Electrocatalytic oxidation of methanol and carbon monoxide at platinum in protic ionic liquids

abstract Article history:Received 6 June 2012Received in revised form 16 July 2012Accepted 17 July 2012Available online 24 July 2012Keywords:Direct methanol fuel cellsElectrocatalysisProtic ionic liquid Oxidation of H 2 O, CO and CH 3 OH was investigated at Pt as a function of temperature in the protic ionic liquiddiethylmethylammonium triﬂuoromethanesulfonate. Trace H 2 O oxidation in the ionic liquid results in cover-ageofthePtwithadsorbedoxides.Increasingthetemperaturesigniﬁcantlyreducesthepotentialatwhichthisreaction occurs.CH 3 OHandCOoxidationkineticsincreased signiﬁcantlywithincreasingtemperatureandox-idationofeachspeciescoincidedwithcoverageofthePtsurfacebytheoxide.Theseobservationsindicatethatsurface oxides are required for complete oxidation of CH 3 OH to CO 2 in the protic ionic liquid, in a similar waytothatobserved inconventionalaqueouselectrolytes.Whiletheoverpotential forCH 3 OHoxidationwasdras-tically higher than that observed in purely aqueous electrolytes, itdecreased with increasing water content ofthe ionic liquid. The results described here have implications for the development of protic ionic liquid elec-trolyte fuel cells and these implications are discussed.© 2012 Elsevier B.V. All rights reserved.


Introduction
The oxidation of methanol in water using Pt electrocatalysts is the anode reaction in direct methanol fuel cells (DMFCs) and is a complex reaction involving the transfer of six electrons and six protons: The formation of adsorbed intermediates such as CO on the Pt surface results in a methanol oxidation overpotential of several hundred millivolts that reduces the efficiency of DMFCs [1,2]. One approach to solving the problem of Pt poisoning is to include into the catalyst a metal such as Ru, which readily forms surface oxides that can facilitate the complete oxidation of CH 3 OH to CO 2 [3]. Alternatively, one can raise the temperature, T, of the cell but the Nafion® membranes usually used in DMFCs must be hydrated to conduct protons. Consequently, T is usually kept below 100°C to ensure the presence of liquid H 2 O in the electrolyte [4,5].
A particularly promising approach towards the development of fuel cells operating above 100°C is to use protic ionic liquid (PIL) electrolytes, which can conduct protons in the absence of liquid water and have very low vapour pressures. PILs have been used as electrolytes in H 2 fuel cells and remarkably high rates of O 2 reduction and H 2 oxidation have been observed at elevated T [6]. The high rates of O 2 reduction in PILs are particularly promising, generating open circuit potentials (OCPs) exceeding that achievable in conventional H 2 -fuelled polymer electrolyte membrane fuel cells containing hydrated electrolytes [8].
The aim of this work is to explore the feasibility of using PILs as electrolytes for DMFCs. If CH 3 OH oxidation does occur rapidly above 100°C in PILs, it is possible that an efficient DMFC could be built with a higher OCP than that of conventional DMFCs. We studied CH 3 OH oxidation electrocatalysis at Pt in diethylmethylammonium trifluoromethanesulfonate, [dema] [TfO], which was chosen because O 2 reduction occurs rapidly in this liquid [6]. After discussing the electrochemistry of [dema][TfO] at Pt electrodes as a function of T (and in particular the role of trace H 2 O oxidation at the Pt electrode), we describe the oxidation of CO ads at Pt in [dema] [TfO]. These observations are then correlated with observations made during the oxidation of CH 3 OH, allowing us to study the role of CO and H 2 O during CH 3 OH oxidation in this PIL. Finally, we discuss the prospects for the use of PIL electrolytes in DMFCs and make some recommendations for the development of electrocatalysts for CH 3 OH oxidation in this PIL.

Reagents and apparatus
All chemicals were reagent grade and were used as received. [dema] [TfO] was prepared according to published procedures and dried by stirring at 70°C under vacuum (6× 10 −2 mbar) for 48 h [9]. The PIL's water content was measured using a coulometric Karl Fischer apparatus (Mitsubishi CA100). Electrochemical measurements were performed using a model 760C potentiostat (CH Instruments, Austin, TX).

Electrochemical measurements
The sealed electrochemical cell contained a 2 mm diameter Pt disk working electrode, a Pt flag counter electrode and Ag/Ag + reference electrode, which was constructed by immersing an Ag wire in 10 mM [Ag] [TfO] in [dema] [TfO]. Controlled T measurements were performed by immersing the entire cell in an oil bath on a hot plate. The Pt disk electrode was polished using an aqueous suspension of 0.05 μm alumina on a felt polishing pad, cleaned ultrasonically in water, rinsed with water and dried under Ar.
Prior to use, the electrochemical cell was charged with~5 mL of [dema][TfO] and dried overnight. CO stripping experiments were performed by first holding the potential of the Pt electrode at 0.1 V vs. Ag/Ag + in CO-saturated [dema][TfO] for 10 min. The PIL was then purged of CO by bubbling with Ar for 30 min (while holding the Pt electrode at 0.1 V) and CO was stripped from the Pt surface during an anodic potential sweep. CH 3 OH oxidation experiments were performed after addition of the appropriate amount of CH 3 OH and/or H 2 O to the cell (see below for further details) while under an Ar atmosphere. Hydrodynamic voltammograms were obtained by vigorously stirring the PIL with a magnetic stirrer bar while sweeping the potential of the working electrode. The anodic peaks between 0.5 and 1.0 V were due to oxidation of trace H 2 O in the PIL resulting in the formation of an adsorbed oxide layer on Pt, which we call Pt-OH ads , and the cathodic peaks were due to reduction of Pt-OH ads to Pt [7,10]:

Electrochemistry of [dema][TfO] at Pt
The appearance of this process demonstrates that, even after heating the PIL under vacuum at 70°C for 48 h and isolating the cell from the atmosphere, the concentration of trace H 2 O (244 ppm) was sufficient to cause the formation of an adsorbed oxide on the Pt surface. The peak-to-peak separation for OH ads formation/removal was especially large at low T, suggesting that Pt-OH ads formation/removal was slow. However, as T increased, the peak-to-peak separation decreased, demonstrating that the Pt-OH ads formation/removal was activated with increasing T.

CO oxidation in [dema][TfO]
In conventional aqueous electrolytes, CH 3 OH oxidation to CO 2 involves a surface reaction between adsorbed OH ads , which forms due to H 2 O oxidation (Eq. (2) above), and adsorbed methanol oxidation products. The most recalcitrant poisoning species is CO ads and complete oxidation of CO ads to CO 2 occurs via reaction with OH ads [3]: Pt À CO ads þ Pt À OH ads →2Pt þ To investigate whether a similar mechanism occurs during oxidation of CH 3 OH in [dema][TfO], the electrochemistry of CO was studied at Pt. Fig. 1B shows linear sweep voltammograms obtained during stripping of CO ads from Pt in [dema][TfO] at a series of temperatures. When the potential was scanned in a positive direction at each temperature, an anodic peak was observed, which we attribute to oxidation of CO ads on the Pt surface. Fig. 1B shows that, as T increased, the stripping peak narrowed and moved to more negative potentials. It is possible that changes in the adsorption mode of CO on the Pt surface (a factor that has yet to be studied in PILs) could be responsible for the observed behaviour. However, this is unlikely as the potential was controlled during pre-concentration of CO on the Pt surface. Furthermore, the shift in the CO oxidation onset as T increased mirrored the shift in the wave due to OH ads formation in Fig. 1A, suggesting that CO ads oxidation only occurred in the PIL when the Pt surface was covered with OH ads . This behaviour is similar to that observed in aqueous electrolytes, given by Eq. (4). [TfO] containing 0.5 M CH 3 OH. In each case, a broad peak was observed during the forward sweep and comparison of these CVs with the blank CVs (Fig. 1A) demonstrates that the peaks were solely due to CH 3 OH oxidation in [dema] [TfO]. During the negative sweep, the current rapidly decayed to a negligible value until a potential of approximately 0.2 V, when a small anodic peak appeared that increased in height with increasing T (Fig. 2A inset). The anodic peak appeared at the potential where Pt-OH ads removal from the Pt surface occurred (Fig. 1A), suggesting that, during the negative sweep, CH 3 OH oxidation restarted as some OH ads was removed from the surface [11]. However, the significant decrease in the current upon switching the potential scan direction differs from the behaviour observed in aqueous solution, in which a large oxidation peak is observed near the switching potential. To explore this effect further, chronoamperometric measurements were used to follow the evolution of the catalytic current as a function of time. Interestingly, the methanol oxidation current decreased rapidly to a negligible value (data not shown), suggesting that catalyst deactivation was occurring at positive potentials. This deactivation does not appear to be due to the limited amount of H 2 O in the PIL, as increasing the H 2 O content does not remove the effect (for example, see Fig. 2B). The decrease in activity during the negative sweep does appear to be associated with poisoning of the electrode as the magnitude of the drop in activity during the reverse sweep depended on the positive switching potential and further work will be required to understand these effects fully.

CH 3 OH oxidation in [dema][TfO]
Comparison of Figs. 1B and 2A reveals that the onset potentials for CO ads oxidation and CH 3 OH oxidation were similar at each T, suggesting that CH 3 OH oxidation occurred in [dema][TfO] at an appreciable rate when CO ads reacted with OH ads on the Pt surface, i.e., by a similar mechanism to that given in Eqs. (2)-(4). In other words, it appears that the availability of OH ads on the Pt surface governs the onset potential for complete CH 3 (Fig. 2B). It is clear that adding successively higher concentrations of H 2 O to the PIL resulted in a progressive shift of the CH 3 OH oxidation wave to less positive potentials. This result demonstrates that CH 3 OH oxidation becomes more facile as the H 2 O content of the PIL increases, which is presumably due to the increased coverage of the Pt surface with OH ads at lower potentials as the concentration of H 2 O increased [10]. This result could have implications for the development of DMFCs containing PIL electrolytes. On one hand, the use of PILs allows one to access higher T and enhance O 2 reduction kinetics at the cathode. On the other hand, our results demonstrate that loss of H 2 O from the electrolyte will result in a large overpotential for CH 3 OH oxidation that will reduce the cell performance. In separate measurements, we have measured the potential at which H 2 oxidation occurs in [dema][TfO] [7], which corresponds to a potential of −0.63 V vs. Ag/Ag + , demonstrating the very high overpotential for CH 3 (Fig. 2B). One possible solution to this problem of high overpotential for CH 3 OH oxidation in [dema][TfO] is to use alloy electrocatalysts such as PtRu, which should lower the potential for OH ads formation. As our work also shows, increasing T will probably not reduce the effects of poisoning of protic ionic liquid electrolyte fuel cell anodes with CO from H 2 fuel supplies (Fig. 1B) and it is likely that the use of PtRu catalysts will also aid in mitigating such poisoning.

Conclusions
The electrochemical oxidation of CO and CH 3 OH has been studied at Pt electrodes in the protic ionic liquid [dema][TfO]. Our results demonstrate that oxidation of trace H 2 O, which is unavoidably present in this liquid, provides the adsorbed oxygen species necessary for complete oxidation of CH 3 OH. However, our results suggest that catalyst deactivation occurs readily in this liquid and that the overpotentials for both CO oxidation and CH 3 OH oxidation are very large. Future work in our group will focus on studying the electrochemical stability of Pt catalysts in protic ionic liquids. In addition, the inclusion of readily oxidised metals such as Ru to the Pt catalyst may aid in reducing the overpotential for methanol oxidation and we are currently exploring this possibility.