# Nanosize Copper Dispersed Ionic Liquids As an Electrolyte of  New Dye-Sensitized Solar Cells

Authors: Fu-Lin Chen; I.-Wen Sun; H. Paul Wang; C.-H. Huang
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

To enhance the electrical conductivity of the electrolyte for a newly developed
dye-sensitized solar cell (DSSC), metallic copper (Cu) encapsulated within the carbon shell
(Cu@C) nanoparticles dispersed in a room temperature ionic liquid (RTIL)
(e.g., [bmim+][PF6−]) has been studied in the present work. By the pulsed-field gradient spin-echo NMR method, the self-diffusion coefficients of cations and anions of the RTIL have been determined. The self-diffusion coefficient of the [bmim+] cations in the RTIL dispersed with 0.08% of Cu@C nanoparticles is increased by 35%. The electrical conductivity of the Cu@C dispersed RTIL is also increased by 65% (1.0 → 2.3 ms/cm). It is very clear the nanosize Cu@C dispersed RTIL with a relatively greater diffusion coefficient and electrical conductivity can be a very effective electrolyte especially utilized in DSSCs.


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## Body

## 1. Introduction

Crystalline silicon solar cells have relatively high
efficiencies, nevertheless, limited by the high manufacturing cost and long
payback period [1, 2].  On the
contrary, organic solar cells have arisen more attentions simply due to the high
possibility of creating extremely lightweight, easily integration, low-cost,
and flexible solar cells [3].
One of the drawbacks on the performance of dye-sensitized solar cells
(DSSCs) is the mediocre stability of the liquid electrolyte in the cells.Room temperature ionic liquids (RTILs) are of
increasing interest and importance in clean industrial processes such as liquid
solvents for chemical reactions and extractions and in electrochemical
applications as electrolytes for DSSCs, fuel cells and lithium batteries [4, 5].  Room temperature ionic liquids can also be utilized
in gas sensors and biosensors  [6].
To increase photovoltaic properties and stability of a DSSC, a composite
ionic liquid electrolyte of mixing silica nanoparticles in the 1-methyl-3-propylimidazolium
iodide (MPII) has been used to replace the conventional electrolyte  [7–9].  An ionic liquid electrolyte containg
dispersed carbon nanoparticles, carbon nanotubes, or titatium oxide
nanoparticles in the 1-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
imide had a better photocurrent density and voltage [10].In the separate
experiments, size-controllable Cu@C nanoparticles were synthesized by
carbonization of Cu2+-starch or -cyclodextrins complexes at 673–873 K [11].  In the present work,
a very small amount of Cu@C nanoparticles with particle sizes as small as 7–20 nm were dispersed in the RTIL.  Their
self-diffusion coefficients were determined by pulsed-field gradient spin-echo
NMR.


## 2. Experimental

The RTILs were synthesized
by the modified procedures reported by Cammarata et al.  [12], Tokuda et al.   [13, 14] and Yeon et al.  [15].  The nanosize Cu (7, 14, and 20 nm in
diameter) encapsulated in the carbon shell (Cu@C) was obtained by carbonization
of the Cu2+-starch complexes at 673 K for two hours.  The 1H
NMR chemical shifts and self-diffusion coefficients were measured using a double-layer
tube in which deteriumoxide–d2 (Aldrich) and the sample were filled in
the inner tube (closed system) and outer tube, respectively [16].  Electrolyte conductivities of the Cu@C
dispersed RTIL were measured on the Suntex sc-170 at
300 K.  Images of the Cu@C dispersed RTIL
were also determined by high resolution transmission electron microscopy (TEM)
(Philps CM-200).The K-edge X-ray absorption near edge structure (XANES)
and extended X-ray absorption fine structure (EXAFS) spectra of copper in the Cu@C
(0.08%) dispersed RTIL were collected on the Wiggler beam line at the Taiwan National
Synchrotron Radiation Research Center.
The electron storage was operated at 1.5 GeV (current of 300 mA).  The beam energy was calibrated by the
adsorption edge of a copper foil at energy of 8979 eV.  The EXAFS data were analyzed using the UWXAFS
3.0 and FEFF 8.0 simulation programs [17, 18].  The background of the X-ray absorption
spectra was justified by the AUTOBK program [17].  The isolated EXAFS data were normalized to
the edge jump and converted to the wavenumber scale.  The Fourier transform of the spectra was
performed on the k3-weighted
EXAFS oscillations in the range of 3.5 to 11.5 Å-1.  Empirical fits of model compounds have an
error of ±0.01 Å in radius and ±10% in coordination number (CN) for the first
shell atoms.


## 3. Results and Discussion

Table1 shows
the self-diffusion coefficients of the [bmim+] cations and [PF6−]
anions in the Cu@C dispersed RTIL determined by 1H and 19F nuclei NMR spectroscopy at 300.  The diffusion coefficient of the [bmim+]
cations in the RTIL dispersed with 0.08% of Cu@C nanoparticles is increased by 25–35%
if compared with the plain RTIL.  Dispersion
and insertion of the Cu@C nanoparticles (Cu sizes = 7–20 nm) in the matrix of
the RTIL may cause a reduction of the ionic bonding energy between its cations ([bmim+])  and
anions ([PF6−]), leading to an increase of the self-diffusion
coefficient of the [bmim+] cations from 5.1 to 6.9 ×  10−12m2s−1 and a decrease of the RTIL’s viscosity from 178
to 164 centipoise (cp). It seems that
the Cu@C nanoparticles interact more preferably with the [bmim+]
cations of the RTIL as the self-diffusion coefficient of the [PF6−] anions
influenced by the dispersed Cu@C nanoparticles is relatively insignificant.Table 1
The self-diffusion coefficient and
electrical conductivity of the  Cu@C dispersed RTIL ([bmim+] [PF6−])
measured at 300 K.
Cu size (nm)Cu@C (wt%)Self-diffusion  coefficientsElectrical conductivityViscosity
(cp)(10−12⁢m2s−1)(ms/cm)[bmim+][PF6−]05.15.32.017870.086.45.53.1168140.086.75.53.4165200.086.95.63.3164In Table1, in addition to the increase of the
self-diffusion coefficients of the [bmim+] cations in the RTIL dispersed with
the Cu@C nanoparticles, an increase (up to 70%) of the RTIL’s electrical
conductivity is also found.  It is worth
noting that the uniform-size Cu@C nanoparticles are well dispersed in the RTIL
(see Figure 1).  The metallic copper (Cu)
having sizes of 7–20 nm is encapsulated in the carbon shell which is consisted
of diamond and graphite carbons with a sp3/sp2 of  0.5–0.7 (determined by Raman spectroscopy [11]).  The greater electrical conductivity of the
ionic liquid electrolyte for the new DSSC may be attributed to the electron-rich
carbon shell surfaces of the Cu@C nanoparticles that interact with the [bmim+]
cations in the RTIL.Figure 1
The TEM image of the Cu@C (Cu size  = 14 nm) dispersed
in the RTIL ([bmim+] [PF6−]).Molecular-scale
data of Cu coated with the carbon shell dispersed in the RTIL in terms of the bond
distance and coordination number (CN) of near neighbor atoms can be determined
by EXAFS spectroscopy.  An over 99%
reliability of the EXAFS data fitting for copper species in the RTIL is shown
in Table2.  Their Debye-Waller factors are
less than 0.01.  In the RTIL, Cu (7–20 nm)
in the Cu@C nanoparticles has Cu–Cu bond distances of 2.530–2.535 Å, and its CNs increase from 6.2 to 8.7
as the Cu sizes increase.Table 2
Structure
parameters of copper in the Cu@C (0.08%) dispersed  RTIL ([bmim+] [PF6−]).
Size (nm)ShellBond distance (Å)CoordinationDybe-Wellernumberfactor (σ2)7Cu–Cu2.5336.20.007914Cu–Cu2.5356.30.008120Cu–Cu2.5308.70.0075Figure2 shows the XANES spectra of copper in
the Cu@C nanoparticles dispersed RTIL. Their
preedge XANES spectra exhibit a very weak 1s-to-3d transition (8975–8980 eV)
which is forbidden by the selection rule in the case of perfect octahedral
symmetry.  A shoulder at (8984–8988 eV)
and an intense band at(8995–9002 eV) is due to the 1s to 4p transition that
indicates the existence of the Cu(II) species.
The XANES spectrum of copper in the Cu@C is very similar to that in the
Cu@C dispersed in the RTIL.
Nevertheless, about 11% of Cu originally in the core of the core-shell Cu@C
nanoparticles (dispersed in the RTIL) is oxidized to CuO.  Note that the fraction of surface Cu atoms to
the total atoms for the Cu nanoparticle having a size of 7 nm is 11%.  It is very likely that in the RTIL the
encapsulated Cu may involve in oxidation with residual oxygen in the RTIL on
the surfaces of the Cu nanoparticles.
The CuO layer on the surfaces of the core Cu in the Cu@C may also
facilitate the electron transport limited to the surfaces of the carbon shell
for a better efficiency.The least-square fitted XANES spectra of
copper in (a) nanosize Cu and Cu@C (0.08%) having Cu sizes of (b) 7, (c) 14,
and (d) 20 nm dispersed in the RTIL ([bmim+] [PF6−]).
(a)(b)(c)(d)


## 4. Conclusion

Dispersion of a very small amount (0.08%) of
Cu@C nanoparticles in the RTIL can increase the diffusion coefficient of the cations
in the RTIL by 35%.  The diffusion
coefficient of the anions in the Cu@C dispersed RTIL is relatively less influenced.  An increase (up to 70%) of the Cu@C dispersed
RTIL’s electrical conductivity is also found.
The greater electrical conductivity of the ionic liquid electrolyte for
the new DSSC may be attributed to the electron-rich carbon shell surfaces of
the Cu@C nanoparticles that interact with the [bmim+] cations in the
RTIL.  The CuO layer on the surfaces of
the core Cu in the Cu@C may also facilitate the electron transport limited to
the surfaces of the carbon shell for a better efficiency.



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*Source: 472950-2009-03-30.xml*