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Cite this: Energy Environ. Sci.,
2020, 13,1725
Efficient direct seawater electrolysers using
selective alkaline NiFe-LDH as OER catalyst
in asymmetric electrolyte feeds†
So
¨ren Dresp, ‡Trung Ngo Thanh,‡Malte Klingenhof, Sven Bru
¨ckner,
Philipp Hauke and Peter Strasser *
Direct seawater electrolysis faces fundamental catalytic and process
engineering challenges. Here we demonstrate a promising seawater
electrolyser configuration using asymmetric electrolyte feeds.
We further investigated the faradaic O
2
efficiency of NiFe-LDH in
alkalinized Cl
-containing electrolytes in comparison to commercial
IrOx-based catalysts. Other than IrOx, NiFe-LDH prevents the oxida-
tion of Cl
and appears highly selective for the oxygen evolution
reaction in alkalinized seawater even at cell potentials beyond
3.0 V
cell
.
The storage of renewable solar or wind electricity is a major
challenge in our efforts to build a sustainable future energy
system. More and more focus is placed on water electrolysers
that split water into O
2
and H
2
(2H
2
O + energy -2H
2
+O
2
) with
H
2
being the energy storage/platform molecule of interest.
Water electrolysers operate on highly purified water, which is
incompatible with wind parks at offshore ocean locations
or photovoltaic plants in ocean-side arid deserts. There, the
direct use of seawater in water electrolysers would be highly
desirable.
1
But splitting seawater faces chemical and engineering
challenges. The competing reactions between the desired oxygen
evolution reaction (OER) (4OH
-O
2
+2H
2
O+4e
;E
0
=
1.23 V
RHE
) and the undesired chloride oxidation reactions,
i.e. the Chlorine evolution reaction (ClER) (2Cl
-Cl
2
;E
0
=
1.36 V
RHE
) at low pH and the formation of hypochlorite (OCl
)
(Cl
+2OH
-OCl
+H
2
O+2e
;E
0
=1.71V
RHE
)athighpH
restrict the operating electrolyser cell voltage.
2
The two design
criteria for seawater splitting mandate an anode potential o1.72
V
RHE
and an electrolyte pH 47.5 to prevent ClO
formation to
ensure a 100% selective OER.
2
Despite the favourable natural pH
of seawater (pH
seawater
B8),
3,4
alkaline pH buffering is needed to
avoid local pH changes at the anode surface.
5
Indeed, at high pH,
NiFeOxHy layered double hydroxide (NiFe-LDH) has revealed its
full performance as anode catalyst for seawater electrolysers.
6
However, at high pH, the cathode faces Mg(OH)
2
deposition
blocking catalytic active sites for the hydrogen evolution reaction
(HER).
7
In addition, the separator/membrane strongly affects the
overall electrolyser performance:
8
saline water constituents such
as Cl
reduce the ionic conductivity of the anion exchange
membrane (AEM) by ion exchange and can result in performance
losses. Efficient seawater splitting without making it more
alkaline has therefore remained a challenge. Typical seawater
electrolyser use a symmetric feed of alkalinized seawater to
achieve efficiencies comparable to state of the art electrolysers.
7
Here, we report how asymmetric electrolyte feeds increase
the performance of seawater electrolysers and avoid the undesired
alkalinisation of seawater. We demonstrate our approach in
an electrolyser operating directly on natural seawater. Previous
designs of seawater electrolysers included identical electrolyte
compositions (alkalinized seawater) on anode and cathode,
6,8–10
but the limiting anode potential of 1.72 V
RHE
to ensure 100%
oxygen selectivity resulted in low electrolyser cell current densities
limited to about 200 mA cm
2
. More efficient and compact
electrolysers should reach cell performances and current densities
of at least up to 1 A cm
2
. To circumvent the limiting potential
range, we developed a new electrolyser feed scheme and com-
pared it to conventional electrolyte feed schemes of seawater and
alkaline electrolysers. Fig. 1 shows the various feed schemes
discussed here in more detail. While Fig. 1a and d show conven-
tional symmetric electrolyte feeds, Fig. 1b and e illustrate – in
analogy but opposite to PEM electrolysers – the electrolyte feed at
the cathode only. Fig. 1c and f demonstrate our novel approach
for efficient future seawater electrolysers using asymmetric feed
scheme compositions.
The electrolyser studies were performed using a customized
test station (Fig. S1, ESI†), commercial cell assemblies (Fig. S2,
ESI†) and membrane electrode assemblies (MEAs) including an
anion exchange membrane (AEM) spray coated with crystalline
Ni
0.66
Fe
0.34
-LDH (Fig. S3, ESI†) as anode catalyst material and
commercially available cathode catalyst. Compared to other
3d-transition metal catalysts, NiFe-LDH has shown outstanding
Department of Chemistry, Technical University Berlin, Straße des 17. Juni 124,
†Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee01125h
‡S. D. and T. N. T. contributed equally.
Received 10th April 2020,
Accepted 20th May 2020
DOI: 10.1039/d0ee01125h
rsc.li/ees
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OER activity and could proof its suitability as seawater oxidi-
zing catalyst.
2,6,8,11,12
The microwave-assisted solvothermal
synthesis and properties of NiFe-LDH are described in the ESI†
and elsewhere.
8
All configurations were initially tested using a
specific measurement protocol (Fig. S4, ESI†), consisting of
subsequent polarization curves and potentiostatic tests with
1.5 h and 12 h at 1.7 V
cell
.
Initial performance test
Fig. 2 shows a 12 h potentiostatic test at 1.7 V
cell
. Consistent to
prior results,
8
the symmetric 0.5 M KOH electrolyte feed, that is
the conventional alkaline electrolysis on highly purified water,
showed the best performance. Alkaline seawater feed on either
side (Fig. 1d) showed very low performance.
8
Surprisingly, the
asymmetric feed schemes with distinct fed compositions on
cathode and anode (Fig. 1c and f) showed superior cell perfor-
mances compared to the symmetric mixed electrolytes (Fig. 1d).
Even the saline seawater feed at the cathode only (Fig. 1e),
outperformed the mixed electrolyte feed scheme of Fig. 1d
significantly.
While initial electrolyser polarization curves differed sharply
among the schemes, the cell characteristics after 12 h con-
verged somewhat (see Fig. S5, ESI†) in line with the 12 h
stability measurement in Fig. 2. It appears as if the membrane
needs a longer break-in time for dry anodes, which is further
supported by the trends in the high frequency resistances
(HFR) of Fig. S6 (ESI†), in which the HFR decreases after each
long-term test.
However, the intriguing possibility of using only untreated
seawater directly (Fig. 1e) caused us to extend the stability test
beyond 12 h (Fig. S7a, ESI†). The current density gradually
increased, until after 18 h the cell performance dropped to
almost zero current. The performance loss was attributed to
strong corrosion processes at the catalyst and the porous
transport layer (PTL): a dark green NiO
x
solution accumulated
in the anode reservoir (Fig. S7c, ESI†), while the PTL sharply
darkened (Fig. S7d, ESI†). Without a buffer or an alkaline
electrolyte, the local pH has likely decreased drastically such
that the anode materials suffered strong acidic dissolution.
This led us to the conclusion that the anodic feed of KOH
is critical. Hence, a continuously circulating KOH anolyte
combined with pure single-pass seawater feeding at the
cathode appeared as the desired scheme (Fig. 1f). For this
asymmetric electrolyte feed scheme, we analysed the electro-
catalysis and transport of Cl
ions across the membrane and
detected a total Cl
amount of 6.36 0.04 mmol after our
standardized testing protocol (Fig. S3, ESI†). As shown below,
this low level of Cl
cross over into the circulating KOH anolyte
does not affect the anode catalysis thanks to the high catalytic
selectivity of NiFe-LDH for the OER.
Seawater electrolysis selectivity
A chemically selective anode catalyst is important for durable
seawater electrolysers in order to prevent component degrada-
tion caused by the oxidation of Cl
to wet Cl
2
(acidic) or to
hypochlorite (OCl
) (neutral/alkaline). Faradaic efficiencies
of O
2
were obtained from inline mass spectrometry (MS)
conducted during a galvanostatic test protocol (Fig. S8, ESI†).
To detect hypochlorite (OCl
), we used an iodometric titration
method.
Fig. 3a shows the sequence of chronopotentiometric (CP),
applied current steps that made up a full polarization curve
(0.05–1.0 A, steps labelled in red). Due to residual O
2
from the
preceding break-in CP step at 0.2 A, faradaic efficiencies of O
2
(FE
O
2
) below 0.25 A exceed their theoretical value. Even though
the measured potential varied for the different electrolytes,
Fig. 1 Schemes for anion exchange membrane (AEM) electrolysers using
independent electrolyte feeds with different electrolyte composition.
(a) Conventional symmetric 0.5 M KOH feeds (b) sole KOH cathode feed
(c) asymmetric 0.5 M KOH feed at cathode and 0.5 M NaCl feed at anode
(d) symmetric alkalinized 0.5 M NaCl feed (e) single NaCl feed at the
cathode (f) asymmetric 0.5 M NaCl feed at the cathode and 0.5 M KOH
feed at the anode.
Fig. 2 12 h electrolyser stability measurement at constant cell potential of
1.7 V
cell
of various electrolyte feeds using an active area of 5 cm
2
and
commercial Pt/C (48.5 wt%) with loading of 0.5 mg
Pt
cm
2
as cathode and
crystalline Ni
0.66
Fe
0.34
-LDH with loading of 2.0 mg
cat
cm
2
as anode
catalyst. The feed schemes of Fig. 1 are indicated in parentheses behind
the electrolyte composition.
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all their FE
O
2
remained almost identical and close to 100%,
indicating no significant Cl
oxidation at any time. Compared
to the symmetric mixed NaCl/KOH feed scheme of Fig. 1d, the
cell potential using asymmetric, separated KOH and saline
feeds (Fig. 1f) displayed significantly lower, and thus favour-
able cell voltages resulting in a superior cell performance.
Interestingly, all electrolyte feed schemes showed stable
activities over the 12 h (Fig. 3b). Even at cell voltages 41.8 V
cell
(Fig. 3b green) the FE
O
2
stayed at 94%, suggesting the absence
of significant Cl
oxidation. Iodometric titration confirmed
that no OCl
was formed at any time. In order to convince
ourselves that OCl
formation is possible and detectable, how-
ever, we tested Ir/TiO
2
and Ir-black as shown in Fig. 3c and d.
While iridium black (magenta) showed an undesired larger
cell voltage, the FE
O
2
was identical to NiFe-LDH (green).
In contrast, Ir/TiO
2
(purple) showed a sudden drop in FE
O
2
at
E
cell
42.4 V
cell
pointing to the electro-catalytic formation of
OCl
. Thus, 2.4 V appears to be the onset potential of OCl
formation. Looking at the CP at 1.0 A (200 mA cm
2
) in Fig. 3d,
Ir-black showed a dramatic increase of E
cell
surpassing 2.4 V
(Fig. 3d) resulting in a drop of O
2
concentration and thus in
FE
O
2
. Iodometry after both Ir-black and Ir/TiO
2
confirmed the
formation of OCl
. In case of the Ir/TiO
2
400 mmol and Ir/black
320 mmol OCl
could be determined.
Despite the change in FE
O
2
for E
cell
42.4 V
cell
, it remained
unclear whether OCl
was formed over a short time window or
whether it formed gradually over the entire testing period.
It appears kinetically interesting that the two electron transfer
reaction to OCl
in alkaline requires such a large overpotential,
while the formation of Cl
2
showed comparably small over-
potential in acidic environment as presented by Vos et al.
13
Conclusively, Ir-based catalysts in our tests showed inferior
performances as seawater anode catalysts compared to NiFe-
LDH. To investigate the OER/ClO
x
selectivity of NiFe-LDH under
higher loads, larger cell potentials appear necessary to enter a
regime where the OCl
formation is kinetically preferred.
In that context, the absolute current while using NiFe-LDH
as anode catalyst was increased up to 4.0 A (800 mA cm
2
)
resulting in cell potentials larger than 2.4 V (Fig. S10, ESI†).
All faradaic efficiencies ranged around 94%, which is con-
sistent with our former tests. Finally, at 4.0 A, the potential
suddenly increased to 4.0 V showing reduced FE
O
2
of 84%. But
again, no OCl
was detected by iodometry. This led us to
believe that the lower FE
O
2
wascausedbytheshorterretention
time. In essence, NiFe-LDH appeared to successfully suppress
Cl
oxidation even at large currents, which highlights it
favourable catalytic OER selectivity and suitability for seawater
electrolysis. The excellent OER selectivity of NiFe-LDH at very
high voltages and currents underlines that the previously
reported Design Criteria of seawater electrolysis are of thermo-
dynamic, but not kinetic nature. In fact, high current densities
at cell voltages of up to 2.5 V
cell
in Cl
-containing electrolyte
is feasible. This also supports the suitability of NiFe-LDH
electrodes as investigated by Kuang et al.
6
and underscores
the potential of our proposed asymmetric KOH/NaCl feed
configuration.
Fig. 3 Faradaic efficiency of O
2
(FE
O
2
) using various anode catalysts with an active area of 5 cm
2
and a loading of 2.0 mg
cat.
cm
2
.O
2
was analysed by
mass spectrometry under 100 ml min
1
N
2
carrier gas flow. The feed schemes of Fig. 1 are indicated in parentheses, the applied currents are in red
numbers, the green dotted line represents an O
2
concentration corresponding to 100% FE
O
2
. (a and b) Ni
0.66
Fe
0.34
-LDH as anode catalyst using the
following electrolyte feed schemes: symmetric 0.5 M KOH (black), symmetric 0.5 M (KOH + NaCl) (green) and asymmetric 0.5 M KOH at the anode and
0.5 M NaCl at the cathode. (a) Stepped constant current tests with current holds of 15 min each (b) subsequent 12 h stability test at applied current density
of 200 mA cm
2
(I= 1.0 A). (c and d) Commercial Ir-black (magenta) and TiO
2
supported Ir (purple) anode benchmark catalysts versus NiFe-LDH (green)
using symmetric 0.5 M (KOH + NaCl) electrolyte feed scheme. (c) Stepped constant current tests with current holds of 15 min each (d) subsequent
stability test at 200 mA cm
2
(I= 1.0 A).
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Long-term stability
To investigate the stability of the NiFe-LDH gas diffusion
electrode deployed in seawater electrolysers with asymmetric
electrolyte feed, we extended the initial selectivity test by six
additional repetitive cycles between the galvanostatic polariza-
tion curve and the 12 h galvanostatic test (protocol see Fig. S4,
ESI†) to obtain a total test time of B100 h consisting of eight
polarization curves and seven 12 h tests at 200 mA cm
2
(Fig. 4).
Although the measured electrode potential slightly increased
for each cycle, the FE
O
2
values remained essentially identical.
This applies for all polcurves (Fig. 4a) and at any time during
each of the seven 12 h stability tests (Fig. 4b). Starting from
1.7 V
cell
at 200 mA cm
2
, the cell voltage increased by roughly
8–10 mV per 12 h stability cycle, resulting in B100 mV after the
entire 100 h test. Given the stability of NiFe-LDH electrodes,
6
we
attribute this electrolyser performance loss to anode catalyst,
current collectors or membrane components. Indeed, AEM mem-
branes have remained one of the bottlenecks of durable seawater
electrolysers. Further, Pt-free seawater cathode HER catalysts
will become important in the future, because Pt is particularly
vulnerable in Cl
containing electrolytes due to the formation of
Cl-complexes. Mn-doped NiO/Ni hetero-structured cathodes
exhibiting Pt-like performances in both neutral electrolytes
and natural seawater
14
or CoMoP@C electrocatalysts with even
superior activities to Pt/C are currently under investigations
and will be incorporated in the current flow schemes in the
future.
15
Conclusions
We have developed and reported a new asymmetric electrolyte
feed concept for direct seawater electrolyser. This approach
enables direct feed of neutral seawater at the cathode in a
single pass, while circulating pure KOH electrolyte at the
anode. In Cl
containing alkaline electrolyte, NiFe-LDH showed
superior catalytic activity and OER selectivity compared to
Ir-based benchmark catalysts up to unprecedented cell voltages
of up to 4.0 V
cell
. Even though trace amounts of Cl
crossed the
membrane to the anode compartment, the NiFe-LDH anode
catalyst remained fully selective for OER without any oxidation
of Cl
, which makes it an excellent catalyst for continuous,
high-power direct seawater electrolysis at high cell voltages and
high current densities. The catalytic mechanism behind the
ClER suppression by using NiFe-LDH remain unclear and
require further investigations in a future work.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support by the federal ministry for economic affairs
and energy (Bundesministerium fu
¨r Wirtschaft und Energie,
BMWi) under grant number 03EIV041F in the collaborative
research project ‘‘MethQuest’’ in the group ‘‘MethFuel’’ and the
German Research Foundation (DFG) through grant reference
number STR 596/12-1 are grateful acknowledged.
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Fig. 4 Electrolyser stability: measured FE
O
2
during repetitive seven stability
test cycles consisting of polcurves #1 to #8 and seven 12 h stability
measurements using Ni
0.66
Fe
0.34
-LDH as anode catalyst and an asymmetric
electrolyte feed of 0.5 M NaCl at the cathode and 0.5 M KOH at the anode.
O
2
concentration was detected by mass spectrometry with a 100 ml min
1
N
2
carrier gas flow. (a) O
2
concentration (top) and measured electrolyser
cell voltages (E
cell
, bottom) during polarization curves #1–#8 with
applied current steps held for 15 min. Between consecutive polcurves a
galvanostatic 12 h stability test at 200 mA cm
2
was applied. (b) O
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concentration and E
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over time during the combined 100 h galvanostatic
stability test consisting of seven intermittent 12 h CP tests at 200 mA cm
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Before and after each CP one polcurve (2 h) was measured. The red
numbers D#1D#7 indicate the slope of the cell voltage after 12 h as a
measure of degradation.
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