Fast-switching electrochromic Li+ -doped NiO films by ultrasonic spray deposition

Article abstract of DOI:10.1149/1.3279992

A low cost, high throughput deposition method for films of nickel oxide (NiO) and lithium-doped nickel oxide with improved electrochromic performance is demonstrated. This method is based on ultrasonic spray deposition of aqueous-based precursor solutions in air at atmospheric pressure, which represents a significant cost savings compared to vacuum deposition methods. The resultant materials are characterized by X-ray diffraction, Raman spectroscopy, electron microscopy, and electrochemical measurements. Electrochromic performance is demonstrated with in situ optical transmission measurements during electrochemical characterization. Nickel oxide materials color anodically and are thereby ideally suited to be used as counter electrode for the well-known tungsten oxide (W O₃) system in smart window applications. The coloration of nickel oxide materials is known to be slow when compared to W O₃ and thereby limits the overall response time of a NiO/W O₃ tandem device. The analysis of potential step response data shows that our lithium-doped nickel oxide material achieves 90% of its total coloration change in 29 s, which is comparable to reported measurements for W O₃. These results significantly mitigate a potential bottleneck to the adoption of metal oxide electrochromic windows not only by demonstrating similar performance between NiO and W O₃, but by achieving this result via low cost, highly scalable processing methods.

Full text of DOI:10.1149/1.3279992

H318
Journal of The Electrochemical Society, 157 ͑3͒ H318-H322 ͑2010͒
0
013-4651/2010/157͑3͒/H318/5/$28.00 © The Electrochemical Society
+
Fast-Switching Electrochromic Li -Doped NiO Films
by Ultrasonic Spray Deposition
z
Robert C. Tenent, Dane T. Gillaspie,* Alex Miedaner, Philip A. Parilla,
Calvin J. Curtis, and Anne C. Dillon*
National Renewable Energy Laboratory, Golden, Colorado 80401, USA
A low cost, high throughput deposition method for films of nickel oxide ͑NiO͒ and lithium-doped nickel oxide with improved
electrochromic performance is demonstrated. This method is based on ultrasonic spray deposition of aqueous-based precursor
solutions in air at atmospheric pressure, which represents a significant cost savings compared to vacuum deposition methods. The
resultant materials are characterized by X-ray diffraction, Raman spectroscopy, electron microscopy, and electrochemical mea-
surements. Electrochromic performance is demonstrated with in situ optical transmission measurements during electrochemical
characterization. Nickel oxide materials color anodically and are thereby ideally suited to be used as counter electrode for the
well-known tungsten oxide ͑WO ͒ system in “smart” window applications. The coloration of nickel oxide materials is known to
3
be slow when compared to WO3 and thereby limits the overall response time of a NiO/WO tandem device. The analysis of
3
potential step response data shows that our lithium-doped nickel oxide material achieves 90% of its total coloration change in 29 s,
which is comparable to reported measurements for WO . These results significantly mitigate a potential bottleneck to the adoption
3
of metal oxide electrochromic windows not only by demonstrating similar performance between NiO and WO , but by achieving
3
this result via low cost, highly scalable processing methods.
©
2010 The Electrochemical Society. ͓DOI: 10.1149/1.3279992͔ All rights reserved.
Manuscript submitted August 31, 2009; revised manuscript received November 25, 2009. Published January 19, 2010.
4
3-45
Internal environmental control of buildings is responsible for a
significant portion of overall electricity use in the U.S. Energy-
films,
and none to date have studied the performance of such
1
films in EC applications. Furthermore, the literature on spray coat-
ing of EC NiO films has predominantly focused on the KOH-based
aqueous electrolyte system.
Our deposition process is based on the ultrasonic spraying
method, which holds several key advantages over more widely em-
ployed pressure-driven spraying methods. Among these are lower
materials usage and improved uniformity due to a narrower size
distribution of droplets in the atomized material. In brief, a liquid
precursor solution is pumped to an ultrasonically excited tip surface.
Standing waves form in the liquid layer on the nozzle surface. These
waves become unstable at their peaks and collapse, leading to the
generation of fine droplets of the precursor solution. The atomized
material is carried by a controlled nitrogen flow to a heated substrate
where it undergoes thermal decomposition to yield the desired film.
We have used this technique to spray deposit both NiO and lithium-
doped NiO films. Furthermore, although NiO has previously been
shown to act successfully as a counter electrode for the WO sys-
tem, ion movement into and out of the NiO structure is reported to
be slower than for WO , thereby limiting the switching speed and
overall device performance. Here, we have also demonstrated im-
proved kinetics for our lithium-doped NiO film, indicating that these
films are more promising as a counter electrode when employing
WO3 as the active electrode.
efficient “smart” windows that can change transmission characteris-
tics in response to external stimuli are poised to make a tremendous
impact by reducing the energy needed to maintain comfortable
building environments as well as providing controllable daylighting
for building occupants. One method to accomplish this is to employ
electrochromic ͑EC͒ metal oxide films, where the optical transmis-
sion may be modulated to pass or block incident sunlight by apply-
ing a small voltage. However, widespread implementation of smart
windows depends critically on developing cost-effective techniques
that can also be integrated into large-scale manufacturing. EC metal
oxide films are typically deposited by vacuum deposition methods,
4
6
2-7
primarily sputtering, although several alternate processes have
6,8-16
been demonstrated.
It is believed that a simple spray-coating
deposition method, which can be performed using aqueous-based
liquid precursor materials in air, is a candidate for an even more
cost-effective and widely implementable production method.
The operation of conventional inorganic EC devices depends on
3
+
a reversible electrochemical double injection of positive ions ͑H ,
3
+
+
+
47
Li , Na , and K ͒ and electrons into the host lattice of multivalent
17,18
transition-metal oxide materials.
most established EC material for the active ͑coloring͒ electrode.
Tungsten oxide ͑WO ͒ is the
3
1
9-23
It shows cathodic coloration with electron injection and charge-
balancing ion insertion. Alternately, some EC materials color anodi-
2
4
cally by the removal of electrons and positive ions. The most
frequently studied materials with anodic coloration are nickel oxide
and iridium oxide. By combining an electrode that exhibits cathodic
coloration with a counter electrode that has anodic coloration, it is
possible to fabricate a more efficient and visually appealing EC
device. One such EC system that has seen intense interest of late
Experimental
Chemicals.— Nickel nitrate and lithium nitrate salts were ob-
tained from Aldrich and used as received. All solutions for spray
deposition were made with deionized water. Premade electrolyte
solutions for electrochemical testing were obtained from Ferro Cor-
poration ͑Zachary, LA͒.
uses WO5-27as the active electrode and NiO as a counter
3
2
electrode.
Here, we present spray deposition methods for nickel oxide and
lithium-doped nickel oxide films for use as a complementary and
Spray apparatus.— The ultrasonic spray system was from Sono-
Tek Corporation and consisted of a model 8700-120 spray head that
operated at a frequency of 120 kHz. A Sono-Tek Broadband Ultra-
sonic Generator was used to apply ultrasonic excitation to the spray
nozzle, which had a 0.230 in. diameter conical tip and a 0.015 in.
diameter orifice that was fitted with the impact system for gas-driven
spray delivery. A controlled gas flow was supplied by a model FMA
1818 mass flowmeter from Omega Engineering, Inc., which deliv-
ered the atomized material to the surface. The precursor solution
was pumped to the spray nozzle through a Fluid Metering Inc. VMP
TRI Pulseless “Smoothflow” pump equipped with three model Q1-
CSC-W-LF pump heads.
anodically coloring counter electrode for the well-known WO sys-
3
tem. Our interest in the Li-doped NiO material is based on reported
longer device lifetimes for Li-ion-based EC devices that employ
7
nonaqueous electrolytes. Several processes have been demonstrated
28-42
for spray-coating NiO films.
Significantly fewer papers have
discussed spray deposition methods for lithium-doped NiO
*
Electrochemical Society Active Member.
E-mail: Robert.Tenent@nrel.gov
z
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Journal of The Electrochemical Society, 157 ͑3͒ H318-H322 ͑2010͒
H319
Substrate materials and preparation.— All films for electro-
optical characterization were deposited on fluorine-doped tin oxide
FTO͒ films. Films for X-ray diffraction ͑XRD͒ and Raman analysis
͑
were deposited on glass microscope slides. Before deposition, all
substrates were cleaned with an isopropanol-soaked clean-room
wipe, blown dry with nitrogen, and then placed in an oxygen plasma
͑
800 mTorr, 155 W͒ for 5 min.
XRD.— Data were collected using a Bruker AXS D8 Discover
with a HiStar area detector. For the XRD measurements, the sample
was illuminated with X-rays from a copper target ͑40 kV, 35 mA͒
using a Göebel mirror and a 1 mm circular collimator. The recorded
XRD signals contained both K␣1 and K␣2 components, but no K␤
component as it was filtered out by the Göebel mirror. The two-
dimensional area detector data were integrated in chi using GADDS
software to produce a more conventional XRD intensity vs 2␪ data
plot. Background data for FTO substrates were collected and sub-
tracted from sample films.
Raman spectroscopy.— Spectra were collected using 2.54 eV
͑
488 nm͒ laser excitation. The backscattered light was analyzed
with a Jobin Yvon 270M spectrometer equipped with a liquid-
nitrogen-cooled Spectrum One charge-coupled device and a holo-
graphic notch filter. A Nikon 55 mm camera lens was employed both
to focus the beam on the sample to an ϳ0.25 mm spot and to
collect the Raman scattered light. Averaging three 60 s scans was
sufficient to obtain high intensity, well-resolved Raman spectra.
Figure 1. Thermogravimetric data for NiNO and LiNO under a pseudoair
mixture.
3
3
2
Figure 2 shows XRD data and Raman spectra for films sprayed
from nickel nitrate solutions with and without 5 wt % LiNO added.
3
Electrochemical analysis.— Cyclic voltammetry and potential
cycling measurements were made using a BioLogic VMP3 multi-
channel potentiostat. Samples were examined in a two-electrode
testing mode vs a Li metal counter electrode in 1 M Li perchlorate
dissolved in propylene carbonate. All electrochemical testing was
performed inside a controlled atmosphere glove box from Vacuum
Atmospheres Corporation. All voltammetric scans were collected at
The lithium nitrate loading level was fixed based on separate experi-
ments that demonstrated no significant improvement in film perfor-
mance for loadings higher than 5% ͑data not shown͒. This agrees
with the recent paper of Joseph et al., who spray-coated lithium-
45
doped NiO films for use in dye-sensitized solar cells. Figure 2a
shows XRD data for films sprayed with and without lithium doping.
The film sprayed from NiNO clearly shows peaks indicating the
2
3
a 20 mV/s scan rate and all electrodes tested were 1 cm in geomet-
ric surface area.
formation of nickel oxide ͑JCPDS file no. 47-1049͒, with the ͑200͒
peak being dominant as expected for a randomly oriented film. XRD
data for the Li-doped material show identical peaks for NiO forma-
tion as well as other peaks that can be attributed to unreacted lithium
nitrate ͑JCPDS file no. 08-0466͒ ͑denoted with an asterisk͒. No
peaks are observed that can be correlated to previously reported
LiNiO crystal structures. Figure 2b shows Raman spectroscopy data
for the same samples. The spectra of both the NiO and lithium-
Transmission measurements.— In situ optical transmission mea-
surements employed a diode laser operating at 670 nm as the source
and a Thor Labs, Inc. DET100A large-area silicon detector. Clean,
uncoated, FTO substrates were used to set the 100% transmission
level of the detector before obtaining sample measurements.
Results and Discussion
−1
doped NiO films exhibit a peak at ϳ500 cm , which is consistent
with previously reported excitation of the NiO stretching
Deposition chemistry.— Figure
1
shows thermogravimetric
5
2-54
vibrations.
No apparent shift is observed upon adding lithium
analysis ͑TGA͒ data for both nickel and lithium nitrate salts col-
lected under a “pseudo” air mixture ͑80% nitrogen, 20% oxygen͒.
Decomposition of NiNO ·6H O appears to come to completion near
ions to the film. The improved EC performance of the lithium-doped
materials ͑discussed below͒ combined with the XRD and Raman
data raises the suggestion that both amorphous LiNiO and NiO crys-
tallites may be present in our material. Lithium–cobalt spinel struc-
tures have been prepared under similar conditions by reaction of
3
2
325°C. The mass loss up to 250°C is consistent with dehydration of
the starting material, whereas the mass loss finishing at 325°C is
consistent with loss of nitrate ion. The final percentage of the origi-
nal mass is below what would be expected for the conversion of the
nickel salt to NiO under these conditions, which may indicate sub-
stoichiometry in the final product. Although TGA data are useful for
predicting optimal deposition temperatures for the spray process,
they do not necessarily indicate the exact products that will form.
Figure 1 also shows the TGA data for lithium nitrate that does not
show significant mass loss until ϳ600°C, although differential ther-
mal analysis confirmed a melting point of ϳ260°C. Attempts to
produce films at ϳ600°C led to the deformation of the underlying
FTO-coated glass substrate. It is unclear if this was due to glass
deformation or damage to the FTO layer itself. Gordon reported
5
5
CoOOH and LiNO3.
EC performance.— Figure 3 shows cyclic voltammetry coupled
with optical transmission data collected at 670 nm for films sprayed
from nickel nitrate with and without lithium nitrate added. Although
both films were sprayed to approximately the same thickness, the
Li-doped material clearly shows both higher charge insertion
upon cycling and, subsequently, a larger transmission change. The
voltammetry for the Li-doped material also shows more defined
peaks for lithium intercalation and deintercalation than for the un-
doped NiO material. The better-defined peaks and the more rapid
increase in current as a function of potential indicate a larger and
faster ion intercalation into the doped material as compared to NiO
only.
48
instability for FTO in this temperature range. Based on these re-
sults, the spray depositions reported here were performed at 330°C,
which was a sufficiently high temperature to decompose the nickel
nitrate as well as to melt the additional lithium nitrate. This is po-
tentially an important point as several papers have reported im-
proved performance for ceramic materials sintered in the presence
Figure 4 shows transmission data for the same samples as shown
+
in Fig. 3 under potential step cycling from 2.25 to 4.25 V vs Li/Li .
The Li-doped material shows a significantly higher transmission
49-51
modulation ͑ϳ83 to ϳ33%͒ than the accompanying NiO material
of a liquid phase.
͑
36–19%͒. These values are slightly different from those observed in
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H320
Journal of The Electrochemical Society, 157 ͑3͒ H318-H322 ͑2010͒
Figure 4. ͑a͒ Optical transmission response from the same samples dis-
cussed in Fig. 3 under potential step cycling between 4.25 and 2.25 V vs
+
Li/Li . ͑b͒ Normalized transmission as a function of time extracted from one
cycle of the data shown in ͑a͒. Transmission was normalized to the final
transmission obtained after 5 min at potential.
Figure 2. ͑a͒ XRD and ͑b͒ Raman spectroscopy data for as-deposited films
sprayed from NiNO with and without LiNO added.
3
3
terial and is defined as the change in optical density ͑OD͒ per unit
inserted charge ͑Q͒, i.e., CE = ⌬͑OD͒/⌬Q. Analysis of the data for
Fig. 3 due to longer times ͑5 min͒ spent at potential extremes than
were employed for the voltammetric scans. Coloration efficiency
2
the Li-doped material yields a CE of 33 cm /C, which is compa-
͑
CE͒ is one of the most important metrics for selecting an EC ma-
2
rable to that measured for sputtered films. The transmission modu-
lation of the NiO film is already starting to degrade in this limited
number of potential cycles. In contrast, the Li-doped material ap-
pears to show improved cycling stability compared to the NiO ma-
terial. However, longer-term cycling ͑50 cycles͒ demonstrated a sig-
nificant, albeit slower, decrease in transmission modulation for the
5
6
Li-doped films, which is well known for NiO materials and re-
mains a weakness of the present system ͑data not shown͒.
In addition to the larger transmission change, the Li-doped ma-
terial has a faster coloration change than the NiO material. This is
clearly demonstrated in Fig. 4b, which shows normalized transmis-
sion data for samples sprayed from nickel nitrate with and without
lithium nitrate. These data are for the coloration of the NiO-based
devices, and the switching speed is reported as the time to achieve
9
0% of the total coloration change. NiO shows a typical slow col-
oration process, reaching 90% of its change in 115 s. However, the
Li-doped film achieves the same coloration change in about 29 s in
addition to having a significantly larger change in transmission.
Similar measurements for the bleaching step show that the Li-doped
Figure 3. Cyclic voltammetry for films spray-coated from NiNO3 ͑a͒ with
and ͑b͒ without LiNO present. The vertical axes are identical for both plots.
3
The vertical axes in the center of the figure have been deleted to allow a less
cluttered view of the data.
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Journal of The Electrochemical Society, 157 ͑3͒ H318-H322 ͑2010͒
H321
and Raman spectroscopy. Furthermore, the switching speed for col-
oration and bleaching of the film is comparable to that for WO3,
thereby demonstrating excellent potential for use as a counter elec-
trode in an EC device. Attempts to modulate film morphology by
changing precursor concentration indicated that higher concentra-
tions deposited quickly yield better EC performance. When lower
concentrations are employed, excessively long deposition times are
required that lead to particle agglomeration and poor performance.
Acknowledgments
The authors acknowledge Dr. Maikel van Hest for assistance
with the ultrasonic spray deposition and useful discussions on po-
tential precursor materials. This work was funded by the U.S. De-
partment of Energy ͑DOE͒ under subcontract no. DE-AC36-
0
8GO28308 through the DOE Office of Energy Efficiency and
Renewable Energy, Office of Building Technologies Program.
Figure 5. ͑a͒ Cyclic voltammetry for lithium-doped nickel oxide films
sprayed from differing precursor concentrations and SEM micrographs of the
National Renewable Energy Laboratory assisted in meeting the publica-
tion costs of this article.
same films sprayed from ͑b͒ 10 mM and ͑c͒ 1 M NiNO . Both samples had
3
5
wt % LiNO added.
3
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