Amorphous titanic acid electrode: Its electrochemical storage of ammonium in a new water-in-salt electrolyte

Article abstract of DOI:10.1039/c8cc04713h

We report an amorphous titanic acid of TiO_1.85(OH)_0.30·0.28H₂O as a new electrode for aqueous ammonium-ion batteries, which operates in a new water-in-salt electrolyte - 25 m NH₄CH₃COO. The titanic acid electrode exhibits a specific capacity nearly 8 times that from the crystalline TiO₂ electrode. In electrochemical reactions, the amorphous titanic acid provides abundant storage sites in its disordered structure and affords strong H-bonding toward the inserted NH₄⁺ ions.

Full text of DOI:10.1039/c8cc04713h

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Amorphous Titanic Acid Electrode: Its Electrochemical Storage of
Ammonium in a New Water-in-Salt Electrolyte
Received 00th January 20xx,
Accepted 00th January 20xx
John J. Holoubek, Heng Jiang, Daniel Leonard, Yitong Qi, Galo C. Bustamante, Xiulei Ji*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report an amorphous titanic acid of TiO1.85(OH)0.30·0.28H2O
Among materials that could use the Ti4+/Ti3+ redox couple,
as a new electrode for aqueous ammoniumꢀion batteries, which there exist a class of “titanic acids” with a general formula of
operates in a new waterꢀinꢀsalt electrolyte—25 m NH4CH3COO. TiOx(OH)4ꢀ2x, which exhibit a variety of structures.9,10 In fact,
The titanic acid electrode exhibits a specific capacity nearly 8 titanic acids have been widely employed as precursors for the
times that from the crystalline TiO2 electrode. In electrochemical TiO2 synthesis,11ꢀ13 ion exchange/absorption,14,15 and
reactions, the amorphous titanic acid provides abundant storage catalysis.16,17 It was not until very recently that titanic acids
sites in its disordered structure and affords strong Hꢀbonding have demonstrated relevance in battery tests, where it was
toward the inserted NH4+ ions.
aimed to utilize the abundant structural defects in them to
achieve a high capacity.18,19 However, the operation potentials
of titanic acids are often too low to exhibit reversible ion
storage properties in aqueous electrolytes.
Recently, the concept of WiSE has been furthered from
lithiumꢀbased electrolytes to ones that are based on earthꢀ
abundant metal cations, such as NaFSI5, and Zn(CF3SO3)2.20
To date, most reported WiSE utilize expensive fluorinated salts
except for 30 m KCH3COO (KAc).7,21
In batteries, the choice of ion charge carriers essentially
defines the properties of devices. Despite the debuts of nearly
all relevant metal ions, nonmetal ions such as proton,
hydronium, and ammonium have received little attention
although these ions may offer rich binding chemistry with
electrodes.22
To explore the potential of WiSE based on nonꢀmetal
cations, herein, we demonstrate an aqueous solution of 25 m
NH4CH3COO (AmAc) as the electrolyte, in which a titanic acid
of TiO1.85(OH)0.30·0.28H2O exhibits an ammonium storage
capacity nearly 8 times that of the crystalline TiO2. As one
distinct factor, we observed the evidence of strong hydrogen
bonding of the stored NH4+ within the titanic acid electrode but
not with crystalline TiO2 electrode. In addition, titanic acid has
a lower density, a higher surface area, and a defectꢀrich
amorphous structure, which may relate to the performance
disparity between the two electrode materials as well.
To enable wide utilization of renewable energy, i.e., wind
and solar, stationary storage solutions are the missing enablers,
where electrochemical energy storage (EES) systems should
play a critical role. Considering safety, EES devices utilizing
nonꢀflammable aqueous electrolytes are particularly promising.
For aqueous EES, the electrolyte potential window that only
spans 1.23 V thermodynamically has been widened up to 3 V
via extremely concentrated aqueous electrolytes, known as
“WaterꢀinꢀSalt”
electrolytes
(WiSE),
e.g.,
lithium
bis(trifluoromethane)sulfonimide (LiTFSI).1 In WiSE the fewer
water molecules available are tightly bound in the primary
solvation sheath of electrolyte ions. This reduced solvation
number decreases the HOMO level of water, which renders
water splitting reactions more difficult, particularly, the oxygen
evolution reaction (OER), and the lower activity of water would
also ‘delay’ its splitting reactions, including the hydrogen
evolution reaction (HER) according to the Nernst equation.1ꢀ3
In addition, anions in WiSE, e.g., TFSIꢀ, are more reactive by
having
a lower LUMO, thus readily forming the solid
electrolyte interphase (SEI) on anodes, which further expands
water’s electrochemical stability window.1,2,4 This not only
promises higher energy densities of aqueous batteries, but more
importantly engenders the development of new electrode
materials that were previously deemed unusable in aqueous
systems, such as materials with the Ti4+/Ti3+ redox couple.5ꢀ8
We prepared the amorphous titanic acid and TiO2 by a
facile solꢀgel method at room temperature (see Supporting
Information), where their preparation only differs in the final
annealing temperature for the solꢀgel precipitate —110 °C and
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wide electrochemical stability window.2,24 As shown in Figure
2A, the 25 m AmAc electrolyte shows a potential window of
2.95 V in contrast to 2.25 V of the 1 M counterpart. Much of its
wider range comes from the greater anodic stability
characteristics. However, as observed in previous works
electrolytes’ concentration has little effect on water’s cathodic
stability.2,25 In fact, the 25 m AmAc electrolyte is slightly less
stable cathodically than the 1 M AmAc on the bare surface of
Al current collector despite having a higher pH value of 7.7, in
contrast to 1 M AmAc, which exhibits a pH of 6.9. This
phenomenon is likely due to the mechanism: 2NHꢇꢍ ꢊ 2eꢈ
→
2NHꢎ ꢊ Hꢉ based on the Le Chatelier principle. This
+
phenomenon was previously observed when increasing NH4
concentration at a constant pH value.26 However, it is worth
noting that when the titanic acid electrode is coated onto the Al
current collector less HER occurs in 25 m AmAc than in 1 M
AmAc, as displayed in Figure 2B, opposite to the stability
trend observed on the blank Al electrode. Recently, it has been
observed that when a WiSE is used, it typically raises the
operation potential of its cationꢀinsertion electrode due to the
increased activity of the cations.2 In this case, the reduction
potential for the titanic acid becomes higher in WiSE than in a
dilute electrolyte. Thus, in WiSE, the titanic acid electrode gets
reduced before the NH4+ ions do, and the HER behavior on the
titanic acid in WiSE looks mitigated.
Figure 1. (A) Powder XRD patterns of the titanic acid and TiO2. FTIR spectra of
(B) pristine titanic acid, and TiO2 powder, (C) (i) pristine TiO2 electrode, (ii)
ammoniated TiO2 electrode, (iii) pristine titanic acid electrode, and (iv)
ammoniated titanic acid electrode.
450 °C for titanic acid and TiO2, respectively.
Thermogravimetric analysis (TGA) of titanic acid reveals mass
loss of 9.7% from 50 ꢀ 450 °C in a distinct threeꢀslope profile
The titanic acid electrode exhibits a galvanostatic chargeꢀ
discharge (GCD) specific capacity of 84 mAh gꢀ1 in the WiSE
of 25 m AmAc and 61 mAh gꢀ1 in 1 M AmAc. These are much
(
Fig. S1A), where we attribute to removal of physisorbed
water, lattice water, and the conversion from TiOx(OH)4ꢀ2x to
ꢄ
ꢆ
ꢇꢈꢉꢃ
TiO2 through the reaction of
ꢀꢁꢂꢃ ꢂꢅ
→ ꢀꢁꢂꢉ ꢊ
higher than the capacity of TiO2 — 11 mAh gꢀ1 (Figure 3A, B)
.
ꢄ
ꢆ
2 ꢋ ꢌ ꢅꢉꢂ
. The formula of the titanic acid is, thus,
Furthermore, the 25 m AmAc electrolyte provides a higher
specific capacity for the titanic acid electrode than the 1 M
AmAc at 1 A gꢀ1, which we believe is due to the increased
redox potential of the titanic acid when tested in the WiSE,
where at the same lower cutoff potential, more capacity could
be released.
It is worth noting that there are slightly larger current and
more charges in the cathodic process than the anodic process in
CV and GCD measurements, which we believe is due to HER.
This may cause the consumption of the aqueous electrolyte
during cycling and lead to the potential failure of the full cells
in long cycling.
determined to be TiO1.85(OH)0.30·0.28H2O.
°
The TiO2 formed at 450 C exhibits a highly crystalline
anatase structure, whereas the titanic acid is completely
amorphous, as revealed by their Xꢀray diffraction (XRD)
patterns (Figure 1A). Scanning electron microscopy (SEM)
imaging of the titanic acid shows large variance of particle sizes
(
Figure S1B, C). The hysteresis of the N2 sorption isotherms of
the titanic acid is characteristic of a mesoporous structure,
where it exhibits the specific Brunauer–Emmett–Teller (BET)
surface area of 214 m2/g and the pores size distribution (PSD)
under 10 nm (Figure S2A, B). The TiO2 has a BET surface
area of 80 m2/g and the PSD from 10 to 60 nm (Figure S2C,
D)
.
In Fourier transform infrared spectroscopy (FTIR), both
titanic acid and TiO2 display the vibration of the TiꢀO bond at
1700 cmꢀ1, and titanic acid shows a peak of TiꢀOH bond at
1400 cmꢀ1 (Figure 1B) 23
Instead of having a single peak at
.
3300 cmꢀ1 assigned to the surface adsorbed water on the TiO2
particles, titanic acid exhibits multiple peaks for OꢀH vibrations
near 3000 cmꢀ1, which indicates the existence of crystal water
molecules and hydroxide groups inside the titanic acid in
addition to the physisorbed water.
The electrochemical performance of all samples was
evaluated in threeꢀelectrode cells using a freeꢀstanding film of
activated carbon (AC) as the counter electrode and an Ag/AgCl
reference electrode. Aluminum and titanium rods were used as
anode and cathode current collectors, respectively, to provide a
Figure 2. (A) Cyclic voltammetry (CV) curves recorded in 1 M and 25 m AmAc
electrolytes at 1 mV sꢀ1. Onset potentials were selected when the current is 0.01
mA. (B) CV curves of the titanic acid electrode in 1 M and 25 m AmAc
electrolytes at 1 mV sꢀ1.
2 | J. Name., 2012, 00, 1-3
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To understand the capacity disparity between the titanic
acid and TiO2, we first measured the density of titanic acid and
TiO2 by the Archimedes principle as 2.41 g cmꢀ3 and 3.00 g cmꢀ
3, respectively. The results suggest there are more available
+
sites for NH4 storage in the titanic acid’s structure. Another
aspect accounting for the capacity difference may arise from the
abundant defects in the amorphous structure of the titanic acid
+ 18
for binding NH4 . This trend is widely observed in the
capacity difference among the storage of Li+, Na+, and K+
ions.19
+
As another critical point, NH4 forms very strong chemical
bonds with local groups in the titanic acid but not with TiO2.
We employed ex situ FTIR spectroscopy to investigate pristine
and ammoniated titanic acid and TiO2 freeꢀstanding film
electrodes. Figure 1C displays the broad TiꢀO bond peak from
1550 to 1600 cmꢀ1, which undergoes a significant redꢀshift after
the ammoniation of the titanic acid at ꢀ1.3 V vs. Ag/AgCl. This
redꢀshift of the peak indicates that the TiꢀO bond is weakened
+
due to the interaction via the NH4 ꢀOꢀTi hydrogen bond. For
the TiO2 electrode, ammoniation does not cause a significant
peak shift, which certainly relates to the small storage capacity
but also to the lack of a chemical environment in TiO2 for
hydrogen bonding.
Furthermore, the titanic acid electrode also exhibits good
rate performance, exhibiting a reversible storage capacity of 70
mAh gꢀ1 and 50 mAh gꢀ1 in 25 m and 1 M AmAc, respectively,
at 5 A gꢀ1
(Figure 3C and D). Additionally, the 25 m AmAc
Figure 3. Typical GCD profiles of the titanic acid and TiO2 electrodes at 1 A gꢀ1
in (A) 25 m AmAc, (B) 1 M AmAc. Rate performance of the titanic acid electrode
at various specific current in (C) 25
m AmAc, (D) 1 M AmAc. Cycling
performance of the titanic acid electrode at 1 A gꢀ1 in (E) 25 m AmAc, (F) 1 M
AmAc.
This indicates that in the 1 M AmAc HER dominates,
corroborating the observed stability trend in Figure 2B, and
further demonstrates the advantage of AmAc WiSE over the
dilute electrolytes. The slightly faster fading in 25 m AmAc is
not well understood at this stage.
displays a higher capacitive nature of the storage, measured to
be 79.1%, where it was 52.3% for the 1M AmAc system
(Figure S5A, B)
.
Note that there is a lack of conductivity disparity between 25 m
and 1 M AmAc (Figure S3A). Figure 3E and 3F show that the
titanic acid electrode in the 25 m AmAc electrolyte retains 80%
capacity after 125 cycles, whereas in 1 M AmAc its CE
deteriorates rapidly despite exhibiting less capacity decay.
In summary, an amorphous titanic acid with the formula of
TiO1.85(OH)0.30·0.28H2O was synthesized through a facile solꢀ
gel method. In the AmAc WiSE, the titanic acid electrode
exhibits enhanced cathodic stability, delivers a higher specific
capacity, and shows a greater capacitive contribution of ion
storage compared to the case of the 1 M electrolyte. In addition,
+
the titanic acid exhibits the capacity of NH4 storage nearly 8
times that of the crystalline TiO2 at the same conditions, where
the higher capacity is attributed to the lower density of the
titanic acid, its amorphous structure, and the strong hydrogen
+
bonding between the hydroxide structure and the inserted NH4 .
This work represents an addition to the aqueous energy storage
by introducing one rarely known electrode material—titanic
+
acid that accommodates NH4 in a new AmAc WiSE system
that by itself may be interesting to various fields.
Acknowledgements:
X. Ji thanks the financial supports from the National Science
Foundation of the United States, Award Number: 1551693.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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+
Amorphous titanic acid reversibly stores NH4 in a new AmAc WiSE system.