Fuel cell anode catalyst performance can be stabilized with a molecularly rigid film of polymers of intrinsic microporosity (PIM)

Article abstract of DOI:10.1039/c5ra25320a

There remains a major materials challenge in maintaining the performance of platinum (Pt) anode catalysts in fuel cells due to corrosion and blocking of active sites. Herein, we report a new materials strategy for improving anode catalyst stability based on a protective microporous coating with an inert and highly rigid (non-blocking) polymer of intrinsic microporosity (PIM-EA-TB). The "anti-corrosion" effect of the PIM-EA-TB coating is demonstrated with a commercial Pt catalyst (3-5 nm diameter, 40 wt% Pt on Vulcan-72) and for three important fuel cell anode reactions: (i) methanol oxidation, (ii) ethanol oxidation, and (iii) formic acid oxidation.

Full text of DOI:10.1039/c5ra25320a

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Fuel cell anode catalyst performance can be
stabilized with a molecularly rigid film of polymers
Cite this: RSC Adv., 2016, 6, 9315
of intrinsic microporosity (PIM)
Daping He,a Yuanyang Rong,a Mariolino Carta,b Richard Malpass-Evans,b
Received 28th November 2015
Accepted 14th January 2016
b
a
*
Neil B. McKeown and Frank Marken
DOI: 10.1039/c5ra25320a
www.rsc.org/advances
There remains a major materials challenge in maintaining the perfor- electro-corrosion via Pt dissolution and colloidal mobility,
mance of platinum (Pt) anode catalysts in fuel cells due to corrosion aggregation, and poisoning by intermediates and corrosion
and blocking of active sites. Herein, we report a new materials strategy products during small organic fuel molecule oxidation.12–14
for improving anode catalyst stability based on a protective micro- A simple materials solution such as a protective coating is
porous coating with an inert and highly rigid (non-blocking) polymer highly desirable.
of intrinsic microporosity (PIM-EA-TB). The “anti-corrosion” effect of
Employing polymer functionalization to optimize the
the PIM-EA-TB coating is demonstrated with a commercial Pt catalyst performance of commercial Pt/C catalysts and to design new
(3–5 nm diameter, 40 wt% Pt on Vulcan-72) and for three important nano-composite catalyst systems are both promising strategies
fuel cell anode reactions: (i) methanol oxidation, (ii) ethanol oxidation, for fuel cell catalyst development and engineering. Some
and (iii) formic acid oxidation.
advanced polymers, such as polyaniline (PANI)13,15–17 or PTFE/
peruorosulfonic acid (PFSA)18–20 have received special atten-
tion in fuel cell catalyst stabilization, because of their unique
conductive and/or proton mobility properties. Interfacial Pt-
catalyst engineering for example with pyridine-containing
poly-benzimidazole on multi-walled carbon nanotubes has
been reported to improve catalyst performance for methanol
fuel cells.21 These materials lead locally to high electrical and
proton conductivity and good chemical stability in acidic envi-
ronments. However, due to poor rigidity these materials also
strongly interact with catalyst surfaces. Challenges are still
considerable in precisely controlling the thickness of these
solid polymer over-layers, which also increase the resistance
towards reaction species transport to the catalyst surface and so
detrimentally affect catalytic activity. A polymer with more rigid
molecularly pre-dened microporosity could be a better alter-
native. The rigid structure of the polymer can maintain
a permanent microporosity, which facilitates the transportation
of the reaction species whilst suppressing morphological
changes.
Fuel cells powered by oxidation of small organic molecules
(such as methanol, ethanol, formic acid, etc.) can be highly
efficient, environmentally friendly, convenient in terms of fuel
storage and infrastructure, and relatively simple in terms of
device manufacturing.1–4 A signicant challenge remaining in
the large-scale implementation of low temperature fuel cells
(such as polymer-electrolyte or PEM fuel cells for cars) is the
oen rapid degradation of catalysts (depending on operational
conditions) in both anode and cathode compartments due to
catalyst migration, Ostwald ripening, carbon substrate corro-
sion, and loss of catalyst from the surface.5,6 These losses are
oen linked to short operational bursts or switching during
power generation. This leads to the irreversible decay of fuel
performance associated with additional costs of mainte-
nance.1,7–9 Conditions during operational deterioration of cata-
lysts can be simulated by accelerated degradation testing (ADT)
based on repeated potential cycles.
Due to the insufficient activity of non-noble metal catalysts,
the most commonly used fuel cell catalysts to date are 3–5 nm
diameter Pt nanoparticles, which are supported on commercial
carbon supports such as Vulcan-72.10,11 The well-known
detrimental corrosion processes for Pt nanocatalysts include
Recently, the novel class of intrinsically microporous polymer
(PIM) materials has been developed primarily for a range of
applications in gas membrane technology.22–26 The structurally
highly rigid PIM backbone achieves open packing to generate
novel properties due to permanent microporosity. In our recent
publications we have demonstrated benets of PIM materials also
in aqueous electrolyte media with potential applications emerging
in electrochemical technology27,28 and in solution phase electro-
catalysis.29–32 Particularly useful is a novel amine-containing PIM
aDepartment of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
E-mail: f.marken@bath.ac.uk
bSchool of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, EH9
3FJ, UK
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(PIM-EA-TB), synthesised via a polymerisation reaction involving
¨
the formation of Troger's base with high molecular mass (Mw
>
70 000 g molÀ1) and high BET surface area (1027 m2 gÀ1). This
PIM material was shown to be applicable as an effective protection
agent against Pt/C catalysts corrosion in fuel cell cathodes32 (for
oxygen reduction). In this report, it is demonstrated that the same
methodology can be applied much more generally also for fuel
cell anode catalysts in methanol, ethanol, or formic acid
oxidation.
The rigid microporous PIM-EA-TB lm is employed here as
a coating to encapsulate the Pt/C catalysts (see Fig. 1a) whilst
maintaining effective diffusion channels of 1–3 nm diameter26
for water and reaction species. The coating is applied on top of
the catalyst composite layer by casting of a PIM-EA-TB solution
in chloroform. A layer of 10 mg PIM-EA-TB is applied to a 6 mm
diameter glassy carbon electrode with underlying catalyst to
give a lm of approximately 40 nm average thickness (or lower
when taking into account surface roughness of the catalyst
layer). The TEM image of commercial Pt/C catalyst (40 wt% on
Vulcan-72) in Fig. 1b shows a size range of 3 to 5 nm for Pt
nanoparticles supported on carbon black. Fig. 1c and d show
SEM images of Pt/C (c) and PIM@Pt/C (d) catalysts. The PIM-EA-
TB membrane layer was found distributed over catalyst layer
surface (appearing as “haze” in the SEM image), which suggests
a successful modication of the PIM-EA-TB lm on top of the
Pt/C catalyst.
Fig. 2 (a) Cyclic voltammograms in N2-saturated 0.1 M HClO4 (50 mV sÀ1
)
and (b) cyclic voltammograms for methanol oxidation in 0.1 M HClO4
+ 1 M CH3OH for Pt/C and for PIM@Pt/C. (c and d) Cyclic voltam-
mograms for Pt/C (c) and for PIM@Pt/C (d) showing accelerated
degradation testing from À0.25 to +0.95 V vs. SCE. (e) Plot of elec-
trocatalytic oxidation peaks for Pt/C and for PIM@Pt/C for the forward
peak current for ethanol electro-oxidation over 200 cycles. (f) Chro-
noamperometric curves of Pt/C and PIM@Pt/C at +0.6 V vs. SCE.
Data in Fig. 2a shows cyclic voltammetry responses for Pt/C
(black) and for PIM@Pt/C (red) immersed in N2-saturated
0.1
M HClO4 at room temperature. Both voltammetric
responses are dominated by the classic platinum surface signals
and essentially identical. That is, under these conditions there
is no direct effect of the PIM-EA-TB surface layer on these measuring the charge collected in the hydrogen adsorption/
signals. The rigid molecular structure prevents blocking of desorption region (see QH) and assuming 210 mC cmÀ2
.
33
surface sites on the platinum. The electrochemically active Values obtained in this way are 53.4 m2 gÀ1 for Pt/C and 52.5 m2
surface area (ESA) for the catalyst can be calculated from gÀ1 for PIM@Pt/C. This provides strong evidence that only few
surface Pt atoms are inactivated by bonding to the amine
functionality within the PIM-EA-TB, which is due to the highly
rigid and conformationally locked structure of the polymer.
Methanol electro-oxidation (which is important in direct
methanol fuel cells34) was investigated in aqueous 0.1 M HClO4
+ 1 M CH3OH as shown in Fig. 2b. The process is known to yield
predominantly carbon dioxide. The Pt/C and PIM@Pt/C cata-
lysts show very similar onset potential and peak currents for
methanol electro-oxidation, at least initially. Then, the long-
term electrocatalytic stability for PIM@Pt/C and commercial
Pt/C were tested by repeating the cyclic voltammetry potential
sweeps for 200 cycles (see Fig. 2c and d).
Whereas Pt/C catalyst (Fig. 2c) suffers signicant degrada-
tion and decay in the catalytic current (due to the harsh accel-
erated degradation conditions), the PIM@Pt/C catalyst (see
Fig. 2d) remains stable under these conditions. Fig. 2e shows
the normalized forward oxidation peak current versus the
number of potential cycle for both catalysts, which demon-
strates that the decrease of the forward peak current of
PIM@Pt/C is much slower compared to that for the commercial
unprotected Pt/C catalyst. Aer 200 potential cycles, 88% of the
initial forward oxidation peak current was maintained for
Fig. 1 (a) Schematic illustration of PIM-EA-TB rigidly encapsulating
and protecting Pt/C catalyst without blocking the surface. (b) TEM
image of Pt/C catalyst. SEM images for Pt/C (c) and PIM@Pt/C (d) on
glassy carbon electrode substrate.
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PIM@Pt/C as compared to only 48% for Pt/C. Additionally,
chronoamperometry test measurements were performed in
aqueous 1 M methanol + 0.1 M HClO4 at the oxidation potential
of 0.6 V vs. SCE (see Fig. 2f). Aer 4000 s, the current is 0.48 mA
for PIM@Pt/C and only 0.21 mA for Pt/C – again indicative of
enhanced catalyst stability during methanol oxidation. The
results show signicant benets even at shorter times. This
suggests that the polymer microporosity, in addition to
enhancing durability due to corrosion protection, may help to
maintain surface activity due to prevention of impurity
adsorption and blocking.
Ethanol electro-oxidation (of interest for direct ethanol fuel
cells34) was investigated under similar conditions (see Fig. 3a–
d). Products for the process include carbon dioxide and acetic
acid. Measurements are reported for PIM@Pt/C and Pt/C in the
aqueous 0.1 M HClO4 + 1 M ethanol at room temperature.
Very similar cyclic voltammetry responses are obtained for
ethanol electro-oxidation on both types of catalysts (Fig. 3a). For
the accelerated durability testing, cyclic voltammetry potential
scans were performed for 500 continuous cycles. Fig. 3b and c
contrast the behaviour observed with/without the PIM-EA-TB
lm coating during durability testing. A signicant decay in
performance is seen only for the commercial Pt/C catalyst.
Fig. 3d shows the forward oxidation peak current density as
a function of cyclic voltammetry scan number and demon-
strates that the forward oxidation peak current density of
PIM@Pt/C catalyst reaches the maximum (3.9 mA) at the 100th
cycle, and then remains relatively constant. The initial increase
could be linked to some relaxation in the polymer lm. In
contrast, a slow but monotonous decrease in catalyst perfor-
mance with the number of potential cycle is seen on the
commercial Pt/C catalyst. This behaviour is usually attributed to
surface poisoning by CO-like species as well as the dissolution
loss of Pt from the surface. Aer 500 potential cycles, 98% of the
Fig. 4 (a) Cyclic voltammograms for formic acid oxidation at room
temperature in a 0.1 M HClO4 + 1 M HCOOH (50 mV sÀ1). (b and c)
Cyclic voltammograms for Pt/C (c) and for PIM@Pt/C (d) for acceler-
ated degradation testing by potential cycling from À0.25 to +0.95 V vs.
SCE. (d) Plot of oxidation peak currents for Pt/C and for PIM@Pt/C for
the forward peak for formic acid electro-oxidation over 200 cycles.
initial forward oxidation peak current was still maintained for
PIM@Pt/C, as compared to the decrease to 85% of initial activity
for Pt/C.
Formic acid electro-oxidation (for direct formic acid fuel
cells35) activity was tested in aqueous 0.1 M HClO4 + 1 M
HCOOH (see Fig. 4a). The process is known to lead to carbon
dioxide as the principle product. Initially, both the Pt/C and the
PIM@Pt/C catalysts show very comparable activities for formic
acid electro-oxidation. The accelerated degradation testing by
cyclic voltammetry and monitoring electrocatalytic perfor-
mance for PIM@Pt/C and for commercial Pt/C are shown in
Fig. 4b and c. It can be seen that current peaks for commercial
Pt/C catalyst (Fig. 4b) change signicantly, while the equivalent
experiments with PIM@Pt/C (Fig. 4c) demonstrate much greater
robustness. The backward potential scan oxidation peak drops
dramatically.
The result for the forward scan oxidation peak currents
versus number of potential cycles for both Pt/C and PIM@Pt/C
are shown in Fig. 4d. Aer 200 potential cycles, 85% of the
initial forward oxidation peak current was still maintained for
PIM@Pt/C, whereas only 43% activity remained for Pt/C. This
again demonstrates enhanced durability during accelerated
degradation testing when the PIM-EA-TB coating is applied to
the catalyst.
Conclusions
A signicant improvement in catalyst durability and perfor-
mance (under accelerated degradation conditions) has been
achieved by applying a PIM-EA-TB lm as protective over-
coating to commercial fuel cell anode catalyst materials. It is
suggested that the molecularly rigid and microporous nature of
the PIM-EA-TB material are crucial in (i) mechanically
Fig. 3 (a) Cyclic voltammograms for ethanol oxidation recorded at
room temperature in 0.1 M HClO4 + 1 M CH3CH2OH (50 mV sÀ1). (b
and c) Cyclic voltammograms for Pt/C (c) and for PIM@Pt/C (d) for
accelerated degradation testing from À0.25 to +0.95 V vs. SCE. (d) Plot
of electrocatalytic peak currents of Pt/C and PIM@Pt/C referring to the
forward peak current for ethanol electro-oxidation over 500 cycles.
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stabilizing both carbon substrate and platinum nano-catalyst,
(ii) preventing electrophoretic mobility, (iii) not directly inter-
acting with the platinum nano-catalyst surface sites, whilst (iv)
preventing larger adsorbates reaching the catalyst surface.
Enhanced stability does not come at the cost of reduced
performance as the ow of reagents and charges through the
microporous PIM-EA-TB environment is sufficient for good
catalysis. Materials properties at the molecular level are
important for catalyst performance improvement for a range of
potential fuel cell technologies (and beyond). We believe our
methodology provides a new and general route to increase fuel
cell anode catalyst performance and lifetime.
References
1 H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang and
P. Wilkinson, J. Power Sources, 2006, 155, 95.
2 S. K. Kamarudin, F. Achmad and W. R. W. Daud, Int. J.
Hydrogen Energy, 2009, 34, 6902.
3 M. Z. F. Kamarudin, S. K. Kamarudin, M. S. Masdar and
W. R. W. Daud, Int. J. Hydrogen Energy, 2013, 38, 9438.
4 X. Yu and P. G. Pickup, J. Power Sources, 2008, 182, 124.
5 J. Park, H. Oh, T. Ha, Y. I. Lee and K. Min, Appl. Energy, 2015,
155, 866.
6 L. Li, L. P. Hu, J. Li and Z. D. Wei, Nano Res., 2015, 8, 418.
7 X. Zhao, M. Yin, L. Ma, L. Liang, C. P. Liu, J. H. Liao, T. H. Lu
and W. Xing, Energy Environ. Sci., 2011, 4, 2736.
8 F. Vigier, C. Coutanceau, A. Perrard, E. M. Belgsir and
C. Lamy, J. Appl. Electrochem., 2004, 34, 439.
Experiment section
Chemical reagents
Commercial Pt/C (40 wt% on Vulcan-72) catalysts and Naon
(5 wt%) were obtained from Johnson Matthey and Dupont,
9 C. Rice, S. Ha, R. I. Masel and A. Wieckowski, J. Power
Sources, 2003, 115, 229.
respectively. Isopropanol, chloroform, and perchloric acid (70– 10 N. C. Cheng, M. N. Banis, J. Liu, A. Riese, X. Li, R. Y. Li,
72%), were purchased from Aldrich or Fisher Scientic and S. Y. Ye, S. Knights and X. L. Sun, Adv. Mater., 2015, 27, 277.
used without further purication. PIM-EA-TB was prepared 11 N. C. Cheng, M. N. Banis, J. Liu, A. Riese, S. C. Mu, R. Y. Li,
following a literature recipe.36 Solutions were prepared with
T. K. Sham and X. L. Sun, Energy Environ. Sci., 2015, 8, 1450.
ltered and deionized water of resistivity 18.2 MW cm from 12 E. G. Ciapina, S. F. Santos and E. R. Gonzalez, J. Electroanal.
a Thermo Scientic water purication system (ELGA).
Chem., 2010, 644, 132; S. Chen, Z. Wei, X. Qi, L. Dong,
Y. Guo, L. Wan, Z. Shao and L. Li, J. Am. Chem. Soc., 2012,
134, 13252.
Instrumentation and procedures
13 M. W. Breiter, J. Electroanal. Chem. Interfacial Electrochem.,
1967, 15, 221.
14 M. Zhiani, B. Rezaei and J. Jalili, Int. J. Hydrogen Energy, 2010,
35, 9298.
15 D. P. He, C. Zeng, C. Xu, N. C. Cheng, H. G. Li, S. C. Mu and
M. Pan, Langmuir, 2011, 27, 5582.
16 M. Zhiani, B. Rezaei and J. Jalili, Int. J. Hydrogen Energy, 2010,
35, 9298.
17 D. P. He, S. C. Mu and M. Pan, Carbon, 2011, 49, 82.
18 D. P. He, K. Cheng, H. G. Li, T. Peng, F. Xu, S. C. Mu and
M. Pan, Langmuir, 2012, 28, 3979.
19 Z. Q. Tian, S. P. Jiang, Z. Liu and L. Li, Electrochem. Commun.,
2007, 9, 1613.
Electrochemical measurements were performed with a mAuto-
lab III system in a conventional three electrode cell with KCl-
saturated calomel (SCE) reference and platinum wire counter
electrode. A Pine AFMSRCE electrode rotator was used for
rotating disk electrode (RDE) experiments. Data obtained in
stationary solution and with rotation were essentially identical
and therefore only stationary experiments are reported here.
Morphologies of the prepared catalysts were analyzed with
a JEOL FESEM 6301F scanning electron microscopy (SEM) and
a JEOL 2010 high-resolution transmission electron microscope
(HRTEM).
Procedures for electrode preparation
20 N. B. McKeown and P. M. Budd, Macromolecules, 2010, 43,
5163.
2 mg of Pt/C catalyst and 100 mL of 5 wt% Naon solution were
dispersed in 1 mL of isopropanol, followed by a sonication for at
least 15 min to form a homogeneous catalyst ink. A volume of 8 mL
of the ink was loaded onto a glassy carbon (GC) disk electrode with
a diameter of 6 mm. The catalyst layer was allowed to dry under
ambient conditions before a CV measurement. Polymer with
intrinsic microporosity coating Pt/C (PIM@Pt/C) was conducted as
follow: a solution of 1 mg cmÀ3 PIM-EA-TB was prepared in
chloroform and 10 mL was applied directly by coating the Pt/C
catalyst layer followed by drying under ambient conditions.
21 T. Fujigaya, M. Okamoto, K. Matsumoto, K. Kaneko and
N. Nakashima, ChemCatChem, 2013, 5, 1701.
22 N. B. McKeown, B. Gahnem, K. J. Msayib, P. M. Budd,
C. E. Tattershall, K. Mahmood, S. Tan, D. Book,
H. W. Langmi and A. Walton, Angew. Chem., Int. Ed., 2006,
45, 1804.
23 C. G. Bezzu, M. Carta, A. Tonkins, J. C. Jansen, P. Bernardo,
F. Bazzarelli and N. B. McKeown, Adv. Mater., 2012, 24, 5930.
24 S. V. Adymkanov, Y. P. Yampolskii, A. M. Polyakov,
P. M. Budd, K. J. Reynolds, N. B. McKeown and
K. J. Msayib, Polym. Sci., 2008, 50, 444.
Acknowledgements
25 P. M. Budd, N. B. McKeown, B. S. Ghanem, K. J. Msayib,
D. Fritsch, L. Starannikova, N. Belov, O. Sanrova,
Y. P. Yampolskii and V. Shantarovich, J. Membr. Sci., 2008,
325, 851.
D. H. thanks the Royal Society for a Newton International Fellow.
F. M. and N. M. thank the Leverhulme Trust for support (RPG-
2014-308). F. M. thanks Dr Robert Potter from Johnson-Matthey
for support with catalyst materials and for helpful discussion.
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26 E. Madrid, Y. Rong, M. Carta, N. B. McKeown, R. M. Evans, 31 D. P. He, Y. Y. Rong, Z. K. Kou, S. C. Mu, T. Peng, R. Malpass
G. A. Attard, T. J. Clarke, S. H. Taylor, Y. Long and
F. Marken, Angew. Chem., 2014, 40, 10927.
Evans, M. Carta, N. B. McKeown and F. Marken, Electrochem.
Commun., 2015, 59, 72.
27 E. Madrid, P. Cottis, Y. Rong, A. T. Rogers, J. M. Stone, 32 C. E. Hotchen, G. A. Attard, S. D. Bull and F. Marken,
R. Malpass-Evans, M. Carta, N. B. McKeown and
Electrochim. Acta, 2014, 137, 484.
F. Marken, J. Mater. Chem. A, 2015, 3, 15849.
33 S. Wasmus and A. Kuver, J. Electroanal. Chem., 1999, 461, 14.
28 F. J. Xia, M. Pan, S. C. Mu, R. Malpass-Evans, M. Carta, 34 Z. Y. Zhou, Z. Z. Huang, D. J. Chen, Q. Wang, N. Tian and
N. B. McKeown, G. A. Attard, A. Brew, D. J. Morgan and
S. G. Sun, Angew. Chem., Int. Ed., 2010, 49, 411.
F. Marken, Electrochim. Acta, 2014, 46, 26.
35 X. W. Yu and P. G. Pickup, J. Power Sources, 2008, 182, 124.
29 Y. Y. Rong, R. M. Evans, M. Carta, N. B. McKeown, 36 M. Carta, R. Malpass-Evans, M. Croad, Y. Rogan, J. C. Jansen,
G. A. Attard and F. Marken, Electroanalysis, 2014, 26, 904.
30 Y. Y. Rong, R. M. Evans, M. Carta, N. B. McKeown, G. A. Attard
and F. Marken, Electrochem. Commun., 2014, 46, 26.
P. Bernardo, F. Bazzarelli and N. B. McKeown, Science, 2013,
339, 303.
This journal is © The Royal Society of Chemistry 2016
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