Monodisperse nickel nanoparticles supported on multi-walls carbon nanotubes as an effective catalyst for the electro-oxidation of urea

Monodisperse nickel nanoparticles (NPs) were synthesized by reduction of nickel acetylacetonate with oleylamine in 1-octadecene, and uniformly dispersed on multi-walls carbon nanotubes via sonication. The electro-oxidation activity and stability of the catalyst were investigated by cyclic voltammetry and chronoamperometry respectively. Characterization results indicate that the 80%Ni/MWCNT (80% represent the wt% of Ni) catalyst shows the highest electro-oxidation activity (1866 mA cm^?2 mg^?1) and stability for urea electro-oxidation, which is higher than most reported Ni-based catalysts. In addition, the urea electro-oxidation process is a mixed control of diffusion and kinetic limitation, as demonstrated by the effects of scan rate on the peak current density and peak potential. Subsequently, the impact of diffusion for the different catalysts varies with the change of Ni loading, which is verified by experiments.     

Fabrication of electrospun nickel sulphide nanoparticles onto carbon nanofibers for efficient urea electro-oxidation in alkaline medium

To design and synthesize a noble-metal free electrocatalyst with increased efficiency and stability during urea electro-oxidation in alkaline solution is still an important challenge in the electrocatalytic field. In this work, carbon nanofibers were decorated with nickel sulphide nanoparticles [NiS@CNFs] through the electrospinning technique with subsequent heating into an argon atmosphere at 900 °C for 2 h. This formed nanomaterial was extensively characterized through X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), Raman spectroscopy and N₂ adsorption-desorption measurements. A conductive network of intertwined CNFs was clearly detected by FE-SEM analysis technique with varied diameters in the range of 0.6–1 μm. A highly porous nature could be suggested after incorporating NiS nanospecies resulting in increased specific surface area and valuable electrocatalytic activity for urea molecules electro-oxidation. The pore size distribution curves showed a decreased average pore diameter for NiS@CNFs nanocomposite by 2.53 folds when compared to that at CNFs. The electroactivity of NiS@CNFs nanomaterial for catalyzing urea electro-oxidation was investigated using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy measurements. Increased activity of this nanocatalyst was registered when urea molecules were added in increased concentrations into KOH solution. Lowered resistance values were also obtained describing the charge transfer process to confirm the feasibility of the studied reaction at NiS@CNFs surface. Moreover, its drawn chronoamperogram showed a stable performance during operation for long periods revealing a lowered catalytic decay. Accordingly, the aforementioned results of our fabricated nanomaterial could provide a good guide for fabricating suitable electrocatalysts for various electrocatalytic purposes.     

NiCo layered double hydroxide/hydroxide nanosheet heterostructures for highly efficient electro-oxidation of urea

Urea oxidation is an important reaction for direct urea fuel cells as well as hydrogen production and/or water remediation via electrolysis using urea-rich wastewater. The key to efficient urea oxidation is to explore a well-designed high-performing catalyst. Herein, NiCo layered double hydroxide/hydroxide (NiCo LDH/NiCo(OH)₂) microspheres composed of ultrasmall nanosheets have been grown on Ni foam by a solution method at room temperature. The NiCo LDH/NiCo(OH)₂ heterostructures have been confirmed by TEM and XRD analysis. The high activity with a small onset potential of 0.29 V vs. Hg/HgO is mainly attributed to the rich NiCo LDH-NiCo(OH)₂ interfaces and the bimetallic nature of the catalysts. The NiCo LDH/NiCo(OH)₂ heterostructures can be promising catalysts for urea oxidation and offer new insights into the design of high-performance nickel-based catalysts.     

Carbon supported Pd-Sn and Pd-Ru-Sn nanocatalysts for ethanol electro-oxidation in alkaline medium

Carbon supported Pd-Sn and Pd-Ru-Sn nanocatalysts were prepared by the chemical reduction method, using sodium borohydride and ethylene glycol mixture as the reducing agent. The catalytic activity towards ethanol electro-oxidation in alkaline medium was studied by cyclic, carbon monoxide stripping voltammetries and chronoamperometry. The current density obtained for the electro-oxidation was affected by varying ethanol concentration between 0.25 and 4 M. Raising ethanol concentration up to 3 M increased the coverage of the adsorbed ethoxy (CH₃CO_ads) species on the nanocatalyst surface, thus yielding an increase in current density. Pd-Sn/C displayed better electrocatalytic activity and stability towards poisoning than Pd-Ru-Sn/C and Pt-Ru/C (E-TEK Inc.) nanocatalysts. Transmission electron microscopy results showed that Pd-Sn and Pd-Ru-Sn nanoparticles were uniformly dispersed on carbon support. The average particle size of Pd-Sn was 7 ± 0.5 nm in diameter while for Pd-Ru-Sn was 6 ± 0.7 nm.     

Ag/Co3O4 as an effective catalyst for glycerol electro-oxidation in alkaline medium

Herein, we report two different modes of combustion synthesis to prepare Ag/Co3O4 for glycerol electro-oxidation reaction. The synthesis technique allows us to deposit and disperse cobalt on the surface of silver and understand the effect of silver-cobalt interactions on the electrocatalytic performance. The bimetallic AgCo catalyst was synthesized via conventional solution combustion (AgCo-SCS); in addition, a novel second wave solution combustion synthesis (AgCo-SWC) was also used to synthesize a sample with a higher degree of surface alloying between the elements. XRD results of AgCo-SWC show peak shifts in 2θ to indicate the formation of a solid solution alloy that could improve the structural and electronic characteristic of the material. In AgCo-SCS, smaller Ag particles are found to be more on the surface, while in AgCo-SWC, Ag particles were largely covered with cobalt particles. The electro-oxidation results indicate that AgCo-SWC has improved electrochemical properties in terms of higher oxidation current and lower onset potential, which makes it an ideal candidate for glycerol oxidation reaction.     

Self-supported iron-doped nickel sulfide as efficient catalyst for electrochemical urea and hydrazine oxidation reactions

The electrochemical oxidation of urea and hydrazine over self-supported Fe-doped Ni₃S₂/NF (Fe–Ni₃S₂/NF) nanostructured material is presented. Among the various reaction conditions Fe–Ni₃S₂/NF-2 prepared at 160 °C for 8 h using 0.03 mM Fe(NO₃)₃ shows the best results for the hydrazine and urea oxidation reactions. The potential values of 0.36, 1.39, and 1.59 V are required to achieve the current density of the 100 mA cm^?2 in 1 M hydrazine (Hz), 0.33 M urea, and 1 M KOH electrolyte, respectively. The onset potential in 1 M KOH, 0.33 M Urea +1 M KOH, and 1 M Hz + 1 M KOH values are 1.528, 1.306, and 0.176 respectively. The Fe–Ni₃S₂/NF-2 shows stable performance at 10 mA cm^?2 until 50 h and at 60 mA cm^?2 over the 25 h. A cell of PtC//Fe–Ni₃S₂/NF-2 requires the potential of 0.49, 1.46, and 1.59 V for the hydrogen production in 1 M Hz + 1 M KOH, 0.33 M Urea +1 M KOH, and 1 M KOH electrolyte, respectively, at a current density of 10 mA cm^?2, and almost 90% stable for the hydrogen production over the 80 h in all electrolytes. The improvement of the chemical kinetics of urea and hydrazine oxidation is due to the synergistic effect of the adsorption and fast electron transfer reaction on Fe–Ni₃S₂/NF-2. The doped Fe ion facilitates the fast electron transfer and the surface of Ni₃S₂ support to the urea and hydrazine molecule adsorption.     

Ni2Zn0.5Fe-LDH modified carbon paste electrode as an efficient electrocatalyst for water oxidation in neutral media

In this study, the ternary-component nickel-zinc-iron layered double hydroxides (Ni_xZn_yFe_z-LDHs) were synthesized by co-precipitation process and characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, diffuse reflectance spectroscopy and atomic absorption spectroscopy. The Ni_xZn_yFe_z-LDHs were then used as the efficient electrocatalyst materials to fabricate the modified carbon paste electrodes (CPEs) for water oxidation in neutral media. The effect of Zn²⁺ ions on the electrocatalytic activity of the ternary-component Ni_xZn_yFe_z-LDHs modified CPE was studied under suggested optimum condition. In this condition, the ternary-component Ni₂Zn_0.5Fe-LDH modified CPE displayed improved electrocatalytic activity in comparison with other ternary-component LDHs, and also binary-component LDHs, Ni₂Fe-LDH and ZnFe-LDH modified CPEs due to high crystallinity and low band gap energy which enhanced the conductivity of the synthesized LDH. Furthermore, the ternary-component Ni₂Zn_0.5Fe-LDH modified CPE had good structural stability with no significant deactivation of the electrocatalytic properties in neutral media after 13 h oxygen evolution reaction at room temperature.     

Enhanced electrocatalytic activity of NiO nanoparticles supported on graphite planes towards urea electro-oxidation in NaOH solution

Nickel oxide nanoparticles are fabricated onto graphite planes [NiO/Gt] by chemical precipitation of Ni(OH)₂ particles with consecutive calcination at 400 °C. The formed electrocatalysts are characterized using X-ray diffraction (XRD) and Transmission electron microscopy (TEM). TEM images demonstrate the deposition of NiO nanoparticles on graphite surface through their crystallite lattice fringes with spacing values of 2.45 Å (111), 2.10 Å (200) and 1.48 Å (220). The electrocatalytic activity of NiO/Gt electrocatalyst is examined towards urea electro-oxidation in NaOH solution using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Urea oxidation peak current density is observed at NiO/Gt electrocatalyst containing 15 wt% NiO [NiO/Gt?15] at a potential value of +640 mV (Ag/AgCl) with a current density value of 17.63 mA cm^?2. The loading amount of NiO in the prepared electrocatalyst significantly affects its electrocatalytic performance. NiO/Gt?15 exhibits the highest urea oxidation current density with the desired stability. The lower Tafel slope, charge transfer resistance and the higher exchange current density and diffusion coefficient values of urea molecules at NiO/Gt?15 surface elect its application as a promising electrocatalyst material during urea oxidation reaction in fuel cells.     

Performance of carbon nanofiber supported Pd-Ni catalysts for electro-oxidation of ethanol in alkaline medium

Carbon nanofibers (CNF) supported Pd-Ni nanoparticles have been prepared by chemical reduction with NaBH₄ as a reducing agent. The Pd-Ni/CNF catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical voltammetry analysis. TEM showed that the Pd-Ni particles were quite uniformly distributed on the surface of the carbon nanofiber with an average particle size of 4.0 nm. The electro-catalytic activity of the Pd-Ni/CNF for oxidation of ethanol was examined by cyclic voltammetry (CV). The onset potential was 200 mV lower and the peak current density four times higher for ethanol oxidation for Pd-Ni/CNF compared to that for Pd/C. The effect of an increase in temperature from 20 to 60 °C had a great effect on increasing the ethanol oxidation activity.     

PdBi/C electrocatalysts for ethanol electro-oxidation in alkaline medium

PdBi/C electrocatalysts (Pd:Bi atomic ratios of 95:05, 90:10, 80:20 and 70:30) were prepared by borohydride reduction using Pd(NO₃)₂.2H₂O and Bi(NO₃)₃.5H₂O as metal sources and Vulcan XC72 as support. The electrocatalysts were characterized by energy-dispersive X-ray analysis, X-ray diffraction, transmission electron microscopy and cyclic voltammetry. The activity for the ethanol electro-oxidation in alkaline medium was investigated at room temperature by chronoamperometry and the results were compared with Pt/C and PtBi/C electrocatalysts. PdBi/C (95:05) electrocatalyst showed a significant increase of performance for ethanol electro-oxidation compared to Pd/C and others PdBi/C electrocatalysts. The final current value after holding the cell at −0.4 V versus Ag/AgCl electrode for 30 min in alkaline medium for PdBi/C (95:05) electrocatalyst was about eleven times higher than the current value of the Pt/C electrocatalyst and 1.5 times higher than PtBi/C (50:50).     

Controlled synthesis of CrxPy-a/ComPn-b nanoarray on nickel foam as efficient electrocatalyst for overall urea splitting

Developing highly efficient bifunctional urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) catalysts for urea splitting to hydrogen are one of the strategies to cope with the energy crisis. Here, a series of Cr_xP_y-a/Co_mP_n-b composites were synthesized on Ni foam through hydrothermal and low-temperature phosphorization process for the first time. It is worth noting that Cr_xP_y-1/Co_mP_n-3@NF exhibited excellent UOR performance (1.331 V at 100 mA cm^?2) and HER performance (0.299 V at 100 mA cm^?2) in an electrolyte of 1 M KOH and 0.5 M urea due to the synergistic effect of Cr–Co. The Cr_xP_y-1/Co_mP_n-3@NF||Cr_xP_y-1/Co_mP_n-3@NF two-electrode system call for only 1.52 V to provide current density of 10 mA cm^?2, which is one of the best electrochemistry performances reported up to now. Experimental analysis show that the promoted electrochemistry performances is assigned to faster charge transfer rate, the exposure of more reaction site and better properties of metals. Density Functional theory (DFT) results demonstrate that the presence of the Co_mP_n material accelerates the kinetics of hydrogen production and the Cr_xP_y material improves the properties of metals for the electrode. The work provides a new idea to develop the environmentally friendly and low cost overall urea splitting catalyst with transition metals instead of noble metals.     

Promoting effect of CeO2 in the electrocatalytic activity of rhodium for ethanol electro-oxidation

The promoting effect of ceria in the electrocatalytic activity of rhodium for ethanol electro-oxidation in alkali media has been studied. Rh/C, CeO₂/C and RhCeO₂/C catalysts were synthesized and characterized by TEM, XRD, XPS, TG-MS, H₂-TPR and XAS. The electrocatalytic activity was studied by Cyclic Voltammetry (CV) and chronoamperometry. The onset potential of oxidation on RhCeO₂/C was shifted negatively as compared to that on Rh/C, despite ceria itself does not show any electrocatalytic activity. The promoting effect of ceria has been attributed to the improved rhodium dispersion, and differences in the oxidation state of rhodium between Rh/C and RhCeO₂/C were not found. The carbon support reduces rhodium species to Rh⁰, and also partially reduces ceria, during the samples preparation, and the surface of the carbon support is oxidised.     

Low-pressure-plasma-processed NiFe-MOFs/nickel foam as an efficient electrocatalyst for oxygen evolution reaction

A NiFe bimetallic metal organic framework (MOF) deposited on nickel foam and processed by low-pressure plasmas with 95%Ar+5%H₂, pure Ar, and 95%Ar+5%O₂ gases is used as an electrocatalyst for the oxygen evolution reaction. An alkaline solution (1 M KOH) with 95%Ar+5%H₂ plasma processed NiFe-MOFs/NF exhibits the best electrocatalytic performance with the lowest overpotential of 149 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1. Furthermore, electrical impedance spectroscopy and cyclic voltammetry show that after 95%Ar+5%H₂ plasma treatment, the interfacial impedance greatly reduces, and the electrical double-layer capacitance slightly increased.     

Phosphorus-doped nickel sulfides/nickel foam as electrode materials for electrocatalytic water splitting

Hydrogen production from electrocatalytic water splitting is viewed as one of the most promising methods to generate the clean energy. In this work, we successfully prepared an electrode material by growing phosphorus-doped Ni₃S₂ (PNi₃S₂) on nickel foam substrate (NF) under hydrothermal conditions. The phosphorus-doping has an obvious effect on the morphology of Ni₃S₂ on the surface of the nickel foam, which probably results in more active sites, higher electrical conductivity and faster mass transfer. The resulting electrode material displays excellent electrocatalytic activities and stability towards both OER (oxygen evolution reaction) and HER (hydrogen evolution reaction). A relatively low overpotential of 306 mV is required to reach the current density of 100 mA cm^?2 for OER and 137 mV at 10 mA cm^?2 for HER in 1 M KOH solution. When PNi₃S₂/NF was used in an electrolyzer for full water splitting, it can generate a current density of 10 mA cm^?2 at 1.47 V with excellent stability for more than 20 h.     

Needle-like CoP/rGO growth on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

In this work, CoP/NF is synthesized at different temperature (250 °C, 300 °C, 350 °C) (denoted as CoP/NF-T, T = 250, 300, 350). Then, CoP/NF-300 with the best performance towards hydrogen evolution reaction (HER), is used to synthesize compounds with different ratio of reduced graphene oxide (rGO) (CoP/rGO/NF-X, X (quality ratio of rGO/CoP) = 1,3,5). In terms of morphology, under the synergistic effect of rGO, uniform and dense CoP provides the possibility to increase the electrochemical area. While CoP/rGO/NF-3 shows the minimum overpotential of 136 mV to drive 50 mA/cm, and the smallest Tafel slope 135 mV/dec among as-synthesized materials. Furthermore, CoP/rGO/NF-3 has good stability during at least 25 h. These result can be construed as the large electrochemical active area, high conductivity and long-time stability.     

Hierarchical NiMo alloy microtubes on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

Developing efficient, non-noble electrocatalysts for hydrogen evolution reaction (HER) is of high significance for future energy supplement, but challenging. NiMo alloy is a non-noble-metal-based efficient catalyst for HER due to its appropriate hydrogen binding energy and excellent alkali corrosion resistance. Herein, for the first time, we report the preparation of radially aligned NiMo alloy microtubes on Ni foam (NiMo MT/NF). The synthesized NiMo alloy catalyst was composed of the Ni₁₀Mo phase; notably, this hierarchically structured material possessed abundant active sites and a high surface area, and exhibited efficient electronic transport properties. The NiMo MT/NF electrode exhibited a low overpotential of 119 mV at 10 mA/cm² in a base solution, which was 50 mV less than that of NiMo alloy nanoparticles on NF (169 mV).     

Defects-rich nickel nanoparticles grown on nickel foam as integrated electrodes for electrocatalytic oxidation of urea

Designing highly efficient, low cost and long-term stable electro-catalysts is the key step for the commercial applications of fuel cells. Electro-oxidation of urea, a hydrogen-rich fuel, is the anodic reaction of direct urea fuel cells. Herein, defects-rich nickel nanoparticles grown on nickel foam as integrated electrodes have been designed and easily fabricated by incomplete reduction of Ni(OH)₂. The Ni²⁺ defects coupled with oxygen vacancies are proposed to be mainly present in the form of amorphous NiO_x, which is the island phase in the metallic nickel nanoparticles and confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. The synergistic effect between metallic metal with high conductivity and numerous defects with good affinity to O contributes to the high catalytic activity towards oxidation of urea with an onset potential of 0.35 V vs Hg/HgO in 2 M KOH +0.33 M urea. Additionally, the defects-rich nickel nanoparticles present good long-term stability.     

MoS2/Mo2TiC2Tx supported Pd nanoparticles as an efficient electrocatalyst for hydrogen evolution reaction in both acidic and alkaline media

Development of electrocatalytic hydrogen production technology is the key to solving environmental and energy problems. Two-dimensional material Mo₂TiC₂T_x (T_x = –OH, –F) has shown great potential in electrocatalytic hydrogen evolution because of its excellent conductivity and hydrophilicity. However, due to the lack of sufficient active sites of Mo₂TiC₂T_x itself, its practical applications in electrocatalytic hydrogen evolution are limited. In this work, a highly-efficient hydrogen evolution electrocatalyst, namely Pd@MoS₂/Mo₂TiC₂T_x, is prepared through a simple pyrolysis method. In such a composite, the MoS₂ nanoflowers hybridized with the ammonia-treated Mo₂TiC₂T_x (MoS₂/Mo₂TiC₂T_x) are used as a substrate for loading a small number of Pd nanoparticles (4.27 at.%). Notably, the introduction of Pd nanoparticles into MoS₂/Mo₂TiC₂T_x provides abundant active sites for the hydrogen evolution reaction, improves the conductivity of the electrocatalyst, speeds up the adsorption and desorption of hydrogen, and induces a synergistic effect with the MoS₂. As a result, the Pd@MoS₂/Mo₂TiC₂T_x catalyst exhibits excellent electrocatalytic performance and remarkable stability in both acidic and alkaline media. In a 0.5 mol/L H₂SO₄ electrolyte, the overpotential of Pd@MoS₂/Mo₂TiC₂T_x was 92 mV with a Tafel slope of 60 mV/dec at a current density of 10 mA/cm². Meanwhile, the catalyst displayed an overpotential of 100 mV associated with a Tafel slope of 80 mV/dec at the current density of 10 mA/cm² in a 1 mol/L KOH electrolyte. This work shows the great potential of using Mo₂TiC₂T_x-based material in the field of electrocatalysis.     

Enhanced performance of urea electro-oxidation in alkaline media on PtPdNi/C,PtNi/C,and Ni/C catalysts synthesized by one-pot reaction from organometallic precursors

In this work, one-pot synthesis of metal nanoparticles from organometallic precursors namely Pt₂(dba)₃, Pd(dba)₂ and Ni(cod)₂ employing octylamine as stabilizer and carbon Vulcan XC-72R as support was carried out to obtain Ni/C, PtNi/C, and PtPdNi/C catalysts. Transmission Electron Microscopy (TEM) analysis of the as-synthesized materials shows semi-spherical particles with good dispersion on the carbon Vulcan having sizes of 3.2, 2.5 and 2.2 nm for Ni, PtNi, and PtPdNi, respectively. X-ray diffraction (XRD) confirms the presence of Pt, Pd, and Ni FCC crystal structure and reveals the formation PtNi and PtPdNi solid solutions. Additionally, XPS Pt4f core-level spectra shown a shift to lower binding energy at the alloy formation due to the addition of nickel and palladium evidencing a solid solution. The tri-metallic PtPdNi/C catalyst exhibits better performance than PtNi/C and Ni/C catalysts toward urea oxidation reaction in alkaline conditions suggesting an activity enhancement due to the oxidized Ni species from NiOOH. Using differential electrochemical mass spectroscopy (DEMS), it has been demonstrated that species such as NH₃ (m/z = 17), CO₂ -or N₂O- (m/z = 44) and NO (m/z = 30) are generated during polarization at 4 mV s^?1. Concomitant pollution control, this study demonstrated that urea could be used as a source of protons for energy generation in fuel cells systems.     

Nitrogen-doped mesoporous carbon nanosheet network entrapped nickel nanoparticles as an efficient catalyst for electro-oxidation of glycerol

The design and synthesis of nitrogen-doped (N-doped) carbon nanosheets network offer a tremendous electro-catalytic activity due to their high surface area and tunable porous structures for the development of direct alcohol fuel cells (DAFCs). In this work, nickel (Ni) nanoparticles entrapped onto the N-doped mesoporous carbon nanosheets network such as Ni–C-1, Ni–C-2, and Ni–C-3 are fabricated and characterized using SEM, TEM, XRD, XPS, and electrochemical methods. The physicochemical characterization of synthesized nanocomposite material confirmed that sodium chloride (NaCl) plays a major role in the formation of porous nanostructure during carbonization process. Under optimized conditions, all the above carbon samples are tested as potential electro-catalyst towards the electro-oxidation of glycerol for DAFCs application. Among them, the Ni–C-2 catalyst displays enhanced catalytic current density (~2.6 mA) and low onset oxidation potential (0.11 V) for electro-oxidation of glycerol than the Ni–C-1, and Ni–C-3 catalysts in alkaline electrolyte. In addition, the mesoporous carbon network structure containing Ni nanoparticles delivers substantial longer durability/robustness when compared to the conventional Pt/C-catalyst towards stable- and efficient electro-oxidation of glycerol for the first time. Thus, the obtained electro-catalytic performance revealed that Ni–C-2 is considered as a promising earth abundant non-noble low-cost catalyst material for DAFCs application.