Sodium Manganese Oxide Nanobelts with a 2 × 4 Tunnel Structure: One-Step Hydrothermal Synthesis and Electrocatalytic Properties

Sodium manganese oxide nanobelts with a 2 × 4 tunnel structure (Na-2 × 4) have been one-step hydrothermally synthesized at 200 °C for a relatively short time (16 h). The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The Na-2 × 4 tunnel manganese oxide nanobelts have the thickness of about 20 nm, the width with ranges from 50 to 200 nm and the length up to several micrometers. The synthetic temperature is critical to the crystal structure of the final products and Na-birnessite is obtained at 120 °C. The electrocatalytic activities of the above products are studied for oxygen reduction reaction in aqueous basic medium. The Na-2 × 4 tunnel manganese oxide nanobelts exhibit higher oxygen reduction activity (12.8 mA cm⁻² or 9.09 A g⁻¹) than that of Na-birnessite (5.6 mA cm⁻² or 3.98 A g⁻¹) at −0.44 V. The Na-2 × 4 tunnel manganese oxide nanobelts could be potentially used as air electrode materials for catalytic reduction of O₂ in alkaline fuel cells and metal/air batteries.