Six polyoxotungstate-based transition metal compounds for electrochemical capacitor application and a comparative analysis of factors affecting capacitances.

Six different polyoxotungstate-based transition metal complexes were synthesized, namely [Cu 5 (2,2'-bpy) 5 (μ 2 -Cl) 2 (PO 4 ) 2 (H 2 O) 2 ][HPW 12 O 40 ]·2H 2 O (1), [Cu 1.5 (2,2'-bpy) 1.5 (inic) 2 (H 2 O) 1.5 ] 3 [H 1.5 PW 12 O 40 ] 2 ·16.25H 2 O (2), [Cu(2,2'-bpy) 2 ] 2 [SiW 12 O 40 ]·10H 2 O (3), [Zn(phen) 3 ] 2 [PW V WVI11O 40 ]·5H 2 O (4), [Zn(phen) 2 (H 2 O)] 2 [SiW 12 O 40 ]·2H 2 O (5), and [Zn(2,2'-bpy) 2 ] 2 [SiW 12 O 40 ] (6) (2,2'-bpy = 2,2'-bipyridine, inic = isonicotinic acid, phen = 1,10-phenanthroline). Compound 1 is based on [HPW 12 O 40 ] 2- anions, which are accommodated within the open channels of a supramolecular network formed by novel Cu-P-Cl coordination clusters. Compound 2 is constructed from [H 1.5 PW 12 O 40 ] 1.5- and novel [Cu 1.5 (2,2'-bpy) 1.5 (inic) 2 (H 2 O) 1.5 ] + coordination fragments, and polyoxoanions are encapsulated within the pores created by the copper coordination fragments, resulting in a unique three-dimensional supramolecular architecture. Compound 3 is a two-dimensional structure formed through the covalent linkage between [SiW 12 O 40 ] 4- and [Cu(2,2'-bpy) 2 ] 2+ . Compound 4 is a supramolecular architecture formed by [PW V WVI11O 40 ] 4- and [Zn(phen) 3 ] 2+ coordination fragments, while compound 5 is a supramolecular structure based on POM bi-supported Zn coordination complexes. Compound 6 is a two-dimensional framework structure constituted by [SiW 12 O 40 ] 4- and [Zn(2,2'-bpy) 2 ] 2+ via covalent interactions. In addition, electrochemical measurement results show that the copper-based tungstate compounds 1-3 and zinc-based tungstate compounds 4-6 exhibit different performances and durabilities as electrochemical capacitors (compound 1 shows the highest specific capacitance of 94.0 F g -1 at 1.5 A g -1 , whereas compound 6 maintains the best cycling stability with the capacity retention of 80.7% after 1000 cycles at 4 A g -1 .). This study contributes to the development of POM-based transition metal complexes with high capacitance by providing insights into the design and synthesis process.