Capacitive deionization (CDI) is a novel technology that has been developed for removal of charged ionic species from salty water, such as salt ions. The basic concept of CDI, as well as electrosorption, is to force charged ions toward oppositely polarized electrodes through imposing a direct electric field to form a strong electrical double layer and hold the ions. Once the electric field disappears, the ions are instantly released back to the bulk solution. CDI is an alternative low-energy consumption desalination technology. Graphene-like nanoflakes (GNFs) with relatively high specific surface area have been prepared and used as electrodes for capacitive deionization. The GNFs were synthesized by a modified Hummers' method using hydrazine for reduction. They were characterized by atomic force microscopy, N2 adsorption at 77 K and electrochemical workstation. It was found that the ratio of nitric acid and sulfuric acid plays a vital role in determining the specific surface area of GNFs. Its electrosorption performance was much better than commercial activated carbon (AC), suggesting a great potential in capacitive deionisation application. Further, the electrosorptive performance of GNFs electrodes with different bias potentials, flow rates and ionic strengths were measured and the electrosorption isotherm and kinetics were investigated. The results showed that GNFs prepared by this process had the specific surface area of 222.01 m²/g. The specific electrosorptive capacity of the GNFs was 23.18 µmol/g for sodium ions (Na+) when the initial concentration was at 25 mg/L, which was higher than that of previously reported data using graphene and AC under the same experimental condition. In addition, the equilibrium electrosorption capacity was determined as 73.47 µmol/g at 2.0 V by fitting data through the Langmuir isotherm, and the rate constant was found to be 1.01 min⁻¹ by fitting data through pseudo first-order adsorption. The results suggested that the chemically synthesized GNFs can be used as effective electrode materials in CDI process for brackish water desalination.

body motion monitoring, covering subtle and large-strain ranges.Capacitive strain sensor has a simple structure with two electrodes separated by a dielectric material. Metals [19] and semiconductors [20] can be used as electrodes for constructing capacitive sensors. However, these electrode materials have poor mechanical properties, limiting the sensing range of capacitive strain sensors. [21] To solve the problem, geometrical engineering of rigid materials into buckled, [22][23][24] wrinkled, [16,25] and kirigami [26] forms is widely employed. For example, Someya's group shaped gold film into a wrinkled form for developing capacitive strain sensors, with a stretchability up to 140%. [16] In addition to the geometrical engineering, the usage of intrinsic stretchable materials as electrode for soft capacitive strain sensors is the other strategy widely adopted. These intrinsic stretchable materials include liquid metal, [27] nanomaterial/elastomer composites, [28] and conductive textiles. [29] Dickey and co-workers developed a liquid-metal-based capacitive strain sensor having a high stretchability (≈100%). [8] A capacitive strain sensor using conductive textile electrode was developed by Walsh's group, with a stretchability of 100% and a sensitivity of 1.23. [29] Cohen et al. reported a carbon nanotube (CNT)/elastomer-based capacitive sensor, owning a stretchability of 100% and a sensitivity of 0.99. [7] Although the above strategies can provide capacitive sensors with high stretchability, most of the electrode materials are not intrinsically self-healable. To fully mimic the functionality of human skin, the self-healability for sensory devices is highly demanded. [30] Hydrogels which consist of 3D networks with large quantity of water or ionic liquid have been proposed as electrode materials for soft electronics, due to their large stretchability, self-healability, and biocompatibility. [31,32] For sensing applications, hydrogel-based resistive strain sensors have been widely studied. [33][34][35][36] These devices have achieved large sensing limits, but the hydrogels have relatively long self-healing time and low self-healing efficiency. [33,36,37] Additionally, some of them needs external heating to accelerate the self-healing process. [35,38] Through the dynamic complexing interaction between metal ion and OH group, polyvinyl alcohol (PVA) based hydrogel has been investigated as active materials for constructing sensors. [39,40] CNT/PVA-based resistive strain Capacitive strain sensors could become an important component of electronic skin (E-skin) due to their low hysteresis and high linearity. However, to fully mimic the functionality of human skin, a capacitive strain sensor should be stretchable and self-healable. The development of such a sensor is limited by electrode materials which generally lack self-healability and/or stretchability. A highly stretchable and self-healing MXene (Ti 3 C 2 T x )/polyvinyl alcohol (PVA) hydrogel electrode is developed for use in capacitive strain sensors for E-skin. The...

Graphene has been synthesized by the modified Hummers method and used as electrosorptive electrodes for capacitive deionization. Batch-mode experiments in NaCl solutions at low voltage (#2 V) are conducted in a continuously recycling system to investigate the electrosorption performance of graphene. The results show that the graphene exhibits a high electrosorption capacity of 1.85 mg/g. The ion sorption follows a Freundlich isotherm, indicating monolayer adsorption. And the electrosorption of NaCl onto graphene electrodes is driven by a physisorption process by taking into account the thermodynamic parameters.

Metal-organic frameworks (MOFs) with high porosity and a regular porous structure have emerged as a promising electrode material for supercapacitors, but their poor electrical conductivity limits their utilization efficiency and capacitive performance. To increase the overall electrical conductivity as well as the efficiency of MOF particles, three-dimensional networked MOFs are developed via using preprepared conductive polypyrrole (PPy) tubes as the support for in situ growth of MOF particles. As a result, the highly conductive PPy tubes that run through the MOF particles not only increase the electron transfer between MOF particles and maintain the high effective porosity of the MOFs but also endow the MOFs with flexibility. Promoted by such elaborately designed MOF-PPy networks, the specific capacitance of MOF particles has been increased from 99.2 F g for pristine zeolitic imidazolate framework (ZIF)-67 to 597.6 F g for ZIF-PPy networks, indicating the importance of the design of the ZIF-PPy continuous microstructure. Furthermore, a flexible supercapacitor device based on ZIF-PPy networks shows an outstanding areal capacitance of 225.8 mF cm, which is far above other MOFs-based supercapacitors reported up to date, confirming the significance of in situ synthetic chemistry as well as the importance of hybrid materials on the nanoscale.

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