PECDEMO is a Collaborative Project co-funded by FCH JU under the call SP1-JTI-FCH.2013.2.5.
GA n°: 621252. Start date: April 1st, 2014. Duration: 36 months.
Project Final Report
FCH JU Grant Agreement number: 621252
Project acronym: PECDEMO
Project title:
Photoelectrochemical
Demonstrator Device for Solar
Hydrogen Generation
Funding Scheme: FP7-JTI-CP-FCH
Date of latest version of Annex I against
which the assessment will be made: 07/07/2016
Period covered: From Apr 2014 to Mar 2017
Name, title and organisation of the scientific
representative of the project’s coordinator:
Helmholtz-Zentrum Berlin
Prof. Dr. Roel van de Krol
Tel: +49 30 8062 - 43035
Fax: +49 30 8062 - 42434
E-mail: [email protected]
Project website address: http://pecdemo.epfl.ch/
PECDEMO Final Publishable Report
TABLE OF CONTENTS
1.1. Executive summary ........................................................................................................ 1
1.2. Summary description of the project context and the main objectives .............................. 2
1.2.1. Context ............................................................................................................................ 2
1.2.2. Approach and main objectives ........................................................................................ 2
1.3. Description of the main S & T results/foregrounds ......................................................... 4
1.3.1. Work Package 1 ........................................................................................................... 4
1.3.2. Work Package 2 ............................................................................................................... 8
1.3.3. Work Package 3 ............................................................................................................. 13
1.3.4. Work Package 4 ............................................................................................................. 17
1.3.5. Work Package 5 ............................................................................................................. 22
1.3.6. Work Package 6 ............................................................................................................. 24
1.4. Potential impact (including socio-economic impact and wider societal implications) and
the main dissemination activities and exploitation of results ................................................... 29
1.4.1. Potential impact ............................................................................................................ 29
1.4.2. Dissemination activities ................................................................................................. 30
1.5. Public website and relevant contact details .................................................................. 33
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1.1. Executive summary
PECDEMO’s main aim was to develop a photoelectrochemical (PEC) water splitting
device based on low-cost and abundant materials that shows a solar-to-hydrogen
(STH) efficiency of 10%, a stability of 1000 hours, and an active area of at least 50 cm2.
PECDEMO has addressed these challenges by focussing its efforts on three metal oxide
photoelectrode materials (Fe2O3, BiVO4, and Cu2O) and by combining them with a
silicon- or perovskite-based photovoltaic (PV) cell in a tandem configuration. To
improve the efficiency and stability of the metal oxides, modifications were made by
doping, application of protection layers, nanostructuring, and surface
functionalization with co-catalysts for hydrogen or oxygen evolution. Fe2O3 is the most
stable material; lab tests showed negligible performance decrease after 1000 h
operation. A new hydrogen treatment method significantly improved the
performance of BiVO4 photoanodes, resulting in a 9.2% STH efficiency for a small-area
dual BiVO4/Fe2O3 photoanode/Si PV tandem cell. PECDEMO’s highest efficiency
achieved for small-area devices was 16.2%, obtained for a Ga2O3/Cu2O nanowire
photocathode coupled to a silicon PV cell using a dichroic mirror for photon
management. The highest large-area photocurrent densities were obtained for Cu2O,
giving an unprecedented 3.5 mA/cm2 for a 50 cm2 photoelectrode.
Various large-area cell designs for were studied, resulting in an optimized design that
features an open path for sunlight from the front to the back window, with counter
electrodes placed at both sides of the central photoelectrode. CFD simulations were
used to ensure an optimal flow path of the electrolyte, resulting in efficient removal of
gas bubbles and good thermal management; the temperature of the cell did not
increase above 55°C even under 17-suns concentrated light. Based on this design, a
modular array of four PEC cells of 50 cm2 each was constructed for field tests on the
SoCRatus facility at DLR in Cologne. The cell design showed limited cross-over of H2,
but the efficiencies for BiVO4 and Fe2O3 were modest under concentrated sunlight –
presumably due to poor carrier transport in these materials.
Two conceptually new innovations were made to further improve the PEC concept. A
power management scheme that allows co-generation of electricity and hydrogen;
in combination with active light management, the PEC efficiencies can exceed those
of PV-electrolyzer systems. The second one is the use of auxiliary NiOOH/Ni(OH)2
electrodes, which avoids the need to separate H2/O2 reaction products within the
same cell. This significantly reduces the overall complexity and costs of the concept.
Plant design studies showed that cooling is a crucial issue, especially under
concentrated sunlight. Life-cycle analyses revealed that the PEC-PV approach is
potentially best in class in terms of global warming potential. Economic analysis
showed that PEC-PV generation can compete with PV-driven electrolysis. However,
STH efficiencies higher than 8%, solar concentration factors > 30, cell temperatures
above 60°C, and active areas approaching 1 m2 should be pursued.
Finally, all PECDEMO targets (10% efficiency, 1000 h stability, 50 cm2) have been
individually achieved, but meeting them simultaneously with a single system remains
a major challenge to be addressed.
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1.2. Summary description of the project context and the main
objectives
1.2.1. Context
Sunlight is by far the largest sustainable source of energy, and there is little doubt that
it will play a major role in any conceivable future energy scenario. One of the main
challenges for the large-scale use of solar energy is its intermittent nature, which
requires intermediate storage solutions. An attractive pathway to achieve this is by
directly converting an abundant resource, such as water, into hydrogen using sunlight.
The hydrogen can then be used directly as a fuel, or further processed into liquid
hydrocarbons. These ‘solar fuels’ have up to 100 times higher energy and power
densities than the best batteries and can be stored indefinitely.
PECDEMO aimed at developing a PhotoElectroChemical DEMOnstrator that
splits water into hydrogen and oxygen under solar irradiation. By integrating the light
absorption and electrolysis functionalities into a single device, significantly lower
balance-of-systems costs than coupled photovoltaic-electrolysis systems are, in
principle, possible. Efficient and cost-effective solar hydrogen production would thus
solve one of the major challenges for a solar-driven society, i.e., that of efficient large-
scale storage of solar energy. However, before this dream becomes reality, some hard
technological and economic targets have to be met. As outlined in the call that
PECDEMO addressed, solar-to-hydrogen energy conversion efficiencies of 8-10% have
to be achieved and lifetimes of more than 1000 h need to be demonstrated. Only
then will there be a realistic chance to meet the FCH-JU’s cost target of 5 € per kg H2
and can this technology have a significant impact on society.
1.2.2. Approach and main objectives
Building on the breakthroughs achieved in the highly successful EU project
“NanoPEC”, PECDEMO partners aimed to develop a module-sized hybrid tandem
device for solar water splitting based on a stable metal oxide photoelectrode as a
wide-bandgap top absorber and an efficient photovoltaic solar cell as a small-
bandgap bottom absorber. Based on earlier work by the partners, three candidates
were selected as promising metal oxide photoelectrode materials: Fe2O3, BiVO4, and
Cu2O. The stability and durability of the photoelectrodes was planned to be
enhanced through functionalization with efficient electrocatalysts, by applying
selective transport layers and protective coatings, and selection of suitable electrolyte
solutions and operating conditions. The photovoltaic cells were to be optimized for
tandem operation with the metal oxide photoelectrodes. Here, silicon-based
photovoltaic cells and the emerging class of perovskite PV cells have been selected
as the most suitable candidates.
The second aim was to demonstrate the scalability of this technology by
combining multiple devices into a larger water splitting module. Nearly all previous
efforts in the field of photoelectrochemical water splitting have been done on < 1 cm2
cells, with only very few exceptions. At such small length scales, ion transport between
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the electrodes is sufficiently fast. At larger length scales, however, resistive losses due
to mass transport limitations in the electrolyte quickly start to dominate the overall
performance. Innovative cell designs are needed to minimize these losses and to
manage the transport of photons, electrons, and ions in the water splitting system.
To achieve the project goals, five science and technology objectives were defined:
1. To demonstrate a chemically stable and highly efficient stand-alone hybrid
water splitting cell based on a metal oxide photoelectrode in tandem with a
photovoltaic solar cell
2. To develop deposition technologies that are suited for fabricating components
for large-area hybrid PEC-PV devices
3. To design, construct, and test complete large-area hybrid PEC-PV devices for
water splitting
4. To carry out extensive techno-economic and life-cycle analyses based on the
devices’ demonstrated performance characteristics, and evaluate the
potential for large-scale commercialization
5. To build a prototype module consisting of an array of large area devices and
to test this prototype in the field
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1.3. Description of the main S & T results/foregrounds
1.3.1. Work Package 1
The first work package (WP1) focuses on the development of an efficient and
chemically stable small-area hybrid PEC-PV water splitting device, by utilizing metal
oxides as the PEC materials. Three metal oxides have been chosen to be our main
focus: iron oxide (Fe2O3), bismuth vanadate (BiVO4) and cuprous oxide (Cu2O).
Activities in the last three years include the development of: (1) small-area metal oxide
photoelectrodes, (2) optically-transparent counter electrodes, (3) PEC-PV without
photon management (1st generation device), (4) effective light management
strategies, and (5) PEC-PV including photon management (2nd generation device).
The summary of these activities and achievements within WP1 is as follows.
Fe2O3 photoanode
Prior to PECDEMO, the champion Fe2O3 photoelectrode utilized a resonant light
trapping structure, by depositing hematite on Ag-coated glass substrates.2 One of the
limitation, however, is the oxidation of Ag during the high-temperature deposition of
hematite, which in turn loses its specularity. In order to overcome this, within PECDEMO,
we reversed the order of deposition of the films, so that the Ag layer is deposited after
hematite. In short, hematite was deposited on a Si substrate, and a Ag layer was
deposited on top of this
hematite film. The film is then
flipped over and attached
to another Si substrate.
Finally, the first Si substrate
was removed by dry
etching, thereby exposing
the hematite layer at the
surface. This process
therefore allows Ag layer to
be deposited at low
temperature and in
vacuum, which is ideal for
specular mirror deposition.
The improved results can be seen in the photographs shown in Figure 1a and b.
In addition, we have also developed heteroepitaxial hematite photoanodes with high
crystalline quality. We first deposited a platinum layer (as a bottom contact and a
reflector) on top of (0001) basal plane sapphire, followed by growth of the hematite
layer with pulsed laser deposition. The in-plane alignment of the film stack is
investigated by azimuthal φ-scans of the off axis peaks as shown in Figure 1c and
verifies the epitaxial growth of the layers. We are still in the process of optimizing the
heteroepitaxial films for photoelectrochemical performance, and have achieved
photocurrents of 1.8 mA/cm2. Noteworthy, the flat-band potential of heteroepitaxial
Ti-doped hematite films was found to be ~0.2 VRHE, considerably lower than reported
values for polycrystalline hematite photoanodes that typical range between 0.4 and
0.6 VRHE.3 This may open up another route to reduce the potential of hematite
photoanodes. Our efforts in heteroepitaxial hematite can be found in our recent
publication.4
Figure 1. Photographs of (a) the champion ultrathin films
hematite photoanode from 2013 and (b) one of the most recent
photoanodes obtained by the film transfer process. (c) Off-axis
φ scans of Al2O3(104), Fe2O3(104), and Pt(200) reflections.
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BiVO4 photoanode
We have explored the possibility of depositing BiVO4 with various techniques, such as
drop casting and reactive magnetron sputtering, but the performance of our Co-Pi
catalysed, spray-pyrolysed, W-doped BiVO4 photoanode is still superior.5 At the
beginning of PECDEMO, in the efforts of transitioning towards large scale BiVO4, we
have also improved the reproducibility of our spray pyrolysed BiVO4; we have now a
sample-to-sample photocurrent reproducibility of ~5-10%. To improve the
photocurrent further,
we have explored
several other dopant.
Calcium-doped
BiVO4 was first studied
in order to produce a
p-type conductivity,
but the improvement
is limited due to
segregation of
calcium. A successful
treatment is to anneal
BiVO4 in a mild
hydrogen
atmosphere (2.4% H2
in Ar) at relatively low
temperature of 300
C. The onset potential and plateau photocurrent were both improved by hydrogen
treatment, as shown in Figure 2a. An AM1.5 photocurrent of 4 mA/cm2 was achieved
at 1.23 V vs RHE for a hydrogen treated W-doped BiVO4. At this point, we were limited
by the absorption in our thin film BiVO4. The absorption can be simply improved by
increasing the thickness of the film, but unfortunately the carrier diffusion in BiVO4 (<
100 nm) does not allow efficient carrier transport in a thick film. We therefore
implemented a dual photoelectrode approach (see Figure 2b), where an additional
BiVO4 photoelectrode was placed behind the same BiVO4. This simple approach—
which is commonly used in the field of organic PV but not to large extent in PEC water
splitting—resulted in further photocurrent improvement to 4.8 mA/cm2 at 1.23 V vs RHE
and > 5.4 mA/cm2 at 1.7 V vs RHE. This satisfied the deliverable 1.1 (first generation
metal oxides with a photocurrent of at least 5.4 mA/cm2) of PECDEMO. The 20%
photocurrent improvement (i.e., from 4 to 4.8 mA/cm2) can also be replicated by
implementing a photon management strategy: we deposited a distributed Bragg
reflector (DBR) at the back-side of the substrate and therefore removing the necessity
of having dual photoelectrode (deliverable 1.3 — proof of enhanced performance
using a photon management strategy).
Cu2O photocathode
In the case of Cu2O photocathode, we have focused on developing a
transparent Cu2O photocathode toward the ultimate goal of designing an efficient
PEC-PV stacked tandem configuration. For the efficient transparent Cu2O
photocathode, we optimized the thicknesses of Au underlayer and Cu2O flat film (see
Figure 3a for the structure of the photoelectrode), because these are crucial
Figure 2. (a) AM1.5 photocurrent-voltage curves of W-doped BiVO4
(black), hydrogen-treated W-doped BiVO4 (blue), and dual hydrogen-
treated W-doped BiVO4 (red). The respective labels indicate the
photocurrent value at 1.23 V vs RHE. The schematic of dual BiVO4
photoelectrode is shown in (b).
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parameters affecting the incident light to the PV device placed behind the
photocathode. Consequently, a photocurrent density was reached up to 5.4 mA cm- 2
at 0 V vs RHE (deliverable 1.1 PECDEMO) in pH 5 electrolyte under 1 sun condition using
3 nm Au underlayer and 260 nm Cu2O thickness (Figure 3b). This performance was
comparable to that achieved on standard photoelectrodes on thick Au substrates,
with the advance that the device showed transmittance of around 35 % for
wavelengths longer than 550 nm. We improved the transparency further by
developing an alternative underlayer in the form of Cu-doped NiO. The transmittance
was further increased by ~10-20 % for wavelengths longer than 550 nm, while
maintaining the PEC performance.
The performance of transparent Cu2O photocathode was still low compared
to the theoretical performance. It is mainly attributed to the imbalance of the carrier
diffusion length and the light absorption depth of Cu2O.6,7 Nanostructuring is a
promising way to solve this problem, enabling the further improvement of Cu2O
photocathode performance. We recently succeeded to prepare well characterized
Cu2O nanowire array photocathodes through electrochemical anodization and
thermal annealing. The Cu2O nanowire arrays were high-quality and pure and were
adapted into the photocathode device structure by depositing AZO/TiO2 overlayers
and RuOx catalyst (Figure 3c and d). Remarkably, photocurrent densities up to 8 mA
cm-2 at 0 V vs RHE were reached in pH 5 electrolyte under 1 sun condition, with
Figure 3. (a) False-color cross-section SEM image of a Cu2O-based photocathode, indicating the
different underlayers and overlayers. (b) PEC performance of Cu2O photocathode with different
thicknesses on 3 nm Au underlayer substrates. (c) Scanning electron microscope image of Cu2O
nanowire with AZO/TiO2 overlayers. Inset shows transmission electron image of a single composite of
Cu2O/AZO/TiO2/RuO2 nanowire. (d) Linear sweep voltammetry scan under chopped illumination (1
sun) in the pH 5 electrolyte of Cu2O nanowire photocathode with AZO (black) and Ga2O3 (red)
overlayers.
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photocurrents exceeding 10 mA cm-2 at more negative potentials. With the
photocurrent level improved, we finally enhanced the performance of Cu2O
photocathode by replacing the AZO overlayer with Ga2O3. Ga2O3 has a minimal
conduction band offset with Cu2O, resulting in a maximized built-in potential and an
anodic shift of the onset potential up to 1.0 V vs RHE, as shown in Figure 3d.
Our improvement efforts on Cu2O photocathodes can be found in our recently
published articles.8-10
PEC and PV stability
The stability of our photoelectrodes have also been investigated under long-term
AM1.5 exposure. Fe2O3 photoanode is known to be highly stable, and we show for the
first time a stability data of up to 1000 hours (See D1.4), achieving our deliverable 1.4
target (less than 10% performance decrease after 100 hours of operation). No
noticeable degradation was observed; full details on this study were recently
published.11 BiVO4 photoanode is expected to be stable in neutral pH electrolyte, but
we observed a photocurrent decrease within the 100-hour measurement period (See
D1.4). The decrease is, however, not related to material degradation, but due to sub-
optimal PEC cell design. Bubbles formed rapidly and trapped at the surface of BiVO4,
causing decreased effective surface area. More optimal cell design is expected to
fully resolve this issue. In alkaline environment, protection layer consisting of TiO2 and
Ni successfully improves the stability, although it is still limited to less than one hour.
Cu2O photocathodes’ stability is shown to be enhanced with the protection layer
strategy that we developed. Although 10% performance decrease is observed within
~55-60 hours, the improvement in stability is unprecedented for Cu2O photocathodes.
For the PV, perovskite solar cell shows increasing efficiency within the first 500 hours of
measurement, with no noticeable change of efficiency afterwards, up to more than
2000 hours of operation. HIT silicon solar cell shows stable short-circuit current and
open-circuit potential within 100 hours, and a slight decrease of fill factor is observed.
Overall, the efficiency decreases only by less than 4%. Detailed description of our
stability tests and results have been published in a public deliverable 1.4 report.
PEC-PV devices
We then combined the photoelectrodes and PV cells developed within
PECDEMO to form a hybrid tandem water splitting device. First, for the first generation
device (i.e., stacked tandem configuration), we have initially simulated the expected
Figure 4. (a) Schematic setup of our 2nd generation PEC-PV tandem configuration, consisting of a
Cu2O photocathode, an IrO2 anode, a HIT solar cell and a dichroic mirror. (b) Chopped AM 1.5 short-
circuit photocurrent density and calculated STH efficiency of the real PEC-PV tandem device. The
green dashed line is the 10% STH efficiency target.
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solar-to-hydrogen (STH) efficiency from a combination of our photoelectrodes and
multiple multi-junction solar cells. Based on the simulation, we then fabricated a solar
water splitting device consisting of the dual BiVO4 photoelectrode described above
(see Figure 2a) and a 3-HIT series silicon solar cell delivering an STH efficiency of 7.5%
(Figure 5a). To improve the efficiency further, we attempted to address the
fundamental limitation of BiVO4, which is the relatively large bandgap of 2.4 eV. While
our efforts in decreasing the bandgap of BiVO4 is still ongoing, we have simply
extended the light harvesting ability of the dual photoelectrode by combining a BiVO4
and a Fe2O3 photoanode (Figure 5b). As a result, we obtained STH efficiency of 9.2%,
which fulfils the deliverable 1.2 target (1st generation device showing 8% efficiency)
and is already very close to the deliverable 1.5 target (2nd generation device showing
10% efficiency). Detailed results on this dual BiVO4-Fe2O3 photoanode can be found in
our recent publication.1
Finally, for our 2nd generation device, we assembled a hybrid PEC-PV tandem cell
employing a Cu2O–Ga2O3 NW photocathode (see Figure 3d), a HIT PV cell and an
IrO2 anode. For this demonstration, we adopted the tandem configuration with a 600
nm cut-off wavelength dichroic mirror (as shown in Figure 4a). As a result, we
observed operating AM1.5 short-circuit photocurrent of 6.98 mA/cm2 with 2 HIT PV
cell and 10.96 mA/cm2 with 3 HIT PV cell (Figure 4b). The corresponding STH
efficiencies are 10.3 % (2 HIT PV) and 16.2% (3 HIT PV). This result therefore successfully
fulfil our deliverable 1.5 target, as well as the final PECDEMO WP1 target of a device
showing efficiency larger than 10%. Additional information on these PEC-PV devices
can be found in our public deliverable 1.5 report (see PECDEMO website).
1.3.2. Work Package 2 The second work package (WP2) aims to guide the optimization efforts of PEC-PV
tandem cells (WP1) and modules (WP4) by identifying material degradation processes
and efficiency losses; quantifying their effect on the long-term stability and efficiency;
scrutinizing materials compatibility for stable long-term operation with minimal
degradation and efficiency losses; and optimizing the optical and electrical coupling
of the PEC and PV cells. WP2 has five main tasks.
Figure 5. Schematic illustration of a dual (a) BiVO4-BiVO4 and (b) BiVO4-Fe2O3 photoelectrode used in
tandem configuration with a silicon solar cell. STH efficiencies of 7.5% and 9.2%1 have been achieved
with these configurations, respectively.
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Task 2.1: Develop and implement diagnostic methods to identify materials
degradation processes. This task involved work on Cu2O photocathodes which are
known to be intrinsically unstable in aqueous electrolyte solutions, as well as on BiVO4
photoanodes which, although kinetically stable in neutral aqueous electrolyte
solutions, are unstable in either acid or base solutions. Fe2O3 photoanodes are
thermodynamically stable in base (alkaline) aqueous electrolyte solutions and
typically they do not degrade even after repeated long-term operation in alkaline
solutions. The efforts to stabilize Cu2O photocathodes and BiVO4 photoanodes were
carried out by EPFL and HZB, using passivation overlayers deposited by ALD, as
described in the WP1. Here in WP2 we present our efforts to diagnose the effectiveness
of the passivation overlayers. In order to identify the root cause for Cu2O and BiVO4
photoelectrode degradation, different techniques were used such as:
electrochemical measurements, SEM, TEM, XPS and more. An example of Cu2O
photocathode characterization is presented as Figure 6. The TiO2 overlayer is shown
by the green color in the TEM image.
These characterisation techniques enable us to develop efficient passivation
overlayers, which supress the degradation process in Cu2O and BiVO4 photo-
electrodes and achieve high stability as published recently in Nano Letters9 and
Nature Communication5. Further information on these characterization techniques
can be found in these publications.
Task 2.2: Develop and implement diagnostic methods to identify and quantify
efficiency losses. Within this task, two new diagnostic methods were developed in
order to identify and quantify efficiency losses due to charge separation and
recombination processes. Operando diagnostics of Fe2O3 photoanodes, carried out
at IIT, provides the photocurrent and photovoltage generated by the photoanode, as
presented in Figure 7.
Figure 6. Cu2O photocathode characterisation by SEM (Left), TEM (right) and
electrochemical measurement (inset).
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This information is used to extract
the maximum power point and the
intrinsic solar to chemical
conversion (ISTC) efficiency of the
photoanode. This information is
important for the design of the
electrical coupling between the
PEC and PV cells (Task 2.4). Further
information about this diagnostics
can be found in the Journal of
Physical Chemistry Letters12.
Photoelectrochemical
spectroscopy combines PEIS
(Photo Electrochemical
Impedance Spectroscopy), IMPS
(Intensity Modulated Photocurrent
Spectroscopy) and IMVS (Intensity
Modulated Photovoltage
Spectroscopy) to provide new
insights on the charge carrier
dynamics involved in the water
photo-oxidation process. This
method was applied at IIT to investigate charge carrier dynamics in Fe2O3
photoanodes and allowed us to quantify the hole current reaching the hematite –
electrolyte interface and the recombination current at interface as presented in
Figure 8. Further information on this method can be found in Physical Chemistry
Chemical Physics13. Similar analysis was carried out for BiVO4 photoanodes at HZB. We
investigated the effect of Co-Pi catalysts on the surface of BiVO4. It has been shown
by many research groups—including HZB—that Co-Pi effectively improves the
performance of BiVO4 photoanode, due to an increased charge injection efficiency.
However, the true nature of the improvement is not clear. Our IMPS study revealed
that the surface recombination rate decreases by more than 1 order of magnitude
upon deposition of Co-Pi on BiVO4 surface. Surprisingly, the charge transfer rate is not
really affected by the introduction of Co-Pi; in fact, it is slightly decreased. This result is
intriguing since it implies that Co-Pi does not function as a “true” catalyst when
deposited on the surface of BiVO4. Instead, it acts as a surface passivation layer,
Figure 7. A typical current density (J) vs. potential (U)
voltammogram of thin film Fe2O3 photoanodes
measured in 1M NaOH aqueous solution in the dark
(dashed) and under AM1.5G solar simulated
illumination (full).
Figure 8. Deconvolution of the water photo-oxidation current (black dots) of Fe2O3 photoanode into
the hole current (red dots) and recombination current (blue dots).
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reducing the surface recombination. Further information on this study and the
implication can be found in our recent Chemical Science article14.
Task 2.3: Modeling the optical coupling of the PEC and PV cells in PEC-PV tandem cells.
Detailed modeling of the optical
coupling between the PEC and PV
cells in PEC-PV tandem cells was
carried out for the conventional
stacked cells configuration as well as
for advanced configurations that
employ spectral splitting between
the PEC and PV cells in order to
improve the solar to hydrogen
conversion efficiency of the tandem
cell. For example, at HZB we
modeled and tested BiVO4
photoanodes coupled in tandem
with amorphous and nano-crystalline
silicon PV cells. The results are
presented in Figure 9. Further
information can be found in recent
articles published by HZB1,15,16 and
EPFL17.
Several spectral splitting schemes with passive or active light management were
explored in the PECDEMO project, in order to tailor the degrees of freedom of the
tandem system to make it more efficient (but also more complicated). One example
of an active light management design is presented in Figure 10. When the PEC cell is
active, during hydrogen generation, the incident light is splitted between the PEC
(Absorber 1) and the PV (Absorber 2) as presented in the left figure (A). On the other
hand, if only power generation is required the system turns to its “off” state, presented
in the right figure (B). This active light management design allows the system to actively
control the portions of the light that go to the PEC and PV cells in order to tailor
hydrogen and power productions (see Task 2.4 below). This scheme is especially
attractive for tandem cells that co-generate both hydrogen and electrical power as
explained in ACS Energy Letters18.
Figure 9. The EQE curves of the a-Si:H/nc-Si:H solar
cell, the a-Si:H top junction (black), and the nc-Si:H
bottom junction (red). Dashed curves indicate EQE
spectra of the PV junctions under full AM 1.5
simulated solar illumination; solid curves indicate the
EQ
Figure 10. Active light management design in a two-absorber tandem system. A) Absorber 1 (PEC)
is active (“on”). B) Absorber 1 (PEC) is inactive (“off”).
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Task 2.4: Modeling the electrical coupling of the PEC and PV cells. Modeling and
testing the electrical coupling of the PEC and PV cells started from simple coupling by
series connection of the two cells1,15-17. This scheme leads to a large loss of electric
power generated by the PV cell, which is typically much larger than the power used
by the PEC cell. In order to rectify this loss, we invented a new design that splits the
power generated by the PV cell between the PEC cell and another consumer, i.e.,
co-generation of hydrogen and power. The electrical power can power auxiliary
systems such as cooling,
flow and compression,
or be sold to the grid.
Further optimization can
be achieved using
power convertors that
enable continuous
tracking of the
maximum power point
of the PEC-PV tandem
system. Applying the
power splitting
approach enables to
overcome the
efficiency of PV-
electrolysis systems, as
presented in Figure 11
and explained in ACS
Energy Letters18
Task 2.5: Predictive
modeling of large-area
PEC-PV tandem cells. The series resistance loss due to the resistance of the transparent
electrode that collects the photocurrent from the photoelectrode in the PEC cell
becomes critical in large-area PEC-PV tandem cells. Figure 12 presents an analysis of
the effect of the series resistance on the photocurrent as a function of the size of the
photoelectrode. The analysis was done for a transparent electrode with a sheet
resistance of 15 /square, which is
typical for FTO-coated glass
substrates (TEC15). It clearly shows the
adverse impact of the series
resistance on large area cells. This
effect must be rectified using metallic
grid lines, as implemented in WP4.
Solar plant design. One of the
greatest challenges in large-scale
solar water splitting plants is the
separation of the hydrogen from the
oxygen and the collection and
transport of the hydrogen from
millions of PEC cells distributed in the
solar field to a central hydrogen
distribution facility. This involves an
immense sealing and piping
constructions that puts a heavy
Figure 12. Calculated photocurrent density as a
function of the photoanode radius for a series
resistance of 15 /square.
0 5 10 15 201
1.5
2
2.5
3
3.5
4
Photoanode Radius [cm]
J (
U=
1.5
V v
s R
HE
) [m
A]
Figure 11. Calculated figure of merit (FOM, the ratio between the
total power produced by the PEC-PV tandem system and the power
produced by the PV system alon) as a function of the fraction of the
total power production that goes toward chemical power
generation
PECDEMO Final Publishable Report
13
burden on the hydrogen production economy. To overcome this challenge we
invented a new PEC cell design with separated oxygen and hydrogen cells.
According to this design, the PEC solar cells produce oxygen that is simply discharged
to the atmosphere, whereas the hydrogen is produced elsewhere in another cell. The
ion exchange between the two cells is mediated with a set of auxiliary electrodes
made of NiOOH and Ni(OH)2 that undergo a reversible redox reaction exchanging
OH- ions with the primary electrodes (i.e., the photoanode and cathode) in the oxygen
and hydrogen cells. This enables centralized hydrogen production far away from the
solar field, as explained in Nature Materials.19
1.3.3. Work Package 3
Fabrication of large area TCO-coated glass substrates.
In order to fabricate large-area transparent conducting oxide (TCO) electrodes spray
pyrolysis based fluorine-doped tin oxide (FTO) deposition was developed. This process
allows to prepare custom-made conductive substrates with the resistivity and
transmittance directly related to the FTO thickness deposited (Figure 13). It is a flexible
process that allows coating a large variety of substrates as long as the raw material is
able to withstand the high
temperatures. TCOs on up to
35 x 35 cm² were deposited
and characterized including
double-side coating, e.g.
suitable for tandem devices.
Fabrication of large-area
photovoltaic (PV) cells
A mesoscopic
methylammonium lead
iodide perovskite/TiO2
heterojunction solar cell with
low-cost carbon counter
electrode and full screen
printable process was built
on a monolithic design
without any extra organic hole conducting material (Figure 14). These cells were
claimed to be air stable under illumination. Such a perovskite, fully printable
mesoscopic solar cell was deposited on an FTO covered glass. The mesoporous layers
were infiltrated with perovskite by drop-casting from solution through an 8 µm thick
carbon layer printed on top of the ZrO2. The dense TiO2 layer deposited on the FTO
conducting glass prevents the valence band holes from reaching the FTO-covered
front electrode. With a complete set of new screen printable materials: (1) Ti-Nanoxide
Figure 14. Left: SEM cross section of a monolithic perovskite test cell. Right: Perovskite PV Module
Figure 13. Sheet resistance of FTO with various thicknesses.
Insert: Examples of FTO coated glass.
PECDEMO Final Publishable Report
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BL/SP (50 nm TiO2 based
electron blocking layer); (2) Ti-
Nanoxide T600/SP (600 nm
nano-TiO2 scaffolding layer);
(3) spacer layer: Zr-Nanoxide
ZT/SP (1 µm nano-ZrO2 layer);
(4) highly conducting carbon
layer: Elcocarb B/SP (5 ~10
µm, 10 ~ 20 ohm/sq resistivity) cell efficiencies over 13 % were achieved on lab scale
(1.2 cm2). Monolithic modules (64 cm2 aperture area) with an optimized design to limit
the electrical losses showed an efficiency of 11% (Jsc =16.3 mA/cm2, Voc = 7.1V). The
perovskite deposition process was automated to improve safety, reproducibility and
productivity. By means of a DC-DC converter the appropriate voltage for biasing the
photo-electrode can be obtained. For direct integration in PEC+PV tandem devices
silicon heterojunction (SHJ) modules were developed, fabricated and analyzed, e.g.
upon various illumination condition conditions. Such SHJ based modules offer the
advantages that high PV efficiencies exceeding 20% are possible and the excellent
near-Infrared light absorption and high voltage makes it appropriate for application
as bottom cell in such a tandem configuration with a wide-gap photo-electrode (PE)
in front. In order to match the required voltage to bias the PE two PV cells have been
interconnected (Figure 15). Modules reached efficiencies of 15.7% under one sun
(Figure 16), which allow for STH efficiency of complete PEC-PV tandems devices
exceeding 8%.
Evaluation of deposition techniques for fabrication of counter-electrodes
Spray coating, a well-known method to prepare thin particle layers on substrates (e.g.
varnishing), was adapted for fabrication of counter-electrodes for the
electrochemical hydrogen and the oxygen evaluation reaction (HER/OER). An
automated setup was built with an airbrush moveable in two axis. The developed ink
contains the catalyst, a volatile solvent mixed of water/ethanol and an additional ion-
conductive binder on basis of Nafion®. Different catalysts (iridium dioxide, platinum
on carbon, Evonik OER rare metal free) were deposited in different loadings on a
porous nickel foam (1.6 mm thickness) as substrate. Ex-situ testing was done in a three-
electrode setup similar to the common rotating disk electrode (RDE) technique. For
measurements at high current densities, electrodes were mounted to a motor, so that
the catalyst coated porous substrate can rotate around its own axis. This leads to an
efficient electrolyte flow right through the electrode enhancing the mass transport,
especially the removal of the produced gas. The polarization curves in Figure 17 show
that the spray-coated electrodes are suitable for the electrochemical water splitting.
Figure 15. Cross section of PE + PV tandem device with two
interconnected Silicon heterojunction cells.
Figure 16. Left: 50 cm² PV module consisting of two Si cells. Right: IV date of SHJ cells and modules
Samplearea
(cm²) (%)
Jsc
(mA/cm²)
Voc
(V)
FF
(%)
Vmpp
(V)
Impp
(mA)
Median of 14 cells (reference) 4 21.0 36.5 728 79.2
Best 5x5 cm² cell cut out of wafer 25 16.0 34.1 699 67.0
Best 2-cell module 50 15.7 17.4 1.40 64.7 1.0 15.3
Module with hematite PE 50 10.6 11.7 1.36 66.8 1.0 10.3
PECDEMO Final Publishable Report
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Beside the known good activities of state-of-the-art catalysts is the activity of a rare
metal free catalyst similar to IrO2.
Large-scale Cu2O photocathode
Large-area, transparent photocathode consisting of electrodeposited Cu2O thin film,
atomic-layer deposited AZO/TiO2 over-layers and photo-deposited RuOx catalysts
were deposited on up to 5 x 10 cm2 area. For enhanced charge collection we applied
metal grids and contacts on the Cu2O photocathode. By optimizing metal grids and
contacts on edges, we demonstrated that sputtered Cu grids and Paste-based Ag
contacts on edges are effective to improve the charge collection on the large-scale
Cu2O photocathode. Especially, Ag contacts on edges improved the resistive PEC
performance, while Cu grids assisted to enhance photocurrent density. We could get
a photocurrent density of 3.7 mA cm-2 at 0V vs RHE, corresponding to a STH efficiency
of 4.6 % using Cu grids and Ag contacts in pH 5 electrolyte under 1 sun illumination
from LED light source (Figure 18 a, c). We finally introduced the Ga2O3 overlayer to the
Figure 18. (a) A transparent large-scale AZO overlayered Cu2O photocathode with Cu grids. (b) A
transparent large-scale Ga2O3 overlayered Cu2O photocathode with an active area of 5 x 10 cm2.
(c) Linear sweep voltammetry scans in the pH 5 electrolyte under 1 sun illumination from LED light
source of large-scale Cu2O photocathode with AZO overlayer (black), AZO overlayer/Cu grids (blue)
and Ga2O3 overlayer (red).
Figure 17. Polarization curves of deposited counter-electrodes with OER-catalysts (left) and HER-
catalyst (right). Electrodes were made by spraying a catalyst ink on a nickel foam as porous
substrate. The measurements were done in a potentiostatic mode vs. an Ag|AgCl 3M KCl
reference in 1M KOH at 25 °C. At each point, the ohmic cell resistance was determined and IR
correction of applied potential was performed. The geometrical area of the deposited electrode
was 1 cm².
PECDEMO Final Publishable Report
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large-scale Cu2O photocathode instead of AZO overlayer. Consequently, the
optimized Ga2O3 film by atomic layer deposition was homogeneous on the large-
scale, resulting in an anodic shift of the onset potential up to 0.9 V vs RHE.
(Figure 18 b, c).
Large-scale W:BiVO4 Photo-anodes
Large-scale photo-anodes consisting of TEC 15TM FTO substrate with a
electrodeposited Ni grid, a spray pyrolysis deposited thick 1% Tungsten doped BiVO4
(W:BiVO4) absorber (~ 200 nm), and photo-deposited CoPi catalyst were fabricated
on up to 7 x 12 cm2 area, with an active area of 5 x 10 cm2. In order to limit the ohmic
losses, 200 nm thick and 2 mm wide Ni lines were electrodeposited onto treated FTO
substrates spaced 9 mm apart prior W:BiVO4 deposition, and Paste-based Ag contacts
and Cu tape were placed along the edges. As with the large scale Cu2O
photocathodes, the Cu contacts on edges improved the resistive PEC performance,
while Ni grids assisted to enhance photocurrent density. We finally introduced a rapid
annealing step of the W:BiVO4 photoanodes in 2% H2 Ar 98% atmosphere at 320 oC for
10 mins which shifted the photocurrent onset potential from ~0.4 V to 0.3 V vs RHE and
a maximum photo current density of 1.8 mAcm-2 at 1.25 V vs RHE can was achieved
with W:BiVO4 without CiPi in pH 7 1.0 M KPi, 0.5M Na2SO3 (hole scavenger) electrolyte
(Figure 19). For the optimised large area W:BiVO4 photoanodes with CoPi catalyst and
an active area of 5 x 10 cm2 we could achieve a photocurrent density of 1.5 mAcm2
at 1.23 V vs RHE corresponding to a STH efficiency of 1.85 %, using TEC 15 FTO, Ni
gridlines, and Cu contacts in a pH 7, 2.0 M KPi electrolyte, with a 3 electrode setup,
under 1 sun illumination from a quartz tungsten halogen lamp (Figure 19).
Figure 19. W:BiVO4 photoanode deposited on to TEC 15TM FTO with Ni gridlines b) without Ni
gridlines, with the edge coated with Paste-based Ag contacts and Cu tape, protected with Kapton
tape. c) Photocurrent densities against potential vs. RHE, in a 3 electrode configuration (WE, CE and
Ref.) under 1 sun, in hole scavenger electrolyte (1.0 M KPi, 0.5M Na2SO3). Presented is a comparison
of the performance of the W:BiVO4 photo-anodes for a small scale sample (black), a 50 cm2
sample without gridlines (blue), with gridlines (green) and with gridlines + hydrogen annealing (red).
PECDEMO Final Publishable Report
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Large area integrated PV-PEC devices were fabricated consisting of a 2 x HIT Silicon
module with an area of 5 x 10 cm2 and one large scale CoPi coated W:BiVO4 photo-
anodes as the front window of the electrochemical cell. The integrated PV-PEC device
achieved an initial photocurrent density of 1.20 mAcm-2 at a potential of 1V vs RHE,
corresponding to a STH efficiency of 1.48 % (Figure 20). Although lifetime studies for a
range of samples showed that over 24 hrs the photocurrent density of the integrated
cell decreased and reached a plateau of ~0.6 mAcm-2 at a potential of 1.1 V
corresponding to a STH efficiency of 0.74 %. It is proposed that the loss in photocurrent
is due to a combination of, the degradation and loss of CoPi catalyst and, the partial
degradation W:BiVO4, which visibly remained on the photoanodes after weeks of
testing.
1.3.4. Work Package 4
Within PECDEMO, WP4 was focused on the
design, optimization and assessment of a 50
cm2 PEC cell for efficient water splitting and on
evaluating its potential use under
concentrated solar radiation. To accomplish
such goal four main tasks were initially
identified: the development of both “angled”
and “vertical” PEC devices (Tasks 4.1 and 4.2);
their adaptation for use under concentrated
solar radiation conditions (Task 4.3); and, by the
end of the project, the selection of the optimal
device design (Task 4.4).
The key challenge of designing a 50 cm2 PEC
device is to significantly reduce ionic and electronic resistances keeping the cell/unit
price competitive. These resistances depend on the geometry and volume of the cell,
which were optimized to have the lowest ohmic losses. As mentioned, WP4 strategy
relied on the development of two different designs: i) the so-called “angled” PEC cell
comprising an innovative system to separate the evolved gases, with exemption of
the vertical diaphragm and where the photoelectrode may be back or front
illuminated; and ii) the so-called “vertical” PEC cell with a vertical diaphragm, and
where a zero gap distance between electrodes was pursued. Figure 21 displays the
“angled” and “vertical” PEC devices developed within tasks 4.1 and 4.2, respectively.
Figure 21. “Angled” (left) and “vertical”
(right) PEC devices developed within task
4.1 and task 4.2, respectively.
Figure 20. a) The voltage and current overtime for the integrated PV-PEC device under 1 sun whilst
the 2.0 M KPi pH: 7 electrolyte is stirred. d) Photo of the Large scale PV-PEC cell consisting of 2HIT silicon
PV’s, two platinum coated counter electrode and a CoPi coated W:BiVO4 photoanode with an
active area of 50 cm2.
PECDEMO Final Publishable Report
18
“Angled” PEC Cell Design
The developed
“angled” PEC cell took
into account a set of
important requirements in
its design and
construction: i) ionic and
electronic resistances
were minimized to attain
a maximum voltage drop
of 50 mV; ii) the cell
weight and size were
optimized; and iii) its
capability to operate under different working conditions (temperature, irradiances
and tilted positions) assured. Based on previous assumptions, the PEC cell detailed in
Figure 22 was designed and built.
The cell body is made of transparent acrylic assuring resistance to corrosive
electrolytes and a very acceptable range of operating temperatures20. Two metallic
frames (front and back) screwed to the acrylic embodiment against an O-ring assure
the proper sealing of the electrolyte container. The one placed in the front side is
lacquered in black allowing an illumination area of 5 × 10 cm2 through a transparent
glass or synthetic quartz window Figure 22-a4. The interior side of this front window is
spin-coated with a thin film of TiO2, developed at UPorto, which allows the evolved
gases bubbles to easily slip over the cell’s window increasing the amount of light
reaching to the
photoelectrode up to
9 %.21
Another important
feature of this cell is that
the photoelectrode
works simultaneously as
the cell back window,
allowing a narrower
construction. Thus, a
conductive path
between both sides of
the TCO substrate was
developed and
implemented without
changing the dimensions
of the housing area. To fulfill this requirement a conductive silver frame was printed in
both sides of the substrate and in its lateral area. Thus, the back metallic frame that
holds the photoelectrode works as external contact. Such strategy was never reported
before Figure 23. By placing the metallic counter-electrodes side-by-side to the
photoelectrode it is possible to have an open path for the sunlight to reach the PV cell
in the backside of the photoelectrode in a tandem configuration.
The cell is sealed with a transparent acrylic cap screwed on the top. This cap has
three separated compartments, two for hydrogen collection (internally connected)
and one for oxygen collection Figure 22-a3. External tubes can be directly connected
to these cambers. Between the top cap and the transparent body a Teflon®
diaphragm is placed to avoid liquid passage to the gas-collecting chambers due to
Figure 22. “Angled” PEC cell: a) disassembled in 3D project, b) final
device. 1 – acrylic body; 2 – Teflon membrane; 3 – acrylic cap with
separated chambers for oxygen and hydrogen collection; 4 – front
side black metallic frame; 5 – electrolyte inlet; 6 – right side
electrolyte outlet; 7 – one group of acrylic plates.
a) b)
Figure 23. Scaled-up photoelectrode assembly for the “angled” PEC
cell: a) photo of the in-house printed Ag metal frame on the front
and back sides of the TCO glass substrate; b) 5 × 10 cm2 bare
hematite photoelectrode ready to be assembled in the back side of
the “angled” PEC cell.
PECDEMO Final Publishable Report
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its high hydrophobicity Figure 22-a2. This membrane imposes, however, a pressure
drop to permeate the evolved gases (between ca. 5.5 mbar and 10 mbar) that the
feeding electrolyte has to compensate; this membrane allows the continuous supply
of electrolyte to the cell. The electrolyte inlet is located in the bottom of the cell to
force the upward movement of produced gas bubbles - Figure 22-a5.
“Vertical” PEC Cell Design
The “vertical” PEC cell
design was based in a PEC
device previously developed
by UPorto – the PortoCell.22
Taking advantage of some
important features from the
PortoCell, it comprises a
reservoir holding the
electrolyte wherein the two
electrodes are immersed and
physically separated by a
diaphragm Figure 24. The main
advantage of this cell is the
existence of a vertical
diaphragm that separates the
oxygen and hydrogen
evolution compartments. This
membrane allows placing
both electrodes very close to
each other – near zero gap configuration, minimizing ionic resistances. The dimensions
of the photoelectrode were set at 7.1 × 7.1 cm2 by the PECDEMO consortium in
accordance with the the original proposal of having a 50 cm2 PEC cell.
In this PEC device the photoelectrode also works as cell window taking advantage
of the new strategy implemented in the “angled” PEC cell. Depending on the
diaphragm transparency and the counter-electrode shape different arrangements
could be exploited to maximize the light reaching the photoelectrode and the PV cell
in a tandem configuration as detailed in D4.2 report. Still, this design does not allow
the existence of an open path for the sunlight to reach the PV cell, which is a major
drawback in comparison to the “angled” PEC cell.
Similarly, to the “angled” PEC cell, this cell is made of transparent acrylic and it has
a cap that allows placing a Teflon® diaphragm to collect the evolved gases
separated from the electrolyte. Alternatively, without the Teflon membrane, this cap
allows collecting the evolved gases together with electrolyte. The cell was engineered
considering continuous electrolyte feeding and gas collection in separate chambers.
Concentrated Solar Radiation
Different concentrator concepts for use with PEC-PV devices were developed and
assessed in the scope of Task 4.3. One of the favourite concepts is the Two-axis tracking
Linear Fresnel Reflector realised before in DLR’s test platform SoCRatus (Solar
Concentrator with a Rectangular Flat Focus ).23 Thus, it was decided to employ the
SoCRatus, which provides homogeneously concentrated sunlight with a
concentration ratio of about 17.5, as the solar concentrator for the final prototype. In
order to prepare the development of the final cell design, the “angled” and “vertical”
PEC cells were tested on the SoCRatus. Their general behavior under varying
Figure 24. The “vertical” PEC cell: a) disassembled in 3D
project, b) final prototype. 1 – acrylic body; 2 – diaphragm
holder; 3 – acrylic cap with separated chambers for oxygen
and hydrogen collection; 4 – front and back side black
metallic frames; 5 – Photoactive electrode; 6 – back window;
7 – metallic counter-electrode; 8 – left side electrolyte inlet; 9
- left side electrolyte outlet.
8
9
a) b)
1
2
3
4 4
56 7
10
PECDEMO Final Publishable Report
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irradiances and tilt angles was assessed with respect to gas mixing, electrolyte and
cell temperature, electrolyte flow stabilization, and gas separation. The experiments
resulted in the identification of cell specific advantages and drawbacks under
practical conditions. The tested devices mounted in the focal plane of the SoCRatus
are shown in Figure 25.
Final Design
The knowledge and experience gained during the initial tasks of WP4 allowed
designing and building the final prototype that best fulfills the targets of the project,
and which is a combination of both cell designs24. A demonstration module
comprising four identical PEC cells of 50 cm2, combining key features of the "angled"
and "vertical" PEC cells and going beyond, is presented in Figure 26. Each cell has an
open path for the
sunlight, from the front
to the back window,
allowing the use of a
tandem PEC/PV
arrangement in which
the PV-cell is placed in
the back of the
photoelectrode. Ray-
tracing simulations
confirmed the
applicability of the
prototype design for
operation with the
SoCRatus concerning
concentration profiles
on the relevant cell
surfaces, i.e. on
photoelectrodes and
PV modules. The
counter-electrodes (CE) are placed side-by-side to the working-electrode (WE), but
physically separated by an anion exchange membrane to avoid gas mixture. The
modular prototype embodies an acrylic skeleton in which the components detailed
in Figure 26 are assembled. In this arrangement, the active area of each
photoelectrode is 5 × 10 cm2 based on the project target ensuring minimum values of
Figure 26. Single PEC cell identical to the 4 units of the sub-module
prototype used in the field tests. 1 – Photoelectrode (back window); 2
– stainless steel frame with an in-built screw for electrical contact; 3 –
platinized-Ti meshes (CE) fixed against the ionic exchange membrane;
4 – screws for electrical contact with the CE; 5 – electrolyte inlets (Ø =
10 mm); 6 – electrolyte outlets (Ø= 3 mm).
Figure 25. The “vertical” PEC cell (left) and the “angled” PEC cell (right) mounted in the focal plane
of the SoCRatus.
PECDEMO Final Publishable Report
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ionic and electronic resistances. The electrolyte flowpath inside each compartment
of the cell was optimized in a CFD-based simulator to improve heat dissipation and to
allow efficient collection of gases at the top, preventing its accumulation inside the
reservoir. The final and optimized geometry in terms of fluid pattern is presented in
Figure 27.
Two main inlets located at
the bottom of the cell force the
upward movement of
produced gas bubbles – Figure
27. Inside the cell body there
are two inlet manifolds: i) 10
inlets located in the bottom of
the WE compartment (5 close
to the back window and 5 close
to the front window Figure 27-7
and Figure 27-8, respectively);
and ii) 2 inlets located at the
bottom of each CE
compartment ( = 8 mm, Figure
27-5). The electrolyte flow
pattern created by the inlets
located close to both windows
is important to assure the
bubbles detachment. In this
optimized design the top Teflon
membrane was not considered
to avoid the drawbacks
reported on deliverables D4.1,
D4.2 and D4.3. Alternatively,
without the Teflon membrane,
the evolved gases are
collected together with
electrolyte through the four outlets located in the cell cap, two on top of the WE
Figure 28 CFD simulations of the PEC cell: a) Non-optimized
and b) optimized design in terms of temperature profile
[electrolyte: water; total flow rate: 500 ml·min-1; external
temperature: 25 °C under 10-SUN irradiance] and
electrolyte flow distribution [electrolyte: water; total flow
rate: 500 ml·min-1; external temperature: 25 °C].
Figure 27. The optimized design for the electrolyte container of the individual PEC cell that is part of
the modular prototype: a) front, side and tilted views; b) sectional plans considered for the CFD results.
1 – CE compartments; 2 – WE compartment; 3 – main electrolyte inlet ( = 10 mm); 4 – electrolyte
outlets ( = 3 mm); 5 – inlets of the counter-electrode compartments ( = 8 mm); 6 – inlets of the WE
compartment close to the back window ( = 6 mm); 7 – inlets of the WE compartment close to the
front window ( = 4 mm).
PECDEMO Final Publishable Report
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compartment and one on top of each CE compartment ( = 3 mm, Figure 27-4). A
transparent acrylic plate (2 mm thick) was placed in the middle of the WE
compartment to enhance the fluid flow towards the outlets. This arrangement of inlets
and outlets allowed creating a uniform upward flow with an efficient collection of the
evolved gases. Figure 28 shows the differences between a non-optimized and
optimized design in terms of temperature profile and electrolyte distribution. Following
the same strategy applied to the “angled” and “vertical” PEC cells, photoelectrodes
work simultaneously as front or back windows. In both configurations the
photoelectrode must have an electrical collecting frame at the semiconductor side,
connected to the back of the substrate where the electrical cables are connected or
the PV cell is installed.
The ultimate goal of WP4 was successfully accomplished; a sub-modular prototype
composed by four individual PEC cells, each with an active area of 50 cm2 was
designed, optimized, built and tested under artificial solar conditions and
concentrated solar radiation in field tests.25 Complementary results are presented
hereafter within the framework of WP6, modular prototype and field tests.
1.3.5. Work Package 5
The fifth work package addresses design of process, pilot plant and infrastructure
following by inventory analysis and component sizing. Finally, Life-Cycle-Analysis
(LCA), cost estimation for hydrogen production using PEC-PV technology as well as
benchmarking with common H2 production technologies like steam methane
reforming, coal and biomass gasification, wind and PV electrolysis were performed
and evaluated.
Task 5.1. Design of process, pilot plant and infrastructure
The generation of hydrogen with photoelectrochemical-photovoltaic (PEC-PV)
tandem devices via water splitting finally has to be economically viable and
industrially applicable. The PEC-PV system has to be embedded in suitable processes
and plants. Three hydrogen production and application scenarios were considered:
a single home application (SHA), a hydrogen refuelling station (HRS), and an industrial
process (IP). The SHA refers to a decentralised approach of hydrogen production
rated at 1 kg/d @ 6 bar and subsequent use in fuel cells to provide electrical power
needed in small buildings. The HRS offers hydrogen at a nominal production rate of
400 kg/d @ 810 bar to fuel vehicles such as cars and buses, which carry a pressurised
hydrogen tank, whereas the introduced IP features a nominal production rate of
4,000 kg/d @ 20 bar and addresses utilisation of hydrogen as a feedstock for diverse
processes.
Numerous criteria are relevant for the choice of a suitable location for the hydrogen
production plants (weather conditions, politics, terrain, infrastructure, etc.). However,
the most important criterion for the provision of hydrogen at reasonable costs is a high
level of global irradiance. Thus, Seville (Spain) and Negev (Israel) were identified as
promising locations.
Appropriate plant designs for the three scenarios were elaborated, which comprise
beside the PEC-PV system major components such as pumps, heat exchangers,
compressors, tanks, and blowers. The latter component refers to an air cooling system
that controls the temperature of the electrolyte, which flows through the PEC-PV
system, in order to avoid critical temperatures with respect to efficiency and stability.
According to the project targets a solar-to-hydrogen efficiency (STH) of 8% based on
the higher heating value of hydrogen was considered. Collector sizes between 89.1 m2
PECDEMO Final Publishable Report
23
and 378,139 m2 were calculated depending on the location and the scenario. Active
power management (APM)18 allows in-situ generation of excess electricity and was
implemented in the plant models with 5% solar-to-electricity efficiency. Passive cooling
due to convective and radiative heat transfer from the PEC-PV system and the
electrolyte piping to the ambience is relevant and was taken into account. Moderate
concentration ratios up to 30 were considered. The solar plant should be operated at
temperatures as high as reasonable in terms of stability and efficiency, since higher
temperatures clearly decrease the needed active cooling capacity because of
enhanced passive cooling.
Task 5.2. Inventory analysis and component sizing
Mass and energy flows regarding the main plant components were estimated for 60°C
maximum temperature of the PEC-PV system and 8 K temperature increase between
inlet and outlet of the PEC-PV system. Average operation conditions as well as severest
operation conditions concerning ambient temperature and solar input were
considered in order to assess a) representative mean operating modes of the solar
plants and their mean demands with respect to electricity and water and b) the
required maximum operating capacity of the main components of the solar plants. In
both cases negligible influence of wind was assumed.
Cooling is a crucial issue and the implemented blower of the air cooling system
dominates the electricity demand under severest conditions. However, even under
severest conditions and concentrated sunlight APM completely or to a large extend
covers the electricity demand of the entire plant. Since the electricity demand of the
plant increases only moderately for a concentration ratio of 30 compared to 10 or 20
a concentration ratio of 30 was chosen for further investigation related to the HRS and
the IP.
Task 5.3. Life-cycle-analysis (LCA)
To quantify the environmental impact associated with all the stages during the life of
the product, i.e. from the raw material extraction until disposal or recycling (so-called
“cradle-to-gate” cycle), the LCA was performed in accordance with ISO 1404026 using
GaBi 7.0 software27. The focus of LCA was set on the global worming potential (GWP),
which is a measure of the amount of heat trapped by a certain mass of the gas in
question to the amount of heat trapped by a similar mass of carbon dioxide expressed
as kg CO2 eq per kg of produced hydrogen.
It was found that if grid electricity from local sources is used to meet the electricity
demand of plant components, in all analysed scenarios the GWP impact of the PEC-
PV technology is higher than the “best in class” wind electrolysis technology (1.0 kg
CO2 eq kg-1 H2). However, if no external electricity is needed due to APM, the GWP
impact of PEC-PV technology can be lowered up to 1.4 kg CO2 eq kg-1 H2 assuming
implementation of 1 m2 PEC-PV cells. Moreover, if solar concentration (C = 30) is
combined with APM a new state of the art technology with lowest reported to date
GWP impact of 0.4 kg CO2 eq kg-1 H2 could be obtained.
Task 5.4. Cost estimation
Economic analysis was performed using H2A Hydrogen Production model (version 3.1)
provided by the US Department of Energy (DoE) on its web page28 and levelised costs
of hydrogen production (LCHP) were estimated for three different hydrogen
production scenarios. The H2A Hydrogen Production model is based on process design
assumptions, which were verified by an international H2A team. Required input
parameters to the H2A models include capital and operating costs, efficiencies of
PECDEMO Final Publishable Report
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used process, plant life as well as financial parameters such as the type of financing,
discounted cash flow rate and desired internal rate of return.
Using H2A Hydrogen Production model levelised costs of hydrogen production were
estimated. It was found out that H2 production using 1 m2 PEC-PV cells will lead to LCHP
values of 9 € kg-1 for SHA scenario, LCHP of 19-23 € kg-1 for HRS scenario, and LCHP of
16-20 € kg-1 for IP scenario. In the case of HRS and IP scenarios higher LCHP values
correspond to the case when electricity from the grid is used and no solar
concentration is applied, while lower LCHP values correspond to the case when no
electricity from the grid is used due to implementation of APM and solar concentration
with C = 30 is used. Higher LCHP values for HRS scenario are caused by high electrical
energy consumption due to compression of H2 to 810 bars, while comparably low
LCHP value for SHA scenario is mainly caused by rather simple hardware required. The
specified conditions of hydrogen at the outlet of the plant have a great influence on
the LCHP.
An increase of STH from 8% to 12% or even 15% would have a significant influence on
the LCHP value. Additionally, selling of generated by PV module excessive electrical
energy would generate a revenue and would lead to further decrease of the LCHP
value.
Task 5.5. Benchmarking
Hydrogen production via PEC-PV water splitting was assessed in the context of
alternative hydrogen production technologies. Steam reforming of methane, which
uses as coal gasification fossil feedstocks and therefore inherently involves the
generation of carbon dioxide, is the dominating hydrogen production technology
today. Prominent technologies which use renewable feedstocks are biomass
gasification and electrolysis powered by electricity produced by wind turbines or PV
modules. Respective global warming potentials and H2 production costs29-35 were
analysed and compared.
PEC-PV water splitting could potentially reach lowest reported to date GWP impact
of 0.4 kg CO2 eq kg-1 H2 followed by wind electrolysis (1.0 kg CO2 eq kg-1 H2) and PV
electrolysis (2.5 kg CO2 eq kg-1 H2), biomass gasification (8.0 kg CO2 eq kg-1 H2), steam
methane reforming (14.5 kg CO2 eq kg-1 H2), and coal gasification (23.7 kg CO2 eq
kg- 1 H2).
Comparison of H2 production costs showed that the hydrogen production costs for all
three considered scenarios (9-23 € kg-1 H2) are higher than estimated costs for steam
reforming of methane (0.8-3.0 € kg-1 H2), coal (0.9-2.1 € kg-1 H2) and biomass gasification
(1.0-4.3 € kg-1 H2), as well as wind electrolysis (4.2-6.4 € kg-1 H2). In the case of PV
electrolysis, which shows most similarity to PEC-PV water splitting since it uses the same
feedstocks, a rather broad range of 5.3-22.4 € kg-1 of costs has been estimated, that
shows a wide overlap with cost ranges determined here for PEC-PV hydrogen
production technology. Though, solar concentration with C = 30, active power
management, and large active area per PEC-PV device are promising approaches
to reduce hydrogen production costs and should be pursued, further efforts have to
be made to reach economic viability. Cost figures could effectively be enhanced by
higher STH maintained at higher operating temperatures (aiming at superseding the
active cooling system) and higher concentration ratios.
1.3.6. Work Package 6
WP6, Modular Prototype and Field Tests, as a demonstration work package was
focused on the final assessment of the sub-modular PEC-PV prototype, which
PECDEMO Final Publishable Report
25
embraces four identical 50 cm2 compartments. The optimized cell design with respect
to optical path and electrolyte flow characteristics was developed in the scope of
WP4 as presented above. The performance of the prototype under practical
conditions was evaluated in particular regarding efficiency and stability. The final
demonstration phase began in Nov 2016 meeting MS7 – Start of field tests of prototype
module.25
Two sets of experiments were simultaneously conducted to test the optimized
device design: i) with the 1 x 4 demonstration module array under concentrated solar
radiation at DLR and ii) with an individualized 50 cm2 cell, identical to the four cells
comprised in the sub-modular prototype, under non-concentrated artificial sunlight at
UPorto. Each set of experiments was divided into two campaigns. Bare hematite
photoelectrodes produced by UPorto were used in the first campaign, whereas
bismuth vanadate (BiVO4) photoelectrodes with cobalt oxide/phosphate (CoPi)
catalyst prepared at HZB within the framework of WP1 were installed in the second
campaign. Hematite was the semiconductor selected for the first campaign due to its
high stability under continuous operation.11 HIT silicon mini modules manufactured by
HZB/PVcomB, connected in series to the photoelectrodes, delivered bias voltage to
promote the water splitting reactions.
Tests under Concentrated Sunlight
The experiments under concentrated sunlight were conducted employing DLR’s test
facility SoCRatus23 in Cologne. The prototype was mounted in the rectangular focus
of the two-axis tracking solar concentrator and provided with homogeneous, about
17.5-fold concentrated sunlight. The developed PEC-PV device was implemented in
the set-up using two fluid cycles of the SoCRatus. They both fed the inlets of the
prototype, where the flow was distributed to the hydrogen and oxygen chambers
connected to Fluid Cycle 1 and Fluid Cycle 2 respectively.
The first experimental campaign was carried out with front illuminated hematite
photoelectrodes, whereas in the second campaign two BiVO4 photoelectrodes
equipped with grid lines were installed in each compartment – the first one being back
illuminated as part of the front window, the second one being front illuminated as part
of the back window. HIT silicon mini modules with an active area of 50 cm2 each were
placed behind the back windows realizing a true tandem configuration and delivered
bias voltage to the system. In case of hematite partly an additional bias of +325 mV
Figure 29. Modular prototype equipped with BiVO4 photoelectrodes irradiated with concentrated
sunlight in the focal plane of the SoCRatus with reflective shields to protect sensitive parts of the
setup.
PECDEMO Final Publishable Report
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was applied to reach a total bias of about 1.6 V. The prototype operating with BiVO4
photoelectrodes under concentrated sunlight in the focal plane of the SoCRatus can
be seen in Figure 29. The total irradiation on the prototype, the achieved average
current density, and the estimated molar flow of generated hydrogen relative to
respective mean values for Campaign 1 and 2 are shown in Figure 30. The hydrogen
flow generally follows the current density with a certain delay due to mixing and
saturation effects in the fluid cycle. With hematite a total experimental time of about
15 h was reached, thereof more than 8.5 h without additional bias and close to 6.5 h
with 325 mV additional bias. A total irradiance of 12.4 kW m-2 on average and of
14.0 kW m-2 in peak time was applied. Current densities of about 0.2 mA cm-2 and
0.5 mA cm-2 as well as maximum hydrogen flows of 924 µmol h-1 and 2,078 µmol h-1
were achieved without and with the additional bias respectively. Efficient product gas
separation was obtained. The daily solar-to-hydrogen efficiency (STH) based on the
higher heating value of hydrogen reached 0.059% with additional bias. Within the
duration of operation the prototype featured stable performance.
Campaign 2 with BiVO4 photoelectrodes covered about 48 h. An applied total
irradiance of 7.85 kW m-2 on average and of 16.5 kW m-2 in peak time was estimated.
The mean current density was calculated to 0.87 mA cm-2 while a maximum value of
1.88 mA cm-2 was obtained on Day 2 at about 13 kW m-2. Hydrogen was produced at
rates up to 6,741 µmol h-1. The daily STH reached 0.42% on Day 5 at comparably low
levels of irradiance. Though a certain degradation of the photoelectrochemical
system could be observed within the duration of the campaign, even after 48 h
operation under demanding conditions the BiVO4 system was still active. Since in both
campaigns a non-proportional dependency of hydrogen formation on the solar input
became apparent, further efforts have to be made in order to allow efficient
exploitation of concentrated sunlight.
a) b)
Figure 30. Total irradiation on the prototype (smoothed ± 30 s), average current density (smoothed
± 30 s), and hydrogen flow relative to respective mean values as well as average solar-to-hydrogen
efficiencies (STH) of the particular days associated with a) Campaign 1 (hematite photoelectrodes,
1 M KOH, 25 °C, 1.9 l min-1, membrane: Fumasep® FAA-3-PK-130) and b) Campaign 2 (BiVO4
photoelectrodes, 0.5 M K2SO4 + 0.1 M K2HPO4/KH2PO4, 30 °C, 1.7 l min-1, membrane: Fumasep® FAA-3-
PK-130 + Nafion® NE-1110 / Nafion® NE-1110 / none / none).
PECDEMO Final Publishable Report
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Tests under Non-Concentrated Sunlight
To assess the stability of the optimized hybrid PEC-PV device under non-concentrated
sunlight an experimental
setup was assembled at
UPorto applying the sulphur
plasma lamp system AS 1300
V 2.0 (Plasma International
GmbH) which provides
1,000 W m-2. In this setup the
PEC cell operates at a
constant bias potential
provided by a HIT Si-PV
module. The medium term
test was performed in two
experimental campaigns: i)
with bare hematite
photoelectrodes and ii) with
BiVO4 photoelectrodes. A
constant electrolyte feeding
of ca. 200 ml min-1 was
promoted using a peristaltic pump and a water bath was used to keep the electrolyte
temperature constant under operation.
Again, hematite was the first photoelectrode tested due to its high stability under
continuous operation; in this experiment the water bath was set to operate at ca.
45 °C.20 At the initial instant a photocurrent density of 0.62 mA cm-2 was produced by
the hematite photoelectrode at 1.6 V in a 2-electrode configuration. Accordingly, to
supply the hematite photoelectrode with the necessary bias potential of ca. 1.6 V, two
50 cm2 PV modules were connected in series and an Autolab potentiostat was used
for continuous monitoring the photocurrent produced by the semiconductor over
1,000 h – Figure 31.
The presented results show that the PEC-PV device remained stable over 1,000 h
(approximately 42 days) delivering an average photocurrent density of ca.
0.43 mA cm-2. The photocurrent oscillations along the polarization curve are due to
periodic interruptions to obtain the J-V curves; a slight decrease on the photocurrent
Figure 31. Polarization curve of the hematite photoelectrode
obtained under a constant bias of 1.6 V and simulated solar
irradiance. Enlarged view of the polarization between 96 h and
192 h.
a) b) Figure 32. a) J-V characteristics of the 50 cm2 hematite photoanode prepared by spray pyrolysis,
before starting the stability test, 0 h ( ), and after 1,005 h ( ) under simulated sunlight; b) J-V
characteristic curves for the two 50 cm2 Si-PV modules connected in series before starting the
stability test, 0 h ( ), and after 1,005 h ( ) under simulated sunlight.
0.0
0.2
0.4
0.6
0.8
1.0
0.8 1.3 1.8
Ph
oto
cu
rre
nt D
en
sit
y /
mA
·cm
2
Applied Potential / V
0 h
1005 h
0
2
4
6
8
10
12
14
16
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Ph
oto
cu
rre
nt D
en
sit
y /
mA
·cm
2
Applied Potential / V
0 h
1005 h
PECDEMO Final Publishable Report
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density before stabilization was always observed at the initial instants of each testing
period. The latter, can be easily seen in the zoom out of the polarization curve in Figure
31. This initial photocurrent decay, before stabilization, should be related to the
generation of long-lived holes during the formation of a space charge layer, which
can oxidize water on a timescale of around 1 s.36 Figure 32 plots the J-V curves
obtained at the initial instant and at the end of the stability test for the two
components that comprise the hybrid PEC PV device. Both components, the hematite
photoelectrode and the PVs modules, presented very similar performances before
and after 1,000 h of operation with no evidence of corrosion or degradation.
BiVO4 photoelectrodes were tested at UPorto in a second experimental campaign; in
this case the electrolyte was kept at ca. 25 °C. Considering the characteristic
performance of BiVO4 (Figure 33-a) a single 50 cm2 PV module was enough to provide
the necessary bias to the PEC cell for promoting water electrolysis. In this test the setup
operated at the interception point of the J-V curves of BiVO4 photoelectrode and HIT
PV module – Figure 33-a). The photocurrent history over 24 h is plotted in Figure 33-b).
During this period an average photocurrent density of 0.41 mA cm-2 was recorded at
1.28 V, corresponding to a STH efficiency of 0.61%. Over this time the BiVO4
continuously decreased with a current density loss rate of 2 nA cm-2 s-1.
Similar to the test with hematite, the individual performance of the BiVO4
photoelectrode and the PV module was assessed at the beginning and at the end of
the test; the performance of the PV modules remained unchanged after 24 h of
operation. However, from Figure 33-c it can be extracted that the performance of the
BiVO4 photoelectrode continuously decreased after operating 24 h. The latter may be
explained by the material detachment observed during the stability test.
a) b) c)
Figure 33. a) J-V characteristics curves: 50 cm2 BiVO4 photoelectrode (• ) under simulated sunlight and in 0.1
M KPi, obtained in a 2-electrode configuration; single 50 cm2 Si-PV HIT module (• ) under simulated sunlight;
b) Polarization curve of the BiVO4 photoelectrode obtained in a 2-electrode configuration under simulated
sunlight; c) J-V characteristics curves of 50 cm2 BiVO4 photoelectrode, before starting the stability test, 0 h
( ), and after operating 24 h ( ) under simulated sunlight and in 0.1 M KPi, obtained in a 3-electrode
configuration.
PECDEMO Final Publishable Report
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1.4. Potential impact (including socio-economic impact and
wider societal implications) and the main dissemination
activities and exploitation of results
1.4.1. Potential impact
By achieving its main project goals, PECDEMO has made an important step forward in
the development of efficient, stable, and scalable water splitting concepts. The small-
area solar-to-hydrogen efficiencies1 of up to 9.2% (BiVO4/Fe2O3/Si-HIT) and even 16.2%
(Cu2O/3-HIT) are amongst the highest ever reported for this concept, and have put
Europe at the forefront of efforts in this field. Moreover, PECDEMO has demonstrated
the very first large-area (50 cm2) metal oxide-based PEC-PV water splitting systems that
are based on a true tandem design, i.e., with a wide-bandgap absorber in front of a
smaller-bandgap PV cell. These activities have attracted the interest of Toyota; as a
direct result of the PECDEMO project, one of the project partners (HZB) has recently
started a small seed project with (and funded by) Toyota to further explore
photoelectrochemical water splitting devices.
Although the project represents a significant step forward, we are still far away from a
viable PEC-based technology for solar water splitting. Specifically, the efficiencies for
the large-area devices are still modest. Moreover, fulfilling all three requirements
(efficiency, stability, and scalability) within a single system remains a major challenge.
Nevertheless, with our scaling work we pushed the limit for real application and
performed important pioneering work to reveal (and overcome) limitation
mechanisms and paved the way for solutions, which are of great importance for future
work and coming projects towards commercial PEC-PV applicability.
On the systems level, cooling turned out to be an important aspect that has received
little attention in the field. While all these technical issues can be addressed, the
inherent complexity of the overall process tends to drive up the costs, and makes it
challenging to compete with alternative approaches that make use of mature
technologies, such as PV-driven electrolysis. While this can be partly remedied by
developing more efficient materials, especially light absorbers, innovative new
concepts may be needed in order to achieve the necessary breakthroughs.
PECDEMO has proposed several innovative solutions that may help achieve these
breakthroughs. Examples are the PEC-PV power management strategy (i.e., co-
generation of electricity and hydrogen) and the auxiliary electrode concept. These
concepts have been published in high-ranking journals and are likely to have a
1 STH efficiencies in PECDEMO are based on the enthalpy of hydrogen, 286 kJ/mol (1.48 eV)
PECDEMO Final Publishable Report
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significant impact on future efforts in the field. Continued efforts by multi-disciplinary
teams consisting of materials scientists, chemical engineers, plant designers, and
business developers are needed to further develop photoelectrochemical water
splitting into a viable technology that has a substantial impact on society.
1.4.2. Dissemination activities
Dissemination activities concentrated on four tasks:
- To effectively communicate PECDEMO’s innovative research
- To establish and maintain a web database to foster communication within the
consortium
- To organize two international workshops
- To conduct outreach activities
For Task 1 the following list compiles some relevant publications from PECDEMO
J. Luo et al. (2014), Water photolysis at 12.3% efficiency via perovskite
photovoltaics and Earth-abundant catalysts, Science Vol. 345/Issue 6204,
26/09/2014 1593-1596
J.H. Kim et al. (2016), Hetero-type dual photoanodes for unbiased solar water
splitting with extended light harvesting, Nature Communications Vol. 7 Nature
Publishing Group, 14/12/2016 13380
Landman et al. (2017), Photoelectrochemical water splitting in separate
oxygen and hydrogen cells, Nature Materials N/A Nature Publishing Group,
13/03/2017
J- Luo et al. (2016), Cu 2 O Nanowire Photocathodes for Efficient and Durable
Solar Water Splitting, Nano Letters Vol. 16/Issue 3, American Chemical Society,
09/03/2016 1848-1857
M-K Son et al. (2017), A copper nickel mixed oxide hole selective layer for Au-
free transparent cuprous oxide photocathodes, Energy and Environmental
Science Vol. 10/Issue 4, Royal Society of Chemistry, 01/01/2017 912-918
Tin oxide as stable protective layer for composite cuprous oxide water-splitting
photocathodes
J. Azevedo et al. (2016), Nano Energy Vol. 24, Elsevier Netherlands 01/06/2016,
10-16
P. Dias et al. (2015), Transparent Cuprous Oxide Photocathode Enabling a
Stacked Tandem Cell for Unbiased Water Splitting, Advanced Energy
Materials Vol. 5/Issue 24, Wiley 01/12/2015
J- Luo et al. (2015), Targeting Ideal Dual-Absorber Tandem Water Splitting
Using Perovskite Photo voltaics and CuIn x Ga 1- x Se 2 Photocathodes,
Advanced Energy Materials Vol. 5/Issue 24, Wiley 01/12/2015
J. Luo et al. (2015), Solution Transformation of Cu 2 O into CuInS 2 for Solar
Water Splitting, Nano Letters Vol. 15/Issue 2, American Chemical Society,
11/02/2015 1395-1402
L. Steier et al. (2015), Low-Temperature Atomic Layer Deposition of Crystalline
and Photoactive Ultrathin Hematite Films for Solar Water Splitting, ACS Nano
Vol. 9/Issue 12, American Chemical Society, 22/12/2015 11775-117 83
J. Azevedo et al. (2014), On the stability enhancement of cuprous oxide water
splitting photocathodes by low temperature steam annealing, Energy and
Environmental Science Vol. 7/Issue 12, Royal Society of Chemistry, 01/01/2014
4044-4052
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G. Morales-Guio et al. (2015), An Optically Transparent Iron Nickel Oxide
Catalyst for Solar Water Splitting, Journal of the American Chemical Society
Vol. 137/Issue 31, American Chemical Society, 12/08/2015, 9927-9936
J. Luo et al. (2016), Bipolar Membrane-Assisted Solar Water Splitting in Optimal
pH, Advanced Energy Materials Vol. 6/Issue 13, Wiley, 01/07/2016
P. Dias, Extremely stable bare hematite photoanode for solar water splitting,
Nano Energy Vol. 23 Elsevier, 01/05/2016 70-79
J. D. Costa et al. (2016), The effect of electrolyte re-utilization in the growth
rate and morphology of TiO2 nanotubes, Materials Letters Vol. 171 Elsevier,
01/05/2016 224-227
Rothschild et al. (2017), Beating the Efficiency of Photovoltaics Powered
Electrolysis with Tandem Cell Photoelectrolysis, ACS Energy Letters Vol. 2/Issue
1, American Chemical Society, 13/01/2017 45-51
G. Segev et al. (2016), High Solar Flux Concentration Water Splitting with
Hematite (#-Fe 2 O 3 ) Photoanodes, Advanced Energy Materials Vol. 6/Issue
1, Wiley 01/01/2016
H. Dotan On the Solar to Hydrogen Conversion Efficiency of Photoelectrodes
for Water Splitting, Journal of Physical Chemistry Letters Vol. 5/Issue 19,
American Chemical Society, 02/10/2014 3330-3334
S. Kirner et al. (2016), Architectures for scalable integrated photo driven
catalytic devices-A concept study, International Journal of Hydrogen Energy
Vol. 41/Issue 45, Elsevier, 01/12/2016 20823-20831
S. Kirner et al. (2015), Quadruple-junction solar cells and modules based on
amorphous and microcrystalline silicon with high stable efficiencies, Japanese
Journal of Applied Physics Vol. 54/Issue 8S1, Japan Society of Applied Physics,
01/08/2015 08KB03
F. F. Abdi et al. (2014), Plasmonic enhancement of the optical absorption and
catalytic efficiency of BiVO4 photoanodes decorated with Ag@SiO2 core–
shell nanoparticles, Physical Chemistry Chemical Physics Vol. 16/Issue 29, Royal
Society of Chemistry, 01/01/2014 15272
S. Kirner et al. (2016), Wafer Surface Tuning for a-Si:H/µc-Si:H/c-Si Triple Junction
Solar Cells for Application in Water Splitting, Energy Procedia Vol. 102 Elsevier
BV Netherlands 01/12/2016 126-135
Zachäus Photocurrent of BiVO 4 is limited by surface recombination, not
surface catalysis, Chemical Science Vol. 8/Issue 5 Royal Society of Chemistry,
01/01/2017 3712-3719
In addition, PECDEMO was represented at conferences with oral and poster
presentations as listed below (most important)
HZB, Oral presentation to a scientific event, Direct current magnetron sputtering
of photoactive BiVO4: Role of stoichiometry on grain size, structure, carrier
mobility and lifetime, 28/11/2016 MRS Fall 2016,Boston, USA
HZB, Oral presentation to a scientific event, Photoelectrochemical Water
Oxidation of BiVO4 Photoanodes with 50 cm2 Active Area 18/04/2017 MRS
Spring 2017,Phoenix, USA
HZB, Oral presentation to a scientific event, Surface and bulk recombination in
spraydeposited BiVO4, 07/04/2015 MRS Spring 2015, San Francisco, USA
EPFL, Oral presentation to a scientific event, Large Scale Cuprous Oxide
Photocathode toward PEC-PV Tandem Demonstrator for Solar-Driven Water
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Splitting – From Design to Characterization, 28/11/2016 MRS Fall 2016, Boston,
USA
EPFL, Oral presentation to a scientific event, Using potential-dependent
quantum efficiency measurements to probe device characteristics in
photoelectrodes for solar fuels generation, 01/04/2016 MRS Spring 2016,
Phoenix, USA
UPORTO, Oral presentation to a scientific event, Solar Photoelectrochemical
Hydrogen – Technological Advancements, 02/12/2016, MRS Fall 2016, Boston,
USA
UPORTO, Oral presentation to a scientific event, Up Scaled Photoelectro
chemical Device for Solar Water Splitting-Development and Characterization
of a New Design, 02/12/2016, MRS Fall 2016, Boston, USA
Regarding Task 2, the web domain www.pecdemo.eu was obtained and a
comprehensive project website was built, hosted by EPFL. The site went live on July
15th, 2014. The website features many sections, including “About PECDEMO” (Project
Details, Project Description, Consortium), “Partners”, “Activities” (Meetings,
Deliverables, Demonstrations), and “Dissemination” (Publications, Presentations). The
website was continuously updated with current news and publications from the
project. Pictures of the demonstrator device were published on the website on
November 30th 2016.
For Task 3, the first goal of organizing an international conference was successfully
accomplished by realizing the IPS-20 meeting in Berlin in 2014. The meeting, titled “20th
International Conference on Photochemical Conversion and Storage of Solar Energy”
was organized by HZB and chaired by Prof. Roel van de Krol. The conference was a
great success, attracting over 430 participants from 36 countries and featuring 14
plenary speakers, 19 keynote speakers, and hundreds of contributed talks and posters.
Link: http://www.helmholtz-berlin.de/events/ips20/
The second part of the task was to organize a symposium on solar fuels conversion at
a large international conference. To this end, members of the PECDEMO consortium
have co-organized the “Symposium EC4 – Materials, Devices and Systems for
Sustainable Conversion of Solar Energy to Fuels” at the “2016 Materials Research
Society Fall Meeting” in Boston. The five-day symposium took place November 28 –
December 2, 2016, and featured 21 invited speakers, 73 contributed oral
presentations, and 21 poster presentations. The four co-organizers were Roel van de
Krol (HZB), Avner Rothschild (Technion), Matthew Mayer (EPFL), and Todd Deutsch
(NREL), which were able to recruit symposium support by ACS Energy Letters, ACS
Publications, Helmholtz-Zentrum Berlin für Materialien und Energie, Journal of Physics
D: Applied Physics, IOP Publishing, Nature Energy, and Macmillan Publishers Ltd. The
symposium was well-attended and during the presentation of Harry Atwater, the
meeting room was even filled beyond capacity. Especially the PECDEMO project was
well-represented within the symposium, with 16 oral presentations and 5 posters
contributed by members of the project. For detailed information, see the links:
http://www.mrs.org/fall2016/call-for-papers?Code=EC4 (call for papers)
http://www.mrs.org/fall2016/fall-2016-symposia/?code=EC4 (program)
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Outreach activities were mainly undertaken in the form of teaching. PECDEMO’s main
materials of interest (Fe2O3, BiVO4, and Cu2O) and overall approach were extensively
discussed during the following courses and summer/winter schools:
MSc course on “Photo-Electrochemical Energy Conversion”, taught at the TU
Berlin in the winter semester of 2014, 2015, and 2016
Two-hour seminar on “Solar Fuels and Photocatalysis” as part of the MSc course
on “Modern Developments in X-Ray and Neutron Methods for Science and
Technology“, taught at the Free University of Berlin in 2015, 2016, and 2017
Seminar (1/2 day) taught in August 2015 for students of the German Academy
for Renewable Energy and Environmental Technology
(http://www.germanacademy.net/)
QuantSol Summer School, Hirschegg, Austria (September 2015)
EPFL hosted a one-day research symposium “SwissPEC” on the topic of
photoelectrochemical energy conversion, hosted by EPFL on 11 November
2016 In Lausanne
1.5. Public website and relevant contact details
www.pecdemo.eu
Prof. Roel van de Krol
Helmholtz-Zentrum Berlin für Materialien und Energie
Institute for Solar Fuels (EE-IF)
Hahn-Meitner-Platz 1,
14109 Berlin, Germany
Tel. +49 30 8062 - 43035
Fax: +49 30 8062 - 42434
Mail: [email protected]
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References
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Nat. Commun. 2016, 7, 13380.
(2) Dotan, H.; Kfir, O.; Sharlin, E.; Blank, O.; Gross, M.; Dumchin, I.; Ankonina, G.;
Rothschild, A. Nat Mater 2013, 12, 158.
(3) Hankin, A.; Alexander, J.; Kelsall, G. Phys. Chem. Chem. Phys. 2014, 16, 16176.
(4) Grave, D. A.; Dotan, H.; Levy, Y.; Piekner, Y.; Scherrer, B.; Malviya, K. D.;
Rothschild, A. Journal of Materials Chemistry A 2016, 4, 3052.
(5) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R. Nat.
Commun. 2013, 4, 2195.
(6) Paracchino, A.; Brauer, J. C.; Moser, J. E.; Thimsen, E.; Graetzel, M. J. Phys.
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(7) Musselman, K. P.; Marin, A.; Schmidt‐Mende, L.; MacManus‐Driscoll, J. L. Adv.
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Hagfeldt, A.; Mendes, A.; Grätzel, M. Advanced Energy Materials 2015, 5.
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