| Nowadays,environmental pollutions and depletions of energy resources have become serious.The development of energy storage devices with the advantage of environmentally sustainability has been greatly stimulated as one of the hot spots to meet the current energy research.Owing to the advantages such as low price and high theoretical capacitance,Ni/Co-based compounds considered as promising electrode materials for supercapacitors.Meanwhile,Ni/Co-based compounds are found to exhibit electrocatalytic activity toward OER in alkaline solution.Inspired by the highly electrochemical activity,the controllable synthesis and performance optimization of Ni/Co-based electrodes have recently drawn increased attentions.In this work,a series of Ni/Co-based hydroxide/oxide electrode was synthesized,and their capacitance behaviors and OER performances were studied.Firstly,the Ni-Co carbonate hydroxide nano wire arrays precursor grown on carbon cloth?CC?substrate was synthesized by hydrothermal process.Because of the hydrophobic nature of carbon materials,attaining the direct growth of hydroxides with a large mass loading on carbon fibers is difficult."Two step" growth is an attractive approach for further increasing the loading of active material,and an initial pre-activation before the second step is of fundamental importance to the final rate performance and mass loading of the electrode.The as-prepared precursor electrode was soaked in 6 mol·L-1 KOH aqueous solution,and the evolution of phase,morphology and electrochemical properties with the soaking time were characterized.After soaked for 16 h,a thermodynamically favoring conversion from carbonate to hydroxide was taken place.A nanowire-supported nanoflake arrays with high specific area,active site and better mass transfer channel was also achieved,suggesting the high capacitive performance.The areal capacity of the as-soaked free-standing Ni-Co hydroxide electrode increased from 1712 to 6056 mF·cm-2 at 2 mA·cm-2 after soaking for 16 h.The coulombic efficiency was up to 99%,and the capacity retention reached to 75%when increasing current density from 2 to 30 mA·cm-2.In addition,an asymmetric supercapacitor?ASC?was assembled using an activated carbon/CC as the anode and the soaked electrode as the cathode in 2 mol·L-1 KOH aqueous solution.The device possessed a high areal capacitance of 1.47 F·cm-2 at 1 mA· cm-2,a large energy density of 6.7 mWh·cm-3 at 11.6 mW·cm-3.Moreover,its energy density remained 2.2 mWh·cm-3 even at a higher power density of 350 mW·m-3.These results demonstrate that alkali conversion method can open up facilely the new possibilities for hydroxide materials in high-energy supercapacitor applications.Secondly,the effects of NaBH4 in in-situ alkali conversion of Ni-Co precursor was studied.The evolution of composition and morphology of active materials during alkali conversion was accelerated after the addition of NaBH4,which might be attributed to the change of local micro-environment of electrode.A gas-released reaction was taken place after the addition of NaBH4,speeding the ion movement and shortening the alkali conversion process.A nanowire-supported nanoflake arrays with network of nanosheets were formed,which increased greatly the surface area and active sites of the electrode.Since the gas-released reaction carried out in internal space of nanoarray,many open channels between nanosheet network were formed for gas migration,which optimized the mass transfer kinetics process.As a result,the areal capacity of electrode achieved 3904 mF·cm-2 and 2472 mF·cm-2 at current densities of 2 mA cm-2 and 30 mA cm-2?63%of capacity retention?after soaked in KOH-NaBH4 solution for 16 h,respectively.Meanwhile,the coulombic efficiency of electrode was above 99%at different current densities.The capacitance retained 83.4%of the initial capacitance after 4000 cycles.The capacitance properties of electrode soaked in KOH-NaBH4 solution was more superior than those of the electrode soaked in KOH solution.The valence state of metal element and the composition of surface oxygen components did not show great change after soaking in KOH-NaBH4 solution.Finally,NiCo-O electrode after partial alkali conversion and post-calcination was fabricated.Meanwhile,NiCoFe-O electrode was synthesized via partial alkali conversion,dipping in Fe?III?sulfate solution and post-calcination.The results showed that NiCo-O electrode was mainly composed of spinel mixed oxide.Due to the porous structure,NiCo-O electrode showed high electrical double-layer capacitor and improved OER performance.For NiCoFe-O electrode,the covered film formed after soaking in Fe?III?sulfate solution tend to crack during calcination,resulting in a microporous nanoarray structure.This microporous structure enhanced the transfer of electrlyte and gas,and boosted OER activity.For NiCo-O and NiCoFe-O electrode,the overpotentials at 10 mA·cm-2 were 487 and 439 mV,and Tafel slope were 195 and 229 mV·dec-1,respectively.The large Tafel slope of NiCoFe-O electrode was attributed to the deposition of Fe oxide on the surface of catalyst,limiting the synergistic effect between multi-metallic ion. |
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