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.
► CuO nanosheet arrays on nickel foam are prepared via a facile one-step method. ► Nanosheet array CuO exhibits a high specific capacitance of 569
F
g
−1 in KOH solution. ► CuO nanosheet arrays ...electrode has porous structure and high utilization of CuO.
CuO nanosheet arrays freely standing on nickel foam are prepared via a template-free growth method. The morphology of CuO nanosheet arrays is examined by scanning and transmission electron microscopy and the phase structure of nanosheets is analyzed by X-ray diffraction spectroscopy. The supercapacitance of CuO nanosheet arrays is investigated by cyclic voltammetry, galvanostatic charge/discharge test and electrochemical impedance spectroscopy. The results show that the array of CuO nanosheets forms a uniform film of around 5
μm in thickness on nickel foam skeleton. The film is composed of clusters of arrays of nanosheets with a thickness up to around 150
nm. The CuO nanosheet arrays exhibit a specific capacitance of 569
F
g
−1 at a current density of 5
mA
cm
−2 in 6.0
mol
dm
−3 KOH electrolyte. The capacitance loss is less than 17.5% after 500 charge/discharge cycles at 10
mA
cm
−2 and with columbic efficiency higher than 93%.
•Ni-Co NWAs electrode was fabricated by polycarbonate template.•Ni-Co NWAs electrode with 10% of Co molar ratio shows best catalytic activity.•Direct urea/H2O2 fuel cell shows high output performance ...with Ni-Co NWAs anode.
Nickel-cobalt nanowire arrays (Ni-Co NWAs) electrode is prepared by one-step galvanostatic electrodeposition with a polycarbonate membrane as the template. By adjusting the Co proportion in the Ni and Co bath solution into 10%, the optimal Ni-Co NWAs electrode in terms of relatively lower onset potential and highest current density towards urea electro-oxidation is obtained. Its catalytic performance is investigated by constructing single direct urea/hydrogen peroxide (H2O2) fuel cell. Results show that a peak power density of 7.4mWcm−2 and an open circuit voltage of 0.92V are achieved at room temperature when 9.0molL−1 KOH and 0.33molL−1 urea are used as the anolyte, H2SO4 and H2O2 as the catholyte. Additionally, the urea/H2O2 fuel cell also demonstrates excellent stability during short term duration test.
Display omitted
•Both indirect oxidation and direct urea electro-oxidation paths exist on nickel.•Rate determining step depends on polarization potential and KOH concentration.•Nickel catalyst is ...poisoned by the CO2 intermediate.
Urea electro-oxidation reaction in alkaline medium is systematically analyzed by electrochemical impedance spectroscopy (EIS). The effects of polarization potential and KOH concentration on the impedance appearance are investigated. In the presence of urea, it is found that Nyquist plots exhibit two depressed semicircles, with one at higher frequencies stably locating in the first quadrant while the other’s location at lower frequencies varying between the first and second quadrant as the polarization potential changes. Results show both indirect and direct pathways proceed in urea electro-oxidation reaction. A mathematical model indicates the reverse loop in the Nyquist plots is attributed to CO2 poisoning on the catalyst, which is also validated by the followed chronoamperometric method. Moreover, the rate determining steps of urea electro-oxidation reaction is dependent on KOH concentration. The EIS technique gives a new sight to interpret the poor stability of urea electro-oxidation on nickel catalyst, and thus helps to explore a CO2-insensitive catalyst.
Display omitted
•Low-cost MgFexMn2-xO4 nanoparticles were prepared via a facile so-gel route.•MgFe1.33Mn0.67O4 exhibited good rate ability and cycling stability.•MgFe1.33Mn0.67O4 displays Mg-ions ...diffusion coefficients of 2.55 × 10−7 cm2 s−1.
Aqueous Mg-ion batteries attract lots of attention due to its high safety, low cost and potential application for large scale energy storage system. Although spinel-type metal oxides display their capable Mg ions storage behavior, low diffusion ability extremely hinder their practical application. Herein, low-cost MgFexMn2-xO4 (x = 0.67, 1, 1.33, 1.6) nanomaterials are prepared by a facile sol–gel method. Their electrochemical performance is affected by molar ratio of iron to manganese. The optimized MgFe1.33Mn0.67O4 exhibits excellent electrochemical cycling performance and rate capability. Even at high current density of 1000 mA g−1, a specific capacity 88.3 mAh g−1 is obtained after 1000 charge–discharge cycles. The stable structure of MgFe1.33Mn0.67O4 promise a cycling stability. Moreover, hydrogen evolution and oxygen evolution of the electrode material during charging-discharging process can be effectively suppressed by regulating the atom ratio of iron to manganese. In addition, it presents a high magnesium diffusion coefficient at two oxidation peaks, leading to a good rate ability.
Incorporation of reduced graphene oxide (rGO) optimizes the interfacial properties of MgMn2O4 and improves the Mg2+ diffusion in the electrode. This results in enhanced specific capacity and rate ...capability. MgMn2O4/rGO nanocomposites show a large discharge specific capacity of 140.1 mA h g−1 at a current density of 1000 mA g−1.
In this report, graphene nanosheets (GNS)/nickel sulfide (NiS) based material for high-performance supercapacitor is prepared by "dip and dry" and electrodeposition methods. Commercial flexible ...make-up cottons (MCs) are chose as skeletons to construct homogeneous three-dimensional (3D) interconnected graphene-wrapped macro-networks, which can support structures for high loading of active electrode materials and facilitate the access of electrolytes to active electrode materials. The hybrid GNS/NiS based MCs (denoted as MCsatGNSatNiS) electrode yields relatively high specific capacitance of 775 F g super(-1) at a charge/discharge specific current of 0.5 A g super(-1) and good capacitance retention of 88.1% after 1000 cycles at 2 A g super(-1). Furthermore, the MCsatGNSatNiS electrode delivers a high energy density of 11.2 Wh kg super(-1) at even a high power density of 1008 W kg super(-1). Therefore, such low-cost and high-performance energy MCs based on GNS/NiS hierarchical nanostructures offer great promise in large-scale energy storage device applications.
Ni(OH)2 nanosheets are directly grown on nickel foam by a simple template-free growth process. Their microstructure and surface morphology are studied by X-ray diffraction spectroscopy and scanning ...electron microscopy. The XRD and SEM results show that Ni(OH)2 has a beta -phase structure and covers the nickel foam skeleton with nanosheets. This beta -Ni(OH)2/Ni-foam electrode exhibits a high specific electric quantity of 790.3 C g-1 approaching the theoretical value (1040.6 C g-1) and high electrochemical activity. Asymmetric supercapacitor has been fabricated successfully using beta -Ni(OH)2/Ni-foam nanosheets as positive electrode and activated carbon as negative electrode in a KOH aqueous electrolyte. The electrochemical capacitances of this supercapacitor are investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. An asymmetric supercapacitor AC/6 mol L-1 KOH/ beta -Ni(OH)2/Ni-foam could be cycled reversibly in the high-voltage region of 0-1.6 V and displays intriguing performances with a specific capacitance of 105.8 F g-1 and high energy density of 36.2 W h kg-1. Importantly, this asymmetric supercapacitor device exhibits an excellent long cycle life along with 92% specific capacitance retained after 1000 cycles.
