Tuning the active sites and optimizing the d-spacing value in CoFe-LDH by ex situ intercalation of guest anions: an innovative electrocatalyst for overall water splitting reaction

Abstract

Tuning and stabilizing the surface-active sites is an important strategy to enhance electrocatalysts for the total water splitting reaction. Here, we present an interesting approach to modifying the interlayer distance via intercalating guest anions (I⁻, Br⁻ and Cl⁻). Guest-anion-induced CoFe-LDHs were prepared by a simple wet chemical method. Among them, the largest anion (I⁻) provides a better OER, HER and total water splitting activity as compared to the smaller anions (Br⁻, Cl⁻). The as-synthesized I⁻ anion intercalated CoFe-LDH shows the highest activity in the OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) with lower overpotentials of 264 mV and 108 mV at a 50 mA cm⁻² current density with the lowest Tafel values of 56.34 and 111.97 mV dec⁻¹. The redox transformation from the cyclic voltammetric study reveals that the iodine anion allows more redox transformation, which leads to an increase in the active phase (CoOOH). In addition to this superior total electrode activity, the modified CoFe-LDH delivers high specific and intrinsic activity as compared to bare CoFe-LDH and similar reported catalysts, which was measured in terms of TOF calculation. After obtaining this remarkable response for the OER and HER, the CoFe-LDH/I⁻ (as anode and cathode) was used in a two-electrode system in 1 M KOH solution, and required a cell voltage of 1.6 V to reach 10 mA cm⁻² current density. In addition, XPS results suggest that cobalt loses its electron, suggesting that the electronic state of the cobalt ion increases upon intercalation of the I⁻ anion. Thus, this reveals that the I⁻ anion stabilizes the higher oxidation state of the cobalt ion via electron coupling.