An efficient Ag-ionomer interface for hydroxide exchange membrane fuel cells

Article abstract of DOI:10.1039/c2cc34862d

An efficient Ag-phosphonium ionomer interface is discovered in HEMFCs, helping enhance oxygen reduction and improve mass transports simultaneously. As a result, a completely-precious-metal-free HEMFC has been fabricated, which shows a cost-normalized power much higher than that of a Pt-based PEMFC benchmark (117 vs. 7.7 W US^-1. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc34862d

Chemical Communications
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Volume 49 | Number 2 | 7 January 2013 | Pages 101–200
ISSN 1359-7345
COMMUNICATION
Yushan Yan et al.
An efficient Ag–ionomer interface for hydroxide exchange membrane fuel cells

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An efficient Ag–ionomer interface for hydroxide
exchange membrane fuel cells†
Shuang Gu,ab Wenchao Sheng,ab Rui Cai,a Shaun M. Alia,ab Shuqin Song,c
Kurt O. Jensenab and Yushan Yan*ab
Cite this: Chem. Commun.,
2013, 49, 131
Received 7th July 2012,
Accepted 11th September 2012
DOI: 10.1039/c2cc34862d
www.rsc.org/chemcomm
An efficient Ag–phosphonium ionomer interface is discovered in For example, higher ORR activity (e.g., 2.4 times higher exchange
HEMFCs, helping enhance oxygen reduction and improve mass current density) can be obtained at the Ag–HEM interface than at
transports simultaneously. As a result, a completely-precious-metal- its Ag–NaOH counterpart,14 being attributed to the reduced oxygen/
free HEMFC has been fabricated, which shows a cost-normalized hydroxide adsorption on the Ag surface at Ag–HEM. By contrast,
power much higher than that of a Pt-based PEMFC benchmark Pt–HEM and Pt–KOH interfaces were shown to behave roughly
(117 vs. 7.7 W US$À1).
the same.15 Ag–ammonium ionomer interfaces have been most
commonly utilized in HEMFCs but the cell performances they
Hydrogen polymer proton exchange membrane fuel cells delivered are limited (8.1–50 mW cmÀ2 of peak power density and
(PEMFCs) have been demonstrated to have high efficiency 32–160 mA cmÀ2 of maximum current density10–13). Here we show
and high power density; however their heavy dependence on an efficient Ag–phosphonium ionomer interface that drastically
precious metals (typically Pt) as cathode catalysts makes them enhances the ORR specific activity (7.3 times that of the RDE test)
possibly uneconomical.1 By contrast, simple non-precious and mass transports for the Ag electrode simultaneously. As a
metals have long been known to have good activity and result, a high-performance completely-precious-metal-free HEMFC
durability in liquid alkaline fuel cells (AFCs).2,3 For instance, has also been successfully fabricated, promising towards affordable
Ag (ca. 1% cost of Pt) has shown a reasonably high oxygen HEMFC technologies.
reduction reaction (ORR) activity with ca. 10% exchange current
A phosphonium based ionomer (TPQPOH) was recently
density of Pt, based on the ex situ rotating disk electrode (RDE) developed to improve Pt electrode performance in HEMFCs.16
measurements4 and high durability (even better than Pt).5,6 When the TPQPOH ionomer is simply applied to the Ag catalyst
Unfortunately, AFCs suffer from complex handling of the layer of an HEMFC, the Ag–phosphonium ionomer interface
corrosive liquid electrolyte and severe electrode degradation brings a surprisingly high cell performance (Fig. 1): 208 mW cmÀ2
due to carbonate precipitation.7,8 By combining the favourable of peak power density and 770 mA cmÀ2 of maximum current
features of AFCs and PEMFCs, i.e., non-precious metal catalysts density (0.5 mgAg cmÀ2, 80 1C and 250 kPa back pressure), which is
of AFCs and solid polymer electrolyte of PEMFCs, polymer more than double those of the Ag–ammonium ionomer interface
hydroxide (OHÀ) exchange membrane (HEM) fuel cells counterpart under the same test conditions (95 mW cmÀ2 and
(HEMFCs) have the potential to provide affordable high-performance 360 mA cmÀ2, Fig. S1, ESI†). Also they are remarkably higher than
fuel cell technologies.9 As expected, Ag has been demonstrated those of Ag–ammonium interfaces reported albeit different test
to be a feasible cathode catalyst for HEMFC.10–13 It has also conditions used: [8.1–30.1 (0.5–2.0 mgAg cmÀ2, 60 1C, and ambient
been shown that catalyst–electrolyte interfaces control the pressure)11, 19 (0.5 mgAg cmÀ2, 50 1C, and ambient pressure),12 48
electrochemical reactions and mass (ions and water) trans- (4.0 mgAg cmÀ2, 50 1C, and ambient pressure),10 and 50 mW cmÀ2
ports, significantly impacting the electrode performance. (1.0 mgAg cmÀ2, Ni-based anode, 60 1C, and 130 kPa back
pressure)13], and the corresponding maximum current densities
(32–130,11 120,12 and 160 mA cmÀ210). To the best of our knowledge,
a Department of Chemical and Environmental Engineering, University of California-
this is the highest Ag electrode-based cell performance reported,
and in fact it is even slightly higher than the Pt electrode-based
counterpart (196 mW cmÀ2 of peak power density and
560 mA cmÀ2 of maximum current density,16 under the same
test conditions) with the same metal loading, in spite of the
larger particle size for Ag than for Pt (30 nm vs. 5 nm). This
comparison between Ag and Pt cathode performance in
Riverside, Riverside, CA 92521, USA. E-mail: yanys@udel.edu; Fax: +1 302 8312582
b Department of Chemical and Biomolecular Engineering, Center for Catalytic Science
and Technology, University of Delaware, Newark, DE 19716, USA
c State Key Laboratory of Optoelectronic Materials and Technologies, School of
Physics and Engineering, Sun Yat-Sen University, Guangzhou, Guangdong Province
510275, China
† Electronic supplementary information (ESI) available: Experimental methods,
Fig. S1–S6, Table S1, and Scheme S1. See DOI: 10.1039/c2cc34862d
c
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obtained for Ag and Pt (2.83 and 2.77 mA cmÀ2, respectively)
while Ag’s ex situ (RDE) ORR specific activity is approximately
only 10% of Pt’s (0.39 and 4.07 mA cmÀ2, at the same tempera-
ture of 80 1C, Fig. S3 and S4, ESI†). Opposite to the reduction of
the ORR specific activity (by 32%) for Pt, Ag has a significant
increase of ORR specific activity (7.3 times) from RDE (ex situ)
to HEMFC (in situ), suggesting much more active Ag surfaces at
the Ag–phosphonium ionomer interface than at the Ag–KOH
one. Equally importantly, slower voltage-drop (starting at B600
vs. B300 mA cmÀ2 for Ag vs. Pt, Fig. S2A, ESI†) and lower
internal-resistance (0.21 vs. 0.38 O cm2 for Ag vs. Pt, Fig. S2B,
ESI†) are observed at the same time for the Ag cathode than for
the Pt one, both of which indicate much improved hydroxide
and water transport across the Ag–phosphonium interface over
the Pt–phosphonium one (instead of oxygen transport, water
transport had been identified to be mostly responsible for the
mass-transport cell voltage loss18). The most plausible explana-
tion for enhanced ORR activity and improved mass transport is
that the Ag–phosphonium interface is tighter and thus more
efficient than the Pt–phosphonium one. Beside improving the
mass transport directly, the tighter Ag–phosphonium interface
may either tune the Ag–O/Ag–OH binding energy or alter the
catalytic mechanism to enhance Ag’ ORR activity intrinsically.19
The tighter Ag–phosphonium interface may be a result of
stronger attractive forces between the Ag surface and the
phosphonium functional groups. Ag has the capability of
adsorbing oxygen-containing species, such as carbon dioxide,20
phosphates,21 and silicate,22 and the oxygen-containing species
adsorptions have been found to be much stronger on Ag than
on Pt.23 It is thus speculated that the nine oxygen atoms-
containing quaternary phosphonium functional group in the
TPQPOH ionomer may be more strongly attracted to the Ag
surface than Pt, leading to the tighter and more efficient
catalyst–ionomer interface. To verify this assumption, a small
model phosphonium molecule, benzyl tris(2,4,6-trimethoxy-
Fig. 1 Polarization curves (open symbols) and power density curves (solid
symbols) of the Ag–phosphonium cathode interface (
,
) and
Pt–phosphonium cathode interface (&, ’) incorporated HEMFCs. Anode for
both cases: Pt black (Premetek Co., 5 nm), Pt loading of 0.5 mg cmÀ2, and
TPQPOH ionomer with 20 wt% content. Cathode: metal loading of 0.5 mg cmÀ2
and TPQPOH ionomer with 20 wt% content for both Pt (Pt black, Premetek Co.,
5 nm) and Ag (Ag nanoparticle, Quantum Sphere Inc., 30 nm) cathodes. HEM:
FAA commercial membrane (Fuma-Tech, 70 mm thickness, and 17 mS cmÀ1
hydroxide conductivity at 20 1C). Test conditions: cell temperature of 80 1C,
humidifier temperatures of 70 1C and 80 1C for H2 and O2, respectively, flow rate
of 0.2 L minÀ1 and back pressure of 250 kPa for both H2 and O2.
Table 1 Electrochemical surface area and ORR specific activity of Ag and Pt
catalyst in HEMFC and RDE
ESAa/m2 gÀ1
SAb/mA cmÀ2
HEMFC
(in situ)
RDE
(ex situ)
HEMFCc
(in situ)
RDEd
(ex situ)
Catalyst
Ag
Pt
3.6
13
5.1
21
2.83
2.78
0.39
4.07
a
b
ESA, electrochemical surface area. SA, specific activity for the oxygen
c
reduction reaction. Obtained at 0.85 V internal resistance-free cell
voltage in the HEMFC test. Obtained at 0.85 V vs. RHE in the RDE test.
d
HEMFCs is particularly interesting in that Ag provides almost phenyl) quaternary phosphonium chloride (BTQPCl, Scheme
negligible performance compared with Pt in PEMFCs (e.g., peak S1, ESI†), was synthesized and added into the supporting
power density: 0.6 mW cmÀ2 for Ag vs. 300 mW cmÀ2 for Pt; and electrolyte to investigate its adsorption on both Ag and Pt bulk
maximum current density: 20 mA cmÀ2 for Ag vs. 1200 mA metal surfaces in ORR RDE tests (Fig. S5, ESI†). At 0.85 V vs.
cmÀ2 for Pt).10
RHE, the ORR specific activity drops by 66% and 40% for the Ag
Consistent with the Ag’s larger particle size, the Ag catalyst and Pt electrode, respectively, when 1 mM BTQPCl was in the
has smaller electrochemical surface areas (ESA) than Pt in both 0.1 M KOH supporting electrolyte, and the ORR specific activity
ex situ (RDE) and in situ (HEMFC) cases (Table 1). Ag has a decreases by 87% and 73% for Ag and Pt, respectively, when
higher ESA ratio of the in situ to the ex situ measurements than 10 mM BTQPCl was added. Taking into account the strong
Pt, but overall their numbers are close (71% and 62% for Ag chloride adsorption on Pt which penalizes Pt’s ORR activity, the
and Pt, respectively), suggesting that they have similar electrode RDE data clearly indicate that the phosphonium cation has
microstructure and catalyst utilization. On the other hand, in much stronger adsorption on Ag than on Pt. Note that while in
the high cell voltage region of an HEMFC, the hydrogen the RDE experiments, the small phosphonium molecules adsorb
oxidation reaction (HOR) over-potential is very small (e.g., at on the catalysts surface, and thus stronger adsorption leads to
0.85 V cell voltage, HOR has an over-potential that is one order more severe catalyst poisoning, the phosphonium groups in the
of magnitude smaller than ORR in an HEMFC17), and the mass ionomer inside the catalyst layer are restrained by the polymer
transport over-potential is also negligible. As such, both HOR chains and thus stronger attractive forces only bring them closer
and mass-transport cell voltage losses can be neglected, and the to the catalyst surface to create a tighter Ag–phosphonium
in situ (HEMFC) ORR specific activity of Ag and Pt can be interface, facilitating the electrochemical reaction.
estimated based on the internal resistance-free cell voltage
As the most active non-precious metal for HOR, nickel has
curves (Fig. S2A, ESI†) and the in situ ESA measured (Table 1). been successfully used as a hydrogen electrode catalyst for
Surprisingly, very close in situ ORR specific activities are AFCs, electrolyzers,2,8 and recently HEMFCs.13 With the Ag
c
132 Chem. Commun., 2013, 49, 131--133
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experiment, helping enhance ORR activity and improve mass
transport simultaneously. Based on such an interface, a high-
performance completely-precious-metal-free HEMFC has been
successfully fabricated and shown to have a cost-normalized
power much higher than a Pt based PEMFC benchmark, a
promising sign to affordable HEMFC technologies. Our find-
ings also reveal the fundamental impact of the polymer electro-
lyte on the catalyst activity, possibly by means of either tuning
the binding energies of metal-oxygen and metal-hydroxyl, or
altering the ORR mechanism completely.
This work is supported by the ARPA-E of the US Department
of Energy.
Notes and references
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In summary, an efficient Ag–phosphonium ionomer interface
is discovered in HEMFCs, confirmed by an ex situ absorption
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 131--133 133