Hydrogen production via water electrolysis, comprising the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is vital for a clean energy cycle. While noble metals like Pt are efficient HER catalysts, the OER's high overpotential causes substantial energy loss. Platinum group metals and their oxides (e.g., IrO2, RuO2) show good durability in acidic media, but improving OER efficiency is key. Single-crystal Pt electrodes, with their defined surface atom arrangements, allow precise active site identification. Many techniques, including scanning probe microscopy and X-ray diffraction, have been employed to study interfacial structures, particularly the outer Helmholtz plane. Pt oxidation creates various oxidation states (PtOH, PtO, PtO2), with adsorbed OH and O detectable through in situ spectroscopy. However, surface oxidation disrupts the ordered surface atom arrangement via place exchange with subsurface atoms, leading to dissolution and restructuring. Although complex oxide layers (α-PtO2, β-PtO2) form at OER potentials, their detailed atomic structure remains poorly understood. Recent studies using scanning probe microscopy have provided insights into complex oxide layers at higher potentials, revealing surface roughening due to Pt oxide formation/reduction during potential cycling. X-ray diffraction confirms irreversible oxidation accompanied by subsurface Pt atom place exchange. Previous research investigating OER activity on Pt(111) and Pt(100) at constant potentials (0.8–1.7 V vs RHE) showed activity decreased with increasing positive potentials due to highly oxidized species formation, while activity at negative potentials depended on crystal orientation, with Pt(100) more active than Pt(111). This highlights the substrate's atomic arrangement influence on Pt oxide structure. This study investigates the OER's response to structural changes induced by electrochemical oxidation/reduction cycles on single-crystal Pt electrodes, using high-index planes with defined step-terrace structures and X-ray crystal truncation rod (CTR) measurements to determine electron density profiles, including the subsurface layer. The aim is to determine how surface atomic defects from potential cycling affect OER activity.

Extensive research has been conducted on the oxygen evolution reaction (OER) and its dependence on catalyst material and structure. Studies using single-crystal platinum electrodes have been particularly insightful, allowing researchers to probe the relationship between surface atomic arrangements and catalytic activity. Early work established the importance of the crystallographic orientation of the platinum surface, with different facets exhibiting varying OER activity. Later studies utilized in situ techniques such as X-ray absorption spectroscopy and scanning tunneling microscopy to investigate the formation of platinum oxide layers during the OER and their impact on the reaction kinetics. This body of work emphasized the role of surface oxidation in both enhancing and inhibiting OER activity, depending on the extent and nature of the oxide layer. The dynamic nature of the platinum surface under OER conditions, including the processes of place exchange and surface roughening, has also been established. However, many questions remain regarding the exact atomic-scale structure of the active sites and the mechanisms by which potential cycling modifies the catalytic properties of the surface. This study builds upon this prior research by employing a combination of electrochemical methods and surface-sensitive X-ray techniques to gain a deeper understanding of the active site structure.

The study utilized single-crystal Pt electrodes prepared using Clavilier's method for voltammetry experiments and a Pt(111) disk electrode (from Mateck, Germany) for X-ray measurements. Electrolyte solutions of perchloric acid (HClO4) and sulfuric acid (H2SO4) were prepared using ultrapure water. Electrodes were annealed using an H2/O2 flame, then cooled and transferred to an electrochemical cell with a reversible hydrogen electrode (RHE) as the reference. Two potential protocols were employed: Protocol I involved a negative scan to 0.05 V after holding the potential above 1.4 V, while Protocol II held the potential above 1.1 V without reduction. OER activity was evaluated by measuring anodic current density at 1.6 V. Electrochemical surface area (ECSA) was determined using charge density in H2SO4. Potential cycling involved oxidation/reduction scans between 0.05 and 1.60 V. Specific OER activity (jOER) was calculated by normalizing current density at 1.6 V (after background current subtraction). High-index planes of Pt (n(111)-(111), where n is the terrace width) were used to investigate the effect of (111) terrace width. Cyclic voltammetry was performed to analyze the impact of step structures on OER activity. X-ray crystal truncation rod (CTR) measurements were carried out at BL13XU (SPring-8) and BL3A (KEK PF) using a multi-axis diffractometer, utilizing X-ray beam energies of 20 and 14 keV. A drop cell with RHE and Au counter electrodes was used. Specular and non-specular CTRs were collected. Data was corrected for irradiated surface area, X-ray path length, and Lorentz factor. Structural optimization used the least-squares method and the ANAROD program. Models incorporated adsorbed oxygen, Pt oxide layers, and subsurface Pt layers, optimizing atomic coordinates, occupancy factors, and anisotropic Debye-Waller factors. The study analyzed 419-440 reflections for each cycle and used 131-135 independent reflections for structural optimization, assuming a P3 space group. The study also analyzed hard sphere models of different atomic-sized vacancies.

The study revealed a significant enhancement of OER activity on Pt(111) after repeated potential cycling (0.05–1.6 V vs RHE). The OER current density on Pt(111) peaked in the third cycle, reaching nine times the initial cycle's value. Protocol I (including Pt oxide reduction) showed six times higher j1.6V on Pt(111) than Pt(100), indicating the importance of higher Pt oxidation state reduction. ECSA increased 1.2-fold for Pt(111) from cycle 1 to 50. However, jOER on Pt(100) sharply decreased after the initial scan, while Pt(110) remained relatively unchanged. After several cycles, Pt(111) had ~five times higher jOER than Pt(100) or Pt(110). A characteristic redox peak at 0.31 V appeared after potential cycling, attributed to single atom vacancies from subsurface Pt atom place exchange. This peak also appeared in HClO4 (Fig. 2c). The OER activation correlated with (111) terrace width: surfaces with terrace widths > 5 atomic rows showed OER enhancement, while narrower terraces did not. X-ray CTR measurements showed surface roughening from oxidation/reduction cycles and indicated that defects in the second subsurface Pt layer activated the OER. The electron density profile showed an expansion of the spacing between the first and second Pt layers after oxidation. The occupancy of the subsurface Pt2 layer decreased (from 1.00 to 0.96 after the second cycle), showing defects are formed in the subsurface Pt2 layer. The study concluded that bowl-shaped surface roughening, created by vacancies and islands, produced high coordination-number Pt atoms at cavity bottoms, enhancing OER activity. The analysis of the high-index planes revealed that vacancies larger than V3-0 are needed for OER activation.

The findings demonstrate that the OER activity on Pt(111) can be dramatically enhanced by carefully tailoring the surface structure through electrochemical potential cycling. The observed enhancement is not simply due to an increase in surface area, as the specific OER activity (jOER) shows a significant increase. The formation of atomic-scale vacancies and the associated surface roughening play a crucial role in creating high-coordination-number Pt sites that are highly active for the OER. The results highlight the importance of subsurface defects in determining the catalytic activity and provide valuable insights into the design of improved OER electrocatalysts. The study's focus on the relationship between terrace width and OER activity offers guidance for optimizing the design of platinum-based catalysts. The identification of the second subsurface layer as the location of the active sites offers new avenues for exploring alternative catalyst materials and structures.

This study reveals a significant increase in OER activity on Pt(111) electrodes achieved through controlled potential cycling, resulting in the formation of specific surface defects. The crucial role of subsurface defects in enhancing OER activity was demonstrated, and the size and location of these defects were determined. The insights gained provide valuable guidance for designing improved OER electrocatalysts. Future research could explore the impact of other surface treatments or alloying to further enhance OER activity.

The study focuses on single-crystal platinum electrodes, which might not directly translate to the behavior of polycrystalline or nanoparticle catalysts used in practical applications. The use of a specific potential cycling protocol may limit the generalizability of the findings to other electrochemical conditions. Furthermore, while the X-ray CTR measurements provide detailed structural information, obtaining atomic-resolution images of the active sites remains challenging and requires advanced techniques.