Eight sets of IPMC samples with different Pt electrode surface structures were prepared by varying the number of impregnation-reduction cycles (referred to as primary platings) from 1 to 8 in electroless plating process. A detailed description is provided in the Methods section. The number of subsequent chemical deposition cycles (referred to as secondary platings) was kept constant and one plating was applied to equalize the electrode conductivity of the samples. The notation of the prepared IPMCs and applied platings are listed in Table 1.

Table 1 Notation of IPMC actuators prepared in the study Full size table

Electrode surface structure

Figure 1 shows the cross-sectional scanning electron microscope (SEM) images of the prepared IPMC samples. The micrographs were taken at 1k × magnification from one side of the cross-section and show the deposited Pt electrode layer at the surface of the polymer membrane in case of each sample. It can be seen that the electrode layer grows in thickness with increasing the number of primary plating cycles from 1 to 8 (samples Pt(1)…Pt(8)). Also, noticeable changes in the electrode surface profile can be observed. The sample Pt(1) has a rather uniform and flat electrode layer, while the samples prepared with higher number of Pt platings exhibit large bumps at the inner surface of the electrode. It should be noted that the uneven and rough regions on the polymer are due to fracturing the samples in liquid nitrogen prior to SEM observations. Also, it is important to note that due to fracturing the cross-section cuts are not perfectly perpendicular to the IPMC surface. Therefore, when the cross-section is aligned perpendicular to the electron beam, the image can also reveal electrode outer surface from a side, making the electrode appear thicker, such as in case of Pt(3).

Figure 1 Cross-sectional morphology of IPMCs. SEM cross-sectional micrographs for different IPMC samples (Pt(1)...Pt(8)) at 1k × magnification, showing the deposited Pt electrode layer at the surface of ionomer membrane. Full size image

Figure 2 provides a detailed view of the polymer-electrode interface for different IPMC samples at 30k × magnification. It can be seen that the electrode interface is composed of Pt nanoparticles that gradually become larger with increasing the primary plating cycles. Starting from 5th plating (sample Pt(5)), the platinum particles with multiple sharp tips and edges – called ‘nanothorn assemblies’ are formed that become larger and more developed with further increment of plating cycles. The IPMC prepared with eight platings (sample Pt(8)) already shows rather large nanothorn assemblies with well-developed structure at the electrode interface (Fig. 2b). There are very few reports concerning Pt nanoparticles with sharp tips. Tian et al. have prepared similar nanostructures on a glassy carbon substrate using electrodeposition method27. Our as-synthesized nanothorn assemblies are larger and more developed and created electrolessly via impregnation-reduction process at the polymer-electrode interface. To the best of our knowledge, such nanostructured Pt assemblies with sharp tips and edges have not been synthesized through electroless plating method or used in EAP application before.

Figure 2 Electrode surface structure. (a) SEM cross-sectional images showing the polymer-electrode interface of IPMCs Pt(1)…Pt(8) at 30k × magnification. (b) Detailed view of nanothorn assemblies at 100k × and 110k × magnification for sample Pt(8). Full size image

The mechanism of formation of Pt nanothorn assemblies is not well understood. However, the dendritic crystal structures generally form due to growth instabilities when the growth rate is limited by the rate of diffusion of atoms to a surface28. There also has to be a concentration gradient from a solution to the equilibrium at the interface. Deposited metal particles by nature contain surface defects and imperfections such as bumps and tips. A corner or tip on the deposited particle results in a steeper concentration gradient and thereby increases the diffusion rate, leading to a faster growth of the structure at the tip and eventually formation of peak28. The growth of Pt nanothorn assembly with consecutive impregnation-reduction cycles is illustrated in Fig. 3.

Figure 3 Growth of “nanothorn” assembly. Schematic illustration of the growth of Pt nanothorn assembly with consecutive primary plating cycles. The dashed arrows indicate faster growth rate at the corners of the particle. Full size image

To fundamentally understand the role of electrode surface structure in IPMC transduction, a series of finite element simulations was carried out with different electrode surface profiles using our recently developed physics-based electromechanical model29,30,31. The Koch fractal geometry25 was implemented in the model to mathematically describe the polymer-electrode interface in IPMC. The classic Koch fractal algorithm is based on dividing a line segment into three and replacing the middle segment by two sides of equilateral triangle. This procedure is recursively repeated for each of new segments. The fractals were designed random directional, i.e. 50% chance for each triangle to be constructed on top or bottom of the segment. The generation of random directional fractal electrode surfaces (generations noted as gen 1 …gen 3 based on the fractal depth 1…n) is illustrated in Fig. 4a. Due to the complicated geometry of gen 2 and gen 3 fractals, the domain width for the calculations was chosen very narrow – only 40 µm. Figure 4b shows the simulated transported charge in case of electromechanical transduction of IPMC with an applied voltage of 1 V for flat and fractal electrodes. It can be seen that the transported charge in case of fractal electrodes is considerably higher compared to the flat electrodes and increases with the generation depth of fractals. Due to the complicated geometry of the electrodes and resulting computational cost, the body force and displacement were not considered. However, based on the model29,30,31, higher charge density at the electrodes results in a larger displacement. Thus, the simulations suggest that increasing the electrode interfacial surface area by generating fractals (or dendrites) can improve the electromechanical output of IPMC.

Figure 4 Fractal electrode simulations. (a) Fractal electrode surfaces used in the simulations (domain width = 40 µm), (b) Calculated transported charge in case of flat vs. gen 1 …gen 3 fractal electrodes at an applied DC voltage of 1 V. Full size image

Electromechanical performance of IPMC actuators

Figure 5a shows the measured voltage, current, transported charge and corresponding displacement responses in time for IPMCs at ±1 V AC square-wave input at 0.1 Hz. Although the IPMCs of this type are typically operated at input voltages between 2–4 V, a lower voltage (1 V) was used in order to avoid electrochemical processes (i.e. electrolysis of water) – the lost current, to allow more exact analysis of the data in terms of the transported charge. It can be seen that the current as well as the transported charge and displacement are the lowest in case of actuator Pt(1) and increase significantly with the number of plating cycles (or growth of nanothorn assemblies). The peak displacement of actuator Pt(3) is already three-fold compared to that of Pt(1). However, it can be noticed that after the 4th plating the displacement reaches plateau, while there is still a noticeable increase in the transported charge with the number of plating cycles (see Fig. 6a). This is related to low input voltage (1 V) and resulting low actuation force which apparently is too low to overcome the increasing flexural stiffness of the samples with higher number of Pt platings (see Table 1). Overall, the measurements show that the growth of nanothorn assemblies with repeated impregnation-reduction of Pt leads to a higher transported charge and larger displacement of IPMC. These results are in good agreement with our theoretical prediction (see Fig. 4)24.

Figure 5 Actuation performance of IPMCs. Measured voltage, current, transported charge and displacement responses for IPMCs with different electrode surface structures (a) at 0.1 Hz, ±1 V and (b) at 0.1 Hz, ±3 V AC square-wave input. Full size image

Figure 6 Transported charge/displacement correlation. Peak-to-peak displacement versus transported charge (a) at 0.1 Hz, ±1 V and (b) at 0.1 Hz, ±3 V AC square-wave input. Full size image

The displacement measurements were also performed at higher voltages in order to see how the transported charge correlates with the actuation performance at normal operating voltages (2–3 V). Figure 5b shows the measured voltage, current, transported charge and displacement responses at ±3 V AC square-wave at 0.1 Hz. As can be seen, the differences in the measured responses in case of different samples are more apparent compared to measurements at ±1 V AC input (Fig. 5a). The peak currents are higher and decay at a slower rate as the number of platings is increased, indicating a higher double-layer charging. It should be noted that the measured current response at a given voltage also includes the charge transfer associated with faradaic processes – electrolysis of water32, which does not contribute to the actuation of IPMC. Therefore, exact evaluation of the actuation performance in terms of transported charge is complicated at higher input voltages (>1.8 V). Nonetheless, the measured current/transported charge data correlates well with the displacement response of the samples (Fig. 6b). As compared to the measurements at ±1 V AC (Fig. 6a), the differences in the transported charge and displacement are more pronounced, especially for the IPMCs with higher number of primary platings, i.e. actuators Pt(4)…Pt(8). While there is a steady increment in the charge transport with the number of added platings, the displacement increases up to seventh plating and the actuator with eight platings (Pt(8)) already shows slightly less displacement than Pt(7). This can be related to the increasing flexural stiffness of the material that starts to limit the actuation performance (see Table. 1). Overall, the data shows that increasing the number of impregnation-reduction cycles from 1 to 7 results in an improvement of displacement amplitude more than 3 times.

Figure 7 shows the transported charge and corresponding peak-to-peak displacement of Pt-IPMCs as a function of frequency from 0.05 to 5 Hz at ±3 AC square-wave input. The transported charge and the displacement values were calculated by taking an average over minimum of 6 actuation cycles at each measured frequency. As can be seen, the transported charge data correlates well with the actuation performance of IPMCs in the tested frequency range. It can be noticed that at higher frequencies (>1 Hz) the effect of plating cycles on the transported charge and corresponding displacement response is relatively low. This is due to the limited charging time at higher actuation frequency that prevents utilizing the larger interfacial area of the electrodes in case of the samples with higher number of platings. At 0.5 Hz and below, there is a notable increase in the charge transport and displacement response with increasing the number of plating cycles. The actuator response time is suitable for various biomimetic applications, such as for artificial muscle fins and control surfaces used for locomotion and maneuvering of bio-inspired autonomous underwater robotic systems7.

Figure 7 Actuation performance at different frequencies. Frequency dependences of (a) total transported charge and (b) corresponding displacement of IPMCs at ±3 V AC square-wave input between 0.05–5 Hz. Full size image

Figure 8a shows the bending strain of the actuator Pt(7) as function of frequency from 0.05–5 Hz at ±3 V AC square-wave input, in comparison with other ionic EAP actuators reported in the literature33,34,35,36,37. It is important note that the actuators fabricated in this study and elsewhere differ greatly in dimensions, particularly in thickness that ranges from 70 µm to 0.6 mm. Since the displacement output of the EAPs is highly dependent on the intensity of the applied electric field, the different actuators are also compared in terms of the effective field strength (V/mm), taking into account the actuator thickness (across which the voltage is applied). It can be seen that IPMC with nanothorn assembly electrodes operated at relatively low electric field (5 V/mm) exhibits notably higher actuation performance compared with previously reported EAP actuators. In addition to outstanding displacement performance, the new IPMC actuators with nanothorn electrodes also offer a long cycle life. Figure 8b demonstrates the long-term durability of IPMC with nanothorn electrodes under continuous operation in water at ±2 V AC square-wave input at 1 Hz. The actuator shows repeated actuation over 23000 cycles with no decrease in the displacement amplitude.

Figure 8 Comparison with other EAPs and cycle life of the new IPMC actuator. (a) Frequency dependence of the bending strain for IPMC with nanothorn electrodes compared with other ionic EAP actuators reported in the literature. The parameters in the brackets indicate the applied voltage and field strength across the actuator thickness. (b) Cycle life of IPMC with nanothorn electrodes (Pt(7)) under continuous operation at ±2 V AC square-wave input at 1 Hz. Full size image

The blocking force, which characterizes the generated electromechanical force at IPMC tip at zero displacement, was examined at different driving voltages (1–3 V DC). Figure 9a shows the measured voltage, current, transported charge and corresponding blocking force response in time for different IPMC samples at 1 V DC input. The data shows that increasing the impregnation-reduction cycles leads to a notable increase in the total transported charge and resulting blocking force performance of IPMC. These results are in accordance with the displacement measurements discussed earlier. Figure 9b presents the peak blocking force of IPMCs as a function of primary plating cycles at different input voltages (1–3 V DC). As can be observed, there is a significant increase in the peak blocking force with the growth of nanothorn assemblies, especially at the higher driving voltages (2–3 V). However, the most dramatic increase in the measured force response occurs from the 2nd to 5th plating cycle. The overall improvement in the blocking force performance is more than 5 times in case of 1 V DC and more than 3 times in case of 3 V DC input with growth of nanothorn assemblies.

Figure 9 Blocking force performance of IPMCs. (a) Voltage, current, transported charge and blocking force responses in time at 1 V DC input. (b) Peak blocking force vs. the number of plating cycles at different input voltages (1–3 V DC). Full size image

Electrochemical properties

The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were employed to evaluate the interfacial surface of different IPMC electrodes in terms of capacitance. The electric double-layer capacitance describes directly the electrode interfacial area and its effectiveness to accommodate charges at the polymer-electrode boundary. Figure 10a shows the frequency dependence of the differential capacitance for different IPMC samples, obtained from EIS measurements in the frequency range of 0.1–100 Hz with an AC perturbation of 10 mV and DC bias of 0.1 V in two-electrode cell. The measured data shows that increasing the primary plating cycles and thereby developing the nanothorn assemblies at the electrode interface leads to a notable increase in the double-layer capacitance of IPMC at the lower frequencies (f < 10 Hz), indicating an enlarged interfacial surface area of nanostructured electrodes. A higher charge accumulation in case of more developed surface geometry is in accordance with the fractal electrode simulations (see Fig. 4).

Figure 10 Capacitive properties of Pt nanothorn assemblies. (a) Frequency dependence of differential capacitance for IPMCs with different electrode surface morphologies at 0.1 V DC bias with a 10 mV AC perturbation. (b) An alternative representation of the data: EIS capacitance vs. frequency vs. primary plating cycles. (c) Cyclic voltammograms measured at a potential scan rate of 50 mV/s. (d) Charging-discharging capacitance of IPMCs vs. primary plating cycles. Full size image

The charging-discharging capacitance of IPMCs with different electrode surface structures was determined using cyclic voltammetry in a potential range of −0.5 to 0.5 V at the scan rate of 50 mV/s in two-electrode cell. The measured cyclic voltammograms in Fig. 10c indicate non-faradaic capacitive current behavior. IPMC in principle is similar to electrochemical capacitor that stores energy in electric double layer using high surface area electrodes and electrolyte. It can be seen in Fig. 10c that increasing the number of plating cycles leads to a considerably higher current densities during the charging and discharging, which can be expected as more charges are involved in the double-layer formation due to the larger interfacial surface area of electrodes. The charging-discharging capacitance values were determined from the measured cyclic voltammograms according to the equation (3) and are plotted against the number of plating cycles, as shown in Fig. 10d. As can be seen, there is a steady and almost linear increase in the capacitance with the increment of plating cycles (growth of nanothorn assemblies). The improvement in the capacitance is six times (0.02 mF/cm2 to 0.121 mF/cm2) with increasing the plating cycles from 1 to 8. These results are consistent with EIS measurements as well as with the SEM data and are in good agreement with electromechanical performance data of IPMCs. The capacitance measurements demonstrate that careful control of the synthesis parameters of electroless plating process allows good control over the interfacial area of Pt nanothorn assembly electrodes.