J Solid State Electrochem (2009) 13:1241–1250
DOI 10.1007/s10008-008-0653-9
ORIGINAL PAPER
The NiFe
2
O
4
− MgFe
2
O
4
series as electrode materials
for electrochemical reduction of NO
x
F. Bræstrup · K. K. Hansen
Received: 16 May 2008 / Revised: 12 August 2008 / Accepted: 12 August 2008 / Published online: 5 September 2008
© Springer-Verlag 2008
Abstract Solid solutions of spinel-type oxides with the
composition Ni
1−x
Mg
x
Fe
2
O
4
(x = 0.0, 0.3, 0.5, 0.6, 1.0)
were prepared with the glycine-nitrate combustion syn-
thesis (x = 0.0, 0.3, 0.5, 0.6) and the citric-acid com-
bustion synthesis (x = 1.0). The oxides were used as
electrode materials in a pseudo-three-electrode setup
in the temperature range of 400–600
◦
C. Cyclic voltam-
metry and electrochemical impedance spectroscopy
were used to characterize the electrochemical behav-
ior in 1% NO and 10% O
2
. Measurements show that
NiFe
2
O
4
has relatively high cathodic activity in both
NO and O
2
, whereas MgFe
2
O
4
shows much higher
activity in NO compared to O
2
. MgFe
2
O
4
was also mea-
sured with cyclic voltammetry in 1% NO
2
and different
gas mixtures of NO and O
2
at 300 and 400
◦
C. Results
show that the cathodic activities (−0.6 V) are relatively
high with current ratios, I
NO
x
+O
2
/ I
O
2
, ranging from
10.1–167.7 and with a maximum at 400
◦
C. Dilatometry
measurements were performed on the materials in air
up to 1,000
◦
C, and they showed that the Curie tem-
perature could be detected for all samples. Four-point
DC resistivity measurements at elevated temperatures
show that Ni
0.4
Mg
0.6
Fe
2
O
4
has the highest conductivity,
whereas Ni
0.7
Mg
0.3
Fe
2
O
4
and NiFe
2
O
4
have the highest
conductivity at lower temperatures.
Keywords Spinel-type oxide · Conductivity ·
Powder diffraction · NO
x
· O
2
F. Bræstrup (B ) · K. K. Hansen
Fuel Cells and Solid State Chemistry Department,
Risø National Laboratory for Sustainable Energy,
Technical University of Denmark, Copenhagen, Denmark
e-mail: frantz.braestrup@risoe.dk
Introduction
One of the main polluting agents from internal com-
bustion processes in diesel-fired engines is NO
x
(NO
and NO
2
) gases. Presently, the three-way catalytic
converter (TWCC) is employed for treating gasoline
engine emissions; however, the TWCC requires stoi-
chiometric conditions to simultaneously catalyze oxi-
dizing and reducing reactions. This makes it unsuitable
in lean burn and diesel engine applications. Several
attempts have been made to solve the problem with
NO
x
emission, and the one that has gained the most
attention is the system based on the selective catalytic
reduction process (SCR). In SCR, the reduction of NO
x
is performed by injecting a reducing agent, such as
ammonia, urea, or a hydrocarbon into the exhaust
gas upstream of the SCR catalyst. Although the SCR
technology has been shown to reduce NO
x
by 65–99%
[1] over a range of diesel operating conditions, one of
the disadvantages with mobile SCR technology is that
the reduction agent must be carried onboard in a tank,
which places some requirements on packing the sub-
stance. It also has to be durable and function-effective
in a diverse range of a truck engines. Moreover, systems
must be designed to prevent unreacted reducing agents
to escape out of the tailpipe of the vehicle, as well as
N
2
O formation.
An alternative route is to reduce NO
x
electrochem-
ically in an all solid-state reactors [2]. The NO
x
gases
are reduced at the cathode following the overall Eqs. 1
and 2:
4e
−
+ 2NO → N
2
+ 2O
2−
(1)
4e
−
+ NO
2
→ 0.5N
2
+ 2O
2−
. (2)