Silver-montmorillonite-conducting polymer composite materials as low-cost oxygen reduction catalysts

: Throughout the globe, researchers are in quest of electro-catalysts for oxygen reduction reaction (ORR) in order to develop cathode materials for fuel cells (FCs). Although the anodic half-reactions of FCs are fast enough for oxidation of any fuel on cheap catalysts, ORR at the cathode is a slow process even with expensive platinum-based catalysts. Development of low-cost electro-catalysts with reasonably fast kinetics for (ORR) is desirable if fuel cell power is to be utilised in practice. The present study is based on the preparation and characterisation of silver-based electro-catalysts towards ORR. Simple chemical processes were developed to prepare three different electronically conducting nano-composites. Aniline, pyrrole or 3,4-ethyldioxythiophene (EDOT) in between the layer space of silver [Ag (I)] ion exchanged montmorillonite (MMT) undergoes spontaneous polymerisation. During the polymerisation process, Ag(I) cations are reduced to form metallic silver, while aniline, pyrrole or EDOT monomers are converted to polyaniline (PANI), polypyrrole (PPY) and poly (3,4-ethylenedioxythiphene) (PEDOT), respectively. The prepared composites were extensively characterised by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), X-ray photoelectron spectroscopy (XPS), conductivity measurements, AC-impedance and electrochemical analyses. Electrochemical studies showed that composites are good candidates towards ORR in alkaline electrolyte medium. Ag/MMT/PEDOT showed the best performance in catalytic properties. All three composites showed better performances than that of typical Ag/C composites with a similar mass loading of silver. Although the performances of the cheap composites are somewhat lower when compared to commercially available Pt/C, the performance is sufficient to use the materials as cheap alternatives for FC cathodes.


INTRODUCTION
The foreknown usable energy crisis is an important problem for the present day scientists to pursue solutions. Fuel cell (FC) is a good alternative energy source that can efficiently generate electricity by consuming fuels and oxygen. FC has fuel flexibility. Hydrogen is the cleanest fuel with highest energy density in terms of mass. Both hydrogen and oxygen can be synthesised by water splitting using photo-catalytic processes and fuels like methanol can be produced from bio-degradable garbage. This means that solutions for the usable energy crisis can be sought from our own surroundings instead of depending on crude oil. However, there are some drawbacks in FC systems. The major problem is the cost of production due to expensive platinum-based catalysts used for both electrodes, particularly, as the cathode for oxygen reduction. Reduction of platinum content is an important step forward in reducing the cost of FCs. Cutting edge research on platinum alloys or platinumfree alloys together with other transition metals are highly active and these catalysts have performed better than naked platinum as revealed by many publications (Wang et al., 2012). Moreover, nano-particles of metals or metal alloys have been prepared with supporting materials to increase the surface area and to minimise metal loading (Chung et al., 2013;Perini et al., 2015).

December 2019
Journal of the National Science Foundation of Sri Lanka 47 (4) Abundance of silver on Earth crust is around 0.08 ppm (John, 2001), which is 25 times higher than that of Pt and around 27,300 tonnes had been produced for world market in the year 2015 (George, 2016). Hence, silver is considerably cheaper and more abundant compared to Pt (Holewinski et al., 2014). Moreover, silver-carbon black and silver-polypyrrole composites have been synthesised and used as ORR electrocatalysts (Singh & Buttry, 2012;Senarathna et al., 2015). MMT is a good catalyst due to its structural arrangement in gas trapping and better binding properties (Senarathna et al., 2016). Hence, MMT conducting polymer (CP) composites have been used as catalyst for ORR and there are several publications reporting their activities towards ORR (Rajapakse et al., 2010;Senarathna et al., 2016).

Purification of bentonite
Bentonite clay was purified by the procedure given below. Twenty grams of bentonite clay was suspended in 500 mL distilled deionised water by shaking for 12 h at room temperature to remove water soluble impurities. The dispersion was centrifuged and the supernatant was discarded. The remaining solid was washed two more times by repeating the same procedure. The solid was then suspended in 1.0 M HNO 3 and the above procedure was repeated three times. The solid mass obtained was then suspended in 6 % H 2 O 2 and the procedure was repeated thrice. This has resulted in pure MMT with H + Substitutes for cations in interlayer space (Zhao et al., 2016;Senarathna & Rajapakse, 2018).

Preparation of Ag(I) ion-exchanged montmorillonite [Ag(I)/MMT]
AgNO 3 (1.71 g) was dissolved in 0.10 mol dm -3 HNO 3 solution (10.0 mL) in a 100 mL volumetric flask and diluted up to the mark to prepare 0.10 moldm -3 Ag + (aq) solution. Purified MMT (0.50 g) was added to the prepared 0.10 mol dm -3 Ag + (aq) solution (10 mL) and stirred for 48 h. The suspension obtained was centrifuged and the slurry of Ag(I)/MMT was collected by discarding the supernatant. The solid mass was washed thrice with distilled water, dried under ambient conditions and powdered in an agate mortar and pestle.

Preparation of Ag/MMT/conducting polymer composites
For the preparation of these types of composites, the amount of silver nitrate was selected so that the Ag content is equal to ~20 % w/w of the total mass of the final product. Then, the amount of monomer was selected so as to have 1:1 molar ratio of Ag: monomer.

Preparation of Ag/MMT/PANI and Ag/MMT/PPY composites
Purified MMT (0.30 g) was added to a solution prepared by mixing 0.10 M Ag(I) solution (10.0 mL) and 0.10 M HNO 3 (10.0 mL). Mixture was sonicated for 2 min and stirred for 48 h. A 0.10 M solution of aniline in 0.10 M HNO 3 (10.0 mL) was added and the mixture was again sonicated for 15 min. Then the reaction mixture was stirred again for 24 h and left for three weeks to allow for the polymerisation of aniline to PANI while shaking it once a day to re-disperse the sediments formed. A green coloured Ag/MMT/PANI composite sample was formed in the container. The composite was separated by discarding the supernatant after centrifugation. It was washed with 0.10 M HNO 3 solution first and then with acetone. The sample obtained was allowed to dry under ambient conditions and powdered in an agate mortar and pestle (theoretical silver content ~21.4 % w/w). The same procedure was followed with 0.10 M pyrrole in 0.10 M HNO 3 solution (10.0 mL) instead of aniline for the preparation of Ag/MMT/PPY composite. The composite was black in colour (theoretical silver content ~22.7 % w/w). The same procedure was followed with EDOT (110 mL) in ethanol (20.0 mL) instead of aniline and blue coloured Ag/MMT/PEDOT composite was obtained (theoretical silver content ~ 19.6% w/w).

Synthesis of Ag/C composite of 20 % w/w Ag
To prepare the Ag/C composite, Vulcan XC-72R (0.430 g) was added to a 0.05 mol dm -3 Ag + (aq) solution (10 mL). Mixture was sonicated for 2 min and then stirred for 48 h. Sodium borohydride (0.11 g) was added and the mixture was again sonicated for 15 min. Then, the reaction mixture was stirred again for 5 h and the black coloured sample formed was separated by discarding the supernatant after centrifugation. It was washed with water and then with acetone. The sample was allowed to dry under ambient conditions and powdered in an agate mortar and pestle (theoretical silver content ~20.0 % w/w).
Journal of the National Science Foundation of Sri Lanka 47(4) December 2019

Characterisation of products
XRD analysis was carried out to identify the crystalline phases of both raw materials and synthesised materials using Siemens D5000 X-ray powder diffractometer (Cu K α radiation λ = 0.154 nm/scan rate of 1° or 2° per minute /2θ range 3°-80°). Each sample was dried at 150 °C, for 2 h in order to produce its anhydrous form and the XRD analysis was carried out again (2θ range 3°-14°) at 150°C. Particle size, shapes of silver and morphological studies of products were carried out from Hitachi SU6600 Scanning Electron Microscope (SEM) [acceleration voltage =10 kV / Leo 1530 VP Field Emission Gun Scanning Electron Microscope (FE-SEM)]. FT-IR spectra were examined for KBr/ sample pellets (diameter = 13 mm, pressed at a pressure of 5 tonnes, mass ratio of sample : KBr is 1:40). XPS of synthesised materials were characterised using Axis Ultra DLD X-ray Photoelectron Spectrometer and the data were re-plotted and analysed using CasaXPS 2.3 and Origin pro 8.0 software. Powdered samples were pressed at a pressure of 7 tonnes to prepare pellets (surface area of 0.143 cm 2 /thickness was varied) to study the DC conductivity. Prepared pellet was sandwiched between two stainless steel rods and voltage of 0.1 V was applied to the two terminals and the current passed through the pellet was measured and conductivity was calculated using Ohms law. The four-probe techniques proposed by van der Pauw et al. was also used (Yao et al., 2012). To carry out the electrochemical studies, catalytic ink was prepared by separately dispersing Ag/PANI/MMT, Ag/PPY/MMT or Ag/PEDOT/MMT samples (10 mg each) in a solution formed by mixing ethanol (2.0 mL) and Nafion solution (10 mL). Then the mixture was sonicated for 10 min. The ink (10 mL) thus obtained was deposited on a cleaned and polished glassy carbon (GC) electrode of 0.384 cm 2 active surface area (Jia, 2014). This electrode was dried and used as the working electrode to run cyclic voltammogrammes (CVs) using Metrohm Potentiostat 101 and NOVA 1.7 software. The above procedure was repeated for the preparation of platinum carbon (Pt/C) electrode and silver carbon (Ag/C) electrode using commercially available Pt/C (20% w/w) and prepared Ag/C, respectively. Potentials were applied with respect to the saturated calomel electrode (SCE) and a Pt rod was used as the counter electrode. Scan rate used was 50 mV s -1 and 0.10 M KOH (aq) was used as the electrolyte solution. CVs were recorded in N 2 -purged and O 2 -saturated solutions. Then the electrodes prepared were used as the working electrodes to run a LSV at rotating speeds from 1600 rpm and 5 mV s -1 scan rate in oxygen saturated 0.10 M KOH (aq) electrolyte solutions. Polarizing curves were drawn using Nova 1.7 software. All graphs are re-plotted using Origin 8.0 Pro software (Fadley, 2010   room temperature. Thus, the XRD data provides indirect evidence to the exchange of H + by Ag + and also the formation of polymers in the interlayer spaces when monomers are introduced. This so happens due to the oxidative polymerisation of pyrrole or aniline by Ag + cations thus forming Ag(0) as shown in equations 1, 2 and 3. The presence of Ag(0) in Ag/MMT/PPY is verified by its XRD spectrum recorded in the full range of 2θ values from 0° to 80° which is shown in Figure 1

XPS analysis
High resolution XPS spectra of Ag 3d range for Ag(I)/ MMT, Ag/MMT/PANI and Ag/MMT/PPY compounds are depicted in Figure 2(a). In both Ag/MMT/PANI and Ag/MMT/PPY materials, Ag(I) ions were used to initiate the polymerisation process. Hence, PANI and Ag particles generate simultaneously during the reaction process. Binding energies of Ag 3d 5/2 and 3d 3/2 are 366.9 eV and 373.0 eV, respectively, for Ag(I)/MMT and theoretical values of Ag 3d 5/2 appears around ~368 eV. (Sivanesan et al., 2014) This deviation of binding energy is due to the Ag(I) being bonded to the MMT layers. In Ag/MMT/ PANI, binding energies of Ag 3d 5/2 and 3d 3/2 are shifted to 367.5 eV and 374.5 eV, respectively. These values are also different from those of naked silver metal, which appear at 368.2 eV and 374.2 eV, respectively. This may be due to the formation of dative bonds from N atoms of amine units of PANI with silver particles or ions in MMT layers. For Ag/MMT/PPY, both binding energies of Ag 3d 5/2 and 3d 3/2 are shifted to 368.2 eV and 374.2 eV, respectively, (Wei et al. 2010) as depicted in Figure 2(b). These values correspond to those of the silver metal, confirming the presence of Ag metal in the composite.  Figure 2(c) shows the XPS N 1s core-level spectra of Ag/MMT/PANI nanocomposite and it consists of five peak components of binding energies (BEs). Bands at BEs of 399.6 eV and 400.7 eV are attributed to the -NHand -NH 2 + -species and components at 398.3 eV and 402.8 eV are due to the uncharged deprotonated imine (=N-) nitrogen and charged imine (=NH + -) groups of PANI. Hence, the ratio of protonated N atoms to deprotonated N atoms is 1: 1.2. According to the XPS data, =N-to -NH-ratio of Ag/MMT/PANI is 1:4 and this result indicates the presence of ES form of PANI.
Component at around 406.8 eV is usually due to the oxidised N atoms. The possible oxidised N species in this composite is NO 3 ions. However, no other anions exist in the composite and percentage value of N of NO 3 ions is half of that of total protonated atoms, hence other protonated N atoms exist in the bonded form with the negatively charged MMT layers. Figure 2(d) shows the XPS N 1s core-level spectra of Ag/MMT/PPY nanocomposite and it consists of three peak components as typical of PPY. BEs at 399.15 eV and 400.80 eV are attributed to the -NH-and -NH + -species (Menon et al., 1996). Hence, ratio of the areas of these nitrogen peaks (1.2:1.0) correlates with the doped state of the PPY (Bala et al., 2000). Component at around 406.45 eV is usually due to the oxidised N species and hence this composite also contains NO 3 ions. Percentage values of N of NO 3 ions is half of that of protonated N atoms hence other protonated atoms exist in the bonded form with the negatively charged MMT layers.
According to the above characterisations, possible reactions can be suggested for the polymerisation process. Ag(I) initiates the polymerisation of aniline according to the reaction given below. Hence, the metallic silver particles are formed due to the reduction of Ag(I) as shown in equation 1. ... (1) Further oxidation of PANI into its Emeraldine and Pernigraniline forms takes place with the amount of oxidant. Ag(I) can induce similar reactions with pyrrole and EDOT as shown in following chemical equations 2 and 3.
| 11 Equation 1 Further oxidation of PANI in to its Emeraldine and Pernigraniline forms takes place with the amount of oxidant. Ag(I) can induce similar reactions with pyrrole and EDOT as show in following chemical Equations 2 and 3.

FTIR analysis
FTIR spectra of the samples are depicted in Figure 3. All samples have a band at the 1030 cm -1 , which is assigned to the Si-O stretching of MMT sheets. Hence, other bands are compared with respect to this band. The Si-O-Si and Si-O-Al deformation vibrations, respectively, appear at 523 cm -1 and 466 cm -1 and are deformed and shifted due to the effect of exchanged cations (Sogandares & Fry, 1997). All composites show H-O-H bending mode at 1640 cm -1 (Cursino et al., 2011). However, Ag/MMT/PANI, Ag/MMT/PPY and Ag/MMT/PEDOT show weaker absorptions at 1640 cm -1 than that of pure MMT. This is because the polymerisation process leads to removal of water inside the MMT layers as proven by the XRD data. Hence, they are water-repellant materials when compared to pure MMT. Corresponding bands for PANI appears in the Ag/MMT/PANI composites also. However, some bands assigned to PANI are masked by the higher absorbance of characteristic MMT bands. Similarly, Ag/MMT/PPY and Ag/MMT/PEDOT show characteristic bands of PPY and PEDOT, respectively.

Equation 3
FTIR analysis  Figure 4. Images reveal that the MMT particles are covered by PPY. MMT stacked structures are not clearly seen in the images due to the coverage by polymers. Every image has bright spots around 10 to 200 nm scale. This may be due to the conducting Ag particles or spherical Ag clusters attached with the polymers. The particle sizes match with the data from XRD. Circled area of the image in Figure 4(c) shows a hexagonal silver particle of 160 nm diagonal length. Although the Ag(I) ions are intercalated into the MMT layers, it appears that the Ag metal particles are present outside of MMT. When the pyrrole is introduced to the suspension of Ag(I)/MMT, Ag(I) ions can exist both outside and inside of the MMT layers and the sonication process would lead to the exfoliation of the MMT layers. Ag(I) attached MMT planes are then combined with pyrrole molecules and pyrrole is polymerised while Ag(I) is reduced to Ag particles.

DC conductivity studies of composites
The DC conductivity of Ag(I)/MMT is 1.03 × 10 -4 S m -1 and it is higher than that of pristine MMT (1.24× 10 -6 S m -1 ) due to the ionic conductivity caused by mobile silver ions. Ag/MMT/PANI, Ag/MMT/PPY and Ag/MMT/ PEDOT show conductivities of 5.02, 2.36 and 2.86 S m -1 , respectively. This increment of conductivity is due to the presence of conducting polymers and silver nano particles in the composite. Ag/MMT/PANI has the highest conductivity.

AC impedance analysis
EIS spectra for catalyst-deposited GC electrodes in 0.10 M KOH electrolyte are depicted in Figure 5(a) and Figure 5(c) is the modified Randles equivalent circuit suggested for the Nyquist plots obtained.
Catalyst loading was 0.18 mg cm -2 for all electrodes. In the circuit R s , R int and R ct , denote the bulk solution resistance, interfacial resistance and charge-transfer resistance, respectively. C int and C dl , respectively, represent the inter-facial capacitance and the double layer capacitance. Z w is Warburg impedance. The Nyquist plots at higher frequency ranges are zoomed and displayed in the Figure 5(b). Nova 1.10 software was used for curve fitting and calculating component values. Calculated values for important circuit components are summarised in Table 2. R int depends on the conductivity of the materials hence lowest R int is displayed for Ag/MMT/PANI. However, Ag/MMT/ PEDOT shows the lowest R ct . Ideal catalysts should have low R ct and low R int . Hence, all these composites have good catalytic activities. When comparing the values of C dl , Y 0 values of composites are larger than those of Pt/C and GC and N values are smaller than "1". It indicates the deviations from the ideal capacitance nature and is due to the roughness of the surfaces.

Cyclic voltammetric studies
ORR kinetics is faster in alkaline electrolytes than in acid electrolytes and Pt, Ag or some other metals help hydroxide ions to decompose rapidly, and able to increase the potential close to the theoretical potentials of the oxygen electrode (Singh & Buttry, 2012). Moreover, Ag particles are not much stable in the acidic medium, but they are stable catalysts towards ORR in alkaline medium (Liu & Chen, 2013). Hence, in this study, the characteristics of oxygen reduction at silver-based catalyst electrodes are studied in alkaline media. Figure 6(a) depicts the CV of Ag/MMT/PANI in nitrogensaturated 0.10 M KOH solution at a potential scan rate of 50 mV s -1 . It is shown in the potential window of -1.0V to + 1.0 V. In the CV, four anodic current peaks are clearly observed at +0.31 V, +0.41 V, +0.74 V and +0.80 V as indicated by A1 to A4, respectively. The small peak A1 could be attributed to the formation of a monolayer of AgOH by oxidizing Ag(0) to Ag(I) species and the peak A2 is attributed to the formation of inner hydrous oxide layers (Liu & Chen, 2013). More compact outer oxide layers and Ag 2 O form at A3 and A4 anodic peaks which are characteristic to silver electrodes. A5 is for the water oxidation (oxidation of OHto O 2 ). In the reverse scan, 3 cathodic peaks appear at +0.40 V, +0.05 V and -0.40 V which are labelled as C1 to C3, respectively. C1 and C2 are attributed to the reduction of the silver oxides formed during the anodic potential scan to form AgOH and Ag(0), respectively. Such characteristic CV featured from silver indicate that the surface clean Ag nano-particles that are supported on MMT/polymer matrix exhibit high electrochemical activity. C3 is for the reduction of oxygen formed at A5. CV for Ag(I)/MMT is depicted in Figure 6(b). CV shows all characteristic peaks corresponding to silver due to the presence of Ag(I). Higher current gains than that of Ag/MMT/PANI are observed for all characteristic peaks which may be due to higher amount of silver content present in Ag/MMT/PANI. However, the peaks are shifted to positive values in anodic sweep and shifted to negative potentials in cathodic sweep of the CV and ORR peak is very low when compared to that of Ag/MMT/PANI. The stability of Ag/MMT/PANI has also been evaluated by CV measurements.
According to Figure 6(c), there is only around 20 % change of the current density of CV profile even after 1000 cycles. This indicates the considerable electrochemical stability of the Ag/MMT/PANI catalyst. Considering the ORR catalytic features, Figure 6(d) shows the CVs for Ag/MMT/PANI in N 2 -saturated and O 2 -staurated 0.10 M KOH solution between +0.20 V and −1.0 V at 50 mV s -1 scan rate. The CV of the N 2 -purged system is featureless. This means that the reduction peaks observed are not due to reduction of the conducting polymer. This is clearly understandable because PPY, PANI and PEDOT are converted to their reduced forms well before -0.40 V. However, when the electrolyte is saturated with oxygen gas, the oxygen reduction is clearly observed in the negative potential range after -0.40 V with respect to the saturated calomel electrode. The cathodic wave with a peak potential at around −0.42 V is attributed to O 2 reduction. As such, our Ag/MMT/PANI catalyst acts as a good catalyst for the ORR that is very much suitable to be utilised in fuel cells. The procedure was repeated in order to confirm the catalytic property and shows the same performance. CV is similar to that of Ag/MMT/PANI. However, the peak assigned to formation of monolayer of AgOH is diminished and only 3 anodic current peaks are clearly observed at +0.35 V, +0.74 V and above +0.80 V, which are labelled as A1, A2 and A3, respectively. A1 and A2 could be attributed to the formation of AgOH and Ag 2 O, respectively, and A3 is for the water oxidation to form O 2 . In the reverse scan, usual cathodic peaks appeared at +0.40 V, +0.00 V and -0.45 V, which are labelled as C1, C2 and C3, respectively. C1 and C2 are attributed to the reduction of the silver oxide back to AgOH and Ag(0), respectively, and C3 is for the ORR.
Considering the ORR catalytic features, Figure 7(b) shows the CVs for an Ag/MMT/PPY in N 2 -saturated and O 2 -saturated 0.10 M KOH between potentials +0.2 V and −1.0 V at 50 mV s -1 scan rate. Compared to the featureless CV in the N 2 -saturated electrolyte, a large reduction current peak around −0.42 V is seen in the O 2 -saturated conditions, indicating the electrocatalytic activity of the composite for the ORR. Peak current density is considerably higher than that of Ag/MMT/ PANI and shifted to the positive direction. Ag/ MMT/PPY. However, the peaks are broader than that of Ag/ MMT/PPY and A1 and A2 are shifted to +0.42 V and +0.75 V, respectively and A3 peak for the water oxidation is also observed in the CV. In the cathodic sweep of the CV, usual C1, C2 and C3 cathodic peaks appeared at +0.40 V, −0.08 V and -0.50 V, respectively.
Journal of the National Science Foundation of Sri Lanka 47(4) December 2019 Figure 7(d) shows the CVs of the Ag/MMT/PEDOT electrode in 0.10 M KOH saturated with N 2 or O 2 in the potential range from -1.0 V to +0.2 V at scan rate of 50 mV s -1 . Compared to the CV in the N 2 -saturated electrolyte, a large reduction current peak around −0.32 V from ORR can be seen in the O 2 -saturated electrolyte, indicating the high electrocatalytic activity of the composite for the ORR. Peak current density is considerably higher than that of Ag/MMT/PANI and Ag/MMT/PPY and peak potential is shifted to positive potentials showing lower over-potentials for ORR. In order to compare the catalysts with standard electrodes for their activities towards ORR, we have repeated the experiments with O 2 -saturated KOH solutions with just the bare glassy carbon electrode, Ag/C (20% Ag w/w) and Pt/C (20% Pt w/w) as the working electrodes and keeping all the other parameters essentially the same as those used in CV experiments done with Ag/MMT/ PANI, Ag/MMT/PPY and Ag/MMT/PEDOT working electrodes. Figure 7(e) shows the CVs of all synthesised silver-based composites together with Ag/C and Pt/C in O 2 saturated 0.10 M KOH solution between potential range -1.0 V to +0.2 V. Ag/MMT/PPY and Ag/MMT/ PEDOT composites show the positive shift of the onset potential values. Cathodic peak current density values of Ag/C, Ag/MMT/PANI, Ag/MMT/PPY, Ag/MMT/ PEDOT and Pt/C are 1.8, 1.7, 1.9, 3.0 and 4.5 mA cm -2 and these data reveal that our composites have better performance than that of Ag/C composites with the same silver loading. However, the novel catalysts show lower ORR activities than that of commercially available Pt/C.

I-V polarization curves
ORR performance is further studied with the linear sweep voltammetry (LSV) measurements on a rotating disk electrode (RDE) of Ag/MMT/PEDOT electrode along with the naked GC, Ag/C (20% Ag w/w) and Pt/C (20% Pt w/w) electrodes. LSV is run under O 2 saturated condition in 0.10 M KOH solutions at a scan rate of 5mV s -1 and a rotation rate of 1600 rpm within the range of + 0.5 V to -1.0 V. As shown in Figure 7(f), the onset potentials for oxygen reduction at the Pt/C, Ag/MMT/ PEDOT, Ag/C and GC are +0.05 V, -0.05 V, -0.15 V and -0.24 V, respectively. The limiting diffusion current densities at -0.20 V for the Pt/C, Ag/MMT/ PEDOT, Ag/C and GC electrodes are 2.43, 1.71, 0.02 and 0.00 mA cm -2 , respectively. At -0.6 V, current values have been changed to 5.5, 4.6, 2.8 and 1.0 mA cm -2 , respectively. These results are consistent with the CV data and confirm the significant contributions to the ORR electro-catalytic activity of Ag/MMT/PEDOT.

CONCLUSIONS
The Ag/MMT/PANI, Ag/MMT/PPY and Ag/ MMT/PEDOT composites were prepared and their morphological and chemical behaviours as well as conducting properties were characterised. XPS and XRD data confirm that the Ag(I) ions participate in the polymerisation and are reduced to Ag(0). The Ag/MMT/PANI composite contains Ag nanoparticles and emeraldine salt form of PANI intercalated within the interlayer space of MMT. Ag/MMT/PPY composite also shows the similar morphological structure. However, MMT layers are partially exfoliated and Ag particles around 10 nm to 200 nm diameter sizes are bonded with the PPY outside the MMT layers. In the Ag/MMT/ PEDOT composites, MMT layers are totally exfoliated in the successfully polymerised PEDOT matrix. All composites show high conductivity and the highest is 5.02 S m -1 for Ag/MMT/PANI which is the highest recorded value for clay / electronically conducting polymer composites. Electrochemical analysis shows that all composites have good catalytic activity towards ORR and have enhanced properties Ag/C. Ag/MMT/ PEDOT shows the highest performance among these three composites and it has competitive performance with commercially available Pt/C. Therefore, all three composites are suitable for application as low-cost fuel cell cathodes; Ag/MMT/PEDOT being the best.