Voltage Generation in Optically Sensitive Supercapacitor for

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Voltage Generation in Optically Sensitive Supercapacitor for Enhanced Performance Buddha Deka Boruah, and Abha Misra ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01248 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Applied Energy Materials

Voltage Generation in Optically Sensitive Supercapacitor for Enhanced Performance Buddha Deka Boruaha and Abha Misraa*

a

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore,

Karnataka, India 560012 *Corresponding

Author: Prof. Abha Misra Email: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT Optically sensitive solid-state supercapacitor (SSC) is presented for direct utilization of optical energy for charge storage in a compact opto-electrochemical system. Optically and electrochemically active, energy efficient heterostructure nanomaterial of zinc cobalt oxide and zinc oxide (ZCZO) nanorods (NRs) were used as a SSC, where the energy storage performance of the as-fabricated SSC displays highly sensitive behavior towards ultraviolet (UV) illumination, where the photoinduced electrons under UV radiation notably participate in the energy storage process that boosts the overall charge storage capacity (174%) of the SSC. The photogenerated areal capacitance and energy density under UV were found to be 150 µF/cm2 and 11.8 × 10-3 µWh/cm2. Moreover, SSC can be charged by exposing to UV radiation without any integration of electrical power source, where the self-generated voltage in the SSC was measured to be 350 mV under the UV illumination, which is much higher than so-far presented selfpowered SSCs. KEYWORDS: optically driven, self-powered, heterostructure nanomaterial, supercapacitor, opto-electrochemical system.

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The realization of advanced renewable and sustainable energy storage devices has become a necessity due to increased demand of energy consumption. Supercapacitor (SC) has drawn attention for its exceptional energy storage performance such as high power density than rechargeable batteries, high energy density than capacitors, rapid charge and discharge rates, environmental compatibility, long cycle stability.1-7 Energy storage performance of SC is mainly determined by its electrode material and cell voltage.8 Therefore, higher specific capacitance materials such as carbon nanotubes, reduced graphene oxide, activated carbon, etc have widely been used in symmetric SCs.9,10 On the other hand, the cell voltage of symmetric SCs are limited that restricts their energy storage performance.11,12 Asymmetric SCs have been developed to widen the cell voltage by introducing battery-type materials or pseudocapacitive materials for positive electrode, whereas carbon or its composite materials for negative electrode.11,12 However, both as-developed symmetric or asymmetric SCs are driven by integrating an external electrical power source during the charging process, thus, results in the large consumption of energy with overall heavy circuitry. Therefore, it is essential to develop the energy efficient selfpowered SCs for the elimination of external electrical power source during energy storage process. Self-powered SC offers the advantages such as save of unwanted energy losses, continuous storage of self-generated energy for the autonomous device application, increase in lifetime of SCs, simple device circuitry as well as the reduction of overall cost. Hence, selfpowered SCs have been developed, e.g., Wang’s group fabricated self-powered SC composed of manganese dioxide nanowire electrodes and a polyvinylidene difluoride (PVDF)-zinc oxide (ZnO) piezoelectric separator, where the self-generated voltage in the SC was measured to be ~110 mV.13 Likewise, Song et al reported functionalized carbon cloth electrodes and PVDF separator based piezoelectrically driven self-powered SC.14 A self-generated voltage of ~100 mV 3



was measured in the SC under the compressive force in the absence of external electrical power source. It is clear that the self-generated voltage in the reported piezoelectrically driven selfpowered SCs is very limited. In addition, the direct introduction of piezoelectric separator in SC may hinder the electrolyte ions permeation efficiency under charge storage process, which may reduce the overall energy storage performance of the SC. Therefore, an alternative approach is essential for the self-powered SCs to boost the self-generated voltage response. Recently, our group developed UV sensitive SC by introducing optically and electrochemically active electrode material. The significant enhancement in the energy storage response of the SC was measured under UV illumination as compared to absence of UV because of photo-charging effect originated due to photogenerated electrons and holes.15 However, the development of the optically driven self-powered SC requires both the photogenerated electrons and holes to be internally separated without any external driving force. Thus, in this work the heterostructure of zinc cobalt oxide and ZnO (ZCZO) nanorods (NRs) provides both the photo-charging effect along with strong built-in electric field in between junction of heterostructure electrode for selfcharging in SC. Herein, for the first time, direct utilization of optical energy is exploited in solid-state SC (SSC) for self-charge storage. Heterostructure nanomaterials composed of ZCZO NR array were used for the electrodes because of electrochemically active and optically sensitive properties.16 The optical property of ZCZO NRs was directly explored to enhance the overall electrochemical energy storage response of SSC composed of ZCZO NRs electrodes. Nearly three times enhancement in the energy storage response of the SSC was measured upon UV illumination as compared to the absence of UV. The photogenerated areal capacitance and energy density originated were measured to be 150 µF/cm2 and 11.8 × 10-3 µWh/cm2. As-fabricated SSC can 4


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directly be charged by UV without any integration of external electrical power source. SSC displays much higher self-generated voltage response of 350 mV under UV illumination. Moreover, self-generated voltage of the SSC increases linearly with UV illumination intensity that directly converts the optical energy into electrochemical energy in opto-electrochemical system. Furthermore, the self-powered photodetector based on ZCZO NRs shows an efficient UV illumination conversion efficiency in absence of external driving force. This study reveals the fabrication of optically sensitive SSC that opens up avenues for the direct utilization of optical energy for charge storage in the modern self-powered opto-electrochemical system. An SSC was fabricated with electrodes comprised of ZCZO NRs electrodes grown on transparent indium tin oxide (ITO) coated glass substrates for optically driven self-powered SC. Figure 1a shows the steps involved in the fabrication of self-powered SSC. For the growth of ZCZO, vertically ZnO NRs was grown on the ITO coated glass substrate followed by the hydrothermal process. Hydrogenation of as-grown ZnO NRs was employed to increase the conductivity of NRs for efficient electron transportation along c-axis of NRs. Electrochemically active ZnCo2O4 NRs was directly grown on hydrogenated ZnO NRs for heterostructure. The hydrothermal synthesis of nanomaterials offers direct integration of both optically and electrochemically active materials. In addition, an optimum contact resistance in between heterostructure materials can be obtained on a template-free synthesis process. Therefore, ZCZO heterostructure introduces direct electron transportation path as well as optically active medium for electrochemically active sites in ZnCo2O4 NRs. As a result, both the optically and electrochemically active properties of ZCZO NRs can be implemented for novel self-powered SC applications.

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a

Hydrogenated ZnO NRs

ZnO seed layer

ITO

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ZnCo2O4 NRs

Separator with PVA-KOH

SSC assemble

b

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500 nm

2 µm

e

2 µm

20 µm

1 µm

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Figure 1. (a) Schematic demonstration of fabrication processes involved in the self-powered SSC. (b) Microstructures of ZnO NRs at both low and high magnifications. (c) Cross-sectional SEM image of as-grown ZnO NRs and (d) microstructures (at both low and high magnifications) of the as-grown ZnCo2O4 NRs on ZnO NRs. (e) Elemental analysis of the ZCZO NRs. (f) Crystal structures of as-grown ZnO and ZCZO NRs. Microstructure of ZnO and ZnCo2O4 NRs was investigated by scanning electron microscopy (SEM), where Figure 1b depicts as-grown ZnO NRs at both low and high (inset in Figure 1b) magnifications. The cross-sectional view of ZnO NRs (Figure 1c) demonstrates that 6


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ZnO NRs are vertically aligned on the substrate. The diameter of of as-grown ZnO NRs ranges from 80 to 150 nm and length is around 1.5 μm. Microstructure of as-grown ZnCo2O4 NRs on ZnO NRs is depicted in Figure 1d, where diameter of ZnCo2O4 NRs ranges from 150 to 500 nm, whereas length is around 5 μm. Figure S1 shows the cross-sectional view of as-grown ZCZO NRs on ITO-coated glass substrate. Elemental analysis of ZCZO NRs (Figure 1e) reveals that assynthesized ZCZO contains no external impurities. Table S1 depicts the weight and atomic percentages of the zinc, cobalt and oxygen elements of ZCZO NRs. X-ray diffraction (XRD) measurement revealed the wurtzite crystal structure. The attributed XRD diffraction peaks of ZnO NRs and ZCZO NRs in Figure 1f correspond to the diffraction planes of (100), (002), (101), (102), (103), (112) and (201) of ZnO NRs. The presence of (002) diffraction plane reveals that most of ZnO NRs are oriented along c-axis. The cubic spinel phase structure of ZnCo2O4 NRs was confirmed from the diffraction planes (311), (222), (400), (511), and (440).17 Moreover, the observed smaller diffraction peak at around 52o could be attributed from the ITO substrate of the sample.18,19 The electrochemical performances of the as-fabricated SSC is then evaluated both in presence and absence of UV exposure of wavelength 365 nm, where Figure 2a shows the schematic to illustrate the UV illumination on the SSC. Under charge storage process, the reaction mechanism of the positive electrode in PVA-KOH electrolyte can be expressed as follows, ZnCo2O4 + OH − → ZnCo2O4 / OH + ZnCo2O4 − OH ; where, ZnCo2O4 / OH represents the formation of electrical charge double layer and ZnCo2O4 − OH is the reaction between the hydroxyl ions with electrode material, whereas on the negative electrode, only electrical charge double layer formation ( ZnCo2 O4 / K ) takes place.16 In the heterostructure ZCZO NRs electrode, 7



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ZnCo2O4 provides electrochemically active sites whereas ZnO NRs offers the direct conduction path for electron along the c-axis. Moreover, in ZnCo2O4 electrode, some of the Co2+ are partially replaced by Zn2+ in the crystal structure that introduces better conductivity and electrochemical activity as compared to the pristine Co3O4. The detailed electrochemical performance of the heterostructure of ZCZO NRs electrode material can be found elsewhere.16 The CV profiles of the SSC at UV illumination intensity of 3 mW/cm2 are shown in Figure 2b and the asymmetric CV profiles of the SSC could be mainly because of the involvements of surface related reactions on the positive electrode and electrical double layer on the negative electrode of the electrolyte ions. Interestingly, no distinct redox peaks are observed to the Faradaic redox reaction related to Co(OH)2/CoOOH both in absence and presence of UV in the full cell configuration. It is noticed from the results that upon UV illumination, area exclosed by CV of the SSC is enhanced by 52% as compared to absence of UV illumination. Figure 2c and d show the CV profiles at different scan rates ranging from 10 to 50 mV/s both in absence and presence of UV illumination. At all the scan rates, a significant enhancement in the area under CV is noticed in the presence of UV exposure. Moreover, galvanostatic charge-discharge plots of the SSC both in absence and presence of UV over a working voltage of 0.6 V at a constant current density of 2 μA/cm2 are shown in Figure 2e. The discharge times of the SSC were measured to be 25.67 s and 46 s, respectively in the absence and the presence of UV. Significant increase in the discharge time (79.2%) upon exposure of UV as compared to the absence of UV reveals the enhancement of energy storage capacity of the SSC. Figure 2f and g depict the charge-discharge curves with respect to current density both in absence and presence of UV illumination. Likewise, at all the current densities, the discharge time is enhanced significantly in the presence of UV illumination. 8


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a

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Figure 2. (a) Schematic representation of the as-fabricated SSC. (b) CV comparison both in absence and presence of UV at 20 mV/s. (c) and (d) are the CV responses at different scan rates both in absence and presence of UV illumination of intensity, 3 mW/cm2. (e) Charge-discharge comparison plots at the current density of 2 μA/cm2. (f) and (g) charge-discharge responses at different current densities both in absence and presence of UV illumination intensity of 3 mW/cm2. (h) Schematic illustrates the working mechanism of the SSC under exposure of UV (VB: valence band and CB: conduction band).

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Figure 2h shows the schematic representation of the working mechanism of the SSC under the presence of UV, where the electrolyte ions (anions and cations) are driven toward the respective electrodes to accumulate on the electrode-electrolyte surfaces. Under UV illumination, the photogeneration of electron and hole takes place ( h → e− + h + ; where, h is the photon energy, e − and h + are the photogenerated electron and hole), where electrons transport from ZnO to the ITO electrode while holes from ZnO to ZnCo2O4 due to the built-in electric fields in the junctions of ITO, ZnO and ZnCo2O4. Thereby, both the free electrons and holes preferably participate in the anions or cations separation process for charge storage under UV illumination. Preferably, the anions of the electrolyte movement take place towards the positive electrode to form both the electrical charge double layer and the reaction between the hydroxyl ions with electrode material for self-charging response under light illumination. Moreover, the photogenerated electrons and holes not only participate in the charge storage, but it also significantly increases the electron transportation efficiency of the electrodes. The areal capacitance of the SSC was then evaluated followed by charge-discharge plots and Figure 3a shows the areal capacitance plots of the SSC with respect to current density both in absence and presence of UV illumination intensity of 3 mW/cm2. Notably, superior increase in the areal capacitance, nearly 174% enhancement at the current density of 1.2 μA/cm2 was measured under the presence UV as compared to that in the absence of UV. The photogenerated areal capacitance ( C A, photo ) originated because of the photoinduved electrons and holes is evaluated by the relation as follows, C A, photo = C A,UV − C A ; where, C A and C A,UV are the areal capacitances both in absence and presence of UV illumination.

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a

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Figure 3. (a) Areal capacitance comparison both in absence and presence of UV illumination. (b) and (c) are the photogenerated areal capacitances at different current densities and UV light intensities. (d) Ragone plots both in absence and presence of UV illumination. (e) and (f) are the photogenerated energy density profiles at different current densities and UV illumination intensities. Figure 3b shows the photogenerated areal capacitance curve with respect to current density of the SSC. At the current density of 1.2 µA/cm2, the measured photogenerated areal capacitance was 150 µF/cm2, which decreases gradually with current density ( I ) followed by a bi-exponential decay function as follows, C A, photo( I ) = C A0 + A1  exp(− I / t1 ) + A2  exp( − I / t2 ) ; where, A1 , A2 ,

t1 and t 2 are constants and C A0 is the capacitance equal to 8.95 µF/cm2, respectively. At the lower current densities, the photogenerated areal capacitance sharply increases, then gradual decrease is observed at the relatively higher current densities. It is concluded that the 11



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photogenerated electrons and holes are effectively participate in the electrolyte ions separation process at the lower current densities as compared to that at the relatively higher current densities. As a result, the photogenerated areal capacitance of the SSC displayed a bi-exponential decay function with current density. Also, photogenerated areal capacitance of the SSC was further studied at different UV illumination intensities ranging from 0.3 to 3 mW/cm2 at a constant current density of 1.2 μA/cm2 as shown in Figure 3c. It is observed that with the increase in UV radiation intensity, the photogenerated areal capacitance of the SSC increases linearly as follows, C A, photo( P) = C A/ 0 + a  P ; where, a is a constant, P is the UV radiation intensity and C A/ 0 is the areal capacitance equal to 6.4 µF/cm2. The increase in the photogenerated areal capacitance of the SSC with the UV radiation intensity is mainly due to the increase in the electron-hole pairs generation efficiency.20-22 Thereby enhancement in the electrolyte ions separation efficiency, which is because the electron-hole pairs generation efficiency of ZCZO NRs is proportional to the UV illumination which is discussed in later. Moreover, the areal energy density and power density of the SSC were evaluated both in absence and presence of UV illumination. The comparative Ragone plots of the SSC are shown in Figure 3d. At the current density of 1.2 µA/cm2, the areal energy density of the SSC increases from 4.3 × 10-3 to 11.8 × 10-3 µWh/cm2 (174% enhancement) under UV illumination of intensity 3 mW/cm2 as compared to in the absence of UV illumination. Figure 3e shows the variation in the photogenerated areal energy density ( EA, photo = EA,UV − EA ; where, E A and E A,UV are the areal energy densities both in the absence and the presence of UV) with respect to the current density of the SSC, which also follows a bi-exponential decay function on the current density as follows, E A, photo( I ) = E A0 + B1  exp( − I / f1 ) + B2  exp( − I / f 2 ) ; where, B1 , B2 , f1 and 12


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f 2 are constants and E A0 is equal to 4.47 × 10-4 µWh/cm2, respectively. Furthermore, the photogenerated areal energy density of the SSC was measured at different UV illumination intensities of 0.3 to 3 mW/cm2 as depicted in Figure 3f. The photogenerated areal energy density of the SSC was 0.85 × 10-3 µWh/cm2 at UV illumination of 0.3 mW/cm2, which is increased to 7.51 × 10-3 µWh/cm2 at 3 mW/cm2. The photogenerated areal energy density of the SSC follows a linear relation on the UV illumination intensity, such as E A, photo( P) = E A/ 0 + b  P ; where, b is a constant and E A/ 0 is the areal energy density equal to 3.2 × 10-4 µWh/cm2. Likewise, the increase in the photogenerated areal energy density with UV illumination intensity is mainly because of the increase in photoinduced electron-hole pairs.20-22 In order to study the capacitance stability of the SSC, cyclic charge-discharge measurement was done for 4000 cycles as shown in Figure S2. Interestingly, the negligible capacitance losses of only 2% and 4% were observed in the SSC in absence and presence of UV illumination. The increase in the capacitance loss (2%) under the presence of UV illumination as compared to the absence of UV illumination could mainly because of the generation of internal heat under continuous exposure of UV light.15 In general, ZnO based one-dimensional materials show transient increase in temperature under UV illumination because of the pyro-phototronic effect which is discussed later, which may affect in capacitance degradation of SSC. Figure S3 shows the Coulombic efficiency plots of the SSC both in the absence and the presence of UV illumination. At relatively lower current density of 1.6 µA/cm2, the observed Coulombic efficiency was 50.13% which is increased to 89.04% at current density of 12 µA/cm2 in the absence of UV. On the other hand, 44% and 89% of Coulombic efficiencies were observed at 1.6 µA/cm2 and 12 µA/cm2 under the presence of UV illumination. The relatively lower Coulombic efficiencies of the SSC both in the presence and 13



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the absence of UV illumination could be originated because of the solid-state device configuration of the as-fabricated SSC. However, no any distinct change in the state of the SSC device was observed after UV illumination (Figure S4). Moreover, to deliver required voltage and capacitance for direct practical realization, three SSCs were integrated both in the series and parallel configurations, where in series, the voltage of the supercapacitor increases from 0.6 to 1.8 V as depicted in Figure S5a. Likewise, the overall capacitance of the assembled SSC increases in parallel combination of three SSCs (Figure S5b). The self-charging performance of the SSC was then evaluated both in absence and presence of UV illumination, where self-generated voltage in the SSC was measured under the presence of UV illumination in the absence of external electrical power source. Figure 4a shows the self-generated voltage versus time response of the SSC and inset depicts the schematic diagram of the SSC. In the absence of UV, negligible voltage is generated in the SSC and 350 mV of the self-generated voltage is measured with the UV illumination of intensity, 3 mW/cm2 as depicted in Figure 4b and inset shows the schematic representation of the SSC under UV illumination in the absence of external electrical power source. As mentioned earlier that the increase in the self-generated voltage in the SSC under the presence of UV is mainly due to the generation of electrons and holes that directly participate in the electrolyte ions separation process for charge storage. After switched off the UV, the self-generated voltage response of the SSC decreases slowly with time by maintaining a self-generated voltage response of 180 mV in the absence of external power source (Figure 4b). The decrease in the self-generated voltage of the SSC after switched off UV is mainly because of the random movement of the electrolyte ions under light illumination and maintained equilibrium in absence of light. Moreover, the cyclic self-generated voltage response of the SSC is shown in Figure 4c. 14


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a

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Figure 4. (a) Voltage-time response of the SSC in absence of UV. (b) Self-generated voltage response of the SSC with respect to UV radiation. (c) Cyclic self-generated voltage response under periodically UV off and on states of the SSC. (d) Self-generated voltage response with respect to the UV radiation intensity. (e) Self-generated voltage comparison plot of the SSC with reported piezo-electrically driven self-powered energy storage systems.

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Figure 4d shows the self-generated voltage responses at different UV illumination intensities of the SSC, which increases linearly as follows, V ( P) = V0 + c  P ; where, c is a constant and V0 is the voltage equal to 13.63 mV. The measured self-generated voltage in the SSC under UV illumination is much larger than the previously reported self-powered systems (Figure 4e), e.g., 110 mV for piezoelectrically driven self-powered SSC composed of MnO2 nanowires electrodes and a polyvinylidene PVDF-ZnO separator,13 ~100 mV for piezoelectrically modulated selfpowered SSC based on functionalized carbon cloths electrodes and PVDF separator,14 151 mV for NiCoOHCuO@Cu and RGO@Cu electrodes with fish swim bladder separator based SSC driven by piezoelectric response,23 120 mV at 30 N for all-solid-state self-charging power cell (PC) based on PVDF-LiPF6 film as piezo-electrolyte,25 350 mV at 34 N for for all-solid-state self-charging PC based on PVDF separator26 etc. Moreover, SSC shows the low leakage current of 30 nA (Figure S6a) along with the slow self-discharge voltage response as depicted in Figure S6b. Hence, results suggest that as-fabricated SSC can be directly charged by UV light without any integration of external electrical power source. Moreover, the UV photoresponse behavior of the as-grown ZCZO NRs was further evaluated by fabricating a self-powered UV photodetector. During the fabrication of the selfpowered photodetector, ZCZO NRs were selectively grown on cleaned ITO coated glass substrate. Afterwards, 20 µl PEDOT:PSS was spin coated on the top surface of as-grown ZCZO NRs for hole transportation medium to form a ZCZO NRs and PEDOT:PSS junction. Finally, silver (Ag) paste was used to make contacts on the top of PEDOT:PSS to measure the UV photoresponses of the self-powered photodetector. The the schematic representation of the processes involved in the self-powered photodetector is shown in Figure S7. The absorotoin 16


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spectrum of the as-grown ZnO NRs as depicted in Figure S8 that implies visible-blind UV sensitive response. The photo-charge effect of ZnO in the presence of external bias voltage under UV illumination as depicted in Figure S9a, where the self-charging photoresponse of ZCZO in absence of external voltage is shown in Figure S9b. Figure S10a depicts the schematic representation of the as-fabricated self-powered UV photodetector. The UV photoresponse of the photodetector was measured in presence of UV radiation of wavelength 365 nm. Figure S10b depicts the cyclic photoresponses of the self-powered photodetector, where the response current,

I = IUV − I dark ; where I dark is the dark current and IUV is the current upon exposure of UV, is plotted with respect to UV radiation time for nine cycles in the absence of bias voltage. Under the presence of UV radiation, the response current of the self-powered photodetector increases sharply to 800 nA, then gradually decreases to maintain a constant value of nearly 400 nA. Once UV illumination is off, the response current sharply decreases to generate negative response current and then maintained a constant zero response current. Therefore, three stages of response current behavior such as i, ii and iii were observed for the self-powered photodetector (Figure S10c). The sudden increase in the response current of the photodetector under UV illumination in stage i is mainly due to the combined contribution of currents originated from both the photocurrent (Iph) (originated because of the photoinduced electrons and holes) and the pyrocurrent (Ipy). The origin of the pyro-current is mainly because of transient increase in temperature in the ZCZO NRs under UV illumination. In stage i, the response current (ΔI) of the photodetector can be written as, ΔI = Iph + Ipy.28,29 After certain period of UV illumination, the temperature in ZCZO NRs is remained same to maintain negligible pyro-current (Ipy ≈ 0). Hence, the contributed response current in this stage ii is mainly originated from the photocurrent (ΔI = Iph). Once the UV is switched off, the photocurrent goes to zero and then negative response 17



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current originates mainly due to the reverse flow of pyro-current to maintain the equilibrium temperature in stage iii (ΔI = -Ipy) and the detail photoresponse mechanism can be found elsewhere.28 Moreover, UV photoresponses of the photodetector were evaluated at different UV radiation intensities varying from 0.3 to 3 mW/cm2 as depicted in Figure S10d. With the increase in UV radiation intensity, the response current of the photodetector increases followed by a power dependent relationship on UV illumination intensity as follows, I  P x ; where, P is the UV radiation intensity and x is a constant equal to 0.92. The x value provides information related to recombination kinetics of photogenerated charge carriers of the self-powered photodetector, where the higher value of x closure to unity demonstrates efficient electron and hole transfortation rates under UV illumination. The increase in the response current of the photodetector with UV illumination intensity is mainly because of the increase in the electronhole pairs generation efficiency. Also, the power dependent relation of the response current on UV illumination intensity is mainly because of the involvement of electron-hole generations, charge trapping, recombination, etc processes under UV illumination.30,31 Therefore, this result suggests reproducible photoresponse behavior of the as-fabricated self-powered photodetector. Cyclic photoresponse of the photodetector was measured at a 5 Hz optical chopper frequency as shown in Figure S10e. It is observed that as-fabricated photodetector can detect the UV signal in the order of ms in the absence of external bias voltage. The UV photon detection speed of the photodetector was further analyzed and Figure S10f shows the response time and the recovery time calculation of the photodetector.32 Faster response time (τr = 0.029 s) and recovery time (τr = 0.0334 s) were measured for the photodetector, these valuse are faster than most of the reported photodetectors, e.g., τr < 0.2 s and τr < 0.2 s for rGO-ZnO,33 τr < 1 s and τd < 1 s for ZnO Micor/NW,34 τr = 0.23 s and τd = 0.21 s for p-NiO/ZnO-NRs,35 τr = 0.14 s and τd = 0.52 s for Ag18


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ZnO NWs,36 τr = 9.5 s and τd = 38 s for ZnO NWs/graphene foam,32 τr = 32.2 s and τd = 7.8 s for ZnO-SnO2 nanofibers,37 τr = 50 s and τd = 50 s for ZnS-ZnO bilayer film,38 etc. Therefore, assynthesized ZCZO NRs is not only suitable for the energy storage application, it also displays an efficient photoresponse performance towards UV illumination. Optically sensitive SSC is successfully fabricated for direct utilization of optical energy for charge storage in a compact opto-electrochemical system. Electrochemically and optically active, energy efficient heterostructure nanomaterial of ZCZO NRs was synthesized to fabricate the SSC. As-synthesized ZCZO NRs showed higher energy storage capacity and UV illumination sensitive due to the synergistic effect. Hence, the energy storage response of the SSC displayed efficient sensitivity towards UV illumination, where 2.7 times (174%) enhancement in the energy storage capacity of the SSC was measured under the exposure UV as compared to that in absence of UV illumination. The photoinduced electrons and holes under UV radiation directly participate in the electrolyte ion separation process to boost the overall capacitive response of the SSC, where the photogenerated areal capacitance and energy density were measured 150 µF/cm2 and 11.8 × 10-3 µWh/cm2. Moreover, as-fabricated SSC can be directly charged by exposing of UV without any additional integration of external electrical power source. The self-generated voltage in the SSC under 3 mW/cm2 of UV illumination intensity was 350 mV, which is much higher than previously reported piezoelectrically driven self-powered SSCs. Moreover, the self-powered UV photoresponse performance of the ZCZO NRs photodetector demonstrated an efficient photo conversion efficiency of the ZCZO NRs heterostructure nanomaterial. Therefore, present study demonstrates the fabrication of optically sensitive SSC for direct utilization of optical energy for energy storage, which displays novel development of self-powered opto-electrochemical energy storage system. 19



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Synthesis of vertically aligned ZnO NRs: First, ZnO seed layer was deposited on cleaned ITO coated glass substrate (surface resistivity 8-12 Ω/sq) followed by the procedure as follows, zinc acetate dihydrate (40 mM) was dissolved into ethanol and then spin coated the solution on top of cleaned ITO coated glass substrate. The process was repeated for several times to get uniform seed layer and heated at 200 oC for 1 h. ZnO seed layer deposited sample was then dipped into zinc nitrate hexahydrate (25 mM) and hexamethyleneteramine (25 mM) solution and maintained at 85 oC for 6 h. Finally, sample was cleaned with ethanol and DI water and then dried at 200 oC for 1 h. For the hydrogenation of the sample, as-grown ZnO NRs sample was kept inside a quartz tube and then annealed at 350 oC for 2 h in presence of both argon gas (800 sccm) and hydrogen (400 sccm). Synthesis of ZCZO NRs: Hydrogenated ZnO NRs sample was dipped into the growth solution of zinc nitrate hexahydrate (1 mmol), cobalt nitrate hexahydrate (2 mmol), urea (6 mmol) and ammonium fluoride (5 mmol) in 30 ml DI water, which was then maintained at 120 oC for 5 h. The mass loading for the ZCZO NRs was around 3.7 to 4 mg/cm2. Fabrication of SSC: Two identical ZCZO NRs samples were dipped into PVA-KOH gel electrolyte along with a separator for 10 min. Afterward, the electrodes were assembled with the separator in sandwich-type pattern and then left overnight under airflow chamber. Gel PVAKOH electrolyte was prepared as follows, 1 g PVA was dissolved into 10 ml DI water by stirring at 95 oC and then added 0.59 g KOH. Fabrication of photodetector: Vertically aligned ZCZO NRs was selectively grown on ITO coated glass substrate. Afterwards, PEDOT:PSS (20 μl) was spin coated on the top surface of the

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as-grown ZCZO NRs and dried at 90 oC for 1 h. Finally, Ag paste was paint on the PEDOT:PSS surface to fabricate the self-powered photodetector. Electrochemical measurement of the SSC: The electrochemical responses of the SSC was measured by using an electrochemical workstation (CHI 660E). A UV illumination source of 365 nm wavelength (3W high power LED) was used to study of energy storage performance of the SSC. Photoresponse measurement of the photodetector: The photoresponses of as-fabricated selfpowered photodetector was studied using UV source of 365 nm wavelength (3W high power LED), which was measured using Keithley source meter (2611B) and Agilent (B1505A) device analyzer. Moreover, an optical chopper (C-995) was used to measure the photoresponse of the photodetector at the frequency of 5 Hz. All the UV photoresponses of the photodetector were evaluated in absence of external bias voltage. Supporting Information: Cross-sectional SEM image of as grown ZCZO NRs; Elemental compositional percentages of the elements; Capacitance retention plots of the SSC; Digital images of the SSC; CV profiles of SSC integrated in series and parallel configurations; Leakage current and self-discharge characteristics of the SSC; schematic representation of the processes involved in the self-powered photodetector; UV-Vis-NIR spectrum of as-grown ZnO NRs; Photocharge effect of ZnO in the presence of external bias voltage under UV illumination; Selfcharging photoresponse of ZCZO in absence of external voltage and Self-powered UV photoresponses of the photodetector.

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Table of Content (TOC):

UV

V

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Voltage Generation in Optically Sensitive Supercapacitor for

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