Cobalt(II) phosphonate coordination polymers: Synthesis, characterization and application as oxygen evolution electrocatalysts in aqueous media and water-saturated hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid

Abstract

Four cobalt phosphonate coordination polymers Co[(CH3PO3)(H2O)] (1), Co[(C2H5PO3)(H2O)] (2), Co[(C4H9PO3)(H2O)] (3) and Co[(C6H5PO3)(H2O)] (4) have been synthesized using a hydrothermal method. Their crystal structures have been characterized by X-ray diffraction and all exhibit two-dimensional polymer structures. The water oxidation catalytic activities of these polymers have been studied for the first time using electrochemical methods in both aqueous solutions and water-saturated [BMIM][PF6] ionic liquid (BMIM=1-butyl-3-methylimidazolium). The results reveal that these polymers are able to catalyze water oxidation in aqueous media but are not fully stable in this environment. However, they are more stable and retain significant catalytic activity towards water oxidation in water-saturated [BMIM][PF6] and addition of a buffer is not required. The catalytic activity decreases in the sequence of 1>2>3>4. Consequently, water-saturated [BMIM][PF6] provides a good medium for catalytic water oxidation with this type of coordination polymer.