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Chen, Chubai

Publications and source records attributed to Chen, Chubai.

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Operando probing dynamic migration of copper carbonyl during electrocatalytic CO2 reduction

Single crystals and shape-controlled nanocrystals are well known to exhibit facet-dependent catalytic properties. However, few studies have investigated how those nanocrystals evolve and (de)activate during reactions, calling for the development of nanoscale time-resolved operando methods. In this context, we have designed Cu nanocubes as a model system to elucidate the underlying driving force of dynamic nanocatalyst reconstruction during the CO2 reduction reaction (CO2RR). Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) and synchrotron-based X-ray spectroscopy reveal the size- and potential-dependent complete transformation from (100)-oriented Cu@Cu2O nanocubes to polycrystalline metallic Cu nanograins under CO2RR conditions. In addition, machine learning-assisted operando four-dimensional STEM reveals that large Cu nanograins derived from nanocubes form mainly crystalline domains, while their smaller counterparts are more amorphous due to faster evolution kinetics. In situ Raman spectroscopy and density functional theory calculations suggest that CO drives the ejection of single Cu atoms, resulting in few-nanometre Cu clusters and the surface migration of highly mobile copper carbonyl (Cu–CO) species. Combined, these multimodal operando methods and theoretical approaches pave the way for understanding the complex structural evolution of energy-related nanocatalysts under electrochemical conditions.

Yang, Yao↗
DOE OSTI2024

Dynamic Evolution of Copper Nanowires during CO 2 Reduction Probed by Operando Electrochemical 4D-STEM and X-ray Spectroscopy

Nanowires have emerged as an important family of one-dimensional (1D) nanomaterials owing to their exceptional optical, electrical, and chemical properties. In particular, Cu nanowires (NWs) show promising applications in catalyzing the challenging electrochemical CO 2 reduction reaction (CO 2 RR) to valuable chemical fuels. Despite early reports showing morphological changes of Cu NWs after CO 2 RR processes, their structural evolution and the resulting exact nature of active Cu sites remain largely elusive, which calls for the development of multimodal operando time-resolved nm-scale methods. Here, in this study, we report that well-defined 1D copper nanowires, with a diameter of around 30 nm, have a metallic 5-fold twinned Cu core and around 4 nm Cu 2 O shell. Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) showed that as-synthesized Cu@Cu 2 O NWs experienced electroreduction of surface Cu 2 O to disordered (spongy) metallic Cu shell (Cu@Cu S NWs) under CO 2 RR relevant conditions. Cu@Cu S NWs further underwent a CO-driven Cu migration leading to a complete evolution to polycrystalline metallic Cu nanograins. Operando electrochemical four-dimensional (4D) STEM in liquid, assisted by machine learning, interrogates the complex structures of Cu nanograin boundaries. Correlative operando synchrotron-based high-energy-resolution X-ray absorption spectroscopy unambiguously probes the electroreduction of Cu@Cu 2 O to fully metallic Cu nanograins followed by partial reoxidation of surface Cu during postelectrolysis air exposure. This study shows that Cu nanowires evolve into completely different metallic Cu nanograin structures supporting the operando (operating) active sites for the CO 2 RR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Operando High-Energy-Resolution X-ray Spectroscopy of Evolving Cu Nanoparticle Electrocatalysts for CO 2 Reduction

Advances in electrocatalysis research rely heavily on building a thorough mechanistic understanding of catalyst active sites under realistic operating conditions. Only recently have techniques emerged that enable sensitive spectroscopic data collection to distinguish catalytically relevant surface sites from the underlying bulk material under applied potential in the presence of an electrolyte layer. Here, we demonstrate that operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) is a powerful spectroscopic method which offers critical surface chemistry insights in CO 2 electroreduction with sub-electronvolt energy resolution using hard X-rays. Combined with the high surface area-to-volume ratio of 5 nm copper nanoparticles, operando HERFD-XAS allows us to observe with clear evidence the breaking of chemical bonds between the ligands and the Cu surface as part of the ligand desorption process occurring under electrochemical potentials relevant for the CO 2 reduction reaction (CO 2 RR). In addition, the dynamic evolution of oxidation state and coordination number throughout the operation of the nanocatalyst was continuously tracked. With these results in hand, undercoordinated metallic copper nanograins are proposed to be the real active sites in the CO 2 RR. In conclusion, this work emphasizes the importance of HERFD-XAS compared to routine XAS in catalyst characterization and mechanism exploration, especially in the complicated electrochemical CO 2 RR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Chemical and Structural Evolution of AgCu Catalysts in Electrochemical CO 2 Reduction

Silver–copper (AgCu) bimetallic catalysts hold great potential for electrochemical carbon dioxide reduction reaction (CO 2 RR), which is a promising way to realize the goal of carbon neutrality. Although a wide variety of AgCu catalysts have been developed so far, it is relatively less explored how these AgCu catalysts evolve during CO 2 RR. The absence of insights into their stability makes the dynamic catalytic sites elusive and hampers the design of AgCu catalysts in a rational manner. Here, in this study, we synthesized intermixed and phase-separated AgCu nanoparticles on carbon paper electrodes and investigated their evolution behavior in CO 2 RR. Our time-sequential electron microscopy and elemental mapping studies show that Cu possesses high mobility in AgCu under CO 2 RR conditions, which can leach out from the catalysts by migrating to the bimetallic catalyst surface, detaching from the catalysts, and agglomerating as new particles. Besides, Ag and Cu manifest a trend to phase-separate into Cu-rich and Ag-rich grains, regardless of the starting catalyst structure. The composition of the Cu-rich and Ag-rich grains diverges during the reaction and eventually approaches thermodynamic values, i.e., Ag 0.88 Cu 0.12 and Ag 0.05 Cu 0.95 . The separation between Ag and Cu has been observed in the bulk and on the surface of the catalysts, highlighting the importance of AgCu phase boundaries for CO 2 RR. In addition, an operando high-energy-resolution X-ray absorption spectroscopy study confirms the metallic state of Cu in AgCu as the catalytically active sites during CO 2 RR. Taken together, this work provides a comprehensive understanding of the chemical and structural evolution behavior of AgCu catalysts in CO 2 RR.

Chen, Peng-Cheng↗
DOE OSTI2023

Operando Electrochemical Liquid-Cell Scanning Transmission Electron Microscopy (EC-STEM) Studies of Evolving Cu Nanocatalysts for CO 2 Electroreduction

The design and synthesis of nanocatalysts with well-defined sizes, compositions, and structures have revolutionized our accessibility to tunable catalyst activity and selectivity for a variety of energy-related electrochemical reactions. Nonetheless, establishing structure-(re)activity correlations requires the understanding of the dynamic evolution of pristine nanocatalysts and the identification of their active states under operating conditions. We previously communicated the operando observation of Cu nanocatalysts evolving into active metallic Cu nanograins for CO 2 electroreduction (Yang et al. Nature 2023, 614, 262–269). Here, we expand our discussion to the technical capabilities and further research applications of operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM), which enables quantitative electrochemistry while tracking dynamic structural evolution of sub-10 nm Cu nanocatalysts. The coexistent H 2 bubbles, often disruptive to operando spectroscopy, are an effective approach to create a thin-liquid layer that significantly improves spatial resolution while remaining electrochemically accessible to Cu nanocatalysts. Operando four-dimensional (4D) STEM in liquids provides insights into the complex structure of active polycrystalline metallic Cu nanograins. With continuous technical developments, we anticipate that operando EC-STEM will evolve into a powerful electroanalytical method to advance our understanding of a variety of nanoscale electrocatalysts at solid/liquid interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Octahedral Distortion and Excitonic Behavior of Cs 3 Bi 2 Br 9 Halide Perovskite at Low Temperature

The metal halide ionic octahedron, represented as [MX 6 ] $n$- (M = metal cation, X = halide anion), serves as the basic structural unit in halide perovskites and plays a crucial role in determining their optoelectronic and chemical properties. Thus, it is possible to correlate the responses of metal halide perovskites to various environmental stimuli with the dynamic behaviors of the [MX 6 ] $n$- octahedra. In this study, with the temperature-dependent single-crystal X ray diffraction (SCXRD) measurements on Cs 3 Bi 2 Br 9 2D halide perovskites, we can identify two classes of distortions through the lowering of temperature: intraoctahedral distortion, which is the off-centering of Bi 3+ cation within a [BiBr 6 ] 3– octahedron due to the Bi 3+ 6s 2 lone pair electrons, and interoctahedral distortion, which is the collective misalignments among the [BiBr 6 ] 3– building blocks. Free exciton (FE) and self-trapped exciton (STE) models are used to study the relationship between the distortion of octahedra in Cs 3 Bi 2 Br 9 and the corresponding changes in its optoelectronic properties, which transform from dominating blue emission above 100 K to red emission at 4 K. In conclusion, this work provides new insights into the excitonic behaviors of perovskites and suggests a possibility that we can design and rationalize the optical properties of halide perovskites by regulating the environmental stimuli based on the knowledge of behaviors of the individual [MX 6 ] $n$- building blocks.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Operando studies reveal active Cu nanograins for CO 2 electroreduction

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals. Although Cu enables CO 2 -to-multicarbon product (C 2+ ) conversion, the nature of the active sites under operating conditions remains elusive. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu 2 O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO 2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C–C coupling. Quantitative structure–activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C 2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C 2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. Importantly, the correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Exploration of the bio-analogous asymmetric C–C coupling mechanism in tandem CO 2 electroreduction

C–C coupling is a critical step of CO 2 fixation in constructing the carbon skeleton of value-added multicarbon products. The Wood–Ljungdahl pathway is an efficient natural process through which microbes transform CO 2 into methyl and carbonyl groups and subsequently couple them together. This asymmetric coupling mechanism remains largely unexplored in inorganic CO 2 electroreduction. Here we experimentally validate the asymmetric coupling pathway through isotope-labelled co-reduction experiments on a Cu surface where 13 CH 3 I and 12 CO are co-fed externally as the methyl and the carbonyl source, respectively. Isotope-labelled multicarbon oxygenates were detected, which confirms an electrocatalytic asymmetric coupling on the Cu surface. We further employed tandem Cu–Ag nanoparticle systems in which *CH x and *CO intermediates can be generated to achieve asymmetric C–C coupling for a practical CO 2 electroreduction. We found that the production of multicarbon oxygenates is correlated with the generation rate of two intermediate indicators, CH 4 and CO. By aligning their rates, the oxygenates generation rate can be maximized.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Heterostructured Au–Ir Catalysts for Enhanced Oxygen Evolution Reaction

Electrochemical water splitting operated under acidic conditions provides a clean approach to generate hydrogen fuels. Currently, the sluggish kinetics of oxygen evolution reaction (OER) at the anode is a bottleneck limiting the acidic water splitting. Here, we report that the OER activity of Ir, one of the most commonly used OER catalysts, can be boosted by forming phase boundaries with Au. Mixed Au and Ir catalysts were synthesized on carbon paper electrodes, which underwent structural evolution under the OER environment to form an Au-Ir interface-rich structure. Compared with Ir catalyst, which requires an overpotential of ~393 mV to achieve a current density of 10 mA/cm 2 in 0.1 M HClO 4 electrolyte, the evolved Au-Ir catalyst shows a lower overpotential at ~351 mV. X-ray photoelectronic spectrum (XPS) study, in conjunction with electrochemical analysis on the surface-site-normalized activity, reveals that the improved OER performance is due to the presence of Au/Ir interfaces in the catalysts. In addition to Ir, the strategy of accelerating OER via Au/Ir interfaces has been further applied to IrCo, IrNi, and IrCu alloy catalysts. Here, with the broad applicability of the strategy demonstrated, this study opens a new route to design OER catalysts for efficient acidic water splitting.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Ligand removal of Au 25 nanoclusters by thermal and electrochemical treatments for selective CO 2 electroreduction to CO

Undercoordinated metal nanoclusters have shown great promise for various catalytic applications. However, their activity is often limited by the covalently bonded ligands, which could block the active surface sites. Here, we investigate the ligand removal process for Au 25 nanoclusters using both thermal and electrochemical treatments, as well as its impact on the electroreduction of CO 2 to CO. The Au 25 nanoclusters are synthesized with 2-phenylethanethiol as the capping agent and anchored on sulfur-doped graphene. The thiolate ligands can be readily removed under either thermal annealing at ≥180°C or electrochemical biasing at ≤-0.5 V vs reversible hydrogen electrode, as evidenced by the Cu underpotential deposition surface area measurement, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure spectroscopy. However, these ligand-removing treatments also trigger the structural evolution of Au 25 nanoclusters concomitantly. The thermally and electrochemically treated Au 25 nanoclusters show enhanced activity and selectivity for the electrochemical CO 2 -to-CO conversion than their pristine counterpart, which is attributed to the exposure of undercoordinated Au sites on the surface after ligand removal. This work provides facile strategies to strip away the staple ligands from metal nanoclusters and highlights its importance in promoting the catalytic performances.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Sulfur-doped graphene anchoring of ultrafine Au 25 nanoclusters for electrocatalysis

The biggest challenge of exploring the catalytic properties of under-coordinated nanoclusters is the issue of stability. We demonstrate herein that chemical dopants on sulfur-doped graphene (S-G) can be utilized to stabilize ultrafine (sub-2 nm) Au 25 (PET)18 clusters to enable stable nitrogen reduction reaction (NRR) without significant structural degradation. The Au 25 @S-G exhibits an ammonia yield rate of 27.5μgNH 3 ∙ mg Au -1 ∙ h -1 at -0.5 V with faradic efficiency of 2.3%. More importantly, the anchored clusters preserve ~ 80% NRR activity after four days of continuous operation, a significant improvement over the 15% remaining ammonia production rate for clusters loaded on undoped graphene tested under the same conditions. Isotope labeling experiments confirmed the ammonia was a direct reaction product of N 2 feeding gas instead of other chemical contaminations. Ex-situ X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy of post-reaction catalysts reveal that the sulfur dopant plays a critical role in stabilizing the chemical state and coordination environment of Au atoms in clusters. Further ReaxFF molecular dynamics (RMD) simulation confirmed the strong interaction between Au nanoclusters (NCs) and S-G. Furthermore, this substrate-anchoring process could serve as an effective strategy to study ultrafine nanoclusters’ electrocatalytic behavior while minimizing the destruction of the under-coordinated surface motif under harsh electrochemical reaction conditions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Size Transformation of the Au 22 (SG) 18 Nanocluster and Its Surface-Sensitive Kinetics

For many applications of well-defined gold nanoclusters, it is desirable to understand their structural evolution behavior under working conditions with molecular precision. Here we report the first systematic investigation of the size transformation products of the Au 22 (SG) 18 nanocluster under representative working conditions and highlight the surface effect on the transformation kinetics. Under thermal and aerobic conditions, the consecutive and pH-dependent transformation from Au 22 to both well-defined clusters and small Au(I)SR species was identified by ESI-MS and UV-vis spectroscopy. By introducing a perturbation onto the Au 22 surface, significant changes in the activation parameters were determined from the kinetic study of the Au 22 transformation. This indicates the sensitivity of the nanocluster transformation pathway to the cluster surface. Finally, the systematic study of cluster transformation and the sensitivity of cluster transformation to the surface revealed herein has significant implications for future attempts to design gold nanoparticles with adaptation to the working environment and the regeneration of active nanoparticles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗