NiM/β zeolites (M = Co, Cu, and Zn) as bifunctional oxygen electrocatalysts

Abstract

A series of Ni/β and NiM/ β zeolites (where M =Co, Cu, and Zn) were synthesized by a low-cost aqueous ion exchange procedure from hydrogen form β zeolite (SiO<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> = 38). After calcination and morphological examination, the materials were examined for oxygen evolution (OER) and reduction reaction (ORR) in alkaline media. Namely, all zeolites exhibited good OER and ORR activity. For OER, NiZn/β exhibited the lowest onset potential (E<inf>onset</inf>) of 1.63 V, followed by NiCo/β (1.66 V). Overpotentials at onset (η<inf>onset</inf>) decreased in the order: Ni/β (630 mV) > NiCu/β (610 mV) > NiCo/β (460 mV) > NiZn/β (430 mV). NiCo/β gave the highest OER current density (34.2 mA cm⁻²), followed by NiZn/β (19.4 mA cm⁻²), while NiCu/β (3.0 mA cm⁻²) and Ni/β zeolites (2.1 mA cm⁻²) gave significantly lower values. These results confirmed that NiCo/β and NiZn/β showed the highest activity for OER. For ORR, NiZn/β and Ni/β displayed the lowest Tafel slopes, indicating favorable kinetics. The half-wave potentials (E<inf>1/2</inf>) of 0.69, 0.68, 0.66, and 0.65 V were observed for Ni/β, NiZn/β, NiCu/β, and NiCo/β, respectively. NiZn/β demonstrated excellent stability and gave a number of exchanged electrons from 3.0 to 3.3, which shows mixed direct and indirect pathways of ORR, while the rest of the NiM/β zeolites predominantly showed indirect pathways of ORR. These results confirmed that NiCo/β exhibits the best activity for OER, while NiZn/β demonstrated similar OER activity to NiCo/β but showed the highest ORR activity.