The electrochemical CO2 reduction reaction (CO2RR) can convert widely available CO2 into value-added C2 products, such as ethylene and ethanol. However, low selectivity toward either compound limits the effectiveness of current CO2RR electrocatalysts. Here, we report the use of pulsed overpotentials to improve the ethylene selectivity to 67% with >75% overall C2 selectivity on (100)-textured polycrystalline Cu foil. The pulsed CO2RR can be made selective to either ethylene or ethanol by controlling the reaction temperature. We attribute the enhanced C2 selectivity to the improved CO dimerization kinetics on the active Cu surface on predominately (100)-textured Cu grains with the reduced hydrogen adsorption coverage during the pulsed CO2RR. The ethylene vs ethanol selectivity can be explained by the reducibility of the Cu(I) species during the cathodic potential cycle. Our work demonstrates a simple route to improve the ethylene vs ethanol selectivity and identifies Cu(I) as the species responsible for ethanol production.
In this work, we found that the side reactions of both the Li anode and cathode with the electrolyte can be obviously alleviated at low temperature. This favorable merit enables long cycle life of the Li-O2 cells at low temperature. At 0 °C, the cells can sustain stable cycling of 279 and 1025 cycles at 400 mA g-1 with limited capacities of 1000 and 500 mA h g-1, respectively. Even at -20 °C, the cell can be stably cycled for 83 cycles at 200 mA g-1 with a limited capacity of 500 mA h g-1.
Journal Article 4D-STEM Mapping of Nanoscale Structural Ordering in Cathode Materials Get access Wenxiang Chen, Wenxiang Chen Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Search for other works by this author on: Oxford Academic Google Scholar Xun Zhan, Xun Zhan Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Search for other works by this author on: Oxford Academic Google Scholar Reliant Yuan, Reliant Yuan Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Search for other works by this author on: Oxford Academic Google Scholar Saran Pidaparthy, Saran Pidaparthy Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Search for other works by this author on: Oxford Academic Google Scholar Zhichu Tang, Zhichu Tang Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Search for other works by this author on: Oxford Academic Google Scholar Jian-Min Zuo, Jian-Min Zuo Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Corresponding author: jianzuo@illinois.edu; qchen20@illinois.edu Search for other works by this author on: Oxford Academic Google Scholar Qian Chen Qian Chen Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Corresponding author: jianzuo@illinois.edu; qchen20@illinois.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2608–2609, https://doi.org/10.1017/S1431927622009916 Published: 01 August 2022
MgH2 contains a high content of 7.6 wt % H2. However, its poor kinetics and high thermodynamic stability exhibit unacceptably low energy efficiency. Catalyst doping is deemed as one of the most effective strategies to improve its kinetic performance. In this work, porous rod-like TMTiO3 (TM = Ni and Co) samples are designed and introduced into an MgH2 system for the first time. TMTiO3 exhibits a high catalytic effect on the hydrogen desorption performance of MgH2. In particular, MgH2–6% NiTiO3 possesses excellent catalytic efficiency with a relatively low dehydrogenation temperature (235 °C) and fast dehydrogenation rate (∼0.1842 wt %/min at 235 °C), and the sample exhibits wonderful cycling stability with respect to both capacity (6.4 wt %) and kinetics (∼0.64 wt %/min at 300 °C). Mechanistic research shows that the in situ-formed Mg2Ni/Mg2NiH4 phases are regarded as catalytically active species, which work as a "Hydrogen Pump" and make the hydrogen release easier. Meanwhile, the interconversion within multivalent titanium (Ti4+, Ti3+, and Ti2+) that acts as a carrier for electron transformation leads to easy H formation from H–. The synergetic catalytic effects between Mg2Ni/Mg2NiH4 and multivalent titanium result in favorable and lasting catalytic efficiency for the enhanced hydrogen desorption properties of the MgH2 system.
Self-assembly of three-dimensional structures with order across multiple length scales—hierarchical assembly—is of great importance for biomolecules for the functions of life. Creation of similar complex architectures from inorganic building blocks has been pursued toward artificial biomaterials and advanced functional materials. Current research, however, primarily employs only large, nonreactive building blocks such as Au colloids. By contrast, sulfur-bridged transition metal clusters (<2 nm) are able to offer more functionality in catalytic and biochemical reactions. Hierarchical assembly of these systems has not been well researched because of the difficulty in obtaining single-phase clusters and the lack of suitable ligands to direct structure construction. To overcome these challenges, we employ a rigid planar ligand with an aromatic ring and bifunctional bond sites. We demonstrate the synthesis and assembly of 1.2 nm sulfur-bridged copper (SB-Cu) clusters with tertiary hierarchical complexity. The primary structure is clockwise/counterclockwise chiral cap and core molecules. They combine to form clusters, and due to the cap–core interaction (C–H···π), only two enantiomeric isomers are formed (secondary structure). A tertiary hierarchical architecture is achieved through the self-assembly of alternating enantiomers with hydrogen bonds as the intermolecular driving force. The SB-Cu clusters are air stable and have a distribution of oxidation states ranging from Cu(0) to Cu(I), making them interesting for redox and catalytic activities. This study shows that structural complexity at different length scales, mimicking biomolecules, can occur in active-metal clusters and provides a new platform for investigation of those systems and for the design of advanced functional materials.
Here we present our recent understandings and engineering opportunities on the two-faceted nature of the size effect of cathode particles on electrochemically driven phase transformation pathways and reaction mechanisms. We have been using spinel λ-MnO2 particles as a model cathode material and Mg- and Zn-ion insertion as our focus of multivalent ion battery systems. We find that small, nanoscale cathode particles consistently favor a solid-solution-type phase transition and uniform ion distribution upon discharge. This phase transformation pathway facilitates fast charge insertion kinetics and mechanical stability compared to the multiphase transition pathway in large, micron-sized particles. Meanwhile, when it comes to the electrochemical reaction mechanism, the cathode particle size effect diverges for different systems. Whereas nanoscale cathode particles exhibit superior discharge capacity and cycling performance for Mg-ion-insertion systems, they suffer from a severe side reaction of Mn dissolution in aqueous Zn-ion batteries. Micron-sized λ-MnO2 particles instead show enhanced cycling performance for Zn-ion insertion because of decreased side reaction sites per mass and accommodation of an interpenetrating network of amorphous MnOx nanosheets. Regarding the mechanistic understanding of the size effect, we discuss insights provided by high-resolution imaging methods such as scanning transmission electron microscopy and scanning electron diffraction, which are capable of monitoring structural changes in cathode particles upon multivalent ion insertion. Together we highlight the opportunities in both fundamentally understanding the electrochemically driven phase transformation in insertion materials and engineering high-performance electrode materials, not by composition variation but by tailoring of the "size"─and potentially the shape, exposed facets, surface chemistry, and mesoscale assemblies─of the cathode particles. The particle size effects are transferrable and have potential applications in both multivalent and monovalent ion batteries.
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