Sulfur-Rich Colloidal Nickel Sulfides as Bifunctional Catalyst for All-Solid-State, Flexible and Rechargeable Zn-Air Batteries

Earth-abundant and high-performance catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are highly desirable in development of energy-efficient rechargeable Zn-air batteries. In this work, sulfur-rich colloidal nickel sulfides (NiS _x ) are prepared as OER/ORR bifunctional catalysts via a two-step hydrothermal process. The NiS _x nanoparticles (NPs) with large surface area show high OER activity and excellent stability, as evidenced by low overpotential of 301 mV, small Tafel slope of 41 mV dec ⁻¹ and high stability over 20 h of chronopotentiometry test. Due to their sulfur-rich nature (i. e. Ni ₃ S ₄ and NiS ₂ ), the obtained NiS _x also exhibit good ORR activity. The introduction of graphene oxide (GO) in the starting materials leads to the formation of a composite catalyst composed of conductive sulfur-doped reduced graphene oxide (S-rGO) and NiS _x . A high ORR onset potential of 0.91 V (vs. RHE) is obtained from the sulfur-rich NiS _x NPs coupled with the S-rGO which facilitates the electron-transfer and furnishes the bifunctional catalytic activity. Rechargeable Zn-air batteries with NiS _x /S-rGO bifunctional catalyst deliver stable charge and discharge voltages of 2.1 and 1.1 V over 590 cycles. Furthermore, all-solid-state and foldable Zn-air batteries using pliable and robust air cathodes of NiS _x /S-rGO show similar voltage profile as their non-foldable counterparts. The foldable batteries exhibit stable cycling performance for up to 120 discharge/charge cycles at either flat or folded state, proving their high electrochemical and mechanical stability.