David A. Harrington

When a molecule or sub-molecular entity is ordered on a surface in a manner that can be described by a tilt- and azimuthal-angle distribution, the projection of the molecular hyperpolarizability into the laboratory frame can be described using spherical harmonics. We illustrate that this lends itself to the construction of ten achiral order parameters associated with vibrational sum-frequency generation. We describe how these order parameters can be extracted from spectral data, first ignoring and then including the dispersion of the local electric fields. We then use these order parameters to determine the most probable orientation distribution without any assumptions about the surface alignment and in the absence of any other type of experimental data. This constitutes a flexible framework for describing a wide array of surface orientation distributions.

The molecular structure of the electrochemical double layer is an important fundamental topic in interfacial electrochemistry. It has received renewed interest in the last years, because of its influence on many electrochemical reactions and the emergence of ab initio molecular dynamics simulations that allow a detailed description of the interface structure and dynamics [1,2]. In the measurements presented here, we investigated the properties of the electrochemical double layer on Pt(111) via in situ surface X-ray diffraction (SXRD), a technique that is well-established for single crystal studies [3-7]. Platinum electrodes are widely used as electrocatalysts, for example as cathodes in proton exchange membrane fuel cells due to their high ORR reactivity. The single-crystalline Pt(111) surface, which is a well studied model system, is especially interesting for studies of the electrochemical double layer. Recent electrochemical measurements revisited this system and found evidence for a double layer structure that strongly deviated from the traditional Gouy-Chapman-Stern model and indicated a complex structural influence of water [8,9]. The water structure within the electrochemical double layer is also of major interest in computational studies. For non-adsorbing, aqueous electrolyte the electrode surface charge can be partly compensated by a layer of oriented water molecules due to their dipole moment. For potentials around the potential of zero charge (PZC), a potential-dependent orientation of water molecules with respect to the electrode surface was suggested [1,2,10]. To investigate the arrangement and orientation of the water molecules near the Pt(111) surface we used in situ SXRD methods, focussing on X-ray reflectivity (XRR) and crystal truncation rod (CTR) analysis. These methods allow to acquire detailed information about the electrode surface structure as well as the electrochemical double layer. By measuring multiple CTR sets at different surface potentials, information on the potential-dependence of the interface processes can be obtained. The experiments were performed at beamline ID31 of the European Synchrotron Radiation Facility using high photon energies of 75 keV, which provide very large data sets. This allows significantly improved structural modeling as compared to conventional SXRD. The studies employed an electrochemical hanging-meniscus X-ray diffraction cell that allows for SXRD measurements of fast dynamic systems in a well-defined electrochemical environment. We will report results on the water structure in the double layer obtained in perchloric acid at potentials around the PZC. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1. References [1] A. Groß, S. Sakong, Chem. Rev. 122 (2022) 10746. [2] R. Sundararaman et al., Chem. Rev 122, (2022) 10651. [3] J. Drnec et al, Electrochim. Acta, 224 (2017) 22. [4] M. Ruge et al., J. Electrochem. Soc., 164 (2017) H608. [5] J. Drnec et al., Current Opinion Electrochem., 4 (2017) 69. [6] J. Drnec et al., Electrochem. Comm., 84 (2017) 8577, ibid 100, 4699. [7] Y. Liu, et al., J. Phys. Chem. Lett. 9, (2018) 1265. [8] K. Ojha et al., PNAS 119 (2022) e2116016119. [9] K. Ojha et al., Angew. 59 (2020) 711. [10] A. Bouzid et al., J. Phys. Chem. Lett. 9, (2018) 1880.

Platinum electrocatalysts are still widely used as cathode material in proton-exchange membrane fuel cells (PEMFC) because of their high ORR activity. However, their long-term stability is limited due to degradation processes. This degradation is partly caused by Pt oxidation and oxide reduction, which lead to irreversible surface restructuring and subsequently Pt dissolution. To increase the lifetime of PEMFCs it is mandatory to understand the mechanisms behind the degradation on an atomistic level. This requires studies by structure sensitive in situ methods, in particular, also studies at the elevated temperatures typically employed in PEMFCs operation. Previous studies revealed that in the initial stage of oxide formation Pt atoms are extracted out of the surface and form an ultra-thin oxide layer directly above the surface. This extraction is long known as place exchange [1], but the atomic-scale details of the surface oxide structure, the extraction mechanism, and especially the kinetics of the initial oxide growth were only recently clarified [2-7]. It was found that the structural arrangement of the Pt atoms within the surface oxide on the Pt(111) and the Pt(100) surface is distinctly different, leading to differences in the oxidation/reduction kinetics, the reversibility of the surface oxidation/reduction process, and the amount of Pt dissolution into the electrolyte. Recently, we performed fast operando high-energy surface X-ray diffraction (HESXRD) studies, in which we investigated the initial extraction of Pt atoms on Pt(111) during potential cycles. Surprisingly, it was found that the Pt extraction during the anodic sweep is largely independent of the potential sweep rate, although the peak cyclic voltammograms (CVs) associated with the charge transfer during surface oxidation showed a pronounced shift [7-9]. This suggests that the Pt extraction is a fast process, rather driven by potential and not directly coupled to the slow formation of surface oxygen species. However, the Pt extraction during the anodic sweep and the re-insertion during the cathodic sweep still exhibit significant hysteresis, indicating a complex multistep oxidation kinetics. Previous electrochemical studies of Pt(111) electrodes revealed that the described influence of oxidation/reduction kinetics and the reversibility of the charge transfer processes increase at elevated temperature [10]. However, the temperature dependence of the Pt extraction behavior is currently unknown, because all previous structural studies of oxide growth and reduction have been performed exclusively at room temperature. For the experiments at elevated temperatures we developed a new setup for HESXRD studies of single crystals, which combines a hanging meniscus electrochemical cell for operando X-ray diffraction measurements with an inductive heating device in a joint Ar-filled compartment. This setup allows direct (re-)preparation of the aligned Pt single crystals on the diffractometer. By this, the time between preparation and measurement is reduced down to a few minutes and sample transfer through air is rigorously avoided. Temperature-dependent HESXRD measurements at temperatures up to 343 K were performed at beamline ID31 of the European Synchrotron Radiation Facility. The simultaneously measured electrochemical current and X-ray intensity indicate kinetic shifts in the charge transfer but a rather temperature independent Pt extraction process. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1. References [1] H. You et al, J. Chem. Phys., 100 (1994) 4699 [2] T. Fuchs et al., Nat. Catal., 3 (2020) 754 [3] J. Drnec et al, Electrochim. Acta, 224 (2017) 22 [4] M. Ruge et al., J. Am. Chem. Soc., 139 (2017) 4532 [5] M. Ruge et al., J. Electrochem. Soc., 164 (2017) H608 [6] J. Drnec et al., Current Opinion Electrochem., 4 (2017) 69 [7] J. Drnec et al., Electrochem. Comm., 84 (2017) 8577, ibid 100, 4699 [8] M. Luo et al., Nat. Catal., 5 (2022) 615 [9] T. Fuchs et al., J. Phys. Chem. Lett., 14 (2023) 3589 [10] N. Marković et al., J. Phys. Chem. B, 103 (1999) 8568

Pt-based electro-catalysts are the most common cathode material in proton exchange membrane fuel cells given their high oxygen reduction reaction activity and stability. However, practical applications are limited by catalyst performance loss and degeneration. A main cause for this is nanoscale Pt surface restructuring, which occurs upon formation and reduction of a Pt surface oxide. During oxidation, a fraction of the Pt surface atoms are extracted in a process known as “place-exchange” from the electrode and are bound at specific sites in an oxide about 1 – 4 Å above the surface [1]. Upon subsequent oxide reduction, these atoms as well as the vacancies from which they originate can agglomerate, leading to the nucleation of adatom and vacancy islands. If this process is repeated, nanoislands with a well-defined lateral size of about 20 – 60 Å are grown [2-6]. Detailed studies of this growth process have been performed on the Pt(111) electrode [2-6], revealing two distinct growth regions which are characterized by different vertical scaling exponents [3]. For the development of catalysts with superior long-term stability, a more detailed knowledge of the restructuring of the other low index planes of Pt is required. In our recent study of the Pt oxides on Pt(111) and Pt(100) we already showed a strong dependence of the structural stability of the surface on differences in the atomic-scale oxide structure formed on those two surface orientations [1]. However, the influence of surface structure on the nanoisland growth after repeated oxidation and reduction is largely unclear. We present a comparative operando grazing incidence small angle X-ray scattering (GISAXS) study of the nanoisland formation on Pt(111), Pt(100) and Pt(110) in acidic electrolyte. The GISAXS measurements of the nanoisland growth were performed at a high photon energy of >70 keV, which lowered background scattering from the electrolyte, enabling us to resolve the islands after the very first ox./red. cycle. Differences in island shape and growth behaviour on the low index planes will be discussed. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497, MA 1618/23-1 and the BMBF via 05K19FK3. References Fuchs et al., Nat. Catal. 3, 754–761 (2020) Sugawara and K. Itaya, J. Chem. Soc., Faraday Trans. 185, 1351-1356 (1989) Jacobse et al., Nat. Mater. 17 (3), 277-282 (2018) J. Rost, et al., Nat. Commun. 10, 5233 (2019) Ruge et al., J. Am. Chem. Soc. 139, 4532 (2017) Jacobse, et al. ACS Cent. Sci. 5 (12), 1920–1928 (2019)

Kinetic analysis of catalytic reactions is a powerful tool for mechanistic elucidation but is often challenging to perform, limiting understanding and therefore development of these reactions. Establishing order in a catalyst is usually achieved by running several reactions at different loadings, which is both time-consuming and complicated by the challenge of maintaining consistent run-to-run experimental conditions. Continuous addition kinetic elucidation (CAKE) was developed to circumvent these issues by continuously injecting a catalyst into a reaction, while monitoring reaction progress over time. For reactions that are mth order in a single yield-limiting reactant and nth order in catalyst, a plot of reactant concentration against time has a shape dependent only on the orders m and n. Therefore, fitting experimental CAKE data (using open access code or a convenient web tool) allows the reactant and catalyst orders, rate constant, and the amount of complete catalyst inhibition to be determined from a single experiment. Kinetic information obtained from CAKE experiments showed good agreement with the literature.

The degradation of Pt‐containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO₄ by combining operando high‐energy surface X‐ray diffraction, online mass spectrometry, and density functional theory. Our atomic‐scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe‐like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO₂ and starts to grow when the coverage of the stripe‐like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential‐independent after the stripe‐like oxide has reached its saturation coverage.

The degradation of Pt‐containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO₄ by combining operando high‐energy surface X‐ray diffraction, online mass spectrometry, and density functional theory. Our atomic‐scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe‐like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO₂ and starts to grow when the coverage of the stripe‐like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential‐independent after the stripe‐like oxide has reached its saturation coverage.

The first step of electrochemical surface oxidation is extraction of a metal atom from its lattice site to a location in a growing oxide. Here we show by fast simultaneous electrochemical and in situ high-energy surface X-ray diffraction measurements that the initial extraction of Pt atoms from Pt(111) is a fast, potential-driven process, whereas charge transfer for the related formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. It is concluded that potential plays a key independent role in electrochemical surface oxidation.