Functionalized Carbon Nanotubes with Ni(II) Bipyridine Complexes as

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Functionalized carbon nanotubes with Ni(II) bipyridine complexes as efficient catalysts for the alkaline oxygen evolution reaction Mohammad Tavakkoli, Magdalena Nosek, Jani Sainio, fatemeh davodi, Tanja Kallio, Pekka M. Joensuu, and Kari Erik Laasonen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02878 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Functionalized carbon nanotubes with Ni(II) bipyridine complexes as efficient catalysts for the alkaline oxygen evolution reaction †

‡

§

†

†

Mohammad Tavakkoli, Magdalena Nosek, Jani Sainio, Fatemeh Davodi, Tanja Kallio, Pekka M ‡

Joensuu, and Kari Laasonen

†

†,∗

Physical chemistry Group, Department of Chemistry and Material Sciences, School of Chemical

Engineering, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland.

‡

Organic chemistry Group, Department of Chemistry and Material Sciences, School of Chemical

Engineering, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland.

§

Department of Applied Physics, School of Science, Aalto University, P.O. Box 15100, FI 00076 Aalto,

Finland.

Abstract

Among current technologies for hydrogen production as an environmentally friendly fuel, water splitting has attracted increasing attention. However, the efficiency of water electrolysis is severely limited by the large anodic overpotential and sluggish reaction rate of oxygen evolution reaction (OER). To overcome this issue, the development of efficient electrocatalyst materials for OER has drawn much attention. Here, we show that organometallic Ni(II) complexes immobilized on the sidewalls of the multi-walled carbon nanotubes (MWNTs) serve as the highly active and stable OER electrocatalysts. This class of electrocatalyst materials are synthesized by covalent functionalization of 1

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the MWNTs with organometallic Ni bipyridine (bipy) complexes. The Ni-bipy-MWNT catalyst generates a current density of 10 mA cm-2 at overpotentials of 310 and 290 mV in 0.1 and 1 M NaOH, respectively with a low Tafel slope of ~ 35 mV dec-1, placing the material among the most active OER electrocatalysts reported so far. Different simple analysis techniques have been developed in this study to characterize such class of electrocatalyst materials. Furthermore, density functional theory calculations have been performed to predict the stable coordination complexes of Ni before and after OER measurements. KEYWORDS: carbon nanotubes, functionalization, bipyridine, organometallic Ni complex, oxygen evolution reaction

1. Introduction

Water splitting is a promising technology to produce hydrogen and store intermittent electrical energy from renewable resources such as solar and wind energy in the form of H2 fuel.1 However, the efficiency of water electrolysis is severely limited by the large anodic overpotential and sluggish reaction rate of oxygen evolution reaction (OER, 4OH− → 2H2O + 4e− + O2 in alkaline media).2 Minimizing the OER overpotential (ŋ), the required additional voltage to carry out the reaction in excess of the OER equilibrium potential of 1.23 V vs reversible hydrogen electrode (RHE), is a critical parameter in maximizing the efficiency of water splitting for producing hydrogen fuel. Reaching this goal is especially important when the OER is catalyzed through non-precious

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electrocatalysts rather than the high performing but expensive and rare active OER catalysts such as Ir or Ru-based catalysts.3

Metal−organic catalysts have been recently proposed as robust and active new electrocatalysts

for

OER.4-6 Organometallic

complexes

have

shown

efficient

performance for electrocatalytic reactions.7-10 Organometallic nickel complexes have been reported recently as efficient catalyst materials for photocatalytic11-12 and electrocatalytic7,10 hydrogen evolution and electrocatalytic OER9. The electrocatalytic activity of organometallic transition metal complexes can be improved when they are decorated on conductive supports with high-surface area because the number of active catalytic sites can be increased and the charge transfer is facilitated. Among such catalyst

supports,

carbon

nanotubes

(CNTs)

are

excellent

candidates

for

heterogeneous catalysts because of their high-surface area, high thermal and chemical stability, excellent electrical conductivity, insolubility in most solvents, and commercial availability.13-15 Recently, we have also shown that the exceptional structure of the CNTs allows us to use them as catalyst support without any functionalization or pretreatment to decorate metal nanostructures for making highly active and stable electrocatalyst materials.16-18 Moreover, CNTs are highly stable in the harsh conditions of OER in alkaline media, making them interesting catalyst supports for OER.15,18-21 Therefore, decorating organometallic nickel complexes on the CNTs can make a highly active and stable electrocatalyst for OER.

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Incorporating the Ni complexes onto the sidewalls of the CNTs via covalent immobilization can produce a stable hybrid material with efficient charge transfer between the CNTs and the complexes. Among different strategies for covalent functionalization of CNTs, diazonium salt addition to the surface of the single-walled carbon nanotubes (SWNTs) has been widely used because of its high efficiency and versatility.22-24 Diazonium salt-functionalization has been also performed to coordinate metal based catalysts to the SWNTs24 as the support to develop stable hybrid materials. However, the sp2 hybridization of the hexagonal structure of the sidewalls of the SWNTs is disturbed by the covalent functionalization and therefore the conductivity of the SWNTs is dramatically degraded. Consequently, the covalently functionalized SWNTs are not suitable catalyst supports for highly active electrocatalysts, where the high conductivity of the support is required. Brozena et al.25 have shown double-walled CNTs (DWNTs) after oxidation retain electrical conductivity up to 65% better than similarly functionalized SWNTs, attributing to enhanced electrical percolation through the intact inner tubes of the DWNTs. Therefore, by increasing the number of layers in CNTs, the degradation of electrical conductivity after chemical functionalization can be reduced. For covalent functionalized CNTs in electrochemical reactions, multi-walled carbon nanotubes (MWNTs) are highly promising catalyst supports since the covalent modification does not significantly degrade their high electrical conductivity. In MWNTs, the outer graphitic layer is used for the covalent functionalization and provides reaction sites for the electrocatalytic reaction, while the intact inner tubes serve as the highly conductive pathway, which significantly promote charge transfer process at the active sites on the surface of the outer tube.19 Therefore, the covalent immobilization of

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organometallic Ni complexes on the sidewalls of the MWNTs is a promising method to enhance the electrocatalytic activity of the Ni complexes. The following reaction mechanism has been proposed previously26-28 for the covalent functionalization of a graphene sheet with aryl diazonium salts, and the same mechanism can be also proposed for the other sp2 carbon structures such as CNTs. First, the aryl diazonium cation is reduced by a delocalized electron spontaneously transferred from the carbon support, resulting in an aryl radical by evolution of a molecule of N2. This radical is so reactive and thus the activation energy barrier of the reaction is quite low.

26,28

Therefore, this extremely reactive radical covalently bonds to a sp2 carbon atom in the support and changes the hybridization to sp3.27

Here, a synthesis method is presented to covalently functionalize MWNTs with bipyridine (bipy) ligands. Subsequently, Ni(II) is grafted on the Bipy-MWNT material and organometallic Ni-bipy complexes are synthesized on the sidewalls of the MWNTs (denoted as Ni-bipy-MWNT). The Ni-bipy-MWNT system is reported as a new class of low-cost, highly stable and active hybrid catalyst material for water oxidation.

2. RESULTS AND DISCUSSION

2.1. Synthesis of Catalyst Materials

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Synthesis of organometallic Ni complexes covalently attached on the CNTs Scheme 1 represents the synthesis process of covalent functionalization of the CNTs with bipy ligands and subsequently grafting Ni(II) on the ligands to make Ni-bipy-CNT material. The summary of the growth procedure is explained below (see Supporting Information for additional details of the synthesis process).

Step I: Functionalization of the MWNTs with bipyridine (Bipy-MWNT) 4-amino-2,2’-bipyridine is mixed with the MWNTs, dispersed in acetic acid, under argon atmosphere. Then sodium nitrite is added and mixed. The reaction is exothermic and performed in an open flask for the rapid evolution of N2. After a few minutes, the gas bubbling ceases. The mixture is stirred for 1 hour at room temperature under argon atmosphere. The mixture is diluted by adding dimethylformamide (DMF). The BipyMWNT material is filtered and washed with DMF and ethyl acetate (EtOAc).

Step II: Procedure for decoration of Ni(II) on Bipy-MWNT (Ni-bipy-MWNT) Bipy-MWNT produced in Step I, is suspended in DMF under argon atmosphere. The suspension is sonicated for a few minutes. Nickel(II) acetate, Ni (OAc)2, dissolved in DMF is added to the Bipy-MWNT solution via a syringe over a 20 minute period and the mixture was further sonicated for 11 hours. Afterward, the obtained dispersion is filtered through a PTFE filter (dark orange filtrate) and washed with DMF, EtOAc and ethanol.

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Scheme 1. The schematic representation of synthesis process of the Ni-bipy-MWNT hybrid material. The two possible Ni coordination complexes, Ni-bipy-(OAc)2-MWNT and Ni-(bipy)2-(OAc)2-MWNT, are represented. In the schematic structure of the MWNTs only the outer layer, which is used for functionalization, is depicted.

2.2. Oxygen Evolution Activity and Discussion The electrocatalytic activity of the functionalized MWNTs is investigated for the OER by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements using a standard three and four-electrode systems, respectively, in 0.1 and 1 M NaOH. Figure 1a exhibits polarization curves for OER on the Ni-bipy-MWNT catalyst compared with the pristine MWNT, Bipy-MWNT, and RuO2 electrodes in 0.1 M NaOH. All the catalysts are deposited on a glassy carbon (GC) electrode with a similar loading of ~ 0.2 mg cm-2. The OER activity follows the trend Ni-bipy-MWNT˃ RuO2˃ MWNT˃ BipyMWNT. Figure 1b demonstrates the activity of the Ni-bipy-MWNT in 0.1 and 1 M NaOH solutions (pHs of ~13 and ~14, respectively). The Ni-bipy-MWNT catalyst requires only the overpotentials of 310 and 350 mV in 0.1M NaOH and 290 and 320 mV in 1M NaOH to reach 10 and 60 mA cm−2 (denoted as ŋOER,10 and ŋOER,60), respectively. The pristine MWNT shows almost no catalytic activity for OER at low overpotentials (less than 50 mV). The OER activity slightly decreases when the MWNTs are covalently functionalized with bipyridine ligands. After functionalization of the surface of the MWNTs with bipy, the surface composition is changed. This change can affect surface structure or electronic properties of active sites as the most effective parameters on 7


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catalytic activity29, causing the slight change in the activity of the MWNTs after the functionalization.. However, the OER activity of Bipy-MWNT is significantly improved when Ni ions are grafted on the bipy ligands (Ni-bipy-MWNT sample).

In the cyclic voltammogram (CV) of the Ni-bipy-MWNT sample (Figure S1), a redox feature was observed around 1.38–1.45 V. A similar feature within the same potential range is observed previously for the Ni2+ oxidation state, which has been assigned to the reversible Ni(OH)2/NiOOH redox reaction.30-34 It is noteworthy that the OER performance of the nickel-based electrocatalysts has been commonly seen to be enhanced upon cycling in the OER region and it is attributed to Ni hydroxide formation.35-38 Likewise, herein the acetate (OAc) groups in Ni(II) acetate in the complex (Scheme 1) can be replaced with hydroxyl groups. Therefore, the all OER measurements presented here are reported after the OER performance has been stabilized (as also discussed in section 2.1). For the Ni-bipy-MWNT catalyst, an improvement in the OER activity is observed during the first ~ 50 cycles in the OER potential region and during these cycles the OER performance is stabilized. Figure S2 shows the OER performance of the Ni-bipy-MWNT before the OER current stabilization (initial anodic potential sweep) and after 50 OER cycles where the OER performance has been stabilized.

The kinetics of the reaction is assessed by Tafel plots derived from the OER polarization curves in Figure 1a. The potential region which is selected for the Tafel fit does cover neither the high potentials at which oxygen bubble evolution causes mass 8


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transport limitations nor the low potentials at which the redox transition for Ni occurs. As Figure 1c shows the Tafel slopes of 35, 60, 95, and 123 mV dec-1 are calculated for Nibipy-MWNT, RuO2, MWNT, and Bipy-MWNT, respectively. The Tafel slope of Ni-bipyMWNT (35 mV dec−1) is close to 40 mV dec−1 that has been also found for aged nickel, oxidized Ni, Co and Fe electrodes in alkaline electrolytes.34,38-40 The Tafel slope of Nibipy-MWNT is also similar to that of nickel−iron layered double hydroxide (NiFeLDH)/CNT (35mV dec−1 in 0.1M KOH)21, which is the state-of-the-art electrocatalyst for OER. Under alkaline mechanism with the assumption of a single-site mechanism, the following OER mechanism can be proposed based on the literature:41 Step I: S + OH− → S-OH + e-

(1)

Step II: S-OH + OH- → S-O + H2O + e-

(2)

Step III: S-O + OH- → S-OOH + e-

(3)

Step IV: S-OOH + OH- → S-OO- + H2O

(4)

Step V: S-OO- → S + O2 + e-

(5)

where S represents an active site on the catalyst. The Tafel slope is related to reaction mechanism and hence, differences in the slopes can be attributed to a change in the rate-determining step (RDS). Therefore, the significant difference of the Tafel slopes, 123 mV dec-1 for Bipy-MWNT and 35 mV dec-1 for Ni-bipy-MWNT, can be attributed to a change in the RDS of the OER mechanism. A Tafel slope of 30mV dec−1 has been observed when equation 2 is the RDS, while a Tafel slop of ≈ 123 mV dec-1 can exhibit that equation 1 is the RDS.41 Therefore, the significant reduction in Tafel slope in our 9


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study upon adsorption of Ni to the Bipy-MWNT can show that Ni species lower the first step adsorbing energy of hydroxide and suggests that Ni in the Ni-bipy complex functions as the active site for OER in Ni-bipy-MWNT.

The observed anodic currents at the potentials of more than ~ 1.48 V can be resulted from the desired four electron transfer (OER pathway: 4OH- → O2 + 2H2O + 4e-) or from the undesirable two electron transfer reaction (3OH– → HO2– + 2e- + H2O) resulting in peroxide formation. Moreover, at high potentials (i.e. more than 1.4 V vs RHE) catalyst and/or support oxidation/corrosion might be occurred, generating unwanted anodic oxidation currents in OER potential region. To investigate the origin of the measured anodic current on the Ni-bipy-MWNT catalyst, the rotating ring-disk electrode (RRDE) technique18,42-44 in N2-saturated 0.1 M NaOH has been performed as shown in Figure 1d. In this technique, the oxygen evolved at the catalyst covered glassy carbon disk is subsequently reduced at the surrounding Pt ring electrode hold at 0.4 V through oxygen reduction reaction (ORR). Likewise, to monitor the production of H2O2, the Pt ring is held at 1.4 V to oxidize any produced H2O2 on the disk.43-44 As Figure 1d shows when the applied potential on the disk exceeds ~ 1.48 V, the ORR current on the Pt ring is observed and this current increases by increasing the OER current on the Ni-bipyMWNT disk electrode. Furthermore, no H2O2 oxidation current is observed at the Pt ring during anodic potential sweep of the disk, revealing that no detectable amount of H2O2 is produced at the Ni-bipy-MWNT disk electrode. The approximate OER Faradaic efficiency (ε) is calculated through the equation of jORR / jOER × N, where jORR and jOER are current densities measured on the Pt ring and the GC disc, respectively, and N is the

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collection efficiency of the RRDE. For parallel RRDE measurements, ε ˃ 90% is calculated for the OER on the Ni-bipy-MWNT electrode where the OER current is less than 2.5 mA cm-2. At higher OER currents, the local oxygen saturation and bubble formation at the disk electrode interfere the ORR current on the disk and ε is decreased.42 It should be noted that small errors in the ring current and collection efficiency can cause large errors in ε.42 The value of ε is also affected by the oxygen bubble formation on the disc, hindering O2 transfer to the ring, and the error in the measured geometric surface area of the disc which is caused by inhomogeneous catalyst dispersion on the glassy carbon disk. Therefore, small deviation from 100% efficiency does not necessarily show the presence of other anodic currents than the oxygen evolution. However, RRDE measurements are still useful for fast screening the approximate Faradaic efficiency.42 Furthermore, the pristine MWNT and bipy-MWNT show almost no anodic current at the potentials < 1.7 V where the OER activity of the Ni-bipy-MWNT has been reported (Figure 1a). Therefore, the anodic current of the OER polarization curve of the Ni-bipy-MWNT is not attributed to the oxidative corrosion of MWNT or bipy-MWNT support. Therefore, the current measured in the OER polarization curve of the Ni-bipy-MWNT originates almost only from the oxygen evolution and for the Ni-bipy-MWNT catalyst the dominant process for water oxidation is via the 4-electron transfer pathway.

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Figure 1. Electrochemical characterization of Ni-bipy-MWNT for OER. (a) RDE polarization curves obtained with Ni-bipy-MWNT (black), Bipy-MWNT (green), MWNT (blue), and RuO2 (red) in 0.1 M NaOH solution. (b), OER polarization curves of Ni-bipy-MWNT in 0.1 (black) and 1 M (red) NaOH solutions. (c), Tafel plots derived from Figure a. (d), Detection of O2 and H2O2 generated from Ni-bipy-MWNT catalyst in N2 saturated 0.1 M NaOH solution using RRDE measurements, inset shows the schematic of RRDE detection for ORR on the Pt ring caused by OER on the disk. The ring potential was kept at 0.4 and 1.4 V for monitoring O2 and H2O2 production at the disc electrode, respectively, during the OER sweep. The -1

polarization curves were measured at a scan rate of 5 mV s and a rotation of 1600 rpm.

The Ni-bipy-MWNT catalyst shows a low OER onset overpotential, small ovepotentials to reach the high currents, and a small OER Tafel slope. The OER activity of the Nibipy-MWNT is close to that of observed for the most active OER electrocatalysts reported recently

9,21,36-37,45-58

(see details of the OER catalytic activity comparisons with

several highly active catalysts in Table S1). In comparison to the recently reported 12


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organometallic Ni complex of Ni6(PET)12 (PET = phenylethyl thiol) with OER onset overpotential of 314 mV, ŋOER,10 of ~ 430 mV and the OER Tafel slope of 69 mV dec-1 in 0.1 M KOH9, the Ni-bipy-MWNT indicates a significant improvement for utilizing organometallic Ni complexes as active OER electrocatalysts. These results represents the high OER electrocatalytic activity of the Ni-bipy-MWNT, indicating the high potential application of organometallic Ni complexes immobilized on the conductive CNT support for electrocatalysis.

Figure 2 shows OER polarization curves for Ni-bipy-MWNT and RuO2 electrodes before and after 1,000 potential cycles between 1 and 1.65 V at a scan rate of 50 mV s-1 in 0.1 M NaOH. The upper limit for the OER cycling stability measurement (1.65 V without iR compensation) corresponds roughly to a high current density of ~ 34 mA cm-2 for the Nibipy-MWNT electrode. While the OER activity of Ni-bipy-MWNT is almost completely preserved after 1,000 cycles RuO2 demonstrates a relatively significant degradation, revealing the high stability of the Ni-bipy-MWNT catalyst. It is noteworthy that Figure 2 can only compare the structural stability of the Ni-bipy-MWNT to that of RuO2 and thus it is not necessarily shows that Ni(II), for instance, is more stable and/or even more active than RuO2 for OER. The OER stability measurement has been further carried out in 0.1 M NaOH by chronoamerometry measurements for 10 h at a static potential of 1.54 V where the current density is ~ 10 mA cm-2 (Figure S3). Based on the protocol for measuring

the

stability

of

heterogeneous

electrocatalysts

for

OER

(half-cell

measurements),42 such time dependent measurements (chronoamperometry or chronopotentiometry) are usually reported over the course of ca. 2 h. The Ni-bipy-

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MWNT is almost stable during the first 2h of the chronoampertometry measurements and after 10 h ~ 96% of the initial OER current is retained. The slight current decrease can be also attributed to the observed partial detachment of the MWNTs from the GC because of the poor adhesion of the CNTs on the GC substrate.59 Ir/C and state-of-theart NiFe-LDH/CNT OER electrocatalysts have shown more degradation rate than that of the Ni-bipy-MWNT, when they kept at a constant current density of 10 mA/cm2.21

Figure 2. The OER polarization curves of the Ni-bipy-MWNT before (black solid line) and after (red dotdashed line) 1,000 stability cycles between 1 and 1.65 V vs RHE at a scan rate of 50 mV s

-1

in 0.1 NaOH,

compared to RuO2 before (green solid line) and after (blue dot-dashed line) the same stability cycles.

To study the intrinsic activity of the Ni-bipy-MWNT, the turnover frequency (TOF) was estimated. The TOF is defined as the moles of generated O2 per moles of total metal content (Ni) evolved per second (s−1). Here, TOF is calculated at the overpotential of 300 mV where the OER current is ca. 16 mA cm-2 in 1 M NaOH. TOF is estimated to be 0.427 s-1 (see TOF calculations in the Supporting Information) which is higher than the previously reported TOF values for some Ni-based catalysts,35,37 and is also about 48-

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fold higher than that that reported for IrOx catalyst (0.0089 s-1).35 This high value of TOF reflects the high catalytic activity of the Ni-bipy-MWNT catalyst. Since the Ni is only located on the outermost surface of the MWNTs (with several coaxial carbon nanotube layers), the real amount of the Ni on the surface can be more than what measured even with the XPS as a surface analysis technique. Therefore, the reported TOF which is attributed to the Ni wt% on the catalyst surface is an approximate value.

2.3. Density functional theory (DFT) calculations Density functional theory (DFT) calculations were performed to predict the stable coordination complexes of Ni in neutral and basic environments, before and after the electrochemical OER measurements, (see details of calculations in the Supporting Information and Tables S2 and S3). The calculations show that the most stable coordination in neutral environment (before OER) is Ni-bipy-(OAc)2 which can be changed to Ni-bipy-(OH)2 in basic environment (after OER in alkaline media). In neutral conditions, when the two bipy ligands are close enough to each other, Ni can coordinate to two bipy ligands with different Ni-N distances (see Table S3) and form the most stable Ni-(bipy)2-(OAc)2 complex. However, in alkaline media Ni is likely to coordinate to only one bipy ligand and two or more OH- groups as presented in Figure S4 and Scheme S1.

2.4. Characterization of catalytic OER active sites of the Ni-bipy-MWNT catalyst

2.4.1. X-ray photoelectron spectroscopy (XPS) analysis

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XPS was carried out on the MWNT starting material, Bipy-MWNT and Ni-bipy-MWNT before and after OER measurements. Figure 3 shows carbon 1s, oxygen 1 s, nitrogen 1s and nickel 2p regions for the samples. Survey spectra (Figure S5) and atomic concentrations (Table S4) are provided in the Supporting Information.

Figure 3a shows the C 1s spectra of all the samples. In all cases, a typical graphitic carbon peak is observed at about 284.4 eV. In addition, samples with bipy ligands show slightly increased intensity in the C 1s peak shoulder region at a binding energy of 285288 eV. This could be due to the pyridine-type C-N bonds in bipy at a binding energy of 285.9- 286.0 eV,60 but can be also attributed to carbon-oxygen single bonds.61 The C 1s spectrum of Ni-bipy-MWNT shows the highest intensity in this region and after OER this shoulder is decreased. Less additional intensity is found in all samples around 288-289 eV which would correspond to the –COO acetate bond,61 however the OAc binding energy upon complexation with nickel might differ slightly from that of a regular O=C-O bond.

Figure 3b shows O 1s spectra for all samples. After the introduction of nickel acetate, the amount of oxygen is increased from 1 and 3.4 atomic percent (at-%) in the MWNT and the Bipy-MWNT samples, respectively, to about 7.4 at-% in the Ni-bipy-MWNT material. In the Bipy-MWNT sample the increase in the oxygen amount could be attributed to residual NOx species from the use of sodium nitrate and OH-groups originating from the utilized solvents and washing agents. In Ni-bipy-MWNT, an increase in two components at roughly 532 eV and 534 eV can be seen in the O 1s 16


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spectrum which can be related to C=O and C-O and/or OH-bonds respectively, assuming that the NOx species are removed in this step. The higher binding energy component at ~ 534 eV could be assigned to the single bonded oxygen carbon bond of the OAc ion.61 After OER, the latter component diminishes and a main O 1s peak at ~ 531.9 eV is observed. This shift would indicate changes in the OAc bonding. This observation is consistent with the replacement of the OAc groups with hydroxyl groups during OER (Scheme S1).62 Louie and Bell38 observed that for as-deposited Ni(OH)2 surface layer on a Ni film, O 1s spectrum shows a single peak at 531.0 eV (attributable to Ni-O-H). However, they observed that a new O 1s peak appears at 529.0 eV (attributed to Ni-O species) after OER, which reveals the formation of NiOOH during OER. In contrast, herein the O 1s spectrum of Ni-bipy-MWNT after OER does not show any peak, which can be attributed to Ni hydrated phase (Ni-O-H) in NiOOH.

The XPS N1s spectra for all samples is shown in Figure 3c. The nitrogen spectrum of the Bipy-MWNT shows a rather broad feature peaked around 399 eV which after introduction of Ni (OAc)2 shifts to around 400 eV. After OER both of these contributions are visible in the N 1s spectrum. To investigate this further, the N 1s peak of BipyMWNT has been deconvoluted assuming the presence of four peaks at 398.9 (N1), 400.0 (N2), 401.2 (N3), and 405.4 eV (N4). The N1 peak is attributed to the typical characteristic peak for pyridinic nitrogen.22,24,63 The N2 peak is assigned to azo/azoxy groups, resulting from the side reactions of the diazonium salt64-65

and/or possible

amino-groups either left over from incompletely reacted amino-bipyridine or from reduction of nitro groups under the X-ray beam.61,65 The N3 peak can be attributed to 17


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protonated pyridinic nitrogen (pyridinium salt)22,63 formed in the acetic acid. The N4 peak is assigned to nitro/nitrite species,60 probably originated from sodium nitrite precursor. Because of the large number of possible nitrogen functionalities, these peak assignments should be considered as tentative, however, they offer a reasonable explanation and evolution of the observed spectra.

After grafting of Ni(II) ions (Ni-bipy-MWNT sample), the pyridinic (N1) peak is mostly shifted to a higher binding energy of ~ 400.1 eV (see Table S5). This shift can be interpreted as metal bonding to the bipy ligands and thus the formation of the organometallic complexes.66-67 We assign this peak at 400.1 eV (N5) to the Ni-bipy complex. Because of the overlap of N5 and N2 peaks, the proportion of the complex is somewhat uncertain. However, the N 1s spectrum of Ni-bipy-MWNT seems to indicate that most of the bipy moieties form complexes with nickel. After OER cycles, the amount of N5 functionalities decreases and the pyridinic N1 peak is observed again. This might occur when a Ni complex coordinated with two adjacent bipy ligands on the MWNT breaks to a Ni complex coordinated with 1 bipy ligand and a free bipy (see scheme S1 in the Supporting Information). This assumption is supported by the atomic concentrations derived from XPS including the peak deconvolution of the nitrogen region (Tables S3 and S4). The relative amount of nickel remains more or less the same for Ni-bipy-MWNT before (Ni/C ratio ≈ 0.015) and after OER (Ni/C ratio ≈ 0.016), indicating that nickel ions are strongly grafted on the surface of the bipy–functionalized MWNTs. The total amount of nitrogen decreases slightly in the process from an N/C ratio ≈ 0.017 for bipy-MWNT to N/C ratio ≈ 0.011 after OER. This decrease can be 18


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attributed to the decrease in N3 and N4 (see table S5) due to the deprotonation of the pyridinium salt and the removal of the residual nitro/nitrite species from the surface of the MWNTs, respectively. The total amount of nitrogen in free bipy (N1) and in complexes (N5) remains, however, rather constant. These observations are consistent with the model proposed on the basis of the DFT calculation.

Figure 3. XPS characterization of the materials. Photoelectron spectra of (a) carbon 1s (for a better comparison, C 1s spectra are normalized to the maximum intensity ), (b) oxygen 1s, (c) nitrogen 1s, the dashed lines show the deconvoluted components N1 (blue dashed line), N2 (olive dashed line), N3 (orange dashed line), N4 (grey dashed line), and N5 (cyan dashed line) peaks; and (d) nickel 2p regions for MWNT (blue lines), Bipy-MWNT (green lines), Ni-bipy-MWNT before (black lines) and after (red lines) OER cycles.

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Figure 3d shows the Ni 2p region for Ni-bipy-MWNTs before and after OER. In both cases, the shape of the 2p doublet with its satellite peaks is consistent with Ni(II) compounds.68-69 The Ni 2p spectrum of the Ni-bipy-MWNT shows two major peaks at 856.9 and 874.6 eV, which can be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. This relatively high binding energy for a Ni2+ compound could be related to the organometallic complex or to the starting material nickel (OAc)2. After OER, the major Ni 2p peaks shift down to binding energies of 856.3 eV (Ni 2p3/2) and 873.9 (Ni 2p1/2). This shift could be due to the formation of Ni (OH)270-72 when the OAc ions are replaced with hydroxyl groups.

It is shown73 that Fe impurities in the electrolyte can incorporate into layered structure of Ni(OH)2/NiOOH films during electrochemical experiments, improving the OER catalytic activity. For the Ni-bipy-MWNT, XPS measurements did not detect Fe on the catalyst after OER measurements as shown in Table S4, indicating that the improvement in the activity of this catalyst during the first OER cycles is not ascribed to Fe adsorption from the electrolyte to the catalyst.

2.4.2. Electron microscopy characterization The resulting N/Ni ratio from the XPS data is ≈ 1 which would mean that at least half of the nickel did not form the organometallic complex. The higher measured amount of Ni than what was expected can be attributed to the decoration of Ni(II) acetate on the defect sites of the MWNTs. An aberration corrected scanning transmission electron 20


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microscopy (STEM) was utilized to observe the possibility of the formation of Ni(II) nanoparticles on the MWNTs. Through STEM, the individual heavy atoms or small metallic nanoparticles in the catalyst material can be discerned in the high-angle annular dark-field (HAADF) images.74-75 By this method, the high-Z Ni nanoclusters on the low-Z MWNT can be observed through the light spots in the images. Since there is no detected metal impurities in the pristine MWNTs used in this work (Table S4), thus the presence of Ni(II) nanoparticles on the MWNTs by the STEM and HAADF images can be easily detected. Figure 4 shows the STEM and HAADF images of the Ni-bipyMWNT sample. Through HAADF images the presence of small amount of Ni impurities (most possibly from the decoration of Ni(II) acetate on the MWNTs) is visible. These decorated Ni(II) nanomaterials on the MWNTs as well as the Ni organometallic complexes contribute to the detected amount of Ni in the Ni-bipy-MWNT sample and cause the N/Ni ratio of ≈1.

Figure 4. STEM images of the Ni-bipy-MWNT material. Bright field and the corresponding HAADF images (a) from three adjacent tubes and (b) from an isolated MWNT. The HAADF images show the dispersion of Ni (bright areas) on the MWNTs.

In order to show the contribution of those Ni(II) nanoparticles in the OER activity of the Ni-bipy-MWNT electrode shown in Figure 1a, the same synthesis process to that of Ni21


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bipy-MWNT but without bipy ligands was performed to produce MWNTs modified with Ni (OAc)2. As Figure S6 shows for the sample without bipy ligands, the activity is significantly lower than the Ni-bipy-MWNT.Figure S6 exhibits that at potentials < 1.58 V vs. RHE, where the OER curve of the Ni-bi-MWNT is reported, the activity of the Nibipy-MWNT sample is mainly attributed to organometallic Ni complexes covalently attached to the MWNTs. Furthermore, the oxidation peak of Ni at ~ 1.45 V observed in Figure 1a is significantly reduced in the absence of bipy ligands (Figure S6). This suggests that in the absence of the bipy ligands the dispersion and coverage of the Ni(II) nanoparticles on the surface of the electrode is very low that cannot be detected by a sharp Ni oxidation peak.

2.4.3. Raman spectroscopy characterization Raman spectroscopy is a powerful method to investigate changes in the structure and properties of carbon nanotubes. The Raman spectrum of CNTs demonstrates the major Raman modes of the defect induced D-mode at 1300–1360 cm-1, the graphitic mode (G-mode) at around 1600 cm-1, and the overtone of the D-band (G´ or 2D) at ~2500– 2800 cm-1.76-77 Raman studies in doped CNTs have shown that the disorder-induced D band and the G′-band are the modes most affected by the doping of CNTs. Figure 5 shows the Raman characteristic peaks (D-, G-, and G´-modes) of the MWNTs before and after the functionalization of the MWNTs with bipy and Ni. A G band at around1600 cm-1 and a D band at around 1327 cm-1 are observed and attributed to the vibration of sp2 carbon atoms in the graphitic structure of the CNTs and the presence of defects in

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the tubes or amorphous carbon materials, respectively.78 The D band is originated from the presence of sp3 defects, and thus, the covalent functionalization of CNTs, which induces re-hybridization of sidewall C atoms, increases the ratio of the D to G band (ID/IG).77,79 The ID/IG ratio is increased from ~ 1.4 in the pristine MWNT to 1.9 in the BipyMWNT, indicating the formation of functional groups on the sidewall of CNTs. After grafting Ni(II) ions on the Bipy-MWNT the ID/IG ratio is further increased to 2.3. This latter increase might be due to the decoration of Ni(II) on the defect sites of the MWNTs as discussed above in connection with the XPS and electron microscopy results.

The so-called G´ band is observed in the Raman spectra of all kinds of sp2 carbon materials, which is strongly sensitive to any perturbation to the π electronic structure.77,80 The G´-mode is the most sensitive band to charge transfer between dopants and nanotubes,81-82 and is used to assign p- and n-type doping in CNTs.80 As shown in Figure 5, covalent functionalization of the MWNTs with bipy causes an upshift of 6 cm-1 in G´ band (from 2654 cm-1 in the MWNT to 2660 cm-1 in the Bipy-MWNT), indicating charge transfer between the MWNTs and the attached bipy species. This blue-shift in G´ peak suggests that the bipy ligands act as electron acceptors (p-type doping) for the MWNTs.83-84 Grafting Ni(II) on the Bipy-MWNT and forming the Ni-bipyMWNT complex makes a downshift of 2 cm-1 in the G´ peak, revealing charge transfer between Ni and functionalized MWNTs with bipy.

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Figure 5. Raman spectra for pristine Ni-bipy-MWNT (black line), Bipy-MWNT (red line), and MWNT (blue line).

2.4.4. Electrochemical characterization by cyclic voltammetry Cyclic voltammetry (CV) measurement with the MWNTs shows a featureless voltammetric response (Figure 6). However, after functionalization of MWNTs with bipy (Bipy-MWNT sample), a unique electrochemical response with redox peaks at an equilibrium potential, E1/2 (Epa + Epc)/2, where Epa and Epc are anodic and cathodic peak potentials, respectively) of 0.465 V vs RHE is observed (Figure 6). Non-covalent functionalized MWNTs with bipy have shown non-response electrochemical behavior85. For functionalized MWNTs, a similar CV feature has been reported recently when a strong interaction between MWNT and phenanthroline as a bidentate ligand with similar coordination property to that of bipy, is acquired.85 Likewise, here we could not observe such a CV feature for bipy deposited on the GC. Interestingly, the characteristic CV peaks of Bipy-MWNT is almost diminished after grafting Ni(II) in Ni-bipy-MWNT. This change in the CV feature can be attributed to the complete formation of organometallic Ni complex. 24


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Figure 6. Cyclic voltammetry response of the materials. Cyclic voltammograms for Ni-bipy-MWNT (black solid curve), Bipy-MWNT (red dot-dashed curve), Bipy (green dashed curve), and MWNT (blue dotted -1

curve) in N2 purged 0.1 M NaOH solution with a scan rate of 50 mV s .

3. CONCLUSIONS

In summary, a facile synthesis method is utilized in this work to covalently immobilize Ni-bipy complexes on the conductive and high surface area MWNT support. This study shows for the first time that organometallic Ni complexes linked to the MWNTs are very promising candidates to be used as highly active and durable electrocatalyst materials for OER. The Ni-bipy-MWNT demonstrates a high activity and stability toward OER with superior performance to that of the highly active OER catalysts reported so far. This catalyst demonstrates a superior activity and stability toward OER to that of the wellknown RuO2 electrocatalyst, which is rare and expensive in comparsion to Ni and carbon-based materials. The covalently immobilized bipy ligand on the conductive, high surface area MWNT support demonstrates an essential role to accommodate Ni(II) active site to improve the catalytic activity for OER. The formation of the bipy ligands and organometallic complexes on the MWNTs are characterized by the XPS, Raman 25


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and cyclic voltammetry analysis. Furthermore, the structural changes of the catalyst during the OER is investigated, indicating the high stability of the catalyst. This unique approach to prepare the organometallic complexes covalently grafted on the CNTs for efficient water oxidation, opens the door to a new class of highly active and costeffective electocatalysts.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

ORCID

Kari Laasonen: 0000-0002-4419-7824

Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

Supporting Information

Supplementary figures for electrochemical measurements, XPS, DFT calculations, TOF calculations, additional details about electrochemical procedures and the synthesis process of the materials, comparison of the OER activity, and related references (PDF)

ACKNOWLEDGMENTS This work has been supported by the Strategic Research Council at the Academy of Finland, Closeloop project (grant number 303452), and Aalto University (MOPPI project in AEF program). This work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises.

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Graphical abstract

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Functionalized Carbon Nanotubes with Ni(II) Bipyridine Complexes as

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