ChemCatChem
Supporting
Information
Accelerated
Characterization
of Electrode-Electrolyte
Equilibration
Kevin
Kan,
Dan
Guevarra,
Lan
Zhou,
Ryan
J. R. Jones,
Yungchieh
Lai,
Matthias
Richter,
and
John
M. Gregoire*
Wiley
VCH
Mittwoch,
06.03.2024
2406
/ 332707
[S.
271/271]
1
1 ADSS flow chart
Fig. 1 represents the sequence of processes in an ADSS experiment as a flow chart. For the Bi-V system,
Processes 1 and Y were executed, resulting in Conclusion YY. For the Ni-Sb system, Processes 1, N,
and NY were performed, resulting in Conclusion NYNY. Then, Process NY was repeated to ensure
reproducibility.
Figure 1: To characterize the nature of the corrosion, passivation, and/or equilibration of the electrode
and electrolyte, evaluation of a given electrode proceeds as a series of processes that each involve
measuring the dissolved metals concentrations as a function of time during electrochemical operation.
After the initial process, subsequent processes can involve changes to the electrolyte or changes to the
electrode, with decisions dictated by analysis of prior results. The high-level decisions and resulting
conclusions for the present operation of the ADSS are illustrated as flow chart. We envision that this
decision tree will evolve to include additional types of processes and analyses.
2
2 Tracking total dissolved metals
From an initial electrolyte volume
V
0
, aliquots of volume
δV
(assumed to be a consistent volume)
are taken at times
t
i
(
i
= 1,2,..,N), where ICP-MS analysis of each aliquot reveals that the metals
concentration at each time was
M
A
;
i
for element
A
and
M
B
;
i
for element
B
. To establish the total
dissolved moles of each element at each time point, we can consider the moles of metal removed and
left behind by each aliquot, which we show here for element
A
with the analogous expressions holding
for element
B
. The volume of electrolyte remaining after aliquot
i
is
V
i
=
V
0
−
iδV
, if no injection
=
V
0
−
iδV
+
V
inject
;
i
, if injection
,
where
V
inject
;
i
is the volume of fresh electrolyte at a time between
t
i
and
t
i
−
1
, which is assumed to be
well mixed such that each aliquot samples a uniform electrolyte.
The dissolved
A
at each aliquot
i
is split between the aliquoted portion,
δV M
A
;
i
, and the portion
remaining in electrolyte,
V
i
M
A
;
i
. The total amount of dissolved
A
up to time
t
i
includes contributions
from the aliquoted and remaining electrolyte associated with aliquot
i
, the aliquoted portion of all
prior aliquots, and a negative term corresponding to the amount of
A
in electrolyte prior to contacting
the electrode:
m
A
;
i
= (
V
i
+
δV
)
M
A
;
i
+
δV
X
j
=1
,..,i
−
1
M
j
−
V
0
M
A
;0
,
(1)
where
M
A
;0
is the concentration of A in the starting electrolyte volume
V
0
.
Each aliquot can be characterized by ICP-MS multiple times, and for the present work each aliquot
was sampled 3 times. Instead of inferring an uncertainty in each metals concentration from these
repeated measurements, we establish uncertainty in the calculated
m
A
;
i
(and
m
B
;
i
) by evaluating Eq.
1 for every combination of the ICP-MS values, which for the
i
th
aliquot corresponds to 3
i
values if the
starting electrolyte has no dissovled metals, or 3
i
+1
values if there are 3 measured values for
M
A
;0
.
Taking the mean and standard deviation of this set of values for the
i
th
aliquot provides the value and
uncertainty for
m
A
;
i
, with analogous calculations providing values for
m
B
;
i
.
3 Model for electrode surface evolution
For a metal oxide electrode with 2 cation elements
A
and
B
, the as-prepared electrode has composition
A
x
S
B
1
−
x
S
O
z
, where
x
S
is the pseudo-binary composition parameter for the solid electrode. Consider
the parameterization of the ADSS experiment wherein the electrode of area
α
is analyzed to determine
the total amount of prior dissolved
A
and
B
at each time point
i
. The composition of
A
(compared
to
B
) of the dissolution to-date (the analogy of
x
S
for the liquid instead of the solid phase) is
x
i
=
m
A
;
i
m
A
;
i
+
m
B
;
i
.
Without loss of generality, we choose the identity of
A
such that the liquid phase is enriched with
A
compared to the solid phase, i.e.
x
i
> x
S
. This assumption implies that the solid phase is enriched
with
B
compared to the initial state of the electrode. To gain an intuition for the consequences of
the dissolution up to the time point
t
i
, we can assume a quintessential corrosion passivation structure,
wherein a thickness
τ
dissolved
of the as-prepared electrode dissolves into electrolyte and the remaining
portion of the as-prepared electrode is coated by a layer of a
B
-containing material of thickness
τ
coating
.
To model the evolution in the electrode structure, we can use the time-dependent measurements of
m
A
;
i
and
m
B
;
i
to calculate
τ
dissolved
;
i
and
τ
coating
;
i
given the molar density of the as-prepared electrode
(
ρ
S
) and of the passivation layer (
ρ
B
). The corresponding unit is moles of metal (or cation) atoms
per unit volume, which requires renormalization of molar densities calculated on a formula unit or
total-atom basis.
The value of
τ
dissolved
;
i
corresponds to the amount of electrode that dissolved stoichiometrically
with respect to the compositions of
A
and
B
. Since we assume that
A
is the element that dissolves
super-stoichiometrically,
τ
dissolved
;
i
can be determined by
m
B
;
i
, i.e. the amount of the as-prepared
electrode that must have dissolved to give rise to
m
B
;
i
. Equating the moles of
B
in the
τ
dissolved
;
i
3
portion of the electrode and in the electrolyte gives
τ
dissolved
;
i
=
m
B
;
i
(1
−
x
S
)
ρ
S
α
.
(2)
To determine
τ
coating
;
i
we consider the difference between the moles of
A
that are observed to be
dissolved and the moles of
A
that are accounted by the dissolution of
τ
dissolved
;
i
of the as-prepared
electrode:
δ
A
;
i
=
m
A
;
i
−
x
S
ρ
S
ατ
dissolved
;
i
.
Substituting the prior equation gives
δ
A
;
i
=
m
A
;
i
−
x
S
1
−
x
S
m
B
;
i
.
Based on the as-prepared composition, the moles of
B
left behind by the dissolution of
δ
A
;
i
moles of
A
is
δ
B
;
i
=
δ
A
;
i
1
−
x
S
x
S
=
x
S
1
−
x
S
m
A
;
i
−
m
B
;
i
.
The formation of this excess solid-state
B
into a phase of molar density
ρ
B
gives
τ
coating
;
i
=
δ
B
ρ
B
α
=
1
ρ
B
α
x
S
1
−
x
S
m
A
;
i
−
m
B
;
i
.
(3)
The uncertainty in each inferred thickness is calculated using the ICP-MS-based sampling uncer-
tainty for
m
A
;
i
and
m
B
;
i
, as described in the previous section, and propagating the uncertainty through
Eqs. 2 and 3.
4 Electrochemical measurements
The working electrode potential (
E
we
) was measured with respect to a Ag/AgCl LF2 reference electrode
from Innovative Instruments. The values were converted to the reversible hydrogen electrode scale
(
E
rhe
) as follows:
E
rhe
=
E
we
+ 0
.
210
V
+
offset
+ 0
.
0591
V
∗
pH,
where
pH
is the measured electrolyte pH, 13.6; 0.210 V is the standard reference offset for a 3.0 M
KCl Ag/AgCl reference electrode; and
offset
is the measured offset between the LF2 and a standard
3.0 M KCl Ag/AgCl reference electrode (BASi Research MF-2052), measured prior to each experiment
using a digital voltmeter. The values of
offset
for the Bi-V and the Ni-Sb experiments were 0.02 V
and 0.001 V, respectively.
5 XRD and data tables
The XRD patterns for each photoanode are shown in Fig. 2. The above model requires the molar
densities of the photoanode phase and its presumed passivaiton layer. For BiVO
4
and it’s computa-
tional Pourbaix-identified passivation layer, Bi
4
O
7
, the molar densities from ICDD entries are 0.042
and 0.039 mol cm
−
3
, respectively. Per the structural characterization of the amorphous NiSbO
y
film
(Ref. 27), we approximate the molar density of this film to be the average of the molar densities of
NiO and Sb
2
O
5
, yielding the value 0.056 mol cm
−
3
. The value for the NiOOH coating is 0.044 mol
cm
−
3
.
4
Figure 2: The XRD patterns with ICDD stick patterns for representative electrodes. The conducting
SnO
2
:F layer (FTO) is apparent in both samples. The BiVO
4
sample exhibits crystalization into
the expected monoclinic phase. The NiSbO
y
sample exhibits little contribution from the photoanode
film, in agreement with our previous XRD and electron diffraction study (Ref. 27) revealing that
the sputter synthesis and annealing condition used herein produces a largely amorphous film with
nanoscale precipitates of rutile NiSb
2
O
6
.
Table 1: All ICP-MS data and values derived from them for the BiVO
4
experiment in main paper Fig.
2.
t
i
V
i
M
A
M
B
τ
dissolved
τ
coating
(min)
(mL)
ICP-MS V conc. (
μ
M)
ICP-MS Bi conc. (
μ
M)
m
A
(nM)
m
B
(nM)
(nm)
(nm)
0.00
4.0
3.08
3.9
0.42
0.42
0.43
0.24
0.25
0.24
1.7
±
0.0
1.0
±
0.0
1.46
±
0.01
0.59
±
0.01
5.52
3.8
0.83
0.83
0.85
0.45
0.46
0.46
5.0
±
0.0
2.8
±
0.0
4.14
±
0.03
1.79
±
0.04
14.50
3.7
2.01
2.01
1.99
1.82
1.79
1.79
10.9
±
0.1
8.6
±
0.1
12.99
±
0.09
1.84
±
0.07
23.50
3.6
2.34
2.40
2.36
2.13
2.12
2.12
16.5
±
0.1
14.8
±
0.1
22.20
±
0.09
1.39
±
0.09
26.51
3.5
2.82
2.73
2.74
2.56
2.56
2.57
19.0
±
0.2
17.3
±
0.0
26.06
±
0.05
1.37
±
0.14
27.45
7.0
30.49
6.9
1.62
1.62
1.67
1.56
1.58
1.58
22.0
±
0.2
20.7
±
0.1
31.12
±
0.09
1.02
±
0.18
38.52
6.8
1.92
1.90
1.93
1.76
1.75
1.77
25.5
±
0.2
23.9
±
0.1
35.85
±
0.11
1.34
±
0.15
41.51
6.7
1.89
1.88
1.85
1.75
1.72
1.75
27.0
±
0.1
24.8
±
0.1
37.29
±
0.17
1.74
±
0.15
43.89
6.6
2.22
2.16
2.15
2.01
2.00
2.00
28.5
±
0.2
26.3
±
0.1
39.51
±
0.15
1.82
±
0.22
Table 2: All ICP-MS data and values derived from them for the NiSbO
y
experiment in main paper
Fig. 3. The sample column is an internal identifier for the electrode sample. the 3 panels in main
paper Fig. 3 correspond to the time series for the 3 samples noted in this table.
t
i
V
i
M
A
M
B
τ
dissolved
τ
coating
sample
(min)
(mL)
ICP-MS Sb conc. (
μ
M)
ICP-MS Ni conc. (
μ
M)
m
A
(nM)
m
B
(nM)
(nm)
(nm)
23605
0.00
3.0
23605
6.61
2.9
0.044
0.045
0.043
0.013
0.014
0.011
0.132
±
0.003
0.038
±
0.005
0.043
±
0.005
0.067
±
0.004
23605
9.02
2.8
0.070
0.071
0.071
0.013
0.010
0.014
0.337
±
0.003
0.074
±
0.007
0.083
±
0.007
0.189
±
0.005
23605
18.04
2.7
0.121
0.122
0.120
0.016
0.015
0.012
0.548
±
0.003
0.077
±
0.007
0.086
±
0.008
0.338
±
0.005
23605
27.03
2.6
0.167
0.166
0.165
0.015
0.013
0.013
0.798
±
0.003
0.079
±
0.006
0.088
±
0.006
0.516
±
0.005
23605
30.04
2.5
0.167
0.168
0.164
0.012
0.014
0.012
0.905
±
0.005
0.074
±
0.004
0.083
±
0.004
0.596
±
0.004
23605
30.98
5.0
23605
34.05
4.9
0.109
0.108
0.110
0.015
0.017
0.014
1.019
±
0.006
0.117
±
0.007
0.131
±
0.008
0.647
±
0.007
23605
42.04
4.8
0.111
0.110
0.111
0.018
0.016
0.015
1.146
±
0.004
0.164
±
0.008
0.184
±
0.009
0.704
±
0.007
23605
45.05
4.7
0.114
0.118
0.116
0.017
0.014
0.013
1.169
±
0.007
0.158
±
0.009
0.177
±
0.010
0.724
±
0.008
23605
47.44
4.6
0.123
0.123
0.123
0.017
0.017
0.015
1.214
±
0.007
0.156
±
0.009
0.175
±
0.010
0.758
±
0.008
20522
2.28
4.5
0.171
0.174
0.171
0.014
0.015
0.015
0.228
±
0.007
-0.006
±
0.006
-0.007
±
0.006
0.167
±
0.006
20522
4.70
4.4
0.165
0.167
0.169
0.012
0.013
0.013
0.980
±
0.011
0.052
±
0.006
0.058
±
0.006
0.665
±
0.009
20522
13.68
4.3
0.213
0.214
0.212
0.016
0.016
0.014
1.141
±
0.009
0.052
±
0.007
0.059
±
0.008
0.781
±
0.008
20522
22.69
4.2
0.215
0.215
0.217
0.015
0.016
0.016
1.332
±
0.006
0.064
±
0.007
0.072
±
0.008
0.909
±
0.007
20522
25.70
4.1
0.232
0.233
0.233
0.018
0.014
0.014
1.392
±
0.005
0.063
±
0.010
0.071
±
0.011
0.953
±
0.008
20522
34.42
4
0.264
0.263
0.259
0.015
0.015
0.012
1.561
±
0.010
0.054
±
0.011
0.060
±
0.013
1.081
±
0.011
7617
2.54
3.9
0.280
0.284
0.286
0.017
0.016
0.019
0.086
±
0.013
0.013
±
0.008
0.015
±
0.009
0.052
±
0.011
7617
4.98
3.8
0.304
0.305
0.311
0.014
0.014
0.015
1.281
±
0.017
0.070
±
0.008
0.079
±
0.009
0.868
±
0.014
7617
13.99
3.7
0.369
0.371
0.368
0.019
0.020
0.018
1.580
±
0.015
0.075
±
0.007
0.084
±
0.008
1.079
±
0.012
7617
22.98
3.6
0.407
0.408
0.404
0.025
0.017
0.020
1.918
±
0.012
0.095
±
0.014
0.106
±
0.015
1.308
±
0.013
7617
25.95
3.5
0.386
0.379
0.385
0.015
0.017
0.019
1.933
±
0.016
0.086
±
0.015
0.096
±
0.017
1.324
±
0.016
7617
28.31
3.4
0.387
0.388
0.382
0.019
0.018
0.016
1.820
±
0.017
0.076
±
0.010
0.085
±
0.011
1.251
±
0.014
5