subsequent proton coupled electron transfer reactions. Nitrogen penetration is identified to promote prior formation of Ni2+δO(OH)ads and push up the d band center of Ni-NCNT, enhancing urea adsorption and subsequent molecule cleavage reactions. As a result, Ni-NCNT exhibits superior UOR performance. This work supplies valuable insights for the rational design and construction of efficient nickel-based catalyst for driving UOR.

Engineering hierarchical quaternary superstructure of an integrated MOF-derived electrode for boosting urea electrooxidation assisted water electrolysis

Controllable design of the catalytic electrodes with hierarchical superstructures is expected to improve their electrochemical performance. Herein, a self-supported integrated electrode (NiCo-ZLDH/NF) with a unique hierarchical quaternary superstructure was fabricated through a self-sacrificing template strategy from the metal–organic framework (Co-ZIF-67) nanoplate arrays, which features an intriguing well-defined hierarchy when taking the unit cells of the NiCo-based layered double hydroxide (NiCo-LDH) as the primary structure, the ultrathin LDH nanoneedles as the secondary structure, the mesoscale hollow plates of the LDH nanoneedle arrays as the tertiary structure, and the macroscale three-dimensional frames of the plate arrays as the quaternary structure. Notably, the distinctive structure of NiCo-ZLDH/NF can not only accelerate both mass and charge transfer, but also expose plentiful accessible active sites with high intrinsic activity, endowing it with an excellent electrochemical performance for urea oxidation reaction (UOR). Specially, it only required the low potentials of 1.335, 1.368 and 1.388 V to deliver the current densities of 10, 100 and 200 mA cm−2, respectively, much superior to those for typical NiCo-LDH. Employing NiCo-ZLDH/NF as the bifunctional electrode for both anodic UOR and cathodic HER, an energy-saving electrolysis system was further explored which can greatly reduce the needed voltage of 213 mV to deliver the current density of 100 mA cm−2, as compared to the conventional water electrolysis system composed of OER. This work manifests that it is prospective to explore the hierarchically nanostructured electrodes and the innovative electrolytic technologies for high-efficiency electrocatalysis.

Tragacanth Gum Hydrogel-Derived Trimetallic Nanoparticles Supported on Porous Carbon Catalyst for Urea Electrooxidation

The fabrication of electrocatalysts with high catalytic activity, high durability and low cost towards urea oxidation by a facile method is a great challenge. In this study, non-precious NiCoFe trimetallic supported on porous carbon (NiCoFe@PC) was prepared via gelation and pyrolysis method, presenting a remarkable electrocatalytic activity with low onset potential for urea oxidation in an alkaline medium. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to clarify the morphology of the NiCoFe@PC nanostructure and its nanoparticle size of 17.77 nm. The prepared catalyst with the composition ratio of 24.67, 5.92 and 5.11% for Ni, Fe and Co, respectively, with highly crystalline nanoparticles, fixed on porous carbon, according to energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis. The FeCoNi@PC catalyst showed a catalytic activity of 44.65 mA/cm2 at 0.57 V vs. Ag/AgCl and a low onset potential of 218 mV, which is superior to many other transition bi/trimetallic-based catalysts previously reported.

Ionic liquid crystals/nano-nickel oxide-decorated carbon nanotubes composite for electrocatalytic treatment of urea-contaminated water

Electrochemical treatment is an efficient approach for urea-decontamination from aqueous media. Reliable electro-catalysts are therefore crucial for water decontamination applications. We introduce in this work a newly developed composite by modifying a nickel substrate with nickel oxide nanoparticles (NiO), multiwalled carbon nanotubes (CNTs) and ionic liquid crystals (ILCs). The composite modifier (Ni/NiO + CNTs) exhibit an outstanding specific catalytic current for 0.33 M urea electro-oxidation in 1 M KOH of 1303 A·g−1·cm−2. The optimum regime of the catalyst administration to the substrate surface is a layer of ILCs followed by a mixture of CNTs and NiO with a ratio of 1:1 by mass (Ni/ILCs/NiO + CNTs (1:1)). The inclusions of ILCs assist the ionic exchange of the electrode surface and increase substantially the performance of the catalyst. The change in the carbon-based component of the catalyst affects the electro-catalytic current; reduced graphene oxide (RGO) exhibit relatively lower value compared to CNTs. The Tafel slope, the exchange current density, charge transfer coefficient, diffusion coefficient, and heterogeneous rate constant in 0.33 M urea/1.0 M KOH for the optimized electrode (Ni/ILCs/NiO + CNTs (1:1)) are: 33.0 mV·dec−1, 2.89 × 10−5 A·cm−2, 0.91, 1.70 × 10−3 cm2·s−1, 1.51 × 105 mol−1·L·s−1, respectively. The calculated activation energy of the electrochemical conversion of urea is 5.28 kJ·mol−1. The electrode shows high stability when used for extended time to constant applied potential of about +0.5 V for urea electro-oxidation. The conversion efficiency increased in the order Ni/ILCs/NiO + CNTs > Ni/NiO + CNTs > Ni/NiO compared to the Ni surface.

Urea electrooxidation-boosted hydrogen production on nitrogen-doped porous carbon nanorod-supported nickel phosphide nanoparticles

Urea electro-oxidation reaction (UEOR)-boosted water electrolysis can supplant the kinetics-restricted oxygen evolution reaction (OER) and provide an energy-saving method of hydrogen generation. However, low UEOR activity and the poisoning issue of the catalyst limit its practical application. Herein, a simple coordination reaction is used to synthesize the dimethylglyoxime-NiⅡ complex (DMG-NiⅡ), which efficiently serves as the initial precursor to synthesize nitrogen-doped carbon nanorod-supported nickel phosphide nanoparticle (Ni2P/N-Cnanorods) nanocomposites. The density functional theory calculations and electrochemical results reveal that nitrogen doping can weaken the adsorption of hydrogen and the generated CO2, resulting in an enhancement of hydrogen evolution reaction (HER) and UEOR activity. In addition, N-doping can also promote the generation of Ni3+, which can further promote the UEOR and HER performance. Concretely, the overpotential for the HER on Ni2P/N-Cnanorods-2h nanocomposites is only 201 mV at 10 mA cm−2, and the onset potential of the UEOR on Ni2P/N-Cnanorods-2h nanocomposites is only 1.34 V. Additionally, the Ni2P/N-Cnanorods nanocomposites also show excellent long-term stability due to the introduction of nitrogen-doped carbon material. Consequently, the symmetric Ni2P/N-Cnanorods-2h||Ni2P/N-Cnanorods-2h urea electrolyzer requires 1.41 V of electrolysis voltage for urea electrolysis, which can be applied in energy-saving H2 production and environment purification.

Effect of Mn Doped Ni−Co Mixed Oxide Catalysts on Urea Oxidation

AbstractUrea as an energy generation and storage fuel has attracted increased attention for its stability, non‐toxicity, and availability. Urea possesses 10.1 wt.% of hydrogen. Nevertheless, its electrochemical oxidation is generally sluggish and inefficient. Ni‐based catalysts which showed efficient activity towards urea electrooxidation in the alkaline medium were studied for Direct Urea Fuel Cell (DUFC) anodes. In the present study, we investigated urea oxidation on a novel three‐dimensional mixed oxide catalyst, NiCo2O4. Mn‐doped NiCo2O4 catalyst exhibited urea oxidation activity of 45.88 mA cm−2 (at 1.30 V vs SHE), fifteen times higher than NiO and Co3O4 and twice that of bare NiCo2O4 spinel. Chronoamperometry curves of the Mn‐doped NiCo2O4 catalyst demonstrated stable urea oxidation for over 3 hours. The significantly higher mass activity of Mn‐doped NiCo2O4 catalyst is attributed to the least nano‐crystallite size, better dispersion of Ni nuclei onto the spinel crystal matrix, enhanced Ni electrochemical active surface area, and lower bandgap.

Tuning interface density and electronic structure of NiS/Ni3S4 by Mo, Co co-doping for efficient urea electrooxidation reaction

Mo, Co co-doped NiS/Ni 3 S 4 with high activity, low onset potential, fast kinetics, more active sites, excellent intrinsic catalytic activity and stability demonstrates excellent catalytic activity toward alkaline urea electrolysis. • Mo, Co co-doped NiS/Ni 3 S 4 heterojunction with 3D flower-like shape assembled by 2D ultrathin nanosheets are successfully prepared. • The amount of heterointerfaces and the electronic interaction are simultaneously modified by adjusting experimental parameters. • Mo, Co co-doped NiS/Ni 3 S 4 catalysts exhibit rich exposed active sites and facilitated electron transportation. • The assembled direct urea hydrogen peroxide fuel cell (DUHPFC) employing Mo 0.05 ,Co-NSH has an open-circle voltage (OCV) of 0.86 V and a maximum power density ( P max ) of 5.96 mW cm −2 . Electrocatalysts exhibiting impressive catalytic capability and strong stability for urea oxidation reaction (UOR) are highly desired for direct urea fuel cells (DUFCs). Herein, a series of flower-like Mo, Co co-doped NiS/Ni 3 S 4 heterostructure (Mo, Co-NSH) are controllable synthesized by a one-step hydrothermal method and utilized as catalysts for UOR. Taking advantages of massive heterojunctions, tunable electronic configuration and hierarchical flower-like topological structure together, the optimized component demonstrates advanced UOR performance compared to pure NiS/Ni 3 S 4 and mono Co/Mo element doped NiS/Ni 3 S 4 , it delivers 147.1 mA cm −2 at 1.6 V vs. RHE and works for more than 18 h without decay. The assembly direct urea hydrogen peroxide fuel cell (DUHPFC) with Mo 0.05 , Co-NSH catalyst has an open-circuit voltage of 0.86 V and a maximum power density of 5.96 mW cm −2 at 27 °C. The work provides a promising avenue for exploiting efficient electrocatalysts on both multi-metal doping and heterojunction engineering for electrochemical energy conversion and sewage treatment applications.