Concurrent oxygen evolution reaction pathways revealed by high-speed compressive Raman imaging

The oxygen evolution reaction (OER) is a crucial half-reaction in water electrolysis, vital for producing clean fuels like hydrogen. Improving OER kinetics is essential for enhancing the efficiency of water electrolyzers and CO₂ electrolyzers. The OER mechanism is intricately linked to the catalyst's structure. In amorphous and electrolyte-permeable catalysts, all metallic sites are redox-active and exchange ions with the electrolyte. In contrast, crystalline catalysts typically only show surface site redox activity. Recent evidence suggests that some crystalline catalysts, such as α-Li₂IrO₃ and LiCoO₂, exhibit bulk cation exchange during OER, but the details of this charge compensation are unclear. Understanding these dynamics requires in-situ characterization with high spatiotemporal and chemical resolution. Techniques like spontaneous Raman spectroscopy offer chemical selectivity but often lack the necessary spatial and temporal resolution. This study employs state-of-the-art compressive Raman imaging to overcome these limitations and provide a detailed understanding of charge compensation pathways in dense, crystalline OER catalysts.

Numerous studies have explored the OER mechanism using various techniques, including Raman spectroscopy. However, many of these studies face limitations in temporal and spatial resolution, especially for in-situ, real-time imaging. The authors cite several studies that have investigated OER mechanisms in various catalyst types, highlighting both the successes and limitations of prior methods, particularly in resolving the contributions of surface versus bulk reactions within crystalline materials. This literature review underscores the gap in knowledge regarding the dynamic charge compensation processes within crystalline, dense electrocatalysts that the current work seeks to fill.

The researchers used a custom-built confocal Raman microscope with a 532 nm laser for excitation. Two detection methods were employed: conventional Raman spectroscopy with an EMCCD camera for spectral resolution and compressive Raman imaging with a single-pixel detector (SPAD) for high-speed imaging. α-Li₂IrO₃ particles were dispersed onto conductive ITO-coated cover slides with Nafion. A three-electrode setup was used for electrochemical measurements in 1 M KOH electrolyte. Cyclic voltammetry (CV) and potentiostatic holding experiments were conducted, with Raman spectra and images acquired synchronously. Scanning electron microscopy (SEM) provided structural information, and bright-field imaging tracked oxygen gas bubble evolution. Density functional theory (DFT) calculations aided in interpreting the vibrational modes observed in the Raman spectra.

Operando Raman spectroscopy revealed distinct changes in the 640 cm⁻¹ and 550 cm⁻¹ Raman modes of α-Li₂IrO₃ during CV cycling in KOH. These changes correlated with Li⁺ deintercalation and K⁺ intercalation, respectively. Compressive Raman imaging showed potential-dependent spatial patterns of cation (de)intercalation. During the first anodic scan, a shrinking core pattern was observed, indicating delithiation from the surface inward. In subsequent cycles, a core-shell pattern emerged, with the core losing K⁺ while the shell remained K⁺-rich. This core-shell pattern was not observed in LiOH, indicating that K⁺ intercalation and deintercalation are crucial. Bright-field imaging of gas bubbles revealed that O₂ evolution rates increased with increasing potential, reaching a maximum around 1.5 V vs RHE. Pseudo phase-front velocities for cation (de)intercalation were extracted from Raman images, showing approximately constant values for K⁺ (de)intercalation in subsequent cycles, irrespective of potential, but significantly faster K⁺ exchange as compared to Li⁺. Potentiostatic holding experiments showed that at high overpotentials (1.6 and 1.7 V vs RHE), the OER occurs at the surface while bulk cation exchange cannot occur fast enough to compensate. At lower potentials, the surface remains K⁺-rich due to both deintercalation and reaction with the electrolyte.

The study's findings demonstrate that charge compensation during OER in α-Li₂IrO₃ can occur via two competing pathways: surface redox reactions and bulk cation exchange. At high current densities, the fast O₂ evolution rate limits cation exchange, restricting charge compensation to surface sites. Conversely, at low overpotentials, slow O₂ evolution allows for sufficient time for bulk cation exchange to contribute to charge compensation. The concurrent nature of these pathways explains the observed core-shell patterns in Raman images. The results suggest that similar dual pathways could exist in other layered electrocatalysts.

This research presents a detailed study of charge compensation pathways during OER in α-Li₂IrO₃ using high-speed compressive Raman imaging. The study reveals concurrent surface and bulk charge compensation mechanisms dependent on the applied potential and OER rate. These findings challenge the conventional understanding of OER in crystalline materials and emphasize the power of advanced imaging techniques for resolving complex electrochemical processes. Future work could extend this methodology to other layered electrocatalysts and explore the effects of different electrolytes and acidic media.

The study primarily focuses on α-Li₂IrO₃ and LiCoO₂, leaving the generalizability of the findings to other materials open for further investigation. The analysis of phase-front velocities relies on approximations for a simplified model of the complex agglomerate structure. The high OER activity and bubble formation also limited the scan rates achievable and complicates the kinetic interpretation.