As a self-powered EC device

The device consists of a PB film electrochemically deposited on a piece of indium tin oxide (ITO) coated glass (the PB electrode) and a strip of Al sheet attached on another piece of ITO glass (the Al electrode) using 3 mol l−1 KCl aqueous electrolyte. Considering the application of the device as an EC smart window, the strip of Al sheet is attached onto only one side of the ITO glass, which leaves most area of the as-prepared device transparent. The original blue colour (Fig. 1a) of the device is from the PB film. Without applying any external bias, the blue device can be quickly bleached to colourless (Fig. 1b) by connecting the PB and Al electrodes (a video for the self-bleaching of the device is available in Supplementary Video 1). The corresponding transmittance change is shown in Fig. 1c. The maximum optical modulation of the device is 52.2% at 670 nm, which is actually high enough considering that light has to go through two sheets of ITO glass and electrolyte. A similar phenomenon was observed when replacing Al with Fe (Supplementary Fig. 1), clearly implying that the colour bleaching of the device is related to the reduction of PB by strongly reducing metals.

Figure 1: Characterization of the self-powered PB/Al EC device. (a) Optical photo of the as-prepared EC device. (b) The bleached state by connecting the PB and Al electrodes, and (c) the corresponding transmittance change of the EC device. Full size image

The bleached device can spontaneously recover its blue colour by disconnecting the PB and Al electrodes, as shown in Fig. 2a,b (the bleaching and recovery of PB film in a beaker is shown in Supplementary Fig. 2). This is quite interesting because it means that the as-prepared PB/Al cell can be used as a self-powered EC device; the bleaching is realized by connecting the PB and Al electrodes and subsequent colouration is achieved by the spontaneous recovery. The transmittance change (Fig. 2c) of the bleaching/colouration (recovery) switching of the self-powered EC device was measured in situ using an ultraviolet–visible spectrophotometer by connecting the working PB electrode and Al electrode for 10 s and then disconnecting for 590 s for the recovery of the device. The transmittance is increased by 43.9% in the first 10 s (connecting PB and Al electrodes) and decreased by 21.3% in the next 590 s (disconnecting PB and Al electrodes), showing a characteristic of fast bleaching and slow colouration (recovery). For 90 and 70% transmittance changes (within the total 43.9% transmittance change obtained in the first 10 s), the corresponding bleaching times are only 3.3 and 2.3 s, respectively.

Figure 2: Characterization of the colour restoration process of the PB/Al EC device. (a) Optical photo of the bleached EC device, and (b) the EC device recovered for 1 h. (c) In situ transmittance measurement of the EC device connected for the first 10 s and then disconnected for next 590 s. Full size image

The transmittance spectra for the progressive recovery of the bleached device are shown in Fig. 3a. At 670 nm, the transmittance of the device is decreased by 15.7, 22.4, 29.2, 38.5 and 45.8% after a recovery time of 5, 15 min, and 1, 2, and 4 h, respectively, showing a gradually decreasing colouration rate (or recovery rate). The transmittance decrease within the first 5 min is more than that of the next 55 min. Within the first 2 h, the transmittance change is 38.5%. However it is only 7.3% for the next 2 h. Besides the spontaneous recovery, the as-prepared PB/Al cell can also be coloured by an external bias, which will greatly accelerate the colouration rate. By alternately connecting the PB and Al electrodes for 10 s and applying an external bias of 2 V for 10 s, the PB/Al cell is reversibly switched between high and low transmittances, as shown in Fig. 3b. The recoverable high and low transmittance implies good reversibility of the bleaching/colouration switching processes. The calculated bleaching and colouration time is 4.1 and 4.6 s, respectively.

Figure 3: Spontaneous colour restoration and reversible bleaching/colouration switching. (a) Transmittance spectra of the original self-powered PB/Al EC device (original curve) and the bleached device at various recovery times from 0, 5 min to 4 h. (b) Reversible bleaching/colouration switching driven by alternately connecting the Al and PB electrodes for 10 s and supplying an external bias of 2 V for 10 s. Full size image

As a self-rechargeable battery

Another interesting function of the as-prepared self-recoverable PB/Al cell is its potential use as a self-rechargeable battery. The measured open circuit potential of the PB/Al battery with 3 mol l−1 KCl electrolyte is 1.26 V, which is significantly higher than that of previously reported EC materials-based batteries27,28,29. To light up a red light-emitting diode (LED), we connected two PB/Al cells in series, which can supply a voltage of 2.52 V. The LED is brightest when powered by the freshly prepared cells (Fig. 4a,b) (the corresponding videos are available in Supplementary Videos 2 and 3).

Figure 4: Photos of the as-prepared two PB/Al devices connected in series acting as self-rechargeable batteries. (a) In coloured state with two electrodes disconnected, and (b) the connected circuit powering a LED. (c) In bleached state with connected PB electrode and Al electrode, and (d) connected circuit showing no light from the LED. (e) After recovering for 1 h with circuit disconnected, and (f) connected circuit powering the LED. Full size image

After being bleached to colourless (Fig. 4c), the LED cannot be lighted up (Fig. 4d), implying the discharged state in the bleached device. However, after disconnecting the PB and Al electrodes for 1 h, the PB/Al cells are recovered to bluish (Fig. 4e) and able to light up the LED again (Fig. 4f), indicating that the battery was partially recharged. The recovery from colourless to blue colour of the PB/Al cell corresponds to the charging process of the battery; a deeper colour means a higher charging extent. In this sense, the amount of stored electricity of the self-rechargeable PB/Al battery can be clearly indicated by its colour.

The discharge process (as a battery) of the PB/Al cell corresponds to its bleaching process (as an EC device). The amount of charge (that is, battery capacity) involved during the discharge process and the subsequent charge (colour restoration in the EC device) process can be accurately measured by an electrochemical workstation. Figure 5 shows the specific accumulative discharge and charge capacity of the original and recovered PB/Al cells. The discharge capacity at −2 V for 30 s is 63.6 mA h g−1 (Fig. 5a) and the charge capacity at 2.5 V for 30 s (Fig. 5b) is 64.1 mA h g−1, which are comparable to the capacity of recently reported copper hexacyanoferrate battery29. The discharge and charge capacities are very close, implying a good reversibility in the charging/discharging of the as-prepared PB/Al battery. After spontaneous recovery for 24 h, the recovered PB/Al battery is able to deliver a discharge capacity of 39.4 mA h g−1 (Fig. 5c), that is 61.9% of the total dischargeable amount of the original PB/Al cell. Though the specific capacity is relatively low compared with those of lithium batteries11,12,13,14, the as-prepared PB/Al battery is potentially capable of providing a reasonably large total capacity in a limited volume and a high voltage with the thin-layered structure. If an external bias is applied to charge the bleached device instead of self-charging in air, both of the charge and re-discharge capacities can be increased by ~27.8% (Supplementary Fig. 3).

Figure 5: Specific capacities of the original and recovered PB/Al cells. (a) Discharging capacity curve at −2 V, and (b) charging capacity curve at 2.5 V for the original PB/Al cell and (c) discharging capacity curve at −2 V for the recovered PB/Al cell with a 24 h recovery in air. Full size image

Further battery characterizations

We further characterize the battery characteristics of the PB/Al cell. The cyclic voltammetric response of the PB film electrode is first investigated using a three-electrode system (Supplementary Fig. 4), in which the PB film on ITO was used as the working electrode and the Al foil and standard calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The measured open circuit potential of the half cell is 0.4 V versus SCE. Consistent with a previous report, the cyclic voltammetric behaviour is accompanied by two redox couples in accordance with the following reactions33:

Reaction (1) with a potential of 0.18 V versus SCE (the first pair of peaks) is referred to the reduction of PB to PW via one-electron transfer to the low-spin iron atom. Reaction (2) at 0.96 V versus SCE is referred to the oxidation of PB to so-called ‘Berlin green’ (BG) via one-electron transfer oxidizing high-spin iron in PB lattice.

As for the two-electrode PB/Al cells (that is, Al foil served as both the counter electrode and the reference electrode), the cell reaction is only related with the bleaching and recovery of PB. Considering the potential difference of Al versus SCE (−0.86 V), the bleaching process might occur around 1.0 V (that is, 0.18+0.86 V). Meanwhile, the potential corresponding to the oxidation of PW to PB might shift to even higher position to overcome the overpotential. Herein, the electrochemical measurements using the two-electrode configuration are performed in a wider voltage range between 0.8 and 2.1 V to focus on the redox reactions between PB and PW.

The cyclic voltammetric response of the PB/Al cell is shown in Fig. 6a. One pair of peaks (cathodic, anodic) observed at the potentials (V) of (1.0, 1.9) can be attributed to the reduction of PB to PW (cathodic scan) and oxidation of PW to PB (anodic scan). In agreement with the CV result, one charge plateau (~1.9 V) and one discharge plateau (~1.1 V) can be observed in the charge/discharge curves, which can be assigned to the Fe(III)/Fe(II) redox couple in PB and PW (Fig. 6b). The initial discharge and charge capacities are 72.2 mA h g−1 and 72.1 mA h g−1, respectively. Figure 6c shows the discharge–charge cycling performance evaluated between 0.8 and 2.1 V using the two-electrode configuration. When cycled at a current density of 2000, mA g−1, a capacity of 75 mA h g−1 is obtained in the first cycle, and 81% of this capacity (61 mA h g−1) can be retained after 50 cycles. Moreover, the PB films can deliver higher capacities at a lower current density of 400 mA g−1 (Supplementary Fig. 5). The spontaneous self-recharging process of the PB/Al cell was investigated with a 12-h recovery in deionized water bubbled with O 2 gas. After cycling for three times, the recovered PB/Al battery can still deliver a discharge capacity of 56.6 mA h g−1 (Fig. 6d), with a total capacity loss of 20% compared with the first discharge process (71.9 mA h g−1). Furthermore, the internal resistance of the cell was investigated through the electrochemical impedance spectroscopy measurements (Supplementary Fig. 6). The cell behaves like a resistance at high frequencies. Whereas, the imaginary part of the impedance sharply increases and the plot appears to be a vertical line characteristic of capacitive behaviour at low frequencies34. A possible explanation of the high resistance shown by the cell with the decreasing frequency might be related to the high impedance of the electrolyte30,35.