Enhancing Energy Storage via TEA‐Dependent Controlled Syntheses: Two Series of Polyoxometalate‐Based Inorganic‐Organic Hybrids and their Supercapacitor Properties

Abstract

To enhance energy storage performance via modulation of crystal structure on the molecular level, two series of triethylamine (TEA)‐dependent polyoxometalate (POM) based inorganic‐organic hybrid compounds, [CuII(btx)2]2[SiW12O40] (SiW‐1), [CuI(btx)]4[SiW12O40] (SiW‐2), [CuI4(btx)3][SiW12O40] ⋅ 2H2O (SiW‐3), [{CuII6(btx)7(H2O)12}H4⊂(W12O40)2] ⋅ 12H2O (W‐1), [{CuII7(btx)8(H2O)10}H2⊂(W12O40)2] ⋅ 2H2O (W‐2) and [{CuII10CuI2(btx)11(H2O)16}H2⊂(W12O40)3] ⋅ 6H2O (W‐3) (btx=1,4‐bis(triazol‐1‐ylmethyl)benzene) have been synthesized and explored as pseudocapacitor electrode materials. Compared to other compounds, SiW‐2 shows a specific capacitance of 110.3 F g−1 at 3.0 A g−1 and only experiences a capacitance loss of 13 % after 1000 cycles at a current density of 18.0 A g−1. More importantly, the two series of compounds can be considered as a model for studying the effects of the molecular structures on the pseudocapacitor performance. The result verifies that besides the type of POM, the capacitance ability of the POM‐based compounds is mainly dominated by the connecting modes of adjacent POM building blocks and the dimension of covalent networks formed by POM building blocks. Thus, this work may open a new avenue to optimize the performance of POM‐based capacitors.

Supercapacitor electrodes based on metal‐organic compounds from the first transition metal series

“…So far, reported copper‐organic compounds and their composites with SC properties can be divided into copper carboxylate MOFs ( 140 ‐ 157 ), copper polyoxometalate organic frameworks (Cu‐POMOFs, 158 ‐ 176 ), copper polyamine or polyphenol MOFs ( 177 ‐ 183 ), copper porphyrin compounds ( 184 ‐ 188 ), etc. Copper can exhibit electrochemical activity through the redox reaction between Cu(0), Cu(I), and Cu(II), and its organic compounds have been widely studied in SCs 73,77,78,80,116,159‐183 . Relevant conversion process can be expressed by the following equations 164,167,174 : …”

“…Another important type of copper‐organic compounds is the POMOFs that has attracted recent research interests in the field of SC ( 158 ‐ 176 ) 28,38,77,169‐174 . POMOFs have many exceptional advantages for SC applications, such as long cycling life due to their poor solubility in water and in common inorganic and/or organic solvents, and crystalline forms which are suitable for investigating their effect on SC properties.…”

“… 169,171,172 Chai and coworkers synthesized two Cu‐POMOFs ( 159 and 160 ) based on Keggin‐type POM H 4 SiW 12 O 40 and (NH 4 ) 6 W 12 O 40 via regulating the amount of triethylamine in the POM/Cu/btx reaction system (Figure 13C). 169 They also studied the relationship between crystal structure on the molecular level and their capacitance performance. It is deduced that the different capacitances may be dominated by the modes of direct connection between POM building blocks and covalent networks.…”

Can polyoxometalates enhance the capacitance and energy density of activated carbon in organic electrolyte supercapacitors?

“…where i is the maximum peak current obtained at a specific scan rate v, and a and b are adjustable parameters. The current response of a linear dependence on scan rate (b = 1) usually means charge is stored by a fast response mechanism, such as surface capacitive mechanism and redox reactions which could perform at fast rate [31]. Otherwise, if any slow diffusion of ion or electron transfer limits the reaction, the dependence could deviate from linearity (b < 1).…”

“…Most achievements until now have resorted to inorganic salts of POMs (or their acids) to construct hybrid materials in order to boost capacitance in aqueous electrolytes. Recently, a few studies have been carried out on organic-inorganic POMs-based hybrid materials, but they are still applied for supercapacitors with aqueous electrolytes [31,32], or for lithium ion batteries [33].…”