The Oxygen Reduction Reaction (ORR) serves as the fundamental reaction in electrocatalysis systems like fuel cells, metal-air batteries, and hydrogen peroxide (H2O2) electro-generation. The ORR reaction mechanism encompasses two potential pathways: a two-electron (2e-) process and a four-electron (4e-) process, which respectively result in the reduction of oxygen to H2O2 or water (H2O). While fuel cells ideally prefer a direct 4e- ORR pathway, a sequential pathway involving the exchange of 2e- can be an environmentally friendly approach for decentralized H2O2 production [1, 2]. H2O2 holds significant importance for various applications and has been recognized as one of the top 100 most significant chemicals. Current methods for H2O2 production, such as the anthraquinone process or direct synthesis, face challenges due to their complex and hazardous nature. Therefore, electrochemical H2O2 synthesis via 2e- ORR presents a more environmentally friendly, safer, and faster alternative to existing technologies. Carbon-based materials modified with heteroatoms, specifically nitrogen-doped carbon (N-C), are considered promising substitutes for expensive noble metal catalysts like Pt for ORR in fuel cells and Au for ORR to H2O2 [1, 3]. The synthesis of N-C typically involves a carbonization process performed at temperatures ranging from 500-1200°C for a duration of one to tens of hours, under various gas atmospheres such as air, nitrogen, argon, ammonia, etc. [4]. The thermal carbonization process is characterized by high energy consumption and long preparation time.

Alternatively, microwave (MW) irradiation offers a more energy-efficient and environmentally sustainable method for synthesizing porous carbon-based materials, making it economically viable as well. This study investigates the impact of both thermal and microwave heating on the properties of porous carbon derived from polyaniline. Additionally, the electrocatalytic performance of these porous carbons in alkaline media for the ORR is examined. The objective is to identify the optimal carbonization conditions during MW heating, resulting in a structure with a chemical composition that complements that obtained through conventional thermal pyrolysis methods.

Under optimal conditions of 450 W power and 140 seconds of exposure, a one-step MW synthesis yields disordered graphite structures with nitrogen and oxygen functionalities. This sample exhibits a comparable chemical structure to the polyaniline sample thermally carbonized at a temperature of 700-800°C, as confirmed by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Both the MW (450 W, 140 s) and thermally (700°C, 1 min) prepared samples demonstrate catalyzed electrochemical ORR, with a selectivity of approximately 58±1% towards H2O2.

By replacing thermal carbonization with microwave-based methods, the overall synthesis time and energy consumption for porous carbon derived from polyaniline can be reduced by a factor of 71 and 93, respectively.

The results indicate that the C-450W_140s sample demonstrates comparable performance in alkaline ORR to the thermally prepared samples, highlighting the effectiveness of the MW-carbonization synthesis approach. These findings pave the way for expanding the application of MW carbonized samples in diverse fields such as electrocatalysis, supercapacitors, and zinc-air batteries, thereby complementing sustainable synthesis methods.