Nickel-borate oxygen-evolving catalyst that functions under benign conditions

Abstract

Thin catalyst films with electrocatalytic water oxidation properties similar to those of a recently reported Co-based catalyst can be electrodeposited from dilute Ni(2+) solutions in borate electrolyte at pH 9.2 (B(i)). The Ni-B(i) films can be prepared with precise thickness control and operate at modest overpotential providing an alternative to the Co catalyst for applications in solar energy conversion.

Fig. 1.

CVs using a glassy carbon working electrode, 50 mV/s scan rate, of aqueous 1 mM Ni²⁺ in 0.1 M B_i electrolyte, pH 9.2. Successive scans show an increase of the anodic peak current for the redox event centered at ∼1.0 e. (Inset) First (- - -) and second (─) CV scans using a glassy carbon working electrode, 50 mV/s scan rate, of aqueous 1 mM Ni²⁺ solutions in 0.1 M B_i electrolyte, pH 9.2. CV trace in the absence of Ni²⁺ is shown as a black trace.

Fig. 2.

CVs scans of a Ni borate catalyst film at pH values of 7.82, 8.30, 8.68, 8.88, 9.16, 9.42, 9.72, and 9.98 in. going from left to right, respectively. The inset shows a linear fit for the 8.7–10.0 pH region, with a slope of 96 mV/pH unit. The CVs were taken at scan rates of 50 mV/s.

Fig. 3.

Current density trace for bulk electrolysis at 1.30 V in 0.1 M B_i electrolyte, pH 9.2, 1 mM Ni²⁺ using an ITO anode. Irregularities are due to bubble formation and to stirring. The inset shows an SEM photograph of a film obtained by passing 10 C/cm² at 1.30 V.

Fig. 4.

Tafel plot, η = (V_appl - iR - E⁰), of a Ni-B_i catalyst film operated in 0.2 M B_i, pH 9.2 where η is the overpotential, iR accounts for the uncompensated solution resistance, and E⁰ is the thermodynamic potential for water oxidation at this pH (0.69 V vs. NHE). The slope of the line is 58 mV/decade.