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, 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) is reported. 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 hours 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.

Strategies to Improve Electrical Conductivity in Metal–Organic Frameworks: A Comparative Study

“…At neutral pH, the OER overpotentials are 0.77 and 0.46 mV for the 2D and 1D MOFs, respectively, for a current density of 1 mA cm –2 . These values reduce to 0.36 and 0.25 mV at pH = 14 . Yang et al have reported OER activity of MIL-53­(Fe)-2OH at an overpotential of 215 mV with a current density of 10 mA cm –2 in 1 M KOH .…”

“…These values reduce to 0.36 and 0.25 mV at pH = 14. 194 Yang et al have reported OER activity of MIL-53(Fe)-2OH at an overpotential of 215 mV with a current density of 10 mA cm −2 in 1 M KOH. 195 Tang et al have shown electrocatalytic OER activity of ultrathin nanosheets of 2D NiCo-UMOFN at an overpotential of 180 mV with a current density of 10 mA cm −2 in 1 M KOH.…”