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Enhanced Lithium-Oxygen Battery Performances with Pt Subnanoclusters Decorated N-doped Single wall Carbon Nanotube Cathodes Venkateswara Rao Chitturi, Mahbuba Ara, Wissam Fawaz, Ka Yuen Simon Ng, and Leela Mohana Reddy Arava ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01016 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 2, 2016
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Enhanced Lithium-Oxygen Battery Performances with Pt Subnanoclusters Decorated N-doped Single wall Carbon Nanotube Cathodes Venkateswara Rao Chitturia,b, Mahbuba Arab, Wissam Fawazb, K. Y. Simon Ngb,* and Leela Mohana Reddy Aravaa,* a
b
Department of Mechanical Engineering, Wayne State University, Detroit, MI 48202, USA Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA
ABSTRACT Achieving dramatic improvement in long-term cycling performance in Li-O2 batteries continues to remain a challenge, which is imperative for turning its alluring promise into reality. Developing a bifunctional cathode material capable of reducing overpotentials and enhancing the long-term cycling performances holds the key. Herein, bifunctional cathodes for Li-O2 battery were prepared by electrodeposited Pt subnanoclusters on pristine as well as nitrogen-doped single-walled carbon nanotubes (SWCNTs) using rotating disk electrode voltammetry technique. Diffraction, microscopic and spectroscopic techniques were used to characterize the prepared materials and phase purity of the materials was confirmed. Microscopic analysis depicted a fine dispersion of ≤2 nm sized Pt nanoclusters on single-walled carbon nanotubes. Rotating disk electrode voltammetry measurements indicated low overpotentials as well as high catalytic ORR/OER activities with Pt nanoclusters decorated SWCNTs compared to Pt-free SWCNTs. Among the investigated cathodes, Pt/N-SWCNTs exhibited a high discharge capacity of 7685 and 5907 mAh/g at 100 and 500 mA/g respectively, and also good capacity retention. Moreover, a stable capacity of 3000 mAh/g with 100% Coulombic efficiency at 500 mA/g was
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demonstrated under repeated cycling conditions. Based on the ex-situ Raman spectroscopic studies, the high Li-O2 battery performance of Pt/N-SWCNTs cathode was attributed to the high decomposition activity of Li2O2 and negligible amount of Li2O2 accumulated on the electrode surface during cycling.
KEYWORDS: Electrocatalysis; metal-oxygen batteries; bifunctional catalysts; single-wall carbon nanotubes; nanoclusters
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1. INTRODUCTION Depletion of fossil fuels and global demand for energy triggered researchers to search and develop highly efficient energy storage systems for various applications. In recent years, rechargeable lithium-oxygen batteries (LOBs) garnered unprecedented interest due to their high specific energy (~1700 Wh/kg) comparable to gasoline and much higher than extant rechargeable Li-ion batteries (~160 Wh/kg).1-3 Li-oxygen batteries are characterized by the reduction of oxygen by lithium ions to form lithium (per)oxides via 2Li+ + 2e- + O2 ↔ Li2O2(solid) and/or 4Li+ + 4e- +O2 ↔ 2Li2O(solid) during discharge, and decomposition of lithium oxides to form lithium and oxygen during charge processes. The standard potential for the discharge reaction is given by the thermodynamics of the reaction as 2.96 V vs Li/Li+.4,5 However, practically the generated voltage is less than 2.96 V mainly due to the overpotentials of oxygen reduction reaction (ORR) and oxygen evolution reaction (ORR) during discharge and charge processes respectively. Apparently an applied voltage larger than 2.96 V (~4.0 V) is required to drive the reverse electrochemical reaction during charging process.5 Therefore, it is essential to develop an effective electrocatalyst to catalyze both ORR and OER with enhanced kinetics.6-8 Various kinds of cathode materials have been investigated to reduce potential hysteresis and thus to improve the round-trip efficiency of Li-oxygen batteries. The materials include nanostructured carbons,9-16 metals,17-30 transition metal oxides,31-46 transition metal nitrides,47,48 and transition metal carbides49-51. All the investigated catalysts have limitations such as low onset potentials, low limiting currents, and poor stabilities under non-aqueous conditions and the battery performances need to be improved further to make Li-O2 battery technology as reliable. The studies on cathode electrocatalysts indicated that the charge voltage, the discharge capacity,
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and the capacity retention are substantially dependent on the type of electrocatalyst used at cathode, selection of electrolyte and experimental conditions. Among all the investigated cathodes in non-aqueous Li-oxygen battery configuration, metal/carbon nanohybrids exhibited better performances and seem to be promising. In the present study, sub-nanometer Pt clusters were electrodeposited onto long strands of N-doped single wall carbon nanotubes (N-SWCNTs) and used them as cathodes in Li-O2 batteries. The rationale in designing the Pt/N-SWCNTs hybrid cathode is as follows: (i) A combined theoretical/experimental study on different noble metal systems indicated Pt and Pd as best cathode electrocatalysts for non-aqueous Li-O2 batteries based on the Li+-ORR activity and oxygen adsorption strength,18,52 (ii) N-doping in carbon lattice not only improve the metal particle dispersion and lower particle size but also facilitate the ORR/OER processes and diffusion of ionic species during discharge/charge processes,53-55 (iii) cathodes with nano-sized particles deliver good discharge performances due to the decreased diffusion path length,21,27,30 and (iv) Pt interact more strongly with the discharged product, Li2O2 than C sites in SWCNTs.6,18,52,56 The strong interaction between Li2O2 and the Pt nanoclusters can improve the reversibility of Li2O2 formation during cycling. Overall, Pt/N-SWCNTs is anticipated to show low polarization losses and increase the proportion of active sites at the surface thereby better LiO2 battery performances. For comparison, Li-O2 battery performances with pristine SWCNTs, N-doped SWCNTs and Pt/SWCNTs cathodes are also investigated. To the best of our knowledge, this is the first-ever report dealing with the growth of N-doped SWCNTs using pyrimidine as source, electrodeposition of sub-nanometer Pt clusters onto N-doped SWCNTs using potentiodynamic technique and Pt/N-SWCNTs as bifunctional cathode in Li-O2 batteries. Systematic electrochemical studies with galvanostatic discharge-charge cycling performances of
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the materials are presented. Ex-situ Raman spectroscopic studies are also performed to provide more insights about the interactions between catalyst and discharged species.
2. EXPERIMENTAL METHODS 2.1. Synthesis of pristine and nitrogen-doped SWCNTs In a typical catalytic chemical vapor deposition (CCVD) synthesis57, n-hexane with a given composition of ferrocene (20 mg/ml) was introduced into the reactor at a rate of 0.5 ml/ min for 30 min after heating the reactor to the pyrolysis temperature (1323 K), with a gas mixture of argon (800 sccm) and hydrogen (300 sccm) as the carrier gas. For the synthesis of nitrogen-doped SWCNTs, pyrimidine was used as a nitrogen source. A small amount of thiophene with Fe:S atomic ratio of 10:1 was also used as an additive to improve nitrogen content in the SWCNTs. The as-grown SWCNTs were purified by treating with concentrated acid solutions. 2.2. Electrodeposition of Pt on the SWCNTs Pt/SWCNT nanocatalysts were prepared by hydrodynamic rotating disk electrode (RDE) technique using a three-compartment electrochemical cell separated by glass frits.58 Briefly, an ultrasonically dispersed slurry suspension consisted of 50 mg of SWCNTs and 50 ml of 0.1 M H2SO4 was taken in the center of a three-electrode cell and then 2 mL of a 5 mM K2PtCl6 solution was added. The electrochemical cell was sealed and purged with inert gas for 30 min before starting electrodeposition process. Glassy carbon disk mounted in a RDE (Pine Instrument Co.) introduced in the slurry solution of SWCNTs and H2SO4 was used as the working electrode. Graphite rod and Ag/AgCl electrode were used as counter and reference electrodes, respectively. Electrodeposition of Pt was conducted by rotating the electrode at 900 rpm and applying a constant potential of -200 mV vs Ag/AgCl for 120 min using a potentiostat (BAS Epsilon). The 5 ACS Paragon Plus Environment
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whole process was repeated three times with the addition of 5 mM K2PtCl6 in each repetition. Finally, the suspension was filtered, washed with deionized water and dried at 343 K for 5 h under vacuum. The amount of Pt in the Pt/SWCNTs and Pt/N-SWCNTs is estimated to be 18.5 and 18.7 wt.% respectively. 2.3. Characterization techniques X-ray diffraction (XRD) measurements were performed on Rigaku Miniflex using a Cu Kα source operated at 40 kV and 40 mA. Scanning electron and transmission electron microscopic images were recorded with field emission JEOL-7500F SEM and high-resolution JEOL-2010 TEM systems operated with an accelerating voltage of 20 and 200 kV, respectively. Micro-Raman scattering experiments were performed on a Jobin Yvon T6400 FT-Raman at room temperature. The excitation source used was an Ar-ion laser operating at 514.5 nm. X-ray photoelectron spectra (XPS) were obtained using a PHI instrument equipped with Mg monochromatic X-ray (hν = 1253.6 eV) source at a power of 350 W. 2.4. Electrochemical measurements A thin-film rotating disk glassy carbon electrode (Pine Instruments Inc., USA) technique was employed to study the bifunctional activity of the 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, lithium foil as the counter electrode, and Ag wire immersed in 0.1 M TBAPF6 and 0.01 M AgNO3 in TEGDME [0.0 V (vs. Li/Li+) ≈ 3.52 V (vs. Ag/Ag+) as the reference electrode. 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)
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and 50 µL 5 wt % Nafion for 30 min. The sealed test system was first purged with Ar for 15 mins prior to each experiment. Subsequently, the electrolyte solution (1M LiCF3SO3 in tetraethylene glycol dimethyl ether (TEGDME)) was purged with O2 for 30 mins and LSVs for ORR were recorded at a scan rate of 5 mV/s over a rotation rate of 500 rpm. For OER experiments, Li2O2 was electrochemically deposited by holding the voltage at 2.2 V for 1 h and then LSVs were recorded at a scan rate of 5 mV/s over a rotation rate of 500 rpm. 2.5. Fabrication of gas diffusion cathodes and Li-oxygen cell assembly Gas diffusion electrodes (GDEs) were fabricated by spray-coating a homogeneous catalyst ink consisted of active material, Nafion binder, and isopropanol solvent. Weight ratio of the active material and binder was fixed to be 2:1. The fabricated GDEs with SWCNTs and NSWCNTs contains 0.26 mgcarbon/cm2 where as GDEs with Pt/SWCNTS and Pt/N-SWCNTs contains (0.26 mgcarbon+0.057 mgPt)/cm2. Li-oxygen cell tests were conducted at room temperature using 1 M LiCF3SO3 in TEGDME electrolyte, lithium foil anode, quartz microfiber separator, and spray coated electrocatalyst on Toray carbon paper (E-TEK) cathode. Li-O2 cells were assembled in the following order: i) placing a lithium foil onto the cell’s stainless steel current collector, ii) placing one quartz microfiber soaked in the electrolyte onto the lithium foil, iii) placing the electrocatalyst-coated carbon paper, and iv) adding on top a cathode current collector (nickel foam and spring). Afterwards, cells were sealed and tested galvanostatically at various rates ranging from 100 to 500 mA/gcarbon with a low voltage limit of 2.0 V vs Li/Li+ and upper limits of 4.5 V vs Li/Li+ to avoid electrolyte decomposition.
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3. RESULTS AND DISCUSSION 3.1. Structural and microscopic analyses Powder X-ray diffraction patterns recorded for the materials showed characteristic diffraction peaks of hexagonal carbon lattice and face-centered cubic Pt [Supporting Information, Figure S1].59-60 Average crystallite size of Pt on SWCNTs was determined to be 100) indicated highly graphitic nature of the prepared SWCNT materials. Xray photoelectron spectroscopic measurements were performed to gain insights about the nature of nitrogen and Pt species and the recorded spectra in the N1s and Pt 4f regions were presented in Figure 1. Binding energies of the N1s peaks centered at 398.5 and 401.5 eV indicated the existence of pyridinic- and graphitic-type of nitrogen species in the prepared N-doped SWCNTs and Pt/N-SWCNTs.62,63 The total surface nitrogen content in N-SWCNTs was determined to be 2.3 atom%. In the case of Pt-containing SWCNTs, two peaks at binding energies of 71.3 and 74.6 eV corresponding to the Pt 4f7/2 and 4f5/2 lines respectively were observed.60 The position of peaks revealed the existence of Pt in metallic state. Microscopic examination of the prepared SWCNTs is carried out using field-emission scanning electron microscopic (FESEM) and high resolution transmission electron microscopic
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(HRTEM) techniques. FESEM images (Figures S3a-d) showed long strands of carbon nanotubes with large packing density and several microns in length. Close examination of the SEM images recorded for Pt/SWCNTs (Figure S3c) and Pt/N-SWCNTs (Figure S3e) revealed fine dispersion of metallic Pt nanoparticles on the carbon nanotubes. EDX analysis performed on Pt/SWCNTs and Pt/N-SWCNTs further confirmed Pt distribution on the nanotube surfaces (Figure S3d and S3e). HRTEM images (Figures 2a and b) showed the aligned array of SWCNTs without any impurities. Open-ended, single wall nature of the materials is evident from the images shown in the inset of Figures 2a-d. Tube diameter is estimated to be in the range of 0.9-1.5 nm. HRTEM images recorded for the Pt/SWCNTs (Figure 2c and 2c1) and Pt/N-SWCNTs (Figures 2d and 2d1) depicted the well-dispersed and well-separated Pt atoms (appeared as bright spots in the Figures 2e and 2f) as well as atomic clusters of size
