Advanced Spatiotemporal Voltammetric Techniques for Kinetic Analysis and Active Site Determination in the Electrochemical Reduction of CO2

Delivery of carbon dioxide to an electrode surface using a nanopipette

We have developed a new scanning probe approach for the delivery of a gas-phase reactant to the surface of an electrocatalyst through a self-replenishing bubble located at the end of a scanning probe. This approach enables local electrocatalytic rates to be detected under very-high mass transport rates due to the small distance between the gas-phase reactant in the bubble and the electrocatalyst surface. Here we report experiments for the delivery of carbon dioxide to a gold ultramicroelectrode surface using a micron-scale nanopipette. The approach curve profiles that we measure suggest a complex interplay between carbon dioxide reduction and hydrogen evolution which is mediated by both the probe-electrode distance and the potential of the gold ultramicroelectrode.

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Applying Nuclear Forward Scattering as In Situ and Operando Tool for the Characterization of FeN4 Moieties in the Hydrogen Evolution Reaction

Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and is applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron-nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate-determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal-nitrogen-carbon (MNC) catalysts.

Structure-dependent CO2 reduction on molybdenite (MoS2) electrocatalysts

Scanning electrochemical cell microscopy (SECCM) is employed to directly identify the structure-dependent electrochemical CO2 reduction reaction (eCO2RR) activity of molybdenite (MoS2) electrocatalysts in an aqueous imidazolium-based aprotic ionic liquid electrolyte. Nanoscale defects, where the edge plane (EP) is exposed, are directly targeted, revealing heightened overall activity (eCO2RR + the competing hydrogen evolution reaction, HER) over the relatively inactive basal plane (BP). In addition, certain types of defects (e.g., step edges) only exhibit heightened activity under a CO2 atmosphere (i.e., compared to N2), indirectly confirming higher selectivity at these surface sites. Overall, this work will guide the bottom-up design of earth-abundant electrocatalysts for use in large-scale CO2 electrolysis.

Local reaction environment in electrocatalysis

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Confinement of ionomer for electrocatalytic CO2 reduction reaction via efficient mass transfer pathways

Gas diffusion electrodes (GDEs) mediate the transport of reactants, products and electrons for the electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assemblies. The random distribution of ionomer, added by the traditional physical mixing method, in the catalyst layer of GDEs affects the transport of ions and CO2. Such a phenomenon results in elevated cell voltage and decaying selectivity at high current densities. This paper describes a pre-confinement method to construct GDEs with homogeneously distributed ionomer, which enhances mass transfer locally at the active centers. The optimized GDE exhibited comparatively low cell voltages and high CO Faradaic efficiencies (FE > 90%) at a wide range of current densities. It can also operate stably for over 220 h with the cell voltage staying almost unchanged. This good performance can be preserved even with diluted CO2 feeds, which is essential for pursuing a high single-pass conversion rate. This study provides a new approach to building efficient mass transfer pathways for ions and reactants in GDEs to promote the electrocatalytic CO2RR for practical applications.

Fuelling electrocatalysis at a single nanoparticle by ion flow in a nanoconfined electrolyte layer

We explore the possibility of coupling the transport of ions and water in a nanochannel with the chemical transformation of a reactant at an individual catalytic nanoparticle (NP). Such configuration could be interesting for constructing artificial photosynthesis devices coupling the asymmetric production of ions at the catalytic NP, with the ion selectivity of the nanochannels acting as ion pumps. Herein we propose to observe how such ion pumping can be coupled to an electrochemical reaction operated at the level of an individual electrocatalytic Pt NP. This is achieved by confining a (reservoir) droplet of electrolyte to within a few micrometres away from an electrocatalytic Pt NP on an electrode. While the region of the electrode confined by the reservoir and the NP are cathodically polarised, operando optical microscopy reveals the growth of an electrolyte nanodroplet on top of the NP. This suggests that the electrocatalysis of the oxygen reduction reaction operates at the NP and that an electrolyte nanochannel is formed - acting as an ion pump - between the reservoir and the NP. We have described here the optically imaged phenomena and their relevance to the characterization of the electrolyte nanochannel linking the NPs to the electrolyte microreservoir. Additionally, we have addressed the capacity of the nanochannel to transport ions and solvent flow to the NP.

Spatiotemporal Mapping of Local Heterogeneities during Electrochemical Carbon Dioxide Reduction

The activity and selectivity of a copper electrocatalyst during the electrochemical CO₂ reduction reaction (eCO₂RR) are largely dominated by the interplay between local reaction environment, the catalyst surface, and the adsorbed intermediates. In situ characterization studies have revealed many aspects of this intimate relationship between surface reactivity and adsorbed species, but these investigations are often limited by the spatial and temporal resolution of the analytical technique of choice. Here, Raman spectroscopy with both space and time resolution was used to reveal the distribution of adsorbed species and potential reaction intermediates on a copper electrode during eCO₂RR. Principal component analysis (PCA) of the in situ Raman spectra revealed that a working electrocatalyst exhibits spatial heterogeneities in adsorbed species, and that the electrode surface can be divided into CO-dominant (mainly located at dendrite structures) and C-C dominant regions (mainly located at the roughened electrode surface). Our spectral evaluation further showed that in the CO-dominant regions, linear CO was observed (as characterized by a band at ∼2090 cm^-1), accompanied by the more classical Cu-CO bending and stretching vibrations located at ∼280 and ∼360 cm^-1, respectively. In contrast, in the C-C directing region, these three Raman bands are suppressed, while at the same time a band at ∼495 cm^-1 and a broad Cu-CO band at ∼2050 cm^-1 dominate the Raman spectra. Furthermore, PCA revealed that anodization creates more C-C dominant regions, and labeling experiments confirmed that the 495 cm^-1 band originates from the presence of a Cu-C intermediate. These results indicate that a copper electrode at work is very dynamic, thereby clearly displaying spatiotemporal heterogeneities, and that in situ micro-spectroscopic techniques are crucial for understanding the eCO₂RR mechanism of working electrocatalyst materials.

Electrochemical Visualization of Membrane Proteins in Single Cells at a Nanoscale Using Scanning Electrochemical Cell Microscopy

The electrochemical visualization of proteins in the plasma membrane of single fixed cells was achieved with a spatial resolution of 160 nm using scanning electrochemical cell microscopy. The model protein, the carcinoembryonic antigen (CEA), is linked with a ruthenium complex (Ru(bpy)₃²⁺)-tagged antibody, which exhibits redox peaks in its cyclic voltammetry curves after a nanopipette tip contacts the cellular membrane. Based on the potential-resolved oxidation or reduction currents, an uneven distribution of membrane CEAs on the cells is electrochemically visualized, which could only be achieved previously using super-resolution optical microscopy. Compared with current electrochemical microscopy, the single-cell scanning electrochemical cell microscopy (SECCM) strategy not only improves the spatial resolution but also utilizes the potential-resolved current from the antibody-antigen complex to increase electrochemical imaging accuracy. Eventually, the electrochemical visualization of cellular proteins at the nanoscale enables the super-resolution study of cells to provide more biological information.

Imaging and Quantifying the Chemical Communication between Single Particles in Metal Alloys

The communication within particle agglomerates in industrial alloys can have a significant impact on the macroscopic reactivity, putting a high demand on the adaptation of wide-field methodologies to clarify this phenomenon. In this work, we report the application of correlated optical microscopies probing operando both local pH and local surface chemical transformation correlated with identical location scanning electron microscopy to quantify in situ the structure reactivity of particle agglomerates of foreign elements in the Al alloy. The optical operando analyses allow us (i) to reveal and quantify the local production of OH^- from proton and oxygen reduction at individual Si- or Fe-rich microparticles and (ii) to quantify (and model) the chemical communication between these active sites, within a few micrometer range, on the local chemical transformation of the material. Wide-field image analysis highlights the statistical importance of chemical communication that may introduce a new conceptual framework for the understanding of the mechanisms in related fields of charge transfer, electrocatalysis, and corrosion.

High-Spatiotemporal-Resolution Electrochemical Measurements of Electrocatalytic Reactivity

The real-time measurement of the individual or local electrocatalytic reactivity of catalyst particles instead of ensemble behavior is considerably challenging but very critical to uncover fundamental insights into catalytic mechanisms. Recent remarkable efforts have been made to the development of high-spatiotemporal-resolution electrochemical techniques, which allow the imaging of the topography and reactivity of fast electron-transfer processes at the nanoscale. This Perspective summarizes emerging powerful electrochemical measurement techniques for studying various electrocatalytic reactions on different types of catalysts. Principles of scanning electrochemical microscopy, scanning electrochemical cell microscopy, single-entity measurement, and molecular probing technique have been discussed for the purpose of measuring important parameters in electrocatalysis. We further demonstrate recent advances in these techniques that reveal quantitative information about the thermodynamic and kinetic properties of catalysts for various electrocatalytic reactions associated with our perspectives. Future research on the next-generation electrochemical techniques is anticipated to be focused on the development of instrumentation, correlative multimodal techniques, and new applications, thus enabling new opportunities for elucidating structure-reactivity relationships and dynamic information at the single active-site level.

Selective Electrochemical Hydrogenation of Phenol with Earth-abundant Ni-MoO2 Heterostructured Catalysts: Effect of Oxygen Vacancy on Product Selectivity

Herein, we report highly efficient carbon supported Ni-MoO₂ heterostructured catalysts for the electrochemical hydrogenation (ECH) of phenol in 0.10 M aqueous sulfuric acid (pH 0.7) at 60 °C. Highest yields for cyclohexanol and cyclohexanone of 95 % and 86 % with faradaic efficiencies of ∼50 % are obtained with catalysts bearing high and low densities of oxygen vacancy (O_v ) sites, respectively. In situ diffuse reflectance infrared spectroscopy and density functional theory calculations reveal that the enhanced phenol adsorption strength is responsible for the superior catalytic efficiency. Furthermore, 1-cyclohexene-1-ol is an important intermediate. Its hydrogenation route and hence the final product are affected by the O_v density. This work opens a promising avenue to the rational design of advanced electrocatalysts for the upgrading of phenolic compounds.

Thousand-fold increase in O2 electroreduction rates with conductive MOFs

Molecular materials must deliver high current densities to be competitive with traditional heterogeneous catalysts. Despite their high density of active sites, it has been unclear why the reported O₂ reduction reaction (ORR) activity of molecularly defined conductive metal-organic frameworks (MOFs) have been very low: ca. -1 mA cm^-2. Here, we use a combination of gas diffusion electrolyses and nanoelectrochemical measurements to lift multiscale O₂ transport limitations and show that the intrinsic electrocatalytic ORR activity of a model 2D conductive MOF, Ni₃(HITP)₂, has been underestimated by at least 3 orders of magnitude. When it is supported on a gas diffusion electrode (GDE), Ni₃(HITP)₂ can deliver ORR activities >-150 mA cm^-2 and gravimetric H₂O₂ electrosynthesis rates exceeding or on par with those of prior heterogeneous electrocatalysts. Enforcing the fastest accessible mass transport rates using scanning electrochemical cell microscopy revealed that Ni₃(HITP)₂ is capable of ORR current densities exceeding -1200 mA cm^-2 and at least another 130-fold higher ORR mass activity than has been observed in GDEs. Our results directly implicate precise control over multiscale mass transport to achieve high-current-density electrocatalysis in molecular materials.

Screening Surface Structure-Electrochemical Activity Relationships of Copper Electrodes under CO2 Electroreduction Conditions

Understanding how crystallographic orientation influences the electrocatalytic performance of metal catalysts can potentially advance the design of catalysts with improved efficiency. Although single crystal electrodes are typically used for such studies, the one-at-a-time preparation procedure limits the range of secondary crystallographic orientations that can be profiled. This work employs scanning electrochemical cell microscopy (SECCM) together with co-located electron backscatter diffraction (EBSD) as a screening technique to investigate how surface crystallographic orientations on polycrystalline copper (Cu) correlate to activity under CO₂ electroreduction conditions. SECCM measures spatially resolved voltammetry on polycrystalline copper covering low overpotentials of CO₂ conversion to intermediates, thereby screening the different activity from low-index facets where H₂ evolution is dominant to high-index facets where more reaction intermediates are expected. This approach allows the acquisition of 2500 voltammograms on approximately 60 different Cu surface facets identified with EBSD. The results show that the order of activity is (111) < (100) < (110) among the Cu primary orientations. The collection of data over a wide range of secondary orientations leads to the construction of an "electrochemical-crystallographic stereographic triangle" that provides a broad comprehension of the trends among Cu secondary surface facets rarely studied in the literature, [particularly (941) and (741)], and clearly shows that the electroreduction activity scales with the step and kink density of these surfaces. This work also reveals that the electrochemical stripping of the passive layer that is naturally formed on Cu in air is strongly grain-dependent, and the relative ease of stripping on low-index facets follows the order of (100) > (111) > (110). This allows a procedure to be implemented, whereby the oxide is removed (to an electrochemically undetectable level) prior to the kinetic analyses of electroreduction activity. SECCM screening allows for the most active surfaces to be ranked and prompts in-depth follow-up studies.