Electrochemically driven mechanical energy harvesting

Abstract

Efficient mechanical energy harvesters enable various wearable devices and auxiliary energy supply. Here we report a novel class of mechanical energy harvesters via stress-voltage coupling in electrochemically alloyed electrodes. The device consists of two identical Li-alloyed Si as electrodes, separated by electrolyte-soaked polymer membranes. Bending-induced asymmetric stresses generate chemical potential difference, driving lithium ion flux from the compressed to the tensed electrode to generate electrical current. Removing the bending reverses ion flux and electrical current. Our thermodynamic analysis reveals that the ideal energy-harvesting efficiency of this device is dictated by the Poisson's ratio of the electrodes. For the thin-film-based energy harvester used in this study, the device has achieved a generating capacity of 15%. The device demonstrates a practical use of stress-composition-voltage coupling in electrochemically active alloys to harvest low-grade mechanical energies from various low-frequency motions, such as everyday human activities.

Figure 1. The working principle behind the mechanical energy harvesting device.

(a) Thermodynamic perspective on bending-unbending cycle. On introducing different stress states by bending, a chemical potential difference develops between two electrodes. When the electrodes are connected by an external circuit, new equilibrium under the stress states are established by Li⁺ migration. Once the stresses are removed, the lithiation states return back to the original equilibrium state. The area covered by this cycle in red measures the energy output obtained. (b) Schematics of the cross-section of the device in operation.

Figure 2. A prototype of the mechanical energy harvester.

(a) Schematic view of the device design. Compressed region is illustrated in red while the tensile region is illustrated in blue. Lithium ions migrating from the compressed plate to the tensile plate are shown with arrows. The electrolyte soaked separator is drawn in yellow. (b) An image of the actual device with a bending unit. Both scale bars indicate 1 cm. (c) Cross sectional image of the device showing polypropylene electrolyte layer (A in the figure), Li_xSi electrode on Ag current collector (B in the figure) and polyimide adhesion layer (C in the figure). The scale bars on the left and right indicate 40 and 2 μm, respectively.

Figure 3. Open circuit voltages measured during bending tests.

(a) The open-circuit voltage measured from simple bending of the device. The measured values show clear voltage peaks during bending and releasing the device, each with 30 s interval. Each alphabetical points correspond to the bending geometry illustrated in Fig. 1b. (b) The predicted open-circuit voltage and hydrostatic stress according to the radii of curvature, operated in the elastic regime. s.e. resulted from at least five measurements for each radius of curvature is included. The measured voltage values agree well with the predicted values.

Figure 4. Short circuit current density during bending tests.

Bending was maintained at 2.0 mm radius of curvature with 10 s intervals. The positive peaks correspond to the current during bending and negative peaks to the current during unbending.

Figure 5. Electricity generated during repeated bending tests.

(a) The open-circuit voltage at 10 mm radius of curvature and (b) short-circuit current collected during repeated bending tests at 4.0 mm radius of curvature. The nested figure shows the zoomed-in view of the 800th–807th bending cycle.