Ion conduction, as a vital behavior for all living matter including plants, animals, and human beings, governs a series of biologic functions such as nutrition supply, neural signal transmission, and even liver metabolism. (1−4) Recent decades have witnessed an increasing integration of ionic conduction processes into electronics such as thermoelectrics, (5−9) batteries, (10−13) ionotronics, (14−18) salt gradient power generators, (19,20) and multifunctional sensors. (14,21−25) Since ionic conductor (IC) substantially impacts the functional performance of these devices, engineering IC with optimized performance has been considered one key step for achieving high caliber electronics. However, even though many efforts have been made, IC with integrated electrical, mechanical, environmental, and sustainable properties has yet to be developed. For example, ICs confining aqueous electrolytes possess high ionic conductivity, but their working voltage is severely limited by the narrow operating voltage range of aqueous electrolytes, and good mechanical performance is not readily achievable. (26,27) Conversely, solid ICs can operate at higher voltages, but they normally deliver much lower ionic conductivity. (28) Additionally, most of these ICs are fabricated from petroleum-based materials with non or low biodegradability, which inevitably leads to resource depletion, waste accumulation, and greenhouse gas (GHG) emission, jeopardizing the commitment established by the Paris Agreement to reduce 30% of the CO2 emissions by 2030. (29−31) Therefore, it is desirable and valuable to fabricate ICs with all-around characteristics based on renewable and environmentally sustainable materials.

In this context, current research on ICs has been focusing on using cellulose, the most abundant biopolymer on Earth, as the basic building block. (32−34) The past 10 years have witnessed the historical development of cellulose-based ICs in a wide range of applications, from simple polymer electrolytes, flexible supercapacitors, to contemporary ionic cables and ion intercalated ionic conductors (Figure 1). Cellulose is widely found in plants or bacteria and it has been traditionally employed to manufacture pulp and paper, packaging materials, or textiles. (35,36) The unique molecular structure and configuration equip cellulose with integrated characteristics including high mechanical property, adaptive surface modification chemistry, fibrous network, and programmable structural features. These features make cellulose a promising green building block for the development of next-generation electronics, clearly reflected by the increasing trend of publications. (37−39) There have been several reviews that summarize the knowledge accumulated on the role of cellulose-based conductors in the fabrication of electrical devices, with a special focus on the electrical conduction. (39−41) The functions of cellulose in the context of ionic conductors, to our best knowledge, have not been comprehensively summarized in spite of the pivotal role that ionic conductor plays in many modern devices...

Figure 1. Historical development and milestone of cellulose-based ICs...

Figure 2. Structure of this review, including introduction, classification of cellulose-based ICs, fabrication and engineering strategies, properties and performance, applications, and also environmental impact assessment.

Figure 3. Three main categories of cellulose-based ICs (gel ionic conductors, ion reservoirs, and solid ionic conductors) and their representative characteristics, advantages, and disadvantages.