Scalable production of carboxylated cellulose nanofibres using a green and recyclable solvent

Produced from plant renewable resources, cellulose nanofibres are an emerging class of sustainable materials with favourable mechanical properties. Once carboxylated, they can be used even more widely thanks to the possibility of surface chemical modifications. However, the current fabrication processes for carboxylated cellulose nanofibres (C-CNFs) either require harsh reaction conditions or severely impair the high aspect ratio of products, resulting in low yields, environmental impacts and poor practical value. Here we address these limitations by using a hydrated multi-carboxylic acid deep eutectic solvent comprised of only choline chloride, citric acid and water to produce ultrafine and fairly long C-CNFs. The resultant C-CNFs possess fine diameters of ~3.4 nm, high aspect ratios up to 2,500, a high carboxyl content of 1.5 mmol g⁻¹ and a high mass yield of 90.12%. Superior stability of the C-CNFs suspensions even at high concentrations allows for easy storage, transportation, processing and utilization. Moreover, the solvent exhibits a tenfold increase in its reusability, thereby highlighting its recyclability and economic viability. We further show large-scale production of C-CNFs for preparing large-area, high-performance structural materials. These unique advantages open a new avenue to the production of functional C-CNFs at an industry compatible scale.