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.

In a typical heterogeneous electrocatalytic reaction, for the given active sites, the electronic structure plays a determining role in electron transfer between the active sites and reactant molecules, which impacts the reaction efficiency. Besides the electronic properties of the electrocatalysts, the reaction interface at which the charge transfer occurs plays an important role in the reaction kinetics. Moreover, the accessibility of the active sites to the reactant molecules also affects the reaction efficiency. However, a well-balanced effective strategy for electronic structure optimization that improves not only the activity but also stability and cost-effectiveness is needed. Besides, a robust model specifically tailored to investigate the kinetics of the electrocatalytic reaction is required to exclude the interference of thermodynamic factors. A feasible characterization technique for probing the complex interfacial process is also required.

To address these remaining challenges in the three aspects above, this thesis proposed the strategies to optimize the electrocatalytic reaction processes as follows:

(1) Tuning the electronic structure of the active sites by engineering coordination environment and introducing strain effect. Specifically, Ni single atom was constructed to engineer the coordination environment, and the electrocatalytic performance with the tuned electronic structure was examined towards hydrazine oxidation reaction. The strain effect was created by introducing Cu single atom to BiOCl substrate, and the optimized electronic structure was investigated;

(2) Optimizing the interfacial HER kinetics targeted by proposing a specific Pt model catalyst with a channel-opening modifier. The interfacial water structure was studied by in situ surface-enhanced Raman technique, and the role of this promoting modifier was elucidated by ab initio molecular dynamic simulation;

(3) Improving the local concentration of CO₂ for electrochemical CO₂ reduction reaction with a poly(ionic liquid) modifier, with Au as the model catalyst and the targeted characterization techniques.