Activity, Selectivity, and Anion-Exchange Membrane Fuel Cell

Jan 23, 2012 - Young Jin Sa , Dong-Jun Seo , Jinwoo Woo , Jung Tae Lim , Jae Yeong Cheon , Seung Yong Yang , Jae Myeong Lee , Dongwoo Kang , Tae Joo S...
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Activity, Selectivity, and Anion-Exchange Membrane Fuel Cell Performance of Virtually Metal-Free Nitrogen-Doped Carbon Nanotube Electrodes for Oxygen Reduction Reaction Chitturi Venkateswara Rao* and Yasuyuki Ishikawa* Department of Chemistry and the Chemical Physics Program, University of Puerto Rico, P.O. Box 23346, San Juan, Puerto Rico 00931-3346, USA S Supporting Information *

ABSTRACT: Virtually metal-free, aligned nitrogen-doped carbon nanotubes (N-CNTs) are prepared by an alumina template technique and characterized using XRD, Raman spectroscopy, SEM, and XPS. Highly uniform and aligned nanotubes are identified from SEM. The surface nitrogen concentration in the N-CNTs is estimated to be ∼8.0 at.%. Electrocatalytic oxygen reduction reaction (ORR) activity in alkali media is investigated by rotating disk electrode (RDE) voltammetry and compared with commercial Pt/C (E-TEK). RDE measurements indicated the comparable ORR activity of nitrogen-doped carbon nanotubes with Pt/C. Kinetic analysis reveals that the ORR on nitrogen-doped carbon nanotubes follows the four-electron pathway leading to water. Moreover, during the ORR, nitrogen-doped carbon nanotubes exhibited substantially higher ethanol tolerance than Pt/C. A maximum power density of 37 and 62 mW/cm2 is observed with N-CNT and Pt/C cathodes, respectively, under anion-exchange membrane fuel cell conditions. Durability measurements performed under fuel cell operating conditions for 30 h indicate the good stability of the catalysts.

1. INTRODUCTION Anion-exchange membrane fuel cells (AEMFCs) appear to be one of the alternate energy sources. These fuel cells are characterized by the reduction of oxygen at the cathode to produce OH−, which transfers through the anion exchange membrane to the anode side and reacts with hydrogen to produce water.1−3 Carbon-supported platinum is the active and efficient catalyst for ORR at cathode in fuel cells;2,4 however, the usage of high amounts of Pt for ORR at the cathode increases the cost of the device and hinders commercialization. In addition, Pt electrocatalysts has several drawbacks, such as high overpotential and sluggish kinetics, for ORR in addition to the cost issue.5,6 In recent years, there have been significant efforts to find suitable non-Pt-based catalysts that exhibit a similar activity of Pt,7−11 and the progress has been reviewed elsewhere.12−15 Nitrogen-containing carbon materials appear to be one of the promising catalysts for ORR. A wide variety of nanostructured, nitrogen-containing carbon materials in the form of nanotubes,9,11,16−27 nanofibers,28−30 layered sheets,31−35 nanocapsules,36 and xerogels37 have been prepared and investigated as ORR electrocatalysts in aqueous media. The experimental results indicate that the structure of nitrogen species (pyridinic, graphitic, or pyrrolic ) and the atomic percentage of nitrogen in the carbon lattice are crucial for optimizing the electrochemical ORR performance. Considerable efforts have been devoted to the determination of the composition and the structure of the ORR active site in the nitrogen-doped carbon materials.11,38−43 Moreover, it has been © 2012 American Chemical Society

reported that the nitrogen-doped carbon materials are highly selective toward ORR and more tolerant to species such as methanol, CO, glucose, formaldehyde, etc.9,30 Among the investigated materials, vertically aligned carbon nanotubes exhibit low overpotential and high ORR activity in alkaline electrolyte conditions.9 This is due to the high external surface area and favorable diffusion properties; however, the extant methods employed for the preparation of nitrogendoped CNT electrocatalysts involve the thermal decomposition of nitrogen-containing hydrocarbons over metal particles in a chemical vapor deposition technique. As a result, it is believed that the ORR performance does not solely come from the microstructured CNx materials. One way to prepare aligned CNx materials without the involvement of metal particles is the so-called template technique.44 This method entails synthesis of a desired material by pyrolyzing a suitable polymer precursor within the cylindrical and uniform-sized pores of an alumina membrane so that the tubular morphology will remain for the final carbon material. The objective of this work is to fabricate virtually metal-free, nitrogen-doped carbon nanotube electrodes and investigate their electrochemical ORR performance. To the best of our knowledge, this is the first ever report on the evaluation of the template-synthesized aligned nitrogen-doped carbon nanotubes Received: November 10, 2011 Revised: January 17, 2012 Published: January 23, 2012 4340

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Alkaline fuel cell tests were conducted using a homemade fuel cell test station. Gas diffusion electrodes (GDE) and membrane-electrode assembly (MEA) were fabricated according to the procedure reported in the literature,2,4,45−47 and 20 wt % Pt/C (E-TEK) and the prepared N-CNT catalysts were used to fabricate the anode and cathode, respectively. A commercially available anion-exchange membrane (FAA, Fuma-Tech GmbH, ∼130 μm thickness under wet conditions, 1.2 meq/g ion-exchange capacity) was used for the fabrication of MEA. FAA was pretreated by immersing in 1 M KOH solution to ensure a complete replacement of chloride ions in the polymer with hydroxide ions and washed with deionized water until the pH of the residual water was neutral. Homogeneous catalyst ink was prepared by dispersing the catalyst powder, ionomer (5 wt % OH-type FAA in dimethylacetamide, DMAc), and DMAc in the required amounts to attain a concentration of FAA of 30 wt % (dry basis). The catalyst ink was then sprayed onto a Toray carbon paper to make gas diffusion electrodes. The anode consisted of 0.5 mgPt/cm2 (or 2.5 mg of 20 wt % Pt/C). The cathode fabricated with N-CNT consisted of 5 mg/cm2. To compare the performance of N-CNT with Pt catalysts, cathodes containing 0.5 mgPt/cm2 were fabricated using 20 wt % Pt/C (E-TEK). Finally, the single-cell MEA was fabricated by sandwiching the OH-type FAA membrane between the cathode and anode by hot pressing at 333 K and 90 psi for 3 min. Fuel cell measurements were performed at 323 K by passing humidified H2 (95% RH, flow rate = 500 mL/min) at the anode and humidified O2 (95% RH, flow rate = 1000 mL/min) at the cathode. Before the steady-state polarization curves were recorded, the cell was left under open circuit conditions for 1 h (MEA conditioning). A polarization curve was then recorded by varying the applied potential.

toward ORR activity in alkaline media, selectivity toward ORR in the presence of ethanol, and AEMFC performance. The performance of the N-CNT is compared with commercial Pt/C (E-TEK).

2. EXPERIMENTAL SECTION 2.1. Preparation of Aligned Nitrogen-Doped Carbon Nanotubes. The aligned N-doped carbon nanotubes are prepared using the polymer, PMVI, according to our earlier report.11 In a typical synthesis procedure, 3.5 g of poly(2methyl-1-vinylimidazole) is dissolved in 100 mL of methanol and impregnated directly into the pores of the alumina template (Anodisc47, Whatman Inc.) by applying a vacuum from the bottom. The solvent was evaporated slowly, and the membrane was dried in a vacuum at 343 K. The polymer/ alumina composite was then polished with fine alumina powder to remove the surface layers and ultrasonicated for 20 min to remove the residual alumina used for polishing. The membrane was then placed in a quartz tube kept in a tubular furnace and carbonized at 1273 K for 2 h under argon gas atmosphere. The resulting carbon/alumina composite was then treated with 6 M NaOH to dissolve the alumina template and free the carbon nanotubes. Finally, the carbon nanotubes are washed with deionized water and dried. 2.2. Characterization Techniques. Powder XRD patterns were obtained on a Siemens D5000 diffractometer operating with Cu Kα radiation (λ = 1.5408 Å) generated at 40 kV and 30 mA. Micro-Raman scattering experiments were performed on a ISA Jobin-Yvon Inc. model T64000 FT-Raman at room temperature in a quasi-backscattering geometry with parallel polarization incident light. The excitation source used was an Ar-ion laser operating at 514.5 nm. The morphology of the CNTs was examined using a field emission scanning electron microscope (FESEM; JEOL JSM-7500F). X-ray photoelectron spectroscopy (XPS) spectra of the catalysts were obtained using a PHI Quantera instrument equipped with Mg monochromatic X-ray (hν = 1253.6 eV) source at a power of 350 W. A transmission electron microscopic (TEM) image of Pt/C was obtained using a JEOL 2010 system operated with an accelerating voltage of 200 kV. 2.3. Electrochemical Measurements and Anion-Exchange Membrane Fuel Cell Tests. A thin-film rotating disk glassy carbon electrode (Pine Instruments Inc., USA) technique was employed to study the ORR activity and kinetics on the NCNT and Pt/C catalysts. The measurements were performed at room temperature in a one-compartment electrochemical glass cell assembled with a catalyst-coated RDE disk as the working electrode, Ag/AgCl (+0.205 V vs NHE) as the reference, and Pt foil as the counter electrodes, respectively. The working electrodes were fabricated by casting the required amounts of catalysts as a thin film onto a RDE substrate (0.196 cm2 area) with 5 wt % Nafion as the binding agent. Homogeneous catalyst inks were prepared by ultrasonically dispersing the catalysts in 1950 μL of an isopropyl alcohol−water mixture (1:1 v/v) and 50 μL 5 wt % Nafion for 30 min. Thereafter, a required volume of the catalyst ink was pipetted onto the mirror-polished RDE disk and dried at room temperature. The specific loading of N-CNT and Pt/C in the fabricated electrodes was 0.25 and 0.1 mgcatalyst/cm2, respectively. The LSVs for ORR were recorded in O2-saturated 0.1 M KOH solution at a scan rate of 5 mV/s over the rotation rate 0−3600 rpm. Stability of the electrodes was ascertained by chronoamperometry.

3. RESULTS AND DISCUSSION The commercially obtained alumina membrane contains uniform-sized pores of diameter 100 nm and length 60 μm. The pores and channels of the membrane will be effectively utilized for the polymerization and subsequent carbonization leads to the formation of vertically aligned carbon nanotubes. Since no catalyst has been used for the synthesis of nitrogencontaining carbon nanotubes, it is worth pointing out that the nanotubes produced by the template synthesis under the experimental conditions are virtually free from metal impurities. 3.1. X-ray Diffraction, Raman Spectroscopy, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, and Transmission Electron Microscopy Analyses. Powder XRD pattern of the N-CNT exhibited characteristic (002), (100)/(101), and (004) diffraction peaks at 2θ values around 25, 43, and 52°, respectively (Supporting Information, Figure S1a). The appearance of the (004) diffraction peak at a 2θ value around 52° indicates the good crystallinity of the material and, specifically, the high average number of graphenes stacked within the coherent domains. First-order Raman spectra of the N-CNT show the characteristic D and G bands at ∼1355 and 1582 cm−1, respectively (Supporting Information, Figure S1b). The D band arises from the disordered sp2 hybridized carbon, whereas the G band is associated with the tangential stretching mode of highly ordered pyrolytic graphite and indicates the presence of crystalline graphitic carbon.11,18 The significant features, such as a marked broadening of the fwhm of the D band, high ID/IG ratio (>1), and asymmetric tailing of the D band at ∼1355 cm−1 4341

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Figure 1. (a) Low-magnification, (b−f) high-magnification SEM images of N-CNTs, (g) deconvoluted XP N1s spectrum of N-CNTs (inset shows the survey scan spectrum), (h) typical CNT structure with nitrogen atoms in the possible geometrical positions, and (i) TEM image of commercial Pt/C.

Figure 2. Cyclic voltammograms of (a) N-CNT and (b) Pt/C (E-TEK) electrodes in an Ar-saturated 0.1 M KOH; scan rate, −100 mV/s.

CNTs is shown in Figure 1g. The peaks at binding energies of 398.5 ± 0.1 (N1), 400 ± 0.1 (N2), and 401.5 ± 0.1 (N3) eV are attributed to the pyridinic, pyrrolic, and quaternary type nitrogen functionalities, respectively, in the carbon lattice.11,18 The typical CNT structure with N1, N2, and N3 sites is shown in Figure 1h. It indicates the disruption of π conjugation on the outer layers of the carbon lattice due to nitrogen doping. The calculated atomic percentages of N1, N2, and N3 are 4.6, 0.2, and 3.3, respectively. The transmission electron microscopic image of the Pt/C (E-TEK) shown in Figure 1i depicts the well-dispersed Pt fringes of average size 3.8 nm. 3.2. Activity, Durability, and Selectivity of N-Doped Carbon Nanotube Electrodes. Cyclic voltammograms recorded for N-CNT and commercial Pt/C (E-TEK) catalysts in deaerated 0.1 M KOH at a scan rate of 100 mV/s are shown in Figure 2a and b, respectively. As can be seen, featureless voltammetric currents within the potential range between +0.2 and −1.2 V were observed for N-CNT. It indicates the absence of metal impurities. In the case of Pt/C, well-defined reversible

observed for the as-prepared N-CNTs may be due to the presence of nitrogen atoms in the carbon lattice. Low- and high-magnification SEM images of the prepared NCNTs are shown in Figures 1a-f. Low-magnification SEM images (Figure 1a and Supporting Information, Figure S2) show the uniform, cylindrical, and aligned nanotubes with an outer diameter of 100 nm and length of 60 μm that almost matches the pore dimensions of the alumina template. Highmagnification SEM images (Figure 1b−f) show the defects in nanotubes as well as distorted nanotubes. This is due to the introduction of C−N bonding, which is shorter than the C−C bonds. The existence of different types of nitrogen species in the carbon lattice is analyzed using XPS, and recorded spectra are shown in Figure 1g. XPS survey-scan spectrum shows predominant C1s and N1s peaks that appear at around 284 and 400 eV in the nitrogen-doped CNTs (inset in Figure 1g). The observed C1s peak mainly represents graphitic carbon.18 The surface nitrogen concentration in the N-CNT is determined to be ∼8.0 at. %. The deconvoluted XP N1s spectrum of the N4342

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Figure 3. Steady-state polarization (I−V) curves at different rotation rates recorded in O2-saturated 0.1 M KOH electrolyte for (a) N-CNT and (b) Pt/C electrodes, respectively; scan rate, 5 mV/s. Corresponding Koutecky−Levich plots for ORR on (c) N-CNT and (d) Pt/C electrodes, respectively, and (e) chronoamperometric responses for the N-CNT and Pt/C electrodes at −0.25 V in O2-saturated 0.1 M KOH.

hydrogen desorption/adsorption peaks in the region of −0.45 and −1.0 V and irreversible preoxidation/reduction peaks in the region of +0.05 and −0.45 V were observed. The voltammetric features of N-CNT and Pt/C are in good agreement with literature reports.9,30 To evaluate the electrocatalytic ORR activity and kinetic parameters, thin-film electrodes were fabricated and steadystate polarization curves were recorded at a scan rate of 5 mV/s over a range of rotation rates of 0−3600 rpm. Linear sweep voltammograms (LSVs) recorded for N-CNT and Pt/C catalysts are shown in Figure 3a and b, respectively. As can be seen, an increase in the catalytic current with rotation rate and a single reduction peak for the ORR is observed for both the materials. At the applied potential lower than −0.4 V, the ORR is under a diffusion-controlled regime where mass transfer of O2 to the electrode determines the ultimate current density. The ORR falls into the kinetics-controlled regime at the potential at or above −0.4 V, at which the catalytic activity of the catalysts will be benchmarked. The onset potential for ORR (the potential where the current density increases to 20 μA/

cm2) at N-CNT was approximately −0.05 V, close to that determined for Pt/C. It indicates the similar adsorption/ reduction processes on both these catalysts. The kinetic currents measured from the curves recorded at 1600 rpm are ∼0.91 mA/cm2 for N-CNT and ∼0.95 mA/cm2 for Pt/C at −0.1 V. In addition, substantially enhanced steadystate diffusion current was observed over a large potential range for the aligned N-CNTs with respect to the Pt/C. The good ORR activity in the N-CNT catalyst is ascribed to the conjugation effect of the nitrogen lone-pair electrons on the nitrogen and graphene π-system, which leads to an increase in the bulk electrical conductivity, work function, and the density of states at the Fermi level.43,48 These results demonstrate that aligned N-CNT catalyzes the ORR in facile manner under alkaline conditions. The overall current density (j) in the reaction via RDE is the resultant of contributions from the kinetic current density (jk) and diffusion-limiting current density through the solution boundary layer (jd) according to the equation: (1/j) = (1/jk) + (1/jd).49 The value for jd can be represented as jd = 4343

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Figure 4. Polarization curves for (a) N-CNT and (b) Pt/C catalysts in 0.1 M KOH + x M ethanol (x = 0, 0.025, and 0.05) under oxygen atmosphere at a scan rate of 5 mV/s and a rotating speed of 1600 rpm.

Figure 5. (a) Anion-exchange membrane fuel cell performance of the MEA with N-CNT and Pt/C as cathodes at 323 K (open and closed symbols corresponding to the cell voltage and power density, respectively) and (b) corresponding cell voltage−time curves at 20 mA/cm2 for 30 h.

0.62neFACD2/3υ−1/6ω1/2, where ne is the number of electrons transferred in the reaction, F is the Faraday constant, A is the geometric area of electrode, C is the concentration of oxygen in the solution at atmospheric pressure (1.26 × 10−3 M), D is the diffusion coefficient (1.9 × 10−5 cm2/s), υ is the kinematic viscosity of the electrolyte (0.01 cm2/s), and ω is the electrode rotation rate. According to this equation, a plot of the inverse of the current density at constant potential versus ω−1/2 ought to result in a straight line with an intercept corresponding to the inverse of the purely kinetic current density (jk) and a slope (B, Levich constant) defined by 0.62neFACD2/3υ‑1/6. To determine the number of electrons transferred in the reaction, the overall current density (j−1) was plotted against the square root of angular velocity (ω−1/2). The corresponding Koutecky−Levich (K−L) plots are shown in Figure 3c and d. The experimental value of slope B corresponds to the average number of electrons transferred in the reaction (ne). The values of ne are determined to be ∼3.8 and ∼4.0 for N-CNT and Pt/C catalysts, respectively. Chronoamperometry was used to characterize the durability of the electrodes. The current density−time plots of N-CNT and Pt/C electrodes at −0.25 V in O2-saturated 0.1 M KOH is shown in Figure 3e. As can be seen, N-CNT exhibited a slow attenuation, and a relative current of 89% persists after 7200 s. In contrast, Pt/C exhibited a gradual decrease with a current loss of 80% after 7200 s. This result demonstrates the extreme durability of N-CNT compared with Pt/C. To examine the selectivity for the ORR on N-CNT and Pt/C catalysts, steady-state polarization curves for oxygen reduction are recorded in ethanol-free and ethanol-containing electrolyte. Figure 4 shows the polarization curves obtained for the catalysts in O2-saturated 0.1 M KOH + x M ethanol (x = 0.025 and

0.05) at a rotating speed of 1600 rpm. In the presence of 0.025 and 0.05 M ethanol, no obvious change in the ORR activity for N-CNT was observed (Figure 4a), whereas the activity was diminished over Pt/C (Figure 4b). The overpotential and low activity on Pt/C catalysts was attributed to the formation of a mixed potential, which was caused by simultaneous ethanol oxidation and oxygen reduction at the surface of Pt/C.50 The higher ethanol tolerance of N-CNT during oxygen reduction compared with Pt/C indicates that it may function as an ethanol-tolerant cathode in alkali-based ethanol fuel cells. 3.3. Anion-Exchange Membrane Fuel Cell Performance of N-Doped Carbon Nanotube and Pt/C Cathodes. Fuel cell performance of the electrocatalysts is an important evaluation criterion for practical applications. The steady-state polarization curves with the N-CNT and commercial Pt/C (ETEK) catalysts as cathode in AEMFC under the identical testing conditions are shown in Figure 5a. The MEA with NCNT and Pt/C as cathode shows an open circuit voltage of ∼0.87 and 1.0 V, respectively. The N-CNT exhibited Pt-like behavior in the current density region of 0−50 mA/cm2 but with low cell voltage. At 0.7 V, N-CNT and Pt/C electrodes yielded a current density of 11 and 38 mA/cm2. Alkaline anionexchange membrane fuel cell tests depicted a maximum power density of 37.3 and 61.7 mW/cm2 with N-CNT and Pt/C as cathodes. The power density observed with 0.5 mgPt/cm2 at both the electrodes is in good agreement with the earlier report.2 Figure 5b shows the cell voltage−time curves of the MEA with N-CNT and Pt/C cathodes recorded at 20 mA/cm2 for 30 h. The cell with the N-CNT and Pt/C exhibited stable voltage with low polarization losses within the period of 30 h. However, the MEA with N-CNT as cathode exhibited a lower cell voltage compared with Pt/C. 4344

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Although the performance observed with nitrogen-doped carbon nanotubes as cathode is low compared with Pt/C, there exist possible approaches, such as optimization of the electrode architectures (including the anode and cathode with the intention of reducing the loss of electrochemically generated water into the anode gas flow), increasing the number of catalytic active sites to maximize the electrokinetics, modifying fuel cell conditions (temperature, humidifying conditions, flow rate of gases, and pressure), strategic fabrication of MEA, and minimizing the thickness of the AAEM to facilitate the transfer of conductive hydroxide ions and the back-transport of water molecules from the anode to the cathode to improve the performance comparable to Pt/C. Further studies are underway to improve the performance using different anion-exchange membranes and modifying the electrode architectures.

4. CONCLUSIONS Aligned carbon nanotubes containing nitrogen content of ∼8.0 at. % were synthesized using an alumina template technique. Microscopic images depict the defect and slight distortion of the nanotubes 100 nm in diameter and 60 μm in length. Steady-state linear sweep voltammetric measurements indicate the comparable ORR activity of N-CNTs with Pt/C under alkaline conditions. Moreover, N-CNTs are highly selective toward ORR in the presence of ethanol. The performance of the MEA fabricated with the N-CNT and Pt/C catalysts as cathode in a single-cell AEMFC exhibited a maximum power density of 37 and 62 mW/cm2, respectively, at 323 K.

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ASSOCIATED CONTENT

S Supporting Information *

Powder XRD pattern, Raman spectrum and low-magnification SEM image of N-doped carbon nanotubes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1-787-764-0000 (ext. 5908). Fax: +1-787-756-7717. E-mail: [email protected] (C.V.R.), yishikawa@uprrp. edu (Y.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the support of NASA-EPSCoR under Grant Nos. NNX08AB12A and NNX09AV05A.

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