Author: Milena Setka (National Institute of Chemistry) - Nitrogen-doped carbon-based catalysts (NCCs) have emerged as efficient, metal-free alternatives to noble metals for the electrochemical two-electron oxygen reduction reaction, a promising route for decentralized hydrogen peroxide (H₂O₂) production. Here, we present a controlled approach to synthesizing NCCs via microwave (MW) heating of polyaniline (PANI) in single-mode rectangular cavity designed to operate at 915 MHz. This setup offers significant improvements over traditional domestic microwave ovens, which suffer from uncontrolled power delivery and lack insight into reaction parameters.
We observed that PANI exhibits a strong interaction with MW, undergoing a rapid temperature rise beyond 700–900 °C once a threshold temperature of ~290 °C is reached. When a fixed MW power is applied, thermal runaway occurs, characterized by hot spots and unstable temperature oscillations. To overcome this, we implemented a dynamic power control strategy. By reducing MW power as the temperature approaches 220 °C and subsequently modulating it to maintain the target temperature of 600 °C, we successfully stabilized the heating profile. This enabled precise tuning of the carbonization process and prevention of localized overheating. The synthesized NCCs demonstrated uniform morphology and nitrogen incorporation, critical for catalytic performance. The electrochemical evaluation was conducted in O₂-saturated 0.1 M KOH using a rotating ring-disk electrode (RRDE) setup with a Pt ring and glassy carbon disk. The NCC samples synthesized under controlled MW heating achieved a H₂O₂ selectivity of 93%, a diffusion-limited current density of 2.6 mA cm⁻², and a ring current of 3.9 mA cm⁻² comparable with the commercially available carbon catalyst.
This study demonstrates that by using a TE101 cavity for MW-assisted synthesis, it is possible to finely control the thermal environment during the carbonization of PANI, enabling reproducible and high-performance nitrogen-doped catalysts.