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Electrochemical Properties of Barium Cerate Doped with Zinc for Methanol Oxidation.

Byline: Naveed Kausar Janjua, Mahwish Jabeen, Mehrosh Islam, Azra Yaqub, Sana Sabahat, Sadia Mehmood, Misbah Mumtaz, Ayesha Mujtaba, Rizwan Raza and Ghazanfer Abbas

Summary: Barium cerate and its zinc doped series, BaCe1-xZnxO3 with 0.02 = x = 0.16, were synthesized using ammonia as a co-precipitant. The influence of zinc on phase and morphology was characterized using XRD and SEM, respectively. XRD revealed orthorhombic crystallinity for x= 0 to 14 mol% but distorted hexagonal phase for x = 16 mol%. SEM images revealed homogeneity of synthesized powders. The synthesized materials were then tested for their function as electrocatalyst for model analyte, methanol. Methanol electro-oxidation in acidic media was studied by modifying platinum electrode with BaCeO3 materials using cyclic voltammetry. Kinetic (ks,h, Do) and thermodynamic parameters (Ea, G, H, S) were estimated for methanol electroxidation.

The deduced value of diffusion coefficient of order of 10-10 cm2s-1 indicated faster kinetics of methanol oxidation using BaCeO3 than zinc doped sample in H2SO4 medium. The increasing temperature and methanol concentration enhanced the peak currents which pointed to the suitability of these materials as electrocatalysts. The stability and catalytic activity of these materials at various temperatures also aided to their potentiality in PEMFC application.

Key words: Zinc doped BaCeO3; SEM; XRD; Methanol oxidation; Cyclic voltammetry; Thermodynamics.

Introduction

To meet energy demands across the globe, different energy harvesting ways are very crucial. The limited amount of fossil fuels and increasing energy demands emphasize the use of cheaper and environmental friendly energy technologies. There has been renewed interest in fuel cell technology. Fuel cell is primarily an energy convertor directly providing electricity by extracting chemical energy of a fuel [1, 2]. Among fuel cells, direct methanol fuel cells (DMFCs) are considered as promising power sources for portable electronic devices owing to low operational temperature, high energy density, simple construction, and environmental benignity. The methanol oxidation is one of the key reactions in DMFCs where it is oxidized to carbon dioxide at the anode.

To date, Pt has been found to be the most active catalyst towards methanol oxidation reaction (MOR). The electrochemical oxidation of methanol to CO2 is governed by 6 electron process as shown by the equation:

CH3OH + H2O CO2 + 6H+ + 6e- (Eo = 0.02 V), [3].

In this process, the soluble intermediates are continuously being formed at anode and their formation has been demonstrated experime ntally by various techniques like FTIR, liquid chromatography. These intermediates adsorbed onto the Pt electrode and reduce its active area. The electrode poisoning by adsorbed species (CO and CHnO) decreases the catalytic activity of Pt towards MOR and thus affects the methanol fuel cell efficiency. Considering the high cost of Pt and its poisoning, the most important challenge is to find the ideal catalyst that promotes the oxidation of fuel and to develop low-temperature synthetic route for these fuel cells. To deal with this, platinum electrodes modified with perovskites have gained much popularity.

Perovskites having general formula AIIBIVO3 like barium cerate, barium zirconate, and strontium cerate where A and B are cations of different charges have got attention in this regard. BaCeO3 (BCO) poses high conductivity but this material is chemically unstable in CO2 rich atmosphere

[4, 5]. Barium cerate properties can be modified by changing its stoichiometry or by doping with impurities [6, 7]. It has been found in literature that partial substitution of cerium by a dopant improves the conductivity as well as sufficient chemical and thermal stability over a wide range of conditions relevant to fuel cell operation.

BaCeO3 and its analogues have found a key position in material research of perovskites for particularly use as hydrogen sensor and proton conductor and their electronic and ionic conductivities offer a versatile role in fuel cell technology [6, 7].

In the present work, we report the synthesis and characterization of BaCeO3 and its Zn doped analogues. Wet chemical method [8-12] was used to synthesize micro scale perovskite oxides with fine homogeneity of particles and controlled particle size. Pure barium cerate and its doped samples with different concentrations of zinc (BaCe1-xZnxO3) with x = 0.0 to 0.16 were synthesized through ammonia co-precipitation [13, 14] method and were characterized by XRD, FTIR, and SEM techniques. Drop casting technique is a simple method in which specific volume of suspension of electrocatalytic material is deposited on working electrode after ultrasonication in a suitable solvent. After depositing, binder is added to it so that active material gets bound to the working electrode [15, 16]. The electrochemical properties were tested by modifying Pt electrode with these synthesized samples using drop casting technique.

To understand the electrocatalytic behavior of barium cerate and its analogues, study of kinetic and thermodynamic parameters is important which was done in the present scheme of work. To our knowledge, this study is the first time report on the applications of barium cerates as in proton exchange membrane fuel cells (PEMFC) type technology Experimental

Synthesis of pure BaCeO3 and zinc doped powders

Synthesis process of pure BaCeO3 nanoparticles and zinc doped barium cerate (BaCe1- xZnxO3) powders is shown in scheme 1.

Starting materials were Ba(NO3)2 (Sigma- Aldrich), Ce(NO3)3.6H2O (Sigma-Aldrich) and Zn(NO3)2.6H2O (Sigma-Aldrich). The appropriate molar ratio of precursors was mixed separately each in 50 ml of deionized water by continuous stirring at 80 oC until transparent solutions were obtained. These solutions were dissolved together and concentrated ammonia (Sigma Aldrich) was added to this mixture until yellowish precipitates formed at pH 11-12. During ammonia addition, solution was stirred continuously without heating. The precipitates were then dried on hot plate under stirring at 300 oC. Fumes were emitted during this stage, indicating the emission of nitrates. Finally, the precipitates were heated at 350 oC in furnace for spontaneous combustion. Then powdered sample obtained was sintered at 800 oC for 2 hours. Zinc doped series was also fabricated by similar method.

The calcined powder obtained was then mechanically ground with acetone to ensure thorough mixing and g e t uniform particles. The BaCeO3 and zinc doped barium cerate powders are coded as shown in Table-2.

Table-2: The studied samples with molecular formula, codes, and formula weight.

###Formula###Sample code###-1

###Formula wt. (g mol)

###BaCeO3###BCO###325

BaCe0.98Zn0.02O3###BCZO-1###324

BaCe0.96Zn0.04O3###BCZO-2###323

BaCe0.94Zn0.06O3###BCZO-3###321

BaCe0.92Zn0.08O3###BCZO-4###319

BaCe0.90Zn0.10O3###BCZO-5###318

BaCe0.88Zn0.12O3###BCZO-6###316

BaCe0.86Zn0.14O3###BCZO-7###315

BaCe0.84Zn0.16O3###BCZO-8###313

Electrode Preparation

The incorporation of noble metals like silver, gold and platinum wires or meshes as interconnects for electron transport might be helpful for their specific catalytic activity in modern fuel cell technologies. Also platinum (Pt) or glassy carbon modified with Pt nanoparticles act as a reproducible signal possessing electrode for electroxidation of alcohols [17, 18]. These materials possess efficient and stable signal during the oxidation of methanol [19]. A high accessible surface area and support in electron transfer process makes Pt a better choice to be used as working electrode for electrochemical studies. Bare and modified platinum electrode with geometric area of 0.07 cm2 was used as working electrode.

The platinum electrode was modified through drop casting method [20], where 1 mg of synthesized powder was dispersed ultrasonically in 0.1 mL solvent; acetone to form a suspension. 2 L was deposited on platinum electrode using micropipette in the form of thin layer and dried in open air at room temperature for solvent to evaporate. Then, 1 to 2 drops of 5 wt% Nafion (Sigma Aldrich) solution in ethanol were added onto it and dried further in an oven for 5 minutes.

Material Characterization

Powder X-ray diffraction (XRD) patterns of synthesized samples were recorded on an X-ray diffractometer using CuKa as the radiation source ( = 1.541 A ) at room temperature in 2 range of 20 to 80o. The Fourier transform infrared spectroscopy (FTIR) was used to characterize the final perovskites structured metal oxides. The surface morphology of as synthesized powders was observed by scanning electron microscope (SEM).

Electrochemical Measurements

Electrochemical studies, cyclic voltammetry (CV) have been carried out in three electrode electrochemical cell of 50 mL capacity using Gamry framework version 6. The platinum wire was used as counter and Ag/AgCl (sat'd KCl) was used as reference electrode. CV was carried out between 0.1 V - 0.8 V potential scans in 0.5 M H2SO4 with methanol as an analyte. Before recording the final voltammogram, the electrode was cycled for 5 runs at potential scan of 100 mV s-1 in acidic medium.

Electro-oxidation of methanol was studied by varying temperature (20 oC, 25 oC, 30 oC, 35 oC, 40 oC, 45 oC, 50 oC), final concentration of methanol (0.25 M, 0.5 M, 0.75 M, 1.0 M, 1.25 M, 1.5 M, 1.75 M, 2.0 M, 2.25 M) and scan rate of 10 through 1000 mV s-1 was used in each separate experiment. The data was averaged over triplicate results.

Results and Discussion

XRD analysis

Fig. 1 presents X-ray diffraction patterns of synthesized BaCeO3 (BCO) and its zinc doped analogues (BCZO-1, BCZO-8). The XRD pattern of BaCeO3 is comparable with standard reference card no 00-022-0074 which belongs to orthorhombic structure, Pbnm [21]. All the doped samples (0.14 = x = 0.02) were single perovskite phases and iso- structural to pure barium cerate indicating entrance of zinc into lattice instead of accumulation into the grain boundaries [22].

The average crystallite size was estimated using Scherrer's formula [23-25].

Equation

where D is the average crystallite size in nanometers, k is a constant or shape factor and equal to 0.9, is the wavelength of the radiation (0.154 nm), AY is the peak width at half maximum intensity and is the peak position. The calculated average crystalline size was found in the range of 23-55 nm for all powders.

FTIR Studies

The infrared spectra of BCO and Zn doped powders calcined at 800 oC are shown in Fig. 2.

FTIR of pure BCO (Fig. 2a) exhibited narrow peaks showing maximum interaction of barium and cerium with oxygen. The bands located from 452 cm-1 and 858 cm-1 indicated the presence of Ce-O vibration and Ba-O vibration, respectively [26, 27].

In FTIR spectra of BCZO-1 (Fig. 2b) and BCZO-8 (Fig. 2c), the broad bands located at 722, 728, 1350, and 1540 cm-1 showed the existence of Zn-O vibration [28, 29]. The FTIR of all doped barium cerates were broad indicating the interaction of zinc with oxygen in place of cerium in the perovskite lattice. All other synthesized samples with x from 0.04 to 0.14 showed similar FTIR spectra as that of minimum zinc doped barium cerate. The other bands located between 3000 and 3500 cm-1 corresponded to the O-H stretching which could originate from measurement environment.

Cyclic Voltammetric Investigation

To investigate the electrocatalytic activity, the methanol oxidation was studied using cyclic voltammetry in an assembly combining direct methanol fuel cell (DMFC) and PEMFC strategies. The catalyst material was drop-casted onto Pt active area and coated with Nafion (a proton conductor polymer) thus formulating a PEMFC set-up.

Methanol was used as the direct fuel (as analyte) for cyclic voltammetric data analysis of its electro- oxidation thus leading to a false-DMFC type of cell testing. The results are presented here for BCO and maximum Zn-doped sample, BCZO-8. The Pt working electrode was modified with BCO or BCZO and kinetic and thermodynamic parameters were evaluated from the effects of scan rates, temperatures and concentration on the voltammetric profiles Voltammetric Profiles for Scan Rate Effect Electrochemical oxidation of methanol was studied while keeping its concentration constant by varying the scan rates from lower values like 10 through 100 to higher value up to 1000 mV s-1 on modified Pt working electrode in a positive potential range from 0 to 0.8 V and reversing the scan direction in a cyclic voltammetric manner. The voltammetric profile of methanol oxidation at different scan rates is shown in Fig. 3.

All the BCO- and BCZO-modified electrodes could show the stable and steady voltammograms for methanol oxidation under the given set of experimentation.

The diffusion coefficient demonstrates the facility of the diffusion of the analyte towards the working electrode and of the resulting charge transfer process. Thus an estimate of it can lead to an understanding the role of the active material (BCO or BCZO in present case) in the electro-kinetics of the analyte. To calculate the diffusion coefficient for methanol redox system, the Randles-Sevcik equation was used [20]:

Equation

In this equation, Ip is the peak current (A), is the scan rate (V s-1), C is the concentration (mol cm-3), A is the area of electrode (cm2) while Do it the diffusion coefficient in cm2 s-1. From the slope of plot of Ip vs. 1/2, Do was calculated.

The potential of BCO and its doped analogues is demonstrated in the facile kinetic parameter that is the higher peak currents with the scan rates (Fig. 4a) and the resultant diffusion coefficient. The diffusion coefficient of methanol was found to be 1.44x10-10 cm2 s-1 and 7.16 x10-11 cm2 s-1 using BCO and BCZO-8 modified electrodes, respectively. The obtained values in this study were found to be more than literature reported value (in order of 10-12 cm2 s-1) [30]. Zinc doping is manifested in the mechanical and chemical stability of the BCO samples as these were found stable in an environment of acidic oxidation of methanol and the temperature effect up to 50 oC could not disturb the measurement system.

The electroxidation of methanol was facilitated using the zinc doped barium cerates which are the advanced materials for future technologies such as fuel cells. The higher diffusion coefficient of methanol using BCO could be due to the availability of more active sites at BCO-modified platinum electrode for adsorbed species to oxidize into CO2 in comparison to zinc doped samples.

Voltammetric Profiles for Concentration Effect

To investigate the effect of methanol concentration on its electro-oxidation, cyclic voltammograms were taken at 100 mV s-1. Fig. 4 represents the effect of methanol concentration on its voltammetric profile using BCO modified Pt electrode. One could see the increase in peak current with the increase in methanol concentration (up to 2 M). However, on increasing the concentration of methanol beyond 2 M, the peak current was decreased. The result could be attributed to the blocking of the electrode sites due to more adsorbed intermediates which then inhibit the oxidation of methanol [4, 31].

The reverse scans also showed enhanced peak currents with methanol concentration which case may be attributed to the fast and feasible oxidation of the adsorbed CO type species [32]. Overall redox processes seemed to be diffusion controlled and facilitated due to the proton conducting nature of doped or undoped barium cerate materials [6, 7].

Similar results were obtained for BCZO-8 modified electrode, however the peak current obtained was a little less as compared to that for BCO modified electrode as could be seen from the graph showing comparative peak currents (Fig. 4b).

The catalytic properties of the materials can be invoked by observing the heterogeneous kinetics involved in the redox processes. The suitability of the synthesized barium cerates was probed for the electro-oxidation of methanol. The better value of the standard heterogeneous rate constant manifests the facilitated electron transfer process and it would demonstrate the electrode material activity that is catalysis. Reinmuth equation was used to estimate the standard heterogeneous rate constant, ksh [33]:

Equation

In this equation, ksh is the heterogeneous rate constant while F is the Faraday's constant. Using Reinmuth plots, the value of rate constant, ksh for methanol oxidation could be obtained from the slopes of the linear graphs between Ip and methanol concentration as 0.45x10-5 cm2 s-1 and 0.51x10-5 cm2 s-1 using BCO and BCZO-8 modified Pt electrodes, respectively, Fig. 5:

Although the rate constants are close to each other using either of the modified electrodes, the facile kinetics was obvious for the electro-oxidation of methanol which is the direct fuel source for PEMFC technology. These results demonstrated the suitability of the BCO and BCZO materials as electrocatalysts.

Voltammetric Profiles for Temperature Effect

The voltammetric profiles of the electro- oxidation of methanol were also observed in a low temperature range, from 25 to 50 C which is a working range for PEMFCs. The increasing temperature was found to cause a significant increase in the anodic peak current in all the systems (only two samples are shown here). This indicated that the increasing temperature can enhance the rate of electro-oxidation of methanol by facilitating faster electron transfer.

Fig. 6 shows the temperature dependence of methanol oxidation using modified electrodes.

From the temperature variation study, we can deduce that the electro-oxidation is facilitated with increasing temperature and the cell system remains stable in acidic medium. The fortifying back current in these CV profiles also manifested the occurrence of various electrochemical reactions at the electrodes which produce many intermediate species that block the active site of electrode surface when adsorbed. The intermediate species formed in methanol oxidation process can be removed and/or desorbed by increasing temperature providing fresh surface which can contribute to the enhancement of peak current.

Moreover, this pronounced EC behaviour can also be attributed to decrease in viscosity (thus more diffusion) at higher temperature. The temperature variation in low temperature range under the given set of experimental conditions, again hints clearly towards the effectiveness of the barium cerate materials for PEMFC applications.

Arrhenius plots of logarithm of heterogeneous rate constant (ks) versus the reciprocal of temperature (T1) provided the apparent activation energy, Ea, Fig. 7.

Activation energies for the electro-oxidation of methanol were found to be 0.44 eV and 0.34 eV using BCO and BCZO materials as electrocatalysts, respectively. The low value of Ea in case of BCZO-8 can be correlated to the high values of peak currents obtained in this case (compare Fig. 6a and 6b). The increase in peak current with temperature on BCO or BCZO modified electrodes manifests an enhanced conductive behavior and finds application in intermediate temperature fuel cells.

Evaluation of Thermodynamic Parameters

Different thermodynamic parameters were also deduced for the electro-oxidation of methanol using BCO and BCZO materials as electrocatalysts, with the help of Marcus equation [3438]:

Equation

(eq.) at various temperatures can be derived using the following equation.

Equation

In above equation, M stands for the molar mass of the analyte (methanol in this case). Equation 3 can be re-arranged as

Equations

Equation 8 is a straight line equation and gives value of (Eq.) from the slope of the plot of ln [ks/Zhet] vs. 1/T while (eq.) can be estimated from the intercept. The respective plots are shown in Fig. 8 for the electro-oxidation of methanol using both BCO and BCZO modified Pt electrodes. The obtained values of H#, S#, and G#, are tabulated in Table 1.

Table-1: Thermodynamic data for the electro- oxidation of methanol on modified electrodes.

###Modified Electrodes###H####S####G#

###(kJ mol-1)###(J mol-1K-1)###(kJ mol-1)

###BCO/Pt###43.48###33.75###32.58

###BCZO-8/Pt###30.94###-10.30###34.27

The observed positive values of G#, show the electro-oxidation of methanol to be non- spontaneous and endothermic in all cases.

Morphological Investigation

SEM analysis of calcined powders of BCO, BCZO1, BCZO3, and BCZO8 synthesized by co- precipitation method was carried out at different resolutions to study their morphology.

The SEM morphological studies of synthesized powders showed fine homogeneity. From these images, it was also confirmed that BCO, BCZO1, BCZO3 had less porosity (and more density) and less agglomeration of particles compared to that of BCZO8. At zinc doping level of 16 mol% (BCZO8), ZnO may have formed additional grain boundaries as also indicated by more porosity.

Conclusion

Barium cerate and zinc doped barium cerates of different zinc contents were prepared by co-precipitation method and investigated by using XRD, SEM, FTIR techniques and catalytic activity towards methanol oxidation in acidic medium was done by using cyclic voltammetry. The effect of dopant incorporation into perovskite lattice of the calcined powder was studied. SEM images displayed fine homogeneity of the particles. With increase in concentration of zinc, agglomeration occurred. A single orthorhombic phase perovskite was obtained for BaCeO3 (BCO) indicated by XRD.

The electrochemical oxidation of methanol by modifying Pt electrode with BCO and BCZO-8 was found to be irreversible giving only anodic peaks at 0.4 0.5 V. The current increased almost 10 times on increasing scan rate from 100 to 1000 mV s-1 with multistep followed by re-oxidation of the adsorbed intermediate species on the working electrode. The shift in potential from more positive to less positive ensured the irreversibility of methanol oxidation.

The diffusion coefficient for methanol oxidation using BaCeO3 was higher as compared to zinc doped BaCe1-xZnxO3 due to comparative more availability of active sites for adsorption of intermediate species on the modified platinum electrode. The increasing temperature and analyte concentration (up to 2 M) increased peak current due to more diffusion and enhancement in electro-oxidation rate. The methanol oxidation assembly was stable over the entire methanol concentration, the scan rates, and the working temperature range. This study is the first time report for such an application of the perovskite materials in PEMFC conditions. The cell stability and resulting catalytic activity of the used perovskite materials pointed strongly to their potentiality as electrode materials in fuel cell technologies [6, 18, 22, 39-41].

Acknowledgement

The corresponding author is thankful to Quaid-i-Azam University Islamabad for experimental facilities and HEC for the Project# 20-1718/R and D/2010 (Gamry workstation).

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