of 7
Accelerated
Characterization
of
Electrode-Electrolyte
Equilibration
Kevin
Kan,
[a]
Dan Guevarra,
[a]
Lan Zhou,
[a]
Ryan
J. R. Jones,
[a]
Yungchieh
Lai,
[a]
Matthias
Richter,
[a, b]
and John
M. Gregoire*
[a]
Operational
durability
is poorly
characterized
by traditional
(photo)electrocatalyst
discovery
workflows,
creating
a barrier
to
scale-up
and deployment.
Corrosion
is a prominent
degradation
mechanism
whose
thermodynamics
depend
on the concen-
tration
of corrosion
products
in electrolyte.
We present
an
automated
system
for characterizing
the equilibration
of
(photo)electrodes
with
dissolved
metals
in electrolyte
for a
given
electrode,
pH, and electrochemical
potential.
Automation
of electrode
selection,
electrolyte
preparation,
and electrolyte
aliquoting
enables
rapid
identification
of self-passivating
elec-
trodes
and estimation
of the equilibrium
dissolved
metals
concentrations.
The technique
is demonstrated
for metal
oxide
photoanodes
in alkaline
electrolyte,
where
BiVO
4
is found
to
continually
corrode,
in agreement
the literature.
An amorphous
Ni
Sb
O photoanode
is found
to passivate
with
a Ni-rich
coating
on the order
of 1 monolayer
with less than
1
μ
M total
dissolved
metals
in electrolyte,
demonstrating
its suitability
for
durable
photoelectrochemical
operation.
The automation
and
throughput
of the instrument
are designed
for incorporation
in
accelerated
electrocatalyst
discovery
workflows
so that durabil-
ity can be considered
on equal
footing
with activity.
Introduction
Solar
photoelectrochemical
coupling
of the anodic
oxygen
evolution
reaction
(OER)
with
cathodic
fuel synthesis
is a
nascent
electrochemical
technology
limited
by both
the
durability
and activity
of (photo)electrocatalysts.
[1]
Traditional
approaches
to electrocatalyst
research
relegate
durability
to the
systems-engineering
phase
of technology
development.
Corro-
sion is central
to electrode
durability
and is a molecular-level
process
that is as fundamental
to electrocatalyst
performance
as the targeted
electrochemical
molecular
transformation,
motivating
a new
regime
in electrocatalysis
science
where
corrosion
processes
are studied
on equal
footing
with
catalysis.
[2,3]
Beyond
the underreporting
of corrosion
properties
in the electrocatalysis
literature,
the field is also hampered
by
the challenges
in experimental
characterization
of corrosion
kinetics
and
thermodynamics.
From
the experiment-driven
corrosion
cartography
of Pourbaix,
[4]
to modern
computational
assessment
of Pourbaix
diagrams,
[5–9]
Pourbaix
analysis
of a
solid-state
electrode
considers
the bulk thermodynamic
driving
force
towards
corrosion
(or dissolution)
at a given
pH, applied
bias,
and electrolyte
composition.
The concentration
of corro-
sion
species,
typically
represented
as the dissolved
metals
concentration,
is central
to this thermodynamic
assessment.
Accelerating
experimental
characterization
of Pourbaix
dia-
grams
requires
a platform
with
direct
observation
of the
equilibration
of operating
electrodes
with the dissolved
metals
in electrolyte.
Herein,
we report
the accelerated
durability
screening
system
(ADSS),
which
addresses
this
need
by
measuring
the time evolution
of dissolved
metals
concentration
in a miniature
electrochemical
reactor.
ADSS
is demonstrated
herein
for characterization
of metal
oxide
photoanodes
for solar
fuels applications.
While
the differences
in corrosion
behavior
in polymer
and
liquid
electrolytes
remains
a critical
consideration
in translating
catalyst
discoveries
to commercializable
systems,
[10,11]
discovery
research
for heterogeneous
electrocatalysis
is typically
con-
ducted
in liquid
electrolyte.
Automated
techniques
for measur-
ing and identifying
electrochemical
signatures
of corrosion
excel
at identifying
materials
with
small
corrosion
current,
[12]
but
in situ
characterization
of electrocatalysts
requires
disambig-
uation
of the desired
electrochemical
current
and the undesired
electrode
dissolution.
Online
analytical
characterization
of
electrolytes
has been
well demonstrated
with
electrochemical
flow
cells
by injecting
the electrolyte
into
an inductively
coupled
plasma
with
mass
spectroscopy
(ICP-MS)
or optical
emission
spectroscopy
(ICP-OES)
detection.
[13,14]
Each
technique
measures
corrosion
products
as well as dissolved
or suspended
particles,
for example
from
nanoparticle
detachment.
[15]
Cou-
pling
the flow cell with working
electrode
illumination
enables
rapid
characterization
of photo-driven
corrosion.
[16–19]
These
[a]
K. Kan,
D. Guevarra,
Dr. L. Zhou,
R. J. R. Jones,
Dr. Y. Lai, Dr. M. Richter,
Prof.
J. M. Gregoire
Division
of Engineering
and Applied
Science
and Liquid
Sunlight
Alliance
California
Institute
of Technology
91125
Pasadena,
CA (USA)
E-mail:
gregoire@caltech.edu
Homepage:
https://gregoire.people.caltech.edu/
[b]
Dr. M. Richter
present
address:
DeepXscan
GmbH
01324
Dresden
(Germany)
Supporting
information
for this
article
is available
on the WWW
under
https://doi.org/10.1002/cctc.202301300
This
publication
is part
of a Special
Collection
on “Holistic
Development
of
Electrochemical
Processes
for Industrial
Systems”
© 2023
The Authors.
ChemCatChem
published
by Wiley-VCH
GmbH.
This
is
an open
access
article
under
the terms
of the Creative
Commons
Attribution
License,
which
permits
use,
distribution
and
reproduction
in any
medium,
provided
the original
work
is properly
cited.
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Research
Article
doi.org/10.1002/cctc.202301300
techniques
rapidly
characterize
corrosion
under
continuous
supply
of fresh
electrolyte.
Coupled
with
measurement
of
catalytic
rates,
metrics
such
as the stability-number
[20]
can
quantify
catalyst
activity
vs. corrosion,
which
has aided
comparison
of literature
reports
with
various
electrochemical
cells and procedures.
To meet
the decades-long
operational
time scales
targeted
by industrial
electrolytic
processes,
the stability-number
needs
to above
a million.
[20]
Such
a condition
can be reached
if the
electrode
can equilibrate
with dissolved
metals
species,
wherein
the corrosion
rate equals
the precipitation
rate for each element
in the electrocatalyst.
This opportunity
has been
recognized
in
so-called
self-healing
catalysts,
but these
typically
require
more
than
10
5
M of dissolved
metals
species,
[21,22]
which
is a high
concentration
with respect
to both
the typical
solubility
limit of
corrosion
products
and the levels
of dissolved
metals
concen-
tration
that
may
compromise
device
durability.
[23]
Catalysts
should
therefore
be co-designed
for not only activity
but also
operational
durability,
which
is most
rapidly
assessed
by
observing
equilibration
with a dissolved
metals
concentration.
[2]
The ADSS
platform
uses electrolyte
recirculation
combined
with
robotic
aliquoting
of the working
electrolyte
to rapidly
charac-
terize
electrode
corrosion
and its propensity
to equilibrate
with
dissolved
metals
species
at sub-
μ
M
concentrations.
Results
and
Discussion
The ADSS
platform
(Figure
1a) enables
testing
of flat, compact
electrodes
via an o-ring
seal to a 3-electrode
electrochemical
cell equipped
with a bipolar
membrane
to mitigate
crossover
of
dissolved
metals
species
between
the working
and counter
electrodes
(Figure
1b). While
the working
electrode
chamber
volume
is 0.3 mL with
no headspace,
an additional
13 mL
recirculation
reservoir
with open
headspace
provides
flexibility
in the choice
of electrolyte
volume
and enables
exhaust
of
gasses
such as the O
2
from
the oxygen
evolution
reaction
(OER)
studied
in the present
work.
The electrolyte
volume
and flow
rate can be varied
with
typical
values
being
4 mL of working
electrolyte
recirculated
at 0.2 mL/s,
providing
an electrolyte
cycle
time
of 20 s. Workflow
automation
is implemented
via
HELAO-async
[24]
and includes
automated
exchange
of working
electrode
via translation
stages
and automated
liquid
handling
such
that
a series
of experiments
may
be automatically
Figure
1.
a. A schematic
of the ADSS
instrument.
The electrochemical
cell is sealed
to a selected
location
on the electrode
library,
with electrolyte
handling
and aliquoting
via pumps
and the robotic
liquid
handler.
Aliquots
are stored
in vials within
the sample
tray, where
they undergo
nitric
acid digestion
followed
by manual
transfer
to the ICP-MS
autosampler
(not shown).
b. The flow scheme
for preparing,
operating,
and aliquoting
electrolyte
(not to-scale).
The
electrolyte
syringe
pump
is automatically
filled
from a reservoir
and is used
to infuse
the desired
volume
into the working
electrolyte
recirculation
cell, both at
the beginning
of each experiment
and at programmed
infusion
times
during
electrochemical
operation.
The syringe
insertion
point
for the robotic
solution
handler
is at the center
of the recirculation
cell. c. The schematic
for the electrode-electrolyte
interface
(not to-scale).
The synthesized
A
B
O electrode
is
operated
in contact
with a working
electrode,
leading
to dissolved
concentrations
of A and B. The alterations
to the electrode
that are commensurate
with
the amount
of dissolved
A and B are calculated
as the thickness
of corroded
electrode
as well as the thickness
of a B
O coating
that results
from super-
stoichiometric
dissolution
of A (compared
to the as-synthesized
composition).
Wiley
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06.03.2024
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/ 332707
[S.
265/270]
1
ChemCatChem
2024
,
16
, e202301300
(2 of 7)
© 2023
The Authors.
ChemCatChem
published
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GmbH
Che
mCa
tChem
Research
Article
doi.org/10.1002/cctc.202301300
18673899, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202301300 by California Inst of Technology, Wiley Online Library on [09/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
executed,
as envisioned
by Materials
Acceleration
Platforms.
[25]
Furthermore,
experiments
can be performed
with
scheduled
injection
of electrolyte
via a syringe
pump
and scheduled
aliquoting
of the working
electrolyte
via a syringe.
The robotic
liquid
handling
system
can also process
the
aliquot,
for example
the nitric
acid digestion
used
in the present
work
to dissolve
any metal-containing
precipitates
so that ICP-
MS quantifies
the total
dissolved
metals
concentrations.
While
mechanical
coupling
of the ADSS
system
to the ICP-MS
would
be required
for autonomous
decision
making
based
on
measured
dissolved
metals
concentrations,
for the present
effort
of characterizing
the equilibration
of photoelectrodes
with
the electrolyte,
the tray of liquid
samples
is manually
transferred
to the ICP-MS
autosampler
where
each
aliquot
is
measured
in triplicate.
Corrosion
of
Bismuth
Vanadate
Bismuth
vanadate
(BiVO
4
) is a prominent
photoelectrode
material
due to its efficient
conversion
of photons
(with
energy
higher
than
its 2.4 eV band
gap) to OER photocurrent.
[22,26]
The
photocorrosion
of BiVO
4
in alkaline
electrolyte
has been
well
documented,
[27]
motivating
our selection
of this photoanode
material
in 0.3 M NaOH
electrolyte
(pH 13.6)
for initial
demon-
stration
of ADSS
operation.
ICP-MS
characterization
of the as-
prepared
electrolyte,
using
the same
digestion
method
as for
the aliquots
during
electrochemical
operation,
resulted
in the
following
values
(mean
and standard
deviation
from
3 ICP-MS
injections)
for the elements
characterized
in the present
work:
0.006
0.001
μ
M Bi, 0.033
0.001
μ
M V, 0.005
0.001
μ
M Ni,
0.001
0.0001
μ
M Sb. These
values
are all well below
those
observed
during
electrochemical
operation.
Figure
2 shows
the evaluation
of a sputtered
BiVO
4
working
electrode
with
a SnO
2
:F back
contact.
The electrochemical
measurements
include
1 min open
circuit
potentiometry
(CP)
measurement
before
and after
a 37 min chronoamperometry
(CA)
measurement.
The series
of 9 aliquots
characterize
the
time
evolution
of the dissolved
Bi and
V concentrations.
Figure
1c illustrates
a physical
model
of the alterations
to the
working
electrode,
which
are parameterized
as 2 thickness
values
that we calculate
using
the equations
derived
in the
Supporting
Information.
For each
aliquot,
the model
provides
the equivalent
thickness
of the BiVO
4
electrode
that
has
dissolved
in electrolyte.
Furthermore,
since
the data indicates
a
preferential
dissolution
of V at the beginning
of CA, the
operational
electrode
surface
is coated
with a Bi-rich
film. We
model
this film thickness
using
the molar
density
of Bi
4
O
7
(0.039
mol cm
3
), which
is the stable
species
indicated
by the
computational
Pourbaix
diagram.
[9]
The data in Figure
2 indicate
that
in the first
few
minutes
of operation,
preferential
dissolution
of V leads
to a 1–2 nm coating
of a Bi
4
O
7
-like
material
(Figure
2). Subsequently,
BiVO
4
corrodes
nearly
stoi-
chiometrically
at a rate of ca. 1 nmmin
1
, similar
to the previous
report
for photocorrosion
(1.2 to 1.3 nmmin
1
) for the average
corrosion
over 20 min at 1.23 V vs RHE in pH 12.3 electrolyte.
[27]
We note
that
the dissolved
and
coating
thicknesses
calculated
in the present
work
use the geometric
electrode
area
and bulk
molar
densities.
For a specific
surface
area
ratio
(roughness
factor)
of
X
, the thicknesses
would
be lower
by a
factor
of
X
, which
means
the reported
thicknesses
can be
considered
as upper
limits.
While
the dissolved
metals
concen-
trations
do not depend
on surface
roughness
or the uniformity
of the corrosion
processes,
we note
that
all corrosion
and
passivation
thickness
in the present
work
characterize
the
equivalent
changes
to the electrode
under
the approximation
of uniform
corrosion
across
the electrode
surface.
The red dashed
line in Figure
2 indicates
that at 24 min into
the CA measurement,
3.5 mL of fresh
electrolyte
was injected
to
Figure
2.
The results
from ADSS
characterization
of BiVO
4
in pH 13.6 electro-
lyte at OCP and 1.23 V vs RHE under
385 nm illumination.
The electro-
chemical
and aliquot
data acquired
during
OCP at the beginning
and end of
the experiment
are shown
in orange,
with all CA-associated
data in blue.
The
potential
and current
are measured
by the potentiostat,
the Bi and V
concentrations
are measured
by ICP-MS,
where
for each element
and each
time point,
there
are 3 markers
corresponding
to triplicate
measurement
of
each aliquot.
The markers
are often
overlapping
due to consistency
in the
measurements.
The time evolution
of the dissolved
metals
concentrations
along
with the variation
in the triplicate
measurements
are used
to model
the value
and uncertainty
in the dissolved
BiVO
4
thickness
as well as a Bi
4
O
7
-
like coating,
where
each thickness
value
is an upper
limit due to the
assumption
of a flat electrode.
As illustrated
in the legend,
the thickness
value
corresponding
to each aliquot
is shown
as a vertical
bar representing
the mean
plus and minus
the standard
deviation
of the distribution
of values
derived
from the ICP-MS
data.
The height
of the bar is often
smaller
than the
line width
of the light blue trend
line. The dashed
red line traversing
all
panels
indicates
the time of infusion
of fresh
electrolyte
to double
the
working
electrolyte
volume,
which
instantaneously
lowers
the dissolved
metals
concentrations
and proportionally
lowers
the increase
in concen-
trations
with time for a given
corrosion
rate.
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/ 332707
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1
ChemCatChem
2024
,
16
, e202301300
(3 of 7)
© 2023
The Authors.
ChemCatChem
published
by Wiley-VCH
GmbH
Che
mCa
tChem
Research
Article
doi.org/10.1002/cctc.202301300
18673899, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202301300 by California Inst of Technology, Wiley Online Library on [09/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
double
the working
electrolyte
volume.
This perturbation
is
designed
to evaluate
whether
and
how
the electrode
is
equilibrating
with the electrolyte.
The steady
corrosion
rate of
BiVO
4
indicates
that
the corrosion
kinetics
are not strongly
affected
by the dissolved
metals
concentrations
in the 1–3
μ
M
range
observed
in Figure
2. This observation
is consistent
with
the recent
report
that
BiVO
4
corrosion
in borate-buffered
electrolyte
was suppressed
via addition
of 0.1 M of V
5
+
in
electrolyte,
[22]
i.e. the equilibrium
dissolved
metals
concentra-
tion of V is much
larger
than
the
μ
M-level
concentrations
observed
in the ADSS
experiment.
While
electrode-electrolyte
equilibration
at such
high
dissolved
concentrations
can in
principle
be characterized
by the ADSS
technique,
the intention
of the technique
is to demonstrate
equilibration
at low
dissolved
metals
concentration.
Passivation
of
Nickel
Antimonate
We recently
reported
an amorphous
Ni
0.5
Sb
0.5
O
y
electrode
with
excellent
operational
durability
in pH 10 electrolyte.
[28]
Starting
with
an electrolyte
volume
of 3 mL, Figure
3 shows
the
application
of the Figure
2 ADSS
protocol
to evaluate
the
durability
of NiSbO
y
in pH 13.6 electrolyte.
The data
indicates
dissolution
of ca. 0.1 nm of film within
the first few minutes
of
operation,
after which
no further
stoichiometric
corrosion
of the
film is observed
during
the subsequent
20 min of operation.
Upon
the doubling
of the electrolyte,
the amount
of dissolved
NiSbO
y
also doubles
over the next 10 min. These
data strongly
suggest
that the electrode
is approximately
at equilibrium
with
electrolyte
containing
ca. 15 nM of both Ni and Sb.
The super-stoichiometric
corrosion
of Sb in this first experi-
ment
on the NiSbO
y
electrode
indicates
a Ni-rich
coating,
which
we model
as a NiOOH
(0.044
mol cm
3
) film based
on the
established
identification
of this phase
of Ni under
alkaline
OER
conditions.
[29]
While
the coating
thickness
is less than
1 nm for
the initial
experiment,
the concomitant
formation
of this
Figure
3.
The results
from ADSS
characterization
of 3 NiSbO
y
electrodes
in pH 13.6 electrolyte
at OCP and 1.23 V vs RHE under
385 nm illumination.
The first
column
corresponds
to the analogous
ADSS
experiment
of Figure
2 with potential
and current
measured
by the potentiostat,
dissolved
metals
concentrations
measured
in triplicate
for each aliquot
by ICP-MS
(there
are 3 data points
for each concentration
but they are often
strongly
overlapped),
and the resulting
thickness
under
a flat-electrode
assumption
for the dissolved
NiSbO
y
film as well as a NiOOH-like
coating
(see legends
and labels
in Figure
2). The red dashed
line corresponds
to the electrolyte-doubling
injection,
which
resulted
in an approximate
doubling
of the dissolved
film thickness.
Combined
with the
subsequent
stabilization
of the dissolved
metals
signals,
these
data indicate
a quasi-equilibrium
of the electrolyte
with the NiSbO
y
electrode
coated
with less
than 1 nm of an NiOOH-like
material.
To evaluate
whether
this type of coating
is required
for equilibration
with the electrolyte,
the light orange
arrows
in
between
each column
of panels
indicate
the transfer
of working
electrolyte
from the end of the ADSS
experiment
on one electrode
to the subsequent
experiment
on a fresh,
as-synthesized
NiSbO
y
electrode.
This process
is repeated
twice,
resulting
in the second
and third
columns.
The sub-0.1
nm dissolution
thickness
and ca. 1 nm NiOOH-like
coating
in these
latter
2 experiments
reveal
the importance
of the NiOOH-like
coating
in establishing
a quasi-equilibrium
with the electrolyte
under
these
photoelectrochemical
conditions.
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2024
,
16
, e202301300
(4 of 7)
© 2023
The Authors.
ChemCatChem
published
by Wiley-VCH
GmbH
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mCa
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Research
Article
doi.org/10.1002/cctc.202301300
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passivation
layer
and the near-equilibrium
dissolved
metals
concentrations
in electrolyte
precludes
us from
inferring
whether
this coating
would
form
even
if the electrolyte
contained
the dissolved
metals
prior
to first contact
with
the
electrode.
To probe
the behavior
of the electrode
with pre-dissolved
metal
in electrolyte,
we performed
additional
experiments
wherein
the electrolyte
is reused
with
a fresh,
nominally
duplicate
electrode.
For each
of the 2 additional
experiments
shown
in Figure
3, an as-synthesized
Ni
0.5
Sb
0.5
O
y
electrode
was
used,
while
the electrolyte
from
the end
of the previous
experiment
was used
as the initial
electrolyte.
In each
of these
measurements,
since
the starting
electrolyte
contains
dissolved
metals
near
the equilibrium
value,
less than
0.1 nm of film is
dissolved,
while
ca. 1 nm of NiOOH-like
coating
is consistently
formed,
indicating
that this coating
acts as a passivation
layer
to suppress
further
Sb corrosion.
Regarding
the persistent
corrosion
of a small
amount
of NiSbO
y
from each as-synthesized
electrode,
we offer
two possible
explanations.
The surface
layer
of the as-synthesized
electrode
may
be more
susceptible
to
corrosion
than
the underlying
film. Alternatively,
since
a thin
NiOOH-like
passivation
layer
is required
to passivate
Sb
corrosion,
some
Sb must
dissolve
from
the surface,
which
creates
undercoordinated
Ni atoms,
some
of which
are prone
to
dissolution
before
they can re-organize
into a passivation
layer.
Disambiguating
these
transient
and dynamic
processes
are
beyond
the scope
of the initial
ADSS-based
assessment,
whose
focus
is on accelerated
characterization
of electrode-electrolyte
equilibration.
In our previous
electron
microscopy
characterization
of
NiSbO
y
films
prepared
by the same
method
as those
used
herein,
we observed
an apparent
roughness
factor
of more
than
10,
[28]
suggesting
that the passivation
layer
thickness
for the
electrodes
in Figure
3 is on the order
of 0.1 nm,
which
approximately
corresponds
to a monolayer
of NiOOH
coating
the NiSbO
y
semiconductor.
Given
the intrinsic
catalytic
activity
of NiOOH,
this
in situ
formation
of a thin protective
and catalytic
coating
on a visible-band
gap
light
absorber
is an ideal
architecture
for a photoelectrode.
These
results
strengthen
the
promise
of NiSbO
y
as a photoelectrode
that can achieve
long-
term
operational
durability,
motivating
continued
work
to
improve
the carrier
transport,
absorptivity,
and
radiative
efficiency
of this amorphous
semiconductor.
Conclusions
We report
the accelerated
durability
screening
systems
(ADSS)
for characterization
of corrosion
and
self-passivation
of
(photo)electrodes.
The
automation
of the ADSS
enables
durability-focused
down-selection
in accelerated
electrocatalyst
discovery
workflows.
We demonstrate
that within
an hour
of
experimentation
we can differentiate
between
non-passivating
(BiVO
4
) and passivating
(NiSbO
y
) electrodes
while
also determin-
ing the approximate
equilibrium
dissolved
metals
concentra-
tions
for passivating
electrodes,
which
informs
their
suitability
for device
implementation.
Electrolyte
recirculation
is central
to
this accelerated
screening
technique
and motivates
electrolyte
characterization
via a handful
of scheduled
aliquots,
as opposed
to quasi-real-time
monitoring.
The additional
capability
of
scheduling
electrolyte
infusions
is critical
to validating
whether
the system
is approaching
thermodynamic
equilibrium,
and we
envision
that this technique
can be expanded
to observe
the
response
of an electrode
to a wide
variety
of electrolyte
or
electrode
perturbations.
Finally,
we highlight
the value
of ADSS
in answering
a pervasive
question
in electrocatalysis
research:
“Do we measure
on what
we synthesized?”
[30]
By using
the
dissolved
metals
concentrations
to determine
the effective
thickness
of the dissolved
material
and the passivation
layer,
the ADSS
experiment
provides
a model
for the operational
surface
of the electrode
without
the need
for
operando
surface
characterization.
The measurement
of equilibrium
dissolved
metals
concentrations
at a given
electrochemical
condition
also
enables
unprecedented
experimental
mapping
of Pourbaix
diagrams
for multi-element
electrodes.
Experimental
Electrode
Synthesis
and
Characterization
Thin
film electrodes
were
prepared
using
ratio-frequency
(RF)
magnetron
co-sputtering
of metal
targets
onto
100 mm-diameter
soda
lime
glass
(with
SnO
2
:F conducting
layer)
substrate
in a
custom-designed
combinatorial
sputtering
system
(Kurt
J. Lesker,
CMS24)
described
in detail
previously.
[31]
The deposition
proceeded
in a mixed
O
2
(0.9 mTorr)
and Ar (5.1 mTorr)
gas with 10
8
Torr base
pressure
with
2 in. magnetron
sources
containing
metal
targets
(ACI Alloys)
operated
at powers
of 11, 142, 100, and 35 W for Bi, V,
Ni, and Sb, respectively.
The thin film libraries
were
annealed
at
610
°
C in air.
Identification
of the appropriate
samples
on the sputter
composi-
tion library
thin films
proceeded
by x-ray
fluorescence
(XRF,
EDAX
Orbis)
and x-ray
diffraction
(XRD,
Bruker
DISCOVER
D8 diffractom-
eter with Cu K
α
radiation
from
a I
μ
S source)
characterization.
The
metal
characteristic
peak
intensities
were
extracted
from
the Orbis
software
and
converted
to metal
contents
and
normalized
compositions
using
the sensitivity
factor
for each
element
cali-
brated
by commercial
thin
film
standards
with
ca. 10 at.%
uncertainty.
The oxygen
stoichiometry
was not measured
in the
sputtered
composition
libraries.
The XRD acquisition
was performed
with a 0.3 mm collimator,
and patterns
were
indexed
using
entries
in the Powder
Diffraction
File from the International
Crystallography
Diffraction
Database
(ICDD)
[32]
via the Bruker
EVA software.
The
identifications
for the phases
of tested
materials
are shown
in
Supporting
Information.
Electrolyte
and
Electrochemistry
The custom
electrochemical
cell was machined
from
polyether
ether
ketone
with
o-ring
seals
(Viton,
McMaster
Carr).
The o-ring
seal to the working
electrode
provided
ca. 0.317
cm
2
of electrode-
electrolyte
contact.
OCP
and CA measurements
were
performed
using
a 3-electrode
Gamry
1010
potentiostat
with a miniature
Ag/
AgCl
reference
electrode
(LF2,
Innovative
Instruments).
Electrolyte
was prepared
by dissolving
0.3 M NaOH
Macron
ACS pellets
in
deionized
water
(Millipore
Advantage
A10)
The pH of the electro-
lyte was measured
to be 13.6. The electrolyte
reservoir
was coupled
to a 50 mL syringe
pump
(KD Scientific)
with
a second
syringe
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VCH
Mittwoch,
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/ 332707
[S.
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1
ChemCatChem
2024
,
16
, e202301300
(5 of 7)
© 2023
The Authors.
ChemCatChem
published
by Wiley-VCH
GmbH
Che
mCa
tChem
Research
Article
doi.org/10.1002/cctc.202301300
18673899, 2024, 6, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202301300 by California Inst of Technology, Wiley Online Library on [09/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
pump
containing
deionized
water
for system
cleaning
between
measurements.
Upon
syringe
pump
infusion
of electrolyte
into the
working
electrode
chamber,
the working
electrolyte
was continually
bubbled
with 1 atm O
2
gas. Recirculation
through
Puri-flex
tubing
with
an in-line
Masterflex
CL peristaltic
pump
was performed
on
both
the working
electrolyte
and counter
electrolyte.
The counter
electrolyte
was also 0.3 M NaOH
with a Pt wire counter
electrode
and was
separated
from
the working
electrode
by a bipolar
membrane
(FumaSEP).
The Pt concentration
in the as-prepared
electrolyte
was measured
by 3 ICP-MS
injections
to be 0.0022
0.0001
μ
M, and
the corresponding
value
for the 144 aliquots
acquired
during
electrochemical
operation
is 0.0018
0.0001
μ
M,
demonstrating
that the Pt concentration
is consistently
low and
that no crossover
of Pt from
the counter
electrode
was observed.
During
OCP and CA measurements,
the electrode
was illuminated
with 385 nm light
from
a fiber-coupled
light
emitting
diode
(Thor
Labs).
The illumination
profile
was dictated
by the natural
profile
from
the termination
of the optical
fiber
within
the working
electrode
compartment
approximately
0.4 cm from
the working
electrode
surface.
Without
electrolyte
present,
the total illumination
through
the working
electrode
port was measured
to be ca. 19 mW
(Newport
Model
1918-R
Power
meter
with a Model
818-UV
sensor).
Aliquots
of 0.1 mL volume
were
extracted
from
the recirculation
reservoir
via a robotic
solution
handling
system
(Thermo
Scientific)
operated
via the PAL Sample
Control
software.
Electrode
selection
and mounting
use a 3-axis
translation
stage
(Dover
Motion)
with a
Galil controller.
Computer
control
of all components
proceeded
via
custom
software
developed
in the HELAO-async
[24]
framework.
Electrolyte
Characterization
Each
0.1 mL electrolyte
underwent
nitric
acid digestion
by 0.9 mL
of 4% nitric
acid.
The resulting
1.0 mL vial for each
aliquot
was
loaded
into the autosampling
ICP-MS
(Thermo
Fisher
Scientific
iCAP™
RQ) where
3 measurements
were
performed
per vial to
determine
the concentration
of dissolved
metals.
The instrument
was calibrated
using
calibration
solutions
wherein
0.3 M NaOH
was
diluted
1:10 with
4% nitric
acid
and injected
with
elemental
standards
comprising
0.5, 1, 5, and 10 ppb for each of the following
elements:
Ag, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Tl, V, Zn, Sb, Pt.
Supporting
Information
A flowchart
for ADSS
experiments;
the model
for determining
the thickness
of the corrosion
and coating
layers,
as well as the
associated
uncertainties;
and tables
of all ICP-MS
data,
as well
as their analysis,
are included
as Supporting
Information.
Acknowledgements
This material
is primarily
supported
by the U.S. Department
of
Energy,
Office
of Science,
Office
of Basic
Energy
Sciences,
under
Award
DE-SC0023139.
Fabrication
of the ADSS
instrument
was
supported
by the Liquid
Sunlight
Alliance,
which
is supported
by the U.S. Department
of Energy,
Office
of Science,
Office
of
Basic
Energy
Sciences,
Fuels
from
Sunlight
Hub under
Award
DE-SC0021266.
Conflict
of Interests
J. M. G. is a consultant
for companies
that aim to accelerate
catalyst
discovery.
Data
Availability
Statement
The data
that support
the findings
of this study
are openly
available
in CaltechData
at https://doi.org/10.22002/1n669-
dte30,
reference
number
1n669-dte30.
This
data
package
contains
data files and the Python
code
for processing
ICP-MS
data into the modelled
thicknesses,
including
the rendering
of
figures,
and
is also
available
at https://github.com/High-
Throughput-Experimentation/ADSS-SI.
Computer
drawings
of
the ADSS
instrument
are available
at https://data.caltech.edu/
records/m6btr-szr71
(DOI:
10.22002/m6btr-szr71).
Keywords:
corrosion
·
solar
fuels
·
electrocatalysis
·
oxygen
evolution
reaction
·
durability
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Manuscript
received:
October
17, 2023
Revised
manuscript
received:
November
21, 2023
Accepted
manuscript
online:
November
22, 2023
Version
of record
online:
December
14, 2023
Wiley
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Mittwoch,
06.03.2024
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/ 332707
[S.
270/270]
1
ChemCatChem
2024
,
16
, e202301300
(7 of 7)
© 2023
The Authors.
ChemCatChem
published
by Wiley-VCH
GmbH
Che
mCa
tChem
Research
Article
doi.org/10.1002/cctc.202301300
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