Efficient and Stable Low Iridium Loaded Anodes for PEM Water Electrolysis Made Possible by Nanofiber Interlayers [original]

E ﬃ cient and Stable Low Iridium Loaded Anodes for PEM Water
Electrolysis Made Possible by Nano ﬁ ber Interlayers
Friedemann Hegge, Florian Lombeck, Edgar Cruz Ortiz, Luca Bohn, Miriam von Holst,
Matthias Kroschel, Jessica Hu  bner, Matthias Breitwieser, Peter Strasser, * and Severin Vierrath *
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s ı Supporting Information
ABSTRACT: Signi ﬁ cant reduction of the precious metal catalyst loading is one of the key challenges for the commercialization of
proton-exchange membrane water electrolyzers. In this work we combine IrOx nano ﬁ bers with a conventional nanoparticle-based
IrOx anode catalyst layer. With this hybrid design we can reduce the iridium loading by more than 80% while maintaining
performance. In spite of an ultralow overall catalyst loading of 0.2 mg Ir /cm 2 , a cell with a hybrid layer shows similar performance
compared to a state-of-the-art cell with a catalyst loading of 1.2 mg Ir /cm 2 and clearly outperforms identically loaded reference cells
with pure IrOx nanoparticle and pure nano ﬁ ber anodes. The improved performance is attributed to a combination of good electric
contact and high porosity of the IrOx nano ﬁ bers with high surface area of the IrOx nanoparticles. Besides the improved performance,
the hybrid layer also shows better stability in a potential cycling and a 150 h constant current test compared to an identically loaded
nanoparticle reference.
KEYWORDS: PEM water electrolyzers, ultralow loading, durability, catalyst morphology, nano ﬁ bers
1. INTRODUCTION
Polymer electrolyte membrane (PEM) water electrolysis is a
key technology for a sustainable hydrogen economy, but costs
still have to be reduced to be competitive with hydrogen
production from fossil resources.
1 − 3
Currently, bipolar plates
and porous transport layers make up for more than half of the
PEM water electrolyzer stack costs.
4 , 5
However, with
decreasing costs for these components the noble metal based
catalysts necessary for high conversion rates and e ﬃ ciencies
become a major cost driver.
5
The platinum required for the
hydrogen evolution reaction (HER) can generally be reduced
to values below 0.1 mg/cm 2 without signi ﬁ cantly losing
performance.
6
In contrast, reducing the amount of iridium for
the oxygen evolution reaction (OER) remains a key challenge
for PEM water electrolysis.
2
In state-of-the-art electrolysis
around 0.5 kg of iridium is required per megawatt installed
electrolyzer power.
7
Considering the current iridium produc-
tion of only 5 t per year, it becomes evident that with state-of-
the-art loadings the installed PEM water electrolysis capacity
will not break any time soon into the required terawatt scale.
2
For this reason, reducing the iridium loading at the anode from
the current state-of-the-art (1 − 3 mg/cm 2 ) to values below 0.5
mg/cm 2 is a primary focus of current research and develop-
ment.
5
However, when reducing the loading, two major
challenges arise: low durability and low conversion e ﬃ ciency.
8
The lack of durability can be explained by the intrinsic
mechanical instability of the very thin anode layers (in the
lower micrometer range), which in the current state-of-the-art
consist of iridium-based nanoparticles and a binding ionomer
applied onto the ionomer membrane. Also, iridium per se
slowly dissolves during operation, leading to a lower
catalytically active surface area and electrically isolated catalyst
material.
9 , 10
The uneven current distribution creates local
activity hot spots, which may lead to accelerated degradation of
the membrane electrode assembly (MEA).
11
The same
mechanism is the cause for poor e ﬃ ciencies of low iridium
Received: April 2, 2020
Accepted: August 10, 2020
Published: August 10, 2020
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loaded anodes. Recent studies show that low loadings and
therefore thin electrodes lead to poor electrical contact to the
outer porous transport layer (PTL), whic h provides fo r
electron, water, and oxygen transport to and from the anode.
As a consequence, isolated parts of the anode catalyst layer
(CL) do not contribute to the OER, which is the main cause
for the high overpotential and poor e ﬃ ciency.
12 , 13
The challenges outlined above suggest that the solution to a
signi ﬁ cant reduction of Ir loading without loss of performance
and durability lies, ﬁ rst, in a better catalyst utilization by
improving electric contact and accessibility to the catalyst
material and, second, in increasing the in-plane conductivity of
the catalyst layer. In fact, three approaches have been shown to
partly mitigate the losses of performance and durability of low
loaded anodes. It is worth noting that besides the e ﬀ ect of each
individual approach a combination of various approaches is
viable and presumably enables further performance improve-
ments. The ﬁ rst approach is applying a microporous interlayer
(MPL) between PTL and CL. Lettenmeier et al. ﬁ rst proposed
a Ti interlayer that was fabricated via vacuum plasma
spraying.
14
In their work, it is shown that the interlayer
reduces mass transport losses of the PTL. However, as
reported in a recent study, bulk mass transport of current state-
of-the-art PTLs plays only a minor role with respect to the
overall mass transport losses.
13
Schuler et al. showed that a
major part of the improved performance of the MPL approach
stems from an increased catalyst utilization due to a better
contact of the transport layer to the catalyst. Besides kinetic
and ohmic bene ﬁ ts, the higher catalyst utilization is assumed to
reduce the local mass transport resistance at or near the
cat alys t surf ac e. Th e se cond ap pro ach i s inc rea sing t he
conductivity and accessibility in the catalyst layer itself by
applying a support material resulting in a thicker catalyst layer.
In this regard, introducing a conductive support material like
titanium has successfully demonstrated increased performance
and durability.
15
The third approach is modifying the CL
microstructure. Performance enhancements could be achieved
by using core − shell catalysts and other microstructure
modi ﬁ cations, which enabled higher electron conductivity
and catalyst accessability.
16 − 18
The most successful approaches
are based on increasing the aspect ratio, which is the ratio
between the shortest and the longest dimension of the catalyst
particles. In 2015, Lewinski et al. presented whisker-shaped
iridium oxide (IrOx) nano ﬁ bers (nanostructured thin ﬁ lms) as
an OER catalyst, clearly outperforming IrOx nanoparticle
catalysts in PEMWE cells.
19
Recently, several groups presented
iridium-based nano ﬁ bers for the OER,
20 − 22
with the work of
Alia et al. demonstrating very high activities in operating
electrolyzer cells.
21
However, using only high aspect ratio
nano ﬁ bers a s a cat alys t laye r inc reas es acc es sibil ity a nd
electrical conductivity but comes with the downside of a
reduced electro chemically act ive surface area (ECSA ) in
comparison to high surface nanoparticles.
In this work, we use the advance in electrospun IrOx
nano ﬁ bers to apply an interlayer with high in-plane
conductivity on a conventional high surface IrOx nanoparticle
catalyst layer. With this novel hybrid design we can
signi ﬁ cantly reduce the iridium loading while maintaining
performance and durability.
2. EXPERIMENTAL SECTION
Nano ﬁ ber Synthesis. Electrospinning was performed on a device
from IME Technologies with rotating drum collector and climate
cham ber. A 1 .2 g sam ple of p oly( vinyl a lcoh ol) ( PVA, f rom
Bratachem) and 400 mg of iridium(III) choride − hydrate (from
Strem Chemicals Inc.) were dissolved in N , N -dimethylacetamide
(DMAc, from Carl Roth) to give a 10 wt % solution with respect to
the polymer. The solution was stirred for 12 h at 140 ° C and cooled
to ambient temperature before the electrospinning process was
initialized. The precursor nano ﬁ bers were fabricated in a 14 kV
electric ﬁ eld with a tip-to-collector distance of 15 cm; the ﬂ ow rate of
solution was ﬁ xed at 90 μ L/h. After successful electrospinning, the
nano ﬁ ber mat was placed in an oven under an ambient atmosphere
and heated to 370 ° C for 4 h with a heating rate of 1 K/min. The
result was a brittle IrOx nano ﬁ ber mat, which could be directly used
for ink preparation, as it collapses into individual ﬁ bers during the
following ultrasonication step.
MEA Fabrication. Two di ﬀ erent inks were prepared containing
IrOx catalyst in 40 wt % Na ﬁ on D520 dispersion in a 1:1 DI water to
isopropanol solution, as reported in an earlier work.
23
The IrOx
nanoparticle inks contained 1 wt % solids (Alfa Aesar iridium(IV)
oxide, Premion) while the IrOx nano ﬁ ber inks contained 0.5 wt %
solids. All suspensions were ultrasonicated for 30 min before use. Both
the nanoparticle catalyst and nano ﬁ ber interlayer were deposited via
spray-coating directly onto half catalyst coated membranes with 0.5
mg pt Pt/C cathodes and Na ﬁ on N115 ( ∼ 125 μ m thickness) and
Na ﬁ on NR 212 membranes ( ∼ 50 μ m thickness). For spray-coating a
benchtop ultrasonic spray coater (Sonocell SNR-300) equipped with
a 130 kHz ultrasonic nozzle was applied. The coating was conducted
in an intercrossed pattern with 1.5 mm pitch between the spray paths.
The spray head speed was adjusted to 170 mm/s, the ink ﬂ ow rate to
0.35 mL/min, and the hot plate temperature to 90 ° C. The Ir loading
was determined by weighing a reference substrate in a high precision
scale (Sartorius ME-36S) and correcting to the pure iridium content.
While the reference anodes where sprayed in one process step, the
nano ﬁ ber interlayer MEAs were fabricated by ﬁ rst spraying a loading
of 0.1 mg Ir /cm 2 nanoparticles onto the membrane and further
depositing 0.1 mg Ir /cm 2 nano ﬁ bers on top.
Crystallography. X-ray di ﬀ raction (XRD) patterns were meas-
ured in a Bruker D8 Advance di ﬀ ractometer (Bruker AXS, Cu K α
radiation) between 20 ° and 80 ° 2 θ with an increment of 0.05 ° and a
measuring time of 6 s per step.
Nitrogen Adsorption Analysis. N 2 physisorption isotherms
were obtained at 77 K by using an Autosorb-1 (Quantachrome). The
samples were initially ﬁ lled in a glass tube; to reduce the dead volume,
a glass rod and glass wool were inserted. The samples were degassed
under vacuum at 90 ° C for 24 h to remove adsorbed gas. Adsorption
and desorption isotherms were recorded in a range of 10 5 ≤ p / p 0 ≤
0.995 with p 0 being the saturation pressure and p the gas pressure.
The Brunauer − Emmett − Teller (BET) method
24
was applied to
evaluate the overall surface area. A multipoint ﬁ t was used in the range
of 0.1 ≤ p / p 0 ≤ 0.3.
In-Plane Resistivity. The sheet resistance of the catalyst layers
was determined by a transfer line method using a similar setup as
Ahadi et al.
25
Catalyst ﬁ lms with a width of 1 cm were prepared by
spray-coating onto an insulating glass substrate. Carbon paper with a
microporous layer (MPL) (Freudenberg H24C5) and a width of 5
mm was pressed with 0.7 N, MPL-side ﬁ rst, onto the ﬁ lm, to
electronically contact the entire width of the ﬁ lm. The resistance was
measured for contact distances of 1, 2, 3, and 4 cm at ambient
conditions (25 ° C, 50% RH) by using a FLUKE 175 multimeter. By
plotting the resistance against the distance between measuring points
and ﬁ tting linearly, we determined the sheet resistance from the slope.
Thus, we de ﬁ ne the electronic in-plane resistance reported in this
work as R sheet =d R /d x · w , where R is the measured ohmic resistance, w
is the width, and x is the measuring distance between the contact
points of the test sample.
Rotating-Disk Electrode. The experiments were conducted in a
three-electrode cell with a catalyst loading of 17.8 μ g/cm 2 on a glassy
carbon electrode in N 2 -purged 0.05 M H 2 SO 4 with an electrode
rotation of 1600 rpm. Cyclic voltammetry (CV) was conducted
between 0.35 and 1.4 V at a scan rate of 50 mV/s. The charge was
obtained by averaging the integral of the anodic and cathodic sweeps.
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The OER activity was evaluated by sweeping the potential from 1.0 V
to the potential where 10 mA/cm 2 was reached with 5 mV/s. All
potentials are reported vs RHE, and the results were averaged over six
measurements.
Electron Microscopy and Energy-Dispersive X-ray Spec-
troscopy. To investigate morphology and material composition
electron microscopy and energy-dispersive X-ray spectroscopy (EDX)
were conducted in a FEI Scios 2 focused ion beam scanning electron
microscope (FIB-SEM) with an AMETEK EDAX Elite Super EDX
detector. The scanning electron microscopy images were recorded
with an acceleration voltage of 5 kV. The material composition was
determined in the EDX at 30 kV.
Polarization Curves. Cell polarization was measured in a 5 cm 2
single cell setup by using a Scribner 857 redox ﬂ ow potentiostat. The
cell was operated at ambient pressure, 80 ° C, and a deionized water
ﬂ ow rate of 40 mL/min at the anode and cathode side. A sintered
titanium ﬁ ber PTL (Bekaert 2GDL40-1.0) was used on the anode
side and a carbon paper (Freudenberg H24C5) on the cathode side.
On both sides, gold-coated parallel type titanium ﬂ ow ﬁ elds with 5
cm 2 area, 1 mm channel width, and 1 mm land width were applied.
The high frequency resistance (HFR) free voltage was measured
during polarization measurement at 1 kHz. Polarization curves were
obtained by holding constant current steps of 120 s for the N115
MEAs and 30 s for the N212 MEAs.
Accelerated Stress Tests. Accelerated stress testing (AST) was
conducted according to a protocol suggested by Spo  ri et al.,
9
which is
partly based on the results of Cherevko et al.
26 , 27
The AST protocol
varies the voltage between 0.05 and 1.4 V in square wave cycles (3 s
each potential) and was designed to achieve a maximum dissolution
rate of IrOx. Enhanced Ir dissolution was observed during transient
operation compared to prolonged galvanostatic holds by using a
scanning ﬂ ow cell coupled with an inductively coupled plasma mass
spectrometer. The highest dissolution rates were obtained at upper
potential limits of 1.3 − 1.4 V.
27
The ASTs where conducted in the
same 5 cm 2 single cell setup used for the polarization measurements.
The cell was also operated at ambient pressure, 80 ° C and a ﬂ ow rate
of 40 mL/min for the anode and cathode side. The conductivity of
the process water was measured, and the water was replaced every 24
h to ensure low ion contamination con ﬁ rmed by conductivity values
below 3 μ S/cm. Prior to the AST the cell was cycled until a stable cell
operation was observed, since in the case of the nanoparticle MEA
initial cycling led to a performance improvement, possibly due to a
promoted porosity by initial dissolution.
9
Cell polarization was
measured after 5000, 14000, and 40000 AST cycles.
Constant Current Holds. Constant cu rrent holds were
conducted at 2 A/cm 2 , ambient pressure, 80 ° C, and a ﬂ ow rate of
40 mL/min for the anode and cathode side. Cell polarization curves
were measured at the beginning and every 50 h. The process water
was replaced every 24 h to ensure low ion contamination.
3. RESULTS AND DISCUSSION
Mate rials C harac teri zati on. As the cat alyt ic act ivit y
largely depends on the material composition, the nano ﬁ bers
and nanoparticles used in this study were analyzed with
energy-dispersive X-ray spectroscopy (EDX). The EDX
measurements yield a mass composition of 85% Ir and 15%
O 2 for the IrOx nano ﬁ bers, corresponding to the weight
fractions of pure IrO 2 . The composition of the commercial Alfa
Aesar nanoparticles was measured to be 79% Ir and 21% O 2 .
Excess oxygen of the Alfa Aesar IrOx was also reported by
Pfei ﬀ er et al.
28
and can be attributed to a surface hydration.
Besides the composition, also crystallinity has a strong e ﬀ ect
on the act ivit y.
29 − 33
Ther efore, X-ra y di ﬀ racti on (X RD)
patterns were measured ( Figure 1 ). The sharp re ﬂ ections at
40.8 ° , 47.4 ° , and 69 ° found in both samples can be assigned to
a metallic iridium phase,
34
the mean crystallite size of which
appears smaller for the nano ﬁ bers as indicated by the broader
re ﬂ ect ions.
35
The pa ttern of the com merc ial Alfa Aes ar
nanoparticles is fully consistent with earlier reports.
28
The
signi ﬁ cant amount of oxygen, as seen in both EDX measure-
ments, indicates an oxide phase in addition to the metallic
iridium phase. In the XRD pattern, the IrO 2 re ﬂ ections of the
nano ﬁ bers are either absent or too broad to see, while the
nanoparticles show weak broad IrO 2 re ﬂ ections. Thus, the
oxide phase is rather amorphous for both samples.
29
The electrochemical activity is further governed by the
electrochemically active surface areas (ECSA). Even though
the ECSAs cannot be accurately estimated from the capacitive
charge under CV measurements for metal oxides,
36
the charge
can be used to qualitatively compare the active surface area,
when material and sample loading are similar. When estimated
from the area of the cyclic voltammograms ( Figure 2 a), the
normalized interfacial charge of the nanoparticles (4.2 ± 0.7
mC/cm 2 )i s ∼ 7 times higher than that of the nano ﬁ ber sample
(0.6 ± 0.2 mC/cm 2 ). A larger surface area of the IrOx
nanoparticles is also observed in BET measurements, which
yield 57 m 2 /g for the nanoparticles and only 28 m 2 /g for the
nano ﬁ bers ( Figure 2 c). The di ﬀ erence between the CV and the
BET measurements could be explained by the additional
dependency of the charge from the IrOx calcination temper-
ature.
26
Rotating disk electrode (RDE) measurements show
that the required potentials to reach 10 mA/cm 2 (geometric
area) are 1.65 ± 0.01 V for the nanoparticles and 1.67 ± 0.03
V for the nano ﬁ bers ( Figure 2 b). Thus, the nanoparticles have
a higher geometric activity with the catalyst loading (both 17.8
μ g/cm 2 ), which can be explained by a higher surface area due
to their spherical shape and smaller particle size ( Figure 3 a,b).
The nano ﬁ bers are in the range of 50 − 400 nm in diameter and
1 − 3 μ m in length ( Figure 3 b), while the mean agglomerate
diameter of the nanoparticles is ∼ 100 nm.
37
The electro-
chemical results obtained with the RDE and additional activity
metrics are summarized in Table 1 . As shown in the table, the
mass activity of the nanoparticles at 1.6 V is 45% higher than
the activity of the nano ﬁ bers, which again can be attributed to
their higher surface area.
With this in mind, morphological properties of conventional
nanoparticle catalyst layers and those with nano ﬁ ber interlayer
shall be investigated. As discussed in the Introduction , recent
studies show that the in-plane conductivity and connectivity of
the particles have a strong e ﬀ ect on the overall performance of
the catalyst layer. To quantify the in-plane conductivity, the
electronic sheet resistances were determined for both layers. As
Figure 1. X-ray di ﬀ raction patterns of the IrOx nano ﬁ bers (red
pattern) and the commercial Alfa Aesar IrOx nanoparticles (black
pattern). Characteristic locations of Bragg re ﬂ ections of Ir (squares)
and IrO 2 (lower triangles) are indicated at the bottom.
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shown in Figure 2 d, the sheet resistance of an IrOx particle
layer with nano ﬁ ber interlayer is signi ﬁ cantly lower than that of
the pure IrOx particles, although both samples have the same
total IrOx loading of 0.2 mg Ir /cm 2 . It is to note that the
samples for the conductivity measurements were deposited on
a glass substrate. Even though the same spray parameters were
used as for MEA fabrication, the wettability of the substrate
can have a signi ﬁ cant impact on the morphology of the formed
layer. Moreover, the conductivity of both samples was
measured in dry conditions. In electrolysis operation, the
wetting of the ionomer decreases the conductivity of the
catalyst layer. This e ﬀ ect is due to swelling and thus additional
spatial separation of the catalyst material.
13
Because both
samples are a ﬀ ected in a similar way, we assume a similar
outcome in electrolysis operation.
The morphology of both catalyst layers (CL) is depicted in
Figure 3 , where Figure 3 a shows the top view of the ultralow
loaded (0.2 mg Ir /cm 2 ) nanoparticle anode. The IrOx nano-
particle catalyst layer mostly contains regions with a
homogeneous coating (1) but also certain regions, where the
catalyst material is electronically disconnected from the rest of
the catalyst layer (2). This observation con ﬁ rms the recent
ﬁ ndings of Bernt et al.,
12
reporting disconnected islands in low-
loading anode catalyst layers. As shown in Figure 3 b, the
nano ﬁ ber i nter la yer o f the hy br id an ode (t ot al loa di ng
including the interlayer is 0.2 mg Ir /cm 2 ) contains ﬁ bers with
diameters in the range 50 − 400 nm and lengths in the range 1 −
3 μ m. The particle layer is visible through the nano ﬁ ber
interlayer , indicating h igh porosity and low total laye r
thickness. In contrast to the pure nanoparticle CL, the hybrid
Figure 2. Cyclic voltammograms (a) and OER activity (b) of IrOx
nanoparticles and IrOx nano ﬁ bers, measured on a rotating disc
electrode in 0.05 M H 2 SO 4 with a rotation speed of 1600 rpm. (c)
BET surface area of the IrOx particles and the IrOx nano ﬁ bers. (d)
Sheet resistance of an IrOx nanoparticle sample compared to an IrOx
hybrid sample (IrOx nano ﬁ ber interlayer on IrOx nanoparticles)
measured in ambient air. Both samples were prepared by spray-
coating the catalyst materials on a glass substrate with a total loading
of 0.2 mg Ir /cm 2 .
Figure 3. (a) Top view on low loaded (0.2 mg Ir /cm 2 ) nanoparticle
catalyst layer with representative homogeneous region (1) and some
electronically disconnected islands (2). Scale bar is 2 μ m. (b) Top
vi ew on na no ﬁ be r int er lay er of t he Ir Ox h ybr id a nod e wi th
nanoparticle catalyst layer visible in background. Total loading is
0.2 mg Ir /cm 2 . The scale bar is 2 μ m. Both catalyst layers were
deposited on Na ﬁ on N115 membranes. (c) Schematic of a low loaded
catalyst layer (CL) with catalyst that is electronically disconnected
from the porous transport layer (PTL). (d) Schematic of the hybrid
IrOx anode comprising a low loaded CL with nano ﬁ ber interlayer that
distributes electrons to all regions of the CL and enhances stability.
Table 1. Summary of the RDE Results Measured in 0.05 M
H 2 SO 4 with a Catalyst Loading of 17.8 μ g/cm 2 : Potential to
Reach 10 mA/cm 2 (Geometric Area), Normalized Charge
( Q ), and Various Metrics at 1.6 V (Geometric Surface Area
j geo , Mass-Based j mass , and BET Speci ﬁ c Activity j spec )
Ir NP Ir NF
E at 10 mA/cm 2 geo (V) 1.65 ± 0.01 1.67 ± 0.03
Q (mC/cm 2 ) 4.2 ± 0.7 0.6 ± 0.2
j geo at 1.6 V (mA/cm 2 ) 4.5 3.3
j mass at 1.6 V (A/g Ir ) 318 219
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CL contained no disconnected regions, which is a potential
indication for the stabilizing e ﬀ ect of the nano ﬁ bers on very
thin layers.
Figure 3 c,d shows schematic representations of the observed
e ﬀ ects. As the pure nanoparticle CL contains electrically
disconnected regions, this catalyst material does not contribute
to the overall performance, which also constricts the proton
ﬂ ow in the membrane. In the case of the hybrid CL, the
nano ﬁ ber interlayer connects all regions increasing the overall
utilization. The lower sheet resistance was a ﬁ rst indication for
this theory. However, to further elucidate this, the electro-
chemical performance of both layers was evaluated in depth.
Electrochemical Performance. The performance of the
low loaded IrOx hybrid MEA was compared in cell polarization
tests against three reference MEAs, two with identical loading
(0.2 mg Ir /cm 2 , pure nanoparticle and pure nano ﬁ ber) and one
with a high anode loading (1.2 mg Ir /cm 2 ) as a state-of-the-art
performance benchmark. As shown in Figure 4 a, the low
loaded nanoparticle MEA performed signi ﬁ cantly worse than
the high loaded reference MEA, featuring a cell voltage of 2.12
V (70% voltage e ﬃ ciency) compared to 2.32 V (64% voltage
e ﬃ ciency) at 4 A/cm 2 . A major part of the losses is caused by
an increased high frequency resistance (HFR). The higher
HFR potentially stems from an increased membrane resistance
due to a reduced e ﬀ ective membrane cross section for the
proton transport and a higher contact resistance between the
CL and the PTL because of less contact. This current
constriction is a consequence of an uneven activity
distribution, which is caused by the poor in-plane electron
conductivity of the low loaded CL. This e ﬀ ect of nonuniform
activity distribution with high activities in regions close to the
PTL metal ( Figure 3 c) was ﬁ rst found by Mo et al.
38
and
investigated for low loaded anodes by Bernt et al.
12
To compare kinetic e ﬀ ects, Tafel plots of the HFR free cell
potential were analyzed ( Figure 4 b). The similar Tafel slopes
(depicted by the dashed line) indicate only marginal
di ﬀ erences in kinetics of the low loaded and the high loaded
MEA. Hence, for the IrOx nanoparticle MEAs, investigated in
this work, a di ﬀ erence in kinetic activity does not cause the low
performance when reducing the loading. Experimental results
from the literature show increased kinetic overpotentials for
ultralow loadings
12
for some catalysts but in other cases no
signi ﬁ cant change in kinetic overpotentials
15 , 39 , 39
even when
the loading was decreased to values as low as 0.1 mg/cm 2 .
Thus, we conclude that the in ﬂ uence of kinetics when reducing
the loading plays only a minor role especially for highly active
catalyst materials and is decisively depending on the properties
of the catalyst. Besides the increased HFR, we assume that the
uneven activity distribution is also responsible for the increased
mass transport loss due to increased local ﬂ uxes. The mass
transport losses can be estimated by the di ﬀ erence between the
extrapolated Tafel ﬁ t and the HFR free cell voltage
40
as
depicted by the gray area in Figure 4 b. It is to note that this
estimation also includes the proton transport losses in the CL,
which however are assumed to have a minor e ﬀ ect due to the
small thickness of the low loaded anodes.
41 , 42
With 125 mV vs
80 mV at 4 A/cm 2 , the mass transport overpotential of the low
loaded reference MEA is signi ﬁ cantly higher in comparison to
the 1.2 mg Ir /cm 2 MEA.
When moving from the low loaded pure nanoparticle to a
pure nano ﬁ ber MEA, a signi ﬁ cant reduction of the over-
potentials is achieved ( Figure 4 a). The HFR is comparable to
the high loaded reference MEA, which we attribute to the
higher conductivity due to better connection of the nano ﬁ bers
compared to the particles. The mass transport overpotential is
with 91 mV at 4 A/cm 2 signi ﬁ cantly lower compared to the
equally loaded particle reference. Besides the good connection
of the nano ﬁ bers, the reduction of mass transport resistance
(MTR) possibly stems from a more porous CL structure.
However, the kinetics of the nano ﬁ ber anode are worse as
observed in the low current density region of the Tafel plot
( Figure 4 b). The worse kinetics of the pure nano ﬁ ber catalyst
layer is in line with the lower ECSA and the lower activity
(e spec i all y in th e low c urr ent d ens it y reg ion) in RD E
measurements as discussed in the Materials section.
Applying a nano ﬁ ber interlayer on top of an ultralow loaded
parti cle an ode (0.1 mg Ir /cm 2 nano ﬁ bers and 0 .1 mg/c m 2
nanoparticles) combines the advantages of the nanoparticles,
i.e., high activity and ECSA, and the nano ﬁ bers, i.e., high
conductivity and low mass transport resistance. Furthermore,
the highly active nanoparticles are closely located to the
membrane interface, which is in general assumed to be the
region with the highest activity in through-plane direction.
43
In
fact, the MEA prepared in this fashion yields the best
performance and even outperforms the 1.2 mg/cm 2 nano-
particle MEA ( Figure 4 a,b).
Figure 4. Comparison of cell polarization (a) and Tafel plot of the
HFR free polarization curves (b) of the 0.2 mg Ir /cm 2 hybrid MEA
(nano ﬁ bers on nanoparticle anode) vs equally manufactured reference
MEAs with 0.2 mg Ir /cm 2 pure nano ﬁ ber (NF) anode, 0.2 mg Ir /cm 2
nanoparticle (NP) anode, and nanoparticle anode with state-of-the-art
loading of 1.2 mg Ir /cm 2 . All MEAs are based on Na ﬁ on N115
membranes and measured at ambient pressure and a temperature of
80 ° Ci na5c m
2 cell. The Tafel slope a was determined between 10
and 100 mA/cm 2 .
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To con ﬁ rm the excellent performance of the low loaded
interlayer MEAs, the results were compared to the state-of-the-
art performance range, gathered from ∼ 200 publications in a
recent study by Bender et al.
44
In addition to the Na ﬁ on 115
MEA ( Figure 5 a), a Na ﬁ on NR 212 MEA was fabricated and
included in the comparison ( Figure 5 b). It should be noted
that the reported cell performance values signi ﬁ cantly deviate
from each other due to di ﬀ erent catalyst material compositions
(e.g., Ir − Ni, Ir − Co, IrRuOx, etc.), high Ir loadings (beyond 1
mg/cm 2 ), and cell assemblies including performance enhanc-
ing measures like pretreating the membrane or sputter coating
the porous transport layers. With this in mind, it is even more
astonishing that despite the simple IrOx catalyst material and
the ultralow loading used in this study, the polarization curves
are close to the most active region of the state-of-the-art cell
voltage range, demonstrating the very competitive performance
of the present ultralow loaded hybrid MEAs.
Durability. Besides performance, durability of the anode is
vital for long-term operation of the PEM water electrolyzer. To
support the theory of the stabilizing e ﬀ ect of the nano ﬁ ber
interl ayer (see the In troduction ), acc elerated str ess tes ts
(ASTs) were performed. The applied AST protocol was
chosen to achieve a maximum iridium dissolution.
9
Because
the protocol was originally developed for RDE measurements
and the voltage was signi ﬁ c a n t l yl o w e rt h a ni nt y p i c a l
electrolyzer operation, additional constant current holds at 2
A/cm 2 were performed. Figure 6 a shows the cell polarization
curves of the ultralow loaded IrOx hybrid MEA and the
ultralow loaded nanoparticle MEA before and after 40000 AST
cyc les . Figu re 6 b sho ws th e cel l po lari za tio n cu rve s of
identically prepared samples before and after 150 h constant
current holds at 2 A/cm 2 .
After both durability tests, the overvoltage of the hybrid
MEAs is signi ﬁ cantly lower than the overvoltage of the aged
nanoparticle MEAs. At a current density of 2 A/cm 2 the hybrid
MEA shows a minimal performance loss of 14 mV over 40000
AST cycles ( Figure 6 a). In contrast, the nanoparticle MEA
shows a signi ﬁ cant performance loss, with a voltage increase of
68 mV. The overvoltages after the constant current holds
con ﬁ rm the higher stability of the hybrid MEA. The voltage of
the low loaded nanoparticle layer signi ﬁ cantly increased by 104
mV after 150 h of constant current hold at 2 A/cm 2 , while the
hybrid MEA shows a voltage drop of 29 mV ( Figure 6 b). The
average voltage degradation rates are 0.2 mV/h for the hybrid
MEA and 0.7 mV/h for the NP MEA ( Figure S1 ). The higher
stability of the hybrid layer MEA in comparison to the
nanoparticle MEA seems surprising due to the similar IrOx
type used in both samples. However, as discussed in the
Introduction , the dissolution rates not only depend on the
crystallinity but also on the morphology of the catalyst layer. It
is assumed that besides the material properties, also surface
are a, poro sity, m ass tra nspo rt, and el ect ron and pr oton
Figure 5. Comparison of cell polarization curves of the 0.2 mg Ir /cm 2
hybrid MEA on a Na ﬁ on 115 membrane (a) and on a Na ﬁ on 212
membrane (b) vs state-of-the-art performance ranges of PEMWE
cells
44
for identical membrane type and operating conditions of 80 ° C
and ambient pressure. It is to note that the literature data in this
comparison also contain values from MEAs with highly active mixed
oxide and supported catalysts with loadings >1 mg/cm 2 , while the
hybrid MEAs of this study used pure IrOx.
Figure 6. (a) Comparison of cell polarization curves of the 0.2 mg/
cm 2 IrOx hybrid MEA vs a 0.2 mg/cm 2 nanoparticle MEA before and
after 40000 AST cycles. (b) Comparison of cell polarization curves of
the 0.2 mg/cm 2 IrOx hybrid MEA vs a 0.2 mg/cm 2 nanoparticle MEA
before and after 150 h constant current hold at 2 A/cm 2 . Both MEAs
are based on Na ﬁ on 115 membranes.
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conductivity have an impact on the durabili ty.
44
The
conducted stability tests indicate a higher stability of the
hybrid CCM compared to equally loaded particle CCMs.
However, to fully understand the reasons for the higher
stability of the hybrid CCM, extensive material character-
ization should be conducted in future studies.
4. CONCLUSIONS
We present an IrOx nano ﬁ ber interlayer anode for PEM water
electrolysis. With this novel anode architecture, we were able
to reduce the iridium loading by more than 80% in comparison
to a state-of-the-art IrOx anode while maintaining performance
and durability. We showed that superior performance is
achieved by combining the advantage of good electric contact
and high porosity of the IrOx nano ﬁ bers with high surface area
of IrOx nanoparticles. To investigate the advantages of the
nano ﬁ ber interlayer anode, nanoparticles and nano ﬁ bers where
characterized with respect to composition, crystallinity, catalyst
surface area, conductivity, and microstructure. EDX measure-
ment s showed a s imil ar com posit ion, and X RD spe ctra
exhibited a predominantly amorphous crystal structure for
both samples, indicating that the major part of the perform-
ance di ﬀ erences in fact have to stem from the particular
morphologies of the anodes. The catalyst surface area of the
IrOx nanoparticles was found to be signi ﬁ cantly higher than
that of the nano ﬁ bers, as determined by BET analysis with 57
and 28 m 2 /g, respectively. The ex-situ sheet resistance of a
nano ﬁ ber/nanoparticle hybrid anode was found to be
signi ﬁ cantly lower than that of a pure nanoparticle anode,
indicating an improved electric contact of the catalyst ﬁ bers.
The better connectivity of the catalyst material was also
observed in SEM images. In contrast to an equally loaded (0.2
mg Ir /cm 2 ) IrOx nanoparticle anode, the nano ﬁ ber interlayer
anode exhibited no electrically isolated islands. In polarization
experiments CCMs with IrOx nano ﬁ ber interlayer anodes with
a loading of 0.2 mg Ir /cm 2 outperformed equally fabricated
state-of-the-art nanoparticle MEAs with 0.2 and 1.2 mg Ir /cm 2
loading as well as 0.2 mg Ir /cm 2 pure IrOx nano ﬁ ber MEAs. In
comparison to state-of-the-art performance of higher loaded
MEAs from the literature, the nano ﬁ ber interlayer MEAs
showed above average performance. Besides performance,
durability was assessed by accelerated stress tests and constant
current holds at 2 A/cm 2 . While an equally loaded nanoparticle
MEA showed a signi ﬁ cant performance loss of 68 mV at 2 A/
cm 2 , the overvoltage of the hybrid MEA increased by only 14
mV over the complete test length of 40000 cycles. The results
of the constant current holds con ﬁ rmed the improved stability
of the hybrid MEA with an average voltage degradation of 0.2
mV/h at 2 A/cm 2 over 150 h vs 0.7 mV/h for the nanoparticle
MEA.
The improved performance can be attributed to combining
the advantages of nano ﬁ bers and nanoparticles. The high
porosity and the good electrical and mechanical contact of the
IrOx nano ﬁ bers lead to low mass transport losses and high
catalyst utilization, while the IrOx nanoparticles contribute
with high surface area and activity.
In this work, we intended to demonstrate the advantages of
the electrode structure of the nano ﬁ ber interlayer anodes.
Therefore, pure IrOx was used as a simple and established
anode material. In future studies mixed oxides and iridium
based alloys with higher activity or smaller IrOx nanoparticles
with an increased surface area can be used to even further
increase the performance or durability of the novel hybrid
catalyst layer architecture.
■ ASSOCIATED CONTENT
*
s ı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaem.0c00735 .
Voltage evolution over time during 150 h constant
current hold of 0.2 mg/cm 2 IrOx hybrid MEA and 0.2
mg/cm 2 nanoparticle (NP) MEA ( PDF )
■ AUTHOR INFORMATION
Corresponding Authors
Severin Vierrath − Electrochemical Energy Systems, IMTEK -
Department of Microsystems Engineering, University of Freiburg,
79110 Freiburg, Germany; Hahn-Schickard, 79110 Freiburg,
Germany; orcid.org/0000-0002-4505-2803 ;
Email: [email protected]
Peter Strasser − The Electrochemical Energy, Catalysis, and
Materials Science Group, Department of Chemistry, Technical
University Berlin, 10623 Berlin, Germany; orcid.org/0000-
0002-3884-436X ; Email: [email protected]
Authors
Friedemann Hegge − Electrochemical Energy Systems, IMTEK
- Department of Microsystems Engineering, University of
Freiburg, 79110 Freiburg, Germany
Florian Lombeck − Hahn-Schickard, 79110 Freiburg, Germany
Edgar Cruz Ortiz − Electrochemical Energy Systems, IMTEK -
Department of Microsystems Engineering, University of Freiburg,
79110 Freiburg, Germany
Luca Bohn − Electrochemical Energy Systems, IMTEK -
Department of Microsystems Engineering, University of Freiburg,
79110 Freiburg, Germany
Miriam von Holst − Hahn-Schickard, 79110 Freiburg,
Germany
Matthias Kroschel − The Electrochemical Energy, Catalysis, and
Materials Science Group, Department of Chemistry, Technical
University Berlin, 10623 Berlin, Germany
Jessica Hu  bner − The Electrochemical Energy, Catalysis, and
Materials Science Group, Department of Chemistry, Technical
University Berlin, 10623 Berlin, Germany
Matthias Breitwieser − Electrochemical Energy Systems,
IMTEK - Department of Microsystems Engineering, University
of Freiburg, 79110 Freiburg, Germany; Hahn-Schickard, 79110
Freiburg, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsaem.0c00735
Notes
The authors declare no competing ﬁ nancial interest.
■ ACKNOWLEDGMENTS
The authors gratefully acknowledge ﬁ nancial support of this
work by the Federal Ministry of Education and Research in
Germany within the project NeutroSense (grant: 05KI9VFA)
and the Ministry of Economy of Baden-Wuerttemberg within
the project DirectMEA.
ACS Applied Energy Materials www.acsaem.org Article
https://dx.doi.org/10.1021/acsaem.0c00735
ACS Appl. Energy Mater. 2020, 3, 8276 − 8284
8282

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