Electrochemical performance of Mo 2 C @ PtRu synthesized by electrochemical deposition method on methanol oxidation

The electrodeposition of Platinum and Ruthenium nanoparticles into Molybdenum carbide/ glassy carbon electrodes and their catalytic activity for the oxidation of methanol are described. These Mo2C@PtRu electrodes exhibit good activity with respect to the catalytic oxidation of methanol. The electrodes exhibited excellent long term stability in the acidic methanol solutions.


Introduction
We report herein an electrochemical study to demonstrate the feasibility of using Mo2C@PtRu catalyst synthesized trough electrochemical deposition method to replace platinum (Pt) as anode electrocatalysts for the electrooxidation of methanol.
An effective anode electrocatalyst in direct methanol fuel cell (DMFC) should be active for the decomposition of methanol while remaining stable under the relatively harsh anode environment. It should also have high activity for the oxidation of carbon monoxide (CO), which is a reaction intermediate that poisons the surface by occupying the active sites. Although the Pt/Ru bimetallic alloy is currently the most effective anode electrocatalyst, both Pt and Ru are expensive due to limited supplies and both are susceptible to CO poisoning due to the strong bonding of CO on the active surface [1][2][3].
Consequently, the use of a supporters may influence the methanol oxidation, by decreasing the amount of Pt/Ru catalysts used and their contamination by CO. The discovery of less expensive and more CO tolerant alternative electrocatalysts is of both fundamental and practical importance for the understanding and development of DMFC [4].
Hence, efforts are being channeled toward obtaining a support material that simultaneously optimizes the catalyst dispersion, loading, and electrocatalytic efficiency. An ideal support-catalyst assembly should ensure facile molecular transport of reactants and products, have good electronic conductivity, and possess high surface reactivity; these would enhance the molecular conversion. Molybdenum carbides, with their resemblance in electronic structure to noble metals, have similar catalytic properties to noble metal catalysts in various organic chemical reactions. It makes molybdenum carbides to be a promising support of Pt catalysts [5,6].
In this work, the Mo2C@PtRu electrocatalysts prepared by spontaneous electrochemical deposition of Ru and Pt on Glass Carbon supported Molybdenum carbide particles were tested for methanol oxidation using Cyclic voltammetry and Amperommetric techniques.

Chemicals
Reagent-grade Molybdenum carbide and spectro-grade methanol (Fisher Scientific),Ruthenium Chloride and potassium hexachloroplatinate (Aldrich) were used as received. Water was distilled and subsequently passed through a Millipore Q water purification system prior to use. Aqueous solutions were prepared daily with the Millipore Q water.

Electrodes, Electrochemical Cells, and Instrumentation.
A silver/silver chloride (saturated KC1) electrode was used as the reference electrode and potentials are reported with respect to this reference. The glass carbon electrodes were hand-polished successively with 600 grit, 1.0, 0.3, and 0.05-pm alumina slurries with copious rinsing with Millipore Q water and ultrasonicated between subsequent polishing stages. The counter electrode was a Pt wire, which was isolated by a frit.
All electrochemical measurements were performed with either a PAR Model 760 potentiostat. Data were acquired with a HP 7035 B X-Y recorder (cyclic voltammetry) or a Houston Omniscribe recorder (chronoamperometry).

Preparation of the Mo 2 C/GC electrode.
The Mo2C/GC electrode were prepared as follow: 5mg of Mo2C were dispersed in 2ml of water under ultrasonication agitation for 30 min to form the electrocatalyst ink. 20μl of ink was deposited on GC electrode and dried at room temperature. After six hours,5 μl of ethanol and 15 μl of 5wt% of Nafion were added to the electrodes and dried at room temperature.

Evaluation of the Mo 2 C@PtRu electrodes
The catalytic activities of the Mo2C@PtRu were evaluated with regard to the generation of hydrogen from 0.  which is caused by desorption of hydrogen (the first region). During the backward sweep, another peak is present at -0.13 V which is caused by adsorption of hydrogen (the second region). The presence of these peaks indicates that Mo2C@PtRu electrode strongly bonds to hydrogen atoms on the surface-active sites [8]. In the second region, it is known that Pt oxide formation on the electrode surface. The Pt oxide formation is more complex than hydrogen adsorption. During the verse sweep, we observed a single peak in the third region one normally assigned to Pt oxide reduction. As shown in this figure, the Pt oxide reduction peak potential was 0.52 V on the Mo2C@PtRu electrode. After 100 repetitive cycles, the remaining current of Pt oxide reduction on the Mo2C@PtRu electrode was 96.4% of the initial current. This indicated that the Mo2C@PtRu electrode is highly stable in acidic medium.
The Mo2C@PtRu electrocatalysts have a very defined hydrogen oxidation in the potential range of (-0.2 to 0V),which is common for all platinum based catalysts, and the double layer region(0 to 0.4V) are large which characterize the At elevated temperatures, the methanol oxidation reaction at platinum and platinum ruthenium electrodes have been studied extensively [6]. Generally, on pure Pt electrodes the current density of MeOH change with temperature and the peak of MeOH oxidation shifts towards more negative potentials with increasing temperature thus indicating that the methanol oxidation mechanism is affected by the temperature referring to Fig.2.A. However, to obtain real performance of the electrocatalysts a substraction of the background currents is necessary. Fig.2  The electrooxidation of carbon monoxide on platinum in aqueous acidic electrolytes is one of the most extensively studied electrochemical reactions over the past decades. The main interest is centred on two practical problems pertaining to the application of platinum based catalysts; (i) the search for catalysts for DMFC´s and (ii) the development of CO-tolerant anodes for DMFCs. For both types of fuel cells, it is essential to oxidize and remove CO from the platinum surface at lower over potentials. Fig.3 displays the CV of Carbon monoxide oxidation on Mo2C@PtRu,when the scan rate changed from 10 to 100mV/s.It is observed that the carbon monoxide mixed with HClO4 influences the variation of current as the scan rate changed and the synthesized catalyst may be contaminated with CO at high potential 0.7V. which is related to the methanol oxidation. At the oxidation potential of 0.5V the current suddenly drops to a low steady state current value. The origin of this low current is mostly rooted from the presence of strong adsorbed intermediates such as CO. The activity shown by Mo2C appears to coincide with the Mo2C@PtRu surface and marks a higher current than PtRu commercial catalyst. This enhancement in activity is attributed to an improved CO tolerance of the Mo2C surface, which is consistent with the observation of lower CO desorption [9].

Conclusion
Our results show that Mo2C@PtRu synthesized trough electrochemical deposition is active to methanol oxidation and is a promising alternative electrocatalyst of Pt catalyst at voltages up to 0.8 V. At the same time, our findings also indicate that Mo2C@PtRu is more stable for applications with significantly increasing the activity. In our view the potential applications of Mo2C@PtRu rooted from the strong interaction of Mo2C support with the PtRu. By Comparing with the PtRu commercial catalyst, where only the surface Pt and Ru atoms of the Pt and Ru particles are used for electrocatalysis, all Pt and Ru atoms in Mo2C@PtRu remain in the surface, which should lead to a significant reduction in the noble metals loading.

Conflicts and Interest
The authors declare that there is no conflict of interest.