CO₂ sensors are very important; however, their performance is limited by stability and selectivity. This study unveils a capacitive CO₂ sensor with a dielectric layer comprised of amine-functionalized cellulose nanofibril (CNF) foam, significantly enhanced by the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The core innovation of this research lies in the strategic use of CNF-based foam, which leads to a substantial increase in sensor capacitance, setting a new standard in CO₂ monitoring technologies. The sensor showcases exceptional performance under ambient conditions, with marked improvements in sensitivity towards CO₂. The advancements are attributed to the chemisorption properties of the aminated CNFs combined with the DBU enhancement, facilitating more effective CO₂ capture. By integrating these materials, we present a sensor that opens new avenues for environmental monitoring, healthcare diagnostics, and industrial safety, establishing a new benchmark for capacitive CO₂ sensors in efficiency and environmental sustainability.

The true promise of MXene as a practical supercapacitor electrode hinges on the simultaneous advancement of its three-dimensional (3D) assembly and the engineering of its nanoscopic architecture, two critical factors for facilitating mass transport and enhancing an electrode’s charge-storage performance. Herein, we present a straightforward strategy to engineer robust 3D freestanding MXene (Ti₃C₂T_x) hydrogels with hierarchically porous structures. The tetraamminezinc(II) complex cation ([Zn(NH₃)₄]²⁺) is selected to electrostatically assemble colloidal MXene nanosheets into a 3D interconnected hydrogel framework, followed by a mild oxidative acid-etching process to create nanoholes on the MXene surface. These hierarchically porous, conductive holey-MXene frameworks facilitate 3D transport of both electrons and electrolyte ions to deliver an excellent specific capacitance of 359.2 F g^–1 at 10 mV s^–1 and superb capacitance retention of 79% at 5000 mV s^–1, representing a 42.2% and 15.3% improvement over pristine MXene hydrogel, respectively. Even at a commercial-standard mass loading of 10.1 mg cm^–2, it maintains an impressive capacitance retention of 52% at 1000 mV s^–1. This rational design of an electrode by engineering nanoholes on MXene nanosheets within a 3D porous framework dictates a significant step forward toward the practical use of MXene and other 2D materials in electrochemical energy storage systems. 

This thesis presents a comprehensive study on stimuli-responsive materials derived from cellulose nanofibrils (CNFs), focusing on their synthesis, characterization, and performance evaluation in various applications. Renowned for their biodegradability, renewability, and robust mechanical properties, CNFs are explored in three primary contexts: moisture-responsive actuators, voltage-responsive actuators, and CO₂-responsive sensors.

The unique properties of CNFs, such as high tensile strength and surface area, are leveraged to achieve effective motion in response to moisture exposure. Specifically, CNFs are utilized to create bilayer, torsional, and tensile actuators. These actuators exhibit controllable and dynamic responses, making them suitable for applications in soft robotics and wearable technology.

In the realm of voltage-responsive actuators, this study investigates the impact of various electrolytes and counteranions on positively charged CNFs. It uncovers the critical role of electrolyte choice, ion migration and the plasticization effect within the CNFs matrix, resulting in volumetric expansion, which is pivotal to the actuation mechanism. These insights pave the way for CNFs applications requiring precise control of motion and flexibility in shape, such as in soft robotics.

The third area of application involves the development of a capacitive CO₂ sensor using CNFs-based foams functionalized with primary amines to enhance CO₂ capture through chemisorption. This functionalization turns the CNFs-based foam into an efficient dielectric layer (DE) for sensor applications. The addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to the DE further expands the scope of sensor's capacitance change in response to CO₂ exposure, underscoring its potential in environmental monitoring and CO₂ detection.

Overall, this thesis emphasizes the versatility and adaptability of CNFs as a sustainable biomaterial for developing stimuli-responsive devices. The insights gained from studying CNFs in these varied applications contribute significantly to materials science and open new avenues for research in sustainable, bio-based materials.

Cellulose-based electro-actuators have enormous potential in various applications, e.g. artificial muscles, soft grippers, medical devices, just to name a few, owing to their high mechanical strength, lightness and natural abundance. However, significant challenges remain in the fabrication of such electro-actuators featuring low operating voltage and fast response kinetics. We report here a facile fabrication route towards high-performance electro-actuators composed of CNFs films doped with ionic liquids or lithium salts and sandwiched by two thin film gold electrodes. Large bending motion at voltages as low as 3.0 V could be observed. The size effect of both anions and cations on the actuation was comprehensively investigated. CNF-TFSI@LiTFSI and CNF-BF4@EMIM-BF₄ electro-actuators presented the best bending strain under an AC voltage of 3.0 V. This work provides new inspiration in the design of natural polymer-based high-performance electro-actuators.

Soft actuators capable of large and swift locomotion in combination with biocompatibility hold great potential in medicine and healthcare applications. In this contribution, a polypeptide-type poly(ionic liquid) (termed “PLL-PMIM-Tf2N”) was designed and synthesized by post-polymerization modification of poly-L-lysine; Next, via controlled electrostatic complexation with poly(acrylic acid) (PAA) or poly(L-glutamic acid) (PLG), porous membrane actuators termed PLL-PMIM-Tf2N/PAA or PLL-PMIM-Tf2N/PLG, respectively, were tailor-made. Hemolytic tests support excellent blood cell compatibility of the fully polypeptide-based PLL-PMIM-Tf2N/PLG actuator. The as-made soft actuators could bend significantly up to 500°with a maximum curvature of 7 cm−1 in response to solvent vapor in a fast actuation process within 2 s. Their high sensitivity towards solvent molecules equips them with skills to distinguish solvent isomers, as exemplified by butanol isomers, in terms of bending curvature due to varied molecular interactions between the actuator and the isomer. To be highlighted is an unusual irreversible actuated tubular state of the fully polypeptide-based PLL-PMIM-Tf2N/PLG actuator that is stable in shape and resembles bionic blood vessels. Such polypeptide-type poly(ionic liquid) actuators are of significant value in future development of biocompatible devices for medical and healthcare applications.

Bilayer actuators, traditionally consisting of two laminated materials, are the most common types of soft or hybrid actuators. Herein, a nanomaterial-based organic–inorganic humidity-sensitive bilayer actuator composed of TEMPO-oxidized cellulose nanofibrils (TCNF-Na⁺) and reduced graphene oxide (rGO) sheets is presented. The hybrid actuator displays a large humidity-driven locomotion with an atypical fast unbending. Cationic exchange of the anionically charged TCNF-Na⁺ and control of the layer thickness is used to tune and dictate the locomotion and actuator's response to humidity variations. Assembly of a self-oscillating electrical circuit, that includes a conductive rGO layer, displays autonomous on-and-off lighting in response to actuation-driven alternating electrical heating.

Single-atom catalysts (SACs), on account of their outstanding catalytic potential, are currently emerging as high-performance materials in the field of heterogeneous catalysis. Constructing a strong interaction between the single atom and its supporting matrix plays a pivotal role. Herein, Ti₃C₂T_x-MXene-supported Ni SACs are reported by using a self-reduction strategy via the assistance of rich Ti vacancies on the Ti₃C₂T_x MXene surface, which act as the trap and anchor sites for individual Ni atoms. The constructed Ni SACs supported by the Ti₃C₂T_x MXene (Ni SACs/Ti₃C₂T_x ) show an ultralow onset potential of −0.03 V (vs reversible hydrogen electrode (RHE)) and an exceptional operational stability toward the hydrazine oxidation reaction (HzOR). Density functional theory calculations suggest a strong coupling of the Ni single atoms and their surrounding C atoms, which optimizes the electronic density of states, increasing the adsorption energy and decreasing the reaction activation energy, thus boosting the electrochemical activity. The results presented here will encourage a wider pursuit of 2D-materials-supported SACs designed by a vacancy-trapping strategy. 

A series of organic phosphonic acids (OPAs) were applied as multifunctional spacers to enlarge the inner space of carbide MXene (Ti3C2Tx) laminates. A synergistic improvement in permeance, rejection and stability is achieved via introducing OPA to create pillared laminates. This strategy provides a universal way to regulate transport channels of MXene-based membranes.

Ti₃C₂T_x (MXene), a thriving member of the two-dimensional (2D) materials family, has shown increasing potential in a myriad of applications, ranging from printable electronics to energy storage and separation membranes. Nevertheless, the dilemma of its oxidative instability and the easy disintegration of its assemblies in contact with water has been restricting its real-life use. Here, we report the benefits of tartaric acid, a natural source, as a non-innocent additive in the MXene composite. In water, it can, above all, inhibit oxidation of Ti₃C₂T_x and hold individual components in the composite Ti₃C₂T_x/poly(3,4-ethylenedioxy thiophene):polystyrene sulfonate) (Ti₃C₂T_x/PEDOT:PSS) firmly together; equally important, it can boost 4-fold the composite’s electron conductivity in comparison to the additive-free equivalent. To showcase its practical value, a tartaric-acid-treated, water-stable MXene/PEDOT:PSS conductive coating is made, which serves as electrodes for an ultrafast supercapacitor; among all 2D materials-based assemblies, the designed supercapacitor delivers, to our knowledge, the record-high performance in an alternating-current filtering application.