Showing papers by "Miaofang Chi published in 2020"
TL;DR: In this article, an electrocatalyst with confined reaction volume by coating Cu catalysts with nitrogen-doped carbon layers was developed, which achieved an ethanol Faradaic efficiency of (52 ± 1)% and an ethanol cathodic energy efficiency of 31%.
Abstract: The carbon dioxide electroreduction reaction (CO2RR) provides ways to produce ethanol but its Faradaic efficiency could be further improved, especially in CO2RR studies reported at a total current density exceeding 10 mA cm−2. Here we report a class of catalysts that achieve an ethanol Faradaic efficiency of (52 ± 1)% and an ethanol cathodic energy efficiency of 31%. We exploit the fact that suppression of the deoxygenation of the intermediate HOCCH* to ethylene promotes ethanol production, and hence that confinement using capping layers having strong electron-donating ability on active catalysts promotes C–C coupling and increases the reaction energy of HOCCH* deoxygenation. Thus, we have developed an electrocatalyst with confined reaction volume by coating Cu catalysts with nitrogen-doped carbon. Spectroscopy suggests that the strong electron-donating ability and confinement of the nitrogen-doped carbon layers leads to the observed pronounced selectivity towards ethanol. The electroreduction of CO2 to ethanol could enable the clean production of fuels using renewable power. This study shows how confinement effects from nitrogen-doped carbon layers on copper catalysts enable selective ethanol production from CO2 with a Faradaic efficiency of up to 52%.
286 citations
TL;DR: High-nickel layered oxides, such as LiNi0.8Co0.1Mn 0.1O2 (NCM-811), offer higher energy density than their low-Nickel counterparts at a given voltage and are gaining major traction in automotive lit...
Abstract: High-nickel layered oxides, such as LiNi0.8Co0.1Mn0.1O2 (NCM-811), offer higher energy density than their low-nickel counterparts at a given voltage and are gaining major traction in automotive lit...
117 citations
86 citations
TL;DR: Using an eigenforce model to predict and experiments to verify the strain-induced catalysis enhancement of the oxygen reduction reaction (ORR) in the presence of L10-CoMPt NPs to tune the performance of nanoparticle catalysts by controlling the NP surface strain.
Abstract: Tuning the performance of nanoparticle (NP) catalysts by controlling the NP surface strain has evolved as an important strategy to optimize NP catalysis in many energy conversion reactions. Here, we present our new study on using an eigenforce model to predict and experiments to verify the strain-induced catalysis enhancement of the oxygen reduction reaction (ORR) in the presence of L10-CoMPt NPs (M = Mn, Fe, Ni, Cu, Ni). The eigenforce model allowed us to predict anisotropic (that is, two-dimensional) strain levels on distorted Pt(111) surfaces. Experimentally, by preparing a series of 5 nm L10-CoMPt NPs, we could push the ORR catalytic activity of these NPs toward the optimum region of the theoretical two-dimensional volcano plot predicted for L10-CoMPt. The best ORR catalyst in the alloy NP series we studied is L10-CoNiPt, which has a mass activity of 3.1 A/mgPt and a specific activity of 9.3 mA/cm2 at room temperature with only 15.9% loss of mass activity after 30 000 cycles at 60 °C in 0.1 M HClO4.
58 citations
TL;DR: In this paper, a transition metal oxide catalyst with high temperature stability for CO oxidation was obtained through a synergistic interfacial interaction at the interface of a heterostructure between high-entropy oxides and CuCeOx (catalytic site).
Abstract: Designing high-performance catalysts that can stabilize catalytic active sites against sintering to deactivation at temperature higher than 900 °C is significant but challenging. Here we report a new strategy to obtain a transition metal oxide catalyst with high temperature stability for CO oxidation. This is achieved through a synergistic interfacial interaction at the interface of a heterostructure between high–entropy oxides (HEO, high temperature stability) and CuCeOx (catalytic site). The catalytic site (CuCeOx) for CO oxidation is realized by dissolving an amount of Cu species in HEO into CeO2 via an entropy–driven mechanochemical process. In situ XRD and HAADF–STEM have confirmed the high temperature stability of the heterostructure CuCeOx–HEO, which can remain its CO oxidation catalytic activity at elevated temperatures. It should be expected that this innovative will offer the potential to the synthesis of catalysts with high temperature stability in industry.
57 citations
41 citations
01 May 2020
35 citations
TL;DR: In this article, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb$_2$Te$_4$ and Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic.
Abstract: The van der Waals compound, MnBi$_2$Te$_4$, is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of same types of magnetic defects, the isostructural compound MnSb$_2$Te$_4$ is a better model system for a systematic investigation of the connections among magnetic, topology and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb$_2$Te$_4$. Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site-mixing of MnSb$_2$Te$_4$ crystals depend on the growth parameters, which can lead to $\approx$40\% of Mn sites occupied by Sb and $\approx$15\% of Sb sites by Mn in as-grown crystals. Single crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site-mixing favors a FM interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Our results suggest a long range magnetic order of Mn ions sitting on Bi sites in MnBi$_2$Te$_4$. The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of MnTe(Bi$_2$Te$_3$)$_n$, its Sb analogs and their solid solution.
31 citations
TL;DR: Inorganic Na-ion superionic conductors play a vital role in all-solid-state Na batteries that operate at room temperature as discussed by the authors, and they can be found in all Na batteries.
Abstract: Inorganic Na-ion superionic conductors play a vital role in all-solid-state Na batteries that operate at room temperature. Sodium thioantimonate (Na3SbS4), a popular sulfide-based solid electrolyte...
24 citations
TL;DR: In this paper, the first successful development of single gold (Au) atom catalysts supported on high-surface-area hierarchical perovskite oxides was reported, and the resulting Au single-atoms are extremely stable at calcination temperatures up to 700°C in air and under reaction conditions.
Abstract: Single-atom catalysts (SACs) have shown great potential for achieving superior catalytic activity due to maximizing metal efficiency. The key obstacle in developing SACs lies in the availability of supports that can stabilize SACs. Here we report the first successful development of single gold (Au) atom catalysts supported on high-surface-area hierarchical perovskite oxides. The resulting Au single-atoms are extremely stable at calcination temperatures up to 700 °C in air and under reaction conditions. A high catalytic activity for CO oxidation and distinct self-activating property were also achieved. Furthermore, evidenced by theoretical calculations and experimental studies including X-ray absorption fine structures and in situ Fourier-transform infrared spectra, the surface Au active sites are confirmed to be predominately positively charged. This work provides a generalizable approach to fabricating highly stable Au single-atom catalysts with tunable catalytic performance, and we anticipate that this discovery will facilitate new possibilities for the development of single atom catalysts.
24 citations
TL;DR: In this paper, the authors outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques, and provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.
Abstract: Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.
TL;DR: The systematic analysis suggested that the enhancement in activity can be attributed to the combination of small, uniform size and well-defined {111} facets, which holds promise for applications in proton-exchange membrane fuel cells.
Abstract: We report a facile and scalable synthesis of Pt–Co truncated octahedral nanocrystals (TONs) by employing Pt(acac)2 and Co(acac)2 as precursors, together with CO molecules and Mn atoms derived from the decomposition of Mn2(CO)10 as a reductant and a {111} facet-directing agent, respectively. Both the composition and yield of the Pt–Co TONs could be varied through the introduction of CHCl3. When tested at 80 °C using membrane electrode assembly (MEA), the 4 nm Pt2.6Co TONs gave a mass activity of 294 A gPt−1 at beginning-of-life (BOL) and it increased to 384 A gPt−1 during recovery cycles. The mass activity at BOL only dropped by 24% after 30 000 voltage cycles at end-of-life (EOL) in a metal dissolution accelerated stress test. The Pt2.6Co/C catalyst outperformed the commercial TKK Pt3Co/C (230 A gPt−1 at BOL and 40% loss after 30 000 cycles at EOL) in terms of both activity and durability. Our systematic analysis suggested that the enhancement in activity can be attributed to the combination of small, uniform size and well-defined {111} facets. This new class of catalysts holds promise for applications in proton-exchange membrane fuel cells.
TL;DR: In this article, the authors studied a model reaction, isopropanol conversion, on three differently shape-controlled nanocrystals: cube, truncated cube and dodecahedra, and concluded that the crystal facets impose an indirect effect on the catalysis of STO via controlling the degrees of surface reconstruction.
TL;DR: In this article, the partial or complete blockage of active sites of metal nanoparticles (NPs) on supported-metal catalysts has been of interest for tuning the stability, selectivity, and rate of reactions.
Abstract: The partial or complete blockage of active sites of metal nanoparticles (NPs) on supported-metal catalysts has been of interest for tuning the stability, selectivity, and rate of reactions. Here, w...
TL;DR: In this paper, a combination of advanced electron microscopy and X-ray diffraction is applied to show that the binary Mo/Co carbide catalyst forms uniformly mixed amorphous nanoparticles.
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TL;DR: In this article, the impact of antisite defects on the magnetism and electronic structure of MnSb$_2$Te$_4, an isostructural compound of the van der Waals compound, was studied.
Abstract: The van der Waals compound, MnBi$_2$Te$_4$, is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as axion insulator state and quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. Here, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb$_2$Te$_4$, an isostructural compound of MnBi$_2$Te$_4$. Mn/Sb site mixing leads to a complex magnetic structure and tunes the interlayer magnetic coupling between antiferromagnetic (AFM) and ferromagnetic (FM). The detailed nonstoichiometry and site-mixing of MnSb$_2$Te$_4$ crystals depend on the growth parameters, which can lead to $\sim$40\% of Mn sites occupied by Sb and $\sim$15\% of Sb sites by Mn in as-grown crystals. Single crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn/Sb site-mixing favors a FM interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion for a nontrivial topology. Our results highlight the importance of investigating the presence, distribution, and effects of magnetic impurities on the exotic physical properties of intrinsic magnetic topological insulators.
TL;DR: In this paper, a facile synthesis of Ag@Pd core−shell icosahedral nanocrystals for the development of cost-effective electrocatalysts toward formic acid oxidation was reported.
Abstract: We report a facile synthesis of Ag@Pd core−shell icosahedral nanocrystals for the development of cost‐effective electrocatalysts toward formic acid oxidation. With 12.4 nm Ag icosahedra serving as seeds, Pd shells of controlled thicknesses in the range of 3.6–5.8 atomic layers are grown by adjusting the experimental parameters. When examined as catalysts toward formic acid oxidation, all the Ag@Pd nanocrystals exhibit enhanced mass activities relative to a commercial Pd/C catalyst, with the Ag@Pd4.2L nanocrystals showing enhanced mass activity that is almost twice that of a commercial Pd/C. The chronoamperometry measurements indicate that all the Ag@Pd/C catalysts are more robust than the Pd/C, with the catalyst based on Ag@Pd4.2L nanocrystals showing a mass activity greater than that of the pristine Pd/C after holding in a mixture of HCOOH and HClO4 at 0.75 V for 1,000 s. We believe that the strategy demonstrated here can also be extended to the development of other types of advanced electrocatalysts.
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TL;DR: In this paper, the relative positions of atoms in separate layers of few-layer materials were measured with nm-scale resolution, enabling local measurement of pm-scale in-plane lattice distortions and providing information about the stacking sequence in certain cases.
Abstract: Van der Waals materials composed of stacks of individual atomic layers have attracted considerable attention due to their exotic electronic properties that can be altered by, for example, manipulating the twist angle of bilayer materials or the stacking sequence of trilayer materials. To fully understand and control the unique properties of these few-layer materials, a technique that can provide information about both local structural deformations and the stacking sequence of these materials is needed. Here, we demonstrate an interferometric four-dimensional scanning transmission electron microscopy technique that allows the relative positions of atoms in separate layers of few-layer materials to be measured with nm-scale resolution. Not only does this enable local measurement of pm-scale in-plane lattice distortions, but it also provides information about the stacking sequence in certain cases, providing a means to better understand the interplay between the electronic properties and precise structural arrangement of few-layer 2D materials.
17 Jul 2020
TL;DR: Li phosphorus oxynitride (LiPON) is one of the very few solid electrolytes that have demonstrated stability against Li metal, and performed extended cycling with high coulombic efficiency as mentioned in this paper.
Abstract:
Li phosphorus oxynitride (LiPON) is one of a very few solid electrolytes that have demonstrated stability against Li metal, and performed extended cycling with high coulombic efficiency. However, theoretical calculations show that LiPON does react with Li metal. Here, we utilize in situ electron microscopy to directly observe the dynamic evolutions at the LiPON-Li interface upon contacting and under biasing at the nanometer scale. We reveal that a thin interface layer (~60nm) develops at the LiPON-Li interface upon contact. This interface layer is conductive and robust, serving as an effective passivation layer keeping the interface stable over time and under biasing up to 5 V. Our results explicate the excellent cyclability of LiPON and reconciles the existing debates regarding the stability of LiPON-Li interface. This work demonstrates that glassy solid-state electrolytes with a sufficient ionic conductivity, though may not have a perfect initial electrochemical window with Li metal, may excel in future applications for ASSBs.
TL;DR: In this article, a two-dimensional (2D) materials have attracted a large amount of attention in both basic and applied fields, and scanning transmission electron microscopy (STEM) is often uniquely well-suited for characterizing the atomic-scale structure of these materials.
Abstract: Two-dimensional (2D) materials have attracted a large amount of attention in both basic and applied fields, and scanning transmission electron microscopy (STEM) is often uniquely well-suited for characterizing the atomic-scale structure of these materials [1-4]. As a result, STEM is poised to significantly impact progress on platinum group metal (PGM)-free catalysts, which are currently under intense development to enable low-cost, commercially viable hydrogen fuel cells [5]. While recent advancements have resulted in fuel cell performance comparable to Pt catalysts by some measures [6], cell durability remains a significant challenge, limiting practical applications [7]. Catalytically active sites in PGM-free materials
TL;DR: In this article, an interferometric four-dimensional scanning transmission electron microscopy technique was proposed to extract precise structural information from few-layer materials with nm-scale resolution, which enables measurement of local pm-scale in-plane lattice distortions as well as twist direction and average interlayer spacings in bilayer and trilayer graphene.
Abstract: Van der Waals materials composed of stacks of individual atomic layers have attracted considerable attention due to their exotic electronic properties that can be altered by, for example, manipulating the twist angle of bilayer materials or the stacking sequence of trilayer materials. To fully understand and control the unique properties of these few-layer materials, a technique that can provide information about their local in-plane structural deformations, twist direction, and out-of-plane structure is needed. In principle, interference in overlap regions of Bragg disks originating from separate layers of a material encodes three-dimensional information about the relative positions of atoms in the corresponding layers. Here, we describe an interferometric four-dimensional scanning transmission electron microscopy technique that utilizes this phenomenon to extract precise structural information from few-layer materials with nm-scale resolution. We demonstrate how this technique enables measurement of local pm-scale in-plane lattice distortions as well as twist direction and average interlayer spacings in bilayer and trilayer graphene, and therefore provides a means to better understand the interplay between electronic properties and precise structural arrangements of few-layer 2D materials.