Beyond conventional supercapacitors: Hierarchically conducting polymer-coated 3D nanostructures for integrated on-chip micro-supercapacitors employing ionic liquid electrolytes

Synthetic Metals 247 (2019) 131–143

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Beyond conventional supercapacitors: Hierarchically conducting polymercoated 3D nanostructures for integrated on-chip micro-supercapacitors employing ionic liquid electrolytes David Aradilla

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, Saïd Sadki, Gérard Bidan

T

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Univ. Grenoble Alpes, CEA, CNRS, INAC-SyMMES, F-38000 Grenoble, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Pseudo-supercapacitors Conducting polymers 3D nanostructures Ionic liquid

The potential of electroactive conducting polymers (ECPs), as innovative pseudocapacitive materials, has recently emerged as a promising strategy to design new and advanced concepts in the field of electrochemical energy storage devices. From this point of view, the synergistic effect produced between high electroactivity ECPs (faradaic behaviour) and large active surface 3D nanostructured materials (electrochemical double layer behaviour) in terms of their high specific capacitance, high energy and power density as well as long lifetime, has awakened an enormous interest in the domain of (micro)-supercapacitors. Accordingly, in this work, we have originally analyzed the conformal electrochemical deposition of three different π-conjugated heterocyclic polymers [poly(3,4-ethylenedioxythiophene, polypyrrole and polyaniline] onto various vertically-oriented 3D nanoarchitectures, as for example silicon nanowires (SiNWs), diamond nanowires (DiNWs) or graphene nanosheets (GNs) among others. Next, the resulting as-grown hybrid materials were evaluated as attractive electrodes in symmetric supercapacitors (sandwich configuration) in presence of organic, aqueous, aprotic and protic ionic liquid (IL) electrolytes respectively. An exhaustive characterization of their corresponding electrochemical performances was investigated according to our previously reported works, confirming the remarkable cycling stability of ECPs in ILs. In addition, a detailed state-of-the-art dealing with other types of (micro)-supercapacitors in the literature was reported to corroborate the great impact of such devices to pave the way towards the elaboration of next generation high performance supercapacitors in the coming years.

1. Introduction Since Alan MacDiarmid, Hideki Shirakawa and Alan Heeger were awarded the Noble Prize in Chemistry in 2000 for the discovery of conducting polymers [1], our society has witnessed a great revolution in a wide range of technological applications from chemical sensors (e.g. biosensors), molecular electronic devices (e.g. diodes and field effect transistors), biomedical engineering, electrochromic devices, photovoltaic and dye sensitized solar cells to biofuel cells respectively. During the last decade, the inherent properties of electroactive conducting polymers (ECPs) have also attracted a great deal of attention in the field of electrochemical energy storage devices, comprising mainly battery and supercapacitor devices. Within this context, ECPs have demonstrated an enormous potential as supercapacitive electrodes due to their peculiar characteristics in terms of high specific capacitance (values ranging from 300 to 800 F/g depending on synthesis and

electrochemical performance conditions), high conductivity (up to 103 S cm−1), light weight, flexibility, relative fast charge-discharge processes as well as environmental friendliness, easy processing and relative low-cost. Thus, from the synthesis perspective, the electrochemical deposition of ECPs by means of electrochemical techniques based on galvanostatic, potentiostatic and potentiodynamic methods has recently opened up new perspectives to design and develop a great variety of different polymer morphologies under environmentally friendly synthesis conditions, which represents also an enormous advantage by comparison to other carbonaceous materials [2,3]. Among the different types of electrochemical supercapacitors, commonly categorized into electrochemical double layer capacitors (EDLCs), pseudocapacitors and hybrid supercapacitors respectively, ECPs and other pseudocapacitive materials such as transition metal oxides, hydroxides or nitride complexes play a key role in the category of pseudocapacitors due to their particular energy storage mechanisms associated to

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Corresponding authors. E-mail addresses: [email protected] (D. Aradilla), [email protected] (G. Bidan). 1 Present address: University of Goettingen, Institute of Inorganic Chemistry, Tammannstrasse 4, 37077 Goettingen, Germany. https://doi.org/10.1016/j.synthmet.2018.11.022 Received 4 September 2018; Received in revised form 20 November 2018; Accepted 29 November 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.

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characteristics resulting critical to envisage future concepts in the field of flexible, transparent and smart micro-supercapacitors [52–54]. Furthermore, other nanostructures such as ECP-based hydrogels have recently aroused a great deal of attention maintaining outstanding electrochemical performances for supercapacitor applications [55–57]. From this regard, it is clearly demonstrated that the synergistic effect between nanostructuration and pseudocapacitive properties will be crucial to design ultra-high performance supercapacitors in the coming years [58,59]. Finally, the last strategy has a special emphasis on the research of novel electrolytes based on ionic liquids (ILs) [60–62] or polymer gel electrolytes [63]. Accordingly, the synthesis of ECPs, by using ILs, can serve as platform to enhance the cycling stability of ECPs since such electrolytes are able to avoid the irreversible degradation of polymers caused by radical hydroxide attack from conventional solvents or electrolytes (known commonly as over-oxidation) [60,64,65]. In this direction, it should be noticed the pioneering works reported by Genies et al. using a fluorinated eutectic electrolyte for the PAni electrosynthesis [66] and the PPy electrosynthesis by Pickup et al. in ambient temperature molten salts [67]. This perspective has been successfully demonstrated in several cases concerning the synthesis of PPy [68], PEDOT [69–71] or PEDOT-PPy copolymers [72]. Thus, the synthesis of a thin PEDOT film in presence of 1-butyl-3-methylimidazolium tetrafluoroborate and its subsequent evaluation as supercapacitive electrode allowed to obtain a capacitance retention of approximately 80% after 70 000 cycles [70]. Based on the aforementioned tendencies reported nowadays in the literature, important progress are being carried out through the combination of multiple strategies in order to obtain the maximal benefits of each component and element in the field of supercapacitor devices. A clear example is reflected by the combination of pure EDLCs materials, as for example carbonaceous nanostructures, delivering high power density and long cycling stability with nanometric ECP coatings able to provide large capacitance values and energy density. This perspective has already been studied by using conducting polymer coatings onto aligned carbonaceous nanostructures (e.g. PPy-CNTs [73,74], PEDOTCNTs [75] or poly(3-methylthiophene-CNTs [76]). The next steps will be addressed to widen the knowledge of the different well known strategies by combining new ionic liquid electrolytes to synthesize advanced conducting polymers onto multiple hierarchized 3D nanostructures. From this point of view, in this work we propose different strategies by combining all those elements as a proof of concept. Over the past years, Thissandier et al. reported pioneer works regarding the enormous potential of highly doped silicon nanowires (SiNWs) in the field of (micro)-supercapacitor devices due to their interesting properties in terms of excellent EDLC behaviour (e.g. quasiideal rectangular shape cyclic voltammograms), high power density (maximal power density of 225 mW cm−2), excellent coulombic efficiency (99%) and extraordinary cycling stability in presence of organic solvent electrolytes (e.g. a propylene carbonate solution containing 1 M tetraethylammonium tetrafluoroborate) [77–80]. However, in spite of these impressive results, the lower obtained areal capacitance values (e.g. 10–30 μF cm−2 depending on the morphological characteristics of SiNWs and electrochemical conditions) ascribed to the low SiNW mass deposited by chemical vapor deposition (CVD) (conversion of ∼ 1 F/g), by comparison to other carbonaceous micro-supercapacitors [81,82], remained still a serious obstacle for some industrial technological applications, which exhibit severe technical specifications at capacitance level (e.g. autonomous sensors in the domain of aerospatial industry). Within this context, the European Nanowires for Energy STorage (NEST; 2012–2016) project encompassed various different approaches to enhance the electrochemical performances of SiNW-based microsupercapacitors through the following main strategies [83]: (i) the use of aprotic and protic IL electrolytes to enlarge the operating cell voltage and widen the working temperature range, (ii) the functionalization of SiNWs by using pseudocapacitive materials (e.g. transition metal oxides and ECPs) and other promising high active surface EDLC materials (e.g.

faradaic reactions, allowing to obtain higher capacitance values compared to pure EDLCs (non-faradaic reactions). More specifically, conducting polymer-based pseudocapacitors can be classified into three different types, which are described as follow: (i) Type I corresponding to a symmetric configuration where both electrodes are made of a pdopable active polymer, (ii) type II corresponding to an unsymmetrical configuration based on two different p-dopable active polymers and finally (iii) the type III deals with a configuration based on one electrode composed of a p-doped polymer and the another one composed of a n-doped polymer [4–6]. The type III represents one of the most promising strategies to develop high performance pseudocapacitors in terms of wide operating cell voltage (3 V), power and cycling stability [7], however the research of innovative n-doped polymers is presently still a big challenge in this domain since they are less conducting and more unstable than p-doped polymers, limiting their usage as supercapacitive electrode material. Concerning the conducting polymer structure, π-conjugated heterocyclic polymers based on polythiophene (PTh), polypyrrole (PPy), polyaniline (PAni) and their corresponding derivatives (e.g. PEDOT and PPrODOT) have been the most investigated in the literature owing to their relative easy synthesis and high conductivity [8]. Despite of the numerous advantages of CPs, as mentioned previously, one of the most important drawbacks of CPs concerns their electrochemical stability over electrochemical cycling (commonly hundreds cycles in aqueous or organic electrolytes, compared to millions of cycles in the case of carbon-based EDCLs), which is mainly ascribed to the attack by nucleophilic species (mainly hydroxide anion or hydroxyl radicals) on the radical cations of the doped ECPs backbone [9]. This interrupts the conjugation, which shortens the conjugation length and consequently the redox activity. In addition, some mechanical degradations (e.g. swelling, shrinking, peeling off the electrode) are promoted by the volumetric changes during the doping/ dedoping of ions, as a result of insertion and de-insertion of ions in the charge-discharge processes. Consequently, the potential use of CPs in technological applications is significantly compromised [10]. In order to overcome such hurdles, numerous strategies have been investigated along these years as follow: the first approach relies on the concept of hybridization through different device configurations, as for example hybrid devices denoted also commonly as asymmetric devices in the literature (e.g. one electrode composed of activated carbon and the another electrode composed of a conducting polymer), which allows to enhance the energy and power density. In this direction, thiophene derivatives exhibited promising electrochemical performances compared to pure double layer carbonaceous supercapacitors [e.g. poly(3(4-fluorophenyl)thiophene, p-PFPT, poly(3,4-ethylenedioxythiophene, PEDOT, [11–13], poly (3-methylthiophene) [14,15] or parafluorophenylthiophene [16]. The second strategy consists on the elaboration of nanocomposites made of ECPs and carbon nanostructures such as carbon nanotubes (CNTs) (including single wall carbon nanotubes and multiwalled carbon nanotubes) [17–23], graphene [24–29] or combining both nanostructures [30,31], graphite nanofiber [32], carbon nanocoil [33], porous carbon [34], carbon nanofiber [35], graphene oxide [36,37] or other nanostructures as for example silicon nanoparticules [38] or metal oxides (e.g. Fe2O3) [39]. In overall, the results showed an improved mechanical integrity, higher electronic and ionic conductivity, and larger specific capacitance and cycling stability compared to bare polymer. Accordingly, new tendencies towards ternary composites [40] or the deposition of carbonaceous coating on conducting polymers [41] have recently shown new perspectives in this domain. The third important strategy developed in recent years is focused on the design of advanced conducting polymer nanoarchitectures [42–44]. Among them, a special emphasis has been devoted to the concept of nanowires (e.g. PAni or PPy) [45–48], nanotubes (e.g. PEDOT) [49,50] or nanofibers [51] since they combine on one hand the large pseudo-capacitance properties associated to their faradaic reactions and on the other hand the large electrochemical active surface with an optimal ion diffusion path in the ordered nanostructure. These 132

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Fig. 1. Chemical structure of a) PEDOT, b) PPy and c) PAni.

diamond), and (iii) the implementation of derivate silicon nanowire structures for their integration and compatibility with the microelectronic industry (e.g. solder reflow process). Among all the aforementioned strategies, precisely, one of the most important technical challenges of the NEST project concerned the functionalization of 3D hierarchized silicon and diamond nanostructures by using pseudocapacitive materials in presence of IL electrolytes. Particularly, in this work, a special focus will be addressed to the electrochemical deposition of three different ECPs, employed in the NEST project, poly(3,4ethylenedioxythiophene) [PEDOT], polypyrrole (PPy) and polyaniline (PAni) (Fig. 1) on SiNWs, hyperbranched SiNWs (hereafter denoted as silicon nanotrees, SiNTrs), diamond-coated SiNWs and diamond nanowires (DiNWs) architectures respectively, as well as a detailed description about their extraordinary potential in the field of supercapacitors. This research activity represented an important breakthrough in the field of Si-based micro-supercapacitors in order to design the next generation high performance supercapacitors in the coming years according to their interesting areal/volumetric capacitances, energy and power densities, and excellent lifetime. Thus, the main achievements obtained in this part of the NEST project during the last five-year period are summarized in the next sections as follow:

Fig. 2. Schematic representation of our home-made 3-electrode electrochemical cell employed for the corresponding electro-polymerizations.

were estimated to lengths varying from 20 to 50 μm and diameters ranging from 20 to 200 nm respectively, whereas the branches for SiNTrs were mainly characterized by shorter lengths (approximately 2 μm).

2.3. Electrochemical deposition of ECPs on SiNWs and SiNTrs using ILs (CEA-Grenoble) PEDOT, PPy and PAni were deposited electrochemically onto asgrown SiNW and SiNTr nanoarchitectures by means of a 3-electrode cell configuration. More specifically, the electro-polymerization was conducted in a home-made Teflon cell where SiNWs and SiNTrs were employed as working electrodes (geometric surface of 0.4 cm2), Pt wire as counter electrode and a non-aqueous Ag+/Ag reference electrode composed of a silver wire immersed in a 10 mM silver trifluoromethanesulfonate (AgTf) solution in N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR13 TFSI) respectively (Fig. 2). Fig. 3 shows the respective chemical structures for the electropolymerization of PEDOT and PPy (PYR13 TFSI electrolyte) and PAni (diethylmethylammonium trifluoromethanesulfonate; DEMA OTF electrolyte) respectively. In all the cases, pure PYR13 TFSI or DEMA OTF solutions containing 0.1 M EDOT, Py or Ani monomers were employed for the corresponding electro-polymerization conditions. In this regard, the electrochemical deposition of PEDOT and PPy was carried out by potentiostatic methods using a constant potential of 0.4 V (vs Ag+/Ag) under a polymerization charge of 750 mC cm−2 controlled by the chronocoulometry technique [89,90]. PAni films were electrochemically deposited by applying a constant potential of 1 V (vs Ag+/ Ag) at a polymerization charge of 1250 mC cm−2.

2. Experimental 2.1. Materials and reagents Highly n and p doped 100 mm < 111 > silicon wafers (< 5 mΩ cm, As or B doped) were obtained from Silicon Materials Inc. Anhydrous propylene carbonate (PC), acetonitrile (ACN), tetrabutylammonium tetrafluoroborate (TBABF4), lithium perchlorate (LiClO4), 3,4-ethylenedioxythiophene, pyrrole, aniline and silver trifluoromethanesulfonate were purchased from Sigma Aldrich. The respective ionic liquids (ILs) presented in this work were provided from IOLITEC (Ionic Liquids Technologies GmbH, Germany, NEST consortium) and used without further purification. 2.2. Growth of doped SiNWs and SiNTrs (CEA-Grenoble, France, NEST consortium)

2.4. Growth of diamond on SiNWs (Fraunhofer Institute for Applied Solid State Physics (FHG-IAF), Freiburg, Germany, NEST consortium)

SiNWs and SiNTrs were grown in a chemical vapor deposition (CVD) reactor (Easy-Tube 3000 First Nano, a Division of CVD Equipment Corporation) by using gold catalyzed vapor-liquid-solid (VLS) method on highly doped n-Si (111) substrate according to the numerous previous works reported by P. Gentile and coworkers [84–87]. Briefly, the synthesis was generally performed at 600 °C, under 6 Torr total pressure, with 40 sccm (standard cubic centimeters) of SiH4, 100 sccm of PH3 gas (0.2% PH3 in H2), 100 sccm of HCl and 700 sccm of H2 as supporting gas. The doping level (dl) of SiNWs was managed by the pressure ratio: dopant gas/SiH4, which was evaluated in previous works (dl: 4·1019 cm−3) [84]. Concerning the growth of SiNTrs, the branches were initially obtained by depositing a 1 nm evaporated gold film on SiNWs. Subsequently, the experimental growth conditions were slightly different from those related to SiNWs (trunks) as follow: 50 sccm of SiH4, 100 sccm of PH3 gas, 100 sccm of HCl gas and 500 sccm of H2 as the carrier gas respectively. The SiNTr’s conditions were previously optimized according to our previous works [88]. The morphological characteristics of the corresponding SiNWs (trunks)

Boron-doped diamond was deposited on p-doped SiNWs by using a micro-wave chemical vapor deposition (MWCVD) system in presence of a hydrogen/methane gas mixture. The diamond doping was carried out by adding trimethylborane to the H2/CH4 mixture. More specifically, the growth parameters are indicated as follow: 4% methane in hydrogen, microwave power: 2750 W, gas pressure: 40 mbar and substrate temperature: 675 ± 10 °C. A diamond film thickness of 240 nm was estimated after the corresponding growth conditions [91,92].

Fig. 3. Chemical structure of the corresponding aprotic and protic ionic liquids used for the polymerization of PEDOT and PPy a) PYR13 TFSI and PAni b) DEMA OTF. 133

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Fig. 4. Example of elaboration of a coin cell device. a) Round electrode of SiNWs grown on stainless steel substrate, b) Electrodeposition of PEDOT on SiNWs according to the description in Section 2.3, c) Digital image of the components of a coin cell (cases, wave spring, and stainless steel spacer respectively), d) Assembling of a coin cell in a symmetric configuration (e.g. 2 electrodes made from PEDOTcoated SiNWs) and e) Schematic representation of a supercapacitor device.

and non-aqueous Ag+/Ag reference electrode (0.01 M AgNO3 in acetonitrile). Copper current collector was used to connect the working electrode. PEDOT was coated on VOGNs using a series of 10 pulses of 1 s on-time at 0.9 V with 10 s off-time at 0 V in between by using the NPV technique.

2.5. Growth of diamond nanowires (DiNWs) (FHG-IAF) Firstly, a boron-doped diamond film with a thickness of 8 μm was deposited via microwave enhanced chemical vapor deposition (MWCVD) on a 3-inch Si (100) wafer according to the following parameters: 2% methane in H2, a microwave power of 3200 W, a gas pressure of 50 mbar and a substrate temperature of 750 ± 10 °C. Subsequently, a Ti film of thickness of 3 nm was deposited on the diamond surface via direct current (DC) sputtering at 1000 °C under N2 atmosphere during 1 min. Finally, nanowires were grown using 3 min inductively coupled plasma (ICP) etching in oxygen plasma with the following parameters: 1200 W ICP power, 300 HF power, 3 Pa and 50 sccm of O2 [93].

2.9. Morphological characterization The morphology of the resulting hybrid (functionalized) and bare electrodes in this work was examined by using a ZEISS Ultra 55 scanning electron microscope equipped with energy dispersive X-ray spectrometry (EDX) element mapping analysis at an accelerating voltage of 4 kV and 10 kV respectively.

2.6. Electrochemical deposition of ECPs on diamond-coated SiNWs and DiNWs (CEA-Grenoble)

2.10. Elaboration of supercapacitor devices Coin cell devices (CR2032) were assembled using an electric crimper machine (MSK-160D, MTI, USA) in a symmetric configuration. The device was composed of coin cell cases, wave spring, stainless steel spacer, separator (Whatman type), and two round electrodes (1.76 cm2) made of the corresponding nanostructures presented in this study. The separator is soaked by the corresponding electrolyte (ionic liquid or organic solvent) using a total volume of 220 μl, which is placed between the two electrodes [100]. A schematic representation of the different elements involved in a supercapacitor coin cell device and its working principle is displayed in Fig. 4.

The electrochemical conditions to deposit ECPs on diamond-coated SiNWs were found identical to bare SiNWs and SiNTrs (See Section 2.3) [94]. Regarding the diamond nanostructure of DiNWs, PEDOT was deposited by using the normal pulse voltammetry (NPV). More specifically, a series of 15 pulses of 1.3 s on time at 1.2 V with 8 s off-time at 0 V in between was applied. 2.7. Growth of VOGNs (CEA-Grenoble) VOGNs were synthesized by electron cyclotron resonance-chemical vapor deposition (ECR-CVD) through a reactor built at INAC institute (CEA-Grenoble) by M. Delaunay and coworkers [95,96]. Briefly, The ECR-CVD reactor consists of a 2.45 GHz microwave power injection, two permanent magnets providing the required magnetic field for electron resonance and plasma confinement, a gas inlet and a substrate heater. The experimental conditions for the synthesis of VOGNs have been reported in our previous works [97–99]. Thus, as a summary, VOGNs were grown on highly doped Si or nickel foam substrates using 50 sccm C2H4 gas flow at a temperature of 620 °C during 220 min at 2·10−3 mbar and a microwave power of 280 W.

2.11. Electrochemical characterization and performances of supercapacitor devices The electrochemical characterization of a single electrode was evaluated by cyclic voltammetry (CV) curves in a 3-electrode configuration using a non-aqueous Ag/Ag+ reference electrode and Pt foil as a counter electrode. A pure ionic liquid solution or an organic solution (PC containing 0.1 M LiClO4 or 0.1 M TBABF4) were employed as electrolytes. The areal capacitance (AC) was calculated using the following equation: AC = Q/(ΔVA), where Q is the average voltammetric charge, which is determined by integrating either the oxidative and reduction scans of the corresponding CV curve, ΔV is the potential range and A is the geometric surface of the electrode (0.4 cm2). Regarding the supercapacitor devices, CV, galvanostatic charge-discharge (GCD) cycles and electrochemical impedance spectroscopy (EIS) were performed using a multichannel VMP3 potentiostat/galvanostat with Ec-Lab software (Biologic, France). The AC values of the device were calculated from the charge-discharge curves using the following equation AC = i/A(dV/dt), where i is the discharge current, A is the

2.8. Electrochemical deposition of ECPs on VOGNs/Si substrate (CEAGrenoble) PEDOT was deposited electrochemically on VOGNs from an acetonitrile solution containing 0.01 M EDOT and 0.1 M lithium perchlorate. The polymerization was conducted in a three-electrode cell configuration according to Fig. 2. More specifically, VOGNs was employed as the working electrode, platinum wire as counter electrode 134

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could be achieved. Thereby, in order to highlight the synergistic effect of the polymer coating on Si nanostructures, optimal electrochemical recipes (Section 2.3) were established by using nanometric coatings ranging from 80 to 350 nm. This tendency was found similar in the case of higher active surface Si nanostructures (e.g. SiNTrs) [90]. In this work, the electrochemical performance of ECP-coated Si nanostructures was devoted to the particular example of PPy-coated SiNTrs. Fig. 7a shows clearly the pseudocapacitive effect induced by the polymer coating through the faradaic reactions associated to its oxidation-reduction processes, which led to a clear enhancement of the charge storage compared to bare SiNTrs. This effect was also confirmed in a supercapacitor device as displayed in Fig. 7b. Regarding the capacitive properties, the GCD cycles displayed in Fig. 7c reflected a slight distortion compared to the GCD cycles of bare SiNWs (Fig. 5c), which was attributed to the pseudocapacitive nature of ECPs. Thus, an AC value of 14 m F cm−2 at a current density of 1 mA cm-2 was obtained, whereas an AC of approximately 0.1 m F cm−2 was displayed in the case of bare SiNTrs [101]. This result highlighted the enormous potential of ECPs to enhance the AC and E of supercapacitor devices. Accordingly, Fig. 5d illustrates the Ragone plot of both PPy-coated SiNTrs and PPy-coated SiNWs where again the effect of a higher nanostructuration (SiNTrs) combined with the use of ECPs resulted in a clear improvement of the energy density. Nevertheless, an important loss of electrochemical stability (thousands of GCD cycles with a capacitance retention of 80%) was attributed probably to the swellingshrinkage of the polymer during the charge-discharge processes. In order to compare the electrochemical performances of the three different ECPs (PEDOT, PPy and PAni) a comparative study is provided in Table 1. As can be seen, PAni exhibited the best electrochemical performances in terms of specific capacitance, E and P densities as well as a good cycling stability. Therefore, PAni-coated SiNTrs showed the best platform nanostructure to be employed as supercapacitive electrode in the field of supercapacitors. In this direction, Table 2 provides a complete study concerning the state-of-the-art of SiNW-based micro-supercapacitors and their derivatives (e.g. SiNWs functionalized by carbon, transition metal oxides and ECPs) in terms of capacitance, energy, power and cycling stability. Accordingly, ECP-coated SiNWs/ SiNTrs exhibited better AC (8–12 m F cm-2) and E (9 −15 mJ cm-2) values than bare SiNWs (from 0.010 to 0.050 m F cm−2 and from 0.03 to 0.3 mJ cm-2 respectively), however the lifetime and power density of ECP-coated SiNW devices resulted lower than bare SiNW devices due to its working principle. Another important characteristic between both devices relied on their electrochemical windows. Thus, ECP-coated SiNW microsupercapacitors displayed working cell voltages of approximately 2 V, whereas SiNWs microsupercapacitors were able to operate at larger cell voltages (4 V) using IL electrolytes.

area of the electrode and dV/dt corresponds to the slope of discharging curve. The energy density (E) and power density (P) values were calculated by using E = 0.5AC(ΔV)2 and P = E/t, where t is the total time of discharge. Coulombic efficiency (η) was evaluated as the ratio between the discharging and charging time (η = td /tc). In some particular cases in this study, the capacitive properties were expressed in terms of active mass of the electrode (m) instead of A employing the same equations described previously. Thus, the mass was estimated by subtracting the difference before and after electrodeposition on the corresponding nanostructure using a METTLER Toledo balance (precision of 0.01 mg). All electro-polymerization tests and electrochemical characterization of single electrodes were carried out using an Autolab PGSTAT302 potentiostat/galvanostat equipped with a FRA2 module in an argonfilled glove box with oxygen and water levels less than 1 ppm at room temperature. 3. Results and discussion 3.1. Electrochemical performances and characterization of the resulting hybrid electrodes (ECP-coated SiNWs/SiNTrs) Fig. 5a shows the morphology of SiNWs and SiNTrs grown by CVD according to the experimental conditions described in section 2.2, which were afterwards employed as electrodes for their corresponding surface functionalization by ECP and diamond coatings respectively. The electrochemical characterization of such nanostructures was evaluated by EIS, CV and GCD cycles. More specifically, a particular example regarding the electrochemical performances of symmetric SiNWbased microsupercapacitors in presence of N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide is displayed in Fig. 5b–d. The main characteristics are described as follow: a) Excellent EDLC behaviour within a large cell voltage of 4 V. This feature was confirmed by EIS (vertical line at medium-low frequencies) and GCD cycles (symmetric and linear profiles), which implies a pure adsorption-deadsorption of ions during the charge and discharge processes at the electrode/electrolyte interface (Fig. 5b and c). b) Areal capacitance (AC) values ranging from 25 to 80 μF cm−2 leading to energy and power densities ranging from 0.1 to 0.3 mJ cm−2 and from 0.1 to 6 mW cm−2 respectively. c) Extraordinary cycling stability after millions of GCD cycles keeping a pure EDCL behaviour. Again, quasi-ideal rectangular shape cyclic voltammograms evidenced the pure EDLC capacitive nature at high scan rates (> 10 Vs−1) as well as an excellent coulombic efficiency (∼99%) as illustrated in Fig. 5d.

3.2. Electrochemical performances and characterization of the resulting hybrid electrodes (ECP-coated D@SiNWs and ECP-coated DiNWs)

It is worth noting that this behaviour was generalized by using other different aprotic (e.g. N1114 TFSI, EMIM TFSI or PYR13 TFSI) [100–102] or protic (NEt3H TFSI) [103] ILs among others. Regarding the nanostructure of SiNTrs, the deposition of branches on SiNWs (Fig. 5a) allowed to increase the active surface and as a consequence the capacitive properties in terms of AC and E, keeping the same electrochemical behaviour described for SiNWs (Fig. 5e). Interestingly, the use of mixtures based on ILs and organic solvents (e.g. ACN solution containing 0.1 M TBABF4) allowed to overcome the main drawbacks of ILs concerning their low conductivity and high viscosity and hence their enhancement of capacitive properties, especially for energy and power density [104–106]. These strategies resulted a key factor to accomplish the further challenges of the NEST project. Fig. 6 shows the morphology of SiNWs functionalized by PEDOT (Fig. 6a), PPy (Fig. 6b), PAni (Fig. 6c) and PPy-coated SiNTrs (Fig. 6d). As can be seen, uniform, adherent and homogeneous polymer coatings were obtained in all cases. It is worth noting that by controlling the polymerization charge different nanometric and micrometric thickness

Diamond, as a robust supercapacitive carbonaceous material, exhibits unique properties related to its chemical inertness, high overvoltage, wide electrochemical window, good environmental stability and easiness to tune different nanostructures, which make him an excellent candidate to be employed in the field of micro-supercapacitors, especially for aqueous supercapacitor devices [107,108]. For that purpose, in the NEST project, diamond was chosen as a promising EDLC material to enhance the capacitive properties of SiNW microsupercapacitors and develop new 3D nanostructures (e.g. diamond nanowires) in this domain. Regarding the first approach, the deposition of diamond onto Si nanostructures could be obtained only onto SiNWs since in the case of SiNTrs a non-uniform diamond coating was achieved. Thus, an illustrative example was conducted for the system based on PEDOT-coated D@SiNWs. Fig. 8a shows the morphology of the deposition of PEDOT onto diamond-coated SiNWs. As can be seen, the surface characteristics were clearly identified for both systems. 135

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Fig. 5. a) SEM image of cross-sectional view of SiNWs. Inset shows the morphology of SiNTrs. Scale bar: 1 μm. Example of the electrochemical performance of a symmetric SiNW-based micro-supercapacitor using N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide at a wide cell voltage of 4 V. b) Nyquist plot over a frequency range from 400 kHz to 10 mHz at open circuit potential. c) GCD cycles at a current density of 1 mA cm−2. d) Lifetime testing using 3·106 complete GCD cycles at a current density of 1 mA cm−2. Inset shows the CV curves recorded at a scan rate of 20 Vs-1 after several cycling conditions, 500 000 GCD cycles (black line), 2·106 GCD cycles (red line) and 3·106 GCD cycles (green line) respectively, and e) Ragone plot (P vs E) of SiNWs and SiNTrs supercapacitor devices in presence of the ionic liquid and its corresponding mixture with ACN (50% w.t). Comparison was established using identical morphological characteristics for the trunks (SiNWs).

showed equally the distortion of the linearity and symmetry of the profiles due to the pseudocacapacitive effect of the polymer, which led to AC values ranging from 10 m F cm-2 at 0.1 mA cm-2 to 8 m F cm-2 at 2 mA cm-2 (Fig. 8c). A study of the capacitive properties of PEDOTcoated D@SiNWs in terms of E and P (Ragone plot) was compared to other systems reported in this study (e.g. PEDOT-coated SiNWs, PAnicoated SiNWs, PPy-coated SiNTrs/SiNWs). Thus, Fig. 8d reflected the

Next, the potential of this hierarchized nanostructure was evaluated in a symmetric device using butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1114 TFSI) as electrolyte. Firstly, the potential of the electrode was evaluated in a 3-electrode cell using the CV technique according to the Fig. 8b. The cyclic voltammogram shows the pseudocapacitance effect of PEDOT at a scan rate of 10 mVs−1, as evidenced also in the case of PPy-coated SiNTrs (Fig. 7a). GCD cycles 136

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Fig. 6. SEM images of a) PEDOT-coated SiNWs, b) PPy-coated SiNWs, c) PAni-coated SiNWs and d) PPy-coated SiNTrs. Insets show an individual ECP coated SiNW and the functionalization of the branch of PPy-coated SiNWs respectively. Scale bar: 200 nm.

Fig. 7. a) CV curves of SiNW (blue line) and PPy-coated SiNTr (purple line) single electrodes using a scan rate of 10 mVs−1. Initial and final potential: -1.5 V; reversal potential: 0.5 V (vs Ag/Ag+). Example of the electrochemical performance of PPy-coated SiNTrs based micro-supercapacitors using PYR13 TFSI electrolyte at a cell voltage of 1.5 V.b) CV curve of a full device at a scan rate of 5 mVs−1. c) GCD cycles at a current density of 0.25 mA cm-2 and d) Ragone plot of PPy-coated SiNWs (blue triangle) and PPy-coated SiNTrs (orange square) devices at current densities ranging from 0.1 to 1 mA cm-2.

electrochemical tendencies. In this direction, the deposition of ECPs on diamond-coated SiNWs, according to the strategy followed in the previous section using SiNWs and SiNTrs, might serve as an interesting platform to boost the aforementioned capacitive properties. According to the Table 2, the results presented in this configuration were considered as one of the best electrochemical performances for CVD-grown SiNW micro-supercapacitors.

synergistic effect of diamond-coated SiNWs and ECPs to enhance the capacitive properties improving its cycling stability. From this point of view, a capacitance retention of approximately 80% was obtained after 10 000 GCD cycles at a current density of 1 mA cm-2 preserving the structural and mechanical properties of the electrodes as illustrated in Fig. 8e. According to the state-of-the-art, other carbonaceous structures deposited on SiNWs, such as porous carbon, reflected similar 137

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as it was previously evidenced in the framework of the NEST project for SiNWs. In the following section, a detailed description of ECPs deposited on VOGNs and its potential as pseudocapacitive hybrid material will be discussed.

Table 1 Comparison of electrochemical performances dealing with ECP-coated Si nanostructures (e.g. SiNWs) in the NEST project. Systema

Spec. Cap. (F g−1)

E (Wh kg−1)

P (kW kg−1)

Cycling Stabilityb

3500 (20) 3000 (15) 4000 (16)

PEDOT-coated SiNWs PPy-coated SiNWs

32

10

3.3

47

15

5.1

PAni-coated SiNWs

74

21

9.2

Electrolyte

3.3. Electrochemical performances and characterization of the resulting hybrid electrodes (ECP-coated VOGNs)

PYR13 TFSI

The resulting morphology of VOGNs was only investigated in this study by SEM, although other morphological, structural and physicalchemical characterization techniques such as TEM, Raman, XPS, XRD and 4-probe conductivity were already reported to characterize such nanostructure [97,98]. Fig. 10a depicts the SEM image of VOGNs directly grown on highly doped Si substrates. A particular morphology based on the perpendicular growth to the substrate is observed, which results very interesting for supercapacitor applications since the gap between the nanosheets ensure a good accessibility of the electrolyte over the whole thickness (∼ 1 μm). In addition, its high surface active, porosity and conductivity allowed to obtain very promising capacitive properties. Accordingly, we reported interesting electrochemical performances at a wide operating cell voltage of 4 V using N1114 TFSI electrolyte with an excellent EDLC behaviour as well as a high AC value of ∼ 2 m F cm−2 combined with a high volumetric power density (250 W cm-3) and an outstanding cycling stability (e.g. capacitance retention of 80% after 300 000 GCD cycles at a current density of 4 mA cm−2) preserving entirely the morphological structural even after such cycling conditions [97,98]. Fig. 10b shows successfully the electrochemical deposition of a 50 nm PEDOT coating on VOGNs, which was confirmed by EDX (Inset Fig. 10b). The electrochemical performance of PEDOT-coated VOGNs was evaluated by CV curves in a potential range from -0.4 to 1.2 V vs Ag/Ag+ using a 3-electrode cell configuration (data not shown). An AC value of 6 m F cm-2 and 0.5 m F cm−2 were obtained by PEDOT-coated VOGNs and PEDOT-coated Si substrates (thick film of 500 nm) respectively. Based on these preliminary results, VOGNs can pave the way to explore other nanostructured carbonaceous materials, apart from diamond, in the field of high performance supercapacitors. A more detailed investigation of this study is nowadays under investigation.

PYR13 TFSI DEMA OTF

SiNWs with a length of 20 μm and a diameter of 50 nm. Cycling stability measured by applying GCD cycles at a current density of 1 mA cm−2. Values between parentheses refer to the loss of capacitance retention (%). a

b

The second approach of this section concerns the system of DiNWs and its corresponding functionalization by PEDOT. Fig. 9a shows the morphology of DiNWs grown by MWCVD whose properties were characterized by a density of 4·1010 NWs cm−2 with a length of 240 nm and a diameter of 20 ± 2 nm respectively [93]. After the corresponding electropolymerization of PEDOT on DiNWs, a nanometric PEDOT coating was deposited along all DiNWs as observed in Fig. 9b. Subsequently, the capacitive properties of PEDOT-coated DiNWs and DiNWs were evaluated in a 3-electrode cell configuration using a triethylammonium bis(trifluoromethylsulfonyl)imide (Et3NH TFSI) solution as electrolyte. Fig. 9c shows a cyclic voltammogram corresponding to the faradaic process associated to the oxidation-reduction reactions of PEDOT on DiNWs. Thus, two clear peaks at 0.27 V vs Ag/Ag+ (oxidation) and −0.42 V vs Ag/Ag+ (reduction) were recorded at a scan rate of 5 mVs-1, leading to an areal capacitance value of 20 m F cm2 . The potential of this hetero-nanostructure compared to bare DiNWs was analyzed by using a CV curve recorded at a scan rate of 100 mVs-1 according to the Fig. 9d. The plot shows an evident improvement of the charge storage regarding PEDOT-coated DiNWs, which displayed an AC value of 3.3 m F cm-2 whereas DiNWs exhibited only a value of 83 μF cm-2. Again, the pseudocapacitive nature induced by PEDOT on DiNWs allowed to enhance the capacitive properties of DiNWs as analyzed for ECP-coated SiNWs or D@SiNWs in the previous sections. Concerning the electrochemical stability of PEDOT-coated DiNWs, a capacitance retention of approximately 70% was estimated after applying 10 000 successive CV curves at a scan rate of 1 Vs-1. This value was found to be lower than DiNWs (e.g. loss of capacitance of approximately 12% after 150 000 cycles under the same experimental conditions reported for PEDOT-coated DiNWs). Consequently, these results evidence one more time the excellent potential of ECPs onto nanostructures to be employed as high performance supercapacitive electrodes. In this direction, it is also important to highlight the excellent electrochemical performances of bare DiNWs compared to SiNWs (Table 2) as well as a good capacitive behaviour. This analysis is presently object of a study (data not published) to confirm the emergence and novelty of DiNWs as a very attractive EDLC nanoarchitecture in terms of lifetime and wide operating temperature range for supercapacitor applications. In parallel to the NEST project, other research activities dealing with the development of new high performance micro-supercapacitors based on carbonaceous nanostructures emerged at INAC institute several years ago. Among them, vertically oriented graphene nanosheets (VOGNs) and carbon nanotubes (CNTs) demonstrated their enormous potential as attractive material architectures in the field of EDLC microsupercapacitors owing to their peculiar properties regarding its high conductivity, ion access facility and open structure with high surface area [109–112]. Our original approaches concerning VOGNs, compared to the state-of-the-art, were established according to a new synthesis procedure focused on a derivative CVD technique, known as electroncyclotron resonance (ECR-CVD), and the use of ionic liquid electrolytes,

4. Conclusions and perspectives The rapid and growing demand of sophisticated miniaturized portable devices in a wide range of technological applications, from medicine to aerospatial industry, has recently triggered a great revolution in the field of energy storage. In this direction, the need to find reliable solutions to satisfy their severe technical requirements in terms of high capacitance and energy density as well as maintaining an excellent lifetime will become crucial in the next years. Among the different possibilities reported in the literature, micro-supercapacitors have emerged as promising energy storage units able to provide such challenging responses. During the NEST project, we have opened up new perspectives in this domain by means of the employment of IL electrolytes, the investigation of functionalization concepts based on electroactive materials and the research of high performance advanced hierarchized nanoarchitectures, which provided new insights especially in the field of Si-based micro-supercapacitors. Thus, electrodes based on pseudocapacitive coatings deposited on carbonaceous silicon nanostructures displayed excellent capacitive properties. More specifically, ECPs-coated D@Si nanostructures exhibited the best capacitive properties using aprotic IL electrolytes. At the same time, the design of other nanostructures such as DiNWs or VOGNs and their promising potentials in the domain of supercapacitors, inspired by the scientific strategies of NEST project, has opened a new dimension in this domain to be explored deeply. Regarding the main object of this study, tremendous efforts are being conducted to address the main technical challenges of Si-based supercapacitors. Among them, a special attention is presently 138

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Table 2 Summary of main electrochemical performances dealing with SiNWs and functionalized SiNWs based planar micro-supercapacitors reported in the literature. AC, E, P values were obtained according to the electrochemical method, unless specified otherwise, as a function of the geometrical surface of the electrode. Only cited works refereed in terms of surface have been reported for a better comprehension and comparison. System

Method Si growth

AC (mF cm-2)

E (mJ cm-2)

P (mW cm-2)

Cycling Stability

Electrochemical Method

Electrolyte

Ref.

PEDOT/D@SiNWs

CVD

8.5

26

1.3

15 000 (80%)

GCD (2.5 V 1 mA cm-2)

N1114 TFSI

[94]

PEDOT/SiNWs

CVD

8

9

0.8

3500 (80%)

GCD (1.5 V 1mA cm-2)

PYR13 TFSI

[89]

PPy/SiNWs

CVD

10

11

0.8

3000 (85%)

GCD (1.5 V 1 mA cm-2)

PYR13 TFSI

[90]

PPy/SiNTrs

CVD

14

15

0.8

10000 (70%)

GCD (1.5 V 1 mA cm-2)

PYR13 TFSI

[90]

MnO2/SiNWs

CVD

13 GCD (2.2V) 0.4 mA cm-2

32 GCD (2.2V) 0.4 mA cm-2

0.4 GCD (2.2V) 0.4 mA cm-2

5 000 (91%)

GCD (2.2 V 1mA cm-2)

PYR13 TFSI/ LiClO4

[113]

Diamond/SiNWs

CVD

1.5

11

25

1·106 (65%)

GCD (4 V 10 mA cm-2)

Et3NH TFSI

[92]

Diamond/SiNWs

CVD

0.4

1.8

50

2·106 (70%)

GCD (3 V 10 mA cm-2)

H2O (0.1 M LiClO4)

[114]

C/SiNWs

Etching

75

100

1

5000 (80%)

GCD (2.7 V 3 mA cm-2)

EMIM TFSI

[115]

C/SiNWs

Etching

25.6 GCD (0.7V) 0.1 mA cm-2

6.3 GCD (0.7V) 0.1 mA cm-2

0.04 GCD (0.7 V) 0.1 mA cm-2

25000 (75%)

GCD (0.7 V 1 mA cm-2)

1M Na2SO4

[116]

SiC/SiNWs

Etching

1.7 CV (0.8V) 50 mVs-1

0.850 CV (0.8V) 50 mVs-1

n.d

1000 (95%)

CV (0.8 V 50 mVs-1)

1M KCl

[117]

TiN/SiNWs

Etching

2.6

5.2

1.2

n.d

GCD (2 V 1mA)

0.5 M TEABF4/PC

[118]

Graphene/SiNWs

Etching

n.d.

3.6

1

n.d

GCD (2.7 V 1.5 A/g)

EMIMBF4

[119]

rGO/SiNWs

HWCVP

0.24

0.20

n.d

1000 (99%)

GCD (1.3V 10 μA)

EMIM TFSI

[120]

SiNTrs

CVD

0.058

0.26

n.d

1·106 (84%)

GCD (3 V 1 mA cm-2)

EMIM TFSI PC (1 M)

[88]

SiNTrs

CVD (Electroless)

0.35

2.5

5

1·106 (80%)

GCD (4 V 1 mA cm-2)

EMIM TFSI

[101]

SiNTrs

CVD

0.37 (EIS; 1Hz)

0.8

1

n.d

GCD (3V 1 mA cm-2)

EMIM TFSI/ AMIM TFSI

[121]

SiNWs

HWCVP

0.13

0.001 GCD (0.5V) 0.4 mA cm-2)

0.1 GCD (0.5V) 0.4 mA cm-2)

1000 (98%)

CV (0.5V) (100 mVs-1)

EMIM TFSI

[122]

SiNWs

CVD

0.023

0.19

2

8·106 (75%)

GCD (4 V 1 mA cm-2)

PYR13 TFSI

[102]

SiNWs

CVD

0.007 0.051

0.003 0.037

n.d

2000 (98%)

GCD (1 V 10 μA cm-2)

1M NEt4BF4/PC

[123]

small redox molecules or new electroactive p-and n doped polymers is of vital importance to overcome this drawback. In addition, a better understanding of the electrochemical performances by modulating the working temperature at extreme operating ranges (-80 °C to 80 °C) through of advanced polymeric electrolytes (e.g. biredox ionic liquids) is necessary. Finally, the real development and integration of high

focused on the comprehension of native silicon oxide layer formed on the surface of SiNWs, which plays a key role at the electrode-electrolyte interface, by using for example advanced in-situ techniques (e.g. acelectrogravimetry). Another important aspect related to the poor electrochemical stability of functionalized SiNWs by using organic materials should be addressed properly. The rational design of innovative 139

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Fig. 8. a) SEM image corresponding to the electrochemical deposition of PEDOT on diamond-coated SiNWs. b) CV curve of a PEDOT-coated D@SiNW single electrode using a scan rate of 10 mVs−1. Initial and final potential: -1.75 V; reversal potential: 0.25 V (vs Ag/Ag+). Example of the electrochemical performance of a symmetric PEDOT-coated D@SiNW based micro-supercapacitor using butyltrimethylammonium bis(trifluoromethylsulfonyl)imide at a cell voltage of 2.5 V. c) GCD cycles at different current densities (0.1, 0.25, 0.50, 0.75, 1 and 2 mA cm-2 respectively). d) Comparative of Ragone plots dealing with ECP-coated SiNW/SiNTr and PEDOT-coated D@SiNWs at current densities ranging from 0.1 to 1 mA cm-2. e) Lifetime testing using 10 000 complete GCD cycles at a current density of 1 mA cm-2. Inset shows a SEM image of PEDOT-coated D@SiNWs after the cycling test.

authors are gratefully acknowledged to M. Delaunay, P. Gentile and D. Gaboriau for the synthesis of VOGNs and Si nanostructures (SiNWs and SiNTrs) respectively. The authors would also like to thank to Mr. Buhagiar for the results concerning the deposition of PEDOT on VOGNs. In this direction, J. Wimberg, B. Iliev and T. Schubert from IOLITEC (Ionic Liquids Technologies GmbH, Germany) are indebted for providing us the ionic liquids. F. Gao, G. Lewes-Malandrakis and W. Müller-Sebert from FHG institute (Freiburg, Germany) deserve also a special gratitude for the elaboration of diamond samples. Finally, the French Agence Nationale de la Recherche (ANR), the Direction General de l’Armement (DGA) and the French Alternative Energies and Atomic Energy Commission (CEA) are indebted for financial support. This work has been performed with the use of the Hybriden facility at CEAGrenoble (France).

performance on-chip supercapacitors based on SiNWs for technological applications need a deep comprehension in terms of miniaturization, encapsulation, surface functionalization by precision controlled-surface technique (e.g. deposition of conformal oxides and nitrides coatings by atomic layer deposition) and other electrochemical characteristics (selfdischarge, leakage current among others). In that way, the 3D microsupercapacitors based on the hierarchical electrodes could be connected together in series and/or parallel combinations to form a matrix of micro-supercapacitors with a high operating voltage. Acknowledgments This project has received funding from the European Union’s Seventh Program for Research, Technological Development and Demonstration under Grant agreement no. 309143 (2012 – 2016). The 140

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Fig. 9. SEM image of a) DiNWs and b) PEDOT-coated DiNWs. c) CV curve of PEDOT-coated DiNWs at a scan rate of 5 mV s−1 using a 3-electrode cell configuration and d) Comparative CV curves of PEDOT-coated DiNWs and DiNWs at a scan rate of 1 Vs−1.

Fig. 10. SEM image of a) VOGNs grown by ECR-CVD on silicon substrates and b) PEDOT-coated VOGNs. Inset shows the corresponding EDX analysis regarding the deposition of PEDOT.

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