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Remediation and Control Technologies
Pore Structure-dependent Mass Transport in Flow-through Electrodes for Water Remediation YUJUN ZHOU, Qinghua Ji, Huijuan Liu, and Jiuhui Qu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01728 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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Pore Structure-dependent Mass Transport in
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Flow-through Electrodes for Water
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Remediation
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Yujun Zhou†, §, Qinghua Ji†, ‡, *, Huijuan Liu†, ‡, Jiuhui Qu†, ‡
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†
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
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‡
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School of Environment, Tsinghua University, Beijing 100084, China
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§
Key Laboratory of Drinking Water Science and Technology, Research Center for
State Key Joint Laboratory of Environment Simulation and Pollution Control,
University of Chinese Academy of Sciences, Beijing 100049, China
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11
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*Corresponding author:
[email protected] 15
School of Environment, Tsinghua University, Beijing 100084, China
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Tel: (8610)6279-0565; Fax: (8610)6292-3558
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ABSTRACT
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Hierarchical three-dimensional architectures of granphene-based materials with
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tailored microstructure and functionality exhibit unique mass transport behaviors and
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tunable active sites for various applications. The micro- /nano-channels in the porous
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structure can act as micro- /nano- reactors, which optimize the transport and
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conversion of contaminants. However, the size-effects of the micro- /nano-channels,
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which are directly related to its performance in electrochemical processes, have not
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been explored. Here, using lamellar-structured graphene films as electrodes, we
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demonstrate that the interlayer spacing (range from ~84 nm to ~2.44 µm) between
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graphene nanosheets governs the mass transport and electron transfer in
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electrochemical processes; subsequently influence the water decontamination
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performances. The microchannel (interlayer spacing = ~2.44 µm) can provide higher
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active surface areas, but slow reaction kinetics. Densely packed graphene nanosheets
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(interlayer spacing = ~280 nm), which possessed better electron conductivity and
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could provide higher surface-area-to-volume ratio in narrow nanochannels (7.14 µm-1),
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achieved the highest reaction kinetics. However, the ion‐accessible surface area was
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decreased in highly dense films (interlayer spacing=~84 nm) due to serious interlayer
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stacking of graphene nanosheets, thereby leading poor reaction kinetics. These results
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demonstrate the size-effect of nanochannels in porous materials and highlight the
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importance of controlling mass transport and electron transfer for optimal
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electrochemical performance, enabling a deep understanding of the benefits and
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utilization of these hierarchical three-dimensional architectures in water purification.
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Keywords: pore structure, flow-through, electrosorption
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TOC Art
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INTRODUCTION
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Electrochemical technology has gained increasing interest in the water treatment
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industry in recent years1, taking account of its high efficiency, low cost, and
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multi-functional and process-controllable characteristics. The electrochemical process
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can be adapted to various environmental conditions and easily be combined with other
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technologies2. However, the classical cell structure, described as immersed electrodes
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in a tank cell, often faces problems with poor mass transfer, since the electrochemical
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reactions mainly occur at the bulk electrode/solution interface and thus do not fully
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utilize the active sites inside the porous electrode material. Meanwhile, electrostatic
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repulsion can hinder the migration of charged species from bulk solution to the
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electrode surface. Accordingly, such poor mass transfer leads to low current
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efficiency and high energy demand, and limits the application of the technology.
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Flow-through electrodes, in which water flows perpendicular to and through the
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porous electrodes, can be employed to address the problem of mass transfer
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limitation3,4. By optimizing the flow field inside the porous electrodes, flow-through
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operation enables maximal utilization of the active sites, therefore significantly
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increasing the mass transfer and promoting electron transfer-5,6. Because they
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demonstrate higher current efficiency, enhanced mass transfer and improved
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volumetric rates for electrochemical reactions, flow-through electrodes show great
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promise for a range of applications in various electrochemical processes7. Liu and
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Chad completed a detailed investigation of aqueous dye oxidation by use of a 4 ACS Paragon Plus Environment
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flow-through cell and found that the reaction kinetics increased up to 6-fold compared
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to conventional electrodes8. Convection-enhanced mass transfer to the electrode
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surface was considered to be the key factor for achieving increased current density
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and organic oxidation during this process. Hilary discussed the removal of chlorine by
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a novel flow-through electrode cell9. The reduced resistance of the reactor allowed the
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electrodes to produce chlorine with improved current efficiency. Liu et al. utilized
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graphite felt electrodes in a flow-through system for ferrocyanide removal, and the
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electrochemical kinetics were up to 15-fold faster than those of the classical batch
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system10.
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Graphene-based carbons have been considered to be excellent electrode
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materials due to their high electricity conductivity, superior mechanical flexibility,
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uniquely high chemical and thermal stability, and especially large specific surface
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areas. Monolithic graphene with a three-dimensional macroporous structure and open
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channels between graphene sheets provides an ultrahigh ion-accessible surface area
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and allows electrolytes to freely diffuse inside and through the material, thus holding
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great promise for utilization as a flow-through electrode11-14. Typically, graphene
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hydrogel, in which flexible graphene sheets partially overlap in 3D space and form an
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interconnected porous structure, has a unique hierarchical architecture for both
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electron transfer and water flow15-19. However, little information is available on
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exploring the pore structure and size effects of nanochannels and their impact on the
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electrochemical performance of the flow-through electrodes, which is essential for the
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design and utilization of high efficiency electrode materials for water remediation.
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Here, lamellar-structured N-functionalized reduced graphene oxide hydrogels
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(LSG) with different pore sizes and structures were employed as electrodes to study
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the size effects of nano- /micro- channels in selective electrosorption of heavy metals
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(Scheme 1). As a typical electrochemical method, electro-sorption is a process
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whereby charged species are transported through the pores and removed by
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electrochemical reactions with specific active sites. This process is mainly governed
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by the mass transport of contaminants in the pores, the active surface area and
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electron transfer in the graphene network. In the study, Pb2+ is selected because it is of
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high toxicity, non-degradability, and bio-accumulativity and also ubiquitous
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speciously in the environment in related to mining, refining, ore processing as well as
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welding and battery manufacturing20, 21. A flow-through cell equipped with LSG film
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was fabricated and employed to study the electro-sorption performance toward Pb2+
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ion. The interlayer spacing in the LSG films was demonstrated to be an important
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factor influencing the kinetics and adsorption capacity for Pb2+. For the mechanistic
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study, we investigated the A.C. impedance, the surface and pore structures, as well as
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the electrochemically active surface area of the LSG films. A computational fluid
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dynamics (CFD) study was performed to investigate the mass transport and the
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utilization of the active surface in the “nano-/micro- reactors”.
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SCHEME 1
MATERIALS AND METHODS
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Chemicals and materials: Potassium permanganate (KMnO4), Sodium sulfate
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(Na2SO4), sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric anhydride
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(P2O5), sodium nitrate (NaNO3), lead nitrate (PbNO3), dicyandiamide (C2H4N4),
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potassium persulfate (K2S2O8), nitric acid (HNO3), and hydrogen peroxide (H2O2)
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were purchased from Sinopharm (China). Synthetic graphite powder ( 0.9 revealed that
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large open-pore structures and channels were maintained in the larger layer-spacing
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films. The amount of nitrogen adsorption increased gradually as the layer spacing
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increased, indicating that more micropores existed when the total surface area rose
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from 122.1 to 364.43 m2 g-1 19, 30. It is conjectured that this higher surface area could
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result in increased numbers of active sites, which is favorable for ion adsorption. The
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pore size distribution analysis was obtained based on NLDFT mode and is shown in
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Figure 4B. The average pore diameter of the 84 nm-layer-spacing electrodes was
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mainly centralized around 18 nm, and mostly less than 20 nm. These pores
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presumably arise from the narrowed nano-channels in the densely packed films. When
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the layer spacing increased, the average pore diameter increased synchronously,
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which may facilitate the mass transfer process. A secondary pore size peak appeared
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for the 2.44 µm -layer-spacing film, and the large pores of around 30 nm are attributed
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to the presence of ample interlayer micro-channels. The rather high BET surface area
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and large pore size strongly support the fact that more interconnected channels result
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in additional active sites in the large-layer-spacing films, which thus may change the
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electrochemical characteristics of the materials29.
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Cyclic voltammetry characterization (CV) of the five films was performed at
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different scan rates in order to evaluate the capacitive performance31 , which is related
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to electro-active area and mass transfer effects (Figure 4c-d). The typical
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rectangle-shaped curve obtained at 10 mV s-1 demonstrated that the films were highly
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capacitive and conductive14, while the large area of the CV profile indicates higher
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electrochemical active and low contact resistance. At low scan rate, the CV curves of
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films with narrowed layer spacing exhibit a smaller rectangular area, and minimal 16 ACS Paragon Plus Environment
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current at all potentials could be seen for the 84 nm-layer-spacing film. This is mainly
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because the surfaces of graphene sheets were overlapped in the compact films and
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active sites could not be utilized28. Limited by the ion diffusion rate when higher scan
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rate was applied, the CV configuration displayed some distortion, as the shape
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changed from rectangular to oval. Similarity, the electrodes with narrower layer
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spacing exhibited much smaller capacitive currents and CV areas. As previously
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reported, the narrowed layer spacing and small pore diameter of the films may hinder
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ion diffusion and delay EDL formation, thus less electrochemically active surface area
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was exposed in materials with compacted structure and narrowed nano-channels
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31-33
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ions can easily penetrate into the LSG layers and fully utilize active sites, which
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corresponds well to the shortened Warburg region on the Nyquist plot. The CV
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characterizations well reflect the layer-spacings’ influence on the electrochemical
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active surface area.
28,
. The broadened micro-channels provide low internal mass transfer resistance, and
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FIGURE 4
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The surface-area-to-volume ratio (SA: V) in a reactor affects the rate of diffusion
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in bulk solution and is related to the efficiency of electrochemical reactions34.
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Microfluidic structures have inherently high SA: V due to the small volume of fluid in
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the channels35,36. In this flow-through cell, the micro- or nano-channels in the LSG
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films could be considered as “reactors”, in which the SA: V ratio can be calculated 17 ACS Paragon Plus Environment
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using SA: V = 2/d, where d is the interlayer spacing of graphene nanosheets in the
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LSG film (Figure 5). Among the five LSG films, the 84 nm-layer-spacing film had the
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highest SA: V ratio (23.81 µm-1), nearly 30 times higher than that of the 2.44
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µm-layer-spacing film (0.82 µm-1), and the SA: V ratio inversely correlates with the
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size of the layer spacing. Despite having the highest SA: V ratio, the 84
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nm-layer-spacing film did not show advantages in either reaction kinetics or
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adsorption capacity towards Pb2+. This finding differs from the traditional
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understanding of how the SA: V ratio affects reaction kinetics, which holds that a
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higher SA: V ratio enhances the utilization of the electrode surface and accelerates the
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electrochemical reaction37. In a nano-sized channel, the high SA: V ratio is partially
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attributed to the development of nanopores that are inaccessible to ions due to the
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small size of the pores. However, the 280 nm-layer-spacing film with appropriate SA:
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V ratio (7.14 µm-1) and active surface area achieved the fastest reaction kinetics. It
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proves that by tuning the nano-/mico- channel with a proper SA: V ratio to optimize
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the electron and ions diffusion is conducive to improve the reaction rates 38.
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FIGURE 5
3.4 CFD Simulation Study
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As the behavior of a fluid is greatly influenced by the geometry of the flow
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channel31 , CFD simulations have been carried out to study the flow pattern and
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velocity field distribution in lamellar micro-/nano-channels39. The hydrodynamic 18 ACS Paragon Plus Environment
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performance in 2.44 µm-layer-spacing and 280 nm-layer-spacing films was simulated
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as they represent the two extremes in terms of Pb2+ ion adsorption kinetics. When the
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layer spacing in films was enlarged, the pore structures showed a slit-shaped
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geometry for the 280 nm-layer-spacing and a honeycomb geometry for the 2.44
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µm-layer-spacing pores. As shown in Figure 6, the velocity magnitude contour in the
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280 nm-layer spacing film demonstrates that the nanofluidic flow is homogeneously
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distributed in the interspace of the film. This distribution of flow field enables optimal
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utilization of the active sites, and thus Pb2+ ions could be driven to the surface of
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graphene sheets easily, which enhances the mobility of reactants and greatly
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facilitates mass transfer34, 40. The flow pattern and velocity field distribution of 84
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nm-layer-spacing is of similarity with that of 280 nm-layer spacing film (Figure S2).
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In contrast, a large dead volume area formed in the 2.44 µm-layer-spacing film (blue
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region). Additionally, higher flow velocity is shown at the junction of layers,
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demonstrating that the uneven flow paths also resulted in short-circuiting. Despite the
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higher surface area of the 2.44 µm-layer-spacing film, the dead volume with rather
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low flow velocity and short-circuiting could cause insufficient retention time, thus
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leading to slow reaction kinetics. The improved hydrodynamic performance in the
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280 nm-layer-spacing film may be attributed to the modification of the layer spacing,
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so that more active sites are utilized.
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FIGURE 6
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In our study, the flow-through cell equipped with LSG films showed an excellent
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performance on Pb2+ ions removal since this operation eliminating the long diffusion
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time required with classical immersed modes, thereby enhances the current efficiency
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and reduces of energy loss. The steady-state operating current density is
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approximately 3.2 A m-2 (Vca = 0.8 V) and the energy demand is calculated to be just
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0.2 kWh g-1. The fast kinetics shown by the 280 nm-layer-spacing film on Pb2+
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adsorption is the result of optimization of electron transfer, ion diffusion and mass
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transfer (Scheme 2). The differences in layer spacing changed the intrinsic
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characteristics of the LSG films. When the layer spacing narrowed, highly compacted
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graphene sheets would stack together and the paths provided for electrons passing
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through were shortened, thereby facilitating the electron transfer, as proved by the
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four-point probe resistance. However, based on the EIS measurements and pore size
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distribution, the narrowed nano-channels retarded ion diffusion and thus hindered the
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Pb2+ ion electro-adsorption rates. Notably, available active sites are vital for ion
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sorption. As shown by surface analysis and electrochemical CV measurements, the
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surface area was decreased in films with narrowed layer spacing. Compared to the
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nano-channels in densely packed films, micro-channels exhibit low internal mass
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transfer resistance and more exposed surface area, which is favorable for mass
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transfer. However, from the CFD simulation, it was found that in films with
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broadened layer spacing, fluids were distributed unevenly and tended to bypass some
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fully and effectively utilized. A low SA: V ratio also represents less opportunity for
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Pb2+ ions to interact with active sites and is inversely correlated to mass transfer. It
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can be concluded that, when the layer spacing is narrowed, the removal rates is
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limited by ion diffusion and inactive sites in the nanochannels despite of good
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conductivity and high SA: V ratio. However, uneven flow paths and low electron
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transfer efficiency inversely affect the removal rates when enlarging layer-spacing.
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Thus,
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lamellar-structured electrodes is of great significance and directed to the better
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electrochemical performance in water remediation.
tuning
layer
spacing
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and
optimizing
the
nano/micro-channels
in
Scheme 2
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This work demonstrates the effects of the pore structure and size of
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nano-/micro-channels in porous materials based on a LSG flow-through electrode for
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water remediation. The results here reveal the merits of electrochemical technology
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and highlight the importance of regulating the pore structure to increase the
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electro-sorption efficiency, as it enables the electrodes to optimize mass transport and
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electron transfer. The findings lay the foundation toward promising practical
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applications in electro-oxidation, electro-reduction and other electrochemical
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technologies for advanced water purification.
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AUTHOR INFORMATION
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Corresponding Author 21 ACS Paragon Plus Environment
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*Phone: (8610)6279-0565. Fax: (8610)6292-3558. Email:
[email protected] 408
ACKNOWLEDGMENT
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This work was supported by the National Natural Science Foundation of China (Grant
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No. 51608516, 51738013 and 51438011) and National Key R&D Program of China
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2016YFC0400502.
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Electrode Material's Performance for Ultracapacitors. Energy Environ. Sci. 2010, 3,
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integrated visible-light source. Lab Chip 2012, 12 (20), 3983-3990.
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photocatalytic microreactor using electrospun nanofibrous TiO2 as a photocatalyst.
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Nanocubes with High Surface-to-Volume Ratio for Accurate Magnetic Resonance
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Y. Enhanced Peripheral Nerve Regeneration by a High Surface Area to Volume Ratio
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Scheme 1. (a) Illustration of the pore structures of the LSG films with different
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interlayer spacings. (b) Schematic diagram of the flow-through cell.
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Figure 1 Morphology characterization of the LSG films, (a) Photographs of graphene
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hydrogel electrode before pressing. Cross-sectional high-magnification SEM images
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of graphene electrode under (b) 0.2 Mpa, (c) 0.5 Mpa, (d) 2.5 Mpa, (e) 7.5 Mpa, (f) 20
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Mpa
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Figure 2 Kinetic investigations of Pb(II) ion removal by electrodes with different
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layer spacings at (a) 0 V, (b) -0.4 V, (c) -0.8 V and (d) -1.2 V. (e) Kinetic
551
investigation of Pb (II) ion removal at different flow rates. (f) Pb (II) adsorption
552
isotherm for flow-through cell with electrodes of different layer spacings at -0.8 V.
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The solid lines are Langmuir model simulation, and the dashed lines are Freundlich
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model simulation. (Initial concentration=20 ppm, volume of solution=100 mL,
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temperature=25 oC, flow rate =500 L h-1 m-2, pH=5.0, NaNO3= 10 mmol L-1) 31 ACS Paragon Plus Environment
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Figure 3 (a) Resistance of LSG film electrodes with different layer spacing tested by
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four-point probe resistance test. (b) Nyquist plots of four electrodes with different
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layer spacing in the frequency range of 0.01 Hz to 100 kHz. (Electrolyte was 0.5 mol
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L-1 Na2SO4)
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Figure 4 (a) N2 adsorption-desorption isotherms (b) NLDFT Pore-size distribution of
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the five electrodes. Cyclic voltammetry (CV) curves for five LSG films with different
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layer spacings at scan rates of (c) 10 mv s-1 and (d) 50 mv s-1 in cell voltage range
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from -1.2 to 0 V. (Temperature=25 oC, pH=5.0, electrolyte 0.5 mol L-1 Na2SO4)
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Figure 5 Surface area-to-volume ratios for different layer-spacings electrode
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Figure 6 Views of the flow behaviors in 280 nm-layer-spacing LSG film and 2.44
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um-layer-spacing film LSG film obtained from CFD simulations. (a) The velocity
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magnitude contour of water flow in the 280 nm-layer-spacing LSG film. (b) The
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nanoscale velocity vector field in the nano-channels of the 280 nm-layer-spacing LSG
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film. (c) The velocity magnitude contour of water flow in the 2.44 um-layer-spacing
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film. (d) The nanoscale velocity vector field in the micro-channels of the 2.44
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um-layer-spacing film. The details of the models used for CFD simulation are given
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in Figure S1.
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Scheme 2 Mechanism of electrochemical performance for films with different layer
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spacing.
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