One Step Deposition of PEDOTâPSS on ALD Protected Silicon

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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One Step Deposition of PEDOT−PSS on ALD Protected Silicon Nanowires: Toward Ultrarobust Aqueous Microsupercapacitors Anthony Valero,†,‡ Adrien Mery,† Dorian Gaboriau,†,‡ Pascal Gentile,‡ and Saïd Sadki*,† †

Univ Grenoble Alpes, CEA, CNRS, INAC-SyMMES, 38000 Grenoble, France Univ Grenoble Alpes, CEA, INAC-Pheliqs, 38000 Grenoble, France

‡

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 84.54.57.23 on 01/20/19. For personal use only.

S Supporting Information *

ABSTRACT: Herein, we propose a fast and simple deposition method of a highly robust pseudocapacitive material based on a straightforward drop-cast of a commercial PEDOT:PSS solution onto 3 nm alumina-coated silicon nanowires. The composite material produced (PPSS-A@ SiNWs) displays, remarkable capacitive behavior with a specific capacitance of 3.4 mF·cm−2 at a current density of 2 A·g−1 in aqueous Na2SO4 electrolyte. Moreover microsupercapacitor (MSC) devices based on this material exhibits outstanding lifetime capacity retaining 95% of its initial capacitance after more than 500 000 cycles at a current density of 0.5 A·g−1, a specification which exceeds by far most of the stability of conducting polymers previously reported in the literature. In term of pure energy storage performances, the system is able to reach excellent specific energy and power values of 8.2 mJ·cm−2 and of 4.1 mW·cm−2, respectively, at a high current density of 2 A·g−1. Results are systematically compared to both the state-of-the-art silicon based aqueous on-chip supercapacitors and to that of the pristine alumina-coated silicon nanowires (A@SiNWs) to highlight the contribution of the conductive PEDOT:PSS polymer (PPSS in this study). Hence, the aforementioned one-step deposition represents a simple, cheap and scalable method to thoroughly increase the cycling stability of a well-known conductive polymer, PEDOT−PSS, while drastically increasing the electrochemical performances of an existing technology, the Si NW-based MSCs using aqueous electrolytes. KEYWORDS: microsupercapacitors, silicon nanowires, conductive polymer, ultrarobust, aqueous electrolyte, nanocomposite

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INTRODUCTION In recent years, the supercapacitor technology has brought a great deal of interest because of its promising storage performances, low cost, and high reliability. Nowadays the demand has shifted toward low power on board electrochemical energy storage devices with the rise of new technologies, such as autonomous sensor network, wearable gadgets (Internet of things, biomedical implants, ...).1 For such applications, energy storage units are required to exhibit both high energy and high power densities, while being easy to miniaturize and low cost to produce with low impact on the environment. Among the storage devices, microsupercapacitors (supercapacitors with a footprint in the range of the micrometer scale as defined by Beidaghi and Gogotsi2) are very promising solutions as they combine features from both conventional batteries and capacitors in term of energy and power densities, respectively. Major advantages of supercapacitors are their high power density and their fast reversible charge/discharge during cycling. However, even state-of-theart MSCs (based on graphene, onionlike carbon, carbon nanotubes, carbide-derived carbon) still exhibit insufficient energy density performances compared to what can be achieved by current lithium ion microbatteries.3 Indeed at © XXXX American Chemical Society

the moment, a very wide majority of small scale integrated devices are powered by systems using battery electrode materials, such as thin film batteries and microbatteries. Yet, these technologies are facing two main issues hindering their widespread applications, namely they share the limited lifetime and low power performances of their large counterpart. Moreover, power limitations also hinder microbatteries to be implemented for applications, where high spikes of current are required for the system to fulfill its task. In case of large scale electronic devices, this problem is avoided by oversizing the batteries, a solution that is not viable for integrated microsystems. Although being still at their early stage of development, microsupercapacitors are rising as a viable solution for on ship power sources and many studies have been carried out over the recent years to increase their performances.4 Similar to the most used and known materials for their macro relative electrodes, carbon and its derivatives have been widely investigated. Over the past decades different forms of Received: August 31, 2018 Accepted: December 24, 2018 Published: December 24, 2018 A

DOI: 10.1021/acsaem.8b01470 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Scheme illustrating the microsupercapacitor electrode manufacturing process including the initial chemical vapor deposition of the 50 μm long silicon nanowires, the atomic layer deposition of the 3 nm alumina layer and the drop-cast of the commercial PEDOT: PSS solution.

tunable, exhibiting promising specific power and electrochemical stability performances. However, they still lag behind carbon derived materials in term of pure capacitive performances. Multiple strategies have been investigated to further increase capacitive specifications of SiNWs. In parallel other strategies involve the development of advanced systems combining both pseudocapacitive and common capacitive materials to overcome the energy density limitations of carbon-based and silicon-based supercapacitors.27 A large variety of transition metal oxides combined with carbon derived anodes have been studied, with oxide mainly derived from manganese,28−30 nickel,31 ruthenium,32,33 cobalt,34 and vanadium.35,36 In addition, pseudocapacitive materials have been carried on silicon-derived electrode materials with the deposition of conductive polymers such as poly(3,4ethylenedioxythiophene)37,38(PEDOT), polypyrrole39 (PPy), and transition metal oxides with MnO240 and NiO.41 Following this strategy, in this study, we have selected the (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)), PEDOT:PSS one of the most promising conducting polymer combined with SiNWs as MSCs electrode material for supercapacitor applications. Commercially available at remarkably low cost and extremely versatile for both electrochemical storage and energy conversion devices, PEDOT:PSS has become an essential material for the organic electronic device industry.42 Among other conductive polymers, PEDOT:PSS is a material of choice for energy storage and conversion being capacitive, highly stable and able to reach remarkable conductivity when cunningly tuned.43 Over the past ten years, numerous studies have been performed using PEDOT:PSS as electrode material for large scale supercapacitor electrodes for flexible integrated applications,44 free-

nanostructured carbon materials have been developed and studied as MSC constitutive electrode material including graphene,5−7 Onionlike carbon (OLC),8,9 porous carbon,10,11 reduced graphene oxide (rGO),12,13 carbide-derived carbons (CDC),14−16 and carbon nanotubes (CNT).17,18 Besides carbon derivatives a wide range of innovative materials have been studied as either alternative or composite with carbon materials as MSCs electrodes to further increase footprint normalized specifications. Among others, silicon and its derived nanostructures, namely, SiNWs,19−21 silicon nanoparticles,22 and silicon carbide nanowires,23−25 have recently attracted a strong interest as MSC electrodes because of their capacitive performances coupled with their excellent electrochemical and thermal stability, compatibility with microelectronic, morphological control, and abundance. More specifically, SiNWs have been shown to exhibit promising capacitive behavior using ionic liquids as electrolytes with excellent maximal power density of 225 mW·cm−2, extremely high lifetime capacity with millions cycles of stability, wide operating window and fast charging rate. Still, a major silicon weakness, the native uncontrolled growth of silica when subjected to oxidative atmosphere, was hindering their usage with aqueous electrolytes. The recent addition of a Al2O3 layer by atomic layer deposition (ALD) was shown to protect the nanostructure covering thoroughly even high aspect ratio scaffold and being pinhole free.26 Indeed alumina protected Si NTs system was shown to retain 99% of its initial capacitance after 2 billion galvanostatic charge−discharge cycles at high current density of 0.5 mA·cm−2 in an aqueous electrolyte of Na2SO4, thus opening new opportunities for their use in aqueous medium. Undoubtedly, silicon nanowires have proven to be a material of choice for on-chip integration being morphologically B


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ACS Applied Energy Materials standing film devices45 and hydrogel electrolyte-based electrochemical capacitors.46 However, only few analyses have selected this material as core for aqueous MSC devices, and to this day, PEDOT:PSS electrochemical functioning principles remain far from being known.47 To the best of the writer knowledge, no study has been published yet involving a composite made of contemporary PEDOT:PSS conducting polymer and SiNWs for aqueous microsupercapacitors. In this work, we have achieved a fast, simple, scalable and low cost deposition process of PEDOT:PSS on alumina protected SiNWs for aqueous MSC applications. The successfully obtained nanocomposite shows promising electrochemical performances including excellent specific energy and specific power. In addition, the unique feature of the electroactive material developed is its extraordinary ability to withstand charge−discharge cycles while maintaining its charge storage performances.

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Samples are dried overnight in a chemical hood and weighted the day after. The active polymer layer has a dark blue color with and high coverage because of the high mass loading on the silicon nanostructure. Each coated batch includes selected samples to be characterized by scanning electron microscopy at 5 kV to ensure the integrity of the organic polymer layer. The mass of the polymer layer is weighted after every deposition and taken into account to derive storage performances from electrochemical measurements. Crosssection of the nanostructure is studied using cutter equipment. Samples are washed by water dipping before any electrochemical measurement. Electrochemical Characterizations of the Microsupercapacitor. All electrochemical measurements were carried out in a 0.5 M Na2SO4 aqueous electrolyte. The electrochemical characterization tests were conducted with a potentiostat/galvanostat (VMP3, Biologic, France). Results are systematically compared to that of the pristine alumina coated silicon nanowires (A@SiNWs) to highlight the contribution of the conductive PEDOT:PSS polymer. The investigation of the composite material itself was performed in a homemade Teflon cell in a three electrode configuration using a twisted platinum wire as counter electrode, an aqueous RE-1B Ag/ AgCl reference electrode from BAS, Inc., and the sample on a 500 nm Au-coated copper collector as working electrode. The electrolyte was injected inside each cell using a 1 mL syringe under the consistent basis of roughly 400 μL for each test. Multiple characterizations were carried out with samples from different PEDOT:PSS deposition batches, CVD growth and ALD coatings under the same conditions displaying good reproducibility. Cyclic voltammetry tests were conducted with various potential windows (ranging from −0.7 to 0.5 V vs Ag/AgCl reference electrode) and at different scan rates (ranging from 10 mV·s−1 to 1 V·s−1). Afterward electrochemical impedance spectroscopy measurements (EIS) at different key potentials (0.2, 0.4, −0.4 V, ...) were performed using 70 mV amplitude signal at frequencies ranging from 10 mHz to 200 kHz. A symmetric two electrode configuration was also used to study the behavior of the PEDOT:PSS covered Si NWs electrodes as supercapacitor under operating conditions. Samples were assembled in an ECC-Std hermetic EL-CELL cell in a “sandwich” configuration with a Whatman no. 41 paper separator soaked with roughly 200 μL of 0.5 M Na2SO4 electrolyte. An open-circuit potential of 0.035 mV was reported before the testing. Cyclic voltammetry testing at a scan rate of 20 mV·s−1 was first performed to ensure the stability window of 1.2 V in this configuration. Thereafter, galvanostatic charge− discharge measurements were conducted at multiple current densities (ranging from 0.1 to 1 A·g−1, at a cell voltage of 1.2 V). Gravimetric capacitance, areal energy, and areal power were derived from the slope of the galvanostatic measurement taking into account the weight of the overall nanostructure (0.6 mg·cm−2 with the mass of pristine A@ SiNWs being 25 μg·cm−2), including both the ALD passivated Si NWs mesh and the polymer layers. Capacitance retention of the supercapacitor configuration was investigated through long-term galvanostatic charge−discharge tests at a current density of 0.5 A·g−1. Calculation. Equations 1−4 were used to determine the electrochemical performances. Areal capacitance of the composite Ca1 and Ca2 in mF·cm−2 in two and three electrode cell respectively were determined using the eqs 1 and 2

MATERIALS AND METHODS

Figure 1 exhibits all the manufacturing processes of an aqueous PPSSA@SiNWs microsupercapacitor electrode merging multiple nanostructuration techniques. Silicon Nanowires Growth. Highly doped Si NWs are grown in an hot wall Chemical Vapor Deposition (CVD) reactor used as growth chamber (EasyTube 3000 first Nano, CVD Equipment Corporation) using the vapor liquid solid method (VLS) as defined by our co-workers48−51 based on silane precursor gas reactions with gold catalyst (gold colloidal solutions with disperse 50 nm). CVD using VLS mechanism (vapor−liquid Solid) allows a remarkably vertically guided growth of epitaxial SiNWs on silicon substrates: highly n-doped Si < 111> 10 × 10 mm2 square pieces diced from 100 mm diameter wafers (Silicon Materials Inc., As doped, < 5 mΩ·cm). In operando donor or acceptor doping of the nanostructure is easily performed by adding doping gases, such as phosphine or diborane during material growth. The use of CVD allows the fast and controllable growth of highly dense silicon nanowires, up to 2 × 108 cm−2, with high specific ratio.20 Alumina Protective Layer Deposition. Consecutively, conformal alumina protective layer is deposited using atomic layer deposition (ALD). After deoxidation by HF (50%) vapor, samples are inserted in a thermal ALD commercial reactor (Fiji200 reactor, Cambridge Nanotech). Trimethylaluminum (TMA) and H2O are used as precursors to carry out the deposition of a pinhole free 3 nm thick Al2O3. Cycling deposition is performed at 250 °C, under 10−2 Torr using argon as purge gas until the target thickness is reached according to a recipe described in our previous work.52 This coating step is crucial to expand the application scope of silicon nanostructures to aqueous based devices in the electrochemical storage field. Indeed by removing the uncontrolled, native and inherent Si oxide layer using fluoridric acid treatment and replacing it with alumina, the silicon nanostructure is protected from any oxidative stress from aqueous electrolyte contact. Facile PEDOT:PSS Deposition. The conducting polymer layer PEDOT:PSS (purchased from Hereaus, with a raw conductivity of 1 S·cm−1) is deposited after sample cleaning by successive ethanol and water dipping, dried overnight and weighted with a 0.01 mg accurate scale. As for the coating, the drop-casting technique has been chosen for its low manufacturing cost, straightforwardness, and quickness. The deposition is conducted by a simple drop of ethanol beads from a 5 mL glass Pasteur pipet on the Si NWs, followed by three droplets from a 1 mL syringe of the commercially purchased PEDOT:PSS solution. Each drop-cast is performed at a consistent height of 1 cm to ensure the integrity of the nanostructure and the reproducibility of the deposition. PEDOT:PSS solution is homogenized before being dropcast and kept refrigerated between depositions. Ethanol is used as dispersing agent to increase both the wettability between the polymer and the Si NWs and the conformity of the PEDOT:PSS deposited layer ensuring great reproducibility between deposition batches.

E

Ca1 =

∫E 2 ia(E) × dE 1

(1)

2v × S

for three electrode cells, where E1 and E2 represent the cutoff potentials in cyclic voltammetry, ia(E) is considered as the instantaneous current, ∫

E2

E1

ia(E) × dE is the voltammetric charge by

integration of the positive and negative sweeps in the cyclic voltammograms, ν is the scan rate in V·s−1, and S stands for the surface area of the electrode. Ca2 = C

Ia × t × mt ΔV × 2 × S

(2) DOI: 10.1021/acsaem.8b01470 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. SEM cross section images of pristine A@SiNWs (a) and gelatinous-like PPSS-A@SiNWs composite (b and c) on highly n-doped silicon substrate at different magnifications with a 5 kV accelerating tension. Panels a−c were recorded with 90° tilt angle. (d) TEM image of 50 μm thick silicon nanowire covered by a 3 nm thick alumina layer embedded in a polymer matrix. (e and f) EDX scans of scratched piece of PPSS-A@SiNWs composite at a magnification of 15000 and a 5 kV accelerating tension at a working distance of 3.5 mm. for two electrode symmetric cells where Ia is the gravimetric applied current density in GCPL mode, while t is the discharge time, ΔV the voltage range, mt the overall mass of the active material of the two electrodes, and S stands for the surface area of the electrode. In case of PPSS-A@SiNWs, the mass mt used to derive the areal capacitance from eq 2 takes into account the mass of both the conducting polymer and the A@SiNWs nanostructure deposited on the silicon substrate. Areal energy in mJ·cm−2 and power in mW·cm−2 were derived from eqs 3 and 4, respectively Ea =

1 × Ca2 × ΔV 2 2

Pa =

Ea t

(4)

where ΔV the voltage range and t represents the discharge time.

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RESULTS AND DISCUSSION Morphological aspects of the composite material are identified by SEM technic to investigate the interconnection between the two material components. Figure 2b and c display SEM micrographs of the PPSS-A@SiNWs composite materials at different magnifications with a 5 kV accelerating tension.

(3) D


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Figure 3. PPSS-A@SiNWs electrochemical characterization. (a) Three-electrode cell cyclic voltammograms of pristine A@SiNWs (orange curve) and PPSS-A@SiNWs (blue curve) at a scan rate of 20 mV·s−1 with a potential window of 1.2 V using Na2SO4 0.5 M electrolyte. (b) Cyclic voltammograms of PPSS-SiNWs at different scan rates in three electrodes cell in Na2SO4 0.5 M electrolyte. (c) Areal capacitance of a threeelectrode cell configuration of PPSS-A@SiNWs and pristine A@SiNWS at different scan rates. (d) Magnification of the areal capacitance of pristine A@SiNWS.

Images show a mesh of highly doped 50 μm long A@SiNW embedded inside a 5 μm thick PPSS gelatinous-like matrix ensuring an excellent electrical connection between the two materials. One of the two having nanometric dimensions, the overall structure can be identified as a nanocomposite made of conducting polymers and nanowires. High-magnification pictures show the interconnection of different A@SiNWs through the PEDOT:PSS gelatinous film. For some years now, high aspect ratio silicon nanostructures, such as SiNWs and SiNTs, have been demonstrated as promising electrode material for supercapacitors. However, one of their weaknesses, which has been identified for microsupercapacitors applications where the areal foot print of the device is critical, is the low mass to surface ratio of electrochemically active material. Here, the strategy is to fill voids inside the A@SiNW mesh with an active material in which the electrolyte is easily able to percolate through. Si NWs achieve rather low capacitance compared to pseudocapacitive materials because they display pure electrochemical double layer capacitance. However, being both highly doped, up to 1020 dopant·cm−3, good electrical conductor and epitaxially grown onto silicon wafer, Si NWs are an appropriate structural material to host a distinct highly electrochemically active second one. Thus, inspired by battery electrode common architecture, here, we consider A@SiNWs to be the conductive binder of the electrode material while PEDOT:PSS is the electrochemically active material. In contrast with highly specific and conformal coverage method, such as atomic layer deposition or selective chemical synthesis processes, the drop cast of PEDOT:PSS solution does not

cover specifically the Si nanowires but rather interweave them with PEDOT:PSS into a gelatinous-like network. As shown by Figure 2, the coating of the polymer flattens the nanostructure from 50 to 5 μm. In contrast, Figure 2a shows pristine A@SiNWs before the conducting polymer deposition. The nanostructure is well vertically orientated with nanowires length ranging from 50 to 55 μm. After deposition, the flexible nanowires interlocked each other’s. Although the NWs network has been bowed down under the weight of the aqueous commercial PPSS solution during the drop-cast deposition, the flexibility of the epitaxially grown A@SiNWs ensure the proper electrical and mechanical connection between the n-doped growth substrate and the electrochemically active composite material while allowing a high PEDOT:PSS mass load of the nanostructure without breaking it. The composite film is actually able to withstand the mechanical stress of the cleavage process required to observe the cross-section of the material by SEM. Even though the commercial PEDOT:PSS solution is soluble into water under stirring conditions, the composite PPSS-A@SiNWs does not solubilize and does not tear off from the substrate even after being immerge one month into the aqueous Na 2 SO 4 electrolyte. Figure 2d displays TEM measurement of an alumina coated silicon nanowires covered by a layer of an amorphous material, which we identify as PEDOT−PSS. The nanowire is clearly embedded in a gelatinous-like polymer thus highly mechanically connected. We have chosen to scratch pieces of the composite at stake, to investigate the inner part of the composite, and dispersed it on a carbon grid (for more insights E


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the shifting of the redox peaks, the current density plateau is also shifted by the same extent toward the maximum of the potential window decreasing its width thus the relative energy stored. The areal capacitance of the PPSS-A@SiNW composite in a three-electrode cell at different scan rates is exposed in Figure 3c and compared with that of pristine A@SiNWs. The layout of the curve is typical of a pseudocapacitor with the exponential decrease of the capacitance with the increase of the scan rate. Thus, the electrochemical performance of the nanocomposite in question is a time dependent phenomenon. In terms of specifications, the areal capacitance of the PPSSA@SiNW composite is ranging from 8.4 mF.cm−2 to 2.15 mF.cm−2 with scan rate going from 5 to 500 mV·s−1. Remarkably at high scan rates, the areal capacitance of the pseudocapacitive composite reaches a plateau around 2.0 mF· cm−2, while the pristine material experience a performance degradation up to 0.01 mF·cm−2 (see the zoomed curved for pristine A@SiNWs Figure 3d). In terms of pure capacitive performances, MSCs made of PPSS-A@SiNW composite outreach pristine A@SiNWs one in Na2SO4 0.5 M electrolyte from a factor up to 100. Eventually, at the sight of these first electrochemical characterizations, the facile deposition of PEDOT:PSS onto SiNWs clearly has a major positive impact on the electrochemical properties of the material. Figure 4a exhibits a Nyquist plot of a nanocomposite in three electrode cell over a frequency range of 200 kHz to 10

on TEM measurement please refers to the Supporting Information, Figure S1 in particular). Figure 2e and f display an EDX imaging of a scratched piece of PPSS-A@SiNWs composite. A layer of the composite has been selected to reduce the shadowing effect of neighbor atoms which happens in case of a thick layer of composite. The sulfur atom has been chosen in particular for being a specific marker for PEDOT−PSS. The chemical composition of the additive material displayed onto silicon nanowires matchs the chemical composition of PEDOT−PSS. (For further insight, please refer to Figure S2.) The electrochemical impact of the addition of PEDOT:PSS on SiNWs is investigated in a three electrode cell configuration. The electrochemical response displays a clear highly capacitive evolution. An electrochemical window of 1.2 V was initially chosen as it was demonstrated to be the stability domain of the material at issue in this configuration using Na2SO4 0.5 M electrolyte. Such value is excellent for an aqueous nonhighly concentrated electrolyte with the thermodynamic potential window of pure water being known to be only 1.22 V. Figure 3a benchmarks the electrochemical response of the pristine alumina protected SiNWs (orange curve) against that of the PPSS-A@SiNWs composite (blue curve) within a potential window from −0.7 V to 0.5 V vs Ag/ AgCl reference electrode. The composite shows an impressive capacitive response compared to the pristine A@SiNWs with current density performances increased by a factor of 500 reaching current densities in the order of the tenth of mA·cm−2 at a scan rate of 20 mV·s−1. Considering the cyclic voltammogram, the low resistivity of the nanocomposite in Na2SO4 0.5 M electrolyte implies that the electronic conductivity between the conducting polymer and the nanowires, through the 3 nm thick alumina layer, is effective. The large thickness of the active material does not impact the capacitance behavior of the electrochemical response. It is also worth noting the Faradaic oxidation peak (around −0.2 V vs Ag/AgCl) and the corresponding reduction signal (around 0.4 V vs Ag/AgCl) showing the pseudocapacitive behavior of the PEDOT:PSS coating (blue curve). The current density plateau after the oxidation peak ranging from −0.2 to 0.6 V vs Ag/ AgCl is typical of a Faradaic p-doping process of a conducting polymer where anions intercalate inside the polymeric structure of the PEDOT:PSS. By contrast, as demonstrated in previous papers51 pristine A@ SiNWs displays quasi rectangular and symmetric curve shapes which is the signature of pure electrochemical double layer capacitance involving non-Faradaic reaction. Thus, the overall electrochemical signal of the composite PPSS-A@SiNW is a combination of Faradaic and non-Faradaic storage reaction with a clear dominance of the polymer contribution resulting in the pseudocapacitive behavior observed. Figure 3. b) displays three-electrode cell cyclic voltammograms of PPSS-A@SiNW composite at a variable scan rate, from 20 to 500 mV·s−1 within the potential window aforementioned. The impact of the increase of the potential regime on the electrochemical response of the PPSSA@SiNWs is highlighted. A classical behavior of pseudocapacitive material is observed with an increase of the electrochemical response and a slight shift of redox peaks to higher potential versus Ag/AgCl reference electrode with the increase of the scan rate. The system is thus quasi-reversible with a hysteresis property typical from conducting polymers. Even at high scan speed the p-doping process of PEDOT:PSS is undeniably distinct without degradation of the polymer. With

Figure 4. (a) Nyquist plot of the nanocomposite material in a three electrode-cell measured within the frequency range from 200 kHz to 10 mHz at −0.6 and 0.4 V vs Ag/AgCl reference electrode. The equivalent circuit designed by fitting the Nyquist plots of the material is shown in panel b. The inset c shows a magnified version of the Nyquist plot at 0.4 V vs Ag/AgCl at high frequency. The orange curve represents the model fitting of the composite impedance behavior by EC-Lab software.

mHz at both −0.6 and 0.4 V vs Ag/AgCl reference electrode. The potentials have been chosen to compare the impedance response of the material between the neutral and the highly capacitive regions of the plot displayed previously in Figure 3a. The equivalent series resistance (ESR) is given by the intersection between the Nyquist plot and the x-axis representing the real part of the impedance of the material. F


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Figure 5. PPSS-A@SiNWs sandwich-like device characterization in working conditions: (a and b) Two-electrode cell galvanostatic charge/ discharge plot of Pristine A@SiNWs (red curve) and PPSS-A@SiNWs (blue curve) at a current density of 0.1 and 1 A·g−1, respectively, under a potential window of 1.2 V e. (c) Galvanostatic charge/discharge plot of PPSS-A@SiNW composite under different current densities in two electrodes cell. (d) Areal capacitance of a two-electrode cell configuration of PPSS-A@SiNWs (blue spheres) and A@SiNWs (orange spheres) at different scan rates.

The ESR, estimated to be around 25 Ω·cm−2, is the sum of the resistance of the nanocomposite itself together with the ionic movement resistance of the electrolyte, the resistance of the current collector and the resistance of the electrolyte/material interface. At high frequencies between EIS at 0.4 V and −0.6 V, the diameter of the semicircle is widely increased meaning that the resistance to the charge transfer (R2 in this study) increase to the same extent. This behavior corresponds to the electrochemical response reported in the Figure 3a, where the conducting polymer p-doping process is happening at potential higher than −0.3 V vs Ag/AgCl. In the low frequencies region the EIS at 0.4 V displays a straight line with an alpha angle close to 1, which is typical from a purely capacitive behavior. However, at −0.6 V the curved line with an angle close to 0.5 is the signature of a high resistivity to the ion diffusion consistent with the observation done before in the Figure 3. a). Using an impedance fitting software (EC-Lab, Z-fit tool with Randomize + simplex method) an equivalent circuit, Figure 4b, modeling the electronic behavior of the nanocomposite and its interfaces has been built describing the material in both regimes. The error index, χ2, for this modeling was found to be lower than 1.17 × 10−3, (more information about the fitting process can be found in Figure S1). In the equivalent circuit below, R1 represents the ESR (mainly the resistance of the electrolyte and the global resistance of the cell). Q2 is assigned to the double layer capacitance at the surface of the electrode in parallel with R2 corresponding to the charge transfer resistance at the interface electrode/electrolyte. W models the Warburg impedance in parallel with R3, the resistance of the ionic diffusion at the surface of the material. Q4 can be attributed to the overall capacitance of the electrode material

in parallel with R4, accounting for the resistance to the diffusion of ions inside the porosity of the electrode material. Figure 5a and b display the two-electrode cell galvanostatic charge/discharge curves of A@SiNWs and PPSS-A@SiNWs microsupercapacitors under low and high current regime respectively within a relative potential window of 1.2 V using Na2SO4 0.5 M electrolyte. In either conditions the siliconpolymer composite as well as the pristine A@SiNWs display a triangular shaped curve typical of a capacitive behavior. It is worth noting that a high charge/discharge current (Figure 5b) causes a more important ohmic drop than a low charge/ discharge current (Figure 5a) for PPSS-A@SiNWs microsupercapacitor mainly because of the higher internal ESR of the composite compared to pristine A@SiNWs. However, the discharge time for the PPSSA@SiNWs system is more than 20 times higher at 0.1 A·g−1 than that of A@SiNWs system what will induce a superior capacitance for the PPSS-A@SiNWs device. At high current density of 1 A·g−1, the difference is even more significant with the polymer nanocomposite device being 50 times more capacitive than the pristine one. Therefore, even though the internal resistance of the PPSSA@SiNWs sandwich-like device is higher than the pristine one, the expected value of the maximum areal energy and power allowed by the microsupercapacitor is significantly higher for the PPSS-A@SiNWs device. The impact of the increase of the current scan rate on the electrochemical response of the PPSSA@SiNWs is then highlighted in Figure 5b. The triangular shape of the response typical of a capacitive material is consistent throughout the various scan regimes. Figure 5c displays two-electrode cell galvanostatic charge/discharge of PPSS-A@SiNW composite at a variable charge/discharge currents, from 0.10 to 2.0 A·g−1 within the potential window G


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Figure 6. Ragone plots comparing areal energy and areal power performances of the A@SiNWs MSCs (blue curve) and PPSS-A@SiNWs (orange curve) symmetric microsupercapacitors in Na2SO4 0,5 M electrolyte with the state of the art aqueous silicon based microsupercapacitors reported in the literature.

Figure 7. Capacitance retention and Coulombic efficiency of PPSS-A@SiNW symmetric supercapacitor under working conditions as a function of the number of galvanostatic charge/discharge cycles at 0.5 A·g−1.

capacitance of A@SiNW device is 100 times lower than that of the aforementioned one. The Ragone plots display the evolution of energy densities of the systems as a function of power densities. This chart is well-known in the energy storage field for its ability to highlight the operation areas of each storage unit. A comprehensive study of the specifications of an energy storage device has to include a Ragone plots reported in operating conditions. Supercapacitors, or microsupercapacitors in particular are generally known for their high power densities but quite low energy densities. For the past ten years, the aim of the electrochemists has been to increase their energy levels to fill the specification gap remaining with batteries while keeping charge/discharge stability and power densities at high level. Figure 6 presents the Ragone plots of the two studied

aforementioned. The triangular shape of the silicon-polymer response is consistent over the charge/discharge current spectra even at intense charge/discharge currents. The overall electrochemical charge rate of PPSS-A@SiNW microsupercapacitor at 1.0 A·g−1 is comparable to that of A@SiNWs microsupercapacitor at 0.1 A·g−1, which clearly emphasize the intake of the way of depositing PEDOT:PSS for supercapacitor applications. The positive impact of PEDOT: PSS on SiNWs mentioned before is clearly confirmed in the Figure 5d, which presents the evolution of the areal capacitance of microsupercapacitors as a function of current densities. Areal capacitance of PPSS-A@SiNWs device in two electrode cell reaches 3.3 mF·g−1 at 0.1 A·g−1 and stabilizes 2.2 mF·g−1 at high charge/discharge current of 2 A·g−1. In contrast, areal H


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Table 1. Summary of the Electrochemical Performances of Silicon-Based Aqueous Microsupercapacitors Reported in the Literature to Date electrode material

areal capacitance (mF·cm−2)

energy density (mJ·cm−2)

A@Si NTrs

0.960

0.082

SiC/SiNWs SiC NW

1.7 0.440

0.85 0.041

C/SiNWs diamond/ SiNWs Si/TiN/MnO2 PPSS-A@Si NWs pristine A@ SiNWs

25.64 0.400 41.3 (at 200 mV·s−1) 8.4 (at 20 mV·s−1) 0.55 (at 20 mV·s−1)

6.3 1.5 NP 11.4 (at 0.1 A·g−1) 0.13 (at 0.1 A·g−1)

power density (mW·cm−2)

aqueous electrolyte

coulombic efficiency (%)

cycling stability

99

2 × 10 (99%) 1000 (95%) 2 × 104 (95%) 25000 (75%) 2 × 105 (70%) 5000 (85%) 5 × 105

0.2 M Na2SO4

0.84 NP 0.040

1 M KCl 3.5 M KCl

NP NP

1 M Na2SO4 0.1 LiClO4

NP NP

NP 4.05 (at 1 A·g−1)

1 M Na2SO4 0.5 M Na2SO4

NP

0.23 (at 1 A·g−1)

0.5 M Na2SO4

0.04 50

99 99

6

2 × 106 (99%)

refs In Press 20 23 53 54 58 This work This work

conducting polymers inside aqueous electrolyte to be limited making this achievement more valuable. Even though the capacitance of the Si NWs is low compared to that of the conductive polymer, the input to the charge/discharge resilience of the polymer is high. In the meantime, the comparison between the pristine material and the nanocomposite in term of pure capacitance performances is unequivocal, with performances 100 times higher for the latter one. Finally, the conductive A@SiNWs backbone was chosen as a support material which enables the polymer to cope with the mechanical stress endured during the cycling. The synergy between the conductive polymer and A@SiNWs seems to be efficient leading to an uncommon stability for a conductive polymer for MSC applications. Interestingly, the areal capacitance of the PPSS-A@SiNW is not invariable during the cycling but rather progressive. The polymer-nanowire composite capacitance is increasing over the 80 000 first cycles. This behavior can be explained by looking at the percolation of the electrolyte inside the active material layer and the consecutive rearrangement of the polymeric matrix. Considering the thickness of the composite layer being 5 μm, throughout the cycling the electrolyte is percolating inside the composite reorganizing the polymeric matrix by the successive ionic intercalations and disinsertions. The areal capacitance is thus increasing until the whole nanocomposite is soaked with the electrolyte where it reaches a stable plateau. This slow process, already observed in literature,56 leads the capacitance of the composite to slowly increase above 100% even reaching 115% after 100k cycles corresponding to 2 weeks of cycling. The capacitance is then evolving between 90% and 100% of its initial value while being stable over 500k cycles. Undergoing successive swelling and deflation cycles the composite areal capacitance fluctuates after 200 000 cycles throughout the experiment until 500 000 cycles. We believe that the polymer undergoes mechanical stress during the cycling and rearrange itself following this stress through slow conformational relaxation processes.57 To further investigate this evolution the morphological aspect of the electrode after cycling can be found in supplementary of this Article (Figure S6). Table 1 summarizes electrochemical performances of PPSSA@SiNW devices compared to Pristine A@SiNW devices and every aqueous silicon-based microsupercapacitors reported in the literature up to this day. Areal capacitance of PPSS-A@ SiNW nanocomposite in three-electrode configuration reaches 8.5 mF·cm−2 at 20 mV·s−1 which matches the standards of the

symmetric systems (A@SiNWs and PPSS-A@SiNWs) systematically compared with the state of the art aqueous silicon based microsupercapacitors reported in the literature. The different energies and power densities were calculated from current densities between 0.1 A·g−1 and 2 A·g−1. For the same current density, the maximum energy density achieved by the PPSS-A@SiNWs supercapacitor is 10 mJ·cm−2 against 0.12 mJ·cm−2 for the A@SiNWs system. Thus, with the addition of PEDOT:PSS the energy density is almost 100 times higher. Moreover, the maximum power density reached by the PPSSA@SiNWs supercapacitor is 4 mW·cm−2 against 0.25 mW· cm−2 for the A@SiNWs supercapacitor. Considering gravimetric energy storage performances (given in supplementary of this paper Figure S4), the A@SiNWs match PPSS-A@SiNWs in terms of gravimetric power but concede on decade when considering gravimetric energy. It is also worth noting that energy densities stay stables with the increase of the power density for the PPSS-A@SiNWs supercapacitor which is in agreement with the previous evolution of the areal capacitance with the current density. Indeed, an energy density of 9 mJ· cm−2 is still developed at a power density of 4 mW·cm−2 (and 10 mJ·cm−2 at 0.2 mW·cm−2). Compared with the literature the author was able to find on the subject, PPSS-A@SiNWs microsupercapacitors exhibit the highest areal energy performances among the silicon based aqueous MSCs with areal power being higher than that of carbon coated Si NWs51 and silicon carbide nanowires.23 These results clearly highlight the exceptional contribution of the PEDOT:PSS conductive polymer onto SiNWs for the increase of the energy densities of the supercapacitor.23,51,52 Further in this study, the areal capacitance retention has been measured under extreme cycling conditions for a pseudocapacitive microsupercapacitor. Figure 7. exhibits the specific capacitance of a sandwich-like symmetric supercapacitor (pristine A@SiNW and PPSS-A@ SiNW) as a function of cycle numbers. A major weakness of conducting polymers which still restricts their market applications is their insufficient stability during cycling. Indeed the mechanical stress undergone by the conducting polymers during the multiple cycles of charge/discharge leads to their deficiency after a given period.55 Here the PEDOT:PSS gelatinous-like matrix reinforced by the A@SiNWs network is able to withstand 500 000 charge/discharge cycles under a current density of 0.5 A·g−1 with a capacitance fade of only 5%. To the writer knowledge, this result is one of the all-time high for conductive polymers considering also the stability of I



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ACKNOWLEDGMENTS The authors would like to thank the Direction Générale de l’Armement (DGA), the European InnoEnergy PhD School program, and the CEA (Commissariat à l’É nergie Atomique et aux É nergies Alternatives. Ministère de la Défense, Direction Générale de l’Armement European CommissionHorizon 2020 Framework ProgrammeH2020 European Institute of Innovation and Technology) for financial support.

state-of-art silicon-based MSCs. More interestingly, PPSS-A@ SiNW microsupercapacitors display the highest energy density of silicon-based aqueous MSCs combined with a top tier power density compared to alternative materials. Cycling stability is also a major upper hand of the devices at issue with an extraordinary capacitance fade of only 5% after 500 000 cycles. Even diamond protected SiNWs54 devices, while exhibiting higher power density and being known for their high cycling stability, do not match PPSS-A@SiNW MSCs in terms of energy density and charge/discharge resilience. Herein, we have successfully designed a silicon-based aqueous MSC exhibiting both state-of-the-art energy and power density while being ultrastable in working conditions, a feature which is the very essence of the supercapacitor technology.

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ABBREVIATIONS MSC = microsupercapacitor PPSS = PEDOT: PSS polymer PPSS-A@SiNW = PEDOT: PSS solution onto 3 nm alumina coated silicon nanowires A@SiNWs = alumina coated silicon nanowires OLC = onion like carbon rGO = reduced, graphene oxide CDC = carbide derived carbons CNT = carbon nanotubes ALD = atomic layer deposition PEDOT = poly(3,4-ethylenedioxythiophene) PPy = polypyrrole CVD = chemical vapor deposition TMA = trimethylaluminum EIS = electrochemical impedance spectroscopy measurements EDLC = electrochemical double layer capacitor ESR = equivalent series resistance SEM = scanning electron microscopy Si-NWs = silicon nanowires Si-NTrs = silicon nanotrees

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CONCLUSION In summary, we have demonstrated a simple, cost-effective, and reproducible coating technique to deposit a versatile conducting polymer, PEDOT:PSS on silicon nanowires, a strategy which may provide new insights into scalable pseudocapacitive devices production. A combination of epitaxially grown and highly doped silicon nanowires, covered with a protective alumina ALD deposited layer, embedded in a gelatinous-like polymer matrix was successfully achieved. This new nanocomposite material exhibits excellent performances as microsupercapacitor electrode in aqueous electrolyte in terms of areal capacitance with 8.5 mF·cm−2 at 20 mV·s−1 in three electrode system, energy density of 11.4 mJ·cm−2 at 0.1 A·g−1 and power density of 4.05 mW·cm−2 at 1 A·g−1. Therefore a film of PEDOT:PSS stabilized by a backbone of SiNWs represents a viable and promising alternative for microintegrated energy storage devices based on aqueous electrolyte. In addition and unprecedented for a conductive polymer, a massive galvanostatic charge−discharge stability has been proven with a capacity fade of only 5% after not less than 500 000 cycles. Further studies include a deeper understanding of the electrolyte swelling process inside the polymer layer together with enlarging the scope of such nanomaterials to allsolid, asymmetric, and flexible devices.

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REFERENCES

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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/acsaem.8b01470. TEM images of the composite material, EDX analysis via SEM, impedance equivalent circuit, Ragone plot in gravimetric units, and SEM images after cycling (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anthony Valero: 0000-0002-6057-0625 Pascal Gentile: 0000-0002-1547-4247 Saïd Sadki: 0000-0002-4187-6039 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. J


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