Adjusting organic ligands to construct four 3D metal-organic frameworks with Co2N2O8 SBUs as efficient bifunctional electrocatalysts for OER and HER

Abstract

Multidentate ligands can be used to construct structurally adjustable metal-organic framework materials due to their abundant coordination sites. MOFs 1 − 4 with different configurations are synthesized by solvothermal method based on the symmetrical tetracarboxylic acid H4L main ligand (H4L = 5′-(4-carboxyphenyl)-[1,1′:2′,1′'-terphenyl]-4,4′,4′'-tricarboxylic), named [Co2(L)(bimb)]n (MOF 1, bimb=(1,1′-(1,4-butanediyl) bis (imidazole)), [Co2(L)(bix)]n, (MOF 2, bix=1,4-bis(imidazol-1-ylmethyl) benzene), [Co4(L)2(bidpe)2]n (MOF 3, bidpe=4,4′-bis(imidazolyl)diphenyl ether) and {[Co4(L)(bibdp)2(C3H7NO)]·2(C3H7NO)}n (MOF 4, bibdp=4,4′-bis(1-imidazolyl) biphenyl), respectively. The rich coordination sites of H4L carboxylic acid ligands bring diverse coordination modes, forming Co2N2O8 secondary construction units. Based on it, MOFs 1 − 4 exhibits pillar-layered 3D porous supramolecular structure. Electrochemical measurements support that MOFs 1 − 4 exhibits excellent OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) activities, especially MOF 3, which only requires overpotentials of 180 and 196 mV to reach the current density of 10 mA·cm−2. Remarkable catalytic performance of MOF 3 is attributed to the abundant and dispersed reactive active sites brought by the 3D interpenetrated structure assembled based on tetranuclear metal clusters. The HER occurs through Volmer-Heyrovsky mechanism, meanwhile, possible mechanisms of OER have been predicted. This investigation provides inspiration for exploring multi-core structured MOF materials as efficient HER/OER bifunctional catalysts for electrolytic overall water splitting.