Search Results (21)

Transition metal-based catalysts designed for efficient urea oxidation reactions (UOR) are essential for hydrogen production via urea-assisted water electrolysis. A series of amorphous nickel–cobalt boride catalysts supported on nickel foam were in situ synthesized via a stepwise chemical deposition method (SCDM). The systematic investigation focused on the relationships between synthesis parameters (deposition cycles, reactant feed ratio), morphological characteristics, and UOR performance. Notably, the optimized Co-NiB@NF catalyst exhibits a porous hierarchical architecture composed of metallic nanoparticles encapsulated by surface-wrinkled nanosheets, forming abundant exposed active sites. Electrochemical measurements demonstrate that this catalyst requires a low cell potential of 1.29 V to achieve a current density of 10 mA cm⁻². Moreover, it maintains 83% of the initial current density after 10 h of continuous electrolysis, highlighting its superior durability. The structural-property relationship revealed here provides valuable insights into the rational design of efficient amorphous boride catalysts for urea-assisted hydrogen production. Full article

In this study, a sulfur-doped cobalt–iron catalyst (CoFeS/NF) was synthesized on a nickel foam (NF) substrate via a facile one-step electrodeposition method, and its performance in urea electrolysis for hydrogen production was systematically investigated. Sulfur doping induced significant morphology optimization, forming a highly dispersed nanosheet structure, which enhanced the specific surface area increase by 1.9 times compared with the undoped sample, exposing abundant active sites. Meanwhile, the introduction of sulfur facilitated electron redistribution at the surface modulated the valence states of nickel and cobalt, promoted the formation of high-valence Ni³⁺/Co³⁺, optimized the adsorption energy of the reaction intermediates, and reduced the charge transfer resistance. Electrochemical evaluations revealed that CoFeS/NF achieves a current density of 10 mA cm⁻² at a remarkably low potential of 1.18 V for the urea oxidation reaction (UOR), outperforming both the undoped catalyst (1.24 V) and commercial RuO₂ (1.35 V). In addition, the catalyst also exhibited excellent catalytic activity and long-term stability in the total urea decomposition process, which was attributed to the amorphous structure and the synergistic enhancement of corrosion resistance by sulfur doping. This study provides a new idea for the application of sulfur doping strategy in the design of multifunctional electrocatalysts, which promotes the coupled development of urea wastewater treatment and efficient hydrogen production technology. Full article

Urea shows promise as an alternative substrate to water oxidation in electrolyzers, and replacing OER with the Urea Oxidation Reaction (UOR, theoretical potential of 0.37 V vs. RHE) can significantly increase hydrogen production efficiency. Additionally, the decomposition of urea can help reduce environmental pollution. This paper improves the inherent activity of catalytic materials through morphology and electronic modulation by incorporating tungsten (W), which accelerates electron transfer, enhances the electronic structure of neighboring atoms to create a synergistic effect, and regulates the adsorption process of active sites and intermediates. NiCoW catalytic materials with an ultra-thin nanosheet structure were prepared using an ultrasonic-assisted NaBH₄ reduction method. The results show that during the OER process, NiCoW catalytic materials have a potential of only 1.53 V at a current density of 10 mA/cm², while the UOR process under the same conditions requires a lower potential of 1.31 V, demonstrating superior catalytic performance. In a mixed electrolyte of 1 M KOH and 0.5 M urea, overall water splitting also shows excellent performance. Therefore, the designed NiCoW electrocatalyst, with its high catalytic activity, provides valuable insights for enhancing the efficiency of water electrolysis for hydrogen production and holds practical research significance. Full article

Urea stands as a ubiquitous environmental contaminant. However, not only does urea oxidation reaction technology facilitate energy conversion, but it also significantly contributes to treating wastewater rich in urea. Furthermore, urea electrolysis has a significantly lower theoretical potential (0.37 V) compared to water electrolysis (1.23 V). As an electrochemical reaction, the catalytic efficacy of urea oxidation is largely contingent upon the catalyst employed. Among the plethora of urea oxidation electrocatalysts, nickel-based compounds emerge as the preeminent transition metal due to their cost-effectiveness and heightened activity in urea oxidation. Ni(OH)₂ is endowed with manifold advantages, including structural versatility, facile synthesis, and stability in alkaline environments. This review delineates the recent advancements in Ni(OH)₂ catalysts for electrocatalytic urea oxidation reaction, encapsulating pivotal research findings in morphology, dopant incorporation, defect engineering, and heterogeneous architectures. Additionally, we have proposed personal insights into the challenges encountered in the research on nickel hydroxide for urea oxidation, aiming to promote efficient urea conversion and facilitate its practical applications. Full article

This study presents the successful synthesis of a cesium–nickel–vanadium fluoride (CsNiVF₆) pyrochlore nano-sheet catalyst via solid-phase synthesis and its electrochemical performance in green hydrogen production through urea electrolysis in alkaline media. The physicochemical characterizations revealed that the CsNiVF₆ exhibits a pyrochlore-type structure consisting of a disordered cubic corner-shared (Ni, V)F₆ octahedra structure and nano-sheet morphology with a thickness ranging from 10 to 20 nm. Using the CsNiVF₆ catalyst, the electrochemical analysis, conducted through cyclic voltammetry, demonstrates a current mass activity of ~1500 mA mg⁻¹, recorded at 1.8 V vs. RHE, along with low-resistance (3.25 ohm) charge transfer and good long-term stability for 0.33 M urea oxidation in an alkaline solution. Moreover, the volumetric hydrogen production rate at the cathode (bare nickel foam) is increased from 12.25 to 39.15 µmol/min upon the addition of 0.33 M urea to a 1.0 KOH solution and at a bias potential of 2.0 V. The addition of urea to the electrolyte solution enhances hydrogen production at the cathode, especially at lower voltages, surpassing the volumes produced in pure 1.0 M KOH solution. This utilization of a CsNiVF₆ pyrochlore nano-sheet catalyst and renewable urea as a feedstock contributes to the development of a green and sustainable hydrogen economy. Overall, this research underscores the potential use of CsNiVF₆ as a cost-effective nickel-based pyrochlore electrocatalyst for advancing renewable and sustainable urea electrolysis processes toward green hydrogen production. Full article

The production of green hydrogen using water electrolysis is widely regarded as one of the most promising technologies. On the other hand, the oxygen evolution reaction (OER) is thermodynamically unfavorable and needs significant overpotential to proceed at a sufficient rate. Here, we outline important structural and chemical factors that affect how well a representative nickel ferrite-modified graphene oxide electrocatalyst performs in efficient water splitting applications. The activities of the modified pristine and graphene oxide-supported nickel ferrite were thoroughly characterized in terms of their structural, morphological, and electrochemical properties. This research shows that the NiFe₂O₄@GO electrode has an impact on both the urea oxidation reaction (UOR) and water splitting applications. NiFe₂O₄@GO was observed to have a current density of 26.6 mA cm⁻² in 1.0 M urea and 1.0 M KOH at a scan rate of 20 mV s⁻¹. The Tafel slope provided for UOR was 39 mV dec⁻¹, whereas the GC/NiFe₂O₄@GO electrode reached a current of 10 mA cm⁻² at potentials of +1.5 and −0.21 V (vs. RHE) for the OER and hydrogen evolution reaction (HER), respectively. Furthermore, charge transfer resistances were estimated for OER and HER as 133 and 347 Ω cm², respectively. Full article

Energy shortage and environmental pollution have become the most serious problems faced by human beings in the 21st century. Looking for advanced clean energy technology to achieve sustainable development of the ecological environment has become a hot spot for researchers. Nitrogen-based substances represented by urea are environmental pollutants but ideal energy substances. The efficiency of urea-based energy conversion technology mainly depends on the choice of catalyst. The development of new catalysts for urea oxidation reaction (UOR) has important application value in the field of waste energy conversion and pollution remediation based on UOR. In this work, four metal–organic framework materials (MOFs) were synthesized using ultrasound (NiCo-UMOFs) and hydrothermal (NiCo-MOFs, Ni-MOFs and Co-MOFs) methods to testify the activity toward UOR. Materials prepared using the hydrothermal method mostly form large and unevenly stacked block structures, while material prepared using ultrasound forms a layer-by-layer two-dimensional and thinner structure. Electrochemical characterization shows NiCo-UMOFs has the best electrocatalytic performance with an onset potential of 0.32 V (vs. Ag/AgCl), a Tafel slope of 51 mV dec⁻¹, and a current density of 13 mA cm⁻² at 0.5 V in a 1 M KOH electrolyte with 0.7 M urea. A prolonged urea electrolysis test demonstrates that 45.4% of urea is removed after 24 h. Full article

The very slow anodic oxygen evolution reaction (OER) greatly limits the development of large-scale hydrogen production via water electrolysis. By replacing OER with an easier urea oxidation reaction (UOR), developing an HER/UOR coupling electrolysis system for hydrogen production could save a significant amount of energy and money. An Al-doped cobalt ferrocyanide (Al-Co₂Fe(CN)₆) nanocube array was in situ grown on nickel foam (Al-Co₂Fe(CN)₆/NF). Due to the unique nanocube array structure and regulated electronic structure of Al-Co₂Fe(CN)₆, the as-prepared Al-Co₂Fe(CN)₆/NF electrode exhibited outstanding catalytic activities and long-term stability to both UOR and HER. The Al-Co₂Fe(CN)₆/NF electrode needed potentials of 0.169 V and 1.118 V (vs. a reversible hydrogen electrode) to drive 10 mA cm⁻² for HER and UOR, respectively, in alkaline conditions. Applying the Al-Co₂Fe(CN)₆/NF to a whole-urea electrolysis system, 10 mA cm⁻² was achieved at a cell voltage of 1.357 V, which saved 11.2% electricity energy compared to that of traditional water splitting. Density functional theory calculations demonstrated that the boosted UOR activity comes from Co sites with Al-doped electronic environments. This promoted and balanced the adsorption/desorption of the main intermediates in the UOR process. This work indicates that Co-based materials as efficient catalysts have great prospects for application in urea electrolysis systems and are expected to achieve low-cost and energy-saving H₂ production. Full article

Urea, a prevalent component found in wastewater, shows great promise as a substrate for energy-efficient hydrogen production by electrolysis. However, the slow kinetics of the anodic urea oxidation reaction (UOR) significantly hamper the overall reaction rate. This study presents the design and controlled fabrication of hierarchically structured nanomaterials as potential catalysts for UOR. The prepared MnO₂@NiCo-LDH hybrid catalyst demonstrates remarkable improvements in reaction kinetics, benefiting from synergistic enhancements in charge transfer and efficient mass transport facilitated by its unique hierarchical architecture. Notably, the catalyst exhibits an exceptionally low onset potential of 1.228 V and requires only 1.326 V to achieve an impressive current density of 100 mA cm⁻², representing a state-of-the-art performance in UORs. These findings highlight the tremendous potential of this innovative material designing strategy to drive advancements in electrocatalytic processes. Full article

Considering the renewable electricity production using sustainable technologies, such as solar photovoltaics or wind turbines, it is essential to have systems that allow for storing the energy produced during the periods of lower consumption as well as the energy transportation through the distribution network. Despite hydrogen being considered a good candidate, it presents several problems related to its extremely low density, which requires the use of very high pressures to store it. In addition, its energy density in volumetric terms is still clearly lower than that of most liquid fuels. These facts have led to the consideration of ammonia as an alternative compound for energy storage or as a carrier. In this sense, this review deals with the evaluation of using green ammonia for different energetic purposes, such as an energy carrier vector, an electricity generator and E-fuel. In addition, this study has addressed the latest studies that propose the use of nitrogen-derived compounds, i.e., urea, hydrazine, ammonium nitrate, etc., as alternative fuels. In this study, the possibility of using other nitrogen-derived compounds, i.e., an update of the ecosystem surrounding green ammonia, has been assessed, from production to consumption, including storage, transportation, etc. Additionally, the future challenges in achieving a technical and economically viable energy transition have been determined. Full article

To address a decarbonized cement production process (DCPP), a calcium looping process is connected to an industrial cement production process (CPP) for capturing CO₂ by 93.5~96%. Since the captured CO₂ purity is up to 99.9 wt%, the carbon capture and utilization (CCU) process is connected to generate the additional products of urea and methanol. An integration of DCPP and CCU, named the DCPP-based polygeneration system, is being developed for three scenarios. To meet the power demand for producing high-purity hydrogen and oxygen, Scenario 1 adopts water electrolysis and the full green electricity grid; Scenario 2 adopts the Cu-Cl thermochemical cycle and the partial green electricity grid; and Scenario 3 adopts water electrolysis and the heat recovery steam generator (HRSG). Through the techno-economic analysis and comparisons, the CO₂ avoided costs of three scenarios are estimated between 16.53 and 21.42 USD/ton, which are lower than the conventional DCPP of around 40 USD/ton. It is due to the fact that the polygeneration scheme could reduce the LCOP (levelized cost of producing 1 ton of clinker) due to the production of valorized products. It is noted that Scenario 2 is superior to other scenarios since the RenE2P cost in Scenario 2 is lower than it is in Scenario 1 and the captured CO₂ rate in Scenario 2 is lower than it is in Scenario 3. Full article

Water electrolysis represented a promising avenue for the large-scale production of high-purity hydrogen. However, the high overpotential and sluggish reaction rates associated with the anodic oxygen evolution reaction (OER) posed significant obstacles to efficient water splitting. To tackle these challenges, the urea oxidation reaction (UOR) emerged as a more favorable thermodynamic alternative to OER, offering both the energy-efficient hydrogen evolution reaction (HER) and the potential for the treating of urea-rich wastewater. In this work, a two-step methodology comprising nanowire growth and phosphating treatment was employed to fabricate Cu₃P nanowires on Cu foam (Cu₃P-NW/CF) catalysts. These novel catalytic architectures exhibited notable efficiencies in facilitating both the UOR and HER in alkaline solutions. Specifically, within urea-containing electrolytes, the UOR manifested desirable operational potentials of 1.43 V and 1.65 V versus the reversible hydrogen electrode (vs. RHE) to reach the current densities of 10 and 100 mA cm⁻², respectively. Concurrently, the catalyst displayed a meager overpotential of 60 mV for the HER at a current density of 10 mA cm⁻². Remarkably, the two-electrode urea electrolysis system, exploiting the designed catalyst as both the cathode and anode, demonstrated an outstanding performance, attaining a low cell voltage of 1.79 V to achieve a current density of 100 mA cm⁻². Importantly, this voltage is preferable to the conventional water electrolysis threshold in the absence of urea molecules. Moreover, our study shed light on the potential of innovative Cu-based materials for the scalable fabrication of electrocatalysts, energy-efficient hydrogen generation, and the treatment of urea-rich wastewater. Full article

In this study, noble metal-free Co(OH)F and CoP nanorod electrocatalysts were prepared and explored as bifunctional oxygen electrodes (BOE) in anion exchange membrane-unitized regenerative fuel cells (AEM-URFCs). A CoP nanorod was synthesized from Co(OH)F via the hydrothermal treatment of cobalt nitrate, ammonium fluoride, and urea, followed by phosphorization. The crystal structures, surface morphologies, pore distributions, and elemental statuses of the obtained catalysts were analyzed to identify the changes caused by the incorporation of fluorine and phosphorus. The presence of F and P was confirmed through EDS and XPS analyses, respectively. Using these catalysts, the AEM-based URFCs were operated with hydrogen and oxygen in the fuel cell mode and pure water in the electrolysis mode. In addition, the electrocatalytic activities of the catalysts were evaluated using cyclic voltammetry and electrochemical impedance spectroscopy. In the AEM-URFC test, the CoP catalyst in the BOE delivered the best performance in the fuel cell mode (105 mA cm⁻² at 0.3 V), and Co(OH)F was suitable for the water electrolyzer mode (30 mA cm⁻² at 2.0 V). CoP and Co(OH)F exhibited higher round trip efficiency (RTE) and power densities than the conventional Co₃O₄ catalyst. Full article

Photoelectrochemical (PEC) urea splitting is of great significance for urea wastewater remediation and hydrogen production with low energy consumption simultaneously. Nickel hydroxides as electrocatalysts have been widely investigated for urea electrolysis. However, it is an open question how to synthesize highly catalytic Ni(OH)₂ for the PEC urea splitting. Herein, we take advantage of the instability of metal–organic frameworks (MOFs) to perform an in situ synthesis of Ni(OH)₂ catalysts on the surface of TiO₂ nanorod arrays. This transformed Ni(OH)₂ (T-Ni(OH)₂) possesses a superior PEC catalytic activity for water/urea splitting in comparison to the Ni(OH)₂ prepared by the impregnation method. The in situ transition of a Ni-MOF is accomplished through an electrochemical treatment under AM1.5G illumination in a KOH-and-urea mixed electrolyte. The specific transition mechanism of Ni-MOFs is the substitution of ligands with OH⁻ in a 1 M KOH electrolyte and the successive phase transition. The T-Ni(OH)₂@TiO₂ photoanode delivers a high photocurrent density of 1.22 mA cm⁻² at 1.23 V_RHE, which is 4.7 times that of Ni(OH)₂@TiO₂ prepared with the impregnation method. The onset potential of T-Ni(OH)₂@TiO₂ is negatively shifted by 118 mV in comparison to TiO₂. Moreover, the decline of photocurrent during the continuous test can be recovered after the electrochemical and light treatments. Full article

Urea electrolysis is regarded as a prospective method for energy-saving hydrogen production. However, the practical application of this technology is limited by the lack of high-performance bifunctional catalysts for hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Herein, a heterostructure catalyst composed of NiFe layered double hydroxide (LDH) and sulfides (NiFe LDH-NiFeS_x/NF) catalysts is prepared via a simple one-step hydrothermal approach. Remarkably, the prepared NiFe LDH-NiFeS_x/NF required 138 mV and 1.34 V to achieve 10 mA cm⁻² for HER and UOR in 1 M KOH and 0.33 M urea, respectively. Furthermore, when NiFe LDH-NiFeS_x/NF is used as a cathode for urea electrolysis, only 1.44 V is required at 10 mA cm⁻², which is much lower than the 1.53 V needed for overall water splitting. Full article