The development of well-characterized cellulose nanofibrils (CNFs) has allowed
for the development of the bottom-up engineering of a wide range different
materials with tailored properties. A common denominator in these materials
is the need for a controlled self-assembly of the CNFs in the wet state and a
subsequent preservation of the so created structure in the dry form. In this
respect the preparation of foams and aerogels has attracted a lot of
attention since the arrested states of the fibrils in the foams or in the
liquid state can basically be preserved with these techniques with a
subsequent freeze drying, controlled liquid exchange or different approaches
using chemical crosslinking. In our work we have been using basically all
these different techniques and by combining high quality CNF dispersions with
nano-CaCO3 and alginate, a freezing in a standard freezer at -18°C and
thawing using acidic acetone it was possible to prepare aerogels/foams with
as low density as 2 kg/m3 (1). This is also a scalable technique that
basically has been used to prepare low density foam materials used for
fish-guidance in hydroelectric power plants. In another approach, specially
prepared cationic fibrils were combined with nano-MOFs (nZIF-8), the use of
directional freezing using liquid nitrogen followed by lyophilization we were
able to prepare aerogels with excellent mechanical properties at a
composition of 10 % CNFs and 90 % MOFs. The so prepared aerogels had a
specific surface areas of 1470 m2/g and could be used for gas separation,
water cleaning and for fire retardancy (2). After carbonization at 800-100 °C
these aerogels also could be used as symmetrical electrodes in two electrode
capacitors and showed to have a capacitance of 270 F/g at 50 mV/s and a very
good stability over 20 000 cycles (2).