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Synergistic combination of Pd and Co catalyst nanoparticles over self designed MnO2 structure: Green synthetic approach and unprecedented electrode kinetics in direct ethanol fuel cell Abhishek De, and Jayati Datta ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01251 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Sustainable Chemistry & Engineering

Synergistic combination of Pd and Co catalyst nanoparticles over self designed MnO2 structure: Green synthetic approach and unprecedented electrode kinetics in direct ethanol fuel cell Abhishek De †, ‡ and JayatiDatta †, ‡,* †

Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, West Bengal, India ‡

Department of Chemistry, Renewable Energy Research Centre, Heritage Institute of Technology, Kolkata-700107, West Bengal, India

* Corresponding author e-mail:[email protected]

KEYWORDS. PdCo nanoparticles, manganese dioxide, sonochemical technique, ethanol oxidation reaction, oxidation product analysis.

ABSTRACT. The present study deals with sonochemical synthetic approach in fabricating PdCo/MnO2 catalyst for the study of ethanol oxidation reaction in alkali medium. SEM and TEM images reveal that, MnO2 morphology is changed from nanowire to nanorod during intercalation of Pd, Co NPs in the support materials. Charge transfer across the electrodeelectrolyte interface becomes facile due to (2 x 2) pore tunnels of α MnO2. Lowering of Pd loading around 40% in the catalyst matrix by Co, not only makes the catalyst cheaper but enhances the ethanol oxidation current by 66.3 % compared to Pd/C. Extensive electrolysis on Pd61Co39/MnO2 over a considerable span of potential is reflected in the voltammetric features, typically demonstrating the synergistic effect of the Pd-Co ad atoms ingrained on α MnO2 structure. Chrono-amperometric analysis for an extended period indicates excellent

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sustainability of Pd61Co39/MnO2 in the oxidation environment. The yield of oxidation products on Pd61Co39/MnO2, derived from ion chromatography, further substantiates the considerable extent of the anodic reactions delivering acetate and carbonates, on an average, twice that on Pd/MnO2 and four to five times that on Pd/C. Finally, 43.88 mW cm-2 power density was achieved during performance testing of the DEFC using the binary catalyst formulation on the non-carbon support.

INTRODUCTION The quest for catalyst materials for the anodic oxidation in alcohol based fuel cell has led to the emergence of tailor made electro-catalysts having desired energy efficient functional properties. Since the cost factor is always associated with the fabrication of fuel cell components, the recent trend in course of catalyst selection is focused primarily on replacing Pt and Pt based nano-materials by cost effective catalyst particles equally efficient for oxidation of alcohols. With regard to the choice of fuel, ethanol has received the most attention due to its easy availability, comparable energy density (29.7 MJ/kg,) with gasoline and its positive response towards electrochemical dissociation releasing CO2 as the ultimate product on stringent catalyst materials.1 So far, Pd (1/2 price of Pt)2 based catalyst on carbon support have been best reported

56,57

for the electro-catalytic studies on alcohol oxidation in

alkaline environment; however such catalyst materials are less explored for anodic oxidation in direct ethanol fuel cell (DEFC), particularly with cheaper transition metal/metal oxide as additives in the Pd matrix and using supports other than graphitic materials.3-11 Considering the above points, the present investigation attempts to introduce Co into the base material Pd forming PdCo nanoparticles (NPs) where Pd loading is reduced by almost 40% and it is expected that the incorporation of the transition metal induces downward d band shifting, prohibiting strong adsorption of intermediates and refreshing the surface for further adsorption.12,53


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Further, taking into consideration the temperature and pH environment of the fuel cell, the metal- support combination has become a critical factor, particularly to achieve durability of the device as well as to obtain an additional catalytic assistance in course of the anodic reaction in DEFC. Since long, graphitic carbon was the common choice as support material; but due to its poor corrosion resistance in aggressive electrochemical conditions, alternate 4,11 materials are sought for. Elimination of carbon can be achieved by using different category materials like ceramic oxide (TiO2, CeO2,ZrO2),

13,14,15

transition metal oxide (MoO3, WO3,

MnO2) 1,4,16,20 and polymer composites (polyaniline, polypyrrole, poly N vinyl carbazole) 1,1718,54

. These materials are trying to find their ways as befitting catalyst support depending

upon their compatibility to the electrolytic media. Regarding the choice of support materials in our case, the transition metal oxide MnO2 has been selected in view of its low cost, environment friendliness, easy fabrication and wide range of electrochemical applications in sensors 21, batteries 22, super capacitors 23-24 etc. Amongst the various phases, α, β, γ and δ, in which MnO2 can exist, the α phase contains larger (2 x 2) pore tunnels with approximately 4.6 Å diameter.19 The self designed tunnels in MnO2 structure presumably act as corridor for proton and electron transport within the matrix, strengthen metal / support interactions as well as directly or indirectly take part in the catalytic sequences by intervention of the variable oxidation states of Mn existing in the catalyst matrix.4 The present study involves a completely green synthetic approach for producing α MnO2 by solid phase solvent free method and subsequently decorating Pd and Co NPs over the MnO2 support by sono-chemical method without using any chemical reagents other than the precursor salts. The physico-chemical and electrochemical studies were carried out to elucidate the structure, morphology, electro-catalytic activities and stability of the synthesized Pd61Co39/MnO2 catalysts. Reaction intermediates were also determined to gauge the extent of ethanol oxidation reaction (EOR) and to suggest probable mechanistic pathways



for the oxidation either to intermediates formation or to proceed towards completion. Finally, the performance output of the catalysts were recorded in a fuel cell test station using anion exchange membrane sandwiched between the synthesized catalysts as anode and Pt/C as cathode. EXPERIMENTAL SECTION

Synthesis of MnO2. In the solid phase synthesis of MnO2, calculated amount of solid precursors Mn(OAc)2,4H2O and KMnO4were mixed and heated at 80°C for 4 hours. During synthesis diffusion is followed by the contact of reactant molecules with each other, finally leading to the product formation. The prepared MnO2was washed with de-ionized water and centrifuged. Finally the residue was dried at 100 °C for 12 hrs.

Decoration of Pd & PdCo on the synthesized MnO2. The synthesized MnO2 was taken in proportionate amount and dispersed thoroughly in ethanol solution. Subsequently respective metal precursor solutions (PdCl2 and CoCl2, Arora Matthey Ltd. Each 0.002 M) were added to the MnO2suspension with constant stirring and sonication so as to obtain the PdCo NPs over MnO2 support maintaining 40:60 catalyst- support ratios. The reduced suspension was centrifuged thoroughly and subsequently washed with de-ionized water till the filtrate became chloride free. The residue was dried under vacuum at 100°C for 24 hrs. Pd/MnO2 was also prepared using similar sonochemical method, while the Pd/C catalyst (used for comparison) was synthesized by borohydride reduction method, as because growth of Pd NPs over carbon support could not be obtained under the sonochemical preparative technique.1 It may be noted that the optimized catalyst loading of 40% with respect to support has been maintained throughout the set of catalysts under study.

Material characterization. X-ray diffraction (XRD) studies for all the prepared catalysts were carried out between 20 to 90 degrees with the scans at 10 min-1 for 2θ values using SEIFERT 2000 diffractometer operating under CuKα radiation (λ = 0.1540598 nm)


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generated at 35 kV and 30 mA. Considering (111) peak of the Pd face centered cubic (fcc) structure, particle size of the electro-catalysts was determined using Debye Scherrer equation. The elemental ratio of the catalyst layers were derived from EDX analysis using Link ISIS EDX detector (Oxford Instruments, U.K.) attached with the scanning electron microscope. In order to obtain the morphology, TEM analysis was done by suspending the catalyst NPs on Cu grid (300) mesh and employing JEOL JEM 2010 operated at an accelerating voltage of 200 kV. Surface compositional analysis was done and the oxidation states of the different metals present in the catalyst Pd61Co39/MnO2 were identified using X-ray photoelectron spectroscopy (XPS) taking the help of Omicron Nanotechnology Instrument (Serial No. 0571). The surface area and pore size distribution of the catalyst matrices were determined at 77 K by the BET method using nitrogen as the adsorbate, in Quantachrome Autosorb Instrument (Model AS1-CT). Thermo-gravimetric analysis (TGA) was performed by the help of Perkin Elmer TGA-7 instrument with a heating rate of 10˚C min-1 in Nitrogen flow. Inductively coupled plasma-mass spectroscopy (Varian 820-MS) has been used to determine the composition of the PdCo/MnO2 catalyst.

Electrochemical characterization. In order to fabricate the electrode component, catalyst ink was prepared using 5wt% Nafion solution and isopropanol. The electrolytic solution contained 1 mol L–1ethanol (AR grade, Merck, Germany) & 0.5mol L-1NaOH and 0.65 cm2 geometric area for each of the catalyst matrices was exposed to the working solution during electrolysis while catalyst loading was maintained at 0.77 mgcm-2 in each case. Nitrogen gas (XL grade, BOC India Ltd.) was purged through the working solutions before each of the electrochemical experiments and the measurements were recorded using computer controlled PG STAT 12 (potentiostat / galvanostat) coupled with frequency response analyzer module (Metrohm, Netherlands).



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Estimation of oxidation products. After continuous electrolysis for 1hour, the aliquots were subjected to the estimation of EOR products by the help of Ion Chromatography (Metrohm’s Advanced Modular Ion Chromatography) associated with a L7100 pump (Metrohm Ltd.) and a conductivity detector Metrosep A Supp 4-550 organic acid column.The chronoamperometric measurements were done at a constant potential of -0.3 V for a span of 3600 seconds.

Performance testing of DEFC. Full cell testing was carried out in an in house fabricated cell consisting of membrane electrode assembly (MEA) with exposed area 1cm2 using alkaline exchange membrane (Tokuyama, Japan), Pt/C cathode and the synthesized catalysts as anode on either side of the membrane. Performance screening of all the synthesized catalysts were carried out at 40°Cwith the help of fuel cell testing station (Fuel Cell Technologies, Inc.). Oxygen was fed into the cathode chamber at a flow rate of 100 standard cubic centimeters per minute (sccm) while flow of alkaline ethanol solution to the anode was maintained at 1.0 ml min-1.

RESULT AND DISCUSSION In situ sono-chemical reduction. Under sono-chemical wave perturbation water splitting,

25

(H2O→H●+ OH● ) occurs generating H● radicals which reduce Pd+2 and Co+2

species to the respective metals Pd and Co (M+2 +H● → M+ 2H+). During the sono-chemical process, recombination reactions may also take place: (i) H●+ OH●→H2O and (ii) OH● +OH●→H2O2. The hydrogen peroxide thus produced, immediately reacts with MnO2 by the following reactions whereby MnO2 is reverted back 21: + → () +

..(1)

() + → + 2

..(2)


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Structure, morphology and composition of catalyst matrices. Crystalline structure of the synthesized MnO2 were obtained from XRD patterns (Figure 1) where the peaks are attributed to MnO2(301), MnO2(211), MnO2(301), MnO2(411), MnO2(521), MnO2(002), MnO2(541)and MnO2(312) indicating the presence of α MnO2. The low intensity peaks corresponding to MnO2 (301), MnO2 (211), MnO2 (411) and MnO2 (521) planes are exhibited even when MnO2 is decorated with metallic NPs. For each of the catalysts, Pd/MnO2 and PdCo/MnO2, peaks are indexed with (111), (220, (200), (311) and (222) planes as shown in Figure 1. In case of the binary catalyst 2θ values of the corresponding planes are shifted to higher angles due to lattice contraction which also translates to the formation of alloyed phases of PdCo NPs. The percent of alloying was found to be 14 % as calculated using Vegards law. 1 However, appearance of a small peak at 44.63° for the binary catalyst is due to the presence of metallic Co along in the catalyst matrix along with the alloyed phases. It is worth noting that lattice parameter increases when the graphitic support is changed (Pd/C, 3.893 A°) to transitional metal oxide (Pd/MnO2, 3.900 A°). This may be due to dilation effect and the structural changes occurring out of strong interactions between Pd and MnO2. 26 Incorporation of Co drastically changes the average crystalline size as observed by the reduction of 13.25 nm value for Pd/MnO2 to 8.13 nm for PdCo/MnO2 as shown in Table 1.27 This kind of shrinkage also reflects a strong interaction between Pd and Co NPs, embedded within the MnO2 matrix. It is evident from the SEM image (Figure 2a) that the synthesized MnO2 bears typical nanowire (NW) morphology with diameter of NW ranging between 15-20 nm and length of the NW may extend up to few micrometers. It is predicted that MnO2 acquires the NW morphology through ‘rolling mechanism’ as shown in Scheme 1a.

4, 28-29

The crystalline

arrangement is build up by the unidirectional propagation of MnO6octahedral units leading to the formation of large (2 X 2) tunnels as shown in Scheme 1b. 19



However after decoration with Pd and Co NPs, the NW morphology changes. The SEM images (Figure 2b and 2c) reveal that longer tubes of MnO2 (Figure 2a) are broken into nanorods (NR) when metal NPs are deposited on MnO2. Due to the influence of sonochemical wave perturbation and the strong interaction between Pd and Co NPs, the metal particles are partially agglomerated within the MnO2 matrix and the growth apparently exhibit spherical morphology as observed in Figure 2b and 2c. The EDAX analysis shows that the weight ratio of Pd and Co is 61:39 while the complete elemental ratio of the matrix PdCo/MnO2 is found to be 50.39(Mn): 33.81(O): 10.06(Pd): 5.73 (Co). In this respect plausible explanations have been given in the supporting information file based on the results of TGA (Figure S1) and ICP-MS analysis for PdCo/MnO2 catalyst. TEM images (Figure 3a and 3b) of Pd61Co39/MnO2 exhibit broken morphology of the NW structure and confirm the co-existence of spherical and NR MnO2 structures that leads to localized thickening of the MnO2, matrix depending upon the population density of metal NPs. The length of the MnO2 structure changes drastically30 from micrometer to nanometer scale while the diameter of the NR on an average decreases from 15-20 nm to 6- 11 nm as indicated by Figure 3b. The average size of metal NPs are 4-5 nm in the high population area (Figure 3a) while the distribution remains mostly within 1.5-2.5 nm at low population density zone (Figure 3b). In some cases parallel adherence of the NR leads to the formation of nanosheet as observed in Figure 3b. Some of the regions in the catalyst matrix become indistinct, due to the presence of hydroxide moieties; as a result, precise particle size determination becomes difficult and difference in the values are observed with those obtained from the respective XRD and TEM analysis. Fringe pattern of (211), (200) and (310) planes in Figure 3c confirms the presence of MnO2 in α form. FFT images of the corresponding fringe pattern in Figure 3d indicates the presence of Pd (111), Co (111) planes with respective interplaner spacing of 2.22 nm and


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2.06 nm. All these structural evidences corroborate with those obtained from the XRD patterns. The Scheme 2 represents the overall morphological changes of MnO2occurring before and after metal NPs decoration, under sonochemical perturbation, (i) short time ‘Oswald ripening’ of the MnO6 units resulting in spherical morphology of MnO2 and (ii) one dimensional growth of MnO6 units leading to the nanorod morphology.29,31 The presence of Mn 2p1/2 and Mn 2p3/2 peaks with a spin energy separation of 11.5 eV strongly indicates that Mn (IV) is the predominant oxidation state in the MnO2 nanostructures.32 On deconvoluting the 2p3/2 peak three different peaks are obtained at 640.79, 641.23 and 642.57 eV corresponding to Mn (II), Mn (IV) and Mn (III) species respectively. Among the three different oxidation states, predominant existence of Mn (IV) is revealed by the area ratio (68%) of the three peaks.33 On deconvoluting the Co 2p XPS pattern, it is found that Co exists in three different forms in the catalyst matrix when synthesized by sono-chemical approach. The lower binding energy 2p3/2 and 2p1/2 peaks appearing at 779.37 and 794.33 eV attribute to the presence of Co in metallic form. As reported in literature 2p3/2 binding energy of metallic Co is 778.5 eV;52 however in our case the increase of 2p3/2 binding energy for the binary catalyst indicates the coexistence of alloyed phases and metallic Co in the catalyst matrix, as also evidenced in XRD analysis. Considerable amount of Co oxide is also present in the matrix as revealed from the 2p3/2 and 2p1/2peaks at 780.69 and 795.46 eV respectively. Further peaks 2p3/2 and 2p1/2 appearing at 782.51 and 796.94 eV respectively are indicative of the existence of Co hydroxide in the catalyst matrix.34 Two satellite peaks obtained in the higher binding energy may correspond to the final state effects.35,55,58 The O 1s spectrum can be deconvoluted into three different components. The main peak located at 529.2 eV represents the typical O atom of the anhydrous linkage (M-O-M, M= Mn, Co) of MnO2 and CoO oxides and peak appearing at



531.26 eV are assigned to the hydrous (M-OH, M= Mn, Co) linkage of MnO2 and CoO oxides. The peak at 533.48 eV indicates the presence of residual water (H-O-H bond) in the composite matrix.36 The peaks at 334.74 and 340.06 eV correspond to the Pd (0) state in the Pd spectra and are ascribed to 3d5/2 and 3d3/2, respectively while the relatively low intensity peaks at 336.04 and 341.30 eV are due the presence of 39% Pd in oxidation state (II) as revealed from the area ratio in the XPS spectrum. As both Pd/MnO2 and PdCo/MnO2 catalysts were prepared in the same reducing environment, it implies that the respective oxidation states of Pd and Mn, co-exist as Pd(0), Pd(II) and Mn(II), Mn(III), Mn(IV), in Pd/MnO2 also. Figure 5 represents the BET plots where MnO2, Pd/MnO2 and Pd61Co39/MnO2are found to have type IV, type II and type II nitrogen adsorption and desorption isotherm with H1, H3 and H3 hysteresis loop respectively59. In case of MnO2, H1 hysteresis indicates the presence of mesopores with different geometries like tubular capillary and narrow-necked bottle type.37 For, Pd/MnO2 and Pd61Co39/MnO2, H3 loop indicates the presence of aggregated particles forming slit-like mesopores. The lowering of BET surface area and total pore volume of the catalyst matrix (Pd/MnO2 and Pd61Co39/MnO2) is ascribed to the deposition of metal NPs on MnO2. Notably reduction in BET surface area and total pore volume in case of Pd/MnO2 is observed compared to the binary catalyst and this may be due to the presence of larger Pd NPS (as evidenced by XRD). However, for Pd61Co39/MnO2 the decrease in BET surface area and total pore volume compared to MnO2 may be a combined effect of the change in morphology as indicated by the TEM images and the intercalation of Pd and Co NPs into MnO2 structure, since α MnO2 consists of (2x2) pore tunnels of approximately 4.6 Å diameter. The MnO2, Pd/MnO2 and Pd61Co39/MnO2show pore size distribution with diameter ranging mostly between of 2-25nm. The BET parameters are summarized in Table 2.


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Electrocatalytic studies of Pd61Co39/MnO2. As a typical feature, the hydrogen adsorption desorption (HAD) and to oxide formation regions for Pd on carbon support is depressed in alkali medium, as shown in Figure 6. However by using MnO2 support both the regions are much elevated and expanded as well as positive shifted. This is more pronounced with the addition of Co in the Pd61Co39/MnO2catalyst. In the cathodic sweep a peak appears at ~0.2 mV for the binary catalyst, representing transition metal (Co) oxide reduction, while proceeding toward more negative potential, the adsorbed oxide reduction for the base metal Pd occurs as usual. Similar trend is reflected in the ECSA values calculated from the HAD and the adsorbed oxide reduction (AOR) regions as summarized in Table 3.38 For MnO2 supported catalysts, the ECSA values are significantly high, compared to the carbon supported catalysts. In the HAD region, the respective values are 3.7 and 5.9 times higher for Pd/MnO2 and Pd61Co39/MnO2catalyst while in the AOR region the values are found to be 5.6 and 10.6 times higher than those for Pd/C catalyst. The ascending trend within the total span of double layer charging region for the catalyst comprising Co indicates commencement of surface oxide formation at much lower potential on MnO2 structure compared to using carbon as the support. In fact the robust appearance of the HAD, oxide formation, AOR in case of Pd61Co39/MnO2 reflects stronger surface functional features in this catalyst. Considering the AOR regions in the voltammograms (Figure 6), the sharp peaks and high charges observed for Pd and PdCo on MnO2 may be interpreted as follows. As evidenced by XPS analyses, the MnO2 supported catalysts are enriched with oxides/hydroxides of Pd, Co and Mn which are manifested in the anodic sweep in the oxide/ hydroxide formation region (0.2V −0.8V). The oxide/hydroxide storage in the catalyst matrix becomes responsible for the significant current obtained during reduction in the cathodic sweep. The inset a. of Figure 6



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highlights the amplified CV features exhibiting the surface changes occurring on bare MnO2 according to the surface reactions in the alkaline environment. The equilibrium between Mn(IV)/ Mn(III) during anodic and cathodic sweep in the voltammogram, with increased charge in the oxide formation and reduction (AOR) region.40 Further, it may be noted that the reduction peaks in the AOR region for single Pd and binary PdCo catalysts suffer negative shift on MnO2 compared to the carbon support (Pd/C), as represented by the inset b. plot of Figure 6. The remarkable feature of the Pd- Co combination over MnO2 is also manifested by the widening of AOR peak in case of the binary catalyst 44 reasonably due to the small-scale alloying of Pd and Co as also evidenced from XRD and XPS analyses. At this point, it may be summarized, that the existence of Mn(OH)2[II], MnOOH [III], MnO2[IV] signifies the transition between the multiple oxidation states of Mn and facilitate – OH coverage on the metal NPs decorated MnO2, particularly in presence of Pd. Furthermore, the Co oxy/ hydroxy species play proactive role in creating favorable reaction centers for electro-catalysis which has been further elucidated in term of mechanistic interpretations. The beneficial role of MnO2 structure in EOR kinetics may be highlighted here. The Mn (III) is known to undergo John Teller distortion due to the unsymmetrical electron filling in Eg level while Mn (IV) possesses symmetrical structure. It may be predicted from the forward and backward sweep in the voltammograms (inset a. of Figure 6) that equilibrium exists between Mn (III) and Mn (IV),

39-41

corresponding to distorted and undistorted

structure of the respective species. This equilibrium is expected to have a positive impact for facilitating the adsorption and desorption of the complex C-2 molecule (ethanol) and its reaction intermediates, thus making the oxidation reaction kinetics faster.

41

Each O atom of

the MnO6 unit can form weak coordinate bond with water and ethanol (Scheme 3a), thus strengthening the catalyst-electrolyte interaction and making the ethanol activation on the MnO2 support more fruitful than on the carbon support.


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Moreover it may be noted that pore size of α MnO2 (4.6 Å x 4.6 Å) is very much compatible with the size of ethanol molecule42 (Scheme 3b) and as ethanol does not contain high charge, the size compatibility may induce the reactant species to reach the tunnel inside the MnO2building blocks thus making it more accessible to the Pd and Co NPs and the EOR can take place from close proximity (Scheme 3b) as discussed in the next section.43 It is understood that the EOR sequence involves a number of critical steps which are majorly (i) dehydrogenation/ dissociative adsorption, (ii) water activation (M-OH formation), (iii) C-C bond cleavage and (iv) bifunctional or ligand effect for COads oxidation. In the present investigation, the dehydrogenation process is facilitated by the intervention of MnO2 in course of generating MnOOH which maintains a rapid equilibrium with MnO2, as discussed earlier. In contrast, graphitic (carbon) support is not at all electro-active in the process of water activation. On the other hand by the presence of PdCo on MnO2, the catalyst gets flooded with OHads which effectively can influence the bifunctional mechanism, during EOR, described in our earlier reports.45 Stabilized voltammograms of the catalyst regime in presence of ethanol are characterized by well-defined oxidation peaks in the anodic sweep and renewed oxidation peak in the cathodic sweep as shown in Figure 7. It is interesting to note that EOR feature is clearly discernable on the bare MnO2 support although feeble oxidation current of 10.34 mA cm-2 is produced. The peak current densities for Pd/MnO2 (100.67 mA cm-2) and Pd61Co39/MnO2 (101.42 mA cm-2) are as high as 65.1-66.3% compared to the Pd/C catalysts, suggesting better electro-catalysis on MnO2 support. In fact the EOR kinetics requires much less over-potential when the metal NPs are supported on MnO2as recorded by the onset potential, taken at an arbitrary current output of 5 mA cm-2, shown in Figure 7b. In addition, Figure S2 represents the cyclic voltammograms of EOR where the current densities are expressed in mass specific (mAg-1) unit.



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Thus MnO2 can act as the platform for abating surface poisoning by COads species on metal NPs (M=Pd, Co) and lead to ultimate CO2 production in alkaline medium (equation 3). This intern activates the metal surface for further ethanol anchoring and decomposition, yielding oxidation products to a satisfactory level.4 + − + → + + + …. (3) The distinct advantage of combining Co with Pd is that the degree of ethanol oxidation in terms of peak current densities remains the same even after 40% lowering of Pd loading by Co in the binary catalysts. A much broader area under the CV plot in the anodic sweep is observed with Pd61Co39/MnO2 compared to Pd/MnO2. At this point we quantitatively evaluate the ratio of charge (area ratio) due to ethanol oxidation and it is found to be 1.6:1.0 for Pd61Co39/MnO2 versus Pd/MnO2 catalysts, indicating significant contribution of Co towards extensive electrolysis throughout the potential range. Although the oxidation reaction occurs vigorously over the binary catalyst, the renewed oxidation peak current (iren=45.36 mAcm-2) is found to be only 9 mAcm-2 greater than the MnO2supported single catalyst (iren =38.77 mAcm-2). Hence Co addition to the Pd NPs significantly help promoting EOR kinetics by the ample existence of CoO and Co(OH)2 species prohibiting poisoning of the surface, at the premature stage, during the oxidation process. + − → + + … . (4) It transpires that the conjugate effort of the support and metal NPs as the catalyst components

significantly

accelerates

the

oxidation

kinetics.

The

hybrid

structurePd61Co39/MnO2 in fact becomes more cheap and efficient, replacing the Pd loading by 40% Co NPs in the matrix. In order to obtain the discrete kinetic features of EOR throughout a substantial potential range, Tafel plots (Figure S3) were derived from the quasisteady polarization studies (linear sweep voltammetry) carried out in the working solutions and demonstrated in the supporting information file.


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The stability of the synthesized catalysts is determined through chrono-amperometric studies Figure 8. The rapid initial fall is arrested at a later time for all the catalysts as revealed in Figure 8. The poisoning rate follows the order Pd/C >Pd/MnO2>>Pd61Co39/MnO2. The role of MnO2 and the metal NPs towards poison removal has already been discussed in earlier sections. The reduced poisoning rate forPd61Co39/MnO2to a considerable extent seems to be analogous to the BET data (Table 2). It is observed that the BET surface area and total pore volume for the binary catalyst is decreased compared to bare MnO2,while the average pore size is increased in case of the former. The pores having comparatively larger size forPd61Co39/MnO2 appear to be more befitting for the transit of reactant and intermediates during the oxidation process leading to easy removal of the carbonaceous residues from the surface. In order to determine the long term stability of the catalyst, Pd61Co39/MnO2 was further subjected to chrono-amperometric analysis for extended period of 24400 sec in stages (3600 sec for four consecutive stages and 10000 sec in the final stage as shown in Figure S4). The poisoning rate are found to be 0.0040 for 3600 sec, 0.0041 for 7200sec, 0.0042 for 10800sec, 0.0045 for 14400 sec and finally,0.0047 until 24400 sec is completed, indicating appreciable poison tolerance of the catalyst toward EOR. The electrochemical impedance spectra for the respective catalysts in the alkaline solution of ethanol were recorded at –0.3 V and shown in Figure 9.The equivalent circuits applicable to these plots are shown in the inset of Figure 9 and the corresponding values are summarized in Table 4. In these circuits Rs, Rct, CPEdl represent solution resistance, charge transfer resistance and constant phase element corresponding to the double layer capacitance, respectively, while Rads and CPEads, are related to adsorption of reaction intermediates.60 Nyquist plots derived from the spectra resembling semicircles are assigned to the oxidation process and the diameters of the semicircle represent the charge transfer resistance, Rct. The



drastic fall in the Rct values for the MnO2 supported catalysts indicate the kinetic input of the support material toward EOR. In case of carbon supported catalyst, the poisoning intermediates form a pre-electrode layer which reduces the charge transfer kinetics, whereas MnO2 containing catalysts are capable of alleviating intermediate product accumulation by intervening in the oxidation sequences.38, 46 The high CPEdl values in case of Pd61Co39/MnO2 compared to Pd/MnO2 may be correlated with the size contraction of binary catalyst NPs, offering more available sites for accumulation of charges at the grain boundaries. It is found that the resistance related to the adsorption process, Rads, is reduced for the binary catalyst compared to Pd/MnO2 indicating the ease of ethanol anchoring process over the binary catalyst. It is clearly observed from the Table 4 that huge surface charge (CPEads) is exhibited by the carbon supported Pd catalyst more than 10 times with respect to MnO2 supported catalyst. This reflects the sluggish kinetics over Pd/C while much lower value of CPE on PdCo/MnO2 stands for rapid equilibrium between adsorption –desorption on the surface making the surface poison free. The Bode plots (Figure 10) derived from the EIS records display downward shift of phase angle maxima (- Φ) for the binary catalyst indicating the transformation of capacitive to resistive behavior of the electrode-electrolyte interface which in effect translates to favorable charge transfer kinetics on the Pd61Co39/MnO2 catalyst NPs. The end products of the electrolysis were estimated through ion chromatographic analysis of the chronoamperometric aliquots taken at –300 mV after 1 hr. It is observed from the Table5 that the yield of both acetate and carbonate on Pd/MnO2 is double that obtained on Pd/C. Interestingly, the production of acetate on Pd61Co39/MnO2 is increased 2.2 times that on Pd/MnO2, whereas the yield of carbonate is increased 1.6 times using the binary catalyst. It has already been reported that the Pd based electrodes prefer the acetate pathway during EOR sequences47 through the formation of (CH3CO)ads along with abundant (OH)ads coverage of the Pd61Co39/MnO2 surface.


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The proactive role of MnO2 in the oxidation process can be further supported by the possible formation of alkoxy and hyper oxo-linkages. There are chances of ethanol directly reacting with MnO2 by donating the lone pair of the oxygen atom to the lewis acid site of the metal oxide42 and thereby forming alkoxide species, as reported for alcohols. This ultimately leads to the formation of acetaldehyde as the main product, which in alkali media is transformed to acetic acid. In fact, bare MnO2 has shown the EOR features in the voltammogram (Figure7a), producing never the less and at least ~ 10 mA cm-2 current output. − − − − − + → ( ) − − − − − + () . . (5)

− − − − − + 2() → ( ) + − − − − − + . . (6) ( ) + () → + . . (7)

Bai et al reported on the influence of temperature on the catalytic activity of MnO2 towards ethanol oxidation.48 In presence of MnO2, ethanol is partially converted to CO2 at room temperature, while complete conversion of ethanol to CO2 is achieved only above 140° C. It has been discussed in our earlier article1 that the CH3CO produced as an intermediate species of EOR, can interact with transition metal oxide to form methoxy linkage and (CO)ads, thus dragging the reaction towards CO2/CO32- formation depending on the pH condition. ( ) + − − − → − − − + ( ) … (8)

− − − + () → − − − − … . (9)

− − − − + 5 → + − − − + 4 + 5 … (10)

As revealed from the XPS, Co exists in 0 and +2 states in the catalyst matrix. For the PdCo catalyst, the dehydrogenation pathway may be facilitated Co at the reaction sites as shown in equation 11. CH3CO species generated by the oxidation of ethanol is further oxidized to acetic acid (equation 12) ultimately leading to carbonate formation (equation 13-15) in alkali media as already mentioned in one of our earlier article.49 The presence of Co+2 in PdCo



Page 18 of 46

formulation, initiates the oxidation reaction to go through the acetaldehyde intermediate (equation 16) which further proceeds to form acetate (equation 7) and carbonate (equation 13-15) in alkali medium.50-51 − ( ) + 3 → − ( ) + 3 + 3 [ = "#, ] .. (11) − ( ) + → − + … (12) + 3 → ( ) + + 3 + 3

… (13)

− ( ) + 3 → ( ) + 3 + 3 … ( 14) − ( ) + () + 3 → + 2 +

.. (15)

− ( ) + () → − ( ) + + 2 … (16) It may be summarized that MnO2 has pronounced effect on the propagation of EOR in alkaline medium and the combinatorial formulation of Pd61Co39 on MnO2 active centres orients the oxidation kinetics to proceed toward multiple pathways yielding acetaldehyde, acetic acid and carbonates as shown in Schematic 4. In fact, the electrochemical oxidation of ethanol majorly delivers acetate at the end, almost 2.73 times than carbonate. However, with the introduction of Co as ad-atom to Pd NPs embedded on MnO2 structure, both the acetate and carbonate yields surpass 2.23 and 1.6 times respectively that obtained on single Pd catalyst over MnO2 indicating the assistance of various oxides present in the catalyst matrix, in driving the oxidation reaction to an appreciable extent.

Fuel cell performance study. Performance screening of the Pd/C, Pd/MnO2 and Pd61Co39/MnO2catalysts was carried out using single cell test fixture in the fuel cell test station. Figure 11 represents the I-V curve and power density plots of the catalyst regime. Pd61Co39/MnO2 exhibits highest OCV (open circuit voltage) of 0.77 V which is 0.07 V ahead of the OCV obtained with Pd/MnO2 (0.70 V). Power density values obtained for the Pd/C, Pd/MnO2 and Pd61Co39/MnO2catalysts are 20.66 mW cm-2, 33.78 mW cm-2 and 43.88mW cm-2 respectively. Thus the beneficial role of MnO2 support toward EOR is reflected by the


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63.5 % increase in power density value compared to using carbon as the support material. Further, around 30% increase in power density value is observed using the co-metal Co with Pd in the catalyst matrix. This indicates the energy efficient role of the 40% Co incorporation in the catalyst partially replacing Pd in the catalyst structure. Thus the conjugate effort of non-carbon support and the non Pt mixed metallic catalyst NPs brings about synergy in driving the anodic reaction to a considerable extent as observed in the fuel cell I-V parameters and is analogous to the results obtained from the half-cell studies.

CONCLUSIONS In summary, we have developed the Pd61Co39/MnO2catalyst NPs, by adopting a green synthetic approach based on ultrasonic perturbation and validated for the ethanol oxidation study. The fabrication technique was absolutely solvent free in case of fabricating MnO2 while no chemical reagents other than precursor solutions have been used for decorating MnO2 by the Pd and Co NPs. Interestingly, the catalyst-support development procedure is associated with the transformation of oxidation states of the metallites as well as the support MnO2 changes its shape from nanowire to nanorod due to strong interaction with the metallites, formulating a structure conducive to ethanol oxidation reaction. The multivalent oxidation states of Mn, Co and Pd, as obtained from XPS study, are found to have fabulous effect on the EOR kinetics producing high voltammetric currents. It is to be noted that around 40% reduction in Pd loading by Co not only makes the catalyst cheaper but at the same time the combinatorial formulation exhibits excellent catalytic efficiencies in terms of onset potential, poison tolerance, charge transfer kinetics, I-V characteristics and power density output etc. From the oxidation product analysis it implies that the ethanol oxidation prefers the acetate pathway; however with the intervention of MnO2 and Co in the matrix considerable amount of carbonate is also produced. The hybrid Pd61Co39/MnO2 catalyst NPs



therefore emerge as the promising energy efficient materials for direct ethanol fuel cell with the involvement of transitional metals and their oxides at the same time eliminating carbon form the matrix.


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(a)

(b)

Figure 1(a) X-ray diffraction patterns of MnO2, Pd/MnO2, PdCo/MnO2 electro-catalysts, (b) Magnified view of (111) plane with the shifting in peak position.




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(a) (b)

(c)

(d)

Figure 3. HRTEM image of Pd61Co39/MnO2, (a) high population density, (b) low population density, (c) Fringe pattern of MnO2 and(d) Pd, Co decorated MnO2catalyst.




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Figure 5. Nitrogen adsorption (filled) and desorption (unfilled) isotherm for MnO2, Pd/MnO2 and Pd61Co39/MnO2, Inset: pore size distribution.


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Figure 6. Cyclic voltammograms of Pd/C, Pd/MnO2 and Pd61Co39/MnO2 catalysts in 0.5 M NaOH. Scan rate, 50 mV s

−1

. Inset a. Cyclic voltammogram of MnO2 support and b. AOR

peak potential of the respective catalysts.



Page 28 of 46

(a)

(b)

Figure 7.(a) Cyclic voltammograms of EOR on Pd/C, Pd/MnO2 and Pd61Co39/MnO2 in 0.5 M NaOH solution containing 1.0 M ethanol at room temperature (scan rate, 50 mV s−1); (b) comparison of onset potential at 5.00 mA cm−2 current output.


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Figure 8.Chronoamperograms recorded for 1 h at the potential of -0.3V (vs vs. Hg.HgO) in solution containing 1.0 M EtOH and 0.5 M NaOH on Pd/C, Pd/MnO2 and Pd61Co39/MnO2catalysts. Inset: Poisoning rate.



Page 30 of 46

(a)

(b)

Figure 9(a). Nyquist plots in solution containing 1.0 M EtOH and 0.5 M NaOH on Pd/C, Pd/MnO2 and Pd61Co39/MnO2catalysts at -0.3 V (vs vs. Hg.HgO). Insets: equivalent circuit diagram. (b) Bar plot for charge transfer resistance (Rct).


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Figure 10. Bode plot for Pd/C, Pd/MnO2 and Pd61Co39/MnO2 catalysts.

Figure 11. Polarization and power density plots of the AEM DEFC with Pd/C, Pd/MnO2 and Pd61Co39/MnO2 anode catalysts.



Scheme 1a. Pathways for α MnO2 nanowire formation, Scheme 1b. α MnO2 bearing (2 X 2) tunnel structure.

(a)

(b)

Scheme 2. Morphological changes of MnO2 occurring before and after Pd, Co decoration.


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Scheme 3a. Interaction of MnO6 unit with water and ethanol molecule, Scheme 3b. Interaction of ethanol molecule inside (2x2) tunnel of MnO2.

(a)

(b)

Scheme 4. Ethanol oxidation on Pd61Co39/MnO2 catalyst.



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Table 1. Physical properties of the different catalysts as derived from XRD. Catalyst Pd/MnO2 Pd61Co39/MnO2

Interplanar Distance (A° ) 2.252 2.248

Lattice Parameter (A° ) 3.900 3.893

Table 2. Physical properties as derived from BET plot. Catalyst MnO2 Pd/MnO2 Pd61Co39/MnO2

BET surface area(m2/g) 191.33 59.88 83.20

Total pore volume (cc/g) 0.5885 0.3784 0.4692

Average pore size x10-4 (Å) 0.0373 0.0427 0.0519

Table 3. ECSA (HAD, AOR) for the different catalysts. Catalyst Pd/C Pd/MnO2 Pd61Co39/MnO2

ECSA(m2/g) HAD region AOR region 13.71 12.55 50.74 70.44 82.19 133.49


Average crystallite size (nm) 13.25 8.13

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Table 4. Electrochemical impedance parameters for the different catalysts.

EC parameters

Pd/C

Pd/MnO2

Pd61Co39/MnO2

Rs/ Ω Rct/ Ω CPEdl / F cm–2

5.78 29.67 0.9913

5.00 8.24 0.6495

5.11 7.95 0.6966

Rads/ / Ω

33.96

14.87

11.76

CPEads / F cm–2

0.1280

0.0075

0.0114

Table 5. Analysis of the product formed during ethanol oxidation in alkaline media on Pd/C, Pd/MnO2 and Pd61Co39/MnO2catalysts at -300 mV (vs. Hg.HgO) for 1 hour.

Catalyst Pd/C Pd/MnO2 Pd61Co39/MnO2

Acetate(ppm) 110 233 520

Cabonate (ppm) 55 120 190



ASSOCIATED CONTENT Supporting Information. TGA analysis, Cyclic Voltammetric plot (in mass specific unit), Tafel analysis, Extended Chronoamperometry of Pd61Co39/MnO2 catalyst. AUTHOR INFORMATION Corresponding Author. Jayati Datta, E-mail:[email protected]

ACKNOWLEDGMENTS Financial support by Ministry of New and Renewable Energy (MNRE) (102/80/2010-NT, 05.01.2012), New Delhi, Govt. of India and Council of Scientific and Industrial Research (CSIR) [01(2847)/16/EMR-II, 12.05.2016], New Delhi, India is gratefully acknowledged. A. De wishes to acknowledge the Senior Research Fellowship from the Council of Scientific and Industrial Research (CSIR), New Delhi, India. The authors acknowledge Dr. Abhijit Dutta, Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland for the ICP-MS and TGA studies. REFERENCES 1. De, A.; Datta, J.; Haldar, I.; Biswas, M. Catalytic Intervention of MoO3 toward Ethanol Oxidation on PtPd Nanoparticles Decorated MoO3−Polypyrrole Composite Support, ACS

Appl. Mater. Interfaces 2016, 8, 28574–28584, DOI: 10.1021/acsami.6b07455. 2. Li, Z.; Ji, S.; Pollet, B.G.; Shen, P.K. A Co3W3C promoted Pd catalyst exhibiting competitive performance over Pt/C catalysts towards the oxygen reduction reaction, Chem.

Commun.2014, 50, 566-568, DOI:10.1039/C3CC48240E. 3. Zhang, L.; Chang, Q.; Chen, H.; Shao, M. Recent advances in palladium-based electrocatalysts for fuel cell reactions and hydrogen evolution reaction, Nanoenergy 2016, 29, 198219, doi.org/10.1016/j.nanoen.2016.02.044.


Page 36 of 46

Page 37 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Meher, S. K.; Rao, G. R. Morphology-Controlled Promoting Activity of Nanostructured MnO2 for Methanol and Ethanol Electrooxidation on Pt/C, J. Phys. Chem. C 2013, 117, 4888−4900, DOI: 10.1021/jp3093995. 5. Chen, Z.; Zhang, J.; Zhang, Y.; Liu, Y., Han, X.; Zhong, C.; Hu, W.; Deng, Y. NiOinduced synthesis of PdNi bimetallic hollow nanocrystals with enhanced electrocatalytic activities toward ethanol and formic acid oxidation, Nanoenergy 2017, 42, 353-362, doi.org/10.1016/j.nanoen.2017.11.033. 6. Jiang, R.; Tran, D. T.; McClure, J. P.; Chu, D. A Class of (Pd−Ni−P) Electrocatalysts for the Ethanol Oxidation Reaction in Alkaline Media, ACS Catal. 2014, 4, 2577−2586, DOI: 10.1021/cs500462z. 7. Wang, L.; Zeng, Z.; Ma, C.; Liu, Y.; Giroux, M.; Chi, M.; Jin, J.; Greeley, J.; Wang, C. Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts,

NanoLett.2017, 17, 3391−3395, DOI: 10.1021/acs.nanolett.7b00046. 8. Du, W.; Mackenzie, K.E. D.; Milano, F.; Deskins, N. A.; Su, D.; Teng, X. Palladium−Tin Alloyed Catalysts for the Ethanol Oxidation Reaction in an Alkaline Medium,

ACS Catal. 2012, 2, 287−297, DOI: 10.1021/cs2005955. 9. Tan, J. L.; Jesus, A.M.D.; Chua, S. L.; Sanetuntikul, J.; Shanmugam, S.; Tongol, B. J. V.; Kim, H. Preparation and characterization of palladium-nickel on graphene oxide support as anode catalyst for alkaline direct ethanol fuel cell, Appl. Catal., A 2017, 531, 29-35, doi.org/10.1016/j.apcata.2016.11.034. 10. Ahmed, M. S.; Jeon, S. Highly Active Graphene-Supported NixPd100−x Binary Alloyed Catalysts for Electro-Oxidation of Ethanol in an Alkaline Media, ACS Catal.2014, 4, 1830−1837, DOI: 10.1021/cs500103a.



Page 38 of 46

11. Hayes,E. T.; Bellingham,B. R.; Mark Jr, H. B.; Galal, A. An amperometric aqueous ethanol sensor based on the electrocatalytic oxidation at a cobalt-nickel oxide electrode,

ElectrochimicaActa.1996, 41, 337-344, doi.org/10.1016/0013-4686 (94)00358-8. 12. Liu, S.; Xiao, W.; Wang, J.; Zhu, J.; Wu, Z.; Xin, H.; Wang, D. Ultralow content of Pt on Pd–Co–Cu/C ternary nanoparticles with excellent electrocatalytic activity and durability for

the

oxygen

reduction

reaction,

Nanoenergy

27,

2016,

475–481,

doi.org/10.1016/j.nanoen.2016.07.038. 13. Cao, C.; Hu, C.; Tian, J.; Shen, W.; Wang, S.; Liuc, H. Pt Nanoparticles Supported Inside TiO2 Nanotubes for Effective Ethanol Electrooxidation, J. Electrochem. Soc., 2013, 160, H793-H799, doi: 10.1149/2.011311jes. 14. Kumar, P. S.M.; Thiripuranthagan,S.; Imai, T.; Kumar, G.; Pugazhendhi, A.; Vijayan, R.; Esparza, R.; Abe, H.; Krishnan, S. K. Pt Nanoparticles Supported on Mesoporous CeO2 Nanostructures Obtained through Green Approach for Efficient Catalytic Performance toward Ethanol Electro-oxidation, ACS Sustainable Chem. Eng. 2017, 5, 11290−11299, DOI: 10.1021/acssuschemeng.7b02019. 15. Bai, Y.; Wu, J.; Xi, J.; Wang, J.; Zhu, W.; Chen, L.; Qiu, X. Electrochemical oxidation of

ethanol

on

Pt–ZrO2/C

catalyst,

Electrochem.

7,

Commun.2005,

1087–1090,

doi.org/10.1016/j.elecom.2005.08.002. 16. Xi, Z.; Erdosy, D.P.; Mendoza-Garcia, A.; Duchesne, P. N.; Li, J.; Muzzio, M.; Li, Q.; Zhang, P.; Sun, S. Pd Nanoparticles Coupled to WO Electrochemical

Oxidation

of

Formic

Acid,

2.72Nanorods

NanoLett.

DOI: 10.1021/acs.nanolett.7b00870.


2017,

17,

for Enhanced 2727−2731,

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. De, A.; Adhikary, R.; Datta, J. Proactive role of carbon nanotube-polyaniline conjugate support for Pt nano-particles toward electro-catalysis of ethanol in fuel cell, Int. J. Hydrogen

Energy 2017, 42, 25316-25325, doi.org/10.1016/j.ijhydene.2017.08.073. 18. Datta, J.; Dutta, A.; Biswas, M. Enhancement of functional properties of PtPdnano catalyst in metal-polymer composite matrix: Application in direct ethanol fuel cell,

Electrochem. Commun.2012, 20, 56–59, doi.org/10.1016/j.elecom.2012.02.022. 19. Davis, D. J.; Lambert, T. N.; Vigil, J. A.; Rodriguez, M. A.; Brumbach, M. T.; Coker, E. N.; Limmer, S. J. Role of Cu-Ion Doping in Cu-α-MnO2 Nanowire Electrocatalysts for the Oxygen

Reduction

Reaction,

J.

Phys.

Chem.

C

2014,

118,

17342−17350,

DOI: 10.1021/jp5039865. 20. Zhao, G.; Li, H. Electrochemical oxidation of methanol on Pt nanoparticles composited MnO2

nanowire

arrayed

electrode,

Appl.

Surf.

Sci.

2008,

254,

3232–3235,

doi.org/10.1016/j.apsusc.2007.10.086. 21. Bai, Y.; Zhang, H.; Xu, J.; Chen, H. Relationship between Nanostructure and Electrochemical/Biosensing Properties of MnO2Nanomaterials for H2O2/Choline, J. Phys.

Chem. C 2008, 112, 18984–18990, DOI: 10.1021/jp805497y. 22. Lee, S.; Choi , B.; Hamasuna, N.; Fushimi, C.; Tsutsumi, A. Characterization of MnO2 positive electrode for Fuel Cell/Battery (FCB), J. Power Sources 2008, 181, 177–181, doi.org/10.1016/j.jpowsour.2008.02.083. 23. Yuan, Y.; Wang, W.; Yang, J.; Tang, H.; Ye, Z.; Zeng , Y.; Lu, J. Full synergistic contribution of electrodeposited three-dimensional NiCo2O4@MnO2nanosheet networks electrode

for

asymmetric

supercapacitors,

Nanoenergy

doi.org/10.1016/j.nanoen.2016.08.013.


2016,

27,

627–637,


Page 40 of 46

24. Tompsett, D. A.; Parker, S. C.; Islam, M. S. Surface properties of a-MnO2: relevance to catalytic and supercapacitor behaviour, J. Mater. Chem. A 2014, 2, 15509–15518, DOI:10.1039/C4TA00952E. 25. Bang, J. H.; Suslick, K. S. Applications of Ultrasound to the Synthesis of Nanostructured

Materials,

Adv.

22,

Mater.2010,

1039–1059,

doi.org/10.1002/adma.200904093. 26. Teranishi, T.; Miyake, M. Size Control of Palladium Nanoparticles and Their Crystal Structures, Chem. Mater.1998, 10, 594-600, DOI: 10.1021/cm9705808. 27. Zhou, C.; Wang, H.; Peng, F.; Liang, J.; Yu, H.; Yang, J. MnO2/CNT Supported Pt and PtRuNanocatalysts for Direct Methanol Fuel Cells, Langmuir 2009, 25, 7711–7717, DOI: 10.1021/la900250w. 28. Yin, B.; Zhang, S.; Jiang, H.; Qu, F.; Wu, X. Phase-controlled synthesis of polymorphic MnO2 structures for electrochemical energy storage, J. Mater. Chem. A, 2015, 3, 5722-5729, DOI:10.1039/C4TA06943A. 29. Wang, X.; Prof., Y. L. Synthesis and Formation Mechanism of Manganese DioxideNanowires/Nanorods,

Chem.

Eur.

J

2003,

9,

300–306,

DOI:10.1002/chem.200390024. 30. Poonguzhali, R.; Shanmugam, N.; Senthilkumar, R. G. A.; Shanmugam, R.; Sathishkumar,K. Influence of Zn doping on electrochemical capacitor behavior of MnO2nanocrystals, RSC Adv.2015, 5, 45407-45415, DOI:10.1039/C5RA01326G. 31. Yuan, H.; Deng, L.; Qi, Y.; Kobayashi, N.; Hasatani, M. Morphology-dependent Performance of Nanostructured MnO2 as an Oxygen Reduction Catalyst in Microbial Fuel Cells, Int. J. Electrochem. Sci.2015, 10, 3693 – 3706.


Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32. Zhang, Z.; Xiao, F.; Qian, L; Xiao, J ; Wang, S .; Liu, Y. Facile Synthesis of 3D MnO2 –Graphene and CarbonNanotube–Graphene Composite Networks for High-Performance, Flexible, All-Solid-State Asymmetric Supercapacitors, Adv. Energy Mater.2014, 4, 1400064, doi.org/10.1002/aenm.201400064. 33. Wang, H.; Chen, J.; Hu, S.; Zhang, X.; Fan, X.; Du, J.; Huang, Y.; Li, Q. Direct growth of flower-like 3D MnO2 ultrathin nanosheets on carbon paper as efficient cathode catalyst

for

rechargeable

Li-O2

batteries,

RSC

Adv.

2015,

5,

72495-72499,

DOI:10.1039/C5RA15464B. 34.Łukasiewicz, M. I.; Wojcik-Głodowska, A. ; Guziewicz, E.; Wolska, A.; Klepka, M. T.; Dłuzewski, P.; Jakieła, R.; Łusakowska, E.; Kopalko, K.; Paszkowicz, W.; Ł. Wachnicki, ; Witkowski, B. S.; Lisowski, W. ; Krawczyk, M.; Sobczak, J. W.; Jabłonski, A.; Godlewski, M. ZnO, ZnMnO and ZnCoO films grown by atomic layer deposition, Semicond. Sci.

Technol.2012, 27, 074009, DOI:10.1088/0268-1242/27/7/074009. 35. Li, C.; Zhang, X.; Wang, K.; Zhang, H.; Sun , X.; Ma, Y. Dandelion-like cobalt hydroxide nanostructures: morphological evolution, soft template effect and supercapacitive application, RSC Adv.2014, 4, 59603-59613, DOI:10.1039/C4RA10393A. 36. Xia, H.; Zhu, D.; Luo, Z.; Yu, Y.; Shi, X.; Yuan ,G. ; Xie, J. Hierarchically Structured Co3O4@Pt@ MnO2 Nanowire Arrays for High-Performance Supercapacitors. Sci. Rep.2013, 3, 2978, DOI: 10.1038/srep02978. 37. Kaneko, K. Determination of pore size and pore size distribution 1. Adsorbents and catalysts, J. Membr. Sci.1994, 96, 59-89, doi.org/10.1016/0376-7388(94)00126-X. 38. Dutta, A.; Datta, J. Significant role of surface activation on Pd enriched Pt nano catalysts in promoting the electrode kinetics of ethanol oxidation: Temperature effect,



Page 42 of 46

product analysis & theoretical computations, Int. J. Hydrogen Energy 2013, 38, 7789–7800, doi.org/10.1016/j.ijhydene.2013.03.137. 39. Khilari, S.; Pandit, S.; Ghangrekar, M. M. ; Das, D.; Pradhan, D. Graphene supported α-MnO2 nanotubes as a cathode catalyst for improved power generation and wastewater treatment in single-chambered microbial fuel cells, RSC Adv.2013, 3,7902-791, DOI:10.1039/C3RA22569K. 40. El-Deab, M.S. Platinum nanoparticles–manganese oxide nanorods as novel binary catalysts

for

formic

acid

oxidation,

j.

adv.

res.

2012,

3,

65-71,

doi.org/10.1016/j.jare.2011.04.002. 41. Zhang, K.; Han,X. ; Hu, Z. ; Zhang, X.; Tao, Z.; Chen, J. Nanostructured Mn-based oxides for electrochemical energy storage and conversion, Chem. Soc. Rev.2015, 44, 699728, doi: 10.1039/c4cs00218k. 42. Li, J.; Wang, R.; Hao, J. Role of Lattice Oxygen and Lewis Acid on Ethanol Oxidation over OMS-2 Catalyst, J. Phys. Chem. C 2010, 114, 10544–10550, DOI: 10.1021/jp102779u. 43. Chen,S. ; Zhu, J.; Wu, X.; Han, Q.; Wang, X. Graphene Oxide-MnO2Nanocomposites for Supercapacitors, ACS Nano 2010, 4, 2822–2830, DOI: 10.1021/nn901311t. 44. Savadogo, K.O. Electrochemical investigation of Pd–Co thin films binary alloy for the oxygen reduction reaction in acid medium, J. Electroanal. Chem, 2013, 703, 108-116, DOI: 10.1016/j.jelechem.2013.04.006. 45. Datta, J.; Sen Gupta, S.; Singh, S.; Mukherjee, S.; Mukherjee, M. Significant Role of RuOxide Present in the Pt-Ru Alloy Catalyst for Ethanol Electro-Oxidation in Acid Medium, J.

Mater. Process. Manuf. Sci.2011, 26, 267-271, doi.org/10.1080/10426914.2010.520796.


Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


46. Dutta, A.; Datta, J. Outstanding Catalyst Performance of PdAuNi Nanoparticles for the Anodic Reaction in an Alkaline Direct Ethanol with Anion-Exchange Membrane Fuel Cell, J.

Phys. Chem. C 2012, 116, 25677–25688, DOI: 10.1021/jp305323s. 47. Datta, J.; Dutta, A.; Mukherjee, S. The Beneficial Role of the Co metals Pd and Au in the Carbon-Supported PtPdAu Catalyst toward Promoting Ethanol Oxidation Kinetics in Alkaline Fuel Cells: Temperature Effect and Reaction Mechanism, J. Phys. Chem. C 2011, 115, 15324–15334, DOI: 10.1021/jp200318m. 48. Bai, B.; Li, J.; Hao, J. 1D-MnO2, 2D-MnO2 and 3D-MnO2 for Low-Temperature Oxidation

of

Ethanol,

Appl.

Catal.

B

164,

Environ.2015,

241–250,

doi.org/10.1016/j.apcatb.2014.08.044. 49. Mahapatra, S. S. ; Dutta, A. ; Datta, J. Temperature effect on the electrode kinetics of ethanol oxidation on Pd modified Pt electrodes and the estimation of intermediates formed in alkali

medium,

Electrochimica

Acta

2010,

55,

9097-9104,

doi.org/10.1016/j.electacta.2010.08.029. 50. Li, M.; Chen, J.; Wang, G. Reaction Mechanism of Ethanol on Model Cobalt Catalysts: DFT

Calculations,

J.

Phys.

Chem.

C

2016,

120,

14198–14208,

DOI: 10.1021/acs.jpcc.6b04036. 51.Rostami, H.; Rostami, A. A.; Omrani, A. Investigation on ethanol electrooxidation via electrodeposited Pd-Co nanostructures supported on graphene oxide, Int. J. Hydrogen

Energy, 2015, 40, 10596-10604, doi.org/10.1016/j.ijhydene.2015.06.151. 52. Xue, H.; Tang, J.; Gong, H.; Guo, H.; Fan, X.; Wang, T.; He, J.; Yamauchi, Y., Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three-Dimensional Ordered NDoped Porous Carbon as an Efficient Catalyst for Oxygen Reduction Reaction, ACS Appl.

Mater. Interfaces, 2016, 8, 20766–20771, DOI: 10.1021/acsami.6b05856.



Page 44 of 46

53. Ding, L-X.; Wang, A-L.; Li, G-R.; Liu, Z-Q.; Zhao, W-X.; Su, C-Y.; Tong, Y-X.; Porous Pt-Ni-P Composite Nanotube Arrays: Highly Electro-active and Durable Catalysts for Methanol

Electrooxidation,

J.

Am.

Chem.

Soc.

2012,

134,

5730−5733,

DOI: 10.1021/ja212206m. 54. Wang, A-L.; Xu, H.; Feng, J-X.; Ding, L-X.; Tong, Y-X.; Li, G-R.; Design of Pd/PANI/Pd Sandwich-Structured Nanotube Array Catalysts with Special Shape Effects and Synergistic Effects for Ethanol Electrooxidation, J. Am. Chem. Soc., 2013, 135, 29, 10703– 10709, DOI: 10.1021/ja403101r. 55. Wang, A-L.; He, X-J.; Lu, X-F.; Xu, H.; Tong, Y-X.; Li, G-R.; Palladium–Cobalt Nanotube Arrays Supported on Carbon Fiber Cloth as High-Performance Flexible Electrocatalysts

for

Ethanol

Oxidation,

Angew.

Chem.

2015,127,

3740-3744,

doi.org/10.1002/anie.201410792. 56. Wu, T.; Fan, J.; Li, Q.; Shi, P.; Xu, Q.; Min, Y.; Palladium Nanoparticles Anchored on Anatase Titanium Dioxide-Black Phosphorus Hybrids with Heterointerfaces: Highly Electroactive and Durable Catalysts for Ethanol Electrooxidation, Adv. Energy Mater. 2017, 1701799, doi.org/10.1002/aenm.201701799. 57. Liu, Q.; Fan,J.; Min,Y.; Wu, Lin, T-Y.; Xu, Q.; B, N-codoped graphene nanoribbons supported Pd nanoparticles for ethanol electrooxidation enhancement, J. Mater. Chem. A, 2016, 4, 4929, DOI:10.1039/C6TA01136E. 58. Wu, J.; Liu, W-W.; Wu, Y-X.; Wei, T-C.; Geng, D.; Mei, J.; Liu, H.; Lau, W-M.; Liu, L-M., Three-dimensional hierarchical interwoven nitrogen-doped carbon nanotubes/CoxNi1x-layered double hydroxides ultrathin nanosheets for high-performance supercapacitors,

Electrochim. Acta, 2016, 203, 21–29, doi.org/10.1016/j.electacta.2016.04.033.


Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


59. Wei, H.H.; Zhang, Q.; Wang, Yu.; Li, Y. J.; Fan, J. C.; Xu, Q. J.; Min, Y. L.; Baby Diaper-Inspired Construction of 3D Porous Composites for Long-Term Lithium-Ion Batteries, Adv. Funct. Mater. 2017, 1704440, doi.org/10.1002/adfm.201704440. 60. Autolab Application Note EIS04, 2011, Metrohm Autolab B.V.



Synopsis: Green sonochemical synthetic approach toward fabricating efficient DEFC catalyst PdCo/MnO2, enabling elimination of Pt and carbon, with effective catalyst-support interaction. Graphical Abstract:


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Synergistic Combination of Pd and Co Catalyst ... - ACS Publications

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