Pore Structure-Dependent Mass Transport in Flow-through Electrodes

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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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Environmental Science & Technology

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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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*Corresponding author: [email protected]

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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.

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. 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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pore areas, and the existence of dead volume meant that the active sites could not be 20 ACS Paragon Plus Environment

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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]

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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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Microfluidic reactors for photocatalytic water purification. Lab Chip 2014, 14 (6),

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integrated visible-light source. Lab Chip 2012, 12 (20), 3983-3990.



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Nanoscale 2013, 5 (11), 4687-4690.

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J. L.; Gao, J. H.; Zhao, Z. H. Activated Surface Charge-Reversal Manganese Oxide

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Nanocubes with High Surface-to-Volume Ratio for Accurate Magnetic Resonance

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Tumor Imaging. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm.201700978.

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540 541

Scheme 1. (a) Illustration of the pore structures of the LSG films with different

542

interlayer spacings. (b) Schematic diagram of the flow-through cell.



543 544

Figure 1 Morphology characterization of the LSG films, (a) Photographs of graphene

545

hydrogel electrode before pressing. Cross-sectional high-magnification SEM images

546

of graphene electrode under (b) 0.2 Mpa, (c) 0.5 Mpa, (d) 2.5 Mpa, (e) 7.5 Mpa, (f) 20

547

Mpa


Page 30 of 36

Page 31 of 36


548 549

Figure 2 Kinetic investigations of Pb(II) ion removal by electrodes with different

550

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.

553

The solid lines are Langmuir model simulation, and the dashed lines are Freundlich

554

model simulation. (Initial concentration=20 ppm, volume of solution=100 mL,

555

temperature=25 oC, flow rate =500 L h-1 m-2, pH=5.0, NaNO3= 10 mmol L-1) 31 ACS Paragon Plus Environment


556 557

Figure 3 (a) Resistance of LSG film electrodes with different layer spacing tested by

558

four-point probe resistance test. (b) Nyquist plots of four electrodes with different

559

layer spacing in the frequency range of 0.01 Hz to 100 kHz. (Electrolyte was 0.5 mol

560

L-1 Na2SO4)


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Page 33 of 36


561 562

Figure 4 (a) N2 adsorption-desorption isotherms (b) NLDFT Pore-size distribution of

563

the five electrodes. Cyclic voltammetry (CV) curves for five LSG films with different

564

layer spacings at scan rates of (c) 10 mv s-1 and (d) 50 mv s-1 in cell voltage range

565

from -1.2 to 0 V. (Temperature=25 oC, pH=5.0, electrolyte 0.5 mol L-1 Na2SO4)



566

567 568

Figure 5 Surface area-to-volume ratios for different layer-spacings electrode


Page 34 of 36

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569 570

Figure 6 Views of the flow behaviors in 280 nm-layer-spacing LSG film and 2.44

571

um-layer-spacing film LSG film obtained from CFD simulations. (a) The velocity

572

magnitude contour of water flow in the 280 nm-layer-spacing LSG film. (b) The

573

nanoscale velocity vector field in the nano-channels of the 280 nm-layer-spacing LSG

574

film. (c) The velocity magnitude contour of water flow in the 2.44 um-layer-spacing

575

film. (d) The nanoscale velocity vector field in the micro-channels of the 2.44

576

um-layer-spacing film. The details of the models used for CFD simulation are given

577

in Figure S1.



578 579

Scheme 2 Mechanism of electrochemical performance for films with different layer

580

spacing.


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Pore Structure-Dependent Mass Transport in Flow-through Electrodes

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