N-Doped-carbon/cobalt-nanoparticle/N-doped-carbon multi-layer sandwich nanohybrids derived from cobalt MOFs having 3D molecular structures as bifunctional electrocatalysts for on-chip solid-state Zn-air batteries.

The unsatisfactory energy efficiency and leakable liquid electrolytes in conventional Zn-air batteries with intrinsic semi-open structures greatly limit their opportunity to be a safe micropower source for flexible/wearable electronics. Herein, N-doped-carbon/cobalt-nanoparticle/N-doped-carbon (NdC-CoNP-NdC) multi-layer sandwich nanohybrids were first synthesized via the pyrolysis of a well-designed Co-MOF precursor with a 3D molecular framework. Profiting from the synergistic effect enabled by the interlayer-confined growth of monodispersed cobalt nanoparticles having high activity/stability and a thousand-layer-cake porous N-self-doped carbon skeleton of high conductivity and additional active sites as well as the reasonable design of a multi-layer sandwich interface structure between them that acts as an interconnected nanoreactor, the as-obtained NdC-CoNP-NdC multi-layer sandwich nanohybrids exhibit excellent bifunctional catalytic activity of a small ORR/OER gap (0.83 V). We followed a planar electrode configuration design with an interdigital carbon cloth coated with NdC-CoNP-NdC as the air cathode and an interdigital Zn foil as the metal anode as well as the introduction of a polyacrylamide-co-polyacrylic/6 M KOH alkaline gel as an incombustible solid-state electrolyte. Thus, on-chip all-solid-state rechargeable Zn-air batteries (OAR-ZABs) are further developed, achieving a cycle life up to 150 cycles per 50 h, high power density/specific capacity as much as 57.0 mW cm-2/771 mA h g-1, respectively, and excellent coplanar integration capability and mechanical flexibility for working steadily under bending deformation. Eventually, as an additional advancement, an autonomous smartwatch powered by the coplanar integrated OAR-ZABs is demonstrated, which possesses excellent integrity and flexibility and is comfortably wearable for timing and step counting dynamically; this demonstrates the successful application of assembling OAR-ZABs into highly integrated wearable electronics as a compatible micropower source.