Borate Buffer as a Key Player in Cu-Based Homogeneous Electrocatalytic Water Oxidation

Abstract

Borate buffer was found to have both structural and functional roles within a low-cost tri-copper electrocatalyst for homogeneous water oxidation that exhibits a high turnover frequency of 310 s^-1 . The borate buffer was shown to facilitate the catalytic activity by both bridging the three Cu ions and participating in O-O bond formation. Phosphate and acetate buffers did not show such roles, making borate a unique player in this catalytic system.

Keywords: borate buffer; copper; electrocatalysis; trinuclear; water oxidation.

Figure 1

(a) molecular structure and (b) ORTEP view of CuBor crystalized from borate buffer at pH 9. The guest molecules (perchlorate and water) in the crystal structure are omitted for clarity.

Figure 2

CVs with and without 0.5 mM (a) CuBor, (b) [Cu(BPy)₂(ClO₄)]⁺, and (c) CuAce in 0.2 M borate, phosphate, and acetate buffer solutions at pH 9, respectively, at scan rate 50 mV/s; (d) CVs (solid) and DPVs (dashed) of 1.5 mM [Cu(BPy)₂(ClO₄)]⁺ in 0.2 M borate buffer and 0.5 mM in other 0.2 M buffer solutions at pH 9, at scan rate 50 mV/s. All the measurements were done with glassy carbon as working electrode (0.07 cm²), Ag/AgCl as reference electrode.

Figure 3

(a) Current density of 0.25 mM CuBor and blank in 0.2 M borate buffer at pH 9 for CPE experiments at +1.5 V; (b) corresponding accumulated oxygen; the dashed line represents the end of electrolysis. All the experiments were done using ITO (1.0 cm²) as the working electrode and Ag/AgCl as the reference electrode.

Figure 4

(a) The plot of i _p vs. v ^1/2 of CuBor in 0.2 M borate buffer at pH 9.0; (b) the plot of k _cat vs. borate concentration [B] for catalyst CuBor; the ion strength, I, was maintained as 0.2 M by addition of solid KNO₃.