Enhanced Nitrate Production via Electrocatalytic Nitric Oxide Oxidation Reaction over MnO2 with Different Crystal Facets

The synthesis of nitrate (NO3−) via electrocatalytic nitric oxide oxidation reaction (NOOR) is a green and efficient strategy for nitrogen fixation, which has great advantages over conventional nitrate synthesis. Notably, it also presents a promising solution for the remediation of NO pollutants. In this study, the structure–performance correlations of α-, β-, and δ-MnO2 catalysts were investigated. These three polymorphs of MnO2 exhibited disparate surface chemistries, NO adsorption capabilities, and NOOR catalytic activities. In a 1.0 M KOH electrolyte, α-MnO2, characterized by its large-sized (2 × 2) one-dimensional tunnel structure, demonstrated the most outstanding NOOR catalytic performance, which achieved a remarkable NO3− yield of 665.2 mg·h−1·mgcat−1 at a potential of 1.9 V, along with excellent stability and durability. Furthermore, a Zn-NO system was constructed, employing α-MnO2 as the anode and a Zn plate as the cathode. This innovative setup integrated an energy storage system with NO electrochemical capture, yielding a NO3− production rate of 265.5 mg·h−1·mgcat−1. The density functional theory calculations confirm the NO adsorption performance and catalytic activity of three different crystal forms of MnO2.