Research progress of non-metallic electrode materials for electrochemical actuators

Electrochemical actuators have the remarkable ability to efficiently convert electrical or chemical energy into mechanical work. This capability has captured the attention of researchers in the fields of artificial muscles, bionic robotics, and miniaturized medical devices. A standard electrochemical actuator typically comprises electrode and electrolyte layers. The electrode layers play a pivotal role in determining the actuator's driving capability and electrochemical performance. Traditionally, metal materials were used for these electrodes, but they exhibited limited flexibility and poor cycle stability. Consequently, an increasing number of researchers are now exploring non-metallic electrode materials as a promising alternative. This paper presents a comprehensive overview of recent advancements in non-metallic electrode materials for electrochemical actuators. To begin, we introduce the device structures and operational principles of electrochemical actuators. When compared with other types of flexible actuators, including thermal, optical, and pneumatic actuators, electrochemical actuators offer distinct advantages such as lightweight construction, low voltage requirements, exceptional controllability, rapid response times, and extended durability. Subsequently, we delve into an examination of various electrode materials, encompassing conductive polymers, carbonbased materials, novel two-dimensional substances, and their composite counterparts. We thoroughly evaluate the advantages and disadvantages associated with non-metallic electrode materials deployed in these actuators. Conductive polymer materials are distinguished by their high ionic activity, expansive strain range, and excellent film-forming characteristics. However, they tend to exhibit relatively lower electrical conductivity, diminished strength, and reduced stiffness. In contrast, carbon materials offer superior electrical conductivity, cycle stability, and robust mechanical properties. Nonetheless, they possess a weaker ion storage capacity and can be more cost-intensive. It is noteworthy that although the conductivity of conductive polymers can be enhanced through doping, carbon materials generally surpass them in terms of conductivity. Furthermore, the Young's modulus of carbon materials significantly surpasses that of polymers. However, while carbon materials demonstrate a double-layer capacitance, conductive polymer materials exhibit a pronounced Faraday capacitance effect. This effect reinforces interactions with ions and facilitates ion insertion and removal. Emerging two-dimensional materials exhibit commendable electrical conductivity, structural order, and solvent compatibility. Nevertheless, they often necessitate integration with other materials to serve as effective film binders. Additionally, porous framework materials, increasingly harnessed to augment porosity and specific surface area, are now being incorporated into actuator electrode materials. Finally, we offer insight into the future trajectory of electrochemical actuators and electrode materials. The ongoing development of electrochemical actuators continues to elevate the demands placed on related materials and structural designs. By orchestrating composite integration of diverse materials, leveraging the unique strengths of individual components, we anticipate realizing optimal outcomes across electromechanical performance, mechanical properties, and electrochemical characteristics. Furthermore, ion actuators have transitioned from rudimentary electrochemical driving mechanisms to multifactor-driven systems. They have evolved from singular actuators to integrated arrays, culminating in sophisticated circuitry systems that incorporate various other electrically driven components, such as flexible sensors and memristors. This comprehensive integration endeavors to replicate human sensory perception, cognitive judgment, and physical action. As a result, it holds immense potential for applications in interactive human-machine interfaces and the advancement of intelligent robotics.