Multinuclear Transition Metal Sandwich-Type Polytungstate Derivatives for Enhanced Electrochemical Energy Storage and Bifunctional Electrocatalysis Performances

Different transition metal (TM) units are introduced into a trivacant Keggin cluster to form three sandwich polytungstate derivatives, (H 2 en)[{K(H 2 O) 0.5 } 2 {K 2 (H 2 O) 3 }{Ni(H 2 O)(en) 2 } 2 {Ni 4 (H 2 O) 2 (PW 9 O 34 ) 2 }] ( 1 ), [Cu 6 (Himi) 6 {As III W 9 O 33 } 2 ]·5H 2 O ( 2 ), and (H 2 btp) 4 [Fe III 2 Fe II 2 (H 2 O) 2 (AsW 9 O 34 ) 2 ]·4H 2 O ( 3 ) (en = ethanediamine; imi = imidazole; btp = 1,3-bis(1, 2, 4-triazol-1-yl) propane). Compound 1 is a 2,3,8-connected 3D network with {4 3 } 2 {4 6 ·6 6 ·8 3 ·6 12 ·8}{6} 2 topology based on bisupported tetra-Ni sandwich phosphotungstate and two kinds of potassium connection units. Compound 2 is a dense 12-connected 3D supramolecular network with {3 24 ·4 36 ·5 6 } topology based on hexa-Cu(imi) sandwiched arsenotungstate. Compound 3 represents the first mixed valence tetra-Fe substituted sandwich arsenotungstate assembly. Compounds 1 – 3 show enhanced supercapacitor performance (618.2, 603.4, and 504.6 F·g –1 at a current density of 2.4 A·g –1 with 91.5%, 89.3%, and 87.8% of cycle efficiency after 5000 cycles, respectively) compared to their maternal polyoxometalates (POMs) and most reported POM-based electrode materials, which suggests that the introduction of multinuclear TM into vacant POMs is an effective method to improve the energy storage performance of POMs. In addition, compounds 1 and 3 exhibit dual-functional electrocatalytic behaviors in the reduction of iodate and the oxidation of dopamine for introduction of {Ni 4 } and {Fe 4 } units.