Super-activated biochar from poultry litter for high-performance supercapacitors

Microporous and Mesoporous Materials 285 (2019) 161–169

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Super-activated biochar from poultry litter for high-performance supercapacitors

T

Daniele Pontirolia,∗, Silvio Scaravonatia, Giacomo Magnania, Laura Fornasinia, Danilo Bersania, Giovanni Bertonib,c, Chiara Milanesed, Alessandro Girellad, Francesca Ridie, Roberto Verucchif, Luciana Mantovanig, Alessio Malcevschig, Mauro Riccòa a

Nanocarbon Laboratory, Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Università di Parma, Parco Area delle Scienze, 7/a, I-43124, Parma, Italy CNR - Istituto Nanoscienze, Via Campi 213/A, I-41125, Modena, Italy c IMEM – CNR, Institute of Materials for Electronics and Magnetism, Parco Area delle Scienze 37/A, I-43124, Parma, Italy d Pavia Hydrogen Lab, C.S.G.I. & Dipartimento di Chimica, Sezione di Chimica Fisica, Università di Pavia, Viale Taramelli 16, I-27100, Pavia, Italy e Dipartimento di Chimica “Ugo Schiff” & C. S. G. I., Università di Firenze, Via della Lastruccia, 3, I-50019, Sesto Fiorentino (FI), Italy f IMEM – CNR, Institute of Materials for Electronics and Magnetism, Trento Unit, Via alla Cascata 56/C, 38123, Povo (Tn), Italy g Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università di Parma, Parco Area delle Scienze, 11/a and 157/a, I-43124, Parma, Italy b

A R T I C LE I N FO

A B S T R A C T

Keywords: Biochar Porous carbon Chemical activation Supercapacitor Graphene

We report on the preparation of a novel hierarchically-porous super-activated carbon originating from organic waste with specific surface area exceeding 3000 m2/g, obtained starting from biochar derived by the pyrolysis of poultry litter. The chemical activation process proved to be efficient to remove the majority of impurities other than carbon, stabilizing a highly porous hierarchical structure with local graphene-like morphology. The presence of P and S with concentration below 0.1 wt% distinguishes this activated carbon from the usual ones obtained from vegetal sources. Thanks to these features, the obtained porous compound demonstrated to behave as an excellent electrode material for high-performance symmetric supercapacitors, reaching high specific capacitance up to 229 (13) F/g. Remarkably, the devices also supply high current density of 10 A/g without using any conducting additives and display high power density and reliability. Moreover, these optimal performances have been obtained operating by using simple eco-friendly electrolytes, like KOH and Na2SO4 aqueous solutions. The availability, the biocompatibility and the inexpensiveness of the starting materials, together with the low environmental impact of the electrolyte, suggest possible large-scale applications for such devices, for example in the field of transportation or in renewable energy-grids, but also in the field of bio-medicine.

1. Introduction Supercapacitors (SCs) are rather promising devices for energy conversion and storage, capable to bridge the gap between conventional capacitors and rechargeable batteries. The most important energy storage mechanism in SCs arises from the reversible electrostatic charge accumulation at the surface of highly porous electrodes. Electrical double-layer capacitors (EDLCs) exploit the very large specific surface area (SSA) of polarizable porous electrodes (typically 1000–2000 m2/g) and the very short distance between charges at the electrode/electrolyte interface (of the order of few nanometers) to bring the resulting capacitance to values much higher than flat plate capacitors. With respect to secondary batteries, SCs show much longer cycle life (> 100000 cycles) and higher power densities (> 1 kW/kg), although

∗

energy density values are still lower (of the order of few Wh/kg) [1]. The ideal material used as porous electrode in SCs is carbon, thanks to its relative low cost, good electrical conductivity, high polarizability, very good mechanical strength, large surface area and availability in a rich variety of dimensional forms [2]. An important aspect of the design of high performance devices is the control of the porosity of carbon electrodes at the nanoscale, since, to increase the electrode/electrolyte interface, pores size should match the size of solvated electrolyte [3]. A hierarchical mesoporous and microporous structure is highly desirable to guarantee high power density and good reversibility in SCs [4]. So far, large efforts have been spent in research on innovative carbon nanomaterials, such as graphene, carbon nanotubes, or carbon onions [5], but cost is still the main barrier for mass adoption of these advanced carbon nanostructures.

Corresponding author. E-mail address: [email protected] (D. Pontiroli).

https://doi.org/10.1016/j.micromeso.2019.05.002 Received 5 February 2019; Received in revised form 3 April 2019; Accepted 2 May 2019 Available online 06 May 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.

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wood shavings and saw dust, bird excreta, feather and feed spills. Poultry litter-derived biochar (PLB) was produced in a pilot plant gasifier for the syngas production at the nominal temperature of 550–570 °C for 45–60 min. Potassium hydroxide in form of pellets (reagent grade) was purchased by Carlo Erba. Hydrochloric acid (36–38%, reagent grade) was provided by Sigma Aldrich; glass microfiber filters were provided by Whatman; sodium sulphate (Na2SO4, 99.5% purity) was provided by Prolabo; ethanol (99.8% purity) was provided by Fluka; Ni foam (0.5 mm thick, 100 ppi) was provided by TMAX.

Recently, highly porous activated carbons obtained from biochar, namely the carbon side-product in the pyrolysis/gasification of residual waste biomasses, started to receive a widespread attention in the field of the electrical energy storage [6–15]. This thanks to its hierarchical porous structure inherited from biomass precursors, its excellent chemical and electrochemical stability, high conductivity, high surface area and inexpensiveness [16–21]. So far, biochar produced in thermochemical conversion power plants has been mainly used as a soil amendment [22]. However, biochar converted to activated carbon (SSA > 1000 m2/g) through a chemical activation appears now to be a new potential cost-effective and environmentally-friendly carbon material with great application prospect in many fields [23]. Moreover, recent studies highlighted that the carbonization routes of biomasses and waste materials, also combined with KOH activation, can promote the graphitization and even the production of porous graphene-based nanostructures, thus significantly advancing the properties of the starting materials [24,25]. For example, porous graphene-like nanosheets with a large surface area have been efficiently synthesized from coconut shells [26] and large few-layered graphene has been obtained from carbonization and mechanical exfoliation of peanut shells [27]. Such advanced materials, obtained from natural and renewable sources, have been successfully exploited to develop devices with scalable and sustainable techniques, with a much lower environmental impact, as compared with conventional chemical processes required to produce graphene-based materials [28,29]. Moreover, the intrinsic biocompatibility of functionalized carbon, obtained from natural waste materials, suggests the opportunity to realize fully-biocompatible systems; for example, in the case of SCs, biochar-based electrodes can be coupled with environment-friendly electrolytes to realize low-cost devices for applications in the field of wearable, implantable or even “edible” electronics [30]. Finally, the interest of using biochar and other waste materials as precursors for porous electrodes in supercapacitors arises also from the ability to incorporate heteroatoms in the carbon framework, such as N, S, P and O, naturally present in the biomasses, during the activation process, which were found to increase the performances of devices [31–35]. In this manuscript, we report the production of a novel hierarchically porous super-activated carbon with SSA exceeding 3000 m2/ g starting from the biochar generated by the pyrolysis of poultry litter, which is the solid waste resulting from chicken rearing. The chemical activation, performed with KOH and subsequent HCl washing, proved to be efficient to remove all impurities other than carbon present in the raw material and to enhance the fraction of sp2 carbon, which locally organizes to form few-layered graphene-like nanosheets structures. The presence of graphene signature seems to be peculiar of activated carbon from poultry litter, since porous carbons produced with the same synthesis protocol starting from biochar of different origin (i.e. wood) did not show similar features. The very large SSA, together with the optimal hierarchical mesoporous and microporous structure of this material, allowed to reach remarkable performances in SCs operating either with KOH, or with Na2SO4 environmental-friendly electrolyte, which delivered specific capacitances of up to 229 (13) F/g. The maximum specific energy and power density of active materials is respectively 5.1 (3) Wh/kg and 74 (10) kW/kg (KOH based electrolyte), and 2.05 (8) Wh/kg and 22 (1) kW/kg (Na2SO4 based electrolyte). To our knowledge, this is the first extensive study of activated biochar of poultry litter origin which has been investigated for applications in supercapacitors.

2.2. Synthesis of super-activated carbon from biochar Activation of PLB (A-PLB) was performed at high temperature in presence of KOH and inert atmosphere, following a facile and scalable synthesis route. Specifically, about 1 g of PLB was finely ground in an agate mortar and mixed with about 2 g of KOH. The reagents were then placed in an alumina crucible, hence inserted in a quartz vial. The heat treatment was performed in Argon (Ar) flux; first, the temperature was brought at 200 °C with a thermal ramp of 5 °C/min and let at this temperature for 1 h. Then, the temperature was increased up to 800 °C with a thermal ramp of 5 °C/min and with a dwelling time of 1 h. The sample was then cooled at room temperature in Ar flux. The product was then washed with HCl solution to reach pH 7 and dried in oven at 60 °C overnight. Typically, the final weight of the activated product was about 1/4 of the initial amount of PLB. 2.3. Fabrication of supercapacitors The active materials have been dispersed in a H2O Millipore/ ethanol solution (1:1 in volume) and the suspension has been stirred until it became homogeneous. 7 mm diameter Nickel foam disks were punched and activated by 30 min sonication in a 0.1 M solution of HCl in H2O Millipore. The slurry was then drop cast on Nickel foam disks. Electrodes were then dried in a vacuum oven at 60 °C and pressed at a pressure of 50 bar using a flat-plate press. Symmetric SCs were assembled in standard coin cells (CR-2032) using glass microfiber separator soaked in 7 M KOH or 1 M Na2SO4 aqueous electrolyte respectively. 2.4. Characterization techniques Powder x-ray diffraction (PXRD) investigation was performed using a Bruker D2 PHASER diffractometer, equipped with a LYNXEYE detector (based on silicon strip technology) and CuKα radiation (λ = 1.54178 Å). Diffraction measurements were taken in steps of 0.02° over 2θ range from 10 to 60°, with a counting rate of 2 s per step. Diffraction patterns were interpreted using the Bruker software EVA and the Crystallography Open Database for comparison. Raman spectroscopy was performed using a Jobin Yvon LabRam micro-spectrometer (300-mm focal length), equipped with an Olympus BX40 confocal microscope, in a backscattering geometry. A continuouswave single frequency Nd:YAG diode pumped laser at 473.1 nm was used for excitation, with a spectral resolution of about 3.0 cm−1. The backscattered Raman light was dispersed by a 1800 grooves/mm holographic grating onto a Peltier cooled CCD (1024 × 256 pixels). The laser power was adjusted by density filters to avoid heating effects on the sample. Spectra were collected using the x50 microscope objective in the range 100–2000 cm−1. The system was calibrated with the 520.6 cm−1 Raman band of silicon. Acquisitions were recorded for 60 s with 3 repetitions for each measurement. The data analysis was performed by LabSpec 5 built-in software. A third degree polynomial curve was subtracted as baseline to remove the fluorescence background. Biochar contributions in the spectra were deconvoluted with four Gaussian-Lorentzian curves. Scanning electron microscopy (SEM) analysis was performed with a Zeiss EVO MA10-HR microscope on loose powders deposited on C tapes

2. Experimental section 2.1. Materials and chemicals Biochar derived materials investigated in this work were produced starting from poultry litter, obtained from a local company, as waste product; this consists in a variable mix of bedding materials, such as 162

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stuck on aluminum stubs and sputtered with gold. High-resolution transmission electron microscopy (HRTEM) was performed with a JEM-2200FS microscope (JEOL Ltd., Japan) equipped with a Schottky gun working at 200 kV (point resolution 0.19 nm), an in-column energy filter (Ω-type), a CCD high resolution camera, STEM detectors, and an EDS detector. A small amount of sample was dispersed in isopropanol and a droplet of the suspension was deposited onto holey carbon coated nickel grids. The morphology and the pore size distribution were studied also by Scanning Transmission Microscopy (STEM), taking advantage of the different contrast achievable in this configuration. The carbon sp2/sp3 ratio in the samples was evaluated from the carbon K-edge by means of electron energy loss spectroscopy (EELS). X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) were performed in an Ultra High Vacuum (UHV) μ-metal chamber. XPS spectra have been acquired using Mg kα 1253.6 eV photon energy, while UPS has been performed by means of the He I photon at 21.22 eV. The electron energy analyser (VSW HSA100 hemispherical analyser with PSP electronics) has a total energy resolution of 0.82 eV for XPS and about 0.1 eV for UPS. The binding energy (BE) scale for XPS was calibrated by using the Au 4f 7/2 peak at 84.00 eV as a reference, while for UPS we considered the Fermi level of the same Au clean substrate. The secondary electron cut-off (SECO) spectra were measured with a sample bias of −7.0 V. XPS spectra were background subtracted using a Shirley background, line shape analysis was performed using Voigt functions. Typical uncertainty for the peak energy positioning is ± 0.05 eV, while the full width at half maximum (FWHM) and the area evaluation uncertainties are less than ± 5% and ± 2.5%, respectively. Material stoichiometry. i.e. atomic element percentages, have been evaluated using the relative sensitivity factors approach. XPS analysis has been performed only on A-PLB. Nitrogen adsorption-desorption analysis was performed at 77 K with a Coulter SA 3100. The total specific surface area was calculated with the Brunauer Emmett Teller (BET) method (verifying that the Rouquerol criteria were encountered, see plots in Fig. S4) [36], and the external specific surface area was evaluated with the t-plot analysis method, using the Statistical Thickness Surface Area (STSA) carbon black equation [37]. The pore size distribution was evaluated with the Barrett-Joyner-Halenda (BJH) method, desorption branch. The BJH pore size distribution was compared with those obtained by Non Localized Density Functional Theory (NLDFT) calculation, performed with SAIEUS software from Micromeritics. Cyclic voltammetry measurements (CV) were performed on the SCs with a two electrode configuration using a Keithley Sourcemeter 2400 at 1, 5, 10 mV/s in the applied potential difference range 0–0.8 V when using KOH-based electrolyte and in the applied potential difference range 0–0.6 V when using Na2SO4-based electrolyte. Galvanostatic charge/discharge measurements (GCD) were performed using either a Kikusui DC Voltage/Current Standard Model 101 direct current source coupled with a Keithley Multimeter 2000 connected to a PC via RS-232, or a Landt CT2001A testing system. DC conductivity as a function of temperature (T range 50–300 K) was performed with a Keithley Current Source 6221 coupled in Delta configuration with a Keithley 2182A Nanovoltmeter, whereas the sample cooling was obtained with a CCRCTI Cryodyne 22C. Electrochemical impedance spectroscopy (EIS) measurements were performed with a HP 4192A impedance analyser in the frequency range 5 Hz–20 kHz, by applying a sine wave with amplitude of 10 mV. From GCD specific capacitance CSP (F/g) was calculated from the total capacitance C of the supercapacitor:

CSP = 4

E=

2 C Umax 2 mAM 3600

where Umax (V) is the maximum electrochemical stability region. Maximum theoretical specific power P (kW/kg) that the material can supply is obtained by:

P=

2 Umax 4 ESR mAM

where ESR (Ω) is the equivalent series resistance of the supercapacitor. Effective specific power can also be calculated at different current densities, by using:

P=

E t

where t is the total time of discharge [1]. For each cycle, Coulombic efficiency has been calculated as:

ηC =

CD CC

where CD and CC are respectively the discharge and charge capacity for a given cycle. In order to take into account of possible faradaic chargestorage effects, the energy efficiency, given by the ratio between the area under the GCD curve during discharge and during charge, has been also calculated:

ηE =

t (D ) ∫t0 (fD) U (t ) dt t (C ) ∫t0 (fC ) U (t ) dt

where t0 (s) and tf (s) are the initial and final time during a galvanostatic discharge or charge, and U (V) is the voltage as a function of time [38]. 3. Results and discussion 3.1. Powder x-ray diffraction PLB and A-PLB were analyzed by means of PXRD to have a first identification of the amorphous and/or crystalline phases present in samples. PXRD pattern of the PLB shows rather large amount of crystalline phases (estimated at about 60 ± 10% throughout the BrukerEVA software). Phase identification, made by comparison with literature database ICSD, show the presence of K-chlorides, K-sulphates (K2SO4), Ca-carbonates (CaCO3), quartz (SiO2) and mixed Ca-phosphates Ca9MgNa(PO4)7. To note, despite the identified phases correspond to those show in Fig. 1, they may contain chemical impurities and cation exchanges, stressing the heterogeneity of the material for crystallinity and stoichiometry. These features are indeed expected in poultry litter biochar, which is known to present a higher ash fraction, than i.e. biochar from wood, as a consequence of the significant presence of minerals, arising also from chemical treatments performed during chicken rearing [39]. On the contrary, A-PLB does not give any diffraction peaks, thus indicating the substantially amorphous nature of the material on the macrometer scale (see Fig. 1). 3.2. Raman spectroscopy Raman spectroscopy performed on samples confirms the heterogeneity of PLB, as reported by PXRD results. Besides the presence of two broad bands in the region 1200-1600 cm−1, typical of carbon [40] (see below), some narrower peaks reveal the presence of impurities, which have been identified mainly in the form of amorphous contribution as calcium phosphate (Ca3(PO4)2) and crystalline phases as carbonates and sulphates (see Fig. 2). On the contrary, the Raman spectrum of A-PLB shows only features

C mAM

where mAM is the total mass of the active material. Specific energy E (Wh/kg) of the material in the supercapacitor is given by: 163

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Fig. 1. PXRD pattern of PLB (in black), with the identification of the main diffraction peaks, and of the completely amorphous A-PLB (in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2. Raman spectra of (a–c) PLB (in black) and (d) A-PLB with carbon features (C, in red). The wavenumbers of the identified phase are shown: (a) aCa3(PO4)2, (b) Na2CO3, (c) K2SO4. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

due to carbon. Here, the Raman signal mainly consists of the wellknown D and G bands of carbon materials, the former associated to structural defects and disorder, while the latter due to the E2g phonon mode arising from the sp2 graphitic domains, giving the contributions at 1360 and 1600 cm−1, respectively. At a closer inspection, the deconvolution of the spectrum in four bands (∼1200 cm−1, ∼1530 cm−1, D at 1360 cm−1 and G at 1600 cm−1, as exemplified in Fig. 2c and d), evidenced a slight reduction, amounting to 10%, in the 1530 cm−1 peak intensity, from the raw material to the activated one. Since that contribution can be related to the presence of amorphous carbon structures [41], the observed reduction suggests an increase of the sp2 ordered carbon fraction after the activation process. A decrease in the ID/IG intensity ratio, from 1.8 (6) to 1.5 (1) was also observed from the PLB to A-PLB, as expected when the graphitization level increases [42]. Finally, the rather high value of this ratio suggests an overall large level of disorder of the carbon fraction in these materials.

Fig. 3. Up: SEM image of PLB. The sample appears largely inhomogeneous, with the presence of porous carbon agglomerates and more regularly-shaped chips. Down: SEM image of A-PLB. The sample appears more homogeneous, with large porosity at the micrometer scale.

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Table 1 Elemental composition (in mass%) of PLB and A-PLB, estimated with EDS analysis. Errors are expressed in brackets.

PLB A-PLB

C

O

Na

Mg

Si

P

S

Cl

K

Ca

73.18 (1) 96.63 (1)

11.24 (1) 1.72 (1)

1.72 (1) 0.16 (1)

1.27 (1) –

0.27 (1) 1.30 (1)

2.91 (1) 0.02 (1)

0.86 (1) 0.04 (1)

0.43 (1) 0.13 (1)

6.06 (1) –

2.04 (1) –

data and indicating that the KOH activation process, besides removing the mineral impurities from the sample, also promotes a clear increase of the graphitic fraction, which organizes mainly in form of porous fewlayer graphene-like nanosheets. Some STEM images from the activated samples have shown the presence of SiO2 micro- and nanoaggregates, whose mass fraction was estimated not exceeding 2–3% of the overall mass of the sample, although its presence is not revealed by PXRD (as it is probably below the detection limits). We attributed those aggregates to a SiO2 fraction already present in the raw material, which has not been removed by the activation process. 3.4. XPS analysis XPS analysis performed on A-PLB revealed the presence of carbon, oxygen and silicon with atomic percentages of 88.7%, 8.5% and 2.8% respectively (see Fig. S3). No traces of other elements have been found, at least within the detection limit of about 0.1%, typical for XPS analysis. These values are in good agreement with EDS data, also considering the different depth sampled by the two techniques, i.e. few nanometers for XPS and few microns for EDS, suggesting a good homogeneity of the activated material. C 1s core level analysis shows presence of four different components (see Fig. 5a). The main one is located at 284.37 eV, representing 62.1% of the whole total C 1s area. Its asymmetric shape is typical of sp2 hybridized carbon atoms, a multilayer graphene/graphitic structure as also suggested by presence of a plasmon loss structure at 290.18 eV

Fig. 4. a) HRTEM image of raw biochar shows the presence of solid amorphous carbon at the nanometer scale, while in activated biochar samples b) there are clear evidences of a porous and layered carbon arrangement, due to an increase of the sp2/sp3 fraction. c) and d) STEM images collected on activated samples evidence a hierarchical macro-, meso- and microporous structure. In d), the presence of mesoporosity is clearly distinguishable (for clarity, we indicated two pores about 3 nm in diameter with white circles).

3.3. Electron microscopy SEM analysis on PLB evidence a highly inhomogeneous character, due to the presence of flakes and chips-like agglomerates clearly visible already at low degree of magnification (see Fig. 3, up). This is also confirmed by TEM and EDS analysis, which shows the presence of several elements other than carbon in the sample, compatible with the presence of the mineral phases already detected by PXRD and Raman (see Table 1). Furthermore, HRTEM performed on the PLB indicated that the carbon fraction is substantially in amorphous form on the nanometer scale (Fig. 4a). On the contrary, after the KOH activation process, the sample probed with SEM appears much more homogeneous and with a peculiar porous structure (see Fig. 3, down). Also, HRTEM shows a distinct different behavior in A-PLB, where the presence of meso- and micro-porosity is supported by the presence of higher structural order in the carbon arrangement, locally forming few-layered graphene-like nanosheets structures (see Fig. 4b). STEM images (see Fig. 4c and d) highlighted that the porosity of A-PLB appears at different level of magnification, clearly indicating a hierarchical structure of pores. EELS investigation, performed on biochar materials before and after KOH activation, allowed to give an estimation of the percentage of sp2 carbon, which can be considered as a rough method to estimate the graphitization level of the material [43]. We found (see Fig. S1) that the amount of “graphitic-like” sp2 carbon increased from 65% in the raw material up to 76% in the activated one, thus confirming the Raman

Fig. 5. XPS analysis of the activated material for C 1s (a), O 1s (b) and Si 2p (c) core levels. Components (coloured lines) are described in the legends, spectra are background subtracted. d) UPS analysis of the activated material. The valence band region is shown in the 0–12 eV range, with a magnified ( × 10) portion of the region close to the Fermi energy (EF). On the left part, the secondary cut off region (SECO) is shown with reduced intensity ( × 1/10). 165

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mainly contribute to the overall porosity. The BET specific surface area reaches the noticeable value of 3035 (100) m2/g, with a pore volume of 1.73 (1) mL/g. We performed the t-plot analysis using the STSA carbon black equation [37] to calculate the micropore surface area of activated material. The STSA carbon black equation is:

(8.0% C 1s area) [44–46]. The peak at 285.45 eV (18.1% of C 1s area) can be associated to sp3 carbon atoms, while the fourth component at 287.86 eV (11.8% of C 1s area) is related to carbon bound to oxygen groups. The high intensity of the loss structure, as well as of sp3 aggregates suggest a defected structure, in good agreement with HRTEM results, with a relevant fraction of oxidized species. If we compare the weight of sp2 and sp3 carbon coordinated species, that is considering only peaks at 284.37 and 285.45 eV, we found values of 77.4% and 22.6%, in good agreement with EELS results [43]. O 1s core level is characterized by a broad shape, with presence of two main components (see Fig. 5b). One is located at 533.67 eV (36.6% of the total O 1s peak area) and is associated to C–O groups, in agreement with analysis of C 1s core level. The second component at 532.91 eV (63.4% of O 1s area) stems from oxygen in SiOx silica compounds, as confirmed by the presence of a Si 2p signal at 103.53 eV (see Fig. 5c). The O/Si ratio is about 1.8 instead of the stoichiometric 2, thus suggesting the presence of defected silica as reported in STEM analysis. Valence band spectrum shows broad bands in the 2–4 and 6–10 eV regions (see Fig. 5d) that are typical of sp2 coordinated carbon atoms [45,47,48]. This is further confirmed by the semi-metallic behavior near Fermi energy level (EF) and by the material work function of 4.39 eV (as evaluated by analyzing the secondary electron cut off, SECO), strictly similar to the ∼4.4 eV value for graphene and graphite. Such a result is probably at the origin of the high electrical conductivity found in electrochemical analysis (see below).

( )

t [nm] = 0.088 P P0

2

( )

+ 0.645 P P0 + 0.298

where t is the statistical layer thickness del carbon black, P is the measured manometer pressure and P0 is the saturation vapor pressure of nitrogen (both expressed in kPa). The plot Vads vs t plot was fitted in the range corresponding to 0.2 < P/P0 < 0.5, according to Ref. [37], and the slope M was used to calculate the STSA, which is defined as:

STSA = M ∗ 15.74 where 15.74 is the constant for the conversion of nitrogen gas to liquid volume, and conversion of units to m2/g. The STSA for A-PLB turns out to be 1127 (35) m2/g. The micropore surface area, estimated by the difference of the total and the external surface area, is:

Smicropore = SBET − STSA = 1908(105) m2/ g This value confirms that A-PLB is endowed with a very large microporosity, which is a fundamental characteristic. The BJH pore size distribution (Fig. 6c and d), however, shows that the overall porosity in not limited to the micropore range (whose unique presence should be expected to limit the further insertion of the adsorbate species, once filled [49]): the most part of the pores has a diameter below 6 nm, while another smaller peak is present in the diameter range between 20 and 80 nm. These features are in agreement with the presence of an overall hierarchical porous structure, also observed with electron microscopy, which is indeed expected to facilitate the ion intercalation and de-intercalation process of the solvated ions. In order to accurately evaluate the micropore size distribution, a NLDFT analysis was also performed and it is reported in Fig. S6.

3.5. Sorption analysis Sorption analysis was performed both on PLB and A-PLB. In the first case, the sample displayed a BET specific surface area of 3.8 (4) m2/g and a pore volume of 0.03 (1) mL/g. The isotherm obtained on PLB is reported in Fig. S5. Pore size distribution (see Fig. 6a) shows that the majority of pores exceeds 20 nm diameter, laying thus in the range of meso- and macropores. After activation, the sorption analysis shows a type I isotherm (see Fig. 6b), typical of microporous systems, with H4 hysteresis, indicating the presence of narrow slit-like micropores which

Fig. 6. a) Pore size distribution histogram for PLB. b) Adsorption/desorption isothermal curve for A-PLB. c) Pore size distribution histogram for A-PLB. d) Pore volume and area distribution curve for A-PLB. 166

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Fig. 7. a) Cyclic voltammetry of symmetrical SCs obtained from A-PLB at different scan rates for devices with KOH-based electrolyte and b) with Na2SO4-based electrolyte. c) Galvanostatic charge and discharge at different currents for devices with KOH-based electrolyte and d) with Na2SO4-based electrolyte. e) Discharge specific current dependence of average specific capacity and energy for a SC with KOH-based electrolyte (in blue) and with Na2SO4-based electrolyte (in red). Specific capacity and energy are the average of 10 cycles at each different specific current. Error is smaller than marker height. f) Capacity retention of a SC with KOH-based (at 1 A/g) and Na2SO4-based (at 0.5 A/g) electrolyte. Inset: Nyquist plot of the EIS performed on char-based SCs with different electrolytes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

symmetrical SCs with electrodes made with A-PLB, with KOH-based and Na2SO4-based electrolytes are displayed. KOH-based SCs show a reasonably rectangular-shaped curve, while Na2SO4-based ones show a rectangular shape with rounded corners, suggesting a higher ESR for that devices. In both cases, nearly rectangular-shaped CV aspect is typical of porous carbon-based electrochemical capacitors [2]; however, while the former device shows an electrochemical stability up to 0.8 V,

3.6. Electrochemical analysis Figure S7 shows the electrical conductivity of A-PLB. The high conductivity of 4.6 S/cm at room temperature, achieved after the activation process, made A-PLB suitable for its use as electrode material without conductive additives. In Fig. 7a and b two-electrodes CV measurements performed on 167

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during the activation process is expected to produce rather large empty voids in the carbon matrix, thus favoring the further penetration of KOH and enhancing the activation effect. On the other hand, the presence of heteroatoms, such as N, S and P, in the activated carbon network, provided at low concentration, has been already identified as an effective way to improve the electrochemical performances [53,54], by increasing the redox pseudocapacitive effects, the electrical conductivity and the wettability of the carbon materials to the electrolyte solution [55,56]. The presence in traces of P and S elements, detected by EDS (see Table 1 and Fig. S2), but not by XPS, actually suggests the presence of heteroatoms at the surface of the super-activated biochar material at level below 0.1 wt%. Indeed, the good wettability towards polar solvents shown by A-PLB, as compared with other activated carbons, seems to support this hypothesis. This scenario hence would also suggest that activated carbons originated by biochar of animal waste origin, which is in principle richer of other components providing heteroatoms than, i.e., wood derived biochar, should display optimized performances in supercapacitor devices. In addition to this, it is worth also noting that the activation process in PLB, not only led to an increase of the SSA and to the optimization of the porosity of the material, but also caused a chemical reorganization of carbon atoms, as shown by EELS analysis, indicating a significant increase of the sp2 carbon fraction in the activated sample. HR-TEM images evidenced that this increase is a consequence of the presence of few-layered graphene-like nanosheets structure, which is believed to contribute to the enhancement of the observed microporosity in the activated material [57]. The mechanism of the activation process promoting the increase of the ordered graphene-like fraction in the A-PLB carbon matrix is still not completely understood. However, further investigations, aimed to disentangle the role played either by chemical reaction with KOH in presence of other contaminants present in the raw material, or by the high temperature reached during the activation treatment, or the combination or both, are currently in progress. Photoemission analysis confirmed the presence of a disordered and defected multi-layered graphene/graphitic network, showing semi-metallic and not insulating behavior. The electronic properties of A-PLB resulted also significantly improved, in particular the electrical conductivity significantly increased upon KOH activation, making possible the manufacturing of electrodes without using conducting additives (i.e. carbon black). As compared with other bio-char derived carbon based supercapacitors, our devices, thanks to a superactivated hierarchical porous structure with local graphene-like morphology, good wettability and low internal resistance, achieved excellent specific capacities and power densities in combination with a reasonably good energy density (see Table S3).

the latter can be considered stable only up to 0.6 V. Typical charge/discharge profiles appear with triangular shape (Fig. 7c and d), suggesting that the major contribution to the capacitance should arise from charge accumulation on the double layers. Na2SO4-based devices show a slight deviation from the ideal triangularshaped charge-discharge profile, which may be ascribable to the presence of pseudo-capacitance effects. The highest specific capacity for the SCs with KOH-based and the Na2SO4-based electrolyte reaches the noticeable value of respectively of 229 (13) and 164 (7) F/g (the corresponding areal capacity is equal to 1.36 (6) and 1.11 (5) F/cm2, see Fig. 7e). The maximum energy is respectively of 5.1 (3) and 2.05 (8) Wh/kg, and the maximum specific power theoretically available is respectively of 74 (10) and 22 (1) kW/kg. The highest specific power experimentally measured is respectively of 7.7 (5) and 3.01 (12) kW/kg at the current density of 10 A/g. Specific energy and power at each current density is shown in the Ragone plot in Fig. S9. Mean Coulombic Efficiency, calculated as the average on 5000 cycles, is 99.5% for a KOH-based SCs and 99.6% for a Na2SO4-based SCs (See Fig. S10). In order to take into account the non-ideal behavior shown by Na2SO4 GCD curves, energy efficiency was also calculated. Mean energy efficiency is 98.2% for a KOH-based supercapacitor and 94.4% for a Na2SO4-based one (Fig. S11). The small discrepancy between the two values of efficiency for a KOH-based device, for which the GCD curve is nearly ideal, can be explained as heat dissipation. On the contrary, a larger discrepancy is observed for a Na2SO4-based device, probably due to non-negligible faradaic reactions, which could also explain the reduced electrochemical stability window for Na2SO4-based devices. For both electrolytes, efficiency values are typical of EDLCs [38]. Maximum specific power was calculated using the equivalent series resistance (ESR) given by EIS measurement. The Nyquist plots (Fig. 7f, inset) for the EIS performed on SCs with KOH-based and Na2SO4-based electrolyte show a rather low equivalent series resistance (ESR), respectively of 1.27 (5) Ω and 1.56 (11) Ω, indicated by the real part of the impedance at the intercept with the x-axis (model and fit parameters are shown in Fig. S8 and Table S2) [50]. Capacity retention measured over 5000 cycles indicates that performances of the SC with Na2SO4-based electrolyte are almost unaffected after this large number of charge/discharge cycles; on the contrary, capacity retention of the KOH-based SC appears slightly less stable, although fluctuations over 5000 cycles are within 10% (Fig. 7f). 4. Discussion Thanks to an optimized KOH activation process, performed on raw biochar material obtained from poultry litter, we managed to obtain a highly porous carbon material with a SSA exceeding 3000 m2/g, which falls in the range of the super-activated carbons. The same process demonstrated to be effective in purifying the PLB from compounds other than carbon, as shown by EDS analysis performed during the electron microscopy sessions and by XPS. The presence of impurities, mainly in form of carbonates, phosphates, sulphates and silicates, is likely in raw PLB, as a consequence of the biological processes of chicken's metabolism and of the already discussed heterogeneous nature of the starting waste material. For these reasons, their concentration is generally higher as compared to just wood derived biochar [51]. Such compounds, depending also on the pyrolysis treatment of the litter, are differently incorporated in biochar, with more volatile fractions (N based compounds) decreasing their amount, while less volatile compounds mainly crystallizing in the carbon matrix during pyrolysis [52]. Although, as stressed before, such compounds are substantially absent in the activated material, we suggest that their presence in the raw material, in particular the large amount of phosphorous and sulphur, could play a role in the activation process itself, by increasing the effectiveness of the activation and by favoring the interaction between the activated surface of A-PLB and the electrolyte [31]. In fact, on the one hand, the removal of impurities

5. Conclusions In summary, we found that the activation process with KOH of biochar derived by poultry litter allowed to obtain a superactivated carbon displaying a specific surface area exceeding 3000 m2/g, with hierarchical porous structure and local graphene-like morphology. The activation process revealed also effective in purifying the material from elements other than carbons. The very good electrical conductivity, together with an optimized pore size distribution of the activated material allowed to use it directly as electrode in symmetric supercapacitors without any conducting additives, operating either with KOH, or with Na2SO4 eco-friendly electrolytes and to obtain excellent performances. Such findings hence disclose to direct applications of a novel class of cheap and largely available waste carbon materials in the field of the energy storage, through the manufacturing of “all green” supercapacitor devices.

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Acknowledgements

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