Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging

Proton-Controlled Electron Injection in MoS2 During Hydrogen Evolution Revealed by Time-Resolved Spectroelectrochemistry

Monolayer MoS2 is an effective electrocatalyst for the hydrogen evolution reaction (HER). Despite significant efforts to optimize the active sites, its catalytic performance still falls short of theoretical predictions. One key factor that has often been overlooked is the electron injection from the conductive substrate into the MoS2. The charge transfer behavior at the substrate-MoS2 interface is nonclassical, exhibiting a liquid-gated electron injection behavior, the underlying mechanism of which remain under debate. To investigate this, we employ nanosecond time-resolved spectroelectrochemistry to probe the electron injection dynamics into monolayer MoS2 under operando HER conditions. Simultaneously, transient current measurements provide insights into the electron density at the substrate. By combining the electron density obtained from the MoS2 through spectroelectrochemical analysis with the electron density at the conductive substrate derived from transient current measurements, we explore the electron injection dynamics and characterize the current density potential (J-E) behavior at the substrate-MoS2 interface. Our findings show that the electron injection barrier and capability correlate strongly with proton concentration in the electrolyte. This relationship likely reflects the electron concentration-dependent conductivity of MoS2, where higher proton concentrations lead to fewer stray electrons before injection begins.

Theoretical understanding of water splitting by analyzing nanocatalyst photoabsorption spectra

Photons can be used to either monitor or induce catalysis by acting as photoexcited holes or quasi particles, which aid in water splitting reaction leading to a major step towards sustainable energy. However, the mechanism of catalysis using nanocatalysts under photo-illumination is not fully understood because of the complexity involved in three major steps during the oxygen evolution reaction: photoabsorption on nanocatalyst, hole transport to the surface, and the reaction kinetic barriers at the surface. In a photoelectrochemical cell used for water splitting, the surface states of optically and chemically dominant species affect the catalysts' performance. For instance, the signature of the dominant absorption peak at 580 nm in the observed spectra of Fe2O3 photoanode can shed light on the oxygen evolution reaction mechanism since each reaction intermediate affects the absorption spectrum, and the absorption coefficient in turn affects the photocurrent. In the recent decade, a combination of different theoretical methods starting from density functional theory up to Bethe-Salpeter equation accounting for excitonic effects helped to establish that the *O intermediate is the rate limiting step in agreement with experimental data. Therefore, this perspective focuses on the complexity and variety of fundamental phenomena involved in water splitting mechanism and various theoretical methods applied to address these and also suggests how the predictive capability of these methods can be used to understand mechanisms beyond water splitting, such as CO2 reduction.

Transient Absorption Microscopy Maps Spatial Heterogeneity and Distinct Chemical Environments in Photocatalytic Carbon Nitride Particles

Limitations in solar energy conversion by photocatalysis typically stem from poor underlying charge carrier properties. Transient Absorption (TA) reveals insights on key photocatalytic properties such as charge carrier lifetimes and trapping. However, on the microsecond timescale, these measurements use relatively large probe sizes ranging in millimetres to centimetres which averages the effect of spatial heterogeneity at smaller length scales. A home-built Transient Absorption Microscopy (TAM) setup is reported and used to study single particles of carbon nitride (CNx), an emerging photocatalyst. For the first time, to the best of the authors' knowledge, µs-s timescales are explored within individual particles to gain a more complete understanding of their photophysics. The dynamics of trapped charges are monitored, enabling measurement and quantification of heterogeneity in the transient absorptance signal of individual CNx particles and within them. Particle-to-particle heterogeneity in the trapped charge density is observed, while spatial heterogeneity in lifetimes within a particle is revealed using a smaller probe beam with a ≈5 µm diameter. Overall, the observations suggest that contributions from different local environments independently influence charge trapping at different timescales. TAM on the micron and microsecond spatiotemporal resolution will aid in tackling design questions about optimal chemical environments for the promotion of photoactivity.

Design and regulation of defective electrocatalysts

Electrocatalysts are the key components of electrochemical energy storage and conversion devices. High performance electrocatalysts can effectively reduce the energy barrier of the chemical reactions, thereby improving the conversion efficiency of energy devices. The electrocatalytic reaction mainly experiences adsorption and desorption of molecules (reactants, intermediates and products) on a catalyst surface, accompanied by charge transfer processes. Therefore, surface control of electrocatalysts plays a pivotal role in catalyst design and optimization. In recent years, many studies have revealed that the rational design and regulation of a defect structure can result in rearrangement of the atomic structure on the catalyst surface, thereby efficaciously promoting the electrocatalytic performance. However, the relationship between defects and catalytic properties still remains to be understood. In this review, the types of defects, synthesis methods and characterization techniques are comprehensively summarized, and then the intrinsic relationship between defects and electrocatalytic performance is discussed. Moreover, the application and development of defects are reviewed in detail. Finally, the challenges existing in defective electrocatalysts are summarized and prospected, and the future research direction is also suggested. We hope that this review will provide some principal guidance and reference for researchers engaged in defect and catalysis research, better help researchers understand the research status and development trends in the field of defects and catalysis, and expand the application of high-performance defective electrocatalysts to the field of electrocatalytic engineering.

Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging

Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.