| Hydrogen is commonly acknowledged as one of the most potential renewable resources for its high energy density and carbon-free combustion products.Among the existing hydrogen production technologies,electrochemical water splitting is a simple and reliable approach,which can be recyclable and achieve mass production of high-purity hydrogen as well.However,it still has some drawbacks,for instance,the huge power consumption and limited electrolytic rate.To a certain extent,such drawbacks should ascribe to the oxygen evolution reaction?OER?with intrinsic sluggish kinetics and large overpotential.Therefore,the OER electrocatalysts are crucial to enhance the efficiency of water electrolysis.On the basis of the requirement of activity,stability and cost in industry,Co3O4,a kind of the transition metal oxides,has been extensively preferred by researchers.In order to enhance the catalytic performance of Co3O4 significantly and make it comparable to the commercial noble metal oxides,a series of improvement measures were taken in this study.Urchin-shaped Co3O4 spheres were successfully grown on conductive nickel foam by hydrothermal synthesis and heat treatment.Unique three-dimensional urchin spheres possessed the higher specific surface area accompanying with the hierarchical pore structures,which could expose more active sites,benefit for the penetration of the electrolyte and meanwhile,reduce the resistance of mass transport effectively.Through characterizing the structure and morphology of the precursor and controlling the hydrothermal parameters,it was found that the formation of urchin-like structures mainly depended on the hydrothermal stage instead of the annealing stage.The growth pattern of the precursor was in line with the principle of “nucleation-dissolution-recrystallization”and it presented the transformation process of“disks-flowers-urchins”.Finally the precursor was oxidized to Co3O4 without further morphology change during heat treatment.Urchin-like Co3O4 exhibited considerable catalytic properties in alkaline solution.It showed a low overpotential of308 mV at the current density of 20 mA·cm-2.Moreover,such material had a small Tafel slope of 82.1 mV·dec-1 and good long-term stability.Co3O4 would remain its urchin structures during electrochemical scanning and maintain the large specific surface area all the time,providing enormous reaction zones for catalysis.A “magnetic field-assist”strategy was applied to improve the catalytic performance of Co3O4 based on the electrocatalysis procedure and the influence mechanism of magnetic field on the charge.Constant magnetic field was imposed outside the catalysis system during the electrochemical tests.In this case,magnetic field strength and direction would affect the catalytic activities of Co3O4 evidently.The overpotential and Tafel slope of Co3O4 were both reduced with the increase of the magnetic field strength?0-125 mT?and the included angle?0°-90°?between the orientation vectors of magnetic field and electric field.When the magnetic field was125 mT and perpendicular to the electric field at the same time,the Tafel slope could decrease to 26.7 mV·dec-1 and the overpotential was just 252 mV at 20 mA·cm-2,even superior to RuO2.In magnetic field,the intrinsic activities of active sites in Co 3O4were enhanced and the stability of the catalyst did not weaken.According to the experimental phenomena and theoretical analysis,it was concluded that magnetic field might influence the liquid phase mass transfer and electron transport by inducing the magnetohydrodynamic effect and changing the electron spin.These factors had combined action,which not only alleviated anodic polarization,but also accelera ted the electron transfer,contributing to the required activation energy for redox reaction.Thus the catalytic properties of Co3O4 were optimized.The applied magnetic field upgrades the catalytic ability of Co3O4 for oxygen evolution reaction markedly,which provides a novel idea for exploiting actually implemented catalysts. |
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