Research Article Cite This: ACS Catal. 2017, 7, 8033-8041
pubs.acs.org/acscatalysis
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 S Supporting Information *
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 the oxygen evolution reaction (OER). To overcome this issue, the development of efficient electrocatalyst materials for the OER has drawn much attention. Here, we show that organometallic Ni(II) complexes immobilized on the sidewalls of multiwalled carbon nanotubes (MWNTs) serve as highly active and stable OER electrocatalysts. This class of electrocatalyst materials is synthesized by covalent functionalization of 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 a 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 for photocatalytic11,12 and electrocatalytic7,10 hydrogen evolution and electrocatalytic OER.9 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 supports without any functionalization or pretreatment to decorate metal nanostructures for the manufacture of highly active and stable electrocatalyst materials.16−18 Moreover, CNTs are highly stable under the harsh conditions of the OER in alkaline media, making them
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 the oxygen evolution reaction (OER; 4OH− → 2H2O + 4e− + O2 in alkaline media).2 Minimizing the OER overpotential (η), the required additional voltage to carry out the reaction in excess of the OER equilibrium potential of 1.23 V vs the 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 nonprecious 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 the 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 © XXXX American Chemical Society
Received: August 25, 2017 Revised: October 5, 2017 Published: October 16, 2017 8033
DOI: 10.1021/acscatal.7b02878 ACS Catal. 2017, 7, 8033−8041
Research Article
ACS Catalysis interesting catalyst supports for the OER.15,18−21 Therefore, decorating organometallic nickel complexes on the CNTs can create a highly active and stable electrocatalyst for the OER. 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 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 that double-walled CNTs (DWNTs) after oxidation retain electrical conductivity up to 65% better than similarly functionalized SWNTs, attributed to enhanced electrical percolation through the intact inner tubes of the DWNTs. Therefore, by an increase in the number of layers in CNTs, the degradation of electrical conductivity after chemical functionalization can be reduced. For covalent functionalized CNTs in electrochemical reactions, multiwalled 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 a highly conductive pathway, which significantly promotes charge transfer processes at the active sites on the surface of the outer tube.19 Therefore, the covalent immobilization of organometallic Ni complexes on the sidewalls of 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 aryldiazonium salts, and the same mechanism can be also proposed for other sp2 carbon structures such as CNTs. First, the aryldiazonium 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 quite 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.
Scheme 1. Representation of Synthesis Process of the Nibipy-MWNT Hybrid Materiala
a The two possible Ni coordination complexes, Ni-bipy-(OAc)2MWNT 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.
subsequently grafting Ni(II) on the ligands to make the Nibipy-CNT material. The summary of the growth procedure is explained below (see the Supporting Information for additional details of the synthesis process). 2.1.2. Step I: Functionalization of the MWNTs with Bipyridine (Bipy-MWNT). 4-Amino-2,2′-bipyridine was mixed with the MWNTs, dispersed in acetic acid, under an argon atmosphere. Then sodium nitrite was 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 ceased. The mixture was stirred for 1 h at room temperature under an argon atmosphere. The mixture was diluted by adding dimethylformamide (DMF). The Bipy-MWNT material was filtered and washed with DMF and ethyl acetate (EtOAc). 2.1.3. Step II: Procedure for Decoration of Ni(II) on BipyMWNT (Ni-bipy-MWNT). Bipy-MWNT produced in step I was suspended in DMF under an argon atmosphere. The suspension was sonicated for a few minutes. Nickel(II) acetate (Ni(OAc)2) dissolved in DMF was added to the Bipy-MWNT solution via a syringe over a 20 min period, and the mixture was further sonicated for 11 h. Afterward, the obtained dispersion was filtered through a PTFE filter (dark orange filtrate) and washed with DMF, EtOAc, and ethanol. 2.2. Oxygen Evolution Activity and Discussion. The electrocatalytic activity of the functionalized MWNTs has been investigated for the OER by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements using standard three- and four-electrode systems, respectively, in 0.1 and 1 M NaOH. Figure 1a exhibits polarization curves for the OER on the Ni-bipy-MWNT catalyst in comparison with the pristine MWNT, Bipy-MWNT, and RuO2 electrodes in 0.1 M NaOH. All of the catalysts were 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 > Bipy-MWNT. Figure 1b demonstrates the activity of the Ni-bipy-MWNT in 0.1 and 1 M NaOH solutions (pH values of ∼13 and ∼14, respectively). The Ni-bipy-MWNT catalyst requires only overpotentials of 310 and 350 mV in 0.1 M NaOH and 290 and 320 mV in 1 M NaOH to reach 10 and 60 mA cm−2 (denoted as ηOER,10 and ηOER,60), respectively. The pristine MWNT shows almost no catalytic activity for the 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 the surface structure or electronic properties of active sites as the most effective parameters for catalytic activity,29 causing a slight change in the activity of the MWNTs after the functionalization. However, the OER activity of Bipy-MWNT is significantly improved
2. RESULTS AND DISCUSSION 2.1. Synthesis of Catalyst Materials. 2.1.1. 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 8034
DOI: 10.1021/acscatal.7b02878 ACS Catal. 2017, 7, 8033−8041
Research Article
ACS Catalysis
Figure 1. Electrochemical characterization of Ni-bipy-MWNT for the 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 (a); (d) detection of O2 and H2O2 generated from Ni-bipy-MWNT catalyst in N2-saturated 0.1 M NaOH solution using RRDE measurements. The inset shows the schematic of RRDE detection for ORR on the Pt ring caused by OER on the disk. The ring potentials were kept at 0.4 and 1.4 V for monitoring O2 and H2O2 production at the disk electrode, respectively, during the OER sweep. The polarization curves were measured at a scan rate of 5 mV s−1 and a rotation of 1600 rpm.
RuO2, MWNT, and Bipy-MWNT, respectively. The Tafel slope of Ni-bipy-MWNT (35 mV dec−1) is close to the 40 mV dec−1 value that has been also found for aged nickel, oxidized Ni, Co, and Fe electrodes in alkaline electrolytes.34,38−40 The Tafel slope of Ni-bipy-MWNT is also similar to that of nickel−iron layered double hydroxide (NiFe-LDH)/CNT (35 mV dec−1 in 0.1 M KOH),21 which is the state of the art electrocatalyst for OER. Under alkaline conditions with the assumption of a singlesite mechanism, the following OER mechanism can be proposed on the basis of the literature:41
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 in the Supporting Information), a redox feature was observed around 1.38−1.45 V. A similar feature within the same potential range was 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 this 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 in the Supporting Information shows the OER performance of the Ni-bipyMWNT 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 not cover either the high potentials at which oxygen bubble evolution causes mass transport limitations or the low potentials at which the redox transition for Ni occurs. Figure 1c shows the Tafel slopes of 35, 60, 95, and 123 mV dec−1 calculated for Ni-bipy-MWNT,
step I:
step II:
S + OH− → S−OH + e−
(1)
S−OH + OH− → S−O + H 2O + e−
(2)
step III:
S−O + OH− → S−OOH + e−
(3)
step IV:
S−OOH + OH− → S−OO− + H 2O
(4)
step V:
S−OO− → S + O2 + e−
(5)
where S represents an active site on the catalyst. The Tafel slope is related to the reaction mechanism, and hence, differences in the slopes can be attributed to a change in the rate-determining step (RDS). Therefore, the significant difference in 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 30 mV dec−1 has been observed when eq 2 is the RDS, while a Tafel slope of ∼123 mV dec−1 can determine that eq 1 is the RDS.41 Therefore, the significant reduction in Tafel slope in our study 8035
DOI: 10.1021/acscatal.7b02878 ACS Catal. 2017, 7, 8033−8041
Research Article
ACS Catalysis
the OER catalytic activity comparisons with several highly active catalysts in Table S1 in the Supporting Information). In comparison with the recently reported organometallic Ni complex of Ni6(PET)12 (PET = phenylethanethiol) with an OER onset overpotential of 314 mV, ηOER,10 value of ∼430 mV, and OER Tafel slope of 69 mV dec−1 in 0.1 M KOH,9 Ni-bipyMWNT indicates a significant improvement for utilizing organometallic Ni complexes as active OER electrocatalysts. These results represent the high OER electrocatalytic activity of the Ni-bipy-MWNT, indicating the high potential application of organometallic Ni complexes immobilized on a conductive CNT support for electrocatalysis. Figure 2 shows OER polarization curves for Ni-bipy-MWNT and RuO2 electrodes before and after 1000 potential cycles
upon adsorption of Ni to the Bipy-MWNT can show that Ni species lower the first step of adsorbing energy of hydroxide and suggests that Ni in the Ni-bipy complex functions as the active site for the OER in Ni-bipy-MWNT. The observed anodic currents at potentials of more than ∼1.48 V can result 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 occur, generating unwanted anodic oxidation currents in the OER potential region. To investigate the origin of the measured anodic current on the Nibipy-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 the 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 an 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 disk, respectively, and N is the 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 with 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 disk, hindering O2 transfer to the ring, and the error in the measured geometric surface area of the disk which is caused by inhomogeneous catalyst dispersion on the glassy-carbon disk. Therefore, a small deviation from 100% efficiency does not necessarily show the presence of anodic currents other than the oxygen evolution. However, RRDE measurements are still useful for quick screening of the approximate Faradaic efficiency.42 Furthermore, the pristine MWNT and bipyMWNT show almost no anodic current at potentials
