Electrodepositions on Tantalum in Alkali Halide Melts
J. H. von Barnera, A. H. Jensenb and E. Christensenb
a
b
DTU Chemistry, Technical University of Denmark, DK 2800 Lyngby, Denmark
DTU Energy Conversion,Technical University of Denmark,DK 2800 Lyngby, Denmark
Surface layers of tantalum metal were electrodeposited on steel
from K2TaF7-LiF-NaF-KF melts. With careful control of the oxide
contents dense and adherent deposits could be obtained by pulse
plating. In NaCl-KCl-NaF-Na2CO3 and NaCl-KCl-Na2CO3 melts
carbonate ions seems to be reduced to carbon in a single 4 electron
step. By electrolyses at a constant potential of – 1.4 V vs. Pt in a
NaCl-KCl-NaF-Na2CO3 melt at 800 oC coherent carbon containing
surface layers could be obtained on tantalum substrates, when a
CO2 atmosphere was applied.
Introduction
We have previously developed plating processes with refractory metals, e.g. niobium and
tantalum, from both fluoride (1) and chloride melts (2). To avoid acidic corrosion we
apply tantalum coatings on the stainless steel felt used as gas diffusion layer at the anode
and on the bipolar plates in our PEM electrolyser cells. A draw back may be that the
Ta2O5 layer, naturally formed on tantalum surfaces, is non-conductive and may cause
efficiency losses. Further tantalum is rather soft. It is therefore of interest to form a harder
and more electrical conductive surface layer on the tantalum coating. In this connection
tantalum carbide is a promising candidate.
Massot et al. (3) has recently demonstrated that coatings of tantalum carbide can be
obtained by reaction of electro-deposited carbon with a tantalum substrate. Carbon was
formed by electrochemical reduction of carbonate ions in molten fluoride mixtures.
However fluoride melts are toxic and often difficult to remove from the samples. Thus
we decided to investigate the possibilities to prepare tantalum carbide coatings by use of
mixed chloride-fluoride melts and pure chloride melts.
Experimental
The alkali fluorides (Analytical grade from Merck) were purified by recrystallization
from the molten state. K2TaF7 was prepared by dissolving high purity Ta2O5 and KF in
hot hydrogen fluoride and subsequent cooling. Anhydrous alkali chlorides were obtained
by treating analytical-grade salts in the molten state with dry HCl gas. The purified
chemicals were kept in sealed ampoules. Handling and weighing were done in a nitrogen
filled glove box and the experiments were performed under controlled atmosphere (argon
or carbon dioxide). Eutectic LiF-NaF-KF melts (FLiNaK) were the supporting liquid for
the tantalum depositions. NaCl-KCl-NaF (22.5:60.5: 17 mole %) and NaCl-KCl (50:50
mole %) melts were used as solvents for Na2CO3. Glassy carbon crucibles served as
containers for the melts. The samples were heated in a homemade closed furnace, where
a controlled atmosphere could be applied.
The voltametric measurements and the depositions were carried out with a
VersaSTAT4 potentiostat. Pt, Au and Ta wires (Goodfellow, 99.99 %) were used as
indicator electrodes in the voltammetrtic experiments. Tantalum foils (10x25x0.2 mm,
99.9% from Goodfellow) served as substrates for the depositions from the carbonate
containing melts. The counter electrodes were of glassy carbon and Pt wires (d=1mm)
were applied as reference electrodes.
The scanning electron micrographs were obtained with a Zeiss EVO MA 10 electron
microscope. The microscope was further equipped with an EDX unit.
Results and Discussion
Tantalum Deposition
It has previously been shown (4) that TaF72- in FLiNaK melts is reduced to tantalum
metal in one step i.e.
TaF72- + 5e- → Ta + 7 F-
[1]
If pulse plating is used high quality surface layers of tantalum metal can be plated
from FLiNaK melts e.g. by applying Ta(V) concentrations in the vicinity of 3.5 mole %
and oxide contents of less than 0.5 mole %. An example of such a surface layer can be
seen on Figure 1.
Figure 1. SEM micrograph of a tantalum surface on a rod of 316 steel pulse plated at
700oC from a LiF-NaF-KF eutectic melt with 3.5 mole % TaF5.
The pulse plating program consists of 3 phases:
1) Temperature equilibration phase, where the subject and the tantalum anode are
submersed in the melt for approximately 30 minutes.
2) Nucleation phase consisting of two separated periods with cathodic pulses (totally
5 seconds) followed by a shorter period with anodic polarization.
3) Growth phase with cathodic and anodic pulses for 0.5 and 0.1 second respectively.
The current densities were typically 47 mA/cm2 and 60 mA/cm2 for the cathodic and
anodic pulses, respectively.
Depositions from melts with carbonate present
Two subsequent linear sweep voltammograms performed with Pt working electrodes
on NaCl-KCl-NaF-Na2CO3 melts at 800 oC under an argon atmosphere showed a
significant diminishing of the signals from carbonate reduction. It seems likely that
carbonate is lost due to evaporation of CO2 as stated in Eq. 2.
CO32- → CO2 + O2-
[2]
In contrast the signals were more constant with time when a CO2 atmosphere was
applied. For both NaCl-KCl-NaF-Na2CO3 and NaCl-KCl-Na2CO3 melts two reduction
waves were typically observed: a rather weak wave near –1.0 V vs. Pt and a stronger one
starting at around -1.4 V vs. Pt. A wave similar to the latter has previously (5) been
ascribed to a one step reduction of carbonate ions to elementary carbon:
CO32- + 4 e- → C + 3 O2-
[3]
Figure 2 shows a typical cyclic voltammogram of this type, here obtained with a tantalum
indicator electrode. It can be noticed, that a nucleation loop appears at negative potentials.
Figure 2. Cyclic voltammogram of Na2CO3 (2.3 wt %) dissolved in a NaCl-KCl-NaF
melt at 800 oC. Scan rate: 0.8 V·s-1. Working electrode: tantalum; reference: Pt.
Both galvanostatic and potentiostatic depositions from carbonate containing melts on
tantalum substrates were attempted. Electrolysis at constant current under argon
atmosphere gave no deposits. The best results were obtained from melts under CO2
atmosphere at constant potentials of – 1.4 V or more negative (vs. Pt). The morphology
of such a deposit, obtained with NaCl-KCl-NaF as supporting melt, is shown in Figure 3.
Figure 3. Scanning electron micrograph of a deposit on tantalum from a NaCl-KCl-NaFNa2CO3 melt at 800 oC. Potentiostatic deposition for 15 min. at -1.4 V vs. Pt.
As can be seen the tantalum substrate is covered by a coherent layer with some nodules
present. An EDX analyses showed signals due to tantalum and carbon; but also some
oxygen seemed to be present.
References
1. J. H. von Barner, E. Christensen and N. J. Bjerrum, Eur.Pat.578605 (1994).
2. B. Gillesberg, J. H. von Barner, N. J. Bjerrum and F. Lantelme, J. Appl.
Electrochem., 29, 939 (1999).
3. L. Massot, P. Chamelot and P. Taxil, J. Alloys Comp., 424, 199 (2006).
4. L. P. Polyakova, E. G. Polyakov, F. Matthiesen, E. Christensen and N.J. Bjerrum,
J. Electrochem. Soc. 141, 2982 (1994).
5. L. Massot, P. Chamelot, F. Bouyer and P. Taxil, Electrochim. Acta, 47, 1949
(2002).