PCCP This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Enhancement of the supercapacitive properties of laser deposited graphene-based electrodes Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. Cite this: Phys. Chem. Chem. Phys., 2019, 21, 25175 through carbon nanotube loading and nitrogen doping† Ángel Pérez del Pino, *a Marta Rodrı́guez López,a Mohamed Ahmed Ramadan, ab Pablo Garcı́a Lebière,a Constantin Logofatu,c Immaculada Martı́nez-Rovira,d Ibraheem Yousef d and Enikö György ae Several technological routes are being investigated for improving the energy storage capability and power delivery of electrochemical capacitors. In this work, ternary hybrid electrodes composed of conducting graphene/reduced graphene oxide (rGO), which store charge mainly through electric double-layer mechanisms, covered by NiO nanostructures, for adding pseudocapacitance, were fabricated through a matrix assisted pulsed laser evaporation technique. The incorporation of multiwall carbon nanotubes (MWCNTs) provokes an increase of the porosity and thus, a substantial enhancement of the electrodes’ capacitance (from 4 to 20 F cm3 at 10 mV s1). Volumetric capacitances of 34 F cm3 were also obtained with electrodes containing just carbon nanotubes coated with NiO nanostructures. Moreover, the use of nitrogen containing precursors (ammonia, urea) for laser-induced N-doping of the nanocarbons also provokes a notable increase of the capacitance. Remarkably, Received 30th July 2019, N-containing groups in rGO–MWCNTs mainly add electric double layer charge storage, pointing to an Accepted 18th October 2019 increase of electrode porosity, whereas redox reactions contribute with a minor diffusion fraction. It was DOI: 10.1039/c9cp04237g also observed that the loading of carbon nanotubes leads to an increase of diffusion-controlled charge storage mechanisms versus capacitive ones in rGO-based electrodes, the opposite effect being rsc.li/pccp observed in graphene electrodes. 1. Introduction capability for energy storage but at relatively low power, SCs disclose lower energy density but the ability to provide much The urgent need of exploitation of renewable energy sources, higher power besides longer cycling stability. The continuous the electrification of transportation and industry sectors, and enhancement of SC performance, increasing the energy storage the expansion of portable electronics require the development density without substantial detriment of power delivery and of new technologies for energy storage. Great research and lifetime, is allowing a rapid expansion of SCs in diverse sectors, innovation efforts are being carried out for the advance in such as consumer electronics, wind energy, electricity grid electrochemical energy storage devices, mainly as rechargeable management, electric vehicles, and high power medical and batteries and electrochemical capacitors (also known as super- industrial systems.3,4 capacitors, SCs).1,2 Electrochemical energy storage systems have Generally, the electric energy in SCs can be stored in two specific performances that make them suitable for different ways: (i) by means of electrostatic adsorption of electrolyte ions types of applications. Thus, while batteries exhibit high on the electrode’s surface (electric double layer, EDL), and (ii) through surface faradaic oxidation and reduction of electro- a Instituto de Ciencia de Materiales de Barcelona, Consejo Superior de chemically active entities (pseudocapacitance). Electrostatic Investigaciones Cientı́ficas (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain. mechanisms (EDL) are appropriate for delivering moderate E-mail: aperez@icmab.es; Tel: +34 935801853 energy at high power. However, surface faradaic reactions b Faculty of Engineering, Helwan University, Helwan, Egypt (which add pseudocapacitance) exhibit the storage of much c National Institute for Materials Physics, PO Box MG 7, 77125 Bucharest, Romania d higher energy density but at lower power. One strategy with ALBA Synchrotron, Carrer de la Llum, 2-26, 08290 Cerdanyola del Vallès, Spain e National Institute for Lasers, Plasma and Radiation Physics, PO Box MG 36, great potential for improving the SC performance is the fabrication 77125 Bucharest, Romania of hybrid electrode materials, in which both types of energy storage † Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp04237g mechanisms are combined. This kind of electrode is commonly This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 | 25175 View Article Online Paper PCCP formed by a porous composite of nanostructured carbon (EDL) allows the opening of ionic transport paths that lead to a coated with pseudocapacitive materials as electroactive polymers substantial increase of the electrodes’ volumetric capacitance. or transition metal oxides.4–8 In these materials, nanostructured Also, the influence on the functional properties and capacitive/ carbon acts as a conductive scaffold for pseudocapacitive nano- diffusion-limited nature of the electrodes by doping of the This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. materials, being able to transport electrons between the pseudo- carbon nanostructures with nitrogen and replacing rGO by capacitive material and the current collector, and supply EDL graphene precursor was investigated. Noteworthily, and differing capacitance. The doping of carbon materials with heteroatoms to the conventional interpretation, the obtained results point out such as oxygen, nitrogen, sulphur or boron is also used for that the increase of capacitance obtained by N-doping of rGO adding pseudocapacitance by means of redox reactions and by flakes/CNTs would essentially arise from an augment of the enhancement in electrical conductivity.9–13 On the other hand, porosity rather than by redox reactions. Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. the morphology (distribution of pores) of the material is a key factor for its energy storage performance since it determines the ability of electrolyte ions to reach active sites on the electrode’s 2. Experimental surface contributing to capacitance.14–17 Indeed, there are also works that challenge the ‘‘conventional view’’ of pseudo- For the MAPLE targets, aqueous dispersions were obtained by capacitance mechanisms behind the enhancement of capacitance mixing graphene oxide (GO) powder (NanoInnova Technologies) in an N-doped graphitic structure, which point to the increase of with NiO nanoparticles (NPs, Sigma-Aldrich) and multiwall EDL contribution due to the formation of ultramicropores.18 carbon nanotubes (Sigma-Aldrich) doped with carboxylic groups Therefore, the use of one-dimensional carbon nanotubes (CNTs) for enhancing their dispersibility. GO powder is composed of to physically separate two-dimensional graphene sheets, which less than 15 layer thick sheets and ca. 1 mm2 lateral size, NiO tend to aggregate, appears as a remarkable method for obtaining NPs are about 50 nm in diameter and MWCNTs are about 20 nm high surface area nanocarbon electrodes.19–22 Further improve- in diameter and up to 1 mm in length. Deionized water was used ment of the capacitance of the graphene–CNT composites can be as a solvent. Analogous dispersions were also synthesized by obtained through doping with nitrogen-containing chemical replacing GO by graphene (G) powder (NanoInnova Technologies). groups or by decorating them with pseudocapacitive materials The concentration of the GO, G and NiO NP precursors was chosen (ternary hybrids).23–26 as 5 wt%, and that of MWCNTs was varied up to 2 wt% to avoid Different approaches are also being investigated for the synthesis precipitation and too high viscosity of the dispersions. Further of new nanostructured materials with improved performance.8,27,28 dispersions were obtained by adding nitrogen-containing mole- In this respect, laser-based techniques are revealing high cules (ammonia, urea and melamine from Sigma-Aldrich) which potential for the rapid and versatile fabrication of SC electrodes have been previously proven to provoke laser-induced chemical and devices.29–31 Besides triggering of photochemical processes, reactivity and nitrogen doping.38 The concentration of ammonia laser methods are based on the fast heating of the irradiated and urea was set to 2 M, whereas the melamine concentration was materials, even up to thousands of degrees, leading to extreme, 0.3 M for avoiding precipitation. Next, the dispersions were stirred out of thermodynamic equilibrium, chemical reactions and and sonicated during 20 min at room temperature for good phase changes. Taking advantage of these exceptional proper- homogeneity and low aggregation. The MAPLE targets were ties, a deposition method based on the matrix assisted pulsed prepared by filling 3 mL aluminium holders with the dispersions. laser evaporation (MAPLE) technique was developed for the Afterwards, the targets were flash frozen by immersion in liquid fabrication of hybrid composite films formed by reduced N2, placed inside the deposition chamber and preserved frozen graphene oxide (rGO) coated with transition metal oxide nano- during the deposition process by using a liquid N2-cooled holder. structures.32–35 The complex laser–matter interactions occurring The films’ depositions were accomplished by submitting during deposition lead to the synthesis of a rich variety of 6000 laser pulses (266 nm wavelength, B5 ns pulse duration, compounds with different functional properties, even by slight 10 Hz repetition rate) with 0.4 J cm2 laser fluence to the frozen modifications of the laser parameters, the target composition or targets by means of a Brilliant B Nd:YAG laser system (Quantel). the chemical environment.36,37 In particular, the reactive During the irradiation process, the laser beam was kept scanning inverse MAPLE (RIMAPLE) method was able to accomplish the the target surface with an incident angle of 451. Flexible and simultaneous chemical conversion and deposition of the irradiated conducting 1  1 cm2 substrates made of (i) 0.5 mm-thick copper nanomaterials, allowing the fabrication of functional NiO-coated and (ii) 0.1 mm-thick polypropylene (PP) films coated with sput- nitrogen-doped rGO-based SC electrodes.38,39 In these studies, the tered Au (20 nm)/Cr (5 nm) layers, were sited in front of the target composites containing NiO nanostructures exhibited higher at a separation distance of 4 cm. The films’ growth was carried out stability than those not containing NiO layers, though lower in a 20 Pa N2 background gas atmosphere after evacuation to a volumetric capacitance probably due to differences in compactness residual pressure of 0.1 Pa. of the films. The morphology of the obtained layers was characterized by In the present paper, and for the first time, ternary hybrid SC extreme high resolution scanning electron microscopy (XHR electrodes composed of rGO/graphene and multiwall carbon SEM) through a Magellan 400L system (FEI). The composites’ nanotubes (MWCNTs) decorated with NiO nanostructures were structure was analysed at the nanoscale by means of high fabricated by RIMAPLE. The addition of the carbon nanotubes resolution transmission electron microscopy (HRTEM) and 25176 | Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 This journal is © the Owner Societies 2019 View Article Online PCCP Paper high angle annular dark field scanning TEM (HAADF-STEM) (3 M NaCl internal solution) and a Pt wire counter electrode. by means of a Tecnai F20 microscope (FEI). The chemical 1 M Na2SO4 aqueous solution was used as electrolyte. CV studies composition of the layers was also investigated by X-ray photo- were performed at different sweep rates in the 10–150 mV s1 electron spectroscopy (XPS) and synchrotron-based trans- range. Galvanostatic charge–discharge cycles were obtained at This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. reflection mode Fourier transform infrared microspectroscopy different current densities (0.03–6 mA cm2). Electrochemical (SR-FTIRM). A SPECS spectrometer based on a Phoibos 150 electron impedance spectroscopy (EIS) measurements were also per- energy analyser operating in a constant energy mode was used for formed through a Hioki IM3590 chemical impedance analyser. XPS analyses, an aluminium anode (Al Ka 1486.74 eV) being used as Impedance values were acquired in the 1 Hz–200 kHz frequency a monochromatic X-ray source. Wide range spectra over 1400 eV range at open circuit potential with a perturbation amplitude of binding energies were recorded using a 50 eV analyser pass energy. 5 mV. An average of 5 measurements per data point was taken in Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. High resolution spectra were also acquired over 20 eV ranges at all the electrochemical analyses for minimizing the experi- 10 eV pass energy with an energy resolution of 0.7 eV. All the mental error. Regression of EIS data to equivalent circuit studies were performed in an ultra-high vacuum (B107 Pa). models was carried out using EIS analyser software. SR-FTIRM analyses were carried out at the infrared beamline MIRAS of ALBA Synchrotron using a Hyperion 3000 microscope coupled to a Vertex 70 spectrometer (Bruker, Germany). The 3. Results and discussion microscope, equipped with a liquid N2-cooled 50 microns MCT mercury cadmium telluride detector, used a 36 Schwarzschild The UV pulsed laser irradiation of the frozen MAPLE targets objective (NA = 0.65). The spectra were acquired using a single leads to the prompt heating of the carbon and NiO nanostructures, masking aperture size of 10  10 mm2. A collection of 9 spectra leading to the explosive boiling of the surrounding water ice and per sample was recorded in the 4000–700 cm1 mid-infrared the deposition of the nanostructures on the facing substrate.38 The range at 4 cm1 spectral resolution with 1024 co-added scans per obtained films, about a few micrometres thick, present homoge- spectrum. Baselines were subtracted to the spectra for accurate neous black colour and remain adherent to the substrate after identification of bands. bending. Hereafter, the samples composed by GO–CNT–NiO will The electrochemical properties of the electrodes were inves- be referred to as (wt% GO/wt% MWCNT/wt% NiO) for identifying tigated by means of cyclic voltammetry (CV) and galvanostatic the relative concentrations of the different components used in the charge–discharge (GCD) cycling in the voltage range of [0, 0.8] V corresponding MAPLE target. The morphology of the GO–CNT– by means of a Keithley 2450-EC Electrochemistry Lab System. NiO samples was analysed by means of SEM. Fig. 1a shows the The layers deposited on Au/Cr/PP substrates were used as morphology of the electrode fabricated without CNTs, (5/0/5), working electrodes in a three-electrode cell. The analyses were revealing a rough structure composed of arbitrarily distributed carried out in a 0.5 cm2 sample area using a plate material GO sheets and tens of to a hundred nm nanoparticles (NiO). The electrochemical cell (Bio-Logic), an Ag/AgCl reference electrode addition of carbon nanotubes provokes a clear change in the film’s Fig. 1 High-resolution SEM images of (a) GO–CNT–NiO (5/0/5) and (b) (5/1/5). (c) HAADF-STEM images of a GO flake and MWCNT in sample GO–CNT– NiO (5/1/5). (d) High-resolution TEM of sample (5/1/5); (e) zoomed-in area indicated in (d). Inset in (e): FFT of the area indicated by the rectangle. This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 | 25177 View Article Online Paper PCCP morphology even with 1 wt% of CNTs (Fig. 1b). Randomly oriented neither an increase of the content of CNTs nor the presence CNTs, about 0.5–1 mm in length and ca. 20 nm in diameter, are of N-containing precursors in the MAPLE target provoke a visible mixed with GO sheets and NiO nanoparticles leading to significant variation of the reduction degree. Nevertheless, the an increase of the composite’s porosity. Higher resolution N-doping degree experiences a noteworthy increase with the This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. HAADF-STEM studies reveal the formation of Ni-containing respective presence of ammonia, urea and melamine precursors nanostructures (bright regions in Fig. 1c) as small as 1 nm for a given relative composition of GO–CNT–NiO (5/1/5) account- covering the GO and MWCNT surfaces. The formation of NiO ing for a high chemical reactivity of the precursors. Characteristic nanostructures with sizes larger and smaller than the initial NiO high resolution XPS spectra of C1s and N1s signals are shown in NP average diameter (50 nm) is respectively attributed to Fig. 2c. The recorded C1s peaks are deconvoluted to one carbon– coalescence and dewetting mechanisms of molten NiO. High carbon bond component (284.6 eV, C1), mainly composed of sp2 Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. resolution TEM characterization shows that the nanostructures configuration probably with some contribution of sp3, as well as covering both GO flakes and CNTs are crystalline (Fig. 1d and e). three carbon–oxygen components centred at 285.7, 286.9 and Fast Fourier transform analyses of such nanometric regions 288.5 eV respectively attributed to hydroxyl-epoxy (C2), carbonyl reveals interplanar distances of 0.21, 0.14, 0.12, and 0.10 nm (C3) and carboxylic (C4) functionalities.40 As observed in Fig. S2a that are respectively attributed to (200), (220), (222) and (400) (ESI†), the integrated area percentage of the C1s components crystallographic planes of cubic NiO (Bunsenite; JCPDS 00-047- slightly varies between the samples. Though the reduction degree 1049). No morphologic differences are observed in samples of GO cannot be calculated due to the presence of MWCNTs, obtained with ammonia or urea precursors, though the ones previous studies allow us to assume the partial reduction of GO obtained with melamine display slightly greater compactness. It during the deposition of the films.38,41 The high resolution N1s is worth noticing that the composites obtained from (0/1/5) signal in the samples obtained from targets without N-containing MAPLE targets (without GO), show randomly oriented CNTs precursors reveals a very weak and wide peak (Fig. S2b, ESI†), and totally covered by NiO nanostructures to a higher extent than is mainly attributed to physisorbed nitrogen from the N2 gas those observed in GO-containing samples (Fig. S1, ESI†). In environment during deposition. The signal is much more intense this sample, the NiO nanostructures exhibit similar crystalline in samples obtained with N-containing precursors (Fig. 2c). nature to those of GO-containing samples. However, hundreds of In these cases, the N1s signal is deconvoluted into four nanometres compact NiO aggregates are visible in many locations peaks centred at ca. 398.1, 399.5, 401.2 and 404.5 eV which between CNTs (Fig. S1b, ESI†), not visible in GO-containing are respectively associated with the presence of pyridinic N–C samples. (N1), pyrrolic N–C or amine (N2), graphitic N (N3) and pyridinic Compositional analyses of the GO–CNT–NiO samples were NOx bonds or p excitations (N4).36,38 Consequently, there must performed by means of XPS and SR-FTIRM. A typical wide scan be a contribution of pyridinic and pyrrolic N–C bonds to the C2 XPS spectrum is depicted in Fig. 2a, showing C1s, N1s, O1s, and and C3 peaks of the C1s spectrum (binding energies at ca. 285.9 Ni2p and 3p peaks as well as some Auger signals. From these and 287.3 eV),42,43 which may lead to a reduction of the relative XPS surveys, the overall reduction (at% C/at% O) and nitrogen quantity of hydroxyl-epoxide and carbonyl groups in these doping (100 at% N/[at% C + at% N]) degrees of the nanocarbons samples. As observed in Fig. S2c (ESI†), pyrrolic N–C or/and were calculated for all the samples (Fig. 2b). As observed, amine (N2) groups are the dominant contribution to N-doping Fig. 2 (a) Wide scan XPS signal of a GO–CNT–NiO (5/1/5)-ammonia sample. (b) Reduction and N-doping degrees of all the samples obtained from XPS surveys. (c) C1s and N1s high-resolution XPS spectra of GO–CNT–NiO (5/1/5)-melamine and (0/1/5)-ammonia samples, respectively. (d) Characteristic FTIRM normalized spectra of raw GO, MWCNT urea and melamine powders as well as deposited samples with (5/1/5) relative concentration. 25178 | Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 This journal is © the Owner Societies 2019 View Article Online PCCP Paper in all the samples. Furthermore, the sample (0/1/5)-ammonia, unreacted molecules can be neglected. These results are in not containing GO, shows a similar C–N bond distribution concordance with the significant nitrogen incorporation and similar to that of samples having GO. bonding type reported by XPS. Neither C–H nor C–C sp3 bands Additional study of the composition of the samples was were detected possibly accounting for the minor formation of This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. carried out by means of synchrotron trans-reflection mode-FTIRM. structural defects in rGO/CNTs and alkyl chains. Representative spectra of raw precursors and GO–CNT–NiO Electrochemical energy storage performance of the electro- (5/1/5) samples are shown in Fig. 2d. The list of the observed des was studied through cyclic voltammetry in the 0.0–0.8 V bands and their assignment are presented in Tables S1 and S2 voltage and 10–150 mV s1 sweep rate ranges. Typically, CV (ESI†).44–60 As witnessed, raw GO powder reveals very weak plots present a quasi-rectangular shape with no presence of characteristic bands probably due to high scattering. The prominent peaks (Fig. 3a). The elevation recorded at the highest Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. spectrum of CNT material mainly reveals the bands of CO, potential is triggered by oxygen-evolution reaction in the aqueous COH and C–C, whereas urea and melamine spectra show the electrolyte. The rectangular-like nature of the electrodes’ voltammo- contribution of typical bands of their N-containing groups. The grams, without noticeable redox waves, essentially points toward deposited GO–CNT–NiO (5/1/5) films display reflection bands capacitive (surface) charge storage processes.61,62 Therefore, the corresponding to different types of C–C, C–O and C–N bonds volumetric capacitance (Cv) of the electrodes can be calculated from (Table S2, ESI†). It is worth noting that a wide-range wavy signal CV data at different sweep rates according to the equation: is superimposed on the FTIRM bands due to interferences of IR Þ IðVÞdV radiation passing through the studied films. The spectrum of Cv ¼ (1) 2DVsv GO–CNT–NiO (5/1/5) mainly shows weak bands corresponding to CQC, and C–O–C, besides the contribution of CQO and where the integral at the numerator represents the area enclosed in C–N. Spectra of GO–CNT–NiO (5/1/5) samples obtained with the voltammogram plot (anodic–cathodic carriers), DV is the voltage N-containing precursors reveal the presence of additional range (0.8 V), s is the voltage sweep rate and v is the volume of bands ascribed to several types of carbon–nitrogen chemical the active material during the measurements. As observed in groups. Besides, the spectrum of (0/1/5)-ammonia shows the Fig. 3b, the maximum Cv of the (5/0/5) electrode (without CNTs) presence of C–N bands, similarly to that of GO–CNT–NiO is about 4 F cm3 (at 10 mV s1). Remarkably, the volumetric (5/1/5)-ammonia samples with some minor differences, proving capacitance experiences a considerable increase with the the effective nitridation of MWCNTs (Fig. S2d and Table S3, incorporation of MWCNTs into the material structure. The max- ESI†). The patterns of the deposited samples do not follow the imum values of Cv rise to ca. 10 and 20 F cm3 (at 10 mV s1) in spectra of the precursor ones, so it is assumed that most of samples fabricated with (5/1/5) and (5/2/5) relative concentration, the precursors did react between them and the presence of respectively. This means a 500% increase of Cv just by addition of Fig. 3 (a) Cyclic voltammetry plots of the GO–CNT–NiO (5/1/5) sample at different sweep rates. (b) Volumetric capacitance of GO–CNT–NiO samples with different relative concentrations versus sweep rate. (c) Volumetric capacitance of GO–CNT–NiO samples synthesized in the presence of N-containing precursors. (d) Calculated volumetric capacitance versus applied current density of GO–CNT–NiO samples. Inset: Galvanostatic charge–discharge curves of the GO–CNT–NiO (5/1/5) sample at various applied current densities. (e) Coulombic efficiency and capacitance retention (inset) of various GO–CNT–NiO electrodes obtained at ca. 400 mA cm2. (f) Nyquist plots of GO–CNT–NiO samples. Insets: Zoomed-in area at the high frequency region and equivalent circuit used as a model. The arrow indicates the direction of increasing frequency. This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 | 25179 View Article Online Paper PCCP 2 wt% CNTs to the (5/0/5) target. Furthermore, a capacitance with time, leading to the typical triangular-like shape of capacitive of 34 F cm3 (850% greater) is even reached in the electrode materials (Fig. 3d, inset). As expected, an increase of the current with no presence of rGO (0/1/5). The obtained volumetric capa- density leads to a decrease of the charge/discharge time. The citances could seem quite modest as compared to values of volumetric capacitance of the electrodes was also calculated This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. ca. 200–300 F cm3 reported in the literature.21 However, taking from GCD data by means of eqn (2): into account that the mass loading of the obtained films is about Is Dtd 10 mg cm2, the corresponding gravimetric capacitances range Cv ¼ (2) DVd from about 100–300 F g1, being very competitive values.19,23,26,63 Since the nanostructure and composition of the samples do not being Is the current density, Dtd the discharge time, DV the significantly differ, we attribute the great change in capacitance voltage range (0.8 V) and d the thickness of the film. As observed Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. to the increase of the film’s porosity. A quantitative measurement in Fig. 3d, the capacitance drops rapidly with current density at of the porosity cannot be carried out by the Brunauer–Emmett– low values of Is, the decay being much slower at currents larger Teller (BET) method due to the tiny mass loading of the films. than 0.5 mA cm2. The maximum capacitance, 84 F cm3, is However, SEM studies clearly reveal the presence of randomly recorded in the (5/1/5)-ammonia electrode at a current density oriented NiO-coated CNTs inside the films that open channels of 30 mA cm2. Besides, the stability of the electrodes was between rGO sheets improving the electrolyte’s ionic access to tested applying 1000 consecutive charge–discharge cycles at larger surface area of the material (Fig. 1).7,16,17 Consequently, the ca. 0.4 mA cm2. As witnessed in Fig. 3e, the electrodes reveal augmentation of CNT loading leads to an increase of capacitance, notable performance since their coulombic efficiency (ratio up to a certain extent (Fig. 3b). Additionally, the chemical between charged and discharged carriers) remains at 100% in composition takes a complex role in the capacitance of the films the whole cycling range, whereas their capacitive retention (Fig. 3c). GO–CNT–NiO (5/1/5) samples deposited with the slightly decreases with cycling to about 90% after 1000 cycles. presence of ammonia and urea precursors reveal an increased Electrochemical impedance spectroscopy measurements were volumetric capacitance of 20–21 F cm3 at 10 mV s1 (increase of also carried out for the electrodes. Fig. 3f presents the Nyquist ca. 200%) even showing different N-doping nature of their plots of the samples, which reveal the characteristic straight-line structure. Nevertheless, the (5/1/5)-melamine sample, which shows shape at low frequencies with partial semicircle at high frequencies the highest N-doping percentage (essentially pyrrolic N-amine), of carbon-based porous electrodes in aqueous electrolytes.39 shows lower capacitance than the (5/1/5) sample. A similar effect The equivalent circuit depicted in Fig. 3f, based on Randles was observed previously in rGO–NiO and rGO–NiO–melamine cell, was used for fitting the following technical parameters: an electrodes and was associated with decreasing porosity of the equivalent series resistance (ESR), which includes the electric composite material.38 Moreover, it has been also reported that resistance of the electrode, electrolyte and circuit contacts; the combined action of O- and N-containing groups such as a constant phase element (CPE) related to non-ideal EDL quinone-, and pyridinic–pyrrolic N improves the electrochemical capacitors; a resistor (R) in parallel with the CPE accounting energy storage.9 Besides, graphitic-N (not present in melamine for leakage of charges in the capacitor due to electrochemical sample) has been reported to facilitate the transport of electrons reactions (charge transfer); and a Warburg element (W) in series through carbon materials and add strong electron-acceptor with the capacitor, related to diffusion of ions in the electrolyte. nature.10–12 All these facts would also contribute to increasing Table S4 (ESI†) presents the fitted values of these parameters. the capacitance of the electrodes obtained with ammonia and The ESR of the electrode–electrolyte system ranges from 23–37 O, urea. Remarkably, an increase of the relative amount of CNTs in the lowest value belonging to the (5/1/5) and (5/1/5)-ammonia the ammonia-based targets, (5/2/5)- and (0/1/5)-ammonia, leads samples. The n-exponent, related to CPE impedance, indicates the to lower capacitance as compared to their counterparts synthe- ideality degree of the capacitor.39 In all the samples, the n sized without ammonia. However, Fig. 2 and Fig. S2 (ESI†) exponent is larger than 0.9 revealing a nearly ideal EDL capacitor reveal similar carbon–oxygen and carbon–nitrogen bonding performance. The charge transfer (or polarization) resistance (R) is configuration in these materials. This fact could be caused by: high in all cases (450 kO) accounting for low loss of stored (i) differences in film morphology (porosity), due to different charges, as indicated by the coulombic efficiency measurements thermal conditions developed at the MAPLE targets during laser in GCD cycling tests. Interestingly, R rises with the loading of irradiation; (ii) different redox behaviour or charge-transfer MWCNTs, and the (5/1/5)-ammonia sample shows practically efficiency of similar N-containing groups depending on their no leakages (R 4 1 MO). Indeed, the fitted parameters of the base material, either rGO or MWCNTs; and (iii) subtle differences (5/1/5)-ammonia electrode point toward a nearly ideal EDL in the structure of N-containing groups in reduced graphene capacitor behaviour. It is also worth noting that the EIS data oxide and carbon nanotubes, not detected by XPS and FTIRM of all the samples were fitted with a semi-infinite Warburg measurements, triggered by differences of reactivity of ammonia impedance element, except that of the (5/1/5)-ammonia electrode with oxygen-containing groups in GO and carboxylic-doped which had to be fitted using a finite Warburg impedance element MWCNTs during laser irradiation.64,65 for obtaining reliable results. Galvanostatic charge–discharge analyses of GO–CNT–NiO In the rGO–CNT–NiO hybrid composite that constitutes the samples were performed in the 0.03–6 mA cm2 current density electrodes, two types of mechanisms (capacitive and diffusion- range. The obtained curves display a quasi-linear voltage response controlled) are expected to contribute to the charge storage. 25180 | Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 This journal is © the Owner Societies 2019 View Article Online PCCP Paper Capacitive mechanisms comprehend EDL and pseudocapacitive especially at larger voltages (Fig. 4 and Fig. S3, ESI†). The enhanced charge storage on, respectively, graphene domains and electro- diffusion nature of NiO-coated CNTs, even enabling larger porosity chemically active sites in (N-doped) rGO flakes and NiO nano- in the composites than rGO, could be related to thicker structures. These processes are superficial and fast in nature. NiO-coating on CNTs than on rGO sheets besides the presence This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. On the other hand, diffusion-controlled charge storage encom- of compact NiO aggregates (as inferred from Fig. 1 and Fig. S1, passes volume mechanisms related to the insertion of species ESI†). Indeed, the broad and weak waves observed in the inside the active material. The capacitive and diffusion compo- diffusion component of the (0/1/5) sample at about 0.5–0.6 V nents of the recorded current during CV measurements are could be attributed to redox conversions between NiO and proportional, respectively, to s and s1/2.62 Therefore, the total NiOOH species.66,67 Therefore, the capacitive or diffusion- current can be expressed as: limited nature of the material would be somehow driven by Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. the morphology of NiO, which acts as an extrinsic pseudo- i(s,V) = icap + idif = k1s + k2s1/2 (3) capacitive material. Besides, typical voltammograms with the Hence, the parameters k1 and k2 can be calculated by fitting of calculated capacitive and diffusion-controlled components i(s,V)/s1/2 versus s1/2. These parameters allow establishing the of (5/1/5) samples obtained with and without N-containing amount of capacitive or diffusion current in the voltammograms precursors are displayed in Fig. S4 (ESI†). As observed, the at any given voltage and sweep rate. Fig. 4 displays calculated capacitive component in the composites obtained with N-containing capacitive and diffusion components of voltammograms recorded precursors increases (Fig. S3, ESI†), being more significant in in GO–CNT–NiO (5/0/5), (5/2/5) and (0/1/5) electrodes. As observed, composites obtained with ammonia and urea, which are the the main contribution in the (5/0/5) sample comes from the ones showing the highest capacitance (Fig. 3c and d). Moreover, capacitive component in the whole voltage range. The addition of the capacitive component increases practically in the whole CNTs leads to increased contribution of the diffusion component, voltage range as compared to the (5/1/5) sample. Since pyridinic as witnessed in the voltammogram of the (5/2/5) sample. Indeed, and pyrrolic N are associated with edges and nanoholes in the the evolution of the integrated area ratio of diffusion over capacitive rGO backbone, this augment of the capacitive contribution components, shown in Fig. S3 (ESI†), reveals an increasing trend could be attributed to higher porosity, i.e., accessible surface with the addition of CNTs. As observed, the (0/1/5) sample, which area, and associated EDL capacitance.18 Under comparison, the is composed of only NiO-covered CNTs, shows the greatest sample obtained with melamine, which displays slightly greater contribution of the diffusion component over the capacitive one, morphological compactness and the presence of just pyrrolic N-amine groups, reveals somewhat less capacitive component. Furthermore, the diffusion component in N-containing composites is revealed to be slightly larger at low voltages (Fig. S4, ESI†). This effect has been previously appreciated in electrodes fabricated by RIMAPLE using an imidazole molecule as an N-doping precursor,41 and it could be attributed to a diffusion-limited nature of the insertion/extraction processes of active species during faradaic reactions taking place in N-containing functional groups. In summary, N-containing groups add capacitive nature to the electrodes, probably by increasing the active surface area, whereas redox reactions contribute with a minor diffusion-like capacitance mainly at low voltages. A similar study was accomplished by using graphene (G) sheets instead of GO. Fig. 5a reveals the substantial alteration of the surface morphology of G–CNT–NiO samples with relative concentration of (5/0/5)-without CNTs-, and (5/2/5)-addition of 2 wt% of CNTs to the target. As observed, the (5/0/5) composite discloses the formation of a compact film with large particles of NiO, up to a few hundred nm in size, covering the graphene sheets. These large NiO structures, not observed in GO-based materials, would be produced after laser-induced melting, coalescence and recrystallization of aggregated NiO NPs. The aggregation of NiO NPs in the aqueous MAPLE dispersion prior to freezing would be triggered by the hydrophobic nature of the graphene sheets.68 Instead, the hydrophilic behaviour of GO flakes would favour their dispersion besides NiO NPs and, thus, Fig. 4 Decomposition of cyclic voltammetry plots of samples GO–CNT– the formation of much less compact layers with smaller NiO NiO (5/0/5), (5/2/5) and (0/1/5) taken with 100 mV s1 into capacitive and structures on rGO sheets after the laser irradiation process. diffusion components. Interestingly, the addition of MWCNTs to the G-NiO NP dispersion This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 | 25181 View Article Online Paper PCCP This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. Fig. 5 Study of G–CNT–NiO samples. (a) SEM images of (5/0/5) and (5/2/5). (b) STEM image of (5/2/5). (c) Volumetric capacitance of (5/0/5) and (5/2/5) samples. Inset: Characteristic cyclic voltammetry curves of (5/2/5) at 10–150 mV s1 sweep rate range. (d) Decomposition of cyclic voltammetry plots of (5/0/5) and (5/2/5) samples taken with 100 mV s1 into capacitive and diffusion components. (e) Galvanostatic charge–discharge cycles of (5/0/5) and (5/ 2/5) samples at an applied current of 50 mA cm2. Inset: Calculated volumetric capacitance versus applied current density. (f) Nyquist plots of (5/0/5) and (5/2/5) samples, being the inset a zoomed-in area of the high frequency data. leads to the formation of much more porous films composed of Cyclic voltammetry analyses reveal a volumetric capacitance NiO-coated graphene sheets and MWCNTs (Fig. 5a). In this of the G-CNT-NiO (5/0/5) electrode of about 3 F cm3 (at 10 mV s1 case, the average size of the NiO nanostructures is smaller, sweep rate) (Fig. 5c). This value is similar to that of the equivalent about tens of nanometres (Fig. 5b). The formation of smaller GO–CNT–NiO (5/0/5) sample (Fig. 3b). As expected, the volumetric NiO nanostructures would be linked to the hydrophilic nature capacitance substantially increases after the inclusion of CNTs, of carboxylic-doped MWCNTs, that would prompt a better reaching ca. 20 F cm3 at 10 mV s1, about a 660% increase, which dispersibility of the graphene sheets and NiO NPs. Remarkably, is very similar to that of the equivalent GO–CNT–NiO sample. after laser deposition, the graphene sheets show a similar Therefore, it seems clear that the main limiting factor for the structure to that of the G precursor (Fig. S5, ESI†). No signifi- energy storage of these type of layers is the morphology-structure, cant formation of laser-induced structural defects is identified. i.e., the layer porosity. It should be noted that the deposition This fact totally differs from the GO material, which experi- of G–CNT–NiO material with the presence of N-containing ences a significant structural modification with the appearance precursors (ammonia, urea, melamine) did not improve the of a great quantity of wrinkles and, often, nanoholes after UV capacitance values due to low reactivity of the precursors laser irradiation.38,41,45 with graphene. This fact would indicate that the mechanisms 25182 | Phys. Chem. Chem. Phys., 2019, 21, 25175--25186 This journal is © the Owner Societies 2019 View Article Online PCCP Paper leading to the nitrogen incorporation into the graphene structure obtained by using either rGO or graphene sheets with equivalent are indeed promoted by oxygen-containing functionalities of relative material concentrations. Besides, the study of the electro- GO.64,65 Fig. 5d shows the capacitive and diffusion-limited com- chemical performance of the samples suggests that the augment ponents of the voltammograms of (5/0/5) and (5/2/5) G–CNT–NiO of capacitance obtained by N-doping of rGO flakes and MWCNTs This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. samples taken with 100 mV s1. As observed, and to the contrary of would mainly come from an increase of the accessible surface area the effect observed in the GO–CNT–NiO set of samples, the to the active species. Therefore, and contrarily to the most diffusion-controlled component decreases with the addition of extended conviction, the capacitance of N-doped nanocarbon films CNTs (Fig. S3, ESI†). This effect could be explained as the result of would predominantly arise from EDL mechanisms rather than less aggregation of NiO material (shorter diffusion paths) and pseudocapacitive ones. The results also demonstrate that the NiO more contribution of capacitance from graphene surfaces (mainly material acts as an extrinsic pseudocapacitive material, adding Open Access Article. Published on 06 November 2019. Downloaded on 7/14/2020 4:14:41 PM. through double-layer energy storage) with the addition of CNTs. battery-like behaviour in aggregated form and pseudocapacitive Indeed, by comparison of the capacitive and diffusive components nature when crystallized in a nanometric size. of (5/2/5) GO–CNT–NiO and G–CNT–NiO samples (Fig. 4, 5d and Fig. S3, ESI†), it is clear that the diffusion contribution of GO–CNT–NiO is larger than that of G–CNT–NiO. Since the size Conflicts of interest and crystallinity of NiO nanostructures are similar in both samples, this fact would indicate that remaining oxygen functionalities and There are no conflicts to declare. structural defects present in rGO sheets, but not in G, promote diffusion-limited electrochemical processes. Similar to GO-based samples, galvanostatic charge–discharge Acknowledgements measurements of G-CNT-NiO electrodes show quasi-linear The authors acknowledge the financial support of the Spanish voltage response (Fig. 5e). However, a detailed inspection of the Ministry of Economy, Industry and Competitiveness under the cycles corresponding to the (5/0/5) sample reveals nonlinearities project ENE2017-89210-C2-1-R, and support from AGAUR of with some levelling at about 0.2 V, which could be an indicator Generalitat de Catalunya through the project 2017 SGR 1086. of bulk faradaic reactions (Fig. S6, ESI†), not observed in the ICMAB acknowledges financial support from the Spanish samples containing carbon nanotubes. These faradaic reactions Ministry of Economy and Competitiveness, through the ‘‘Severo would take place in NiO aggregates as already indicated from Ochoa’’ Programme for Centres of Excellence in R&D (SEV- studies of capacitive and diffusive components calculated from 2015-0496). FTIRM experiments were performed at the MIRAS cyclic voltammetry. As expected, the volumetric capacitance beamline at the ALBA Synchrotron with the collaboration of calculated from the charge–discharge data decreases with the ALBA staff. The authors acknowledge support of the publication applied current density, being about threefold higher in the fee by the CSIC Open Access Publication Support Initiative (5/2/5) sample than in the (5/0/5) one. Fig. 5f presents the Nyquist through its Unit of Information Resources for Research (URICI). plots of the same samples, revealing slight differences in behaviour. Using the same equivalent circuit as with the GO–CNT–NiO samples, the calculated ESR is about 25 O and References 48 O in the (5/0/5) and (5/2/5) electrodes, respectively. Besides, their respective n exponent related to the constant phase 1 X. Zhang, X. Cheng and Q. 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