Ionic covalent organic framework based electrolyte for fast-response ultra-low voltage electrochemical actuators

Electrically activated soft actuators with large deformability are important for soft robotics but enhancing durability and efficiency of electrochemical actuators is challenging. Herein, we demonstrate that the actuation performance of an ionic two-dimensional covalent-organic framework based electrochemical actuator is improved through the ordered pore structure of opening up efficient ion transport routes. Specifically, the actuator shows a large peak to peak displacement (9.3 mm, ±0.5 V, 1 Hz), a fast-response time to reach equilibrium-bending (~1 s), a correspondingly high bending strain difference (0.38%), a broad response frequency (0.1–20 Hz) and excellent durability (>99%) after 23,000 cycles. The present study ascertains the functionality of soft electrolyte as bionic artificial actuators while providing ideas for expanding the limits in applications for robots.

Brunauer−Emmett−Teller (BET) surface areas were calculated from N2 sorption isotherms at 77 K using a Micromeritics ASAP 2020 surface area and pore size analyzer. Before measurement, the samples were degassed in vacuum at room temperature for 24 h. By using the non-local density functional theory (NLDFT) model, the pore size distribution was derived from the sorption curve. Particle size analysis was performed with a particle size analyzer (SZ-100, HORIBA).
Ionic Conductivity measurement. Ionic conductivity measurements were performed on sample pellets using CHI 760E workstation over a frequency range from 1 MHz to 1 Hz and with an input voltage amplitude of 10 mV (30 % RH, 25 o C). The sample pellets were tightly connected between two platinum electrodes by means of spring, to ensure good contact between sample and each electrode.  (1). To a mixture of 1,4dimethoxybenzene (5.0 g, 36.1 mmol) and paraformaldehyde (1.5 g, 50 mmol) in 1,4dioxane (20 mL), formaldehyde solution (37 wt.%, 6 mL) was introduced. The resulting mixture was heated to 90 ºC and then concentrated HCl (10 mL) was added in drops within 20 min. After being heated for another 1 h, HCl (37 wt.%, 15 mL) was added and the resulting mixture was cooled to room temperature. The resulting white precipitate was collected by filtration, washed with water, and dried under vacuum, which was further recrystallized from acetone to give product 1 as a white powder.

Synthesis of 2,5-dimethoxyterephthalaldehyde (2).
A mixture of 1 (2.0 g, 8.5 mmol) and hexamethylenetetramine (2.5 g, 17.5 mmol) in chloroform (30 mL) was refluxed at 90 ºC for 24 h. After being cooled to room temperature, the pale yellow precipitate was collected by filtration, washed with CHCl3, dried, and dissolved in water. The aqueous solution was acidified with concentrated HCl (5 mL) and heated at 90 ºC for another 24 h. The mixture was cooled to room temperature, extracted with dichloromethane, and the organic phase was dried over anhydrous MgSO4. After solvent evaporation, the residue was recrystallized from ethanol to yield compound 2 as a yellow needle-shaped solid. Yield: (0.8 g, 40%

1,3,5-tris(4-nitrophenyl)benzene (3)
. 4-Nitroacetophenone (50 g), toluene (200 mL), and CF3SO3H (2.0 mL) were added to a flask equipped with a water separator and a cooling condenser. The mixture was refluxed for 48 h, during this time the formed water was eliminated as a toluene azeotrope. After being cooled to room temperature, the mixture was filtered and washed with DMF under refluxing to yield a grey-green solid product after drying. This product is insoluble in any common solvent.

Supplementary Equation 1
Ionic conductivity. The ionic conductivity ( ) can be calculated with the following equation 1: (1) where R is the ionic resistance, and l and A are the thickness and area of the pellet.

Supplementary Equation 2
Bending strain difference (%). The bending strain difference ( , %) generated in the actuator was estimated by the following equation 2: (2) where d, δ, and l are the thickness, the tip displacement, and the free length of the actuator.