A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped

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A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped to Carbon Nanotubes Forest for Efficient Oxygen Evolution Francesco Malara, Sonia Corallo, Enzo Rotunno, Laura Lazzarini, Elpida Piperopoulos, Candida Milone, and Alberto Naldoni ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 9, 2017

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ACS Catalysis

A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped to Carbon Nanotubes Forest for Efficient Oxygen Evolution Francesco Malara,*,† Sonia Carallo,‡ Enzo Rotunno,§ Laura Lazzarini,§ Elpida Piperopoulos,ǁ Candida Milone,ǁ Alberto Naldoni*,†,≠ †

CNR-Istituto di Scienze e Tecnologie Molecolari, Via Golgi 19, 20133 Milan, Italy Istituto di Nanotecnologia CNR-Nanotec, Polo di Nanotecnologia c/o Campus Ecotekne, Via Monteroni 73100 Lecce, Italy § IMEM-CNR, Parco Area delle Scienze 37/A, 43100 Parma, Italy ǁ Dipartimento di Ingegneria, Università di Messina, 98166 Messina, Italy ≠ Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacky University, Slechtitelu 11, 783 71 Olomouc, Czech Republic ‡

ABSTRACT: The development of highly active, cheap and stable electrocatalysts for overall water splitting is strategic for industrial electrolysis process aiming at a sustainable hydrogen production. Here, we report the impressive electrocatalytic activity for the oxygen evolution reaction of Al-doped Ni(OH)2 deposited on a chemically-etched carbon nanotubes forest (CNT-F) supported on a flexible polymer/CNTs nanocomposite. Our monolithic electrode generates a stable current density of 10 mA/cm2 at an overpotential (η) of 0.28 V toward the oxygen evolution reaction in NaOH 1 M and reaches approximately 200 mA/cm2 at 1.7 V vs RHE in KOH 6 M. The CNT-F/NiAl electrode shows also an outstanding activity for the hydrogen evolution reaction in alkaline conditions. When CNT-F/NiAl is used both at the anode and at the cathode, our device is able to sustain the overall water splitting reaching 10 mA/cm2 at η = 1.96 V. The high electrocatalytic activity of the CNT-F/NiAl hydroxide is due to the huge surface area of the CNT forest, to the high electrical conductivity of the nanocomposite substrate, as well as to the interactions between the NiAl catalyst and the CNTs. KEYWORDS. Water splitting, electrocatalysis, carbon nanotubes, NiOx, support-catalyst interaction.

INTRODUCTION The growing global energy demand requires the industrial implementation of efficient processes that convert renewable energy sources into sustainable chemical fuels.1,2 Among many chemical reactions, water splitting is considered the most promising one, holding the potential to produce highly pure hydrogen without the formation of additional harmful byproducts.3 Electrolysis, namely electrocatalytic water splitting, has been used since decades although its spreading to largescale industrialization has been hampered by the low efficiency of traditional electrocatalysts. This is especially exacerbated for the oxygen evolution reaction (OER), which is the half reaction of water splitting with a sluggish kinetics and that introduces high overpotential losses.4 For long time, the most active catalysts for OER (e.g., RuO2 and IrO2) have been based on precious metals.5-8 However, oxides and oxy-hydroxides of abundant transition metals are excellent alternatives to precious catalysts presenting high electrocatalytic activity for OER. Zhang and coworkers showed that a homogeneous dispersion of FeCoWOx can reach 10 mA/cm2 with only 0.191 V of overpotential.9 Trotochaud et al. investigated oxides and oxy-hydroxides of Ni, Fe and Co and their mixing reporting an overpotential of

0.297 V to produce 1 mA/cm2 for Ni0.9Fe0.1Ox.10 Smith and coworkers studied FeCoNiOx and FeCoOx, highlighting the high performance of amorphous metal oxides for OER.11 In general, Ni- and Co-based oxides result very effective both in OER and in hydrogen evolution reaction (HER), except for oxygen evolution in acid media.5,12 In addition, by taking advantage of metal-support interactions it is possible to further enhance the performance of the electrodes based on Ni, Co and Mn oxides.13,14 The catalyst-substrate interaction may modify the binding energy of the reaction intermediates through the perturbation of the electronic structure of catalytic sites thus decreasing the OER overpotential, as observed in the case of NiCeOx on Au electrode.15,16 Such results were reached thanks to the recent developments of both powerful computational tools and synthetic routes that allow a precise control over nanoscale morphology of electrocatalysts.9,13,17-19 Recently, the use of carbon-based nanostructures for electrocatalytic water splitting have attracted a great interest. CNTs can reach high current density even without the application of further treatments, with their high active surface area being a key contributor to their high electrochemical activity.20,21 However, the most active electrocatalysts include a combination of carbonaceous materials, metal or multi-metal

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oxides.17,22,23 Outstanding performance for both OER and HER24 have been reported by using electrode materials comprising N-doped CNTs,25-28 vertically-aligned CNTs,29 oxidized CNTs,30 CNTs covered by electrocatalysts or metal oxides22,23,31-34and carbonaceous hybrid structure.35 Gong et al. reached the impressive result of 20 mA/cm2 applying a voltage of 1.5 V by using NiO/Ni-CNT as water reduction catalyst and a high-performance NiFe-layered double hydroxide (NiFe LDH) water oxidation catalyst.33 Nevertheless, such materials usually are prepared as inks or liquid suspensions and may present issues related to long-term stability and synthetic scalability compromising the ability to be marketed. A possible solutions to overcome this long-standing issue is to design 3D monolithic carbon based electrodes.26 For instance, Balogun et al. characterized a 3D porous N-doped carbon electrodes in KOH 1 M showing an overpotential for the overall water splitting reaction of 1.85 V to produce 10 mA/cm2.21 Sharifi et al. produced a composite composed by maghemite nanorods anchored on a 3D nitrogen-doped CNTs substrate as scalable and efficient electrode for water oxidation.23 Here, we report the extraordinary performance for overall water splitting of robust and monolithic electrodes based on a CNT forest transferred on a flexible nanocomposite (NC) of CNTs and polypropylene. The bare CNT-F electrode scaffold exceeds the performance of typical metal oxides in alkaline solution, reaching 1.6 mA/cm2 at 0.35 V of overpotential.5 Etching the CNT-F by plasma or by wet chemical approaches introduces organic functional groups (e.g., carboxyl, carbonyl and hydroxyl) onto the CNTs surface thus decreasing the energetic barriers for water splitting. The performance increases considerably after electrodeposition of Ni-based hydroxides onto the CNT-F surface. In particular, the electrodeposition of Al-doped Ni(OH)2 on the etched CNT-F allows to reach 10 mA/cm2 of current density at the overpotential of 0.28 V. We also report the electrochemical characterizations of our nanocomposite electrodes in KOH 6 M and in a two-electrode configuration under both alkaline and acidic conditions. Finally, we propose a schematic of a possible continuous industrial process for the large-scale production of CNTs-based electrodes.

EXPERIMENTAL SECTION Synthesis of Carbon Nanotubes Forest. An aluminium foil was sonicated in ethanol for 30 s to remove organic contaminations from its surface. A fresh solution of Fe(NO3)3·9H2O (Sigma-Aldrich, >98%), Co(NO3)3·9H2O (Sigma-Aldrich, >98%) and Al(NO3)3·9H2O (Sigma-Aldrich, ≥ 99%) was prepared with a concentration respectively of 0.05 M for both salts of Fe and Co and 0.1 M for the Al salt, by using anhydrous ethanol (Fluka, 99.8%). The solution was deposited on the aluminium foil by drop casting and dried at room temperature. Afterwards, the substrate was introduced into a horizontal quartz-tube CVD reactor flooded with helium (200 sccm, standard cubic centimetre per minute) and hydrogen (45 sccm) and heated up to 645 °C in 2 h (heating rate: 5 ºC min-1). After reaching the reaction temperature, acetylene (12 sccm) as the carbon source was added to the gas stream for 20 minutes. After the reaction time, the reactor was cooled down to room temperature under a He/H2 flow. The sodeposited aluminium foil was taken out from the reactor and

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characterized. The multi-walled carbon nanotubes (MWCNTs) have a diameter of about 20 nm and a length of 35 ± 3 µm.36 Preparation of CNT-F on flexible electrodes. Polypropylene (PP) (PP 352-08R) with a Metal Flow Index (MFI) of 8 was bought from Dow Chemicals. Baytubes C150P MWCNTs were purchased from Bayer Material Science AG and had a carbon purity of 95%, a bulk density of 140–160 kg/m3, an outer and an inner mean diameter of 13–16 nm and 4 nm, respectively, and 3–15 walls. All these materials were used as received. The master batch contained 15 wt% MWCNTs in PP. The composites were processed by twin screw extrusion on a Leistritz ZSK 27 HP twin screw extruder with an L/D ratio of 52. The processing methodology was carried out at an extruder barrel temperature of 220 ºC, a processing speed of 1000 rpm and a throughput of 10 kg/h. The material extruded was subsequently pelletized for further use. Nanocomposite pellets were then modelled by compression moulding (temperature: 210 ºC, pressure: 50 MPa for 10 min) in a thin plate with a thickness of about 360 µm. Subsequently, CNT-F was transferred onto NC plates through a properly set up hot pressing process. As described later, the top surface of the NC plate was previously subjected to an oxygen plasma etching treatment (RIE) for 5 min, with the aim of effectively removing the insulating polymeric cover layer and allowing the randomly oriented CNTs to partially protrude from the top surface of the NC plate. The transfer process was executed at a temperature of 150 °C while a constant pressure of about 50 MPa was applied for one minute. The underlying aluminium foil was subsequently removed by peel-off and consequently the CNT-F film was stably embedded onto the flexible NC plate.36 This electrode was labelled FNC. Reactive ion etching and chemical etching on FNC electrodes. Reactive ion etching was performed into an IONVAC inductively coupled plasma (ICP) reactor (PGF 600 RF HUTTER). The FNC samples were placed into the chamber, followed by evacuation to 30 mTorr. O2 gas was introduced at a flow rate of 20 sccm and the glow discharge was ignited at 50 W on both sides of the NC (5 minutes) and at 25 W only on the CNT-F side (15 minutes). Alternatively, CNTs surface of the FNC electrode was chemically etched by means of a piranha solution for 2 h (30 ml H2O2 with 70 ml concentrated H2SO4 solution).36 We will refer to the electrodes treated with RIE as FNC-RE, and those treated with piranha solution as FNC-WE. After etching treatments, the back of the electrodes was covered with silver paste, dried under an IR lamp for 60 minutes and covered with paraffin. In this way, we create an effective back-contact to collect the charges, while preventing the direct contact between silver and the electrolyte solution. Electrodeposition of metal hydroxides electrocatalysts. Ni(OH)2, Fe-doped Ni(OH)2 and Al-doped Ni(OH)2 were deposited onto FNC or fluorine-doped tin oxide (FTO) electrodes through electrodeposition. Ni, NiFe and NiAl catalysts were prepared by starting from 100 mL of a 0.1 M solution of Ni(NO3)2 or by adding 0.2 mmol of either FeCl3·6H2O or AlCl3·6H2O to the Ni(NO3)2 solution, respectively. Prior to electrodeposition, the pH of the solutions was adjusted to 6.6 and the system was purged with nitrogen gas for 1 h. Electrodeposition was carried out in a three-electrode cell configuration at -16 mA/cm2 for 10 s, by using a Ag/AgCl reference electrode (RE) and a Pt mesh as counter electrode (CE).37 Morphological and structural characterization. Scanning electron microscopy (SEM) was performed by a Zeiss Gemini field emission microscope at an accelerating voltage of 15 kV.


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Scheme 1. Schematic representation of the strategy adopted to increase CNTs functionalization and OER activity: (a) no-treated CNTs, (b) etched CNTs, (c) NiAl-based hydroxides catalysts deposited on etched CNTs. The open circuit potential (OCP) values show moderate sensitivity to superficial treatments.

Large area chemical investigations were carried out by means of a S360 Cambridge SEM, operating at 20kV, equipped with an Oxford Instruments X-ray microanalysis (EDX) mapping system. The signal was integrated over a 1 mm2 region under continuous electron beam illumination in order to maximize it. Analytical and conventional transmission electron microscopy (TEM) studies were performed in a highresolution (HR) (0.18 nm) field emission JEOL 2200FS microscope operating at 200 kV, equipped with in-column Ω energy filter, 2 high-angle annular dark-field (HAADF) detectors and EDX. The CNTs were mechanically removed from the device and dispersed on holey carbon grids for the observation. Electrochemical characterization. Electrochemical measurements were performed with a PGSTAT204 Autolab potentiostat (Metrohm). The electrodes were electrochemically characterized in a three-electrode system where the RE was an Ag/AgCl electrode, while a high-surface-area Pt mesh was the CE. The potential (E) was referred to the reversible hydrogen electrode (RHE) scaled through the Nernst equation: / 0.197 0.059 (1) where EAg/AgCl is the measured electrode potential vs the reference electrode and 0.197 V is the reference electrode standard potential vs the normal hydrogen electrode. In the manuscript, all measurements will be reported with respect to the RHE. Unless otherwise noted, the measurements were carried out in NaOH 1 M aqueous solution at pH 13.6. The cell was purged for ∼20 min with N2 prior to each set of experiments. During cyclic voltammetry measurements, the solution was continuously bubbled with O2 and magnetically stirred.37 The uncompensated resistance of the cell was measured with a single-point high-frequency impedance measurement and IR drop was compensated at 85% through positive feedback using the NOVA software. Our typical electrochemical cells had Ru = ∼20 Ω with FTO support and Ru = ∼3 Ω for both NC and FNC electrodes. Cyclic voltammetry (CV) curves were measured at a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) data were gathered using a 10 mV amplitude perturbation at frequencies between 0.01 Hz and 1 MHz. At least five electrodes of each type were fabricated and tested. To refer and compare the CNT-based electrodes activity to the scientific literature, the electrochemically active surface area (ECSA), the roughness factor (RF), the specific current density (js) and the current density per geometric area (jg) were determined. ECSA was estimated from the electrochemical double-layer capacitance of the catalytic surface calculated both by CV measurements at different voltage rates and by EIS at three potentials around the open circuit potential (OCP). All electrodes showed similar characteristics with a deviation

from reported data of ±20 %. RF was calculated dividing the ECSA values by the geometric area of the electrode; js and jg were calculated as reported by Jaramillo et al.37 The average current density per geometric area (jg,η=0.35V) and the corresponding specific activity (js,η=0.35V) were measured at an overpotential of 0.35 V. The values for each catalyst were fully investigated with standard deviations by at least three independent measurements.

RESULTS AND DISCUSSION The FNC electrode is prepared through a hot pressing process that assures CNT-F to be stably embedded onto the flexible NC plates. Our procedure guarantees a strong mechanical adhesion since the CNTs are physically attached to the NC, resulting in a robust and flexible electrode (FNC) that does not release CNTs into the electrolyte under operating conditions thus overcoming intrinsic limitations of spin coating or drop casting deposition. To increase the CNTs-water interactions and to thus enhance the electrodes activity towards OER, we first modify with chemical functional groups (Scheme 1b) the CNTs surface by using a wet etching treatment – (FNC-WE) or a reactive ion etching (FNC-RE), then we deposit Ni-based hydroxides (e.g. NiOx, NiFeOx, NiAlOx) catalysts through electrodeposition (Scheme 1c). Figure 1 shows the SEM morphological investigation of the FNC samples. The carbon nanotubes forest protruding by the NC support does not evidence a preferential orientation (Figure 1a). The wet etching treatment does not produce strong modification of the superficial morphology of the electrode (Figure 1b and S1a). After the RIE process, the CNTs superficial morphology is compromised (Figure S1b) showing broken structures and open terminations (Figure 1c). Figure 1d shows that FNC-WE/NiAl is composed by three distinct layers that starting from the bottom to the top can be identified as the NC film, the CNT-F and the NiAl hydroxide catalyst, respectively. Figure 1e shows more in detail that the NiAl catalyst is deposited as a thin film that wraps (like a glue) the CNT-F surface. Figure 1f shows the EDX spectrum of the FNC-WE/NiAl electrode and the corresponding quantitative analysis. As expected, Ni is detected only on FNC-WE/NiAl at a concentration around 12 %at., while Al, Co and Fe are present in similar %at. both on FNC and FNC-WE/NiAl electrodes (Figures S2). This can be explained considering that Al, Co and Fe were used to synthesize CNTs and the subsequent electrodeposition of NiAl hydroxide does not change significantly the Al amount. Similar compositions were obtained for both FNCWE/Ni and FNC-WE/NiFe samples. Nevertheless, the


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Figure 1. SEM images of the carbon nanotubes forest protruding from NC. (a) No-treated CNTs, (b) after wet etching and (c) after reactive-ion etching. (d) Low magnification and e) high magnification SEM micrographs showing the morphology of the NiAl hydroxide morphology on the FNC-WE/NiAl sample. (f) EDX spectrum and composition analysis for the FNC-WE/NiAl electrode. A high amount of Na is detected since the electrode was analyzed after electrochemical characterization in NaOH.

contribution of the Al electrodeposition on FNC surface highly affects the electrochemical activity as it will be shown below. To provide further structural details of our composite catalysts, we report TEM and HAADF-STEM analysis of representative samples. Figure S3a shows a TEM micrograph of an isolated multi-walled CNT formed by approximately 20 walls and reaching 20 nm in lateral thickness. Inside the cavity of some CNTs we observe also the presence of trapped multiwalled carbon nanoparticles grown during the CNTs synthesis. Importantly, many catalyst nanoparticles used for CNTs growth remain trapped inside the CNTs structure even after applying the etching treatments, as evidenced by both HAADF-STEM image (brighter spots in Figure 2a) and EDX maps (Figure 2c) as well as by TEM analysis (Figures S3b). This isolates the metallic nanoparticles from the electrolyte thus blocking their contribution to OER.20 From the SI maps Fe and Co show an almost identical elemental distribution confirming that they origin from the catalyst used in CNTs synthesis. The distribution of Al closely resembles that one of O, suggesting that the Al deposits are residues from the CNTs transfer process to the NC and that they undergo oxidation. Interestingly, Ni elemental map is very similar to both O and Al distributions. EDX (Figure S4) reveals the presence of about the 17 %at., in agreement with SEM-EDX analysis. To have a fair comparison with other documented OER electrocatalysts as well as to reveal the intrinsic catalysts performance, we follow the benchmarking protocol suggested by Jaramillo’s group and report the figure-of-merits and electrochemical properties of our electrodes in Table 1.37 First, we compare the electrochemical properties of three different supports, namely the fluorine-doped tin oxide coated

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glass (FTO), the nanocomposite NC, and the FNC monolithic electrode. The potenziostat offsets the IR-drop of the FTO electrode with 20 Ω, instead for both the NC and the FNC electrodes, the resistive drop is compensated by only about 3 Ω (as confirmed by the EIS measurements reported below) confirming the high conductivity of our CNTs electrode platforms. Anodic cyclic voltammetry (CV) scans (Figure 3a) highlight that the NC electrode reaches almost 0.5 mA/cm2 at η = 0.35 V, while the FTO performance is almost 5 times lower. A strong increase of the current flowing up to 1.6 mA/cm2 is further observed when the CNT-F is transferred to the NC substrate (FNC electrode), outperforming benchmark electrocatalysts like NiOx (1.1 mA/cm2) and CoOx (0.9 mA/cm2).37 Notably the overpotential to sustain 10 mA/cm2 drops from 0.62 V for FTO to 0.45 V for the FNC electrode. The hot transferring of the CNTs is very effective and produces both a very low electric resistance through the electrode, ascribable to an intimate interaction between the NC and CNT-F, and a strong superficial activity that considerably reduces the resistance at the electrode/electrolyte interface. This is corroborated by electrochemical impedance spectroscopy (EIS) measurements carried out at 1.58 V (Figure 3b). To fit EIS data we use a Rs(RctC) equivalent circuit, where Rs is the series resistance that includes the internal resistance of the electrode, Rct is the charge transfer resistance and C is the capacitance (a constant phase element is used to fit the capacitive element of the NC electrode). The parameters extracted from the data elaboration are summarized in Table S1 and show very low values of Rs for the nanocomposite electrodes (4 and 2.6 Ω*cm for NC and FNC, respectively) when compared to FTO (27.8 Ω*cm). Most importantly, Rct shows an impressive drop of 3 orders of magnitude passing from FTO (100920 Ω*cm) to NC (1123 Ω*cm) and finally to the best performing electrode, that is FNC (135.5 Ω*cm). Interestingly, FTO and NC electrodes show a capacitance of 1.69*10-5 and 9.14*10-5 F/cm2, while the capacitance of FNC electrode is two order of magnitude higher (6.2*10-3 F/cm2) suggesting that the very high surface area of FNC underlies the improved reactivity. To enhance the activity of FNC-based electrodes, we apply both a physical, by means of reactive ion etching, and a wet oxidation treatment, by using a piranha solution.30,36 These treatments generate functional groups (e.g., -COOH, -OH, CO) onto the outer surface of the CNTs and “build a bridge” between the C-based electrocatalyst and water, thus reducing the energetic barrier for OER and improving its catalytic activity.30,38 The functional groups on FNC electrodes are measured by Boehm titration, whose methodology is reported elsewhere.39 Pristine FNC electrodes present 1.6 mmol of acidic oxygen-containing surface groups per gram of CNT material. After chemical etching by piranha solution, FNC-WE show a functional groups increase of about 10%, while the functionalization degree on FNC-RE increases up to about 30%, in agreement with literature data.40,41 Carrying out the CV scans, we observe a difference between the first and the second scan. During the first cathodic sweep, the curve shows one big reduction peak (at 0.61 V) that disappears in the second sweep (Figure S5a). This phenomenon is also present in the FNC-WE/Ni and FNC-WE/NiAl electrodes (Figure S5b and S5c) and can be attributed to the reduction of the carboxylic, hydroxyl and oxygen species produced during the etching treatment in piranha solution and initially


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Figure 2. (a) TEM micrograph and (b) related elemental distribution EDX map of a CNTs bundle of a representative FNC-WE/NiAl electrode.

Figure 3. (a) Cyclic voltammetry and (b) Nyquist plot measured at 1.58 V for the FTO (black curve), the NC (red curve) and the FNC (blue curve) electrodes in NaOH 1M.

physisorbed on the CNTs surface.42 On the contrary, FNC-RE electrodes do not show any evident difference in the two initials CV scans (Figure S5d) suggesting that the RIE treatment modifies the CNTs structure much strongly than the piranha solution, forming only functional groups chemically bonded to the CNT-F structure. Cyclic voltammetry scans (Figure 4a) clearly highlight that both etching treatments boost the performance with the currentdensity generated at η = 0.35 V that reaches about 4 mA/cm2 and 4.5 mA/cm2 for the FNC-RE and FNC-WE electrodes, respectively (Figure 4b), and an η (10 mA/cm2) = 0.36 - 0.37 V for both FNC-RE and FNC-WE (Figure 4c). The NC electrode (dark line in Figure 4c) shows a noisy signal ascribable to the presence of bubbles at the electrode surface. Such bubbles significantly disappear after etching treatments probably due to the presence of more active carbon edges exposed towards the electrolyte.

Figure 4. (a) Cyclic voltammetry, (b) Chronoamperometry at η = 0.35 V and (c) Chronopotentiometry at j = 10 mA/cm2 for the NC (black curve), the FNC (red curve), the FNC-RE (blue curve) and the FNC-WE (green curve) electrodes in Na(OH) 1M. After a period of stabilization, the FNC-WE electrode slightly increases from the point of view of both current density and overpotential.


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Figure 5. (a) Cyclic voltammetry, (b) chronoamperometry at η = 0.35 V and (c) chronopotentiometry at 10 mA/cm2 in NaOH 1 M, for the FNC-WE, the FNC-WE/Ni, the FNC-WE/NiFe and the FNC-WE/NiAl electrodes. (d) CVs and chronoamperometry (inset) of FNCWE/Ni, FNC-WE/NiFe and FNC-WE/NiAl electrodes at η = 0.35 V for 24 h in KOH 6 M. Table 1. Electrochemical parameters of the electrodes tested in NaOH 1M.

a e

Electrodes

ECSA (cm2)a

RFb

ηt=0 (V)c

ηt=2h (V)d

NC FNC FNC-RE FNC-WE FNC-WE/Ni FNC-WE/NiFe FNC-WE/NiAl FTO FTO/Ni FTO/NiFe FTO/NiAl

3.5 ± 0.4 171 ± 18 139 ± 15 450 ± 35 390 ± 22 325 ± 12 334 ± 40 0.23 ± 0.01 0.22 ± 0.01 0.21 ± 0.01 0.23 ± 0.01

5.8 ± 0.3 570 ± 42 464 ± 27 2250 ± 35 1444 ± 45 812 ± 55 939 ± 34 0.24 ± 0.01 0.31 ± 0.02 0.35 ± 0.02 0.34 ± 0.01

0.55 ± 0.01 0.42 ± 0.01 0.39 ± 0.01 0.37 ± 0.01 0.32 ± 0.01 0.31 ± 0.01 0.28 ± 0.01 0.65 ± 0.02 0.55 ± 0.01 0.51 ±0.01 0.53 ± 0.04

0.58 ± 0.04 0.45 ± 0.02 0.37 ± 0.17 0.36 ± 0.01 0.32 ± 0.01 0.31 ± 0.01 0.28 ± 0.01 0.62 ± 0.02 0.53 ± 0.02 0.52 ± 0.02 0.52 ± 0.03

jg,η=0.35V (mA/cm2)e 0.56 ± 0.03 1.58 ± 0.14 3.85 ± 0.25 4.51 ± 0.41 14.54 ± 0.03 16.28 ± 0.04 19.72 ± 1.70 0.12 ± 0.01 0.85 ± 0.01 1.80 ± 0.02 1.6 ± 0.2

js,η=0.35V (mA/cm2)f 0.096 ± 0.004 0.0028 ± 0.0001 0.0083 ±0.0003 0.0020 ± 0.0001 0.0101 ± 0.0020 0.0200 ± 0.0010 0.0210 ± 0.0010 0.5032 ± 0.0200 3.62 ± 0.12 5.18 ± 0.40 4.63 ± 0.36

Electrochemically active surface area. b Roughness factor. c Overpotential at t = 0 s, d Overpotential at t = 7200 s. Specific current density at η=0.35V. f Current density per geometric area at η=0.35V.

The performance of etched electrodes confirms that the presence of functional groups such as –COOH and –OH effectively improves the OER kinetics.30 The electrocatalytic ac tivity of the etched FNC electrodes suggests that the best CNT-F functionalization is obtained by piranha solution. This mildly wet chemical oxidation assures a good balance between conduction and functionalization of the CNT-F thus increasing the electrode performance. On the contrary, RIE treatment negatively affects the CNTs conduction, compromising their structure, and in turn the elec-

tronic continuity through each CNT (see Figure 1c). To further enhance the OER activity of our electrodes, we modify FNC-WE with Ni(OH)2 (FNC-WE/Ni), Fe-doped Ni(OH)2 (FNC-WE/NiFe) and Al-doped Ni(OH)2 (FNCWE/NiAl) catalysts. The activity of the FNC-WE electrode is similarly enhanced in terms of onset potential and maximum current density for both FNC-WE/Ni and FNC-WE/NiFe, while a further boost in performance is obtained in the case of FNC-WE/NiAl (Figure 5a). Specifically, both Ni and NiFe hydroxides catalysts enhance the current density at η = 0.35 V


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ACS Catalysis

Figure 6. (a) and (b) Cyclic voltammetry curves at two different potential ranges for the FNC-WE, FNC-WE/Ni, FNC-WE/NiFe, FNCWE/NiAl electrodes and platinum mesh in NaOH 1 M. (c) Chrono-potentiometry of FNC-WE/NiAl vs. Pt in NaOH 1M at 10 and 100 mA/cm2. (d) Chrono-potentiometry of FNC-WE/NiAl electrodes used both as anode and cathode at 10 mA/cm2 in NaOH 1 M and H2SO4 0.5 M.

(Figure 5b) of the FNC-WE electrode from 4.5 mA/cm2 to 14.5 and 16.3 mA/cm2, respectively, with a similar 50 mV decrease of overpotential to flow 10 mA/cm2 that reaches 0.31 V in the case of FNC-WE/NiFe. The best electrocatalytic activity is observed for FNC-WE/NiAl, which produces about 20 mA/cm2 at η = 0.35 V and needs only 0.28 V of overpotential to operate the OER at 10 mA/cm2. This performance is among the highest reported in the literature and even better than RuOx electrocatalyst that, in the same alkaline conditions, needs an overpotential of 0.32 V after 2 h. 13,20,21,25,30,32-35,43,44 IrOx is not stable in NaOH solution and an overpotential of about 1.05 V after 2 h is necessary to maintain a current density of 10 mA/cm2.37 Moreover, with respect the electrocatalyst reported above, Nickel compound results cheaper and stable for long time. It is worth noting that the initial performance of the Aldoped electrode is even higher, starting from 32 mA/cm2 and becoming stable after 1 h. To report the chemical stability and OER performance of CNTs-based electrodes close to industrial operating conditions for alkaline electrolysers, we test the performance of FNCWE/Ni, FNC-WE/NiFe and FNC-WE/NiAl electrodes in KOH 6M in a three-electrode configuration for 24 h at room temperature (and ambient pressure).13 The current density of the most performing electrode (FNC-WE/NiAl) starts from about 70 mA/cm2 and reaches a good stabilization after about 4 h. Figure 5d reveals a stable and huge current density

reached already at very low overpotential (η = 350 mV) such as 22, 40 and 45 mA/cm2 for FNC-WE electrodes containing Ni, NiFe and NiAl, respectively (inset of Figure 5d). However, if the entire CV scans are considered (Figure 5d), FNC-WE anodes show impressive performance of about 103 (Ni), 143 (NiFe) and 197 mA/cm2 (NiAl) at 1.7 V vs RHE. This performance is already competitive at an industrial level and could be further improved by testing the materials at both higher temperature and pressure. The specific interactions between the electrolyte, the CNT-F and the Ni-based hydroxides underlie the high activity of our electrodes. To investigate more in detail their electrochemical activity, we determine and report in Table 1 the electrochemically active surface area (ECSA), the roughness factor (RF), the specific current density (js) and the current density per geometric area (jg) for all tested electrodes. Moreover, we prepare reference electrocatalysts with Ni(OH)2, NiFe(OH)2 and NiAl(OH)2 on FTO substrates by electrochemical deposition. Their OER activities are reported in Table 1 and Figure S6 and they are in well agreement with the literature data.5,10 From our measurements, the ECSA value of the NC electrode is 3.5 cm2. After transferring the CNT-F on NC (FNC electrode), the ECSA increases of 50 times (170 cm2), confirming the huge increase of superficial area due to the CNTs forest. The RIE treatment slightly reduces the ECSA to 139 cm2 due to the excessive physical damaging provoked by the


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Figure 7. Schematic representation of the proposed production process for the FNC-NiAl electrodes. Extrusion, lamination, hot-transfer of CNT-F on NC, oxygen plasma treatments and metal spray deposition are combined into a continuum roll-to-roll process.

reactive oxygen species. On the contrary, after wet chemical etching (FNC-WE), the ECSA takes a further leap reaching 450 cm2, that is almost 3 times the value obtained for FNC, and 2 and 3 orders of magnitude higher than ECSA of NC and FTO, respectively. After Ni catalysts deposition, the ECSA tends to decrease by 15-20%. ECSA for FTO is 0.23 cm2, and after hydroxides deposition it remains almost constant in agreement with previous reports.37 All the parameters reported in the Table 1 are tightly correlated: in general, an improvement in overpotential and current density is observed when the ECSA increases. However, the lowest overpotential and the highest current density are not reached when ECSA is maximum and this assigns to catalytic properties of the catalysts the good performance of an electrode. In fact, η (10 mA/cm2) of NC, FNC, FNC-RE and FNCWE decreases from 0.55 to 0.37 V with increasing of ECSA (see Table 1). On the contrary, once the NiAl catalyst is deposited on FNC-WE, the η (10 mA/cm2) further decreases to 0.28 V, although a 20% decrease of ECSA is observed. The current density follows a similar trend. This shows how both ECSA and catalytic properties influence the electrochemical activity of a given electrode. Going more down on this data analysis, we highlight that the current density of the FNC electrodes normalized over the geometric area, jg,η=0.35V, are among the highest reported in the literature, namely up to 19.72 mA/cm2 for FNC-WE/NiAl. As expected, when the current density is normalized over the ECSA, js,η=0.35V, the FNCelectrodes show two orders of magnitude lower specific activity than that of FTO/Ni catalysts (Table 1). This suggests that the high performance of FNC-hydroxides electrodes is ascribable to a combination of the CNT-F high surface area and the catalytic properties of Ni-based catalysts. We note also that for FNC-WE electrodes jg,η=0.35V follows the trend FNCWE/NiAl>/NiFe>/Ni, while in the case of FTO electrodes the activity trend is FTO/NiFe>/NiAl>/Ni. This suggests that NiAl may be activated by specific interactions with the FNCWE support resulting more active than the NiFe hydroxide deposited on FNC, similarly to the gold support-metal hydrox-

ides activation recently observed.13 Such considerations require further studies and are not the focus of the present work. We now investigate the behaviour of the FNC-WE electrodes for the HER taking the platinum mesh as a reference (Figures 6a and 6b). The FNC-WE/NiAl electrode produces a current density of about -4 mA/cm2 at Ε = -0.1 V in NaOH 1 M.42 FNC-WE electrodes reach -10 mA/cm2 at a potential of 0.400 V, -0.375 V, -0.360 V and -0.335 V for bare, Ni/, NiFe/ and NiAl/FNC-WE, respectively, while the Pt electrode can provide -10 mA/cm2 just with and advance of 160 mV with respect to the FNC-WE/NiAl electrode. It is noteworthy that at -0.5 V the curves of FNC-WE/NiAl and Pt cross and beyond this potential value our electrode is more efficient. To observe the electrodes behaviour in a device configuration, we build an electrolyser at two electrodes.21,33 Firstly, Figure 6c reports the chronopotentiometry of a device that uses FNC-WE/NiAl as anode and a platinum mesh as cathode in NaOH 1 M. The applied potential to continuously sustain 10 and 100 mA/cm2 is 1.62 V and about 2.2 V, respectively, among the best in scientific literature (see also Table S2 for comparison with other very active electrodes for OER from the literature).13,21 Secondly, by using FNC-WE/NiAl electrodes as the anode and the cathode we build both an alkaline (pH ~14) and an acidic (pH ~ 0) electrolyser. In this device configuration (Figure 6d), the η necessary to produce 10 mA/cm2 in alkaline or acid conditions is 1.96 and 2.04 V, respectively, and the electrodes result stable after 2 h of operation. When we flow 100 mA/cm2 our electrodes show a good stability in alkaline solution, while in acid conditions the performance falls down within one hour, probably due to corrosion phenomena. Considering the excellent performance, the high chemical stability and the robustness of the monolithic electrode based on CNTs, we propose a possible continuous production process (Figure 7) based on roll-to-roll technology that could enable the large-scale manufacturing of flexible FNC electrodes at competitive costs.46-48 Carbon nanotubes and the polymer are introduced into the extruder obtaining, after a lamination


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process, a laminar NC with optimized thickness.36 At this point, a hot wheel presses the CNT-F/Aluminium foil (coming from a roll) against the NC foil, transferring the CNT-F onto the support (NC electrode).49 Afterwards, a plasma superficial treatment removes the surplus superficial polymer and activates the FNC surface for the subsequent deposition of both the Ni-based catalyst (on the front side) and the silver film (on the back side) needed to collect the charge. Finally, the backside of the electrode is isolated by depositing an insulating material. In this process, we propose the use of plasma etching and spray deposition because they enable an industrial process in continuum, while producing electrodes with similar properties than those obtained when applying wet etching and electrodeposition techniques.

ing Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected] ; [email protected]

Author Contributions The manuscript was written through contributions of all authors. F. M. did the catalysts deposition and electrochemical measurements. S. C., E. P., C. M. synthesized CNT materials. E. R. and L. L. carried out TEM analysis. A. N. designed the paper and supervised the project.

Notes The authors declare no competing financial interest.

CONCLUSIONS We propose a monolithic and cheap nanocomposite electrode based on carbon nanotubes and polypropylene for the overall electrochemical water splitting. Such composite is lightweight and flexible and shows extraordinary electrochemical performance and chemical stability for the oxygen evolution reaction. The carbon nanotubes forest transferred on a carbon nanocomposite, and without further catalyst deposition, is an effective electrode able to produce 1.6 mA/cm2 at η = 350 mV, a higher current density than typical metal oxide catalysts. The high efficiency of the FNC electrode is motivated by the huge electrochemical active surface area and by its excellent electrical conduction. Superficial treatments and electrodeposition of catalysts exceptionally improve the performance of our electrodes. Chemical etching treatment promotes the current density from 1.6 up to 4.5 mA/cm2, while after Al-doped Ni(OH)2 deposition the electrode produces about 20 mA/cm2 at 350 V of overpotential (more than 45 mA/cm2 when tested in KOH 6 M solution), and generates a stable current density of 10 mA/cm2 at an overpotential (η) of 0.28 V. Etching treatments produce an intimate interaction between Al-doped Ni(OH)2 and CNTs that promote the synergic and effective cooperation between the high surface area of CNTs and the high specific activity of the NiAl electrocatalyst. Importantly, our monolithic electrodes demonstrate excellent performance for the hydrogen evolution reaction in alkaline conditions producing -10 mA/cm2 at a potential of -0.335 V. Once we build an alkaline electrolyser using FNC-WE/NiAl monoliths both at the anode and at the cathode, we observe the stable flow of 10 mA/cm2 at 1.96 V. The proposed electrode represents an effective and selfconsistent solution for industrial application, showing a maximum current density of 200 mA/cm2 at only 1.7 V vs RHE. This performance can be further enhanced by doping CNTs with heteroatoms and by optimization of multi-metallic catalysts deposition. Stability, lightness, robustness, low cost and high efficiency make this flexible electrode a promising solution for the realization of competitive industrial alkaline electrolysers based on carbon nanotubes electrodes and earthabundant catalysts.

ASSOCIATED CONTENT Supporting Information Additional SEM, TEM, EDX, cyclic voltammetry and electrochemical data on different CNTs/based electrodes. The Support-

ACKNOWLEDGMENT We gratefully acknowledge financial support from the Italian Ministry of Education, University and Research (MIUR) through the FIRB project “Low-cost photoelectrodes architectures based on the redox cascade principle for artificial photosynthesis” (RBFR13XLJ9). A.N. acknowledge the support by the Operational Programme Research, Development and Education - European Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic.

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SYNOPSIS TOC


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Scheme 1. Schematic representation of the strategy adopted to increase CNTs functionalization and OER activity: (a) no-treated CNTs, (b) etched CNTs, (c) NiAl-based hydroxides catalysts deposited on etched CNTs. The open circuit potential (OCP) values show moderate sensitivity to superficial treatments. 170x40mm (300 x 300 DPI)


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Figure 1. SEM images of the carbon nanotubes forest protruding from NC. (a) No-treated CNTs, (b) after wet etching and (c) after reactive-ion etching. (d) Low magnification and e) high magnifi-cation SEM micrographs showing the morphology of the NiAl hydroxide morphology on the FNC-WE/NiAl sample. (f) EDX spectrum and composition analysis for the FNC-WE/NiAl elec-trode. A high amount of Na is detected since the electrode was analyzed after electrochemical characterization in NaOH. 160x170mm (150 x 150 DPI)


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Figure 2. (a) TEM micrograph and (b) related elemental distribution EDX map of a CNTs bundle of a representative FNC-WE/NiAl electrode. 170x63mm (300 x 300 DPI)


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Figure 3. (a) Cyclic voltammetry and (b) Nyquist plot measured at 1.58 V for the FTO (black curve), the NC (red curve) and the FNC (blue curve) electrodes in NaOH 1M. 120x163mm (150 x 150 DPI)


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Figure 4. (a) Cyclic voltammetry, (b) Chronoamperometry at η = 0.35 V and (c) Chronopotentiometry at j = 10 mA/cm2 for the NC (black curve), the FNC (red curve), the FNC-RE (blue curve) and the FNC-WE (green curve) electrodes in Na(OH) 1M. After a period of stabilization, the FNC-WE electrode slightly increases from the point of view of both current density and overpotential. 70x156mm (150 x 150 DPI)


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Figure 5. (a) Cyclic voltammetry, (b) chronoamperometry at η = 0.35 V and (c) chronopotentiometry at 10 mA/cm2 in NaOH 1 M, for the FNC-WE, the FNC-WE/Ni, the FNC-WE/NiFe and the FNC-WE/NiAl electrodes. (d) CVs and chronoamperometry (inset) of FNC-WE/Ni, FNC-WE/NiFe and FNC-WE/NiAl electrodes at η = 0.35 V for 24 h in KOH 6 M. 219x162mm (150 x 150 DPI)


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Figure 6. (a) and (b) Cyclic voltammetry curves at two different potential ranges for the FNC-WE, FNCWE/Ni, FNC-WE/NiFe, FNC-WE/NiAl electrodes and platinum mesh in NaOH 1 M. (c) Chrono-potentiometry of FNC-WE/NiAl vs. Pt in NaOH 1M at 10 and 100 mA/cm2. (d) Chrono-potentiometry of FNC-WE/NiAl electrodes used both as anode and cathode at 10 mA/cm2 in NaOH 1 M and H2SO4 0.5 M. 218x164mm (150 x 150 DPI)


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Figure 7. Schematic representation of the proposed production process for the FNC-NiAl electrodes. Extrusion, lamination, hot-transfer of CNT-F on NC, oxygen plasma treatments and metal spray deposition are combined into a continuum roll-to-roll process. 299x150mm (150 x 150 DPI)


A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped

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