Preparation of a nickel-nickel oxide composite by hot isostatic pressing and its application for anodes used in electrolytic production of nitrogen trifluoride

Article abstract of DOI:10.1149/1.1568739

The nickel-nickel oxide [Ni-NiO_1+x (0 < × ≤ 0.5)] composites prepared from a mixture of Ni and LiNiO₂ or NiO powders at 900°C under 2000 atm for 2 h by hot isostatic pressing were employed as the anode for electrolytic production of NF₃. In electrolysis of a molten NH₄F·2HF with and without LiF at 100°C and at 25 mA/cm², the anode gas generated at the Ni-NiO_1+x composite anode was composed of N₂, O₂, NF₃, N₂F₂, N₂F₄, and N₂O, and its composition was composition was almost the same as that at the Ni sheet anode. The current efficiency for NF₃ formation on the Ni-NiO composite anode from mixture of NiO and Ni powders was high compared with that on the Ni-NiO_1+x composite anode from the mixture of LiNiO₂ and Ni powders. The best current efficiency for NF₃ formation was ca. 53% on the Ni 5 mol% NiO composite anode, and it was almost the same as that of the Ni sheet anode. The addition of LiF in a molten NH₄F·2HF increased it, presumably because of deposition of Li₂NiF₆ on the anode. On the other hand, the anode consumption of the Ni-NiO composite was much smaller compared with that of the Ni sheet electrode. Also, the oxygen content in the oxidized layer formed on the Ni-NiO composite anode was high compared with that on the Ni sheet anode. The scanning electron microscope observation revealed that the surface of the Ni-NiO composite anode was covered with the compact film having some defects. From these results, it is concluded that the Ni-NiO composite anode is favorable for electrolytic production of NF₃, and that the oxidized layer on the anode has a high resistance to corrosion, because of the compact film containing a higher content of oxygen formed on the anode.

Full text of DOI:10.1149/1.1568739

D108
Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
0
013-4651/2003/150͑6͒/D108/9/$7.00 © The Electrochemical Society, Inc.
Preparation of a Nickel-Nickel Oxide Composite by Hot
Isostatic Pressing and Its Application for Anodes Used
in Electrolytic Production of Nitrogen Trifluoride
a, ,z
a
a
b
Akimasa Tasaka, * Yasuhiro Suzuki, Atsushi Oshida, Atsuhisa Mimoto,
a
a
a
a
Taro Hieda, Toshiyasu Tachikawa, Kazuchika Takao, Hideaki Takemura,
and Osamu Yamaguchia
aDepartment of Molecular Science and Technology, Faculty of Engineering, Doshisha University,
Kyotanabe, Kyoto 610-0321, Japan
b
Semiconductor Material Department, Electronics & Information Material Division, Mitsui Chemicals,
Incorporated, Chiyoda-ku, Tokyo 100-6070, Japan
The nickel-nickel oxide ͓Ni-NiO1ϩx(0 Ϲ x Ͻ 0.5)͔ composites prepared from a mixture of Ni and LiNiO2 or NiO powders at
9
00°C under 2000 atm for 2 h by hot isostatic pressing were employed as the anode for electrolytic production of NF . In
3
2
electrolysis of a molten NH F•2HF with and without LiF at 100°C and at 25 mA/cm , the anode gas generated at the Ni-NiO
4
1ϩx
composite anode was composed of N , O , NF , N F , N F , and N O, and its composition was almost the same as that at the
2
2
3
2
2
2
4
2
Ni sheet anode. The current efficiency for NF formation on the Ni-NiO composite anode from mixture of NiO and Ni powders
3
was high compared with that on the Ni-NiO1ϩx composite anode from the mixture of LiNiO and Ni powders. The best current
2
efficiency for NF formation was ca. 53% on the Ni 5 mol % NiO composite anode, and it was almost the same as that of the Ni
3
sheet anode. The addition of LiF in a molten NH F•2HF increased it, presumably because of deposition of Li NiF on the anode.
4
2
6
On the other hand, the anode consumption of the Ni-NiO composite was much smaller compared with that of the Ni sheet
electrode. Also, the oxygen content in the oxidized layer formed on the Ni-NiO composite anode was high compared with that on
the Ni sheet anode. The scanning electron microscope observation revealed that the surface of the Ni-NiO composite anode was
covered with the compact film having some defects. From these results, it is concluded that the Ni-NiO composite anode is
favorable for electrolytic production of NF , and that the oxidized layer on the anode has a high resistance to corrosion, because
3
of the compact film containing a higher content of oxygen formed on the anode.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1568739͔ All rights reserved.
Manuscript submitted July 29, 2002; revised manuscript received December 10, 2002. Available electronically April 10, 2003.
A large amount of nitrogen trifluoride (NF ) is consumed as a
dry etchant and a cleaner gas for the chemical vapor deposition
for 2 h by HIP. The former and the latter are denoted as Ni-NiO1ϩx
0 Ͻ x Ͻ 0.5) and Ni-NiO composites, respectively, in this paper.
3
(
͑
CVD͒ chamber by the electronic industry in Japan. Pure NF free
3
The concentrations of LiNiO and NiO in the mixture were 0, 2, 5,
2
from carbon tetrafluoride (CF ) can be obtained by the electrolysis
10, and 20 mol %. These samples were used as composite elec-
trodes, and the anodic behaviors of them were investigated in a
4
of NH F•2HF with a nickel anode to meet the demand. However, a
4
relatively large corrosion rate and passivation of the nickel anode
dehydrated melt of NH F•2HF.
4
3
are problems for electrolytic production of NF . According to the
A box-type poly͑tetrafluoroethylene͒ ͑PTFE͒ cell of ca. 0.5 dm
3
previous works,1 the nickel dissolution is diminished by a trace of
-3
in volume was provided for electrochemical and corrosion studies,
1
,3,4
water in the melt whereas the current efficiency for NF formation
and the cell configuration was described in the previous papers.
3
2
decreases. The surface layer on the anode formed in a molten salt
containing water was dense and adhesive, and its oxygen content
A composite of 1 cm in surface area and the nickel sheet having a
large surface area were used as the anode and the cathode, respec-
was high.2 When the oxidized layer on the nickel anode has a high
,3
4
tively. A nickel rod immersed in molten NH F•2HF saturated with
content of nickel oxide, it has a higher electric conductivity and a
NH4NiF3 was used as the reference electrode, and the potential was
corrected to the potential of hydrogen evolution on a Pt plate, des-
ignated as V vs. H2 , in the text. The potential of this reference
electrode determined at 100°C was 100 Ϯ 5 mV vs. H2 during the
electrochemical measurement. When the electrochemical measure-
ment, the corrosion test, and the preparation of the specimen for the
X-ray photoelectron spectroscopy ͑XPS͒ analysis were conducted,
the PTFE cell was positioned in a dry box. Since the chemicals
contained water to some extent, pre-electrolysis was conducted with
4
higher resistance to corrosion. Although a LiNiO -coated Ni sheet
2
anode prepared by atmospheric plasma spraying technique has al-
most same current efficiency for NF formation as that on the Ni
3
sheet anode and a relatively small anode consumption, the LiNiO2
layer having the thickness of 20 to 50 ␮m is dissolved during elec-
trolysis for only 120 h, so that the lifetime of the LiNiO layer in a
2
5
dehydrated melt of NH F•2HF may be insufficient. Hence, a
4
Ni-NiO1
ϩx
(0 Ϲ x Ͻ 0.5) composite was prepared from mixture
2
a carbon anode at ca. 10 mA/cm for about 10 days to reduce the
of Ni and LiNiO or NiO powders by hot isostatic pressing ͑HIP͒.
6,7
3
2
water content to less than 0.02 wt % or ca. 20 mmol per dm prior
to the electrochemical, corrosion, and XPS studies. For the corrosion
test, the nickel-nickel oxide composite anode as well as the Ni sheet
anode were washed with water and methanol before weighing. The
current losses caused by nickel dissolution with an assumption of
two-electron transfer for Reaction 1 were calculated from the weight
loss
This paper deals with the effect of the content of nickel oxide in
the composite on the current efficiency for NF formation and the
3
anode consumption.
Experimental
A mixture of LiNiO ͑particle size of 1 to 3.3 ␮m͒ and Ni ͑purity
2
of 99.9%, particle size of 74 ␮m͒ powders and that of NiO ͑purity of
2
ϩ
Ni → Ni ϩ 2e
͓1͔
97.0%, particle size of 1 ␮m͒ and Ni ͑purity of 99.0%, particle size
of 2.9 to 3.6 ␮m͒ powders were sintered at 900°C under 2000 atm
The cell employed for electrolytic production of NF was a cy-
3
3
1,8
lindrical Ni cell ͑1.5 dm ͒ as described in the previous papers. The
Ni sheet and the composite anodes of 19.2 cm in surface area were
2
*
Electrochemical Society Active Member.
E-mail: atasaka@mail.doshisha.ac.jp
z
employed as the anode. The anode was located at the center of cell
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Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
D109
Figure 1. X-ray diffraction patterns of the ͑a͒ HIP Ni, ͑b͒ the 10 mol %
composite, and ͑c͒ the 20 mol % composite materials prepared by HIP. ͑a͒
HIPed Ni, ͑b͒ Ni-10 mol % NiO1ϩx composite, ͑c͒ Ni-20 mol % NiO1ϩx
Figure 2. X-ray diffraction patterns of ͑a͒ the Ni-5 mol % NiO composite
and ͑b͒ the Ni-10 mol % NiO composite materials prepared by HIP. ͑a͒ Ni-5
mol % NiO composite, ͑b͒ Ni-10 mol % NiO composite. ͑᭺͒ Ni, ͑ᮀ͒ NiO.
composite. ͑᭺͒ Ni, ͑᭝͒ NiO, and ͑छ͒ Li O.
2
Seisakusho Ltd.͒ with an Al-K␣ radiation ͑1400 eV͒ and RINT-2500
͑
Rigaku Electric Co. Ltd.͒ with a Cu K radiation were used for the
␣
whereas the cell wall was utilized as the cathode. The Ni rod of 0.1
cm in surface area with pretreatment of anodic oxidation in a de-
1-4,9,10
XPS and the XRD analyses.
2
hydrated melt of NH F•2HF was used as the reference electrode,
4
Results and Discussion
and it is considered to function as a Ni/NiF electrode. A nickel
2
sheet skirt was provided between the anode and the cathode to sepa-
rate the anode gas from hydrogen generated at the cathode so that
Preparation of Ni-NiO1ϩx and Ni-NiO composite materials by
HIP.—A nickel sample was at first prepared from a nickel powder
by HIP. Its density was 8.8763 g/cm , and was almost the same as
that of the commercial nickel sheet ͑purity of 99.6%, Nilaco Corpo-
ration Co. Ltd.͒. Hence, it is found that the material prepared by HIP
is dense and has a low porosity.
The Ni-NiO1ϩx composites were prepared from a mixture of
LiNiO2 and Ni powder as a binder by HIP at 900°C under 2000 atm
for 2 h. Figure 1 shows the XRD diagrams of the composites pre-
pared. The top figure, labeled a, is the XRD patterns of the nickel
specimen ͑specimen A͒ prepared by HIP, which is denoted as HIP
Ni, in this paper. The middle and the bottom figures, labeled b and c,
are of the Ni-10 mol % NiO1ϩx and the Ni-20 mol % NiO1ϩx com-
posites, denoted as the 10 mol % and 20 mol % composites, respec-
tively, in this paper. The Ni-10 mol % NiO1ϩx composite ͑specimen
B͒ was composed of NiO and Ni, whereas the Ni-20 mol % NiO1ϩx
composite ͑specimen C͒ was composed of NiO and Ni with a small
3
explosion and the loss of NF are prevented. The cell bottom was
3
covered with the PTFE sheet to avoid hydrogen evolution.
Electrolysis was conducted with the nickel cell at 100°C. Al-
though the water content was high before start-up, it might be de-
9
creased by electrolysis to less than 0.02 wt % within 80 h. The
anode gas was treated with NaF to eliminate HF before the chro-
1
matographic and IR spectroscopic analyses. The current efficiencies
for the constituents were evaluated from the results of gas analysis
and the flow rate of anode gas.1
,9,10
In order to prepare the specimen for SEM, XPS, and X-ray dif-
fraction ͑XRD͒ studies, the nickel-nickel oxide composite anode
was electrolyzed at 25 mA/cm2 in
a dehydrated melt of
NH F•2HF at 100°C for 120 h. The test specimen was washed with
4
47% HF aqueous solution to remove adhesive melt on the surface
prior to inspection by XPS, XRD, and SEM. ESCA-1000 ͑Shimadzu
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D110
Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
Figure 4. SEM images of the HIP Ni anode after electrolysis at 5 V in a
dehydrated melt of NH F•2HF at 100°C. ͑a͒ Surface, ͑b͒ cross section.
4
Figure 3. Anodic polarization curves of the ͑a͒ HIP Ni, ͑b͒ 10 mol % com-
posite, and ͑c͒ 20 mol % composite anodes in a dehydrated melt of
NH F•2HF at 100°C by the potential sweep method with a sweep rate of 5
4
mV/s. ͑a͒ HIP Ni anode, ͑b͒ Ni-10 mol % NiO
composite anode, ͑c͒
ϩx
1
E͒. Both composites were composed of NiO and Ni. A small peak at
Ni-20 mol % NiO1ϩx composite anode.
530 eV was found on the O 1s spectra, and hence this material can
be designated as the Ni-NiO composite.
Anodic polarization curves.—The anodic polarization curves of
the specimens ͑specimens A, B, and C͒ were determined in a dehy-
amount of Li O as shown in Fig. 1c. No LiNiO was detected in
2
2
both composites by the XRD analysis, so that LiNiO seemed to be
2
drated melt of NH F•2HF at 100°C by the potential sweep method
4
decomposed and/or reduced by heating with nickel powder accord-
4
with a sweep rate of 5 mV/s, and are shown in Fig. 3
ing to Reactions 2-4, resulting in formation of Li O and NiO
2
The top figure, labeled a, is the anodic polarization curves of a
HIPed Ni ͑specimen A͒. A virgin HIP Ni anode was dissolved in the
potential range of 0 to 1 V and passivated in the range of 1 to 4 V
͑curve 1͒. The current density was increased with sweeping potential
from 4 V to 5 V, probably caused by the fluorination of nickel.
The potential sweep measurement with the same specimen was
repeated in the potential range of 0 V to ca. 8 V. Curve 2 is a trace
of the second run, which differs from curve 1. The peaks caused by
active dissolution of nickel at 0.3 and 0.6 V disappeared because of
the formation of a protective layer composed of nickel fluoride with
nickel oxide or oxyfluoride in the potential range higher than 4 V
during the first run. No current passed at potentials lower than 4 V,
but the current was increased thereafter. The current density was
almost the same as that observed in the first run. Curve 3 represents
the fiftieth run which is almost the same as of the second run
whereas the amperage is somewhat smaller because of the growth of
an oxidized layer during repetition of the sweeping potential.
2
LiNiO ϩ Ni → 3NiO ϩ Li O
͓2͔
͓3͔
͓4͔
2
2
2
LiNiO → Ni O ϩ Li O
2
2
3
2
Ni O ϩ Ni → 3NiO
2
3
The specimen was also analyzed by XPS. Two small peaks at
30 and 532 eV on the O 1s spectra were found, and were assigned
5
to NiO and Ni O , respectively. Hence, this material can be desig-
2
3
nated as the Ni-NiO1ϩx(0 Ͻ x Ͻ 0.5) composite.
The Ni-NiO composites were also prepared from a mixture of
NiO and Ni powders prepared by HIP under the same conditions as
the previous procedure. Figure 2 shows the XRD diagrams of the
composite prepared. The upper figure, labeled d, is the XRD pattern
of the Ni-5 mol % NiO composite ͑specimen D͒, and the lower
figure, labeled e, is of the Ni-10 mol % NiO composite ͑specimen
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Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
D111
Figure 6. SEM images of the 20 mol % composite anode after electrolysis at
Figure 5. SEM images of the 10 mol % composite anode after electrolysis at
5
V for 90 min in a dehydrated melt of NH F•2HF at 100°C. ͑a͒ Surface, ͑b͒
4
5
V in a dehydrated melt of NH F•2HF at 100°C. ͑a͒ Surface, ͑b͒ cross
4
cross section.
section.
the lower photographs, labeled b, are of the cross section. The HIP
Ni specimen ͑specimen A͒ had a dense surface with few pores and
the thickness of the oxidized layer was ca. 5 ␮m. The outer surface
seemed to be dense whereas the inner oxidized layer was rough and
lumpy. Since a specimen treated with dehydrated electrolyte is gen-
erally coated with a loose deposit having a number of crevices and
The middle figure, labeled ͑b͒, and the bottom figure, labeled c,
are of the 10 mol % ͑specimen B͒ and the 20 mol % composites
specimen C͒, respectively. The anodic polarization curves of the 10
͑
mol % composite ͑specimen B͒ were almost the same as those of the
HIP Ni anode ͑specimen A͒ with the exception of a potential shift in
the direction of higher potential, presumably because of the forma-
tion of a thick oxidized layer compared with that formed on the HIP
Ni anode. On the other hand, a new current peak at ca. 2.6 V ap-
peared on the anodic polarization curve of the 20 mol % composite
anode ͑specimen C͒ in the first and second runs, probably due to the
oxidation of a trace of water contained in the melt and/or formed at
the composite anode by reaction of nickel oxide with HF in the melt.
The anodic polarization curves of the Ni-NiO composite anodes
3
pores, dehydration of the electrolyte by pre-electrolysis may have
been conducted insufficiently in this case. The specimen of 10 mol
% composite ͑specimen B͒ also had a dense surface as that of the
HIP Ni. The oxidized layer had a thickness of only 2 to 3 ␮m, and
it was very dense over the entire range of depth. On the other hand,
the specimen of 20 mol % composite ͑specimen C͒ had a rough
surface with a number of crevices and pores, and the thickness of
the oxidized layer was 7 to 9 ␮m. Also, Fig. 7 shows the SEM
profile of the 20 mol % composite anode electrolyzed at 25 mA/cm2
͑
specimens D and E͒ were almost similar to that of specimen C.
in the NH F•2HF melt at 100°C for 120 h. The surface looked
4
Surface morphologies of the HIP Ni, the Ni-NiO1ϩx composite,
smooth, but the cross section revealed that the interior of the oxi-
dized layer had many pores and cavities, presumably caused by
dissolution of NiO1ϩx during electrolysis.
and the Ni-NiO composite after electrolysis.—Figures 4, 5, and 6 are
the SEM profiles of the HIP Ni ͑specimen A͒, the 10 mol % com-
posite ͑specimen B͒, and the 20 mol % composite anodes ͑specimen
Figures 8a, b, and 9 also illustrate the SEM profiles of the Ni
sheet, the Ni-5 mol % NiO composite ͑specimen D͒ and the Ni-10
mol % NiO composite anodes ͑specimen E͒ after electrolysis at 25
C͒ polarized at 5 V in a dehydrated melt of NH F•2HF at 100°C,
4
respectively. The specimens were taken out of the melt under a
steady state when the anodic current became constant. The upper
photographs, labeled a, are of the surface of the oxidized layer, and
2
mA/cm for 120 h. The procedure used for preparation of the speci-
mens was the same as described above. The upper and the lower
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D112
Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
Figure 7. SEM images of the 20 mol % composite anode after electrolysis at
2
2
5 mA/cm for 120 h in a molten NH F•2HF at 100°C. ͑a͒ Surface, ͑b͒ cross
4
section.
Figure 8. SEM images of the Ni sheet and the Ni-5 mol % NiO composite
anodes electrolyzed at 25 mA/cm2 for 120 h in a molten NH F•2HF at
4
100°C ͑a͒ Ni sheet, ͑b͒ Ni-5 mol % NiO composite.
photographs in Fig. 9, labeled a and b, are the surface and the cross
section of the oxidized layers formed on the Ni-10 mol % NiO
composite anode, respectively. The Ni-NiO composite specimens
have the dense and adhesive surfaces of the oxidized layer contain-
ing a higher oxygen content.
result, it becomes evident that a new current peak at ca. 2.6 V
obtained on the potential sweep curve of the 20 mol % composite
anode ͑specimen C͒, as shown in Fig. 3c, would be due to the direct
discharge of water and ammonium ion in the melt.
Electrolysis of a molten NH F•2HF with the HIP Ni, the
4
Table I shows the current efficiencies for the constituents in the
anode gas, the overall generation of anode gas, and hydrogen
evolved at the cathode, and the current loss caused by Ni dissolu-
tion. Electrolysis with an anode of commercial nickel sheet ͑speci-
men A-1͒ was conducted as a control. The current efficiency for NF3
formation on the 10 mol % composite anode ͑specimen B͒ was
small compared with those on the HIP Ni ͑specimen A͒ and the Ni
sheet anodes ͑specimen A-1͒, and the anode consumption of the 10
mol % composite anode ͑specimen B͒ was small compared with
those of specimens A-1, A, and C. In contrast, the current efficiency
Ni-NiO
composite, and Ni-NiO composite anodes.—Electrolysis
ϩx
1
2
of molten NH F•2HF was conducted at 25 mA/cm with the HIP Ni
4
͑
specimen A͒, the 10 mol % composite ͑specimen B͒, the 20 mol %
composite ͑specimen C͒, the Ni-2 mol % NiO ͑specimen D-1͒, the
Ni-5 mol % NiO ͑specimen D͒, and the Ni-10 mol % NiO compos-
ite anodes ͑specimen E͒. The temperature of the electrolyte was kept
constant at 100°C during the experiments. Figure 10 illustrates an
example of the time variations of the gas composition on the 20 mol
%
composite anode ͑specimen C͒ and the cell voltage on the same
anode. The upper figure, labeled a, is the time variation of the gas
composition on the 20 mol % composite anode ͑specimen C͒ and
the lower figure, labeled b, is the time variation of the cell voltage
for NF formation on the 20 mol % composite anode ͑specimen C͒
was small and its anode consumption was high compared with those
of specimens A-1, A, and B. On the other hand, the current effi-
3
on the same anode. The anode gas was composed of N , O , NF ,
2
2
3
ciency for NF formation on the Ni-NiO composite anode ͑specimen
3
N F , N F , and N O. NF was evolved at 20 h after switch-on,
2
2
2
4
2
3
D-1, D, and E͒ electrolyzed in a molten NH F•2HF was 49.5 to
4
because the cell voltage attained 5 V. The current efficiency for NF3
formation reached ca. 48% at 20 h, then it decreased acutely and
became ca. 24% after electrolysis for about 60 h. However, the
anode gas evolved at ca. 3 V for the first 20 h was composed of N2 ,
O , and N O without any fluorine-containing compound. With this
52.8%, and it was almost the same as or somewhat smaller com-
pared with that of the Ni sheet anode. The addition of LiF in molten
NH F•2HF increased the current efficiency for NF formation. The
4
3
anode consumption of the Ni-NiO composite anode was 0.093 to
0.503% and was small compared with that of the Ni sheet anode. In
2
2
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Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
D113
Figure 10. The time variations of the gas composition on the ͑a͒ 20 mol %
composite anode and ͑b͒ the cell voltage on the same anode electrolyzed at
2
2
5 mA/cm in a molten NH F•2HF at 100°C. ͑a͒ Time variation of the gas
4
composition on the Ni-20 mol % NiO1 composite anode. 1 ͑᭹͒; NF , 2
ϩx
3
͑
᭝͒; N , 3 ͑ᮀ͒; O , 4 ͑᭺͒; N F , 5 ͑᭡͒; N F , and 6 ͑᭿͒; N O. ͑b͒ Time
2 2 2 4 2 2 2
variation of the cell voltage on the same anode.
Figure 9. SEM images of the Ni-10 mol % NiO composite anodes electro-
on specimen E is high compared with that on a bare Ni sheet anode
͑specimen A-1͒.
2
lyzed at 25 mA/cm for 120 h in molten NH F•2HF at 100°C. ͑a͒ Surface,
4
͑b͒ cross section.
Figure 12 shows the XRD diagrams of the oxidized layer formed
on the Ni-10 mol % NiO composite anode ͑specimen E͒ electro-
2
lyzed at 25 mA/cm in the NH F•2HF melts with ͑Fig. 12b͒ and
4
the Ni-NiO system, the Ni-5 mol % NiO composite material ͑speci-
without 0.9 mol % LiF ͑Fig. 12a͒. The XRD analysis revealed that
the oxidized layer after electrolysis was composed of NiF2 , NiO,
and Ni. N2O3 was not observed on the XRD patterns. It may be
caused by a very small amount of it. On the other hand, the layer
formed only in the melt containing LiF had LixNiF5 , which was a
product from hydrolysis of LixNiF6 (x ϭ 2 and/or 3͒ during wash-
ing the specimen with 47% HF aqueous solution. Since the solubil-
ity of Li2NiF6 in the NH4F•2HF melt may be very small, Li2NiF6
may deposit on the surface of the Ni-NiO composite anode and may
men D͒ was the best for the current efficiency for NF formation and
3
the resistance to corrosion.
The XPS spectra of Ni 2p3/2 , F 1s, and O 1s levels for the
specimens D-1, D, and E after electrolysis of NH F•2HF melts with
4
2
and without LiF at 25 mA/cm for 120 h were similar to that those
1
3,14
of specimens A, B, and C.
The O 1s spectra have peaks at 531
and 533 eV assigned to NiO and Ni O , respectively. This means
2
3
that the oxidized layer of the Ni-NiO composite anode after elec-
ϩ
9,11,12
trolysis is mainly composed of NiF with a small amount of nickel
also act as a fluorinating agent for NH4 at the anode.
2
oxide having plural oxidation states. Figure 11 shows the depth pro-
files of the oxidized layer formed on the Ni-NiO composite anode
after electrolysis. The ordinate shows the atomic ratio of F/O, which
is evaluated from the XPS intensities of F 1s and O 1s, and the
abscissa is the bombardment time corresponding to the depth. The
open triangular and rectangular points are the data for the Ni-NiO
system ͑specimen E͒, while the open circular points are of a bare Ni
sheet anode ͑specimen A-1͒. The atomic ratio vs. time curve for the
In order to elucidate the results given in Table I, the change of
the anode potential with the time of electrolysis was investigated
and the results obtained on the Ni-NiO composite anodes were
shown in Fig. 13. The top figure, labeled a, is the anode potential of
the Ni sheet ͑specimen A-1͒, and the middle figure, labeled b, is of
the Ni-10 mol % NiO composite anode ͑specimen E͒. Also, the
bottom figure, labeled c, is of the Ni-10 mol % NiO composite
anode ͑specimen E͒ polarized in molten NH F•2HF containing 0.9
4
2
Ni-NiO composite specimens electrolyzed at 25 mA/cm in
mol % LiF. The anode potential of the Ni sheet anode ͑specimen
A-1͒ rose up from the static potential at ca. 0.2 to ca. 6.2 V after a
few minutes of switching, decayed gradually through two maxima
and two minima and tended to a steady state with potential fluctua-
tion between ca. 5.2 and 5.6 V. This behavior is elucidated in the
NH F•2HF with and without 0.9 mol % LiF resembled that of a
4
bare Ni sheet anode electrolyzed at the same current density,
whereas the atomic ratio of the former was low compared with that
of the latter. This means that the oxygen content in the surface layer
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D114
Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
Table I. Current efficiencies for the constituents in the anode gas, the overall anode gas, and the cathode gas, and current loss caused by Ni
2
dissolution „Q ÕQ … on the Ni sheet, the HIP Ni, the Ni-NiO
composite, and the Ni-NiO composite anodes electrolyzed at 25 mAÕcm in a
2
a
t
1¿x
molten NH F"2HF with and without LiF at 100°C. Anode area ϭ 19.2 cm .
4
Current loss
caused by Ni
dissolution
Current efficiency of anode gases and the cathode gas/percent
Duration
͑hours͒
N2
O2
NF3
N2F2
N2F4
N O
Overall
H2
2
Specimen
A-1
Anode material
(Q /Q ) %
a
t
Ni sheet
Ni sheet
HIP Ni
100
100
120
120
120
25
100
115
130
120
109
13.47
16.07
14.96
27.31
38.72
17.31
25.98
17.96
14.94
3.75
12.14
9.85
7.71
4.04
8.03
16.03
15.69
15.73
12.97
13.33
8.34
62.34
51.78
60.80
45.01
24.63
48.40
49.48
52.80
53.85
50.34
53.79
0.47
1.01
1.83
2.66
0.41
1.39
0.73
0.50
1.04
4.52
5.89
2.87
2.37
2.92
3.33
12.96
1.74
0.77
0.78
0.74
6.45
0.95
3.08
4.63
3.69
4.72
3.19
2.81
1.17
2.06
9.11
13.48
13.28
94.36
85.70
91.91
87.07
87.94
87.68
93.81
89.82
92.67
91.87
94.51
91.50
82.40
87.99
84.15
83.53
83.05
—
—
—
—
—
3.22
2.31
2.18
1.14
2.26
—
0.45
0.093
0.263
0.503
0.279
A
B
C
Ni-10 mol % NiO1ϩx composite
Ni-20 mol % NiO1ϩx composite
Ni-20 mol % NiO1ϩx composite
Ni-2 mol % NiO composite
Ni-5 mol % NiO composite
Ni-5 mol % NiO composite ϩ LiF
Ni-10 mol % NiO composite
Ni-10 mol % NiO composite ϩ LiF
D-1
D
E
12.26
previous papers.13,14 The chronopotentiograms of the HIP Ni ͑speci-
men A͒ and the Ni-10 mol % NiO1ϩx composite anodes ͑specimen
B͒ were similar to that of the Ni sheet anode ͑specimen A-1͒. On the
other hand, the anode potential of the Ni-10 mol % NiO composite
oxidized layer mainly composed of NiF , resulting in passivity.
2
Even so, the Ni anode is dissolved to some extent to keep passivity
by formation of the film composed mainly of NiF2 according to
Reactions 1 and 5, and/or 5Ј
͑
specimen E͒ jumped at first from ca. 0.2 to ca. 3.2 V after a few
2
ϩ
Ni → Ni ϩ 2e
͓1͔
minutes of switching and stagnated at ca. 3 V through two maxima
and two minima for about 1 h. The direct discharge of water and
ϩ
4
ammonium ion (NH ) in the melt took place at this potential. At 1
h after switch-on, it jumped again to 5 V and reached also at steady
state with ͑Fig. 13b͒ and without potential fluctuation between 4.6
and 5.2 V ͑Fig. 13c͒. The potential at steady state on the composite
anode was low compared with that on a bare Ni sheet anode. This
fact indicates that the electrical resistance of the surface layer on
specimen E is small compared with that on a Ni sheet anode ͑speci-
ϩ
men A-1͒. The electrochemical fluorination of NH4 took place at 5
V. The behaviors of the anode potential with the Ni-NiO composite
͑
specimens D-1 and D͒ and the Ni-20 mol % NiO1ϩx composite
anode ͑specimen C͒ were almost similar to that of the Ni-10 mol %
NiO composite anode ͑specimen E͒, and the stagnation time at ca. 3
V increased with increasing concentration of nickel oxide such as
NiO and NiO1ϩx as shown in Fig. 14.
A fresh Ni anode is dissolved during stagnation of the potential
at 0.2 V as shown in Fig. 13 and 14 and is then covered with an
Figure 11. Depth profiles of the oxidized layer formed on the Ni sheet and
the Ni-10 mol % NiO composite anodes electrolyzed at 25 mA/cm for 120
Figure 12. XRD patterns of the Ni-NiO composite anodes electrolyzed at 25
2
2
mA/cm for 120 h in a molten NH F•2HF with and without LiF at 100°C.
4
h in a molten NH F•2HF with and without LiF at 100°C. ͑᭺ Ni sheet anode
͑a͒ Ni-10 mol % NiO composite anode after electrolysis in a molten
NH4F•2HF and ͑b͒ Ni-10 mol % NiO composite anode after electrolysis in
a molten NH4F•2HF with 0.9 mol % LiF. ͑᭺͒ Ni, ͑ᮀ͒ NiO, ͑᭝͒ NiF2 , ͑᭡͒
LixNiF5 .
4
electrolyzed in molten NH F•2HF, ͑᭝͒ Ni-10 mol % NiO composite anode
4
electrolyzed in a molten NH F•2HF, and ͑ᮀ͒ Ni-10 mol % NiO composite
4
anode electrolyzed in a molten NH F•2HF with 0.9 mol % LiF.
4
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Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
D115
Figure 13. Chronopotentiograms of the Ni sheet and the Ni-10 mol % NiO
2
composite anodes during electrolysis at 25 mA/cm in a molten NH F•2HF
4
with and without LiF at 100°C. ͑a͒ Ni sheet anode electrolyzed in molten
NH F•2HF, ͑b͒ Ni-10 mol % NiO composite anode in a molten NH F•2HF,
4
4
and ͑c͒ Ni-10 mol % NiO composite anode in a molten NH F•2HF with
4
Figure 14. Effect of the NiO content in the Ni-NiO composite anode
0.9 mol % LiF.
on the stagnation time at ca. 3 V observed on chronopotentiograms of the
2
composite anodes during electrolysis at 25 mA/cm in a molten NH F•2HF
4
at 100°C. ͑a͒ Ni-2 mol % NiO composite anode ͑specimen D-1͒, ͑b͒
Ni-5 mol % NiO composite anode ͑specimen D͒, and ͑c͒ Ni-10 mol % NiO
composite anode ͑specimen E͒.
Ni2ϩ ϩ 2FϪ ꢀ NiF2
͓5͔
Ϫ
Ni ϩ 2F → NiF ϩ 2e
͓5Ј͔
2
NiO1ϩx ϩ 2͑1 ϩ x͒HF → ͑1 Ϫ 2x͒Ni ϩ 2xNi3ϩ
2ϩ
On the other hand, a part of the Ni on the composite anode is also
dissolved according to the same reactions and the surface of the
Ϫ
ϩ 2͑1 ϩ x͒F ϩ ͑1 ϩ x͒H O
2
composite anode is partially covered with NiF . In this case, direct
discharge of water and ammonium ion can take place on part of the
2
͓10͔
Ϫ
nickel oxide such as NiO and NiO1ϩx to form O , N , and N O,
2NiO ϩ 4F → 2NiF2 ϩ O2 ϩ 4e
͓11͔
2
2
2
according to Reactions 6 to 8 as shown in Fig. 10
Ϫ
3ϩ
2
NiO1ϩx ϩ 4͑1 ϩ x͒F → 2͑1 Ϫ 2x͒NiF ϩ 4xNi
2
ϩ
Ϫ
2
2
H O → O ϩ 4H ϩ 4e
͓6͔
2
2
ϩ ͑1 ϩ x͒O ϩ 12xF
2
ϩ
ϩ
ϩ 4͑1 ϩ x͒e
͓12͔
NH4 → N ϩ 8H ϩ 6e
͓7͔
͓8͔
2
After part of the nickel oxide on the surface of the composite anode
is also covered with an oxidized layer composed mainly of NiF2 ,
fluoride ions can discharge and hence the electrochemical fluorina-
ϩ
ϩ
2
NH4 ϩ H O → N O ϩ 10H ϩ 8e
2
2
ϩ
The oxidized layer and nickel oxides such as NiO and NiO1ϩx on
the composite anode are dissolved and/or fluorinated by Reactions 9
to 12
tion of NH4 takes place on it.
Conclusion
The nickel-nickel oxide ͑Ni-NiO and Ni-NiO1ϩx) composites
2
ϩ
Ϫ
NiO ϩ 2HF → Ni ϩ 2F ϩ H O
͓9͔
were prepared from a mixture of Ni and NiO or LiNiO powders at
2
2
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D116
Journal of The Electrochemical Society, 150 ͑6͒ D108-D116 ͑2003͒
900°C under 2000 atm for 2 h by HIP, and were employed as the for Advanced Science and Technology at Doshisha University from
anode for electrolytic production of NF . Since the density of HIP
the Ministry of Education, Japan.
3
Ni was almost the same as that of the commercial Ni sheet, the
material prepared by HIP was dense and had a low porosity. The
surface and the cross section of the HIP Ni and the Ni-NiO and the
Doshisha University assisted in meeting the publication costs of this
article.
2
Ni-NiO1ϩx composite specimens after electrolysis at 25 mA/cm or
References
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4
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2
2
3
2
2
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4
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4
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2
6
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1
formation and preventing the anode from corroding.
Acknowledgment
The authors wish to express many thanks to Morita Chemical
Industries Co. Ltd., for supplying chemicals.
1
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This work was supported by the Aid of Doshisha University’s
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