Zhongxin Chen
Email Address
zxchen@nus.edu.sg
Organizational Units
CHEMISTRY
dept
SCIENCE
faculty
SPECIALTY RESEARCH INST/CTRS
faculty
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Publication Promoted Glycerol Oxidation Reaction in an Interface-Confined Hierarchically Structured Catalyst(WILEY-V C H VERLAG GMBH, 2019-01-01) Chen, Zhongxin; Liu, Cuibo; Zhao, Xiaoxu; Yan, Huan; Li, Jing; Lyu, Pin; Du, Yonghua; Xi, Shibo; Chi, Kai; Chi, Xiao; Xu, Haisen; Li, Xing; Fu, Wei; Leng, Kai; Pennycook, Stephen J; Wang, Shuai; Loh, Kian Ping; Dr Priya Yadav; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; CHEMICAL & BIOMOLECULAR ENGINEERING© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Confined catalysis in a 2D system is of particular interest owing to the facet control of the catalysts and the anisotropic kinetics of reactants, which suppress side reactions and improve selectivity. Here, a 2D-confined system consisting of intercalated Pt nanosheets within few-layered graphene is demonstrated. The strong metal–substrate interaction between the Pt nanosheets and the graphene leads to the quasi-2D growth of Pt with a unique (100)/(111)/(100) faceted structure, thus providing excellent catalytic activity and selectivity toward one-carbon (C1) products for the glycerol oxidation reaction. A hierarchically porous graphene architecture, grown on carbon cloth, is used to fabricate the confined catalyst bed in order to enhance the mass-diffusion limitation in interface-confined reactions. Owing to its unique 3D porous structure, this graphene-confined Pt catalyst exhibits an extraordinary mass activity of 2910 mA mgPt−1 together with a formate selectivity of 79% at 60 °C. This paves the way toward rational designs of heterogeneous catalysts for energy-related applications.Publication A non-dispersion strategy for large-scale production of ultra-high concentration graphene slurries in water(NATURE PUBLISHING GROUP, 2018) Dong, Lei; Chen, Zhongxin; Zhao, Xiaoxu; Ma, Jianhua; Lin, Shan; Li, Mengxiong; Bao, Yang; Chu, Leiqiang; Leng, Kai; Lu, Hongbin; Loh, Kian Ping; Dr Priya Yadav; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERING; CHEMICAL & BIOMOLECULAR ENGINEERINGIt is difficult to achieve high efficiency production of hydrophobic graphene by liquid phase exfoliation due to its poor dispersibility and the tendency of graphene sheets to undergo π-π stacking. Here, we report a water-phase, non-dispersion exfoliation method to produce highly crystalline graphene flakes, which can be stored in the form of a concentrated slurry (50 mg mL-1) or filter cake for months without the risk of re-stacking. The as-exfoliated graphene slurry can be directly used for 3D printing, as well as fabricating conductive graphene aerogels and graphene-polymer composites, thus avoiding the use of copious quantities of organic solvents and lowering the manufacturing cost. This non-dispersion strategy paves the way for the cost-effective and environmentally friendly production of graphene-based materials.Publication Engineering the Electronic Structure of MoS2 Nanorods by N and Mn Dopants for Ultra-Efficient Hydrogen Production(AMER CHEMICAL SOC, 2018-08-01) Tao Sun; Jun Wang; Xiao Chi; Yunxiang Lin; Zhongxin Chen; Xiang Ling; Chuntian Qiu; Yangsen Xu; Li Song |Wei Chen; Chenliang Su; Prof Chen Wei; CENTRE FOR ADVANCED 2D MATERIALS; CHEMISTRY© 2018 American Chemical Society. Developing economical and efficient electrocatalysts with nonprecious metals for the hydrogen evolution reaction (HER), especially in water-alkaline electrolyzers, is pivotal for large-scale hydrogen production. Recently, both density functional theory (DFT) calculations and experimental studies have demonstrated that earth-abundant MoS2 is a promising HER electrocatalyst in acidic solution. However, the HER kinetics of MoS2 in alkaline solution still suffer from a high overpotential (90-220 mV at a current density of 10 mA cm-2). Herein, we report a combined experimental and first-principle approach toward achieving an economical and ultraefficient MoS2-based electrocatalyst for the HER by fine-tuning the electronic structure of MoS2 nanorods with N and Mn dopants. The developed N,Mn codoped MoS2 catalyst exhibits an outstanding HER performance with overpotentials of 66 and 70 mV at 10 mA cm-2 in alkaline and phosphate-buffered saline media, respectively, and corresponding Tafel slopes of 50 and 65 mV dec-1. Moreover, the catalyst also exhibits long-term stability in HER tests. DFT calculations suggest that (1) the electrocatalytic performance can be attributed to the enhanced conductivity and optimized electronic structures for facilitating H∗ adsorption and desorption after N and Mn codoping and (2) N and Mn dopants can greatly activate the catalytic HER activity of the S-edge for MoS2. The discovery of a simple approach toward the synthesis of highly active and low-cost MoS2-based electrocatalysts in both alkaline and neutral electrolytes allows the premise of scalable production of hydrogen fuels.Publication Covalent-Organic-Framework-Based Li–CO2 Batteries(Wiley, 2019-10-14) LI XING; Hui Wang; CHEN ZHONGXIN; Hai-Sen Xu; Wei Yu; LIU CUIBO; Xiaowei Wang; Kun Zhang; XIE KEYU; LOH KIAN PING; CHEMISTRYCovalent organic frameworks (COFs) are an emerging class of porous crystalline materials constructed from designer molecular building blocks that are linked and extended periodically via covalent bonds. Their high stability, open channels, and ease of functionalization suggest that they can function as a useful cathode material in reversible lithium batteries. Here, a COF constructed from hydrazone/hydrazide-containing molecular units, which shows good CO2 sequestration properties, is reported. The COF is hybridized to Ru-nanoparticle-coated carbon nanotubes, and the composite is found to function as highly efficient cathode in a Li–CO2 battery. The robust 1D channels in the COF serve as CO2 – and lithium-ion-diffusion channels and improve the kinetics of electrochemical reactions. The COF-based Li–CO2 battery exhibits an ultrahigh capacity of 27 348 mAh g−1 at a current density of 200 mA g−1 , and a low cut-off overpotential of 1.24 V within a limiting capacity of 1000 mAh g−1 . The rate performance of the battery is improved considerably with the use of the COF at the cathode, where the battery shows a slow decay of discharge voltage from a current density of 0.1 to 4 A g−1 . The COF-based battery runs for 200 cycles when discharged/charged at a high current density of 1 A g−1 .Publication From All-Triazine C3N3 Framework to Nitrogen-Doped Carbon Nanotubes: Efficient and Durable Trifunctional Electrocatalysts(American Chemical Society, 2019-12-01) Zeng, Jian; CHEN ZHONGXIN; Zhao, Xiaoxu; Yu, Wei; Wu, Shaofei; LU JIONG; LOH KIAN PING; Wu Jishan; Dr Priya Yadav; CHEMISTRYA new all-triazine C3N3 framework was synthesized via triflic acid-mediated polymerization of 2,4,6-tricyano-1,3,5-triazine in solution, followed by ionothermal treatment in ZnCl2. Complexation of C3N3 with FeCl3, followed by reduction, gave the Fe3O4/C3N3 composite. Subsequent pyrolysis of the composite afforded the iron-species-encapsulated, nitrogen-doped carbon nanotubes, which turned out to be efficient and durable trifunctional electrocatalysts. The optimized catalyst showed a half-wave potential of the oxygen reduction reaction similar to that of platinum/carbon and higher oxygen evolution reaction activity than IrO2. The catalyst also exhibits good hydrogen evolution reaction activity.Publication Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction(Nature Research, 2020) Yao, C.; Guo, N.; Xi, S.; Xu, C.-Q.; Liu, W.; Zhao, X.; Li, J.; Fang, H.; Su, J.; Chen, Z.; Yan, H.; Qiu, Z.; Lyu, P.; Chen, C.; Xu, H.; Peng, X.; Li, X.; Liu, B.; Su, C.; Pennycook, S.J.; Sun, C.-J.; Li, J.; Zhang, C.; Du, Y.; Lu, J.; PHYSICS; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERINGThe ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au4Pt2 clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au4Pt2/G) with exceptional activity for electrochemical nitrogen (N2) reduction. Our mechanistic study reveals that each N2 molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N2 via an enhanced back donation of electrons to the N2 LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant. © 2020, The Author(s).Publication Single Atom Catalysis: From Simple Reactions to the Synthesis of Complex Molecules(Wiley, 2021-09-12) Zhongxin Chen; Jia Liu; Ming Joo Koh; Kian Ping Loh; CHEMISTRYTo date, the scope of single-atom catalysts (SAC) in liquid-phase transformations is rather limited owing to stability issues and the inability to activate complex substances. This calls for a better design of the catalyst support that can provide a dynamic coordination environment needed for catalytic action, and yet retain robustness against leaching or aggregation. In addition, the chemical orthogonality of SAC is useful for designing tandem or multicomponent reactions, in which side reactions common to metal nanoparticles are suppressed. In this review, the intrinsic mechanism will be highlighted that controls reaction efficiency and selectivity in SAC-catalyzed pathways, as well as the structural dynamism of SAC under complex liquid-phase conditions. These mechanistic insights are helpful for the development of next-generation SAC systems for the synthesis of high-value pharmaceuticals through late-stage functionalization, sequential and multicomponent strategies.Publication Enabling High Performance Lithium Metal Battery by Ion-Selective Nanofluidic Transport in a Conjugated Microporous Polymer Protective Layer(2020) Kun Zhang; Wei Liu; Yuliang Gao; Xiaowei Wang; Zhongxin Chen; Ruiqi Ning; Wei Yu; Runlai Li; Li Li; Xing Li; Kai Yuan; Li Ma; Nan Li; Chao Shen; Wei Huang; Keyu Xie; Kian Ping Loh; ELECTRICAL AND COMPUTER ENGINEERING; CHEMISTRY; CHEMICAL & BIOMOLECULAR ENGINEERINGLithium metal is the “holy grail” anode needed for unlocking the full potential of cathodes in next generation batteries. However, the use of pure lithium anode faces daunting challenges in terms of safety, cycle life and rate capability. Here, we have developed a solution-processable conjugated microporous thermosetting polymer (CMP) that can be made into large-scale membrane with nanofluidic channels (5-6 Å). These channels can serve as facile and selective Li ion diffusion pathways on the surfaces of lithium anodes, thereby ensuring stable lithium stripping/plating even at high areal current densities. As a result, CMP-modified lithium anodes (CMP-Li) exhibit cycle stability of 2550 hours at an areal current density of 20 mA cm−2. Remarkably, CMP is readily amenable to solution-processing and spray coating, rendering it highly applicable to continuous roll-to-roll lithium metal treatment processes. Pouch cells using CMP-Li as anode and LiNi0.8Co0.1Mn0.1O2 (NCM811) as cathode exhibits a stable energy density of 400 Wh kg−1.Publication Iron Single Atom Catalysed Quinoline Synthesis(Wiley, 2021-07-19) Zhongxin Chen; Jingting Song; Xinwen Peng; Shibo Xi; Jia Liu; Wenhui Zhou; Runlai Li; Rile Ge; Cuibo Liu; Haisen Xu; Xiaoxu Zhao; Haohan Li; Xin Zhou; Lu Wang; Xing Li; Linxin Zhong; Alexandre I. Rykov; Junhu Wang; Ming Joo Koh; Kian Ping Loh; CHEMISTRY; MATERIALS SCIENCE AND ENGINEERINGThe production of high-value chemicals by single-atom catalysis is an attractive proposition for industry owing to its remarkable selectivity. Successful demonstrations to date are mostly based on gas-phase reactions, and reports on liquid-phase catalysis are relatively sparse owing to the insufficient activation of reactants by single-atom catalysts (SACs), as well as, their instability in solution. Here, mechanically strong, hierarchically porous carbon plates are developed for the immobilization of SACs to enhance catalytic activity and stability. The carbon-based SACs exhibit excellent activity and selectivity (?68%) for the synthesis of substituted quinolines by a three-component oxidative cyclization, affording a wide assortment of quinolines (23 examples) from anilines and acetophenones feedstock in an efficient, atom-economical manner. Particularly, a Cavosonstat derivative can be synthesized through a one-step, Fe1-catalyzed cyclization instead of traditional Suzuki coupling. The strategy is also applicable to the deuteration of quinolines at the fourth position, which is challenging by conventional methods. The synthetic utility of the carbon-based SAC, together with its reusability and scalability, renders it promising for industrial scale catalysis.Publication Salicylideneanilines-Based Covalent Organic Frameworks as Chemoselective Molecular Sieves(AMER CHEMICAL SOC, 2017-07-05) Ning, Guo-Hong; Chen, Zixuan; Gao, Qiang; Tang, Wei; Chen, Zhongxin; Liu, Cuibo; Tian, Bingbing; Li, Xing; Loh, Kian Ping; Dr Priya Yadav; CHEMISTRY; CHEMICAL & BIOMOLECULAR ENGINEERING© 2017 American Chemical Society. Porous materials such as covalent organic frameworks (COFs) are good candidates for molecular sieves due to the chemical diversity of their building blocks, which allows fine-tuning of their chemical and physical properties by design. Tailored synthesis of inherently functional building blocks can generate framework materials with chemoresponsivity, leading to controllable functionalities such as switchable sorption and separation. Herein, we demonstrate a chemoselective, salicylideneanilines-based COF (SA-COF), which undergoes solvent-triggered tautomeric switching. This is unique compared to solid-state salicylideneanilines' counterpart, which typically requires high energy input such as photo or thermal activation to trigger the enol-keto tautomerisim and cis-trans isomerization. Accompanying the tautomerization, the ionic properties of the COF can be tuned reversibly, thus forming the basis of size-exclusion, selective ionic binding or chemoseparation in SA-COF demonstrated in this work.