In-Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2 : Theory, Practice and Challenges

Structural evolution and catalytic mechanisms of perovskite oxides in electrocatalysis

Electrocatalysis plays a pivotal role in driving the progress of modern technologies and industrial processes such as energy conversion and emission reduction. Perovskite oxides, an important family of electrocatalysts, have garnered substantial attention in diverse catalytic reactions because of their highly tunable composition and structure, as well as their considerable activity and stability. This review delves into the mechanisms of electrocatalytic reactions that use perovskite oxides as electrocatalysts, while also providing a comprehensive summary of the potential key factors that influence catalytic activity across various reactions. Furthermore, this review offers an overview of advanced characterizations used for studying catalytic mechanisms and proposes approaches to designing highly efficient perovskite oxide electrocatalysts.

Alkaline-Metal Cations Affect Pt Deactivation for the Electrooxidation of Small Organic Molecules by Affecting the Formation of Inactive Pt Oxide

The activity of Pt for the electro-oxidation of several organic molecules changes with the cation of the electrolyte. It has been proposed that the underlying reason behind that effect is the so-called noncovalent interactions between the hydrated cations and adsorbed OH (OHad). However, there is a lack of spectroscopic evidence for this phenomenon, resulting in an incomplete understanding at the microscopic level of these electrochemical processes. Herein, we explore the electro-oxidation of glycerol (EOG) on platinum (Pt) in LiOH, NaOH and KOH using in situ surface-enhanced infrared absorption spectroscopy in the attenuated total reflectance mode (ATR-SEIRAS) and in situ X-ray absorption spectroscopy (XAS). Our results show that the electrolyte cation influences the rate and potential at which adsorbed CO (COad), a catalytic poison, is formed and oxidized. We attribute this to the cation-dependent stability of oxygenated species on the metallic Pt surface and the different intensities of the electric field at the electrode/electrolyte interface. We also demonstrate that the formation of an inactive Pt oxide layer is indirectly also cation-dependent: the formation of this layer is triggered by the cation-dependent oxidative removal of reaction intermediates (for instance, CO). This phenomenon explains the well-known cation-induced differences in the voltammetric profiles, of not just glycerol, but generally of alcohols and polyols.

Time-Resolved Infrared Spectroscopic Evidence for Interfacial pH-Dependent Kinetics of Formate Evolution on Cu Electrodes

By deployment of rapid-scan (second time scale) electrochemical FT-IR reflection-absorption spectroscopy, we studied the reduction of CO2 in 0.1 M Na2SO4 in deuterated water at a pD of 3.7. We report on the impact of dynamic changes in the bicarbonate equilibrium concentration in the vicinity of a polycrystalline Cu electrode, induced by step changes in applied electrode potential. We correlate these changes in interfacial composition and concentrations of dissolved species to the formation rate of formate, and provide evidence for the following conclusions: (i) the kinetics for the conversion of dissolved CO2 to formate (formic acid) are fast, (ii) bicarbonate is also converted to formate, but with less favorable kinetics, and (iii) carbonate does not yield any formate. These results reveal that formate formation requires (mildly) acidic conditions at the interface for CO2 to undergo a proton-coupled conversion step, and we postulate that bicarbonate reduction to formate is driven by catalytic hydrogenation via in situ formed H2. Interestingly CO was not observed, suggesting that the kinetics of the CO2 to CO reaction are significantly less favorable than formate formation under the experimental conditions (pH and applied potential). We also analyzed the feasibility of pulsed electrolysis to enhance the (average) rate of formation of formate. While a short positive potential pulse enhances the CO2 concentration, this also leads to the formation of basic copper carbonates, resulting in electrode deactivation. These observations demonstrate the potential of rapid-scan EC-IRRAS to elucidate the mechanisms and kinetics of electrochemical reactions, offering valuable insights for optimizing catalyst and electrolyte performance and advancing CO2 reduction technologies.

Liquid Metal-Enabled Tunable Synthesis of Nanoporous Polycrystalline Copper for Selective CO2-to-Formate Electrochemical Conversion

Copper-based catalysts exhibit high activity in electrochemical CO2 conversion to value-added chemicals. However, achieving precise control over catalysts design to generate narrowly distributed products remains challenging. Herein, a gallium (Ga) liquid metal-based approach is employed to synthesize hierarchical nanoporous copper (HNP Cu) catalysts with tailored ligament/pore and crystallite sizes. The nanoporosity and polycrystallinity are generated by dealloying intermetallic CuGa2 formed after immersing pristine Cu foil in liquid Ga in a basic or acidic solution. The liquid metal-based approach allows for the transformation of monocrystalline Cu to the polycrystalline HNP Cu with enhanced CO2 reduction reaction (CO2RR) performance. The dealloyed HNP Cu catalyst with suitable crystallite size (22.8 nm) and nanoporous structure (ligament/pore size of 45 nm) exhibits a high Faradaic efficiency of 91% toward formate production under an applied potential as low as -0.3 VRHE. The superior CO2RR performance can be ascribed to the enlarged electrochemical catalytic surface area, the generation of preferred Cu facets, and the rich grain boundaries by polycrystallinity. This work demonstrates the potential of liquid metal-based synthesis for improving catalysts performance based on structural design, without increasing compositional complexity.

Recent Advances in Revealing the Electrocatalytic Mechanism for Hydrogen Energy Conversion System

In light of the intensifying global energy crisis and the mounting demand for environmental protection, it is of vital importance to develop advanced hydrogen energy conversion systems. Electrolysis cells for hydrogen production and fuel cell devices for hydrogen utilization are indispensable in hydrogen energy conversion. As one of the electrolysis cells, water splitting involves two electrochemical reactions, hydrogen evolution reaction and oxygen evolution reaction. And oxygen reduction reaction coupled with hydrogen oxidation reaction, represent the core electrocatalytic reactions in fuel cell devices. However, the inherent complexity and the lack of a clear understanding of the structure-performance relationship of these electrocatalytic reactions, have posed significant challenges to the advancement of research in this field. In this work, the recent development in revealing the mechanism of electrocatalytic reactions in hydrogen energy conversion systems is reviewed, including in situ characterization and theoretical calculation. First, the working principles and applications of operando measurements in unveiling the reaction mechanism are systematically introduced. Then the application of theoretical calculations in the design of catalysts and the investigation of the reaction mechanism are discussed. Furthermore, the challenges and opportunities are also summarized and discussed for paving the development of hydrogen energy conversion systems.

Visualization of CO2 electrolysis using optical coherence tomography

Electrolysers offer an appealing technology for conversion of CO2 into high-value chemicals. However, there are few tools available to track the reactions that occur within electrolysers. Here we report an electrolysis optical coherence tomography platform to visualize the chemical reactions occurring in a CO2 electrolyser. This platform was designed to capture three-dimensional images and videos at high spatial and temporal resolutions. We recorded 12 h of footage of an electrolyser containing a porous electrode separated by a membrane, converting a continuous feed of liquid KHCO3 to reduce CO2 into CO at applied current densities of 50-800 mA cm-2. This platform visualized reactants, intermediates and products, and captured the strikingly dynamic movement of the cathode and membrane components during electrolysis. It also linked CO production to regions of the electrolyser in which CO2 was in direct contact with both membrane and catalyst layers. These results highlight how this platform can be used to track reactions in continuous flow electrochemical reactors.

Growth and Electrochemical Study of Bismuth Nanodendrites as an Efficient Catalyst for CO2 Reduction

There is a growing interest in creating cost-effective catalysts for efficient electrochemical CO2 reduction to address pressing environmental issues and produce valuable products. A bimetallic ZnBi catalyst that enhances catalytic activity and stability toward the electrochemical reduction of CO2 is designed. It is based on bismuth nanodendrites grown using a facile, scalable, and low-cost method. The results have shown that the incorporation of bismuth can decrease the charge transfer resistance and facilitate CO2 reduction toward the formation of CO and formate. It was revealed that the ZnBi catalyst exhibited higher catalytic activity compared with that of the pure Zn catalyst for CO2 reduction, with a lower onset potential [-0.75 V vs a reversible hydrogen electrode (RHE) compared with -0.85 V vs RHE for Zn]. In situ electrochemical attenuated total internal reflection Fourier transform infrared spectroscopy was employed to study the reaction mechanism, showing the formation of CO and formate through the adsorbed *COO- intermediates. This study has demonstrated a new approach for the feasible synthesis of high-performance catalysts for large-scale electrochemical CO2 reduction.

Layered Sn-Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement

Operando electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (EC ATR-SEIRAS) is a valuable method for a fundamental understanding of electrochemical interfaces under real operating conditions. The applicability of this method depends on the ability to tune the optical and catalytic properties of an electrode film, and it thus requires unique optimization for any given material. Motivated by the growing interest in Sn-based electrocatalysts for selective reduction of CO2 to formate species, we investigate several Sn thin-film synthesis routes for the resulting SEIRA signal response. We compare the SEIRA performance of thermally evaporated metallic Sn to a series of Sn-based films on top of a SEIRA-active Au substrate (metallic Sn, oxide-derived metallic Sn, and metal oxide SnOx). Using alkanethiol self-assembled monolayers as a probe, we find that electrodepositing metallic catalyst films on top of SEIRA-active Au substrates yield higher signal relative to thermal evaporation as well as higher signal than the independent SEIRA-active Au underlayer. These observations come despite the fact that thermally evaporated Sn has a significantly higher surface roughness (and thus higher adsorbate population), suggesting specific SEIRA-magnifying effects for the stacked films. Finally, we applied these films to observe the electrochemical conversion of CO2. Differences are observed in spectral features based on the composition of the electrode being either metallic or oxide-derived metallic Sn, implying differences in their respective reaction pathways.

Boosting CO2 electrocatalysis through electrical double layer regulations

Interfacial investigation for fine-tuning microenvironment has recently emerged as a promising method to optimize the electrochemical CO2 reduction system. The electrical double layer located at the electrode-electrolyte interface presents a particularly significant impact on electrochemical reactions. However, its effect on the activity and selectivity of CO2 electrocatalysis remains poorly understood. Here, we utilized two-dimensional mica flakes, a material with a high dielectric constant, to modify the electrical double layer of Ag nanoparticles. This modification resulted in a significant enhancement of current densities for CO2 reduction and an impressive Faradaic efficiency of 98% for CO production. Our mechanistic investigations suggest that the enhancement of the electrical double layer capacitance through mica modification enriched local CO2 concentration near the reaction interface, thus facilitating CO2 electroreduction.

Nanostructured Black Silicon as a Stable and Surface-Sensitive Platform for Time-Resolved In Situ Electrochemical Infrared Absorption Spectroscopy

Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial chemical processes. However, SEIRAS-active nanostructured metallic thin films for the in situ analysis of electrochemical phenomena are often unstable under biased aqueous conditions. In this work, we present a surface-enhancing structure based on etched black Si internal reflection elements with Au-coatings for in situ electrochemical ATR-SEIRAS. Using electrochemical potential-dependent adsorption and desorption of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates offer advantages over commonly used structures, such as electroless-deposited Au on Si and electrodeposited Au on ITO-coated Si, due to the combination of high stability, sensitivity, and conductivity. These characteristics are especially valuable for time-resolved measurements where stable substrates are required over extended times. Furthermore, the low sheet resistance of Au layers on black Si reduces the RC time constant of the electrochemical cell, enabling a significantly higher time resolution compared to that of traditional substrates. Thus, we employ black Si-based substrates in conjunction with rapid- and step-scan Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption and desorption kinetics of 4-methoxypyridine during in situ electrochemical potential steps. Adsorption is shown to be diffusion-limited, which allows for the determination of the mean molecular area in a fully established monolayer. Moreover, no significant changes in the peak ratios of vibrational modes with different orientations relative to the molecular axis are observed, suggesting a single adsorption mode and no alteration of the average molecular orientation during the adsorption process. Overall, this study highlights the enhanced performance of black Si-based substrates for both steady-state and time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful platform for kinetic and mechanistic investigations of electrochemical interfaces.

Infrared Spectroscopic Observation of Oxo- and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst

Molecular Ir catalysts have emerged as an important class of model catalysts for understanding structure-activity relationships in water oxidation, a reaction that is central to renewable fuel synthesis. Prior efforts have mostly focused on controlling and elucidating the emergence of active species from prepared precursors. However, the development of efficient and stable molecular Ir catalysts also necessitates probing of reaction intermediates. To date, relatively little is known about the key intermediates in the cycles of the molecular Ir catalysts. Herein, we probed the catalytic cycle of a homogeneous Ir catalyst ("blue dimer") at a Au electrode/aqueous electrolyte interface by combining surface-enhanced infrared absorption spectroscopy (SEIRAS) with phase-sensitive detection (PSD). Cyclic voltammograms (CVs) from 1.4 to 1.7 VRHE (RHE = reversible hydrogen electrode) give rise to a band at ∼818 cm-1, whereas CVs from 1.4 to ≥1.85 VRHE generate an additional band at ∼1146 cm-1. Isotope labeling experiments indicate that the bands at ∼818 and ∼1146 cm-1 are attributable to oxo (IrV═O) and superoxo (IrIV-OO•) moieties, respectively. This study establishes PSD-SEIRAS as a sensitive tool for probing water oxidation cycles at electrode/electrolyte interfaces and demonstrates that the relative abundance of two key intermediates can be tuned by the thermodynamic driving force of the reaction.

Gaining Deep Understanding of Electrochemical CO2 RR with In Situ/Operando Techniques

Electrocatalysis for CO2 conversion has been extensively studied to mitigate the energy shortage and environmental issues, which are gaining ever-increasing attention. However, the complicated CO2 reduction process and the dynamic evolution occurring on electrocatalyst surface make it hard to understand the catalytic mechanism. The development of advanced in situ/operando techniques intelligently coupled with electrochemical cells sheds light on the related study via capturing surface atomic rearrangement, tracing chemical state change of catalysts, monitoring the behavior of intermediates and products, and depicting microenvironment near the electrode surface. In this review, fundamentals of the state-of-the-art in situ/operando techniques are clarified first. Case studies on the in situ/operando techniques performed to probe the CO2 reduction reaction processes are then discussed in detail. Finally, conclusions and outlook on this field are presented.

In-situ spectroscopic probe of the intrinsic structure feature of single-atom center in electrochemical CO/CO2 reduction to methanol

While exploring the process of CO/CO₂ electroreduction (CO_xRR) is of great significance to achieve carbon recycling, deciphering reaction mechanisms so as to further design catalytic systems able to overcome sluggish kinetics remains challenging. In this work, a model single-Co-atom catalyst with well-defined coordination structure is developed and employed as a platform to unravel the underlying reaction mechanism of CO_xRR. The as-prepared single-Co-atom catalyst exhibits a maximum methanol Faradaic efficiency as high as 65% at 30 mA/cm² in a membrane electrode assembly electrolyzer, while on the contrary, the reduction pathway of CO₂ to methanol is strongly decreased in CO₂RR. In-situ X-ray absorption and Fourier-transform infrared spectroscopies point to a different adsorption configuration of *CO intermediate in CORR as compared to that in CO₂RR, with a weaker stretching vibration of the C-O bond in the former case. Theoretical calculations further evidence the low energy barrier for the formation of a H-CoPc-CO^- species, which is a critical factor in promoting the electrochemical reduction of CO to methanol.

Hyphenated DEMS and ATR-SEIRAS techniques for in situ multidimensional analysis of lithium-ion batteries and beyond

A wide spectrum of state-of-the-art characterization techniques have been devised to monitor the electrode-electrolyte interface that dictates the performance of electrochemical devices. However, coupling multiple characterization techniques to realize in situ multidimensional analysis of electrochemical interfaces remains a challenge. Herein, we presented a hyphenated differential electrochemical mass spectrometry and attenuated total reflection surface enhanced infrared absorption spectroscopy analytical method via a specially designed electrochemical cell that enables a simultaneous detection of deposited and volatile interface species under electrochemical reaction conditions, especially suitable for non-aqueous, electrolyte-based energy devices. As a proof of concept, we demonstrated the capability of the homemade setup and obtained the valuable reaction mechanisms, by taking the tantalizing reactions in non-aqueous lithium-ion batteries (i.e., oxidation and reduction processes of carbonate-based electrolytes on Li1+xNi0.8Mn0.1Co0.1O2 and graphite surfaces) and lithium-oxygen batteries (i.e., reversibility of the oxygen reaction) as model reactions. Overall, we believe that the coupled and complementary techniques reported here will provide important insights into the interfacial electrochemistry of energy storage materials (i.e., in situ, multi-dimensional information in one single experiment) and generate much interest in the electrochemistry community and beyond.

Direct Determination of Plasmon Enhancement Factor and Penetration Depths in Surface Enhanced IR Absorption Spectroscopy

Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) is a powerful tool for studying a wide range of surface and electrochemical phenomena. For most electrochemical experiments the evanescent field of an IR beam partially penetrates through a thin metal electrode deposited on top of an attenuated total reflection (ATR) crystal to interact with molecules of interest. Despite its success, a major problem that complicates quantitative interpretation of the spectra from this method is the ambiguity of the enhancement factor due to plasmon effects in metals. We developed a systematic method for measuring this, which relies upon independent determination of surface coverage by Coulometry of a surface-bound redox-active species. Following that, we measure the SEIRAS spectrum of the surface bound species, and from the knowledge of surface coverage, retrieve the effective molar absorptivity, ε_SEIRAS. Comparing this to the independently determined bulk molar absorptivity leads us to the enhancement factor f = ε_SEIRAS/ε_bulk. We report enhancement factors in excess of 1000 for the C-H stretches of surface bound ferrocene molecules. We additionally developed a methodical approach to measure the penetration depth of the evanescent field from the metal electrode into a thin film. Such systematic measure of the enhancement factor and penetration depth will help SEIRAS advance from a qualitative to a more quantitative method.

Operando characterization of continuous flow CO2 electrolyzers: current status and future prospects

The performance of continuous-flow CO₂ electrolyzers has substantially increased in recent years, achieving current density and selectivity (particularly for CO production) meeting the industrial targets. Further improvement is, however, necessary in terms of stability and energy efficiency, as well as in high-value multicarbon product formation. Accelerating this process requires deeper understanding of the complex interplay of chemical-physical processes taking place in CO₂ electrolyzer cells. Operando characterization can provide these insights under working conditions, helping to identify the reasons for performance losses. Despite this fact, only relatively few studies have taken advantage of such methods up to now, applying operando techniques to characterize practically relevant CO₂ electrolyzers. These studies include X-ray absorption- and Raman spectroscopy, fluorescent microscopy, scanning probe techniques, mass spectrometry, and radiography. Their objective was to characterize the catalyst structure, its microenviroment, membrane properties, etc., and relate them to the device performance (reaction rates and product distribution). Here we review the current state-of-the-art of operando methods, associated challenges, and also their future potential. We aim to motivate researchers to perform operando characterization in continuous-flow CO₂ electrolyzers, to understand the reaction mechanism and device operation under practically relevant conditions, thereby advancing the field towards industrialization.

Controlling Water Delivery to an Electrochemical Interface with Surfactants

Most electrochemical reactions require delivery of protons, often from water, to surface-adsorbed species. However, water also acts as a competitor to many such processes by directly reacting with the electrode, which necessitates using water in small amounts. Controlling the water content and structure near the surface is an important frontier in directing the reactivity and selectivity of electrochemical reactions. Surfactants accumulate near surfaces, and therefore, they can be used as agents to control interfacial water. Using mid-IR spectro-electrochemistry, we show that a modest concentration (1 mM) of the cationic surfactant CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a large effect on the interfacial water concentration, changing it by up to ∼35% in the presence of an applied potential. The major cause of water content change is displacement due to the accumulation or depletion of surfactants driven by potential. Two forces drive the surfactants to the electrode: the applied potential and the hydrophobic interactions with the water in the bulk. We have quantified their competition by varying the water content in the bulk. To our knowledge, for the first time, we have identified the electrochemical equivalent of the hydrophobic drive. For our system, a change in applied potential of 1 V has the same effect as adding a 0.55 mole fraction of water to the bulk. This work illustrates the significance of surfactants in the partitioning of water between the bulk and the surface and paves the way toward engineering interfacial water structures for controlling electrochemical reactions.

Structure of the (Bi)carbonate Adlayer on Cu(100) Electrodes

(Bi)carbonate adsorption on Cu(100) in 0.1 M KHCO₃ has been studied by in situ scanning tunneling microscopy. Coexistence of different ordered adlayer phases with ( 2 ${\sqrt{2}}$ ×6 2 ${\sqrt{2}}$ )R45° and (4×4) unit cells was observed in the double layer potential regime. The adlayer is rather dynamic and undergoes a reversible order-disorder phase transition at 0 V vs. the reversible hydrogen electrode. Density functional calculations indicate that the adlayer consists of coadsorbed carbonate and water molecules and is strongly stabilized by liquid water in the adjacent electrolyte.

Towards time resolved characterization of electrochemical reactions: electrochemically-induced Raman spectroscopy

Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.

Metal-Organic Framework-Derived BiIn Bimetallic Oxide Nanoparticles Embedded in Carbon Networks for Efficient Electrochemical Reduction of CO2 to Formate

Bismuth-based catalysts exhibit excellent activity and selectivity for the electroreduction of carbon dioxide (CO₂). However, single-component bismuth-based catalysts are not satisfactory for the electrochemical reduction of CO₂ to formic acid, mainly due to their high hydrogen production, low electrical conductivity, and small catalytic current density. Herein, we used a coordination strategy to recombine Bi and In at the molecular level to form Bi/In bimetallic metal-organic frameworks (MOFs), which were then calcined to obtain MOF-derived Bi/In bimetallic oxide nanoparticles embedded in carbon networks. Thanks to the synergistic effect of bimetallic components, high specific surface area, suitable pore size distribution, and high electrical conductivity of the carbon network, the material exhibits excellent activity and selectivity for electroreduction of CO₂ to formate. In H-type electrolyzers, the formate Faradaic efficiency reaches 91% at -0.9 V (vs RHE) and does not decrease significantly within 48 h. In situ Fourier transform infrared spectroscopy confirms the reaction intermediates and reveals that CO₂ electroreduction is dominant by the *OCHO pathway.

Understanding the complementarities of surface-enhanced infrared and Raman spectroscopies in CO adsorption and electrochemical reduction

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

Infrared and Raman spectroscopies are often assumed to provide similar insights into heterogeneous reaction mechanisms. This study shows that these techniques provide similar data when CO is strongly bound to a surface, yet distinct subpopulations of CO are probed when binding is weaker.

The presence and role of the intermediary CO reservoir in heterogeneous electroreduction of CO2

SignificanceThe electroconversion of CO2 to value-added products is a promising path to sustainable fuels and chemicals. However, the microenvironment that is created during CO2 electroreduction near the surface of heterogeneous Cu electrocatalysts remains unknown. Its understanding can lead to the development of ways to improve activity and selectivity toward multicarbon products. This work introduces a method called on-stream substitution of reactant isotope that provides quantitative information of the CO intermediate species present on Cu surfaces during electrolysis. An intermediary CO reservoir that contains more CO molecules than typically expected in a surface adsorbed configuration was identified. Its size was shown to be a factor closely associated with the formation of multicarbon products.

Copper Carbonate Hydroxide as Precursor of Interfacial CO in CO2 Electroreduction

Copper electrodes are especially effective in catalysis of C₂ and further multi-carbon products in the CO₂ reduction reaction (CO₂ RR) and therefore of major technological interest. The reasons for the unparalleled Cu performance in CO₂ RR are insufficiently understood. Here, the electrode-electrolyte interface was highlighted as a dynamic physical-chemical system and determinant of catalytic events. Exploiting the intrinsic surface-enhanced Raman effect of previously characterized Cu foam electrodes, operando Raman experiments were used to interrogate structures and molecular interactions at the electrode-electrolyte interface at subcatalytic and catalytic potentials. Formation of a copper carbonate hydroxide (CuCarHyd) was detected, which resembles the mineral malachite. Its carbonate ions could be directly converted to CO at low overpotential. These and further experiments suggested a basic mode of CO₂ /carbonate reduction at Cu electrodes interfaces that contrasted previous mechanistic models: the starting point in carbon reduction was not CO₂ but carbonate ions bound to the metallic Cu electrode in form of CuCarHyd structures. It was hypothesized that Cu oxides residues could enhance CO₂ RR indirectly by supporting formation of CuCarHyd motifs. The presence of CuCarHyd patches at catalytic potentials might result from alkalization in conjunction with local electrical potential gradients, enabling the formation of metastable CuCarHyd motifs over a large range of potentials.

Electrochemically driven C-N bond formation from CO2 and ammonia at the triple-phase boundary

Electrosynthetic techniques are gaining prominence across the fields of chemistry, engineering and energy science. However, most works within the direction of synthetic heterogeneous electrocatalysis focus on water electrolysis and CO₂ reduction. In this work, we moved to expand the scope of small molecule electrosynthesis by developing a synthetic scheme which couples CO₂ and NH₃ at a gas–liquid–solid boundary to produce species with C–N bonds. Specifically, by bringing in CO₂ from the gas phase and NH₃ from the liquid phase together over solid copper catalysts, we have succeeded in forming formamide and acetamide products for the first time from these reactants. In a subsequent complementary step, we have combined electrochemical analysis and a newly developed operando spectroelectrochemical method, capable of probing the aforementioned gas–liquid–solid boundary, to extract an initial level of mechanistic analysis regarding the reaction pathways of these reactions and the current system's limitations. We believe that the development and understanding of this set of reaction pathways will play significant role in expanding the community's understanding of on-surface electrosynthetic reactions as well as push this set of inherently sustainable technologies towards widespread applicability.

Electrocatalytic formation of C–N bonds was achieved through the electrolysis of CO₂ and NH₃ over Cu catalysts. A combined analytical and spectroscopic approach gave insights into the reaction mechanism leading to formamide and acetamide products.

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Two-dimensional materials for electrochemical CO2 reduction: materials, in situ/operando characterizations, and perspective

Electrochemical CO₂ reduction (CO₂ ECR) is an efficient approach to achieving eco-friendly energy generation and environmental sustainability. This approach is capable of lowering the CO₂ greenhouse gas concentration in the atmosphere while producing various valuable fuels and products. For catalytic CO₂ ECR, two-dimensional (2D) materials stand as promising catalyst candidates due to their superior electrical conductivity, abundant dangling bonds, and tremendous amounts of surface active sites. On the other hand, the investigations on fundamental reaction mechanisms in CO₂ ECR are highly demanded but usually require advanced in situ and operando multimodal characterizations. This review summarizes recent advances in the development, engineering, and structure-activity relationships of 2D materials for CO₂ ECR. Furthermore, we overview state-of-the-art in situ and operando characterization techniques, which are used to investigate the catalytic reaction mechanisms with the spatial resolution from the micron-scale to the atomic scale, and with the temporal resolution from femtoseconds to seconds. Finally, we conclude this review by outlining challenges and opportunities for future development in this field.

Operando Infrared Spectroscopy Reveals the Dynamic Nature of Semiconductor-Electrolyte Interface in Multinary Metal Oxide Photoelectrodes

Detailed knowledge about the semiconductor/electrolyte interface in photoelectrochemical (PEC) systems has been lacking because of the inherent difficulty of studying such interfaces, especially during operation. Current understandings of these interfaces are mostly from the extrapolation of ex situ data or from modeling approaches. Hence, there is a need for operando techniques to study such interfaces to develop a better understanding of PEC systems. Here, we use operando photoelectrochemical attenuated total reflection Fourier transform infrared (PEC-ATR-FTIR) spectroscopy to study the metal oxide/electrolyte interface, choosing BiVO₄ as a model photoanode. We demonstrate that preferential dissolution of vanadium occurs from the BiVO₄/water interface, upon illumination in open-circuit conditions, while both bismuth and vanadium dissolution occurs when an anodic potential is applied under illumination. This dynamic dissolution alters the surface Bi:V ratio over time, which subsequently alters the band bending in the space charge region. This further impacts the overall PEC performance of the photoelectrode, at a time scale very relevant for most lab-scale studies, and therefore has serious implications on the performance analysis and fundamental studies performed on this and other similar photoelectrodes.

Interfacial Acid-Base Equilibria and Electric Fields Concurrently Probed by In Situ Surface-Enhanced Infrared Spectroscopy

Understanding how applied potentials and electrolyte solution conditions affect interfacial proton (charge) transfers at electrode surfaces is critical for electrochemical technologies. Herein, we examine mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on gold using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). Measurements as a function of the applied potential, the electrolyte pD, and the electrolyte concentration determined both the relative surface populations of acidic and basic forms of 4-MBA, as well as the local electric fields at the SAM-solution interface by following the Stark shifts of 4-MBN. The effective acidity of the SAM varied with the applied potential, requiring a 600 mV change to move the pK_a by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pK_a, ∼90% of the applied potential dropped across the SAM layer. This emphasizes the importance of distinguishing applied potentials from the potential experienced at the interface. We use the measured interfacial electric fields to estimate the experienced potential at the SAM edge. The SAM pK_a showed a roughly Nernstian dependence on this estimated experienced potential. An analysis of the combined acid-base equilibria and Stark shifts reveals that the interfacial charge density has significant contributions from both SAM carboxylate headgroups and electrolyte components. Ion pairing and ion penetration into the SAM also influence the observed surface acidity. To our knowledge, this study is the first concurrent examination of both effective acidity and electric fields, and highlights the relevance of experienced potentials and specific ion effects at functionalized electrode surfaces.

Sub-Second Time-Resolved Surface-Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper

The electrocatalytic carbon dioxide (CO₂ ) reduction reaction (CO₂ RR) into hydrocarbons is a promising approach for greenhouse gas mitigation, but many details of this dynamic reaction remain elusive. Here, time-resolved surface-enhanced Raman spectroscopy (TR-SERS) is employed to successfully monitor the dynamics of CO₂ RR intermediates and Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 V vs. RHE and subsequent surface oxide reduction (below -0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hotspots for TR-SERS, enhanced time resolution (down to ≈0.7 s) and fourfold improved CO₂ RR efficiency toward ethylene. With TR-SERS, the initial restructuring of the Cu surface was followed (<7 s), after which a stable surface surrounded by increased local alkalinity was formed. Our measurements revealed that a highly dynamic CO intermediate, with a characteristic vibration below 2060 cm^-1 , is related to C-C coupling and ethylene production (-0.9 V vs. RHE), whereas lower cathodic bias (-0.7 V vs. RHE) resulted in gaseous CO production from isolated and static CO surface species with a distinct vibration at 2092 cm^-1 .

Nanocapillarity and Nanoconfinement Effects of Pipet-like Bismuth@Carbon Nanotubes for Highly Efficient Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction reaction is regarded as an intriguing route for producing renewable chemicals and fuels, but its development is limited by the lack of highly efficient and stable electrocatalysts. Herein, we propose the pipet-like bismuth (Bi) nanorods semifilled in nitrogen-doped carbon nanotubes (Bi-NRs@NCNTs) for highly selective electrocatalytic CO₂ reduction. Benefited from the prominent capillary and confinement effects, the Bi-NRs@NCNTs act as nanoscale conveyors that can significantly facilitate the mass transport, adsorption,and concentration of reactants onto the active sites, realizing rapid reaction kinetics and low cathodic polarization. The spatial encapsulation and separation by the NCNT shells prevents the self-aggregation and surface oxidation of Bi-NRs, increasing the dispersity and stability of the electrocatalyst. As a result, the Bi-NRs@NCNTs exhibit high activity and durable catalytic stability for CO₂-to-formate conversion over a wide potential range. The Faradaic efficiency for formate production reaches 90.9% at a moderate applied potential of -0.9 V vs reversible hydrogen electrode (RHE).

Morphology and mechanism of highly selective Cu(II) oxide nanosheet catalysts for carbon dioxide electroreduction

Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C2+ products, selectivities (> 400 mA cm-2) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu0 dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C2H4 was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH4 and ethanol. We demonstrate evidence that adsorbed methyl species, *CH3, serve as common intermediates of both CH3H and CH3CH2OH and possibly of other CH3-R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO2 electrolysis on Cu catalysts.

Operando spectroscopy of nanoscopic metal/covalent organic framework electrocatalysts

Metal and covalent organic frameworks (MOFs and COFs) are increasingly finding exceptional utility in electrocatalytic systems. Their chemically defined porous nature grants them key functions that may enhance their electrocatalytic performance relative to conventional molecular or heterogeneous materials. In order to obtain insights into their function, mechanism, and dynamics under electrocatalytic conditions, operando spectroscopy, that which is performed as the catalyst is functioning, has been increasingly applied. This mini review highlights several key works emerging in recent years that have used various operando spectroscopic techniques, namely UV-vis absorption, Raman, Infrared, and X-ray absorption spectroscopy, to investigate electrocatalytic MOFs and COFs. A brief introduction to each technique and how it was applied to investigate MOF/COF-based electrolytic systems is detailed. The unique set of data obtained, interpretations made, and progress attained all point to the power of operando spectroscopy in truly opening the functionality of MOFs and COFs across many aspects of catalysis.

Bridging the Gap in the Mechanistic Understanding of Electrocatalysis via In Situ Characterizations

Electrocatalysis offers a promising strategy to take advantage of the increasingly available and affordable renewable energy for the sustainable production of fuels and chemicals. Attaining this promise requires a molecular level insight of the electrical interface that can be used to tailor the selectivity of electrocatalysts. Addressing this selectivity challenge remains one of the most important areas in modern electrocatalytic research. In this Perspective, we focus on the use of in situ techniques to bridge the gap in the fundamental understanding of electrocatalytic processes. We begin with a brief discussion of traditional electrochemical techniques, ex situ measurements and in silico analysis. Subsequently, we discuss the utility and limitations of in situ methodologies, with a focus on vibrational spectroscopies. We then end by looking ahead toward promising new areas for the application of in situ techniques and improvements to current methods.

Tailored electrocatalysts by controlled electrochemical deposition and surface nanostructuring

Controlled electrodeposition and surface nanostructuring are very promising approaches to tailor the structure of the electrocatalyst surface, with the aim to enhance their efficiency for sustainable energy conversion reactions. In this highlight, we first summarise different strategies to modify the structure of the electrode surface at the atomic and sub-monolayer level for applications in electrocatalysis. We discuss aspects such as structure sensitivity and electronic and geometric effects in electrocatalysis. Nanostructured surfaces are finally introduced as more scalable electrocatalysts, where morphology, cluster size, shape and distribution play an essential role and can be finely tuned. Controlled electrochemical deposition and selective engineering of the surface structure are key to design more active, selective and stable electrocatalysts towards a decarbonised energy scheme.

Probing CO2 Reduction Pathways for Copper Catalysis Using an Ionic Liquid as a Chemical Trapping Agent

The key to fully leveraging the potential of the electrochemical CO2 reduction reaction (CO2RR) to achieve a sustainable solar-power-based economy is the development of high-performance electrocatalysts. The development process relies heavily on trial and error methods due to poor mechanistic understanding of the reaction. Demonstrated here is that ionic liquids (ILs) can be employed as a chemical trapping agent to probe CO2RR mechanistic pathways. This method is implemented by introducing a small amount of an IL ([BMIm][NTf2 ]) to a copper foam catalyst, on which a wide range of CO2RR products, including formate, CO, alcohols, and hydrocarbons, can be produced. The IL can selectively suppress the formation of ethylene, ethanol and n-propanol while having little impact on others. Thus, reaction networks leading to various products can be disentangled. The results shed new light on the mechanistic understanding of the CO2RR, and provide guidelines for modulating the CO2RR properties. Chemical trapping using an IL adds to the toolbox to deduce the mechanistic understanding of electrocatalysis and could be applied to other reactions as well.

In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

In-situ Spectroscopic Techniques as Critical Evaluation Tools for Electrochemical Carbon dioxide Reduction: A Mini Review

Electrocatalysis plays a crucial role in modern electrochemical energy conversion technologies as a greener replacement for conventional fossil fuel-based systems. Catalysts employed for electrochemical conversion reactions are expected to be cheaper, durable, and have a balance of active centers (for absorption of the reactants, intermediates formed during the reactions), porous, and electrically conducting material to facilitate the flow of electrons for real-time applications. Spectroscopic and microscopic studies on the electrode-electrolyte interface may lead to better understanding of the structural and compositional deviations occurring during the course of electrochemical reaction. Researchers have put significant efforts in the past decade toward understanding the mechanistic details of electrochemical reactions which resulted in hyphenation of electrochemical-spectroscopic/microscopic techniques. The hyphenation of diverse electrochemical and conventional microscopic, spectroscopic, and chromatographic techniques, in addition to the elementary pre-screening of electrocatalysts using computational methods, have gained deeper understanding of the electrode-electrolyte interface in terms of activity, selectivity, and durability throughout the reaction process. The focus of this mini review is to summarize the hyphenated electrochemical and non-electrochemical techniques as critical evaluation tools for electrocatalysts in the CO2 reduction reaction.

Electrochemical CO2 reduction on nanostructured metal electrodes: fact or defect?

Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions.