Pt-Nanoparticles on ZnO/Carbon Quantum Dots: A Trifunctional Nanocomposite with Superior Electrocatalytic Activity Bosting Direct Methanol Fuel Cell and Zinc-Air Battery

Architecting efficient, multifunctional, and low-cost nano-electrocatalysts plays a vital role in electrochemical energy conversion and storage systems. Low-Pt hybrid catalysts are in high demand, offering cost-effective solutions for electrode materials in direct methanol fuel cells and Zn-air batteries. Herein, we synthesized a ternary nanocomposite (PtNP-ZnO@CQDs) composed of ultrafine platinum nanoparticles (PtNPs) of below 5 nm on photosensitive ZnO and carbon quantum dots (CQDs) via a simple one-pot hydrothermal process for efficient photoinduced electrocatalytic methanol oxidation reaction (MOR), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) with commendable durability. Comprehensive characterizations through XRD, FT-IR, XPS, BET, SEM, EDX, and HRTEM confirm the nanocomposite's structure and properties. The catalyst attains a MOR current density of 9.1 mA cm-2 in photoinduced electrocatalytic methanol oxidation with high CO tolerance and durability. During OER, the PtNP-ZnO@CQDs catalyst reveals a lower overpotential than the commercial RuO2 at higher current densities over 30 mA cm-2. In ORR, the catalyst showed a higher half-wave potential of 0.96 V, higher limiting current density, mass activity, and chronoamperometric stability than the commercial Pt/C used as a standard here. The PtNP-ZnO@CQDs also exhibited low peroxide yield, a high number of electron transfers, and photoinduced ORR capability, indicating its superiority over commercial Pt/C catalysts. When used in a rechargeable aqueous zinc-air battery (ZAB), the PtNP-ZnO@CQDs air cathode delivered an open circuit potential of 1.55 V with an impressive energy density of 668 Wh/kg and a specific capacity of 532 mAh/g, outperforming ZABs with commercial Pt/C and RuO2. Interestingly, the ZAB composed of PtNP-ZnO@CQDs air cathode shows outstanding long-term cycle stability, maintaining the round trip efficiency of 66.87% after 60 h. The assembled ZABs in series successfully powered LED panels, demonstrating the potential of this low-cost, bifunctional Pt-based electrocatalyst for future ZAB commercialization.