Electrocatalytic water oxidation by hydrolytically stable metal-organic frameworks at both neutral and alkaline medium: inverse relation of dimensionality with catalytic activity

Abstract

Catalyst development for water splitting to afford hydrogen as a green source of energy is one of the major areas of research in the pursuit of sustainable energy technology solution. Herein, we report the electrocatalytic water oxidation behaviour of two different Co-pdc (H2pdc = pyridine-2,5-dicarboxylic acid) based metal-organic frameworks (MOFs) of different dimensionalities (2D and 1D). 2D-MOF {[Co(pdc)(H2O)2]·H2O}n acts as the mother which transforms into daughter 1D-MOF {[Co(pdc)(H2O)2]·7H2O}n via consecutive dehydration and rehydration. Within 2D-MOF, the Co2+ shows six coordinated octahedral geometry with two coordinated water molecules and for the 1D-MOF, Co2+ shows five coordinated square pyramidal geometry having two coordinated water molecules and one open metal site. Both the MOFs show excellent stability over a pH range of 3 to 11 even after 24 hrs and show OER activity from neutral to alkaline medium. The MOFs retain their crystalline structure even after OER at neutral pH but eventually gets decomposed; but they are converted into their (oxy)hydroxides at pH 14. Interestingly, 1D MOF shows superior activity in both neutral and alkaline medium over the 2D framework for OER due to presence of open metal sites, better electrical conductivity and larger electrochemically active surface area.