Role of Metal Ion Sites in Bivalent Cobalt Phosphorus Oxygen Systems toward Efficient Oxygen Evolution Reaction
2021
Alex, Chandraraj | Sathiskumar, Chinnusamy | John, Neena S.
In the recent studies on various cobalt phosphates with different phosphorous oxyanion ligands for oxygen evolution reaction (OER), the influence of ligands in efficient proton abstraction has been highlighted for the observed higher activity. The role of CoO(OH) active sites has not been given sufficient importance to compare the activity in these systems. In order to explain the OER activity in relation to metal ion sites, we have synthesized three different cobalt phosphorous oxyanion systems such as NaCoPO₄ (CP-1), Co₁₁(HPO₃)₈(OH)₆ (CP-2), and Co₃(OH)₂(HPO₄)₂ (CP-3) and compared their OER activity with detailed analysis of cyclic voltammograms. Though CP-2 and CP-3 possess a higher number of proton abstracting ligands per metal site than CP-1, it is observed that CP-1 shows an OER activity similar to CP-3 in terms of onset potential and Tafel slope values. A careful estimation of the effective number of reduction sites in terms of Co⁴⁺/Co³⁺ and Co³⁺/Co²⁺ reduction reveals that CP-1 and CP-3 show higher values in accordance with the observed OER activity trend, CP-1 ∼ CP-3 > CP-2. The average number of CoOₓ(OH)y reduction sites in the Co⁴⁺ ↔ Co³⁺ ↔ Co²⁺ sequence explains the OER performance better than the ratio of reduction current density or Co³⁺ reducibility. The changes in morphology and chemical composition of the abovementioned systems after OER cycles support the formation of active species, cobalt oxyhydroxide during OER. The CP-1, CP-2, and CP-3 catalysts show stable current density at ∼10 mA/cm² up to 10 h.
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