Nickel-iron in the second coordination shell boost single-atomicsite iridium catalysts for high-performance urea electrooxidation

Single-atom catalysts(SACs)with high catalytic activity as well as great stability are demonstrating great promotion in electrocatalytic energy conversion,which is also a big challenge to achieve.Herein,we proposed a facile synthetic strategy to construct nickel-iron bimetallic hydroxide nanoribbon stabilized single-atom iridium catalysts(Ir-NiFe-OH),where the nickel-iron hydroxide nanoribbon not only can serve as good electronic conductor,but also can well stabilize and fully expose single-atom sites.Adopted as catalyst for urea oxidation reaction(UOR),it exhibited excellent UOR performance that it only needed a low operated potential of 1.38 V to achieve the current density of 100 mA·cm^(-2).In-situ Fourier transform infrared spectroscopy,X-ray absorption spectrum,and density functional theory calculations proved that Ir species are active centers and the existence of both Ni and Fe in the local structure of Ir atom can optimize the d-band center of Ir species,promoting the adsorption of intermediates and desorption of products for UOR.The hydrogen evolution reaction(HER)/UOR electrocatalytic cell demanded voltages of 1.46 and 1.50 V to achieve 50 and 100 mA·cm^(-2),respectively,which demonstrated a higher activity and better stability than those of conventional catalysts.This work opens a new avenue to develop catalysts for UORs with boosted activity and stability.

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 anioncompetitive reaction, precisely integrates two nickel-based materials into a heterostructure with Ni(OH)_(2) nanosheets and NiC_(2)O_(4) 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/cm^(2), and a slight Tafel slope of 40.7 m V/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.

Hollow Ni Mo-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 watercycling 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 threedimensional(3D)hollow Ni/NiMoN hierarchical structure with arrayed-sheet surface based on a onepot 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 at10 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.

Porous rod-like Ni_(2)P/Ni assemblies for enhanced urea electrooxidation

The urea oxidation reaction has attracted increasing attention.Here,porous rod-like Ni2P/Ni assemblies,which consist of numerous nanoparticle subunits with matching interfaces at the nanoscale have been synthesized via a simple phosphating approach.Density functional theory calculations and density of states indicate that porous rod-like Ni2P/Ni assemblies can significantly enhance the activity of chemical bonds and the conductivity compared with NiO/Ni toward the urea oxidation reaction.The optimal catalyst of Ni2P/Ni can deliver a low overpotential of 50 mV at 10 mA·cm−2 and Tafel slope of 87.6 mV·dec−1 in urea oxidation reaction.Moreover,the constructed electrolytic cell exhibits a current density of 10 mA·cm−2 at a cell voltage of 1.47 V and an outstanding durability in the two-electrode system.This work has provided a new possibility to fabricate metal phosphides-metal assemblies with advanced performance.

Phytate-Coordination Triggered Enrichment of Surface NiOOH Species on Nickel Foam for Efficient Urea Electrooxidation

Nickel(Ni)-based materials are promising electrocatalysts for the urea electrooxidation reaction, as the in situ formed NiOOH species on their surface during operation are catalytically active sites. In this work, phytate-coordinated Ni foam(PA-NF)is shown to deliver a high catalytic performance, with a potential as low as 1.38 V at 10m A/cm2, a Tafel slope as low as 64.1 mV/dec, and superior catalytic stability. Characterizations revealed that such a high performance was ascribed to the kinetic acceleration of surface reconstruction and the enriched NiOOH active species on the PA-NF surface owing to PA-coordination induced upshift of d-band center of Ni sites.Overall, a novel and simple strategy is provided for designing the efficient as well as universal Ni-based catalyst for the electrooxidation of urea, which can also be extended to other transition-metal-based systems.

Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as highly active and stable catalysts for urea electrooxidation

The developing high-efficiency urea fuel cells have an irreplaceable role in solving the increasingly severe environmental crisis and energy shortages.The sluggish six-electron dynamic anodic oxidation reaction is the bottleneck of the rapid progress of urea fuel-cell technology.To tackle this challenge,we select the NiCr bimetallic system due to the unique synergic effect between the Ni and the Cr.Moreover,better conductivity is assured using carbon nanotubes(CNTs)as the support.Most importantly,we use a simple hydrothermal method in catalyst preparation for easy scale-up at a low cost.The results show that the hybrid catalysts of NiCr_(x)-oxide-CNTs with different Ni/Cr ratios show much better catalytic performance in terms of active surface area and current density as compared to that of Ni-hydro-CNTs.The optimized NiCr_(2)-oxide-CNTs catalyst exhibits not only the largest electrochemically active surface area(ESA,50.7 m^(2) g^(−1))and the highest urea electrocatalytic current density(115.6 mA cm^(−2)),but also outstanding long-term stability.The prominent performance of the NiCr_(2)-oxide-CNTs catalyst is due to the combined effect of the improved charge transfer between Ni and Cr species,the large ESA,along with an elegant balance between the oxygen-defect sites and hydrophilicity.Moreover,we have proposed a synergistically enhanced urea catalytic mechanism.