CHEMICAL ENGINEERING TRANSACTIONS

VOL. 63, 2018

A publication of

The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet

Guest Editors: Jeng Shiun Lim, Wai Shin Ho, Jiří J. Klemeš
Copyright © 2018, AIDIC Servizi S.r.l.
ISBN 978-88-95608-61-7; ISSN 2283-9216

Fabrication and Fuel Cells Performance of Lanthanum-Doped
Cerium Diphosphate Electrolyte

Thanh-Hao Nguyen, Minh-Vien Le*
Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU, 268 Ly Thuong Kiet Street, Distric 10,
Ho Chi Minh City, Vietnam
lmvien@hcmut.edu.vn

Proton conductivity of indium-doped cerium diphosphate (CeP2O7) was investigated to explore its potential for
application as an electrolyte in intermediate temperature fuel cells. The La3+-doped CeP2O7 powder was
synthesized by the digestion of metal oxide in a phosphoric acid solution. The structure and ion conductivity of
La3+ doped CeP2O7 were analysed using X-ray diffraction, scanning electron microscopy (SEM) and
electrochemical impedance spectroscopy (EIS). Under humidified conditions, the La3+-doped CeP2O7
exhibited sufficient conductivity in the intermediate temperature range. The maximum ionic conductivity of
Ce0.95La0.05P2O7 was 2.00  10

-2 Scm-1 at 180 °C. The maximum power density of the fabricated H2/air fuel
cells using the Ce0.95La0.05P2O7 as electrolyte (0.44 mm thickness) was 49.0 mW cm

-2 at 240 °C. The results
indicate that Ce0.95In0.05P2O7 is a promising material for the fabrication of intermediate temperature fuel cells.

1. Introduction

Many attempts on proton conductor based on inorganic materials have been reported. A number of tetravalent
metal diphosphate salts, MIVP2O7 (M = Sn, Ti, Zr, Ce), have been proved the superior conductive properties in
the intermediate temperature range of 100 - 350 °C. This particular temperature range is of interest since it
covers most of operating temperatures ranges for energy conversions and chemical processes, and a few
solid proton conductors offer viable conductivity and stability for practical applications (Jin et al. 2010). Cerium
diphosphate, a class of MIVP2O7 compounds, has been demonstrated as a high proton conductor in the
intermediate temperature range (Le and Tsai, 2017).
Ion conductivity of CeP2O7 was reported of 0.018 Scm

-1 at 200 °C under humidified conditions (Sun et al.,
2009). Singh et al. (2012) reported that a maximum conductivity value of CeP2O7 was 2.1  10

−4 Scm−1 at 175
°C in air with 6 % H2O. Most researches have proven that CeP2O7 family is necessary for humidified
electrolyte to support the number of jump sites in the CeP2O7 matrix which lead to increasing proton
conductivity.
A partial substitution of foreign elements on the cerium site may enhance the proton conductivity and broaden
the temperature window for electrolyte applications (Le et al., 2011). Mg-doped CeP2O7 (Ce0.9Mg0.1P2O7)
exhibited sufficient conductivity in the intermediate temperature range, with the ion conductivity of 4.0  10-2

Scm-1 at 200 C in humidified air with 11.4 % H2O (Le et al., 2011). The Mg doped-CeP2O7 widened the
temperature range from 160 to 280 °C. Sr-doped cerium diphosphate, Ce0.9Sr0.1P2O7, has been proved to
possess a maximum conductivity of 6.3  10−3 Scm−1 at 90 °C in air with 12 % H2O (Singh et al. 2013a);
Ce0.9Gd0.1P2O7 (Singh et al. 2014a) reached 2.91  10

−2 Scm−1 at 190 °C, in humidified air (pH2O = 16.2
kPa); Ce0.9Mn0.1P2O7 (Singh et al. 2014b) reached 2.24  10

−2 Scm−1 at 170 C (pH2O = 16.2 kPa) and 1.4 
10−3 Scm−1 at 100 °C in a wet air atmosphere (pH2O = 7.4 kPa) for Ce0.95Eu0.05P2O7 (Wang et al. 2014). It has
been shown in the literatures that acceptor-doped cerium diphosphates exhibit random variations in electrical
conductivity, without showing any dependence on nature of the dopants (Singh et al. 2013b).
Lanthanum, the rare earth element, which is located in same group of cerium in the periodic table has the
similar chemical and physical properties to those of Cerium. In addition, La3+ cation has a lower valance state
to Ce4+, which make more interesting in the investigation of La3+-doped CeP2O7, which has not been
investigated. In present work, we have synthesised the Ce1-xLaxP2O7 and investigated its proton conductivities

DOI: 10.3303/CET1863002

Please cite this article as: Thanh-Hao Nguyen, Minh-Vien Le, 2018, Fabrication and fuel cell performance of lathanum-doped cerium
diphosphate electrolyte, Chemical Engineering Transactions, 63, 7-12 DOI:10.3303/CET1863002

at various temperatures by electrochemical impedance spectroscopy (EIS). The phase composition and
microstructure of La3+-doped cerium diphosphate were also explored. Fuel cells using La3+-doped cerium
diphosphate were fabricated and tested the fuel cell performance under intermediate temperature and
humidified conditions.

2. Experiments

2.1 Preparation of La
3+

-doped CeP2O7 samples

La3+-doped CeP2O7 powder was obtained in the same manner as reported previously (Le et al., 2011). The
molar ratio of phosphoric acid (85 %, Acros) to metal was controlled to be 2.3 : 1. The resultant yellow paste
after refluxing was calcined at 300 °C for 8 h. The calcined sample was ground in agate pestle and mortar.
Disk samples (Ce1-xLaxP2O7, x = 0, 0.05, 0.1, 0.2) were prepared by uniaxially pressing the ground powder
into a stainless steel mould (diameter 14 mm and thickness 0.5 - 1.2 mm) at hydrostatic pressure of 300 MPa,
then sintered at 400 °C for 8 h.

2.2 Characterisation of La
3+

-doped CeP2O7

The X-ray powder diffraction (XRD) patterns of the samples were obtained at ambient temperature on D2
Phaser, Bruker diffractometer using a CuK radiation source (1.5418 Å) and nickel filter operating at 50 kV
and 200 mA. Diffraction patterns were recorded in the 2θ range of 15 − 70° with the step size of 0.03. The
infrared (IR) spectra were collected between 400 – 4,000 cm-1 using a Fourier-transform infrared
spectrometer, FTIR, (Digilab FTS-3500, Biorad). The IR sample was pelletised into a powdered mixture that
contained 3 wt% cerium phosphate (400 °C calcined) and 97 % KBr. The reported spectra are the average
over 64 scans.

2.3 Impedance measurement

It has been proved that the migration of protons in CeP2O7-based material occurs by hopping from oxygen
vacancy site and water molecule Singh et al. (2012). The CeP2O7 membrane should be humidified to
increases the number of vehicle sites which lead to the facilitated conductivity of CeP2O7. In this research, the
La3+-doped CeP2O7 samples were measured in a sealed stainless steel chamber with tight humidity and
temperature control. The chamber humidity was manipulated by the air flow passing a humidifier, pre-
calibrated to supply nearly saturated humidified air at a specific temperature (93 % relative humidity, <60 °C).
The phosphate disk was polished and printed with a silver layer on both sides. The silver electrodes were
pressed against two 0.2 mm gold wires, then connected to the electrical feedthroughs and then placed in the
chamber. Impedance data were collected with a S1260 A frequency response analyser. The steady
impedance data were fitted with an equivalent circuit to get Rgi and Rgb, the resistance of bulk and grain
boundary contribution, respectively. The value of the proton conductivity  is calculated as Eq(1), where t is
the thickness of the disk and A is the disk area.

 = tA−1(Rgi + Rgb)
−1 (1)

2.4 Fuel cell fabrication and evaluation

The membrane-electrode assembly (MEA) was prepared by compressing a polished Ce1-xLaxP2O7 pellet
(Figure 1), which had been sintered at 400 °C for 8 h, against two circular area of 1.05 cm2 gas diffusion
layers of carbon paper (GDL-10BC, SIGRACET). One side of these two carbon papers was coated with a
catalyst ink which was made by mixing 50.0 wt% of Pt/C catalyst powder (20 wt% Pt supported by Vulcan,
BASF), 35.0 wt% of Ce0.95La0.05P2O7 powder (5CLaP) and 15 wt% of polytetrafluoroethylene (PTFE, 60 wt%,
Aldrich). In order to obtain homogeneous ink, an appropriate amount of glycerol solvent was added to the
above mixture and mixed using mortar and pestle. The cathode and anode were dried at 150 °C to remove
the glycerol solvent. The Pt loading for both anode and cathode was calculated to be 0.6 mg cm-2. The fuel
was humidified hydrogen, 50 % H2 and 50 % N2, while the oxidant was humidified air. The performance of the
hydrogen/air fuel cell was measured by using a Keithley238 source-measure unit.

3. Result and discussion

Figure 2 shows the XRD patterns of Ce1-xLaxP2O7 powder (x = 0, 0.05, 0.1, 0.2) calcined at 300 °C for 12 h.
All detected peaks of Ce1-xLaxP2O7 samples (x = 0, 0.05) at 2θ 17.91, 20.72, 23.16, 25.44, 29.49, 34.71,
47.49 and 55.68 were well indexed to the cubic CeP2O7 structure (Pa-3) by comparison with the JCPDS Card
file No. 16-0584. The impurity phase of CeO2 and other unidentified signals were detected in Ce0.9La0.1P2O7
and Ce0.8La0.2P2O7 samples. Peaks of CeO2 were detected at 2θ of 28.7 and 33.3, whereas peaks of

unidentified phases were detected at 22.8, 25.1, 27.2, 31.4. The peaks of the CeO2 residue intensified as
the La3+ concentration increased. According to the X-ray diffraction patterns, it is clearly shown in Figure 2b
that the peaks shift to low diffraction angle due to the larger ionic size of La3+ 0.113 nm (Ali et al., 2013) than
that of Ce4+. These results indicated that 5 mol% La3+ was the substitutable limiting concentration for Ce4+.

Figure 1: Schematic of single cell assembly

Figure 2: X-ray diffraction patterns of the synthesised Ce1-xLaxP2O7 powder

Similar to 300 °C calcined samples, the XRD patterns of 400 C calcined samples as shown in Figure 3a
indicate identical peaks of CeP2O7 structure. Peak intensities of 400 °C annealed pellets were higher than that
of 300 °C. This implied that the CeP2O7 based material is thermally stable up to 400 °C. All further samples,
therefore, were treated at a maximum of 400 °C for 8 h.
To evaluate the utilisation of La-doped cerium diphosphate for fuel cell application, the synthesised powder
was dry pressed into circular-shape plate with 14 mm in diameter and 0.40 – 1.0 mm in thickness, sintered at
400 °C for 8h before polished both sides to the designed thickness and dried in electric oven. The 400 °C
sintered pellet was microstructure analysed using a field-emission scanning electron microscope (FE-SEM,
JSM6500F,JEOL). Figure 3b to 3e show the top-view image of four pellets (x = 0, 0.05, 0.1, 0.2) with the least
densification, indicating that a number of micron sized pores dispersed in small cubic grains still remained in
the sample. This is similar to the observation of the Mg-doped CeP2O7 (Le et al., 2011). The relative density of
the Ce0.95La0.05P2O7 pellet prepared in this study was estimated to be ~74.7 %, whereas, the Ce0.90La0.1P2O7
pellet (10CLaP) was measured to be 66.1 %, being lower than that reported for Mg2+-doped CeP2O7 (Le et al.,
2011) or In3+-doped CeP2O7 (Le et al., 2017) prepared in a similar heating treatment temperature.

Figure 3: (a) X-ray diffraction patterns and microstructure of the Ce1-xLaxP2O7 pellets calcined at 400 °C for 8 h

(b) x = 0, (c) x = 0.05, (d) x = 0.10, (e) x = 0.20

Figure 4: Variation in conductivity of 400 °C-sintered Ce1-xInxP2O7 vs. temperature under humidified conditions

Another material’s characteristic is proton conductivity property. Humidification is critical to the proton
conductivity in the cerium pyrophosphate by the hopping mechanism (Le et al., 2011). The samples were
needed to obtain a steady-state humidified sintered disk at 80 °C, and the time it took to reach the steady-
state value was shorter as the measurement temperature increased. The conductivity as a function of
temperature at 80 – 260 °C was performed. As shown in Figure 4 the conductivity values of Ce1-xLaxP2O7 (x =
0, 0.05, 0.1) samples are higher than 10-2 Scm-1, a required conductivity for electrolyte applications and higher
than that of Ce0.8La0.2P2O7 sample due to impurity phases, as mentioned in Figure 3a. When the temperature
became higher than 180 °C, the temperature dependence of the samples began to differ from each other.
Conductivity of the undoped sample dropped from 1.71 × 10-2 Scm-1 to 2.51 × 10-3 Scm-1, whereas, the
conductivities of other samples Ce1-xLaxP2O7 (x= 0.05, 0.1) were still nearly constant when temperature
increase up to 220 °C. As a result, the undoped CeP2O7 would be more suitable for low-temperature
applications, in the range of 120 – 180 °C. Figure 4 shows that the maximum proton conductivities of
Ce0.95La0.05P2O7 and Ce0.90La0.10P2O7 were measured to be 2.00  10

-2 Scm-1 at 180 °C and 2.29 × 10-2 Scm-1
at 200 °C. The applicable temperature range of the Ce0.95La0.05P2O7 and Ce0.90La0.10P2O7 samples (  1.0 
10-2 Scm-1) was 120 – 250 °C, wider than that of the un-doped sample. It can be concluded that the 400 °C-
sintered Ce1-xLaxP2O7 (x = 0.05, 0.1) samples had the most conductive composition in this investigation.
The performance of fuel cell Pt/C 10CLaP Pt/C using a 0.43 mm thick electrolyte was evaluated at various
temperatures under humidified H2/air fuel cell conditions as shown in Figure 5a. The open-circuit voltages
(OCVs) at the tested temperatures were between 0.50 V and 0.65 V, considerably lower than the theoretical

value of ~1.1 V. The low OCVs of this cell were due to the lesser densification of electrolyte, resulting in the
physical leakage of hydrogen gas through the electrolyte, as our sensor detected the low of hydrogen
concentration at the cathode side. The peak power density of this cell reaches low value of 28.9 mW cm-2 at
140 °C.

Figure 5: Cell voltage and power density of fuel cells vs. temperature under humidified conditions using

electrolytes, (a) Ce0.9La0.1P2O7 (b) Ce0.95La0.05P2O7 and (c) Ce0.95La0.05P2O7 with different thickness

The higher densification of electrolyte (x = 0.05) measured of 74.7 %, the OCVs value increase, a range from
6.5 to 6.8 V as illustrated in Figure 5b. The Pt/C 5CLaP Pt/C cell performance was strongly dependent on the
operating temperature. The peak power density increased in value with the increasing operating temperature,
reaching the maximum value of 49.0 mW cm-2 at 240 °C, and then decreased to 46.2 mW cm−2 at 260 °C.
Deducing from this, the power density of this intermediate temperature fuel cell was strongly influenced by the
conductivity of the electrolyte, which followed the same trend as the proton conductivity. It is concluded that
this fuel cell is suitable for operating at around 220 °C. Figure 5c shows the cell voltage and power density
curve of the Pt/C 5CLaP Pt/C fuel cells in relation to the differences in the thickness of the electrolyte, from
0.44 to 1.11 mm. Cell performance decrease with increasing thickness of electrolyte due to increasing ohmic
resistance. The performance of this cell Pt/C 5CLaP Pt/C using Ce0.95La0.05P2O7 electrolyte thickness of 0.44
mm was 49.0 mW cm-2 at 240 °C, which was higher than those of the CeP2O7 cell, reported by Sun et al.
(2009), (48.9 mW cm-2 at 140 °C), the H3PO4 doping PTFE/Sn0.95Mg0.05P2O7 cell reported by Wang et al.
(2015). These peak power values were much lower than that of the Sn0.9In0.1P2O7 cell (264 mW cm

-2 at 250
°C) (Heo et al., 2006), and that of the cell fabricated using a composite electrolyte of H3PO4-doped PBI/SAPO,
which was 439.6 mW cm-2 at 200 °C (Jin et al., 2011). The low power of cell could have resulted from the less-
ideal cell potential and thick electrolyte membrane. The result indicates that Ce0.95La0.05P2O7 is a promising
electrolyte for intermediate temperature fuel cells.

4. Conclusions

In this study, La-doped CeP2O7 were synthesised with varying lanthanum doping levels, and the proton
conductivity in the intermediate temperature range was investigated. The result of XRD indicates that single
phase Ce0.95La0.05P2O7 was successfully synthesised. The higher value and wider range of conductivities were
found in Ce0.95La0.05P2O7 sample. The maximum power density of 49.0 mW cm

-2 at 240 °C was generated by
fuel cells fabricated using the Ce0.95La0.05P2O7 electrolyte. The fuel cell can be improved by increasing
densification and decreasing the thickness of 5CLaP electrolyte.

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