Dynamic electrochemical impedance study of methanol oxidation at Pt at elevated temperatures

Highlights

• Methanol oxidation on Pt is studied at temperatures up to 140 °C.

• Dynamic electrochemical impedance spectra are correlated with voltammetry.

• A reaction mechanism is suggested based on the experimental results.

• The reaction mechanism is similar at all temperatures.

• Three potential regions with different rate-determining steps are identified.

Abstract

The methanol oxidation reaction (MOR) is studied at temperatures up to 140 °C by a combination of cyclic voltammetry, chronoamperometry, and dynamic electrochemical impedance spectroscopy (dEIS). A mechanistic analysis of the reaction is done based on the Tafel relation, the calculated activation energy, and the fitting of dEIS data. At the fuel cell relevant potentials, 0–0.80 V vs RHE, the MOR mechanism is similar at all temperatures. The rate-determining step is the adsorption of water at low overpotentials (< 0.50 V vs RHE), a combination of the methanol adsorption reaction and the surface reaction between adsorbed CO and OH at intermediate potentials (0.50–0.65 V vs RHE), and the methanol adsorption reaction at high potentials (> 0.65 V vs RHE). The shoulder on the oxidation peak at 0.60–0.65 V vs RHE corresponds well with where the CO coverage approaches zero.

Introduction

The methanol oxidation reaction (MOR) attracts interest due to its application in direct methanol fuel cells (DMFCs) [[1], [2], [3], [4]]. Platinum is the best mono-metallic catalyst for the MOR and on Pt in acidic solution, the MOR proceeds through a dual pathway mechanism [5]. The pathways are commonly denoted as the indirect pathway, through strongly adsorbed CO and further oxidation to CO₂, and the direct pathway, through short-lived or no adsorbates yielding formaldehyde, formic acid, or CO₂. A high selectivity towards CO₂ production is desirable due to both higher efficiency and because formaldehyde and formic acid are toxic products. An increase in reaction temperature gives higher selectivity towards CO₂ production, up to 100% at 100 °C [6]. Thus, studies at high temperature can help in understanding the MOR at high CO₂ yield and high overall reaction rate.

A simplified reaction mechanism for the indirect pathway is given in Eqs. (1), (2), (3) [7,8]. From this mechanism, it is evident that the adsorbates, PtCO and PtOH (from now called “CO” and “OH”), play a major role in the reaction. CO is known to be present at high coverages at low potentials and may block the MOR [[9], [10], [11]]. The activation of water, Eq. (2), also has a large effect. This is well illustrated by the success of bimetallic catalysts where Pt is combined with a co-catalyst with higher affinity towards water, e.g., Ru, Sn, Cu [12]. It should be noted that the nature of the activated water is not entirely clear, and it can be either OH, O, or some form of adsorbed water [13]. The pH also has a large effect on the reaction rates [14], and might influence the nature of these species and the overall reaction mechanism. Conventional electrochemical methods like cyclic voltammetry and chronoamperometry are commonly used to examine reaction kinetics. However, these methods are often inconclusive when studying multistep reactions involving several adsorbates, e.g., the MOR [15,16]. Therefore, complementary techniques are necessary to distinguish closely-related mechanisms.

$$\text{Pt}+{\text{CH}}_3\text{OH}\to \text{PtCO}+4{\text{H}}^{+}+4{\text{e}}^{-}$$

$$\text{Pt}+{\text{H}}_2\text{O}\leftrightarrows \text{PtOH}+{\text{H}}^{+}+{\text{e}}^{-}$$

$$\text{PtCO}+\text{PtOH}\to {\text{CO}}_2+2\text{Pt}+{\text{H}}^{+}+{\text{e}}^{-}$$

Electrochemical impedance spectroscopy (EIS) has great potential as a tool in mechanistic studies [[17], [18], [19], [20], [21], [22]]. In particular, EIS run over a wide frequency range can help identify processes with different time constants. EIS also has the advantage that it can be run simultaneously with cyclic voltammetry as dynamic EIS (dEIS). For the MOR, EIS and dEIS have been used for mechanistic analysis [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. In particular, work by Sundmacher and coworkers [16,29] demonstrates the way that EIS can be used to provide essential information when elucidating reaction kinetics.

In this work, a recently developed apparatus for high temperature aqueous electrochemistry [34,35] was used to study methanol oxidation between 21 °C and 140 °C. The heating program was optimized from the earlier work to make it possible to run slow experimental techniques, i.e., low sweep rate ($\le$ 5 mV s⁻¹) cyclic voltammetry, chronoamperometry, and dEIS. A mechanistic analysis of data from all three types of experiments is used to suggest a five-step reaction mechanism for the indirect pathway.