Modern life is greatly facilitated by sensor technology, which improves both our comfort and efficiency. In particular, humidity sensors are useful because of the many ways in which they impact our daily life. Humidity sensors can ascertain humidity levels in various sections of an object or in distinct places. The sensing material is a critical component of the device, as it detects humidity. The ability to detect humidity levels in any of the materials varies according to their unique properties. Polymers and polymer-based compounds are widely used as sensing elements in humidity sensors because of their unique properties such as water absorption/adsorption, flexibility, hygroscopicity and high surface area, which are more favorable for the Grotthuss mechanism in humidity sensor operation (H₂O + H₃O⁺ = H₃O⁺ + H₂O). This comprehensive review presents an in-depth examination of polymer and polymer composite materials employed as sensing elements in humidity detection applications. This review highlights the significance of polymers in the context of blending, cross-linking, co-polymerization, composites, and coatings. This review presents an overview of four main kinds of humidity sensors utilizing polymer-based materials: (i) polymers and polymer blends; (ii) polymer-based composites with inorganic materials; (iii) polymer-based composites with organic materials and metal–organic frameworks (MOF); and (iv) MOF, covalent organic frameworks and metallo-supramolecular polymers as sensing materials. To improve polymers and polymer-based combinations for humidity sensor applications, we have also pointed out possible research directions and obstacles.

A direct methanol fuel cell (DMFC) is an excellent energy device in which direct conversion of methanol to energy occurs, resulting in a high energy conversion rate. For DMFCs, fluoropolymer copolymers are considered excellent proton-exchange membranes (PEMs). However, the high cost and high methanol permeability of commercial membranes are major obstacles to overcome in achieving higher performance in DMFCs. Novel developments have focused on various reliable materials to decrease costs and enhance DMFC performance. From this perspective, cellulose-based materials have been effectively considered as polymers and additives with multiple concepts to develop PEMs for DMFCs. In this review, we have extensively discussed the advances and utilization of cost-effective cellulose materials (microcrystalline cellulose, nanocrystalline cellulose, cellulose whiskers, cellulose nanofibers, and cellulose acetate) as PEMs for DMFCs. By adding cellulose or cellulose derivatives alone or into the PEM matrix, the performance of DMFCs is attained progressively. To understand the impact of different structures and compositions of cellulose-containing PEMs, they have been classified as functionalized cellulose, grafted cellulose, acid-doped cellulose, cellulose blended with different polymers, and composites with inorganic additives.