Document [original]

Meta-kinks are key to binder performance of

poly(arylene piperidinium) ionomers for alkaline

membrane water electrolysis using non-noble

metal catalysts†

Richard Weber,

Malte Klingenhof,

Susanne Koch,

Lukas Metzler,

Thomas Merzdorf,

Jochen Meier-Haack,

Peter Strasser,

Severin Vierrath

and Michael Sommer *

Anion-exchange membrane water electrolysis (AEMWE) is a key technology for the production of green

hydrogen at high current densities without the necessity of noble metal catalysts. AEMWE technology

does not only rely on chemically stable and highly hydroxide-conducting membranes, but also on

ionomer binders, to which additional criteria apply related to swelling, mechanical properties, gas

permeability and porosity to form a triple phase boundary with catalyst particles on top of an membrane

electrode assembly (MEA). Here, we investigate seven poly(arylene piperidinium)s (PAPs) with diﬀerent

ratios of meta-/para-terphenyl building blocks as binders for non-noble NiFe-LDH catalysts. We ﬁrst

analyze the materials comprehensively in pristine form and subsequently as binders. With increasing

content of meta-terphenyl, speciﬁc surface area, water uptake, swelling ratio and ion-conductivity

increase continuously, with the latter ranging from 145 to 216 mS cm

−1

at 80 °C. We elucidate binder

performance from rotating disk electrode experiments of oxygen evolution reactions (OER) catalysed by

nickel–iron layered double hydroxides (NiFe-LDH) under AEMWE working potentials. Here, an increasing

content of meta-kinks leads to improved catalyst utilization, superior OER performance and improved

electrode stability. Finally, AEMWE single cell tests show a strong improvement in current density when

altering binders from exclusively para-tometa-terphenyl in the polymer backbone. Current densities as

high as 1000 to 1700 mA cm

−2

at 1.8 V and 3000 mA cm

−2

at 2.0 V are measured for the binder with

exclusive meta-terphenyl kinks. The results highlight the role of the binder for AEMWE performance as

well as the importance of its individual optimization aside from membrane properties.

Introduction

Green hydrogen production using water electrolysis is a key

technology for future industrial decarbonization as well as

establishing an economy with climate-friendly energy storage

and supply.

1,2

Since conventional alkaline water electrolysers

are limited in the achievable current density, the zero-gap

conguration in polymer electrolyte water electrolysis is

highly favourable for eﬀective hydrogen production.

Although

proton-exchange membrane water electrolysis is widely studied

and meanwhile established, the inevitable use of expensive

precious metal electrocatalysts like platinum or iridium makes

this technique less cost-eﬀective. In contrast, anion-exchange

membrane water electrolysers (AEMWE) combine the advan-

tages of alkaline water electrolysis and proton-exchange

membrane water electrolysis, as abundant non-precious metal

catalysts can be used and operation at high current density can

be achieved.

3,4

The central component of AEMWE is the membrane elec-

trode assembly (MEA). The two main MEA designs are catalyst

coated substrates (CCS) and catalyst coated membranes (CCM).

For CCS, the porous transport layers are covered with a catalyst

or activated by growing catalysts on their surfaces.

6,7

A CCM for

AEMWE consists of an anion-exchange membrane (AEM)

Institute for Chemistry, Chemnitz University of Technology, Straße der Nationen 62,

09111 Chemnitz, Germany. E-mail: michael.somm[email protected]

Department of Chemistry, Technical University Berlin, Straße des 17. Juni 124, 10623

Berlin, Germany. E-mail: pstrasser@tu-berlin.de

Electrochemical Energy Systems, IMTEK-Department of Microsystems Engineering,

University of Freiburg, Georges-Koehler-Allee 103, 79110 Freiburg, Germany. E-mail:

[email protected]i-freiburg.de

Hahn-Schickard, Georges-Koehler-Allee 103, 79110 Freiburg, Germany

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden,

Germany

Research Center for Materials, Architectures and Integration of Nanomembranes

(MAIN), Chemnitz University of Technology, Straße der Nationen 62, 09126

Chemnitz, Germany

†Electronic supplementary information (ESI) available. See DOI:

https://doi.org/10.1039/d3ta06916h

Cite this: J. Mater. Chem. A,2024,12,

7826

Received 10th November 2023

Accepted 15th February 2024

DOI: 10.1039/d3ta06916h

rsc.li/materials-a

7826 |J. Mater. Chem. A,2024,12,7826–7836 This journal is © The Royal Society of Chemistry 2024

Journal of

Materials Chemistry A

PAPER

coated on both sides with a mixture of catalysts for anodic and

cathodic reactions and ion-conducting binder polymer. The

membrane separates the half cells, conducts hydroxide ions

and catalyses water electrolysis. As the electrochemical reac-

tions take place at the surface of the catalyst particles, an

optimal contact between membrane, binder, catalyst and

conductive carbon is required.

8–10

The CCM approach typically

leads to lower ohmic resistance compared to the CCS approach

and consequently to higher cell performances.

To achieve an

optimal morphology with a so-called triple-phase-boundary

involving the solid catalyst, the polymeric hydroxide

conductor and the gaseous products, the properties of the

binder ionomer are considered key for long-term stability of the

MEA.

8,11,12

Mechanical properties, interaction with catalyst,

ionic conductivity, water uptake and swelling all play a role.

8,12,13

In addition, gas permeability is important as insuﬃcient gas

removal limits mass transport and nally performance of the

electrolyser.

10,12,13

In addition to these aspects, binder properties

need to be adjusted to allow processing dispersions of catalyst

particles in the mixtures used for membrane coating or catalyst

layer fabrication.

8,11–15

The latter is crucial for homogeneous

distribution of catalyst particles, maximised electrochemically

active surface and eﬀective transport of reactants and

products.

11,13

Progress in AEM materials is impressive,

16–18

yet studies that

explicitly address the role of the binder and its optimization are

much less prevalent.

15,19,20

Diﬀerent binder ionomers based on

polyethylene,

21,22

polystyrene,

19,23

polysulfones,

24–27

poly-

phenylene oxides,

28,29

polybenzimidazoles

30,31

or poly(arylene

alkylene)s

15,20,32

have been used. Beside these, also Naon

33,34

and polytetrauoroethylene

34–36

have been reported.

One prominent class of polymers for AEM applications are

rigid, aromatic polymers prepared via superacid-catalysed poly-

hydroxyalkylation. The synthetic simplicity, commercial avail-

ability of arylene monomers and chemical stability make poly-

(arylene alkylene)s highly attractive. As a result, a variety of poly-

(arylene alkylene)s has been prepared, including poly-(arylene

alkylene)s,

37–40

poly(arylene isatine)s,

41–43

poly(xanthene)s

44,45

poly(arylene piperidinium)s.

40,46–49

Among them, poly(arylene

piperidinium)s are most comprehensively studied as main-chain

anion-exchange polymers. The starting materials involve 4-piper-

idone derivatives and diverse arylenes, most oen meta-

40,50–53

para-terphenyl.

32,40,46,48

Additionally, the inuence of diﬀerently

congurated backbone structures of poly(arylene alkylene)s and

poly(arylene piperidinium)s on properties has been studied using

meta-orpara-terphenyl as arene building blocks. It was found that

the incorporation of exclusively meta-terphenyl and the resulting

kinked backbone structure promotes self-assembly and aggrega-

tion of cationic units, leading to improved hydration and increased

conductivities compared to para-terphenyl analogs.

Partial

substitution of para-terphenyl units in poly(p-terphenyl piper-

idinium)s with rigid dimethyluorene has been further suggested

to increase the free volume leading to increased hydration

numbers and hydroxide conductivities.

Despite this evidence of backbone constitution being

a primary factor for packing density and porosity, the afore-

mentioned works have focused on improving AEM properties.

Detailed studies on the optimization of binder properties are

lacking, despite accumulating evidence that this component of

the MEA is central to performance and durability of

AEMWE.

11,13,18

Binder ionomers must meet criteria of high

dimensional and alkaline stability under aqueous and caustic

conditions in order to gain long term stability of the MEA and

the electrolysis cell. Detachment of catalyst particles due to

chemical degradation or mechanical failure of binder materials

is one main cause of AEMWE performance loss.

With respect

to chemical stability under alkaline conditions, ether-linkage

free backbone structures

17,55–60

functionalized with piper-

idiniums as anion-bearing moiety are benecial. Wang et al.

reported for a poly(arylene piperidinium) only 3% ionic loss

aer 2000 h immersion in 1 M KOH solution at 100 °C.

A similar polymer structure investigated by Olsson et al.

exhibited merely 5% degradation aer 360 h in 2 M KOH

solution at 90 °C.

In addition, taking an AEMWE study by

Lindquist et al. into account the commercial poly(arylene

piperidinium) PiperION as binder and membrane material

outperformed polybenzimidazole and polystyrene based

ionomers.

Herein we present a comprehensive study of the inuence of

backbone constitution of poly(terphenylene piperidinium)s

with varying meta-terphenyl content on pristine as well as

binder properties. Seven diﬀerent statistical quaterpolymers are

prepared with varying meta-/para- ratio, and their mechanical

properties, swelling and ionic conductivity are investigated.

Specic surface area, water uptake and swelling and ionic

conductivity continuously increase with increasing meta-kink

content. The prepared ionomers are excellent binders for

nickel–iron layered double hydroxides (NiFe-LDH) catalysts, as

probed by rotating disk electrode (RDE) experiments. Here, an

increasing meta-kink content improves catalyst utilization and

electrode stability. In AEMWE single cell tests an outstanding

current density of 3000 mA cm

−2

at 2 V is achieved with NiFe-

LDH as catalyst and the binder with exclusive meta-terphenyl

kinks.

Experimental

Details on polymer synthesis, materials and all other methods

and experimental procedures are found in the ESI.†

Rotating disk electrode (RDE) experiments

For electrocatalytic activity testing with rotating disk electrode

(RDE), a NiFe-LDH suspension was prepared from 4 mg NiFe-

LDH, 768 mL de-ionised water, 200 mLi-propanol and 32 mL

5 wt% binder solution. 10 mL of the resulting dispersion was

pipetted on the glassy carbon electrode. Electrocatalytic

performance was evaluated in N

-saturated 0.1 M KOH at

1600 rpm using a Biologic SP-200 potentiostat operating in

a three-electrode setup with a platinum counter electrode (CE)

and a reversible hydrogen (RHE) reference electrode, see also

Tables S1, S2 and Fig. S1.†Reported CVs for evaluation of the

electrochemical activity were iR- and capacity-corrected. The

reported overpotentials necessary to reach a current density of

This journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. A,2024,12,7826–7836 | 7827

Paper Journal of Materials Chemistry A

10 mA cm

−2

(OER) are the average of three measurements.

Stability measurements of the multi-component system were

conducted with the same setup. Here, cycling experiments with

2000 CVs between 1.23 V vs. RHE and 1.63 V vs. RHE were

conducted to periodically evaluate the OER performance.

Preparation of anode inks for bar coating

NiFe-LDH (400 mg) was dispersed in a mixture of i-propanol

(IPA) and water (2.0 g, 1 : 1). Aer addition of the previously

prepared ionomer solution (0.6 g, 5 wt% PAP in DMSO), ZrO

grinding balls (Retsch 22.455.0009) were added. Aer 2 days on

a roll mixer (IKA, Roller 10) the inks were used for electrode

fabrication via bar coating. In some cases, additional solvents

(e.g. 200 mL IPA : water) had to be added on the day of cell

fabrication to make the ink processable.

Preparation of cathode inks for bar coating

Pt/C 50 wt% (300 mg, Umicore Elyst Pt50 0550) was dispersed in

water (1.8 g). Aer adding the previously prepared ionomer

solution (1.8 g, 2.5 wt% Aemion

-HNN8 in MeOH/H

O 10/1),

ZrO

grinding balls were added. Aer 2 days on a roll mixer

the inks were used for electrode fabrication via direct bar

coating.

Preparation of catalyst-coated membranes (CCMs)

A membrane (Aemion

, AP2-HNN8-50-X) was attached to a clean

glass surface and the anode ink was carefully distributed with

a bar coater (Thierry, PG-032-150200). Aer drying, the half-

CCM was covered with an adhesive foil, while a PTFE foil pro-

tected the electrode layer. The backing foil was removed and the

half-CCM was attached to the glass surface with the uncoated

side facing up. The cathode ink was evenly distributed on the

membrane with the help of the bar coater and dried over night.

The catalyst loadings were measured by X-ray uorescence

microscope (mXRF, Bruker, M4 Tornado): the combined anode-

loading of Ni and Fe for the M0 cell was 1.05 mg cm

−2

and

0.97 mg cm

−2

for the M100 cell. The average Pt-loading on the

cathodes was 0.54 mg cm

−2

for the M0 cell and 0.45 mg cm

−2

for the M100 cell.

AEMWE single cell measurements

Single cell measurements were performed according to previ-

ously published work by Koch & Metzler et al. unless otherwise

noted.

Briey, MEAs were immersed in 3 M KOH for 24 h and

subsequently immersed in 1 M KOH for another 24 h to perform

ion-exchange to the hydroxide form. Nickel felt (200 mm,

Bekaert) and carbon paper (H24C5, Freudenberg) were used as

transport layers on the anode and cathode side, respectively.

Using a custom-built cell xture and AEMWE test bench, the

MEAs were measured at 60 °C in 1 M KOH.

The electrolyte ow

rate was set to 40 ml min

−1

using a peristaltic pump and the

electrolyte was pre-heated to 67 °C using a bath thermostat. A

BioLogic VSP-300 potentiostat with two 10 A/5 V boosters was

employed to measure polarization curves via electrochemical

impedance spectroscopy to extract the high-frequency resis-

tance (HFR).

Results and discussion

Polymer synthesis and characterization

The investigated binder polymers were synthesized by

superacid-catalysed Friedel–Cras-type polyhydroxyalkylation

reaction and subsequent quaternization with methyl iodide

according to literature.

48,63

The ratio of the electron-rich arene

monomers meta-terphenyl (m-TP) and para-terphenyl (p-TP) was

varied. The ratio of the ketone monomers 2,2,2-tri-

uoroacetophenone (TFAp) and 4-piperidone monohydrate

hydrochloride (Pip) was set at 15 to 85 for each polymerization

(Scheme 1) as previously reported.

Poly(arylene piperidinium)

s are denoted as Mx, where xis the percentage of m-TP in the

feed (Table 1). The commercially available material PiperION by

Versogen most closely resembles the M0 polymer synthesized

herein. The successful incorporation of the monomers was

conrmed by

H NMR spectroscopy (Fig. S2†). Due to the

overlapping of the proton signals in the aromatic region,

quantitatively distinguishing between m-TP and p-TP was not

possible, however a qualitative evaluation of the signals in the

aromatic region hints at the successful introduction of the

diﬀerent arenes according to their feed ratio (Fig. S3†). Molar

masses and molar mass distributions were evaluated by size

exclusion chromatography (SEC) in N,N-dimethylformamide

(DMF) as eluent (Table 1 and Fig. S4†). In general, high

molecular weight polymers with weight average molecular

weights M

> 50 kDa were obtained suitable for preparing

mechanically stable lms for further characterization.

Water uptake and swelling ratio

In AEMWEs the ionomeric binder is responsible for the trans-

port of reactants (hydroxide ions and water) to the catalyst, as

well as product gases away from the catalyst. Thus, high water

diﬀusivity and gas permeability are required. As a result, a more

porous binder material could improve the kinetics of mass

transport at the reaction sites and thereby enhance the overall

cell performance. In addition, in water electrolysers without

supporting electrolyte suﬃciently high hydroxide conductivities

are crucial, too. An optimal water uptake (WU) facilitates ion-

transport in the polymeric material, whereas excessive WU

leads to excessive swelling and loss of mechanical strength,

possibly followed by delamination of the MEA and leaching of

catalyst particles.

WU and swelling ratio (SR) were determined

as a function of temperature and m-TP/p-TP ratio (Fig. 1). As the

ketone feed ratio was kept constant for all polymers, the ion-

exchange capacity (IEC) was considered to be well comparable

among the diﬀerent materials (see Table 1). However, because

of slight diﬀerences between the IEC values, the hydration

number (l) of each polymer was calculated and chosen for

comparison. As expected, land SR increased with increasing

temperature for both the hydroxide and the chloride forms

(Fig. 1a–d and S5†). Both forms were investigated to provide

a comprehensive overview and enable comparison to the

7828 |J. Mater. Chem. A,2024,12,7826–7836 This journal is © The Royal Society of Chemistry 2024

Journal of Materials Chemistry A Paper

literature where oen one form is investigated only. When

comparing the polymers with each other there is a clear trend

that the gradual introduction of meta-kinks in the polymer

backbones causes a gradual increase in land SR (Fig. 1 and

S5†). For example, M0 has a hydration number of 14.4 at 80 °C

in hydroxide and 9.3 at 90 °C in chloride form whereas the

values for M100 are 32.2 and 11.6, respectively (Fig. 1c and

S5b†). We explain this trend by a decreased backbone rigidity

for increasing meta-kinks in the polymer main-chain, leading to

increasing water uptake. The slopes of the regression lines of

Fig. 1d become steeper with increasing temperature. This is

interpreted as a higher temperature dependency of the water

uptake the more meta-kinks are introduced in the polymer

main-chain. The same trend is also seen in Fig. 1c where M100

shows a large increase in lfrom 60 to 80 °C not observed for the

other polymers, which arises from the more kinked and exible

backbone of M100.

The swelling of the materials was determined by measuring

the dimensional changes aer immersion in water and equili-

bration at several temperatures (Fig. 1a, b and S5a†). First,

swelling of the chloride forms of all polymers is signicantly

lower than of the hydroxide forms, in agreement with the trends

of the corresponding hydration numbers. Second, with

increasing m-TP content, the dimensional swelling increased as

well. For example, M0 shows a through-plane SR of 36% and an

in-plane (areal) SR of 33% at 80 °C, whereas M100 gives values

of 52% and 42%, respectively. This observation is again in

agreement with the observed trends of hydration numbers and

arises from the more kinked backbone when meta-terphenyl

units are introduced. We hypothesize that such behaviour

Scheme 1 Syntheses of ionomer binders with controlled meta-kinks via polyhydroxyalkylation (TFSA: triﬂuoromethanesulfonic acid, DIPEA:

N,N-diisopropylethylamine, NMP: N-methylpyrrolidone, DMSO: dimethylsulfoxide).

Table 1 Molar masses, dispersity and ion exchange capacity (IEC) values of the investigated ionomers

Polymer

m-TP/p-TP

feed ratio 

(kg mol

−1

)

(kg mol

−1

)Đ

IEC(Cl

−

)

(meq. g

−1

)

IEC(OH

−

)

(meq. g

−1

)

BET surface

area (m

−1

)

M100 100/0 74.4 191.1 2.6 2.17 2.26 31.8

M85 85/15 19.6 51.5 2.6 2.16 2.25

M65 65/35 25.7 67.3 2.6 2.15 2.25

M50 50/50 25.6 71.8 2.8 2.14 2.23 24.0

M35 35/65 34.8 70.5 2.0 2.13 2.22

M15 15/85 29.1 74.3 2.6 2.18 2.28

M0 0/100 27.8 65.2 2.3 2.19 2.28 15.5

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Paper Journal of Materials Chemistry A

correlates with changes in free volume, and thus nano/micro-

porosity.

To conrm this hypothesis, specic surface areas

were determined using the Brunauer Emmett Teller (BET)

method (Table 1, for adsorption isotherms see Fig. S6†). The

comparison of M100 and M0 with exclusive meta-terphenyl and

para-terphenyl as the aryl monomer, respectively, clearly

conrms a double specic surface area of the polymer M100

with only meta-kinked arylene units in the backbone. Such

enhanced porosity is expected to facilitate gas permeability of

products formed during the oxygen evolution reaction. Addi-

tionally, it can be derived from Table 1 that the introduction of

meta-terphenyl in the polymer structures leads to a signicantly

high increase of the BET surface area. This could be explained

by the lower probability for dense packing in the presence of

a more kinked backbone structure. Finally, we note that in-

plane and through-plane swelling of the hydroxide forms are

of the same order of magnitude hinting at structurally isotropic

membranes.

Ionic conductivity

The in-plane conductivities under immersed conditions of the

prepared ionomers in chloride (Fig. S7†) and hydroxide forms

(Fig. 2) where determined using a four-probe measurement cell

in deionized water at diﬀerent temperatures.

In addition,

chloride-conductivities were determined in-plane at diﬀerent

temperatures and under a relative humidity of 95%, using

a four-point probe geometry. Trends comparable to those under

immersed conditions were obtained (Fig. S8†). Fig. S7a†shows

the chloride conductivities and their temperature-dependent

behaviour. The conductivity increases with increasing temper-

ature due to the higher mobility of chloride ions. All polymers

feature an Arrhenius-like behaviour (Fig. S7b†), with activation

energies ranging from 23.1 to 26.2 kJ mol

−1

. Furthermore,

similar to changes in WU and SR with increasing m-TP units,

also the conductivity increases from 84.5 to 110.8 mS cm

−1

for

M0 to M100, respectively. We ascribe the higher ion

Fig. 1 Swelling ratios (a and b) and hydration numbers (c and d) of the hydroxide forms of Mx.

7830 |J. Mater. Chem. A,2024,12,7826–7836 This journal is © The Royal Society of Chemistry 2024

Journal of Materials Chemistry A Paper

conductivity of more kinked ionomers to the higher water

content (l) and swelling leading to increased ion mobility.

In Fig. S7c†chloride conductivities at variable temperatures

depending on the related hydration numbers of selected poly-

mers are shown. In Fig. S7d,†the slopes of Fig. S7c†are shown

which decrease for increasing m-TP units. These two plots

suggest that polymers with more meta-kinks in the main-chain

are less eﬃcient ion-conductors regardless of their individual

IEC and overall measured conductivity value, meaning that

lower ion conductivities are obtained for a certain hydration

state.

40,49,66

Conductivities of the hydroxide forms are presented in

Fig. 2.

65,67

Values range between 144.6 to 216.4 mS cm

−1

at 80 °C

which is in accordance with literature.

Conductivities corre-

late with the temperature- and meta-kink-dependent trends of

the WU (Fig. 2a). A higher ion-mobility compared to the chlo-

ride form is indicated by the lower activation energies between

14.1 and 15.7 kJ mol

−1

(Fig. 2b) which are in the range of

previously reported poly(arylene piperidinium)s.

50,52

As for the

chloride forms, the more meta-kinks present in the polymer

backbone, the higher the hydration of the ionomers. Never-

theless, as observed for the chloride forms, the ion conductivity

depending on a certain hydration state decreases with

increasing meta-kinks (Fig. 2c). Fig. 2d shows the slopes of

Fig. 2c which are decreasing with increasing fraction of meta-

terphenyl, hinting at a less eﬃcient ionic conductivity with

increasing content of meta-kinks. Thus, the trends of hydroxide

and chloride conductivities match well and indicate that, when

focussing on high values for ion conductivity without increasing

the IEC, the introduction of meta-kinks is a suitable strategy,

even though the increase is caused by the higher WU.

Thermal stability and mechanical properties

The thermal stabilities of the AEM polymers were determined

using thermogravimetric analysis (TGA, Fig. S9†). Thermal

stabilities were evaluated by the decomposition temperature at

Fig. 2 Hydroxide conductivities of Mxdepending on the number of meta-kinks (a) and Arrhenius plots (b). Hydroxide conductivities depending

on the hydration number (c) and the corresponding slopes depending on the percentage m-terphenyl in the polymers (d).

This journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. A,2024,12,7826–7836 | 7831

Paper Journal of Materials Chemistry A

95% weight (T

d,95

). All synthesized polymers showed suﬃciently

high T

d,95

values ranging from 266 to 303 °C, making them

suitable for AEMWE under working conditions. The thermal

decomposition of all polymers proceeded within three steps: At

approximately 250 °C the quaternary ammonium units might

be degraded, followed by side-chain cleavage at ∼370 °C. Ulti-

mately, decomposition of the aromatic polymer backbone

begins at 500 °C in agreement with reported values.

68,69

Apart

from this behaviour, signicant diﬀerences between diﬀerently

kinked polymers were absent.

To assess the mechanical integrity of the investigated poly-

mers stress–strain experiments were conducted. Dumbbell-

shaped specimens were cut from the membranes and studied

as dry chloride forms under ambient conditions. We note that

such conditions are far from those of AEMWE working condi-

tions, but nevertheless allow to map relative diﬀerences among

the samples. As shown in Fig. S10,†all polymers reach tensile

strengths (T

) of about 50 MPa with elongations at break (E

)of

at least 30%. Apart from M100, signicant diﬀerences in

ductility were not observed. This observation could be attrib-

uted to the incorporation of the stiﬀpara-terphenylene unit in

all of these polymers. In the case of only meta-terphenyl as an

aryl monomer, ductility drastically increased because there are

only kinked meta- and sp

-carbons in the polymer backbone

and the associated increased free volume. The larger molar

mass of M100 compared to all other polymers likely contributes

to the strong increase in E

(108% for M100).

Binder performance from rotating disk electrode (RDE)

measurements

The polymers were investigated as binders for catalysts in the

oxygen evolution reaction (OER) using rotating disk electrode

(RDE) experiments (Fig. 3). Disk electrodes were prepared from

diﬀerent catalysts and M0, M50 and M100 as binders. Naon

Fig. 3 (a) OER activities for diﬀerently kinked binder polymers in RDE with NiFe-LDH catalyst in 0.1 M KOH solution. OER activities for diﬀerently

kinked binder polymers in RDE with NiFe-LDH catalyst in 0.1 M (a) and 1 M KOH solution (b) including error bars. Comparison of OER activities

using either NiFe-LDH or IrOx as catalyst and M100 as binder polymer (c) and using either M100 or Naﬁon as binder polymer and NiFe-LDH as

catalyst (d).

7832 |J. Mater. Chem. A,2024,12,7826–7836 This journal is © The Royal Society of Chemistry 2024

Journal of Materials Chemistry A Paper

was used in addition as a commercial reference and literature-

known binder for RDE-assisted catalyst characterization.

The

RDE measurements were conducted in 0.1 M and 1 M KOH

solution at room temperature and a rotation rate of 1600 rpm.

As one of the greatest strengths of AEMWE is the possibility to

use non-precious metal catalysts, NiFe-LDH was employed.

As seen in Fig. 3a and b the onset of current density increases

at lower potential and with a larger slope with increasing

content of meta-kinks. This trend is explained by enhanced

hydration and the resulting improved ion transport. Since the

investigated electrode reaction produces a gaseous product, gas

permeability is also an important property. Additionally, the

stability of these electrodes over 2000 cycles was investigated

(Fig. S11b and S10d†). While the current density of the electrode

made from M100 (Fig. S11b†) remained in the same range aer

testing, M50 (Fig. S11c†) and M0 (Fig. S11d†) delivered

increasingly smaller values. For an example of Nyquist plots see

Fig. S12.†The decrease in stability during RDE testing is

explained by the formation of microbubbles on the surface of

the electrode, limiting the escape of oxygen from the catalyti-

cally active sites.

Accordingly, the active area is reduced and

the electrochemical reaction is slowed down. We correlate the

superior binder performance of M100 in RDE experiments with

the high content of m-TP-kinks causing a higher nanoporosity,

larger WU and higher conductivity compared to M50 and M0.

Furthermore, the increasing error bars for decreasing meta-

terphenyl content in Fig. 3a might also hint at a decreasing

binding capability, inferior surface homogeneity and non-

optimal interaction of binder and NiFe-LDH catalyst.

Additional RDE experiments were carried out to compare

M100 as binder for NiFe-LDH and iridium oxide (IrOx) as

catalysts (Fig. 3c). Strikingly, the combination of M100 and

NiFe-LDH delivered the best performance, which even out-

performed the electrode prepared from the established and

highly active IrOx. Notably, using M100 as a binder for IrOx

appears to be a rather ineﬃcient combination. Further conr-

mation of the exceptional performance of M100 as binder in

RDE experiments was obtained by using Naon as commercial

binder ionomer (Fig. 3d), with M100 delivering a strongly

improved overall performance. This observation indicates more

eﬃcient gas removal from the catalyst surface, which is

explained by the surface morphologies of the RDE featuring

a homogeneous distribution of catalyst particles (vide supra).

Fig. 4 shows compatibility and morphological aspects of

binder catalyst combinations. Fig. 4a depicts NiFe-LDH ink

dispersions with diﬀerent ionomer binders, demonstrating

Naon dispersions to be unstable compared to PAP. This lead to

Fig. 4 (a) Catalyst ink dispersions prepared with diﬀerent binders. Photographs of the electrodes coated with (b) NiFe-LDH/Naﬁon and (c) NiFe-

LDH/PAP. SEM images of the surface of the electrodes with diﬀerent magniﬁcation with Naﬁon (d and f) and M100 (e and g) as binder.

Fig. 5 Polarization curves and high-frequency resistance extracted

from electrochemical impedance spectroscopy for 1 M KOH-fed

operation in an AEM water electrolyser at 60 °C.

This journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. A,2024,12,7826–7836 | 7833

Paper Journal of Materials Chemistry A

diﬀerences in homogeneities of RDE surfaces (Fig. 4b and c). To

further visualize the impact of dispersion quality and ink

stability on the surface morphology of RDEs, scanning electrode

microscopy (SEM) was employed (Fig. 4d and e). Compared to

the Naon-based electrode (Fig. 4d), the use of M100 as binder

enabled a more homogeneous distribution of catalyst particles

(Fig. 4e), resulting in a more eﬃcient catalyst utilization and

consequently, a higher electrochemically active surface area as

well as a more porous layer with less agglomerates.

AEMWE single cell performance

Finally, the performance of M100 and M0 as binders for

membrane electrode assemblies (MEAs) used for AEM water

electrolysis was evaluated (Fig. 5). The PAP ionomers were

applied in the NiFe-LDH anodes of the MEAs. Catalyst-coated

membranes (CCMs) were fabricated by direct deposition of

the catalyst ink onto a 50 mm monolithic Aemion+ membrane as

reported previously.

For both CCMs a previously established

Pt/C cathode was used. In literature, only PAP ionomers

resembling M0 have been used as binder, as they are

commercially available as PiperION by Versogen.

15,72

To the best

of our knowledge, despite numerous indications of increasing

free volume when introducing meta- kinks

38,40,73

in the polymer

main chain and the accompanying increase in porosity, studies

applying meta-terphenyl (or otherwise kinked units) containing

PAP as AEMWE binders are barely conducted.

32,74

Similar to the results of the RDE, the cells with NiFe-LDH

and with M100 as binder showed signicantly lower over-

potentials compared to the M0 analogs at 60 °C in 1 M KOH

(Fig. 5). At 1.8 V, M100 and M0 delivered current densities of

∼1700 mA cm

−2

and 1000 cm

−2

, respectively. This cannot be

explained by the slightly higher high-frequency resistance

(HFR) of the M0 cell alone, which amounts to 89 mUcm

1000 mA cm

−2

compared to 78 mUcm

at 1000 mA cm

−2

for

M100. Also, the iR-free polarization curve showed lower voltages

at all current densities. This indicates a higher NiFe-LDH

activity in the MEA, potentially due to enhanced catalyst utili-

zation which can be attributed to the higher available surface

area of the more porous M100 (see BET surface areas, Table 1).

In addition, the superior hydration and thus increased ion

conductivity in combination with presumably higher gas

permeability (as deduced from RDE) of the kinked M100

increase mass transport, which is of major importance for

AEMWE operation.

Conclusions

We have conducted an in-depth study on the pristine properties

of a series of poly(arylene piperidinium) ionomers with varying

meta-terphenyl content along with their performance as binders

in anion-exchange membrane water electrolysers (AEMWE). A

series of seven PAP ionomers with varying meta- and para-ter-

phenyl ratio but otherwise comparable properties were synthe-

sized and comprehensively characterized with respect to water

uptake, swelling ratio, porosity and ionic conductivity. All these

properties follow the trend of a continuous increase with

increasing meta-kink content. The central eﬀect of meta-kinks

in the polymer backbone is an increase in the specic surface

area, leading to increased water uptake, which in turn aﬀects all

other properties associated with a higher degree of hydration.

In addition, PAP ionomers turn out to be excellent binders for

NiFe-LDH catalysts, probed by both rotating disk electrode

measurements as well as AEMWE single cell performance. Here,

an increasing content of meta-kinks is highly benecial as well,

for both binder dispersion and electrode quality as well as for

gas permeation. The superior performance of fully meta-kinked

PAP as anode binder matches well with the trends from RDE

experiments. AEMWE single cells yield current densities of

1700 mA cm

−2

at 1.8 V and 3000 mA cm

−2

at 2.0 V, which are

among the highest values obtained at 60 °C for state-of-the-art

AEMWE cells.

3,32,75,76

In summary, this work emphasizes the

importance of individual AEMWE binder optimization next to

membrane development, with the former being a much less

investigated aspect of AEMWE research.

Author contributions

Richard Weber: investigation, methodology, validation, writing

–original dra. Malte Klingenhof: investigation, methodology,

validation, writing –review & editing. Susanne Koch: investi-

gation, methodology, validation, writing –review & editing.

Lukas Metzler: investigation, methodology, validation, writing –

review & editing. Thomas Merzdorf: investigation, method-

ology, Jochen Meier-Haack: investigation, methodology, vali-

dation. Severin Vierrath: conceptualization, funding

acquisition, supervision. Peter Strasser: conceptualization,

funding acquisition, supervision. Michael Sommer: conceptu-

alization, funding acquisition, supervision, writing –review &

editing.

Conﬂicts of interest

There are no conicts to declare.

Acknowledgements

The authors thank Hannes Nederstedt for help in establishing

ion conductivity measurements, Rukiya Matsidik for BET

measurements, Luis Hagner for measuring one of the AEMWE

cells and Patricia Godermajer for TGA measurements. Funding

from the BMBF (AEMready, Grant No. 03SF0613A) is greatly

acknowledged.

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