## 14 Intrinsically Conducting Polymer Aerogels for Electromagnetic Interference Shielding  
Intrinsically conductive polymers (ICPs) refer to a group of organic polymers with unique electrical/optical properties that are similar to those of metals and inorganic semiconductors. The key advantages of ICPs are that they can be easily synthesized in a simple and cost-effective manner using less expensive monomers via a simple electro-oxidative polymerization process. The quest for designing and synthesizing ICPs began in the mid-1950s, but garnered much more attention in the late 1970s thanks to the pioneering works of [PERSON], [PERSON] and [PERSON], who later won the Nobel Prize in chemistry in the year 2000. Some of the well-known ICPs are polyacetylene (PA), polyaniline (PANi), poly[3,4-(ethylenedioxy)thiophene] (PEDOT), polythiophene (PT),  
Figure 16: (a–g) Stepwise demonstration of the preparation of cellulose/rGO/Fe\({}_{3}\)O\({}_{4}\) composite hydrogels, aerogels and films via freeze-drying and air-drying, respectively. Reproduced with permission from [156]. Copyright 2020, American Chemical Society. Figure 17: A demonstration of various steps involved in the fabrication of PANI/CNF aerogels. Reproduced with permission from [157]. Copyright 2020, Elsevier Ltd.  
polypyrrole (PPy) and polyphenylene, which have been widely studied for various applications such as sensors, OLEDs, batteries, supercapacitors and EMI shielding [158; 159]. In the past few decades, ICP aerogels have attracted a great deal of interest owing to their low density, high surface area and excellent electrical conductivity, especially for microwave-absorbing applications. In this view, [PERSON] et al. [160] fabricated ultralight and 3D porous polypyrrole/nano-SiO\({}_{2}\) aerogels via an in situ gelation process, as shown in Figure 18a. Pyrrole monomer was dispersed along with nano-SiO\({}_{2}\) powder in H\({}_{2}\)O/ethanol solution, followed by addition of anhydrous FeCl\({}_{3}\) to initiate polymerization. A dark-colored hydrogel was obtained, which was later dried under vacuum at 50 \({}^{\circ}\)C to form PPy/nano-SiO\({}_{2}\) aerogels. At 20% SiO\({}_{2}\) loading, the maximum effective electromagnetic absorption (EMA) bandwidth could reach 6.0 GHz at an absorber thickness of 2.5 mm [160]. It is envisaged that nano-SiO\({}_{2}\) was added to improve the impedance matching and improve the broadband EMA performance of the aerogels. In order to enhance the microwave absorption performance, magnetic materials can also be incorporated in ICP aerogels. In another study by [PERSON] and coworkers, polydopamine-functionalized carbonyl iron powder-anchored polypyrrole (CIP@PPy) aerogels were examined for their microwave absorption ability. Herein, carbonyl iron powder was mechanically mixed along with a surfactant and dopamine hydrochloride to form polydopamine-coated carbonyl iron powder. This powder was later blended to a pyrrole monomer in H\({}_{2}\)O/ethanol solution by ultra-sonication, and FeCl\({}_{3}\) was added dropwise to start polymerization, as shown in Figure 18b. This resulted in the formation of a black hydrogel, which was washed serval times with distilled water and ethanol to remove the unreacted monomers and impurities. The hydrogel was then then dried at 50 \({}^{\circ}\)C for 12 h to form CIP@PPy aerogels. These aerogels demonstrated two strong absorption peaks (min RL) of \(-38.9\) and \(-39.5\) dB at 12.2 and 14.2 GHz, respectively, at a 2.2 mm thickness [161]. It is also noteworthy that they showcased a broad bandwidth (min RL \(<-10\) dB) of 6.1 GHz in the region of 10 to 16.1 GHz, which infers its commercial applicability in this frequency region. It is envisaged that this broad-bandwidth microwave absorption behavior is due to the presence of carbonyl iron powder in these aerogels  
Figure 18: (**a**) The preparation of PPy/SiO\({}_{2}\) composites. (**b**) Schematic representation of the formation of CIP@PPy aerogels and the interaction of the microwave within the composite.