Cobalt-based Nanostructures As Electrocatalysts In Oxygen Evolution Reaction

Electrochemical water splitting has been regarded as a promising method for the low-cost and clean production of hydrogen,due to the advantages of high efficiency,eco-friendly and utility.Oxygen evolution reaction(OER)is considered as the bottleneck during the overall water splitting reaction because of the sluggish multielectron reaction and O-O bond formation,which lead to a higher overpotential and the restricted speed of water splitting.Hence,it is urgent to develop a high activity electrocatalysis that can address the problem of high energy barriers,reduce the overpotential,accelerate the reaction,and enhance the energy conversion efficiency.Up till now,noble metal materials,such as iridium,ruthenium and their compounds are considered to be the state-of-the-art electrocatalysts,for their stability in all pH values.However,the high price and scarcity impedes their practical applications.Recently,for the sake of developing substitution for non-noble mental electrocatalysts,transition metal compounds have garnered large attention in the field of electrocatalysis.Transition metal compounds own the unique properties inclding low-cost,earth abundant and environment-friendly.Extensive research on Co compounds had been reported and suggested that cobalt-based compounds with different morphologies exhibit superior electrocatalytic activity and stability in alkaline solution.Generally,the performance of electrocatalysis is related to crystal structure,composition,morphology,specific surface area,porous structure and other factors.To exploit highly efficient alternative,three methods including electrodeposition,low temperature precipitation and hydrothermal method were used to achieve cobalt-based nanostructures with different compositions and morphologies.The main work of this paper is as follows:Firstly,novel cobalt carbonate hydroxide nanorods are synthesized by facile low-temperature precipitation using cobalt acetate as raw material and sodium carbonate as precipitant in a glycol aqueous solution.The template-free method excludes the negative effects of post-treatment for template-removing on the catalytic activity of the obtained electrocatalysts.The nanorods are networked to form mesoporous structure with a large surface area(291.4 m2·9-1)and high pore volume(1.06 cm3·g-1).The obtained catalyst reaches a current density of 10 mA cm-2 with an overpotential of a 320 mV and a Tafel slope of 39 mV·dec-1.Moreover,OER activity is improved after stability test,Secondly,by using cobalt nitrate and potassium nitrate as the raw materials,Co(OH)2 nanosheet arrays is grown on graphite paper(GP)via an electrodeposition method.The optimization of the sedimentary conditions such as deposition mode and time which have an influence on OER performance is discussed.Under potentiostatic mode,the electrode deposited for 3 min has the optimal performance.The overpotential(?10)of as-prepared electrode is 382 mV and Tafel slope is 56 mV dec-1 respectively.Meanwhile,GP can act as an active oxygen evolution electrode.However,due to the poor adhesion between active materials and substrate,the stability of the as-prepared CO(OH)2/GP is unsatisfied.The Co(OH)2/GP electrode prepared via pulse electrodeposition exhibits high OER performance dur to the enhanced adhesion,in which a smaller ?10 of 366 mV and a Tafel slope of 45 mV·dec-1 is achieved,respectively.Finally,aligned Ni-doped Co(OH)2 nanoplate arrays are formed on Ni foam substrate via hydrothermal process.The shape of nanoarrays can be tuned by the addition of urea and ammonium fluoride.These hydroxide nanoarrays are grown according to a pattern-growth mode,and all nanoplates align themselves in the same direction to construct a domain coinciding the Ni domains.The effect of substrate on the growth of nanoarrays is discussed.Due to the large electrochemically reactive surface and the strong coupling interaction between Co(OH)2 nanoplate and Ni substrate via substrate-induced effect,the nanoarrays electrode exhibits low overpotentials(275 mV),small Tafel slopes(59 mV·dec-1),and strong stability.Moreover,aligned Co(OH)2 nanoplate arrays reveal promising electrocatalysts for full water splitting with high efficiency,good stability and convenient fabrication.