A Flexible Microsupercapacitor with Integral Photocatalytic Fuel Cell

Jun 21, 2019 - is the energy stored during the discharging process,. 퐼. is the constant. current density,. 푆. is the area of the closed discharged...
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A Flexible Microsupercapacitor with Integral Photocatalytic Fuel Cell for Self-Charging Meijia Qiu,†,‡,§ Peng Sun,†,§ Guofeng Cui,*,† Yexiang Tong,*,† and Wenjie Mai*,‡

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MOE Laboratory of Bioinorganic and Synthetic Chemistry, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, The Key Lab of Low-Carbon Chemistry and Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P.R. China ‡ Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou 510632, P.R. China S Supporting Information *

ABSTRACT: With the rapid advancement in different kinds of portable electronics, self-powered systems with small volume and high-performance characteristics have attracted great attention in recent years. It would be rather exciting if one integrated system can not only convert recyclable energy or waste to electricity but also store energy at the same time. Here, flexible all-in-one energy chips composed of ureabased photocatalytic fuel cells (PFCs) and asymmetric microsupercapacitors (AMSCs) are designed on the same plane for powering small portable electronics. The planar PFC consisting of TiO2 photoanode and Ag counter electrode, utilizing urea as fuel, can produce a stable energy output (highest power density of 3.04 μW cm−2 in 1 M urea solution under a UV intensity of 30 mW cm−2) while purify this wasted water simultaneously. Besides, the AMSC comprised of NiCoP@NiOOH positive electrode and zeolite imidazolide framework derived carbon (ZIF-C) negative electrode achieves a high areal capacitance of 54.7 mF cm−2 at 0.5 mA cm−2 and an excellent energy density of 13.9 μWh cm−2 at the power density of 270.5 μW cm−2. Its stability can be confirmed by 86% capacitance retention after 8000 electrochemical cycles and almost no decay after 500 bending cycles. Four PFCs and two AMSCs can be easily constructed into an energy chip and power small electronics. This eco-friendly and self-sustainable system has great potential in future portable electronics. KEYWORDS: all-in-one, energy chip, microsupercapacitors, eco-friendly, photocatalytic fuel cells ecent developments in portable and flexible electronics have pushed our life closer to a new era filled with wearable products.1−3 For example, FlexEnable successfully produced flexible fingerprint sensors for future smartcards or mobile phones. However, the matching power sources of these flexible electronics now are mostly rigid and hard to be deformed, greatly hindering the way of all-flexible electronics. Thus, developing flexible energy supplied devices gradually occupies the research focus.4 Energy storage facilities with flexibility mainly include flexible batteries and supercapacitors (SCs), both of which own low energy density due to their small mass loading and thus need to be recharged frequently.5 One effective strategy is to integrate energy harvesting and energy storing devices into one system or even on one plane.6,7 These integrated systems, also called selfpowered systems, can sustainably supply energy for all kinds of flexible electronics because of the uninterrupted energy collection and store mode.8−11

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© XXXX American Chemical Society

Several types of energy conversion devices including nanogenerators (from mechanical to electric energy),12,13 solar cells (from light to electric energy),14,15 thermoelectric devices (from heat to electrical energy),16 biofuel cells (from biotic to electric energy),17,18 and others (from water vapor absorption or salinity to electrical energy)19,20 have been extensively studied for solving the urgent problem of energy crisis. Although all of the above solutions are beneficial to saving the nonrenewable energy resource to some extent, they are unable to solve the problem of environmental pollution at the same time. Photocatalytic fuel cells (PFCs), combining photocatalysis and fuel cell technologies, have become a promising choice for treating wastewater or biowaste while producing electricity simultaneously.21−23 Under photoirradiaReceived: May 9, 2019 Accepted: June 21, 2019 Published: June 21, 2019 A

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RESULTS AND DISCUSSION The detailed design process of the AMSCs is shown in Scheme 1. Interdigital ITO conducting layer was first obtained through

tion, PFCs can decompose many organic species through the photogenerated holes and thus consume the waste in polluted water or biowaste.24 Besides, compared with enzymatic and microbial fuel cells, PFCs own distinct advantages including stability, high current efficiency, fast electron transfer, low price, and universal adaptability to almost all kinds of organic waste.25,26 Up to now, few works have reported flexible PFCs in one plane, which is highly compatible with wearable energy storage devices and sensors. Another important unit for self-powered systems is the energy storage module. The most commonly used microbatteries for portable devices suffer the severe shortcomings of poor lifetime and low power density. Supercapacitors can make up for these drawbacks since they are capable of being charged and discharged at large currents in a short time (several minutes) and for over 100,000 times. Compared with traditional sandwiched structure SCs, micro-SCs (MSCs) with two interdigital electrodes on one chip have attracted great interests among the research field due to their tiny size, small thickness, excellent rate performance, and easy integration with other microdevices.27−33 The inner energy storage mechanism of MSCs usually can be classified into two kinds: electric double-layer capacitance (EDLC, through ions absorption/desorption)34−36 and pseudocapacitance (through valence change derived from redox reaction). MSCs with pseudocapacitive property can deliver a much higher capacity than that based on EDLC by sacrificing some charging/ discharging speed. In order to further elevate the energy density (E = 1/2CV2, where E is the energy density, C is the specific capacitance, and V is the voltage window) of MSCs, pseudocapacitive materials and asymmetric design can be introduced.37 Pseudocapacitive materials including metal oxides,38−41 metal hydroxides,42,43 metal phosphides,43,44 conducting polymers,45,46 and so on can largely enhance the stored capacitance via complex redox reactions and fast ion intercalation/deintercalation processes, while the asymmetric structure will broaden the voltage windows of MSCs through choosing two electrodes with a large work function difference or premodulating the surface condition between electrodes and electrolyte.47−49 Further, MSCs combined with energy harvesting devices can be fabricated on one plane to achieve a self-powered system with small volume.50,51 Herein, we proposed a paradigm of a flexible all-in-one energy chip incorporating eco-friendly PFCs and asymmetric NiCoP@NiOOH//zeolite imidazolide framework (ZIF-8) derived carbon MSCs. The whole system was achieved by screen printing and electrochemical deposition technologies, which can be easily produced at a large scale. Planar PFCs with TiO2 nanoparticles photoanode and Ag paste counter electrode printed on indium tin oxide (ITO)/polyethylene terephthalate (PET) realized a stable output current by consuming urea as fuel under an ultraviolet (UV) exposure. Asymmetric MSCs (AMSCs) composed of NiCoP@NiOOH and N-doped carbon derived from ZIF-8 can reach excellent performance including high specific capacitance, large energy density (better than most of previous MSCs), long cycle life, and wonderful bending stability. Through intermittently refilling the urea solution in them, several PFCs in series can charge the as-fabricated MSCs which can power small electronics constantly.

Scheme 1. Fabrication Process of the NiCoP@NiOOH// ZIF-C MSC

a printing ink protection and chemical etching method (Figure S1). After that, the NiCoP layer can be decorated on an ITO/ PET interdigital substrate for enhancing the conductivity, improving the adhesion between active material and the substrate while providing some electrochemical energy storage capacity at the same time. The positive and negative electrodes were achieved through electrodepositing NiOOH and screen printing ZIF-C on two sides of the NiCoP interdigital substrate. Finally, KOH gel electrolyte was utilized to coat two electrodes, and the planar MSC was finished. The morphology and composition analyses for the two electrodes of the MSC are presented in Figure 1. The electroless deposited NiCoP microspheres (around 1.5−2.5 μm) on ITO/PET substrate connect with each other and form a conducting network (Figure 1a). As demonstrated in Figure 1b, after the electrodeposition was applied for 10 min, some NiOOH nanoflakes can be found among the NiCoP and ITO layer, which can greatly enhance the storage performance. ZIFC coated on a negative electrode presents a hexagon structure with a size of 100−300 nm (Figure 1c). From the XRD patterns shown in Figure 1d, there exist several peaks at 2θ = 41.56°, 44.48°, 47.46° and 75.86°, corresponding to the (111), (201), (210), and (212) crystal planes of the NiCoP (JCPDS no. 71-2336), respectively. The electrodeposited NiOOH on NiCoP was further confirmed through the XPS spectra (Figure 1e,f). In the Ni 2p narrow spectrum, four peaks can be found, in which two main ones (873.5 and 855.8 eV) can be assigned to Ni 2p1/2 and Ni 2P3/2 of Ni3+ and the left two (879.6 and 861.6 eV) should belong to shakeup satellite peaks.52 As for the O 1s core spectrum, three peaks can be separated. The peak located at 532.3 eV is usually related to the absorbed O2 on the surface, while the other two ones sitting at 531.5 and 530.8 eV can be attributed to Ni−O−H and Ni−O−Ni.52,53 Also, the XRD pattern for ZIF-C shown in Figure 1g reveals that only one peak from amorphous carbon exists, proving the negative material is mainly composed of carbon. Besides, an optical picture and corresponding magnified optical image of the ITO@NiCoP interdigital substrate are presented in Figure 1h,i. It is obvious that the MSC substrate contains 4 pairs of interdigital “fingers”, each of which owns a width of around B

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Figure 1. Morphology and composition characterizations of the NiCoP@NiOOH positive electrode and ZIF-C negative electrode. SEM images of the (a) NiCoP conductive layer, (b) NiCoP@NiOOH positive electrode, and (c) ZIF-C negative electrode. (d) XRD pattern of the NiCoP layer. Core level of XPS spectra of the (e) Ni and (f) O for the NiOOH layer. (g) XRD pattern of the ZIF-C. (h) Optical picture of the interdigital NiCoP layer substrate and (i) its magnified optical image of the area surrounded by a red dashed box in (h).

Figure 2. Electrochemical performance of the NiCoP@NiOOH positive electrode and ZIF-C negative electrode. (a) CV curves of the NiCoP layer and NiCoP@NiOOH composite electrode. (b) CV curves and (c) GCD curves of the NiCoP@NiOOH positive electrode at different scan rates (10−100 mV s−1) and current densities (1−4 mA cm−2). (d) CV curves and (e) GCD curves the ZIF-C negative electrode at different scan rates (10−100 mV s−1) and current densities (0.5−3.0 mA cm−2) and its (f) Nyquist plot.

946 μm, a length of about 8.35 mm, and a separation width (interval between two adjacent “fingers”) of nearly 400 μm. To study the electrochemical performance of the NiCoP@ NiOOH positive electrode and ZIF-C negative electrode, cyclic voltammetry (CV), galvanostatic discharge−charge (GCD), and electrochemical impedance spectrum (EIS)

measurements were performed. As shown in the CV curves in Figure 2a, the initial capacitance of the NiCoP can reach a value of 3.7 mF cm−2, while the NiCoP@ NiOOH delivers a much higher capacitance of 90.3 mF cm−2 (23 times increase), demonstrating the great performance enhancement of the deposited NiOOH. The reason for the significant improveC

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Figure 3. Electrochemical and mechanical performance of the NiCoP@NiOOH//ZIF-C MSC. (a) Capacity match of the positive and negative electrodes. (b) CV curves of the asymmetric device with different voltage windows at a scan rate of 100 mV s−1. (c) CV curves, (e) GCD curves, and (f) Nyquist plot of the NiCoP@NiOOH//ZIF-C MSC at different scan rates (10−100 mV s−1) and current density (0.5− 3.0 mA cm−2). (f) Mechanical and (g) electrochemical stability of the NiCoP@NiOOH//ZIF-C MSC. Insets in (f) and (g) show the optical pictures of the flat and bent state of the as prepared MSC and several GCD curves (1−8 and 7992−8000) during the electrochemical cycles, respectively. (h) Ragone plot of the NiCoP@NiOOH//ZIF-C MSC compared with other previous MSCs.

different scan rates and GCD measurements under several current densities (Figure 2b,c). With the scan rate gradually declining, two redox peaks become more apparent, indicating the adequate valence change and some diffusion behaviors. The capacitance decay from 10 mV s−1 to 100 mV s−1 is only 27.0% (Figure S4a), demonstrating its great rate capacitive performance. All of the GCD curves in Figure 2c show symmetric shapes, further implying the excellent capacitance performance of the NiCoP@NiOOH. CV and GCD curves of negative electrode (ZIF-C) were also collected, as shown in Figure 2d,e. A nearly rectangular shape can be found in CV curves at scan rates ranging from 10 mV s−1 to 100 mV s−1 (Figure 2d), which implies the excellent capacitive property of the ZIF-C. Perfect rate performance (Figure S4b, 72.6% retention from 10 mV s−1 to 100 mV s−1) can also be achieved. Besides, GCD curves collected at current densities from 0.5 mA cm−2 to 3.0 mA cm−2 demonstrate excellent symmetric triangular shape, further proving its great capacitive performance. The equivalent series resistance (ESR) can be as low as 5.6 Ω, which is beneficial to the ion transfer between active materials and electrolyte (Figure 2f). In order to explore the availability of this asymmetric MSC in an actual application, two electrodes were coated with KOH/PVA gel electrolyte to fabricate a quasi-solid-state planar

ment in performance after the decoration of NiOOH was further analyzed through comparing contact angle and electrochemical surface area (ECSA) of the pure NiCoP on ITO and NiCoP@NiOOH on ITO. As shown in Figure S2, the contact angles of the ITO@NiCoP and ITO@NiCoP@ NiOOH were measured to be 66.22° and 44.40°, respectively, indicating that the decoration of NiOOH can improve the wettability of the electrode and thus elevate the contact probability of active materials and electrolyte. CV method was performed to obtain the electrochemical double-layer capacitances Cedl (proportional to the ECSA) of the two kinds of electrodes. As presented in Figure S3a, a nonfaradaic region can be chosen as −0.2 V to −0.1 V, and the CV curves at different scan rates can be conducted under this potential range (Figure S3b,c). Through linear fitting of the capacitive charging/discharging current and scan rates (Figure S3d), the Cedl of the ITO@NiCoP and ITO@NiCoP@NiOOH was calculated to be 0.18 and 1.09 mF cm−2, indicating the electrodeposition of the NiOOH can effectively increase the number of the active sites for electrochemical energy storage. Considering these two points, it is understandable that the ITO@NiCoP@NiOOH delivers much higher capacitance than the ITO@NiCoP. Further, the electrochemical energy storage process of the NiCoP@NiOOH was explored by CV tests at D

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Figure 4. (a) Mechanism of the energy generation from the PFC based on TiO2 photoanode and Ag counter electrode in urea solution under UV irradiation. (b) Photocurrent response, (c) polarization curves, and (d) corresponding power density curves of the planar PFC device under different UV light intensity ranging from 2 mW cm−2 to 30 mW cm−2 in 1 M urea solution. (e) Photocurrent response, (f) polarization curves, and (g) corresponding power density curves of the planar PFC device in urea solutions with different concentration ranging from 0.1 to 1 M under the UV light intensity of 12 mW cm−2. (h) Photocurrent response of the planar PFC device under different bending cycles. (i) Four times of cyclic experiments for dropping urea after the previous drop of urea has been exhausted.

retention can reach 61% from 0.5 mA cm−2 to 3.0 mA cm−2, indicating the great rate performance (Figure S5). EIS measurement of the NiCoP@NiOOH//ZIF-C was also performed, as shown in Figure 3e. A low ESR of 21.3 Ω can be found, manifesting the excellent ion-transfer ability between electrodes and electrolyte again (Figure 3e). Moreover, this planar and flexible MSC device possesses great mechanical flexibility and stability since it can be bent and recovered to flat states for 500 cycles without any capacitance retention, as exhibited in Figure 3f and Figure S6. After 8000 cycles of GCD test, it can retain 86% of the original capacitance (Figure 3g), indicating the electrochemical stability of the NiCoP@ NiOOH//ZIF-C AMSC is rather great. In addition, the maximum energy density of 13.9 μWh cm−2 at the power density of 270.5 μW cm−2 can be achieved, which shows better performance than most of the MSCs, as presented in Figure 3h.30,34,42,55,56,58,60−62 The energy harvest devices for powering the above AMSCs were realized by the PFCs. As presented in Figure S7, the fabricated process of the PFCs is very similar to those of AMSCs. After the ITO patterns have been finished, Ag paste and TiO2 slurry were decorated on surrounding circuits and the core circle through screen printing, respectively. The

device. The capacity match between NiCoP@NiOOH positive and ZIF-C negative electrodes is shown in Figure 3a, revealing approximate energy storage ability. Due to the asymmetric design, the MSC device can reach the highest voltage window of 1.4 V (Figure 3b). The device exhibits well-shaped CV curves at various scan rates from 10 mV s−1 to 100 mV s−1 and the calculated capacitance can reach 42.2 mF cm−2 at 10 mV s−1. GCD curves at different current densities with a nearly symmetric triangle again demonstrate the great capacitive performance. It can be calculated that under a current density of 0.5 mA cm−2, the capacitance of the MSC can achieve a high value of 54.7 mF cm−2, which is higher than most of the symmetric MSCs and AMSCs reported previously28,31,34,50,54,55 (Table S1: MnO2−PPy MSCs,50 7.3 mF cm−2 at 10 mV−1; MoS2@rGO−carbon nanotube (CNT) MSCs,56 13.7 mF cm−2 at 0.1 mA cm−2; Cu(OH)2@FeOOH MSCs,42 58.0 mF cm−2 at 1 mA cm−2; Cu-hexacyanoferrates (HCF)/graphene//Fe-HCF/graphene AMSCs,29 19.8 mF cm−2 at 0.75 mA cm−2; VN//MnO2 AMSCs,57 16.1 mF cm−2 at 1 mV s−1; Ppy@multiwalled CNT AMSCs,58 21.8 mF cm−2 at 0.1 mA cm−2) and comparable to the Bi2O3//MnO2 MSCs59 (97 mF cm−2 at 1.5 mA cm−2), carbide-derived carbons MSCs35 (100.0 mF cm−2 at 1 V s−1). The capacitance E

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decreases slightly as the cycle number increases. This is probably related to the fast hydrolysis of the urea molecular (NH2CONH2 + H2O → NH2CO2NH4 → 2NH3 + CO2) under continuous exposure of UV irradiation. To confirm the above conjecture, the temperature variation in the urea solution and surrounding environment was collected following the operation time (Figure S10). As the UV light was constantly exposed to the PFC with one drop of urea solution, the recorded temperature elevated gradually. After the first drop of the urea solution was exhausted, the remaining water was removed, and another drop of fresh urea solution was added. It is obvious that the initial temperature of the second cycle is much higher than the first cycle, and thus the average temperature in this cycle becomes higher, too. This factor would accelerate the hydrolysis process of the urea, and thus the amount of the urea molecules in this cycle for PFC conversion reduced. With the uninterrupted refilled cycle under UV irradiation, the average temperature becomes higher and higher and finally reaches a stable state. Hence, the output performance of the PFC degraded slightly in Figure 4i. Besides, it can be seen that the PFC needed 30−60 min to exhaust 20 μL of solution of 1 M urea and constantly outputted a rather stable current (Figure 4i). Thus, the behavior of purifying the wasted water of this PFC should be an additional function of a solar electricity conversion process. The main function of the device is the operation mode of a long-term and stable output of electricity by consuming wasted water and then charging for energy storage devices. Finally, an all-in-one energy chip composed of four PFCs in series and two tandem MSCs was constructed to confirm the feasibility of practical applications in portable electronics (detailed fabrication process can be found in Figure S11). As shown in Figure 5a, a demo experiment was designed, and the corresponding circuits were presented. First, without UV light

simple mechanism of the continuous energy generation from PFCs is shown in Figure 4a. The TiO2 photoanode can generate electron−hole pairs once it is under the exposure of the UV light. The holes moved to the surface of TiO2 and oxidized the absorbed urea molecule, as described in following equation: CO(NH2)2 + O2 + 2h+ → N2 + CO2 + H2O + 2H+; 2H+ + 1/2O2 + 2e− → H2O. The left electrons are transferred to the external circuit for providing sustainable outputs and thus can be used to charge the energy storage devices or power the electronics directly. As the planar PFCs were soaked in 1 M urea solution and under UV irradiation (365 nm) with different intensities, they can respond fast and stably (Figure 4b). Besides, their response current density ΔI reveals good linearity versus incident light intensity P (Figure S8a, ΔI = 0.801 P0.981). The PFC demonstrates a peak responsivity of 1.87 mA W−1 under the wavelength of 365 nm (as presented in Figure S9). Polarization curves of the PFCs under UV light with different intensities were collected for further studying the photocatalytic performance, as presented in Figure 4c. It can be clearly found that under a higher intensity of UV exposure, the open circuit voltage for the PFCs can obtain a higher value (0.719 V at 30 mW cm−2). Also, the short-circuit current densities own the same trend with the chronoamperometry results in Figure 4c. As presented in Figure 4d, power curves based on the polarization curves demonstrate that maximum output power densities of the PFCs under the UV intensity of 2, 4, 12, and 30 mW cm−2 achieve 0.45, 0.66, 1.35, and 3.04 μW cm−2, respectively. Next, the photoelectric properties of the PFCs in urea solution with different concentration were also studied. Under a UV intensity of 12 mW cm−2, the response photocurrent increases following the elevation of urea concentration ranging from 0.1 to 1 M (Figure 4e). However, the relationship between the response photocurrent and urea concentration is nonlinear (as shown in Figure S8b), implying that with the enhancement of the urea concentration, PFCs are hard to catalyze all of them in time. Besides, the polarization curves (Figure 4f) also exhibit excellent accordance to the photocurrent response in Figure 4e, and the corresponding power curves (Figure 4g) demonstrate considerable output power densities. Due to the proper design and fabricated process, the planar PFCs demonstrate perfect flexibility since they can suffer 200 bending cycles (around 90°, as shown in the inset of Figure 4h) and still retain an 87.5% responsive photocurrent in 1 M urea solution under the UV intensity of 12 mW cm−2. One exciting point is that the planar PFCs can be refilled with fresh fuel and continuous output energy. As presented in Figure 4i, the first one drop of 1 M urea solution (20 μL) was added on the top of both photoanode and counter electrode. The PFCs showed an instant current response after the UV light (8 mW cm−2) was on. Then the photocurrent gradually decreased from a current density of around 11 μA cm−2 to 4 μA cm−2 because the urea solution was constantly exhausted to a low concentration. After that, the response current density dropped suddenly to around 1.6 μA cm−2 and decreased slowly to the end. The reason for this special behavior is speculated that when the urea was consumed to some extent, there existed some area without urea molecules on the TiO2 photoanode, thus resulting in an abrupt decrease in response current. This refillable trait was repeatedly tested four times and exhibited excellent stability. It can be found that the output duration time of the PFC under continuous dropping of urea solution

Figure 5. (a) Planar scheme illustrating the self-powered system including in-plane PFCs and MSCs for powering red LED. (b) Photographs demonstrating four tandem planar PFCs charging two MSCs in series, which later power a red LED. (c) Light charge by planar PFCs and galvanostatic discharge performance and (d) three cycle tests of the NiCoP@NiOOH//ZIF-C MSCs. F

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into the NiCoP electroless deposition solution for 20 min under a temperature of 90 °C. After that, the as-prepared sample was washed by DI water and dried in an electric oven for over 1 h. Fabrication of the NiOOH on the ITO/PET@NiCoP Interdigital Substrate. NiOOH was prepared by a simple electrodeposition method. Briefly, 0.2 M Ni(NO)3 and 0.2 M hexamethylenetetramine were dissolved in DI water to form a clear solution. NiOOH was then electrodeposited at 40 °C for 10 min at a constant current of 1 mA cm−2 versus Ag/AgCl reference electrode with the carbon rod as the counter electrode. Finally, the obtained sample was washed with DI water and dried in an electric oven for over 1 h. Screen Printing of the ZIF-C on the ITO/PET@NiCoP Interdigital Substrate. ZIF-8 was first synthesized through a previous method.63 In a typical synthesis process, 24 mM of Zn(NO3)2·6H2O and 80 mM 2-methylimidazole were dissolved in methanol and mixed together under stirring for 5 min. Then the solution was aged in air for 1 day. The as-obtained white powder was gathered by centrifugation and then dried at 60 °C. The sample was then transferred to a tube oven and kept at 800 °C for 2 h with a heating rate of 5 °C min−1 in flowing N2 atmosphere, obtaining the ZIF-C. Next, 0.8 g of ethyl cellulose was added into a spin steaming bottle with 10 mL of alcohol and ultrasonically concussed to be uniformly dispersed. 4.1 g of terpilenol was further put into the above solution and dissolved. Finally, 0.9 g of evenly grinded ZIF-C and 0.1 g of carbon black were added and stirred for another 1 h. The slurry for screen printing was gathered through rotary evaporation of the mixture at a temperature of 70 °C with a rotational speed of 150 rpm for 2 h. The gathered slurry was used to screen print on the ITO/ PET@NiCoP interdigital substrate as the negative electrode. Fabrication of the NiCoP@NiOOH//ZIF-C MSC. The NiCoP@ NiOOH//ZIF-C MSC was fabricated by coating the KOH/poly(vinyl alcohol) (PVA) sol−gel electrolyte on both positive and negative electrodes. The KOH/PVA gel electrolyte was made by adding 6 g of PVA powder into 60 mL of 1 M KOH solution and stirred under 85 °C until the solution became brown and a little viscous. Fabrication of the Planar PFCs. TiO2 slurry was prepared by the same method as that of the ZIF-C slurry, except for replacing the 0.9 g of evenly grinded ZIF-C and 0.1 g of carbon black to 1 g TiO2 nanoparticles (P25, Degussa). The gathered slurry was used to screen print on the ITO/PET substrate as the photoanode. Ag slurry was also used to screen print as the counter electrode. Characterizations. The morphologies of all synthesized materials were gathered by using a field-emission scanning electron microscopy (SEM, Hitachi, S-4800, 10 keV) and optical microscopy (Bocheng, BC1800). The crystal structure and material compositions were determined by X-ray powder diffraction (XRD, Rigaku D/max-2200/ PC) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-ALPHA+). The contact angle of the electrodes was measured by a Drop Shape Analyzer (KRüSSGmbH, DSA100). The average mass loading of NiCoP (one side), NiOOH, ZIF-C, and the whole active layer of the AMSC were weighed to be 1.04 mg, 0.13 mg, 0.56 mg, and 2.77 mg, respectively (shown in Table S2). Electrochemical performances of electrodes were measured by a CHI 660E and a Gamry Reference 600 workstation in a standard three-electrode electrochemical cell, with as-prepared samples as working electrode (WE), a graphite rod as counter electrode (CE), and a Hg/HgO as reference electrode (RE). The electrochemical and photoelectrochemical performances of AMSCs and PFCs were conducted via connecting a positive electrode with WE and connecting negative electrode with both CE and RE. The simulated urine is composed of 1.41 g of NaCl, 0.28 g of KCl, 0.06 g of CaCl2, 0.043 g of MgSO4, 1.73 g of urea, and 0.19 mL of 25% NH3·H2O in 100 mL 0.02 M HCl solution. A UV detector (UVA365, SANPOMETER CORP., China) was used to quantify UV irradiance. LEDs (LUYOR-3130, 365 nm, 20 mW cm−2, USA; SMIYFT UV SS03, 365 nm, 3 W) were used as the light sources. UV irradiance intensity was adjusted via changing the aperture size and distance between the light source and the PFCs.

and with the urea addition, no current was produced by PFCs to charge the MSCs, and thus the light-emitting diode (LED) was not lit. After the UV light (100 mW cm−2) was turned on, MSCs were gradually charged by those PFCs and then reached the voltage for illumining the red LED. The UV illumination was turn off after MSCs were charged for 10 min. Encouragingly, they can still light the LED for several minutes, demonstrating that the MSCs successfully stored some energy and could power the small electronics if there were some emergency requirements. Optical pictures can also be found in Figure 5b, indicating the availability of the above design. PFCs charging and galvanostatic discharging curves collected by two tandem PFCs and one MSC integrated together were further studied, as exhibited in Figure 5c. Under a UV irradiation intensity of 100 mW cm−2, the MSC was charged to 1.2 V after 288 s and discharged for 703 s at a current density of 0.1 mA cm−2. This process can also be cycled for several times (Figure 5d), implying the great potential in continuously supplying energy for portable or flexible electronics.

CONCLUSION In this work, a flexible all-in-one energy chip composed of NiCoP@NiOOH//ZIF-C MSCs and TiO2 PFCs has been successfully fabricated. The fabrication methods for the energy chip consisting of screen printing and electrochemical deposition are easy to operate in an assembly line. The NiCoP@NiOOH//ZIF-C MSC exhibits extraordinary energy storage performance, with a rather high areal capacitance of 54.7 mF cm−2 at a current density of 0.5 mA cm−2 and large energy density of 13.9 μWh cm−2 at the power density of 270.5 μW cm−2, which outperforms most of the previous MSCs. It can be charged and discharged for 8000 times and retain 86% of the original capacitance. After 500 bending cycles, the performance of the MSC shows nearly no decay, indicating the great mechanical stability. The planar PFCs based on TiO2 nanoparticles and Ag slurry were constructed and demonstrated perfect performance using urea as fuel. The output power density can reach as high as 3.04 μW cm−2 in 1 M urea solution under a UV intensity of 30 mW cm−2. Besides, the PFCs are bendable and capable of continuously outputting energy by refilling fresh urea on them. Through integrating two MSCs with the four PFCs in one plane, it can be self-powered for small electronics such as the LED easily and constantly, implying a great potential in future self-powered portable electronics. METHODS Fabrication of the ITO/PET Interdigital Substrate. As shown in Figure S1, several interdigital patterns were first printed on thermal transfer paper by a converted commercial printer (Brother 2140). Next, the ITO/PET was soaked in an acetone solution containing 8 mM (3-mercaptopropyl) trimethoxysilane (MPTMS) for 5 min to enhance the adhesion between ITO and the commercial printing ink. Then, the thiol-modified ITO/PET was covered by the interdigital shape printing ink by heat transferring the pattern from thermal transfer paper through a thermal transfer printer (C1001S, bought from TuYi Technology) under 150 °C for 30 s. After that, the ITO/ PET was dipped into 1 M FeCl3 for 4 min to etch the unprotected ITO and then transferred to the acetone for ultrasonic concussion for 10 min to further remove the protected printing ink. After that the ITO/PET interdigital substrate can be obtained. Electroless Deposition of the NiCoP on ITO/PET Interdigital Substrate. The electroless deposition method is similar to our previous work.44 The ITO/PET interdigital substrate was immersed G

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ACS Nano

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03603. Calculation methods of the MSCs; fabrication processes of the ITO/PET ID substrate and the planar PFC; contact angles of the positive electrode of MSCs; CV curves and corresponding fitting of ITO@NiCoP and ITO@NiCoP@NiOOH; rate performance of the NiCoP@NiOOH, ZIF-C electrodes and NiCoP@ NiOOH//ZIF-C MSC; CV curves of the NiCoP@ NiOOH//ZIF-C MSC before and after 500 bending cycles; net photocurrent varied with different light intensities and different urea concentrations; typical photoresponsivity spectrum of the TiO2-based PFC; temperature record of the four times of cyclic experiment for dropping urea after the previous drop of urea has been exhausted; fabricated process for the all-in-one energy chip; scheme of the calculated area of MSCs and PFCs; comparison and explanation of this work and previous work; contact angles of the ITO@NiCoP and Ni@NiCoP stretchable textile; CV curves and corresponding fitting of ITO@NiCoP and Ni@NiCoP stretchable textile; capacitance comparison table between previous work and this work; mass loading list of NiCoP (one side), NiOOH, and ZIF-C (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guofeng Cui: 0000-0001-7152-8185 Yexiang Tong: 0000-0003-4344-443X Wenjie Mai: 0000-0003-4363-2799 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.X.T. gratefully acknowledges the financial support by the National Key Research and Development Program of China (2016YFA0202604), the National Natural Science Foundation of China (21461162003 and 21773315), and the Science and Tech nolog y Plan Project of Guangzh ou, China (201804020025). W.J.M. gratefully acknowledges the financial support by National Natural Science Foundation of China (51772135), Ministry of Education of China (6141A02022516), and Natural Science Foundation of Guangdong Province (2014A030306010). REFERENCES (1) Gates, B. D. Flexible Electronics. Science 2009, 323, 1566−1567. (2) Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; et al. Epidermal Electronics. Science 2011, 333, 838−843. H

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