Key Takeaways & Executive Findings
- •• Discovery of a novel phenomenon where pseudocapacitance of flexible MXene supercapacitors changes sensitively in response to bending, leading to the development of Pseudocapacitive Sensors. • Pseudocapacitive Sensors repurpose supercapacitors as strain sensors, detecting capacitance changes from shifts between pseudocapacitance and electrical double layer capacitor, achieving a gauge factor of about 1200. • The sensing mechanism relies on local pH changes induced by bending, which switch energy storage kinetics from Faradaic protonation to non-Faradaic adsorption. • This work expands the application of pseudocapacitance from energy storage to multifunctional electronics, with potential in robotics, biomedical devices, and health monitoring.
Abstract
Extensively explored for their distinctive pseudocapacitance characteristics, MXenes, a distinguished group of 2D materials, have led to remarkable achievements, particularly in the realm of energy storage devices. This work presents an innovative Pseudocapacitive Sensor. The key lies in switching the energy storage kinetics from pseudocapacitor to electrical double layer capacitor by employing the change of local pH (-log[H+]) in MXene-based flexible supercapacitors during bending. Pseudocapacitive sensing is observed in acidic electrolyte but absent in neutral electrolyte. Applied shearing during bending causes liquid-crystalline MXene sheets to increase in their degree of anisotropic alignment. With blocking of H+ mobility due to the higher diffusion barrier, local pH increases. The electrochemical energy storage kinetics transits from Faradaic chemical protonation (intercalation) to non-Faradaic physical adsorption. We utilize the phenomenon of capacitance change due to shifting energy storage kinetics for strain sensing purposes. The developed highly sensitive Pseudocapacitive Sensors feature a remarkable gauge factor (GF) of approximately 1200, far surpassing conventional strain sensors (GF: ~1 for dielectric-cap sensor). The introduction of the Pseudocapacitive Sensor represents a paradigm shift, expanding the application of pseudocapacitance from being solely confined to energy devices to the realm of multifunctional electronics. This technological leap enriches our understanding of the pseudocapacitance mechanism of MXenes, and will drive innovation in cutting-edge technology areas, including advanced robotics, implantable biomedical devices, and health monitoring systems.
1. Introduction
MXenes, owing to their unique pseudocapacitance traits, have been extensively researched in the field of energy storage devices, particularly in supercapacitors and microsupercapacitors (MSCs) [1–7]. Ti3C2Tx MXenes, renowned for their remarkable pseudocapacitive properties, exhibit this phenomenon within supercapacitors due to redox reactions at the electrode–electrolyte interface [8–13]. Specifically, these reactions involve the transfer of charge, resulting in the alteration of the titanium oxidation state, and are accompanied by protonation of oxygen functional groups [14–17]. This phenomenon is compared with that of an electrical double layer capacitor (EDLC), which operates through non-faradaic physisorption without involving any redox chemical reaction.
The potential applications of this distinctive feature extend across diverse fields, specifically in energy storage and power supply systems, eliciting substantial interest. MXene has inherent anisotropic mechanical properties. MXene-hydrogel, containing negatively charged unilamellar nanosheets, aligns under mechanical shearing of Ti3C2Tx liquid–crystal phase.
Meanwhile, in the field of mechanical strain sensor, there are still challenges in practical use for motion sensing as human–machine interfaces: sensitivity, responsivity, hysteresis characteristics (response/recovery), linearity (sensitivity to strain). Among these, the most critical limitation is low sensitivity, where gauge factor (GF) is theoretically 1 [18–20]. Xu et al. improved sensor sensitivity by using ionically cross-linked polymer-alginate and covalently cross-linked polymer-polyacrylamide, achieving a gauge factor (GF) of approximately 165 [21]. Rao et al. fabricated a highly sensitive capacitive strain sensor comprising a self-healing polydiacetylene-polyacrylic acid- Cr3+ hydrogel, which exhibits a GF of up to 160 [22]. However, these
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Eunji Kim, Seongbeen Kim, Hyeong Min Jin, Gyungtae Kim, Hwi-Heon Ha, Yunhui Choi, Kyoungha Min, Su-Ho Cho, Hee Han, Chi Won Ahn, Jaewoo Roh, Il-Kwon Oh, Jinwoo Lee, Yonghee Lee (2024). Unlocking Novel Functionality: Pseudocapacitive Sensing in MXene-Based Flexible Supercapacitors. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01567-2
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Frequently Asked Questions
What is a Pseudocapacitive Sensor?
A Pseudocapacitive Sensor is a novel device that repurposes MXene-based flexible supercapacitors as strain sensors. It detects capacitance changes resulting from the shift between pseudocapacitance and electrical double layer capacitance, which is induced by bending and local pH changes.
How does the Pseudocapacitive Sensor achieve such a high gauge factor?
The sensor achieves a gauge factor of approximately 1200, far exceeding conventional strain sensors (GF ~1), by exploiting the sensitive dependence of pseudocapacitance on bending-induced local pH changes, which alter the energy storage kinetics from Faradaic to non-Faradaic processes.
What is the underlying mechanism of pseudocapacitive sensing?
The mechanism involves bending-induced shearing that aligns MXene sheets, increasing the diffusion barrier for H+ ions and raising local pH. This blocks protonation (Faradaic) and shifts to physical adsorption (non-Faradaic), causing a measurable capacitance change.
In which electrolyte is pseudocapacitive sensing observed?
Pseudocapacitive sensing is observed in acidic electrolytes, but it is absent in neutral electrolytes, indicating the critical role of H+ ions in the sensing mechanism.
What are potential applications of Pseudocapacitive Sensors?
Potential applications include advanced robotics, implantable biomedical devices, and health monitoring systems, where high sensitivity and flexibility are required.
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