Electrolytic production of NF3 with a LiNiO2 coated nickel sheet anode prepared by atmospheric plasma spraying technique

Article abstract of DOI:10.1016/S0013-4686(01)00683-1

A nickel sheet coated with LiNiO₂ powder having average particle sizes of 40 and 50 μm in diameter by atmospheric plasma spraying technique was employed as the anode for electrolytic production of NF₃. In electrolysis of a molten NH₄F·2HF at 100°C and 25 mA cm^-2, the anode gas generated at the LiNiO₂ coated Ni sheet anode was composed of N₂, O₂, NF₃, N₂F₂, N₂F₄, and N₂O, and its composition was almost the same as that at the Ni sheet anode. The current efficiency for the NF₃ formation on the LiNiO₂ coated Ni sheet anode was increased to reach the constant value of ca. 55% during electrolysis for 100 h, and it was almost the same as that on the Ni sheet anode. The anode consumption of the LiNiO₂ coated Ni sheet was small compared with that of the Ni sheet. Also, the oxygen content in the oxidized layer formed on the LiNiO₂ coated Ni sheet anode was high compared with that on the Ni sheet anode, and the surface of the LiNiO₂ coated Ni sheet anode was covered with a compact and adhesive film having some defects. Although the bottom of the hollow was covered with a thinner layer, no pore penetrated through the oxidized layer. Hence, the LiNiO₂ coated Ni sheet anode is favorable for the electrolytic production of NF₃, and the oxidized layer on the LiNiO₂ coated Ni sheet anode has the higher resistance to corrosion, because of the compact and adhesive film containing the higher content of oxygen formed on the anode.

Full text of DOI:10.1016/S0013-4686(01)00683-1

Electrochimica Acta 46 (2001) 4349–4358
www.elsevier.com/locate/electacta
Electrolytic production of NF3 with a LiNiO2 coated nickel
sheet anode prepared by atmospheric plasma spraying
technique
Akimasa Tasaka a,*,1, Yasuhiro Suzuki a, Shin-ichiro Sakaguchi a,
Nobuhiro Fukuda b, Tetsuro Tojo c
a Department of Molecular Science and Technology, Faculty of Engineering, Doshisha Uni6ersity, Miyako-dani 1-3, Tadara,
Kyotanabe, Kyoto 610-0321, Japan
b Research and De6elopment Department, Shimonoseki Mitsui Chemicals Co. Ltd, Sakomachi 7-1-1, Hikoshima, Shimonoseki,
Yamaguchi 750-0092, Japan
c Ohnohara Research and De6elopment Center, Toyo Tanso Co. Ltd, Nakahime 2181-2, Ohnohara-cho, Mitoyo-gun,
Kagawa 769-1612, Japan
Received 26 March 2001; received in revised form 11 June 2001
Abstract
A nickel sheet coated with LiNiO2 powder having average particle sizes of 40 and 50 mm in diameter by
atmospheric plasma spraying technique was employed as the anode for electrolytic production of NF3. In electrolysis
of a molten NH4F·2HF at 100 °C and 25 mA cm−2, the anode gas generated at the LiNiO2 coated Ni sheet anode
was composed of N2, O2, NF3, N2F2, N2F4, and N2O, and its composition was almost the same as that at the Ni sheet
anode. The current efficiency for the NF3 formation on the LiNiO2 coated Ni sheet anode was increased to reach the
constant value of ca. 55% during electrolysis for 100 h, and it was almost the same as that on the Ni sheet anode.
The anode consumption of the LiNiO2 coated Ni sheet was small compared with that of the Ni sheet. Also, the
oxygen content in the oxidized layer formed on the LiNiO2 coated Ni sheet anode was high compared with that on
the Ni sheet anode, and the surface of the LiNiO2 coated Ni sheet anode was covered with a compact and adhesive
film having some defects. Although the bottom of the hollow was covered with a thinner layer, no pore penetrated
through the oxidized layer. Hence, the LiNiO2 coated Ni sheet anode is favorable for the electrolytic production of
NF3, and the oxidized layer on the LiNiO2 coated Ni sheet anode has the higher resistance to corrosion, because of
the compact and adhesive film containing the higher content of oxygen formed on the anode. © 2001 Elsevier Science
Ltd. All rights reserved.
Keywords: Nitrogen trifluoride; Nickel oxide coated nickel sheet anode; Atmospheric plasma spraying technique; Current efficiency;
Corrosion
1. Introduction
Large amounts of nitrogen trifluoride (NF3) is con-
sumed as a dry etchant and a cleaner gas for the CVD
* Corresponding author. Tel.: +81-774-65-6592; fax: +81-
chamber by the electronic industry in Japan. Pure NF3
free from carbon tetrafluoride (CF4) can be obtained by
the electrolysis of NH4F·2HF with a nickel anode to
774-65-6841.
E-mail address: atasaka@mail.doshisha.ac.jp (A. Tasaka).
1 ISE member.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(01)00683-1

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A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
meet demand. However, a relatively high corrosion rate
and passivation of the nickel anode are problems for
the electrolytic production of NF3. According to the
previous work [1–3], the nickel dissolution is dimin-
ished by traces of water in the melt, whereas the current
efficiency for NF3 formation decreases. The surface
layer on the anode formed in a molten salt containing
water was compact and its oxygen content was high
[2,3]. When the oxidized layer on the nickel anode has
a higher content of nickel oxide, it has a higher electric
conductivity and a higher resistance to corrosion [4]. To
elucidate the performance of the Ni sheet anode for the
electrolytic production of NF3 in a molten NH4F·2HF,
the properties of the Ni sheet anode covered with an
oxidized layer composed of NiF2 with a small amount
of nickel oxide having multiple oxidation states was
investigated with the thermally oxidized and/or fluori-
nated Ni sheet anode in detail [5]. On the other hand,
the addition of LiF in a molten NH4F·2HF increases
the current efficiency for NF3 formation, and also the
co-existence of the highly oxidized nickel compounds
such as Li2NiF6 and NiF3 in the oxidized layer formed
on the Ni anode cause an increase of it [6–8]. Since the
nickel oxide coated Ni sheet anode applied for electro-
chemical fluorination may be compared to a DSA
electrode for a brine electrolysis, the LiNiO2 coated Ni
sheet was prepared by atmospheric plasma spraying
technique.
was used as the reference electrode, and the potential
was corrected to the potential scale for hydrogen evolu-
tion on a Pt plate, designated as V versus H2, in the
text. The potential of this reference electrode deter-
mined at 100 °C was 10095 mV versus H2 during the
electrochemical measurements. When the electrochemi-
cal measurement, the corrosion test, and the prepara-
tion of the specimen for the XPS 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 a carbon anode at
ca. 10 mA cm−2 for about ten days to reduce the water
content to less than 0.02 wt% [9,10] or ca. 20
mmol dm−3, prior to the electrochemical, corrosion,
and XPS studies. For the corrosion test, Specimens B
and C as well as the Ni sheet anode (Specimen A) were
washed with water and methanol before weighing. The
current losses caused by nickel dissolution were calcu-
lated from the weight loss with the assumption of
two-electron transfer for the reaction (1):
Ni“Ni2+ +2e−
(1)
The cell employed for the electrolytic production of
NF3 was a cylindrical Ni cell (1.5 dm3) as described in
the previous paper [1,11]. Specimens A–C of 19.2 cm2
in surface area were employed as the anode. The anode
was located at the center of the cell whereas the cell
wall was utilized as the cathode. The Ni rod of 0.1 cm2
in surface area provided with pre-treatment of anodic
oxidation in a dehydrated melt of NH4F·2HF was used
as the reference electrode, and it is considered to func-
tion as a Ni/NiF2 electrode. A nickel sheet skirt was
provided between the anode and the cathode to sepa-
rate the anode gas from hydrogen generated at the
cathode so that explosions and the loss of NF3 are
prevented. The cell bottom was covered with a PTFE
sheet to avoid hydrogen evolution.
Electrolysis was conducted with the nickel cell at
100 °C. Although the water content was high before
startup, it might be decreased by electrolysis to less
than 0.02 wt% within 80 h [6]. The anode gas was
treated with NaF to eliminate HF before the chromato-
graphic and IR spectroscopic analyses [1]. The current
efficiencies for the constituents were evaluated from the
results of gas analysis and the flow rate of anode gas
[1,6,7].
In order to prepare the specimen for SEM, XPS, and
XRD studies, the LiNiO2 coated Ni sheet anode was
electrolyzed at 25 mA cm−2 in a dehydrated melt of
NH4F·2HF at 100 °C for 120 h. The test specimen was
washed with 47% HF aqueous solution to remove
adhesive melt on the surface prior to inspection by
XPS, XRD, and SEM. ESCA-1000 (Shimadzu
Seisakusho Co. Ltd.) with an Al–Ka radiation (1400
eV) and RINT-2500 (Rigaku Electric Co. Ltd.) with a
Cu–Ka radiation were used for the XPS and the XRD
analyses, respectively [1–4,6,7].
This paper deals with the effect of the LiNiO2 coat-
ing on the current efficiency for NF3 formation and the
current loss caused by Ni dissolution.
2. Experiment
A Ni sheet was coated with LiNiO2 powders (average
particle sizes of 40 and 50 mm) by atmospheric plasma
spraying technique, and the specimens were prepared
by Tokaro Co. Ltd. The specimens are denoted as the
LiNiO2 (40 mm) (Specimen B) and the LiNiO2 (50 mm)
coated Ni sheets (Specimen C), respectively, as shown
in Table 1. These samples were used as the anode, and
their behaviors were investigated in a dehydrated melt
of NH4F·2HF. The LiNiO2 (40 mm) and the LiNiO2 (50
mm) coated Ni sheet anodes are also denoted as the
anode B and the anode C, respectively, in the text. A
fresh nickel sheet (Specimen A) was employed as a
control.
A box-type PTFE cell of ca. 0.5 dm3 in volume was
provided for the electrochemical and corrosion studies,
and the cell configuration was described in the previous
paper [1,3,4]. The LiNiO2 coated Ni sheet specimen
(Specimens B and C) of 1 cm2 in surface area and a
nickel sheet having a large surface area were used as the
anode and the cathode, respectively. A nickel rod im-
mersed in molten NH4F·2HF saturated with NH4NiF3

Table 1
Current efficiencies for the constituents in the anode gas and the overall anode gas, and the current loss caused by Ni dissolution (Qa/Qt) on the Ni sheet and the LiNiO2 coated
Ni sheet anodes electrolyzed at 25 mA cm−2 in a molten NH4F·2HF at 100 °C
/
Specimen
Materials
Current efficiencies of anode gases (%)
N2 O2 NF3 N2F2 N2F4 N2O Overall
Current loss caused by anodic dissolution (Qa/Qt) (%)
A
B
C
Ni sheet
Ni sheet
l00 h 13.47 12.14
100 h 16.07 9.85
100 h 13.11 11.85
l00 h 25.57 11.88
l00 h 19.76 10.96
100 h 22.06 10.46
62.34 0.47 2.87 3.08 94.36
51.78 1.01 2.37 4.63 85.70
55.15 1.21 5.30 7.32 91.94
49.04 0.37 0.22 5.57 87.52
53.85 2.42 2.18 6.41 95.58
50.41 2.80 3.86 8.78 98.36
56.89 1.50 2.42 3.90 95.21
3.22
2.31
0.85
1.29
0.85
1.08
1.13
LiNiO2 (40 mm) coated Ni
LiNiO2 (40 mm) coated Ni
LiNiO2 (50 mm) coated Ni
LiNiO2 (50 mm) coated Ni
LiNiO2 (50 mm) coated Ni
4
(
439
l00 h 21.00
9.50
Anode area=19.2 cm2.
4

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A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
Fig. 1. XRD patterns of the LiNiO2 coated Ni sheet prepared by atmospheric plasma spraying technique. (a) LiNiO2 (40 mm) coated
Ni sheet (Specimen B) and (b) LiNiO2 (50 mm) coated Ni sheet (Specimen C): (ꢀ) Ni; (ꢁ) LiNiO2; (ꢂ) NiO; (ꢃ) Ni2O3.
3. Results and discussion
Ni and LiNiO2 with a small amount of NiO and Ni2O3.
When a nickel sheet was coated with LiNiO2 particles
by atmospheric plasma spraying technique, a part of
LiNiO2 seemed to be decomposed and/or reduced with
a nickel substrate according to reactions (2)–(4), result-
ing in formation of NiO, Ni2O3, and Li2O [4].
3.1. The LiNiO2 coated Ni sheet specimen prepared by
atmospheric plasma spraying technique
The LiNiO2 coated Ni sheet specimens (Specimens B
and C) were prepared with LiNiO2 powders having
average particle sizes of 40 and 50 mm by atmospheric
plasma spraying technique. Fig. 1 shows the XRD
diagrams of the LiNiO2 coated Ni sheets. The upper
figure, labeled (a), is the X-ray diffraction pattern of
Specimen B, and the lower figure, labeled (b), is of
Specimen C. The XRD analysis indicated that the
surface layer of Specimens B and C was composed of
2LiNiO2“Ni2O3+Li2O
Ni2O3+Ni“3NiO
(2)
(3)
(4)
2LiNiO2+Ni“3NiO+Li2O
Fig. 2 illustrates the XPS spectra of Ni 2p3/2 and O 1s
levels on the surface of specimens. The left figure,
labeled (1), is the XPS spectra of the surface on Speci-
Fig. 2. XPS spectra of Ni 2p3/2 and O 1s levels on the LiNiO2 coated Ni sheet prepared by atmospheric plasma spraying technique.
(1) LiNiO2 (40 mm) coated Ni sheet (Specimen B) and (2) LiNiO2 (50 mm) coated Ni sheet (Specimen C). (a) Outermost surface.
Exposed to Ar+ beam with beam current=20 mA at 2 kV for: (b) 0.5 min; (c) 1 min; (d) 4 min; (e) 20 min.

A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
4353
surface layer of Specimens B and C is composed of
LiNiO2 with a small amount of Ni and nickel oxides
such as Ni2O3 and NiO.
The surface morphologies of the LiNiO2 coated Ni
sheet were observed by means of SEM. Figs. 3 and 4
show the SEM profiles of Specimens B and C, respec-
tively. The upper figures, labeled (a), are the surfaces of
the specimens, and the lower, labeled (b), are the cross-
sections of them. Although the LiNiO2 particles with
average diameters of 40 and 50 mm were used for
coating, the particle size on the surface of Specimens B
and C is 5–20 mm. The SEM observation also revealed
that the surface of the LiNiO2 coated Ni sheets (Speci-
mens B and C) prepared by atmospheric plasma spray-
ing technique was rugged and that the thickness of the
layers on Specimens B and C was 20–50 mm.
Fig. 3. SEM images of the LiNiO2 (40 mm) coated Ni sheet
prepared by atmospheric plasma spraying technique (Specimen
B): (a) surface and (b) cross-section.
men B and the right figure, labeled (2), is of that on
Specimen C. Curve a is the specimen for the outermost
surface while b–e are the data for different exposures,
0.5, 1, 4, and 20 min, respectively. The peaks at 852.7,
854.5, 855.7, and 861.4 eV on the Ni 2p3/2 spectra are
assigned to Ni, NiO, Ni2O3 and LiNiO2, and NiO or
Ni2O3 and LiNiO2, respectively [12]. The peak at 852.7
eV on the Ni 2p3/2 spectrum for the outermost surface
was almost not observed (see Fig. 2-1a) or sometimes
appeared on the outermost surface (see Fig. 2-2a) and
its height was increased with the bombardment time of
Ar+ beam or the depth. This means that the Ni sheet is
covered thoroughly with the layer mainly composed of
LiNiO2 or that a part of the nickel substrate is exposed,
presumably because of the lack of the coverage with
LiNiO2. Also, a large peak at 532 eV on the O 1s
spectra is assigned to LiNiO2 and Ni2O3. The results
from the XRD and XPS analyses describe that the
Fig. 4. SEM images of the LiNiO2 (50 mm) coated Ni sheet
prepared by atmospheric plasma spraying technique (Specimen
C): (a) surface and (b) cross-section.

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A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
3.2. Electrolysis of a molten NH4F·2HF with LiNiO2
coated sheet anodes
Fig. 5 illustrates the chronopotentiograms of a fresh
nickel, Specimen A, the LiNiO2 (40 mm), Specimen B,
and the LiNiO2 (50 mm) coated Ni sheets, Specimen C,
in a dehydrated melt of NH4F·2HF at 100 °C and 25
mA cm−2. The top figure, labeled (a), is the chronopo-
tentiogram of a bare Ni sheet. The middle and the
bottom figures, labeled (b) and (c), are of the LiNiO2
coated Ni sheets, Specimens B and C, respectively. The
potential of bare Ni rose up from the static potential at
ca. 0.2 to ca. 6.2 V after few minutes of switching,
decayed gradually through two maxima and two min-
ima, and tended to a steady state with potential fluctu-
ation between ca. 5.2 and 5.6 V. This behavior is
elucidated in the previous paper [5,13]. In the case of
the LiNiO2 coated Ni sheet specimens, the potential
jumped to ca. 5.8 V soon after switch-on, decayed
gradually, and reached also a steady state with poten-
tial fluctuation between ca. 4.6 and 5.2 V. The fluctua-
tion of potential is presumably caused by repetitive
electrochemical formation and dissolution of surface
oxide and/or fluoride on the Ni substrate [5,13]. The
potential at steady state on the coated anode was low
compared with that on a bare nickel anode. This fact
indicates that the electrical resistance of the surface
layer on Specimens B and C is small compared with
that on a nickel sheet anode (Specimen A) [4].
Fig. 5. Chronopotentiograms of the Ni and the LiNiO2 coated
Ni sheet anodes during electrolysis at 25 mA cm−2 in a
molten NH4F·2HF at 100 °C: (a) Ni sheet (Specimen A); (b)
LiNiO2 (40 mm) coated Ni sheet (Specimen B); and (c) LiNiO2
(50 mm) coated Ni sheet (Specimen C).
Fig. 6 shows an example of the time variation of the
current efficiencies for constituents in the anode gas
evolved on the anode C. Water in the melt was elec-
Fig. 6. The change of current efficiencies for constituents in the anode gas during electrolysis at 25 mA cm−2 in a molten NH4F·2HF
at 100 °C with LiNiO2 (50 mm) coated Ni sheet electrode (Specimen C) as a function of time: (ꢃ) N2; (ꢄ) O2; (ꢅ) NF3; (ꢁ) N2F2;
(ꢂ) N2F4; (ꢀ) N2O.

A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
4355
Fig. 7. XRD patterns of the LiNiO2 coated Ni sheet anodes electrolyzed at 25 mA cm−2 for 120 h in a molten NH4F·2HF at
100 °C: (a) LiNiO2 (40 mm) coated Ni sheet (Specimen B) and (b) LiNiO2 (50 mm) coated Ni sheet (Specimen C). (ꢀ) Ni; (ꢅ) NiF2.
trolyzed in preference to the fluorination of NH+4 for
some 20 h after startup. NF3 was formed after electrol-
ysis only for 5 h, and the gas composition became
constant after electrolysis for 100 h. NF3 was the main
product, followed by N2 with a small amount of O2,
N2O, N2F2, and N2F4.
Table 1 shows the current efficiencies for the con-
stituents in the anode gas and the overall current
efficiency. The current inefficiency caused by the disso-
lution of nickel from the anode is shown in the last
column. The average current efficiency for NF3 forma-
tion on the anode B, was ca. 53% and that on the
anode C (ca. 54%) was similar to that on the anode B.
Since the current efficiency for NF3 formation on a
fresh Ni sheet anode was 52–62%, and hence the
current efficiency for NF3 formation on the coated
anode was almost the same as that on a bare Ni sheet
anode. Although NF3 is generally detected in the anode
gas evolved on a fresh Ni sheet anode after electrolysis
for more than 20 h, it is formed on the coated anode
after electrolysis for only 5 h, as shown in Fig. 6. The
existence of Li+ ions on the anode and/or in the melt
may cause the higher current efficiency for NF3 forma-
tion, presumably because of deposition of Li2NiF6 on
the anode, which may act as a strong fluorinating agent
[6,8,14,15]. On the other hand, the current loss caused
by Ni dissolution on the coated anode (Specimens B
and C) was only 0.85–1.29%. Hence, the oxide coating
on the Ni sheet anode is effective in decreasing the
anode consumption.
Under the conditions of interest, NH+4 and NH3 are
in equilibrium, as shown in Eqs. (5) and (6),
NH4F+nHF X NH+4 +F(HF)n−
NH+4 +mHF X NH3+H(HF)+m
(5)
(6)
where F(HF)−n and H(HF)+m are solvated ions of F−
and H+, respectively, and are now written as F− and
H+ for simplicity. The amount of NH3 in the melt is
generally considered negligible. Nitrogen trifluoride and
nitrogen are evolved at the anode by reactions (7)–(9),
NH+4 +7F−“NF3+4HF+6e−
2NH+4 +8F−“N2+4HF+6e−
2NH3“N2+6H++6e−
(7)
(8)
(9)
where reaction (9) is the direct discharge of NH3 ad-
sorbed on the anode. However, the side reactions (10)–
(14) may also proceed in parallel to form minor
products, e.g. N2F2, N2F4, N2O, and O2,
3.3. Surface morphologies on the LiNiO2 coated Ni
sheet anode after electrolysis of NH4F·2HF melt
2NH+4 +10F−“N2F2+8HF+8e−
2NH+4 +12F−“N2F4+8HF+10e−
2NH+4 +H2O+10F−“N2O+10HF+8e−
2H2O+4F−“O2+4HF+4e−
2H2O“O2+4H++4e−
(10)
(11)
(12)
(13)
(14)
The surface layer on the coated anodes after electrol-
ysis of NH4F·2HF melt at 25 mA cm−2 for 120 h was
analyzed by XRD, and the result obtained was shown
in Fig. 7. The upper figure, labeled (a), is the X-ray
diffraction pattern of Specimen B, and the lower figure,
labeled (b), is that of Specimen C. The XRD analysis
revealed that the surface layer after electrolysis was
where reaction (14) is the direct discharge of water
adsorbed on the anode.

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A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
composed of NiF2 and Ni, and no nickel oxide was
detected in the surface layer. This means that nickel
oxides such as LiNiO2, Ni2O3, and NiO in the layer are
dissolved and/or fluorinated during electrolysis to form
water and oxygen according to reactions (15)–(20). The
formed water is also oxidized to evolve oxygen gas
according to reactions (13) and (14).
Fig. 8 shows the XPS spectra for the anode C after
electrolysis of NH4F·2HF melt at 25 mA cm−2 for 120
h. The XPS spectra for the anode B is similar to those
for the anode C. Curve a shows the features of the
outermost surface, and curve e on the bottom is a
profile near the Ni substrate. The peak at 852.7 eV on
the Ni 2p3/2 spectra is assigned to Ni, and the peak
height is increased with the bombardment time or the
depth. The peaks at 857.7 eV on the Ni 2p3/2 spectrum
and at 686.0 eV on the F 1s spectrum are assigned to
NiF2, and these peaks diminished with the bombard-
ment time. The peak at 532 eV on the O 1s spectrum
appears on the outermost surface (a), and it is assigned
to H2O adsorbed on the surface of the anode. The
peaks at 531 and 533 eV near the Ni substrate are
assigned to NiO and Ni2O3, respectively [5,12,13]. With
these results, the oxidized layer on the coated anode
after electrolysis is composed of NiF2 with a small
amount of NiO and Ni2O3, and it is similar to that on
a bare nickel after electrolysis.
Fig. 9 shows the depth profiles of the constituents in
the surface layer of the test specimens. 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 coated anode (Specimens B and C), respectively,
while the open circular points are of a bare Ni sheet
anode (Specimen A). The atomic ratio versus time
curves for the LiNiO2 coated specimens electrolyzed at
25 mA cm−2 resembled to the bare Ni sheet anode
electrolyzed at the same current density, whereas the
atomic ratios of the former were low compared with
that of the latter. This means that the oxygen content in
LiNiO2+4HF“Li++Ni3+ +4F−+2H2O
Ni2O3+6HF“2Ni3+ +6F−+3H2O
NiO+2HF“Ni2+ +2F−+H2O
(15)
(16)
(17)
(18)
(19)
(20)
LiNiO2+4F−“O2+LiF+NiF3+4e−
2Ni2O3+12F−“3O2+4NiF3+12e−
2NiO+4F−“O2+2NiF2+4e−
Fresh nickel is dissolved during stagnation of poten-
tial at 0.2 V as shown in Fig. 5a and then is covered
with the oxidized layer mainly composed of NiF2,
resulting in passivity. Even so, the Ni anode is dissolved
to some extent to keep passivity by formation of NiF2
according to reactions (1) and (21), and/or (21%).
Ni“Ni2+ +2e−
(1%)
(21)
(21%)
Ni2+ +2F− X NiF2
Ni+2F−“NiF2+2e−
When a part of the nickel on the coated anode is
exposed to a dehydrated melt of NH4F·2HF during
electrolysis, reactions (1) and (21), and/or (21%) also
take place on nickel. The oxidized layer may contribute
to the stability of passivity, but nickel oxides in the
surface layer are dissolved and/or fluorinated by reac-
tions (15)–(20).
Fig. 8. XPS spectra of Ni 2p3/2, F 1s, and O 1s levels on the LiNiO2 (50 mm) coated Ni sheet anode (Specimen C) electrolyzed at
25 mA cm−2 for 120 h in a molten NH4F·2HF at 100 °C. (a) Outermost surface. Exposed to Ar+ beam with beam current=20
mA at 2 kV for: (b) 0.5 min; (c) 1 min; (d) 4 min; (e) 20 min.

A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
4357
Fig. 9. Depth profiles of the oxidized layer on the Ni and the LiNiO2 coated Ni sheet anodes electrolyzed at 25 mA cm−2 for 120
h in a molten NH4F·2HF at 100 °C: (a) Ni sheet (Specimen A); (b) LiNiO2 (40 mm) coated Ni sheet (Specimen B); and (c) LiNiO2
(50 mm) coated Ni sheet (Specimen C).
the surface layer on Specimens B and C is high com-
pared with that on a bare Ni sheet anode (Specimen A).
This may cause the lower electric resistance of the
surface layer on Specimens B and C than that on
Specimen A, as shown in Fig. 5.
Fig. 10 shows the SEM profiles of the surface on a
fresh Ni sheet (Specimen A) and the LiNiO2 (50 mm)
coated Ni sheets (Specimen C) electrolyzed at 25
mA cm−2 for 120
h
in
a dehydrated melt of
NH4F·2HF. The left-top figure, labeled (a), is the sur-
Fig. 10. SEM images of the Ni and the LiNiO2 (50 mm) coated Ni sheets electrolyzed at 25 mA cm−2 in a molten NH4F·2HF at
100 °C: (a) Ni sheet (Specimen A), surface; (b) LiNiO2 (50 mm) coated Ni sheet (Specimen C), surface; and (c) LiNiO2 (50 mm)
coated Ni sheet (Specimen C), cross-section.

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A. Tasaka et al. / Electrochimica Acta 46 (2001) 4349–4358
Kanemaru, N. Iwanaga, M. Aritsuka, J. Electrochem.
Soc. 141 (1994) 1460.
face morphology of Specimen A. The lower figures,
labeled (b) and (c), are the surface and the cross-section
of Specimen C, respectively. The surface of bare nickel
sheet anode was rough and had a number of pores (see
Fig. 10a). In contrast, the coated anode was covered
with the slightly rugged layer, which was compact and
adhesive. Also, the SEM image of the cross-section
indicates that even the bottom of the hollow is covered
with a thinner layer without pores that penetrates
through the oxidized layer. This may cause the lower
anode consumption during electrolysis.
[3] A. Tasaka, Y. Tsukuda, T. Ohashi, S. Yamada, K. Mat-
sushita, A. Kohmura, N. Muramatsu, H. Takebayashi,
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Acknowledgements
The authors wish to express many thanks to Morita
Chemical Industries Co. Ltd for supplying chemicals.
This work was supported by Aid of Doshisha Univer-
sity’s Research Promotion Fund in 1996 and a grant to
Research Center for Advanced Science and Technology
at Doshisha University from the Ministry of Education,
Japan.
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