Electrolyte-Philic Electrode Material with a Functional Polymer Brush

Apr 5, 2019 - (14,15) In principle, fast and efficient diffusion of electrolyte ions in the ..... prepared into an ASC, using 2 M KOH aqueous as the e...
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Electrolyte-philic Electrode Material with Functional Polymer Brush Zhen Wang, Wenlin Zhang, Yongtao Tan, Ying Liu, Ling-Bin Kong, Long Kang, and Fen Ran ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03054 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Materials & Interfaces

Electrolyte-philic Electrode Material with Functional Polymer Brush Zhen Wang b,#, Wenlin Zhangb,#, Yongtao Tan a,b, Ying Liu a,b, Lingbin Kong a,b, Long Kang a,b, Fen Ran a,b,* a

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,

Lanzhou University of Technology, Lanzhou 730050, Gansu, P. R. China b

School of Material Science and Engineering, Lanzhou University of Technology, Lanzhou

730050, Gansu, P. R. China *Corresponding author: Fen Ran ([email protected]; or [email protected]) # These authors contributed equally to this work.

Abstract: The electrode materials with advanced surface structure and architecture that effectively wet electrolyte and promote (ad/de)sorption of electrolyte ions. It remains a great challenge to improve electrode surface wetting properties when submersed in electrolyte. Herein, we report a novel “electrolyte-philic electrode material (EEM)” involving interconnected hierarchically porous carbon (IHPC) and grafted electrolyte-philic polymer chain brush by the surface initiated electrochemical mediated

atom

transfer

radical

polymerization

(SI-eATRP),

named

as

EEM-g-Polymer, which possesses higher capacity, almost 3 times larger than that of pristine one. And the selectivity and responsiveness of polymer brush can be tailored to different electrolyte environments. The EEM-g-PVP (or EEM-g-PSSNa) negative electrodes are combined with Ni(OH)2 positive electrodes for the fabrication of asymmetric supercapacitors. Notably, the device presents a maximum energy density of 27.8 Wh/kg (or 27.5 Wh/kg) when the power density is 351 W/kg (or 314 W/kg), 1

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and the capacitance retention is 70.0 % (or 65.0 %) of the initial capacitance value after 10000 cycles. Keywords:

Electrode

material,

SI-eATRP,

Functional

polymer

brush,

Electrolyte-philic, Electrochemical performance 1. Introduction Significant research has been invested to develop advanced energy storage devices, and state-of-the-art batteries and supercapacitors exhibit high energy and power density,

good

reversibility,

long

environmentally-friendly properties

cycle 1,2.

life,

and

also

in

some

cases

The electrode materials are critical

components of these devices since electrochemical processes take place at the electrode-electrolyte interface, including physical charge accumulation 3,4 and/or some redox processes

5,6

during charging and discharging processes. Thus, the surface

characteristics of electrode materials directly determine the electrochemical behavior of the electrochemical active materials in the electrochemical process. In general, when a potential is applied to an energy storage device, the electrochemical behavior of the electrode process will be influenced by three factors 7-9:

i) migration and diffusion of ions in the electrolyte close to the electrode interface,

ii) electrochemical reactions of electrolyte ions with the electrode surface involving (ad/de)sorption or reversible oxidation-reduction reactions, and iii) carrying charge efficiently across the electrode (conductivity). Based on these basic principles, various methods such as increasing the conductivity of electrode materials hierarchical pore structure

11,

adding mesopores

12,

10,

constructing a

and reducing the dimensions of

2

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electrode materials to the nanoscale 13 have been widely used to modify the electrode materials and obtain high conductivity, enhance the specific surface area, and optimize the pore structure to effectively improve the electrolyte ions dynamics and charge transfer during rapid charging and discharging processes. Nevertheless, the actual capacity of structurally optimized materials is invariably still much less than their theoretical capacity

14,15.

In principal, fast and efficient diffusion of electrolyte

ions in the near-interface region should be the first and key step of a whole electrode process, which largely determines the accompanying electrochemical performance. The efficiency of migration and diffusion of electrolyte ions depends on the surface structure and properties of electrode materials. Herein, we define “electrolyte-philic electrode materials” as electrode materials with advanced surface structure and architecture that effectively wet electrolyte and promote (ad/de)sorption of electrolyte ions. Scientists have noticed the importance of electrode surface wetting properties on performance when submersed in electrolyte, which prompted them to construct electrode materials with functional surfaces

16-18.

Previous reports indicate the

incorporation of heterogeneous atoms of N, O and P, or grafting surface oxygenous groups such as hydroxyl (-OH) and carboxyl group (-COOH) resulted in enhancement of electrode surface wettability, with a concomitant improvement in electrode performance

19-21.

However, these approaches only addressed surface structure at the

(near) atomic scale, which is limited in effectiveness. Significantly, it also should be noted that these kinds of functional surfaces would raise oxidation and reduction

3

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reactions during charging and discharging, i.e., as a pseudo-capacitor, which is good for improving the capacitance performance but might interfere with the surface contribution in other electrochemical processes.

Figure 1 Schematic illustration of the electrolyte-philic electrode material with polymer brush.

Herein, we reported a novel “electrolyte-philic electrode material (EEM)” involving interconnected hierarchically porous carbon (IHPC) and grafted electrolyte-philic polymer brushes. IHPC was used as an essential electrode material because of their outstanding structural features such as interconnected pore structure, highly developed porosity. Moreover, its pore systems can be well controlled. The polymer brush was grafted by the surface initiated electrochemical mediated atom transfer radical polymerization (SI-eATRP), through which the polymer chain architecture can be tailored

22-24.

The schematic illustration and morphology for EEM-g-Polymer is

illustrated in Figure 1. Note that no pseudo-capacitor was introduced and the surface architecture can be designed for different goals by varying the type of polymer used.

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2. Results and Discussions The carbon substrate of IHPC was fabricated by a method involving high concentrated internal phase emulsion polymerization and heat-treatment. As shown in Figure S1a in the Supporting Information, the SEM image of IHPC exhibited obvious interconnected pore structure and highly developed porosity. And it is worth noting that the basic morphology of IHPC did not change after grafting the polymer brushes (EEM-g-Polymer) (Figure 1). In addition, the pore structure parameters of all samples are shown in Table S1 in the Supporting Information, the specific surface area (SBET), total pore volumes (Vtotal), micropores volumes (Vmic), mesopores and macropores volumes (Vmes+mac) of all samples were calculated to be almost consistent, further proved that the morphology of the EEM-g-Polymer did not change and the porous structure was not blocked after grafting the polymer brush compared to that of IHPC, which was important for the charge-discharge process. The TEM image of IHPC has been tested and shown in Figure S1b in the Supporting Information, One can find that a micro-mesoporous structure with the uniform pore size.In addition, Figure S1c in the Supporting Information shows the related energy dispersive X-ray spectroscopy (EDS) of IHPC, and the C and O elements were distributed in the IHPC material. And the XPS spectrum of IHPC is shown in Figure S2a in the Supporting Information.The full XPS spectrum exhibits that the carbon materials contain C and O elements, further confirming the results from the EDS measurements; and Figure S2b in the Supporting Information shows the curve fitting of C 1s core level peak of the XPS spectrum, which has two new peaks at 285.4 and 286.4 eV attributed to C-C and

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C-O bonds. As shown in Figure S3 in the Supporting Information, the pore systems of the all samples were characterized by the nitrogen adsorption-desorption experiments, and the result of all samples were similar. The curves presented standard type IV N2 adsorption-desorption isotherms with close to Y-axis at low relative pressures (0.0< P/P0 K+ >Na+ >OH- >H+. And the fitted parameters for EIS are shown in Table S4 in the Supporting Information, which was the same order as of the ions size. The data revealed smaller Rs, Rct and Rw values in 2M KOH electrolyte, indicating the excellent electrochemical performance of the electrodes. The rate capacitances of EEM-g-PVP and EEM-g-PSSNa remained about 75% and 65% of the initial specific capacitances in a variety of aqueous electrolyte solutions (Figure S9 in the Supporting Information).

Figure 6 a) CV curves, b) GCD curves, and c) the specific capacitances obtained for

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Ni(OH)2||EEM-g-PVP ASC device; d) CV curves, e) GCD curves, and f) the specific capacitances obtained for Ni(OH)2||EEM-g-PSSNa ASC device; and g) the cycling stability at 1 A/g of two devices (the inset shows Ragone plot).

Two asymmetric supercapacitors (ASC) were assembled with Ni(OH)2 and EEM-g-PVP (or EEM-g-PSSNa ) electrode acted as the positive and negative electrodes, respectively. The polarization range of the voltammetry was changed after being prepared into an asymmetric supercapacitor, using 2M KOH aqueous as electrolyte. The potential window of Ni(OH)2 ranged from -0.2 to 0.6 V, which matched well to that of EEM-g-PVP (or EEM-g-PSSNa ) electrode ranged from -1.0 to 0 V. The EEM-g-PVP (or EEM-g-PSSNa) electrode and Ni(OH)2 electrode were tested at a scan rate of 5 mV/s at the potential window ranging from -1.0 to 0 V, and -0.2 to 0.6 V, respectively, as shown in Figure S10a and S10c in the Supporting Information. As such, the Ni(OH)2||EEM-g-PVP and Ni(OH)2||EEM-g-PSSNa ASC can be operated up to 1.4 V without obvious increase of anodic current (Figure S10b and S10d in the Supporting Information). The electrochemical performance of Ni(OH)2||EEM-g-PVP and Ni(OH)2||EEM-g-PSSNa ASC devices were measured in 2M KOH aqueous solution as electrolyte. CV curves of the two devices at scanning rates from 5 to 50 mV/s (Figure 6a and 6d) showed broad oxidation and reduction humps, indicating the pseudo-capacitance properties corresponding to the redox reactions on Ni(OH)2 surface, and the shape of CV curves has no obvious change at different scan rates,which represented an ideal capacitive behavior. The GCD curves of two devices at different current densities from 0.5 to 30 A/g showed non-linear

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symmetric triangular shapes (Figure 6b and 6e), indicating excellent columbic efficiency and remarkable rate performance. The maximum specific capacitance values of Ni(OH)2||EEM-g-PVP and Ni(OH)2||EEM-g-PSSNa ASC devices were 102 and 101 F/g, and when the current density increased to 30 A/g the capacitance still remained 42 and 45 F/g. Figure 6c and 6f show the rate capability of two devices, approximately 43.0% of initial specific capacitance were remained when the current density

increased

from

0.5

to

30

A/g.

The

long-term

stability

of

Ni(OH)2||EEM-g-PVP and Ni(OH)2||EEM-g-PSSNa ASC were investigated by charge-discharge cycling at the current density of 1 A/g. As shown in Figure 6g, after 10000 charging-discharging cycles, the capacitance retention remained approximately 70.0% and 65.0%, respectively. The energy density and power density were two important parameters for the evaluation of the device, and the Ragone plots of two devices

(inset

in

Figure

6g)

were

calculated,

The

energy

density

of

Ni(OH)2||EEM-g-PSSNa device was 27.5 Wh/kg when the power density was 314 W/kg. Even when the power density increased to 20520 W/kg, the energy density still remained 11.4 Wh/kg. For the Ni(OH)2||EEM-g-PVP device, at a power density of 351 W/kg, the device delivered an high energy density of 27.8 Wh/kg, and even when the power density increased to 20455 W/kg, the energy density still remained 12.5 Wh/kg. 3. Conclusions In summary, here we demonstrated a carbon material with different polymer brushes by using SI-eATRP. The obtained EEM-g-Polymer still maintains an interconnected

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pore structure and enhances the electrolyte affinity towards carbon material surfaces due to the rational design of pendant functional polymer brushes. Electrochemical measurements suggest that the EEM-g-Polymer possesses higher capacity, almost 3 times larger than that of pristine IHPC. The selectivity and responsiveness of polymer brush can be tailored towards electrolyte ions as demonstrated with different polymer brush chemistries that were grafted on the substrate material to accommodate different

electrolyte

environments.

Remarkably,

the

ACS

devices

of

Ni(OH)2||EEM-g-PVP (or Ni(OH)2||EEM-g-PSSNa) exhibited a maximum energy density of 27.8 Wh/kg (or 27.5 Wh/kg) at the power density of 351 W/kg (or 314 W/kg), and the capacitance retention was 70.0% (or 65.0%) of the initial one after 10000 cycles. Moreover, polymer brushes can be grafted to different active material substrates, including various carbon allotropes and transition metal oxide/nitride (e.g., NiO and VN). The fundamental understanding of this research provides guidelines for the future design of hierarchically-defined electrode materials with electrolyte-philic surfaces. 4. Experimental 4.1 Preparation of electrolyte-philic electrode material (EEM) The methods and experimental section of the paper refer to our previously published work 25, which mainly included the following three parts: i) Chemicals Materials; ii) Fabrication of carbon-scaffold (named as IHPC); iii) Incorporation of functional polymer brushes on carbon-scaffold (named as EEM-g-Polymer). The detailed process was provided in the Supporting Information.

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4.2 Characterization The morphology and microstructure was characterized using scanning electron microscope (SEM, HITACHI S-4800). the surface chemical composition of the samples were measured by FTIR spectrum ( FT-IR Nexus 670 instrument ) and X-ray photoelectron spectra (XPS, Physical Electronics UK). The porosity and specific

surface

area

of

samples

were

characterized

by

the

Nitrogen

adsorption-desorption isotherms at 77K (Micromeritics ASAP 2010MUSA). For the water contact angle (WCA) measurement, the samples were pressed into pellets under a pressure of 20 MPa for several minutes, the behavior of water on the pellets was captured

by

a

high-speed

digital

camera

(DRS

Technologies).

Aqueous

electrophoretic data for EEM-g-PVP and EEM-g-PSSNa at different electrolytes (0.001M KOH aqueous, 0.001M H2SO4aqueous and 0.001M Na2SO4aqueous) were obtained using a potentiometer (Malvern Zetamaster S), and the Zeta potentials were calculated using Zeta mode V1.51 software. 4.3 Electrochemical measurements The working electrodes and the asymmetric supercapacitors were prepared for electrochemical measurement refers to our previously published work

25,

it mainly

includes the following two parts: (i) The working electrodes; (ii) The asymmetric supercapacitor, see the detailed process provided in the Supporting Information. Regarding the working electrodes test: The electrochemical properties of working eletrodes were tested using an electrochemical workstation (CHI660E, Shanghai, China), using 2 M KOH aqueous, 1 M H2SO4 aqueous and 1 M Na2SO4 aqueous as

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the electrolyte. Cyclic voltammetry (CV) were recorded at different scan rates from 5 to 50 mV/s. Galvanostatic charge-discharge (GCD) were tested at the different current densities from 1.0 to 5.0 A/g within the same potential range as in CV measurements. Electrochemical impedance spectra (EIS) were acquired with a frequency range from10-2 to 105 Hz. Regarding the asymmetric supercapacitor (ASC) test: the asymmetric supercapacitor was assembled with Ni(OH)2 acted as positive electrode and EEM-g-PVP (or EEM-g-PSSNa ) acted as negative electrode. The electrochemical performance of ASC also tested using an electrochemical workstation (CHI660E, Shanghai, China), the 2 M KOH aqueous was used as electrolyte. CV was recorded at different scan rate from 5 to 50 mV/s with a potential windows ranging from 0 to 1.4 V. For GCD measurement, the current density was varied from 0.5 to 30A/g within the same potential range as CV measurements. The cycling test was performed using LAND CT2001A instrument at a current density of 1 A/g. The specific capacitance of electrode can be calculated from the discharging curve at different current densities by according to following the equation (1): C(F/g)= I×t/ (△V× m)..............(1) Energy density of the device was calculated by the equation (2): E (Wh/kg)=C×△V2/ (2×3.6)..............(2) Power density of the device was calculated by the equation (3): P(W/kg)=E×3600/△t..............(3) Where C (F/g) is the specific capacitance, I (A) is the constant discharge current, △t 21

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(s) is the discharge time, △V (V) is the voltage change during discharge (excluding the IR drop), m (g) is the mass of active materials of electrode, E (Wh/kg) is the energy density of device and P (W/kg) is the power density of device. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acssuschemeng.XXXXXX. Additional experimental procedures; estimating the solubility parameter (δ) of different polymer; estimating the grafting density (ρ) of different polymer; pore structure parameters of the different samples; surface properties of different samples; group contributions to Ecoh and V according to Fedors; fitted parameters for EIS obtained by Zswinpwin software of different samples; schematic illustration of the synthesis process for EEM-g-Polymer; SEM images, TEM images, and the related energy dispersive X-ray spectroscopy (EDS) of IHPC; XPS data of IHPC; nitrogen adsorption-desorption isotherms; pore size distribution of the IHPC and EEM-g-Polymer; cyclic voltammogram curves of the electrolyte system for different monomers at a scan rate of 5 mV/s; FTIR spectrum of IHPC-Br; XPS data of EEM-g-PAN, EEM-g-PAM, EEM-g-PVP and EEM-g-PSSNa; the equivalent electrical circuit model of all impedance spectra for different samples; electric double-layer models for different surfaces with grafted the polymer; specific capacitances at different current densities of EEM-Ploymers electrode in different electrolytes (2 M KOH, 1 M H2SO4 and 1 M Na2SO4) in a three-electrode system; CV curves for Ni(OH)2 and EEM-g-PVP electrodes at 5 mV/s in a three-electrodeconfiguration in 2M KOH aqueous, 22

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respectively; CV curve of Ni(OH)2||EEM-g-PVP ASC device at 10 mV/sin 2M KOH aqueous; CV curves for Ni(OH)2 and EEM-g-PSSNa electrodes at 5 mV/s in a three-electrode configuration in 2M KOH aqueous, respectively; and CV curve of Ni(OH)2||EEM-g-PSSNa ASC device at 10 mV/sin 2M KOH aqueous.

■ AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] or [email protected]. ORCID
 Fen Ran: 0000-0002-7383-1265 Notes The authors declare no competing financial interest.

Acknowledgements This work was partly supported by the National Natural Science Foundation of China (51203071, 51363014, 51463012, and 51763014), China Postdoctoral Science Foundation (2014M552509, and 2015T81064), Natural Science Funds of the Gansu Province (1506RJZA098), the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology (J201402), and Joint fund between Shenyang National Laboratory for Materials Science and State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals (18LHPY002). We should also 23

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acknowledge the help of Prof. Christopher M. Bates, of the Department of Materials at University of California, Santa Barbara, for the thoroughly editing and nice recommendation. References 1. Gogotsi, Y.; Simon, P. True Performance Metrics in Electrochemical Energy Storage. Science. 2011, 334, 917-918. 2. Miller, J. R.; Simon, P. Electrochemical Capacitors for Energy Management. Science. 2008, 321, 651-652. 3. Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854. 4. Wang, G. P; Zhang, L.;Zhang, J. J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. 5. Ferrero, G. A.; Fuertes, A. B.; Sevilla, M. N-Doped Microporous Carbon Microspheres for High Volumetric Performance Supercapacitors. Electrochim. Acta. 2015, 168, 320-329. 6.

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