Fast Ionic Actuators with Silver–Silver Chloride Electrodes and a Mixed Ionic Liquid Electrolyte

Excellent conductivity makes silver (Ag) an attractive electrode and current collector material for a variety of devices, also promising faster soft actuators. However, high reactivity challenges the use of Ag in electrochemical systems, as dendritic growth can create undesired conduction pathways and short‐circuit the oppositely polarized electrodes. The challenge of rapid Ag electrochemical migration is addressed using a mixed mixed‐room temperature ionic liquid (RTIL) system (l‐ethyl‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide and trihexyl(tetradecyl)phosphonium chloride) that prevents short‐circuiting by engaging the produced Ag+ ions with Cl− anions. The stability of the system is demonstrated on an ionic actuator using Ag–silver chloride (AgCl) electrodes and a mixed‐RTIL electrolyte. The work demonstrates fast (in 5 s) transfer of a large (0.2 C cm−2) charge to high‐specific‐surface‐area carbon without observable dendritic growth in cycling and good electromechanical performance: 4° s−1 deflection rate at 0.8 V s−1 scan rate. Simple spray‐deposition of the Ag–AgCl electrodes promises scalable and cost‐effective fabrication. The combination of high stability and vast charge capacity encourages the engagement of Ag‐based electrodes for many electrochemical applications involving organic electrolytes also beyond robotics.


Introduction
In plants, changes in the swelling degree of symmetric, layered structures, such as bilayers, are responsible for a large number of passive, i.e., environmentally driven, movement mechanisms. [1,2]ilayer movement has widely been engineered; [3] however, usually, not controlled by environmental humidity levels.A biomimetic control approach has been implemented in soft robotics by electrically creating asymmetric swelling in symmetric electroactive laminates mainly via the displacement of mobile ions in an open-porosity polymer matrix that has a thickness in the range of 100 μm in which the direction of the swelling gradient is determined by the polarity of the electrical field applied across the laminates.The asymmetric swelling gradients induce strain gradients that result in bending of the structures, which is amplified by the laminates' cantilever geometry. [3,4]In such actuators, electrolytes are selected for significantly asymmetric cationic and anionic mobility, which can even be further boosted by asymmetric interaction with the polymeric matrix. [5]he high compliance, of the porous matrix increases the damage tolerance of soft robots and also enables their efficient and safe interaction with their environment, including humans. [6]o electrically excite cross-plane ion mobility in a thin openporosity film with an asymmetric electrolyte filling its pores, the film needs to be covered by compliant electron-to-ion transducers on both sides.Ideal electrodes demonstrate high electron-to-ion transduction capacity, signifying that they displace a large number of ions per area that translates into large actuation magnitude, and high conductance purporting fast charge transfer that, in turn, results in high actuation speed.Two electron-to-ion transduction strategies are available: capacitive or electrical double-layer (EDL) charging, and redox reactions.Both strategies offer high areal charge densities and therefore actuation magnitudes.The EDL charging strategy is attractive for extreme reversibility, whereas redox reactions can be prone to degradation or short-circuiting.Even the best available options for electrodes, like carbon nanotubes, [7] conducting polymers [8] need additional current collector (CC) layers to avoid the bottleneck of lateral electronic charge transfer.To date, only nonreactive gold (Au) or platinum (Pt) foil CCs provide sufficient compromise between conductance and mechanical compliance but they are very fragile and prone to delamination. [9]Silver (Ag) is highly electrically conductive and cost-effective.With a conductivity of 6.30 Â 10 7 S m À1 at 20 °C, Ag enables efficient charge transfer and improved actuation speed and performance.DOI: 10.1002/adem.202300214Excellent conductivity makes silver (Ag) an attractive electrode and current collector material for a variety of devices, also promising faster soft actuators.However, high reactivity challenges the use of Ag in electrochemical systems, as dendritic growth can create undesired conduction pathways and short-circuit the oppositely polarized electrodes.The challenge of rapid Ag electrochemical migration is addressed using a mixed mixed-room temperature ionic liquid (RTIL) system (l-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide and trihexyl(tetradecyl)phosphonium chloride) that prevents short-circuiting by engaging the produced Ag þ ions with Cl À anions.The stability of the system is demonstrated on an ionic actuator using Ag-silver chloride (AgCl) electrodes and a mixed-RTIL electrolyte.The work demonstrates fast (in 5 s) transfer of a large (0.2 C cm À2 ) charge to high-specific-surface-area carbon without observable dendritic growth in cycling and good electromechanical performance: 4°s À1 deflection rate at 0.8 V s À1 scan rate.Simple spray-deposition of the Ag-AgCl electrodes promises scalable and cost-effective fabrication.The combination of high stability and vast charge capacity encourages the engagement of Ag-based electrodes for many electrochemical applications involving organic electrolytes also beyond robotics.
Additionally, Ag is a less expensive alternative to Au or Pt, making it a more practical choice for large-scale manufacturing. [10,11]evertheless, Ag CCs are reactive and suffer in low cycling [12] discouraging research on Ag-based CCs.It is common knowledge for experts within the field of soft actuators, but not explicitly published, that the application of Ag CCs on an l-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide (EMITFSI)containing electrochemical system with activated carbon (AC) electrodes, similar to ref. [13], can cause electrical short-circuiting [14] already within the very first actuation cycle.This is because Ag metal from the CCs reacts [12] and gives soluble Ag þ cations: These ions travel across the transducers during actuation, creating dendrites at the opposing electrode and shorting the electrodes electronically.
The state-of-the-art engineering approaches specifically employ and even accelerate the high mobility of Ag, for example, to increase electrode flexibility due to fractal structure. [15]Ag electromigration is also recently suggested even as a microfabrication technique. [16]We avoided dendritic short-circuiting from the oxidized electrode by preparing ionic actuators that consisted of membranes that were covered with composite electrodes.We utilized spray-painting for the application of reactive electrodes due to its simplicity and high control of layer thicknesses. [17]ur work demonstrates a scalable fabrication method with promising results for using it for automated fabrication. [18,19]irst, the ion-conductive films were coated with AC-AgCl composite layers and then with solid Ag particles as the outmost layers.AC is to provide high specific surface area (SSA) for ion electrosorption, whereas Ag and AgCl with negligible SSA are to facilitate fast interelectrode electron transport.Ag-AgCl electrodes are standard reference electrodes in aqueous media, [20,21] which emphasizes their exceptional stability in time.
The electrochemical subsystem at the electrodes' surface for electron-to-ion transduction via Ag can be summarized as We used a room temperature ionic liquid (RTIL) with Cl À cations, namely, trihexyl(tetradecyl)phosphonium chloride (TTPCl), to immediately immobilize the Ag þ ions forming at the electrodes.Ag þ is precipitated as AgCl before any substantial displacement within the laminates under the applied electrical fields as it has extremely low solubility.
The Joule-Lenz law states that the power of heating W generated by an electrical current I through a conductor is analogous to its resistance R according to However, the use of pure TTPCl as the electrolyte, which has the lowest viscosity among Cl À -containing RTILs, may not be suitable due to its high viscosity resulting in excessive heating during actuation. [22]ur solution therefore utilized a mixture of two RTILs as an electrolyte: TTPCl to provide Cl À anions to undergo reaction (2).EMITFSI to provide mobile EMI þ cations responsible for actuation.During actuation, EMI þ cations are drawn to the cathode, while less mobile TFSI -anions move to the anode.The difference in mobility between the two ions is not necessarily due to size, but rather their interactions with the surrounding area charge, which is influenced by factors such as size, charge, and distribution of charge. [23,24]t should be noted that ethyl-3-methylimidazolium chloride IL was not used in this work as it has a high viscosity at room temperature.
The operation principle of the electrochemical and electromechanical system is shown in Figure 1.The actuation concept featured as a case study in this work, well-documented in existing literature, [25][26][27] operates independently of the Ag þ ions.
The thin open-porosity film of the laminate was based on polyvinylidene fluoride (PVDF) for which N,N-dimethylacetamide (DMAc) was chosen as a solvent while propylene carbonate (PC) was used as a plasticizer because it does not evaporate easily due to its low vapor pressure and high boiling point of 242 °C. [28,29]

Materials
Analytical grade PVDF, DMAc, 4-methyl-2-pentanone (MP), AgNO 3 , and NaCl were obtained from Sigma-Aldrich.PC was purchased from Alfa Aesar.EMITFSI RTIL (99.5%) was purchased from Solvionic.1-hexyl-3-methylimidazolium chloride, 1-methyl-3-octylimidazolium chloride, and TTPCl (≥95.0%)RTILs were acquired from IoLiTec.The commercial Ag ink Laminate with Ag-AC-AgCl electrodes, containing a mixed RTIL composed of EMITFSI and TTPCl.When an electric potential difference is applied between the electrodes two processes occur concurrently.In process a, Ag atoms are oxidized at the anode to Ag þ cations.As the electrolyte is rich in Cl À cations, insoluble AgCl is formed and precipitated nearby, preventing short-circuiting.In process b, mobile EMI þ cations from the electrolyte are drawn to the cathode while less mobile TFSI -anions to the anode.Asymmetric strain in the laminate results in bending.
(metalon HPS-FG57B) (70 wt% Ag content) was bought from Novacentrix.The AC (black Pearls 2000) utilized was purchased by Cabot.The silk fabric (surface density of 11.5 g m À2 ) was produced by Esaki.All chemicals were used without further purification.

Actuator Fabrication and Sample Preparation
The fabrication protocol of the actuator entailed three steps: membrane fabrication, electrode application, and introduction of Cl À -containing RTIL throughout the laminate before preparing the samples.For comparative analysis, two laminate thicknesses were fabricated and tested, type A and type B. The samples were produced in-house with a method similar to that described in ref. [13] (Figure 2).To ensure the practicality of our ionic actuators in real-world applications, we intentionally fabricated them under ambient conditions with a room temperature of 25 °C and a relative humidity of 30-35%.Our goal was to create actuators that can be used in open-air environments, where these conditions are typical.

Membrane Fabrication
For both types A and B, an ion-conductive textile-reinforced polymer membrane was prepared by combining an EMITFSI electrolyte with a passive porous polymer network composed of PVDF.A piece of silk fabric was first inserted into an embroidery frame and stretched taut as a support for the membrane.Under the fume hood, a membrane solution was prepared by dissolving 2 g of EMITFSI and 2 g of PVDF in 18 mL of DMAc and 4 mL of PC.The mixture was then stirred overnight at 60 °C.The membrane solution was subsequently applied to both sides of the silk fabric using a paintbrush.After every painting iteration, the solution was dried with a hot air gun.The membranes were then inspected against backlight for pinholes; if found, they were filled in by subsequent painting cycles.Solvents of the membrane solution partially dissolved the existing membranes; thus, each following cycle was applied as thin as possible and only on dry surfaces.Once continuous films were achieved, the thickness of the membranes was simply tuned by adding more layers to both sides (Figure 2a and 3a).Type A membrane was a result of three consecutive painting iterations on either side of the silk support while type B membrane was a consequence of six painting cycles.

Electrode Fabrication AC-AgCl Preparation
20 w/w% of AgCl, 4.9 g of AC granules were first processed manually in a crucible to break down any aggregates.The AC powder was then mixed with a concentrated AgNO 3 solution (containing 1.6 g of AgNO 3 ) and was later mixed with saturated NaCl solution (containing 0.9 g NaCl).The mixture was washed 10 times with high purity water (Millipore Q) to eliminate NaNO 3 and remaining NaCl, and dried in a vacuum oven for 12 h at 60 °C.Finally, the dry powder was homogenized again with a mortar to a grade suitable for airbrush.AC spheres had been used before in actuators in ref. [30] and were utilized in this project as they have a very high SSA (1500 m 2 g À1 ) compared to other C sources. [31,32]C-AgCl Suspension Preparation: A suspension of 2 g of AC-AgCl powder in 2 g of EMITFSI RTIL and 10 mL of MP was prepared in an Erlenmeyer flask.In parallel, a polymer solution was prepared by dissolving 2 g of PVdF-HFP in 24 mL of MP in another Erlenmeyer flask.Both mixtures were stirred overnight at 60 °C.The final electrode suspension was obtained by mixing the carbon suspension and the polymer solution.
AC-AgCl Layer Application: The AC-AgCl part of the electrodes was sprayed onto both sides of their silk-reinforced membranes using an airbrush (Iwata HP TR-2) at about 20 cm.The compressed air pressure of the airbrush was adjusted at 2 bars, to eject the electrode suspension but not damage the membranes.It was also important to keep the suspension at 60 °C to not form aggregates and clog the airbrush.Once single layers of electrodes had been sprayed onto each side of the membranes, MP was evaporated using a hot air gun.Additional cycles of spraying and drying both sides of the composites was then repeated until the desired thicknesses were reached  (Figure 2b).The process was repeated, on both sides, 4 times for type A and 8 times for type B.
Ag Layer Application: The Ag ink was first diluted in DMAc 1-1 to facilitate spraying.The laminate was coated with the diluted Ag ink on both sides using the same airbrush as above where they underwent alternate spraying and drying cycles (Figure 2c).After the application of two layers on either side of the membrane of actuator type A and three layers of type B, the laminates were kept in vacuum oven at 125 °C, at 2 mb for 48 h, to cure the Ag layers and evaporate any remaining solvents.Type A had a final thickness of 70 μm and type B was 120 μm thick with electrodes 22.5 and 45 μm in thickness on average respectively.
Introduction of the Cl À -Containing RTIL: Finally, the laminates were sprayed with a solution of 10% TTPCl RTIL in PC to introduce further Cl -in the composites.They were thereafter left to dry for 12 h under a fume hood and placed in a vacuum oven at 90 °C at 2 mbars for another 12 h (Figure 2c and 3b).

Actuator Preparation
The laminates were released from their frames and their thicknesses were measured at ten locations with a thickness gauge.Samples of 25 Â 2 mm were then prepared using a sharp scalpel to avoid any short-circuits on the actuators' sides that can be created from an excess of conductive material.

Microstructure
The cross sections of the smart materials were visualized using a table-top scanning electron microscope (SEM) (Hitachi TM3000) equipped with a back-scattered electron detector, at an acceleration voltage of 15 kV.The samples were first prepared by cryofracturing with liquid nitrogen to get a clean cut of their cross sections, and then taped on a sample holder.

Equivalent Bending Modulus Measurement
The laminated actuators were clamped from their narrow ends.These were loaded by adding weights m of either 17, 17.5, or 65.2 mg at the free ends of the cantilevers at distances l of 17.5 mm on average from their bases.A Canon EOS 60D camera registered the laminates' cross-sections as they were in their unloaded initial as well as loaded conditions.The change in curvature that occurred from the loads was determined in postprocessing in LabView by tracing the position of visual markers attached on the samples' tips, as described in. [33]he bending modulus (BM) was thereafter calculated using the following formula for end-loaded cantilever beams, as explained in ref. [34]: where m is the mass of the added load, g is the gravitational acceleration (9.8 m s À2 ), l is the free length of the cantilever beam from the mounting clamp to the load, and θ is the measured angle of deflection of the cantilever beam due to the added load and I is the second moment of area that is I = w Â t 3 /12 where w is the width and t is the thickness of the rectangular crosssectional cantilevered beam sample (Figure 4).

Peak-To-Peak Maximum Actuation Strain and Impedance Measurement
To calculate the actuators' peak-to-peak maximum actuation strain ε max , they were clamped in a cantilever configuration, between a Kelvin clip with 24k Au contacts.Visual markers were attached at their free ends, and their positions recorded.An electrochemistry workstation Biologic BP-300 with software EC-Lab V11.33 in two-electrode configuration was used to determine the samples' electrical as well as electromechanical behavior.Using cyclic voltammetry (CV), the samples were driven by triangular voltage waveforms with scanning rates of 50, 100, 200, 400, and 800 mV s À1 .The first work-in cycles were disregarded from analysis.Their peak-to-peak maximum actuation strain ε max was then calculated as where t is the thickness of the specimen and R max is their radius at maximum deflection. [35]As explained in ref. [36], assuming uniform curvature, R max = l/sin(θ/2) where l is the free length of actuator and θ is the measured angle of deflection.Using electrochemical impedance spectroscopy (EIS), AC impedance spectra were captured prior and after actuation by scanning input signals of 5 mV RMS at frequencies from 200 kHz to 0.01 Hz to measure the samples' cross-sectional impedance (Figure 5).

Full Cycle Charge
The charge of the actuators was calculated by taking the integral of the current in time of a full cycle.

Results and Discussion
A micrograph in Figure 6 reveals that the spray-painted Ag ink of the electrodes, consisting of Ag flakes and additives, was deposited uniformly.This is critical for the functionality of the actuators as excitation depends on the uniform distribution of electrical charge across their surface areas.Moreover, a good mechanical connection was achieved between the Ag and the AC-AgCl regions of the electrodes, as the surface of the latter is rough, which can further prevent delamination.MP solvent was chosen for the AC-AgCl suspension of the electrodes that was subsequently spray-painted on the polymer films, as it does not dissolve the silk reinforcement or the PVDF polymer of the textile-reinforced polymer membrane but it can only swell PVDF.What is more, it also evaporates fast as it has a high vapor pressure and boiling point of 117-118 °C, reducing the risk of causing short-circuits in the membrane during electroding.To introduce Cl À in the laminates, the RTIL TTPCl was chosen as it has a viscosity of 1631 cP at 25 °C (according to IoLiTec) that is lower than 3302 cP at 25 °C of 1-hexyl-3-methylimidazolium chloride and 3690 cP at 35 °C (according to IoLiTec) of 1-methyl-3-octylimidazolium chloride and a lower viscosity RTIL leads to faster ion transfer.
The average BM for type A was 24 MPa and type B was 47 MPa, comparable to soft ionic electroactive actuators previously published in the literature. [19,23,37]igure 7 presents the cyclic strain pattern for both type A and type B samples when subjected to a scanning rate of 50 mV s À1 .Notably, the data illustrates a symmetric and repeatable deformation profile in response to the applied voltage waveform, indicating a consistent and predictable behavior in both sample types during the actuation process.A small time lag was noticed after the scan rate polarity reversal, potentially evidencing the occurrence of two distinct mechanisms of different charge capacities, i.e., processes a and b.In Figure 8, the current responses of type A and type B samples over time were illustrated at two extremes of investigated scan rates: 50 and 800 mV s À1 .The area beneath the current-time curve represents the displaced charge; a larger amount (37% vs 27%) of charge was displaced in actuating a type B sample, compared to type A, at 50 and 800 mV s À1 , respectively.Electromechanically induced strain is shown in Figure 9.The maximum peak-to-peak strain of type A and B samples produced was 0.09% and 0.17%, respectively, at voltage range À1 to 1 V and 50 mV s À1 which is on par to the values reported in ref. [37].At lower scanning rates, more time is given to the cations of the systems to travel through the membranes and therefore to deflect, resulting in higher strains.Thicker electrodes resulted in a higher strain.The charge generated during the second actuation cycle as a function of scanning rate is     presented in Figure 10 and 11.Deformation was charge-dependent as thicker electrodes translated to more moving charge and to more moving charge per electrode thickness.From the peak-to-peak strain data versus charge, exhibited in Figure 12, it is inferred that the swelling gradient was more pronounced at the larger thickness samples as their strain increased per moved charge.With the thinner samples, even if a larger ion displacement were induced, the swelling gradient would not increase proportionally as it would not be possible for liquid to be localized in the small volume of the open-porosity system.The typical hysteresis curve for voltage-dependent strain at 50 and 800 mV s À1 , together with the corresponding CV graph, is given in Figure 13.A 4°s À1 deflection rate can be deducted from the CV graph of the actuator at 800 mV s À1 .Moreover, comparing the CV graph with the movement graph, we hypothesize that there is back-relaxation at maximum voltage values because the current increases but the range of movement already decreases, possibly indicating a significant increase in the movement of anions toward the end of the cycle.
Figure 14 compares the cyclic stability of the mixed Cl Àcontaining RTIL system to one only containing a single non-Cl À RTIL (EMITFSI).As expected, the EMITFSI system exhibits a spiking behavior very quickly (1.18 V, 31.5 s), already within the very first cycle even at a very fast (50 mV s À1 ) voltage scan rate.This spiking behavior is attributed to the electroformation of dendritic bridges due to electrical activity within the system.These metallic structures create shortcuts for the electric current, causing abrupt and significant surges.Moreover, these bridges are subject to Joule-failure, a process where the heat generated by the current passing through the resistance of these bridges is substantial enough to cause damage or destruction.This phenomenon underscores the volatility of the single non-Cl À RTIL system, accentuating the critical role that Cl À plays in maintaining the stability of the system.

Conclusions
Our work demonstrates that Ag is a promising alternative to other electrode materials due to its high electrical conductivity and cost-effectiveness.By adding spray-painted nongassing AC-AgCl layers and a combination of RTILs, we have produced stable and fast-bending Ag electrode ionic actuators.Our approach of utilizing a layer-by-layer spray-deposition technique to apply the Ag-AC-AgCl electrodes is promising for future scalable production.By leveraging Cl À anions to create extremelylow-solubility AgCl, we prevent short-circuiting of the laminates during actuation, ensuring long-term stability.Fast (5 s) transfer of a large (0.2 C cm À2 ) charge to high-specific-surface-area carbon yielded a deflection rate of 4°s À1 , and no observable dendritic growth of Ag in cycling.In contrast, the system containing  only EMITFSI RTIL shorted within the first operating cycle.Our results suggest that the proposed electron-to-ion transduction mechanism is encouraging for many electrochemical applications involving organic electrolytes beyond robotics.

Figure 1 .
Figure1.Laminate with Ag-AC-AgCl electrodes, containing a mixed RTIL composed of EMITFSI and TTPCl.When an electric potential difference is applied between the electrodes two processes occur concurrently.In process a, Ag atoms are oxidized at the anode to Ag þ cations.As the electrolyte is rich in Cl À cations, insoluble AgCl is formed and precipitated nearby, preventing short-circuiting.In process b, mobile EMI þ cations from the electrolyte are drawn to the cathode while less mobile TFSI -anions to the anode.Asymmetric strain in the laminate results in bending.

Figure 2 .
Figure 2. Actuator fabrication: a) membrane preparation by painting a taut silk fabric with membrane solution, b) spray coating with AC-Cl and then Ag electrode suspension, and c) spraying Cl À -containing RTIL.

Figure 3 .
Figure 3. Preparation of actuator: a) membrane; b) final actuator after electrode and Cl À introduction.

Figure 4 .
Figure 4. Method for measuring the equivalent BM of the actuator samples.Illustration of sample with video marker at its tip: a) unloaded and b) loaded condition.

Figure 5 .
Figure 5. a) Experimental setup for electrical and electromechanical testing of actuator samples.Positioning of a sample in clamp with a visual marker (in white) attached at the distal part of the cantilever: b) side-view (typical frame observed from camera) and c) top-view.

Figure 6 .
Figure 6.Scanning electron micrographs of the cross sections of the two types of actuators at x800 magnification: a) type A and b) type B.Figure 7. Transient strain for type A and type B samples at a scanning rate of 50 mV s À1 .

Figure 7 .
Figure 6.Scanning electron micrographs of the cross sections of the two types of actuators at x800 magnification: a) type A and b) type B.Figure 7. Transient strain for type A and type B samples at a scanning rate of 50 mV s À1 .

Figure 8 .Figure 9 .Figure 10 .
Figure 8.Current waveform for type A and type B samples for 50 and 800 mV s À1 scan rates.Displayed are a) the lowest (50 mV s À1 ) and b) the highest (800 mV s À1 ) scanning rates.The light gray-shaded region, exemplified under the current-time curve for type A at 50 mV s À1 , signifies the displaced charge during one actuation cycle.

Figure 11 .Figure 12 .
Figure 11.Full cycle charge normalized per total electrode thickness of type A and B samples.

Figure 13 .
Figure 13.CV and corresponding actuation of composite actuator (type A) at a) 50 mV s À1 and b) 800 mV s À1 .

Figure 14 .
Figure 14.Comparative a) CV and b) chronoamperometric analysis of single non-Cl À -containing RTIL (EMITFSI) (red line) and mixed Cl À -containing RTIL (EMITFSI and TTPCl, type B) (black line) laminates at 50 mV s À1 highlighting the spiking behavior (electroformation and Joule-failure of individual dendritic bridges) of the former at 1.18 V, 31.5 s.