of 21
Supporting Information
Effects
of
Bubbles on the
Electrochemical Behavior of
Hydrogen-Evolving Si
Microwire Arrays Oriented
Against
Gravity
Paul A. Kempler
1
, Robert H. Coridan
2
, Nathan S. Lewis
1,3*
1
Division
of
Chemistry
and
Chemical
Engineering,
127-72,
210
Noyes
Laboratory,
California
Institute of
Technology, Pasadena, CA 91125
2
Department
of
Chemistry
and
Biochemistry,
University
of
Arkansas,
Fayetteville,
Arkansas
72701
3
Beckman Institute, California Institute
of Technology, Pasadena, CA 91125
*Corresponding Author: nslewis@caltech.edu
S1
Electronic
Supplementary
Material
(ESI)
for
Energy
&
Environmental
Science.
This
journal
is
©
The
Royal
Society
of
Chemistry
2020
Table
of Contents
Extended experimental methods
Figure S1
– Microwire array dimensions
Figure S2
– Potential
versus
time
behavior
as a
function of
α
Figure S3
– Comparisons of fractional
gas coverage
Figure S4
– Potential
versus
time
data for electrodes
at |
| = 100 – 200 mA cm
-2
2
Figure S5
– Relationship
between gas coverage
and
current
density
Figure S6
– Image comparison of bubble
coverage
at |
| = 10 – 50 mA cm
-2
2
Figure S7
– Image comparison of bubble
coverage
at |
| = 100 – 200 mA cm
-2
2
Figure S8
– Weighted mean
bubble
diameters
Figure S9
– Number density of bubbles vs. time
Figure S10
– Stability of bubbling behavior at |
| = 30 mA cm
-2
2
Figure S11
– Extended electrochemical stability testing
Figure S12
- High speed image sequence
of bubble nucleation at a μW
6|28
electrode
Figure S13
– Map of
departure diameters for a μW
6|14 electrode
and μW 6|28 electrode
Figure S14
– Bubble growth curves measured from high speed microscope videos
Figure S15
– Cyclic voltammogram of Fe
3+
redox couple in 0.50 M H
2
SO
4
(aq)
Figure S16
– Image of
indicator
solutions for Fe
2+
quantification
Figure S17
– Representative
UV-Vis spectra used to quantify
Fe
2+
Figure S18
– Microelectrode measurements
of Fe
2+
and Fe
3+
in
0.50 M
H
2
SO
4
(aq)
Figure S19
– Current vs. time behavior
for Fe
3+
in stagnant 0.50 M H
2
SO
4
(aq)
Figure S20
– Electrochemical impedance spectra for n
+
-Si/Ti/Pt cathodes
Figure S21
– Potential
versus
time
data for n
+
-Si/Ti/Pt cathodes at
α
= 15° and
α
= 180°
Figure S22
– High speed image sequence of
bubble spreading at a
μW 6|28 electrode
Figure S23
– Cell diagram for hydrogen
evolution
testing
at controlled
α.
Movie S1
- Comparison of
gas evolution at
α
=
15°
and |
| = 10 – 50 mA cm
-2
2
Movie S2
– Comparison
of
gas
evolution at
α
=
15°
and |
| = 100 – 200 mA cm
-2
2
S2
Extended experimental
methods
Materials:
All
chemicals
were
commercially
available
and
used
as
received.
Fe(II)
sulfate
heptahydrate
(ACS
Reagent,
>99%),
Fe(III)
sulfate
hydrate
(97%)
and
1,10-phenanthroline
(>99%)
were
obtained
from
Sigma-Aldrich,
gallium-indium
eutectic
(GaIn,
99.99%,
metals
basis)
was
obtained
from
Alfa
Aesar,
and
concentrated
ammonium
hydroxide
(NH
4
OH,
28%-30%)
was
obtained
from
JT
Baker.
Hydrochloric
acid
(HCl,
ACS
grade
36.5-38%),
acetone,
and
isopropyl
alcohol
were
obtained
from
Millipore.
Buffered
oxide
etchant
(6:1
(v/v)
40%
NH
4
F
to 49%
HF)
was
obtained
from
Transene
Inc
and
hydrogen
peroxide
(H
2
O
2
,
ACS
grade
30%)
was
obtained
from
Macron
Chemicals.
Hydroxylamine
sulfate
(>98%)
was
obtained
from
TCI
America.
Sulfuric
acid
(H
2
SO
4
,
TraceMetal
grade)
was
obtained
from
Fisher
Scientific
and
diluted
to
0.50
M
with
water
having
a resistivity
of
18.2
MΩ·cm,
obtained
from
a Millipore
deionized
(DI)
water
system.
N-type
Si wafers
with
a resistivity
< 0.005
Ω-cm
and
diameters
of
100
mm,
thicknesses
of
525
μm,
and
<100>
orientation,
were
obtained
from
Addison
Engineering.
Nafion
TM
117
was
obtained from
Fuel Cell
Store.
Preparation
of
μW
substrates:
Silicon
wafers
were
cleaned
with
acetone
and
isopropyl
alcohol,
exposed
to
a hexamethyldisilazane
primer,
and
spin-coated
with
S1813
photoresist
(Shipley)
at
4000
rpm.
The
resist
layer
was
photolithographically
patterned
via
UV-exposure
through
a chrome
mask
which
had
a square
grid
of
circular
holes
3 or
6 μm
in
diameter
and
11,
14,
or
28
μm
in
spacing.
Al
2
O
3
etch
masks,
125
nm
in
thickness,
were
evaporated
into
the
exposed
hole
array
via
electron
beam
evaporation
at
1 Å
s
-1
.
Substrates
were
cleaved,
mounted
onto
a 6” diameter
Si
carrier
wafer
with
thermally
conductive
oil,
and
loaded
into
an
Oxford
Instruments
Dielectric
System
100
ICP-RIE.
Microwires
were
formed
via
deep
reactive-ion
etching,
DRIE,
of
Si
at -120-
S3
130°C
in
a SF
6
/O
2
plasma
at
a capacitively
coupled
power
of 3-5
W
and
an
inductively
coupled
power
of
900
W.
Silicon
was
cleaned
prior
to
metallization
via
a modified
Radio
Corporation
of
America
(RCA)
standard
clean
1 (5:1:1
(vol)
H
2
O:NH
4
OH:H
2
O
2
at 70-75
°C)
for
at least
10
min
followed
by
an
RCA
standard
clean
2 (6:1:1
(vol)
H
2
O:HCl:H
2
O
2
at 65-70
°C)
for
at least
10
min.
The
samples
were
dipped
in
HF
between
the
cleaning
steps,
which
also
resulted
in
the
removal
of
the Al
2
O
3
etch mask.
Mass
transport
measurements:
The
thickness
of
the
diffusion
layer
was
measured
via
spectrophotometric
determination
of
Fe
2+
in
a Shimadzu
Solid
Spec
3700
ultraviolet-visible
spectrometer,
following
complexation
with
1,10-phenanthroline
in
0.50
M
H
2
SO
4
(aq)
and
mixing
the
solution
with
2.3
mL
of
0.2
M
sodium
acetate
(aq)
to
bring
the
pH
to 4-4.5.
1
The
testing
cell
was
set
up
in
a nominally
identical
manner
to
the
cell
used
for
HER
testing,
with
the
addition
of
8.80
mL
of
0.100
M
Fe
3+
(aq),
as
Fe(III)
sulfate,
in
0.50
M
H
2
SO
4
(aq)
to the
50
mL
electrolyte
prior
to
testing
to
an initial
= 0.0150
M.
The
precise
concentration
of
the
ferric
sulfate
stock
퐹푒
3
+
solution
was
determined
via
spectrophotometry,
following
reduction
with
hydroxylamine,
and
assuming
a molar
extinction
for
tris(1,10-phenanthroline)iron(II)
of
1.10
x 10
4
M
-1
cm
-1
.
1,2
A
diffusion
coefficient
of
Fe
3+
(aq)
of
5.5
x 10
-6
cm
2
s
-1
was
assumed
in calculating
boundary
layer
thicknesses,
assuming
planar
diffusion
(Equation
1).
The
value
of
was
adjusted
to be the
last
퐹푒
3
+
recorded
concentration
of
Fe
2+
(aq),
with
the
concentration
of
Fe
2+
(aq)
not
changing
by
more
than
4% during an individual
electrolysis.
Impedance spectroscopy:
Nyquist
plots were prepared by taking an average
of 2 consecutive
impedance measurements at a given
frequency, at a
sampling
rate of 6 points
per
decade.
Galvanostatic impedance measurements
were performed using a
sinusoidal current wave of
S4
0.100 μA at a frequency of 50 kHz
superimposed over a
constant
. At high
frequencies, Z(Re)
2
was constant but Z(Im) consistently trended towards positive values. Such an
impedance
response, which
is equivalent to the behavior of a
negative capacitance, has previously been
reported for silicon-metal contacts under forward bias at the high frequency
limit.
3
Electrochemical Testing
with
High-Speed Microscopy:
Electrodes for
microscopy
experiments were made by procedures that
were
mutually consistent with those used to
make
electrodes for the other experiments. Si microwire samples were mounted
to
wires by first
scribing In-Ga eutectic into
the back of the chip,
then
adhering the samples to
a coiled wire with
conductive Ag paint (SPI). The wire
was secured in a length of flexible PVC
shrink tubing, then
sealed
with
epoxy (Loctite
Hysol
9460)
to leave
exposed only
the active electrode area. The
electrode area was measured with a
flatbed scanner
in
conjunction with the
ImageJ software
package. High-speed microscopy
experiments
were performed in 0.50 M H
2
SO
4
(aq)
in an
HDPE electrochemical
cell
with a glass cover. The imaging system
consisted of a
microscope
(Olympus BX-53) with a
5x
objective, a LED
reflectance
illuminator (Prior Scientific),
and
a
high-speed camera (Fastec Imaging).
The
working electrode was
positioned under
the
objective
lens with the
microwires oriented
vertically. The electrode depth
was
maintained at 8 mm under
the electrolyte level to maintain a consistent hydrostatic pressure
between experiments. The
counter electrode
consisted of a Pt wire in a glass tube separated from the rest
of the
electrolyte
by a Nafion membrane, to prevent O
2
crossover.
A Ag/AgCl (sat. KCl) reference
electrode was
used to monitor the
working electrode potential
during the chronopotentiometry measurements.
Between microscopy experiments, the electrolyte
was purged by electrolysis
via
a Pt wire
cathode (and
the
counter electrode as the anode) performing H
2
evolution at 300 mA.
S5
Calculations
of
Growth
Coefficients:
Radius
versus
time
data
for
individual
bubbles
measured
via
high-speed
microscopy
were
fit
to
a model
for
diffusive
growth
of
a gas
bubble
in a
supersaturated medium
(Equation S1).
4,5
(S1)
(
)
=
̃
(
2
)
1/2
where
cm
-2
s
-1
is the
diffusivity
of
H
2
in solution
and
is the
dimensionless
growth
2
=
4.5
×
10
5
̃
coefficient.
When
the
driving
force
for
bubble
growth
is
small,
the
effects
of
advection
at
the
growing
surface
can
be
ignored,
such
that
analytical
expressions
can
be
derived
relating
b\tilde
to
.
4
A
self-consistent
requirement
for
neglecting
the
effects
of
advection
is
that
the
Péclet
2
(
푎푞
)
number,
which
expresses
the
ratio
of
advective
and
diffusive
growth,
is < 1. For
> 1 this
condition
̃
does not hold and
growth
coefficients were thus not directly related to
.
4
2
(
푎푞
)
Calculation
of Weighted
Mean
Bubble
Diameter:
The
thickness
of
the
gas
bubble
layer
was
variable
in time
and
with
position
on
the
electrode
surface.
The
mean
bubble
diameter,
d
, weighted
by
the
fraction
of surface
obscured
by
an
individual
bubble,
was
calculated
as
an approximation
of the instantaneous gas bubble layer thickness (Equation S2).
(S2)
̅
=
×
2
4
Where
A
is the
geometric
surface
area
of
the
electrode,
the
surface
is assumed
to
be
obscured
by
the
projected
area
of
the
bubble,
and
the
contact
angle
is assumed
to be
large
such
that
the
height
of the bubble is approximately equal to the
diameter.
S6
Supporting Figures
Figure S1: Representative
cross section SEM images for μW 3|11 (A), μW
6|14
(B), and
μW
6|28 (C)
samples. Statistics for wire
height, H, and tip diameter, D, and pitch,
P,
as measured
from N wires in multiple
SEM images
are
presented
at the
bottom of each image
along with the
standard deviation across the measurements.
S7
Figure S2: (A) Schematic of cell
used
to test electrodes as both upward-facing
and
downward
facing
cathodes. (B) Potential versus time data
for
a planar n
+
-Si/Ti/Pt electrode as a
function of
current density
and
orientation versus the
gravitational
vector in
stagnant,
0.50 M
H
2
SO
4
(aq)
under 1-atm H
2
(g)
(B) Potential versus
time data for
a n
+
-Si/Ti/Pt
μW 6|14 electrode at
= 20
|퐽
2
|
mA cm
-2
.
Figure S3: Comparison of fractional
gas coverage
of downward-facing cathodes, with 15 degrees
of tilt, during
a representative
60 s of a
constant current experiment,
operating
at (A-C)
-10 mA
cm
-2
and (D-F) -70 mA cm
-2
, relative
to the absolute overpotential for the
HER in
0.50 M
H
2
SO
4
(aq) as a
function of time.
S8
Figure S4: IR-corrected chronopotentiometry
data
for
n
+
-Si/Ti/Pt cathodes at
α
= 15° in
0.50 M
H
2
SO
4
(aq) at -100 mA cm
-2
(A) and
-200 mA cm
-2
(B).
0
2
0
4
0
6
0
C
u
r
r
e
n
t
D
e
n
s
i
t
y
/
m
A
c
m
-
2
0
0
.
2
0
.
4
0
.
6
0
.
8
1
G
a
s
C
o
v
e
r
a
g
e
F
r
a
c
t
i
o
n
Figure S5: Relationship between the mean gas coverage and current density for various
downward-facing hydrogen-evolving cathodes in 0.50 M H
2
SO
4
(aq). Planar silicon: black
squares; μW
3|11:
gray
diamonds;
μW 6|14: red x’s; μW 6|28: blue circles.
S9
Figure S6: Representative
images from Movie S1, recorded at downward facing μW 3|11 (A-C),
μW 6|14 (D-F) and
planar (G-I) cathodes
as a
function of |
| at
α
= 15° in
0.50 M H
2
SO
4
(aq).
2
S10
Figure S7: Representative
images from Movie S2, recorded at downward facing μW 3|11 (A-C),
μW 6|14 (D-F) and
planar (G-I) cathodes
as a
function of |
| at
α
= 15° in
0.50 M H
2
SO
4
(aq).
2
S11
Figure S8: Weighted mean bubble diameters (Equation S2)
versus
video time
at |
| = 10-70 mA
2
cm
-2
(A-C)
and |
| = 100-200
mA cm
-2
(D-F) as measured at n
+
-Si/Ti/Pt
planar, (B,E) μW 3|11
2
(A,D),
and μW 6|14 electrodes (C,F) in 0.50 M H
2
SO
4
(aq) at
α
= 15°.
Figure S9: (A) Number density,
N
, of bubbles on the electrode surface as a function
of time
for
planar, μW 6|14, and μW
3|11
n
+
-Si/Ti/Pt electrodes passing -30 mA cm
-2
in
0.50 M H
2
SO
4
(aq)
at
α
= 15° represented by black squares, red
x’s and blue circles respectively. Number density of
bubbles on a
(B) planar and (C)
μW 6|14
n
+
-Si/Ti/Pt
electrode as a function of time and
absolute
current density
towards
H
2
.
S12