Pt-nanoparticles on ZnO/carbon quantum dots: a trifunctional nanocomposite with superior electrocatalytic activity boosting direct methanol fuel cells and zinc–air batteries†

Abstract

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 (DMFCs) and Zn–air batteries (ZABs). Herein, we synthesized a ternary nanocomposite (PtNP-ZnO@CQDs) composed of ultrafine platinum nanoparticles (PtNPs) smaller than 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 characterization through Powder X-ray Diffraction (PXRD), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS), Brunauer–Emmett–Teller (BET), morphological analysis and electrochemical impedance spectroscopy (EIS) confirms 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 the OER, the PtNP-ZnO@CQD catalyst reveals a lower overpotential than commercial RuO₂ at higher current densities over 30 mA cm⁻². In the ORR, the catalyst showed a higher half-wave potential of 0.96 V, higher limiting current density, mass activity, and chronoamperometric stability than commercial Pt/C used as a standard here. PtNP-ZnO@CQDs also exhibited a 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 ZAB, the PtNP-ZnO@CQD air cathode delivered an open circuit potential of 1.55 V with an impressive energy density of 668 W h kg⁻¹ and a specific capacity of 532 mA h g⁻¹, outperforming ZABs with commercial Pt/C and RuO₂. Interestingly, the ZAB composed of the PtNP-ZnO@CQD air cathode shows outstanding long-term cycling stability, maintaining a round trip efficiency of 66.87% after 60 h. ZABs assembled in series successfully powered LED panels, demonstrating the potential of this low-cost, bifunctional Pt-based electrocatalyst for future ZAB commercialization.