POLYOXOTUNGSTATE@CARBON NANOCOMPOSITES AS

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POLYOXOTUNGSTATE@CARBON NANOCOMPOSITES AS OXYGEN REDUCTION REACTION (ORR) ELECTROCATALYSTS Diana Mesquita Fernandes, Hugo C Novais, Revathi R. Bacsa, Philippe Serp, Belén Bachiller-Baeza, Inmaculada Rodriguez-Ramos, Antonio Guerrero-Ruiz, and Cristina Freire Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00299 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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POLYOXOTUNGSTATE@CARBON NANOCOMPOSITES AS OXYGEN REDUCTION REACTION (ORR) ELECTROCATALYSTS

Diana M. Fernandesa*, Hugo C. Novaisa, Revathi Bacsab, Philippe Serpb, Belén BachillerBaezac, Inmaculada Rodríguez-Ramosc, Antonio Guerrero-Ruizd, Cristina Freirea

a

REQUIMTE/Departamento de Química e Bioquímica, Faculdade de Ciências,

Universidade do Porto, 4169-007 Porto, Portugal. b

Laboratoire de Chimie de Coordination UPR CNRS 8241, Composante ENSIACET,

Université Toulouse, 4 allée Emile Monso, 31030 Toulouse, France. c

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Cantoblanco, 28049

Madrid, Spain. d

Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED,

Senda de Rey 9, 28040 Madrid, Spain

*Corresponding authors: Dr. Diana M. Fernandes ([email protected])

Keywords: Oxygen reduction reaction, Electrocatalysis, Graphene flakes, carbon nanotubes, Phosphotungstate, fuel cells

1 ACS Paragon Plus Environment

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Abstract The oxygen reduction reaction (ORR) has a crucial function as the cathode reaction in energyconverting systems, such as fuel cells (FCs), which contributes to a sustainable energy supply. However, the current use of precious Pt-based electrocatalysts (ECs) is a major drawback for the economic viability of fuel cells. Hence, it is urgent to develop cost-effective and efficient electrocatalysts (ECs) without noble metals to substitute the Pt-based ECs. Herein, we report the preparation and application as ORR electrocatalysts of four new nanocomposites based on sandwich-type

phosphotungstate

[(TBA)7H3[Co4(H2O)2(PW9O34)2]

(TBA-Co4(PW9)2)

immobilized onto different carbon nanomaterials (single-walled carbon nanotubes (SWCNT), graphene flakes (GF), carbon nanotubes doped with nitrogen (N-CNT) and nitrogen-doped few layer graphene (N-FLG)). In alkaline medium, the four nanocomposites studied presented comparable onset potentials (0.77 – 0.90 V vs. RHE), which are similar to that observed for Pt/C (0.91 V vs. RHE). Higher diffusion-limiting current densities (jL, 0.26V, 1600rpm = -168.3 mA cm-2 mg-1) where obtained for Co4(PW9)2@N-CNT, as compared to Pt/C electrode -130.0 mA cm-2 mg-1) and the other ECs (-45.0, -50.7 and -87.5 mA cm-2 mg-1 for Co4(PW9)2@SWCNT, Co4(PW9)2@GF and Co4(PW9)2@N-FLG, respectively). All the Co4(PW9)2@CM ECs showed selectivity towards direct O2 reduction to water with the exception of Co4(PW9)2@GF where a mixture of the 2- and 4-electron mechanisms is observed. Furthermore, low Tafel slopes were obtained for all the nanocomposites (68 – 96 mV dec-1). Co4(PW9)2@CM ECs also showed excellent tolerance to methanol with no significant changes in current density, in contrast to Pt/C (decrease of ≈ 59% after methanol addition) and good long-term electrochemical stability with current retentions between 75 – 84%.


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Introduction In the face of challenges such as climate change, significant and continuous increase of environmental pollution and decrease of fossil fuels, the research for new cleaner energies is of vital importance for the forthcoming years. The development of novel renewable energy storage and conversion technologies (metal–air batteries, water splitting, fuel cells), has therefore been extensively studied both from the fundamental to the practical point of view.1-4 Among these technologies, fuel cells have been acknowledged as efficient and viable alternative energy sources. Fuel cells are exceptional power sources as they are able to convert chemical energy directly into electricity with high conversion efficiencies, without making use of combustion processes.5 In general, in these devices a fuel (methanol or hydrogen) is electrocatalytic oxidized at the anode and at the cathode occurs the reduction of oxygen.6-7 Unfortunately, the kinetics of the ORR is very slow and the reaction is not easily trigged electrochemically, implying that high reduction potentials need to be applied, which decreases FC performance.8 So as to overcome these obstacles, the use of an efficient cathode ORR electrocatalyst is essential, which means that the development of efficient ORR electrocatalysts is of great significance. The most effective known ORR electrocatalysts, are the platinum-based ECs, such as carbon (nano)materials decorated with nanoparticles of platinum (Pt/C) which provide large current densities, low ORR overpotentials and are selective to the direct 4-electron reaction.8-9 However, there are still some relevant drawbacks like

scarcity,

high cost, poor

stability/durability, and probable Pt deactivation by methanol poisoning that have hampered the application of Pt-based FCs in a large-scale, motivating the search for other alternative that are more stable and cost-effective. In the last few years, carbon nanomaterials (CMs) have emerged as alternative ORR electrocatalysts. These are widely available nanomaterials with unique characteristics that


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make them ideal for several electrochemical applications.10 CMs have shown high chemical and thermal stability, good electrical properties and high surface areas and porosity.11 There are several types of carbon nanomaterials with the most well-known being carbon nanotubes (CNTs) and graphene (GF). These nanomaterials themselves have been applied in ORR both as metal-free ECs in alkaline media, and as electrocatalyst supports in hybrid nanocomposites.9,

12-14

As electrocatalysts, the most promising alternatives are nitrogen,

sulphur and phosphorous-doped carbon nanomaterials.12-13, 15 Doped-CMs showed enhanced durability when compared to metal electrocatalysts, with N-doped graphene exhibiting the best performance towards ORR among this type of electrocatalysts.16 As electrocatalyst supports, CMs have several advantages, such as increased electroactive surface area, enhanced conductivity, the presence of electrocatalytic active sites closest to the reactant species; these characteristics have resulted in improving the overall electrocatalytic properties and durability of the CM-based electrocatalysts.17 Polyoxometalates (POMs) are among the infinite number of species that can be immobilised on different CMs. They constitute a large class of inorganic oxides based on oxygen atoms and transition metals (d-block), with an infinite range of applications from materials science to electrocatalysis.18-22 These compounds have also been successfully used in the preparation of (multi)functional (nano)materials with enhanced electrocatalytic properties.19-20, 23 One of main advantages of POMs is their rich redox chemistry as they are able to suffer reversible multi-valence redox processes, giving rise to mixed-valence species, which generates favourable electrocatalytic properties for numerous electrochemical reactions of interest. Even though there are several examples in the literature regarding the use of pristine POMs24-25 and POM@CMs hybrids21,

23, 26

for oxygen evolution reaction (OER), few

examples can be found on their use as ORR electrocatalysts. Mbomekalle et al.27-28 reported


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the successful application of different pristine Cu- and Mn-substituted POMs as ORR electrocatalysts in acidic media. Nunes and co-workers reported the incorporation of a vanadium-substituted phosphomolybdate (PMo11V) into different carbon materials (graphene, single-walled carbon nanotubes and carbon black) and applied them as ORR electrocatalysts. The PMo11V@graphene nanocomposite exhibited the most promising performance, with the least negative onset potential (Eonset = 0.18 V vs. RHE).14 Rousseau et al.29 reported a Co7(AlePyZn)2(Co7Zn) immobilized on cost-effective Vulcan XC-72 (Co7Zn/C), which was applied as cathode material for ORR. The electrocatalytic performance was evaluated in basic and neutral electrolyte, and the results showed high selectivity for the 4-electron process, tolerance to the methanol poisoning and greater durability than Pt/C 20 wt% EC. Taking into account that several cobalt-based POM nanocomposites have proven to be highly efficient towards OER30-33, and that several cobalt-based oxides/nanocomposites are usually used as ORR electrocatalysts34-37 with outstanding results we anticipate that the TBACo4(PW9)2 to be an excellent choice as the electroactive species, which can be immobilized on a carbon nanomaterial support. Additionally, since the type of carbon material may also have a great impact on the overall electrocatalyst performance we decided to use not only SWCNT and the GF but also N-doped CNT and FLG. Thus, this work reports the preparation and application as ORR electrocatalysts of four new nanocomposites denoted Co4POM@CM (CM = SWCNT, GF, N-CNT or N-FLG). Their electrocatalytic properties were evaluated in alkaline media using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). All nanocomposites exhibited superior overall electrocatalytic activity than the corresponding carbon materials with onset potentials (0.77 – 0.90 V vs. RHE) comparable to commercial carbon supported Pt (0.91 V vs. RHE). In particular, the Co4(PW9)2@N-CNT presented greater diffusion-limiting current density (1.8 times) than Pt/C. Chronoamperometric tests revealed that the as-prepared nanocomposites


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also rendered a significant improvement in tolerance to methanol (no significant changes in current density) in comparison with Pt/C (where the current density decreased to approximately half of its initial value of current density).

Experimental Section Materials and characterization methods Sodium phosphate monobasic dehydrate (> 99%, Sigma-Aldrich,), sodium tungstate dehydrate (>99%, Sigma-Aldrich,), sodium chloride (99.5%, Merck) and tetra-nbutylammonium bromide (>99%, Merck,) were used directly without modification. Sodium sulphate (99.5%, Prolabo), phosphoric acid (85%, Aldrich), sulphuric acid (95-97%, Merck), acetonitrile (>99.9%, Romil – Pure Chemistry,), single-walled carbon nanotubes, SWCNT (Nanoledge, Batch nº P0329A) and graphene flakes, GF (Graphene Technologies, Lot GTX7/6-10.4.13) were also used directly. A Jasco FT/IR-460 Plus spectrophotometer was used to acquire the Fourier transform infrared (FTIR) spectra (64 scans, resolution of 4 cm-1 and between 400 and 4000 cm-1). The spectra were obtained for samples dispersed in in KBr pellets (spectroscopic grade, Merck) comprising 0.2% Co4(PW9)2@CM. X-ray photoelectron spectroscopy (XPS) measurements were performed at the Centro de Materiais da Universidade do Porto (CEMUP), Portugal, on A VG Scientific ESCALAB 200A spectrometer with non-monochromatized Al Kα radiation (1486.6 eV) was used for Xray photoelectron spectroscopy (XPS) measurements at CEMUP. Potential deviations induced by electric charge of the samples were corrected using, as an internal standard, the C 1s band at 284.6 eV. The analysis of XPS results were performed using the CasaXPS software for spectra deconvolution.


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Scanning electron microscopy (SEM) was carried out using a high resolution (Schottky) environmental SEM with X-ray microanalysis and electron backscattered diffraction analysis (Quanta 400 FEG ESEM/EDAX Genesis X4M), in high-vacuum conditions, at the Centro de Materiais da Universidade do Porto (CEMUP).

Materials preparation The N-CNT were produced by catalytic chemical vapour decomposition using o a previously described method.38 A LECO CHNS-932 equipment was used to determine the N content (4.1%) by elemental analysis. The full characterization of N-CNT can be found in Faba et al.38 paper. The N-FLG was produced by fluidized bed chemical vapor deposition process through decomposition of an ammonia and ethylene mixture in the presence of a ternary oxide powder catalyst at 650 °C.39 The powder of N-FLG was recuperated after washing in 35% HCl and its full characterization is given elsewhere.39 The (TBA)7H3[Co4(H2O)2(PW9O34)2] (TBA-Co4(PW9)2), with TBA = (C4H9)4N) was synthesised using a previously reported procedure.40 The Co4(PW9)2@CM nanocomposites were prepared thru immobilization of TBACo4(PW9)2 onto four different carbon materials (SWCNT, GF, N-CNT and N-FLG) by adapted procedure.41 Briefly, an acetonitrile solution (5 mL) of TBA- Co4(PW9)2 (40 mg) was added to a 50 mL toluene dispersion containing 40 mg of CM. Then, the mixture was for 15h at 25º C. The black precipitates obtained were filtered,washed with toluene and left to dry at 40 ºC under vacuum for 8 h. The nanocomposites were designated as Co4(PW9)2@SWCNT, Co4(PW9)2@GF, Co4(PW9)2@N-CNT and Co4(PW9)2@N-FLG.


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Co4(PW9)2@CM/GCE electrochemical characterization For CV experiments an Autolab PGSTAT 30 potentiostat/galvanostat (EcoChimie B.V.) with a GPES software was used. A three-electrode compartment system was used: 1) reference electrode - Ag/AgCl (sat. KCl) (MF-2052, BAS); 2) working electrode - glassy carbon electrode, GCE, (3 mm diameter, MF-2012, BAS); 3) auxiliary electrode - platinum wire (7.5 cm, MW-1032, BAS). Before measurements, all solutions were degassed with nitrogen for 30 min. Before any type of modification, a cleaning step was performed to the GCE with diamond polishing pastes of 6, 3 and 1 µM (Buehler) on a microcloth pad (BAS), followed by washing with ultra-pure water. The procedure for electrode modification involved the deposition of a 3 µL drop of the electrocatalyst ink onto the GCE and subsequent drying with air flux. The electrocatalyst inks were prepared dispersing 1 mg of Co4(PW9)2@CM in 1 mL of N,N-dimethylformamide (DMF) followed by sonication for 15 min. The solution containing the free Co4(PW9)2 was prepared dissolving 1 mg of Co4(PW9)2 in 1 mL of acetonitrile. Ultra-pure water (18.2 MΩ cm at 25°C, Millipore) was used to prepare the electrolyte solutions for Co4(PW9)2@CM characterization by CV. Buffer solution (pH = 2.5), used as electrolyte, was prepared mixing appropriate amounts of a 0.5 mol dm-3 Na2SO4 solution with a 0.2 mol dm-3 H2SO4 solution. The electroactive surface area of the bare GCE was determined from CV of K3[Fe(CN)6] (1.0 × 10-3 mol dm-3 in KCl 1.0 mol dm-3) with the Randles-Sevcik equation, Equation (1). It was assumed that the electrode process is diffusion-controlled:


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ipc = 2.69 × 105 n 3/2 A Dx 1/2 C ν1/2

(1)

Where ipc = intensity of the cathodic peak current (A), n = involved number of electrons (1 in this case), A = electrode surface area (cm2), Dx = diffusion coefficient (6.30 × 10-6 cm2 s-1 for [Fe(CN)6]3-/4-),42 C = concentration of the specie (mol cm-3), and v = scan rate (V s-1).

ORR electrochemical performance An potentiostat/galvanostat Autolab PGSTAT 302N (EcoChimie B.V.), controlled by the NOVA v2.0 software was used for the CV and LSV tests. As in the previous section, a 3electrode cell was used: 1) reference electrode - Ag/AgCl (3 mol dm-3 KCl, Metrohm); 2) working electrode - glassy carbon rotating disk electrode (RDE) (3 mm of diameter, Metrohm); 3) auxiliary electrode - carbon rod (2 mm of diameter, Metrohm). The modifiedRDE were prepared as follows: 1 mg of Co4(PW9)2@CM or Pt/C were dispersed ultrasonically in a 250 µL ethanol and 20 µL Nafion mixture for 15 min. Next, a 4 × 2.5 µL drop of ECs dispersion was deposited onto the RDE and left to dry under an air flux. Before modification, the electrode was cleaned as depicted described in previous section. Electrochemical tests were carried out in N2- or O2-saturated KOH (0.1 mol dm-3). To achieved this, the electrolyte was degassed for 30 min with the selected gas. CV measurements were performed at 0.005 V s-1 and the LSV ones at 0.005 V s-1 with rotation speeds in the range 400 - 3000 rpm. To obtain the ORR current, the current obtained in N2saturated electrolyte was subtracted from that in O2-saturated. Chronoamperometry (CA) tests were accomplished at a potential (E) of 0.5 V vs. RHE and a rotation speed of 1600 rpm for 20 000 s. Tolerance to methanol was evaluated by CV at 0.005 V s-1 and by CA at E = 0.5 V vs. RHE and 1600 rpm for 2000 s. Onset potential (Eonset) was calculated as described in literature and is defined as the potential at which the reduction of O2 beggins.43 The electrochemical potentials and the Eonset


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values determined vs. Ag/AgCl were transformed to reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Ag/AgCl) + 0.059 pH + Eº(Ag/AgCl)

(2)

where E(RHE) is the potential vs. RHE, Eº(Ag/AgCl) = 0.1976 V (25 ºC) and E(Ag/AgCl) is the potential measure vs. Ag/AgCl.44 The Koutecky-Levich (K-L) equation (3) was used to analyse the LSV data and the number of electrons transferred per O2 molecule (nO2) in the oxygen reduction reaction was determined K-L plot slopes:

1 1 1 1 1 = + = + ⁄

(3)

with j = current density measured, jL = diffusion-limiting current density, jk = kinetic current density and ω = angular velocity. The parameter B is associated to the diffusion limiting current density expressed by the Equation (4): B = 0.2 nO2 F (DO2)2/3 ν-1/6 CO2

(4)

with F = 96 485 C mol-1 , DO2 = O2 diffusion coefficient, v ) electrolyte kinematic viscosity and CO2

=

O2 bulk concentration. For rotation speeds in rpm is adopted a constant 0.2.

Additionally, in the electrolyte used KOH (0.1 mol dm-3): DO2 = 1.95×10-5 cm2 s-1, v = 0.008977 cm2 s-1 and CO2 = 1.15×10-3 mol dm-3.45 Tafel plots (E(RHE) vs. log jk) were obtained after the measured LSV currents were rectified for diffusion to yield the corresponding kinetic current values. The jL parameter, determinated combining equations (3) and (4), was used to make the mass transport. The values of jk obtained were normalized for the total deposited mass of EC.


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Results and Discussion Materials preparation and characterization Figure 1 shows the FTIR spectra for Co4(PW9)2, SWCNT and Co4(PW9)2@SWCNT. The spectra of the other nanomaterial are in Figure S1 in supporting information file (SI). FTIR spectra of Co4(PW9)2 shows the typical vibration bands of the polyoxotungstates in the 1100-700 cm-1 frequency region: the bands at 1058, 953, 883 and 820 cm-1 are attributed to P–O stretching vibration, W–Od vibration, and W–Ob–W and W–Oc–W stretching modes, respectively.46-48 Additionally, the characteristic bands of the TBA cations are observed in the frequency ranges 2964-2866 cm-1 and 1483-1378 cm-1 (C-H stretching and bending vibrations, respectively). The FTIR spectrum of pristine SWCNT is also observed in Figure 1, where the vibration bands are very weak and poorly resolved. Still, several bands are observed and are attributed to the vibrations of atoms within the nanotubes structure and those due to C-O groups, which arise from residual oxidation. At 1580 cm-1 is observed the band corresponding to the C=C stretching vibrations of the aromatic carbon, the band at 1632 cm-1 is attributed to C=O stretching vibration and that at 1385 cm-1 to the phenol groups.49 At 1137 cm-1 is the band corresponding to both the C–C tangential motions and to C–O stretching vibrations of oxygen groups50 and the one observed at 3439 cm-1 is assigned to the OH stretching vibrations from residual adsorbed water and to the hydroxyl groups. Similar to SWCNTs, the FTIR spectra of pristine graphene flakes, N-doped CNT and FLG (Figure S1, SI) also show weak vibrational bands. The FTIR spectrum of GF (Figure S1 (a), SI) presents bands at: ≈ 3440 cm−1 (OH groups stretching vibrations), 1631 cm−1 (C=O stretching vibration)49, 2853 and 2921 cm−1 (aromatic sp2 C–H stretching vibration). Other three bands at 1581, 1378 and 1140 cm-1 correspond to C=C stretching vibrations, vibration of hydroxyls groups and C−O stretching vibration, respectively.51-52


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In the spectrum of N-CNT (Figure S1 (b), SI) vibrational bands are observed at: 3420 (OH groups stretching vibrations), 1560 (C=N or C=C stretching vibrations), 1385 (C–N stretching vibrations) and 1161 cm−1 (C–O stretching vibration).41,

53

Finally, the N-FLG

spectrum (Figure S1 (c), SI) presents seven bands at 3455, 2926, 2855, 1639, 1383, 1533 and 1119 cm-1. The bands at 1639 and 1383 cm-1 correspond to C=N and C–N stretching vibrations, respectively.54 Those at 2926 and 2855 cm-1 to aromatic sp2 C–H, that at 3455 cm-1 to OH stretching vibration and finally that at 1119 cm−1 to stretching vibration of C–O.55 FTIR spectra of the Co4(PW9)2@CM nanocomposites confirms the immobilization of Co4(PW9)2 onto the four CM as verified from the presence of bands due to CM between 1140 and 1631 cm−1, and the characteristic ones from POM between 798 and 1081 cm−1.

SWCNT

Transmitance / a.u.

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Co4(PW9)2

Co4(PW9)2@SWCNT 4000

3000

2000

1000

Wavenumber / cm-1

Figure 1. FTIR spectra of SWCNT (black), Co4(PW9)2 (red) and Co4(PW9)2@SWCNT (blue).

X-ray photoelectron spectroscopy (XPS) was also used to characterize the nanocomposites. The XPS surface atomic percentages of the various elements for each of the components in all the composites are summarised in Table 1. All the deconvoluted XPS spectra are shown in Figures S2-S8. The presence of the elements of each compound, CM (C, O, and N, depending on CM) and POM (P, Co, W, O, N) in the nanocomposites confirms its 12 ACS Paragon Plus Environment

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successful preparation. The atomic ratios obtained (≈ 1:2 and 1:9 for P/Co and P/W, respectively) gives an indication that no changes were observed in the POM structure after immobilization on the CM. The C1s high-resolution spectrum of the pristine SWCNT (Figure S2 (a), SI) was fitted with five peaks: at 284.6 eV atributted to the graphitic structure (sp2, C-C, C=C), a peak at ≈ 286.2 eV attributed to C-O (phenol, epoxy, hydroxyl), a peak at 287.1 eV assigned to C=O (ketones, quinones, aldehydes), a peak at ≈ 288.1 eV related to O-C=O (carboxylic acids, esters), and a peak at 290.6 eV assigned to the π-π* transition.56 Figures S2 (b-d) and S5-8 (a) show the C1s high-resolution spectra of the other CM and of Co4POM@CM; the fitting was similar to that for pristine SWCNT. The major difference was the presence of the peak at ≈ 286 eV, which is the contribution from C-N bonds due to CM doping with nitrogen.57 The additional peaks observed between 293.1 and 296.4 eV for the Co4POM@CM were assigned to the K 2p. The O1s high-resolution spectra of the CM (Figure S3, SI) were fitted with three peaks: one between 530.2 and 531 eV attributed to C=O, one at ≈ 532 eV assigned to C-O and another in the range 533 – 534 eV attributed to O-C=O.50, 57 Figures S5-8 (b) show the O1s high-resolution spectra of the Co4POM@CM and the fitting was similar to the CM in terms of number of peaks but the peak at lower binding energy became higher. This is attributed to the O-W from the POM that appears at the same binding energy as C=O. The N 1 s XPS spectrum of N-CNT (Figure S4 (a), SI) was deconvoluted into four peaks: 398.6, 401.0, 403.6 and 406.1 eV attributed to pyridinic-N, pyrrolic-N, quaternary N, and some oxidized N, respectively.58 The spectrum for the N-FLG is similar and is given in Figure S4 (b). The W 4f high-resolution spectra of Co4POM@CM are presented in Figures S5–8(f).


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For W 4f, the binding energies are resolved into 4f7/2 and 4f5/2 doublets induced by spinorbital coupling. All W 4f spectra were fitted with one pair of peaks with 4f7/2 at binding energies close to 36 eV and 4f5/2 at ≈ 38 eV. The P 2p high-resolution spectra of Co4POM@CM are shown in Figures S5–8 (d) and the binding energies of P 2p can be resolved into 2p3/2 and 2p1/2 doublets. All the spectra were fitted with one pair of peaks with 2p3/2 at binding energies close to 134 eV and 2p1/2 at ≈ 135 eV. The Co 2p spectra (Figures S5–8 (e)) presented two main peaks at ≈ 782 (2p3/2) and ≈ 797 eV (2p1/2). The satellite peaks of 2p Co2+ can also be observed at ≈ 6 eV, which is higher than those for 2p3/2 and 2p1/2.59

(Insert Table 1) Figure 2 shows the scanning electron microscopy (SEM) images of the prepared Co4(PW9)2@CM at magnification of 50000× and in Figure S9 are presented the corresponding ones at 20000×. As it can be seen, their morphologies are quite different. In Figures 2 (a) and (c) it is clearly observed the carbon nanotubes decorated with lighter microsized spots that correspond to polyoxometalate aggregates. Also, for the Co4(PW9)2@N-CNT it is possible to observed CNTs with different widths and some agglomeration of POM clusters. For the Co4(PW9)2@GF (Figure 2b) and Co4(PW9)2@N-FLG (Figure 2d) a more roughened texture is observed. As for Co4(PW9)2@N-CNT, the Co4(PW9)2@N-FLG also presents some POM clusters agglomeration.


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. SEM images of Co4(PW9)2@SWCNT (a), Co4(PW9)2@GF (b), Co4(PW9)2@N-CNT (c) and Co4(PW9)2@N-FLG at magnification 50000×.

Electrochemical behaviour of Co4POM@CM nanocomposites H2SO4/Na2SO4 buffer solutions (pH 2.5) were used to evaluate the electrochemical properties of TBA-Co4(PW9)2/GCE and Co4(PW9)2@CM/GCE. In the range of potentials used (+0.90 to -0.90 V), Co4(PW9)2 revealed two redox processes (Figure 3(a)): Epc1 = -0.531 V and Epc2 = -0.723 V vs. Ag/AgCl. Both peaks are attributed to tungsten reduction processes (WVI → WV).60 The plot of log ip vs. log v for the first pair of peaks can be observed in Figure 3 (b). Both cathodic (ipc) and anodic (ipa) peak currents are directly proportional to scan rate (0.997 ≥ r ≥ 0.989), suggesting surface-confined processes.41 15 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

Figure 3 (c) depicts the CVs of Co4(PW9)2@SWCNT/GCE at different scan rates, as an example (see Figure S10 for the other nanocomposites). As for the free POM, in the same potential range, the Co4(PW9)2@SWCNT shows two redox processes with Epc1 = -0.534 V and Epc2 = -0.712 V vs. Ag/AgCl, attributed to W-centred reduction processes. The other three nanocomposites present similar behaviour except for the Co4(PW9)2@GF, for which the observation of POM peaks is difficult due to the increased capacitive current. This nanocomposite also exhibits a new quasi-reversible redox process with Epc ≈ 0.167 vs. Ag/AgCl, attributed to quinone-like moieties present on GF.61 Nevertheless, all the peaks have higher current intensities in the Co4(PW9)2@CM modified electrodes in comparison with free POM. This suggests that the kinetics of electron-transfer are faster and usually this is related to the CM good electronic properties. In the range of scan rates between 0.02 and 0.5 V s-1, both Epc and Epa changed between 0.010 and 0.041V for Co4(PW9)2@CM. Figure 3 (d) shows the plot of log ip versus log v for the first pair of peaks for Co4(PW9)2@SWCNT/GCE. Cathodic peak currents as well as anodic ones are directly proportional to scan rate (0.993 ≥ r ≥ 0.989), which suggests a surface-confined process.41 The same behaviour is also observed for the other Co4(PW9)2@CM (Figure S11, SI). CV results of the first W reduction process were further used to calculate the electrochemical surface coverages (Γ) of Co4(PW9)2/GCE and Co4(PW9)2@CM/GCE using the equation Γ = (4ipRT) / (n2F2νA), where ip is the peak current (amperes), R is the gas constant, T is the temperature (298K), n is the number of electrons transferred (here is 2), ν is the scan rate (V s-1), F is the Faraday constant and A is the electrode geometric area (0.07068 cm2) .62 The ip of the first tungsten were represented against ν (0.02 to 0.15 V s-1 for Co4(PW9)2, and 0.02 to 0.50 V s-1 for Co4(PW9)2@CM, and the obtained ip/v value was used to estimate the Γ. The obtained values were: Γ = 0.0352 ± 0.0044; 0.1493 ± 0.0148, 0.2500 ±


Page 17 of 34

0.0196 and 0.0749 ± 0.0049 nmol cm-2 for Co4(PW9)2 and Co4(PW9)2@CM with CM = SWCNT, N-CNT and N-FLG, respectively. Electrochemical surface coverages for the Co4(PW9)2@CM are higher (up to 6 times) than that obtained for Co4(PW9)2 modified electrode. This is very advantageous for the development of novel electrocatalysts for electrode modification. Owing to their nanostructures, carbon nanomaterials allowed that higher percentages of electroactive Co4(PW9)2 was immobilized at the GCE surface and consequently this also improved the electrochemical response.

1.5

7.0

(a)

1.0

(b) 6.0

0.5

slope = 1.14

5.0

ipc ipa

-0.5

log or -log ip

i / µA

0.0

-1.0 -1.5 -2.0

4.0

-5.0 slope = 0.82

-2.5

Co4(PW9)2

-3.0 -0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

-3

-6.0

-7.0 -1.8

-1.6

E / V vs. Ag/AgCl (3 mol dm KCl)

-1.4

-1.2

-1.0

-0.8

log v

20.0

7.0

(c)

(d)

10.0

6.0

0.0

5.0

log or -log ip

i / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

-10.0

-20.0

4.0

slope = 0.87

ipc ipa

-5.0 slope = 0.84

-30.0

-6.0

Co4(PW9)2@SWCNT -0.9

-0.6

-0.3

0.0

0.3

0.6 -3

E / V vs. Ag/AgCl (3 mol dm KCl)

0.9

-7.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2

log v


Langmuir

Figure 3. CVs for Co4(PW9)2 (a) and Co4(PW9)2@SWCNT (c) in H2SO4/Na2SO4 pH 2.5 buffer solution for 0.02 ≤ ν ≤ 0.5 V s-1 and respective plots of log ipc and log ipa vs.log ν for 1st pair of peaks (b and d).

Electrocatalytic performance towards ORR The electrocatalytic activity of the Co4(PW9)2@CM nanocomposites towards ORR was firstly assessed by CV in N2- and O2-saturated KOH solution (0.1 mol dm-3). Figure 4 presents CVs in N2- and O2-saturated solutions for the four nanocomposites prepared. In the absence of O2, no electrochemical processes are observed, while in the presence of O2, all the nanocomposites display an irreversible ORR peak at Epc = 0.72 V vs. RHE for Co4(PW9)2@SWCNT, Epc = 0.82 V vs. RHE for Co4(PW9)2@GF, Epc = 0.86 V vs. RHE for Co4(PW9)2@N-CNT, and Epc = 0.83 V vs. RHE for Co4(PW9)2@N-FLG. The pristine CM presented a similar behaviour (Figure S12, SI); however, the ORR peak appeared at less positive potentials with Epc = 0.64, 0.75, 0.81 and 0.80 V vs. RHE for SWCNT, GF, N-CNT and N-FLG, respectively. The Pt/C (20 wt. %) was also measured in ORR (Figure S13 (a), SI), and it presented similar behaviour than the nanocomposites with Epc = 0.88 V vs. RHE.

0.1

(a)

(b) 0.2

0.0

0.0

j / mA cm

-2

-2

-0.1

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

-0.2

-0.3

-0.2

-0.4

N2 O2

-0.4

-0.6

N2 O2

-0.5

-0.8 0.2

0.4

0.6

0.8

1.0

1.2

0.2

E / V vs. RHE

0.4

0.6

0.8

1.0

1.2

E / V vs. RHE


Page 19 of 34

(c)

1.2

(d)

0.1

0.8

0.0

j / mA cm

-2

-2

0.4

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.0 -0.4

-0.1 -0.2 -0.3

-0.8

-0.4 N2

-1.2

O2

N2 -0.5

O2

-1.6 0.2

0.4

0.6

0.8

1.0

1.2

0.2

0.4

E / V vs. RHE

0.6

0.8

1.0

1.2

E / V vs. RHE

Figure 4. CVs of Co4(PW9)2@SWCNT (a), Co4(PW9)2@GF (b), Co4(PW9)2@N-CNT (c) and Co4(PW9)2@N-FLG (d) modified electrodes in N2-saturated (dash line) and O2-saturated (full line) 0.1 mol dm-3 KOH solution at 0.005 V s-1.

To gain further insights into the electrocatalytic behaviour of Co4(PW9)2@CM nanocomposites towards ORR, LSV curves were acquired using a RDE at 1600 rpm in O2saturated KOH solution (0.1 mol dm-3). The ORR LSV of the prepared nanocomposites and Pt/C (20 wt. %) are depicted in Figure 5 (a), while those of the support carbon materials are presented in Figure 5 (b). The onset potential (Eonset), diffusion-limiting current density values (jL, 0.26V, 1600rpm) and jL values normalized by the mass of catalyst (in mA cm-2 mg-1) obtained for all materials are shown in Table 2. The ORR performance of Co4(PW9)2@N-CNT stands out from the other nanocomposites with a considerable higher (80.8%) diffusion-limiting current density (jL, 0.26V, 1600rpm = -8.5 mA cm-2) even when compared to Pt/C electrode (-4.7 mA cm-2). However, when this value is normalized by the mass of catalyst the difference is not so significant. Nevertheless, the Co4(PW9)2@N-CNT still presents better results than

Pt/C

(increase of approximately 29%). Additionally, this nanocomposite exhibits an Eonset = 0.90 V vs. RHE (only 0.001 V more negative than Pt/C). The Co4(PW9)2@GF and Co4(PW9)2@NFLG showed the same onset potential (0.89 V vs. RHE), but higher diffusion-limiting current densities (jL,

0.26V, 1600rpm

= -87.5 mA cm-2 mg-1) were obtained for Co4(PW9)2@N-FLG, in 19 ACS Paragon Plus Environment

Langmuir

contrast to Co4(PW9)2@GF (jL,

0.26V,

1600rpm

= -50.7 mA cm-2 mg-1). In the case of

Co4(PW9)2@SWCNT, Eonset = 0.77 V vs. RHE and jL, 0.26V, 1600rpm = -45.0 mA cm-2 mg-1 were obtained. The differences observed may be due to the individual morphologies and structures of the as-prepared nanocomposites. As observed by the SEM images, the nanocomposites with POM immobilized at the N-doped CM presented higher agglomeration of POM clusters which may be responsible for their better performances. Several works have reported that Ndoping of carbon materials not only improves the immobilization of different species42, 53 but also the electrocatalytic activity towards ORR.12,

63

So, the Co4(PW9)2@N-CNT and

Co4(PW9)2@N-FLG better performances (higher current densities) are a consequence of Ndoping of CMs but also of a better synergy between these CMs and the polyoxometalates. Considering these two nanocomposites the better performance of the former also may be a consequence of the higher amount of N content determined by the XPS analysis. Furthermore, the immobilization of Co4(PW9)2 onto the pristine CMs promotes a clear improvement of the electrocatalytic activity, since there is an increase of 22.6%, 35.6%, 168.0% and 248.6% of the diffusion-limiting current density values for Co4(PW9)2@SWCNT, Co4(PW9)2@GF, Co4(PW9)2@N-CNT and Co4(PW9)2@N-FLG, respectively, compared to the starting carbon materials.

0.0

(a)

(b) 0.0

-1

-90.0

-2

-2

-60.0

j / mA cm mg

-1

-30.0

j / mA cm mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

-120.0 Pt/C POM@GF POM@SWCNT POM@N-CNT POM@N-FLG

-150.0 -180.0 0.2

0.4

0.6

0.8

1.0

-30.0

-60.0

-90.0 Pt/C GF SWCNT N-CNT N-FLG

-120.0

-150.0 0.2

E / V vs. RHE

0.4

0.6

0.8

1.0

E / V vs. RHE


Page 21 of 34

4.5

4.5

(c)

(d) 4.0

4.0

3.5

3.5

3.0

3.0

n O2

n O2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Pt/C Co4POM@GF

2.5

2.5 2.0

Co4POM@SWCNT

2.0

1.5

Co4POM@N-CNT Co4POM@N-FLG

1.5 0.2

0.3

0.4

0.5

0.6

0.7

Pt/C GF SWCNT N-CNT N-FLG

1.0 0.2

0.3

0.4

0.5

0.6

0.7

E / V vs. RHE

E / V vs. RHE

Figure 5. ORR LSV curves acquired using a RDE in O2-saturated KOH solution (0.1 mol dm-3) at 1600 rpm and 0.005 V s-1 of commercial Pt/C (20 wt %) and Co4(PW9)2@CM nanocomposites (a) and pristine CM (b); nO2 at several potential values (c) and (d).

The Koutecky-Levich (K-L) plots were used to evaluate the ORR kinetic parameters at several potentials, which were obtained using the RDE voltammograms acquired at different rotations. The RDE voltammograms of Pt/C, CM and Co4(PW9)2@CM are shown in Figures S13 (b), S15 and S16, respectively, while the corresponding K-L plots (j-1 vs. ω-1/2) are presented in Figures S13 (c), S17 and S18. All materials showed a linear increase in the limiting current with increasing rotation speed, implying that the electron transfer reaction is limited by diffusion. The analysis of the RDE voltammograms of the Co4(PW9)2@CM reveals the existence of three distinct potential regions: at potential values more positive than E = 0.77 V vs. RHE for Co4(PW9)2@SWCNT, or E = 0.88 V vs. RHE for the other nanocomposites. The ORR is kinetically controlled; the region between E = 0.77 V and E = 0.50 V vs. RHE or E = 0.88 V and E = 0.60 V vs. RHE is a mixed kinetic-diffusion region and at potential values more negative than E = 0.50 V vs. RHE for Co4(PW9)2@SWCNT or E = 0.60 V vs. RHE for the other nanocomposites the ORR is controlled by the diffusion of O2. 21 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

In the case of K-L plots, the observed linear relationship between j-1 vs. ω-1/2 for all the Co4(PW9)2@CM suggests a first order electrocatalytic O2 reduction reaction with respect to the concentration of dissolved O2. Additionally, all the K-L plots of the Co4(PW9)2@CM nanocomposites present parallel straight lines with similar slopes over the potential range from 0.26 V to 0.50, 0.60, 0.65 and 0.65 V vs. RHE, for CM = SWCNT, GF, N-CNT and NFLG, respectively. This suggests that the number of transferred electrons per oxygen molecule (nO2) is independent of the potential. These results differ from those obtained for the pristine CM where the K-L plots (Figure S17, SI) exhibit different slopes, suggesting a dependency of nO2 with the potential. In alkaline medium, the ORR process can occur through a direct pathway involving a 4-electron reduction, with the direct reduction of O2 to H2O/HO- (O2 + H2O + 4e- → HO-).12 Alternatively, oxygen may be reduced through an indirect peroxide pathway, which involves 2steps: a first one where O2 is reduced to HO2- (O2 + H2O + 2e- → HO2- + HO-) and, a second where the intermediates are reduced to H2O/HO- (HO2- + H2O + 2e- → 3HO-).12 The transferred electrons per oxygen molecule were estimated by the K-L equation (Eq. 2), and the plot of nO2 vs. potential is depicted in Figure 5 (c) for the Co4(PW9)2@CM nanocomposites. For all the nanocomposites, the nO2 values are almost constant in the range of potentials between 0.26 V and 0.50, 0.60, 0.65 and 0.65 V vs. RHE, with estimated nO2 values of 3.9, 2.7, 4.0 and 3.6 for Co4(PW9)2@CM where CM = SWCNT, GF, N-CNT and N-FLG, respectively. In contrast, for the pristine CM the nO2 values are lower, and dependent on the potential, with the exception of SWCNT for which these only changed from 3.1 to 3.0. The nO2 values decrease from 2.5 to 2.0 for GF, from 2.7 to 2.1 for N-CNT and 3.1 to 2.6 for NFLG, when the potential is changed in the range from 0.26 to 0.60 V vs. RHE. Globally, these results suggest that in the potential range scanned, both GF and N-CNT carbon supports are involved in a 2-electron process, namely, indirect reduction through peroxide pathway while,


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

the SWCNT and the N-FLG appear to be involved in a mechanism which is a mixture of the 2- and 4-electron mechanisms, since the nO2 values are close to 3 electrons. In the case of the nanocomposite, the Co4(PW9)2@GF presents a nO2 value close to 3 electrons, suggesting a mixture of the 2- and 4-electron mechanisms. On the other hand, for the other three nanocomposites, a one-step 4-electron transfer mechanism appears to be the dominating process with Co4(PW9)2@N-CNT and Co4(PW9)2@SWCNT presenting nO2 values of 4.0 and 3.9, respectively. These values are practically the same as the nO2 = 4.0 electrons estimated for the commercial Pt/C (20 wt %). Tafel plots were also used to gain more information regarding ORR mechanism and kinetics of Co4(PW9)2@CM and Pt/C (Figure 6 (a)). In the potential region, from E = 1.00 to 0.75 V vs. RHE, the ORR process exhibits Tafel slopes of 68, 71, 92, 63 and 89 mv dec-1 for Co4(PW9)2@CM where CM = SWCNT, GF, N-CNT and N-FLG, and Pt/C, respectively. Therefore, lower Tafel slopes were obtained for the Co4(PW9)2@CM nanocomposites with the exception of Co4(PW9)2@N-CNT comparatively to Pt/C, suggesting that these hybrids can easily adsorb O2 onto its surface and activate it, promoting a robust electrocatalytic performance towards ORR. Stability/durability is one of the major parameters when evaluating performances of novel

ORR

electrocatalysts.

Hence,

the

long-term

stability

of

Co4(PW9)2@CM

nanocomposites was evaluated by chronoamperometry at Ep = 0.50 V vs. RHE during 20 000s, in O2-saturated KOH solution (0.1 mol dm-3) at 1600 rpm. Chronoamperometric responses of the nanocomposites and Pt/C are depicted in Figure 6 (b). After 20 000 s, the commercial Pt/C catalyst shows a current decline to 87.4%, while the Co4(PW9)2@GF and Co4(PW9)2@N-FLG present similar results with current retentions of 82.8 and 84.3%. For Co4(PW9)2@SWCNT and Co4(PW9)2@N-CNT the values are slightly lower with current


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

retentions of 74.9 and 77.9 %. Still, the values obtained for the Co4(PW9)2@CM nanocomposites are similar to those obtained for commercial Pt/C catalysts. In order to evaluate the effect of long-term stability tests, in alkaline medium, on the polyoxometalate stability, XPS analysis was performed before and after chronoamperometry with the Co4(PW9)2@N-CNT nanocomposite. All the deconvoluted XPS spectra are shown in Figures S21 and S22. The surface atomic percentages obtained for P, Co and W before chronoamperometric tests (0.3, 0.5 and 1.8 %, respectively) are very similar to those obtained after chronoamperometric tests (0.4, 0.6 and 2.2 %, respectively). Also, the atomic ratios obtained (≈ 1:2 and 1:9 for P/Co and P/W, respectively) suggests that the POM maintains its structure. Tolerance of the electrocatalysts to methanol is also extremely important since in methanol fuel cells, the methanol crossover from the anode to the cathode can reduce the cathodic performance, if the electrocatalysts is sensitive to it. Hence, the methanol tolerance of the Co4(PW9)2@CM nanocomposites was assessed by cyclic voltammetry and chronoamperometry. CVs were obtained in in O2-saturated 0.1 mol dm-3 KOH (0.1 mol dm-3), in the absence and presence of methanol (0.5 mol dm-3) for Pt/C and Co4(PW9)2@CM nanocomposites (Figure S23). When methanol is present, the Pt/C (Figure S23 (a)) shows an anodic peak at Epa = 0.82 V vs. RHE ascribed to the oxidation of methanol. This peak shows the low methanol tolerance of Pt since it overlaps with the peak of ORR. It is known that Pt/C electrocatalysts are highly active towards methanol oxidation reaction, greatly affecting their ORR performances and reducing the current output.64 Oppositely, there were no significant variations in the Co4(PW9)2@CM electrocatalysts electrochemical responses after the addition of methanol, which demonstrates their higher selectivity towards ORR in contrast to the methanol electro-oxidation. These results are supported by the chronoamperometric test as shown in Figure 6 (c). The addition of methanol did not cause a current decrease in the j-t


Page 25 of 34

chronoamperometric response of the Co4(PW9)2@CM electrocatalysts, suggesting a selectivity towards ORR and the excellent tolerance towards crossover methanol effect. However, for Pt/C electrocatalyst, there is an abrupt current density decrease of ≈ 59% after the addition of methanol. These results demonstrate that Co4(PW9)2@CM nanocomposites are promising non-precious metal electrocatalysts for ORR with full tolerance to methanol crossover effects, which overcomes an inherent problem of Pt-based ORR electrocatalysts.

120 Pt/C Co4POM@GF

1.1

(b)

(a) 100

Co4POM@SWCNT Co4POM@N-FLG

E / V vs. RHE

0.9

63mV/dec

89mV/dec

71mV/dec

68mV/dec

0.8

Relative current %

Co4POM@N-CNT 1.0

96mV/dec

80

60 Pt/C Co4POM@GF

40

Co4POM@SWCNT

20

Co4POM@N-CNT Co4POM@N-FLG

0

0.7 -1.5

-1.0

-0.5

0.0

0.5

1.0 -2

1.5

2.0

-1

log | jK / mA cm mg |

5000

10000

15000

20000

Time / s

120

(c)

Methanol 100

Relative current %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

80

60

40

20

Pt/C Co4POM@GF Co4POM@SWCNT Co4POM@N-CNT Co4POM@N-FLG

0 100

200

300

400

500

600

700

800

Time / s

Figure 6. ORR Tafel plots obtained from LSV data in Figure 4(a) where current densities are normalized to the mass of each ECs (a); chronoamperometric responses of the ECs at E = 0.46 V vs. 25 ACS Paragon Plus Environment

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RHE, at 1600 rpm, in O2-saturated KOH (0.1 mol dm-3) for 20000 s (b); chronoamperometric responses of the ECs with the addition of 0.5 mol dm-3 methanol after ≈ 500 s, at E = 0.46 V vs. RHE, at 1600 rpm, in O2-saturated KOH (0.1 mol dm-3) (c).

Conclusions Co4(PW9)2@CM nanocomposites based on sandwich-type polyoxotungstate TBACo4(PW9)2 and different carbon materials (SWCNT, GF, N-CNT and N-FLG) were successfully prepared. FTIR and XPS analyses confirmed that POM was immobilized and that its structure integrity is maintained. The nanocomposites prepared showed superior overall ORR electrocatalytic activity in alkaline media than the corresponding pristine carbon materials with onset potentials between 0.77 and 0.90 V vs. RHE, low Tafel slopes (68 – 96 mV dec-1) and good diffusion-limiting current densities. Additionally, the Co4(PW9)2@N-CNT electrocatalyst showed higher value (jL, 0.26V, 1600rpm = -168.3 mA cm-2 mg-1) than the state-of-art Pt/C catalyst. It is likely that the carbon material support plays an important role for sustaining activity by: i) protecting the Co4(PW9)2 from decomposition at high pH, and/or ii) protecting the active cobalt centres of the electrocatalyst from deactivation by surface restructuration. Additionally, N-doped FLG and CNT with nitrogen functionalities act as stabilizing and conducting agents without the need for the addition of any external agent or surface modification, thereby leading to even better performance. The superior surface area of the carbon materials support may also be crucial for the integrity of the electrocatalytic system by offering a good and stable dispersion of Co4(PW9)2. Analysis of Koutecky-Levich plots revealed that the ORR process occurs through a mixture of the 2- and 4-electron mechanism for Co4(PW9)2@GF and a 4-electron mechanism for the other three catalysts. Studies performed in the presence of methanol unambiguously indicate that the Co4(PW9)2@CM nanocomposites have excellent tolerance to 26 ACS Paragon Plus Environment

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methanol crossover. Moreover, the nanocomposites prepared showed good long-term electrochemical stability. This proof-of-concept study resulted in the discovery of costeffective, efficient and durable noble metal-free Co4(PW9)2@CM electrocatalysts with good ORR performance in alkaline medium, which are therefore more attractive for actual market implementation.

Supporting Information FTIR spectra (S1); Deconvoluted XPS spectra (S2-S8); SEM images (S9); CVs of Co4(PW9)2@CM in H2SO4/Na2SO4 pH 2.5 buffer solution (S10); Corresponding plots of log ipc and log ipa vs.log ν (S11); CVs of CM modified electrodes in N2- and O2-saturated 0.1 mol dm-3 KOH solution (S12); CVs of commercial Pt/C modified electrode in N2 and O2saturated 0.1 mol dm-3 KOH solution, ORR polarization curves and the corresponding K-L plots (S13); ORR polarization curves of CM and Co4(PW9)2@CM (S14-S16); Corresponding K-L plots (S17, S18); Number of electrons transferred per O2 molecules (nO2) at several potential values for CM (S19), Tafel plots for CM (S20); Deconvoluted XPS spectra of Co4(PW9)2@N-CNT before (S21) and after (S22) chronoamperometry; CVs for ORR in O2saturated 0.1 mol dm-3 KOH with and without methanol for Pt/C and Co4(PW9)2@CM (S23); Chronoamperometric responses of the ECs with the addition of 0.5 mol dm-3 methanol for Pt/C and Co4(PW9)2@CM (S24).

Acknowledgments This work was financially supported by: Project UNIRCELL - POCI-01-0145-FEDER16422 – funded by European Structural and Investment Funds (FEEI) through - Programa operacional Competitividade e Internacionalização - COMPETE2020 and by national funds through FCT - Fundação para a Ciência e a Tecnologia, I.P. 27 ACS Paragon Plus Environment

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Acknowledgments

are

also

due

to

the

FCT

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-

project

UID/QUI/50006/2013-

POCI/01/0145/FEDER/007265.

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TABLES Table 1. XPS surface atomic percentages for all compoundsa.

Atomic % Sample

a

C 1s

O 1s

N 1s

P 2p

W 4f

K 2p

Co 2p

GF

97.1

2.9

-

-

-

-

-

SWCNT

98.4

1.6

-

-

-

-

-

N-FLG

92.9

3.5

3.6

-

-

-

-

N-CNT

89.4

5.4

5.2

-

-

-

-

Co4POM@GF

90.2

7.2

0.5

0.2

1.2

0.4

0.3

Co4POM@SWCNT

93.5

5.1

0.5

0.1

0.6

-

0.2

Co4POM@N-FLG

77.8

14.4

2.0

0.3

2.6

2.0

0.8

Co4POM@N-CNT

82.8

10.1

3.8

0.2

1.5

1.2

0.4

Determined by the areas of the respective bands in the high-resolution XPS spectra.

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Langmuir

Table 2. Onset potentials (Eonset) and diffusion-limiting current density values (jL, 0.26V, 1600rpm) derived from the ORR polarization curves in O2saturated 0.1 mol dm-3 KOH solution for commercial Pt/C, carbon materials and nanocomposites prepared.

Eonset vs. RHE

jL (mA cm-2)

jL (mA cm-2 mg-1)*

Pt/C (20 wt %)

0.91

-4.7

-130.0

SWCNT

0.72

-2.6

-36.7

GF

0.82

-3.2

-37.4

N-CNT

0.90

-3.2

-62.8

N-FLG

0.86

-2.2

-25.1

Co4(PW9)2@SWCNT

0.77

-3.2

-45.0

Co4(PW9)2@GF

0.89

-3.3

-50.7

Co4(PW9)2@N-CNT

0.90

-8.5

-168.3

Co4(PW9)2@N-FLG

0.89

-4.4

-87.5

Sample

*

normalized by the mass of catalysts

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TOC Graphic


POLYOXOTUNGSTATE@CARBON NANOCOMPOSITES AS

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