Morphology-Controlled Promoting Activity of Nanostructured MnO2 for

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Morphology-Controlled Promoting Activity of Nanostructured MnO2 for Methanol and Ethanol Electrooxidation on Pt/C Sumanta Kumar Meher and G. Ranga Rao* Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, India S Supporting Information *

ABSTRACT: Herein, an evident microstructure effect of MnO2 in the electrooxidation of methanol and ethanol on Pt/MnO2/C electrocatalyst composite is reported. In this context, urchin-like MnO2 composed of identical nanorods and microcubes of MnO2 were selectively synthesized via reduction of KMnO4 by HCl and forced hydrolysis of MnCl2 by urea, respectively. The physicochemical studies showed smaller crystallite size (∼11 nm) and higher BET surface area (∼61 m2 g−1) of MnO2-nanorods as compared to the MnO2-microcubes with crystallite size of ∼15 nm and BET surface area of ∼26 m2 g−1. The HRTEM analysis was performed to evaluate the inherent morphology effect of MnO2 on the size and dispersion of Pt crystallites in Pt/MnO2/C composite, and the results showed miniaturization and higher dispersion of Pt crystallites in the presence of MnO2-nanorods. The cyclic voltammetry studies revealed higher current response and lower overpotential by the MnO2-nanorod-modified Pt/C as compared to the MnO2microcube-modified Pt/C and bare Pt/C during alcohol electrooxidation reaction. The chronopotentiometry and chronoamperometry analyses showed lower alcohol oxidation overpotential and longer polarization time/stability for MnO2-nanorod-modified Pt/C as compared to the MnO2-microcubemodified Pt/C and bare Pt/C. Further, in CO stripping voltammetry study, the MnO2-nanorod-modified Pt/C showed higher current response and stronger negative shift in the CO electrooxidation potential. The MnO2-nanorods provide more triplephase interfaces for better adsorption of oxidizing species which facilitate the oxidation of poisoning species via synergic effect during alcohol electrooxidation reaction. All these results together corroborate enhanced antipoisoning and promoting activity of MnO2-nanorod as compared to MnO2-microcube for methanol and ethanol electrooxidation reactions on Pt/C. The end results in this report hold adequate significance in future development of electrocatalysts based on suitably structured transition metal oxides for fuel cell applications.

1. INTRODUCTION In the context of the growing need for renewable energy production and storage methodologies due to enormous energy demand of modern society, notable effort has been put forward to develop devices which ensure high efficiency with lowest environmental issues.1,2 Fuel cells which convert chemical energy of a fuel into electrical energy through feasible electrochemical reactions are by far the most important due to their high energy conversion efficiency and exceptional portability.3−5 Direct alcohol fuel cells (DAFCs) have significant advantages over several other types of fuel cells, since they do not require a fuel reformer and comprise high operating efficiency, excellent volumetric energy density, and the fewest environmental issues.6,7 Owing to the high volumetric energy density of the fuels in DAFCs, the dimension of the device can be suitably optimized for small- and largescale stationary, portable as well as transport applications. Methanol and ethanol are among the most promising fuels in DAFCs due to their higher efficiency to be oxidized electrochemically.6,7 Methanol possesses disadvantages like slow electrode kinetics and is prone to crossover and toxicity while ethanol is less toxic and bears high energy density. However, ethanol cannot be oxidized fully at the favorable © 2013 American Chemical Society

thermodynamic potential of 0.08 V (vs RHE) due to the formation of various stable intermediate species from resilient C−C bonds.8,9 Hence, in order to improve the performance of methanol- and ethanol-based DAFCs, it is essential to design catalysts which can improve the oxidation kinetics and reduce the poisoning of the electrode by intermediate species. In this context, Pt has been the best catalyst for decades.4,10 However, Pt is very expensive for widespread commercial application.11 In addition, nanostructures of Pt, which are mostly used as electrocatalysts, are highly prone to segregation during longterm operation. This leads to reduction in the number of active sites on Pt, which drastically decreases its efficiency for alcohol electrooxidation.10,11 Besides, during the electrooxidation of alcohols, the major carbonaceous intermediate, CO, gets adsorbed on the active Pt sites and arrests the long-term activity of the catalyst.12−14 However, at a high overpotential, the activation of water generates oxygen-containing species such as OH, which can oxidize the adsorbed CO on the Pt surface. Hence, one of the major challenges in DAFC research Received: September 21, 2012 Revised: February 21, 2013 Published: February 21, 2013 4888

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promoting Pt/C for methanol and ethanol electrooxidation reactions in acidic medium.

is the development of efficient and cost-effective catalysts, which can provide oxygenated species at lower overpotentials for oxidative removal of adsorbed CO.12−14 In this context, the state-of-art approach is the designing of bi-, tri-, and poly metallic Pt-based alloys with suitable metals such as Pd, Ru, Rh, Au, Sn, Ni, Ir, Os, Cu, etc., which effectively provide electronic suitability to make the oxygenated species available for reaction at lower overpotentials.15,16 It is known that the electrooxidation of alcohols is highly site specific. Eventually, catalysts with suitable size and shape provide ample active surface for preferential adsorption of alcohols.17−19 Unlike methanol, the electrooxidation of ethanol is rather complicated. Hence more active and selective catalytic sites are required for the facile electrooxidation of ethanol.20−22 In the above perspectives, model structurization of a polymetallic alloy for dual methanol and ethanol selectivity is of practical importance. However, ideal structurization and commercial scale synthesis of multimetallic architectures for dual methanol and ethanol selectivity is extremely challenging. Lately, the issues in dual methanol and ethanol electrooxidation have been addressed by various metal oxides-, carbides-, and nitrides-promoted electrocatalysts.23−25 Among these various promoters, metal oxides are considered as the best due to their low cost. Further, straightforward synthesis procedures can be adopted to produce high surface area oxides with suitable surface properties which can efficiently promote the methanol and ethanol electrooxidation reactions on Pt/C. Metal oxides also provide suitable functional groups which strongly interact with small Pt crystallites, thereby circumventing their random growth and agglomeration during device operation for longer duration. Metal oxides such as WO3, CeO2, RuO2, V2O5, Nb2O5, MoOx, TiO2, and MnO2 show immense promise in promoting Pt/C for electrooxidation of alcohols.26−39 Among these oxides, MnO2 possesses good proton−electron intercalation properties and is known to show good electrochemical properties under various operating milieu.33−39 Due to the possibility of the Mn4+/Mn3+ redox couple and labile oxygen, MnO2 shows high promoting and antipoisoning activities for alcohol electrooxidation. The majority of the reports which cover the effectiveness of MnO2 as a promoter specify the size, surface area, tunnel structure, and crystal phase as the main reasons for its activity. It is known that oxides with onedimensional (1D) structures such as nanorods, nanowires, and nanotubes possess distinctive crystalline phase states, nonlinear optical properties, and quantum size effects as compared to their bulk counterparts.40,41 It is also known that the interaction between metal crystallites and an oxide surface is very much influenced by the nature of interfacial contact and crystalline characteristic of the oxide.42 In this context, MnO2 with smaller and uniform crystalline orientation as well as suitable surface morphology should offer apposite active sites for facile interaction with Pt crystallites, which can provide optimized synergic effect for alcohol electrooxidation. However, there is no report available in the literature showing the concerted effect of microstructure/morphology of MnO2 on the nature of Pt dispersion on Pt/MnO2/carbon based electrocatalysts. Further, the effect of nanostructured MnO2 has not been differentiated from its bulk counterpart in the context of promoting Pt/C for alcohol electrooxidation reactions. In this perspective, the present work is aimed at the synthesis of tunable morphologies of MnO2 with different physicochemical properties. Further, the effect of nanostructured MnO2 has been clearly differentiated from that of bulk MnO2 in

2. EXPERIMENTAL SECTION 2.1. Synthesis of MnO2 Samples. Analytical grade KMnO4 (SD Fine Chemicals, India), HCl (Rankem, India), MnCl2·4H2O (CDH, India), and urea (Rankem, India) were used as received. In a typical experiment, 22.2 mmol of HCl in 20 mL of water was added dropwise to 180 mL of 5.5 mmol aqueous KMnO4 solution, with constant stirring, to form the precursor solution. The solution was then subjected to hydrothermal treatment at 120 °C for 12 h in a stainless steel autoclave of 250 mL capacity, with an inner Teflon liner. The autoclave was then cooled to room temperature and a dark solid was separated by centrifuging the product at 5000 rpm. The product was then repeatedly washed with water, a mixture of ethanol and water, and pure ethanol several times to remove impurities. The product was then dried at 60 °C under vacuum overnight. The sample was designated as MnO2−NR. In another typical experiment, 20 mmol of urea dissolved in 100 mL of triple-distilled water was added dropwise to 20 mmol of MnCl2·4H2O dissolved in 100 mL of triple-distilled water. The resultant solution was stirred for 1 h and then transferred to a stainless steel autoclave of 250 mL capacity, with an inner Teflon liner, and subjected to heating at 120 °C for 12 h. The autoclave was then allowed to cool to room temperature and after the resultant product was aged for 24 h, a brown precipitate was separated by centrifugation at 5000 rpm, through repeated washing with triple-distilled water followed by a mixture of absolute ethanol and water, and finally with absolute ethanol. The precipitate was then dried under vacuum at 60 °C overnight. The dried precursor sample was then subjected to thermal treatment at 400 °C (10 °C min−1) for 3 h followed by 450 °C for 3 h in flowing O2. The black sample was designated as MnO2−B. 2.2. Preparation of Electrocatalyst Composites. The electrocatalyst composites were prepared by the microwaveassisted polyol process. The mass content of MnO2 in all the electrocatalysts was fixed at 20 wt %. For the preparation of a typical Pt-loaded MnO2/carbon catalyst, 20 mg of MnO2 and 100 mg of Vulcan XC-72 carbon black (C, Cabot Corporation, BET surface area of ∼250 m2 g−1) were dispersed in 100 mL of ethylene glycol (EG)−isopropyl alcohol (IPA) mixture (v/v = 4:1) via ultrasonication for 30 min. To the above mixture, 1.282 mL of 0.1 M H2PtCl6·6H2O (Aldrich, ACS Reagent)−EG solution was added, then the mixture was sonicated for 15 min and then stirred for 6 h to obtain the mixture ink. The pH of the resultant mixture was then adjusted to ∼10 by dropwise addition of 1 M KOH-EG solution. The mixture ink was then treated with microwave irradiation for 50 s in a domestic microwave oven (Sharp NN-S327 WF, 2450 MHz, 1100 W) for complete reduction of H2PtCl6 to Pt. The resulting product was then allowed to cool to room temperature under continuous stirring. The pH of the microwave-treated mixture ink was then adjusted to ∼4 with 0.1 M HNO3 solution and was further stirred for 12 h before the residue was filtered out. The resultant product was repeatedly washed with Millipore water, followed by acetone (AR grade) until it became free from Cl− ion. The product was then dried overnight under vacuum at 60 °C. The Pt/Carbon electrocatalyst was prepared by using a similar procedure as described above, but without MnO2. The theoretical mass of Pt metal in all the composite electrocatalysts was fixed at 20 wt %. The Pt/carbon, Pt loaded 4889

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MnO2−B/carbon, and Pt loaded MnO2−NR/carbon samples were designated as Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/ C, respectively. 2.3. Preparation of Working Electrodes and Electrochemical Measurements. The working electrodes were fabricated by using the ink prepared by ultrasonically dispersing the Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C powders in 1.0 mL of distilled water and 0.1 mL of Nafion solution (5 wt % solution in a mixture of lower aliphatic alcohols, Sigma-Aldrich) for 30 min. A known volume of the composite ink was then spread onto a mirror finished glassy carbon electrode (polished with 5 μm γ-alumina micropolish paste, Buehler, USA) with a diameter of 6 mm (electrode area = 0.28 cm2), using a micropipet tip. The solvent of the ink was then slowly evaporated at 40 °C, under vacuum, to obtain a well-dispersed catalyst layer on the glassy carbon electrode. The Pt loading on each glassy carbon electrode (for all three catalysts) was 28 μg metal cm −2 . The electrochemical measurements were performed in a CHI 7081C electrochemical workstation, using a conventional three-electrode setup at room temperature. A platinum foil (area = 2 cm2) and an Ag/AgCl electrode (BAS Instruments, USA) were used as the counter and reference electrodes, respectively. For a typical methanol electrooxidation reaction, a solution of 1 M CH3OH in 0.5 M H2SO4 purged with high-purity argon for 30 min under constant stirring was used as the test solution. Similarly, for a typical ethanol electrooxidation reaction, a solution of 1 M C2H5OH in 0.5 M H2SO4 purged with high-purity argon for 30 min under constant stirring was used as the test solution. The final measurements were recorded after stable current response from the working electrodes, subjected to continuous voltammetry cycling at 20 mV s −1 . To ensure the reproducibility of the results, freshly prepared electrolyte solutions were used in every electrochemical measurement. The CO stripping voltammetry was performed in 0.5 M H2SO4 solution. After purging the solution with ultrapure Ar for 30 min, gaseous CO (0.1% CO in Ar) was bubbled for 120 min under a fixed potential of 0.0 V vs Ag/AgCl, to promote the formation of a perfect CO adlayer on the surface of the catalyst. Further, excess CO traces from the solution as well as surface of the catalyst were flushed out by purging the solution with ultrapure Ar for 30 min, under the same applied potential of 0.0 V vs Ag/AgCl. The CO stripping voltammetry patterns were recorded at a potential scan rate of 20 mV s−1. 2.4. Physicochemical Characterizations. The PXRD patterns were recorded in a Bruker AXS D8 Advance diffractometer, at a scan rate of 0.01° s−1, using Cu Kα (λ = 0.15408 nm) radiation generated at 40 kV and 30 mA. The crystallite size was approximated by using the Scherrer equation, D = Kλ/(β cos θ), where D is the linear dimension of the particle (particle size), K is the spherical shape factor taken as (0.89), and β is the full width at half-maximum height (fwhm) of the peaks. Multipoint nitrogen adsorption− desorption measurements were carried out by means of an automatic Micromeritics ASAP 2020 analyzer, using the Brunauer−Emmett−Teller (BET) gas adsorption method. The samples were degassed at 100 °C for 2 h followed by 150 °C for 10 h in a dynamic vacuum before physisorption measurements at 77 K. The specific surface area values (SBET) were estimated by using software of the instrument based on the BET equation. The porosity distributions in the samples were generated from desorption branches of the isotherms, using the Barrett−Joyner−Halenda (BJH) method and a

cylindrical pore model. The High Resolution Scanning Electron Microscopy (HRSEM) measurements were carried out using a field emission gun equipped FEI Quanta 200 microscope. The sample powders were dispersed on a carbon tape before mounting on the sample holder for HRSEM analysis. The High Resolution Transmission Electron Microscopy (HRTEM) measurements were performed on a JEOL 3010 HRTE Microscope (lattice resolution of 0.14 nm), fixed with a UHR pole piece and operated at an accelerating voltage of 300 kV. The XPS measurements were carried out by using a multiprobe system (Omicron Nanotechnology, Germany) equipped with a dual Mg/Al X-ray source operated at 300 W and 15 kV, and a hemispherical analyzer operating in constant analyzer energy (CAE) mode. The spectra were obtained with a pass energy of 50 eV. The base pressure in the analyzing chamber was maintained at 10−10 mbar. The data profiles were subjected to a nonlinear least-squares curve-fitting program with a Gaussian− Lorentzian production function and processed with the Casa XPS program (Casa Software Ltd., U.K.). The C1s binding energy of 284.9 eV was taken as the reference binding energy for charge correction. The ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) measurements were performed in a Perkin-Elmer Optima 5300DV ICP-OES analyzer.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Studies. The wide-angle PXRD patterns of the MnO2 samples are shown in Figure 1. The samples synthesized via acid reduction as well as forced hydrolysis methods show characteristic peaks of α-MnO2 (JCPDS: 44−0141). The formation of MnO2 (MnO2−NR) by reduction of KMnO4 with HCl is a one-step process (eq 1). 2KMnO4 + 8HCl → 2MnO2 + 3Cl 2 + 4H 2O + 2KCl (1)

However, the formation of MnO2 (MnO2−B) from forced hydrolysis of MnCl2 involves multistep reactions, as shown in eq 2.43,44 ⎫ ⎪ − − + −⎪ NCO + 3H 2O → HCO3 + NH4 + OH ⎪ ⎪ HCO3− → CO32 − + H+ ⎪ ⎬ ⎪ Mn 2 + + CO32 − → MnCO3 ⎪ 1 ⎪ MnCO3 + O2 ⎪ 2 o o 400 C for 3 h, followed by 450 C for 3 h ⎪ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MnO2 + CO2 ⎭ hydrolysis

NH 2CONH 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NH4 + + NCO−

(2)

The crystallinity of the two MnO2 samples was ascertained from the intensity and broadness of their characteristic PXRD profiles. From Figure 1, the corresponding XRD peak intensities of the MnO2−NR sample are lower than those of the MnO2−B sample. The crystallite sizes of the MnO2−B and MnO2−NR samples were found to be ∼11 and ∼15 nm, respectively, which were estimated from the corresponding diffraction peaks of (110), (200), (310), and (211) planes. The high-temperature thermal treatment during synthesis may be one of several reasons for higher crystallinity of the MnO2−B sample. The FESEM images of the MnO2−B and MnO2−NR samples are shown in Figure 2. The low- and high-resolution 4890

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dimensional sheets, which in the presence of K+ ions at elevated temperature and pressure under hydrothermal condition curl into smaller nanorods. Subsequently, the smaller nanorods grow to larger ones via Ostwald ripening and then become energetically stable by undergoing oriented attachment, which results in the urchin-like morphology.45−47 However, during the forced hydrolysis process, the slow nucleation of MnCO3, in the absence of any structure-directing agent, gives rise to larger-size spherical-shaped crystals (thermodynamically and symmetrically favorable). The larger crystals then undergo condensation to form three-dimensional sheets which are sterically unfavorable for curling. Consequently, under hydrothermal condition at elevated temperature and pressure, the three-dimensional sheets undergo layer-by-layer stacking and Ostwald ripening to form microcubes. In the absence of any structure directing agent, the growth of three-dimensional sheets to microcubes is highly random.50,51 The microcubes of MnCO3 undergo thermal transformation to MnO2 with complete retention of surface morphology. The highly random sizes of the microcubes are clearly seen from the FESEM images in Figure 2A,B. Precisely, the most important factors which control the crystal growth of MnO2-nanorods and MnO2-microcubes are the nature of various ions in the reaction medium and the difference in the precipitation kinetics. The formation pathways of corresponding urchin- and agglomerated microcube-like surface morphologies of MnO2−NR and MnO2−B are portrayed in Scheme 1. The N2 adsorption/desorption analyses (BET) show that the MnO2−NR sample possesses BET surface area and BJH pore volume of 61 m2 g−1 and 0.12 cm3 g−1, respectively, whereas the corresponding values for the MnO2−B sample are 26 m2 g−1 and 0.05 cm3 g−1. The results of BET analysis are presented in Figure S1 in the Supporting Information. It is obvious that unlike MnO2−B, the higher surface area of MnO2−NR may

Figure 1. PXRD profiles of MnO2−B and MnO2−NR samples.

FESEM images (Figure 2A,B) of the MnO2−B sample show truncated cube-like surface morphology, which are micrometer in size and highly agglomerated. However, the low-resolution FESEM image of the MnO2−NR sample shows urchin-like structure (Figure 2C). The high-resolution image (Figure 2D) shows that the urchin structures are made up of uniform nanometer-sized rods. The formation mechanism of this type of nanorod morphology entails a well-known dissolution−crystallization− rolling-phase transformation-oriented attachment-type mechanism.45,46 The appearance of nascent crystal seeds and their geometric arrangement followed by further growth to one- or multidimensional structures occur to minimize the surface energy.47 The final structure is also organized by kinetic growth of the primary structures/anisotropic nanocrystals. In fact, the growth of primary structures/anisotropic nanocrystals is modulated by preferential adsorption of anions, solvent molecules, and different reaction conditions such as temperature, pressure, etc.48,49 During the reduction of KMnO4, the primarily formed nascent crystals of MnOx condense into two-

Figure 2. Low- and high-resolution FESEM images of (A, B) MnO2−B and (C, D) MnO2−NR samples. 4891

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Scheme 1. Plausible Formation Pathways of Urchin- and Agglomerated Microcube-Like Surface Morphologies of MnO2−NR and MnO2−B by (A) Reduction of KMnO4 and (B) Forced Hydrolysis of MnCl2

indicates very high dispersion of MnO2 in the respective composites. Further, the diffraction peaks of Pt in the Pt/ MnO2−B/C and Pt/MnO2−NR/C samples are very broad and significantly lower in intensity. This is attributed to smaller crystallite size and enhanced dispersion of Pt in the MnO2loaded Pt/C samples. Again, the broadness and lesser intensity of diffraction peaks corresponding to Pt are significantly prominent in the MnO2-nanorod modified Pt/C as compared to the MnO2-microcube modified Pt/C sample. This is a clear indication of smaller crystallite size and higher dispersion of Pt particles in the presence of nanostructured MnO2 as compared to bulk MnO2 in the composites. This has been further substantiated from the HRTEM analysis and the corresponding HRTEM images are presented in Figure 4. The HRTEM image of MnO2-nanorod modified Pt/C sample (Figure 4C) shows finer dispersion of Pt crystallites as compared to the MnO2microcube-modified Pt/C (Figure 4B) and bare Pt/C (Figure 4A) samples. It appears that the nanostructured MnO2 promotes miniaturization and very good dispersion of Pt crystallites. This is due the higher surface area and onedimensional surface morphology of MnO2-nanorods, which provide more active surface area/larger number of active surface sites as well as uniformly oriented crystal faces, suitable for hindering the rapid ripening and subsequent agglomeration of the Pt crystallites. The average crystallite sizes of the Pt in the Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples were found to be ∼4.7, ∼3.9, and ∼2.6 nm, respectively, which were estimated from the characteristic reflections from the (220) planes of Pt in the corresponding PXRD profiles. The Pt atoms are known to be adsorbed preferentially on the surface Obridge sites of the negatively charged CeO2 surface.27,52 In this context, the surface ζ potential studies at different pH show that MnO2 possesses high negative surface charge in basic medium.53 Since the reduction of H2PtCl6 to Pt was carried out under highly basic medium in the presence of MnO2 and carbon, the Pt crystallites may preferentially become bonded to MnO2 via a similar mechanism as in the case of CeO2.27,52 Essentially, the nanorods of MnO2 expose more surface with a high degree of structural anisotropy, and rich edges as well as corner atoms which facilitate the dispersion of Pt particles.

provide more active surface for better dispersion of Pt crystallites. MnO2 being known as a promising oxide that promotes Pt/C to electrooxidize alcohols for fuel cell applications, the MnO2− NR and MnO2−B samples with different surface microstructures were assessed for their characteristic promoting efficiency to Pt/C for methanol and ethanol electrooxidation reactions. In this context, MnO2−NR and MnO2−B loaded Pt/ C and bare Pt/C samples were subjected to structural and electrochemical characterizations. The Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples were first subjected to PXRD analysis to generate primary information on the structural nature of the species present in the composites. The PXRD patterns of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples in Figure 3 show major diffraction peaks at 2θ values

Figure 3. PXRD patterns of Pt/C, Pt/MnO2−B/C, and Pt/MnO2− NR/C samples.

of 39.8°, 46.3°, 67.6°, and 81.4°, which are attributed to respective diffractions from (111), (200), (220), and (331) planes of FCC-type Pt crystallites. In addition, broad peaks at 2θ value of 24.9° in all the samples is the signature of (002) diffractions from graphitic Vulcan XC-72. Interestingly in the PXRD patterns of Pt/ MnO2−B/C and Pt/MnO2−NR/C samples, no diffraction peak characteristics of α-MnO2 were observed. This clearly 4892

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Figure 4. HRTEM images of Pt crystallites in (A) Pt/C, (B) Pt/MnO2−B/C, and (C) Pt/MnO2−NR/C samples.

caused by dispersion of Pt and carbon on the surface of MnO2nanorods during preparation of Pt/MnO2−NR/C composite. The large atomic number for Pt as compared to Mn also leads to a poor contrast between Pt and Mn. The typical spin-energy separation of ∼11.8 eV further confirms the presence of Mn4+ species. The deconvoulated XPS profiles in the Mn 2p region do not show any characteristic signatures pertaining to species other than Mn4+, suggesting the exclusive presence of MnO2 in the Pt/MnO2−NR/C composite. Since the Pt/MnO2−B/C composite is also synthesized under similar experimental conditions, the chemical states of Pt and MnO2 in the Pt/ MnO2−B/C sample are expected to be analogous to those in the Pt/MnO2−NR/C composite. 3.2. Catalytic Activity Study. 3.2.1. H2 Electrosorption Study. The promoting efficiencies of MnO2−NR and MnO2−B to Pt/C were evaluated from methanol and ethanol electrooxidation reactions in acidic media, using cyclic voltammetry, chronopotentiometry, and chronoamperometry measurements. A comparison of cyclic voltammetry responses of MnO2promoted Pt/C and bare Pt/C samples measured in 0.5 mol L−1 H2SO4 solution at a scan rate of 20 mV s−1 is presented in Figure 6.

The microwave-mediated reduction of H2PtCl6 to Pt in ethylene glycol medium also promotes faster reduction and better dispersion of Pt particles on the MnO2/C supports.54 The amount of Pt leaching during the preparation of Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C composites was investigated by using ICP-OES analysis. The actual amounts of Pt loadings were estimated to be ∼99.9% to 99.93% of the deliberately loaded Pt in the samples. The oxidation states of Pt and MnO2, and their possible interactions were obtained from XPS analysis of the Pt/MnO2− NR/C sample, and the core level XPS profiles corresponding to Pt 4f and Mn 2p regions are presented in Figure 5, panels A

Figure 5. Core level fitted and deconvoluted XPS profiles of (A) Pt 4f and (B) Mn 2p regions of Pt/MnO2−NR/C sample.

and B, respectively. The core level XPS profile in the fingerprint region of Pt 4f shows peaks centered at 71.2 and 74.55 eV, with a spin−orbit splitting of ∼3.35 eV, which are ascribed respectively to 4f7/2 and 4f5/2 states of Pt0.30,31,54 However, the binding energy of Pt0 is ∼0.3 eV higher as compared to the metallic Pt (70.9 eV for 4f7/2 state), which is attributed to some charge transfer due to Pt−MnO2 interaction. Studies show that nanocrystallites of metallic Pt surface are usually covered with a trivial amount of platinum oxide (PtO). Hence the XPS profile in the Pt 4f region is carefully deconvoluted to identify species with other oxidation states of Pt (other than Pt0) in the Pt/ MnO2−NR/C composite. The deconvoluted XPS profile shows weaker peaks at 72.33 and 75.66 eV which are attributed to the Pt2+ state.30,31,54 The deconvoluted profiles do not show any signature of Pt4+, which clearly suggests complete reduction of H2PtCl6 by EG, under microwave irradiation. However, the characteristic signatures due to Pt2+ in the deconvoluted profiles imply minor surface oxidation of Pt to PtO. The deconvoluted core level XPS profile of the Mn 2p region (Figure 5B) shows peaks centered at 654.4 and 642.6 eV corresponding to Mn 2p1/2 and Mn 2p3/2 states.55,56 The characteristic XPS profile of Mn 2p is highly noisy, which is due to the low exposed surface fraction of MnO2-nanorods. This is

Figure 6. Comparative cyclic voltammograms (scan rate = 20 mV s−1) of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes in 0.5 mol L−1 H2SO4 solution.

The individual cyclic voltammograms (CVs) of Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C samples are shown in Figure S2 (panels A, B, and C) in the Supporting Information. The CVs clearly reveal the characteristic features due to hydrogen adsorption/desorption, double layer charging, oxide (of Pt) formation, and oxide (of Pt) reduction on Pt surface.57 The signatures in the potential region of −0.2 to 0.2 V (vs Ag/ AgCl) during the forward as well as reverse scan correspond to underpotential deposition of hydrogen (Hupd) owing to adsorption/desorption processes (H+ + e− = Hupd).57 Further, 4893

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The QH values were estimated by integrating the potential versus current density plots in the Hupd regions of the individual samples as shown in the insets of Figure S2 (panels A, B, and C) in the Supporting Information. The QH values for Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C samples were found to be 0.246, 0.283, and 0.341 mC cm−2, respectively. The EAS values of each sample owing to active surface sites of Pt nanocrystals were determined by using the QH and Pt loading on the working electrodes (MPt = 0.028 mg cm−2) in eq 4.29

the signatures beyond 0.4 V (vs Ag/AgCl) during the anodic scan, and 0.8 to 0.2 V (vs Ag/AgCl) in the reverse scan are characteristics of respective formation and reduction of PtOx·nH2O/Pt(OH)y·nH2O (x = 1, 2; y = 2, 4) on the Pt surface.58 However, a closer look at the voltammetric behavior in the Hupd regions confirms possible differences in the nature of hydrogen adsorption−desorption processes, which can be correlated to the differences in the structural and crystallographic characteristics of Pt in MnO2-modified Pt/C and bare Pt/C samples.59,60 In particular, the distinctive peaks due to H on (110) and (100) steps, and anions on (111) domain of Pt in the MnO2-modified Pt/C samples demonstrate that the dispersed Pt particles in the presence of MnO2 expose specific crystal facets for interaction with H and other anions. In contrast, the broad and undistinguished peaks during H adsorption/desorption (Hupd region) on Pt in the bare Pt/C sample indicate random Pt crystallites with no specifically exposed crystal facets.58−60 Further, sharpness in the H adsorption/desorption peaks with more current response in the Hupd regions of MnO2 promoted Pt/C samples as compared Pt/C sample clearly suggests a higher number of specifically exposed clean Pt surfaces in the MnO2-promoted Pt/C samples.59,60 Among the two MnO2-promoted Pt/C samples, the MnO2-nanorod-promoted sample shows higher current response (despite the same amount of Pt loading) and sharper H adsorption/desorption peaks in the Hupd region. This essentially shows a higher number of uniformly exposed crystal facets of Pt in the MnO2-nanorod-promoted Pt/C sample as compared to the MnO2-microcube-promoted Pt/C sample. This is due to the fact that larger numbers of active surface sites on MnO2-nanorods induce nucleation of uniform and geometrically controlled Pt nanocrystals with evenly exposed crystal facets. However, lower numbers of surface active sites are exposed on the MnO2-microcubes, which induce geometrically uncontrolled growth of Pt crystals. Further, the anodic and the cathodic peaks are more symmetric in the Hupd regions of MnO2-modified Pt/C samples as compared to the bare Pt/C sample. This indicates good reversibility of hydrogen adsorption/desorption processes and Volmer reactions on the Pt surface.61,62 It signifies interfacial promoting activity of MnO2, in particular nanostructured MnO2 (MnO2−NR), for controlled and specific crystal growth of Pt crystallites which are favorable to electrochemical reactivity. Further, in the Hupd region, anodic as well as cathodic peak current responses are greater for MnO2-promoted Pt/C samples as compared to the bare Pt/C sample. This is attributed to larger numbers of electroactive sites on the Pt nanocrystals in MnO2-promoted Pt/C samples.61,62 A quantitative analysis of the number of electroactive sites on Pt nanocrystals in different samples was performed by estimating the electrochemical active surface area (EAS) of Pt. The EAS values were determined from the corresponding Coulombic charge for hydrogen adsorption and desorption (QH) in the Hupd region of the cyclic voltamograms of the Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples, assuming a hydrogen adsorption stoichiometry of one (i.e., one adsorbed hydrogen per active Pt surface atom). The average Coulombic charge (QH) for hydrogen adsorption as well as desorption was determined from the total charge transfer (Qtotal) in the Hupd and double-layer charging (QDL) regions of Pt and Vulcan XC-72, using eq 3.63 Q H = 0.5(Q total − Q DL)

EAS (m 2 g −1) =

QH 0.21MPt

(4)

Here 0.21 is the theoretical electrical charge (mC cm−2) for monolayer adsorption of hydrogen or charge required to oxidize a monolayer of H on a polycrystalline Pt electrode (assumed surface density of 1.3 × 1015 atoms cm−2). The correponding EAS values of Pt/C, Pt/MnO2−B/C, and Pt/ MnO2−NR/C samples with similar Pt loading were calculated to be 42, 48, and 58 m2 g−1. The higher EAS values of Pt in MnO2-promoted Pt/C samples are essentially due to smaller size and superior dispersion of Pt crystallites. Further, the higher EAS value of Pt in MnO2-nanorod-promoted Pt/C than MnO2-microcube-promoted Pt/C is ascribed to the Pt crystallites with a higher number of active crystal planes, which are grown in a geometrically controlled manner on the uniform MnO2-nanorod surface. The lower EAS of Pt in Pt/C is essentially due to larger size, lower numbers of active crystallographic planes, and high agglomeration of Pt crystallites. 3.2.2. CO Stripping Voltammetry Study. It is known that during electrooxidation of alcohols, many carbonaceous species are formed as intermediates which get adsorbed to the active catalyst sites, in particular on the Pt sites, which deactivate the catalyst.8,9,12 CO is the key species among the lot that becomes linearly bonded to Pt, with a saturation coverage up to ∼0.68.64,65 A Pt site is highly prone to adsorption of CO due to electron donation from the 5σ orbital of CO to Pt followed by back-donation from the Pt d-band to the 2π* molecular orbital of CO.66 The orbital mixing of Pt and CO is very strong, which poisons the Pt sites, and further reaction of alcohol molecules becomes unfeasible. Hence, the adsorbed CO should be simultaneously removed to generate clean and active Pt sites. On a bare Pt surface, the CO desorption occurs via the reaction path first described by Gilman as shown in eq 5.67 ⎫ ⎪ ⎪ COads + OHads ⇄ COOHads ⎬ ⎪ COOHads ⇄ CO2 + H+ + e− ⎪ ⎭ H 2O ⇄ OHads + H+ + e−

(5)

To understand and probe the microstructure/morphology effect of MnO2 on the CO electrooxidation efficiency on Pt/C, CO-stripping voltammetry was performed on the Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C sample electrodes, and the results are presented in Figure S3 in the Supporting Information. The very first cycle of the CO stripping voltammetry profile of the individual samples shows electrooxidation of CO followed by signatures due to typical phenomena in the Hupd region in the second cycle. This essentially shows the complete oxidation of COads during the first voltammetry scan leaving the active Pt surface clean for interaction with hydrogen. The corresponding current responses of MnO2-promoted Pt/C and

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MnO2 + H 2O → MnO2 −OHads + H+ + e− ⎫ ⎪ ⎪ Pt−COads + MnO2 −OHads ⎪ ⎬ → Pt + MnO2 + CO2 + H+ + e− ⎪ and/or ⎪ ⎪ Pt−MnO2 + xCO → Pt−MnO2 − x + xCO2 ⎭

bare Pt/C samples during COads electrooxidation are compared in Figure 7. The results reveal higher current response at lower

(6)

The resultant peaks due to CO stripping from the MnO2promoted Pt/C samples are significantly narrower than that from the bare Pt/C sample, which confirms higher feasibility and stronger promoting activity of MnO2 for CO electrooxidation on the Pt surface. This can be further explicated as the possible formation of labile OH species on the triple-phase interface (interface between the Pt, oxide, and the electrolyte), which provides electronic suitability for the oxidation of CO species on the Pt surface.27,70 Hence the CO tolerance of the MnO2-promoted Pt/C samples is significantly higher as compared to the bare Pt/C sample. In particular, the promotional activity of MnO2-nanorod for CO tolerance is better as compared to the MnO2-microcube, which is schematically represented in Scheme 2. 3.2.3. Methanol Electrooxidation. The microstructure/ morphology-induced promoting activity of MnO2 to Pt/C for the methanol electrooxidation reaction was investigated by cyclic voltammetry, chronopotentiometry, and chronoamperometry measurements in 0.5 mol L−1 H2SO4 + 1 M CH3OH solution, and the results are compared with those of the Pt/C sample in Figure 8. Figure 8A shows typical voltammetry profiles during anodic and cathodic cycling of the electrocatalysts (Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C) at a scan rate of 20 mV s−1. The broad peaks during the anodic potential sweep (positive sweep) are characteristic of oxidation of (CH3OH)ads on the Pt surface (eq 7).52

Figure 7. Comparative CO stripping current response of Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C sample electrodes in 0.5 mol L−1 H2SO4 solution at a scan rate of 20 mV s−1.

oxidation overpotential (negative shift in the CO electrooxidation peak potential) on MnO2-promoted Pt/C samples as compared to the bare Pt/C sample. Further, the MnO2nanorod-promoted Pt/C sample shows marginally higher current response and larger negative shift in the CO electrooxidation peak potential as compared to the MnO2microcube-promoted Pt/C sample. This in principle indicates formation of OHads species on the MnO2 at lower overpotential as compared to the bare Pt/C sample, in particular formation of higher number of OHads species on the MnO2-nanorod at lower overpotential than on the MnO2-microcube.67−69 The OHads species on the MnO2 tend to electronically weaken the Pt−CO bond and oxidize CO to CO2. From the CO-stripping voltammetry it is ascertained that the MnO2-nanorod possesses superior electronic promoting activity as compared to the MnO2-microcube. The superior promoting activity of the MnO2-nanorod may be the mixed contribution of higher surface area and uniform atomic ensembles on the crystallographically uniform nanorod surface, which enhance the sitespecific adsorption of OH species on the MnO2-nanorod surface. The large number of OHads species is responsible for the oxidation of a larger number of adjacent COads species on the Pt surface. This results in the enhanced current response during CO electrooxidation on the Pt/MnO2−NR/C sample. The promotional CO stripping from the Pt surface in the presence of MnO2 occurs via a kind of synergic effect,27 as proposed in eq 6.

CH3OH + Pt → Pt−(CH3OH)ads → 4H+ + 4e− + Pt−CO

(7)

The sharp peaks during the cathodic scan (reverse scan) with high reactivation current are attributed to the subsequent removal/oxidation of adsorbed carbonaceous poisoning species on the catalyst surface and/or reoxidation of MeOH on the platinum oxide surface (formed during the anodic scan).21,23,24,64,71 Further, the area-specific peak current densities of MnO2-modified Pt/C samples are higher than the current response of the bare Pt/C sample (9.3 mA cm−2), which clearly show the promoting activity of MnO2 for methanol electrooxidation. Further, the area-specific peak

Scheme 2. Schematic Representation of COads (on Pt) Oxidation by OHads (on MnO2) by (A) MnO2 Nanorod and (B) MnO2 Microcube

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Figure 8. (A, B) Comparative cyclic voltammograms of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes in 0.5 mol L−1 H2SO4 + 1 M CH3OH solution at a scan rate of 20 mV s−1: Graphs in panels A and B show the current response with respect to geometrical area of the electrode (in cm2) and per unit electroactive surface area (in m2) of the catalyst samples, respectively; (C) potential vs time (chronopotentiometry) profiles of the sample electrodes at a current density of 15 mA cm−2; and (D) current vs time (chronoamperometry) profiles of the samples at an applied potential of 0.7 V.

higher than that of the bare Pt/C sample (0.22 mA EAS−1). In particular, the MnO2-nanorod-modified Pt/C sample shows higher EAS-specific current response (0.27 mA EAS−1) than the MnO2-microcube-modified Pt/C sample (0.25 mA EAS−1), which further corroborates the stronger promoting activity of the MnO2-nanorod. Chronopotentiometry measurements were performed to evaluate the microstructure/morphology effect of MnO2 on the constant current operation efficiency, antipoisoning abilities, and stability of the Pt/C catalyst during the methanol electrooxidation reaction. The chronopotentiometry measurements were carried out at a constant current density of 15 mA cm−2, and the resultant profiles of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples are shown in Figure 8C. All the chronopotentiometry profiles show increase in potential with polarization time, and then a final jump to the limiting potential after a certain interval of time. The potential jump signifies oxygen evolution and poisoning of the catalyst surface.27 The oxygen evolution occurs due to the electrolysis of water at the potential required to compensate the applied anodic current density, after deactivation of the catalyst surface due to complete accumulation of poisonous carbonaceous intermediates. Lower polarization overpotential and longer time taken for the potential jump demonstrate higher activity, antipoisoning ability, and stability of an electrocatalyst.27,29−31 From Figure 8C, it is observed that the oxidation overpotentials for MnO2modified Pt/C samples are lower than that for the bare Pt/C sample. Further, the deactivation of the MnO2-modified Pt/C samples occurs at much longer time as compared to the bare Pt/C sample. These results indicate that the MnO2 provides strong antipoisoning activity and stability to the Pt/C. In particular, the MnO2-nanorod-modified Pt/C sample maintains lower overpotential as well as much longer time (327 min) for deactivation than the MnO2-microcube-modified Pt/C sample (208 min). It further corroborates the stronger promoting

current density of MnO2-nanorod-modified Pt/C sample (15.7 mA cm−2) is higher than that of MnO2-microcube-modified Pt/ C sample (11.9 mA cm−2). In addition, from the inset of Figure 8A, it is seen that the onset potential for methanol electrooxidation on the MnO2-nanorod-modified Pt/C sample is lower than that for the MnO2-microcube-modified Pt/C sample. Basically, during electrooxidation of MeOH, strongly adsorbed carbonaceous species inhibit further adsorption of MeOH on the catalyst surface, which brings about a positive shift in the onset potential and decrease in the resulting current response at a specific potential. Therefore, materials responsible for negative shift in onset potential and increase in current response are better promoters.68,69 From the CV results, it can be corroborated that the MnO2-nanorod is a stronger promoter to Pt/C than the MnO2 -microcube for the methanol electrooxidation reaction. This is in absolute agreement with the CO stripping voltammetry results. The better promoting efficiency of the MnO2-nanorod is a collective contribution of smaller size and highly dispersed Pt crystallites, and enhanced synergic effect of the high surface area MnO2-nanorod. The improved synergic effect is associated with the surface morphology and crystallographic features of MnO2 which manipulate the concentration of triple-phase interfaces and the number of OHads species on the catalyst surface. Unlike disordered MnO2-microcubes, the nanorods possess welldefined as well as long-range surface, uniformly exposed surface planes, and identical atomic ensembles. As a result, the triplephase interfacial region is larger in MnO2-nanorods which provide added OHads for oxidation of more number of COads. To exclusively validate the microstructure/morphology controlled promoting activity of MnO2, the area-specific methanol electrooxidation current densities were normalized by the corresponding EAS of Pt for each sample, and the resultant CV profiles are compared in Figure 8B. The EAS-specific current densities of the MnO2-promoted Pt/C samples are clearly 4896

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Figure 9. (A, B) Comparative cyclic voltammograms of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes in 0.5 mol L−1 H2SO4 + 1 M CH3CH2OH solution at a scan rate of 20 mV s−1: Graphs in panels A and B show the current response with respect to geometrical area of the electrode (in cm2) and per unit electroactive surface area (in m2) of the catalyst samples, respectively; (C) chronopotentiometry profiles of the sample electrodes at a current density of 15 mA cm−2; and (D) chronoamperometry profiles of the sample electrodes at an applied potential of 0.9 V.

makes larger number of oxidizing species available for oxidation of carbonaceous species (attached to the Pt surface) at lower overpotential, which significantly reduces the poisoning of the electroactive Pt surface. This makes the regeneration of sufficiently clean electroactive Pt surface and the unhindered oxidation of methanol (almost constant oxidation current) possible for longer duration. The small current spikes in the chronoamperometry profile of the Pt/MnO2−NR/C sample is due to successive formation of a clean Pt surface by removal of poisoning intermediate species, resulting in sudden jump in current at regular time intervals.73 3.2.4. Ethanol Electrooxidation. Unlike methanol, the complete electrooxidation of ethanol releases 12 electrons per ethanol molecule.74,75 However, formation of intermediate species like acetaldehyde and acetic acid makes the ethanol electrooxidation release two and four electrons, respectively. Hence the ethanol electrooxidation is considered as more complicated, which needs more active and selective catalysts. To evaluate the microstructure/morphology control of MnO2 for promoting Pt/C, the ethanol electrooxidation reactions were performed on Pt/C, Pt/MnO2−B/C, and Pt/MnO2− NR/C sample electrodes in 0.5 mol L−1 H2SO4 + 1 M C2H5OH solution, and the results from cyclic voltammetry, chronopotentiometry, and chronoamperometry measurements are presented in Figure 9. The cyclic voltammetry responses of the Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes in Figure 9A demonstrate the similar trend in activities as observed in the methanol electrooxidation study. The enhanced area-specific peak current densities for the MnO2-modified Pt/C samples, in particular the MnO2nanorod-promoted Pt/C sample (12.5 mA cm−2) as compared to the MnO2-microcube-promoted Pt/C (8.9 mA cm−2) and bare Pt/C (5.2 mA cm−2) samples, demonstrate the promoting activity of MnO2, in particular, MnO2-nanorod, for the ethanol electrooxidation reaction. Further, pronounced decrease in

activity (antipoisoning) of the MnO2-nanorod as compared to the MnO2-microcube for the methanol electrooxidation reaction on Pt/C. An ideal catalyst should be highly stable during the actual operating condition of fuel cells, where current is drawn at constant potential. Hence, chronoamperomety measurements were performed to investigate the microstructure/morphology effect of MnO2 on the antipoisoning activity and stability of the Pt/C catalyst for long duration operation. The chronoamperomety profiles of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C samples in Figure 8D show a typical trend, where the current generated by the samples at a fixed potential of 0.7 V (vs Ag/ AgCl) is not maintained for longer duration (7200 s) due to poisoning of the active Pt sites by carbonaceous intermediates during the methanol electrooxidation reaction.27,72 The polarization potential of 0.7 V (vs Ag/AgCl) was applied here, since it is the nearest anodic peak potential of the Pt/C, Pt/MnO2− B/C, and Pt/MnO2−NR/C samples for maximum oxidation current in the cyclic voltammetry study (Figure 8A). From the chronoamperometry profiles, it is evident that the MnO2modified Pt/C samples show higher current response and lower current decay with polarization time, as compared to the bare Pt/C sample. For instance, the corresponding initial current responses of the Pt/C, Pt/MnO2−B/C, and Pt/ MnO2−NR/C samples are found to be 8.9, 10.4, and 14.2 mA cm−2, which are reduced to 1.6, 3.6, and 11.1 mA cm−2, respectively, after 7200 s. These results confirm stronger promoting and antipoisoning activity of the MnO2-nanorod as compared to the MnO2-microcube. The sharper current drop for the Pt/C sample as compared to the MnO2-promoted Pt/C samples demonstrates serious poisoning of the Pt surface by intermediate carbonaceous species in the absence of a promoter. Further, there is a sharper current drop for the MnO2-microcube-promoted Pt/C sample as compared to the MnO2-nanorod-promoted Pt/C sample. The MnO2-nanorod 4897

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active surface area as compared to the MnO2-microcube based Pt/C and bare Pt/C composites. The microstructure-dependent promoting activity of MnO2 to Pt/C was studied by methanol and ethanol electrooxidation reactions using cyclic voltammetry, chronopotentiometry, and chronoamperometry techniques. The voltammetry results show higher promotional (higher methanol and ethanol oxidation currents) and antipoisoning (higher CO oxidation current at lower overpotential) activity of the MnO2-nanorods. Further, the typical potential, time, and current responses in chronopotentiometry and chronoamperometry studies show that the MnO2-nanorod provides better promoting as well as antipoisoning activity and longer duration stability to Pt/C for methanol and ethanol electrooxidation reactions. The uniformly dispersed smaller Pt crystallites and higher number of triple-phase interfacial active centers on the MnO2-nanorods provide larger number of oxygen-containing species, which facilitate alcohol electrooxidation reactions on Pt/C. On the whole, the present study provides clear evidence of certain improvement in the electrocatalytic activity of Pt/C by suitable microstructures of MnO2. The present findings are crucial in the future development and exploitation of oxides with suitable microstructures as sans-noble metal catalysts for fuel cell applications.

oxidation peak potentials and onsets of oxidation overpotentials (inset of Figure 9A) for the MnO2-modified Pt/C samples corroborate the promoting and antipoisoning activities of the MnO2, in particular, MnO2-nanorod, for the ethanol electrooxidation reaction on Pt/C.76 The higher EAS specific current response of the MnO2-nanorod-modified Pt/C sample (0.22 mA EAS−1) in Figure 9B further demonstrates its high promoting and antipoisoning activity as compared to the MnO2-microcube-modified Pt/C (0.18 mA EAS−1) and bare Pt/C (0.12 mA EAS−1) samples. The Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes were subjected to chronopotentiometry measurements under fixed current densities of 15 mA cm−2 and their corresponding potential as well as time responses are presented in Figure 9C. The chronopotentiometry profiles show a similar pattern as seen in the case of the methanol electrooxidation reaction. Lower oxidation overpotential and longer polarization time for potential jump/deactivation is observed for MnO2modified Pt/C samples as compared to the bare Pt/C sample, which suggests strong promoting and antipoisoning activity of the MnO2 samples. Further, the MnO2-nanorod-modified Pt/C exhibits lower oxidation overpotential and longer polarization time (∼264 min) as compared to the MnO2-microcubemodified Pt/C sample. This confirms stronger promoting and antipoisoning activity of the MnO2-nanorod over the MnO2microcube for ethanol electrooxidation on Pt/C. Chronoamperomety measurements were carried out on the Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C sample electrodes at fixed polarization potential of 0.9 V (vs Ag/AgCl) and the corresponding profiles are shown in Figure 9D. The results demonstrate higher current response and lower current decay for the MnO2-modified Pt/C samples as compared to the bare Pt/C sample. The initial current responses of the Pt/C, Pt/ MnO2−B/C, and Pt/MnO2−NR/C samples are 3.8, 8.3, and 10.8 mA cm−2, which get reduced to 2.7, 3.2, and 5.9 mA cm−2, respectively, after potential polarization for 7200 s. The higher current response and minor current decay after long polarization time further connotes better promoting and antipoisoning activity of MnO2-nanorods over MnO2-microcubes for ethanol electrooxidation on Pt/C. The ethanol electrooxidation results are in precise corroboration with the observations from methanol electrooxidation, which suggest that suitable surface microstructure of MnO2 provides a higher number of triple-phase interfacial active centers, and induce miniaturization as well as uniform dispersion of Pt crystallites for promoting methanol and ethanol electrooxidation reactions on Pt/C. The findings in this work hold significance in the context of designing new oxide materials with controlled microstructures for improved applicability as promoters in direct alcohol fuel cells.

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ASSOCIATED CONTENT

* Supporting Information S

N2 adsorption/desorption analysis, cyclic voltammograms, and CO stripping voltammograms of Pt/C, Pt/MnO2−B/C, and Pt/MnO2−NR/C electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+91) 44 2257 4226. Fax: (+91) 44 2257 4202. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial supports from MNRE, New Delhi, for providing the electrochemical workstation, and SERC division of DST, Ministry of Science and Technology, New Delhi, for providing other instrumental facilities under the FIST Schemes. We thank Mr. A. Narayanan and Mrs. S. Srividya for TGA, BET, and PXRD data collection.

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REFERENCES

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4. CONCLUSIONS Here, synthesis of α-MnO2 samples with nanorod and microcube microstructures by hydrothermal mediated reduction of KMnO4 using HCl and forced hydrolysis of MnCl2 using urea is reported. The physicochemical studies show that the MnO2-microcubes possess larger crystallite size and lower BET surface area than the MnO2-nanorods. The HRTEM study shows that unlike the MnO2-microcubes, the MnO2nanorods promote miniaturization and superior dispersion of Pt crystallites in the Pt/MnO2/C composite. The H2electrosorption study reveals that the Pt crystallites in the MnO2-nanorod based Pt/C composite possess higher electro4898

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