## 1 Introduction
CNCs and CNFs combine important cellulose properties (e.g, a low density, high mechanical properties, the possibility to modify the surface chemistry) with the features of nanomaterials (e.g., high aspect ratio \(l/d\), few internal defects, high specific surface area, etc.) [1; 3; 5; 29; 30]. These cellulosic nanoparticles are used as building blocks for novel biobased engineering materials [29] such as nanocomposites [1; 31; 32], densely packed and transparent nanocellulose films [33; 34], continuous filaments [35; 36], and aerogels [37; 38; 39] and foams [40; 41; 42]. The research interest in nanocellulose-based aerogels and foams is recent but growing rapidly. Porous materials in the form of foams and aerogels exhibit unique properties such as a low density, high specific surface area, excellent deformability, the ability to absorb the shocks and/or to have good sound and thermal insulation properties [43; 44; 45]. These materials are increasingly used in an ever-wider variety of engineering fields. However, foams and aerogels are usually synthesised from polymer or carbon-based materials, most of which are derived from petroleum-based products. The reasons for the growing interest in nanocellulose-based foams and aerogels are explained by the desire to move toward renewable raw materials and more sustainable resources and processes. Nanocellulose-based foams and aerogels display high porosity [37; 46], interesting mechanical behavior characterised by a great deformability under compression [47; 48; 49], high thermal insulation properties [37; 41], and potential bio-compatibility [46; 50; 51], which make them promising solutions for several engineering fields [52]. These materials can be used in the building and car-manufacturing industry as heat/and or sound insulation board, as porous template for storage and generation of energy, as membrane for fluid purification, or even as scaffold for biomedical applications. The forming processes of aerogels and foams commonly involve two main steps, namely (i) the preparation of nanocellulose suspensions/gels or wet foams, and (ii) removal of the suspending fluid. As shown in Figure 2, the removal of the suspending fluid can be carried out using various drying procedures such as convective drying [53; 54], supercritical drying [37; 39; 55] and solidification combined with freeze-drying, also referred to as \"ice-templating\" [40; 56]. Depending on the chosen processing route, various porous structures can be obtained: (i) aerogels of ultra-high porosity (\(>\)90%) and with nanosized pores (average pore size \(d\approx 2\)-50 nm), and (ii) cellular materials in the form of foams of medium to high porosity (\(>\)50%) and with microsized pores (\(d\approx 10\)-500 um) [52]. This review focuses on the fabrication of porous nanocellulose-based materials obtained by ice templating, i.e., after freezing of suspensions and sublimation of the formed ice crystals (freeze-drying). The materials obtained with this approach commonly exhibit a foam-like structure with open porosity and micrometric cells. This technique is interesting owing to its simplicity and versatility, and constitutes an eco-friendly alternative (most of the time the solvent used is water) for the design of biobased lightweight materials. Hence, the main objective of this review is to describe and discuss the various materials, microstructures, and end-use properties that can be obtained, changing or tuning the forming conditions and/or the characteristics of nanocellulose suspensions/gels.