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Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells
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Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells J.A. Prithi a,b, N. Rajalakshmi b, G. Ranga Rao a,* a
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Centre for Fuel Cell Technology (CFCT), International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), 2nd Floor, IIT-M Research Park, Phase-1, 6 Kanagam Road, Taramani, Chennai 600113, India
b
article info
abstract
Article history:
Nitrogen doped mesoporous carbons are employed as supports for efficient electro-
Received 8 January 2017
catalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption
Received in revised form
and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mes-
27 September 2017
oporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal
Accepted 22 November 2017
silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO2) as hard template using
Available online xxx
melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these
Keywords:
materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface
Mesoporous carbon
areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are
Nitrogen doping
609 m2 g1 and 736 m2 g1, respectively. The estimated electrochemical surface areas for Pt/
Carbon support
NMC-1 and Pt/NMC-2 are 73 m2 g1 and 59 m2 g1, respectively. It is found that Pt/NMC-1
ORR
has higher ORR activity with higher limiting current and 44 mV positive onset potential
PEMFC
shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/ NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The proton exchange membrane fuel cell (PEMFC) is considered as a promising energy device due to its advantages like clean energy power source, low operating temperature and high power density [1e3]. Hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are the electrochemical
reactions that occur at anode and cathode catalyst layers of PEMFC, respectively. The ORR kinetics at cathode is more sluggish when compared to HOR kinetics at anode [4]. Platinum supported on carbon is the conventionally used catalyst at both anode and cathode. The carbon is used as support because it possesses good electrical conductivity and high surface area, and thus facilitates dispersion of platinum nanoparticles and enhances the electrochemical activity. The
* Corresponding author. E-mail address: [email protected] (G. Ranga Rao). https://doi.org/10.1016/j.ijhydene.2017.11.137 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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carbon as support material plays an important role in the catalytic activity. The morphology of carbon support, and interaction between support and metal particles are crucial factors which influence the Pt metal nanoparticle dispersion and morphology, and overall stability of the electrocatalyst [5]. For over a decade, mesoporous carbons have been studied as catalytic supports in fuel cells because of their controllable pore sizes, high surface areas and large pore volumes making it more suitable for fuel cell application [6]. The Pt catalyst on mesoporous carbon support shows lower overpotential than the conventional Vulcan carbon supported Pt catalyst [7]. Large pore volume carbons with mesopores and high specific surface areas up to 1800 m2 g1 have been synthesized for fuel cell applications using MCM-48, SBA-1 and SBA-15 silica templates [8,9]. Recently hetero atoms (N, B, O, P and S) are doped into carbon structures to further improve the electrochemical activity [10e13]. Regardless of whether the dopants have a higher (as N) or lower (as B) electronegativity than that of carbon, could create charged sites (Cþ or Bþ) that are favorable for O2 adsorption and subsequent reduction process [14]. Depending on the chemical nature of heteroatoms and the local structures, the doping can either increase the oxygen reduction current or decrease the onset overpotential by facilitating the O2 adsorption, increasing the total number of active sites, and improving the surface hydrophilicity [14]. Among many dopants attempted within the carbon matrix, nitrogen has been attracting significant attention due to its electron accepting ability and superior electrochemical stability. Various types of nitrogen doped materials have been reported which include N-doped carbon nanotubes, graphene, nanocoils, nanoplatelets, ordered mesoporous carbon, and quantum dots [15e20]. Nitrogen doping has been shown to change both the electronic and structural properties of carbon nanostructures. For example, nitrogen doping in CNTs leads to large electron donor states near the Fermi level, improving metallic properties of CNTs [21,22]. The nitrogen doped carbon materials have been synthesized by various methods such as arc discharge, chemical vapour deposition, plasma treatment, electrochemically and thermal annealing method [23e26]. Recently, studies on
nitrogen doping in mesoporous carbons are reported for supercapacitor and fuel cell applications [27e30]. In the present work, we synthesized nitrogen doped mesoporous carbons (NMCs) with high surface area and varying nitrogen contents by colloidal silica assisted sol-gel process. The NMCs are used as supports for Pt to obtain Pt/NMCs electrocatalysts which show good electrochemical performance and ORR activity.
Experimental Synthesis of nitrogen doped mesoporous carbon and electrocatalyst Nitrogen doped mesoporous carbon (NMC) synthesized by colloidal silica assisted sol-gel process, using phenol and formaldehyde as carbon precursors [31,32]. In 50 mL of 0.2 M NaOH, 3.67 g of phenol and 9.50 g of formaldehyde were added and the mixture was stirred at 70 C. The nitrogen precursor melamine (4.92 g) and again 9.50 g of formaldehyde were added to the solution after 40 min at 70 C under stirring. After 1 h, 37.5 g of Ludox AS-40 (40 wt% SiO2) was added and stirred for another hour. The mixture was then transferred to a polypropylene bottle and heated at 80 C for 5 days under autogenous pressure. The resulting gel was dried at 80 C for two days followed by calcination at 800 C in nitrogen atmosphere. Finally, the silica was etched by dissolution of the resultant powder in 2 M NaOH solution at 80 C for 12 h. The silica free solid was washed until the neutral pH with distilled water followed by drying at 100 C for 12 h. The nitrogen content was controlled by varying the mole ratio of melamine to phenol (M/P) from 4 to 1. Hexachloroplatinic acid (H2PtCl6) was used as precursor for platinum reduction on nitrogen doped mesoporous carbons. The polyol method was used for the metal reduction on carbon with ethylene glycol as solvent. The solvent with carbon was ultrasonicated for 1 h followed by stirring at 120 C for 24 h to ensure complete reduction of the precursor to Pt. The sample was then filtered, washed with DI water until neutral pH and dried overnight at 80 C (Scheme 1).
Scheme 1 e Schematic illustration of the procedure followed for the synthesis of nitrogen doped mesoporous carbons. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Physical characterization The BET surface areas of NMCs were calculated from N2 sorption isotherm at 196 C using Quantachrome Autosorb IQ. The samples were evacuated at 250 C for 3 h at a residual pressure of 2 103 torr prior to N2 sorption analysis. Elemental chemical analyser was used to calculate the amount of nitrogen content doped in mesoporous carbons. The ordered nature of NMCs were studied with Raman spectra recorded using Lab RAM HR800 (Jobin Yvon Horiba). FT-IR analyses for the NMC samples were performed using a Bruker Optic (TENSOR-27). All samples were mixed with KBr and determined in the 500e4000 cm1 range and the spectrum. XRD patterns of NMCs and platinum doped NMCs were obtained using a Rigaku SmartLab with Nickel filtered Cu Ka radiation (l ¼ 1.5406 A, 40 kV, 30 mA). The diffractograms were recorded in the 2q range of 10 e90 with 0.0052 step size. The metal loading in the carbon samples are calculated from the residual mass of the TGA curves obtained from NETZSCH STA 449 F1 Zupiter. Morphological structures of NMCs were studied by field emission scanning electron microscope (Carl ZEISS Merlin Compact, Germany). Transmission electron microscope (TEM) images were obtained on a JOEL JEM 3010. The TEM analysis was done on dried samples prepared by placing a drop of sample suspension in ethanol on a carbon coated copper grid. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on S-Probe TM2803, Fisons instruments, USA.
Electrochemical measurements Cyclic voltammetry (CV) studies were performed using a bipotentiostat (Biologic Instruments) to study the electrochemical activity of the catalyst. In order to evaluate the ORR activity of catalysts, linear sweep voltammetry (CH Instruments) was performed at various rotations with rotating ring disc electrode (RRDE, Pine Instruments). The catalyst inks were prepared by dispersing and ultrasonicating the Pt/NMCs in mixture of DI water, IPA and 5 wt% Nafion. An aliquot of the ink was coated on a glassy carbon working electrode with an area of 0.071 cm2. Cyclic voltammograms were recorded for the catalysts by cycling the potential between 0.20 V and 0.8 V. For ORR studies, an RRDE was used as a working electrode which had a glassy carbon disc of area 0.25 cm2 and a concentric platinum ring around the disc. This glassy carbon disc was coated with the catalyst and was swept from a potential of 0.8 Ve0 V and the ring electrode was held at constant potential of 1.0 V. All the electrochemical measurements were performed in a three-electrode electrochemical setup with 0.1 M aqueous HClO4 as electrolyte with a Pt wire as counter electrode. All potentials in the electrochemical studies are reported with respect to reversible hydrogen electrode potential (RHE).
Single cell characterization The PEM fuel cell was assembled with membrane electrode assembly (MEA) sandwiched between graphite bipolar plates, copper current collectors and aluminium end plates. The electrocatalyst was dispersed in 10 wt% Nafion solution (du
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Pont de Nemours & Co), deionised water and 2-propanol to form catalyst ink, which was brush coated uniformly on the carbon cloth used as gas diffusion layer to prepare the electrodes. The anode catalyst used was a commercial 20% Pt/C (Arrora Matthey) and the cathode catalysts used were assynthesized Pt/NMC-1 or Pt/NMC-2 electrocatalysts. The Pt loading in the active surface area of 30 cm2 was 0.25 mg cm2 for anode and 0.5 mg cm2 for cathode. The membrane electrode assembly was prepared by placing the gas diffusion electrodes on either side of the chemically stabilized Nafion membranes (212CS) of 50 mm thickness (du Pont de Nemours) and hot-pressed at 130 C for 150 s, under 100 kg cm2 pressure. A single cell test fixture was employed for evaluating the PEMFC performance. The fuel cell polarization experiments were conducted using Dolphin fuel cell test station. Humidified hydrogen gas at the anode and air or oxygen at the cathode (95% relative humidity) at a constant flow rate of 0.5 slpm of H2, 1 slpm of air/O2 were continuously supplied to the fuel cell. The current voltage (IV) characteristics of the cell were then generated at a cell temperature of 70 C.
Results and discussion Physical characterization Nitrogen doped mesoporous carbons (NMCs) with different amount of nitrogen content were synthesized successfully by varying the mole ratio of precursors (melamine to phenol). The amounts of doped nitrogen content in carbons, measured quantitatively by CHN elemental analyser, along with other physical parameters are given in Table 1. The amount of nitrogen content doped in carbon increased with melamine content in the sol-gel mixture. The BET isotherms and BJH pore size distribution of NMCs are shown in Fig. 1a and b, respectively. These results show that the surface area, pore diameter and pore volume decrease with increased nitrogen content in the carbon matrix [32]. The Raman spectra were recorded to analyse the degree of graphitization and structural defects present in NMC samples. These carbons show characteristic D-band peak at 1332 cm1 due to highly disordered graphitic carbon domains and the Gband peak at 1578 cm1 corresponding to E2g vibrations of sp2 bonded graphitic carbon. The Raman peak intensity ratio, ID =IG is a measure of sp2 graphitic crystallite size. The ratio of disordered band and graphitic band ðID =IG Þ calculated from the Raman spectra of NMC samples are presented in Fig. 2a. The ID =IG ratio is higher for NMC-1 than NMC-2 indicating more induced defects in the former due to increased nitrogen doping [33,34]. The corresponding FTIR spectra of NMC samples are shown in Fig. 2b. The interaction between carbon and nitrogen is observed with a small band at 1122 cm1 corresponding to CeN stretching vibration, and the two broad bands between 3000 and 3500 cm1 are attributed to NeH stretching vibrations. The remaining bands at 689 cm1, 1400 cm1 and 1627 cm1 are the stretching frequencies of CbeH group of alinkage in pyrrole ring, C]C backbone stretching vibrations of aromatic rings and the vibration of phenylene conjugated C] C bonds, respectively [32].
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Table 1 e Summary of physical parameters of NMC-1 and NMC-2 samples. Carbon NMC-1 NMC-2 a b c
M/P ratioa
Surface area (m2 g1)b
Pore volume (cc g1)b
Pore diameter (nm)b
Carbon wt%c
Nitrogen wt%c
4:1 1:1
609 736
0.54 1.09
12 18
68 80
11 6
Melamine (M) to phenol (P) ratio. BET measurements. CHN analysis.
Fig. 1 e (a) BET isotherms and (b) BJH pore size distributions for NMC-1 (11 wt% N) and NMC-2 (6 wt% N) samples.
Fig. 2 e (a) Raman spectra and (b) FTIR spectra for NMC-1 and NMC-2 samples.
The powder XRD for the NMCs and Pt/NMCs are shown in Fig. 3a. Both the mesoporous carbons show two significant broad diffraction peaks at 2q ¼ 25.2 and 43.7 from (002) and (100) planes of carbon, respectively. They indicate the amorphous nature of mesoporous carbons. The additional diffraction peaks are seen from Pt loaded NMCs at 2q ¼ 39.6 , 46.1 , 67.3 and 81.5 . The weight percent of Pt is estimated from the TGA analysis shown in Fig. 3b. The initial weight loss below 150 C is due to the loss of moisture content in the catalyst followed by complete decomposition of carbon up to 500 C. The final residual content of 20 wt% corresponds to the Pt present in the catalysts. The FESEM images of NMC-1 in Fig. 4a show clear carbon sheet like structures, wrinkled together to form porous structure which is not observed for NMC-2 in Fig. 4c. The white
patches seen in Fig. 4b and d are the dispersed Pt present on NMC samples. The TEM analysis has been carried out to examine the porosity of NMC-1 and NMC-2, as well as Pt dispersion on these samples. The TEM images of NMC-1 and NMC-2 are shown in Fig. 5a and e, respectively. The NMC-1 sample shows disordered porous structure compared to NMC-2 sample. The TEM micrographs in Fig. 5b and c as well as Fig. 5f and g shows the Pt dispersion, respectively, in Pt/NMC1 and Pt/NMC-2 samples with corresponding histogram profiles given in Fig. 5d and h. The average Pt particle size deposited on NMC-1 is about 1.9 nm while it is more than 4 nm in NMC-2 sample. The Pt particle size is much smaller and less agglomerated on NMC-1 compared to on NMC-2.
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 3 e (a) Powder XRD pattern of NMC-1, NMC-2, Pt/NMC-1 and Pt/NMC-2; (b) TGA curves of Pt/NMC-1 and Pt/NMC-2 samples.
Fig. 4 e FESEM images of (a) NMC-1, (b) Pt/NMC-1, (c) NMC-2, and (d) Pt/NMC-2 samples.
Electrochemical characterization The cyclic voltammograms of Pt/NMC-1 and Pt/NMC-2 electrodes in0.1 M HClO4 solution at a scan rate of 20 mV s1 are shown in Fig. 5. The well-defined peaks for hydrogen adsorption/desorption (0 V E 0.3 V), the double layer plateau (0.3 Ve0.5 V), and the peaks due to formation (0.7 Ve1 V) and reduction (0.8 Ve0.5 V) of platinum surface oxides are clearly observed. The electrochemical surface areas (ECSA) for the two catalysts were calculated using Equation (1). ECSA ðqH Þ ¼
QH Ptloading 0:21
(1)
where QH ¼ average charge (mC) under the hydrogen adsorption and desorption curve; Pt loading is the platinum content (g) loaded on GC; 0.21 (mC cm2) is the charge required
for removal of one monolayer of hydrogen from bright platinum surface. The calculated ECSAs are 73 m2 g1 and 59 m2 g1 for Pt/ NMC-1 and Pt/NMC-2, respectively from the cyclic voltammetry shown in Fig. 6. The Pt/NMC-1 sample shows larger ECSA compared to Pt/NMC-2. The higher ECSA activity can readily be correlated to the smaller Pt particles of size less than 2 nm dispersed on NMC-1. The ORR activities of the two catalysts are studied by rotating ring disc electrode experiment at 1600 rpm [35]. The Pt/NMC-1 had a higher limiting current when compared to Pt/NMC-2 as shown in Fig. 7. The half-wave potential (E1/2) of Pt/NMC-1 electrode is 0.702 V, which is shifted by 44 mV towards positive side compared to the E1/2 value of 0.658 V for Pt/NMC-2 electrode. This is a clear indication that Pt/NMC-1 has superior ORR activity compared to Pt/NMC-2. Further,
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 5 e TEM images (a) NMC-1, (b, c) Pt/NMC-1 and (d) histogram of Pt particle size distribution in Pt/NMC-1, (e) NMC-2 (f, g) Pt/NMC-2 and (h) histogram of Pt particle size distribution in Pt/NMC-2. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 6 e Cyclic voltammogram profiles of Pt/NMC-1 and Pt/ NMC-2 electrodes in 0.1 M HClO4 electrolyte. the H2O2 production (%) and the number of electrons (n) are calculated at the highest rpm rate of 1600 using Equations (2) and (3) respectively and are depicted in Fig. 7. H 2 O2 % ¼
n¼
200 ID þ
4ID ID þ INR
IR N IR N
(2)
(3)
where ID and IR are the disk and rings currents respectively, N is the collection efficiency of the ring electrode (N ¼ 0.37
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provided by electrode manufacturer, Pine Instruments). The H2O2 production is found to be ~5% and ~3.5% for Pt/NMC-1 and Pt/NMC-2, respectively, which is half of the reported value for Pt/C and slightly less than Pt/activated-ZrC-C electrocatalysts [36]. Similarly, the number of electrons transferred for both the catalysts are estimated to be ~3.9. The RRDE experiments show that Pt/NMC electrocatalysts have good ORR activity with minimum H2O2 formation following the four electron pathway. In order to find the reason for higher ECSA and ORR activity of Pt/NMC-1, we analysed the nature of doped nitrogen in carbon by photoemission spectroscopy. Fig. 8 shows the core level spectra of nitrogen and Pt in Pt/NMC-1 and Pt/NMC-2 samples. The deconvoluted N 1s spectra of Pt/NMC-1 and PtNMC-2 show the presence of pyridinic-N at 397.8 eV, pyrrolic-N at 399.8 eV and oxidized-N 403.9 eV (Fig. 8a and b) [15e17,37]. The percentages of pyridinic-N and pyrrolic-N contents are estimated from the relative areas of integrated peak intensities which are presented in Table 2. The Pt 4f region displays the spin-orbit splitting doublet peaks of 4f7/2 and 4f5/2. The deconvolution of Pt 4f region (Fig. 8c and d) shows the presence of three doublet peaks originating from Pt0, Pt2þ and Pt4þ. The most intense doublet peaks at 70.9 eV and 74.1 eV in Pt/NMC-1 and at 71 eV and 74.1 eV in Pt/NMC-2 are from metallic Pt. The second doublet peaks at 72 eV and 75.3 eV in Pt/NMC-1 and at 72 eV and 75.4 eV in Pt/NMC-2, are ~1.0 eV higher binding energy than Pt0 and ascribed to Pt2þ species. The third set of weak doublets found around 73.5 eV and 76.6 eV in Pt/NMC-1 and around 73.5 eV and 77.5 eV in Pt/ NMC-2 are assigned to Pt4þ present as surface oxide/hydroxide species [38e41]. The relative areas of integrated peak intensities of Pt0, Pt2þ and Pt4þ are tabulated in Table 2. The XPS
Fig. 7 e ORR activity of Pt/NMC-1 and Pt/NMC-2 catalysts. Left panel shows the disc and ring currents. Right panel shows the % H2O2 formation and number of electrons transferred. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 8 e N 1s and Pt 4f XP spectra of Pt/NMC-1 and Pt/NMC-2 electrocatalysts.
Table 2 e The relative areas of integrated peak intensities from the XPS for type of N-bonding and oxidation state of Pt. Electrocatalyst
Type of N-bonding to carbon
Oxidation state of Pt
Pyridinic-N (%)
Pyrrolic-N (%)
Oxidized N (%)
Pt (%)
Pt2þ (%)
Pt4þ (%)
18 12
58 61
24 27
49 48
27 32
24 20
Pt/NMC-1 Pt/NMC-2
data shows similar Pt0 content in both the samples while higher pyridinic nitrogen content is present in Pt/NMC-1. We can conclude that pyridinic-N nitrogen is responsible for higher ECSA and ORR activity of Pt/NMC-1 than Pt/NMC-2.
Single fuel cell measurements The Pt/NMC electrocatalysts are tested in single fuel cell mode performances are tested for two different membrane electrode assemblies using Pt/NMC-1 or Pt-NMC-2 as cathode catalysts and commercial Pt/C as an anode catalyst. The performance of the cell is evaluated using H2/air and H2/O2 feed gases. With H2/air as feed gases, the open circuit potential (OCP) for Pt/NMC-1 and Pt/NMC-2 electrodes are 0.908 V and 0.902 V, respectively. Both the electrodes have activation loss voltage of about 98 mV. The significant difference in the performance is observed in the ohmic region and mass transport region of the IV characteristic curves, shown in Fig. 9a. The Pt/NMC-1 electrode shows higher performance with lower ohmic loss at 0.6 V having current density of 443 mA cm2 compared to 191 mA cm2 for Pt/NMC-2. The maximum power
0
density delivered by Pt/NMC-1 electrode is 313 mW cm2 at 0.5 V which is 1.8 times higher compared to 170 mW cm2 at 0.4 V delivered by Pt/NMC-2. The higher performance of Pt/ NMC-1 at comparatively lower ohmic loss can be attributed to 11 wt% of nitrogen content and also the presence of high pyridinic-N content of about 2% in Pt/NMC-1 catalyst compared to 0.7% in Pt/NMC-2 catalyst. However, the mass transport loss (concentration loss) below 0.4 V was more significant in the case of Pt/NMC-1 electrode compared to the Pt/ NMC-2 electrode. The lower concentration loss observed in the Pt/NMC-2 electrode can be explained in terms of higher surface area of NMC-2 (736 m2 g1) compared to NMC-1 (609 m2 g1). The operating voltage of a fuel cell generally is 0.6 V and hence the best performing catalyst at this voltage is Pt/NMC-1. Similar performance is observed using H2/O2 feed gases as shown in Fig. 9b. The Pt/NMC-1 electrode shows higher performance 0.6 V having current density of 790 mA cm2 compared to 343 mA cm2 for Pt/NMC-2. The maximum power density delivered by Pt/NMC-1 electrode is 515 mW cm2, which is two times higher than the 247 mW cm2 power density delivered by Pt/NMC-2 catalyst.
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 9 e IeV characteristic curves for Pt/NMC-1 and Pt/NMC-2 electrocatalysts at 70 C cell temperature using (a) H2/air and (b) H2/O2 feed gases.
Conclusion Mesoporous carbons with varied nitrogen content of 11 wt% (NMC-1) and 6 wt% (NMC-2) were successfully synthesized by Ludox AS-40 colloidal silica template assisted sol-gel method. The NMCs have higher surface areas with pores in the mesoporous range (12 nm and 17 nm). Among them, Pt/NMC-1 showed higher ECSA, higher limiting current, positive E1/2 potential shift by 44 mV and also improved fuel cell performance. The enhanced performance of Pt/NMC-1 is attributed to higher nitrogen content and higher pyridinic-N content in NMC-1. This study highlights (i) higher pyridinic nitrogen content in carbons helps reducing the ohmic losses as noted in Pt/NMC-1, and (ii) higher surface area and pore size facilitate mass transport at cathode as noted in Pt/NMC-2.
Acknowledgements Prithi would like to thank the Director, ARCI, for granting the research fellowship and Department of Chemistry, IIT Madras, for DST-FIST experimental facilities.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.11.137.
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[19] Shinde DB, Dhavale VM, Kurungot S, Pillai VK. Electrochemical preparation of nitrogen-doped graphene quantum dots and their size-dependent electrocatalytic activity for oxygen reduction. Bull Mater Sci 2015;38:435e42. [20] Kong D, Yuan W, Li C, Song J, Xie A, Shen Y. Synergistic effect of Nitrogen-doped hierarchical porous carbon/graphene with enhanced catalytic performance for oxygen reduction reaction. Appl Surf Sci 2017;393:144e50. [21] Chang H, Joo SH, Pak C. Synthesis and characterization of mesoporous carbon for fuel cell applications. J Mater Chem 2007;17:3078. [22] Stein A. Advances in microporous and mesoporous solidsdhighlights of recent progress. Adv Mater 2003;15:763e75. [23] Du Z, Wang S, Kong C, Deng Q, Wang G, Liang C, et al. Microwave plasma synthesized nitrogen-doped carbon nanotubes for oxygen reduction. J Solid State Electrochem 2015;19:1541e9. [24] Sun L, Wang C, Zhou Y, Zhang X, Cai B, Qiu J. Flowing nitrogen assisted-arc discharge synthesis of nitrogen-doped single-walled carbon nanohorns. Appl Surf Sci 2013;277:88e93. [25] Mohapatra BD, Mantry SP, Behera N, Behera B, Rath S, Varadwaj KSK. Stimulation of electrocatalytic oxygen reduction activity on nitrogen doped graphene through noncovalent molecular functionalisation. Chem Commun 2016;52:10385e8. [26] Ito Y, Christodoulou C, Nardi MV, Koch N, Sachdev H, Mu¨llen K. Chemical vapor deposition of N-doped graphene and carbon films: the role of precursors and gas phase. ACS Nano 2014;8:3337e46. [27] Minamida Y, Noda Z, Hayashi A, Sasaki K. Development of MEAs with Pt/Mesoporous carbon as a cathode catalyst. ECS Trans 2014;64:137e44. [28] Liu S, Wu J. Functionalized mesoporous carbons as platinum electrocatalyst supports for applications in fuel cells. Int J Electrochem Sci 2012;7:8326e36. [29] Liang H, Wei W, Wu Z, Feng X, Mu¨llen K. Mesoporous metal nitrogen-doped carbon electrocatalysts for highly efficient oxygen reduction reaction. J Am Chem Soc 2013;135:16002e5. [30] Liu SH, Chiang CC, Wu MT, Liu SB. Electrochemical activity and durability of platinum nanoparticles supported on
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Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells J.A. Prithi a,b, N. Rajalakshmi b, G. Ranga Rao a,* a
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Centre for Fuel Cell Technology (CFCT), International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), 2nd Floor, IIT-M Research Park, Phase-1, 6 Kanagam Road, Taramani, Chennai 600113, India
b
article info
abstract
Article history:
Nitrogen doped mesoporous carbons are employed as supports for efficient electro-
Received 8 January 2017
catalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption
Received in revised form
and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mes-
27 September 2017
oporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal
Accepted 22 November 2017
silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO2) as hard template using
Available online xxx
melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these
Keywords:
materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface
Mesoporous carbon
areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are
Nitrogen doping
609 m2 g1 and 736 m2 g1, respectively. The estimated electrochemical surface areas for Pt/
Carbon support
NMC-1 and Pt/NMC-2 are 73 m2 g1 and 59 m2 g1, respectively. It is found that Pt/NMC-1
ORR
has higher ORR activity with higher limiting current and 44 mV positive onset potential
PEMFC
shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/ NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The proton exchange membrane fuel cell (PEMFC) is considered as a promising energy device due to its advantages like clean energy power source, low operating temperature and high power density [1e3]. Hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are the electrochemical
reactions that occur at anode and cathode catalyst layers of PEMFC, respectively. The ORR kinetics at cathode is more sluggish when compared to HOR kinetics at anode [4]. Platinum supported on carbon is the conventionally used catalyst at both anode and cathode. The carbon is used as support because it possesses good electrical conductivity and high surface area, and thus facilitates dispersion of platinum nanoparticles and enhances the electrochemical activity. The
* Corresponding author. E-mail address: [email protected] (G. Ranga Rao). https://doi.org/10.1016/j.ijhydene.2017.11.137 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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carbon as support material plays an important role in the catalytic activity. The morphology of carbon support, and interaction between support and metal particles are crucial factors which influence the Pt metal nanoparticle dispersion and morphology, and overall stability of the electrocatalyst [5]. For over a decade, mesoporous carbons have been studied as catalytic supports in fuel cells because of their controllable pore sizes, high surface areas and large pore volumes making it more suitable for fuel cell application [6]. The Pt catalyst on mesoporous carbon support shows lower overpotential than the conventional Vulcan carbon supported Pt catalyst [7]. Large pore volume carbons with mesopores and high specific surface areas up to 1800 m2 g1 have been synthesized for fuel cell applications using MCM-48, SBA-1 and SBA-15 silica templates [8,9]. Recently hetero atoms (N, B, O, P and S) are doped into carbon structures to further improve the electrochemical activity [10e13]. Regardless of whether the dopants have a higher (as N) or lower (as B) electronegativity than that of carbon, could create charged sites (Cþ or Bþ) that are favorable for O2 adsorption and subsequent reduction process [14]. Depending on the chemical nature of heteroatoms and the local structures, the doping can either increase the oxygen reduction current or decrease the onset overpotential by facilitating the O2 adsorption, increasing the total number of active sites, and improving the surface hydrophilicity [14]. Among many dopants attempted within the carbon matrix, nitrogen has been attracting significant attention due to its electron accepting ability and superior electrochemical stability. Various types of nitrogen doped materials have been reported which include N-doped carbon nanotubes, graphene, nanocoils, nanoplatelets, ordered mesoporous carbon, and quantum dots [15e20]. Nitrogen doping has been shown to change both the electronic and structural properties of carbon nanostructures. For example, nitrogen doping in CNTs leads to large electron donor states near the Fermi level, improving metallic properties of CNTs [21,22]. The nitrogen doped carbon materials have been synthesized by various methods such as arc discharge, chemical vapour deposition, plasma treatment, electrochemically and thermal annealing method [23e26]. Recently, studies on
nitrogen doping in mesoporous carbons are reported for supercapacitor and fuel cell applications [27e30]. In the present work, we synthesized nitrogen doped mesoporous carbons (NMCs) with high surface area and varying nitrogen contents by colloidal silica assisted sol-gel process. The NMCs are used as supports for Pt to obtain Pt/NMCs electrocatalysts which show good electrochemical performance and ORR activity.
Experimental Synthesis of nitrogen doped mesoporous carbon and electrocatalyst Nitrogen doped mesoporous carbon (NMC) synthesized by colloidal silica assisted sol-gel process, using phenol and formaldehyde as carbon precursors [31,32]. In 50 mL of 0.2 M NaOH, 3.67 g of phenol and 9.50 g of formaldehyde were added and the mixture was stirred at 70 C. The nitrogen precursor melamine (4.92 g) and again 9.50 g of formaldehyde were added to the solution after 40 min at 70 C under stirring. After 1 h, 37.5 g of Ludox AS-40 (40 wt% SiO2) was added and stirred for another hour. The mixture was then transferred to a polypropylene bottle and heated at 80 C for 5 days under autogenous pressure. The resulting gel was dried at 80 C for two days followed by calcination at 800 C in nitrogen atmosphere. Finally, the silica was etched by dissolution of the resultant powder in 2 M NaOH solution at 80 C for 12 h. The silica free solid was washed until the neutral pH with distilled water followed by drying at 100 C for 12 h. The nitrogen content was controlled by varying the mole ratio of melamine to phenol (M/P) from 4 to 1. Hexachloroplatinic acid (H2PtCl6) was used as precursor for platinum reduction on nitrogen doped mesoporous carbons. The polyol method was used for the metal reduction on carbon with ethylene glycol as solvent. The solvent with carbon was ultrasonicated for 1 h followed by stirring at 120 C for 24 h to ensure complete reduction of the precursor to Pt. The sample was then filtered, washed with DI water until neutral pH and dried overnight at 80 C (Scheme 1).
Scheme 1 e Schematic illustration of the procedure followed for the synthesis of nitrogen doped mesoporous carbons. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Physical characterization The BET surface areas of NMCs were calculated from N2 sorption isotherm at 196 C using Quantachrome Autosorb IQ. The samples were evacuated at 250 C for 3 h at a residual pressure of 2 103 torr prior to N2 sorption analysis. Elemental chemical analyser was used to calculate the amount of nitrogen content doped in mesoporous carbons. The ordered nature of NMCs were studied with Raman spectra recorded using Lab RAM HR800 (Jobin Yvon Horiba). FT-IR analyses for the NMC samples were performed using a Bruker Optic (TENSOR-27). All samples were mixed with KBr and determined in the 500e4000 cm1 range and the spectrum. XRD patterns of NMCs and platinum doped NMCs were obtained using a Rigaku SmartLab with Nickel filtered Cu Ka radiation (l ¼ 1.5406 A, 40 kV, 30 mA). The diffractograms were recorded in the 2q range of 10 e90 with 0.0052 step size. The metal loading in the carbon samples are calculated from the residual mass of the TGA curves obtained from NETZSCH STA 449 F1 Zupiter. Morphological structures of NMCs were studied by field emission scanning electron microscope (Carl ZEISS Merlin Compact, Germany). Transmission electron microscope (TEM) images were obtained on a JOEL JEM 3010. The TEM analysis was done on dried samples prepared by placing a drop of sample suspension in ethanol on a carbon coated copper grid. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on S-Probe TM2803, Fisons instruments, USA.
Electrochemical measurements Cyclic voltammetry (CV) studies were performed using a bipotentiostat (Biologic Instruments) to study the electrochemical activity of the catalyst. In order to evaluate the ORR activity of catalysts, linear sweep voltammetry (CH Instruments) was performed at various rotations with rotating ring disc electrode (RRDE, Pine Instruments). The catalyst inks were prepared by dispersing and ultrasonicating the Pt/NMCs in mixture of DI water, IPA and 5 wt% Nafion. An aliquot of the ink was coated on a glassy carbon working electrode with an area of 0.071 cm2. Cyclic voltammograms were recorded for the catalysts by cycling the potential between 0.20 V and 0.8 V. For ORR studies, an RRDE was used as a working electrode which had a glassy carbon disc of area 0.25 cm2 and a concentric platinum ring around the disc. This glassy carbon disc was coated with the catalyst and was swept from a potential of 0.8 Ve0 V and the ring electrode was held at constant potential of 1.0 V. All the electrochemical measurements were performed in a three-electrode electrochemical setup with 0.1 M aqueous HClO4 as electrolyte with a Pt wire as counter electrode. All potentials in the electrochemical studies are reported with respect to reversible hydrogen electrode potential (RHE).
Single cell characterization The PEM fuel cell was assembled with membrane electrode assembly (MEA) sandwiched between graphite bipolar plates, copper current collectors and aluminium end plates. The electrocatalyst was dispersed in 10 wt% Nafion solution (du
3
Pont de Nemours & Co), deionised water and 2-propanol to form catalyst ink, which was brush coated uniformly on the carbon cloth used as gas diffusion layer to prepare the electrodes. The anode catalyst used was a commercial 20% Pt/C (Arrora Matthey) and the cathode catalysts used were assynthesized Pt/NMC-1 or Pt/NMC-2 electrocatalysts. The Pt loading in the active surface area of 30 cm2 was 0.25 mg cm2 for anode and 0.5 mg cm2 for cathode. The membrane electrode assembly was prepared by placing the gas diffusion electrodes on either side of the chemically stabilized Nafion membranes (212CS) of 50 mm thickness (du Pont de Nemours) and hot-pressed at 130 C for 150 s, under 100 kg cm2 pressure. A single cell test fixture was employed for evaluating the PEMFC performance. The fuel cell polarization experiments were conducted using Dolphin fuel cell test station. Humidified hydrogen gas at the anode and air or oxygen at the cathode (95% relative humidity) at a constant flow rate of 0.5 slpm of H2, 1 slpm of air/O2 were continuously supplied to the fuel cell. The current voltage (IV) characteristics of the cell were then generated at a cell temperature of 70 C.
Results and discussion Physical characterization Nitrogen doped mesoporous carbons (NMCs) with different amount of nitrogen content were synthesized successfully by varying the mole ratio of precursors (melamine to phenol). The amounts of doped nitrogen content in carbons, measured quantitatively by CHN elemental analyser, along with other physical parameters are given in Table 1. The amount of nitrogen content doped in carbon increased with melamine content in the sol-gel mixture. The BET isotherms and BJH pore size distribution of NMCs are shown in Fig. 1a and b, respectively. These results show that the surface area, pore diameter and pore volume decrease with increased nitrogen content in the carbon matrix [32]. The Raman spectra were recorded to analyse the degree of graphitization and structural defects present in NMC samples. These carbons show characteristic D-band peak at 1332 cm1 due to highly disordered graphitic carbon domains and the Gband peak at 1578 cm1 corresponding to E2g vibrations of sp2 bonded graphitic carbon. The Raman peak intensity ratio, ID =IG is a measure of sp2 graphitic crystallite size. The ratio of disordered band and graphitic band ðID =IG Þ calculated from the Raman spectra of NMC samples are presented in Fig. 2a. The ID =IG ratio is higher for NMC-1 than NMC-2 indicating more induced defects in the former due to increased nitrogen doping [33,34]. The corresponding FTIR spectra of NMC samples are shown in Fig. 2b. The interaction between carbon and nitrogen is observed with a small band at 1122 cm1 corresponding to CeN stretching vibration, and the two broad bands between 3000 and 3500 cm1 are attributed to NeH stretching vibrations. The remaining bands at 689 cm1, 1400 cm1 and 1627 cm1 are the stretching frequencies of CbeH group of alinkage in pyrrole ring, C]C backbone stretching vibrations of aromatic rings and the vibration of phenylene conjugated C] C bonds, respectively [32].
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Table 1 e Summary of physical parameters of NMC-1 and NMC-2 samples. Carbon NMC-1 NMC-2 a b c
M/P ratioa
Surface area (m2 g1)b
Pore volume (cc g1)b
Pore diameter (nm)b
Carbon wt%c
Nitrogen wt%c
4:1 1:1
609 736
0.54 1.09
12 18
68 80
11 6
Melamine (M) to phenol (P) ratio. BET measurements. CHN analysis.
Fig. 1 e (a) BET isotherms and (b) BJH pore size distributions for NMC-1 (11 wt% N) and NMC-2 (6 wt% N) samples.
Fig. 2 e (a) Raman spectra and (b) FTIR spectra for NMC-1 and NMC-2 samples.
The powder XRD for the NMCs and Pt/NMCs are shown in Fig. 3a. Both the mesoporous carbons show two significant broad diffraction peaks at 2q ¼ 25.2 and 43.7 from (002) and (100) planes of carbon, respectively. They indicate the amorphous nature of mesoporous carbons. The additional diffraction peaks are seen from Pt loaded NMCs at 2q ¼ 39.6 , 46.1 , 67.3 and 81.5 . The weight percent of Pt is estimated from the TGA analysis shown in Fig. 3b. The initial weight loss below 150 C is due to the loss of moisture content in the catalyst followed by complete decomposition of carbon up to 500 C. The final residual content of 20 wt% corresponds to the Pt present in the catalysts. The FESEM images of NMC-1 in Fig. 4a show clear carbon sheet like structures, wrinkled together to form porous structure which is not observed for NMC-2 in Fig. 4c. The white
patches seen in Fig. 4b and d are the dispersed Pt present on NMC samples. The TEM analysis has been carried out to examine the porosity of NMC-1 and NMC-2, as well as Pt dispersion on these samples. The TEM images of NMC-1 and NMC-2 are shown in Fig. 5a and e, respectively. The NMC-1 sample shows disordered porous structure compared to NMC-2 sample. The TEM micrographs in Fig. 5b and c as well as Fig. 5f and g shows the Pt dispersion, respectively, in Pt/NMC1 and Pt/NMC-2 samples with corresponding histogram profiles given in Fig. 5d and h. The average Pt particle size deposited on NMC-1 is about 1.9 nm while it is more than 4 nm in NMC-2 sample. The Pt particle size is much smaller and less agglomerated on NMC-1 compared to on NMC-2.
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Fig. 3 e (a) Powder XRD pattern of NMC-1, NMC-2, Pt/NMC-1 and Pt/NMC-2; (b) TGA curves of Pt/NMC-1 and Pt/NMC-2 samples.
Fig. 4 e FESEM images of (a) NMC-1, (b) Pt/NMC-1, (c) NMC-2, and (d) Pt/NMC-2 samples.
Electrochemical characterization The cyclic voltammograms of Pt/NMC-1 and Pt/NMC-2 electrodes in0.1 M HClO4 solution at a scan rate of 20 mV s1 are shown in Fig. 5. The well-defined peaks for hydrogen adsorption/desorption (0 V E 0.3 V), the double layer plateau (0.3 Ve0.5 V), and the peaks due to formation (0.7 Ve1 V) and reduction (0.8 Ve0.5 V) of platinum surface oxides are clearly observed. The electrochemical surface areas (ECSA) for the two catalysts were calculated using Equation (1). ECSA ðqH Þ ¼
QH Ptloading 0:21
(1)
where QH ¼ average charge (mC) under the hydrogen adsorption and desorption curve; Pt loading is the platinum content (g) loaded on GC; 0.21 (mC cm2) is the charge required
for removal of one monolayer of hydrogen from bright platinum surface. The calculated ECSAs are 73 m2 g1 and 59 m2 g1 for Pt/ NMC-1 and Pt/NMC-2, respectively from the cyclic voltammetry shown in Fig. 6. The Pt/NMC-1 sample shows larger ECSA compared to Pt/NMC-2. The higher ECSA activity can readily be correlated to the smaller Pt particles of size less than 2 nm dispersed on NMC-1. The ORR activities of the two catalysts are studied by rotating ring disc electrode experiment at 1600 rpm [35]. The Pt/NMC-1 had a higher limiting current when compared to Pt/NMC-2 as shown in Fig. 7. The half-wave potential (E1/2) of Pt/NMC-1 electrode is 0.702 V, which is shifted by 44 mV towards positive side compared to the E1/2 value of 0.658 V for Pt/NMC-2 electrode. This is a clear indication that Pt/NMC-1 has superior ORR activity compared to Pt/NMC-2. Further,
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 5 e TEM images (a) NMC-1, (b, c) Pt/NMC-1 and (d) histogram of Pt particle size distribution in Pt/NMC-1, (e) NMC-2 (f, g) Pt/NMC-2 and (h) histogram of Pt particle size distribution in Pt/NMC-2. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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Fig. 6 e Cyclic voltammogram profiles of Pt/NMC-1 and Pt/ NMC-2 electrodes in 0.1 M HClO4 electrolyte. the H2O2 production (%) and the number of electrons (n) are calculated at the highest rpm rate of 1600 using Equations (2) and (3) respectively and are depicted in Fig. 7. H 2 O2 % ¼
n¼
200 ID þ
4ID ID þ INR
IR N IR N
(2)
(3)
where ID and IR are the disk and rings currents respectively, N is the collection efficiency of the ring electrode (N ¼ 0.37
7
provided by electrode manufacturer, Pine Instruments). The H2O2 production is found to be ~5% and ~3.5% for Pt/NMC-1 and Pt/NMC-2, respectively, which is half of the reported value for Pt/C and slightly less than Pt/activated-ZrC-C electrocatalysts [36]. Similarly, the number of electrons transferred for both the catalysts are estimated to be ~3.9. The RRDE experiments show that Pt/NMC electrocatalysts have good ORR activity with minimum H2O2 formation following the four electron pathway. In order to find the reason for higher ECSA and ORR activity of Pt/NMC-1, we analysed the nature of doped nitrogen in carbon by photoemission spectroscopy. Fig. 8 shows the core level spectra of nitrogen and Pt in Pt/NMC-1 and Pt/NMC-2 samples. The deconvoluted N 1s spectra of Pt/NMC-1 and PtNMC-2 show the presence of pyridinic-N at 397.8 eV, pyrrolic-N at 399.8 eV and oxidized-N 403.9 eV (Fig. 8a and b) [15e17,37]. The percentages of pyridinic-N and pyrrolic-N contents are estimated from the relative areas of integrated peak intensities which are presented in Table 2. The Pt 4f region displays the spin-orbit splitting doublet peaks of 4f7/2 and 4f5/2. The deconvolution of Pt 4f region (Fig. 8c and d) shows the presence of three doublet peaks originating from Pt0, Pt2þ and Pt4þ. The most intense doublet peaks at 70.9 eV and 74.1 eV in Pt/NMC-1 and at 71 eV and 74.1 eV in Pt/NMC-2 are from metallic Pt. The second doublet peaks at 72 eV and 75.3 eV in Pt/NMC-1 and at 72 eV and 75.4 eV in Pt/NMC-2, are ~1.0 eV higher binding energy than Pt0 and ascribed to Pt2þ species. The third set of weak doublets found around 73.5 eV and 76.6 eV in Pt/NMC-1 and around 73.5 eV and 77.5 eV in Pt/ NMC-2 are assigned to Pt4þ present as surface oxide/hydroxide species [38e41]. The relative areas of integrated peak intensities of Pt0, Pt2þ and Pt4þ are tabulated in Table 2. The XPS
Fig. 7 e ORR activity of Pt/NMC-1 and Pt/NMC-2 catalysts. Left panel shows the disc and ring currents. Right panel shows the % H2O2 formation and number of electrons transferred. Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
Fig. 8 e N 1s and Pt 4f XP spectra of Pt/NMC-1 and Pt/NMC-2 electrocatalysts.
Table 2 e The relative areas of integrated peak intensities from the XPS for type of N-bonding and oxidation state of Pt. Electrocatalyst
Type of N-bonding to carbon
Oxidation state of Pt
Pyridinic-N (%)
Pyrrolic-N (%)
Oxidized N (%)
Pt (%)
Pt2þ (%)
Pt4þ (%)
18 12
58 61
24 27
49 48
27 32
24 20
Pt/NMC-1 Pt/NMC-2
data shows similar Pt0 content in both the samples while higher pyridinic nitrogen content is present in Pt/NMC-1. We can conclude that pyridinic-N nitrogen is responsible for higher ECSA and ORR activity of Pt/NMC-1 than Pt/NMC-2.
Single fuel cell measurements The Pt/NMC electrocatalysts are tested in single fuel cell mode performances are tested for two different membrane electrode assemblies using Pt/NMC-1 or Pt-NMC-2 as cathode catalysts and commercial Pt/C as an anode catalyst. The performance of the cell is evaluated using H2/air and H2/O2 feed gases. With H2/air as feed gases, the open circuit potential (OCP) for Pt/NMC-1 and Pt/NMC-2 electrodes are 0.908 V and 0.902 V, respectively. Both the electrodes have activation loss voltage of about 98 mV. The significant difference in the performance is observed in the ohmic region and mass transport region of the IV characteristic curves, shown in Fig. 9a. The Pt/NMC-1 electrode shows higher performance with lower ohmic loss at 0.6 V having current density of 443 mA cm2 compared to 191 mA cm2 for Pt/NMC-2. The maximum power
0
density delivered by Pt/NMC-1 electrode is 313 mW cm2 at 0.5 V which is 1.8 times higher compared to 170 mW cm2 at 0.4 V delivered by Pt/NMC-2. The higher performance of Pt/ NMC-1 at comparatively lower ohmic loss can be attributed to 11 wt% of nitrogen content and also the presence of high pyridinic-N content of about 2% in Pt/NMC-1 catalyst compared to 0.7% in Pt/NMC-2 catalyst. However, the mass transport loss (concentration loss) below 0.4 V was more significant in the case of Pt/NMC-1 electrode compared to the Pt/ NMC-2 electrode. The lower concentration loss observed in the Pt/NMC-2 electrode can be explained in terms of higher surface area of NMC-2 (736 m2 g1) compared to NMC-1 (609 m2 g1). The operating voltage of a fuel cell generally is 0.6 V and hence the best performing catalyst at this voltage is Pt/NMC-1. Similar performance is observed using H2/O2 feed gases as shown in Fig. 9b. The Pt/NMC-1 electrode shows higher performance 0.6 V having current density of 790 mA cm2 compared to 343 mA cm2 for Pt/NMC-2. The maximum power density delivered by Pt/NMC-1 electrode is 515 mW cm2, which is two times higher than the 247 mW cm2 power density delivered by Pt/NMC-2 catalyst.
Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Fig. 9 e IeV characteristic curves for Pt/NMC-1 and Pt/NMC-2 electrocatalysts at 70 C cell temperature using (a) H2/air and (b) H2/O2 feed gases.
Conclusion Mesoporous carbons with varied nitrogen content of 11 wt% (NMC-1) and 6 wt% (NMC-2) were successfully synthesized by Ludox AS-40 colloidal silica template assisted sol-gel method. The NMCs have higher surface areas with pores in the mesoporous range (12 nm and 17 nm). Among them, Pt/NMC-1 showed higher ECSA, higher limiting current, positive E1/2 potential shift by 44 mV and also improved fuel cell performance. The enhanced performance of Pt/NMC-1 is attributed to higher nitrogen content and higher pyridinic-N content in NMC-1. This study highlights (i) higher pyridinic nitrogen content in carbons helps reducing the ohmic losses as noted in Pt/NMC-1, and (ii) higher surface area and pore size facilitate mass transport at cathode as noted in Pt/NMC-2.
Acknowledgements Prithi would like to thank the Director, ARCI, for granting the research fellowship and Department of Chemistry, IIT Madras, for DST-FIST experimental facilities.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.11.137.
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Please cite this article in press as: Prithi JA, et al., Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.11.137