Cheap and environmentally benign electrochemical energy storage and conversion devices based on AlI3 electrolytes

Article abstract of DOI:10.1021/ja057791v

Cheap and environmentally benign electrochemical energy conversion and storage devices, including a dye-sensitized solar cell (DSSC) using an AlI₃-ethanol electrolyte and a new Al/I₂ primary battery, are reported. The AlI₃-ethanol electrolyte can be prepared simply by adding aluminum powder and iodine into ethanol at ambient conditions. The DSSC using this AlI₃-ethanol electrolyte achieved an energy conversion efficiency of 5.9% at AM 1.5 (100 mW/cm-2). In the Al/I₂ battery, AlI₃is formed spontaneously when aluminum and iodine electrodes are brought into contact at room temperature. Then I^- anions transport across the AlI₃ solid electrolyte for further electrochemical reactions. Copyright

Full text of DOI:10.1021/ja057791v

Published on Web 06/17/2006
Cheap and Environmentally Benign Electrochemical Energy Storage and
Conversion Devices Based on AlI3 Electrolytes
Bofei Xue,† Zhengwen Fu,*,‡ Hong Li,*,† Xizhe Liu,† Sunchao Cheng,‡ Jia Yao,‡ Dongmei Li,†
Liquan Chen,† and Qingbo Meng*,†
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
Beijing 100080, People’s Republic of China, and Department of Chemistry & Laser Chemistry Institute,
Shanghai Key Laboratory of Molecular Catalysts and InnoVatiVe Materials, Fudan UniVersity,
Shanghai 200433, People’s Republic of China
Received November 16, 2005; E-mail: zhengwen@sh163.net; hli@aphy.iphy.ac.cn; qbmeng@aphy.iphy.ac.cn
Cheap and environmentally benign electrochemical energy
conversion and storage devices, including primary and secondary
batteries, dye-sensitized solar cells (DSSCs), supercapacitors, fuel
cells, and so on, are in great demand for all kinds of electronic
devices.1 The finding of new electrolytes always plays a key role
for their technical progress.2 Here we report a new Al/I2 primary
battery system. Different than the operating mechanism in other
Al-based batteries, in which an Al electrode is active and dissolved
into aqueous or nonaqueous liquid electrolytes,3 AlI3 is formed
spontaneously in the Al/I2 battery when an aluminum electrode and
Figure 1. Discharge behavior of an Al/I2 cell at a rate of 0.5 C (5 mA/
an iodine electrode are brought into contact at room temperature.
Then I- anions transport across the AlI3 solid electrolyte for further
cm2).
reactions. On the other hand, it is found that an AlI3-ethanol
electrolyte can be prepared simply by adding aluminum powder
and iodine into ethanol at ambient conditions. A DSSC using this
AlI3-ethanol electrolyte achieved an energy conversion efficiency
of 5.9% at AM 1.5 (100 mW/cm-2). This is comparable to the
DSSC using a LiI-nitrile electrolyte, where expensive anhydrous
LiI and poisonous acetonitrile or methoxypropionitrile are used and
the electrolytes are prepared under dry conditions.
An Al/I2 cell was constructed as follows. An Al plate with a
thickness of 0.5 mm was used as an anode directly. Iodine and
graphite with a weight ratio of 8:2 were mixed mechanically and
pressed into a pellet (diameter ) 11.3 mm, thickness ) 0.5 mm)
as a cathode. Then both electrodes were put into a two-electrode
Teflon-lined cell using stainless steel plates as current collectors.
The cells were assembled and measured in an Ar-filled glovebox.
No separator or electrolyte was used in this case.
A typical discharging curve of the Al/I2 cell is given in Figure
1. The discharging voltage starts at 0.79 V and decreases gradually.
The electrochemical reaction could be written as
Figure 2. The Raman shifts of iodine (black line), commercial aluminum
iodide (red line), and a mixture of aluminum powder and iodine (blue line).
even when aluminum powder was mixed with iodine powder
mechanically at room temperature (evidenced by Raman spectra,
Figure 2). Obviously, the reaction mechanism of the Al/I2 cell can
be described as a thin AlI3 electrolyte layer formed once the Al
anode and I2 cathode are in contact. The consequent electrochemical
reaction is related to the diffusion of either Al3+ ions or I- ions
across the AlI3 layer.
To clarify the transport mechanism, an iodide ion conductor, LiI-
(3-hydroxypropionitrile)4 (LiI(HPN)4) with 15 wt % SiO2 (15 nm),
was used as a solid electrolyte sandwiched between Al and I2
electrodes to assemble an Al/LiI(HPN)4-15 wt % SiO2/I2 cell. This
solid electrolyte shows a mono-ion (I-) transport feature with an
ion conductivity of 1.3 × 10-3 S/cm at room temperature.5 The
discharge curve of the Al/LiI(HPN)4-15 wt % SiO2/I2 cell is shown
in Figure 3. It works well even when the cell is discharged at the
current density of 5 mA/cm2. Apparently, the initial polarization
loss in the Al/LiI(HPN)4 + 15 wt % SiO2/I2 cell is higher than that
in the Al/I2-C cell due to the addition of LiI(HPN)4 + 15 wt %
SiO2 electrolyte. This result strongly supports that the operation of
the Al/LiI(HPN)4-15 wt % SiO2/I2 cell is based on the transport
of I- ions. Thus, the Al/I2 cell is demonstrated as an iodide ion
primary battery. Compared with the Li/I2 battery, which is normally
operated below 0.1 mA/cm,2,6 the Al/I2 cell can supply a large
2Al + 3I2 f 2AlI3
(1)
According to the Nernst equation (∆rG ) -nEF), the thermo-
dynamic equilibrium voltage (noted as E) of this reaction is 0.962
V. The large difference between E and the working voltage as well
as the decreasing voltage upon discharging depth indicates that a
high internal polarization exists and increases gradually in this cell.
This agrees well with the reaction model that an ionic conductive
electrolyte film is growing gradually sandwiched between Al and
I2 electrodes during the electrochemical reaction of aluminum and
iodine.
AlI3 could be prepared by heating aluminum and iodine powder
at 300 °C for 24 h.4 It is confirmed here that AlI3 can be formed
† Chinese Academy of Sciences.
‡ Fudan University.
9
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J. AM. CHEM. SOC. 2006, 128, 8720-8721
10.1021/ja057791v CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
AlI3 electrolyte B (commercial AlI3 reagent dissolved into ethanol)
and electrolyte C (see Supporting Information Figure S3).
For comparison, a traditional electrolyte was also prepared, the
composition of which was 0.6 M dimethylpropylimidazolium
iodide, 0.1 M of lithium iodide, 0.5 M TBP, and 0.1 M of iodine
in methoxypropionitrile (MPN).8 A DSSC using the traditional
electrolyte achieved an efficiency of 7.2% with Voc ) 665 mV, Isc
) 16.87 mA/cm-2, and FF ) 0.64. The difference between the
reported best result (η ) 10.4%)8 and our efficiency for the same
system indicates that other components except the electrolyte are
not well optimized. Thus, we are convinced that the efficiency of
DSSC using the aluminum iodide-based electrolyte can be further
improved by optimizing the TiO2 film, other components, and
assembling procedures of DSSC.
Figure 3. Discharge behavior of Al/LiI(HPN)4 + 15 wt % SiO2/I2 cell at
the discharge rate of 0.5 C (5 mA/cm2).
Above preliminary results of DSSC using the AlI3-ethanol
electrolyte system shows a promising future for achieving high
energy conversion efficiencies with the advantages of low cost and
environmental benignity.
In summary, we report cheap, environmentally benign energy
conversion and storage devices, that is, an Al/I2 primary battery
and a DSSC based on AlI3 electrolyte. It is believed that this AlI3
electrolyte, based on iodide transport, can find other applications.
Acknowledgment. The authors appreciate the financial support
from the National “863” and “973” Programs and the “100-talent”
project of CAS.
Figure 4. The photocurrent-voltage characteristics of a DSSC using the
aluminum iodide electrolyte A in ethanol ([I-] ) 0.3 M, [I2] ) 0.03 M,
ethanol/TBP ) 8:1 (v/v), measured under AM 1.5, 100 mW/cm-2).
Supporting Information Available: Al/I2 battery discharged at
different rates, the procedure of TiO2 film preparation and cell
fabrication, the optimization of the concentration of TBP and iodine
in the electrolytes, experiments for determining the dissociated state
of AlI3 in ethanol by UV-vis spectra, FT-Raman spectra, conductivity
measurements of AlI3 and LiI solutions, secondary ion mass spectrom-
etry (Cs+ ion being the incident ion beam), and complete ref 8. This
material is available free of charge via the Internet at http://pubs.acs.org.
discharge rate with discharge current density at an order of mA/
cm2 (see Figure S1). This should be related to fast transport of I-
ions in the AlI3 electrolyte at room temperature, needing further
investigation. The low cost of the Al/I2 system as well as the feature
of environmental friendliness makes this system as an attractive
primary battery.
The AlI3-ethanol electrolyte was prepared as follows. Excess
aluminum powder and iodine (A.R.) were added into ethanol and
stirred for about 2-5 h at ambient conditions. A clear and colorless
solution could be obtained after filtering out excess aluminum
powder. In this way, AlI3 was formed and existed in ethanol as
Al3+ and I- (see Supporting Information). Then, appropriate
amounts of 4-tert-butylpyridine (TBP) and iodine were added into
the solution to form the final electrolyte (A). For comparison, two
similar electrolytes were also prepared by dissolving the commercial
reagents of aluminum iodide (Aldrich, 95%) and lithium iodide
(Aldrich, 99.9%) directly in ethanol. TBP and iodine were also
included in the above electrolytes (B and C, respectively). In all
cases, the concentration of I- was kept at 0.3 M.
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