## 15 Next-Generation 2D Materials for High-Performance Porous EMI Shields  
Over the past decade, researchers have been actively working towards developing novel 2D materials with superior electronic, optical and mechanical properties when compared to existing 2D nanostructures such as graphene, boron nitride and metal dichalcogenides. Very recently, transition metal carbides, carbonitrides and nitrides, known as MXenes, are the latest additions to the world of 2D material. They were discovered by the pioneering works lead by [PERSON] and [PERSON] from Drexel University, USA, in 2011 [162]. MXenes are synthesized by selective etching of certain atomic layers from their layered precursors, MAX phases, which are a family of ternary carbides and nitrides. MXenes have a general formula of M\({}_{n+1}\)X\({}_{n}\)T\({}_{x}\) (\(n\) = 1-3), where M corresponds to an early transition metal (such as Ti, Zr, Hf, Sc, V, Nb, Ta, Cr or Mo), X is C\(/\)N and T\({}_{x}\) stands for the surface terminal groups such as hydroxyl, oxygen or fluorine. MXene showcases a high electrical conductivity of 1400 S.m\({}^{-1}\) with a superior ability to form stable aqueous dispersions, good antibacterial properties and ease of processing. These unique properties makes MXene suitable for a wide range of applications, such as Li-ion, Li-S, Na-ion, Mg-ion batteries, supercapacitors, ultra-thin EMI shielding materials, water purification and biomedical applications. The high electrical conductivity of these 2D materials has encouraged researchers to fabricate porous architectures via suitable incorporation into a polymer matrix. In this view, [PERSON] et al. [163] fabricated a series of lightweight, porous Ti\({}_{2}\)CT\({}_{x}\) MXene/poly(vinyl alcohol) (PVA) composite foams and flexible films via a simple freeze-drying technique and subsequent compressing of these foams, as shown in Figure 19a. Initially, few-layered Ti\({}_{2}\)CT\({}_{x}\) (f-Ti\({}_{2}\)CT\({}_{x}\)) was synthesized via a gentle etching of Ti\({}_{2}\)AlC using LiF and HCl as etching agents. f-Ti\({}_{2}\)CT\({}_{x}\) was then mixed with PVA solutions at different concentrations to form stable hydrosols and subsequently freeze-dried for 48 h in a freeze-dryer at 0.1 Pa. These foams were then pressed under a rolling machine to form MXene/PVA films. The maximum EMI shielding values of these foams and films were 33 and 26 dB at 5 mm and \(\sim\)100 \(\upmu\)m, respectively. In the case of MXene/PVA foams, they exhibited a high specific shielding effectiveness of 5136 dB.cm\({}^{3}\).g\({}^{-1}\) at an ultra-low MXene content of only 0.15 vol.%, with excellent absorption-dominated shielding performance [163]. Upon compression, these foams transform into flexible films that showcase enhanced conductivity (8.0 \(\times\) 10\({}^{-4}\) S/m), which could have resulted due to a decrease in porosity. The absorption- or reflection-dominant shielding behavior depends upon whether the incident EM wave can enter the shield material. In the case of MXene/PVA composite film, the incident EM wave is reflected back from the shield surface due to poor impedance matching, similar to metallic surfaces, whereas for the composite foam, a better impedance matching can be achieved which will minimize EM wave reflection. When the EM wave enters the bulk of foam, it will undergo multiple internal reflection within the porous network and the layered structure of the MXene flakes, which will end up as heat dissipation due to dipolar and interfacial polarization. The possible EMI shielding mechanism of MXene/PVA foam and film is demonstrated in Figure 19b. In order to maintain the structural stability and induce the elastic property in MXene foams, [PERSON] et al. [164] fabricated highly conductive polydimethylsiloxane (PDMS)-coated MXene foams.