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A hygroelectric power generator with energy self-storage
Chemical Engineering Journal xxx (xxxx) xxxx
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
A hygroelectric power generator with energy self-storage Yuyang Hana, Bing Lua, Changxiang Shaoa, Tong Xua, Qianwen Liua, Yuan Liangb, Xuting Jina, ⁎ Jian Gaoa, Zhipan Zhanga, a Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China b College of Materials Science and Engineering, Beijing University of Technology, Beijing 100084, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
hygroelectric power generator with • Aenergy self-storage is fabricated. generator is integrated with • Power energy storage unit by the Au common electrode.
array of devices can output a vol• An tage of 2 V to drive a calculator.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hygroelectric power generator Supercapacitor Energy self-storage Moisture Graphene oxide
Devices that convert ambient energy into electricity and simultaneously store it for future usage are highly desirable. Herein, we have designed a hygroelectric power generator with energy self-storage ability (HPGES) by hybridizing a moist-electric energy harvester with a supercapacitor, thus achieving the conversion of chemical energy into electricity and the storage of electric energy at the same time. An array of ten HPGES devices are able to deliver a voltage output of 2 V and adequately drive a commercial calculator.
1. Introduction The increasing energy demand and ongoing environmental issues have urged mankind to find alternative energy sources other than fossil fuels for sustainable development. To date, new technologies based on solar, wind, tidal and geothermal energies have greatly enriched the energy portfolio. Recently, harvesting electricity from different types of water motions has emerged as a new option to utilize the ambient energy [1–9]. For example, a droplet of ionic solution moving on monolayer graphene could produce a voltage of few millivolts [1]. Our group found that graphene oxide (GO) assembly with an oxygen gradient
⁎
could induce a gradient of protons under moisture ingress and the migration of protons could induce a voltage output of ~40 mV [2]. Moreover, electrical generation could also be achieved by moisture directional stimulation to pristine GO [3]. In addition to graphenebased materials, Zhou el al. found the TiO2 nanowire networks could induce an open-circuit voltage of up to 0.5 V when exposed to the diffusive flow of moisture [4]. Although these hygroelectric power generators have been substantially improved during the past few years, there are still some difficulties impeding its widespread application. For example, almost hygroelectric generators suffer from a limited and pulse energy outputs (in the range of nA–μA), causing the
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.cej.2019.123366 Received 6 August 2019; Received in revised form 11 October 2019; Accepted 2 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yuyang Han, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123366
Chemical Engineering Journal xxx (xxxx) xxxx
Y. Han, et al.
2.3. Preparation of PEDOT:
uncontrollable and intermittent energy output. Moreover, the hygroelectric generators are failing to produce electricity continually keeping in a constant environment for a long time, which due to the helplessness of storing the electricity. The application scenarios are limited due to the occurrence of electricity generation under the condition of humidity change. Hence, we need to develop a new generator that can collect and store energy at the same time, with the features of controllable voltage and current to output. With advantages of fast charge–discharge rate and long cycling life, supercapacitors have the promising development in the field of energy storage [10,11]. In view of the supercapacitor has the ability of charge storage, it can be integrated with power generation device to realize energy conversion and storage in the meanwhile. In this study, we have designed a hygroelectric power generator with energy self-storage ability (HPGES) by integrating a moist-electric energy harvester with poly (3,4-ethylenedioxythiophene)-based supercapacitors on a common electrode, which can deliver the power output in a stable and controllable way. Compared to the pristine hygroelectric generator, the HPGES can even work under dry conditions, which avoid the waste of energy in a great extent. A HPGES delivers an energy density of 23.9 μWh cm−3 and electrical charge of 870 mC cm−3 when discharged at a current density of 5 mA cm−3. In addition, the output power can be easily enhanced by scaling up the HPGES and an array of 10 HPGES devices achieve an output voltage of 2 V to adequately power a commercial calculator.
Poly (3,4-ethylenedioxythiophene) (PEDOT) was electrodeposited on the side of Au-PTFE in a three-electrode system. The precursor solution contained 1.08 wt% LiClO4 and 1.42 wt% EDOT in deionized water. The electrodeposition was carried out at 1 V for 50 s, 100 s, 200 s, 300 s and 600 s, respectively, before the deposited PEDOT was vacuum-dried at 80 °C for 2 h. The area of the film was 0.1 cm2. 2.4. Preparation of MnO2 The MnO2 film was also electrodeposited on the Au-PTFE. The precursor solution contained 0.1 M C6H13MnO8 and 0.5 M Na2SO4 in deionized water. The deposition was carried out at 0.8 V for 300 s and the MnO2 film was dried in the air for 2 h. 2.5. Characterization of the material The morphology and thickness of all samples were measured by scanning electron microscope (SEM, Zeiss SUPRA TM 55 SAPPHIRE, Germany). The relative humidity in the experiment was controlled by regulating the amount of moisture in the closed test chamber. To generate a humid environment, the moisture was brought in with the flow of nitrogen (N2) through deionized water and the relative humidity in the test chamber could be monitored online by a Humidity & Temperature Meter (AR847+, SMART SENSOR®) beside the HPGES. Alternatively, pure N2 was used to purge the moisture from the test chamber for a low humidity environment. Electric signals of the hygroelectric generator were acquired on a Keithley 2400 multimeter. All electrochemical measurement were performed on a CHI-760E workstation. The volumetric capacitance (in F cm−3), C, was calculated by C = I × t/ΔV, where I, t and ΔV were the discharge current density, discharge time and the potential range in the galvanostatic charge/ discharge curves. Meanwhile, the stored electrical charge (in C cm−3) was estimated by Q = I × t and the energy density (in Wh cm−3) was calculated by E = 0.5 × C × (ΔV)2.
2. Experimental methods 2.1. Materials 3,4-Ethylenedioxythiophene, LiClO4, C6H13MnO8 and Na2SO4 were purchased from Aladdin Industries. All chemicals were of analytical grade and used as received.
2.2. Preparation of graphene oxide film (GOF) Graphene oxide (GO) was prepared by a modified Hummers’ method [12]. The GOF was prepared by doctor-blading a GO dispersion (10 mg mL−1, 5 mL) on the poly(tetrafluoro ethylene) (PTFE) plate sprayed with gold (denoted as Au-PTFE). A thin film of 1 cm2 size was formed after drying at 50 °C for 5 h. The area and thickness of GOF could be conveniently adjusted by the size of tapes used in doctorblading.
3. Results and discussion Fig. S1 depicts fabricating procedures of the HPGES. Briefly, a lightweight and flexible Au-PTFE was used as the common electrode. For the supercapacitor moiety, PEDOT was deposited electrochemically on the Au-PTFE and a symmetrical supercapacitor was then assembled by pairing it with a second PEDOT/Au-PTFE electrode and a filter paper (as the separator). For the power generation unit, GO film (GOF) was
Fig. 1. Scheme of the HPGES including an electricity harvester for energy conversion and a supercapacitor for energy storage. (a) Device structure of the HPGES. (b) Top-section and (c) cross-section SEM images of GOF. (d) Top-section and (e) cross-section SEM images of PEDOT. 2
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Fig. 2. The power output of single electricity harvester and single supercapacitor. (a) Schematic diagram of the single electricity harvester. (b and c) The V oc and I sc induced by the interaction between GOF and moisture. (d) The voltage output of the electricity harvester in response to different RH. (e) The CV plot of the supercapacitor under various scan rates of 10–100 mV s−1. (f) The GCD plot at broad current density of 25–200 mA cm−3.
electrodeposited on the Au-PTFE as the electrode material of the supercapacitor owing to its merits of easy preparation, high conductivity and satisfactory stability. The electrodeposition of PEDOT was carried out for a period of 50–600 s (Fig. S4) and the electrochemical capacitance of the obtained supercapacitor was found to improve with the increasing deposition time (Fig. S5). It also should be noted that the capacitance of the supercapacitor unit was inclined to follow a much slower growth under long deposition time. For instance, when the deposition time was doubled from 300 s to 600 s, the capacitance of the supercapacitor was only increased by ~ 50% (7.2F vs.10.6 F cm−3). Therefore, a deposition time of 300 s was selected in current study as the optimal condition owing to the relatively high obtained capacitance and short preparation period. As shown in Fig. 2e, cyclic voltammetry (CV) curves of the supercapacitor unit based on PEDOT electrodes deposited for 300 s showed a rectangular shape between 0 and 0.8 V under different scan rates of up to 100 mV s−1 and its galvanostatic charge/discharge (GCD) curves retained a symmetric triangle shape in the current density range of 25 to 200 mA cm−3, showing characteristic behaviors of double layer capacitors. In addition, the Nyquist plot of its electrochemical impedance spectrum also featured a vertical line in the mid-frequency region (Fig. S6), further suggesting excellent capacitive behaviors of the supercapacitor unit. To rule out the influence of moisture on electrochemical capacitance, GCD curves of the supercapacitor were measured both under the dry and humidity conditions. As shown in Fig. S7, GCD curves basically remain unchanged under different humidity, negating any significant influence of moisture on the electrochemical property of the supercapacitor. The HPGES could simultaneously convert and store ambient energy. Under a RH of ca. 90%, the HPGES based on PEDOT electrodes electrodeposited for 300 s could generate a Voc of 0.2 V (Fig. 3a) and after charging for 1337 s, it delivered a charge of 870 mC cm−3 (an energy density of 23.9 µWh cm−3) under a discharge current density of 5 mA cm−3 when the moisture was removed. We further studied the performance of HPGES assembled with PEDOT electrodes prepared by
doctor-bladed on the other part of Au-PTFE and covered by a perforated gold plate (GP) to form the electricity harvester. Fig. 1a shows the structure of the HPGES. The electricity harvester and the supercapacitor were integrated on a common electrode of sprayed Au and their top electrodes were connected by a single-pole double-throw switch to control the charge/discharge process (Fig. S2). Fig. 1b and Fig. 1c reveal that the GOF featured a smooth surface and layered structure with a thickness of ca. 40 µm. Fig. 1d and 1e confirm the granular structure of the 20-µm-thick PEDOT layer in the supercapacitor moiety and elemental composition results from energy dispersive X-ray spectroscopy suggested the formation of the GOF and PEDOT (Table S1). The electricity harvester operated on interactions between GOF and moisture (Fig. 2a). When a single electricity harvester unit was exposed to the condition of 50% RH, an open-circuit voltage (Voc) of ca. 0.25 V could be observed (Fig. 2b). Due to the presence of holes on the top electrode, moisture entered the GOF vertically, interacting with oxygen-containing functional groups in the GOF and releasing H+. The released H+ moved deeper towards the bottom of the GOF along with the ingress of moisture, with the negatively charged functional groups (e.g., COO−) firmly attached on the carbon backbone. As a result, a potential was induced by the difference of H+ concentration. When the moisture supply was over, the free moving H+ recombined with the Ofunctionalized groups and the voltage accordingly returned to zero. A similar trend was also found in I sc at the 50% RH, suggesting the generation of current was accompanied with voltage induction by moisture absorption and desorption (Fig. 2c). As shown in Fig. 2d, the Voc of the electricity harvester was ~0.1, 0.25, 0.5 to 0.7 V when the RH increased from ca. 20%, 50%, 70%, to 90% (Fig. S3). As the moisture was carried by the flow of N2 through deionized water, increasing the relative humidity to values higher than 90% led to apparent formation of water droplets on the GOF and thus destabilized the signal of the device. Therefore, a relative humidity of 90% was deemed as the optimal test condition for better performance and reproducibility. Meanwhile, to build the supercapacitor unit in the HPGES, PEDOT was 3
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Fig. 3. The excellent performance of HPGES achieving convert and store electricity simultaneously. (a) The charge-discharge curve of a single HPGES. (b) The cycling stability of a HPGES unit for 106 cycles. (c) The charge-discharge curve of two seriesconnected HPGESs. (d) The charging voltage of HPGES arrays in response to the number of HPGESs in series. (e) 10 HPGESs that were connected in series reached 2 V. (f) A photograph of the commercial calculator lighted by the HPGES array. (g) The chargedischarge curve of the HPGES with MnO2 electrode.
electrode material instead of PEDOT and the device delivered 3.75 C cm−3 charge and an energy density of 93.8 µWh cm−3 (Fig. 3g), implying the great potential in further optimizing the performance of the HPGES through future selection of appropriate electrode materials.
electrodeposition for 600 s. As shown in Fig. S8, the device did deliver a larger amount of charge (2081 mC cm−3) under the same discharge current density, but it required a significantly longer time to charge (3805 s, ~3 times as long), further supporting the selection of 300 s as the representing deposition time. Additionally, a range of holding time (the dwelling time before discharging) was used to study its effect on the performance of HPGES (Fig. S9). Due to the self-discharge behavior of supercapacitors, the Voc of the device slowly decayed over the extended holding time, but it remained higher than 0.1 V even after a holding time of 12 h in dry air. As a result, the device delivered a charge of 555 mC cm−3 and 444 mC cm−3 after a holding time of 5 h and 12 h, representing 63.8% and 51% retention of initial electrical charge, respectively. In sharp contrast, when the supercapacitor unit was absent, the voltage of the single moist-electricity harvester dropped to zero within a second upon the removal of moisture (Fig. S10), only supplying 0.2 mC cm−3 charge and a far inferior energy density of 0.02 µWh cm−3. As shown in Fig. 3b, cyclic stability tests showed that the HPGES could survive over 100 continuous charging–discharging cycles. Impressively, the charging time and discharging time essentially remained unchanged in Cycles 1 to 4 and Cycles 101 to 104 (Fig. S11), highlighting the excellent stability of the HPGES. More importantly, the HPGES could be conveniently connected in series to supply a higher power output (Fig. 3c). Two series-connected HPGESs could be charged to 0.4 V with a similar charging/discharging profile to the single HPGES and the operating voltage of the HPGES array was proportional to the number of HPGESs in series (Fig. 3d and Fig. S12). A maximum Voc of 2 V was obtained with 10 HPGESs connected in series (Fig. 3e and Fig. S13) and such an array could sufficiently power a commercial calculator for complicated calculations after moisture-charging (Fig. 3f and Movie S1), while the bare electricity harvester only lighted up the same calculator for few seconds (Movie S2). Finally, to verify the universality of our design, MnO2 film was electrodeposited on the Au-PTFE as the
4. Conclusion In summary, for the first time, we have designed a HPGES that combines a moist-electric energy harvester with a PEDOT-based supercapacitor. The HPGES can harvest and store electricity in a humid environment and discharge under any conditions. For a single HPGES, it delivered an output voltage of 0.2 V and a charge of 870 mC cm−3 during discharge. Additionally, an array of 10 HPGES devices delivered a voltage of 2 V and adequately powered a commercial calculator for complicated calculations, rendering far more stable power supply than the bare electricity harvester. The current device opens up a new route to harness the ambient energy for powering portable and microscale electronic devices.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement We acknowledge the financial support from National Natural Science Foundation of China (NSFC) with grant number of Nos. 21774015 and 21975027. 4
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Appendix A. Supplementary data
1705925. [5] I. Must, U. Johanson, F. Kaasik, I. Poldsalu, A. Punning, A. Aabloo, Charging a supercapacitor-like laminate with ambient moisture: from a humidity sensor to an energy harvester, PCCP 15 (2013) 9605–9614. [6] Z.L. Luo, C.H. Liu, S.S. Fan, A moisture induced self-charging device for energy harvesting and storage, Nano Energy 60 (2019) 371–376. [7] C.X. Shao, J. Gao, T. Xu, B.X. Ji, Y.K. Xiao, C. Gao, et al., Wearable fiber form hygroelectric generator, Nano Energy 53 (2018) 698–705. [8] M.M. Ma, L. Guo, D.G. Anderson, R. Langer, Bio-inspired polymer composite actuator and generator driven by water gradients, Science 339 (2013) 186–189. [9] F.X. Gao, W.W. Li, X.Q. Wang, X.D. Fang, M.M. Ma, A self-sustaining pyroelectric nanogenerator driven by water vapor, Nano Energy 22 (2016) 19–26. [10] A. Muzaffar, M.B. Ahamed, K. Deshmukh, J. Thirumalai, A review on recent advances in hybrid supercapacitors: design, fabrication and applications, Renew. Sustain. Energy Rev. 101 (2019) 123–145. [11] V. Naresh, L. Elias, S.K. Martha, Poly (3, 4-ethylenedioxythiophene) coated lead negative plates for hybrid energy storage systems, Electrochim. Acta 301 (2019) 183–191. [12] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z.Z. Sun, A. Slesarev, et al., Improved Synthesis of Graphene Oxide, ACS Nano 4 (2010) 4806–4814.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123366. References [1] J. Yin, X.M. Li, J. Yu, Z.H. Zhang, J.X. Zhou, W.L. Guo, Generating electricity by moving a droplet of ionic liquid along graphene, Nat. Nanotechnol. 9 (2014) 378–383. [2] F. Zhao, L.X. Wang, Y. Zhao, L.T. Qu, L.M. Dai, Graphene oxide nanoribbon assembly toward moisture-powered information storage, Adv. Mater. 29 (2017) 1604972. [3] Y. Liang, F. Zhao, Z.H. Cheng, Y.X. Deng, Y.K. Xiao, H.H. Cheng, P.P. Zhang, Y.X. Huang, H.B. Shao, L.T. Qu, Electric power generation via asymmetric moisturizing of graphene oxide for flexible, printable and portable electronics, Energy Environ. Sci. 11 (2018) 1730–1735. [4] D.Z. Shen, M. Xiao, G.S. Zou, L. Liu, W.W. Duley, Y.N. Zhou, Self-powered wearable electronics based on moisture enabled electricity generation, Adv. Mater. 30 (2018)
5
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Short communication
A hygroelectric power generator with energy self-storage Yuyang Hana, Bing Lua, Changxiang Shaoa, Tong Xua, Qianwen Liua, Yuan Liangb, Xuting Jina, ⁎ Jian Gaoa, Zhipan Zhanga, a Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China b College of Materials Science and Engineering, Beijing University of Technology, Beijing 100084, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
hygroelectric power generator with • Aenergy self-storage is fabricated. generator is integrated with • Power energy storage unit by the Au common electrode.
array of devices can output a vol• An tage of 2 V to drive a calculator.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hygroelectric power generator Supercapacitor Energy self-storage Moisture Graphene oxide
Devices that convert ambient energy into electricity and simultaneously store it for future usage are highly desirable. Herein, we have designed a hygroelectric power generator with energy self-storage ability (HPGES) by hybridizing a moist-electric energy harvester with a supercapacitor, thus achieving the conversion of chemical energy into electricity and the storage of electric energy at the same time. An array of ten HPGES devices are able to deliver a voltage output of 2 V and adequately drive a commercial calculator.
1. Introduction The increasing energy demand and ongoing environmental issues have urged mankind to find alternative energy sources other than fossil fuels for sustainable development. To date, new technologies based on solar, wind, tidal and geothermal energies have greatly enriched the energy portfolio. Recently, harvesting electricity from different types of water motions has emerged as a new option to utilize the ambient energy [1–9]. For example, a droplet of ionic solution moving on monolayer graphene could produce a voltage of few millivolts [1]. Our group found that graphene oxide (GO) assembly with an oxygen gradient
⁎
could induce a gradient of protons under moisture ingress and the migration of protons could induce a voltage output of ~40 mV [2]. Moreover, electrical generation could also be achieved by moisture directional stimulation to pristine GO [3]. In addition to graphenebased materials, Zhou el al. found the TiO2 nanowire networks could induce an open-circuit voltage of up to 0.5 V when exposed to the diffusive flow of moisture [4]. Although these hygroelectric power generators have been substantially improved during the past few years, there are still some difficulties impeding its widespread application. For example, almost hygroelectric generators suffer from a limited and pulse energy outputs (in the range of nA–μA), causing the
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.cej.2019.123366 Received 6 August 2019; Received in revised form 11 October 2019; Accepted 2 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yuyang Han, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123366
Chemical Engineering Journal xxx (xxxx) xxxx
Y. Han, et al.
2.3. Preparation of PEDOT:
uncontrollable and intermittent energy output. Moreover, the hygroelectric generators are failing to produce electricity continually keeping in a constant environment for a long time, which due to the helplessness of storing the electricity. The application scenarios are limited due to the occurrence of electricity generation under the condition of humidity change. Hence, we need to develop a new generator that can collect and store energy at the same time, with the features of controllable voltage and current to output. With advantages of fast charge–discharge rate and long cycling life, supercapacitors have the promising development in the field of energy storage [10,11]. In view of the supercapacitor has the ability of charge storage, it can be integrated with power generation device to realize energy conversion and storage in the meanwhile. In this study, we have designed a hygroelectric power generator with energy self-storage ability (HPGES) by integrating a moist-electric energy harvester with poly (3,4-ethylenedioxythiophene)-based supercapacitors on a common electrode, which can deliver the power output in a stable and controllable way. Compared to the pristine hygroelectric generator, the HPGES can even work under dry conditions, which avoid the waste of energy in a great extent. A HPGES delivers an energy density of 23.9 μWh cm−3 and electrical charge of 870 mC cm−3 when discharged at a current density of 5 mA cm−3. In addition, the output power can be easily enhanced by scaling up the HPGES and an array of 10 HPGES devices achieve an output voltage of 2 V to adequately power a commercial calculator.
Poly (3,4-ethylenedioxythiophene) (PEDOT) was electrodeposited on the side of Au-PTFE in a three-electrode system. The precursor solution contained 1.08 wt% LiClO4 and 1.42 wt% EDOT in deionized water. The electrodeposition was carried out at 1 V for 50 s, 100 s, 200 s, 300 s and 600 s, respectively, before the deposited PEDOT was vacuum-dried at 80 °C for 2 h. The area of the film was 0.1 cm2. 2.4. Preparation of MnO2 The MnO2 film was also electrodeposited on the Au-PTFE. The precursor solution contained 0.1 M C6H13MnO8 and 0.5 M Na2SO4 in deionized water. The deposition was carried out at 0.8 V for 300 s and the MnO2 film was dried in the air for 2 h. 2.5. Characterization of the material The morphology and thickness of all samples were measured by scanning electron microscope (SEM, Zeiss SUPRA TM 55 SAPPHIRE, Germany). The relative humidity in the experiment was controlled by regulating the amount of moisture in the closed test chamber. To generate a humid environment, the moisture was brought in with the flow of nitrogen (N2) through deionized water and the relative humidity in the test chamber could be monitored online by a Humidity & Temperature Meter (AR847+, SMART SENSOR®) beside the HPGES. Alternatively, pure N2 was used to purge the moisture from the test chamber for a low humidity environment. Electric signals of the hygroelectric generator were acquired on a Keithley 2400 multimeter. All electrochemical measurement were performed on a CHI-760E workstation. The volumetric capacitance (in F cm−3), C, was calculated by C = I × t/ΔV, where I, t and ΔV were the discharge current density, discharge time and the potential range in the galvanostatic charge/ discharge curves. Meanwhile, the stored electrical charge (in C cm−3) was estimated by Q = I × t and the energy density (in Wh cm−3) was calculated by E = 0.5 × C × (ΔV)2.
2. Experimental methods 2.1. Materials 3,4-Ethylenedioxythiophene, LiClO4, C6H13MnO8 and Na2SO4 were purchased from Aladdin Industries. All chemicals were of analytical grade and used as received.
2.2. Preparation of graphene oxide film (GOF) Graphene oxide (GO) was prepared by a modified Hummers’ method [12]. The GOF was prepared by doctor-blading a GO dispersion (10 mg mL−1, 5 mL) on the poly(tetrafluoro ethylene) (PTFE) plate sprayed with gold (denoted as Au-PTFE). A thin film of 1 cm2 size was formed after drying at 50 °C for 5 h. The area and thickness of GOF could be conveniently adjusted by the size of tapes used in doctorblading.
3. Results and discussion Fig. S1 depicts fabricating procedures of the HPGES. Briefly, a lightweight and flexible Au-PTFE was used as the common electrode. For the supercapacitor moiety, PEDOT was deposited electrochemically on the Au-PTFE and a symmetrical supercapacitor was then assembled by pairing it with a second PEDOT/Au-PTFE electrode and a filter paper (as the separator). For the power generation unit, GO film (GOF) was
Fig. 1. Scheme of the HPGES including an electricity harvester for energy conversion and a supercapacitor for energy storage. (a) Device structure of the HPGES. (b) Top-section and (c) cross-section SEM images of GOF. (d) Top-section and (e) cross-section SEM images of PEDOT. 2
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Fig. 2. The power output of single electricity harvester and single supercapacitor. (a) Schematic diagram of the single electricity harvester. (b and c) The V oc and I sc induced by the interaction between GOF and moisture. (d) The voltage output of the electricity harvester in response to different RH. (e) The CV plot of the supercapacitor under various scan rates of 10–100 mV s−1. (f) The GCD plot at broad current density of 25–200 mA cm−3.
electrodeposited on the Au-PTFE as the electrode material of the supercapacitor owing to its merits of easy preparation, high conductivity and satisfactory stability. The electrodeposition of PEDOT was carried out for a period of 50–600 s (Fig. S4) and the electrochemical capacitance of the obtained supercapacitor was found to improve with the increasing deposition time (Fig. S5). It also should be noted that the capacitance of the supercapacitor unit was inclined to follow a much slower growth under long deposition time. For instance, when the deposition time was doubled from 300 s to 600 s, the capacitance of the supercapacitor was only increased by ~ 50% (7.2F vs.10.6 F cm−3). Therefore, a deposition time of 300 s was selected in current study as the optimal condition owing to the relatively high obtained capacitance and short preparation period. As shown in Fig. 2e, cyclic voltammetry (CV) curves of the supercapacitor unit based on PEDOT electrodes deposited for 300 s showed a rectangular shape between 0 and 0.8 V under different scan rates of up to 100 mV s−1 and its galvanostatic charge/discharge (GCD) curves retained a symmetric triangle shape in the current density range of 25 to 200 mA cm−3, showing characteristic behaviors of double layer capacitors. In addition, the Nyquist plot of its electrochemical impedance spectrum also featured a vertical line in the mid-frequency region (Fig. S6), further suggesting excellent capacitive behaviors of the supercapacitor unit. To rule out the influence of moisture on electrochemical capacitance, GCD curves of the supercapacitor were measured both under the dry and humidity conditions. As shown in Fig. S7, GCD curves basically remain unchanged under different humidity, negating any significant influence of moisture on the electrochemical property of the supercapacitor. The HPGES could simultaneously convert and store ambient energy. Under a RH of ca. 90%, the HPGES based on PEDOT electrodes electrodeposited for 300 s could generate a Voc of 0.2 V (Fig. 3a) and after charging for 1337 s, it delivered a charge of 870 mC cm−3 (an energy density of 23.9 µWh cm−3) under a discharge current density of 5 mA cm−3 when the moisture was removed. We further studied the performance of HPGES assembled with PEDOT electrodes prepared by
doctor-bladed on the other part of Au-PTFE and covered by a perforated gold plate (GP) to form the electricity harvester. Fig. 1a shows the structure of the HPGES. The electricity harvester and the supercapacitor were integrated on a common electrode of sprayed Au and their top electrodes were connected by a single-pole double-throw switch to control the charge/discharge process (Fig. S2). Fig. 1b and Fig. 1c reveal that the GOF featured a smooth surface and layered structure with a thickness of ca. 40 µm. Fig. 1d and 1e confirm the granular structure of the 20-µm-thick PEDOT layer in the supercapacitor moiety and elemental composition results from energy dispersive X-ray spectroscopy suggested the formation of the GOF and PEDOT (Table S1). The electricity harvester operated on interactions between GOF and moisture (Fig. 2a). When a single electricity harvester unit was exposed to the condition of 50% RH, an open-circuit voltage (Voc) of ca. 0.25 V could be observed (Fig. 2b). Due to the presence of holes on the top electrode, moisture entered the GOF vertically, interacting with oxygen-containing functional groups in the GOF and releasing H+. The released H+ moved deeper towards the bottom of the GOF along with the ingress of moisture, with the negatively charged functional groups (e.g., COO−) firmly attached on the carbon backbone. As a result, a potential was induced by the difference of H+ concentration. When the moisture supply was over, the free moving H+ recombined with the Ofunctionalized groups and the voltage accordingly returned to zero. A similar trend was also found in I sc at the 50% RH, suggesting the generation of current was accompanied with voltage induction by moisture absorption and desorption (Fig. 2c). As shown in Fig. 2d, the Voc of the electricity harvester was ~0.1, 0.25, 0.5 to 0.7 V when the RH increased from ca. 20%, 50%, 70%, to 90% (Fig. S3). As the moisture was carried by the flow of N2 through deionized water, increasing the relative humidity to values higher than 90% led to apparent formation of water droplets on the GOF and thus destabilized the signal of the device. Therefore, a relative humidity of 90% was deemed as the optimal test condition for better performance and reproducibility. Meanwhile, to build the supercapacitor unit in the HPGES, PEDOT was 3
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Fig. 3. The excellent performance of HPGES achieving convert and store electricity simultaneously. (a) The charge-discharge curve of a single HPGES. (b) The cycling stability of a HPGES unit for 106 cycles. (c) The charge-discharge curve of two seriesconnected HPGESs. (d) The charging voltage of HPGES arrays in response to the number of HPGESs in series. (e) 10 HPGESs that were connected in series reached 2 V. (f) A photograph of the commercial calculator lighted by the HPGES array. (g) The chargedischarge curve of the HPGES with MnO2 electrode.
electrode material instead of PEDOT and the device delivered 3.75 C cm−3 charge and an energy density of 93.8 µWh cm−3 (Fig. 3g), implying the great potential in further optimizing the performance of the HPGES through future selection of appropriate electrode materials.
electrodeposition for 600 s. As shown in Fig. S8, the device did deliver a larger amount of charge (2081 mC cm−3) under the same discharge current density, but it required a significantly longer time to charge (3805 s, ~3 times as long), further supporting the selection of 300 s as the representing deposition time. Additionally, a range of holding time (the dwelling time before discharging) was used to study its effect on the performance of HPGES (Fig. S9). Due to the self-discharge behavior of supercapacitors, the Voc of the device slowly decayed over the extended holding time, but it remained higher than 0.1 V even after a holding time of 12 h in dry air. As a result, the device delivered a charge of 555 mC cm−3 and 444 mC cm−3 after a holding time of 5 h and 12 h, representing 63.8% and 51% retention of initial electrical charge, respectively. In sharp contrast, when the supercapacitor unit was absent, the voltage of the single moist-electricity harvester dropped to zero within a second upon the removal of moisture (Fig. S10), only supplying 0.2 mC cm−3 charge and a far inferior energy density of 0.02 µWh cm−3. As shown in Fig. 3b, cyclic stability tests showed that the HPGES could survive over 100 continuous charging–discharging cycles. Impressively, the charging time and discharging time essentially remained unchanged in Cycles 1 to 4 and Cycles 101 to 104 (Fig. S11), highlighting the excellent stability of the HPGES. More importantly, the HPGES could be conveniently connected in series to supply a higher power output (Fig. 3c). Two series-connected HPGESs could be charged to 0.4 V with a similar charging/discharging profile to the single HPGES and the operating voltage of the HPGES array was proportional to the number of HPGESs in series (Fig. 3d and Fig. S12). A maximum Voc of 2 V was obtained with 10 HPGESs connected in series (Fig. 3e and Fig. S13) and such an array could sufficiently power a commercial calculator for complicated calculations after moisture-charging (Fig. 3f and Movie S1), while the bare electricity harvester only lighted up the same calculator for few seconds (Movie S2). Finally, to verify the universality of our design, MnO2 film was electrodeposited on the Au-PTFE as the
4. Conclusion In summary, for the first time, we have designed a HPGES that combines a moist-electric energy harvester with a PEDOT-based supercapacitor. The HPGES can harvest and store electricity in a humid environment and discharge under any conditions. For a single HPGES, it delivered an output voltage of 0.2 V and a charge of 870 mC cm−3 during discharge. Additionally, an array of 10 HPGES devices delivered a voltage of 2 V and adequately powered a commercial calculator for complicated calculations, rendering far more stable power supply than the bare electricity harvester. The current device opens up a new route to harness the ambient energy for powering portable and microscale electronic devices.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement We acknowledge the financial support from National Natural Science Foundation of China (NSFC) with grant number of Nos. 21774015 and 21975027. 4
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Appendix A. Supplementary data
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