The issue of electromagnetic pollution has become increasingly severe with the development of the electronic communication technology. The excessive electromagnetic waves pose risks to the national security and the human health in daily life. Consequently, wave-absorbing materials have gradually garnered public attention. Biomass, with its inherent network structure, can be used to produce porous carbon for addressing electromagnetic pollution. Among various biomass sources, coconut shells are widely distributed in China and have long been treated as agricultural by-products or waste. Recycling and utilizing coconut shells to prepare wave-absorbing materials not only helps mitigate electromagnetic pollution but also offers a new approach for the high-value application of agricultural by-products such as coconut shells.

The experimental materials included coconut shells purchased from Hainan Wenchang Coconut Shell Co., Ltd.. Potassium hydroxide (KOH), calcium carbonate (CaCO₃), hydrochloric acid (HCl), and paraffin wax (C₂₅H₅₂) purchased from Shanghai Titan Scientific Co., Ltd. The coconut shells were processed into 1-2 cm pieces, cleaned, and dried at 80 ℃ for 24 h. The dried pieces were then ground into powder using a pulverizer and sieved through a mesh with an aperture of 250-300 μm. The coconut shell powder was mixed with CaCO₃ and KOH at mass ratios of 1.0∶1.0∶0.5, 1∶1∶1, 1∶1∶2, and 1∶1∶3, respectively. The mixtures were uniformly ground in a pulverizer to obtain alkalized coconut shell powder, which was subsequently dried. The dried alkalized powder was placed in a tube furnace, which was purged with nitrogen gas (N₂), and then carbonized at 700 ℃ for 2 h. The resulting product was neutralized with hydrochloric acid (HCl) under magnetic stirring for 12 h, washed with deionized water until neutral, and finally dried at 80 ℃ for 24 h , then the coconut shell-based porous carbon was obtained.

In this study, coconut shell was utilized as the carbon source, based on its inherent multi-level network structure and high carbon content. Using KOH and CaCO₃ as dual activators, a one-step carbonization method was employed to prepare coconut shell-based carbon wave-absorbing materials with superior microwave absorption performance. Compared to pure coconut shell carbon and coconut shell carbon activated solely with an equal mass of KOH, the sample prepared with dual activators exhibited more uniform surface pore distribution and hierarchical structure. This specific structure played a critical role in enhancing the electromagnetic wave absorption performance. Consequently, the dual-activator method offered a novel approach for preparing porous carbon materials with complex three-dimensional micro/mesoporous structures. At 700 ℃, the gradual addition of activator resulted in enlarged pores and increased defects in the porous carbon structure, ultimately causing pore collapse. Higher activator concentrations led to larger pore diameters, which reduced electromagnetic wave reflection efficiency and consequently diminished microwave absorption performance.

A coconut shell-based porous carbon material with excellent wave-absorbing performance was successfully prepared via a one-step carbonization method combined with a dual-activator (KOH and CaCO₃) activation process. By adjusting the mass ratios of KOH to CaCO₃, the pore structure of the resulting carbon material was modulated, leading to varied electromagnetic wave absorption properties. The optimal absorption performance was achieved under the conditions of a carbonization temperature of 700 ℃ and a mass ratio of coconut shell powder : CaCO₃∶KOH = 1∶1∶1. The material obtained under these conditions exhibited a minimum reflection loss (RL_min) of -45.79 dB at a sample thickness of 5.0 mm and a frequency of 5.12 GHz. This study utilized a simple one-step carbonization process to produce effective wave-absorbing materials with abundant coconut shell waste, providing valuable theoretical guidance for the development of high-performance absorbers and significantly broadening the application prospects for biomass-derived wave-absorbing materials.