Solution Chemistry Preparation And Electrochemical Properties Of BaCoF₄

The electrode material is the crucial component of a supercapacitor.The electrochemical property of the electrode material directly determines the performance of the supercapacitor.As a new family of supercapacitor electrode materials,transition-metal fluoride BaMF4(M= Mn,Co and Ni)compounds,due to possessing special polarization characteristics and lamellar structures,have raised extensive attention for the promising applications.On the explorations of such material sorts,there are still several fundamental questions to be resolved.For instance,the valence state of the M cation for the as-prepared BaMF4,the valence state variations during electrochemical reaction as well as their impacts on the electrochemical behaviors,are not yet clear.On the other hand,the existing works on the BaMF4 synthesis always lead to quite large particle sizes(normally at micron scale)that are not conductive to achieve high performance for the supercapacitor because of the too less active sites.As a consequence,how to prepared the BaMF4 materials with small sizes and construct the proper channels favoring the electronic and ionic transport in order to greatly improve the active sites are also quite challenging.To address the aforementioned questions,we selected BaCoF4 that shows the best performance in BaMF4 family as the electrode material protype and imitated the studies and investigations on the nanoscale synthesis,microstructural control,electrochemical properties and mechanisms,with giving rise to the following results and conclusion:(1)Through systematical control over the hydrothermal temperatures,duration,concentrations/ratios of the starting chemicals and the F source use,pure-phase BaCoF4 with orthorhombic structure(S.G.:Cmc21)was synthesized with the final conditions:simultaneous utilizations of CoF2,BaF2 and CF3COOH as F sources with an excess of 5at%for CoF2 at 230 0C for 24 hours.The as-prepared BaCoF4 particle presents a bulk form with the size at micron-scale that consists of the multilayer lamellar structures.Electrochemical tests demonstrated the obvious REDOX and pseudo-capacitance behaviors for the present BaCoF4 electrode material with showing a specific capacitance of 121 F·g-1.The REDOX process was confirmed by XPS technique that reveals the valence state variations of the active Co cations(Co2+/Co3+)during the electrochemical reactions.To conclude,electrochemical property and charge storage performance for BaCoF4 are originated from the valence state evolutions of the active Co cations.(2)A strategy of the surfactant utilizations coupled with the aforementioned hydrothermal conditions was proposed to further thin the dimensions of the BaCoF4 electrode material.When 0.4 g(0.0239 mol/L)of the surfactant i.e.sodium dodecyl benzene sulfonate(SDBS)was used for hydrothermal reaction,BaCoF4 with nanosheet structure characterized in the[3-1 0]orientation(crystal zone axis)was synthesized.The as-synthesized BaCoF4(OR-BaCoF4)thus exhibits the feature of ionic tunnels from the crystallographic analysis because this is close to the theoretically ionic tunnel i.e.[100]or a direction of BaCoF4.As a result of the thinning along ionic tunnels that promote more active Co cations to fulfil the electrochemical reactions,the OR-BaCoF4 electrode materials exhibit the excellent energy storage properties with the specific capacitance increasing up to 692.3 F·g-1,which is almost 5 folds higher than that of the bulk BaCoF4 synthesized without adding SDBS.(3)The thermal stability and structural transformations upon high-temperature treatment were investigated with the aim to acquire the fundamental parameters for BaCoF4.Above 300 ℃,both the as-synthesized bulk BaCoF4 and OR-BaCoF4 start to decomposition,with producing BaF2.The BaCoF4 sample color changed gradually from original pink to gray and/or black,which indicates the formation of Co3+ that could enhance the electronic conductance.Heat treatment in protective atmosphere(N2)could slightly improve the thermal decomposition temperatures.Electrochemical tests demonstrated a noticeable enhancement in the charge storage,e.g.the bulk BaCoF4 after heat treatment showed an about 3-fold larger capacitance.However,if BaCoF4 is selected as a high-temperature electrode material candidate,the structural stability should be taken into consideration.