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Effect of alkali cations on Pt based catalyst towards methanol oxidation reaction in acidic medium
Applied Surface Science 489 (2019) 149–153
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Effect of alkali cations on Pt based catalyst towards methanol oxidation reaction in acidic medium Krishnamoorthy Silambarasan, James Joseph, Sundar Mayavan
T
⁎
CSIR-Central Electrochemical Research Institute, Karaikudi-630003, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Methanol oxidation reaction Alkali cations Interface
This communication demonstrates the effect of electrostatically adsorbed alkali cations at electrochemical interface towards methanol oxidation reaction (MOR) in acidic medium. The well-known electrostatic interaction between adsorbed OH/(OHad) species of catalyst surface and hydrated metal cations (M+(H2O)x) at the interface could be accountable for the observed different electrocatalytic activity of Pt/C and f-MWCNT/Pt-Co alloy NPs towards MOR. This cation effect has been found predominantly in weekly adsorbed ClO4− ion at Pt/C and fMWCNT/Pt-Co alloy NPs surface than that of strongly adsorbed SO42− ion. Cyclic voltammetry results reveal that the strongly adsorbed cation at the interface shifts the oxidation potential more positively than that of weekly adsorbed cation.
1. Introduction Understanding the mechanism of electrified catalyst-electrolyte interface is always welcome to the advancement of energy technology [1,2]. Pt and Pt based alloys and core-shell nanoparticles have been extensively used in energy conversion reactions such as methanol oxidation, oxygen reduction reaction, hydrogen peroxide reduction and metal-air batteries [3–6]. Recently, N. M. Markovic group have revealed that non covalent interactions between OH group of Pt (111) surface and hydrated alkali metal cation at the interface play a critical role towards electrocatalytic reactions in alkaline medium [7]. However, they couldn't see this effect in sulphuric acid medium for oxygen reduction reaction, likely due to very low surface coverage of OHad groups on Pt (111) surface [7]. The effect of alkali cations on Pt (111) interface also have been studied by different research group in phosphate buffer solution [8], sulphuric acid and alkali solution [9]. From their investigation, it is well-understood that there is a strong interaction between adsorbed OHad group of Pt surface and non-specifically adsorbed alkali metal cations. However, in all cases, this effect on hydrogen adsorption/desorption region is very less compare to oxide formation region. Thus, the presence of alkali metal cation at interface can be identified in OHP layer and it interacts with covalently formed Pt-OHad surface available in the IHP. This interaction is purely controlled by ion-dipole interaction between OH group and hydrated metal ion clusters (M+(H2O)x). Here, we have interested to investigate the effect of alkali metal
⁎
cations on Pt/C and f-MWCNT/Pt-Co alloy nanoparticles in weekly adsorbed perchlorate ion medium which is not explored in literature. And also, we have intended to study this cation effect on Pt based electrocatalyst in acidic medium towards methanol oxidation reaction (MOR) due to following reasons. i) It is well known that ClO4− anion is weakly adsorbed on Pt surface [10,11] compare to SO42− ion, where the adsorption of OHad/H2O on Pt surface may possible in HClO4 medium. ii) If it is, then there is a possibility of non-specific interaction between alkali metal cation and adsorbed OH group, in which we may encounter different electrochemical activity towards methanol oxidation reaction in presence of alkali cations. Additionally, the formation Pt-OHad or Pt-Oad by dissociation of water molecule on Pt surface in acidic medium, particularly HClO4 medium has been proved by many research groups [3,12]. With this background, we have tried to demonstrate the catalytic activity of commercial 20% Pt/C and fMWCNT/Pt-Co alloy nanoparticles towards MOR in the presence of Li+, Na+ and K+ ions. 2. Experimental procedures 2.1. Chemical and reagents Hydrochloroplatinic acid (H2PtCl4.2H2O), Methanol (99%), Multiwalled carbon nanotubes (MWCNT) are purchased from sigma Aldrich. Cobalt chloride and 20% commercial Pt/C (20%) is purchased from Alfa acer. LiClO4, NaClO4, KClO4 and H2SO4 are analytical in grade and
Corresponding author. E-mail address: [email protected] (S. Mayavan).
https://doi.org/10.1016/j.apsusc.2019.05.265 Received 19 March 2019; Received in revised form 20 May 2019; Accepted 22 May 2019 Available online 29 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 489 (2019) 149–153
K. Silambarasan, et al.
sequence of Pt-OH reduction potential for Pt/C is Li+ > Na+ > K+. The reason may be that Li+ ion can stabilize the Pt-OH more strongly than other cations [7,17]; as a result, more energy is needed to break the PteOH bond. In case of PteCo alloy NPs, the reduction of Pt-OH bond is easier in the presence of alkali cations due to the synergetic effect of PteCo alloy NPs. However, Fig. 2C reveals that there is no drastic change in solution resistance of an electrolyte by addition of alkali metal cations, however, there was a change in capacitance as observed in capacitance plot derived from Nyquist [18] as shown in Fig. 2D. This result has confirmed the presence of alkali cations at the interface which might be responsible for change in charge transfer capacitance. To understand the effect of non-covalent interaction on Pt based catalyst, we have experimented methanol oxidation reaction as a modal in weekly adsorbed ClO4 medium. Fig. 3A shows the methanol oxidation reaction by Pt/C in 50 mM HClO4 solution. The forward peak potential of methanol oxidation has shifted more negatively in the presence of Li+ than that of cation free electrolyte medium. However, in terms of peak current, cation free electrolytes shows more catalytic current compared to cation containing HClO4 solution. This can be correlated by site blocking nature of hydrated metal ion complex available in the interface as described by N M Markovic group [7]. They have demonstrated this cation effect for Pt (111) plan in alkaline medium towards oxygen reduction reaction, MOR and HOR. And also non-covalent interactions towards water electrolysis on Pt(111) surface in acidic medium has been demonstrated by Wolfgang Schuhmann group [19]. According to their discussion, the non-covalent interaction of PteOHad-M+(H2O)x is serve as spectator or partially inhabit the movement of reactant to the catalyst surface. This might be applicable in our case that the presence of hydrated metal ion cluster at the interface partially blocks the movement of reactants to the Pt surface: as a result, we have observed lesser current in the presence of cations compared to cation free solutions. The same trend also has been observed for bimetallic nanoparticles, f-MWCNT/PteCo NPs. The trend in catalytic activity towards methanol oxidation reaction in the presence of different alkali cations have been correlated with hydration energies of metal ions in water. Fig. 3B and C show that the sequence of methanol oxidation peak potential and catalytic current is in order of Li+ > Na+ > K+. The ion-dipole interaction between PteOHad and M+(H2O)x increases with increasing the interaction energy between OH and alkali cations i.e., Li+ > Na+ > K+. As a result, the reaction trend also follows the above sequence as expected [17]. This result reveals that non-covalently adsorbed metal ion cluster at the interface serve as a spectator and also partially inhibit the movement of the reactants towards electrode surface and shows good agreement with the discussion given for non-covalent interaction on Pt surface by N M Markovic group [7,17]. However, this result is different from the result observed for alkaline medium. Moreover, limited experiments have been carried by Antonio Aldaz to understand the effect of cations on Pt (111) surface in sulphuric acid and buffer solution [8,9]. They have interpreted that this effect of cations on voltammetry response of Pt (111) electrode may be due to the lateral interactions of cation with the chemically adsorbed sulphate anion on Pt surface. In order to prove this, we have conducted this cation effect in 50 mM H2SO4 medium instead of HClO4. The cation effect has been seen in voltammogram of Pt/C and f-MWCNT/PteCo NPs in H2SO4 solution. However, Fig. 3D shows that the effect of cation on catalytic oxidation of methanol in H2SO4 solution has not been encountered and this shows good agreement with N M Markovic group who has done ORR on Pt (111) surface in H2SO4 medium and K+ containing H2SO4 solution. This is due to Pt-oxide formation is very less in the strong adsorbed sulphate anion on Pt-surface [7]. However, in HClO4 medium, the formation of subsurface oxygen on Pt surface is possible due to weakly adsorbed ClO4− anion [10]. Fig. 3E shows electrocatalytic cycling process in 0.5 M methanol solution. The enhancement in catalytic current has been observed
used without further purification. MilliQ water (18.2 MQ) was used for electrolyte and solution preparations. 2.2. Synthesis of f-MWCNT decorated platinum‑cobalt alloy nanoparticle Initially, the commercial MWCNT was subjected to surface functionalization using concentrated sulphuric acid (3 M) at 90 °C about 48 h [13]. After that, the product was washed several times with MilliQ water using centrifugation (8000 rpm). The functionalized MWCNT was dissolved in MilliQ water, followed by 3 mM of choloroplatinic acid and 1 mM of cobalt chloride solutions were added slowly at constant stirring about 30 min. After that, 10 mM of freshly prepared sodium borohydrate was added to dropwise into the above precursor solution and the stirring was continued up to 3 h at room temperature. The prepared bimetallic nanoparticle was washed with MilliQ water to remove the unreduced ions by centrifugation at 8000 rpm. 2.3. Physical characterization and electrochemical measurements X-ray powder diffraction (XRD) analysis was conducted by Smart Lab guidance, Rigaku to confirm the bimetallic alloy nanoparticle formation. The morphology of the alloy nanoparticles was performed by Carl Zeiss, SUPRA 55VP. Cyclic voltammetry (CV) experiments were performed with an Autolab PGSTAT 30 potentiostat (Eco chemie Netherlands). The electrochemical impedance spectroscopy (EIS) measurement was carried out at open circuit potential (OCP) condition with Ametek PARSTAT MC Instrument, in the frequency range of 35,000 Hz to 0.1 Hz at 10 mV amplitude. To carry out the CV and EIS experiments, we have used three electrode systems where, nanoparticle modified glassy carbon electrode was used as working electrode, normal calomel electrode (NCE) and platinum foil were used as reference and counter electrode, respectively. The catalytic ink was prepared by dispersing 3 mg of commercial Pt/C in 1 mL of MilliQ water by sonication, and 5 μL of the above ink was coated on glassy carbon electrode (GCE) for catalyst modification. The calculated Pt concentration is 0.214 mg/cm2. The another catalytic ink was prepared by dispersing of 1 mg of f-MWCNT/Pt-Co alloy nanoparticles in 0.5 mL of MilliQ water and 5 μL of the above ink was coated on GCE surface. Both are dried at room temperature. Electrochemical active surface area (ECSA) of Pt was calculated by integrating the hydrogen desorption peak of Pt obtained from CV results in HClO4 medium and the ECSA value for Pt/C is 116.19 cm2 and for f-MWCNT/Pt-Co alloy nanoparticles is 44.52 cm2. 3. Results and discussion Fig. 1a confirms the formation of PteCo alloy nanoparticles decorated on the surface of f-MWCNT by shifting of 2θ value of platinum nanoparticles peak towards higher degrees than that of 20% Pt/C, and this shows good agreement with other reports [13,14]. This higher degree of alloy nanoparticles play an important role in methanol oxidation reaction by minimizing the interaction energy between intermediates formed by methanol dissociation and also other electrocatalytic reactions such as oxygen reduction reaction etc., [14–16] Fig. 1b shows the FE-SEM images of decorated PteCo alloy nanoparticles on the functionalized surface of MWCNT. The spherical shape PteCo alloy nanoparticle was attached on the surface and edges of MWCNT. Fig. 2 summarize the effect of alkali cations on voltammetric response of Pt/C and f-MWCNT/Pt-Co NPs in 50 mM HClO4 medium. Fig. 2A and B shows that addition of cations into the electrolytes shifts the potential of both hydrogen adsorption/desorption region as well as platinum oxide formation/reduction region. For pure Pt/C, both hydrogen desorption and Pt-oxide reduction peak potentials shifts more negatively in the presence of alkali cations, whereas, in case of fMWCNT/Pt-Co NPs, the Pt-oxide reduction peak shift positive side. The 150
Applied Surface Science 489 (2019) 149–153
K. Silambarasan, et al.
Fig. 1. a) XRD pattern of Pt/C and f-MWCNTs/Pt-Co alloy nanoparticles and B) FE-SEM images of f-MWCNTs/Pt-Co alloy nanoparticles.
4. Conclusions
during cycling process in absence of cations due to increase the surface coverage of Pt-oxide formation and this is known as “active oxy groups” [1,20]. This active oxy groups may be responsible for self-cleaning process on Pt surface during cycling process. Whereas, in the presence of cations, the interface is stabilized by hydrated metal ion clusters which may allow almost same number of reactant molecules to the catalytic surface during the cycling process in methanol solution.
In conclusion, the electrochemical activity trend on Pt/C and PtCo alloy NPs in HClO4 medium has been understood by the interaction energy between adsorbed OH species and hydrated alkali metal ions at the interface. CV results reveals that the strongly adsorbed cation at the interface shift the oxidation potential more positive than that of weekly adsorbed cation. However, in the case of catalytic current, it is reversed.
Fig. 2. a) CV response of Pt/C in different cationic electrolyte, b) CV response of f-MWCNT/Pt-Co alloy NPs in different cationic electrolyte solution, c) Nyquist plot of Pt/C in acidic medium with different cationic environment and d) its corresponding capacitance plot. 151
Applied Surface Science 489 (2019) 149–153
K. Silambarasan, et al.
Fig. 3. a) Voltammetric response of methanol oxidation on Pt/C b) Double Y-axis plot of effect of cation towards methanol oxidation on Pt/C, c) Double Y-axis plot of effect of cation towards methanol oxidation on F-MWCNT/Pt-Co alloy NPs, d) CV response of methanol oxidation reaction on Pt/C in H2SO4 medium and e) Cycling effect of methanol oxidation reaction on Pt/C with and without Li+ in HClO4 medium.
References
In H2SO4 medium, neither potential nor current is changed. This implies that the interaction of (M+(H2O)x) with OHad species may not possible at the interface due to the strongly adsorbed SO42− ion on Pt/ PtCo NPs surface.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Effect of alkali cations on Pt based catalyst towards methanol oxidation reaction in acidic medium Krishnamoorthy Silambarasan, James Joseph, Sundar Mayavan
T
⁎
CSIR-Central Electrochemical Research Institute, Karaikudi-630003, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Methanol oxidation reaction Alkali cations Interface
This communication demonstrates the effect of electrostatically adsorbed alkali cations at electrochemical interface towards methanol oxidation reaction (MOR) in acidic medium. The well-known electrostatic interaction between adsorbed OH/(OHad) species of catalyst surface and hydrated metal cations (M+(H2O)x) at the interface could be accountable for the observed different electrocatalytic activity of Pt/C and f-MWCNT/Pt-Co alloy NPs towards MOR. This cation effect has been found predominantly in weekly adsorbed ClO4− ion at Pt/C and fMWCNT/Pt-Co alloy NPs surface than that of strongly adsorbed SO42− ion. Cyclic voltammetry results reveal that the strongly adsorbed cation at the interface shifts the oxidation potential more positively than that of weekly adsorbed cation.
1. Introduction Understanding the mechanism of electrified catalyst-electrolyte interface is always welcome to the advancement of energy technology [1,2]. Pt and Pt based alloys and core-shell nanoparticles have been extensively used in energy conversion reactions such as methanol oxidation, oxygen reduction reaction, hydrogen peroxide reduction and metal-air batteries [3–6]. Recently, N. M. Markovic group have revealed that non covalent interactions between OH group of Pt (111) surface and hydrated alkali metal cation at the interface play a critical role towards electrocatalytic reactions in alkaline medium [7]. However, they couldn't see this effect in sulphuric acid medium for oxygen reduction reaction, likely due to very low surface coverage of OHad groups on Pt (111) surface [7]. The effect of alkali cations on Pt (111) interface also have been studied by different research group in phosphate buffer solution [8], sulphuric acid and alkali solution [9]. From their investigation, it is well-understood that there is a strong interaction between adsorbed OHad group of Pt surface and non-specifically adsorbed alkali metal cations. However, in all cases, this effect on hydrogen adsorption/desorption region is very less compare to oxide formation region. Thus, the presence of alkali metal cation at interface can be identified in OHP layer and it interacts with covalently formed Pt-OHad surface available in the IHP. This interaction is purely controlled by ion-dipole interaction between OH group and hydrated metal ion clusters (M+(H2O)x). Here, we have interested to investigate the effect of alkali metal
⁎
cations on Pt/C and f-MWCNT/Pt-Co alloy nanoparticles in weekly adsorbed perchlorate ion medium which is not explored in literature. And also, we have intended to study this cation effect on Pt based electrocatalyst in acidic medium towards methanol oxidation reaction (MOR) due to following reasons. i) It is well known that ClO4− anion is weakly adsorbed on Pt surface [10,11] compare to SO42− ion, where the adsorption of OHad/H2O on Pt surface may possible in HClO4 medium. ii) If it is, then there is a possibility of non-specific interaction between alkali metal cation and adsorbed OH group, in which we may encounter different electrochemical activity towards methanol oxidation reaction in presence of alkali cations. Additionally, the formation Pt-OHad or Pt-Oad by dissociation of water molecule on Pt surface in acidic medium, particularly HClO4 medium has been proved by many research groups [3,12]. With this background, we have tried to demonstrate the catalytic activity of commercial 20% Pt/C and fMWCNT/Pt-Co alloy nanoparticles towards MOR in the presence of Li+, Na+ and K+ ions. 2. Experimental procedures 2.1. Chemical and reagents Hydrochloroplatinic acid (H2PtCl4.2H2O), Methanol (99%), Multiwalled carbon nanotubes (MWCNT) are purchased from sigma Aldrich. Cobalt chloride and 20% commercial Pt/C (20%) is purchased from Alfa acer. LiClO4, NaClO4, KClO4 and H2SO4 are analytical in grade and
Corresponding author. E-mail address: [email protected] (S. Mayavan).
https://doi.org/10.1016/j.apsusc.2019.05.265 Received 19 March 2019; Received in revised form 20 May 2019; Accepted 22 May 2019 Available online 29 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 489 (2019) 149–153
K. Silambarasan, et al.
sequence of Pt-OH reduction potential for Pt/C is Li+ > Na+ > K+. The reason may be that Li+ ion can stabilize the Pt-OH more strongly than other cations [7,17]; as a result, more energy is needed to break the PteOH bond. In case of PteCo alloy NPs, the reduction of Pt-OH bond is easier in the presence of alkali cations due to the synergetic effect of PteCo alloy NPs. However, Fig. 2C reveals that there is no drastic change in solution resistance of an electrolyte by addition of alkali metal cations, however, there was a change in capacitance as observed in capacitance plot derived from Nyquist [18] as shown in Fig. 2D. This result has confirmed the presence of alkali cations at the interface which might be responsible for change in charge transfer capacitance. To understand the effect of non-covalent interaction on Pt based catalyst, we have experimented methanol oxidation reaction as a modal in weekly adsorbed ClO4 medium. Fig. 3A shows the methanol oxidation reaction by Pt/C in 50 mM HClO4 solution. The forward peak potential of methanol oxidation has shifted more negatively in the presence of Li+ than that of cation free electrolyte medium. However, in terms of peak current, cation free electrolytes shows more catalytic current compared to cation containing HClO4 solution. This can be correlated by site blocking nature of hydrated metal ion complex available in the interface as described by N M Markovic group [7]. They have demonstrated this cation effect for Pt (111) plan in alkaline medium towards oxygen reduction reaction, MOR and HOR. And also non-covalent interactions towards water electrolysis on Pt(111) surface in acidic medium has been demonstrated by Wolfgang Schuhmann group [19]. According to their discussion, the non-covalent interaction of PteOHad-M+(H2O)x is serve as spectator or partially inhabit the movement of reactant to the catalyst surface. This might be applicable in our case that the presence of hydrated metal ion cluster at the interface partially blocks the movement of reactants to the Pt surface: as a result, we have observed lesser current in the presence of cations compared to cation free solutions. The same trend also has been observed for bimetallic nanoparticles, f-MWCNT/PteCo NPs. The trend in catalytic activity towards methanol oxidation reaction in the presence of different alkali cations have been correlated with hydration energies of metal ions in water. Fig. 3B and C show that the sequence of methanol oxidation peak potential and catalytic current is in order of Li+ > Na+ > K+. The ion-dipole interaction between PteOHad and M+(H2O)x increases with increasing the interaction energy between OH and alkali cations i.e., Li+ > Na+ > K+. As a result, the reaction trend also follows the above sequence as expected [17]. This result reveals that non-covalently adsorbed metal ion cluster at the interface serve as a spectator and also partially inhibit the movement of the reactants towards electrode surface and shows good agreement with the discussion given for non-covalent interaction on Pt surface by N M Markovic group [7,17]. However, this result is different from the result observed for alkaline medium. Moreover, limited experiments have been carried by Antonio Aldaz to understand the effect of cations on Pt (111) surface in sulphuric acid and buffer solution [8,9]. They have interpreted that this effect of cations on voltammetry response of Pt (111) electrode may be due to the lateral interactions of cation with the chemically adsorbed sulphate anion on Pt surface. In order to prove this, we have conducted this cation effect in 50 mM H2SO4 medium instead of HClO4. The cation effect has been seen in voltammogram of Pt/C and f-MWCNT/PteCo NPs in H2SO4 solution. However, Fig. 3D shows that the effect of cation on catalytic oxidation of methanol in H2SO4 solution has not been encountered and this shows good agreement with N M Markovic group who has done ORR on Pt (111) surface in H2SO4 medium and K+ containing H2SO4 solution. This is due to Pt-oxide formation is very less in the strong adsorbed sulphate anion on Pt-surface [7]. However, in HClO4 medium, the formation of subsurface oxygen on Pt surface is possible due to weakly adsorbed ClO4− anion [10]. Fig. 3E shows electrocatalytic cycling process in 0.5 M methanol solution. The enhancement in catalytic current has been observed
used without further purification. MilliQ water (18.2 MQ) was used for electrolyte and solution preparations. 2.2. Synthesis of f-MWCNT decorated platinum‑cobalt alloy nanoparticle Initially, the commercial MWCNT was subjected to surface functionalization using concentrated sulphuric acid (3 M) at 90 °C about 48 h [13]. After that, the product was washed several times with MilliQ water using centrifugation (8000 rpm). The functionalized MWCNT was dissolved in MilliQ water, followed by 3 mM of choloroplatinic acid and 1 mM of cobalt chloride solutions were added slowly at constant stirring about 30 min. After that, 10 mM of freshly prepared sodium borohydrate was added to dropwise into the above precursor solution and the stirring was continued up to 3 h at room temperature. The prepared bimetallic nanoparticle was washed with MilliQ water to remove the unreduced ions by centrifugation at 8000 rpm. 2.3. Physical characterization and electrochemical measurements X-ray powder diffraction (XRD) analysis was conducted by Smart Lab guidance, Rigaku to confirm the bimetallic alloy nanoparticle formation. The morphology of the alloy nanoparticles was performed by Carl Zeiss, SUPRA 55VP. Cyclic voltammetry (CV) experiments were performed with an Autolab PGSTAT 30 potentiostat (Eco chemie Netherlands). The electrochemical impedance spectroscopy (EIS) measurement was carried out at open circuit potential (OCP) condition with Ametek PARSTAT MC Instrument, in the frequency range of 35,000 Hz to 0.1 Hz at 10 mV amplitude. To carry out the CV and EIS experiments, we have used three electrode systems where, nanoparticle modified glassy carbon electrode was used as working electrode, normal calomel electrode (NCE) and platinum foil were used as reference and counter electrode, respectively. The catalytic ink was prepared by dispersing 3 mg of commercial Pt/C in 1 mL of MilliQ water by sonication, and 5 μL of the above ink was coated on glassy carbon electrode (GCE) for catalyst modification. The calculated Pt concentration is 0.214 mg/cm2. The another catalytic ink was prepared by dispersing of 1 mg of f-MWCNT/Pt-Co alloy nanoparticles in 0.5 mL of MilliQ water and 5 μL of the above ink was coated on GCE surface. Both are dried at room temperature. Electrochemical active surface area (ECSA) of Pt was calculated by integrating the hydrogen desorption peak of Pt obtained from CV results in HClO4 medium and the ECSA value for Pt/C is 116.19 cm2 and for f-MWCNT/Pt-Co alloy nanoparticles is 44.52 cm2. 3. Results and discussion Fig. 1a confirms the formation of PteCo alloy nanoparticles decorated on the surface of f-MWCNT by shifting of 2θ value of platinum nanoparticles peak towards higher degrees than that of 20% Pt/C, and this shows good agreement with other reports [13,14]. This higher degree of alloy nanoparticles play an important role in methanol oxidation reaction by minimizing the interaction energy between intermediates formed by methanol dissociation and also other electrocatalytic reactions such as oxygen reduction reaction etc., [14–16] Fig. 1b shows the FE-SEM images of decorated PteCo alloy nanoparticles on the functionalized surface of MWCNT. The spherical shape PteCo alloy nanoparticle was attached on the surface and edges of MWCNT. Fig. 2 summarize the effect of alkali cations on voltammetric response of Pt/C and f-MWCNT/Pt-Co NPs in 50 mM HClO4 medium. Fig. 2A and B shows that addition of cations into the electrolytes shifts the potential of both hydrogen adsorption/desorption region as well as platinum oxide formation/reduction region. For pure Pt/C, both hydrogen desorption and Pt-oxide reduction peak potentials shifts more negatively in the presence of alkali cations, whereas, in case of fMWCNT/Pt-Co NPs, the Pt-oxide reduction peak shift positive side. The 150
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Fig. 1. a) XRD pattern of Pt/C and f-MWCNTs/Pt-Co alloy nanoparticles and B) FE-SEM images of f-MWCNTs/Pt-Co alloy nanoparticles.
4. Conclusions
during cycling process in absence of cations due to increase the surface coverage of Pt-oxide formation and this is known as “active oxy groups” [1,20]. This active oxy groups may be responsible for self-cleaning process on Pt surface during cycling process. Whereas, in the presence of cations, the interface is stabilized by hydrated metal ion clusters which may allow almost same number of reactant molecules to the catalytic surface during the cycling process in methanol solution.
In conclusion, the electrochemical activity trend on Pt/C and PtCo alloy NPs in HClO4 medium has been understood by the interaction energy between adsorbed OH species and hydrated alkali metal ions at the interface. CV results reveals that the strongly adsorbed cation at the interface shift the oxidation potential more positive than that of weekly adsorbed cation. However, in the case of catalytic current, it is reversed.
Fig. 2. a) CV response of Pt/C in different cationic electrolyte, b) CV response of f-MWCNT/Pt-Co alloy NPs in different cationic electrolyte solution, c) Nyquist plot of Pt/C in acidic medium with different cationic environment and d) its corresponding capacitance plot. 151
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Fig. 3. a) Voltammetric response of methanol oxidation on Pt/C b) Double Y-axis plot of effect of cation towards methanol oxidation on Pt/C, c) Double Y-axis plot of effect of cation towards methanol oxidation on F-MWCNT/Pt-Co alloy NPs, d) CV response of methanol oxidation reaction on Pt/C in H2SO4 medium and e) Cycling effect of methanol oxidation reaction on Pt/C with and without Li+ in HClO4 medium.
References
In H2SO4 medium, neither potential nor current is changed. This implies that the interaction of (M+(H2O)x) with OHad species may not possible at the interface due to the strongly adsorbed SO42− ion on Pt/ PtCo NPs surface.
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