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Cite this:
Energy Environ. Sci.,
2019,
12
,358
Enhancing the activity of oxygen-evolution
and chlorine-evolution electrocatalysts by
atomic layer deposition of TiO
2
†
Cody E. Finke,
*
abc
Stefan T. Omelchenko,
c
Justin T. Jasper,
abc
Michael F. Lichterman,
a
Carlos G. Read,
bd
Nathan S. Lewis
d
and
Michael R. Hoffmann*
abc
We report that TiO
2
coatings formed
via
atomic layer deposition (ALD) may tune the activity of IrO
2
,RuO
2
,
and FTO for the oxygen-evolution and chlorine-evolutio
n reactions (OER and CER). Electrocatalysts exposed
to
B
3–30 ALD cycles of TiO
2
exhibited overpotentials at 10 mA cm
2
of geometric current density that were
several hundred millivolts lower than uncoated catalysts,
with correspondingly higher specific activities. For
example, the deposition of TiO
2
onto IrO
2
yielded a 9-fold increase in the OER-specific activity in 1.0 M
H
2
SO
4
(0.1 to 0.9 mA cm
ECSA
2
at 350 mV overpotential). The oxidation state of titanium and the potential of
zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation
state, potential of zero charge, and activity of the tuned electrocatalysts.
Broader context
Realizing a low anthropogenic CO
2
emissions future depends on the electrochemical production of fuels and commodity chemicals. In the absence of a
substantial carbon tax, electrochemical production of these materials must be cost competitive with conventional production. The levelized cost o
f
electrochemically produced chemicals depends heavily on operational expenses (OpEx;
e.g.
, buying electricity) and the balance of systems costs, and depends
relatively less on the price of the catalyst.
1
Therefore, one pathway to low cost electrochemical fuel and commodity chemical production is to reduce the OpEx
by fabricating highly active catalysts. Current methods to enhance catalytic activity are limited or rely on computationally-expensive calculati
ons. Simple tools
that can be used to enhance the catalytic activity for a variety of chemical reactions, such as tuning catalysts through atomic layer deposition as pre
sented here,
are essential to developing low-cost electrochemical systems that can meet global energy and chemical demands.
Introduction
Highly active electrocatalysts are required for the cost-effective
generation of fuels and commodity chemicals from renewable
sources of electricity.
2,3
Despite potential advantages (
e.g.
, facile
product separation), the industrial use of many heterogeneous
electrocatalysts is currently limited in part by suboptimal
catalytic activity and/or selectivity. In addition, there are limited
methods to tune the selectivity and activity of heterogeneous
electrocatalysts.
2
Methods and design tools such as doping,
inducing strain, and mixing metal oxides have been used to
improve the catalytic activity of heterogeneous electrocatalysts.
4–7
The activity of heterogeneous electrocatalysts can also be
tuned by applying thin layers of another material, leading to
an altered surface charge density on the resulting composite
material relative to the bulk charge density of either individual
material.
8–13
This approach has been widely used to alter the
catalytic and electronic properties of core/shell nanoparticles,
although additional tuning of the particle support structure
is necessary to create an efficient heterogeneous electro-
catalyst.
14,15
Density functional theory calculations have
shown that a single atomic layer of TiO
2
on RuO
2
should lead
to enhanced selectivity for the chlorine-evolution reaction
(CER) relative to the oxygen
-evolution reaction (OER).
9
Enhanced catalytic activity for the OER has been reported
for WO
3
photocatalysts coated with 5 nm of alumina, with the
activity increase ascribed to an alteration in the electronic
surface-state density.
16
Enhanced catalytic activity has also
been observed at the interface between TiO
2
and RuO
2
,with
a
The Linde Center for Global Environmental Science, Caltech, Caltech, Pasadena,
CA 91125, USA
b
The Resnick Sustainability Institute, Caltech, Caltech, Pasadena, CA 91125, USA
c
Division of Engineering and Applied Science, Caltech, Caltech, Pasadena,
CA 91125, USA. E-mail: finkec@caltech.edu, mrh@caltech.edu
d
Division of Chemistry and Chemical Engineering, Caltech, Caltech, Pasadena,
CA 91125, USA
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ee02351d
Received 12th August 2018,
Accepted 26th November 2018
DOI: 10.1039/c8ee02351d
rsc.li/ees
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359
charge transfer between RuO
2
and TiO
2
resulting in a mixed
phasewithanintermediatechargedensity.
5
Herein, atomic layer deposition (ALD; a stepwise deposition
technique) has been used to tune the surface charge density,
and consequently tune the catalytic activity, of electrocatalytic
systems in a fashion consistent with estimates based on group
electronegativity concepts (see Fig. S1–S5 in the ESI
†
for further
discussion of ALD, surface homogeneity, and group electro-
negativity estimates). To test these predictions, the activities of
the known electrocatalysts, IrO
2
, RuO
2
, and F-doped SnO
2
(FTO) were tuned and evaluated for the chlorine-evolution
reaction (CER) and the oxygen-evolution reaction (OER). The
CER provides a promising approach to infrastructure-free
wastewater treatment as well as for the production of chlorine,
an important industrial chemical whose global annual demand
exceeds seventy million metric tons.
17,18
The OER is the limit-
ing half-reaction for water splitting that could provide hydrogen
for transportation and could also provide a precursor to energy
storage
via
thermochemical reaction with CO
2
to produce an
energy-dense, carbon-neutral fuel.
19
Results and discussion
Each material tested was selected based on its theoretical group
electronegativity (
w
) relative to the group electronegativity of
RuO
2
(
w
E
2.72), the most active catalyst for the OER in the
benchmarking literature (Fig. S5, ESI
†
) as well as the most
active catalyst for the CER.
20
IrO
2
(
w
E
2.78) and FTO (
w
E
2.88)
were also investigated because they have higher electronegativities
than RuO
2
, and therefore using ALD to overcoat these catalysts
with TiO
2
(
w
E
2.62) is expected to shift their surface electronic
properties (
i.e.
, the potential of zero charge,
E
ZC
)andcatalytic
activities towards that of RuO
2
, the optimal single metal oxide
catalyst. These materials were also chosen because TiO
2
,IrO
2
,
RuO
2
, and other materials are commonly used to form mixed
metal oxide electrodes, most notably the dimensionally stable
anode (DSA), in which TiO
2
increases the anode’s stability, but
does not confer enhanced activit
y to the aggregated material.
21
Overpotentials (
Z
; the excess potential beyond the equili-
brium potential required to reach a given current density) were
determined for IrO
2
, RuO
2
, and FTO as a function of the
successive number of TiO
2
ALD cycles (see ESI
†
for additional
details on electrode preparation and testing, and TiO
2
growth
rate) for the OER at 10 mA (cm
geo
)
2
in 1.0 M H
2
SO
4
and for the
CER at 1 mA (cm
geo
)
2
in 5.0 M NaCl adjusted to pH 2.0 with
HCl. Current densities were chosen to produce
4
95% measured
Faradaic efficiency for each catalyst (Table S2, ESI
†
), and current–
potential data were corrected for the solution resistance (
o
2.0 mV
correction) as measured by electrochemical impedance spectro-
scopy (see ESI
†
for details). The three catalysts were prepared on
substrates that had very low roughness to minimize effects in
geometric overpotential measurements due to surface area differ-
ences. Specifically, electroca
talyst samples consisted of a
B
300 nm
metal–oxide film sputter deposited
on a (100)-oriented Si substrate,
in the case of IrO
2
and RuO
2
, or commercially available TEC
15 FTO glass substrates, in the case of FTO-based electrocatalysts.
TiO
2
overlayers were then deposited on top of the electrocatalysts.
The microstructure of a typical IrO
2
-based electrocatalyst is shown
in the cross-sectional scanning electron microscopy (SEM) image
in Fig. 1A. The resulting electrocatalysts were very smooth with low
surface roughness (Fig. 1B) such that the surface area as measured
by atomic-force microscopy (AFM
) was roughly equivalent to the
measured geometric surface areas (Table S1, ESI
†
). Further char-
acterization of the electrocatalysts’ surface topology can be found
in Fig. S1–S4 and Table S1 (ESI
†
).
Geometric overpotentials for these catalysts were consider-
ably higher than geometric overpotentials for identical catalysts
prepared on rougher substrates, however, the measured OER
overpotentials at 10 mA (cm
geo
)
2
for bare RuO
2
and IrO
2
agreed well with values reported for catalysts prepared on
similarly flat surfaces. We are unaware of comparable OER
data for FTO or for CER catalysts.
20,22
The overpotentials for
IrO
2
and FTO, for both the OER and CER, initially showed an
improvement (
i.e.
, reduction) with increasing ALD cycle number,
before exhibiting an inflection point due to an increase in over-
potential at higher ALD cycle numbers (Fig. 2). The triangular
shape observed between the overpotential and the TiO
2
ALD cycle
number is typical of a vo
lcano-type relationship that exemplifies
the Sabatier principle.
23
The overpotential reductions between
bare IrO
2
and FTO catalysts and those at the peak of the volcano
curvefortheOERwere
D
Z
OER
E
200mVat10cyclesand
100 mV
at 30 cycles, respectively. For the CER, the observed overpotential
reductions were
D
Z
CER
E
30 mV at 3 cycles and
100 mV at
10 cycles, for IrO
2
and FTO respectively (Fig. 2). A volcano-type
relationship between cycle number and overpotential was also
observed for RuO
2
facilitating the OER, with
D
Z
OER
E
350 mV
between 0 and 10 cycles. However, for the CER, the over-
potential of the RuO
2
-based catalyst increased with TiO
2
ALD
cycle number (Fig. 2).
The specific activity (
i.e.
, the current density normalized to
the electrochemically active surface area (ECSA)) is a standard
quantity for comparing the OER activity of heterogeneous
electrocatalysts (see Fig. S9–S11, and the ESI
†
for details on
specific activity calculations and additional discussion). For
IrO
2
and RuO
2
catalysts, the OER specific activities of the
uncoated catalysts were in good agreement with previously
reported values.
20
We are unaware of reported specific activities
for FTO for the OER or for any catalyst for the CER. The specific
activities for the OER and CER were characterized by volcano-
type relationships as a function of the TiO
2
ALD cycle number
(Fig. 2). In fact, IrO
2
coated with 10 ALD cycles of TiO
2
showed a
9-fold increase in OER specific activity at
Z
= 350 mV relative to
uncoated IrO
2
. Recently, IrO
x
/SrIrO
3
has been reported as an
especially active catalyst using current normalized to atomic
force microscopy measured surface area (AFMSA) in 0.5 M H
2
SO
4
.
To compare these catalysts, we measured the roughness of our
catalysts using AFM (Table S1, ESI
†
). For our catalysts, bare IrO
2
exhibited a Tafel slope of
B
60 mV dec
1
in good agreement with
previously reported OER catalysts.
24
As the activity of our IrO
2
based catalyst increased from bare IrO
2
to 10 TiO
2
ALD cycles,
the Tafel slope remained constant at
B
60 mV dec
1
while the
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exchange current density (
i
0
)increasedfrom
B
1
10
7
to
B
2
10
5
mA (cm
AFMSA
)
2
. Initially the IrO
x
/SrIrO
3
catalyst
also had an OER Tafel slope of
B
60 mV dec
1
and an
i
0
of
B
7
10
6
mA (cm
AFMSA
)
2
. For the IrO
x
/SrIrO
3
, however, after
a period of activation the Tafel slope improved dramatically to
B
40 mV dec
1
, which indicates a previously unknown OER
mechanism, while the
i
0
deteriorated to
B
3
10
7
mA (cm
AFMSA
)
2
(see Fig. S11, Table S5, and ESI
†
for details on Tafel analysis). In our
case, IrO
2
coated with 10 ALD cycles of TiO
2
exhibited lower
overpotentials than the freshly prepared IrO
x
/SrIrO
3
catalyst at
current densities
o
1mA(cm
AFMSA
)
2
and lower overpotentials
than the activated IrO
x
/SrIrO
3
catalyst at
o
0.02 mA (cm
AFMSA
)
2
,
but substantially higher overpotentials at the more industrially
relevant current densities of
4
10 mA (cm
AFMSA
)
2
.
2,25
Further
discussion on surface roughness, including AFM, and SEM sample
characterization is presented in the ESI
†
(Fig. S1–S4 and Table S1).
To test the longevity of the enhanced catalytic performance
with TiO
2
deposition, we performed 24 h stability testing at
10 mA cm
2
for both the CER and the OER for the uncoated
catalyst and for the most active catalyst for each material
system. The catalysts investigated herein were not optimized
for stability and, as was previously reported for thin IrO
2
and
RuO
2
catalyst depositions,
20,26
the overpotential on uncoated
catalysts for the OER in 1 M H
2
SO
4
degraded rapidly after
o
1h
of operation at 10 mA (cm
geo
)
2
. For thinly coated catalysts
(3–10 cycles) the OER stability improved from about 1 h to
about 4 h, while for thicker TiO
2
coatings (
4
30 cycles) the OER
stability increased to
4
9 h (Fig. S7, ESI
†
). The loss in activity
for the OER for TiO
2
coated samples was associated with a loss
in the TiO
2
coating as illustrated in X-ray photoelectron spectro-
scopy (XPS) measurements of the Ti 2p core level before and
after electrochemical stability testing (Fig. S22, ESI
†
). For the
CER, all catalysts were relatively stable over the 24 h testing
period except for the FTO-based catalysts which followed the
same trend as the OER, with thicker TiO
2
coatings stabilizing
the electrodes. XPS measurements of the stable CER catalysts
indicated that the TiO
2
overcoating was still present even after
24 h of continuous operation (Fig. S23, ESI
†
). These results
indicate that, as prepared here, these catalysts are not long-
term stable, and substantial work is needed to obtain an
industrially relevant catalyst. Similarly prepared catalysts exhibit
enhanced stability by making the ca
talyst material thicker, anneal-
ing the catalyst, or mixing Sb
x
O
y
,TiO
2
,Ta
x
O
y
,orSnO
2
into the
catalyst.
26–28
It is possible that similar techniques could be used to
enhance the stability of the cata
lysts presented in this work.
The enhancement in catalytic performance observed with
deposition of TiO
2
is not readily explained by surface morpho-
logical changes of the electrocatalyst. Deposition of TiO
2
does
not substantially affect the electrochemically active surface
area, a metric believed to be related to active site density, and
changes in the surface area alone do not account for the magni-
tude of the enhancement in the specific activity (Fig. S11, ESI
†
).
Fig. 1
Material characterization of typical electrocatalyst samples. (A) SEM image of an IrO
2
catalyst with 1000 ALD TiO
2
cycles. (B) AFM map of IrO
2
with
10 ALD cycles of TiO
2
. (C) HAADF-STEM image of an IrO
2
-based electrocatalyst with 10 ALD cycles of TiO
2
. The underlying crystalline material is IrO
2
while the hair-like material at the surface is TiO
2
. (D and E) Energy dispersive X-ray spectroscopy (EDS) maps of IrO
2
-based electrocatalysts with 10 and
40 ALD cycles of TiO
2
, respectively. The red color indicates Ir and green indicates Ti. Note that green and red intermix throughout this cross section due
to the inherent roughness of the sample.
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Furthermore, while high-angle annular dark-field scanning trans-
mission electron microscopy (HAADF-STEM) images and STEM
electron dispersive X-ray spe
ctroscopy (EDS) maps of IrO
2
samples
with 10 cycles of TiO
2
(Fig. 1C and D) indicate that the TiO
2
film is
semi-continuous with small areas of the underlying IrO
2
exposed,
deposition of 40 cycles of TiO
2
results in a uniform, continuous
film (Fig. 1E) and catalysis commensurate with the bare IrO
2
samples. These facts suggest the
phenomenon does not arise from
surface morphological effects al
one, instead suggesting that TiO
2
is playing a partial role in enhancing the activity of the active sites.
The idea that TiO
2
maybeabletoplayaroleintheactivesiteis
consistent with both experimental and computational literature
which indicates that TiO
2
may hydrate and evolve both chlorine
and oxygen.
3,29–31
The Tafel slopes for all active IrO
2
and RuO
2
based catalysts agree well with previously reported Tafel slopes
(
B
60 mV dec
1
and
B
30 mV dec
1
for the OER and CER
respectively;TablesS5,S6andFig.S11,ESI
†
),
32
consistent with
expectations that addition of TiO
2
does not fundamentally change
the mechanism or the potential determining step for either
reaction. Hypothesized mechanisms generally involve coordi-
nation of either OOH or OCl groups to unsaturated sites on the
metal oxide in the potential determining reaction steps.
33–35
FTO based catalysts exhibited very large overpotentials for both
the CER and OER and had correspondingly high Tafel slopes in
excess of 190 mV dec
1
, potentially indicating a different, much
less efficient mechanism than the process that controls the
reactivity of the more active catalysts.
To investigate the electrocatalyst
s’ surface electronic properties
the potentials of zero charge (
E
ZC
) of the electrocatalysts were
measured as a function of TiO
2
thickness (Fig. 3).
E
ZC
is the
Fig. 2
Specific activities (
j
s
) and overpotentials (
Z
) for the OER and CER on IrO
2
, RuO
2
, and FTO coated at various ALD cycles of TiO
2
. Overpotentials
were measured at 10 mA (cm
geo
)
2
for the OER and at 1 mA (cm
geo
)
2
for the CER (normalized to geometric surface area). Specific activities for the OER
were measured at 350 mV (IrO
2
and RuO
2
) or 900 mV (FTO). Specific activities for the CER were measured at 150 mV (IrO
2
and RuO
2
) or 700 mV (FTO).
The red squares indicate available literature values.
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