Haile, Sossina M.

Pintauro, Peter N.

English

Caltech Authors - Main

EDITORIAL www.rsc.org/materials | Journal of Materials Chemistry
D
ow
nl
oa
de
d 
on
 0
9 
A
ug
us
t 2
01
0
Pu
bl
ish
ed
 o
n 
06
 Ju
ly
 2
01
0 
on
 h
ttp
://
pu
bs
.rs
c.o
rg
 | d
oi:
10.
103
9/C
0JM
900
35D
View OnlineProton transport for fuel cells
DOI: 10.1039/c0jm90035dInternational efforts in fuel cell materials
research have been largely driven by the
need to develop benign energy technolo-
gies. As is widely recognized, fuel cells
have the potential to convert chemical
energy into electrical energy at excep-
tionally high efficiencies and with almost
undetectable emissions of regulated
pollutants. As electrolytes for fuel cells,
proton conducting materials offer an
important advantage over their counter-
parts based on mobile anions (i.e., OH,
O2, CO3
2) for operation on hydrogen;
because H2O is produced at the cathode
rather than the anode, hydrogen fuel is
not diluted during operation as occurs in
other fuel cell types. However, while fuel
cells based on proton conducting
membranes are well-suited to hydrogen
fuel, they are by no means limited to this
particular energy carrier. Thus, research
in proton conductors for fuel cells
remains vibrant, despite a thoughtful re-
evaluation of the global commitment to
a hydrogen energy economy.
Beyond the technological significance
of proton conducting materials, the very
nature of a proton – a bare atomic nucleus
– renders it unique amongst ionic species.
It is mobile in a broad range of materials,
from acid-functionalized polymers, to
hydrated oxides, to oxyanion acid salts
containing structural protons. In the solid
state it typically resides within the elec-
tron cloud of the anions to which is it
bound (normally oxygen in proton
conductors) and its structural and
dynamical characteristics can be probed
by a host of techniques from nuclear
magnetic resonance spectroscopy and
neutron scattering methods, to isotope
effect measurements and computational
studies. In parallel with advanced char-
acterization, synthesis of proton con-
ducting materials, both new and known,
has become an increasingly sophisticated
endeavor. This is particularly true of
polymeric systems, in which research has
evolved from a hit and miss, brute-force
approach of synthesizing and then char-
acterizing different acid-functionalized
polymers to more sophisticated strategies
that focus onmorphologcialmanipulationThis journal is ª The Royal Society of Chemistryand control, especially at the nano-
scale. As the synthesis of pure proton
conductors matures, efforts are being
directed towards the development of
materials with multiple mobile carriers
(protons plus ‘‘x’’) to create new func-
tionalities. The papers in this themed issue
of Journal of Materials Chemistry exem-
plify nicely these recent trends and present
key advances in fuel cell materials
research. Thus, the results herein provide
the reader with an excellent ‘‘snapshot’’ of
state-of-the-art research within the
protonics community.
Amongst the polymer papers, three in
this issue deal with new membrane
morphologies for moderate temperature
fuel cells (as candidates with enhanced
properties to replace DuPont’s Nafion
perfluorosolfonic acid membranes).
Layer-by-layer (LbL) assembly methods
offer many possibilities for tuning the
structure, composition, and thickness of
next-generation proton conducting fuel
cell membranes. Thus, Hammond and co-
workers at MIT show that one can alter
the ionic crosslink density and properties
of LbL membranes composed of alter-
nating nm-thick layers of poly(diallyl
dimethyl ammonium chloride) and
sulfonated poly(2,6,-dimethyl 1,4-phe-
nylene oxide) by adjusting the ionic
strength of the assembly solution. The
highest ionic conductivity (7.0  102 S
cm1 at 25 C) reported to date for any
LbL membrane was achieved by opti-
mizing the membrane fabrication condi-
tions. The use of block copolymers for
fuel cell membranes has long been an
attractive idea, where tunable phase-
separated polymer morphologies are
generated to promote proton conduc-
tivity while limiting water uptake.
Hickner’s group at Penn State presents
their recent results on such a system,
where triblock copolymer films are
examined. The effects of mid-block
sulfonation degree and hydrophobic
endblock composition on the block
copolymer domain size, the degree of self-
assembly, and the proton conductivity
and water swelling properties of solvent
cast membranes is quantified. Some2010 Jtriblock compositions generated prom-
ising preliminary results, with a
membrane conductivity exceeding that of
Nafion at 30 C for relative humidities
greater than 65%. An alternative to self-
assembled block copolymer systems is
presented by Pintauro et al. in their paper
on nanofiber composite proton conduct-
ing membranes. In this study, a 3-D in-
terconnecting network of electrospun
proton-conducting perfluorosulfonic acid
polymer nanofibers is created and then the
interfiber voids are filled with an inert/
impermeable (uncharged) polymer to
provide mechanical strength, gas barrier
properties, and water swelling control. In
such a forced assembly system, the role of
the mechanical support polymer is de-
coupled from that of the proton con-
ducting material. Membranes made with
3M Corporation ionomer nanofibers and
Norland Optical Adhesive as the inert
polymer matrix material exhibited
a higher proton conductivity than that of
commercial Nafion at 80 C and relative
humidities between 50% and 80%, with
low water swelling and improved
mechanical properties.
Two papers in this themed issue deal
with new polymeric proton conducting
materials for anhydrous fuel cell opera-
tion above room temperature. Matsuda
et al., fabricated and tested a series of
polybenzimidazole (PBI) films that were
loaded with heteropoly acid (HPA), as
a potential replacement for acid-doped
PBI. Inorganic–organic composite
membranes, containing either phospho-
tungstic acid or silicotungstic acid, were
prepared and evaluated for different HPA
loadings. Optimized films exhibited
a high proton conductivity (1.91  102 S
cm1 at 160 C under anhydrous condi-
tions) and performed well in hydrogen/air
fuel cell tests. Roy and co-workers in their
paper pursued a different strategy, where
a liquid crystalline phenylsulfonic acid
compound was examined as a possible
membrane material that conducts
protons under non-humidified condi-
tions. In its smectic A liquid crystal state,
the proton conductivity of 4-(octadecy-
loxy)phenylsulfonic acid at 80 C in dry. Mater. Chem., 2010, 20, 6211–6213 | 6211
D
ow
nl
oa
de
d 
on
 0
9 
A
ug
us
t 2
01
0
Pu
bl
ish
ed
 o
n 
06
 Ju
ly
 2
01
0 
on
 h
ttp
://
pu
bs
.rs
c.o
rg
 | d
oi:
10.
103
9/C
0JM
900
35D
View Onlinenitrogen was very high (1.0  102 S
cm1), as compared to a conductivity of
1.5 107 S cm1 for the same material in
a solid powder state. X-Ray experiments
andmolecular modeling studies suggested
that proton conducting liquid crystals
posses a lamellar structure composed of
bilayer stacks of phenylsulfonic acid. The
authors would like to use this compound
for high temperature anhydrous fuel cell
operation, but first they must find
a method of stabilizing the morphology
above the liquid crystal temperature
range.
Direct methanol fuel cells (DMFCs)
are a viable alternative to fuel cells
powered by hydrogen, especially for
portable power applications. The
membrane in such devices requires
a unique and seemingly contradictory
set of properties: high proton conduc-
tivity (which requires highly hydrophilic
polymer domains) and low methanol
crossover (which is best achieved if the
membrane hydrophilicity is minimized).
The two papers in this themed issue that
deal with DMFC membranes take
a somewhat similar approach: add
a sufficient number of protogenic groups
to the polymer for high conductivity and
then crosslink the polymer to minimize
swelling in aqueous methanol solutions
(where reduced swelling translates into
lower methanol permeability). Thus,
Wang et al. in their paper describe the
synthesis and properties of an amino-
terminated copolymer, with varying
amounts of perfluorosulfonic acid
moieties, that was crosslinked with
a triisocyanate agent. By proper adjust-
ment of polymer ion-exchange capacity,
crosslinked membranes with exception-
ally low methanol permeability and
reasonable proton conductivity were
fabricated. In the second DMFC paper,
Na and co-workers report on a new
class of hybrid membranes composed of
crosslinked sulfonated poly(ether ether
ketone), with an ion-exchange capacity
in the range of 1.18–1.55 mmol g1, that
were loaded with 10–40 wt% heteropoly
acid (phosphotungstic acid). The
approach yields materials with an
attractive combination of sufficient
conductivity, unusually low swelling
(less than 20% at 80 C), good
mechanical properties, and methanol
permeability was is order of magnitude
lower than that in commercial Nafion.6212 | J. Mater. Chem., 2010, 20, 6211–6213The last polymeric proton conductor
paper to discuss for this themed issue is
the molecular modeling study by Paddi-
son et al. Over the years, molecular
simulations have provided valuable
insights into how acid-functionalized
polymers interact with water and how
proton conducting membranes function
at high and low water contents, especially
with regards to perfluorosulfonic acid
polymers membranes. In their present
paper, Paddison and co-workers have
chosen to model water and protons in
a single-walled carbon nanotube func-
tionalized with –CF2SO3H groups, as an
ideal hydrophilic channel in a minimally
hydrated (hypothetical) perfluorosulfonic
acid membrane. Ab initio molecular
dynamic simulations revealed that disso-
ciation of the acidic proton increased with
increasing density of sulfonic acid groups
up to a limit (after which there was proton
trapping) and the presence of fluorine
atoms on the nanotube wall affected the
hydrogen bonding network of water,
where the sulfonic acid groups had less
tightly bound water and an enhanced
proton dissociation.
Amongst proton conducting oxides,
perhaps trivalent doped barium zirconate
and derivatives thereof have received the
greatest attention in recent years due to
their combined characteristics of high
inherent (bulk) proton conductivity,
chemical stability, and mechanical
robustness. The papers presented here
tackle the difficult problems of eluci-
dating transport mechanisms as they
relate to defect chemistry and dynamics
and of high grain boundary impedance.
In a computational study, Islam and co-
workers predict proton trapping in triva-
lent doped BaPrO3 and BaZrO3. In the
series M3+ ¼ Sc, In, Yb, Y and Gd, trap-
ping was most severe for Sc, consistent
with experimental observations that
Sc-doped BaZrO3 has low proton
conductivity. They furthermore find that
trapping is more pronounced for BaPrO3
than BaZrO3. A tantalizing prediction
from this study is that Gd-doped barium
zirconate, which has received little atten-
tion, should have higher conductivity
than the widely studiedY-dopedmaterial.
In a comprehensive NMR study of Sc-
doped BaZrO3, Grey and co-workers
provide a somewhat different picture of
defect distributions than the computa-
tional predictions. These authors findThis journthat, prior to hydration, Sc and O are
randomly distributed so long as the Sc
content is sufficiently below its solubility
limit. Upon hydration, they observe, at
most, a slight tendency of the protonic
defect to order nearby a scandium ion. As
an alternative to trapping effects, these
authors suggest that the O–H dipole re-
orientation barrier may be inherently high
for Sc doped barium zirconate due to
strong hydrogen bonding, giving rise to
the observed low conductivity.
Yugami and co-workers have explored
the origins of high grain boundary
impedance in Y and Gd doped barium
zirconate. Transmission electron micros-
copy reveals the grain boundaries be free
of impurity phases, but slightly enhanced
in dopant concentration relative to the
bulk. Furthermore, the grain boundary
impedance was observed to decrease with
increasing measurement voltage (whereas
the bulk properties remained constant).
These results provide evidence (though
not conclusive) that the high impedance is
due to space charge effects. The grain
boundary barrier height is observed to
decrease with increasing yttrium concen-
tration and upon replacing Y with Gd,
suggesting strategies for enhancing
transport across grain boundaries. The
poor transport properties across the grain
boundaries of doped barium zirconate
has been a recognized obstacle for several
years, a problem exacerbated by inher-
ently poor grain growth leading to a high
number density of grain boundaries. It
the past few years it has become well
recognized that sintering aids such as
ZnO, NiO and CuO enhance grain
boundarymobilities, thereby lowering the
temperatures required for good densifi-
cation and enhancing grain growth. Tong
et al. provide a careful study of the
mechanism by which NiO in particular
aids in the processing of yttrium doped
barium zirconate. Following a reactive
sintering procedure, they find that the
intermediate, transient compound,
BaY2NiO5, plays a major role in densifi-
cation and grain growth.
While proton transport in doped per-
osvkites is generally considered attractive
at temperatures in the range of 400 to 600
C (below 400 C the mobility is low and
above 600 Cdehydration occurs), several
‘hard’ materials display high conductivity
at lower temperatures, competing, in
some sense, with the high temperatureal is ª The Royal Society of Chemistry 2010
Peter N. Pintauro
Department of Chemical and
Biomolecular Engineering, Vanderbilt
University, USA
Sossina M. Haile
Material Science and Chemical
Engineering, California Institute of
Technology, USA
D
ow
nl
oa
de
d 
on
 0
9 
A
ug
us
t 2
01
0
Pu
bl
ish
ed
 o
n 
06
 Ju
ly
 2
01
0 
on
 h
ttp
://
pu
bs
.rs
c.o
rg
 | d
oi:
10.
103
9/C
0JM
900
35D
View Onlinepolymers. In a comprehensive feature
article, Sinistyn provides a review of the
pressure-temperature phase behavior in
MeHAO4 compounds (Me ¼ Cs, Rb,
NH4 and A ¼ S, Se, and all combinations
except NH4HSeO4 are explicitly dis-
cussed). While each material has, of
course, distinct phase behavior, Sinistyn
creates an integrated approximate picture
in which pressure and ion size are equiv-
alent and each of the compounds displays
so-called superprotonic conductivity (in
a phase with orientationally disordered
oxyanion groups) under the appropriate
thermodynamic conditions. Chemically
related to these solid acids are metal
pyrophosphates, MP2O7 materials.
Proton incorporation is attributed to
a hydration reaction, similar that of the
oxides, but with due recognition of a high
electronic hole concentration. Hibino
reports a study of new compositions in
this class, Sn1xAlxP2O7, as well as re-
viewing the technological developments
in fuel cell preparation and testing.
An entirely different approach is
adopted by Kim, who has induced
ambient temperature proton conduction
in yttria-stabilized zirconia through
nanostructuring. At temperatures below
180 C, conductivity monotonically
increases with decreasing grain size (in the
range 17 to 100 nm) and with increasing
water partial pressure, consistent with
high proton transport along grain
boundaries. Bridging the polymeric and
non-polymeric materials is the work of
Yamaguchi, who has prepared nano-
particulate zirconium phosphate, zirco-
nium sulfophenyl-phosphonate, and
zirconium sulfate materials as precursor
components for polymer–inorganic
hybrids. The mild synthesis conditions set
the stage for in situ conversion of zirco-
nium oxide to the material of interest in
the presence of the polymer of interest,
while the reported conductivities even in
the absence of any polymer, are already
attractive.This journal is ª The Royal Society of ChemistryIn any operational fuel cell, the proton
conducting electrolyte is accompanied by
electrodes, which in some regards must
meet a higher set of functional require-
ments than themembrane. It is commonly
recognized that electrode materials that
transport both ionic and electronic
species can broaden the physical zone for
the electrochemical reactions, thereby
enhancing activity. Tricoli describes
a sol–gel route for synthesizing high
surface area WO3 and controlling the
relative magnitudes of protonic and elec-
tronic conductivities. Specific surface
areas of 100 m2 g1 and pore diameters
of 5–10 nm are readily achieved, attrac-
tive values for fuel cell electrodes. Haile
and co-workers present a very different
synthesis method for the preparation of
multi-component electrodes for fuel cells
based on CsH2PO4. The electrospray
approach is demonstrated to yield high
surface area electrodes and reproducible
electrochemical characteristics, and
enables a 30-fold lowering in Pt loading
relative to mechanically milled electrodes
without loss in activity. For higher
temperature fuel cells, Meng reviews in
a feature article the state of the art in
mechanistic understand of cathode
processes and materials development.
The authors argue that composite elec-
trodes formed of high-activity mixed
oxide-ion and electronic conductors with
proton conductors lead to high perfor-
mance; the first component provides the
catalytic activity for oxygen dissociation
while its high mobility brings the reactive
species to the triple phase boundary at
which the reaction occurs. O’Hayre
reports a comprehensive model for treat-
ing multi-component transport in mate-
rials with simultaneous electronic (either
hole or electron), oxide vacancy, and
proton transport, in contrast to
a composite system. A formalism for as-
sessing transport behavior in terms of
a ternary plot (analogous to a ternary
phase diagram) is presented. In addition,2010 Jnew experimental data show that simul-
taneous species transport indeed occurs in
yttrium doped barium zirconate, result-
ing, under some conditions, in steam
permeation through ceramic membranes.
This breadth of activities demonstrates
that extensive opportunities remain for
bringing the promise of protonics for
a clean and sustainable energy future to
realization.. Mater. Chem., 2010, 20, 6211–6213 | 6213


Proton transport for fuel cells

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Crossref