Detecting Intermediates of the Electrochemical Oxygen Reduction Reaction on the Model Semiconductor Germanium

Detecting Intermediates of the Electrochemical Oxygen Reduction on the Model Semiconductor Germanium

Parts of this research project are published [1]

Simantini Nayak, P.U. Biedermann, Andreas Erbe

Max-Planck-Institut für Eisenforschung GmbH, Department of Interface Chemistry and Surface Engineering, Düsseldorf

The electrochemical oxygen reduction reaction (ORR) is of fundamental importance for a number of fields in chemistry, including conversion of chemical to electrical energy and corrosion [2].  However, the lack of understanding of the reaction mechanism is a major bottleneck in the developemnt of efficient fuel cell catalysts and in the prevention of corrosion.  ORR involves the transfer of 4 electrons and 4 protons to the O2 molecule and breaking the formal double bond.  The net reaction in acidic electrolyte is:

Recently, interest in the reaction mechanism has increased due to its role in fuel cells and air batteries.  Several major pathways have been put forward, including a direct mechanism based on dissociative adsorption of O2 on the electrode surface, or different variants of multi-step reaction mechanisms with step-by-step electron transfer, which proceed via superoxide radicals and/or peroxides to the final dissociation of the O-O bond [2].

Intermediates that are stable enough to diffuse away from the electrode surface have been deteted by electrochemical experiments [3,4].  Furthermore, vibrational spectroscopic techniques have been used to detect intermediates during ORR on rough noble metal films relevant to fuel cell applications [5,6].  Vibrational spectroscopy is a powerful tool to analyze the nature of transient oxygen-related species on the surface, as peak frequencies can clearly distinguish between double bonds (~1550 cm-1) as present in O2, single bonds (~800 cm-1) as present in peroxides including H2O2 and bonds with bond order 1.5 (~1200 cm-1) as present in superoxides.

The semiconductor Ge is a model for semiconducting passive films, as under ambient conditions most metallic materials are covered with protective semiconducting oxides.  In this work, ATR-IR spectroscopy and density functional theory (DFT) calculations have been used to study the mechanism of the ORR and the intermediates on Ge(100) in 0.1 M HClO4.

Fig.1 Spectro-electrochemical atenuated total reflection infrared ATR-IR setup used for the in-situ and in-operando detection of ORR intermediates

Experimental:  The setup consists of the optical base of a horizontal ATR unit.  The crystal plate was replaced by a copper piece mounted on PTFE that was used to electrically contact the Ge ATR crystals.  Trapezoidal Sb-doped n-type Ge(100) crystals (10-40 Ω cm) were used as substrates.  The short edges were polished to an angle of 30° leading to an incidence angle of 60° and ca. 100 internal reflections at the Ge/electrolyte interface.  A PTFE basin was screwed onto the base part pressing the Ge crystal against the copper piece and sealing the electrochemical cell, which includes a Pt loop as counter electrode, a double junction Ag/AgCl/ 3 M KCl microreference electrode, gas inlets and outlets for Ar or O2, and 0.1 M HCLO4 electrolyte.  The Ge crystal served as working electrode.  IR spectra were recorded with an FTIR spectrometer in s- and p-polarization and are shown as absorbance relative to a reference spectrum at the initial potential of +0.01 VSHE.  The tilt angle θ of the transition dipol moments (TDM) relative to the surface normal were determined from the dichroic ratio and squared amplitudes of the electric field components via the orientational order parameter S2 assuming a single orientation [7].

Fig.2: CV (50 mV s-1) of n-Ge(100) in O2 (red lines) and Ar (blue lines) saturated 0.1 M HClO4. Arrows indicate the scan direction. Average currents during ATR-IR experiments in O2 (red dots) and Ar (blue dots).

Calculations:  The models for electronic structure calculations were based on small Ge-clusters representing the germanium surface interacting with the solvated ORR intermediates.  For DFT calculations the program package TURBOMOLE version 6.2 was used.  The hybrid density functional B3-LYP was used with the basis set def2-TZVP and the RI-JK approximation.  Vibrational frequencies were scaled by 0.97.  Solvent effects were simulated by a first shell of explicit water molecules and the conductor like screening model COSMO with ε = 78.36.  This approach gives vibrational frequencies in agreement with experiment within a standard deviation of ±35 cm-1 (for details see [1]).  The direction of the TDM was calculated using the keyword $PRTVTM.

Fig.3: ATR-IR absorbance spectra (p-polarisation) in O2 and Ar saturated 0.1 M HClO4 on Ge(100) in the direction of (a) more negative and (b) less negative potential.

Results:  The CV in Ar (Fig.2) shows the hydrogen evolution reaction (HER) at potentials below -0.6 VSHE, oxidation above +0.1 VSHE and a small cathodic peak at -0.3 VSHE attributed to the change in surface termination from Ge-OH to Ge-H [8,9].  In oxygen saturated solution, substantially increased currents are observed in the range -0.3 to -0.7 VSHE.  Figure 3 shows a series of ATR-IR spectra recorded using p-polarisation .  Three vibratonal bands are observed, at 1385 cm-1, 1210 cm-1, and 1030 cm-1 during ORR on Ge(100).  The orientations of the corresponding transition dipol moments are (35±10)°, (28±10)°, and (10±5)°, respectively.  The band intensities increase when stepping to more negative potentials (Fig. 3a, increasing ORR currents) and decrease when stepping to less negative potentials (Fig. 3b).  Under Ar no bands are observed. 

Assignment:  The observed band at 1385 cm-1 is in the range of O-O-H bending modes of peroxides.  It could be due to the asymmetric bending mode of H2O2 in solution (expt: 1380 cm-1, calc. 1359 cm-1 [1]), or due to a surface-bound peroxide Ge-OOH (calc. 1427 cm-1 [1]).  The distinctly upright orientation of the observed TDM would be difficult to explain for a freely rotating solution species, for which a value corresponding to the magic angle of θ=54° would be expected.   However, the orientation of the TDM calculated for the Ge-OOH species (Fig.4a) is in reasonable agreement.

The observed band at 1210 cm-1 is in the range of O-O stretch modes of superoxides.  The calculated frequencies are 1183 cm-1 for the superoxide anion (OO•-), 1196 cm-1 for the peroxide radical (HOO) and 1159 cm-1 for a surface-bound superoxide (Ge-OO).  An assignment based on the small differences in the calculated vibratonal frequencies would be ambiguous, however, the very upright orientation of the observed TDM clearly argues for the surface-bound superoxide (Fig.4b).

The band at 1030 cm-1 is in the range of the symmetric Cl-O stretch mode of HClO4 (expt. 1039 cm-1, calc. 963 cm-1) or of a surface-bound Ge-O-ClO3 (calc. 964 cm-1).  Again the upright orientation of the observed TDM leads to the assignment of a surface-bound species (Fig.4c).

 

Fig.4: Structures of the explicitly solvated surface clusters optimized with COSMO. Important vibrational modes are represented by their atomic velocity vectors. The TDM is indicated in cyan and the surface normal in black. Together with the chemical formula of the cluster, the wavenumber and tilt angle of the TDM are given.
Fig.5: Catalytic cycle with the mechanism proposed for the ORR on Ge. The surface-bound superoxide and peroxide species drawn in red were identified in this work.

Reaction Mechanism:  The observation of a Ge-bound superoxide as an intermediate in the ORR suggests that the ORR starts by the combination of an O2 molecule with a surface radical.  Such surface radicals have been discussed as intermediates in the transformation from the OH- to the H-terminated surface and in the HER [9,10].  Further reduction of the Ge–OO and proton transfer leads to the second intermediate, Ge–OOH.  Based on the Ge-bound intermediates identified in the electrochemical ATR-IR experiments and on the mechanisms for the change in surface termination and HER [9,10], we propose the catalytic cycle shown in Fig. 5.  This mechanism includes the surface radical as a common surface active site in both ORR and HER, thus providing a coupling mechanism between the two reactions.

References:

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http://edoc.mpg.de/644894

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