Solid state protonic conductor NH4PO3-(NH4)2Mn(PO3)4 for intermediate temperature fuel cells

Article abstract of DOI:10.1016/j.electacta.2006.04.040

A new proton-conductive composite of NH₄PO₃-(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm^-1 was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²-10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃-(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃-(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt-C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃-(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt-C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.

Full text of DOI:10.1016/j.electacta.2006.04.040

Electrochimica Acta 51 (2006) 6542–6547
Solid state protonic conductor NH4PO3–(NH4)2Mn(PO3)4 for
intermediate temperature fuel cells
Xilin Chena, Xu Lib, Shuai Jianga, Changrong Xiaa,∗, Ulrich Stimmingc
a Laboratory for Biomass Clean Energy, Department of Materials Science and Engineering, University of Science and
Technology of China, Hefei, 230026 Anhui, China
b Department of Physics, University of Science and Technology of China, Hefei, 230026 Anhui, China
c Department of Physics E19, Technical University of Munich, James-Franck-Strasse 1, 85748 Garching, Germany
Received 11 January 2006; received in revised form 20 April 2006; accepted 22 April 2006
Available online 15 June 2006
Abstract
A new proton-conductive composite of NH4PO3–(NH4)2Mn(PO3)4 was synthesized and characterized as a potential electrolyte for intermediate
temperature fuel cells that operated around 250 ◦C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH4)2Mn(PO3)4
was stable as a supporting matrix for NH4PO3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the
molar ratio of NH4PO3 in both dry and wet atmospheres. A conductivity of 7 mS cm−1 was obtained at 250 ◦C in wet hydrogen. Electromotive forces
measuredbyhydrogenconcentrationcellsshowedthatthecompositewasnearlyapureprotonicconductorwithhydrogenpartialpressureintherange
of 102–105 Pa. The proton transference number was determined to be 0.95 at 250 ◦C for 2NH4PO3–(NH4)2Mn(PO3)4 electrolyte. Fuel cells using
2NH4PO3–(NH4)2Mn(PO3)4 as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm2 was
achieved at 250 ◦C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH4PO3–(NH4)2Mn(PO3)4 electrolyte
is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel
cells.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Composite electrolyte; Intermediate-temperature fuel cells; Protonic conductor; Concentration cell; NH4PO3–(NH4)2Mn(PO3)4
1. Introduction
ities at 250 ◦C under dry and wet atmospheres. In these compos-
ites, APP provides high ionic conductivity, and (NH4)2SiP4O13
and TiP2O7 are used as supporting matrix [6]. The high ionic
conductivity originates in the thermal decomposition of APP
to HPO3 at around 250 ◦C. While the decomposition is nec-
essary to increase APP conductivity to an appreciably level, it
also lowers APP’s melting point and thus softens the electrolyte.
Therefore, APP is usually made into composite to enhance its
mechanical strength. (NH4)2SiP4O13 [7] and TiP2O7 [8], which
have much higher melting points than APP and chemically
compatible with APP, have been recently used as supporting
matrixes.
(NH4)2Mn(PO3)4 is an analogue of APP and stable below
400 ◦C [9]. It may be a good supporting matrix. In this
paper, we report the preparation, proton-transport properties of
APP–(NH4)2Mn(PO3)4 composites, andthecontributionofsup-
porting matrix to conduction at 130–250 ◦C. Further, fuel cells
that incorporate a APP–(NH4)2Mn(PO3)4 composite as elec-
trolytes are evaluated.
While some oxide materials are protonic conductors at tem-
peratures as high as 600 ◦C [1,2], most protonic conductors have
an operating temperature below 150 ◦C. A solid protonic con-
ductor that has a high conductivity at 150–500 ◦C would be most
useful as electrolyte for applications in intermediate temperature
fuel cells [3]. However, only a few studies have been conducted
on electrolytes for use at 150–500 ◦C [4–6] and protonic con-
ductors that operate at this temperature range have not been
developed yet.
Recently, ammonium polyphosphate (APP, NH4PO3) has
been studied as a proton conductive electrolyte [6–8]. Ammo-
nium polyphosphate composites, APP–(NH4)2SiP4O13 [7] and
APP–TiP2O7 [8], were reported to exhibit very high conductiv-
∗
Corresponding author. Tel.: +86 551 3607475; fax: +86 551 3606689.
E-mail address: xiacr@ustc.edu.cn (C. Xia).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.04.040

X. Chen et al. / Electrochimica Acta 51 (2006) 6542–6547
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2. Experimental
(NH4)2Mn(PO3)4 was synthesized using commercial ammo-
nium polyphosphate (APP, NH4PO3) and manganese dioxide
(MnO2) as raw materials. The APP had form I structure as
identified with X-ray diffraction (XRD) pattern [10]. MnO2
was prepared by oxidizing MnCl2 with KMnO4 in solutions,
resulting in a black precipitate. Fine MnO2 powders were
obtained by filtering the precipitate and drying in vacuum. The
MnO2 was then mixed with APP in the molar ratio required to
form (NH4)2Mn(PO3)4. The mixed powders were subsequently
heated at 400 ◦C for 10 h in NH3. The resulting products were
ground into powders.
Composite electrolytes were prepared by mixing
(NH4)2Mn(PO3)4 and APP in various molar ratios as
xAPP–(NH4)2Mn(PO3)4, where x = 2, 4, and 6. The mixtures
were milled, pressed at 250 MPa into pellets and fired at 400 ◦C
for 10 h in NH3. The pellets were 15 mm in diameter and
0.9–2 mm in thickness.
Both the anode and cathode contained two overlapped con-
ductive porous layers: a diffusion layer formed from Toray TGP
090 carbon paper having a thickness of 0.3 mm, a porosity of
76%and wet-proofed withpolytetrafluoro-ethylene(PTFE), and
a catalyst layer formed from APP, Pt–C catalyst (Dalian sunrise
power Co. Ltd) and PTFE. The catalyst ink was prepared by mix-
ing APP, distilled water, 50% Pt–C, glycerol and a 60 wt% PTFE
dispersion (Teflon-30 dispersion-DuPont). The ink was spread
onto the carbon paper, dried at 200 ◦C and sintered at 370 ◦C
for 30 min. Both the PTFE and the APP contents in the catalyst
layer were 23 wt.% and the platinum loading was 0.8 mg/cm2.
The composite pellet was sandwiched by the two electrodes to
form a single cell.
The crystalline structures of composite electrolytes were
examined by XRD patterns (Philips X’pert PROS diffractome-
ter). Thermo gravimetric analysis measurements (TGA) were
performedwithTA-50(SHIMADZU)thermalanalyzerataheat-
ing rate 10 ◦C/min under nitrogen flow of 100 ml/min. Conduc-
tivity measurements were carried out by means of ac impedance
spectroscopy with Zahner IM6e electrochemical workstation
in the frequency range of 1–106 Hz. Single cells of gas (H2),
Pt–C|APP–(NH4)2Mn(PO3)4|Pt–C, gas(H2/ArorO2)werefab-
ricated in order to measure proton transference number and fuel
cell performance. The thickness of the electrolyte was 0.9 mm
and the fuel/oxidant flow was about 50 ml/min.
Fig. 1. The mass loss of (NH4)2Mn(PO3)4 as a function of temperature in nitro-
gen.
subsequent weigh gain is about 0.2%. The weigh gain maybe
corresponds to the intrinsic errors of the apparatus.
Shown in Fig. 2 are XRD patterns for APP–(NH4)2Mn(PO3)4
composites. Fig. 2a shows that (NH4)2Mn(PO3)4 is well-
crystallized when the pattern is compared with that reported
in JCPDS 73-1060. Fig. 2e shows that a form V type of APP is
formed when APP I is heated at 400 ◦C in NH3 [10]. Fig. 2b–d
show that both patterns of APP V and (NH4)2Mn(PO3)4 are
present in the composite. The intensity of the peak assigned to
APP V increases with the increase of APP in the composite. Fur-
thermore, no obvious interaction reactions are observed between
the two components when these patterns are carefully examined.
This implies that (NH4)2Mn(PO3)4 is chemically compatible
with APP when the composites were fired at 400 ◦C in NH3.
The firing is necessary to increase the electrolyte density.
Shown in Fig. 3 are XRD patterns for (NH4)2Mn(PO3)4
and APP–(NH4)2Mn(PO3)4 composites fired at 250 ◦C for 24 h
in air. Comparing Fig. 3a with Fig. 2a it can be found that
3. Results and discussions
The thermal stability of the (NH4)2Mn(PO3)4 has been deter-
mined by means of TGA measurement. As shown in Fig. 1,
the weight decreases gradually around 300 ◦C. After an ini-
tial mass loss of 0.9%, the material is thermally stable below
300 ◦C. This result is consistent with the previous report that
(NH4)2Mn(PO3)4 is thermally stable up to 400 ◦C [9]. The mass
loss corresponds most probably to a loss of H2O which was
absorbed from air into (NH4)2Mn(PO3)4 or/and NH3 which was
absorbed into (NH4)2Mn(PO3)4 when it was treated in NH3.
When increasing the temperature from 150 to 250 ◦C again, the
Fig. 2. XRD diffraction patterns for composites consisting of APP and
(NH4)2Mn(PO3)4. The molar ratio of APP:(NH4)2Mn(PO3)4 is (a) 0:1, pure
(NH4)2Mn(PO3)4, (b) 2:1, (c) 4:1, (d) 6:1, and (e) 1:0, pure APP.

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X. Chen et al. / Electrochimica Acta 51 (2006) 6542–6547
Fig.
3. XRD
diffraction
patterns
of
(NH4)2Mn(PO3)4
and
APP–(NH4)2Mn(PO3)4 composites, which were fired at 250 ◦C for 24 h
in air.
(NH4)2Mn(PO3)4 is stable in air. This agrees well with the TG
analysis. On the contrary, comparing Fig. 3b–d with Fig. 2b–d, it
reveals that structure evolution occurs when the composites are
fired in air. The structure change is due to the partial decompo-
sition of APP, resulting in an amorphous HPO3 which provides
high conductivity [8]. However, (NH4)2Mn(PO3)4 is quite sta-
ble, even when it is fired with APP. Consequently, the stability of
(NH4)2Mn(PO3)4 isnotaffectedbythepresenceofAPP. Inaddi-
tion, no obvious change in shape and size was observed with the
composite pellets after firing in air. Therefore, (NH4)2Mn(PO3)4
seems to be a suitable supporting matrix for APP in terms of
chemical stability and mechanical strength.
Fig. 4. Temperature dependence of conductivities for (NH4)2Mn(PO3)4 and
APP–(NH4)2Mn(PO3)4 composites. The measurements were conducted in dry
nitrogen (a) and in wet nitrogen (3% H2O) (b).
Shown in Fig. 4a and b are Arrhenius plots of conductiv-
ity for (NH4)2Mn(PO3)4 and its composites. The conductivity
was measured at temperature from 130 to 250 ◦C in dry and
wet N2 atmospheres, respectively. The measurements were con-
ducted after each sample was annealed at 250 ◦C for 24 h to
achieve a steady state. As shown in Fig. 4a, the conductivity
was improved by increasing the molar ratio of APP in the com-
posite at each given temperature. Further, the conductivity for
(NH4)2Mn(PO3)4 is more than one magnitude lower than that
for the composite. This indicates that APP is responsible for the
highconductivitywhile(NH4)2Mn(PO3)4 servesasasupporting
matrix. In dry N2, a maximum conductivity of 1.1 mS cm−1 was
obtained at 250 ◦C for the composite with the highest amount of
APP. Under the humidified condition, the conductivity increased
dramaticallycomparedtothatunderthedrycondition. Theeffect
of water is significant, and the conductivity measured in humid-
ified nitrogen is almost an order of magnitude higher than that in
a dry environment. A maximum conductivity of 7 mS cm−1 was
observed at 250 ◦C. The activation energies of conductivity for
the composites under dry and wet condition are listed in Table 1.
The activation energy in wet condition is more than 40% smaller
than that in dry condition.
The proton conduction in APP is believed to be very similar
to that in silica glass reported by Nogami and Abe [11]. The
possible route is a cooperative proton transfer along a linked
chain of hydrogen bonds. In dry environment, POH bonds form
Table 1
Activation energies of conductivity for APP–(NH4)2Mn(PO3)4 composites measured under different conditions
Atmosphere
Ea (kJ/mol)
2APP–(NH4)2Mn(PO3)4
4APP–(NH4)2Mn(PO3)4, N2
6APP–(NH4)2Mn(PO3)4, N2
40 0.6
H2
O2
N2
Dry
Wet (3% H2O)
22
9
1
1
33
17
3
2
30
18
3
3
38
21
2
3
22
2

X. Chen et al. / Electrochimica Acta 51 (2006) 6542–6547
6545
weaklyhydrogenbondingwiththeneighboringPOH. Therefore,
the electrical conductivity is associated with proton dissociation
from POH bonds under an electrical field and the subsequent
hopping of protons from the initial site to a neighboring site. But
inwetenvironment, thedissociatedprotonsfromPOHbondscan
move from an initial site to a neighboring site through molecu-
lar water which is absorbed into APP. Molecular water makes it
possible to decrease the distance for proton hopping, resulting
in accelerating cooperative transfer through water molecules.
Proton conduction in APP containing water molecules is based
on the dissociation of proton from the POH bond and the dis-
sociation equilibrium between the proton and water molecule.
The energy required to dissociate a proton from the OH bond
is reduced by the strong hydrogen bonding between the proton
and water molecule. The dissociated proton moves to a water
molecule forming an activated H2O:H+ state. Thus the activation
energy for proton conduction is reduced markedly when water
molecules participate in the proton conduction. The conductiv-
ity behaviors and activation energies of APP–(NH4)2Mn(PO3)4
composites provide evidence for this supposed proton conduc-
tion mechanism.
oxygen or nitrogen. The conductivity difference under dry and
wet hydrogen conditions is small, and decreases with tempera-
ture. At 250 ◦C, the conductivity in dry hydrogen nearly equals
to that in wet hydrogen. These phenomena might be originated
from the proton conduction since the conductivity of a protonic
conductor increases with hydrogen partial pressure [12]. There-
fore APP–(NH4)2Mn(PO3)4 seems to be a protonic conductor
since a protonic conductor usually shows higher conductivity in
hydrogen than those in oxygen and nitrogen, and the effect of
the water is not remarkable in hydrogen. The lower activation
energy in hydrogen is also an indication of protonic conduction.
The proton transference number can be determined by
measuring the open cell voltage, E, of a hydrogen concen-
tration cell PHI (105 Pa), Pt–C|2APP–(NH4)2Mn(PO3)4|Pt–C,
2
PHII (H2/Ar). Assuming that the electrolyte is a pure protonic
2
conductor, the Nernst potential of the concentration cell is
expressed as: E0 = (RT/2F)ln(PI /PHII ), where R is the gas
H2
2
constant, F the Faraday constant, and PHI and PHII the partial
2
2
+
pressures of the hydrogen. The transference number, tH , can
+
be calculated with measured E, tH = E/E0 [13]. As shown in
Fig. 6, the experimental values agreed well with the theoreti-
cal values. Therefore, 2APP–(NH4)2Mn(PO3)4 composite was
a protonic conductor and its proton transference number was
determined to be 0.95 at 250 ◦C.
Due to its lowest activation energy and highest mechanical
strength, 2APP–(NH4)2Mn(PO3)4 was used as an electrolyte for
the subsequent fuel cell performance evaluation.
Shown in Fig. 5 are Arrhenius plots of conductivity for
2APP–(NH4)2Mn(PO3)4 in various atmospheres. The activation
energies of conductivity for 2APP–(NH4)2Mn(PO3)4 are also
provided in Table 1. The conductivities in oxygen are similar to
those in nitrogen under both dry and wet conditions. Further, the
conductivity behavior in oxygen is similar to that in nitrogen:
higher conductivity and lower activation energy under wet oxy-
gen compared with those under dry conditions. However, the
conductivity behavior in hydrogen is noticeably different from
those in oxygen and nitrogen. Under wet and dry conductions,
the conductivities in hydrogen are always higher than those in
Fuel cell has been tested using 2APP–(NH4)2Mn(PO3)4 as
an electrolyte and Pt–C catalyst as an electrode. Cell voltage
and power density as a function of current density are shown in
Fig. 7. Amaximumpowerdensityof16.8 mW/cm2 wasobtained
at 250 ◦C. The time dependence of cell power density is shown
in Fig. 8. There was no obvious decrease in cell performance
for 10 h. The performance of this fuel cell is comparable to that
using an APP–TiP2O7 as electrolyte [8]. Although the fuel cell
performance is relatively low, much improved performance is
expected if the fuel cell components are further optimized.
Shown in Fig. 9 are TGA curves for the electrolyte com-
ponents, APP and (NH4)2Mn(PO3)4, and also for these com-
Fig. 5. Temperature dependence of conductivity for 2APP–(NH4)2Mn(PO3)4
measured in various atmospheres.
Fig. 6. Emfs of hydrogen concentration cell using 2APP–(NH4)2Mn(PO3)4 as
an electrolyte at 250 ◦C. The thickness of the electrolyte was 1.2 mm.

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X. Chen et al. / Electrochimica Acta 51 (2006) 6542–6547
Fig. 9. Thermal stability measured at 250 ◦C. (a) Pure (NH4)2Mn(PO3)4, (b)
pure APP, (c) (NH4)2Mn(PO3)4 mixed with 50 wt.% Pt–C, and (d) APP mixed
with 50 wt.% Pt–C. The Pt–C weight was taken out when the relative mass loss
was calculated.
Fig. 7. Cell voltage and power density as a function of current density with dry
hydrogen (105 Pa, 50 ml/min) as a fuel and dry oxygen (105 Pa, 50 ml/min) as
an oxidant. 2APP–(NH4)2Mn(PO3)4 was used as electrolyte and the thickness
was 1.0 mm.
the electrolyte and Pt–C electrode should cause high interfacial
polarization resistances, which eventually result in a poor fuel
cell performance as shown in Fig. 7. This might also be the rea-
son that the open circuit voltages were relative low. Therefore it
is critical to develop new electrode materials for the intermediate
temperature fuel cells.
ponents mixed with the electrode materials, Pt–C. As shown
in Fig. 9a, (NH4)2Mn(PO3)4 is pretty stable at 250 ◦C. How-
ever, when (NH4)2Mn(PO3)4 was mixed with Pt–C, the relative
mass loss increased to 6.0% (Fig. 9c). The 6.0% loss is close
to the theoretical mass loss, 8.4%, when (NH4)2Mn(PO3)4 is
fully decomposed. So, Pt–C is not thermally compatible with
(NH4)2Mn(PO3)4. As shown in Fig. 9b, meanwhile, APP is not
stable at 250 ◦C. The mass loss is due to its partial decomposi-
tion, which is consistent with the literature [8]. Fig. 9d shows an
accelerated decomposition rate for APP when it is mixed with
Pt–C. The mass loss within 5 h was about 16.0%, close to the
theoretical loss, 17.5%, when APP is fully decomposed. There-
fore, the electrolyte components, both (NH4)2Mn(PO3)4 and
APP, are not thermally compatible with Pt–C catalyst. Pt was
previously reported to catalyze the thermal decomposition of
solid-acid proton conductors [14]. The incompatibility between
4. Conclusions
A
new proton-conducting electrolyte consisting of
xAPP–(NH4)2Mn(PO3)4 (x = 2, 4, 6) was synthesized
using solid-state reaction. Thermal gravimetric analysis and
X-ray diffraction showed that (NH4)2Mn(PO3)4 was stable
enough to serve as a supporting matrix for APP. The proton
conductivity of the composite electrolyte was improved by
increasing the molar ratio of APP under both dry and wet
atmospheres. For x = 6, the highest proton conductivity was
determined to be 7 mS cm−1 at 250 ◦C under wet hydrogen. The
proton transference number of 2APP–(NH4)2Mn(PO3)4 was
determined to be 0.95 at 250 ◦C using a hydrogen concentration
cell. Fuel cells utilizing 2APP–(NH4)2Mn(PO3)4 as electrolyte
were constructed and tested. Maximum power density of
16.8 mW/cm2 was observed at 250 ◦C when dry hydrogen
and dry oxygen were used as fuel and oxidant, respectively.
These results suggest that (NH4)2Mn(PO3)4 is an excellent
supporting matrix and APP–(NH4)2Mn(PO3)4 composites are
potential candidate electrolytes for intermediate temperature
fuel cells. The catalytic degradation of (NH4)2Mn(PO3)4
and APP indicates that the Pt–C catalyst is not suitable for
APP–(NH4)2Mn(PO3)4 electrolyte and novel catalysts for
APP–(NH4)2Mn(PO3)4 electrolyte should be developed.
Acknowledgement
This work was supported by the Sino-German Center for
Research Promotion of NSFC and DFG (No. GZ 210) and the
Ministry of Education (NCET-04-0580).
Fig. 8. Time dependence of cell power density for a 2APP–(NH4)2Mn(PO3)4
composite fuel cell operated at 250 ◦C while drawing a continuous current of
20 mA cm−2 for 10 h.

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6547
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