Redox insights and OER activity in 3D-MOFs: the role of alkali metal ions

Abstract

Bimetallic 3D MOFs, where coordinated Co(II) ions are entirely surrounded by potassium ions, assist in achieving an exceptional topological arrangement. The formation of 1D chains of potassium ions and their immense attraction to water enable such a system to get stabilized and carry hydroxide ions in solution and facilitate the existence of catalytically active metal hydroxide slices, which is a key factor for the electrocatalytic OER. Therefore, by maintaining proper orientation and concentration of potassium ions in such a bimetallic 3D MOFs-based system, structural stability can be maintained in an alkaline medium and the electrocatalytic OER activity can also be tuned. Herein, it has been demonstrated that the introduction of potassium ions and their different arrangements in the K@Co-carbonate MOF (Set 2) and K@Co-2,6-pc MOF (Set 3) can exhibit excellent potential for the electrocatalytic OER, both in terms of catalytic efficacy and structural stability. Interestingly, K@Co-2,6-pc MOF (Set 3) showed a low overpotential of 292 mV (at 10 mA cm⁻²) and a small Tafel slope of 50 mV dec⁻¹ with a remarkable long-term electrochemical stability of 30 h for the OER. The turnover frequency (TOF) of K@Co-2,6-pc MOF (Set 3) was 1.96 s⁻¹, which is ∼28 times greater than the commercially available cobalt oxide (Co₃O₄) catalyst for the OER performed under identical conditions, and it is the highest value that has been achieved as compared to previously reported MOFs.