Carbon nanotube and graphene-based bioinspired electrochemical actuators.

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Carbon Nanotube and Graphene-based Bioinspired Electrochemical Actuators Lirong Kong and Wei Chen* or chemical reactions), optical, thermal, pneumatic or electrical, depending on different mechanisms. Among these actuators, electrical actuator is mostly investigated owing to its better controllability, higher energy conversion efficiency and potential applications in intelligent robot, industrial micro-operating systems, aerospace and defense technology.[3–5] Currently, electrical actuator materials mainly include shape memory alloys, piezoelectric ceramics, and electroactive polymers (EAPs).[6] As the actuation of inorganic materials usually requires high voltage supply (typically >1 kV) or high operating temperature, EAPs appear to be good candidates for electrical actuators due to their inherent advantages in lightweight, high flexibility, and high fracture tolerance ability.[7] In principle, the deformation of EAPs under electrical stimulation could be classified into “electronic” and “ionic” categories based on different actuation mechanisms. The actuation in electronic EAPs (eEAPs) is driven by electrical field or Coulomb force, while ionic EAPs (iEAPs) are based on the diffusion or transport of ions. Although eEAPs usually exhibit high mechanical densities and large actuation forces, main disadvantage is still the high electric fields or actuation voltage needed (between 10–100 V/μm, which leads to voltages of up to several kV). Comparatively, though electrolytes with mobile ions are required, iEAPs which can be actuated under low voltage (below 1 V up to several volts) are extensively investigated for biomedical devices, bonic robotics, micro-electro-mechanical systems and many other applications.[7–10] Ionic polymer metal composite, which is a typical kind of iEAPs and capable of electromechanical actuation under low applied voltage (1–5 V).[11–14] The traditional IPMC structure usually consists of a semi-permeable ion-exchange polymer membrane covered with two electrode layers.[15,16] During actuation process, ion migration caused by the applied electrical field induces the swelling of one side and shrinkage of the other side, and thus the actuator strip bends.[17] The IPMC bending response under low voltage of about 1–2 V was firstly reported by Oguro and Shahinpoor almost at the same time.[18,19] At that time, the IPMC actuators were mostly operated in water based on water containing electrolyte layer and noble metal electrode

Bio-inspired actuation materials, also called artificial muscles, have attracted great attention in recent decades for their potential application in intelligent robots, biomedical devices, and micro-electro-mechanical systems. Among them, ionic polymer metal composite (IPMC) actuator has been intensively studied for their impressive high-strain under low voltage stimulation and air-working capability. A typical IPMC actuator is composed of one ionconductive electrolyte membrane laminated by two electron-conductive metal electrode membranes, which can bend back and forth due to the electrode expansion and contraction induced by ion motion under alternating applied voltage. As its actuation performance is mainly dominated by electrochemical and electromechanical process of the electrode layer, the electrode material and structure become to be more crucial to higher performance. The recent discovery of one dimensional carbon nanotube and two dimensional graphene has created a revolution in functional nanomaterials. Their unique structures render them intriguing electrical and mechanical properties, which makes them ideal flexible electrode materials for IPMC actuators in stead of conventional metal electrodes. Currently although the detailed effect caused by those carbon nanomaterial electrodes is not very clear, the presented outstanding actuation performance gives us tremendous motivation to meet the challenge in understanding the mechanism and thus developing more advanced actuator materials. Therefore, in this review IPMC actuators prepared with different kinds of carbon nanomaterials based electrodes or electrolytes are addressed. Key parameters which may generate important influence on actuation process are discussed in order to shed light on possible future research and application of the novel carbon nanomateials based bio-inspired electrochemical actuators.

1. Introduction As the materials used in many branches of engineering for actuation applications are typically heavy, nonflexible and require high power supply,[1,2] lightweight and bionic actuator systems are proposed. The bionic actuators refer to those devices which could produce reversible deformation under external stimulation. The stimulus could be chemical (electrochemical L. Kong, Prof. W. Chen i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou, 215123, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201303432

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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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layers. Upon hydration, the migration of cations under electrical field results in the bending motion. As the pressure from strained polymer matrix usually causes water to diffuse out of the cation-gathered areas, the displacement can not be kept and slow relaxation toward the cathode layer happens. When the IPMC actuators are in open air, the cations in the electrolyte layer are mainly crosslinked to the polymer chain, and are not easy to move in dry state. The actuation performance would decline quickly as a result of the evaporation and also electrolysis (>1.23 V) of water molecules. Therefore, they are not fit for air-working devices such as land robotics, biomimetic flying, switches, microsensors and other micro/nano electromechanical systems (MEMs/NEMs).[7–10] In order to increase their working stability in air, non-water IPMC devices in which the hydrated moving ions are replaced by air-stable ionic liquids (IL) appear.[20] As known, IL has low volatility (=0), wide potential windows (ca. ±2–3 V) and high ionic conductivity (1–10 mS cm−1), which are all advantageous for fast and stable response of ionic actuators. According to Akle et al., a lifetime of up to 250000 cycles has been recorded for an ionic liquidcontained IPMC actuator operated in air.[21] Athough Li+ ions have also been reported to enhance the lifetime of water containing actuators due to their strong interaction with water, the stability is still far away from IL based actuators. Except for the electrolyte layer, the strain resistance, low flexibility and high cost of noble metal electrode layers also restrict the wide application of traditional IPMC devices. Conventionally, noble metal electrode layers are made by time-consuming chemical plating to achieve better adhesion with ionic polymer layer.[7,9,22–24] In an obtained electrode layer, the granular noble metals are both buried in and deposited on surface of polymer. When the density of coated noble metal particles is low, the particles would be separated from each other by polymer which will definitely lead to decrease in conductivity of the electrode layer. However, when the density is high, the noble metal particles would contact with each other and even coagulate together which will probably create much higher stiffness and modulus of elasticity for the ionic polymer. Therefore during the bending, the stiff electrode layers add more strain resistance on actuator strip and forces it to bend back.[25] Moreover, the low flexibility of noble metal layer often makes it easy to crack, which may significantly decrease the life time of IPMC actuator. As a result, exploring novel electrode materials with higher flexibility and excellent electromechanical properties are of great importance for next-generation ionic actuator with higher performance. Since the discovery of single walled carbon nanotubes (SWCNTs) by Iijima in 1991,[26,27] conductive carbon nanomaterials, including carbon nanotube (CNT),[28–31] graphene,[32,33] and other carbon materials have attracted great research interest for their light weight, high conductivity, large surface area, unique mechanical and electrical properties. As these properties are all benefit for the electromechanical actuation process, in 1999 Baughman et al. reported the SWCNTs based electrochemical actuator which could exhibit actuation performance in an electrolyte solution.[28] In 2003, Guo et al. reported the exceptionally large axial electrostrictive deformation of SWCNTs as well as graphene sheets formed by opening the CNTs, can be greater than 10% for a field strength within 1 V/Å. The corresponding volumetric and gravimetric work capacities


Dr. Lirong Kong obtained her B.Sc. degree from the Department of Chemistry at Jilin University in 2007 and received her Ph.D. degree from the Institute of Polymer Physics and Chemistry at Jilin University in 2012. She is now a postdoctoral fellow in Prof. Wei Chen’s group in the Suzhou institute of NanoTech and Nano-Bionics. Her current research interests focus on the controlled synthesis, energy storage and transformation application of functional carbon materials and polymer nanocomposites. Prof. Dr. Wei Chen received his PhD degree at the Institute of Solid State Physics, Chinese Academy of Sciences (CAS) in 2001. He is now a Professor at the Suzhou Institute of NanoTech and Nano-Bionics, CAS. His research areas include carbon nanotube/graphenebased functional and intelligent nanocomposites for sensors, actuators, and solar fuels.

are predicted to be three and six orders higher than those of the best known ferroelectric, electrostrictive, magnetostrictive materials and elastomers, respectively.[34] These important results provide us strong theoretical and experimental basis for the potential application of conductive carbon nanomaterials in electromechanical actuators. A great many recent research showed that by introducing carbon nanomaterials into the electrode, the actuation performances including respond speed, actuation displacement and strain rate could be significantly improved.[35–38] By introducing carbon nanomaterials into the electrolyte layer, the actuation performance could also be improved as the doping could increase ionic conductivity and mechanical properties of the electrolyte layer.[22] Typical experimental results are listed in Table 1 to give a much clearer development status for these carbon electrode based actuators.[39–47] Although both intrinsic properties of electrode layer and electrolyte layer would affect the electromechanical properties of actuator, it can be shown from Table 1 that the electrode materials have more complicated structure characteristics and would play much more important role in influencing the final actuator performance. A tiny modulation in structure and properties, such as nanostructure, conductivity, capacitance and mechanical properties, would greatly affect the actuation properties, and the affection may be very complex because a positive change for high strain may be negative for strain rate or


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Electrode

Surface conductivity

Capacitance

Strain

Young’s modulus

1–4 Ω/sq

15–40 μF/cm2

0.5% (±2 V, 1 Hz)

30–60 MPa

[7]

—

48 F/g

0.9% (±3.5 V, 0.01 Hz)

11.1 MPa

[39]

Millimeter-long SWCNT

169 S/cm

45 F/g

0.228% (±2.5 V, 10 Hz)

156 ± 59 MPa

[40]

Vertically aligned CNTs

—

—

8.2% (±4 V)

—

[41]

CNT/PANI (50/10)

8.6 S/cm

0.0852 F/cm2

0.88% (±2 V, 0.1 Hz)

390 MPa

[42]

CNT/CB (50/40)

4.6 S/cm

0.119 F/cm2

1.1% (±2 V, 0.1 Hz)

410 MPa

[42]

MWCNT-COOH

4.6 S/cm

33.2 F/g

0.78–0.8% (±2 V, 0.05–0.005 Hz)

188 MPa

[43]

IPMC (noble metal) SWCNT

Reference

MWCNT/RuO2

> 5 S/cm

> 50 F/g

—

90–125 MPa

[44]

MWCNT/MnO2

7–11 S/cm

35–195 F/g

1.4–1.65% (±2 V, 0.1–0.005 Hz)

80–120 MPa

[45]

40 S/cm

—

0.0125% (±2 V, 1 Hz)

—

[46]

Graphene/MWCNT

135 S/cm

—

0.0287% (±2 V, 1 Hz)

—

[46]

Graphene/Ag

900 S/cm

—

0.053% (±2 V, 8.33 Hz)

—

[47]

Graphene

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Table 1. Summary of IPMC properties with different electrodes from the literature.

IPMC actuators operated in water has been reported in other Young’s modulus. Thus the challenge for development of high reviews before,[48–50] this review will lay particular attention to performance IPMC actuators still exists in large strain and fast the recently developed carbon nanomaterials based air-working response under low voltages, as well as in understanding the IPMC actuators. mechanism on how the intrinsic properties of carbon nanomaterial electrodes affect the actuation performance. The investigation will include not only the physical processes (capaci2. Carbon Nanomaterials Based IPMC tive charge of the electrical double layer, electrophoresis and Electrochemical Actuator: Fundamentals electroosmosis), but also chemical processes (faradaic charge process, rearrangement of double bonds, generation of rad2.1. Structure and Fabrication ical-ions, conformations change and so on) which are all very important for improvement of the actuation performances. The structure of carbon nanomaterials based IPMC actuator Limited by in-situ and dynamic characterization methods for is very similar to traditional IPMC actuator, which contains an these intermediate processes, some chemical processes will electrolyte layer sandwiched by two electrode layers where the need more time to be studied before a systematic theory can electrode materials are changed from noble metal to carbon be proposed. As a result, in this review we put our emphasis nanomaterials (Figure 1a). When actuation performance is mainly on the physical processes during the actuation. Based tested, the tri-layer actuator is clamped between two contact on these considerations, it is necessary to have a systematic metal electrodes which are connected to a power supply. The overview on the achievements of carbon based IPMC actuators, contact metal electrodes are usually made of copper or gold to a detailed analysis on the restrictive factors for high actuation ensure good conductivity and stability.[22,51] In most cases, they performance, and discussion for gaining a guiding direction for their future development. In detail, the review has five sections: after the introduction section, we give a fundamental description of carbon nanomaterials based IPMC actuator in the second section, including their structure, basic setup for actuation measurement, judging parameters for performance and corresponding equivalent circuit modeling. Then the important developments mainly in carbon based electrodes are addressed in the third section. Based on the above presentation, in the fourth section, we give a convincing understanding and description on the key parameters which may influence the actuator performance. Figure 1. (a) Schematic structure of a typiacal actuator strip composed of a polymer-supported Finally, possible future research and appli- electrolyte layer sandwiched by two carbon based electrode layers. (d: the thickness of the cation of the carbon nanomaterials based actuator strip). (b) Characterization platform for bending actuation performance (L: the free IPMC actuators are proposed. As traditional length, δ: displacement).

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are directly attached to the tip of an actuator strip. In some cases, in order to ensure the homogeneous electric field and lower the voltage drop, another thin layer of metal is sputtered on the carbon electrode surface.[41] Common IPMC electrodes can be classified into two categories: carbon based[46,52,53] and carbon-polymer based electrodes.[54–56] The polymer referred here mainly acts as an additive for better film formation and adhesion to electrolyte layer. Usually, the carbon nanomaterials electrodes are prepared by casting, filtration or self-assembly methods.[46,52–57] The whole actuators are then fabricated by hot pressing of the as-prepared electrode and electrolyte layers, or by dip coating of electrolyte layers into the carbon nanomaterials gel.[22,58] The electrolyte layers used here can also be divided into two groups: one is perfluorinated ion-exchange polymers which are mostly used in traditional IPMC actuators.[25] In the ionic polymers, anionic groups are fixed and located at the end of side chains of polymers so as to position themselves in their preferred orientation to some extent, forming hydrophilic nano-channels. When electronic field is applied, the hydrated cations migrate in these channels towards positive pole, causing the actuators bend. The other is polymer-supported internal ionic liquid electrolyte layers, in which actuation is caused by migration of both cations and anions of IL in opposite directions. As the cations are larger than anions, the accumulated volume difference brings considerable bending displacement of actuators. The novel IL electrolyte overcome the drawbacks related to the solvent losing due to evaporation and electrolysis.[22,59–61] 2.2. Measurement and Performance Evaluation A basic set-up for the actuation measurement is shown in Figure 1b. A Potentio/Galvanostat with a waveform generator is used to activate the actuator and measure the electrical parameters simultaneously. Typically, the actuator strip is clamped by two gold disks. The tip displacement at a point 10 mm away from the fixed point (free length) is measured by a laser displacement meter. In bending actuators, tip displacement or deflection, strain, stress, blocking force and energy conversion efficiency can be considered as the main parameters to evaluate their performance.[62] Displacement which is denoted as δ, is the horizontal distance between original position and bending position of the strip tip. The free bending displacement is the maximum displacement at the tip of the sample without any mechanical load. Strain (ε) is calculated based on the displacement and some parameters of the actuator strip including free length (L) and thickness (d). The calculation formula of strain is given below.

g = 2d* /(L2 + * 2 )

(1)

The concept of stress (σ) which is related to intrinsic mechanical property of the IPMC actuator can be calculated from the strain ε and the Young’s modulus Y, as shown in Equation (2).

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Figure 2. Blocking force measurement platform.[63] Reprinted with the permission from John Wiley and Sons. (copyright 2011).

F = Yg

(2)

Blocking force is the force output of actuator generated when the tip is fixed, representing the loading carrying capacity of an actuator.[63] The blocking force can be measured using a load cell as shown in Figure 2. It has been reported that IPMC strips with higher stiffness (higher tensile strength and modulus) will exhibit higher blocking force.[61] Energy conversion efficiency is an important parameter to evaluate performance of the IPMC actuator. However, it is difficult to give a unified definition and calculation formula at present. Actually, few papers reported the results due to the unclear calculation process and some unknown intrinsic parameters. Cottinet et al gave a much detailed and convincing method to calculate the energy conversion efficiency.[64] As reported, the efficiency of an electromechanical actuator is mainly determined by the mechanical and electrical losses within the material.[65] The global efficiency (ηtotal) of the actuator is given by the following equation:

0total = 0electric · 0transduction · 0mechanic

(3)

where ηelectric is the efficiency of the electrical conversion, which is equal to

0electric =

V · Icharge V · Itotal

(4)

The ηtransduction corresponds to the mechanical output power on the electric input power (V·Icharge); ηmechanic is related to the loss tangent of the material, tanδ. Therefore, efficiency of the electrical to mechanical energy conversion is calculated as below:

0transduction =

0.5 · Y · S21 · f · Vol V · Icharge

(5)

in which Y is the Young’s modulus, S1 is the bending strain, f is the frequency of actuation, and Vol is the volume of the actuator. The efficiency of the mechanical conversion can be expressed as:

0mechanical =

1 1 + B · tan*

(6)

with tan δ, a measure of energy lost to energy stored in a cyclic deformation, is a material property defined by the ratio of the loss modulus to the storage modulus and a function of


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2.3. Equivalent Circuit Modeling In order to quantitatively describe the frequency dependence of the generated strain in the IPMC actuator, electrochemical equivalent circuit model is often adopted to understand the electrochemical behavior of the IPMC actuator. Equivalent circuit model can be obtained according to electrochemical impedance spectroscopy (EIS). Because the actuation in the IPMC-type actuators depended on the charge storage in porous electrodes, the information and analysis obtained from EIS data for porous electrode were applicable. Levie first developed a transmission line circuit (TLC) model consisting of solution resistance and double-layer capacitance of the porous electrode. After that, many researchers modified the TLC model by considering the pore shape, redox reactions at the pore wall,[65–67] pore-size distribution,[68] and frequency dispersion of the electric conductivity.[69] In 2009, Baughman et al. further developed the equivalent circuit model for their carbon nanotube yarn actuators in electrolyte solution.[70] For their actuators, the conventional RC circuit model was not fit because the frequency dependence of the electrochemical response is often neither purely capacitive nor proportional to 1 jT . As a result, the electrical capacitor in equivalent circuit model was replaced by a constant phase element (CPE), in which impedance of the √ circuit parameter was in the form of Z = 1/Y0(jω)P. Here, j = −1 , ω = 2πf, and f was the frequency of the electrical excitation. Y0 was a constant combining the resistive and capacitive properties of the electrode. P was a real number, usually close to unity and when it equaled to 1, the circuit parameter could be looked as a capacitor. Moreover, it was found that as the absolute value of potential across the interface increased, some parasitic reactions started occurring at the interface, which shunted the capacitor. As a result, the cell impedance was modeled as a circuit consisting of a resistor in series with parallel combination of a second resistor and the CPE (Figure 3). The resistor R1 represented the electrolyte and contact resistance, and R2 represented the chargetransfer resistance across the interface, leading to partially

Figure 3. Proposed circuit models for the electrochemical behavior of the MWNT yarn electrode using (a) a CPE and (b) an ideal capacitor. The series resistor R1 models the solution and contact resistance and the parallel resistor R2 models the processes resulting in a loss of the stored charge.

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discharging the capacitor. According to their report, only when the applied potential above +0.5 V or below –0.8 V, which were corresponded to the breakdown potentials of water, the parasitic reaction became significant, implying that R2 was caused by the breakdown of water. As a result, in some no water contained systems, such as IL based actuators, R2 might be ignored. Based on the equivalent circuit model for underwater actuator, later, Asaka et al. developed an equivalent circuit model for IL based air-working IPMC actuator.[71] They proposed a much more accurate model (Figure 4) for their actuator in which Rs

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frequency, temperature, environment, and residual stresses and strains. By using dynamic mechanical analysis (DMA), the value of tan δ could be determined. In order to simplify the calculation of energy conversion process, many groups used the value of ηtransduction instead of ηtotal to characterize the energy conversion efficiency, and they deemed the values of ηelectric and ηmechanic equal to approximately 1.[2,62]

Figure 4. Porous electrode model: (a) the total impedance of the system where Zs is the impedance of the gel, Ze that of the porous electrode, and Zc that for the contact between the SWCNT layers and the metal electrode. (b) Schematic drawing of the porous electrode. (c) Equivalent circuit model of the porous electrode consisting of the distributed impedance of the electrolyte in the pore, χ1; that of the electrode, χ2; and that across the pore wall, χ3. (d) Equivalent circuit model of the porous electrode consisting of the distributed resistance of the electrolyte in the pore and the distributed constant phase element. The resistance of the electrode is negligibly small compared to that of the electrolyte in the pore. (e) Equivalent circuit model of the bucky-gel electrode and the gel electrolyte consisting of the resistance of the gel electrolyte layer Rs, the impedance of the porous electrode Ze, the constant phase element Qc, and the resistance Rc for the contact.[71] Reprinted with permission.[71] Copyright 2010, the American Chemical Society.



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and Ze represented the resistance of the gel electrolyte and the impedance of the porous electrode; Qc and Rc were referred to the constant phase element and the resistance for the contact. In all cases in their experiment, the stimulated curves and experimental points were in good agreement with each other. The accurate impedance model of the porous electrode was realized by decomposing it into three elements: the distributed impedance of the electrolyte in the pore, χ1; that of the electrode, χ2; and that across the pore wall, χ3. As the impedance of the electron-conductive film is negligible compared to that of the ion-conductive pore, the equivalent circuit can be simplified (Figure 4d). When the relation between the generated strain and induced charge transfer was investigated, as the Qc and Rc did not contribute to the frequency dependence of the strain in the actuator since the characteristic frequency associated Qc and Rc was very high (more than 10 KHz), the equivalent circuit model was simplified into one containing a series capacitance C and a resistance R. Compared to the above one proposed by Baughman et al., this model was easily to be understood. As IL based electrolyte was used, the solvent decomposition could be avoided and thus the R2 was cancelled. Moreover, the randomly oriented SWCNTs film showed a P value of close to 1, so the CPE was replaced by the capacitor. By investigating the frequency dependent strain response, the proposed model suggested that not only the ionic conductivity in the gel electrolyte, but also the ionic conductivity in the pores of the electrode layer and the pore structure of the electrode layer affected the response of the bucky-gel actuator. Later on, the RC model was also used by Asaka’s group for the prepared actuator with similar IL/polymer gel based trilayer structure. In order to calculate the specific capacitance, they gave a more detailed equivalent circuit model which closely related to the tri-layer structure (Figure 5).[43,72] In this model, C1

Figure 5. Equivalent circuit model of the bucky-gel actuator. (a) The model composed of the two double-layer capacitance C1 and the ionic resistance R. (b) The model in which the double-layer capacitance is represented by C = C1/2. (c) The model composed of the double-layer capacitance C, the ionic resistance R and the electrode resistance Rel. Reprinted with permission.[72] Copyright 2009, Elsevier Limited.

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represented the double-layer capacitance between the electrode and electrolyte layer. As the capacitance of the two electrodes were regarded same to each other, the equivalent circuit model was represented by the two C1 in series connection, and it could be further simplified to that shown in Figure 4b, in which the two C1 were replaced by the capacitance C = C1/2. Accordingly, the specific capacitance Cs = C1/weight of nanocarbon was obtained. In our recently published paper,[46] the equivalent circuit model of graphene electrode based actuator was demonstrated. In our experiment, we found that the Bode plot, phase vs. frequency, exhibited a phase angle close to 0° at a frequency lower than 10 Hz, indicating that the impedance response are then mainly controlled by the resistance at low frequency. As a result, the circuit model did not contain the capacitor. It was consisted of the resistance of the actuator R1 and the elements (Q2/R2) in parallel which represented the contact impedance.

3. Carbon Nanomaterials Based IPMC Electrochemical Actuator: Recent Developments 3.1. Carbon Nanomaterials Based Electrode Materials Recently, applications of carbon nanomaterials to electrochemical devices are very attractive in both scientific and technological fields. In the area of the EAP actuators, it is very promising to use them as high conductive electrodes for the IPMC actuator. The development of carbon based electrode materials has undergone the following sequence: from one dimensional CNT to planar graphene, and finally to graphene/CNT composite based electrode materials (Figure 6). With unique one

Figure 6. Different carbon-based electrode materials for IPMC actuators: A) Reprinted with the permission from Elsevier Limited (copyright 2012);[2] B) Reprinted with permission from The Royal Society of Chemistry (copyright 2012);[76] C) Reprinted with the permission from John Wiley and Sons (copyright 2012),[46] scale bar: 500 nm.


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3.1.1. Single Walled Carbon Nanotubes Based Electrode Materials Since the discovery of single walled carbon nanotubes by Iijima in 1991,[26,27] SWCNTs have been the subject of many studies due to their larger surface area, unique mechanical and electrochemical actuation properties. In SWCNTs, sp2 hybridization occurs, in which each atom is connected evenly to three carbons (120°) in the xy plane, and a weak π bond is present in the z axis. The sp2 set forms the hexagonal (honeycomb) lattice typical of a sheet of graphite.[73] CNT’s Young’s moduli are superior to all carbon fibers with values greater than 1 TPa which is approximately 5 times higher than steel.[74] Moreover, their tensile strength or breaking strain was up to 63 GPa which is around 50 times higher than steel. These properties made SWCNTs firstly used as reinforcing material for metal and polymers. Then, considering their high surface area and conductivity (as high as 104 S/cm), their applications were expanded to the electrochemical field, including electrocatalysis, supercapacitor, electrochemical sensing and electromechanical actuators. As a good candidate for actuator materials, SWCNTs have been used in various actuations, including optically-driven actuation,[77,78] acoustic actuation,[79] electromechanical actuation,[28,80,81] dry and wet electrochemical actuation,[82–84] electrochemical pneumatic actuation.[85] Recently, large scale preparation of electrolyte free muscles made by twist-spun carbon nanotube yarns with fast, high-force, large-stroke torsional and tensile actuation has been realized.[86,87] In 1999, Baughman et al. reported the preparation of the first SWCNTs based electrochemical actuator.[28] By adhesively applying cut strips of carbon nanotube paper to opposite sides of a Scotch Double Stick Tape, the actuator was obtained. As they reported, the actuation of carbon nanotubes could be subscribed into three mechanisms: electronic actuation or ionic actuation or both of them. For electronic actuation which originates from quantum chemical effects, carbon nanotubes demonstrate expansion with electron injection and contraction with

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hole injection (charge injection 10% strain/second) and high actuation strain (>8% strain under 4 V). According to them, the enhanced electroactive device performance was attributed to the following three reasons: creating continuous paths through inter VA-CNT channels for fast ion conduction, minimizing electrical conduction resistance due to continuous CNTs, and by tailoring elastic modulus anisotropically to enhance actuation strain. As reported above, the actuation performance are largely dependent on the stored charge in the actuator composite

Figure 9. Schematic drawing of a) a CNC/ionomer/CNC three-layer bimorph actuator with VA-CNTs in the CNC layers (no voltage applied), b) a bent actuator with excess ions on cathode side with voltage applied (x3 axis in the thickness direction and x1 axis along length direction, i.e., actuation direction). In the actuator, the mobile ions are assumed to be cations, c) tortuous ion transport paths in nanoparticle/Nafion CNCs, where black dots are conductive nanoparticles, and d) direct ion transport paths in VA-CNT/Nafion CNCs. SEM image of e) 1% Vf as-grown CNT forest before densification, and f) 10% Vf CNT forest after densification. Reprinted with permission.[41] Copyright 2010, John Wiley and Sons.

during the bending process. In order to further increase the strain level and stress of carbon electrode based IPMC actuators, many groups tried to compose capacitive materials into the actuator electrode, such as conducting polymer and metal oxides. Recently, Asaka et al. have added polyaniline (PANI) and carbon black (CB) into the electrode to increase its conductivity and capacitance.[42] PANI, one of the most promising conductive polymers not only for EAP actuators,[101–103] but also for supercapacitors,[104] has high specific capacitance and easy synthetic process. By simply mixing PANI/CB composite nanoparticles (NPs) with SWCNTs, IL and PVDF-HFP in DMAc, the gelatinous solution for preparing electrode layer was obtained. Other procedures were similar to those reported by Asaka’s group before and 1-ethyl-2-methylinidozium tetrafluroborate (EMIBF4)/PVDF-HFP was also used as the electrolyte layer in this experiment. In comparison, pure CB NPs were also added into the electrode layer and actuation performance of the corresponded actuator was recorded. According to the reported results, among the eight different types of electrode films investigated here, which were composed of CNT/PANI or



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CNT/CB but with different percentage of PANI or CB in the SWCNTs based electrode material, the strain of CNT/PANI (50/50), (1.9% at 0.005 Hz) was increased to be almost three times larger than that of CNT (50) (0.65%). Accordingly, CNT/ PANI (50/50) showed more than five times the generated stress when compared with CNT (50). For CNT/CB electrode, CNT/ CB (50/40) showed a more than three times larger strain (2.1%) than that of CNT (50) (0.65%) and a more than four times larger stress than CNT (50). The tested capacitance and conductivity values of these electrode films further confirmed that the increased strains were due to the higher capacitance and conductivities of CNT/PANI (0.196 F cm−2, 15 S cm−1) and CNT/ CB (0.119 F cm−2, 4.6 S cm−1) based films than CNT based film (0.0508 F cm−2, 4.1 S cm−1). Based on the above results, it can be clearly seen that the stress increased much more times than strain. This was attributed to the enhanced mechanical property, such as higher Young’s modulus. As the author reported, the increased Young’s modulus resulted from the dispersion of PANI or CB particles onto and into the CNT mesh structure which afforded larger surface area and effectively untangled CNT bundles into finer and intricate mesh structure which was also advantageous for the attainment of larger surface area and higher strength. Except for adding capacitive materials by simply mixing them together, Ricci et al. composed polypyrrole (PPy) with SWCNTs to make the electrode layer by chemical oxidative polymerization of pyrrole directly on the bucky slurry.[105] Actuators prepared with this novel hybrid material showed an increase of the maximum strain which is five times larger than the one prepared with only CNTs electrode. Moreover, for the same applied voltage, the strain generated increased up to twenty-five times. However, the superior performance of CNTs/ PPy electrode only appeared under low frequency, which was similar to those reported by Asaka et al.[42] This was caused by slow process of farradic reactions of PPy which could dramatically increase the strain. 3.1.2. Multiwalled Carbon Nanotubes Based Electrode Materials Though SWCNTs based actuator exhibited great actuation performance in air, the very expensive price limited its wide

application. More importantly, considering the single layer structure of SWCNTs, any chemical modification including carboxylation and amination would greatly decrease its electrical conductivity. As a result, chemically composing SWCNTs with other materials in nanoscale is difficult. With similar nanomorphology and sp2 carbon structure to SWCNTs, MWCNTs, which contained two or more carbon layers, and often ranged in outer diameter from 3 nm to 30 nm, have a much lower price than SWCNTs (ca 1/10 to 1/10000 the price of SWCNTs). On the other hand, different from SWCNTs, the multilayer structure of MWCNTs makes it keep highly conductive even when the surface layer is chemically modified. Accordingly, chemically anchoring nanoscale reinforcing materials on them is much easier. For these two reasons, it attracted more and more interest in application in electric double layer capacitors (EDLCs), sensors catalysis and many other fields. For actuation application, though cellulose based actuators with MWCNTs electrodes were reported previously,[106,107] the dependence of the actuator performance in dry air on its water uptake capacity greatly limited its real application. Moreover, Biso et al. have reported a MWCNTs gel electrode for polymer actuators containing IL.[108] However, their performance did not surpass that of SWCNTs/polymer actuators and this may be due to the lower quantum chemical efficiency and lower surface area (10– 500 m2/g) of MWCNTs when compared to SWCNTs. In order to enhance the actuation properties of MWCNTs electrode based actuators, different methods on modifying their surface or composing them with other materials were reported. One important work on the investigation of MWCNTs electrode based materials was the preparation of chitosan (CS)/ MWCNTs electrode based actuators, which was reported by Chen et al.[55] In this work, CS was chosen as the polymer matrix for electrode and electrolyte layers not only because it could well disperse the MWCNTs by being wrapped on their surface, but also because it was a natural cationic polyelectrolyte and its biocompatibility allowed the actuator to be used in many biological fields. In this work, 1-butyl-3-methylimidazolium tetrafluroborate (BMIBF4)/CS was used to prepare the electrolyte layer and MWCNTs/CS composites with different weight ratios of CS to MWCNTs were used to cast electrode layers (Scheme 1). When the weight percentage of MWCNTs

Scheme 1. Scheme of preparation of the composite actuator.[55] Reprinted with the permission from John Wiley and Sons (copyright 2010).

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by the migration of hydrated cations. In order to assess the relationship between the actuation strain and EDLC, they also tested the capacitance of these three kinds of actuators by using different CNT bucky paper but with same density (22–25 g/m2) and thickness (15 μm) as the electrode layers. Their preparative method of IPMC actuators was a little different from previously reported hot-pressing method: before combining the bucky paper and nafion membranes, approximately nine drops of nafion solution was coated onto the two sheets of bucky papers to increase the interfacial bonding. As they reported, the strains of the finally obtained actuators were in the order of MWCNTs>SWCNTs>aligned SWCNTs. The different strains were caused by different entanglement densities and degrees of CNT interactions in different structures. This point could also be proved by Young’s moduli. In detail, SWCNTs films, which have higher Young’s modulus (695 MPa) than MWCNTs films (470 MPa), has much greater van der waals forces among the smaller diameter nanotubes, which caused them to rope together and more densely pack than that of MWCNTs bucky paper. This blocks the ion exchange to the inner tube and reduces actuation strain. As a result, the lowest strain of aligned SWCNTs film based actuator could be explained in the same way. Moreover, the EDLC magnitude of the three actuators were in the order of MWCNTs (20 F/g) >SWCNTs (17.5 F/g) >aligned SWCNTs (11.2 F/g). As the EDLC related to the accumulated charge as well as ions in the electrode layer, the above theory could be further proved. 3.1.3. Graphene Based Electrode Materials

Figure 10. a) Photographs of a bimorph strip actuator under 3 V applied voltage; b) Bending displacement of an actuator under square wave potential stimulation at 0.5 Hz. Reprinted with permission.[55] Copyright 2010, John Wiley and Sons.

in the electrode layer was 80 wt%, the composite membrane showed conductivity more than 15 times the 2 S cm−1 of aligned SWCNTs biofibers. In addition, the prepared actuators exhibited high response speed of 2 mm s−1 (Figure 10) and high capacity of approximately 15–20 F/g. In 2012, Asaka et al. found that activated MWCNT based actuator could exhibit better actuation performance than nonactivated MWCNT and SWCNTs electrode.[43] This was due to the higher Brunauer-Emmett-Teller (BET) specific surface area, larger pore volume and better wettability of the oxygen functional groups contained MWCNTs. Afterwards, they further increased the actuation performance of MWCNT based actuator by adding capacitive materials,[109,110] including RuO2[44] and MnO2[45] into the electrode gel. When nafion was used as the electrolyte layer, Liang et al. reported that MWCNTs performed better than SWCNTs and aligned SWCNTs bucky papers when they were used as the electrodes for actuators and this could be explained by the higher pore volume as well as electrochemical capacity of MWCNTs bucky paper.[2] In this case, the bending was caused

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In order to further develop the actuation performance of carbon electrode based actuators, new carbon materials need to be applied in the preparation of actuator electrode. Recently, the emergence of graphene has attracted great attention of worldwide researchers. Graphene is single-layered carbon atoms which packed into a two-dimensional (2D) honeycomb lattice.[111] Considering its fascinating electronic and mechanical properties due to its unique structure, it has been extensively explored for applications in a large variety of fields including quantum physics, nanoelectronic devices, transparent conductors, nanocomposite materials, energy research and catalysis.[112–116] The surface area of graphene (high up to 2600–2700 m2/g) is on the same level of SWCNTs and a lot larger than MWCNTs. Theoretical calculation also shows that the insertion of solvated ions into graphene layers leads to the directional large volume expansion (>700%) perpendicular to its basal plane direction. As reported, the graphene layers could expand to >700% when intercalated by solvated ions Li+(PC)y, the expansion may be due not merely to straightforward electrochemical intercalation of Li+(PC)y, and thus to the sites available in the graphite crystal for solvated intercalates, but also to a physical penetration of electrolyte into open fissures between graphene layers and into the pores that have opened up by electro-chemical intercalation.[75] Based on the above two unique properties, graphene is thus of great value for electrochemical actuation since it is basically caused by the migration of ions. However, as there are still some problems existing in the preparation of graphene based electrode for actuators, the relative researches are few. One big problem is the restacking



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IL in the as-prepared electrode. Accordingly, it is proved that pre-expanding RGO by IL could effectively improve the actuation performance. Though the IL contained RGO electrode exhibited larger thickness variation during actuation, its stability was poor. The composite with 66.7 wt% IL could be actuated for only 20 cycles under 2 V electrical stimulation at 0.01 Hz. This is due to the breakdown of electrodes under higher voltage or longer actuation half period, which was as a result of binder deficiency among RGO layers. Considering the instability of IL/graphene hybrid electrodes, to design a porous and networked structure which could provide high stability and large bending actuation performance is of great importance for developing graphene based actuators. Our group then developed a new RGO derived electrode by combining it with MWCNTs (Figure 11),[46] which could effectively prevent the restacking of RGO and also reinforced their electrical conductivity, ion diffusion and electrochemical properties.[118–124] Actuation characterizations demonstrated the better performance of RGO/MWCNTs Scheme 2. Illustration of (A) ion insertion induced graphite expansion, (B) widely adopted actuator under high frequencies (1 Hz and configuration of graphene electrode, in which the plane of graphene is perpendicular to that 10 Hz) and worse performance under low of applied electric field and the ionic transport through the interval between graphene edges, frequencies (0.01 Hz and 0.1 Hz) than (C) newly developed configuration of graphene electrodes. Reprinted with permission.[76] CopRGO actuator. Comparatively, the bending yright 2012, Royal Society of Chemistry. displacement of MWCNTs electrode based actuator was always the lowest no matter of graphene layers, which may greatly decrease the surface area under high frequency or low frequency. The phenomena and actuation performance of the corresponding actuator. As could be explained by the microstructure of the electrode a result, modification of graphene surface to prevent it from materials: as the well aligned anisotropic RGO electrode has restacking is important before it could be applied in actuation a larger quantum mechanical elongation in the 2D aromatic preparation. plane direction than that of the MWCNTs, its higher actuaOur groups have done many works in this area. Firstly, the tion displacement than those of RGO/MWCNTs hybrid and graphene based in plane electrochemical actuator with large MWCNTs actuators under low frequency were attributed to volume variation was constructed.[76] The in plane multilayboth the simple volume expansion and fast charge injection which could induce both quantum mechanical elongation ered reduced graphene oxide (RGO) was prepared by sizing and ion insertion. However, when the applied frequency was one cast RGO membrane into two parallel electrodes. Then, IL increased, the difficulty in ion migration into a neatly paralwas dropped between the two electrode strips as the electrolyte leled RGO electrode made smaller volume expansion and (Scheme 2). The effectively expansion of the graphene interquantum mechanical elongation than that of RGO/MWCNTs layer could favor the ionic insertion and extraction which was electrode. Moreover, the RGO/MWCNTs hybrid actuator also important for realizing the electromechanical actuation of the exhibited high stability as well as RGO electrode, while the stagraphene based ionic type actuator.[33,117] By successfully prebility of MWCNTs was much worse. This was also attributed to incorporation of IL in RGO layers, the volume variation ration the RGO microstructure, in which the plane contact between perpendicular to that of the paralleled graphene membrane RGO nanosheets was much more stable than the point concould reach as high as 98%, which was an order of magnitude tact among MWCNTs in their network. As a result, the RGO higher than that of the CNT actuator. Moreover, along with the and RGO/MWCNTs electrodes exhibited no obvious drop of increase of the weight percentage of IL in the composite electheir actuation displacement after a million cycles (2V, 1Hz) trode from 0% to 50%, and then to 66.7%, the specific capaciwhile the MWCNTs actuator could only preserve 65% actuatance for corresponding actuators has been found to increase tion displacement. from 1.97 F·g−1 to 30.68 F·g−1 and finally to 60.00 F·g−1. The Although the highly porous structure of RGO based elechigh capacitance also confirmed that the varied amount of IL trodes allowed the high capacitance as well as high strain of in the electrode upon expansion and shrinkage during actuaRGO derived actuators, its low conductivity still limit further tion increased along with the increase weight percentage of



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Figure 11. A) TEM image of the RGO/MWCNT hybrid, scale bar 100 nm. B) SEM image of the porous RGO/MWCNT hybrid membrane, scale bar 500 nm. C) Cross section SEM image of the RGO/MWCNT electrode supported actuator. Reprinted with permission.[46] Copyright 2012, John Wiley and Sons.

improvement of their actuation performance. As reported by Oh et al., the defect sites on RGO greatly limit their conductivity and could be compensated by anchoring noble metal particles on these defect sites.[125] The high conductivity and abundant natural storage of Ag make it a good candidate for making actuator electrode. As a result, Ag NPs was composed with RGO to prepare the electrode by our group (Figure 12).[47] The composite electrode exhibited a high electrical conductivity of 900 S cm−1, which was a lot higher than the 45 S cm−1 of a RGO electrode. Moreover, as the Ag NPs played the role of spacer in the RGO film, the capacitance is developed. Accordingly, the hybrid-supported actuator could be actuated in a wide frequency range (0.01–10 Hz) with much larger bending displacement than that of the pure Ag electrode-supported actuator. More importantly, the RGO/Ag hybrid actuator exhibited better actuation stability than pure Ag electrode based actuator because the RGO layers wrapping around the Ag NPs could effectively prevent them from being corroded.

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Besides the above three kinds of carbon materials with similar sp2 carbon structure, other carbon materials were also investigated as the electrode materials for IPMC type actuators.[36,51,54,56,126,127] As they may have larger effective electrochemical area or better dispersibility, the investigation on their electromechanical properties is also of great value. Such kinds of carbon materials include carbon nanofibers,[54,56] carbidederived carbon[36,51,127] and nanoporous carbon[126] etc. Asaka et al. reported the preparation of actuator electrode by using activated carbon nanofiber (ACNF),[54] which had very large electrochemical surface area (1500–2500 m2 g−1) and a good dispersibility. However, due to its low conductivity, they also added SWCNTs in the electrode material to enhance the conductivity and this strategy was also used by other group to improve the conductivity of the electrode as well as the actuation performance of the actuators.[127] The prepared ACNF/ SWCNTs actuator exhibited better frequency response than pure ACNF and ACNF/vapor grown carbon fiber (VGCF) actuator. By optimizing the mixing weight ratio of the two kinds of carbon materials, the performance of the composite actuator could be further improved. Later, by comparing two new kinds of carbon electrode with high surface area (1450 and 1900 m2 g−1) based actuators,[126] Aabloo et al. reported that the actuation properties, such as the strain and strain rate, not only depended on the charge storage in the electrode, but also depended on the stretchability of the electrode materials. Between the two synthesized IL (EMITf) actuators with either carbide-derived carbon (CDC) or coconutshell-based-activated carbon based electrodes, which both exhibited higher strain than RuO2 electrode based actuator, the latter exhibited higher surface area as well as electrochemical capacity but lower stretchability. As a result, its strain and strain rate were restricted. By evaluating their EDLC, Aabloo et al. proposed the concept of supercapacitor-like actuator.[51] They found that the gravimetric capacitance of CDC in actuator electrodes could reach up to 119 F g−1 at 1 mV s−1 sweep rate of the applied triangle voltage. According to them, the requirements for CDC actuator, EDLC and sensor were similar but with minute difference. The optimal CDC sensor device should have a dense structure in which the spatial separation of the oppositely charged ions could easily be excited to a high degree while for actuator and EDLC, unblocked ion transport were required. For all of them, a high capacitance and hence the large surface area and pore volume should be obtained.

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3.1.4. Other Carbon Nanomaterials Based Electrode Materials

3.2. Carbon Nanomaterials as Additive for Electrolyte Layer Not only the electrode layer, but the intrinsic properties of the electrolyte layer also contribute to the actuation performance of the obtained actuator strip. For example, the ionic conductivity of the electrolyte layer greatly affected the response rate of the actuators. As a result, using electrolyte layer with high ionic conductivity were necessary for obtaining high performance actuators. As mentioned in section 2, the electrolyte layer could be divided into two categories based on the different actuation mechanisms. Though both of their actuations were caused by



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actuators which relied on the migration of ILs was more and more investigated. For this kind of actuators, the used kind of IL and its concentration in the electrolyte layer may both contribute to its actuation properties. For both of the two kinds of electrolyte layers, recent research also focused on the ionic conductivity in the polymer matrix which may greatly contributed to its actuation performance. As a result, some additive materials, such as graphene oxide, graphene and MWCNTs et al., were used to modify the polymer matrix to accelerate the ion migration in the electrolyte layers. These results will be reviewed below in more detail. In order to enhance the conductivity of the electrolyte layer, more work has been done on modifying the polymer matrix of the electrolyte layer by adding other materials into them. Graphene oxide (GO) with high dispersibility, was composed with Nafion to prepare the electrolyte layer for IPMC-type actuator by Cao et al. (Figure 13A).[22] In this work, CNTs were used as the electrode materials and after formation of the actuator strip, Li+-IPMC was obtained by ion-exchanging the H+-IPMC in LiOH solution. The obtained GO-Nation composite electrolyte based actuator with 10 wt% GO exhibited a higher strain of nearly 2 times that of pure Nafion and a higher blocking force of 4 times that of pure Nation (Figure 13B). The strain reinforcement was ascribed to the conductivity development by the addition of GO. This was Figure 12. A) Illustration of the deposition of Ag NPs on the GO surfaces and further reduc- also confirmed by the experimental results: tion to RGO/Ag hybrid from solution. B) TEM image of RGO/Ag hybrid; scale bar represents along with the increased added amount of 100 nm. C) SEM image of RGO/Ag hybrid membrane; scale bar represents 10 μm. D, E) GO (from 0.5 wt% to 10 wt%), the conducHigh-resolution SEM images of cross section of the area labeled 1 and side view of the area tivity was increased from 2 × 10−9 S cm−1 to [ 47 ] labeled 2 in (C); scale bar represents 500 nm. Reprinted with permission. Copyright 2012, 2 × 10−5 S cm−1. John Wiley and Sons. Though metal electrodes instead of carbon based electrodes were used, some the migration of the ions, the used ions were totally different. modification to the electrolyte layer by adding functionalized The first category was same to the ones used in conventional MWCNTs or graphene could also get high actuation perforIPMC actuators, which was based on iEAP and the actuation mance and these results were also inspiring for preparing was caused by the migration of the hydrated cations. As the carbon electrode based actuators. As a result, these research polymer matrix, two ionomers were commonly used: Perfluoridevelopments were also addressed here. Pristine MWCNTs, nated alkenes with anionic-group-terminated side chains, such which was difficult to be dispersed in polymer matrix, needed as Nafion (perfluorosulfonate) by Dupont and Flemion (perfluoto be surface modified before being composed with the polcarboxylate) by Asahi Glass, or styrene-divinylbenzene-based ymer matrix. Tang et al. reported the use of poly (sodium polymers with ionic group substitution in the phenyl rings. Due 4-styrene sulfonate-co-acrylic acid) (PSA)-grafted-MWCNTs to the commercial availability, Nafion has been mostly used for as reinforcing material for electrolyte layer.[129] The modified fabricating IPMCs. The first kind of used cations in Nafion was MWCNTs could be homogeneously dispersed in the PSA/polyhydrogen ion. Later, in order to get quicker strain response, vinyl alcohol (PVA) membrane with a loading percentage of Li+ was used to replace H+ for faster migration of hydrated Li+ up to 20 wt%. The corresponded actuator exhibited improved mass transfer of cations and electromechanical coupling. By ions. However, this kind of actuators still has big problems in incorporation of the PSA-g-MWCNTs, the entanglement of the their water uptake. Compared to water, IL is less volatile and polymer chains was strengthened and their slipping under show high ionic conductivity and wide potential windows, external electrical stimulus was weakened. As a result, when which are advantageous for the quick response and high in-air the loading of PSA-g-MWCNTs reached more than 10 wt%, stability of the actuator.[128] As a result, the second category of



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the oxidized CNTs with polyethylene glycol (PEG).[130] By introduction of CNTs into Nafion membrane, its water uptake and ion exchange capacity decreased but the proton conductivity and mechanical performance increased. Thus, when composite containing 0.5% CNTs was used, the maximum generated strain and the blocking force were 2 and 2.4 times higher than neat Nafion actuator respectively (Figure 14). As reported above, the ionic conductivity could be increased by addition of carbon nanofillers into the electrolyte layer. However, the mechanism was not clear. Most groups attributed it to the increase of the high conductivity of the carbon nanofillers. Oh et al. gave a more detailed study on the interaction mechanism between graphene and Nafion, which resulted in great improvements of the actuation performance (Figure 15).[62] According to them, the addition of layered graphene could alter the morphology, the orientation and crystallinity of di-block nafion copolymer microdomains and hence caused the increase of ionic conductivity and mechanical strength. The tensile strength of the graphene-Nafion membrane was improved up to 200% within a 1.0 wt% loading, and the tip displacement of the composite membrane was almost three times higher than that of the recast Nafion actuator.

4. Important Influencing Factors for Electrochemical Actuation 4.1. Electronic and Ionic Conductivity Based on the above reviewed reports, it could be found that the generated strain of the carbon electrode based IPMC-type actuators was proportional to the charge storage in the actuators.[36,50] The charging process was consisted of electron transfer and ion migration.[52,54] The charging rate and the total stored charge, which were corresponded to the strain rate and strain level Figure 13. (A) Schematic representation for the fabrication of GO-Nafion nanocomposite respectively, were determined by the slowest membrane with parallel GO sheets. (B) Displacement photo of an IPMC based on the pure charge transfer step. As fast electron transfer Nafion membrane (a) before test and (b) driven under a 3 V electrical field. The displacement only asked for high conductivity of the elecphoto of an IPMC based on the Nafion hybrid membrane (c) before test and (d) driven under a trode materials of the actuator, in the present 3 V electrical field. (e) Dependence of the generated strain (ε) on the GO content of the memgeometry it appeared that ion movement branes. (f) Dependence of the blocking force on the GO content of the membranes.Reprinted through the film thickness was the rate with permission.[22] Copyright 2010, American Chemical Society. determining step.[46,53,131] Many factors may affect the ion migration rate. For electrolyte the small oscillation in the mechanical output of the actuator materials, the ionic conductivity was controlled by the kind and vanished. nanostructure of the polymer support and the intrinsic properLian et al. also obtained well dispersed CNTs/Nafion comties of the migrated ions, including the ion sizes, velocity, the posite actuator by modifying the CNTs through esterification of molecular force between the polymer matrix and the migrated

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of slightly oblong-shaped EMI+ cation was 0.53–0.95 nm.[36] The values could be referenced when choosing other migrated ions. 4.2. Ion Migration and Charge Storage in Electrode Materials

Figure 14. Displacement of an IPMC based on pure Nafion and FCNT– Nafion composite membranes: (a) 0 V; (b) pure Nafion membrane; (c) 0.5 wt% FCNT–Nafion composite membrane; (d) 2 wt% FCNT– Nafion composite membrane; (e) 6 wt% FCNT–Nafion composite membrane; (f) 10 wt% FCNT–Nafion composite membrane driven under a 3 V electrical field. Reprinted with permission.[130] Copyright 2011, Elsevier Limited.

ions.[37,72] The large ions might result in large volume variation and further lead to large displacement, but their moving rate was low, which may also affect the amplitude of bending displacement under high frequency. As a result, using ions with proper sizes, low velocity and weak interaction with the polymer matrix could improve their ionic conductivity. Recently, most IL based actuators adopted EMIBF4 as the migrated ion pair.[54,56] According to quantum chemical calculations, the diameter of symmetric BF4− anion was 0.47 nm and the lateral dimension

Figure 15. SEM image of graphene (a); graphene–Nafion composites in molds and their membranes (b); morphology of actuators after platinum electroless plating with plating thickness of ∼6 μm. Inset in (c) shows a graphene sheet. Reprinted with permission.[62] Copyright 2011, Elsevier Limited.

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The charge storage amount in the electrode would greatly influence the strain output. The charge storage rate and the total stored charge amount was not only determined by the electronic and ionic conductivity, but also determined by some other factors such as the microstructure of the electrode and electrolyte materials.[36,50,126] The ion resistance in the pores of the electrode layer depended on various factors, including their pore sizes, total pore volume and total surface area, etc. Basically, large total pore volume and large surface area favored the ion migration. However, the proper pore size was the premise for both of them. The pore size should be comparable but larger than the migrated ion sizes. Theoretical calculations also proved that the electrochemical thermodynamics of electrolytes in porous electrodes is quantitatively different from that in the bulk with planar electrodes when the pore size is comparable to the size of the electrolyte ions.[132–135] According to Aabloo et al., micropores (pore diameter

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