Spinel Mn-Co oxide in N-doped Carbon Nanotubes as Bifunctional Electrocatalyst Synthesized by Oxidative Cutting

Transcription

1 Spinel Mn-Co oxide in N-doped Carbon Nanotubes as Bifunctional Electrocatalyst Synthesized by Oxidative Cutting Anqi Zhao 1, Justus Masa 2, Wei Xia 1, Artjom Maljusch 2, Marc-Georg Willinger 3, Guylhaine Clavel 3, Kunpeng Xie 1, Robert Schlögl 3, Wolfgang Schuhmann 2*, Martin Muhler 1* Experimental details Catalyst synthesis Nitrogen-doped carbon nanotubes (NCNTs, outer diameter of nm) were synthesized by catalytic chemical vapour deposition using spinel-type cobalt-manganese-based mixed oxide as catalyst and pyridine as precursor [M. Becker et al., Carbon, 2011, 49, ]. The Mn and Co contents in the NCNTs measured by ICP-OES (PU701) were 1.67 and 1.69 wt.%, respectively (Table S1). The NCNTs were washed in 1.5 M HNO 3 under stirring for 72 h at room temperature to remove residual catalysts on their surfaces. The acid-washed NCNTs were denoted as NCNT. The content of Mn and Co in NCNT was 0.59 and 1.12 wt.% respectively (Table S1). The residual metal catalysts in NCNT had strong interaction with the NCNTs. NCNTs were placed in a quartz boat and loaded into a tubular reactor with an inner diameter of 20 mm. The sample was heated at different temperatures (300, 400, 500 and 600 o C) for 5 min, and then withdrawn from the heating zone for 12 min. Thermal oxidative cutting of NCNTs was achieved by repeating this two-step procedure three times while maintaining the flow rate of the compressed air. The obtained samples denoted as NCNT-300, NCNT-400, NCNT-500 and NCNT-600 were ground in an agate mortar before further applications. S1

2 Characterization Elemental analysis was carried out with an ICP-OES instrument (PU701) from Philips-Unicam. Thermogravimetry was performed with a Cahn TG-2131 thermobalance in air at a heating rate of 10 K min -1. N 2 physisorption measurements were carried out at 77 K using an Autosorb-1 MP Quantachrome system. Prior to the measurements, all samples were degassed at 300 C for 2 h. The surface areas were calculated from the linear part of the Brunauer-Emmett-Teller (BET) plots. The pore volume and the pore size distribution were derived from the desorption profiles of the isotherms using the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) was performed using a PANalytical theta-theta powder diffractometer with a Cu Kα source. Scans were run from 10 to 80 with a step width of 0.03 and a collection time of 20 s per step. Transmission electron microscopy (TEM) was performed on a CM200FEG (Philips) microscope, operated at 200 kv. The samples for TEM observation were prepared in a dry way, i.e., by simply dipping the copper TEM grids into the CNT samples. In general, sufficient amounts of tubes adhere to the holey carbon support film of the TEM grid. Raman spectroscopy measurements were carried out using a Horiba Jobin Yvon LabRam 2 confocal Raman Microscope with a HeNe Laser excitation at 633 nm (1.96 V) and a laser power of 3.5 mw. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum (UHV) set-up equipped with a monochromatic Al Kα X-ray source ( ev; anode operating at 14 kv and 55 ma) and a high resolution Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was maintained at about mbar. The measurements were carried out in the fixed transmission mode with pass energy of 200 ev resulting in an overall energy resolution better than 0.5 ev. A flood gun was applied to compensate for the charging effects. The binding energies were calibrated based on the graphite C 1s peak at S2

3 ev. The CASA XPS program with a Gaussian-Lorentzian mixed function and Shirley background subtraction was used to analyze the XP spectra. The peak positions for all the samples were reproducible along with the fixed Gaussian to Lorenz ratio of 70:30 and FWHM. Electrochemical tests Electrochemical measurements were performed in a conventional three-electrode cell using glassy carbon (Ø 4 mm; HTW, Germany) modified with the catalysts as the working electrode, Ag/AgCl/3 M KCl as the reference electrode and a platinum counter electrode. The reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE). Prior to the experiments, the glassy carbon electrode was polished on a polishing cloth using different alumina pastes ( µm) to obtain a mirror-like surface, followed by ultrasonic cleaning in water. For the preparation of the working electrode, 5.0 mg of the catalyst was dispersed ultrasonically for 30 min in a mixture of water (490 µl), ethanol (490 µl) and Nafion (5 %, 20 µl). 5.3 µl of the resulting catalyst suspension was dropped onto the polished glassy carbon electrode to obtain a catalyst loading of 210 µg cm -2. The electrode was dried in air at room temperature before measurement. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements were carried using an Autolab potentiostat/galvanostat (PGSTAT12, Eco Chemie, Utrecht, The Netherlands) in combination with a rotating disc electrode rotator (EDI101; Radiometer, Villeurbanne, France) and its speed control unit (CTV101). All experiments for the ORR were performed in 0.1 M KOH at room temperature in the potential range of +0.2 to -0.8 V vs Ag/AgCl/3 M KCl at a scan rate of 5 mv s -1 after purging with argon or oxygen for 20 min. Before each RDE measurement, the catalyst was scanned in Ar-saturated electrolyte and the obtained background voltammogram was subtracted from that measured in the O 2 -saturated electrolyte. S3

4 A GC disk and Pt ring electrode (AFE7R9GCPT) from Pine Instrument Co. (Grove City, PA) was employed in the rotating ring-disk electrode (RRDE) measurements. The disk area of the RRDE (glassy carbon) was cm 2 while the area of the ring (Pt) was cm 2. The catalyst loading on the disk was 210 µl cm -2 obtained by pipetting 10.6 µl of the catalyst ink onto the disk. The collection efficiency (N) of the RRDE ranged from 18 to 27%, in contrast to the value of 37 % reported by the manufacturer for the unmodified disk. The yield of hydrogen peroxide yield and the electron transfer number (n) of the samples were calculated from the ring current (I R ) and disk current (I D ) through the following equations: n = 4NI D / (NI D + I R ) H 2 O 2 (%) = 200I R / (NI D + I R ) The durability of the catalyst was assessed by chronoamperometry, switching between oxygen reduction at 0.5 V vs. RHE for 1 hour and oxygen evolution at the potential corresponding to a current density of 4 ma cm -2 for 1 hour in quiescent air saturated KOH (0.1 M). The onset potential of the OER was determined using scanning electrochemical microscopy (SECM) by in situ detection of evolved oxygen. Glass insulated Pt-disk microelectrodes were fabricated using 10 µm Pt wire. Borosilicate glass capillaries (L = 100 mm, D out = 1.5 mm, D in = 0.75 mm) were conically pulled by melting the glass with a heating coil while simultaneously vertically pulling the capillary on one side. Short pieces ( 10 mm) of Pt wire were placed into the pulled part of the capillary and subsequently sealed into the glass using a heating coil. During the sealing procedure an air pump was used to lower the pressure inside the capillary to guarantee tight sealing of the Pt wire in the glass. The sealed end of the capillary was then carefully polished with sandpaper until a disk-shaped surface of Pt was exposed. The unsealed end of the Pt wire was connected to a 0.5 mm thick Cu wire using silver paint. To S4

5 additionally stabilize the Cu wire inside the capillary a drop of two-component epoxy glue was placed at the unsealed end of the capillary. Before each experiment the working electrode was cleaned by polishing with different grades of alumina paste (3 µm, 1 µm, 0.3 µm and 0.05 µm) and subsequently ultrasonicated in 2-propanol and deionized water for 5 minutes. After cleaning, all electrodes were dried in an argon stream. For SECM experiments a home-build SECM set-up was used. A special cell was developed to enable accurate determination of the onset potential for oxygen evolution in an oxygen-free electrolyte. This closed single compartment glass cell enables positioning of the SECM tip in close proximity to the sample surface by maintaining the Ar pressure inside the cell at a value equal to the ambient pressure. All SECM measurements were carried out in oxygen-free 0.1M KOH and in a four-electrode configuration with the sample as working electrode 1 (WE1), SECM tip as working electrode 2 (WE2), a Pt-wire as counter electrode (CE) and a double junction (0.1M KOH) silver-silver chloride (3M KCl) reference electrode (RE). The tip-tosample distance was maintained at 5 µm using the negative feedback mode of SECM by controlling the distance dependence of the cathodic current measured at the tip during approach towards the sample surface. All reported potentials are referred to the reversible hydrogen electrode scale (RHE). The glass-insulated 10 µm Pt disc microelectrode was placed 5 µm above the catalyst layer on FTO and a cathodic potential of -0.6 V vs. Ag/AgCl (3M KCl) which is sufficient for the diffusion-controlled reduction of molecular oxygen was applied at the tip. An anodic potential pulse profile was applied at the sample and the onset potential for oxygen evolution reaction was defined as the sample potential needed to cause a noticeable change of the cathodic current at the tip (Figure S1). S5

6 Figure S1. Schematic representation of the SECM experiment and the corresponding applied potentials. Elemental analysis Table S1. Composition of metals in the NCNTs obtained by ICP-OES. Sample Mn (wt.%) Co (wt.%) Mg (wt.%) Al (wt.%) NCNT NCNT NCNT NCNT NCNT Thermogravimetry (TG) The thermal stability of NCNT was determined by TG (Fig. S2). The initial oxidation temperature of NCNT was approximately 300 o C. S6

7 100 Weight loss (%) NCNT Temperature ( o C) Figure S2. TG weight loss curve of NCNT. Mass loss The weight loss of NCNT-300 was only 2 % which was prepared at the oxidative temperature of 300 o C. At the oxidative temperature above 400 o C, the weight loss of the samples increased dramatically with the increase of oxidative temperature, which is in good agreement with the TG results. Table S2. Description and mass loss of the samples Sample Description Mass loss (%) NCNT acid-washed NCNTs NCNT-300 Three heating cycles at 300 o C, air 2 NCNT-400 Three heating cycles at 400 o C, air 47 NCNT-500 Three heating cycles at 500 o C, air 65 NCNT-600 Three heating cycles at 600 o C, air 80 S7

8 Physisorption As shown in Table S3, the specific surface area decreased from 214 to 175 m 2 g -1. However, the specific surface area of the NCNT-400 sample increased due to the exposed metal oxide on NCNTs. At the oxidative temperature of 600 o C, the agglomeration of metal oxides on NCNTs led to decreasing specific surface area of the NCNT-600 sample. The average pore size decreased from 35.5 to 9.2 nm with increasing oxidative temperature. Furthermore, the pore volume also changed during the thermal oxidative cutting. Table S3. Surface Area and texture parameters of the samples Sample S BET (m 2 g -1 ) average pore size (nm) pore volume (cm 3 g -1 ) NCNT NCNT NCNT NCNT NCNT Raman studies Raman spectroscopy was used to provide information on the degree of the structural defects. All the samples show two bands at about 1331 and 1579 cm -1, assigned to the D-band and G-band of nitrogen-doped carbon structures, respectively (Figure S3). In particular, the intensity ratio of the two bands (I D /I G ) is considered as a key parameter to assess the degree of the structural defects of the samples. A higher ratio indicates more defects in the samples. After the thermal oxidative S8

9 cutting, the I D /I G ratio of all the samples increased to about 1.1, which is higher than the ratio of 1.0 before the cutting. I D /I G Normalized intensity (a. u.) NCNT-600 NCNT-500 NCNT-400 NCNT NCNT Raman shift (cm -1 ) Figure S3. The Raman spectra of NCNT and samples after oxidative cutting at different temperatures. The intensity ratios of D- and G-band are given for comparison. XPS studies High-resolution XPS measurements were carried out using spinel manganese-cobalt oxide supported on NCNTs. The XPS survey spectra of the samples are shown in Figure S4a. There were not any metallic species to be detected in the survey spectrum of NCNT, suggesting that the effect of metallic species on the surface of NCNT can be excluded through the washing procedure in 1.5 M nitric acid. Only C, N and O could be detected in the survey spectrum. When the oxidative temperature was above 400 o C, Mn and Co can be detected in the survey spectra S9

10 due to the formation of spinel Mn-Co oxide during thermal oxidative cutting. The region spectra were normalized to the intensity of the C 1s peak of graphitic carbon at ev. The C 1s spectra of the samples did not show clear change with the increase of oxidative temperature (Figure S4b). The deconvoluted XP N 1s spectra of the samples are shown in Figure S2c. All the samples show two main contributions in the N 1s region, which were decomposed into three peaks at binding energies of ev, ev and ev, assigned to pyridinic-, pyrrolic- and quaternary-type nitrogen groups, respectively. The surface concentration of nitrogen decreased slightly with the increase of oxidative temperature. The results showed that there was not significant change in the surface nature of support. a b Normalized intensity (a. u.) NCNT-600 NCNT-500 NCNT-400 Normalized intensity (a. u.) NCNT NCNT-300 NCNT-400 NCNT-500 NCNT-600 NCNT-300 NCNT Binding energy (ev) Binding energy (ev) Figure S4. XPS survey scans (a) and XP C 1s (b) spectra of the samples. S10

11 CV of NCNT-300 and NCNT-400 The ORR activity of the samples was investigated by cyclic voltammetry in 0.1 M KOH. The measurements were performed in the potential range from to V at a scan rate of 5 mv s -1. All of the samples were measured in argon- and oxygen-saturated electrolyte. The CV cycle of NCNT-300 and NCNT-400 are shown in Figure S5. It can be seen that there were no clear redox features in argon-saturated electrolyte for NCNT-300. However, for NCNT-400, the redox peaks can be observed in argon-saturated electrolyte, ascribed to the redox reactions of metal oxide. After the electrolyte was saturated with oxygen, a steep increase in the reduction current was observed. The CVs of NCNT-500 and NCNT-600 displayed similar features to that of NCNT-400 (not shown). Current density (ma/cm 2 ) 0,0-0,2-0,4 a Saturated Ar Saturated O 2-0,6 0,2 0,4 0,6 0,8 1,0 Potential (V vs. RHE) Current density (ma/cm 2 ) 0,2 0,0-0,2-0,4-0,6 0,2 0,4 0,6 0,8 1,0 Potential (V vs. RHE) Saturated Ar Saturated O 2 Figure S5. CV curves of NCNT-300 (a) and NCNT-400 (b) recorded at a scan rate of 5 mv/s in argon- and oxygen-saturated 0.1 M KOH. b S11

12 The activity of acid-washed NCNTs Figure S6a shows linear sweep voltammograms of NCNT, NCNT-300 and acid-washed NCNT- 300 measured in O 2 -saturated 0.1 M KOH at a scan rate of 5 mv s -1. The acid-washed NCNT- 300 showed lower activity than NCNT-300 but had higher reduction current than NCNT. After the acid-washing, the contents of Mn and Co decreased by 0.14 and 0.16, respectively, suggesting that a few amounts of metallic species can improve the catalytic activity. The similar results can be observed for the acid-washed NCNT-500 in Figure S5b a 0.0 b Current density (ma/cm 2 ) acid-washed NCNT-300 NCNT NCNT-300 Current density (ma/cm 2 ) acid-washed NCNT-500 NCNT NCNT Potential (V vs. RHE) Potential (V vs. RHE) Figure S6. Linear sweep voltammograms in O 2 -saturated 0.1 M KOH of a) NCNT-300 and b) NCNT-500 before and after acid washing, at a scan rate of 5 mv s -1 and rotation rate of 100 rpm. S12

13 Scanning electrochemical microscopy (SECM) Figure S7 represents the currents measured at the tip over thin layers of NCNT (Figure S7a) and NCNT-500 (Figure S7b) samples. Figure S7. Oxygen reduction current measured at the SECM tip placed in close proximity to the NCNT (a) and NCNT-500 (b) samples. The potential applied at the sample was increased stepwise from 1.4 V to 1.6 V vs. RHE in 5 mv steps, while the potential applied at the SECM tip was kept constant at 0.4 V vs. RHE. All experiments were performed in oxygen-free 0.1 M KOH. Durability tests by double potential chronoamperometry The long-term durability of a given catalyst is crucial for its practical application. However, assessment of the durability of gas evolving electrodes is complicated by the accumulation of gas bubbles on the surface of the electrode. The long-term durability of NCNT-500 as a bifunctional catalyst was investigated by double potential chronoamperometry (Fig. S8) and compared with RuO 2 under similar conditions. NCNT-500 shows a similar degradation trend RuO 2 over same time scales suggesting that the catalyst has comparable stability with RuO 2. S13

14 Current density (ma/cm 2 ) NCNT-500 RuO Time (h) Figure S8. Durability measurement of the bifunctional activity of the NCNT-500 catalyst in comparison to RuO 2 by chronoamperometry, switching between oxygen reduction at 0.5 V vs. RHE for 1 hour and oxygen evolution for 1 hour at the potential corresponding to a current density of 4 ma cm -2 for each of the samples. The measurements were recorded at room temperature in quiescent air saturated KOH (0.1 M). S14