Operando Insights Research Articles

Degradation Effects in Li4Ti5O12-Based Cells─Learning from Electrode Potential Profiles.

Li4Ti5O12 (LTO) is a promising negative electrode active material for high-power applications of lithium-ion batteries due to its structural and dimensional stability, high safety properties, and rate capability. However, there are still challenges regarding the cyclic aging behavior of LTO-based cells, especially when it comes to the origin of gas evolution that needs to be resolved in order to enable an optimized cell design and prolonged cycle life. Using a three-electrode setup provides operando insights into the cyclic aging behavior of LiNi1/3Co1/3Mn1/3O2 (NCM111)||LTO cells. The results demonstrated that an initial slight increase in specific capacity followed by an intense decrease after 40 cycles could be attributed to a relative shift of the individual electrode potential profiles caused by loss of lithium inventory and associated electrolyte consumption during cyclic aging. By using higher formation temperatures, the capacity decrease, an associated loss of lithium inventory, and continuous electrolyte consumption were successfully suppressed. Moreover, the results suggested that gas evolution in Li4Ti5O12-based cells majorly originates from the reductive decomposition of moisture residues and the electrolyte at the Li4Ti5O12 electrode surface.

Self-regenerative Ni/SiO2 for dry reforming of methane (DRM): 1000 h-longevity assessment and operando insights to coke removal under N2 atmosphere

Commercializing Ni-catalyst for dry reforming of methane (DRM) has yet to be realized due to two known issues: catalyst sintering and carbon deposition. Significantly, our developed tremella-like Ni/SiO2 has previously demonstrated outstanding resilience against sintering under prolonged heat exposure. The carry-on investigation herein further showcases its interesting self-regenerative behavior, which enables carbon removal and full activity restoration under inert N2-blanket. Better still, the resultant N2-regenerated Ni/SiO2 can immediately re-apply to DRM, contrasted to the traditional oxidative-regenerated counterpart that mandated intermediate reduction process for catalyst re-activation. Characterization results indicate that an appreciable portion (32.05 %) of deposited Ni (7.3 wt%) is reducible into active metallic phase; however, only 1.233 % of them are resided on the surface of Ni/SiO2 and involved in DRM. Despite such low involvement rate, >90 % CO2 and CH4 conversion could still be attained initially, reasonably due to the morphological prominence of the catalyst. Longevity experiment confirmed the adequacy of intermittent N2-regeneration in removing deposited carbonaceous, mechanistically assisted by the labile O-atom migrated from SiO2 support. Upon 4 h N2-regeneration, thorough activity restoration of Ni/SiO2 can be facilely realized, thereafter sustaining DRM for 1000 h with CO2 conversion controlled at > 78 %. The results herein provide important insights to DRM community, where the demonstrated N2-self-regeneration process promises a reduction-free, time- and cost-saving solution to DRM catalysts beyond Ni/SiO2.

Operando Insights on the Degradation Mechanisms of Rhenium‐Doped and Undoped Molybdenum Disulfide Nanocatalysts During Hydrogen Evolution Reaction and Open‐Circuit Conditions

AbstractMolybdenum disulfide (MoS2) nanostructures are promising catalysts for proton‐exchange‐membrane (PEM) electrolyzers to replace expensive noble metals. Their large‐scale application demands high activity for the hydrogen evolution reaction (HER) as well as robust durability. Doping is commonly applied to enhance the HER activity of MoS2‐based nanocatalysts, but the effect of dopants on the electrochemical and structural stability is yet to be discussed. Herein, operando electrochemical measurements to the structural evolution of the materials down to the nanometric scale are correlated by identical location electron microscopy and spectroscopy. The range of stable operation for MoS2 nanocatalysts with and without rhenium doping is experimentally defined. The responsible degradation mechanisms at first electrolyte contact, open circuit stabilization, and HER conditions are experimentally identified and confirmed with the calculated Pourbaix diagram of Re‐doped MoS2. Doping MoS2‐based nanocatalysts is validated as a promising strategy for continuing the improvement of high‐performance and durable PEM electrolyzers.

Operando insights into ammonium-mediated lithium metal stabilization: surface morphology modulation and enhanced SEI development

Lithium-ion batteries (LiBs) with graphite as an anode and lithiated transition metal oxide as a cathode are approaching their specific energy and power theoretical values. To overcome the limitations of LiBs, lithium metal anode with high specific capacity and low negative redox potential is necessary. However, practical application in rechargeable cells is hindered by uncontrolled lithium deposition manifesting, for instance, as Li dendrite growth which can cause formation of dead Li, short circuits and cell failure.The electrochemical behaviour of a protic additive (NH4PF6) in a carbonate-based electrolyte has been investigated by operando confocal Raman spectroscopy, in situ optical microscopy, and X-ray photoelectron spectroscopy, elucidating its functional mechanism. The ammonium cation promotes a chemical modification of the lithium metal anode-electrolyte interphase by producing an N-rich solid electrolyte interphase and chemically modifying the lithium surface morphology by electrochemical pitting. This novel method results in stable lithium deposition and stripping by a decreasing the local current density on the electrode, thus limiting dendritic deposition.

Operando Studies for CO2/CO Reduction in Flow‐Based Devices

AbstractElectrocatalytic CO2 reduction reaction (CO2RR) conducted in a flow‐based device exhibits a substantial enhancement in Faradaic efficiency and catalytic current density compared to a conventional H‐type configuration. This highlights the distinct catalytic environment and behavior inherent in flow cells as opposed to H cells. To investigate the authentic properties of a catalyst within a flow‐based device, customized flow cells have been specifically devised for operando techniques during CO2RR and CORR, rather than resorting to an in‐situ three‐electrode H‐type configuration with its disparate catalytic environment and performance. This approach ensures a catalytic environment identical to that employed in electrochemical measurements. This review delineates the disparities between H‐type and flow‐based cells as well as the operando techniques tailored for flow‐based devices, including X‐ray absorption spectroscopy and Raman spectroscopy, preserving a consistent catalytic environment. It also compiles recent findings on copper‐based systems using operando flow‐based devices. The operando insights reveal a significant augmentation in catalytic current density, impacting both chemical properties and crystal structures. Furthermore, the observation of various catalytic intermediates enriches our comprehension. In essence, the application of operando techniques to flow‐based devices furnishes a comprehensive understanding of the catalytic behavior exhibited by diverse systems, propelling progress toward achieving Net Zero emissions.

Operando insights into correlating CO coverage and Cu-Au alloying with the selectivity of Au NP-decorated Cu2O nanocubes during the electrocatalytic CO2 reduction.

Electrochemical reduction of CO2 (CO2RR) is an attractive technology to reintegrate the anthropogenic CO2 back into the carbon cycle driven by a suitable catalyst. This study employs highly efficient multi-carbon (C2+) producing Cu2O nanocubes (NCs) decorated with CO-selective Au nanoparticles (NPs) to investigate the correlation between a high CO surface concentration microenvironment and the catalytic performance. Structure, morphology and near-surface composition are studied via operando X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy, operando high-energy X-ray diffraction as well as quasi in situ X-ray photoelectron spectroscopy. These operando studies show the continuous evolution of the local structure and chemical environment of our catalysts during reaction conditions. Along with its alloy formation, a CO-rich microenvironment as well as weakened average CO binding on the catalyst surface during CO2RR is detected. Linking these findings to the catalytic function, a complex compositional interplay between Au and Cu is revealed in which higher Au loadings primarily facilitate CO formation. Nonetheless, the strongest improvement in C2+ formation appears for the lowest Au loadings, suggesting a beneficial role of the Au-Cu atomic interaction for the catalytic function in CO2RR. This study highlights the importance of site engineering and operando investigations to unveil the electrocatalyst's adaptations to the reaction conditions, which is a prerequisite to understand its catalytic behavior.

Time-resolved operando insights into the tunable selectivity of Cu–Zn nanocubes during pulsed CO2 electroreduction

Electrochemical CO2 reduction using CuZn nanocubes boosted ethanol selectivity when pulsed in the oxidation regime of zinc, while time-resolved operando techniques uncovered the key roles of dynamic zinc oxide formation and hydroxide coverage.

A Segmented Cell Measuring Technique for Current Distribution Measurements in Batteries, Exemplified by the Operando Investigation of a Zn-Air Battery

A transimpedance amplifier circuit as well as an instrumental amplifier circuit were used to measure current densities of a zinc-air battery with an integrated segmented current collector foil. Error calculation showed that the transimpedance amplifier is superior to the used instrumental amplifier, but both methods provide valuable and consistent results. They both showed comparable results with operando insight into the current distribution of the battery. The knowledge about those distributions is essential to avoid fast degradation of battery materials and irreversible capacity loss due to heterogeneous dissolution of the anode during discharge. In this work we showed that oxygen starvation as well as gas flow rate leads to large current gradients. It was also demonstrated that heterogeneous current distributions on cathode side induces also a heterogenous dissolution behavior on the anode, resulting in irreversible capacity loss.

Complementary operando insights into the activation of multicomponent selective propylene oxidation catalysts

Two Bi–Mo–Co–Fe–O catalysts were synthesized by flame spray pyrolysis and tested for their catalytic performance in selective oxidation of propylene to acrolein. Pronounced structural changes during temperature–programmed oxidation and reaction were observed by operando X–ray absorption spectroscopy, X–ray diffraction, Raman spectroscopy, and thermogravimetric analysis. During oxidative treatment, mainly binary oxide phases (α–Bi2Mo3O12, β–CoMoO4, Fe2(MoO4)3) were observed, but single (MoO3) or ternary (Bi3(FeO4)(MoO4)2) oxides also formed depending on the relative elemental catalyst composition. During propylene oxidation, the reduction of Fe3+ to Fe2+ led to a strong rise in activity and induced further phase transformations. MoO3 was found to be unselective towards acrolein but was essential in binding other single oxides. The formation of β–Co0.7Fe0.3MoO4 and Bi3(FeO4)(MoO4)2 as well as their synergistic interplay with α–Bi2Mo3O12 are key factors for high performance. The combination of complementary operando methods was crucial to reveal new structure–activity/selectivity correlations, therefore bridging the knowledge gap between simplified model systems and complex applied catalysts.

Waste Tire Heat Treatment to Prepare Sulfur Self-Doped Char: Operando Insight into Activation Mechanisms Based on the Char Structures Evolution

Waste tire (WT) can be heat-treated to be high-quality sulfur self-doped char via pyrolysis and K2FeO4-assisted activation processes. This work aimed at further studying the activation mechanisms based on the char structures evolution by operando experimental method. Activation treatment process (from 50 °C to 800 °C and then held for 3 h) was divided into six typical stages (S1–S6) and consisted of carbonization process (S1–S4) and effective activation process (S4–S6). During the carbonization process, the specific capacitance only increased from 0.2 F/g to 12.4 F/g, aromatic ring systems and alkyl-aryl C-C bonds generated, S 2p3/2 (sulphide bridge) was mainly gradually consumed. During the effective activation process, the specific capacitance hugely increased from 12.4 F/g to 112.5 F/g, aromatic ring systems and alkyl-aryl C-C bonds turned to ordered graphitic char. The pores massively generated from S4 to S5, while micropores partly formed to larger and mesopores+macropores fractionally converting to smaller from S5 to S6. Besides, both S 2p3/2 and S 2p5/2 (sulphone bridge) were enriched after S5. Furthermore, the key structural parameters for huge improvement of specific capacitance were found and it further revealed that mesopores+macropores possessed stronger promotion effect than micropores and S 2p3/2 was more beneficial than S 2p5/2.

Highly Reversible Sodiation/Desodiation from a Carbon-Sandwiched SnS2 Nanosheet Anode for Sodium Ion Batteries.

The further improvement of sodium ion batteries requires the elucidation of the mechanisms pertaining to reversibility, which allows the novel design of the electrode structure. Here, through a hydrogel-embedding method, we are able to confine the growth of few-layer SnS2 nanosheets between a nitrogen- and sulfur-doped carbon nanotube (NS-CNT) and amorphous carbon. The obtained carbon-sandwiched SnS2 nanosheets demonstrate excellent sodium storage properties. In operando small-angle X-ray scattering combined with the ex situ X-ray absorption near edge spectra reveal that the redox reactions between SnS2/NS-CNT and the sodium ion are highly reversible. On the contrary, the nanostructure evolution is found to be irreversible, in which the SnS2 nanosheets collapse, followed by the regeneration of SnS2 nanoparticles. This work provides operando insights into the chemical environment evolution and structure change of SnS2-based anodes, elucidating its reversible reaction mechanism, and illustrates the significance of engineered carbon support in ensuring the electrode structure stability.

Operando Insight into the Correlation between the Structure and Composition of CuZn Nanoparticles and Their Selectivity for the Electrochemical CO2 Reduction.

Bimetallic CuZn catalysts have been recently proposed as alternatives in order to achieve selectivity control during the electrochemical reduction of CO2 (CO2RR). However, fundamental understanding of the underlying reaction mechanism and parameters determining the CO2RR performance is still missing. In this study, we have employed size-controlled (∼5 nm) Cu100–xZnx nanoparticles (NPs) supported on carbon to investigate the correlation between their structure and composition and catalytic performance. By tuning the concentration of Zn, a drastic increase in CH4 selectivity [∼70% Faradaic efficiency (F.E.)] could be achieved for Zn contents from 10 to 50, which was accompanied by a suppression of the H2 production. Samples containing a higher Zn concentration displayed significantly lower CH4 production and an abrupt switch in the selectivity to CO. Lack of metal leaching was observed based on quasi in situ X-ray photoelectron spectroscopy (XPS). Operando X-ray absorption fine structure (XAFS) spectroscopy measurements revealed that the alloying of Cu atoms with Zn atoms takes place under reaction conditions and plays a determining role in the product selectivity. Time-dependent XAFS analysis showed that the local structure and chemical environment around the Cu atoms continuously evolve during CO2RR for several hours. In particular, cationic Zn species initially present were found to get reduced as the reaction proceeded, leading to the formation of a CuZn alloy (brass). The evolution of the Cu–Zn interaction with time during CO2RR was found to be responsible for the change in the selectivity from CH4 over Cu-ZnO NPs to CO over CuZn alloy NPs. This study highlights the importance of having access to in depth information on the interplay between the different atomic species in bimetallic NP electrocatalysts under operando reaction conditions in order to understand and ultimately tune their reactivity.

Operando Insight into the Oxygen Evolution Kinetics on the Metal-Free Carbon-Based Electrocatalyst in an Acidic Solution.

Operando insight into the catalytic kinetics under working conditions is important for further rationalizing the design of advanced catalysts toward efficient renewable energy applications. Here, we enable a ubiquitous carbon material as an efficient acidic oxygen evolution reaction (OER) electrocatalyst, synthesized via a facile and controllable "amino-assisted polymerization and carbonization" strategy. This as-developed metal-free amino-rich hierarchical-network carbon (amino-HNC) framework directly supported on carbon paper can catalyze OER at a quite low overpotential of 281 mV and a small Tafel slope of 96 mV dec-1 in an acid solution, and maintain ∼98% of its initial catalytic activity after 100 h oxygen evolution operation. By using the operando synchrotron infrared spectroscopy, a crucial structurally evolved H2N-(*O-C)-C, formed by adsorbing the *O intermediate on the active H2N-C═C moiety, is observed on amino-HNC electrocatalysts during the OER process in the acid medium. Furthermore, theoretical calculations reveal that the optimization of the sp2 electronic structure of C═C by amino radicals could effectively lower the kinetic formation barrier of the *O intermediate on the H2N-C═C moiety, contributing to a prominent acidic oxygen-involved catalysis.

Operando Insights into Nanoparticle Transformations during Catalysis

Nanostructured materials play an important role in today’s chemical industry, acting as catalysts in heterogeneous thermal and electrocatalytic processes for chemical energy conversion and the prod...

Piece by Piece-Electrochemical Synthesis of Individual Nanoparticles and their Performance in ORR Electrocatalysis.

The impact of individual HAuCl4 nanoreactors is measured electrochemically, which provides operando insights and precise control over the modification of electrodes with functional nanoparticles of well-defined size. Uniformly sized micelles are loaded with a dissolved metal salt. These solution-phase precursor entities are then reduced electrochemically-one by one-to form nanoparticles (NPs). The charge transferred during the reduction of each micelle is measured individually and allows operando sizing of each of the formed nanoparticles. Thus, particles of known number and sizes can be deposited homogenously even on nonplanar electrodes. This is demonstrated for the decoration of cylindrical carbon fibre electrodes with 25±7 nm sized Au particles from HAuCl4 -filled micelles. These Au NP-decorated electrodes show great catalyst performance for ORR (oxygen reduction reaction) already at low catalyst loadings. Hence, collisions of individual precursor-filled nanocontainers are presented as a new route to nanoparticle-modified electrodes with high catalyst utilization.

Speciation and Electronic Structure of La1−xSrxCoO3−δ During Oxygen Electrolysis

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.

Operando Insights into CO Oxidation on Cobalt Oxide Catalysts by NAP-XPS, FTIR, and XRD.

Cobalt oxide Co3O4 has recently emerged as promising, noble metal-free catalyst for oxidation reactions but a better understanding of the active catalyst under working conditions is required for further development and potential commercialization. An operando approach has been applied, combining near ambient (atmospheric) pressure X-ray photoelectron spectroscopy (NAP-XPS), Fourier transform infrared spectroscopy (FTIR), or X-ray diffraction (XRD) with simultaneous catalytic tests of CO oxidation on Co3O4, enabling one to monitor surface and bulk states under various reaction conditions (steady-state and dynamic conditions switching between CO and O2). On the basis of the surface-specific chemical information a complex network of different reaction pathways unfolded: Mars-van-Krevelen (MvK), CO dissociation followed by carbon oxidation, and formation of carbonates. A possible Langmuir–Hinshelwood (LH) pathway cannot be excluded because of the good activity when no oxygen vacancies were detected. The combined NAP-XPS/FTIR results are in line with a MvK mechanism above 100 °C, involving the Co3+/Co2+ redox couple and oxygen vacancy formation. Under steady state, the Co3O4 surface appeared oxidized and the amount of reduced Co2+ species at/near the surface remained low up to 200 °C. Only in pure CO, about 15% of surface reduction were detected, suggesting that the active sites are a minority species. The operando spectroscopic studies also revealed additional reaction pathways: CO dissociation followed by carbon reoxidation and carbonate formation and its decomposition. However, due to their thermal stability in various atmospheres, the carbonates are rather spectators and also CO dissociation seems a minor route. This study thus highlights the benefits of combining operando surface sensitive techniques to gain insight into catalytically active surfaces.

Speciation and Electronic Structure of La1 -XSrxCoO3 -Δ during Oxygen Evolution

Transition metal oxides present a promising alternative to noble metals for oxygen electrocatalysis, including cobalt-containing perovskite oxides for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. Investigation of epitaxial oxide thin films allows quantification of the intrinsic activity, as well as examination of their chemical speciation in an aqueous environment using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We couple electrochemical studies of epitaxial La1-xSrxCoO3 -δ films with in situ and operando AP-XPS to investigate the surface chemistry and electronic structure. By quantifying the formation of hydroxyl (OH) groups in situ, we compare the relative affinity of different surfaces for this key reaction intermediate in oxygen electrocatalysis. Operando measurements of La0.8Sr0.2CoO3 -δ in alkaline electrolyte indicate that the surface is stable during OER and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, OH and carbonate species persist on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate-adsorbate interactions. The accumulation of OH species with oxidative potentials suggests that, for covalent oxides with facile charge transfer kinetics, the rate of reaction could be limited by proton transfer kinetics. This operando insight highlights the potential importance of carbonates in oxygen electrocatalysis and will help guide modeling of self-consistent oxide electrocatalysts.

(Keynote) In Situ and Operando Insight into CO2 electroreduction over Size- and Shape-Controlled Nanoparticles and Nanostructured Films

Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in electro-catalysis research. In order to achieve this elusive goal, we must first obtain a fundamental understanding of the structural and chemical properties of these complex systems. In addition, the dynamic nature of the nanostructured films and nanoparticle (NP) catalysts and their response to the electrochemical environment must be taken into consideration, since their working state might not be the state in which the catalyst was prepared, but rather a structural and/or chemical isomer that adapted to the particular reaction conditions. To address the complexity of real-world electrocatalysts, a synergistic approach taking advantage of a variety of cutting-edge experimental methods (EC-AFM, AP-XPS, XAFS) has been undertaken here. This talk will focus on model electrocatalysts (Ag, Cu, Cu-M with M = Zn, Ga, Sn, Co, Ni, Au, Ag) for the electrocatalytic reduction of CO2. We will show how rationally design electrocatalytically active model NPs with narrow size, shape, interparticle distance distributions and tunable oxidation state. Furthermore, we will provide insight into structure/chemical state-reactivity correlations during the electrochemical reduction of CO2 through an in situ and operando approach under reaction conditions. The results are expected to open up new routes for the reutilization of CO2 through its direct selective conversion into valuable chemicals and fuels such as carbon monoxide, methane, ethylene, ethanol, and propanol.

CO2 sensing with chemoresistive Nd2O2CO3 sensors - Operando insights

In this work the sensing of Nd2O2CO3-based chemoresistive CO2 sensor was investigated combining DRIFT and DC-resistance measurements. Besides the already reported effect of CO2 exposure, we found that exposure to CO determines comparable effects and that the presence of oxygen in the background is not important. The humidity significantly influences the response for both gases. The spectroscopic results reveal that the exposure to both CO and CO2 consumes the water-related surface species – the rooted-hydroxyls being the electrically active one – and increases the concentration of carbonates; the decrease of the rooted-hydroxyls is probably the cause of the resistance increase.