Operando Characterization Research Articles

Quasi-operando Transmission Electron Microscopy Diagnostics for Electrocatalytic Processes in Liquids.

The need to relate the mechano-physico-chemical phenomena in liquid-based electrocatalysts to the stages of start-up, operation, and shut-down phases is one of the major challenges that the energy community is facing. Understanding these phenomena will pave the way for the tailor-made design of efficient, commercially viable electrocatalytic systems. Transmission electron microscopy plays an important role in the investigation of local electrocatalytic effects, complementing other operando characterization techniques. Herein, after attempting to define the meaning of operando methodologies in relation to electron microscopy studies, the progress in the field is reviewed in terms of the knowledge gained about the catalysts, the solid-liquid interfaces, and the solid-liquid-gas interfacial phenomena for several electrocatalytic reactions. Finally, the parameters that require consideration in operando ec-LPTEM studies of electrocatalytic systems are discussed.

Structure and Chemical Reactivity of Y-Stabilized ZrO2 Surfaces: Importance for the Water-Gas Shift Reaction.

The surface structure and chemical properties of Y-stabilized zirconia (YSZ) have been subjects of intense debate over the past three decades. However, a thorough understanding of chemical processes occurring at YSZ powders faces significant challenges due to the absence of reliable reference data acquired for well-controlled model systems. Here, we present results from polarization-resolved infrared reflection absorption spectroscopy (IRRAS) obtained for differently oriented, Y-doped ZrO2 single-crystal surfaces after exposure to CO and D2O. The IRRAS data reveal that the polar YSZ(100) surface undergoes reconstruction, characterized by an unusual, red-shifted CO band at 2132 cm-1. Density functional theory calculations allowed to relate this unexpected observation to under-coordinated Zr4+ cations in the vicinity of doping-induced O vacancies. This reconstruction leads to a strongly increased chemical reactivity and water spontaneously dissociates on YSZ(100). The latter, which is an important requirement for catalysing the water-gas-shift (WGS) reaction, is absent for YSZ(111), where only associative adsorption was observed. Together with a novel analysis Scheme these reference data allowed for an operando characterisation of YSZ powders using DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). These findings facilitate rational design and tuning of YSZ-based powder materials for catalytic applications, in particular CO oxidation and the WGS reaction.

Struktur und chemische Reaktivität von Yttrium‐stabilisierten ZrO2‐Oberflächen: Zur Bedeutung für die Wassergas‐Shift‐Reaktion

AbstractDie Oberflächenstruktur und die chemischen Eigenschaften von Yttrium‐stabilisiertem Zirkoniumdioxid (YSZ) waren in den letzten drei Jahrzehnten Gegenstand intensiver Diskussionen. Ein umfassendes Verständnis der chemischen Prozesse, die bei YSZ‐Pulvern ablaufen, ist jedoch aufgrund der fehlenden Erhebung von zuverlässigen Referenzdaten für gut kontrollierte Modellsysteme mit erheblichen Herausforderungen verbunden. Wir stellen hier die Ergebnisse aus der polarisationsaufgelösten Infrarot‐Reflexions‐Absorptions‐Spektroskopie (IRRAS) vor, die für unterschiedlich orientierte, Yttrium‐dotierte ZrO2‐Einkristalloberflächen nach der Adsorption von CO und D2O erhalten wurden. Die IRRAS‐Daten zeigen, dass die polare YSZ(100)‐Oberfläche eine Rekonstruktion erfährt, die durch eine ungewöhnliche rotverschobene CO‐Bande bei 2132 cm−1 gekennzeichnet ist. Berechnungen gemäß der Dichtefunktionaltheorie ermöglichten es, diese unerwartete Beobachtung mit unterkoordinierten Zr4+‐Kationen in der Nähe der durch die Dotierung induzierten O‐Fehlstellen in Verbindung zu bringen. Diese Rekonstruktion führt zu einer stark erhöhten chemischen Reaktivität und zu einer spontanen Dissoziation von Wasser an YSZ(100). Letzteres, eine wichtige Voraussetzung für die Katalyse der Wassergas‐Shift (WGS)‐Reaktion fehlt für YSZ(111), wo lediglich eine assoziative Adsorption beobachtet wurde. Zusammen mit einem neuartigen Analyseschema ermöglichten diese Referenzdaten eine operando‐Charakterisierung von YSZ‐Pulvern mithilfe der DRIFTS (Diffuse‐Reflexions‐Infrarot‐Fourier‐Transform‐Spektroskopie). Diese Erkenntnisse erleichtern das rationale Design und die Abstimmung von YSZ‐basierten Pulvermaterialien für katalytische Anwendungen, insbesondere für die CO‐Oxidation und die WGS‐Reaktion.

Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction.

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to β-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals β-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate β-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared β-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Combustion Growth of NiFe Layered Double Hydroxide for Efficient and Durable Oxygen Evolution Reaction.

NiFe layered double hydroxide (LDH) with abundant heterostructures represents a state-of-the-art electrocatalyst for the alkaline oxygen evolution reaction (OER). Herein, NiFe LDH/Fe2O3 nanosheet arrays have been fabricated by facile combustion of corrosion-engineered NiFe foam (NFF). The in situ grown, self-supported electrocatalyst exhibited a low overpotential of 248 mV for the OER at 50 mA cm-2, a small Tafel slope of 31 mV dec-1, and excellent durability over 100 h under the industrial benchmarking 500 mA cm-2 current density. A balanced Ni and Fe composition under optimal corrosion and combustion contributed to the desirable electrochemical properties. Comprehensive ex-situ analyses and operando characterizations including Fourier-transformed alternating current voltammetry (FTACV) and in situ Raman demonstrate the beneficial role of modulated interfacial electron transfer, dynamic atomic structural transformation to NiOOH, and the high-valence active metal sites. This study provides a low-cost and easy-to-expand way to synthesize efficient and durable electrocatalysts.

A snapshot review of nanostructure synthesis and operando characterization

A snapshot review of nanostructure synthesis and operando characterization

Beam Effects in Synchrotron Radiation Operando Characterization of Battery Materials: X-Ray Diffraction and Absorption Study of LiNi0.33Mn0.33Co0.33O2 and LiFePO4 Electrodes.

Operando synchrotron radiation-based techniques are a precious tool in battery research, as they enable the detection of metastable intermediates and ensure characterization under realistic cycling conditions. However, they do not come exempt of risks. The interaction between synchrotron radiation and samples, particularly within an active electrochemical cell, can induce relevant effects at the irradiated spot, potentially jeopardizing the experiment's reliability and biasing data interpretation. With the aim of contributing to this ongoing debate, a systematic investigation into these phenomena was carried out by conducting a root cause analysis of beam-induced effects during the operando characterization of two of the most commonly employed positive electrode materials in commercial Li-ion batteries: LiNi0.33Mn0.33Co0.33O2 and LiFePO4. The study spans across diverse experimental conditions involving different cell types and absorption and scattering techniques and seeks to correlate beam effects with factors such as radiation energy, photon flux, exposure time, and other parameters associated with radiation dosage. Finally, it provides a comprehensive set of guidelines and recommendations for assessing and mitigating beam-induced effects that may affect the outcome of battery operando experiments.

Two‐Dimensional Layered Hydroxide Materials for Nucleophile Oxidation

AbstractThe global energy crisis highlights the need to transition from fossil fuels to sustainable, renewable sources of energy. Green hydrogen produced via renewable energy‐driven water electrolysis is an emerging alternative due to its zero emissions and high energy density. To address the high energy consumption of water electrolysis, innovative hybrid electrolyzers integrating nucleophile oxidation reactions (NOR) are under activity investigation, reducing energy demands and enabling valuable product synthesis and waste treatment, thereby enhancing the efficiency and economic viability of hydrogen production via electrocatalytic technologies. Recent advancements in layered hydroxide materials (LHMs) have markedly improved the efficiency of alkaline electrochemical conversion processes and are gaining prominence in NORs due to their expansive surface areas and tailorable characteristics through diverse engineering strategies. Further, their layered architecture is readily conducive to in‐situ characterization, using techniques like XAS, XRD, Raman, and IR spectroscopy, providing insights into their anodic NOR mechanisms. This review summarizes the latest developments in LHMs as electrocatalysts for NOR, discusses current design strategies of LHMs, and emphasizes the significance of operando characterization techniques in elucidating the reaction mechanisms of different LHMs. Finally, the future challenges and potential advancements in their scale‐up application in electrocatalytic NOR are put forward.

Operando Spectroscopic Monitoring of Metal Chalcogenides for Overall Water Splitting: New Views of Active Species and Sites

AbstractMetal‐based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X‐ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure–activity relationships of low‐crystalline cobalt sulfide (L−CoS) catalysts toward overall water splitting. The operando results for L−CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S−Co−Co−S moieties as the true catalytically active species. In comparison, during the acidic HER, L−CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co−S−Co configurations. Further operando characterizations highlight the crucial formation of high‐valent Co4+ species in L−CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co‐LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity.

Operando Spectroscopic Monitoring of Metal Chalcogenides for Overall Water Splitting: New Views of Active Species and Sites.

Metal-based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X-ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure-activity relationships of low-crystalline cobalt sulfide (L-CoS) catalysts toward overall water splitting. The operando results for L-CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S-Co-Co-S moieties as the true catalytically active species. In comparison, during the acidic HER, L-CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co-S-Co configurations. Further operando characterizations highlight the crucial formation of high-valent Co4+ species in L-CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co-LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity.

An active, stable cubic molybdenum carbide catalyst for the high-temperature reverse water-gas shift reaction.

Although technologically promising, the reduction of carbon dioxide (CO2) to produce carbon monoxide (CO) remains economically challenging owing to the lack of an inexpensive, active, highly selective, and stable catalyst. We show that nanocrystalline cubic molybdenum carbide (α-Mo2C), prepared through a facile and scalable route, offers 100% selectivity for CO2 reduction to CO while maintaining its initial equilibrium conversion at high space velocity after more than 500 hours of exposure to harsh reaction conditions at 600°C. The combination of operando and postreaction characterization of the catalyst revealed that its high activity, selectivity, and stability are attributable to crystallographic phase purity, weak CO-Mo2C interactions, and interstitial oxygen atoms, respectively. Mechanistic studies and density functional theory (DFT) calculations provided evidence that the reaction proceeds through an H2-aided redox mechanism.

Magnetic Force Dilatometry for Operando Coin Cell Electrochemical Dilation Measurements

Operando characterization of the physical and chemical changes occurring within batteries during electrochemical cycling has become a powerful tool for next generation technology development. In particular, a better understanding of the expansion (dilation) behavior of active materials during charge/discharge is critical for mitigating performance degradation, particularly for high expansion materials like Li or Si. However, current dilatometry devices rely on direct mechanical coupling or line-of-sight measurements with pouch- or custom-cells, which prohibits their use in the most common and accessible research battery format: coin cells. To this end, we propose a novel magnetic force dilatometry (MFD) technique for operando electrochemical dilation measurements. Our custom low-cost dilatometer utilizes magnetic force sensing for contactless expansion measurements via facile replacement of the austenitic spacer with a ferritic spacer within a coin cell. To validate this setup, we demonstrate operando electrochemical dilatometry of a LiNi0.6Mn0.2Co0.2O2 (NMC622) || Li metal full cell in a CR2032 format. Our MFD accurately captures cell expansion/contraction with sensitivities of less than 0.1 μm, and reliability for over hundreds of hours and cycles. This new MFD method is expected to increase the accessibility of electrochemical dilatometry by eliminating the need for pouch cells or other specialized cell expansion measurement configurations.

Operando Characterization and Molecular Simulations Reveal the Growth Kinetics of Graphene on Liquid Copper During Chemical Vapor Deposition.

In recent years, liquid metal catalysts have emerged as a compelling choice for the controllable, large-scale, and high-quality synthesis of two-dimensional materials. At present, there is little mechanistic understanding of the intricate catalytic process, though, of its governing factors or what renders it superior to growth at the corresponding solid catalysts. Here, we report on a combined experimental and computational study of the kinetics of graphene growth during chemical vapor deposition on a liquid copper catalyst. By monitoring the growing graphene flakes in real time using in situ radiation-mode optical microscopy, we explore the growth morphology and kinetics over a wide range of CH4-to-H2 pressure ratios and deposition temperatures. Constant growth rates of the flakes' radius indicate a growth mode limited by precursor attachment, whereas methane-flux-dependent flake shapes point to limited precursor availability. Large-scale free energy simulations enabled by an efficient machine-learning moment tensor potential trained to density functional theory data provide quantitative barriers for key atomic-scale growth processes. The wealth of experimental and theoretical data can be consistently combined into a microkinetic model that reveals mixed growth kinetics that, in contrast to the situation at solid Cu, is partly controlled by precursor attachment alongside precursor availability. Key mechanistic aspects that directly point toward the improved graphene quality are a largely suppressed carbon dimer attachment due to the facile incorporation of this precursor species into the liquid surface and a low-barrier ring-opening process that self-heals 5-membered rings resulting from remaining dimer attachments.

Extending Droplet-Based Microfluidic Tools to Single-Atom Heterogeneous Catalysis.

From energy-related transformations to organic syntheses, single-atom heterogeneous catalysts (SACs) are offering new prospects to tackle sustainability challenges. However, scarce design guidelines and poor mechanistic understanding due to a lack of discovery and operando characterization tools impede theirbroader development. This perspective offers a glimpse into how droplet-based microfluidic technologies mayhelp solve both of these issues, and provides technical considerations for platform design to systematically fabricate SACs and study them under operational conditions during liquid-phase organic syntheses.

Chromium-Induced High Covalent Co-O Bonds for Efficient Anodic Catalysts in PEM Electrolyzer.

The proton exchange membrane water electrolyzer (PEMWE), crucial for green hydrogen production, is challenged by the scarcity and high cost of iridium-based materials. Cobalt oxides, as ideal electrocatalysts for oxygen evolution reaction (OER), have not been extensively applied in PEMWE, due to extremely high voltage and poor stability at large current density, caused by complicated structural variations of cobalt compounds during the OER process. Thus, the authors sought to introduce chromium into a cobalt spinel (Co3O4) catalyst to regulate the electronic structure of cobalt, exhibiting a higher oxidation state and increased Co-O covalency with a stable structure. In-depth operando characterizations and theoretical calculations revealed that the activated Co-O covalency and adaptable redox behavior are crucial for facilitating its OER activity. Both turnover frequency and mass activity of Cr-doped Co3O4 (CoCr) at 1.67V (vs RHE) increased by over eight times than those of as-synthesized Co3O4. The obtained CoCr catalyst achieved 1500mA cm-2 at 2.17V and exhibited notable durability over extended operation periods - over 100h at 500 mA cm-2 and 500h at 100 mA cm-2, demonstrating promising application in the PEMWE industry.

Electrocatalytic hydrogenation of acetonitrile to ethylamine in acid

Electrochemical hydrogenation of acetonitrile based on well-developed proton exchange membrane electrolyzers holds great promise for practical production of ethylamine. However, the local acidic condition of proton exchange membrane results in severe competitive proton reduction reaction and poor selection toward acetonitrile hydrogenation. Herein, we conduct a systematic study to screen various metallic catalysts and discover Pd/C exhibits a 43.8% ethylamine Faradaic efficiency at the current density of 200 mA cm−2 with a specific production rate of 2912.5 mmol g−1 h−1, which is about an order of magnitude higher than the other screened metal catalysts. Operando characterizations indicate the in-situ formed PdHx is the active centers for catalytic reaction and the adsorption strength of the *MeCH2NH2 intermediate dictates the catalytic selectivity. More importantly, the theoretical analysis reveals a classic d-band mediated volcano curve to describe the relation between the electronic structures of catalysts and activity, which could provide valuable insights for designing more effective catalysts for electrochemical hydrogenation reactions and beyond.

Single‐Particle and Single‐Molecule Characterization of Immobilized Enzymes: A Multiscale Path toward Optimizing Heterogeneous Biocatalysts

AbstractHeterogeneous biocatalysis is highly relevant in biotechnology as it offers several benefits and practical uses. To leverage the full potential of heterogeneous biocatalysts, the establishment of well‐crafted protocols, and a deeper comprehension of enzyme immobilization on solid substrates are essential. These endeavors seek to optimize immobilized biocatalysts, ensuring maximal enzyme performance within confined spaces. For this aim, multidimensional characterization of heterogeneous biocatalysts is required. In this context, spectroscopic and microscopic methodologies conducted at different space and temporal scales can inform about the intraparticle enzyme kinetics, the enzyme spatial distribution, and the mass transport issues. In this Minireview, we identify enzyme immobilization, enzyme catalysis, and enzyme inactivation as the three main processes for which advanced characterization tools unveil fundamental information. Recent advances in operando characterization of immobilized enzymes at the single‐particle (SP) and single‐molecule (SM) levels inform about their functional properties, unlocking the full potential of heterogeneous biocatalysis toward biotechnological applications.

Single-Particle and Single-Molecule Characterization of Immobilized Enzymes: A Multiscale Path toward Optimizing Heterogeneous Biocatalysts.

Heterogeneous biocatalysis is highly relevant in biotechnology as it offers several benefits and practical uses. To leverage the full potential of heterogeneous biocatalysts, the establishment of well-crafted protocols, and a deeper comprehension of enzyme immobilization on solid substrates are essential. These endeavors seek to optimize immobilized biocatalysts, ensuring maximal enzyme performance within confined spaces. For this aim, multidimensional characterization of heterogeneous biocatalysts is required. In this context, spectroscopic and microscopic methodologies conducted at different space and temporal scales can inform about the intraparticle enzyme kinetics, the enzyme spatial distribution, and the mass transport issues. In this Minireview, we identify enzyme immobilization, enzyme catalysis, and enzyme inactivation as the three main processes for which advanced characterization tools unveil fundamental information. Recent advances in operando characterization of immobilized enzymes at the single-particle (SP) and single-molecule (SM) levels inform about their functional properties, unlocking the full potential of heterogeneous biocatalysis toward biotechnological applications.

Oxygen-Bridged Cu Binuclear Sites for Efficient Electrocatalytic CO2 Reduction to Ethanol at Ultralow Overpotential.

Electrocatalytic CO2 reduction (CO2RR) to alcohols offers a promising strategy for converting waste CO2 into valuable fuels/chemicals but usually requires large overpotentials. Herein, we report a catalyst comprising unique oxygen-bridged Cu binuclear sites (CuOCu-N4) with a Cu···Cu distance of 3.0-3.1 Å and concomitant conventional Cu-N4 mononuclear sites on hierarchical nitrogen-doped carbon nanocages (hNCNCs). The catalyst exhibits a state-of-the-art low overpotential of 0.19 V (versus reversible hydrogen electrode) for ethanol and an outstanding ethanol Faradaic efficiency of 56.3% at an ultralow potential of -0.30 V, with high-stable Cu active-site structures during the CO2RR as confirmed by operando X-ray adsorption fine structure characterization. Theoretical simulations reveal that CuOCu-N4 binuclear sites greatly enhance the C-C coupling at low potentials, while Cu-N4 mononuclear sites and the hNCNC support increase the local CO concentration and ethanol production on CuOCu-N4. This study provides a convenient approach to advanced Cu binuclear site catalysts for CO2RR to ethanol with a deep understanding of the mechanism.

Hydrogen spillover in Pt/Ni(OH)2/Mo2TiC2Tx electrocatalyst improves pH-universal hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) shows potential to transition from fossil fuels to green renewable energy. Designed for high efficiency and stability electrocatalyst in pH-universal electrolyte is of great significance but challenging. Herein, a novel electrocatalyst of two-dimensional (2D) Mo2TiC2Tx MXene supported Pt/Ni(OH)2 (denoted as Pt/Ni(OH)2/Mo2TiC2Tx) is reported by hydrogen spillover effect for efficient HER in the full-pH range. The operando electrochemical characterization reveals that the hydrogen spillover phenomenon interactions between multi-functional catalytic sites, e.g. Pt, Ni(OH)2 and Mo2TiC2Tx, are used to enhance HER performance and expedite HER kinetics. Moreover, theoretical calculation via density functional theory demonstrates the Pt/Ni(OH)2/Mo2TiC2Tx possess free energy of absorbed hydrogen close to zero (≈– 0.09 eV), which leads to optimal activity. As expected, the electrocatalyst necessitates the overpotential of 25, 33 and 36 mV at 10 mA cm−2 in acidic, alkaline, and neutral conditions, respectively, which are outperforming Pt/C. This work furtherly demonstrates a new way to design a powerful electrocatalyst for pH-universal HER based hydrogen spillover phenomenon.