Document [original]

Impact of Carbon Support Meso-Porosity on Mass

Transport and Performance of PEMFC Cathode Catalyst

Layers

Sebastian Ott,[a] Andreas Bauer,[b] Fengmin Du,[b] Tuan Anh Dao,[b] Malte Klingenhof,[a]

Alin Orfanidi,*[b] and Peter Strasser*[a]

The analysis of the impact of the cathode catalyst layer pore

structure on the membrane electrode assembly (MEA) cell

performance of a PEMFC is presented. In this study, a pristine

CMK-3 catalyst carbon support material with well-defined pore

structure in the 3–6 nm range together with two nitrogen-

doped variants is analyzed against a commercial carbon black

to achieve a better understanding of catalyst layer porosity-

performance relations. We used chemically N-doped CMK-3

catalyst to learn more about the effect of N-doped porous

catalyst supports on the concomitant transport properties and

PEMFC cell performance. Chemical treatment using cyanamide

was conducted to introduce a variety of N-functionalities. A

detailed in-situ electrochemical investigation was combined

with N2-physisorption analysis. Based on their structural proper-

ties, the mesopore fractions and pore openings display a major

role for reducing oxygen transport resistance and enhance Pt

accessibility. We find that hierarchically ordered mesoporosity is

superior to disordered porosity with prevalent micropore

character: Analysis including adsorption electrochemical active

surface area (ECSA), Pt-accessibility, ionomer coverage, pore

geometry, proton resistivity and transport loss we conclude the

importance of a well-defined mesoporous structure for its cell

performance.

Introduction

In the global fight against climate change, the reduction of

greenhouse gas emission is a major strategy. One fraction of

the human made emissions lies in the transport section for

short and long-range transportation. To reduce the CO2

emission of the transport sector, fuel cell technology provides a

big potential as power source for zero net emission in future

economy and infrastructure. Even though fuel cell electrical

vehicle (FCEVs) gathered increased attention due to their short

refueling time and long distance range, their wider commercial

spread is still hindered compared to battery electrical vehicles

(BEVs) and internal combustion engine vehicles (ICEVs). One

major issue lies in the cost intensive production line of the

components especially in the need of noble metals such as Pt.

The high demand of catalytic active sites breaks down to the

sluggish ORR on the cathode side of a fuel cell and transport

limitations throughout the catalytic layer itself. According to the

DOE targets, the Pt loading needs to be reduced below

0.1 mgPtcm2to enable an economic and ecologic efficient

mass scale-up production line of FCEVs.[1] To achieve this target,

many researchers’ efforts were made to increase the catalytic

activity of the metal specie by dealloying, shape controlling

etc.[2] However, the significant role of the supporting material

and their tuning potential towards performance enhancement

has only recently started to gain attention.

High surface area carbon (>800 m2g1) is used to enable an

increased mass activity by minimizing poisoning of the active

sites originating from the sulfonic acid groups of the ionomer,

since direct metal/ionomer contact can be reduced compared

to low surface area carbons.[3] However, utilizing a well-

developed porous support material, where the active metal

sites are mainly located in the interior of the carbon support,

causes mass transport limitations.[4] As a result of this, the

catalytic material suffers from oxygen depletion and con-

comitant oxygen mass transport losses at high current densities.

A number of studies have been put forward in recent years to

analyze and mitigate the detrimental catalyst layer transport

effects. Therein, a number of different aspects were identified

to contribute to the observed performance losses, especially for

low Pt loading based electrodes. The influence of the electro-

chemical active surface area (ECSA) was found to directly

determine the magnitude of the local oxygen flux.[5] In addition,

the catalyst support-ionomer interaction was shown to affect

the spatial distribution of the ionomer, which in turn strongly

affect the proton and oxygen transport characteristics of the

catalyst layers.[6] Kodama et al. provided the first structural

[a] S. Ott, M. Klingenhof, Prof. P. Strasser

Department of Chemistry

Chemical Engineering Division

Technical University of Berlin

Straße des 17 Juni 124

10623 Berlin (Germany)

E-mail: [email protected]

[b] A. Bauer, F. Du, T. A. Dao, A. Orfanidi

BMW Group

80788 Munich (Germany)

E-mail: [email protected]

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/cctc.202101162

an open access article under the terms of the Creative Commons Attribution

License, which permits use, distribution and reproduction in any medium,

provided the original work is properly cited.

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evidence associating the presence of dense ionomer layers over

the catalytic active metal sites with a so-called local oxygen

transport resistance.[7] The importance of the homogenous

ionomer distribution towards improvement of transport phe-

nomena and increase in cell performance was also recently

demonstrated in the previous publication by Ott et al.[6b] At the

same time, the porosity of both the support material and the

resulting catalytic layer plays an important role, building up the

pathway oxygen molecules have to travel trough.

To gain deeper insights into the role of the catalyst support

within the catalytic layer and the influence of their porosity

towards mass activity and oxygen mass transport resistance,

Yarlagadda et al. investigated fine-tuned carbon support

materials.[8] They were able to show that utilizing a carbon

material with accessible pores in a range of 4–7 nm improved

both mass transport and catalyst activity. Based on their

findings, metal active sites within pores of such “accessible”

dimensions would provide an optimized trade-off between

reduced poisoning effect (from the ionomer in direct contact

with the active sites) and improved oxygen transport through

the catalytic layer reaching the metal surface. By contrast,

Ramaswamy et al. focused on the correlation of micropores

(<2 nm) and macropores (here defined as >8 nm) with the cell

performance.[9] They hypothesized a correlation of the micro-

pore surface area with the extension of bottleneck openings

throughout the carbon structure. Whereas a decreased ratio of

macropores would reduce the carbon support surface area

available for ionomer interaction, this may favor a more

homogeneous ionomer distribution. Consequently, both param-

eters, namely micro- and meso/macroporous fraction, would

influence oxygen mass transport phenomena and thus directly

determine the fuel cell performance at high current densities.

These two studies by Yarlagadda et al. and Ramaswamy et al.

provide us valuable guidelines on how to tune the mesopores

of the catalyst support, with the aim to achieve increased mass

activity and improve performance by reducing the micropore

surface area in non-hierarchically ordered porous structured

carbon.

To close the knowledge gap between those two studies, in

the present contribution, we investigated the influence of

pristine and N-doped carbon supports with well-defined

mesoporous structure on the overall PEMFC cell performance.

By using an ordered/templated mesoporous carbon with well-

defined pore structure, in particular CMK-3, we were able to

correlate performance changes with pore properties of the

carbon support. A commercial high surface area carbon

(Ketjenblack) was also used in the present study for comparison

reasons, as it is a widely used as carbon support in fuel cells

applications. A thermal process using a solid N precursor

enabled concomitant pore tuning and N-modification of the

CMK-3 carbons. We conducted electrochemical characterization

of nanoparticle Pt catalysts supported on our tailored N-doped

mesoporous supports and referenced them to the pristine

carbon material. The identification of oxygen mass transport

resistance, ECSA and performance under different relative

humidity (RH) conditions provides information about transport

phenomena within the catalytic layer. In combination with

structural modified carbon material, we are able to deconvolute

the influence of specific porous characteristics towards the

membrane electrode assembly (MEA) performance.

Experimental

Synthesis

For the carbon pre-oxidation step, a batch of 500 mg commercial

CMK-3 (ACS material) was mixed in a 50 mL flask with 25 mL 70%

HNO3(VWR chemicals, AnalaR NORMAPUR). The mixture was

continuously stirred under reflux for 30 min and immersed in a

70°C pre heated oil bath. The resulting powder was filtrated and

washed with water until pH neutrality and dried in a vacuum oven

at 80°C for 17 h. The pre-oxidized carbon was denoted as CMK-

3Ox.

For the N-functionalization, 200 mg CMK-3 and CMK-3Ox were

mixed with 17 mg and 12 mg Cyanamid, respectively. The mixture

was diluted in 4 mL water and sonicated for 15 min to ensure a

homogenous precursor distribution. The suspension was then

frozen in liquid nitrogen and the water content evaporated in a

freeze dryer overnight. For final heat treatment, the resulting

powder was set in a tube furnace (CARBOLITE GERO GmbH & Co

KG, Germany) and treated up to 600°C. After an initial purging step

of 15 min with Ar, a heating ramp of 400 K/h was set until the

temperature reached 600°C and was held for 2 h before naturally

cooled down to room temperature. A constant gas flow rate of

10 L/h was kept for the Ar flow. The obtained powders were

denoted as CMK-3/CA and CMK-3Ox/CA.

A commercial Ketjenblack EC-300j (here denoted as KB) was

purchased from AkzoNobel and used in the present study for

comparison reasons. For Pt-deposition 150 mg of the N-modified

(CMK-3/CA and CMK-3Ox/CA) or pristine carbon (CMK-3 and KB)

support was mixed with 100 mL ethylene glycol (99.8%, Sigma-

Aldrich) and 50 mL deionized water in a 250 mL round bottom

flask.[6a] The mixture was bath-sonicated for 15 min before adding

H2PtCl6solution (0.25 mol/L, obtained from solid H2PtCl6·6H2O, Alfa

Aesar) solution, whose amount was adjusted to match a nominal

loading of 14–20 wt.%. The mixture was continuously stirred under

reflux for 2 h and immersed in a 120°C pre heated oil bath. The

resulting powder was filtrated and washed with water until all Cl-

residues were removed (3 L water per 200 mg catalyst) and

thereafter dried in a vacuum over at 80°C for 17 h. The exact Pt

loading for each catalyst was determined using inductively coupled

plasma-optical emission spectrometry ICP-OES and shown in

Table 2.

Physico-chemical characterization

Elemental analysis was applied to determine the exact carbon

supports composition. Therefore, a Thermo Flash 1112 Organic

Elemental Analyzer (Thermo Finnigan) was used. The samples were

combusted in presence of V2O5(as oxidizer) by dynamic flash at

1020°C. The decomposition takes place in a manually stacked

reactor of WO3/Cu/Al2O3layers. Gas chromatography (GC) deter-

mines and quantifies the resulting gases. The elemental analysis of

C, H, N, and S of different carbon supports is given in Table 1.

The exact Pt content was determined via inductively coupled

plasma (ICP) analysis using a VARIAN 715-ES system. Around 7 mg

of the final catalyst was placed in a microwave tube and mixed

with 2 mL H2SO4(95–98%, VWR chemicals, AnalaR NORMAPUR),

2 mL HNO3(69%, VWR chemicals, AnalaR NORMAPUR) and 6 mL

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HCl (37%, VWR chemicals, AnalaR NORMAPUR). All Pt particles were

dissolved during the microwave (Microwave GO, ANTON PAAR)

heat treatment, where the running protocol comprised of applying

a heating ramp up to 120°C within 10 min and hold for another

10 min before cooling down to room temperature. From the

standard solutions of 1 mg/L, 5 mg/L and 10 mg/L (H2PtCl6in

2 mol/L HCl, 1000 mg/L Pt, MERCK KGAA), a calibration curve was

constructed and the chosen wavelengths for the Pt determination

were 203.646 nm, 204.939 nm, 212.863 nm, 214.424 nm,

217.468 nm and 224.552 nm. The results are depicted in Table 2.

N2physisorption was conducted to investigate the micro and meso

pore structure of the carbon species using Autosorb-1 (Quantach-

rome Instruments). A 4 mm diameter glass tube was stacked with

the sample, glass wool and a glass rod to minimize the dead

volume. The sample weight was adjusted so that the absolute

surface area exceeds 10 m2in order to minimize the errors of the

measurement. To remove all gas and water residues adsorbed at

the carbon surface the sample was degassed under vacuum at

80°C for at least 24 h. The adsorption and desorption isotherms

were recorded in a range of 105�p/p0�0.995 with p0referring to

the saturation pressure and pthe actual gas pressure. To be able to

distinguish between micropores (<2 nm) to mesopores (>2 nm)

and avoid any artificial gaps the choice of a matching Density

Functional Theory (DFT) kernel combined with a fitting model is

important. In case of Ketjenblack (KB), a Quenched Solid Density

Functional Theory (QSDFT) kernel with a slit/cylindrical pores model

was applied for the adsorption branch. In case of CMK-3 based

supports a non-local density functional theory (NLDFT) kernel with

a cylindrical pore of the equilibrium model was applied. The latter

model was considered more suitable as template carbon species of

CMK-3 provides more defined and structured carbon particles as

opposed to the amorphous Ketjenblack carbon.[10]

The effective ionomer film thickness is calculated based on the

equation 5a and 5c of Liu et al.[11] For this calculation and the

investigation of ionomer distribution in the catalyst layers the

catalyst powder (Pt+carbon support) and the catalyst powder

coated with ionomer were also analyzed using N2physisorption

while QSDFT kernels with cylindrical pore model was used for the

adsorption branch. The catalyst void volume below 4 nm correlates

with the volume of intruded ionomer and can be considered as not

taking part in the exterior film thickness. The exterior pore surface

is defined as carbon support surface above 4 nm.

To determine the Pt crystallite size, X-ray powder diffraction (XRD)

measurement was conducted. A BRUKER D8 ADVANCE diffractom-

eter with a single beam and a Cu-Kα1(γ=1.54051 Å) source was

used. The samples were grinded and placed on a Si waver before

scanning from 20–90°without rotation using a scan rate of 0.04°

per 7 s.

Transmission electron microscope (TEM) measurements were con-

ducted using a Tecnai G2 20 s-Twin microscope, equipped with a

LaB6-cathode and a GATAN MS794 P CCD-detector at ZELMI

Centrum, Technical University of Berlin. TEM samples were ultra-

sonicated in i-PrOH and drop-dried on copper grids.

Membrane electrode assembly (MEA) testing

The decal transfer method was employed to manufacture all

MEAs.[12] For the catalyst inks, a low-EW ionomer dispersed in 40%

H2O/60% 1-propanol (3 M™Dyneon™PFSA (725 EW�

gpolymer/molH+)) was mixed with the catalyst powder. A specific

water/1-propanol ratio of 16–25 wt.% H2O in 1-propanol was

applied for Ketjenblack based samples and 12–15 wt.% H2O in 1-

propanol for CMK-3 based ones. The ionomer to carbon ratio (I/C)

was in all cases 0.65. After optimizing the ink recipe and viscosity

to achieve high decal quality, all materials were merged in a 15 ml

HDPE capped bottle containing 26 g of 5 mm ZrO2beads in the

following sequence: catalyst, water, 1-propanol, and finally the

ionomer dispersion. After roll-mixing for 18 h at a speed of 60 rpm

at room temperature, the homogenously mixed ink was coated

onto a virgin PTFE using a Mayer rod coater (ERICHSEN

UNICOATER MODEL 490). The resulting cathode electrode loadings

for each catalytic layer is given in Table 2.

To prevent any influence in voltage loss originating from the anodic

electrode, a commercially available 30 wt.% Pt/graphitized-Ketjen-

black (TEC10EA30E, TANAKA Kikinzoku Kogyo K. K.) was used to

manufacture an anode with a I/C ratio of 0.65 and a Pt loading of

0.17 mgPt/cm2. To determine the exact electrode loading the decal

was weight before and after decal transfer. The decal transfer was

done by hot pressing a 10 μm membrane (GORE MX20.10) between

the anode and cathode decals at 155°C for 3 min with 0.24 kN/cm2

(hereby denoted as CCM). The anode and cathode decals had an

active area of 8.25 cm2. To accurately define the active area the

MEA to 5 cm2, the CCMs were additionally sandwiched between

two subgaskets with an active area window of 5 cm2(CMC

Klebetechnik, type: PEM-Schutzfolie 61325). The lamination process

comprises of a first hot pressing step at 135°C for 10 min under

0.135 kN/cm2, followed by ramping down to 75°C within 10 min

without releasing the applied force.

A modified single-cell hardware from Tandem Technologies con-

taining a 50 cm2active area graphite composite flow field (14

channel serpentine flow field[13] purchased from Nisshinbo) was

used for all electrochemical testing (the flow field and adjustment

of the MEA is shown in Figure S4). A hardstop sealing approach is

used to define the compression of the MEA based on the thickness

of the sealing layers (shown in Figure S3). The incompressible

fiberglass-reinforced PTFE-gaskets (Fiberflon) was adjusted to

obtain a 20% compression of the GDL (29BC with a nominal

thickness of 235–240 μm; SGL Carbon) by applying 9 bar clamping

pressure on the cell. All fuel cell measurements were conducted on

an automated HORIBA FuelCon GmbH (Germany) single cell test

station (typ 200 A FUELCON) equipped with a potentiostat

(ZAHNER-Elektrik GmbH & CoKG) coupled with a booster. Pure

hydrogen (99.999% purity) and compressed air were used as anode

and cathode reactant, respectively. For each type of MEA, two

independent fuel cell measurements were performed and the

Table 1. Elemental bulk composition (CHN) of the different types of

carbons. The oxygen content was estimated as the difference to 100%

assuming only C, N, H and O atoms in the carbon species.

Elemental analysis [wt%]

Sample C N H O

KB 98.57 0 0 1.43

CMK-3Ox 85.07 0.50 0.16 14.27

CMK-3/CA 86.51 0.78 0.24 12.47

CMK-3Ox/CA 94.09 1.29 0.12 4.50

Table 2. Cathode specifications of 5 cm2MEAs: Pt catalyst loading on

different carbon supports and the cathode Pt loading.

Catalyst type Pt catalyst loading Cathode Pt loading

[wt.%] [mgPt/cm2]

Pt/KB 16.1�0.5 0.115�0.008

Pt/CMK-3 20.6 0.140�0.002

Pt/CMK-3/CA 19.3 0.120

Pt/CMK-3Ox/CA 14.2 0.135�0.005

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average value with corresponding error bars as the standard

deviation is depicted in all figures.

All MEAs were activated, using a conditioning protocol from DOE,

were the voltage was cycled 8x times between OCV (5 min), 0.85 V

(10 min) and 0.6 V (45 min) at 80°C cell temperature, 90% RH and

170 kPaabs under differential flows.[6b] The MEA performance was

determined via polarization curves conducted under differential

flows (H2/air=1.0/2.0 Nl/min) at both dry (90°C cell temperature,

30% RH and 170 kPaabs) and wet (60°C cell temperature, 90% RH

and 170 kPaabs) operating conditions.[14] The ECSA was evaluated via

CO stripping at a cell temperature of 80°C at different relative

humidities (30% and 100% RH).[6b] In addition, proton conductivity

of the cathodic catalyst layer was evaluated via EIS with peak-to-

peak perturbation of 20 mV, where the cell was kept under H2flow

(1.0 Nl/min) at the anode and N2flow (2.0 Nl/min) at the cathode,

cell temperature of 80°C, and cathode voltage at 0.5 V.[14] The

determination of the specific proton resistivity (1, in Ωcm) via

fitting of the transmission line model was the same as described in

detail in Du et al.[14] The mass transport resistance was estimated

via limiting current measurement using 2.4% O2/N2(2.045 Nl/min)

at the cathode side and pure H2(1.0 Nl/min) at the anode side,

where the testing conditions were: cell temperature was kept at

80°C with two relative humidities (30% RH and 100% RH).[6b] All

testing was done under differential flows. For extensive details on

the testing protocols, please refer to our previously published

papers cited next to each characterization method, as the exact

same testing conditions were used in the present study.

Results and Discussion

The overarching goal of this work is the description and

improved understanding of the impact of the mesopore

structure of cathode catalyst carbonaceous support materials

on the overall PEMFC cell performance. To achieve this goal, we

used commercial Ketjenblack (EC 300j) as a reference carbon

support and compared it to highly ordered template-based

CMK-3 carbon materials with tuned, yet tailored mesoporosity.

The well- defined mesoporous structure of pristine CMK-3 was

tuned by applying a variety of synthetic N-modifications. The N

modification synthesis step employed solid cyanamide as an N-

precursor during an annealing treatment at 600°C under Ar

atmosphere (more details can be found in Experimental section

above). This N-modified CMK-3 material will be referred to as

“CMK-3/CA”. From “CMK-3/CA”, an oxidized variant of a N-

modified CMK-3 support material was prepared by means of

treatment with aqueous HNO3, henceforth denoted as “CMK-

3Ox/CA”. The thermal protocol and other synthetic parameters

of the N modified samples were modified to control the

resulting weight-based nitrogen content, N wt% (Table 1), to

comparable values in all materials. The Pt-based catalysts

supported on such modified carbons reveal an overall homoge-

nous and similar Pt-particle distribution over the entire carbons'

surface as proven by TEM (Figure S1). Additional XRD evaluation

determine a Pt-crystallite size from 2.0–3.7 nm revealing the

bigger Pt-particles on CMK-3Ox/CA (Figure S2 and Table S1.

Cathode catalyst support porosity

N2adsorption isotherms served to investigate the micro and

mesoscale pore structure of the four differently prepared

carbon supports. Table 1 shows the surface area values derived

from a QSDFT (KB) and NLDFT (CMK-3 based carbons) analysis.

Figure 1 depicts the differential surface area vs pore diameter of

the four different carbon supports. The absolute surface area of

the pristine carbon materials (CMK-3 and KB) are within a similar

range, however, alter after the chemical N-modification. In the

mesoporous region from 2–8 nm, the well-ordered pristine

CMK-3 exposes all of its porosity (Table 3 and inset Figure 1),

whereas Ketjenblack (KB) showed significant contributions of

microporosity <2 nm. The NLDFT surface area of CMK-3 some-

what declined after N-modification (Table 3). The latter might

appear counter-intuitive, given the notion that the cyanamide

annealing is expected to etch and oxidize surface carbon bonds

by forming hydrocyanide vapor, creating additional

microporosity.[15] In fact, this etching effect was evident by the

significant increase in the microporous pore size range (<2 nm)

in Figure 1. However, the overall pore structure was nonetheless

collapsing during the N-modification process, evidenced by the

decline in the integral surface contribution in the mesopore

region. The further decreased surface area of CMK-3Ox/CA

compared to CMK-3/CA appeared to be caused by the reduced

pore size fraction in the range above 8 nm.

The inset of Figure 1 shows the cumulative surface area

indicating a shift of the step from micro to mesopores around

2.5 nm with the N-modification and type of carbon. It is evident

from the Figure 1 inset that the unmodified CMK-3 revealed

almost the entire pore structure in a range of 3–6 nm without

any contribution from micropores. The other three samples

revealed a comparable fraction of surface area within this range

Figure 1. Pore area distributions of the carbon support material with

different types of carbons (unmodified and N-functionalized) calculated by

the DFT model (QSDFT for KB and NLDFT for CMK-3 based carbons) and

based on the adsorption branch. The graph shows the derivative of the pore

surface with respect to the pore diameter vs. the pore diameter for the

different carbons. The inset shows the cumulative surface area with respect

to the pore width in a range up to 30 nm. The absolute cumulative surface

area calculated with this QSDFT-model is given in Table 3.

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from 300 to 350 m2/g. However, a closer look at the pore size

distribution clearly shows their striking difference in mesopore

structure. CMK-3 is a cylindrical constructed carbon thus making

the pores accessible via an opening in a similar size to the

cylindrical shaped pore. This pore opening is determined as the

maximum peak in the pore size distribution plot (Figure 1). This

maximum peak of the pore width is clearly shifted by the N-

modification process. Therefore, it seems that pre-treatment

with acid shrinks the pore openings what can be explained by

creation of some carboxylic groups at the graphene sheet

edges. Consequently, those surface attached functional groups

could partially close or block the pore openings even after

conversion into N-functional groups by reacting with

cyanamide.[6a,16] By contrast, a treatment with cyanimide in

absence of a pre-oxidation step etches the pore openings and

broadens their size, as also reported effect in literature.[17] In

both cases, the pore openings are the most vulnerable sites,

because an edge in the carbon matrix causes unsaturated sites,

making them the weakest point. After ionomer coating and

electrode manufacturing, the altered pore structure of the

support did not change the gravimetric stacking volume of the

catalytic layer significantly (Table S2). Since the layer porosity

throughout the electrode is directly proportional to the

gravimetric stacking volume, it did not change, either. As a

result of this, any changes in the MEA performance cannot be

attributed to mass transport issues on a macroscopic level of

the catalytic layer, but rather must arise from processes at the

micro/nano scale.

N-modification cannot only change the pore structure of a

carbon support but can influence the interaction with the

ionomer during the catalyst layer manufacturing process, as

well. In our recent publications, we demonstrated that N-groups

in the carbon lattice are able to strongly interact with the

negatively charged side chains of the ionomer resulting in more

evenly distributed ionomer layer over the catalyst surface.[6b]

Hence, in the present study to gain a better understanding of

the ionomer distribution and the accessibility of the active sites

as a result of the structural changes of the carbon support, we

conducted CO stripping measurements at different relative

humidities.

Correlation of Pt-accessibility and ionomer coverage

The determination of the ECSA via CO stripping at different

relative humidities gives information about the Pt surface

accessibility.[18] After CO adsorption on the Pt sites, an anodic

scan can electrochemically oxidize CO to CO2only in the

presence of oxygenated surface groups such as -OH. Under dry

operating conditions (<40% RH), -OH groups are available for

electrochemical stripping of CO from Pt, if and only if those

sites are located in close proximity to the ionomer. By contrast,

under wet conditions (high relative humidities >70% RH) water

is omnipresent within the catalytic layer and all the Pt sites are

electrochemical accessible for CO stripping. The ratio of two

experimental ECSA values at different humidity yields the so-

called “Pt-accessibility” in the catalyst layer. Figure 2 shows all

four Pt-accessibilities normalized with respect to the absolute

ECSA value of Pt/CMK-3. It is evident that the catalysts

supported on CMK-3 and CMK-3/CA showed enhanced electro-

chemical utilization over the other two. While the Pt accessi-

bility describes the interface between ionomer and Pt surface, it

fails to provide insight into the ionomer coverage over the

entire catalyst (i.e. the ionomer interface of the ionomer with

carbon/Pt). For this reason, we evaluated the double layer

Table 3. Pore analysis of the DFT surface area (QSDFT for KB and NLDFT for CMK-3 based carbons), mesoporous surface area in the range 2–8 nm (Smeso),

and pore opening on the primary particle.

Sample DFTsurface area Surface area2–8nm Pore opening CL sample Gravimetric stacking volume[a] teff

[b]

[m2/g] [m2/g] [nm] [μm/(mgC/cm2)]

KB 736 301 4.98 Pt/KB 24�2 0.22

CMK-3 696 630 5.67 Pt/CMK-3 27�1 0.45

CMK-3/CA 653 307 6.08 Pt/CMK-3/CA 26�1 0.75

CMK-3Ox/CA 613 346 4.17 Pt/CMK-3Ox/CA 28�1 1.27

[a] Gravimetric stacking volume of the catalyst layers calculated from cross section determination for each type of MEA used in the present study.

[b] Effective ionomer film thickness (teff

i) corrected for the ionomer intruded into the interior voids (pores <4 nm) of the primary particle determined by N-

physisorption.

Figure 2. An overview of the normalized Pt-accessibility factor and the

ionomer coverage descriptor with respect to the different types of catalyst

prepared with different types of carbon support. Pt-accessibility factor (left

axis in black) gives the ratio of the ECSA30RH/ECSA100RH normalized to Pt/CMK-

3 Pt accessibility, whereas the ionomer coverage factor describes CDL30RH/

CDL100RH (right axis in red).

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capacitance at different relative humidities. In the regime of

0.45 V under the operation conditions of H2/N2(anode/

cathode), neither of the ORR, OER, HOR or HER current is

contributing to the cyclic voltammogram. Hence, the electron

transfer originates exclusively from double layer and pseudo-

capacitive currents. This double layer is built up at the interface

between carbon/Pt and water/ionomer. Under dry conditions, it

is reasonable to assume all water to be trapped in the

hydrophilic ionomer, implying that the double layer interfaces

only build up between ionomer and carbon or Pt. Thus, the

double layer capacity could be directly correlated with the

ionomer distribution. On the other hand, under wet conditions,

liquid water will be present over the entire catalyst surface due

to water condensation and capillary forces in pores, which will

then render most of the Pt electrochemically active. The ratio of

the double layer from dry to wet conditions than can be used

as an “ionomer coverage” descriptor, also plotted in Figure 2.

Figure 2 reveals a very similar trend and correlation

between the ionomer coverage and Pt accessibility. As

expected, Pt-accessibility is directly linked with ionomer cover-

age, as it is a direct measure of the ionomer distribution and

the electrochemical accessibility under dry conditions. After N-

modification, a moderate increase and a drastic decrease in

ionomer coverage was discernible for Pt/CMK-3/CA and Pt/

CMK-3Ox/CA, respectively. Based on the latter, we can conclude

that we are looking at a set of catalysts with a homogenous

ionomer distribution (i.e. Pt/CMK-3 and Pt/CMK-3/CA) and

another one with a rather inhomogeneous one (Pt/KB and Pt/

CMK-3Ox/CA). Interestingly, the Pt-accessibility factor does not

go down to the same extent as the ionomer coverage. It

appears that Pt/KB displays a lower than expected accessibility

factor compared to Pt/CMK-3, if one would expect the ionomer

coverage to be the only parameter defining Pt-accessibility.

Hence, we can say that the ionomer coverage influences both

the ECSA under dry conditions and the Pt-accessibility. How-

ever, as we will show now, another parameter must be

considered to account for this discrepancy; reveals that Pt-

accessibility is not linearly correlating with the ionomer cover-

age.

Correlation of dry ECSA with the pore structure in the CL

Figure 3 reveals a surprising, yet clear correlation between the

ECSA at 30% RH and the pore opening diameter. This indicates

that the electrochemically active Pt sites are only those that in

close contact with the ionomer, which serves as the only surface

oxygenate-supplying phase under these dry conditions. A

possible explanation for this correlation lies in the effect that

broader pore openings can enable a better penetration of the

ionomer into the pore structure reaching out for the Pt sites.

This is in agreement with the findings of Yarlaggada et al.

describing a enhanced Pt-utilization and better penetration of

the ionomer into the pores for carbon revealing a higher

fraction of pores in the range of 4–7 nm.[4b] In contrast to this,

smaller pores would tend to be either blocked by larger Pt

particles or the ionomer chain would not be able to penetrate

the pore, thus making the Pt particles located inside the pore

inaccessible. Additionally, the increased Pt-particle size of Pt/

CMK-3Ox/CA reduced the overall ECSA compared to the other

catalysts. The relation of the catalyst layer ECSA with oxygen

mass transport resistance and the resulting performance has

been repeatedly emphasized in recent work.[19] A higher

number of catalytic active sites per mass of catalyst or per

geometric area of catalyst layer limits the local oxygen flux to

the Pt surface under dominating mass transport conditions

present at high current densities.

Correlation of MEA performance and ECSA values

The ECSA value under dry (e.g. 30% RH) operating conditions

depends on the ionomer coverage and Pt accessibility, where it

controls the high current cell performance, as demonstrated in

Figure 4a. The dry ECSA values are a measure of the number of

accessibility active Pt surface sites and evidently correlate with

the carbon porous structure. The choice of 1.0 A/cm2in

Figure 4a aids to facilitate direct correlation with other graphs

in this contribution. However, the impact of the dry ECSA value

becomes more and more pronounced at higher current

densities. As an example, the cell voltage of Pt/CMK-3Ox/CA

drops below that of Pt/KB at 1.2 A/cm2(cf Figure 8). By contrast,

under wet operation conditions (Figure 4b), there is no

significant correlation between cell performance and (wet)

ECSA. This is because liquid water compensates as proton

conducting medium for site disconnecting from the ionomer.

Consequently, all Pt sites will be electrochemically accessible

and the MEA performance no longer scales with ECSA / Pt-

accessibility. However, water accumulation and local flooding

effects form the origin of the observed mass transport losses for

the case of Pt/CMK-3 Ox/CA, due to inhomogeneous ionomer

Figure 3. Correlation of supports’ pore opening with relative ECSA values of

their MEAs, comprising cathodic CLs with various carbon supports, at. 30%

RH. The ECSA values for all the catalysts were normalized to the ECSA value

of Pt/CMK-3, as it exhibited the highest performance and highest ECSA. The

pore openings were extracted from N2physisorption pores area distribution

plot as given in Figure 1.

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distribution (see Figure 2) combined with an unfavorable pore

structure of the catalyst layer.

Catalyst layer (CL) porosity

To characterize the impact of material components on the

catalyst layer properties, and to account for the observations

and conclusions of Figure 4, the pore size distribution of the

cathode catalyst/ionomer layer was investigated by means of N2

adsorption where a QSDFT kernel using cylindrical pore models

was used for analysis. Figure 5 displays the resulting differential

porous surface area distribution, dS(d) [m2/nm/g] against the

pore size (width). Here, the surface area that is directly

represented by the area under the curve.[10a] Figure 5 reveals

the origin of the poor wet MEA performance of the “CMK-3Ox/

CA” support in Figure 4: The high fraction of micropores <2 nm

(red curve in Figure 5) in the CL benefits water accumulation

due to capillary forces. The accumulated water is speculated, to

cause local flooding resulting in increased mass transport

resistance. The flooding of the Pt/CMK-3Ox/CA CL is further

compounded by an unfavorable ionomer coverage (Figure 2).

The reference CL, Pt/KB, showed a similar ratio between micro-

and mesopores across the layer. By contrast, the well perform-

ing CLs, Pt/CMK-3/CA and Pt/CMK-3, were found to have a

more homogenous ionomer distribution. The latter was based

on a more prevalent mesopore ratio near the 5 nm pore size

width, in combination to the minor contributions from micro-

pores (<2 nm pore width).

Oxygen transport resistance

The ECSA values of our four CLs control the observed mass

transport effects and layer resistances.[19] As demonstrated

above, the ECSA values are a strong function of the support

pore structure and the individual ionomer distribution. Limiting

current measurements yield quantitative insight in oxygen

transport resistances within the CL that affect the MEA perform-

ance. Data from limiting current measurements allow for

deconvolution of oxygen transport losses originating from the

CL and GDL, into pressure-dependent (PD) and pressure-

independent (PI) contributions. RPI resistances (originating

mainly in the CL) can be further split into i) a so-called “local”

oxygen diffusion resistance at the ionomer-covered Pt interface,

and ii) into a Knudsen pore diffusion resistance. Harada et al.

provided the first structural evidence of the local transport

resistance. They showed that ionomer films on top of a

Figure 4. Correlation of performance of 5 cm2MEAs, comprising cathodic CLs with various carbon supports, with their relative ECSA values at each respective

relative humidity: a. 30% RH and b. 100% RH. For each RH, the ECSA values for all the catalysts were normalized to the ECSA value of Pt/CMK-3, as the latter

exhibited the highest performance and ECSA values. Cell voltage determined at (a) 1.0 A/cm2at 90°C and 30% RH, while (b) at 2.0 A/cm2at 60°C, 90% RH.

Backpressure of 170 kPaabs were applied for both anode and cathode compartments. Error bars represent the mean absolute deviation from two independent

measurements.

Figure 5. Pore area distributions of the catalyst material with different types

of carbon supports (unmodified and N-functionalized) calculated by the

QSDFT model and based on the adsorption branch. The graph shows the

derivative of the pore surface with respect to the pore diameter vs. the pore

diameter for the different catalyst layers (catalyst+ionomer).

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platinum nanoparticles revealed higher density causing addi-

tional transport resistance for molecular oxygen.[20] The Knudsen

resistance, on the other hand, describes the oxygen transport

through a porous material when a single molecule is rather

restricted in movement by the pore walls instead of interaction

with other molecules. This factor will be influenced by the

ionomer coverages that determines the effective pore structure

within the catalytic layer.[6b]

We conducted limiting current measurements at 30% and

100% RH to mimic dry and wet operating conditions,

respectively. In doing so, we followed Baker et al.’s approach to

estimate the PI oxygen mass transport resistance, RPI, from

limiting current measurements by varying the partial pressure

of oxygen in the cathode feed.[13,21] Figure 6a shows that the

MEA performance under dry conditions at 1.0 A/cm2correlates

very well with estimates of RPI. Interestingly, even though the

KB-based CL had the thinnest ionomer film layer (Table 3), its RPI

is comparably high (Figure 6a). This can be attributed to the

reduced pore opening diameter of KB, offsetting the beneficial

effect of the thin ionomer layer. Surprisingly, the Pt/CMK-3Ox/

CA CL displayed a similar RPI as the Pt/CMK/CA despite its

unfavorable pore opening and ionomer distribution character-

istics (cf Figures 2,3). This can be rationalized based on the

influence of the proton transport under dry operating con-

ditions on the effective pressure independent resistance term,

RPI.[22] Hence, the relatively low RPI value of Pt/CMK-3Ox/CA is

also attributed to its favorably low proton transport resistance

under low RH, which will be discussed further below. Finally,

the CLs Pt/CMK-3 and Pt/CMK-3/CA featuring broader and

better accessible pores in combination with favorable ionomer

distribution and high ECSA values show the lowest RPI values.

Proton conductivity in cathode CL

Good cell performance requires sufficient supply of protons,

especially under dry operating conditions. Proton-conduction

rates across the catalyst layer are controlled by the specific

proton resistivity, 1, extracted from impedance data using the

transmission line equivalent circuit model (Figure S5). Catalytic

layers with varying ionomer distributions and tortuosities can

provide comparable proton resistivity values on a macroscopic

level.[10a] Figure 7 reports the proton transport resistivities of our

four CLs determined from EIS, where the experimental data

Figure 6. (a) Correlation of performance of 5 cm2MEAs, comprising cathodic CLs with various carbon supports, with their pressure independent O2mass

transport term RPI at. 30% RH. Cell voltage determined at 1.0 A/cm2at 90°C and 30% RH with a backpressure of 170 kPaabs were applied for both anode and

cathode compartments (b) Comparison of O2mass transport resistance pressure independent term (RPI). The measurements were conducted at 80°C and at

100% RH (solid bar) or 30 % RH (dashed bar). Absolute pressures were set at 170 kPaabs on both anode and cathode sides. The error bars correspond to the

standard deviation between independent measurements with two different MEAs for each type of catalyst layer.

Figure 7. Effect of relative humidity on the proton resistivity of the cathode

CL comprising different types of catalyst. The proton resistivity was

determined from impedance spectroscopy in H2/N2via fitting with a

transmission line model. Conditions for the impedance are: cell temperature

of 80°C, as well as 100% RH (solid bar) or 30% RH (dashed bar). Error bars

represent the mean absolute deviation from two independent measure-

ments.

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were fitted with a transmission line model equivalent circuit as

illustrated in Figure S6 and S7. The CL comprising Pt/CMK-3Ox/

CA revealed the lowest proton resistivity, even though the

ionomer of this CL is less homogenously distributed compared

to Pt/CMK-3 and Pt/CMK-3/CA. A possible explanation lies in

the “effective” ionomer film thickness. A certain amount of the

ionomer will intrude into internal pore voids of the primary

carbon particle and therefore will not take part in the ionomer

film covering the external catalyst surface. Liu et al. found a

correlation between the intruded ionomer fraction and the

pore volume of pores <4 nm. We note that an intrusion of the

ionomer into small micropores of <1 nm is physically

unlikely.[11] Thus, the larger the ionomer ratio that intrudes, the

thinner the remaining effective ionomer film that covers the

catalyst. As a consequence, the proton resistivity (in Ω·cm) of

the effective ionomer film increases. Among the four CLs under

investigation under dry operating conditions the CL Pt/CMK-3/

Ox revealed the lowest proton transport resistivity at an

effective film thickness of around 1.27 nm. Increased resistivities

were obtained for Pt/KB and Pt/CMK-3 CLs with an effective

ionomer film of thickness <0.5 nm (see Table 3).

Under wet operating conditions, liquid water will compen-

sate the reduced effective ionomer film thickness. Now, proton

resistivity is no longer determined by the film thickness alone.

As a result of this, under dry operating conditions, the pressure

independent mass transport resistance, RPI, is affected by the

additional proton transport resistance and molecular oxygen

mass transport resistance, as shown in Figure 6.

As expected the proton resistivity is lower under 100% RH

compared to 30% RH. The experimental 1-values obtained from

the of the Pt/KB based CL utilizing low EW type ionomer, and

its I/C ratio, is in excellent agreement with literature data under

comparable conditions.[4a] We note that the accuracy of the

transmission line model under high humidity suffers from a

relatively large error. Taking this in consideration, the observed

differences in proton resistivity among the four CLs can be

considered insignificant. In addition, these values cannot

account for the observed changes in performance under wet

operating conditions (cf Figure 4). Therefore, under such high

relative humidity operating conditions, structural or other

physico-chemical properties of the CL may explain the ab- or

presence of electrode flooding.

Effect of pore structure on PEMFC performance

Figure 8 shows the corresponding polarization curves of MEAs

comprising the differently modified catalytic layers. The polar-

ization curves were obtained using 5 cm2MEAs under two

extreme operating conditions, at 60°C and 90% RH, herein

defined as “wet operating conditions”; and at 90°C and 30%

RH, herein defined as “dry operating conditions. All tested

MEAs had an average cathode catalyst loading of 0.129�

Figure 8. MEA performance, comprising of different cathodic catalyst layers, under cell temperature of 60°C with 90% RH (a) and under cell temperature of

90°C with 30% RH (b) with a backpressure of 170 kPaabs,out for both anode and cathode compartments. Polarization curves were measured under constant

differential flow (H2: 1.0 Nl/min and air: 2.0 Nl/min). The voltage was corrected for the monopolar plate resistance (0.0345 Ωcm2). Error bars represent the

mean absolute deviation from two independent measurements.

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0.013 mgPt/cm2and an ionomer to carbon mass ratio (I/C) of

0.65. As also discussed in earlier parts of the manuscript

(Figure 4), a clear flooding tendency of Pt/CMK-3Ox/CA

catalyst layer gets discernible under wet operating conditions

(Figure 7a), which can be mainly attributed to the bad ionomer

coverage leading to blockage of certain pores within the

catalytic layer. Despite the Pt/KB electrode having a slightly

lower Pt loading compared to the other catalyst layers, it

exhibits the highest performance under wet operating con-

ditions. Hence, any observed difference in performance cannot

be rationalized by an alteration in Pt loading of the MEA or by

changes in ESCA (which is already normalized to the Pt

loading). On the other hand, the presence of high fractions of

micropore volume after ionomer coating(Figure 5) benefits

water accumulation, as a result of capillary forces, which

would then result in an increase mass transport resistance at

high current densities. The ionomer and pore size distribution

throughout the catalytic layer of the other three catalysts

prevent a significant cell flooding thus their performance does

not alter significantly from each other.

Under dry operating conditions, the performance of the

MEA strongly depends on the ionomer distribution and ECSA

since there is no excess of water to compensate for any

discontinuity in the ionomer film over the catalysts’ surface.

Figure 7b clearly shows the improved performance of CLs based

on CMK-3 or CMK-3/CA when used as supporting material. The

beneficial pore structure and pronounced porosity in the range

of 3–6 nm enables a good Pt accessibility and combined with a

tailored N-modification a homogenous ionomer distribution, as

opposed to a conventional KB carbon support. This in turn

points out the importance of the structural and physico-

chemical property of the support material itself. A well-defined

pore structure revealing pore openings in the range of 3–6 nm

improve the Pt accessibility due to enhanced ionomer pene-

tration without blocking essential pore structure for oxygen

mass transport. The increased ECSA alongside with reduced

mass transport resistance, optimizes the transport phenomena

within the catalytic layer. Pt/CMK-3 combines those benefits

together with a sufficient protonic conductive pathway on a

macroscopically scale. Generally speaking, the performance

goes up with the pore openings (Figure 3) of the supporting

material. This clearly shows the importance of the supports'

mesoporous structure for both the activity of the catalyst and

also its performance as well.

Conclusion

This experimental study highlights the important role of

controlled mesoporosity of a high performing PEMFC cathode

catalyst support material. A well-defined CMK-3 support was

chemically treated and its pore structure tuned. A detailed

investigation of such structural modified support materials was

conducted under two extreme MEA operating conditions: wet

(at 60°C, 90% RH) and dry (at 90°C, 30% RH). The combination

of these two working conditions as well as a detailed analysis of

the support material and the catalytic layer via N2-physisorption

enables us to explain the improved performance of Pt/CMK-3.

A well-defined pore structure in the range of 3–6 nm

enables a deeper penetration of the ionomer into and over the

surface of the Pt particles. Following this, all active sites tend to

be in close proximity of the ionomer, consequently increasing

ECSA values even under dry operating conditions. Additionally,

mesoporous support material combined with homogenous

ionomer distribution construct a catalytic layer with favored

pore size distribution for oxygen and water transport. Thus,

preventing water accumulation and at the same time reducing

oxygen mass transport resistance towards the active sites.

In the present study, we have demonstrated the importance

of a well-defined mesoporous support structure. Recommenda-

tions for preferable porosity characteristics are provided in the

range of 3–6 nm in absence of microporosity for not only the

increase in catalyst activity, but also towards its overall perform-

ance. Future studies in developing new catalyst materials for

fuel cell applications shall consider the role the support material

in the same extent as the active material itself, including their

interplay, which could crucially influence catalyst layer perform-

ance.

Acknowledgement

This work was supported by the BMW Group. The authors would

also like to thank the members of FC Test Field, FC Technology

Development and Technology Material Analysis of BMW Group for

their support during Fuel cell testing, MEA manufacturing and

decal preparation. Open Access funding enabled and organized by

Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: oxygen reduction reaction ·fuel cell ·mass

transport ·porosity ·catalyst layer

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Manuscript received: August 3, 2021

Revised manuscript received: September 8, 2021

Accepted manuscript online: September 14, 2021

Version of record online: September 27, 2021

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