Electrocatalytic Reduction of Oxygen at Perovskite ( BSCF )-MWCNT Composite Electrodes

A composite paste electrode based on Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF)—initially synthesized by solgel method—and multiwall carbon nanotube (MWCNT) as a cathode in fuel cells is developed. The composite pastes are prepared by the direct mixing of BSCF:MWCNT at 90:10, 80:20 and 70:30 (% w/W). These electrodes are then characterized by the x-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherm, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The XRD and SEM confirm the inclusion and the uniform dispersal of the MWCNT within BSCF, respectively. The nitrogen adsorption isotherm study shows that the porosity of the composite paste electrode has been improved by two-fold from the BSCF electrode. The EIS and CV demonstrate that the higher ratios of MWCNT in the composites are critical in improving the electronic conductivity as well as the kinetics. It is also noticeable that the electrode has increased the catalysis of oxygen in 0.1 M KOH (pH 12.0). Cyclic voltammetric studies on the oxygen reduction reaction (ORR) suggest that the incorporation of MWCNT is vital in improving the electrode (cathode) properties of a fuel cell.


Introduction
Although the oxygen reduction reaction (ORR) is important in variety of electrochemical processes including corrosion inhibition and metal-air batteries, the application of the ORR in fuel cells is of particular interest [1]- [3].In the fuel cell the electro catalytic ORR at various electrode materials have been reported [4]- [10].The electrode process may involve either 2 or 4 electrons depending on the electrode material used [11]- [14].This reaction, however, requires the use of platinum as the catalyst, which then contributes to the high cost of these fuel cells.Highly active transition-metal-oxides such as perovskite-type oxides, ABO 3 , have been used as catalysts in fuel cells [15] [16] and are operable at room temperature [17] [18].By improving its electronic properties, the perovskite materials can be used as catalyst for low temperature fuel cell and metal air-batteries [19] [20].
The Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF) is known to have a high oxygen vacancy concentration.It has exhibited the highest oxygen evolution reaction (OER) activity [21].This is due to the high occupancy of the 3d electrons with an e g symmetry of the transition metal cations in an oxide.Accordingly, BSCF has been proposed as the electrodes in rechargeable metal-air batteries and regenerative fuel cells.However, the application of BSCF for the ORR in aqueous media at room temperature has never been investigated.
It is generally accepted that nanomaterials with high surface area could maximize the surface activity to improve the reaction kinetics of electrode materials, and afford a short distance for ionic mass and charge transportation, which resulted in higher electrochemical reaction.Multiwall carbon nanotube (MWCNT) is known to facilitate the electron transfer process especially in biosensor fabrications [22] [23].MWCNT has also been employed as a useful material to modify electrode surface in order to enhance the electrochemical response [24]- [27].The combined effects of the high specific surface area and good electrical conductivity make MWCNT as a good choice as an electrode material [28] [29].
To the best of our knowledge, the composite of BSCF and MWCNT has not yet been exploited as an electrode for ORR purposes.This report describes the preparation and characterization of BSCF and its composite (BSCF-MWCNT) electrodes and also their applications for ORR at the room temperature.
X-ray diffraction (XRD) measurements were performed using an X-ray diffractometer, Siemens-D5000, Germany.The surface morphologies of the samples were characterized by the field emission scanning electron microscopy (FESEM) LEO SUPRA 55VD ultra-high resolution of Zeiss, Germany.The Brunauer-Emmett-Teller (BET) surface area and porosity properties were measured using multi point BET Quantachrome surface area analyzer of NovaWin 32, USA.Electrochemical measurements were carried out on Potentiostat/ Galvanostat EG & G Model 273A (Princeton Applied Research, USA) complete with the Power Suite program.Platinum wires and an Ag/AgCl (3.0 M KCl) served as the working, the auxiliary and the reference electrodes, respectively.Electrochemical impedance spectroscopic (EIS) measurements were carried out using an EG&G Frequency Response Detector 100 with ZSimpWin 3.22 software of Princeton Applied Research, USA at a frequency range of 100 kHz to 100 mHz and AC voltage amplitude 5 mV.

BSCF and MWCNT Powder Preparation
The BSCF precursor powders was prepared using a sol-gel method which was adapted from the literature [30].The powders produced was then calcined at 1000˚C and ball-milled for 10 hours.The MWCNT was purified by the sonication in 6 molL −1 HCl for 4 h.Later, 50 mg of purified MWCNT was dispersed in a 40 mL mixture of H 2 SO 4 and HNO 3 (v:v = 3:1) prior to the further sonication in a water bath for 7 h at 40˚C.After cooling upto an ambient temperature, the MWCNT was centrifuged and washed to neutral.This pretreatment was subsequently repeated over and again until all the acids are removed.The final solid was then dried to a constant weight.

Electrode Preparation
BSCF-MWCNT composite paste electrode was prepared by hand-mixing BSCF powder and functionalized MWCNT powder with 90:10, 80:20 and 70:30 (w:w) ratio.Appropriate amount of the mineral oil was added and served as a binder.The mixture was homogenized thoroughly prior to packing into a Teflon tube (4 mm id).A copper wire was then inserted into the opposite end of the cathode to establish an electrical contact.The surface was smoothed on a weighing paper.For comparison purposes, the BSCF and MWCNT paste electrodes were also prepared using the same method.

Procedure
The surface area of the modified, pure BSCF and MWCNT electrodes were analyzed by cyclic voltammetry in the applied potential region of −0.2 to 0.8 V vs Ag/AgCl in ferricyanide solutions at various scan rates.For ORR activity, cyclic voltammetric experiments were carried out in 15 mL 0.1 M KOH which has been deaerated by purging N 2 gas into the solution for 10 min.Later, the similar experiments were done by purging O 2 gas for 10 min.These were scanned several times in the applied potential range of −1.1 to 0.2 V to obtain the stable responses.The aqueous solutions at different pH values were prepared; 0.1 M H 2 SO 4 (pH 1.0), 0.1 M H 2 SO 4 + 0.1 M KOH (pH 2.0 -3.0), 0.2 M CH 3 COOH + 0.2 M CH 3 COONa (pH 4 -5), and 0.1 M NaH 2 PO 4 + 0.1 M KOH (pH 6.0 -12.0).The pH of the solutions were adjusted by adding suitable quantities of an acid or alkali and checking the pH of the medium using a pH meter.

Phase Determination
The structural characteristics of the as-prepared BSCF-MWCNT composite have been examined via XRD (Figure 1).The single-phase of the pure BSCF is identified without any noticeable second-phase peaks (Figure 1(a)).It is indexed using a cubic ABO 3 perovskite-type structure and is in good match with the published data [31] [32].The BSCF has high crystalinity as shown by the sharp peaks present.However, with the addition of MWCNT, the peaks become broader indicating transformation to polycrystalline structure (Figures 1(c)-(e)).This happens due to the scattering of X-rays in many directions leading to a large bump distributed in a wide range of 2θ.A new peak is detected at a 2θ value of 25.8˚ which is assigned to (002) diffractions corresponding to the interlayer spacing of the nanotube.This proves the presence of MWCNT and is in good agreement with the previous work [33].
It is noticeable that a higher ratio of MWCNT used in the composite results in a (002) peak with higher intensity.It is also observed that the intensity of BSCF diffraction peaks decreases with the increases of the mass ratio of MWCNT to BSCF.A small shift of the BSCF peaks at 2θ of 47.2˚ (100) to a lower diffraction angles (45˚) have been observed which is due to the incorporation of the MWCNT into the composite structure.The peak is attributed to the 101 diffraction of functionalized MWCNT.The XRD pattern of the functionalized MWCNT (Figure 1(b)) also confirms the assignment of this peak.The BSCF-MWCNT crystallite sizes of 174, 165 and 149 Å are obtained for the BSCF-MWCNT ratio of 90:10, 80:20 and 70:30, respectively.As the BSCF's actual crystallite size is 659 Å, the reduction in the crystallite size can be attributed to the presence of functional groups on the MWCNT surface which hinders the crystallization process.

Surface Morphology
The method of preparation of ceramic material is known to affect the particle size, structure, porosity and connection between the particles.The sol-gel method is more preferred for obtaining the porous particles.The synthesis produces homogeneous material composed of uniform particles.An electrode of sufficient porosity is important for ORR to allow rapid transport of the gaseous reactants and to provide abundant sites for the electrochemical reactions.
The BSCF formed from the sol-gel synthesis route are coarse and round-shaped with the particle diameters of >5 μm (Figure 2(a)).High sintering temperatures (T > 1000˚C) during the synthesis of BSCF resulted in a decreased surface area.The incorporation of MWCNT resulted in the fibrous-like particles with a higher surface area (Figure 2(c)).This result is further confirmed by the nitrogen adsorption study.The MWCNT tend to 2 theta  homogenously attach on the external surface of the BSCF and dispersed uniformly without an aggregation.The morphology can be described as a uniform layer of MWCNT covering the BSCF surface completely.This suggests that the direct mixing is sufficient in dispersing the MWCNT within BSCF.A uniform dispersion ensures a non-disrupt conduction within the electrode.Figure 2(d) indicates the successful modification of the BSCF with MWCNT in forming a conductive porous matrix.A porous fibrous network of composites MWCNT is beneficial for the electrolyte access to the active material [34].

Adsorption Study
The porosity of the composites is further studied by nitrogen adsorption-desorption isotherms.The measurements were carried out at 77 K with a subtracting pore effect (SPE) method using high-resolution αs plots.In Figure 3(a), the BSCF exhibits type-IV Brunauer-Emmett-Teller (BET) sorption characteristics with IUPAC type H3 hysteresis loop.This is the characteristic of the solids with aggregates or agglomerates of particles forming a slit shaped pores with non-the uniform sizes and shapes.The presence of the micropores is expected from the sudden increase at P/P o < 0.01.However, after the addition of MWCNT (Figure 3(b)) the isotherm changes to type II with H1 hysteresis loop together with a higher adsorption volume, which qualitatively translates to an increase of pore volume in the BSCF-MWCNT composites.The H1 hysteresis loop arises due to the capillary condensation of the gas in the mesopores, indicating the transition from the adsorption to the condensation of nitrogen near the saturation vapor pressure [35].
It also implies to the cylindrically shaped pores.This shows that the presence of MWCNT changes the sorption behaviour of the composites.The hysteresis on isotherm curves of the BSCF and BSCF-MWCNT composites suggest the mesoporous structures with lower BET area for BSCF.The larger hysteresis loop of the composites when compared to the BSCF alone indicates an enhanced mesoporosity.
A BSCF-MWCNT composite shows a similar adsorption type to MWCNT as reported [36] [37].The gradual uptake of nitrogen at the medium (P/P o ) and an increase in the adsorption of nitrogen at higher P/P o by BSCF-MWCNT composites can be associated with the multilayer adsorption on the external surface and in the larger mesopores formed in the spaces between the particles [38].The nitrogen adsorption of the BSCF at P/P o < 0.8 may be inferred to the existence of smaller pore volumes.
The pore size distributions of BSCF (Figure 3(c)) and BSCF-MWCNT (Figure 3(d)) composites are calculated using the Barrett-Joyner-Halenda (BJH) analysis of the desorption branch.The mesopore volume was calculated by subtracting the micropore volume from the total pore volume, which was determined from the amount of nitrogen adsorbed at P/P o = 0.98.The BSCF has a wider pore size distribution i.e. 1 -10 nm which may be contributed to the intra-particle cavities.After modification with the MWCNT, the peak intensity increases with the decrease in the pore size distribution i.e. 1 -4 nm, suggesting a more uniform pore size.The BJH analysis for the pore size distributions further indicate that the BSCF and BSCF-MWCNT are essentially made up of mesopores.
Information on pore parameters is summarized in Table 1.The specific surface areas (S BET ), calculated by the BET equation, increase with the presence of MWCNT in the samples.The increase in S BET for BSCF-MWCNT composites is mainly due to the increase of the micropores surface area.However, the Langmuir surface areas indicate a slightly different result since it is calculated on an area covered by one layer of gas.BSCF-MWCNT composites have a greater tendency of adsorption as the strength of a porous material depends directly on the total pore volume.

Electrochemical Impedance Analysis
EIS measures the impedance of a system (Z) as a function of frequency of an applied perturbation.Nyquist plot (imaginary component (Z im ) vs. real component (Z re )) explains the frequency dependence of electrode/electrolyte in the system.It combines three components, namely the electrolyte resistance between working and reference electrodes (R e ), the double-layer capacitance (C dl ) and the Faradaic impedance due to the charge-transfer process (Z f ) at the working electrode-electrolyte interface.Z f is normally subdivided into a charge-transfer resistance (R ct ) and the so-called Warburg Impedance (Z w ), which reflects the influence of the mass transport of the electroactive species on the total impedance of the electrochemical cell.Figure 4(a) indicates that charge transfer resistance (R ct ) is only predominant in BSCF.It decreases as the amount of MWCNT is increased in the composites.Overall, the electrode process of the composite paste electrode is a diffusion limiting process.However, for MWCNT at lower frequencies, the imaginary part of the impedance increases and approaches that of an ideal capacitor.A slight variation from the ideal capacitive behaviour could be attributed to the pore size distribution of MWCNT.The loading of the BSCF has increased the conductivity of the composite where the imaginary part of the impedance for BSCF-MWCNT composite has lower value than of MWCNT.
Figure 4(b) shows that BSCF-MWCNT 70:30 has almost ideal capacitor behaviour, often known as a constant phase element (CPE) which is caused by the porosity of the electrode [39].On the contrary, the BSCF electrode shows resistive behaviour at low frequencies which changes to capacitive behaviour at high frequencies.The BSCF-MWCNT composites exhibit capacitive behaviour at all frequencies with the BSCF-MWCNT of 70:30 w/W is the best.Figure 4(c) clearly reveals the presence of two distinguishable frequency-dependent segments on BSCF-MWCNT composites suggesting a two-step reaction closer to each other.The high-frequency region can be related to the fast charging/discharging process at the electrode/electrolyte interface, and the low-frequency region corresponds to the slow reaction process [40].Therefore the BSCF-MWCNT composite paste electrodes have two CPEs.In Randles equivalent circuit (Figure 4(d)) the BSCF and BSCF-MWCNT composites electrodes are fitted with the model where the capacitors have been replaced with CPEs.The best fit circuit changes from R(C(RW)) for BSCF to R(Q(RW))(CR) for the BSCF-MWCNT electrode.The electron transfer processes at the interfacial layer of BSCF-MWCNT are facile since the apparent standard rate constant (k app ) value [41] is greater by almost three fold (Table 2).This also shows that R ct of BSCF-MWCNT decreases as more MWCNT is added into the composites.Though the decrease in R ct is obvious for all the modified electrodes, the value seems less ideal compared to the MWCNT due to the imperfect coverage of the MWCNT in the composites.The bulk solution resistance (R s ) is found to be small and shows little variation for all the electrodes.Meanwhile, there is no variation in the value of non ideality of the capacitor, n, which indicates the metal-solution reaction near to ideal capacitor.Therefore, it can be concluded that the MWCNT has enhanced the rate of the [Fe(CN) 6 ] 4−/3− in BSCF-MWCNT.By using Equation (1) [41], the respective surface coverage (θ) of the BSCF-MWCNT (90:10), BSCF-MWCNT (80:20) and BSCF-MWCNT (70:30) paste electrodes are 0.41, 0.58, and 0.62.This shows that the composites with a higher ratio MWCNT are better.modified electrode bare electrode Exchange current (i 0 ) is also used as a measure of the rate of electron transfer on BSCF and BSCF-MWCNT composites electrode.The i 0 of electrode system is given by Equation ( 2) where R, F and n are gas constant, Faraday constant and number of electron transferred, respectively.The i 0 values for the redox reaction of [Fe(CN) 6 ] −3/−4 on BSCF, BSCF-MWCNT (90:10), BSCF-MWCNT (80:20) and BSCF-MWCNT (70:30) are 9.83 × 10 −5 , 1.87 × 10 −4 , 2.44 × 10 −4 , and 2.52 × 10 −4 , respectively.It can be deduced that the rate of electron transfer is faster with the presence of MWCNT.Hence, the catalytic effect is due to enhanced surface area and conductivity.

Cyclic Voltammetry
There is no obvious redox by [Fe(CN) 6 ] 3− shown at BSCF electrodes but the cyclic voltammogram (CV) indicates that there is a capacitive behaviour (Figure 5(a)).But, the gradual increase of peak currents (I p ) as the increase of MWCNT loading in the composite paste electrodes implies that the catalytic affects as well as kinetic properties of the electrode.This is due to larger specific surface area in the presence of MWCNT, which provides sufficient reaction sites for enhancing the rate of the electron transfer.Peak separations (ΔE p ) for all composites prepared have converged to the theoretical value (59 mV) according to the Nernst equation which shows excellent reversibility.The BSCF-MWCNT (70:30) has exhibited the most reversible among these electrodes even when compared to the pure MWCNT.To further investigate the characteristics of BSCF-MWCNT/GCE electrode surface, the effect of scan rates on the voltammetric behaviour ofBSCF-MWCNT (70:30) is examined.The results obtained (Figure 5(b)) show the redox peak currents increase linearly with scan rates, which indicates that the redox process is confined to the surface of BSCF-MWCNT (70:30) electrode, confirming that the electrode is stable.The redox peak currents show linear behaviour with the square root of the scan rate, ν 1/2 (Inset: Figure 5(b)) revealing that the electron-transfer process is a diffusion controlled [42].Hence it is obvious that the BSCF-MWCNT (70:30) electrodes possess faster chargetransfer kinetics which is attributed to the presence of MWCNT as a modifier.

Oxygen Reduction Behaviour
From  mV on the BSCF-MWCNT (70:30) as compared with that for BSCF electrode.Moreover, the cathodic peak current (I pc ) has increased four fold which is attribute to the larger specific surface area caused by the higher surface-to-volume ratio.Two irreversible peak currents are observed at MWCNT electrode.The I pc at the low overpotential is attributed to 2e − reduction to H 2 O 2 and at more negative potentials the peroxide intermediate is further reduced.However, for BSCF and BSCF-MWCNT (70:30) electrodes, only one peak is observed.Such catalytic effect can be explained by the high affinity of the BSCF towards oxygen.
The effect of pH on the oxygen reduction reaction (ORR) potential at BSCF-MWCNT (70:30) is shown (Figure 7).There is no significant reduction observed at pH 1 which probably due to the decomposition of O 2 .However, as the pH is increased, reduction peak potential (E pc ) of O 2 is shifted positively and the I pc increases.
Only one E pc is produced in an acidic media indicating irreversibility.On the contrary, there are two pairs of redox peaks observed in a basic media.Overall, the reduction peaks are more dominant with the smaller peak become less important due to its more negative value and usually bypassed by catalysing the 2 HO − decomposition.Depending on the electrode material and its surface properties as well as the solution pH, two steps for ORR are anticipated; it is either via a direct four electrons (Equation ( 3)) or an indirect two step two electrons mechanism leading to water (Equations ( 4) and ( 5)): The larger peak is attributed to the reduction of oxygen to hydrogen peroxide and the minor peak is the reduction to water.The change in the redox peak potential that occurs with the change of pH is attributed to the reaction involving the surface adsorbed species.As the electrocatalysis is minimized as the pH is increased (with less H + available), it is concluded that the mechanism of ORR is a H 3 O + -dependent process in an acidic medium and is predominantly OH − based in alkaline medium.Generally, the best catalytic activity is observed in pH 12.
The CV of the BSCF-MWCNT (70:30) in the presence and absence of oxygen is included in Figure 8(a).It shows that there is a large enhancement of I pc at E pc = −0.37V in the presence of O 2 .A linear relationship in peak current vs the square root of the scan rate (Figure 8 detached from the BSCF-MWCNT electrode.

Conclusion
A simple BSCF-MWCNT electrode has been prepared and optimized for ORR at pH 12.It has exhibited an excellent electrocatalytic activity towards ORR at the room temperature.The higher ratio of MWCNT in the composites is vital for better electrocatalysis.The MWCNT, the modifier material, has improved the performance of the BSCF on its charge-transfer, conductivity, oxygen adsorption and reduction.The application of BSCF-MWCNT electrode as a cathode for ORR in a fuel cell is then proposed.This is the subject for the next report.

Table 1 .
The pore parameters of the electrodes.