The Effect of Anions on the Oxygen Reduction Reaction Activity and Selectivity on a Au Electrode: A Double Disk Electrode Flow Cell ATR-FTIR Spectroscopy Study

The oxygen reduction reaction (ORR) on Au electrode exhibits a sluggish kinetics and its selectivity is sensitive to adsorbed hydroxyl intermediates and the pH [1,2]. Adsorbed anions are known to strongly affect the ORR in the kinetic region via the occupation of surface sites required for the dissociative adsorption of O₂ molecules. Also the ORR selectivity for the reduction to water instead to hydrogen peroxide is affected when going from weakly adsorbing to more strongly adsorbing anions [3,4].

Application of in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS) was demonstrated recently to offer important insights on the nature of adsorbed species / intermediates in the ORR [5,6], which was recently extended to ORR studies using a spectro-electrochemical flow cell [7]. However, the application of in situ ATR-FTIRS, which allows correlation of the ORR activity and the presence of adsorbed species, so far has not provided any direct information on the impact of the adsorbed anions on the ORR selectivity. To pursue this aim, a spectroelectro-chemical dual thin layer flow cell, used for combination of in situ ATR-FTIRS, online mass spectrometry and electrochemical measurements [8], was modified for simultaneous electrochemical detection of hydrogen peroxide at the second (Pt collector) electrode  [9].

A thin (~30 nm) Au film was electrolessly deposited [10] onto the face plane of a hemi-cylindrical Si prism, and then carefully annealed in a butane flame in a N₂ atmosphere to improve the film stability, while maintaining a high IR sensitivity (about three-fold higher linearly adsorbed CO band intensity at 0.0 V in CO saturated solution compared to the literature [5]). A homemade mirror accessory was used within the sample chamber of a Varian 670i FTIR spectrometer, equipped with a p-polarizer and a liquid nitrogen cooled mercury cadmium telluride detector. Measurements were performed at a spectral resolution of 4 cm^-1 and a temporal resolution of 1 s^-1 per spectrum (by co-adding 5 interferograms). The Pt collector was biased at 1.2 V (RHE) for hydrogen peroxide oxidation (collection efficiency of ca. 20%). O₂ saturated solutions of perchloric, sulfuric acids (0.5 M) or sodium hydroxide (1.0 M) were used.

The ORR starts at ca. 0.5, 0.6 and 1.0 V (RHE) in perchloric, sulfuric acids, and alkaline solution, respectively. It does not reach the mass transport limitation up to 0.0 V in acidic solutions, whereas in alkaline medium the mass transport limited ORR current is reached at about 0.5-0.6 V, and slightly decreases at lower potentials due to increasing hydrogen peroxide formation. The hydrogen peroxide yields of 60 to 80% were found at potentials 0.0 to 0.5 V in both perchloric and sulfuric acid solutions, in agreement with gradually increasing integrated band intensities of the corresponding anions. In contrast, in alkaline solution the hydrogen peroxide yields are close to 0% at the onset of the ORR on Au film at high potentials and gradually increase with decreasing potential to ca. 60 % at 0.0 V. In this case, IR spectroscopy is little helpful since it cannot discriminate between adsorbed water, hydroxyl or peroxyl. A correlation between the variation of the ORR activity and selectivity in alkaline solution and the OH adsorption is derived indirectly, by i) comparing the double layer current obtained on a Au film electrode in alkaline solution, and from ii) additional measurements in CO saturated 0.1 M NaOH solution, where the mass transport limited current is reached over a wide potential range (0.5 - 1.2 V). These results strongly support the presence of adsorbed OH in this potential range, which, however, does not favor the ORR to hydrogen peroxide.

Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2)

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