Insight into the Alkaline Stability of N-Heterocyclic Ammonium Groups for Anion-Exchange Polyelectrolytes

Article abstract of DOI:10.1002/anie.202105231

The alkaline stability of N-heterocyclic ammonium (NHA) groups is a critical topic in anion-exchange membranes (AEMs) and AEM fuel cells (AEMFCs). Here, we report a systematic study on the alkaline stability of 24 representative NHA groups at different hydration numbers (λ) at 80 °C. The results elucidate that γ-substituted NHAs containing electron-donating groups display superior alkaline stability, while electron-withdrawing substituents are detrimental to durable NHAs. Density-functional-theory calculations and experimental results suggest that nucleophilic substitution is the dominant degradation pathway in NHAs, while Hofmann elimination is the primary degradation pathway for NHA-based AEMs. Different degradation pathways determine the alkaline stability of NHAs or NHA-based AEMs. AEMFC durability (from 1 A cm^?2 to 3 A cm^?2) suggests that NHA-based AEMs are mainly subjected to Hofmann elimination under 1 A cm^?2 current density for 1000 h, providing insights into the relationship between current density, λ value, and durability of NHA-based AEMs.

Full text of DOI:10.1002/anie.202105231

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 19272–19280
International Edition:
German Edition:
doi.org/10.1002/anie.202105231
doi.org/10.1002/ange.202105231
Fuel Cells
Insight into the Alkaline Stability of N-Heterocyclic Ammonium
Groups for Anion-Exchange Polyelectrolytes
Nanjun Chen+, Yiqi Jin+, Haijun Liu, Chuan Hu, Bo Wu, Shaoyi Xu, Hui Li, Jiantao Fan,* and
Young Moo Lee*
Abstract: The alkaline stability of N-heterocyclic ammonium
(NHA) groups is a critical topic in anion-exchange membranes
(AEMs) and AEM fuel cells (AEMFCs). Here, we report
a systematic study on the alkaline stability of 24 representative
NHA groups at different hydration numbers (l) at 808C. The
results elucidate that g-substituted NHAs containing electron-
donating groups display superior alkaline stability, while
electron-withdrawing substituents are detrimental to durable
NHAs. Density-functional-theory calculations and experimen-
tal results suggest that nucleophilic substitution is the dominant
degradation pathway in NHAs, while Hofmann elimination is
the primary degradation pathway for NHA-based AEMs.
Different degradation pathways determine the alkaline stability
of NHAs or NHA-based AEMs. AEMFC durability (from
1 AcmÀ2 to 3 AcmÀ2) suggests that NHA-based AEMs are
mainly subjected to Hofmann elimination under 1 AcmÀ2
current density for 1000 h, providing insights into the relation-
ship between current density, l value, and durability of NHA-
based AEMs.
platinum-group-metal (PGM)-free catalysts.[1–5] AEMFC
technology has been rapidly advanced in the past four years,
attributed to the discovery of high-performance AEMs and
AEIs. AEMFCs have attained remarkable peak power
densities (PPDs) > 2 WcmÀ2 in H2-O2 and > 1 WcmÀ2 in
H2-air so far, which is comparable to or even surpasses proton
exchange membrane fuel cells (PEMFCs), which have
dominated the field of low-temperature fuel cells for
years.[6–11] Recent studies suggest that the insufficient dura-
bility of AEMFCs has become the most critical issue, which is
strongly associated with the chemistry of their AEMs and
AEIs.[11–14]
As key materials of AEMFCs, AEMs and AEIs consist of
the polymer backbones and suspended cationic groups that
are responsible for conducting OHÀ ions. Although many
cationic groups, such as quaternary ammonium (QA), imida-
zolium (IM), quaternary phosphonium (QP), and organome-
tallic cations,[15–22] have been explored as AEMs and AEIs,
most of them are vulnerable to being exposed to alkaline
media due to severe chemical degradations, such as nucleo-
philic substitution, Hofmann degradation, ring-opening reac-
tion, and ylide degradation. Years of study have revealed that
only a few cationic groups have exhibited promising alkaline
stability (so-called ex-situ durability) under harsh alkaline
conditions and elevated temperatures.[18,22–27] For instance, 6-
azonia-spiro[5.5]undecane (ASU), N,N-dimethyl piperidini-
um (DMP), tetramethyl quaternary ammonium (TMA), large
steric hindrance IMs, bulky QP, and substituted cobaltoce-
nium groups have been reported to possess high and
promising alkaline stability. Typically, some bulky IMs
exhibited outstanding half-life time () over 10,000 h in 3 M
NaOD/D2O/CD3OD at 808C under hydration number (l) =
4.8 conditions.[18] Bulky QP groups also displayed high
alkaline stability in KOH/D2O/CD3OD at 808C.[24] Unfortu-
nately, the application of bulky IM and QP groups in AEMs
and AEMFCs are limited at present by their complicated
chemistry and synthesis.[26–28] On the other hand, previous
studies have suggested that ASU and DMP groups possessed
much higher alkaline stability among QA groups in 6 M
NaOH at 1608C due to the merits of their N-heterocyclic
ring.[23] Systematic studies on the comparison between these
stable cationic groups (ASU, DMP, IMs, or QP) have not been
reported yet.
Introduction
Anion exchange membranes (AEMs) and anion exchange
ionomers (AEIs) have received a surge of interest related to
applications in AEM fuel cells (AEMFCs) and AEM water
electrolyzers (AEMWEs) due to the feasibility of utilizing
[*] Dr. N. J. Chen,[+] C. Hu, Prof. Y. M. Lee
Department of Energy Engineering, College of Engineering, Hanyang
University
Seoul 04763 (Republic of Korea)
E-mail: ymlee@hanyang.ac.kr
Y. Q. Jin,[+] H. J. Liu, B. Wu, Prof. S. Y. Xu, Prof. H. Li, Prof. J. T. Fan
Department of Materials Science and Engineering, Southern Uni-
versity of Science and Technology
Shenzhen, 518055, Guangdong (China)
E-mail: fanjt@sustech.edu.cn
Prof. S. Y. Xu, Prof. J. T. Fan
Academy for Advanced Interdisciplinary Studies of SUSTech, South-
ern University of Science and Technology
Shenzhen, 1088, Guangdong (China)
Prof. S. Y. Xu, Prof. H. Li, Prof. J. T. Fan
Guangdong Provincial Key Laboratory of Energy Materials for Electric
Power
To date, QA groups have been the most-studied cationic
groups, possessing outstanding ion conductivity and AEMFC
performance.[3,29] The realm of AEMFCs has been greatly
advanced by the discovery of high-performance QA-based
AEMs in the past few years. Three types of QA groups-N-
Shenzhen, 518055, Guangdong (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202105231.
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heterocyclic ammonium (NHA), alkyl ammonium, and
benzyl trimethylammonium (BTMA)-based
NHA groups are presented in Figures S21 to S85 and
Schemes S5 to S25.
AEMs[1,4–9,12,30–32] currently dominate AEMFCs. Specifically,
NHA-based AEMs were stable in 1 M NaOH at 808C over
2,000 h.[1,7,12] Alkyl ammonium-based AEMs exhibited prom-
ising alkaline stability in 1 M NaOH at 808C as well.[33]
However, some arguments have been raised in current QA
and QA-based AEMs. For instance, (1) BTMA groups have
been documented to possess poorer alkaline stability than
NHA and bulky IM groups, while some reports indicated that
the BTMA groups may have higher durability than ASU or
bulky IM groups under low l conditions (l < 4).[34] In
addition, few BTMA-based AEMs[31] (such as: BTMA-based
high-density polyethylene-BTMA-HDPE) exhibited excel-
lent AEMFC durability (so-called in situ durability) for
1,000 h at a 0.6 AcmÀ2 current density at 608C. (2) Another
typical example is that ASU groups have been preliminarily
documented to possess the highest alkaline stability among
current QA groups, while some DMP-based AEMs displayed
higher alkaline stability than ASU-based AEMs.[35,36] (3) Dif-
ferent l values certainly show a significant effect on the
alkaline stability of cationic groups, while the relationship
between l values, alkaline stability of QA and AEMs, and in
situ durability still has not been well elucidated to date.[34]
Here, we explored the of 24 representative QA groups in
NaOD/D2O/CD3OD under different l conditions (i.e., 4.8, 7.0
and 10.0), and present several durable and promising NHA
groups for future design of AEMs for AEMFC and AEMWE
research. The electronic effect of substituents on NHA groups
and degradation mechanisms of NHA groups are systemati-
cally investigated by 1H nuclear magnetic resonance
(1H NMR) and density functional theory (DFT) calculations.
High-performance poly(aryl-co-aryl piperidinium) (c-PAP)
AEMs and ionomers[1,7] are used as an example to examine
the ex-situ and in situ durability under different l values and
current densities, intending to disclose the relationship
between l values, and ex-situ and in situ durability.
Alkaline Stability of NHA Groups
l = 4.8 conditions: Figure 1a indicates the original ASU
and DMP groups exhibit outstanding alkaline stability under
l = 4.8 conditions due to highly symmetric structure and
special ring strain. of ASU is greater than 20,000 h (compared
with 13,256 h for DMP), which is comparable to bulky IMs
(> 10,000 h) and is higher than bulky QP compounds.[18,24,25]
This result is consistent with the discovery of Marino et al.
that ASU and DMP possessed excellent alkaline stability in
6 M KOH (l = 9.25) at 1608C.[23] Nevertheless, it is theoret-
ically impossible to use the pristine ASU or DMP groups
directly in polymeric AEMs without substitutions. Therefore,
a study on the effect of substituents on the alkaline stability of
NHA groups is a crucial topic for current AEMs. Using DMP
as an example, the possible chemical structure of NHA-based
AEMs is shown in Figure 1b.[1–9,36–39]
NHAs (DM-IQ, O-DMP, IS-ASU, O-ASU) with electron-
withdrawing substituents (such as phenyls or heteroatoms)
presented in the a- or b-position exhibit much lower than
those of the pristine DMP or ASU (Figure 1a). The electron-
withdrawing substituents activate a-carbons and b-hydrogens
in the NHA ring so as to accelerate the nucleophilic
substitution and Hofmann elimination reactions, respectively,
which dramatically decreases the alkaline stability of NHAs.
Moreover, H-DMP (3,527 h), H-ASU (3,433 h), Bis-TP-DMP
(2,078 h), Bis-TP-ASU (3,777 h) groups with electron-with-
drawing substituents in the g-position display significantly
improved compared to the aforementioned NHAs with
electron-withdrawing substituents in the a- or b-position (2
h-1,157 h) due to the weakening electron-withdrawing effect.
Besides, of Bis-TP-DMP and Bis-TP-ASU are close to or
even higher than the TMA benchmark (2,412 h). DMP or
ASU series display much higher than BTMA (171. 6 h), which
is consistent with previous findings.
Results and Discussion
On the other hand, the of NHA groups containing
electron-donating substituents (such as -CH2-, CH3-, benzyl,
or aliphatic chain) surpasses that of the TMA benchmark,
which is much higher than that of NHAs containing electron-
withdrawing substituents. The electron-donating groups con-
tribute to increasing the steric hindrance and electron density
of N-heterocyclic ring and thus reducing the possibility of
nucleophilic substitution and Hofmann degradation reactions.
Moreover, NHAs with electron-donating substituents in the
g-position (4-M-DMP) exhibited much higher alkaline stabil-
ity ( ꢀ 14,000 h) than NHAs with electron-donating substitu-
ents in the a- or b-position (3-M-DMP: 5589 h; 2-M-DMP:
< 2,000 h) due to the higher geometric symmetry, as shown in
Figure 1a. Recently, Pham et al.[35,36,38] presented a series of
B-DMP and B-ASU-based polyphenylene AEMs with elec-
tron-donating substituents in the g-position. They found that
the alkaline stability of these AEMs significantly increased
compared to polyphenylene-based AEMs with electron-
withdrawing substituents (phenyl groups) in the g-position.
Synthesis of ASUs and DMPs with different substituents
1
was reported along with H NMR analysis in Figures S1 to
S19 and Schemes S1 to S4. The alkaline stability of these
NHA groups was examined in air based on 3 M NaOD/D2O/
CD3OD solution that can effectively dissolve compounds with
many different types of cationic groups.[18,25,35] The molecular
structures of 24 representative QAs along with abbreviations
are shown in Table 1. BTMA and TMA were used as
a benchmark, and benzyl DMP (B-DMP) and hexyltrimethyl
ammonium (HTMA) were used for comparison. Sodium 3-
(trimethylsilyl)propane-1-sulfonate (SDBS) was used as an
1
internal standard for H NMR measurement to calculate the
degradation ratio (Figure S20). To confirm the alkaline
stability of cationic compounds, three l conditions-l = 4.8,
7.0 and 10.0-were selected, and the measuring temperature
was fixed at 808C to match typical AEMFC applications.[1–7]
1H NMR spectra„ and possible degradation pathways of
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Table 1: The chemical structure and abbreviation of different QAs.
This phenomenon is well verified
by the alkaline stability of B-DMP
(3,460 h) and B-ASU (4,156 h) in
this work.
Entry QA
Abbreviation
Entry QA
Abbreviation
1
DMP
2
4
ASU
Having said that, compared to
the pristine ASU and DMP, most
NHAs with electron-donating sub-
stituents still show slightly lower
alkaline stability, which is assumed
to the change in ring strain and
3
5
TMP
DM-ASU
symmetry.
Multiple-substituted
HMP groups only exhibit moder-
ate alkaline stability (of ꢀ 1,570 h),
indicating that the overladen sub-
stituents may not be a good choice
for highly stable AEMs. Moreover,
NHAs with a symmetric structure
(such as: ASU, DMP, Bis-ASU,
Bis-DMP) seem to possess higher
alkaline stability than that of asym-
metric NHAs, such as B-DMP and
B-ASU. Consequently, linking
NHA groups with the polymer
backbone via g-position could be
a rational design for highly stable
AEMs, particularly using electron-
donating substituents. Attention
should be taken to avoid placing
improper substituents in the wrong
position of the NHA ring because
this can be highly detrimental to
the alkaline stability of AEMs.
l = 7.0 and l = 10.0 conditions:
The alkaline stability of NHA
groups was explored under l = 7.0
and 10.0 conditions as well, as
shown in Figure 2. Basically, most
NHAs show much longer under
higher l values, which is quite
natural for QA groups. The elec-
tronic effect of substitutions on
NHA groups under both l = 7.0
and 10.0 conditions are almost the
same as that of l = 4.8. Again,
NHAs containing electron-donat-
ing substituents exhibit much high-
er alkaline stability than that of
NHAs containing electron-with-
drawing substituents. Specifically,
the of g-substituted NHA groups
with electron-donating groups can
reach > 10,000 h, and some of them
can even reach the over 60,000 h
under l = 10.0 conditions. Unfortu-
nately, Bis-TP-DMP and Bis-TP-
ASU groups cannot be well dis-
solved in NaOD/D2O/CD3OD sol-
vents when the l value increases to
7.0 and 10.0.
B-DMP
6
B-ASU
7
9
Bis-DMP
8
Bis-ASU
Bis-TP-DMP
10
Bis-TP-ASU
11
H-DMP
12
H-ASU
13
15
DM-IQ
O-DMP
14
16
IS-ASU
O-ASU
17
19
HMP
18
20
2-M-DMP
4-M-DMP
3-M-DMP
21
23
BTMA
TMA
22
24
BMP
HTMA
t
1/2 >10,000 h
t1/2 >10,000 h
under l=4.8 at
under l=4.8 at
808C[18]
808C[25]
t
1/2 =2,030 h
l=9.3, 18%
under l=4.8 at
deg, 5,000 h[24]
808C[18]
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HTMA and DMP-based
AEMs are both promising
for durable AEMFC appli-
cations. (3) DM-IQ and IS-
ASU with quinoline struc-
ture possess poor alkaline
stability; thus, these struc-
tures may not be a rational
design for AEMs.
Degradation Mechanism of
NHA Groups and NHA-Based
AEMs
Several interesting find-
ings have been noticed in
this study. (1) Based on
1H NMR investigation and
analysis, we found that the
SN2 reaction, instead of the
ring-opening E2 reaction,
dominates the degradation
pathway of most NHA
groups in NaOD/D2O/
CD3OD under different
l conditions, as shown in
Figures 3a and 3b using B-
DMP and B-ASU as exam-
ples. (2) Under lower l con-
ditions (4.8 or 7.0), ASU
and substituted ASU groups
exhibited higher alkaline
stability than DMP or sub-
stituted DMPs. However,
under higher l conditions
(l = 10.0), DMP and substi-
tuted DMPs display compa-
rable alkaline stability to
Figure 1. a) The of different QA groups under l=4.8 conditions in NaOD/D2O/CD3OD at 808C. b) The
possible chemical structure of NHA-based polymers for AEMs. 1H NMR spectra monitored total degradation
ratios of NHAs. The degradation ratio of Bis-DMP and Bis-ASU with diammonium groups is twice that of the
other NHA groups containing a single ammonium group. Therefore, the actual half-life time of Bis-DMP and
Bis-ASU can be predicted by multiplying the first half-life by 2.
Currently, three types of AEMs have been mostly studied
in AEMFCs: BTMA-based AEMs, HTMA-based AEMs, and
DMP-based AEMs.[1,4–9,12,30–32] Some of them displayed ex-
cellent AEMFC performance with power density over
1.5 WcmÀ2 in H2-O2, such as BTMA-based HDPE,[9,31]
HTMA-based polynorbornene,[8] HTMA-based polypheny-
lene,[32] and poly(aryl piperidinium)s (PAPs).[1,7,12,14] Some
additional information can be obtained from this research.
(1) BTMA and BMP exhibit acceptable over 1,100 h under
l = 10.0 conditions, and BMP possesses slightly higher
alkaline stability than BTMA, implying that replacing BTMA
into BMP will be helpful for benzyl ammonium-based AEMs
(such as for the development of DMP-based HDPE). Note
that Biancolli et al.[40] has previously demonstrated BMP-
based poly(ethylene-co-tetrafluoroethylene) (ETFE) ionom-
ers showed higher alkaline stability and AEMFC durability
than BTMA-based ETFE. (2) HTMA groups display com-
parable (2,046 h) to Bis-TP-DMP (2,078 h) groups under l =
4.8 conditions. HTMA shows higher over 6,000 h under l =
7.0 and 10.0 conditions, indicating that state-of-the-art
ASU or substituted ASUs. All these phenomena can be
explained by their different degradation pathways deter-
mined by 1H NMR results and DFT calculation, as discussed
below.
DFT was used to calculate the Gibbs free energy (DG, so
called energy barrier) of degradation in DMP, ASU, diphenyl-
substituted DMP (DP-DMP), and diphenyl-substituted ASU
(DP-ASU) undergoing potentially different degradation
pathways, as shown in Figure 4.[41,42] Calculated results
elucidate that the free energy of SN2 (DGSN2) is lower than
that of E2 (DGE2) in DMP or ASU series under alkaline
conditions, implying that the SN2 is the dominant degradation
pathway in these NHA groups, which is well matching with
1
our H NMR results. Notice that DMP displays free energy
values of 15.4 kcalmolÀ1, 46.36 kcalmolÀ1, and 48.88 kcal
molÀ1 for SN2 (1), SN2 (2), and E2, respectively. Moreover,
DP-DMP shows lower free energy in all degradation path-
ways (SN2 (1): 15.23 kcalmolÀ1, SN2 (2): 22.44 kcalmolÀ1, E2:
35.54 kcalmolÀ1) compared to DMP, particularly in SN2 (2)
and E2, meaning that electron-withdrawing substituents in
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1
Figure 2. The of different QA groups under l=7.0 and 10.0 conditions in NaOD/D2O/CD3OH at 808C. H NMR spectra monitored total
degradation ratios of NHA groups. The degradation ratios of Bis-DMP and Bis-ASU with diammonium groups are twice that of the other NHA
groups with a single ammonium group. Therefore, the actual of Bis-DMP and Bis-ASU can be predicted by multiplying the first half-life by 2.
the g-position of NHA ring can accelerate the degradation
reactions, particularly attacking the inside of the NHA ring.
ASU and DP-ASU display similar degradation behavior to
the DMP series. Note that ASU and DP-ASU show higher
DGSN2 (18.7 kcalmolÀ1 and 17.8 kcalmolÀ1) but lower DGE2
(29.1 kcalmolÀ1and 24.0 kcalmolÀ1) compared to those of
DMP and DP-DMP, implying that the ASU series possesses
higher alkaline stability than the DMP series when SN2 is the
dominant degradation pathway, while DMP may possess
higher alkaline stability when E2 is the dominant degradation
pathway.
According to the features of SN2 and E2 on QA groups,
the SN2 reaction is triggered by the nucleophilic attack on a-
carbon (electron-poor) from the back side. The SN2 reaction
rate is highly dependent on the concentration of nucleophilic
reagent (electron-rich, OHÀ), followed by the reaction rate of
primary carbon (18C) > secondary carbon (28C) > tertiary
carbon (38C) > quaternary carbon (48C). On the other hand,
E2 reaction starts from a b-hydrogen elimination through
a trans and coplanar conformation, followed by the reaction
rate: 18C < 28C < 38C < 48C. Note that ASU contains four 28
a-carbons in the spiro ring, whereas DMP possesses two 18 a-
carbons outside of the ring and two 28 a-carbons in the ring.
Naturally, two 18 a-carbons outside of the DMP ring are more
sensitive to SN2 degradation than 28 a-carbons, followed by
SN2 (1) degradation pathway. Therefore, the ASU series
reasonably possesses higher alkaline stability than the DMP
series when SN2 reaction is the dominant degradation path-
way. However, the half-life time gap between ASU and DMP
series becomes weakened under high l values (or low OHÀ
concentration) because the SN2 reaction rate decreases
significantly at low OHÀ concentration according to the
feature of SN2 reaction.[23,25] On the other hand, ASU series
contains eight b-hydrogens, while the DMP series only
contains four b-hydrogens, meaning that ASU series are
more sensitive to E2. In other words, the alkaline stability of
ASU and DMP series is not always the same, which is
determined by their degradation pathways. Moreover, the
reversibility of these degradation reactions was calculat-
ed[41,42] in Table S1 based on DMP and ASU groups. The
results show that the rate constant of reverse degradations is
much slower than that of forward degradations, suggesting
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Figure 3. a) Typical 1H NMR spectra and of B-DMP in NaOD/D2O/CD3OH under l=4.8 conditions at 808C. b) Typical 1H NMR spectra and of B-
ASU in NaOD/D2O/CD3OH under l=4.8 conditions at 808C. The was estimated by exponential fitting via Origin software.
Figure 4. DFT calculation of a) DMP and DP-DMP and b) ASU and DP-ASU in different degradation pathways.
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that the effect of reverse degradation reactions on the alkaline
stability of NHA groups can be neglected.
4.8) is much lower than that of phenyl-substituted DMPs.
Recent discoveries[7,35–38] found that DMP-based PAP AEMs
possessed higher alkaline stability than ASU-based PAP
AEMs, and E2 was always the dominant degradation pathway
in PAP AEMs. Certainly, the alkaline stability behavior of
NHA-based AEMs does not match well with that of NHAs
due to different degradation pathways. The aforementioned
DFT calculation in Figure 4 also indicates that DGE2 and
DGSN2 can be significantly influenced by substituents. In
addition, Marino et al.[23] pointed out that the balance
between ring strain and transition state energy of the NHA
ring may determine the degradation pathways. Based on the
DFT calculation presented in Figure S85, bond angles near
central g-C (i.e., b-g-b-Cs) of DP-DMP (105.1528) and DP-
ASU (106.0778) are lower than those of DMP (110.478) and
ASU (110.8218), and the ring strain and total strain energy of
diphenyl-substituted DMP or ASU compounds also has
increased. These calculations give us a possible explanation
that the decreased transition state energy and increased ring
strain of NHA groups in PAP AEMs upon substitution, may
contribute to accelerating E2 degradation.
Interestingly, in the case of a-multi-substituted HMP,
HMP groups contain two 48 a-carbons in the NHA ring; thus
SN2 reaction is significantly suppressed. However, monomo-
lecular SN1 reaction is aroused since the rate-determining step
of the SN1 reaction is the formation of carbocation (C+)
intermediate. In addition, based on the features of the SN1
reaction, the reaction rate of SN1 is not related to the
concentration of nucleophilic reagent (OHÀ), but strongly
related with the stability of C+ intermediate. Under high
l value (or low OHÀ concentration), there are more free
water molecules, which can contribute to stabilizing C+
intermediates.[23,25] Therefore, HMP groups exhibit depressed
alkaline stability or when the l value increases (l = 4.8,
1,570 h; l = 7.0, 440 h; l = 10.0, 243 h), which is totally
opposite to other NHA groups.
Degradation behavior in c-PAP AEMs: Notably, c-PAP
AEMs mainly suffered from ring-opening E2[1,7] reaction,
instead of SN2 reaction, as shown in Figures S83 and S84. The
of poly(diphenyl-co-terphenyl piperidinium) (PDTP) AEMs
can be predicated under different l values, as shown in
Figure 5a. Compared to the of Bis-TP-DMP (2,078 h, l =
4.8), the alkaline stability of PDTP AEMs (< 1,100 h, l =
Therefore, the alkaline stability of NHAs and NHA-based
AEMs is not always stereotyped, which is related to specific
degradation pathways. These fundamental discoveries and
1
Figure 5. a) The relationship between half-life of c-PAP AEMs and l values. The Figure was plotted based on H NMR spectra of c-PAP AEMs
after alkaline treatment in NaOH/H2O at 808C under different l (55.5, 11.1, 5.5) conditions. b) In-situ durability of c-PAP-based AEMFCs under
different current densities: H2-O2 808C, 75/100% anode (A)/cathode (C) RH, 200/200 mLminÀ1 A/C flow rate, 0.6/0.6 bar A/C backpressure, Pt-
Ru/C anode and Pt/C cathode along with 0.39 mgcmÀ2 catalyst loading. c) and d) 1H NMR spectra of CCMs after in situ durability testing.
[D6]DMSO was used as solvent, and 10% TFA was added to NMR sample to eliminate the effect of H2O.
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understandings of DMP and ASU effectively answer the
4.8, 7.0 and 10.0) in NaOD/D2O/CD3OD at 808C. (1) NHA
current argument associated with their alkaline stability,
guiding the rational design of highly stable AEMs.
groups with electron-donating substituents exhibited out-
standing alkaline stability among current cationic groups,
whereas electron-withdrawing substituents displayed a detri-
mental effect on the stability of NHA groups. (2) g-
substituted NHAs possess much higher alkaline stability than
a- or b-substituted NHAs. (3) DFT calculation and exper-
imental results demonstrated that different degradation
pathways determine the alkaline stability of NHAs or
NHA-based AEMs. The SN2 reaction is the dominant
degradation pathway in NHAs, while E2 reaction is the
primary degradation pathway for PAP-based AEMs. In-situ
durability of c-PAP-based AEMFCs suggests that E2 is the
dominant degradation pathway of aryl ether-free NHA-based
AEMs under 1 AcmÀ2 current density at 608C for 1,000 h. We
believe that the present work provides insights into current
NHAs and NHA-based AEMs and presents a clear guideline
for design of highly stable AEM or ionomer, contributing to
promoting the development of durable AEMFCs.
Relationship Between l, Ex-Situ Durability, and In-Situ
Durability
Meanwhile, insight into in situ durability of AEMs is still
lacking to date. Determining the realistic alkaline conditions
for operating AEMFCs is crucial for the development of
durable AEMFCs. Previous research[34] simulated the l values
of AEMFCs under different current densities (such as
0.5 AcmÀ2, 1 AcmÀ2, 2 AcmÀ2 may be equal to the l values
of 7, 4, 2, respectively), while recent work hinted that these
predictions may not well match with current AEMFCs. For
instance, BTMA-based HDPE AEMs can be operated stably
under 0.6 AcmÀ2 at 608C for 1,000 h.[9] That is, the real
alkalinity of AEMFCs and the degradation pathway of QA
groups during in situ durability testing should be well verified
by experiment.
Recently, we developed a series of high-performance c-
PAPs[1,7] for AEMs and AEIs, which can be operated under
high current density in AEMFCs. Therefore, the short-term
durability of c-PAP-based AEMFCs under super high current
densities (1 AcmÀ2, 1.5 AcmÀ2, 2 AcmÀ2, 3 AcmÀ2) at 808C
was tested in H2-O2, and the cells were continuously measured
for ꢀ30 h, ꢀ 10 h, ꢀ 8 h, and ꢀ 2 h, respectively. As shown in
Figure 5b, the cell voltage was kept stable at a current density
of 1 AcmÀ2 for the initial 10 hours, whereas the voltage
dropped quickly under high current densities, particularly in
3 AcmÀ2. To confirm the chemical stability of AEMs and
AEIs, all catalyst-coated membranes (CCMs) were detached
Acknowledgements
This research was supported by the Technology Development
Program to Solve Climate Change through the National
Research Foundation of Korea (NRF), funded by the
Ministry of Science and ICT (NRF-2018M1A2A2061979).
Additional funding came from the Technology Innovation
Program (20010955) through the Korea Evaluation Institute
of Technology (KEIT) funded by the Ministry of Trade,
Industry & Energy (MOTIE) of the Republic of Korea, the
National Key Research and Development Program of China
(2018YFB1502503),
Shenzhen
Free
Exploration
1
from the cells and redissolved in [D6]DMSO for H NMR
(JCYJ20190809163611271) and Sustainable Development
Project (KCXFZ202002011010317), and the Center for Com-
putational Science and Engineering at Southern University of
Science and Technology. C. Hu appreciates the financial
support from China Scholarship Council (CSC) [NO:
201906310144]
1
analysis (Figure 5c). H NMR spectra of CCMs indicate that
no degradation signal can be detected under 1 AcmÀ2,
1.5 AcmÀ2, and 2 AcmÀ2 current densities for short time,
meaning that the alkaline environment of AEMFCs is not as
harsh as previously predicted. Therefore, long-term durability
of c-PAP-based AEMFCs was conducted under 1 AcmÀ2
current density at 608C for 1,000 h (Figure 5d). Interestingly,
=
three typical -C C- signals (4.97 ppm, 5.35 ppm, 6.35 ppm)
Conflict of Interest
derived from Hofmann degradation reaction were detected
under 1 AcmÀ2 after 1,000 h ( ꢀ 12% piperidinium group
degradation in AEMs), suggesting that the Hofmann degra-
dation is the dominant pathway of NHA-based AEMs and
AEIs during in situ durability testing. The detailed informa-
tion of long-term durability under 1 AcmÀ2 current density
will be presented in our separate publication. These discov-
eries hint that SN2-dominated AEMs or ionomers may have
advantages in the durability of AEMFCs, which may answer
the reason why the aforementioned BTMA-HDPE[9] can be
operated stably under 0.6 AcmÀ2 for 1,000 h.
The authors declare no conflict of interest.
Keywords: anion-exchange-membrane fuel cells ·
ex-situ durability ·in-situ durability ·N-heterocyclic ammonium
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Manuscript received: April 16, 2021
Revised manuscript received: June 10, 2021
Accepted manuscript online: June 23, 2021
Version of record online: July 20, 2021
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