The electrochemical generation of ferrate at porous magnetite electrode

Article abstract of DOI:10.1016/j.electacta.2004.01.031

In this work, ferrate(VI) was generated by the electrochemical oxidation of porous magnetite electrodes, made by melting pure magnetite grains. Pretreatment of the anode by cathodic polarization was necessary for ferrate(VI) generation and the achievement of high current efficiency. A electrolyte composition was found to be 16M NaOH. In this electrolyte, the effect of anode current density J on Fe(VI) synthesis rate, current efficiency, and internal cell temperature were studied. An optimum result was obtained at J = 3.3mAcm^-2, 30°C in 16M NaOH for 5h.

Full text of DOI:10.1016/j.electacta.2004.01.031

Electrochimica Acta 49 (2004) 3155–3159
The electrochemical generation of ferrate at porous magnetite electrode
a,∗
a
b
Zhe Ding , Changchun Yang , Qiang Wu
a
Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, PR China
b
Department of Chemistry, Fudan University, Shanghai 200433, PR China
Received 15 September 2003; received in revised form 21 January 2004; accepted 26 January 2004
Available online 9 April 2004
Abstract
In this work, ferrate(VI) was generated by the electrochemical oxidation of porous magnetite electrodes, made by melting pure magnetite
grains. Pretreatment of the anode by cathodic polarization was necessary for ferrate(VI) generation and the achievement of high current effi-
ciency. A electrolyte composition was found to be 16 M NaOH. In this electrolyte, the effect of anode current density J on Fe(VI) synthesis rate,
−
2
◦
currentefficiency, andinternalcelltemperaturewerestudied. AnoptimumresultwasobtainedatJ = 3.3 mA cm , 30 Cin16 MNaOHfor5 h.
2004 Elsevier Ltd. All rights reserved.
©
Keywords: Porous magnetite; Ferrate; Cathodic polarization; Current efficiency
1
. Introduction
the formation of ferrate(VI) by anodic oxidation of an iron
electrode in a concentrated alkaline hydroxide solution.
These studies were continued by Haber [24] and Pick [25],
who found that a necessary condition for the formation of
ferrate(VI) was pH ≥ 14; the results were better in NaOH
than in KOH solutions.
Previous studies on the electrochemical synthesis of fer-
rate focused on various iron electrodes: grey cast iron, white
cast iron, steel, mild steel, etc. The anodic oxidation reaction
of an iron electrode is:
Ferrate is an iron species in an unusual valence state
(>3) and therefore, a very powerful oxidizing agent that has
been considered for several applications [1–3]. For instance,
ferrate can be used as an oxidant: (i) in organic synthesis
as a substitute and safer alternative to other highly toxic
oxidizing compounds (e.g. CrO3, K2Cr2O7) and (ii) for the
destruction of organics and for water treatment to replace
chlorine, hydrogen peroxide or ozone. Ferrate has also been
recently used in a new class of batteries, referred to as super-
iron batteries, which use the Fe(VI)/Fe(III) system as cath-
ode material in alkaline medium (with a zinc anode) [2,4–7].
Ferrate can be chemically and electrochemically syn-
thesized. The chemical synthesis involves the oxidation of
Fe(NO3)3 with an oxidant such as alkaline hypochlorite [8]:
Fe + 8OH− → FeO42− + 4H2O + 6e−
(2)
The current yield for ferrate generation on iron electrodes
was in the range 20–60%. An optimum current yield of
about 60% at a low concentration (about 1 mM) was ob-
served at a pressed iron powder electrode for a 1 h elec-
trolysis in 14–16 M NaOH solutions [26]. In the search for
higher current efficiencies, we studied the current yield for
ferrate generation at various electrodes other than iron and
observed an exciting result at magnetite electrode. Ferrate
was successfully generated by the electrochemical oxida-
tion of magnetite electrode in a high concentration alkaline
solution. The anodic oxidation reaction of magnetite is:
−
−
2
Fe(OH)3 + 3ClO + 4OH
2FeO42 + 3Cl + 5H2O
−
−
(1)
→
or of Fe2(SO4)3 with OxoneTM, which is a triple salt mix-
ture of monoperoxosulfate (HSO5 ), containing K2SO4,
KHSO4, and KHSO [9]. Ferrate can also be electrochemi-
cally generated by oxidation of an iron electrode in alkaline
media [10–22]. Poggendorf [23] was the first one to observe
−
5
Fe3O4 + 16OH− → 3FeO42− + 8H2O + 10e−
(3)
∗
Compared with oxidation of iron electrode, Eq. (2), the num-
ber of electrons per ferrate ion produced, Eq. (3), is only
3.33.
Corresponding author. Tel.: +86-371-7765036;
fax: +86-371-7763220.
E-mail address: dingzhe80@zzu.edu.cn (Z. Ding).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.01.031

3
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Z. Ding et al. / Electrochimica Acta 49 (2004) 3155–3159
This study focuses on the electrochemical synthesis of
2. Experimental
ferrate by oxidation of porous magnetite electrodes. The
aim is to use these high surface area magnetite electrodes
to generate ferrate in high yield and in high concentration
compared with iron electrodes. The use of a porous electrode
was successfully applied in previous studies with iron where
it was shown that iron passivation was diminished and iron
dissolution was enhanced upon increasing the porosity of
the iron electrode [26,27]. Similarly, the porous magnetite
electrode influenced the formation of the passive layer in
such a way that it led to an increase in current yield for
ferrate generation and to a higher concentration of ferrate.
The synthesis was carried out in a divided two-compart-
ment aboratory cell separated by sulfonic acid substituted
2
perfluorocarbon polymer membrane (surface area 40 cm ).
The anode was a 30 cm2 slab of porous magnetite pre-
pared by melting pure magnetite grains (FeO 31.01%, Fe O
2
3
2
68.97%). A 30 cm (apparent area) sheet of high surface
area platinum gauze acted as the cathode. The volumes of
the anode and cathode electrolytes were 80 and 150 ml, re-
spectively. Anode pretreatment included 2 min sonication in
1:10 (v/v) H SO , followed by de-ionized water washing to
2
4
60
40
20
0
4
6
8
10
12
14
16
18
20
[NaOH] / M
Fig. 1. Current efficiency as a function of NaOH concentration for 1 h electrolysis of magnetite electrode at a current of 100 mA and room temperature.
1
2
0
8
6
4
2
0
-2
-2
-2
-2
0
.5 mA cm
1
3.3 mA cm
6
1
.6 mA cm
0 mA cm
20
30
40
50
o
Temperature / C
−
1
Fig. 2. Concentration of Fe(VI) as a function of cell temperature in 16 mol l NaOH at different current densities for 1 h electrolysis.

Z. Ding et al. / Electrochimica Acta 49 (2004) 3155–3159
3157
pH 7 and cathodic polarization in the range, 15–20 mA cm 2
for 15 min. All electrolytes were prepared from analyti-
cal grade reagents and de-ionized water. Electrolyte pre-
treatment was carried out as suggested in [28]. The elec-
trolyte in both compartments was 16 M NaOH solution. Cell
−
UV-Vis photodiode array spectrophotometer at λ = 505 nm
[29].
3. Result and discussion
◦
temperature was controlled within ± 1 C by a large wa-
ter bath. Constant direct current was supplied by YJ83/2
DC power sources (Precision & Scientific Instrument Co.
Ltd., Shanghai, China). The stirring near the electrode sur-
face was necessary in the anode compartment. The synthesis
was carried out for 5 h when the formation of a passivation
layer significantly decreased the generation yield of ferrate.
Pretreatment of the magnetite anode by cathodic polar-
ization is a necessary condition for the achievement of high
current efficiency. The passive layer was destroyed and the
electrode was activated during cathodic polarization. A max-
imum current efficiency for 1 h electrolysis was observed
in 14–16 M NaOH solutions, as shown in Fig. 1. The high
current efficiency indicated higher anode corrosion rate and
The solution phase FeO42 concentration was probed by
−
-2
-2
-2
-2
25
20
15
10
5
0
0.5 mA cm
3
6
1
.3 mA cm
.6 mA cm
0 mA cm
50
100
150
200
250
300
Duration of electrolysis / min
Fig. 3. Concentration of Fe(VI) as a function of duration of electrolysis in 16 mol l 1 NaOH at different current densities and at 30 C.
−
◦
100
-2
-2
-2
-2
0
3
6
1
.5 mA cm
.3 mA cm
.6 mA cm
0 mA cm
80
60
40
20
0
0
50
100
150
200
250
300
350
Duration of electrolysis / min
Fig. 4. Current efficiency as a function of duration of electrolysis in 16 mol l 1 NaOH at different current densities and at 30 C.
−
◦

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Z. Ding et al. / Electrochimica Acta 49 (2004) 3155–3159
lower ferrate decomposition rate. An abrupt decrease in cur-
rent efficiency was observed upon increasing the NaOH con-
centration to 18 M. In this study, a 16 M NaOH solution
was used as electrolyte. In this electrolyte, the effect of an-
ode current density J on Fe(VI) synthesis rate, current effi-
ciency, and internal cell temperature were probed. Increas-
ing the cell temperature generally results in faster rates of
Fe(VI) generation and higher current efficiency. However,
this trend is diminished or reversed for temperatures higher
than 30 C, resulting in a maximum at 25–35 C, as shown
in Fig. 2. At higher current densities, the cell potential is so
high that an increase in the rate of ohmic heating is observed.
The cell temperature increasing accelerates the Fe(VI) de-
composition reaction, and the surface of porous magnetite
electrode is passivated rapidly, which leads to a low current
efficiency and gradually decreases the rate of Fe(VI) gener-
ation. Although lower current densities retard the passiva-
tion process and increase the current efficiency for Fe(VI)
generation, lower synthesis rates are obtained. The Fe(VI)
synthesis rate and current efficiencies, as shown in Figs. 3
−
2
−1
and 4, at J = 0.5 and 3.3 mA cm for 1 h are: 1.5 mM h
−
1
and 70.2%; 7.3 mM h and 52.3%, respectively. A current
−
2
density of 3.3 mA cm is observed as an optimal compro-
mise between these phenomena supporting a high synthesis
rate and a high current efficiency.
◦
◦
The porouse magnetite electrode made by melting pure
magnetite grains was initially characterized by scanning
electron microscopy. Fig. 5. shows that these pores were not
distributed evenly and their shape was irregular. The XRD
patterns for a porous magnetite electrode before electrolysis
Fig. 5. Scanning electro micrograph (SEM) of the surface of a porous magnetite electrode.
1
50
00
5
h
1
50
0
10
20
30
40
50
60
70
O
2
/
Fig. 6. X-ray diffraction histograms for a porous magnetite electrode before electrolysis and after 5 h electrolysis: Fe3O4 (᭿); FeO (᭺); Fe2O3 (᭹).

Z. Ding et al. / Electrochimica Acta 49 (2004) 3155–3159
3159
and after 5 h electrolysis were shown in Fig. 6. The XRD
pattern for porous magnetite electrode exhibited some char-
acteristic diffraction peaks. The XRD peaks were very sharp,
which suggested a pure well-crystalline magnetite. Never-
theless, new diffraction peaks were visible at 36.1, 41.94,
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1
[6] S. Licht, B. Wang, S. Ghosh, J. Li, R. Tel-Vered, Electrochem.
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◦
and 60.88 after 5 h, and were characteristic to FeO. A close
[7] S. Licht, B. Wang, Electrochem. Solid State Lett. 3 (5) (2000) 209.
[
8] G.W. Thompson, L.T. Ockerman, J.M. Schreyer, J. Am. Chem. Soc.
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examination of the XRD pattern after 5 h electrolysis might
revealed the presence of Fe2O3. Fe3O4 almost disappeared
after 5 h.
7
[
[
[
[
[
[
[
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4
. Conclusion
The electrochemical generation of ferrate at a porous
magnetite electrode has been demonstrated. The electrolysis
conditions were investigated and optimized. A ferrate solu-
tion about 25 mM was obtained under the optimum condi-
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(
−
2
tions: NaOH concentration (16 M), J = 3.3 mA cm , I =
[
◦
1
00 mA, 30 C, electrolysis duration (5 h). The optimal cur-
7
rent efficiency was 52.3%. The result of this study suggests
that this approach is very promising to improve the yield
for ferrate generation. On the other hand, even under the
optimum conditions tested, the formation of a passive layer
of magnetite electrode was not completely avoided. Thus, it
will be crucial to improve this particular aspect in order to
achieve a more attractive method of ferrate generation.
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