Electro-Deposition Pt Catalysts Supported on Carbon-Nanotubes for Methanol Oxidation

，，，

（Chemical Engineering Institute，Harbin Institute of Technology，Harbin 150001，China）

Electro-Deposition Pt Catalysts Supported on Carbon-Nanotubes for Methanol Oxidation

Hailin Song，Peixia Yang?，Xiaoyu Wen，Maozhong An and Jinqiu Zhang

（Chemical Engineering Institute，Harbin Institute of Technology，Harbin 150001，China）

In order to study the properties of supporting Pt catalysts for methanol oxidation，carbon-nanotubes are used by electrochemical deposition method.Different deposition turns，different cyclic voltammetry scanning speeds and processing time with ascorbic acid are investigated in this paper.The micrographs of Pt／CNTs catalysts are characterized by scanning electron microscopy，the electro-catalytic properties of Pt／CNTs catalysts for methanol oxidation are investigated by cycle voltammetry and chronoamperometry.The results show that the size of platinum will be greater with the faster scanning speed.After dissolution in ascorbic acid，Pt nanoparticles disperse uniformly.The obtained Pt／CNTs catalysts show a high electro-catalytic activity and stability.

Carbon nanotube；Pt catalyst；Cyclic voltammetry；Ascorbic acid

1 Introduction

Direct methanol fuel cells（DMFCs）have attracted increasing attention in portable power and mobile applications because of the high energy density，rapid start-up，and low temperature［1-3］.However，the conversion from chemical to electrical energy still faces several critical challenges，including the durability and their low electro-catalytic activity［4］.Pt is the most effective single metal catalyst for methanol oxidation. Because of the high cost of Pt catalysts，reducing the amount of Pt and enhancing its electro-catalytic activity and stability are still the hot spots of DMFC research［5］.

Variety of catalyst support has been studied in last decades ranging from vulcan carbon，carbon black，carbon nano-fiber and graphene.Among them，carbon nanotubes（CNTs）as catalyst support material have drawn great interest because of their large accessible surface areas，high stability，and high electron conductivity［6-7］.Recently，many methods have been reported such as Liquid-phase chemical reduction method［8］，Sol-gel method［9］，Hydrothermal method，Solid state reaction method，Electro-reduction［10-12］.

Electro-deposition has advantages of simple technology，low cost and high purity coating.By controlling the current，electrolyte components and process parameters，one can control the coating components，grains，grain size.It has been a research area of common concern and has made great progress［13］.

This paper studies the use of CNTs as Pt catalyst support by voltammetry method and processes the obtained Pt particles with ascorbic acid，further reducing nano-particles size.The impact of experimental factors affecting the performance of catalysts have been investigated.Additionally，the better conditions of process preparation is chosen，and the physical properties of obtained catalysts are analyzed.The catalysts show a high durability and electro-catalytic activity for methanol oxidation.

2 Experimental

2.1 Reagents and Apparatuses

Chloroplatinic acid（H2PtCl6·6H2O）was obtained from Shenyang Kedachem co.Ltd.Ascorbic acid was purchased from Tianjin Kemiouchemco.Ltd. All other chemicals were analytical grade.All solutions were prepared with pure water.The pristine CNTs were purchased from Shenzhen Nanotechnologies Port Co. Ltd.All electrochemical measurements were performed with an electrochemical workstation（CHI630B，Shanghai，Chenhua）through a conventional threeelectrode cell.

2.2 Pt／CNTs Catalysts Preparation

Glassy carbon electrode（GCE）was polished carefully by Al2O3powders.Then dry it with high purity nitrogen blow after cleaning.0.1%Nafion solutiondiluted by ethanol was dispersed ultrasonically.Then，5 μL of the ink obtained was pipetted and spread on a polished glassy carbon electrode（GCE，? 4 mm）as the working electrode.Pt／CNTs catalysts were prepared by cycle voltammetry method with three-electrode system.GCE was used as the working electrode，a saturated calomel electrode（SCE）used as the reference electrode and the counter electrode was a bright Pt plate.The voltammetric activation behavior was characterized in 0.5 M H2SO4+5 mM H2PtCl6mixed solutions.The prepared catalysts took square wave method when it was treated by ascorbic acid on the Electrochemical Tester M273，in a 30 mM ascorbic acid+0.1 M H2SO4mixed solutions.

2.3 Electrochemical Measurement

The catalytic activities of catalysts loaded on carbon nano-materials were measured by cycle voltammetry method.The durability of Pt／CNTs was demonstrated by chronoamperometry test at a fixed potential of 0.5 V in 0.5 M CH3OH+0.5 M H2SO4mixed solutions.High-purity argon was purged through the solution for 10 min before the tests were performed. All electrochemical experiments were performed at ambient temperature（25±1℃）.

3 Results and Discussion

3.1 Effect of Deposition Circles on Catalyst

Pt／CNTs catalysts prepared by electrochemical deposition method can control the deposit amount by controlling the cycle turns.So appropriate cycle turns are essential to guarantee the grain size and the deposit amount.SEM image of Pt／CNTs catalysts prepared by different cycle turns is shown in Fig.1 with scanning potential ranging from-0.3 to 1.3 V，scanning speed at 50 mV／s.

Fig.1 SEM images of Pt／CNTs prepared by different deposition turns

As can be seen in Fig.1，Pt particles disperse uniformly on the surface of CNTs and the grain size is about 150 nm.When added to 130 circles，Pt particles began to reunite.When added to 160 circles，Pt particles size is about 250 nm，and covers the whole CNTs.Pt nano-particles grow up with cycle turns addition because the original nucleus begin to form at low cycle turns.And nucleating is slow and disperses evenly.Then Pt particles quickly grow up on the basis of the original nucleus until a reunion at high cycle turns［14］.In this paper，deposition circles at 60 are taken.

As can be seen in Fig.2，the peak current for methanol oxidation increases with deposition circles addition，reaching maximum at 130 circles.The reason is that when the deposition circles were small，Pt particles dispersed evenly on the surface.With cycle turns addition，the effective surface of catalyst increases，and the peak current for methanol oxidation also increases.When the scanning laps reach a certain amount，the active position provided by CNTs will be occupied by Pt particles deposited before.Then，the catalysts will grow up，and the active surface will decrease，creating the catalytic properties cutting down，which is consistent with the SEM images.

3.2 Effect of Cyclic Voltammetry Scanning Speeds on Catalyst

SEM images of Pt／CNTs at different scanning speeds of 25 mV／s，50 mV／s，75 mV／s，respectively，scanning potential ranging from-0.3-1.3 V are shown in Fig.3.

As can be seen in Fig.3，Pt nano-particles disperse uniformly on the walls of CNTs.Pt grain size increases with scanning speeds addition.At 25mV／s，Pt particle reaches about 30 nm.At 50 mV／s，Pt particle reaches about 40 nm.At 75 mV／s，Pt particle reaches about 90 nm.

As can be seen in Fig.4，with the increase of scanning speed，the peak currents for methanol oxidation reaction get smaller and smaller.So both for Pt particles morphology and the catalytic properties，cyclic voltammetry scanning speed in electro-deposition is the smaller the better.In this article，25 mV／s scanning speed is chosen to prepare the catalysts［15］.

3.3 Effect of Ascorbic Acid Treatment on Catalyst Properties

Pt／CNTs catalysts prepared by cyclic voltammetry，scanning potential ranging from-0.3-1.3 V，scanningspeed at 25 mV／s，and running 60 circles have a relatively smaller grain size（30 nm）.But comparing with the commercial catalysts，its grain size is too big. The obtained Pt particles prepared by above method is treated with ascorbic acid，further reducing the catalyst particle size in 30 mM ascorbic acid+0.1 M H2SO4mixed solutions using pulse potential.SEM images of catalysts are treated and untreated by ascorbic acid respectively，as shown in Fig.5.

Fig.2 Curves of peak current for methanol oxidation varying with cycle turn

Fig.3 SEM images of Pt／CNTs catalyst in different cyclic voltammetry scanning speeds

SEM images in Fig.5 reveal some differences between the two images.In Fig.5（a），there are much more Pt particles on the surface，but some particles reunite.And after dissolving in the ascorbic acid，the amount of Pt particles is cutting down，as well as the grain size.Consequently，it is also improved that the grain size of Pt particles can be reduced by cyclic voltammetry method dissolution in ascorbic acid.

Fig.4 CV cures of Pt/CNTs catalysts prepared at different scanning speeds in 0.5 M methanol+ 0.5 M H2SO4

Fig.5 Effects of ascorbic acid dissolyed

As can be seen in Fig.6，the electro-activity decreases a little after dissolution in ascorbic acid because the amount of Pt particles is lower.But Pt particles disperse much evenly than before.And the remaining solutions can be reused again，which can save the amount of Pt.

3.4 Catalyst Stability Test

Chronoamperometry is performed to measure catalytic stability of the electro-catalyst in Fig.7.It is the measurement of its oxidation current with time，done in 0.5 M CH3OH+0.5 M H2SO4at 0.5 V for 7 000 s.

Initial decrement in the current value is due to the formation of intermediates during methanol oxidation. The current of Pt／CNTs is higher compared to other electro-catalysts during the whole testing time，indicating higher stability of the catalysts.Hence one can conclude that the CNTs are a better alternative for anode catalyst support［16］.

Fig.6 CV curves of Pt／CNTs catalyst before and after treatment in ascorbic acid in the methanol solution

Fig.7 Chronoamperometry curve of Pt／CNTs catalyst in 0.5 M H2SO4+0.5 M CH3OH solutions

4 Conclusions

Pt／CNTs catalysts are prepared by electrodeposition using cyclic voltammetry method.Control the amount of deposition turns to decide the amount of Pt particles，and decide the scanning speeds to control the deposition speeds of Pt nano-particles，the grain size of Pt nano-particles and the degree of its dispersion.The obtained catalysts reach about 30 nm by choosing the scanning speeds at 25 mV／s，cycling 60 turns.After dissolution treatment in ascorbic acid，smaller size catalysts have a better catalytic activity and stability.The direct methanol fuel cell performance of the anode and cathode electrodes using these catalysts will be presented in future work.

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TM911.46

：

：1005-9113（2015）05-0085-04

10.11916／j.issn.1005-9113.2015.05.013

2014-04-16.

Sponsored by the Fundamental Research Funds for the Central Universities（Grant No.HIT.ICRST.2010005）.

?Corresponding author.E-mail：yangpeixia＠hit.edu.cn.