## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials  
### Biochar as EMI Shielding Material  
Among conventional EMI shielding materials, metals such as Cu, Ag, Al, etc., are highly efficient EMI shielding materials due to their electrical conductivity and ability to reflect incident EMWs [16]. Regardless, the use of these metals is limited owing to their high density, inconvenient processing, and poor resistance to corrosion [16]. Recently, shielding materials have been investigated in the form of composites with plastic or metals as substrates [34]. As alternative solutions, conductive polymer-based composites, porous conductive foams, and polymers with micro- and nanoscale filters such as carbon material (graphite, graphene, reduced graphene oxide, carbon nanotubes), MXenes, metal nanoparticles, and nanowires have been explored due to their low mass densities, good flexibility, and stretchability [4; 35]. One of the latest approaches in developing new, sustainable, and eco-friendly solutions for EMI shielding is the integration of natural, renewable carbon materials--biochars--into polymers [4]. Biochars originate from various biomass sources, such as bamboo, sugarcane, and cork. To create biochars from biomass, biomass resources are converted to carbon materials by pyrolytic processes in an oxygen-limited environment. A schematic presentation of biochar starting materials and production steps is displayed in Figure 2. After high-temperature carbonization, biochars show extraordinary hardness, excellent thermal stability, and high electrical conductivity, making them suitable fillers for polymers [36]. By combining biochars and polymers, the mechanical, physical, and electrical properties of both materials can be upgraded successfully. Figure 1: Schematic representation of new shielding materials based on biochars, scaffolds, rare earth, and ferrite materials discussed in this review. Part of Figure 1 (lower, left part) is adapted from reference [33]. The electrical conductivity of biochar is closely related to the source properties and carbonization conditions [37]. Nevertheless, some researchers have disregarded the electrical properties of biochars, so their potential application in EMI shielding has rarely been examined [38; 39; 40]. In 2018, [PERSON] et al. conducted an initial research study [39]. Namely, they used commercially available bamboo charcoal as the biochar filler and/or an ultra-high-molecular-weight polyethylene (UHMWPE)/linear low-density polyethylene (LLDPE) blend as a potential EMI shielding composite. The composite with 80 wt.% of biochar was fabricated using the mass-producing extrusion and hot compression methods. Through the carbonization of bamboo charcoal at 1100 \({}^{\circ}\)C, a graphitic-like structure with good electrical conductivity and a high specific surface area was obtained. Namely, an electrical conductivity of 107.6 S/m and EMI shielding effectivity (SE) of 48.7 dB were measured at 1.5 GHz. The specific EMI SE of the composite was 39.0 dB cm\({}^{3}\)/g. This value is four times higher than the specific EMI SE of copper (10.0 dB cm\({}^{3}\)/g) [41]. [PERSON] et al. applied the same approach to prepare a biodegradable two- and three-phase composite of graphite, biochar derived from pine chips, and polylactic acid (PLA) [4]. The EMI shielding performances were tested in the K-band (18-26.5 GHz). The composite showed an outstanding EMI SE of \(>\)32 dB when the materials were prepared in the form of a film with a thickness of 25 \(\upmu\)m and a high specific shielding effectiveness (SSE/t) of \(>\)890 dB cm\({}^{2}\)/g. It can be seen that these composites are convenient for the application of wearable/portable and stationary devices. Shortly after, [PERSON] et al. presented a biochar-iron (BC-Fe) material obtained by the pyrolysis of industrial tea waste biomass and the encapsulation of BC-Fe into polymethyl methacrylate (PMMA) polymer [42].