Direct evidence of the mechanism for the electro-oxidation of urea on Ni(OH)2 catalyst in alkaline medium

Electrochemical Oxidation of Urea on NiCu Alloy Nanoparticles Decorated Carbon Nanofibers

Bimetallic Cu3.8Ni alloy nanoparticles (NPs)-anchored carbon nanofibers (composite NFs) were synthesized using a simple electrospinning machine. XRD, SEM, TEM, and TGA were employed to examine the physiochemical characteristics of these composite NFs. The characterization techniques proved that Cu3.8Ni alloy NPs-anchored carbon NFs were successfully fabricated. Urea oxidation (UO) processes as a source of hydrogen and electrical energy were investigated using the fabricated composite NFs. The corresponding onset potential of UO and the oxidation current density (OCD) were measured via cyclic voltammetry as 380 mV versus Ag/AgCl electrode and 98 mA/cm2, respectively. Kinetic study indicated that the electrochemical oxidation of urea followed the diffusion controlled process and the reaction order is 0.5 with respect to urea concentration. The diffusion coefficient of urea using the introduced electrocatalyst was found to be 6.04 × 10−3 cm2/s. Additionally, the composite NFs showed steady state stability for 900 s using chronoamperometry test.

Tungsten Carbide and Cobalt Modified Nickel Nanoparticles Supported on Multiwall Carbon Nanotubes as Highly Efficient Electrocatalysts for Urea Oxidation in Alkaline Electrolyte

The tungsten carbide and cobalt-modified Ni-based catalyst [Ni-Co-WC/multiwall carbon nanotubes (MWCNTs)], synthesized through a sequential impregnation method, was evaluated for the urea electrooxidation in alkaline electrolyte to reduce the overpotential and increase the current density simultaneously. The as-prepared Ni-Co-WC/MWCNTs catalyst was characterized using scanning electron microscopy-EDX, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Characterization results indicate that Ni, Co, and WC nanoparticles are uniformly distributed on the MWCNTs. For the Ni-Co-WC/MWCNT electrode, the maximum current density for urea electrooxidation is more than 4 times higher than that of the Ni/C catalyst, with a reduction of 120 mV in the onset overpotential. In addition, the Ni-Co-WC/MWCNTs catalyst also shows an enhanced catalytic stability with a continuous higher current density under steady-state conditions.

Recent Advances in the Electro-Oxidation of Urea for Direct Urea Fuel Cell and Urea Electrolysis

This paper provides an overview of recent advances in urea electro-oxidation. Urea sources are abundant from human urine, urea-containing wastewater, and industrial urea, thus becoming an attractive option as anodic fuel for the application in direct urea fuel cells (DUFCs). Besides, as a hydrogen-rich chemical fuel, urea can also be electrolyzed to produce hydrogen for energy storage in the near future. The exact mechanisms of urea decomposition are pretty different in alkaline or neutral mediums and are separately discussed in detail. More importantly, the development of anodic electro-catalysts is of great significance for improving the electrochemical performance of both DUFCs and urea electrolysis cells, which is systematically summarized in our review. Challenges and prospects on the future development of urea electro-oxidation are particularly proposed.

Effect of Urine Compounds on the Electrochemical Oxidation of Urea Using a Nickel Cobaltite Catalyst: An Electroanalytical and Spectroscopic Investigation

Cyclic voltammetry (CV) and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to investigate the effect of major urine compounds on the electro-oxidation activity of urea using a nickel cobaltite (NiCo₂O₄ ) catalyst. As a substrate, carbon paper exhibited better benchmark potential and current values compared with stainless steel and fluorine-doped tin oxide glass, which was attributed to its greater active surface area per electrode geometric area. CV analysis of synthetic urine showed that phosphate, creatinine, and gelatin (i.e., proteins) had the greatest negative effect on the electro-oxidation activity of urea, with decreases in peak current up to 80% compared to that of a urea-only solution. Further investigation of the binding mechanisms of the deleterious compounds using in situ ATR-FTIR spectroscopy revealed that urea and phosphate weakly bind to NiCo₂O₄ through hydrogen bonding or long-range forces, whereas creatinine interacts strongly, forming deactivating inner-sphere complexes. Phosphate is presumed to disrupt the interaction between urea and NiCo₂O₄ by serving as a hydrogen-bond acceptor in place of catalyst sites. The weak binding of urea supports the hypothesis that it is oxidized through an indirect electron transfer. Outcomes of this study contribute to the development of electrolytic systems for treating source-separated urine.

Ammonia Generation via a Graphene-Coated Nickel Catalyst

A novel graphene-coated Ni electrode was developed in this investigation to improve corrosion resistance while unexpectedly enhancing the ammonia generation rate in the electrochemically induced urea to ammonia (eU2A) process, which is an electrochemical onsite ammonia generation method. The development of the electrode is crucial for the eU2A reactions since in the ammonia generation process, the concentration of ammonia is inevitably high on the surface of the electrode, leading to severe corrosion of the electrode and the loss of generated ammonia as well. In this paper, the graphene was derived from raw coal by using the chemical vapor deposition method and self-lifted onto a Ni electrode to form a protective layer for corrosion prevention. Transmission electron microscopy showed the synthesized graphene had few-layers and Raman spectroscopy indicated that the coating of graphene was stable during the eU2A reaction. As a result, the ammonia corrosion of the Ni electrode was dramatically reduced by ~20 times with the graphene coating method. More importantly, a higher ammonia generation rate (~2 times) was achieved using the graphene-coated Ni working electrode compared to a bare Ni electrode in the eU2A process.

A hydrothermally grown CdS nanograin-sensitized 1D Zr:α-Fe2O3/FTO photoanode for efficient solar-light-driven photoelectrochemical performance

A CdS nanograin sensitized 1D Zr:Fe₂O₃ nanorod arrays nanostructure was hydrothermally synthesized and showed an excellent photoelectrochemical performance due to the combined effect of light absorption in CdS and effective charge transport in one dimensional Zr:Fe₂O₃ nanorod arrays.

Facile preparation of three-dimensional Ni(OH)2/Ni foam anode with low cost and its application in a direct urea fuel cell

The nano-sheet Ni(OH)₂/Ni foam electrode exhibits superior catalytic activity and stability towards urea electro-oxidation and shows a good fuel cell performance.

A practical anodic oxidation of aminofurazans to azofurazans: an environmentally friendly route

Nickel oxyhydroxide anode is effective green tools for the electrooxidation of aminofurazans to azofurazans inca. 1% aqueous NaOH at 20 °C. The reaction is simple and convenient, eliminating the use of expensive and toxic organic or inorganic oxidants.

Facile synthesis of mesoporous spinel NiCo₂O₄ nanostructures as highly efficient electrocatalysts for urea electro-oxidation

Mesoporous spinel nickel cobaltite (NiCo2O4) nanostructures were synthesized via a facile chemical deposition method coupled with a simple post-annealing process. The physicochemical properties were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS) and nitrogen sorption measurements. The electrocatalytic performances were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) tests. The obtained NiCo₂O₄ materials exhibit typical agglomerate mesoporous nanostructures with a large surface area (190.1 m(2) g(-1)) and high mesopore volume (0.943 cm(3) g(-1)). Remarkably, the NiCo₂O₄ shows much higher catalytic activity, lower overpotential, better stability and greater tolerance towards urea electro-oxidation compared to those of cobalt oxide (Co₃O₄) synthesized by the same procedure. The NiCo₂O₄ electrode delivers a current density of 136 mA cm(-2) mg(-1) at 0.7 V (vs. Hg/HgO) in 1 M KOH and 0.33 M urea electrolytes accompanied with a desirable stability. The impressive electrocatalytic activity is largely ascribed to the high intrinsic electronic conductivity, superior mesoporous nanostructures and rich surface Ni active species of the NiCo₂O₄ materials, which can largely boost the interfacial electroactive sites and charge transfer rates for urea electro-oxidation, indicating promising applications in future wastewater remediation, hydrogen production and fuel cells.