of 8
ARTICLE
Ef
fi
cient solar hydrogen generation in microgravity
environment
Katharina Brinkert
1,2
, Matthias H. Richter
1,3
, Ömer Akay
4
, Janine Liedtke
2
, Michael Giersig
4,5
,
Katherine T. Fountaine
6,7
& Hans-Joachim Lewerenz
8
Long-term space missions require extra-terrestrial production of storable, renewable energy.
Hydrogen is ascribed a crucial role for transportation, electrical power and oxygen generation.
We demonstrate in a series of drop tower experiments that ef
fi
cient direct hydrogen pro-
duction can be realized photoelectrochemically in microgravity environment, providing an
alternative route to existing life support technologies for space travel. The photoelec-
trochemical cell consists of an integrated catalyst-functionalized semiconductor system that
generates hydrogen with current densities >15 mA/cm
2
in the absence of buoyancy. Con-
ditions are described adverting the resulting formation of ion transport blocking froth layers
on the photoelectrodes. The current limiting factors were overcome by controlling the micro-
and nanotopography of the Rh electrocatalyst using shadow nanosphere lithography. The
behaviour of the applied system in terrestrial and microgravity environment is simulated
using a kinetic transport model. Differences observed for varied catalyst topography are
elucidated, enabling future photoelectrode designs for use in reduced gravity environments.
DOI: 10.1038/s41467-018-04844-y
OPEN
1
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125, USA.
2
Advanced Concepts
Team, European Space Agency, ESTEC, Keplerlaan 1, Noordwijk 2200 AG, The Netherlands.
3
Brandenburg University of Technology Cottbus, Applied
Physics and Sensors, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany.
4
Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin,
Germany.
5
International Academy of Optoelectronics at Zhaoqing, South China Normal University, 526238 Guangdong, China.
6
Resnick Sustainability
Institute, California Institute of Technology, Pasadena, CA 91125, USA.
7
NG Next, Northrop Grumman Corporation, One Space Park, Redondo Beach, CA
90278, USA.
8
Division of Engineering and Applied Science and Joint Center for Arti
fi
cial Photosynthesis, California Institute of Technology, 1200 E. California
Blvd., Pasadena, CA 91125, USA. Correspondence and requests for materials should be addressed to K.B. (email:
brinkert@caltech.edu
)
or to H.-J.L. (email:
lewerenz@caltech.edu
)
NATURE COMMUNICATIONS
| (2018) 9:2527 | DOI: 10.1038/s41467-018-04844-y | www.nature.com/naturecommunications
1
1234567890():,;
O
ur atmosphere on earth is sustained by the photo-
dissociation of water, a fundamental process used by
nature in oxygenic photosynthesis to convert solar energy
into storable, chemical energy
1
. It relies on the so-called Z-
scheme, where photons of different energy are absorbed in a
staggered energy-level system. Presently, arti
fi
cial photosynthesis
systems are being intensively developed
2
6
based on the analogue
of the Z-scheme, e.g. in tandem solar cells
5
. Here, the light-driven
oxidation of water to oxygen at the (photo) anode is accompanied
by the production of so-called solar fuels at the (photo)cathode:
hydrogen, a storable fuel which can be used as a feedstock for fuel
cells generating power for transportation or carbon dioxide
reduction products, holding the promise for converting emissions
back to fuels utilizing renewable energy. Inorganic systems have
yielded the hitherto highest ef
fi
ciencies and stabilities when
combining the tandem absorbers with high-activity
electrocatalysts
7
,
8
.
The ef
fi
cient conversion of abundant sunlight to oxygen and
storable fuels is also a key step in realizing long-term space
missions and
cis
-
lunar research platforms such as the Deep Space
Gateway. Since the early 1960s, water electrolysis cells comprising
of photovoltaic p
n junction solar cells and a separate water
electrolyser system have been employed as a part of a spacecraft
environmental control system for the production of oxygen from
carbon dioxide
9
and are still in use on the International Space
Station (ISS). The involved travel distances on future deep space
missions restrict volume and mass of consumables required for a
voyage of months or years, with a resupply of water and fuel from
Earth becoming impossible. These long-duration trips into space
demand regenerative, reliable and light life support hardware
which repeatedly generates and recycles essential, life sustaining
elements required by human travellers. An ef
fi
cient and stable
monolithic surface modi
fi
ed tandem device structure, capable of
oxidizing water and simultaneously producing hydrogen and/or
reducing CO
2
presents a compact and lighter alternative to the
currently employed photoelectrolysis system. Moreover, analyses
of terrestrial systems show that the fully integrated devices
compare favourably with separate PV-electrolyser units regarding
installation and fuel production costs
10
,
11
. Despite these advan-
tages, direct photoelectrochemical water splitting for hydrogen
and oxygen production
12
in space relevant conditions such as
reduced gravitation has not been explored yet, although micro-
gravity environment has already been employed for the electro-
chemical synthesis of advanced nanomaterials for energy
conversion
13
.
Herein, we describe the development of an ef
fi
ciently operating
semiconductor
electrocatalyst half-cell in microgravity environ-
ment. Experimental conditions are described which are required
for investigating photoelectrochemical hydrogen production in
reduced gravitational environments, realized at the Bremen Drop
Tower. We show that due to missing buoyancy, microgravity has
a signi
fi
cant impact on the mass transfer rate of protons to and
hydrogen from the photocathode surface due to the formation of
so-called gas bubble froth layers. These froth layers are known to
drastically reduce ion and gas transport at electrodes in micro-
gravity and increase the ohmic resistance in proximity to the
electrode surface
14
,
15
. Using shadow nanosphere lithography
(SNL)
16
,
17
, we adjust the shape of the employed electrocatalyst on
the photocathode and demonstrate continuous bubble release
even at high current densities in the absence of buoyancy. The
transfer of our concepts regarding catalyst micro- and nanoto-
pography supported by theoretical analyses can contribute to the
design of ef
fi
cient life revitalization systems and energy genera-
tion in future space missions. Furthermore, they can be imple-
mented in fully integrated devices for unassisted water splitting
currently being realized for terrestrial applications.
Results
Drop tower experimental arrangement
. The investigation of
light-induced hydrogen production on photocathodes was carried
out at the Bremen Drop Tower, Center of Applied Space Tech-
nology and Microgravity (ZARM)
18
, in a minimum
g
level of
10
6
g
with a free fall duration of 9.3 s (Fig.
1
a). The complete
photoelectrochemical potentiostatic experiment, comprising light
sources, electrochemical cells, potentiostats, including analysis
and recording devices was installed in a drop capsule (Fig.
1
b)
and submitted to microgravity in a free-
fl
ight drop tower
experiment. A hydraulically controlled pneumatic piston-cylinder
system launched the capsule upwards from the bottom of the
tower. The capsule was accelerated in 0.25 s to a speed of 168 km/
h closely to the top of the drop tube and then fell down into a
deceleration chamber. The equipment of the 1.34 m tall drop
capsule consisted of the photoelectrochemical setup with a two-
compartment photoelectrochemical cell (Fig.
1
c) which allowed
the simultaneous investigation of two photoelectrodes during free
fall. Two digital cameras recorded the gas bubble evolution
behaviour on the photoelectrode surface for each cell compart-
ment from the front and from the side. In order to avoid sample
contact with the electrolyte prior to reaching microgravity con-
ditions, a pneumatic lifting ramp was installed at the backside of
the cell which allowed immersing and emersing the photoelec-
trodes upon command.
A programmed, automated drop sequence (see upper part in
Fig.
1
and Supplementary Table
1
for more detailed information),
ensured the precise synchronization of potentiostats, light
sources, cameras and the pneumatic lifting ramp during the
experiment.
Two different types of photoelectrodes were investigated in
microgravity environment: in the
fi
rst set of electrodes, p-type
indium phosphide was employed as the photocathode material
onto which rhodium particles were photoelectrochemically
deposited under stroboscopic illumination (further on referred
to as
thin
fi
lm electrodes
)
19
,
20
. In the second set, SNL was
applied to obtain nanocrystalline Rh particles of three-
dimensional arrangement
16
on the p-InP photocathode. This
technique using the self-assembly of hexagonal closed-packed
monolayer of latex spheres was applied on the p-InP electrode to
create masks for the electrodeposition of rhodium. After the latex
spheres were removed, hexagonal unit cell patterns of the
rhodium electrocatalyst were obtained (further on referred to as
nanostructured electrodes
).
Photoelectrochemical behaviour
. Figure
2
a shows the
J
V
measurements of the two different photocathodes types in ter-
restrial (1
g
) and microgravity conditions (10
6
g
) in 1 M HClO
4
and in the presence of 1% isopropanol, which was added to the
electrolyte to lower the surface tension and to favour gas bubble
release
21
. Although both photoelectrodes exhibit similar
J
V
behaviour in terrestrial conditions, conspicuous differences are
observed in the photocurrent
voltage characteristics in micro-
gravity. The short circuit current of the thin-
fi
lm sample was
reduced by almost 70% during free fall, whereas the open circuit
voltage decreased by 25%. Differences in the
V
OC
of the nanos-
tructured and thin-
fi
lm sample in terrestrial conditions could
have been attributed to performance differences of the photo-
electrodes as shown in Supplementary Table
2
. In contrary to the
performance loss of the thin
fi
lm in microgravity, the terrestrial
J
V
characteristics and also the cell ef
fi
ciency remained the same
when the nanostructured p-InP
Rh electrodes where exposed to
microgravity (Fig.
2
a). Similar observations were made in
chronoamperometric measurements of the thin-
fi
lm and nanos-
tructured sample (Supplementary Figure
1
) in microgravity
ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-04844-y
2
NATURE COMMUNICATIONS
| (2018) 9:2527 | DOI: 10.1038/s41467-018-04844-y | www.nature.com/naturecommunications
environment: the current density of the thin-
fi
lm photoelectrode
remained at a constant value of 5 mA/cm
2
, whereas the current
density of the nanostructured sample showed a nearly stable value
of 16 mA/cm
2
. This
fi
nding is also re
fl
ected in the gas bubble
evolution behaviour on the two electrode surfaces (Fig.
2
b): video
recordings during the experiments reveal that although the
electrodes do not show noticeable differences in their hydrogen
evolution behaviour in terrestrial conditions, their microgravity
behaviour differs signi
fi
cantly: here, the evolved hydrogen gas is
not released from the thin-
fi
lm electrode surface and bubbles
coalesce in proximity to the electrode, whereas the gas bubble
release is enhanced on the nanostructured photoelectrodes.
Similar observations have been made in various studies of water
electrolysis in microgravity environments
9
,
15
,
22
,
23
, where the
absence of buoyancy and the suppression of natural convection
caused the coalescence of gas bubbles on the electrode surface and
the formation of froth layers. The resulting mass transfer lim-
itations and the increased ohmic drop in the gas bubble
–120
Drop tube
Potentiostat,
shutter controller
PEC SetUp,
cameras
1
2
3
Lamps, PC
DC/AC converter
Battery
Mirror
CE
1
CE
2
WE
2
WE
1
RE
1
RE
2
Mirror
Beam
splitters
4
5
Drop capsule
Deceleration
container
Catapult
system
120 m
abc
Drop tower
Drop capsule
–105
–90
–12
0
g
(ms
–2
)
5
10
15
15
0
25
20
20
Photoelectrochemical cell
Time (s)
Time (min)
Cameras on
Cameras off
Sample immersed
Samples emersed
Sample loaded,
capsule closed
Capsule loaded
onto catapult
Tower is evacuated
Light on
Light off
Capsule released
Capsule decelerates
Capsule in
starting position
Retrieving capsule
Capsule back
at docking station
Retrieving samples
Potentiostat on
Potentiostat off
Fig. 1
Scheme of the experimental set-up and time line of the photoelectrochemical experiments in microgravity conditions. The inset images show the
drop tower (
a
), the drop capsule (
b
) and the photoelectrochemical cell (
c
). The capsule contained two potentiostats and two shutter control boxes
(platform 1), the photoelectrochemical setup (platform 2) including four digital cameras, two W-I light sources and a Matrox 4Sight GPm computer
(platform 3), a DC/AC converter (platform 4) and a battery for power supply during free fall (platform 5). The photoelectrochemical setup of platform
2
contained four digital cameras which allowed recording of gas bubble formation on the photoelectrode from the front through beam splitters and from t
he
side through mirrors of the photoelectrochemical cell. Illumination of the photoelectrodes occurred through the beam splitters in front of the cell
.A
pneumatic lifting ramp ensured the immersion of the photoelectrodes in the electrolyte immediately before activation of the catapult system. WE is t
he
working electrode, RE is the reference electrode and CE is the counter electrode. The subscript numbers indicate the respective cell compartment. Th
e time
line represents the programmed drop sequence. The inset shows the gravitational
fi
eld according to the time line of the experiment. The colour code
matches the events in the time line with the involvement of drop tower, drop capsule and/or photoelectrochemical cell
NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-04844-y
ARTICLE
NATURE COMMUNICATIONS
| (2018) 9:2527 | DOI: 10.1038/s41467-018-04844-y | www.nature.com/naturecommunications
3
dispersion zone in proximity to the electrode led to a substantial
decrease in current density.
In
fl
uence of surface nanotopography
. In order to elucidate the
role of surface morphology, structure and composition of the
thin-
fi
lm and nanostructured photoelectrodes for their perfor-
mance in microgravity environment, surfaces were characterized
by structural, surface morphological and compositional analyses
using atomic force microscopy (AFM), scanning electron
microscopy (SEM), high-resolution transmission electron
microscopy (HRTEM), energy-dispersive X-ray analyses and X-
ray photoelectron spectroscopy (XPS). The photoelectrodeposi-
tion of rhodium on p-InP resulted in a homogenous layer of a
rhodium grain conglomerate, whereas the application of SNL on
p-InP prior to Rh electrodeposition resulted in a nanosized, two-
dimensional periodic Rh structure (Fig.
3
a
c).
SEM studies con
fi
rmed the homogenous array of holes in the
metallic Rh
fi
lm, in which rhodium exhibited a nanocrystalline
cubic structure (Supplementary Figs.
2
,
3
). For both electrodes,
XPS spectra (Supplementary Fig.
4
) provide evidence for an InO
x
/
PO
x
oxide layer formation on the InP which is more distinct in
the case of the nanostructured electrode, apparent in the larger
InP signal at 128.4 eV. This is not surprising, given the fact that
the PS bead prepared structure leaves open areas of InP which is
accessible by the electrolyte. Despite the fact that Rh was
deposited through the polystyrene sphere mask, the Rh 3
d
3/2
and
3
d
5/2
signal intensities are almost identical, suggesting a similar
overall coverage of Rh on both electrodes with distinctive
differences in the local coverage due to the different surface
topographies.
Discussion
In order to further understand the processes involved in the
current
voltage reduction of the thin-
fi
lm samples in micro-
gravity environments observed here, a terrestrial experiment was
designed, demonstrating the involvement of mass transfer lim-
itations in the current density drop (Supplementary Fig.
5
a): the
photoelectrochemical cell with the photoelectrode was placed
upside down and illumination occurred from the bottom of the
cell via an optical mirror. This set-up allowed trapping the
produced gas bubbles on the electrode surface while simulta-
neously recording the
J
V
characteristics. After initiating the
hydrogen evolution reaction, the photocurrent density dropped
instantaneously, resulting in an overall decrease of about 25%
after 25 min reaction time. The open circuit voltage was also
reduced by 50 mV (Supplementary Fig.
5
b). The initial
J
V
behaviour could be recovered again when the surface tension of
the electrolyte was decreased by addition of 1% (v/v) isopropanol,
causing an enhanced gas bubble detachment from the electrode
surface.
To elucidate the role of mass transfer for the performance of
the thin-
fi
lm electrode in microgravity environments further, the
J
V
characteristics of the investigated devices were theoretically
modelled in terrestrial and reduced gravity environments. A
semi-analytic formalism for PEC devices with nanostructured
catalysts was used that builds on our model developed in previous
publications
7
,
8
,
24
to include mass transport limitations.
The full current
voltage characteristics of the device when the
rate of reaction is determined solely by reaction kinetics and
without any mass transport considerations is captured via Eq. (
1
).
It is an analytic equation for the current
voltage behaviour of a
nanostructured coupled electrocatalyst
semiconductor device, in
which
k
is the Boltzmann constant,
T
(K) is the temperature,
Thin film
Nanostructured
Nanostructured
Thin film
10
–10
0.5
0.5
0
0
00
0
1
1
1
1
2
2
0
50
z
(nm)
z
(nm)
y
(
μ
m)
y
(
μ
m)
x
(
μ
m)
x
(
μ
m)
Rh
Rh
InP
InP
InP
x
O
y
InP
x
O
y
SEM
b
a
c
AFM
Fig. 3
Structural investigations of the thin-
fi
lm and nanostructured
photoelectrodes.
a
Scheme of the two photoelectrode sets which were
investigated in microgravity environment. In both cases, p-InP was
employed as the light-absorbing semiconductor coated with a rhodium
electrocatalyst layer. In the
fi
rst set of electrodes, rhodium was
photoelectrochemically deposited onto the planar p-InP surface. In the
second set, Rh was deposited onto the p-InP surface through a mask of
polystyrene particles, resulting in a hexagonal unit cell pattern of the
rhodium after removal of the latex spheres.
b
SEM and tapping mode AFM
images of the planar and nanostructured catalytic layer of rhodium on p-InP
(also compare Supplementary Fig.
4
c). The scale bars indicate the
resolution of 500 nm of the thin-
fi
lm electrode SEM and AFM images and
2
μ
m and 1
μ
m for the nanostructured electrode SEM and AFM images,
respectively.
c
Three-dimensional surface structures of the thin-
fi
lm and
nanostructured photoelectrode obtained by AFM used for the calculation of
catalyst surface area in the simulations
10
Thin film:
Thin film
Nanostructured:
Nanostructured
10
–6
g
10
–6
g
10
–6
g
1
g
1
g
1
g
ab
5
0
J
(mA cm
–2
)
0.0
U
vs. RHE (V)
0.2
0.4
0.6
0.8
–5
–10
–15
–20
Fig. 2
Results of the photoelectrochemical experiments in microgravity
environment.
a
J
V
measurements of thin-
fi
lm and nanostructured p-
InP
Rh photoelectrodes in terrestrial (1
g
) and microgravity environments
(10
6
g
) at 70 mW/cm
2
illumination with a W-I lamp in 1 M HClO
4
with the
addition of 1% (v/v) isopropanol. Differences in the
V
OC
of the
nanostructured and thin-
fi
lm sample in terrestrial conditions are subject to
performance differences of the photoelectrodes as shown in Supplementary
Table
2
.
b
Images from video recordings of the thin-
fi
lm and nanostructured
photoelectrodes after 9.3 s in terrestrial and microgravity conditions. In
microgravity environment, hydrogen gas bubbles form a froth layer on the
thin-
fi
lm electrodes whereas the bubble adhesion to the electrode surface
is decreased in the presence of the nanostructured Rh layer
ARTICLE
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4
NATURE COMMUNICATIONS
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which is assumed to be 300 K,
q
is the elementary charge,
j
L
is the
light limited current,
j
0
is the dark current,
R
is the universal gas
constant,
n
e
is the number of electrons associated with reaction
which is 2 in this case,
F
is Faraday
s constant,
j
0,cat
is the catalyst
exchange current density and
f
SA
is the catalyst surface area factor
relative to the planar device area.
V
PEC
j
ðÞ
¼
kT
q
ln
j
L

j
j
0
þ
1


2
RT
n
e
F
sinh

1
j
2
j
0
;
cat
f
SA
!
ð
1
Þ
This equation is essentially the difference between the photo-
voltage of the diode, derived from the ideal photodiode equation,
and the overpotential of the catalyst, derived from the
Butler
Volmer equation
24
.
When mass transport plays a role in the reaction rate, the
current of the reaction can be derived from the Koutecky
Levich
equation, Eq. (
2
), in which
j
rr
is the overall current of the reaction,
j
BV
is the kinetic current and
j
mtl
is the mass transport current.
1
j
rr
¼
1
j
BV
þ
1
j
mtl
ð
2
Þ
Essentially, this equation represents the kinetic and mass
transport currents as two parallel current pathways. The kinetic
current is described by Butler
Volmer kinetics, as in Eq. (
1
) for
the non-mass transport limited case. The mass transport current
is described by Eq. (
3
), in which
j
mtl,a/c
are the anodic and
cathodic limiting mass transport current densities, respectively,
e
is the elementary charge and
V
mt
is the overpotential due to
mass transport.
j
mtl
V
mt
ðÞ¼
1

exp

n
e
eV
mt
kT



1
j
mtl
;
a

1
j
mtl
;
c

exp

n
e
eV
mt
kT



1
ð
3
Þ
This equation is derived from Fick
s 1st law for diffusion across
the Nernst boundary layer (Supplementary Fig.
6
a) and the
Butler
Volmer equation. A full derivation of this equation can be
found in Supplementary Information (Supplementary Note
1
).
The current
voltage curve can then be found numerically by
fi
rst
calculating the {
j
PV
,
V
PV
} and {
j
rr
,
V
mt
} pairs from the ideal
photodiode equation (
fi
rst term in Eq. (
1
)) and the
Koutecky
Levich equation, Eq. (
2
), respectively, using equal
current values for both sets of pairs; and secondly, by subtracting
the mass transport overpotential,
V
mt
, from the diode photo-
voltage,
V
PV
.
This process is the numerical equivalent to Eq. (
1
), in which
the second term is replaced with the Koutecky
Levich equation,
Eq. (
3
), (Supplementary Fig.
6
b). Figure
4
summarizes the
simulated results. Whereas nanostructured and thin-
fi
lm sample
shows a nearly equivalent
J
V
behaviour in terrestrial environ-
ments, the assumed mass transfer limitations in microgravity
environment affect the performance of the thin-
fi
lm electrode
signi
fi
cantly, providing strong evidence that this is the main effect
leading to the decrease in photocurrent during free fall. The
V
OC
decrease of the thin-
fi
lm sample is furthermore a result of the
light-induced excess electron accumulation at the photoelectrode
surface which originates from limited mass transfer causing
increased recombination at the electrode surface due to charge
transfer inhibition
25
.Itisre
fl
ected in the simulation by a lower
dark current value,
j
0
(see Methods part). Generally, it is to be
taken into account that for the simulations, the same catalytic
activity for Rh (
j
0,cat
) is assumed in terrestrial and microgravity
environments. Due to the formation of gas bubble froth layers,
some catalytic Rh sites might not be active in microgravity con-
ditions, leading to a lower value for
j
0,cat
and furthermore, a
slower initiation of catalysis close to the
V
OC
. Furthermore, non-
idealities in the photodiode equation which reduce the
fi
ll factor,
such as series resistance (resistance across the InPO
x
layer) and
shunt resistance (incomplete junction) accounting for e.g. inter-
face resistance, are not considered in the simulations. Although
previously considered
8
, they were neglected in the description of
the
J
V
behaviour of this system since a variety of variables
in
fl
uence these experimental parameters which then only operate
as additional
fi
tting parameters.
The results show that under microgravity conditions, the
electrode surface morphology plays a crucial role for the photo-
electrochemical performance. The catalyst micro- and nanoto-
pography have decisive in
fl
uence on the life cycle of bubbles on
the surface. The growth and accumulation of bubbles on the thin-
fi
lm electrode leads to a froth layer also observed in dark elec-
trolysis experiments
9
,
15
,
22
,
23
that seriously inhibits the hydrogen
evolution reaction. Figure
5
a sketches the effect of lateral accu-
mulation of gas bubbles that form a gaseous interphase which
increasingly suppresses hydronium ion transport to the surface.
However, with speci
fi
c nanotopographies one can overcome
microconvectional limitations
26
: the three-dimensional catalyst
structure, depicted schematically in Fig.
5
b, generates hot spots
due to increased local electrical
fi
elds at the tips of the structure
that has been formed by SNL. Bubble generation occurs pre-
ferably at the tips of the catalyst structure. Figure
5
b illustrates the
effect: gas bubbles nucleate and grow at the tips of the Rh deposits
that have been formed at the circumference of the open InP
circles. The removal of the grown bubbles results from weakened
adhesion to the surface due to the small contact area in con-
junction with microconvection. Concentration gradients along
the surface facilitate H
2
transfer to the bubbles upon formation.
In addition to the decreased probability of forming bubble
agglomerates on the electrode surface, the bubble size is further
determined by the stability of the formed gas bubble on the Rh
tip. These morphologic advantages lead to an increased
J
V
performance in microgravity and suggest a
fi
rst design principle
for photoelectrodes employed in this environment for light-
assisted fuel production.
0
0.2
0.4
0.6
0.8
0
5
10
15
20
Thin film:
10
–6
g
1
g
Nanostructured:
10
–6
g
1
g
J
(mA cm
–2
)
U
vs. RHE (V)
Fig. 4
Simulations of the
J
V
characteristics of the thin-
fi
lm and
nanostructured p-InP
Rh photoelectrodes in terrestrial and microgravity
environments. Illumination was assumed to occur at 70 mW/cm
2
through
a W-I lamp. The electrolyte composition was 1 M HClO
4
with the addition
of 1% (v/v) isopropanol. The microgravity environment was arti
fi
cially
created by assuming that the
J
V
characteristics of the thin-
fi
lm
photoelectrode are mass transfer limited (see text for details). The dashed
red line corresponds to the nanostructured sample in 1
g
, the subjacent
yellow line corresponds to the sample in 10
6
g
. The blue dotted line shows
the behaviour of the thin-
fi
lm sample under terrestrial conditions whereas
the cyan dotted-dashed line corresponds to the
J
V
characteristics in 10
6
g
NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-04844-y
ARTICLE
NATURE COMMUNICATIONS
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5
We report ef
fi
cient light-generated production of hydrogen on
InP
Rh photoelectrodes in microgravity environment by per-
forming photoelectrochemistry during drop tower
fl
ights. The
reduction of the photocurrent due to the absence of buoyancy
and therefore inhibited bubble removal is observed on photo-
cathodes with unstructured, rather planar electrocatalysts (thin-
fi
lm electrodes). Mathematical modelling shows
in a straight-
forward extension of the Butler
Volmer equation combined with
a description of coupled diode
electrocatalyst systems
that mass
transport limitations in the electrolyte result in suppression of the
respective photocurrents from the thin-
fi
lm samples. The pre-
paration of a judiciously chosen nanostructured catalyst surface
topography produces highly active Rh
hot spots
preventing
bubble coalescence and also favouring the detachment of pro-
duced hydrogen gas bubbles from the electrode surface. The
developed model reproduces both, the photocurrent
voltage
behaviour of nanotopographic and planar-type thin-
fi
lm samples.
SNL has been used for the design of speci
fi
c, three-dimensional
nanostructures establishing the method as an attractive general
tool for the design of high-activity photodiode
electrocatalyst
systems. Our demonstrated ef
fi
ciently operating half-cell produ-
cing hydrogen in microgravity environment opens up an alter-
native pathway for the improvement and extension of life support
systems for long-duration space travel and
cis
-lunar research
platforms while promoting investigations of currently developed
systems for terrestrial solar fuel production for application in
space. Investigations of phenomena such as gas bubble formation
and evolution in reduced gravity environments can furthermore
also lead to an enhanced understanding of processes at the
electrode
electrolyte interface in photoelectrochemical devices,
complementing ongoing terrestrial studies.
Methods
Preparation of the p-InP photoelectrodes
. Single crystal p-InP wafers with the
orientation (111A) were obtained from AXT Inc. (Geo Semiconductor Ltd.
Switzerland) with a Zn doping concentration of 5 × 10
17
cm
3
. The preparation of
an ohmic back contact involved the evaporation of 4 nm Au, 80 nm Zn and 150 nm
Au on the backside of the wafer which was then heated to 400 °C for 60 s. The 0.5
cm
2
polished indium face of (111A) p-InP was furthermore etched for 30 s in
bromine (0.05% (w/v))/methanol solution, rinsed with ethanol and ultrapure water
and dried under nitrogen
fl
ux. All solutions were made from ultrapure water and
analytical grade chemicals with an organic impurity level below 50 ppb. Subsequent
cyclic voltammetric and chronoamperometric measurements were performed in a
standard three-electrode potentiostatic arrangement whereas a carbon electrode
was used as counter electrode and an Ag/AgCl (3 M) was employed as reference
electrode. All potentials are converted to those vs. reversible hydrogen electrode
(RHE). Moreover, the p-InP surface was photoelectrochemically conditioned in 0.5
M HCl, realized by potentiodynamic cycling under illumination (100 mW/cm
2
)
between
0.44 V and
+
0.31 V at a scan rate of 50 mV/s while purging with
nitrogen of 5.0 purity. Illumination occurred with a white-light tungsten halogen
lamp (Edmund Optics) through a quartz window of the borosilicate glass cell. The
light intensity was adjusted with a calibrated silicon reference photodiode.
A thin Rh layer was photoelectrochemically deposited from a solution of 5 mM
RhCl
3
, 0.5 M NaCl and 0.5 vol% 2-propanol for 5 s at a constant potential of
V
dep
=+
0.01 V and a light intensity of 100 mW/cm
2
using the same settings as for the
photoelectrochemical conditioning procedure. The electrodeposition resulted in
the formation of a nanocrystalline thin
fi
lm or a nanostructured surface
morphology if the rhodium was deposited through a polystyrene mask applying
SNL (see below).
To compare the current
voltage characteristics and solar-to-hydrogen
conversion ef
fi
ciency of the photocathodes under terrestrial and microgravity
conditions, sample electrodes were also tested in 1 M HClO
4
electrolyte solution
upon illumination with a W-I white-light source (100 mW/cm
2
) under terrestrial
conditions in the laboratory in a quartz glass cell. Samples for the tests in the Drop
Tower facility were prepared one week prior to testing and stored under N
2
atmosphere in the dark. XPS analysis of the stored samples did not show changes of
the surface composition in comparison to freshly prepared samples (see below).
Fabrication of rhodium nanostructures
. SNL
16
was employed to fabricate rho-
dium nanostructures on the InP substrate. For creating the masks, mono-dispersed
beads of polystyrene (PS) sized 784 nm obtained at a concentration of 5% (w/v)
from Microparticles GmbH were dissolved in MiliQ water and further diluted. For
the
fi
nal solution of 600
μ
l, 300
μ
l of the PS-beads dispersion was mixed with 300
μ
l
ethanol containing 1% (w/v) styrene and 0.1% sulphuric acid (v/v). The solution
was applied onto the air
water interface using a Pasteur pipette with a curved tip.
In order to raise the area of the monocrystalline structures, the petri dish was
gently turned, resulting in the transformation of multiple smaller domains into
larger ones. The solution was carefully distributed to cover 50% of the water surface
with a hcp monolayer, while leaving place for stress relaxation and avoiding for-
mation of cracks in the lattice during the next preparation steps. The photoelec-
trochemically conditioned p-InP electrodes were delicately placed under the
fl
oating closed-packed PS sphere mask in the petri dish. Residual water was gently
removed by pumping and evaporation with the mask being subsequently deposited
onto the electrode. After the surface was dried with N
2
, rhodium was photoelec-
trochemically deposited through the PS spheres as described above. The samples
were furthermore rinsed with MiliQ water and dried under a gentle
fl
ow of N
2
. The
PS spheres were removed from the surface by placing the electrodes for 20 min
under gentle stirring in a beaker with toluene. The electrodes were further cleaned
by rinsing the sample with acetone and ethanol for 20 s. In order to remove
residual carbon from the surface, O
2
-plasma cleaning was employed for 6 min at a
process pressure of 0.16 mbar, 65 W and gas in
fl
ows of O
2
and Ar of 2 sccm and 1
sccm, respectively.
Structural and optical characterization
. Soft Tapping Mode Atomic Force
Microscopy (TM-AFM) was used for the characterization of the surface mor-
phology after each treatment step using a Bruker Dimension Icon AFM. In order to
optimize the tapping (mode) frequency and experimental parameters such as gain,
set point and cantilever tuning, ScanAsyst mode was used. ScanAsyst-Air tips
(silicon nitride) were employed with a rotated (symmetric) geometry and a
nominal tip radius of 2 nm. Peakforce Quantitative Nanomechanical parameters
provide information on the height, adhesion and deformation of the sample
surface.
Re
fl
ectance spectra of the thin-
fi
lm and nanostructured photoelectrodes were
obtained in air using a Cary 5000 UV/vis/NIR with an integrating sphere that
include diffuse re
fl
ectivity measurement.
SEM images were obtained with a FEI Nova NanoSEM 450 microscope.
HRTEM analysis was performed with a Philips CM-12 electron microscope
with twin objective lenses as well as a CCD camera (Gatan) system and an
Energy-dispersive spectroscopy of X-rays system to measure the sample
composition. For sample preparation, the thin rhodium
fi
lm deposited on the p-
InP substrate was scratched off and placed onto an amorphous carbon-coated (ca.
50 Å thickness) copper grid. The grid was then transferred to an electron
microscope. A point number of grids was prepared from each sample in order to
ensure the reproducibility of the preparative procedure.
Thin film
Nanostructured
p-InP
p-InP
Ru
Ru
1.
2.
2.
1.
3.
3.
H
+
H
+
2H
+
2H
+
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
a
b
Fig. 5
Cross sectional illustration of a gas bubble evolution model on the
thin-
fi
lm and nanostructured photoelectrode. Whereas H
2
is formed at
discretionary nucleation spots on the thin-
fi
lm electrode surface (
a
)
resulting in gas bubble coalescence and the formation of a bubble froth
layer, the nanostructured Rh surface favours the formation of H
2
gas
bubbles at the induced Rh tips, catalytic hot spots (
b
). Here, concentration
gradients along the surface facilitate H
2
transfer to the bubbles upon
formation. The distance between the hot spots prevents the coalescence of
the formed gas bubbles
ARTICLE
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Photoelectron spectroscopy
. XPS analysis was performed using a system from
VG Scienta with a base pressure below 8 × 10
9
mbar equipped with a Scienta
R3000 analyser and a monochromatic Al K
α
(1486.6 eV) X-ray source. The ana-
lyser was operating at a pass energy
fi
xed at 200 eV for survey scans and 50 eV for
regional spectra acquisition. The used slit sizes were 3 mm and 0.4 mm for the
survey and region scans, respectively. The measured surface area was 5 × 1 mm
2
.
The binding energy scale was calibrated by calibrating the position of the C 1
s
peak at 284.8 eV. The background photoelectron intensity was subtracted by the
Shirley method
27
. The area under the principal peaks of each element in the XPS
spectra and atomic sensitivity factors were used for calculations of atomic
concentrations of the elements in approximately top 3
12 nm of the sample
surface, depending on a sample.
Prior to testing the p-InP
Rh photocathodes in the drop tower, it was
investigated whether they could be prepared in advance of the drop experiments
and then stored under nitrogen atmosphere until used. Two samples were prepared
and stored under nitrogen atmosphere by 4 days. The XPS spectra after four days
of storage did not signi
fi
cantly change from the ones of freshly prepared samples.
Furthermore, structural investigations of the electrode surface prior and after the
drop did not suggest any changes of the surface morphology caused by the capsule
deceleration process.
Photoelectrochemical experiments in microgravity
. Microgravity environments
were realized at the Drop Tower facility at the Centre of Applied Space Technology
and Microgravity (ZARM), Bremen. With the ZARM Catapult System, 9.3 s of
microgravity could be generated
18
. Here, the capsule was launched upwards from
the bottom of the tower by a hydraulically controlled pneumatic piston-cylinder
system and was decelerated again in a container which was placed onto the cylinder
system during free fall of the capsule. The approached minimum
g
level was about
10
6
g
.
For the photoelectrochemical experiments in the drop tower, a custom-made
two-compartment photoelectrochemical cell was used (
fi
lling volume of each cell:
250 mL). Each cell consisted of two optical windows made of quartz glass
(diameter: 16 mm) through which the working electrode was illuminated.
Photoelectrochemical measurements in the two cells were carried out in a three-
electrode arrangement with a Pt counter electrode and an Ag/AgCl (3 M) reference
electrode in HClO
4
(1 M) with the addition of 1% (v/v) isopropanol to reduce the
surface tension of the electrolyte and favour gas bubble release. XPS measurements
and photoelectrochemical measurements in terrestrial conditions did not show any
effect of the isopropanol on the (photoelectrochemical) properties of the
photoelectrodes. The light intensity of 70 mW/cm
2
was provided by a W-I
white-light source (Edmund Optics). All experiments were carried out under
ambient pressure.
Two cameras (Basler AG; acA2040-25gc and acA1300-60gm NIR, lens types:
35 mm Kowa LM35HC 1
Sensor F1.4 C-mount and Telecentric High Resolution
Type WD110 series Type MML1-HR110, respectively) were attached to each cell
via optical mirrors (monochromatic camera, side) and beamsplitters (colour
camera, front, see Fig.
1
c) to record the gas bubble formation in microgravity
conditions. Data were stored during each drop on a Matrox 4Sight GPm integrated
unit in the drop capsule. Single pictures were recorded at a frame rate of 25 fps
(front camera) and 60 fps (side camera).
The photoelectrochemical set-up and the cameras were mounted on an optical
board (Thorlabs) attached to the capsule. Power supply in the capsule was provided
by a battery. Prior to each drop and during the evacuation time of the drop shaft
(about 1.5 h), the capsule and the photoelectrochemical cell were set under Ar
atmosphere which was maintained during the drop and during capsule recovery
after the drop (about 45 min).
For the photoelectrochemical measurements, an automated drop sequence was
written which was started prior to each drop. Upon reaching μg conditions, the
sequence started cameras, illumination sources and potentiostats while
simultaneously immersing the working electrode into the electrolyte using a
pneumatic system (see Fig.
1
and Supplementary Figure
1
for more detailed
information). Photoelectrochemical measurements such as cyclic voltammetry and
chronoamperometric measurements were performed during the 9.3 s of
microgravity. At the end of the drop, when the drop capsule was decelerated again
to zero velocity, the sample was emersed from the electrolyte and the cameras,
potentiostats and illumination source were switched off. The pneumatic system
used for immersing and emersing the sample into and out of the electrolyte
ensured that surface morphology changes of the electrode resulted not from long-
term exposure to the electrolyte prior or after each drop. After retrieving the
capsule from the deceleration container and removal of the protection shield, the
samples were removed from the pneumatic stative, rinsed with MiliQ water and
dried under nitrogen
fl
ux. The sample was stored under N
2
atmosphere until the
optical and spectroscopic investigations were carried out.
Theoretical simulations
. Lumerical FDTD, a commercial electromagnetic simu-
lation software package, was used to optically model the system. To apply the above
set of equations to the structures used here, the following set of assumptions is
made. The current
voltage curve incorporating mass transport considerations
(Eqs. (
2
) and (
3
)) is used for the thin-
fi
lm sample in microgravity environments,
and a limiting mass transport current density,
j
mtl
, of ±5 mA/cm
2
is assumed for
the anodic and cathodic current density respectively; for all other current
voltage
curves Eq. (
1
) is used. These assumptions are based on experimental observations,
as discussed above. The catalyst exchange current density,
j
0,cat
, is assumed to be
0.1 mA/cm
2
, which is consistent with experimental reports in literature for Rh as a
hydrogen evolution catalyst
8
. For the InP|Rh Schottky junction, the dark current
(
j
0
) is assumed to be 10
8
mA/cm
2
. Due to the InP
x
O
y
layer, the ideal equations for
the dark current of a Schottky junction did not accurately describe the system,
therefore, this value is based on a
fi
t to the experimentally measured
current
voltage curves. For the thin-
fi
lm sample under simulated microgravity
conditions,
j
0
was assumed to be 10
5
mA/cm
2
, accounting for enhanced charge
recombination processes in the semiconductor due to mass transfer limitations.
The
f
SA
values for the thin-
fi
lm and nanostructured samples are 1.16 and 1.1,
respectively, and are based on the surface areas of the catalyst as determined from
the AFM data (Fig.
3
b).
Due to the nanostructuring of our catalyst, numerical simulations are required
to accurately determine the limiting photocurrent density,
j
L
, which is needed to
apply the above set of equations to our photocatalytic system. Lumerical FDTD was
used to obtain the InP absorption spectrum,
f
A
(
λ
). The InP absorption spectrum
was furthermore weighted with the lamp spectrum which was used in our
experiment and via integration, the absorbed photocurrent,
j
L
, according to Eq. (
4
)
was obtained. Here,
λ
is the wavelength and
λ
Eg
is the wavelength corresponding to
the semiconductor band edge which is 925 nm for InP.
j
L
¼
R
λ
Eg
0
f
A
λ
ðÞ
AM1
:
5G
λ
ðÞ
d
λ
ð
4
Þ
In the optical simulations, the device structure is de
fi
ned as a semi-in
fi
nite layer
of InP coated with an 8 nm layer of InP
x
O
y
and an effective medium layer of Rh
(see XPS data discussion above and refs.
20
,
28
), all embedded in water. The Rh|H
2
O
effective medium layer is assumed to follow the Maxwell Garnett approximation,
whereas the
fi
ll fraction of Rh was 0.4. For the thin-
fi
lm and nanostructured
samples, a layer thickness of 20 and 25 nm, respectively, was used. For the
nanostructured Rh layer, the pattern is based on the assumption that the
polystyrene spheres were hexagonally close-packed on the electrode surface with
each sphere resulting in a cylindrical opening in the Rh layer, possessing a radius of
200 nm. These assumptions are based on previous publications (see above) and
AFM data on the surfaces (Fig.
3
b).
Data availability
. All relevant data are available from the authors upon request.
Received: 7 March 2018 Accepted: 23 May 2018
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Acknowledgements
K.B. acknowledges funding from the fellowship programme of the German National
Academy of Sciences
Leopoldina
, grant LPDS 2016-06 and the European Space Agency.
Furthermore, she would like to thank Dr. Leopold Summerer, the Advanced Concepts
Team, Alan Dowson, Dr. Jack van Loon, Dr. Gabor Milassin, Marcel van Slogteren and
Dr. Robert Lindner (ESTEC), Robbert-Jan Noordam (Notese) and Prof. Harry B. Gray
(Caltech) for their great support. M.H.R. is grateful for generous support from Prof.
Nathan S. Lewis (Caltech). K.B. and M.H.R. acknowledge support from the Beckman
Institute of the California Institute of Technology and the Molecular Materials Research
Center. M.G. acknowledges funding from the Guangdong Innovative and Entrepre-
neurial Team Program titled
Plasmonic Nanomaterials and Quantum Dots for Light
Management in Optoelectronic Devices
(No. 2016ZT06C517). Furthermore, the author
team greatly acknowledges the effort and support from the ZARM Team with Dr.
Thorben Könemann and Dr. Martin Castillo at the Bremen Drop Tower. It is also
thankful for enlightening discussions with Prof. Yasuhiro Fukunaka (Waseda Uni-
versity), Prof. Hisayoshi Matsushima (Hokkaido University) and Dr. Slobodan Mitrovic
(Lam Research). The team would also like to thank Dr. Eser Metin Akinoglu from the
International Academy of Optoelectronics, Zhaoqing, for his help with the SEM char-
acterization of the samples and Dr. Axel Knop-Gericke (Fritz Haber Institute of the Max
Planck Society) for his generous help with XPS measurements.
Author contributions
K.B., M.H.R., J.L. and H.-J.L. planned and carried out the terrestrial experiments and the
experiments at the Bremen Drop Tower. Ö.A., K.B. and J.L. prepared the nanostructured
photoelectrodes under the supervision of M.G. and H.-J.L. K.T.F. carried out the theo-
retical calculations and simulations. K.B., H.-J.L and K.T.F. wrote the manuscript which
is approved by all authors.
Additional information
Supplementary Information
accompanies this paper at
https://doi.org/10.1038/s41467-
018-04844-y
.
Competing interests:
The authors declare no competing interests.
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