- Email: [email protected]
Spectroelectrochemical studies of alkali metal generation from solid state cells
.__ + __ lf!B
s
ELSEVlER
SOLID STATE Solid State Ionics 86-88 (1996) 1371-1378
Spectroelectrochemical
IONICS
studies of alkali metal generation from solid state cells S.C. Roy”‘*, P.G. Bruceb
“Northern Carbon Research Laboratories, Chemistry Department, Bedson Building, University of Newcastle-upon-Tyne, Newcastle-uponTyne, NE1 7RU, England, UK bCentre for Electrochemical and Materials Sciences, School of Chemistry, Purdie Building, University of St Andrews, St Andrew, Fife, KY16 9ST, Scotland, UK
Abstract p-alumina/metal grid, is presented which is capable of producing A solid state electrochemical cell, Na 0 ,,CoO,/sodium a beam of sodium vapour in UHV. Cells such as these have potential applications as a source of dopants in semiconductor fabrication. Electrochemical measurements and a new spectroelectrochemical technique provide mechanistic information regarding sodium electro-generation. Keywords: Electrochemical cell; Solid-state cell; Vapour beam doping; Spectroelectrochemical analysis; Overpotential
1. Introduction Solid state electrochemical cells are a growing part of electrochemistry and have a wide variety of applications [ 11. Common examples include lithiumion batteries and solid oxide fuel cells that generate portable electric power, sensors that detect gases and electrochromic devices such as SMART windows, through which the level of light transmission may be varied electrochemically. A much less familiar application is the use of solid state electrochemical cells to generate high purity beams of vapours that can be used to dope semiconductors. Recently we have demonstrated that electrochemically generated potassium vapour may be used to dope the II-VI semiconductor ZnSe in a *Corresponding author.
controlled fashion, thus forming p-type material during crystal growth by Molecular Beam Epitaxy [2,3]. This offers an alternative to nitrogen doping of ZnSe. The successful formation of p-type ZnSe is a critical goal in the field of optoelectronics, since p-n junctions, based on ZnSe, form the bases of blue LEDs and blue laser diodes [4]. This paper focuses on developing an understanding of the electrochemical mechanism responsible for alkali metal production from solid-state cells. As a model system we have chosen to present results concerning sodium evolution from an all-solid-state Naf -ion cell. After giving details concerning cell fabrication, this account provides laser ionisation mass spectroscopic (LIMA) evidence for sodium production, followed by results of cyclic voltammetric and ac impedance measurements. As part of our studies, we have developed a simultaneous mass
0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved PII SO167-2738(96)00312-S
1372
S.C.
P.G. Bruce
Solid State
spectrometric analysis of the vapour being electrogenerated in UHV during cyclic voltammetty.
2. Experimental 2.1. Preparation
of Na p-alumina
Na,CO, (Fisons) and A&O, (Fluka) were dried and then thoroughly ground together for 15 min in air in a I:5 mole ratio of Na,CO, to Al,O, [5]. The powder was placed in a platinum crucible, calcined in an electric muffle furnace at 650°C for 2 h, in order to decompose the carbonate, and then reacted for 24 h at 1200°C. The resulting material was subsequently quenched in air to room temperature and examined by powder X-ray diffraction using a Stoe STADI/P diffractometer and Cu Ka radiation. The product consisted of a mixture of /? and p” phases. 2.2. Preparation (Na o.,400,)
of sodium cobalt oxide
[61
A 3:2 molar ratio of Na,O, (BDH) to Co,O, (Aldrich) was ground together for 15 min in a MBRAUN argon filled glove box and then transferred to a gold boat, which was placed in a quartz tube fitted with two taps. The tube was removed from the glove box and placed inside a cylindrical furnace where the temperature of the sample was raised at a rate of 4”C/min until it reached 500°C. The mixture was left at this temperature for 2 days. After this period, the temperature was lowered to 25°C again at a rate of 4”C/min. Oxygen gas was permitted to flow across the sample throughout the entire heating period. A powder X-ray pattern of the resulting material, when compared with data from the powder diffraction data base, ASTM 32-1068, indicated that it was
86-88
(1996)
materials were subjected to a pressure of 1 ton for 5 min. The resulting composite pellet was placed with the p-alumina side downwards, in a gold boat lined with Na p-alumina powder and covered with sodium cobalt oxide. The boat was then placed in a silica tube fitted with taps, which again allowed oxygen to flow across the sample. The temperature was raised from 25°C to 500°C at a rate of 4”C/min, left at this temperature for 17 h and then cooled back to 25°C once again at 4”Clmin. This heating induced sintering of the cell and, hence, improved its mechanical properties. On removal from the silica tube, conductive silver paint was applied to the flat sodium cobalt oxide surface to act as a current collector. A stainless steel gauze disk was rubbed with emery paper, boiled in distilled water and thoroughly dried before being placed on the exposed flat surface of the solid electrolyte (Fig. 1). 2.4. Laser ionisation
mass analysis
(LIMA)
This technique involves the ablation of the surface of a solid and analysis of the resulting vapour phase by mass spectrometry. It was used to establish the electrochemical evolution of Na vapour. Measurements were carried out using a Cambridge Mass Spectrometry Ltd. LIMA (401 L) fitted with a Nd:YAG laser (Spectra-Physics DCR- 11) operating at 266 nm. 2.5. Three-electrode mass spectroscopy
measurements
and in-situ
In order to carry out electrochemical studies, a three-electrode cell was designed as shown in Fig. 2.
Na p-alumina
Na,.,,Co% 2.3. Fabrication
of two-electrode
ceramic cell
Nao.75CoO2 Sodium cobalt oxide (0.75g) was placed in a 13 mm die and gently compacted with the plunger by hand. A few drops of acetone were then added followed by 0.3 g of Na p-alumina. The two
Fig. 1. Solid state electrochemical sodium vapour.
’
Stainless Steel Gauze cell capable
of generating
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
M&3X
Gramic Cell H0lder Na p-alum& electrolyte
Cartridge Heaters NaO.7SCoO2 counter electrode stainless steel working electrode UHV CJWlbCr
Fig. 2. Spectroelectrochemical apparatus for monitoring gas evolution from the cell by in-situ mass spectroscopy, whilst recording cyclic voltammetric data. The three-electrode cell is positioned within a stainless steel heating block and the entire assembly is located inside an UHV chamber.
The working electrode consisted of a stainless steel gauze in contact with the p-alumina electrolyte, the counter and reference electrodes were formed from disks of Na,.,,CoO, with the reference electrode being located in close proximity to the working electrode. The counter electrode/electrolyte assembly was constructed in the same fashion as for the two-electrode cell. A Macor ceramic cell holder containing the three-electrode cell was placed in a stainless steel heating block. This was mounted on an UHV flange, which was also fitted with three electrode contacts for the electrochemical measurements, a power supply for the heaters and two thermocouple plugs; one for the heating block and one for the cell holder. Electrochemical measurements could be made up to 350°C and over a range of pressures from 10m3 to lo-* torr. To carry out simultaneous mass analysis along with the electrochemical studies, a quadrupole mass spectrometer head, shown in Fig. 2, was positioned beneath the cell to obtain an in-situ analysis of gas evolution. The ioniser was placed approximately 2 cm from the stainless steel grid surface. Ion abundance, as a function of atomic mass, was initially observed on a Telequipment DlOlO oscilloscope and
(1996)
1371-1378
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then transferred to an x-y chart recorder. Mass spectra were recorded in the range O-50 amu. The signal intensity could be varied by controlling the multiplier current which was measured on an arbitrary scale from O-2.0. 2.6. Electrochemical
techniques
Cyclic voltammetry and chronoamperometric measurements were carried out using a Solartron 1286 potentiostat. AC impedance studies were made by connecting the 1286 potentiostat to a Solar&on 1255 frequency response analyser. An AC perturbation of 10 mV was applied to the cells over a variety of frequency ranges and data were collected and processed by a Zenith microcomputer.
3. Results and discussion 3.1. Laser ionisation
mass analysis
(LIMA)
The two-electrode electrochemical cell was placed in a holder which consisted of a stainless steel disk located 3 mm from the stainless steel grid electrode. Before being placed in the holder, the disk was cleaned by polishing with diamond paste, rinsing with deionised water and drying in an oven at over 100°C. The cell holder was located in a vacuum chamber, the pressure reduced to 10m3 torr and the cell heated to 250°C. The electrodes were connected to a power supply and a potential of 10 V was applied, such that the stainless steel grid was polarised negatively with respect to the Na,,,,CoO, intercalation electrode. After 24 h the disk adjacent to the grid electrode was removed from the cell holder and inserted into the LIMA spectrometer. A 0.1 mm2 section of the disk was irradiated with pulses from the Nd:YAG laser to induce the formation of a plasma. Positive ions were extracted from the plasma and forced into the time-of-flight mass spectrometer, by applying a voltage of 3 kV LIMA spectra recorded from the disk, before and after polarisation of the source, are shown in Fig. 3a and Fig. 3b respectively. The presence of a peak at a mass/charge ratio of 23 confirmed that the cell had evolved vapour containing sodium, which had deposited on the stainless steel disk. The peak arising
1374
SC. Roy. PC. Bruce I Solid State Ionics 86-88
-
1
4
Fe
A .=
2
2 +
Cr I
Lo
0
80
40
1
Fe Na
0
40
Mass/Charge
80
120
Ratio
Fig. 3. (a) LIMA spectrum taken from the surface of the stainless steel disk before polarising the cell. (b) After polarisation. The peaks at mass/charge ratios of 52 and 56 correspond to chromium and iron from the stainless steel disk.
from sodium is split due to signals from different regions of the rough sodium surface. On both spectra there is a cluster of peaks at m/z 50-60 due to elements comprising the stainless steel substrate. A control experiment, carried out under identical conditions, except that the potential difference was not applied, indicated a complete absence of sodium evolution. This result proves that the cell only releases sodium when polarised. The cell reaction involves the deintercalation of sodium ions from Na,,,,CoO,, these then traverse the solid electrolyte forming Na or Na-containing species at the stainless steel grid due to reduction.
(1996) 1371-1378
the pressure in the chamber at 10m3 torr. The working electrode potential was initially swept from 0 to - 3 V, followed by an anodic sweep to + 3 V, both sweeps were at a rate of 100 mV/s. The resulting cyclic voltammogram of the first sweep is shown in Fig. 4a. A loop is evident on the cathodic sweep, which is indicative of a mechanism involving the nucleation and growth of liquid sodium or compounds containing sodium on the working electrode. The later case seems quite possible, at a pressure of 10d3 torr, where many contaminants e.g. 0, and CO,, that react rapidly with sodium, would be available. Comparison of the integrals of the charges bound by both anodic and cathodic waves showed that the coulombic efficiency was 0.4, thus indicating that 40% of the sodium, which was produced during the cathodic scan, could be recovered on subsequent oxidation. Additional cycles showed a reduction in size of the nucleation loop, which reflected the fact that less nucleation could occur on a stainless steel surface already containing sodium. Fig. 4b shows that the source behaves quite differently in UHV under identical cycling conditions. Instead of a nucleation loop at relatively high cathodic overpotentials, all that is observed is a linear variation of current with voltage. This ohmic behaviour suggests that only the iR drop of the Na p-alumina, between reference and working eleca)
u = 100 mV/s
b)
E VS. N~75CoO2
(V)
T = 350°C u = l;x) mV/s
3.2. Cyclic voltammetry The three-electrode cell, shown enclosed in the UHV chamber in Fig. 2, was operated at 350°C with
Fig. 4. (a) Cyclic voltammogram recorded with the cell at lo-’ torr. (b) Cyclic voltammogram recorded with the sodium cell at 1O-8 torr. It shows that the nucleation loop observed at 10e3 torr has been replaced with linear current-voltage behaviour.
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
trodes, is impeding the current flow. It can also be seen that the anodic wave in Fig. 4b is considerably smaller than that observed at 10e3 torr. This clearly indicates that, at the lower pressure, less sodium remains on the gauze after the sweep than at the higher pressure. Repeated cycling in UHV provided results which were identical to the first cyclic voltammogram.
(1996)
1371-1378
1315
result shown in Fig. 5a, there was no observable rise in current due to a nucleation process. For each polarisation potential a constant current was obtained as expected for a cell in which the rate limiting process is the iR drop. Calculating the resistance associated with the current response to each potential pulse gave a value of 2.5 k0. 3.4. AC impedance
3.3. Chronoamperometry The potential of the gauze was stepped from + 2.0 to - 2.5 Vat 10m3 torr and 350°C. The current passing through the cell was monitored as a function of time (Fig. 5a). After initial double layer charging, the current was observed to increase steeply with time, which provided further evidence for a mechanism involving nucleation and growth in agreement with the cyclic voltammetric data presented in Fig. 4a. Attempts at fitting the data in Fig. 5a to various models [7], which represented possible shapes of the nucleation points, were unsuccessful. This was not surprising since cyclic voltammetry had shown that the products of the electrochemical reaction were continuously evaporating from the stainless steel surface, or a possible following chemical reaction between sodium and, for example, oxygen was occurring. The chronoamperometric measurement was repeated in ultrahigh vacuum and Fig. 5b shows results obtained when the potential was stepped from + 2 V to a range of values located on the linear region of the cyclic voltammogram in Fig. 4b. Unlike the
AC impedance has been used to study the Na (liquid)/Na P-A&O, interface, previously by Armstrong [8]. Carrying out AC impedance at a range of cathodic overpotentials, but prior to the linear region of the cyclic voltammogram in Fig. 4b, yielded complex impedance plane plots consisting of two semicircles. Impedance plots for three potentials are shown in Fig. 6. The data could be modelled using the
Fig. 6. Complex impedance plane plots from the cell operating three different cathodic potentials, with respect of Na, ,sCoOz, UHV.
at in
b) p = 10-8 tot-r
a)
T = 350°c
p = 10-3 torr
-i (mA)
T = 350°C 0.3 J 0.2 0.1 LIZ
IO 40 Time (s)
70 Tie
(20 s/div.)
Fig. 5. (a) A rising current-time transient recorded with the cell operating at lo-’ torr. (b) Current-time transients of the cell in UHV when the potential of the working electrode is stepped from + 2.0 V to cathodic potentials on the linear region of the cyclic voltammogram in Fig. 4.
1376
S.C. Roy, P.G. Bruce I Solid State tonics 86-88
(1996) 1371-1378
in UHV (2.5 kLJ), further supporting the view that, at high polarisation potentials, the current is limited by the iR drop across the electrolyte. 3.5. In-situ mass spectroscopy cb
cil
Fig. 7. The equivalent circuit which was used to represent the cell during operation at relatively low cathodic overpotentials in UHV.
equivalent circuit shown in Fig. 7. A computer programme written by MacDonald [9] was used to fit the circuit to the data. R, and C, represent the resistance and capacitance associated with the electrolyte, whilst R,, represents a resistance due to the interfacial process and C,, represents the double layer capacitance. The following evidence supports this interpretation of the impedance data. Firstly, changing the DC potential influenced the size of the low frequency semicircle but not the high frequency semicircle. This suggested that the low frequency semicircle represents the electrode reaction and that the high frequency semicircle represents the bulk electrolyte response. Further proof that the low frequency semicircle was indeed due to the electrode process was obtained by measuring its corresponding capacitance. A value in the microfarad domain, which is typical of a double layer capacitance, was found. Second, a value of 1.12 nF was obtained for C,. While it is difficult to make a definitive interpretation of this number because the cell constant is unknown, due to the three-electrode geometry, it is typical of the capacitance expected for the grain boundaries in Na /?-alumina ceramics, sintered using the conditions in this paper, and certainly the value is too small for an interfacial process. Third, AC impedance measurements have been carried out on two-electrode cells consisting of platinum electrodes, between which was placed a Na /3-alumina pellet sintered using the conditions described in the experimental section. These measurements provided an electrolyte capacitance of 151 pF cm-‘. There seems little doubt that the high frequency semicircle arises from the electrolyte response. Finally it is important to note that the bulk resistance, corresponding to the diameter of the high frequency semicircle in Fig. 6, is approximately 4 k0. This value is comparable to that measured during the potential step experiments
Fig. 8 shows a slow sweep cyclic voltammogram obtained at a rate of 1.66 mV/s in UHV. The potential was swept from 0 to - 2.5 to + 2.5 V vs. Na, _,CoO,. In the cathodic region, the rise of current with potential corresponds to the production of sodium vapour from the source. The absence of an anodic wave indicates that, at this scan rate, all of the sodium produced has entered the gas phase since there are clearly no oxidisable species left on the cathode gauze after scanning cathodically. Mass spectra in Fig. 9a-f show the growth and decay of two peaks whilst the cyclic voltammogram was being recorded. These occurred at mass/charge (m/z) ratios of 23 and 39; the former being undoubtedly due to sodium. Besides these two peaks, there are four others found at mlz = 18, 28, 32 and 44, which remain stationary throughout the entire sweep. These are believed to be due, respectively, to the contaminants H,O, N,, 0, and CO, located within the chamber, and are considered not to be associated with gas evolution from the source. In order to ensure that this was indeed the case, a mass spectrum was recorded under identical conditions but in the absence of the three-electrode cell. The four peaks were still present alongside small contributions from the mlz = 23 and mlz = 39 peaks, which probably arose from minor quantities of sodium left over from the preceeding spectroelectrochemical
i&A
I
E vs.Nag.75CoO? (v)
Fig. 8. Slow-sweep cyclic voltammogram recorded toring of the evolved gases by mass spectrometry.
during moni-
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
8 5
dl
._z
Reverse Cathodic Sweep
l
-1.85 v
1377
(1996) 1371-1378
r Na
d e) Reverse Cathodic Sweep
-I .55v NaO
I-) Reverse Anodic Sweep +I33 mV -
fi
IO
0
20
30
Mass/Charge p=2sx
40
Ratio
IW71orr
T = 350°C
Fig. 9. Mass spectra recorded at several potentials on the cyclic voltammogram decrease during forward and backward cathodic scans respectively.
experiment and deposited on the walls of the UHV chamber. It therefore appeared that the cell generates sodium along with another species possessing a mass/ charge ratio of 39. An obvious interpretation would be that this peak arises from potassium. However, the levels of potassium present in the starting materials used to synthesise the cell components would seem to rule this out. Also, no such evidence of potassium was present in the LIMA experiments. Instead, we suggest that the m/z ratio of 39 corresponds to a NaO fragment, which arises from reaction between the electro-generated sodium and oxygen present in the chamber. This result reinforces
in Fig. 8. The peaks at m/z = 23 and 39 increase
and
our view that it is important to supplement electroanalytical data with simultaneous spectroscopic studies in order to gain a more precise understanding of the electrochemical mechanisms.
4. Conclusions In UHV it appears that Na a minority sodium containing Whether this second species electrochemical reduction or oxygen in the gas phase overpotentials, the kinetics
is generated along with species, possibly NaO. is generated directly by whether Na reacts with is not clear. At low of the electrochemical
1378
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
generation of vapour are observed. At overpotentials more negative than - 2.3 V, the current is limited by the electrolyte resistance only. The situation is more complex at higher pressures ( lop3 torr), with evidence for the nucleation and growth of sodium, and possibly other sodium containing compounds, at high overpotentials. Either the liquid sodium or the sodium containing species, which is directly electro-generated, may evaporate from the surface or take part in a following chemical reaction. There are several possible explanations for the differences observed at different pressure. For example, if the nucleation and growth does involve directly the electrochemical formation of a compound containing sodium with a contaminant from the gas phase (e.g. Na,O), this may be suppressed in UHV Alternatively, the rate of sodium evaporation at lo-* torr may ensure that as Na atoms are formed, they exist on the electrode surface for too short a time to form nuclei and grow into liquid-Na metal. Further work will be required to resolve which of these models is more appropriate.
Acknowledgments PGB is indebted award of a Pickering
to the Royal Society for the Research Fellowship. PGB and
(1996) 1371-1378
SCR are grateful to the EPSRC and the University St Andrews for supporting SCRs studentship.
of
References [l] Solid State Electrochemistry, ed. P.G. Bruce (Cambridge University Press, Cambridge, 1995). [2] P.G. Bruce, I. Abrahams, K.A. Prior and H. Stewart, Solid State Ionics 53 (1992) 1. 131 P.G. Bruce, S. Roy, H. Stewart and K.A. Prior, Mater. Res. Sot. Symp. Proc. 293 (1993) 93. [4] IS. Hauksson, S.Y. Wang, J. Simpson, M.R. Taghizadeh, K.A. Prior and B.C. Cavenett, Physica B 191 (1993) 124. [5] D.J. Dyson and W. Johnson, Trans. J. Brit. Ceram. Sot. 72 (1973) 49. [6] J.-J. Braconnier, C. Delmas, C. Fouassier and P. Hagenmuller, Mater. Res. Bull. 15 (1980) 1797. [7] Southampton Electrochemistry Group, Instrumental Methods in Electrochemistry (Ellis Horwood Limited, Chichester, 1985) p. 283. [8] W.I. Archer and R.D. Armstrong, in: Electrochemistry, Vol.7, ed. H.R. Thirsk (The Chemical Society, London, 1980) p. 188. [9] J. Ross Macdonald, Electrochim. Acta 38 (1993) 1883.
s
ELSEVlER
SOLID STATE Solid State Ionics 86-88 (1996) 1371-1378
Spectroelectrochemical
IONICS
studies of alkali metal generation from solid state cells S.C. Roy”‘*, P.G. Bruceb
“Northern Carbon Research Laboratories, Chemistry Department, Bedson Building, University of Newcastle-upon-Tyne, Newcastle-uponTyne, NE1 7RU, England, UK bCentre for Electrochemical and Materials Sciences, School of Chemistry, Purdie Building, University of St Andrews, St Andrew, Fife, KY16 9ST, Scotland, UK
Abstract p-alumina/metal grid, is presented which is capable of producing A solid state electrochemical cell, Na 0 ,,CoO,/sodium a beam of sodium vapour in UHV. Cells such as these have potential applications as a source of dopants in semiconductor fabrication. Electrochemical measurements and a new spectroelectrochemical technique provide mechanistic information regarding sodium electro-generation. Keywords: Electrochemical cell; Solid-state cell; Vapour beam doping; Spectroelectrochemical analysis; Overpotential
1. Introduction Solid state electrochemical cells are a growing part of electrochemistry and have a wide variety of applications [ 11. Common examples include lithiumion batteries and solid oxide fuel cells that generate portable electric power, sensors that detect gases and electrochromic devices such as SMART windows, through which the level of light transmission may be varied electrochemically. A much less familiar application is the use of solid state electrochemical cells to generate high purity beams of vapours that can be used to dope semiconductors. Recently we have demonstrated that electrochemically generated potassium vapour may be used to dope the II-VI semiconductor ZnSe in a *Corresponding author.
controlled fashion, thus forming p-type material during crystal growth by Molecular Beam Epitaxy [2,3]. This offers an alternative to nitrogen doping of ZnSe. The successful formation of p-type ZnSe is a critical goal in the field of optoelectronics, since p-n junctions, based on ZnSe, form the bases of blue LEDs and blue laser diodes [4]. This paper focuses on developing an understanding of the electrochemical mechanism responsible for alkali metal production from solid-state cells. As a model system we have chosen to present results concerning sodium evolution from an all-solid-state Naf -ion cell. After giving details concerning cell fabrication, this account provides laser ionisation mass spectroscopic (LIMA) evidence for sodium production, followed by results of cyclic voltammetric and ac impedance measurements. As part of our studies, we have developed a simultaneous mass
0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved PII SO167-2738(96)00312-S
1372
S.C.
P.G. Bruce
Solid State
spectrometric analysis of the vapour being electrogenerated in UHV during cyclic voltammetty.
2. Experimental 2.1. Preparation
of Na p-alumina
Na,CO, (Fisons) and A&O, (Fluka) were dried and then thoroughly ground together for 15 min in air in a I:5 mole ratio of Na,CO, to Al,O, [5]. The powder was placed in a platinum crucible, calcined in an electric muffle furnace at 650°C for 2 h, in order to decompose the carbonate, and then reacted for 24 h at 1200°C. The resulting material was subsequently quenched in air to room temperature and examined by powder X-ray diffraction using a Stoe STADI/P diffractometer and Cu Ka radiation. The product consisted of a mixture of /? and p” phases. 2.2. Preparation (Na o.,400,)
of sodium cobalt oxide
[61
A 3:2 molar ratio of Na,O, (BDH) to Co,O, (Aldrich) was ground together for 15 min in a MBRAUN argon filled glove box and then transferred to a gold boat, which was placed in a quartz tube fitted with two taps. The tube was removed from the glove box and placed inside a cylindrical furnace where the temperature of the sample was raised at a rate of 4”C/min until it reached 500°C. The mixture was left at this temperature for 2 days. After this period, the temperature was lowered to 25°C again at a rate of 4”C/min. Oxygen gas was permitted to flow across the sample throughout the entire heating period. A powder X-ray pattern of the resulting material, when compared with data from the powder diffraction data base, ASTM 32-1068, indicated that it was
86-88
(1996)
materials were subjected to a pressure of 1 ton for 5 min. The resulting composite pellet was placed with the p-alumina side downwards, in a gold boat lined with Na p-alumina powder and covered with sodium cobalt oxide. The boat was then placed in a silica tube fitted with taps, which again allowed oxygen to flow across the sample. The temperature was raised from 25°C to 500°C at a rate of 4”C/min, left at this temperature for 17 h and then cooled back to 25°C once again at 4”Clmin. This heating induced sintering of the cell and, hence, improved its mechanical properties. On removal from the silica tube, conductive silver paint was applied to the flat sodium cobalt oxide surface to act as a current collector. A stainless steel gauze disk was rubbed with emery paper, boiled in distilled water and thoroughly dried before being placed on the exposed flat surface of the solid electrolyte (Fig. 1). 2.4. Laser ionisation
mass analysis
(LIMA)
This technique involves the ablation of the surface of a solid and analysis of the resulting vapour phase by mass spectrometry. It was used to establish the electrochemical evolution of Na vapour. Measurements were carried out using a Cambridge Mass Spectrometry Ltd. LIMA (401 L) fitted with a Nd:YAG laser (Spectra-Physics DCR- 11) operating at 266 nm. 2.5. Three-electrode mass spectroscopy
measurements
and in-situ
In order to carry out electrochemical studies, a three-electrode cell was designed as shown in Fig. 2.
Na p-alumina
Na,.,,Co% 2.3. Fabrication
of two-electrode
ceramic cell
Nao.75CoO2 Sodium cobalt oxide (0.75g) was placed in a 13 mm die and gently compacted with the plunger by hand. A few drops of acetone were then added followed by 0.3 g of Na p-alumina. The two
Fig. 1. Solid state electrochemical sodium vapour.
’
Stainless Steel Gauze cell capable
of generating
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
M&3X
Gramic Cell H0lder Na p-alum& electrolyte
Cartridge Heaters NaO.7SCoO2 counter electrode stainless steel working electrode UHV CJWlbCr
Fig. 2. Spectroelectrochemical apparatus for monitoring gas evolution from the cell by in-situ mass spectroscopy, whilst recording cyclic voltammetric data. The three-electrode cell is positioned within a stainless steel heating block and the entire assembly is located inside an UHV chamber.
The working electrode consisted of a stainless steel gauze in contact with the p-alumina electrolyte, the counter and reference electrodes were formed from disks of Na,.,,CoO, with the reference electrode being located in close proximity to the working electrode. The counter electrode/electrolyte assembly was constructed in the same fashion as for the two-electrode cell. A Macor ceramic cell holder containing the three-electrode cell was placed in a stainless steel heating block. This was mounted on an UHV flange, which was also fitted with three electrode contacts for the electrochemical measurements, a power supply for the heaters and two thermocouple plugs; one for the heating block and one for the cell holder. Electrochemical measurements could be made up to 350°C and over a range of pressures from 10m3 to lo-* torr. To carry out simultaneous mass analysis along with the electrochemical studies, a quadrupole mass spectrometer head, shown in Fig. 2, was positioned beneath the cell to obtain an in-situ analysis of gas evolution. The ioniser was placed approximately 2 cm from the stainless steel grid surface. Ion abundance, as a function of atomic mass, was initially observed on a Telequipment DlOlO oscilloscope and
(1996)
1371-1378
1313
then transferred to an x-y chart recorder. Mass spectra were recorded in the range O-50 amu. The signal intensity could be varied by controlling the multiplier current which was measured on an arbitrary scale from O-2.0. 2.6. Electrochemical
techniques
Cyclic voltammetry and chronoamperometric measurements were carried out using a Solartron 1286 potentiostat. AC impedance studies were made by connecting the 1286 potentiostat to a Solar&on 1255 frequency response analyser. An AC perturbation of 10 mV was applied to the cells over a variety of frequency ranges and data were collected and processed by a Zenith microcomputer.
3. Results and discussion 3.1. Laser ionisation
mass analysis
(LIMA)
The two-electrode electrochemical cell was placed in a holder which consisted of a stainless steel disk located 3 mm from the stainless steel grid electrode. Before being placed in the holder, the disk was cleaned by polishing with diamond paste, rinsing with deionised water and drying in an oven at over 100°C. The cell holder was located in a vacuum chamber, the pressure reduced to 10m3 torr and the cell heated to 250°C. The electrodes were connected to a power supply and a potential of 10 V was applied, such that the stainless steel grid was polarised negatively with respect to the Na,,,,CoO, intercalation electrode. After 24 h the disk adjacent to the grid electrode was removed from the cell holder and inserted into the LIMA spectrometer. A 0.1 mm2 section of the disk was irradiated with pulses from the Nd:YAG laser to induce the formation of a plasma. Positive ions were extracted from the plasma and forced into the time-of-flight mass spectrometer, by applying a voltage of 3 kV LIMA spectra recorded from the disk, before and after polarisation of the source, are shown in Fig. 3a and Fig. 3b respectively. The presence of a peak at a mass/charge ratio of 23 confirmed that the cell had evolved vapour containing sodium, which had deposited on the stainless steel disk. The peak arising
1374
SC. Roy. PC. Bruce I Solid State Ionics 86-88
-
1
4
Fe
A .=
2
2 +
Cr I
Lo
0
80
40
1
Fe Na
0
40
Mass/Charge
80
120
Ratio
Fig. 3. (a) LIMA spectrum taken from the surface of the stainless steel disk before polarising the cell. (b) After polarisation. The peaks at mass/charge ratios of 52 and 56 correspond to chromium and iron from the stainless steel disk.
from sodium is split due to signals from different regions of the rough sodium surface. On both spectra there is a cluster of peaks at m/z 50-60 due to elements comprising the stainless steel substrate. A control experiment, carried out under identical conditions, except that the potential difference was not applied, indicated a complete absence of sodium evolution. This result proves that the cell only releases sodium when polarised. The cell reaction involves the deintercalation of sodium ions from Na,,,,CoO,, these then traverse the solid electrolyte forming Na or Na-containing species at the stainless steel grid due to reduction.
(1996) 1371-1378
the pressure in the chamber at 10m3 torr. The working electrode potential was initially swept from 0 to - 3 V, followed by an anodic sweep to + 3 V, both sweeps were at a rate of 100 mV/s. The resulting cyclic voltammogram of the first sweep is shown in Fig. 4a. A loop is evident on the cathodic sweep, which is indicative of a mechanism involving the nucleation and growth of liquid sodium or compounds containing sodium on the working electrode. The later case seems quite possible, at a pressure of 10d3 torr, where many contaminants e.g. 0, and CO,, that react rapidly with sodium, would be available. Comparison of the integrals of the charges bound by both anodic and cathodic waves showed that the coulombic efficiency was 0.4, thus indicating that 40% of the sodium, which was produced during the cathodic scan, could be recovered on subsequent oxidation. Additional cycles showed a reduction in size of the nucleation loop, which reflected the fact that less nucleation could occur on a stainless steel surface already containing sodium. Fig. 4b shows that the source behaves quite differently in UHV under identical cycling conditions. Instead of a nucleation loop at relatively high cathodic overpotentials, all that is observed is a linear variation of current with voltage. This ohmic behaviour suggests that only the iR drop of the Na p-alumina, between reference and working eleca)
u = 100 mV/s
b)
E VS. N~75CoO2
(V)
T = 350°C u = l;x) mV/s
3.2. Cyclic voltammetry The three-electrode cell, shown enclosed in the UHV chamber in Fig. 2, was operated at 350°C with
Fig. 4. (a) Cyclic voltammogram recorded with the cell at lo-’ torr. (b) Cyclic voltammogram recorded with the sodium cell at 1O-8 torr. It shows that the nucleation loop observed at 10e3 torr has been replaced with linear current-voltage behaviour.
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
trodes, is impeding the current flow. It can also be seen that the anodic wave in Fig. 4b is considerably smaller than that observed at 10e3 torr. This clearly indicates that, at the lower pressure, less sodium remains on the gauze after the sweep than at the higher pressure. Repeated cycling in UHV provided results which were identical to the first cyclic voltammogram.
(1996)
1371-1378
1315
result shown in Fig. 5a, there was no observable rise in current due to a nucleation process. For each polarisation potential a constant current was obtained as expected for a cell in which the rate limiting process is the iR drop. Calculating the resistance associated with the current response to each potential pulse gave a value of 2.5 k0. 3.4. AC impedance
3.3. Chronoamperometry The potential of the gauze was stepped from + 2.0 to - 2.5 Vat 10m3 torr and 350°C. The current passing through the cell was monitored as a function of time (Fig. 5a). After initial double layer charging, the current was observed to increase steeply with time, which provided further evidence for a mechanism involving nucleation and growth in agreement with the cyclic voltammetric data presented in Fig. 4a. Attempts at fitting the data in Fig. 5a to various models [7], which represented possible shapes of the nucleation points, were unsuccessful. This was not surprising since cyclic voltammetry had shown that the products of the electrochemical reaction were continuously evaporating from the stainless steel surface, or a possible following chemical reaction between sodium and, for example, oxygen was occurring. The chronoamperometric measurement was repeated in ultrahigh vacuum and Fig. 5b shows results obtained when the potential was stepped from + 2 V to a range of values located on the linear region of the cyclic voltammogram in Fig. 4b. Unlike the
AC impedance has been used to study the Na (liquid)/Na P-A&O, interface, previously by Armstrong [8]. Carrying out AC impedance at a range of cathodic overpotentials, but prior to the linear region of the cyclic voltammogram in Fig. 4b, yielded complex impedance plane plots consisting of two semicircles. Impedance plots for three potentials are shown in Fig. 6. The data could be modelled using the
Fig. 6. Complex impedance plane plots from the cell operating three different cathodic potentials, with respect of Na, ,sCoOz, UHV.
at in
b) p = 10-8 tot-r
a)
T = 350°c
p = 10-3 torr
-i (mA)
T = 350°C 0.3 J 0.2 0.1 LIZ
IO 40 Time (s)
70 Tie
(20 s/div.)
Fig. 5. (a) A rising current-time transient recorded with the cell operating at lo-’ torr. (b) Current-time transients of the cell in UHV when the potential of the working electrode is stepped from + 2.0 V to cathodic potentials on the linear region of the cyclic voltammogram in Fig. 4.
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S.C. Roy, P.G. Bruce I Solid State tonics 86-88
(1996) 1371-1378
in UHV (2.5 kLJ), further supporting the view that, at high polarisation potentials, the current is limited by the iR drop across the electrolyte. 3.5. In-situ mass spectroscopy cb
cil
Fig. 7. The equivalent circuit which was used to represent the cell during operation at relatively low cathodic overpotentials in UHV.
equivalent circuit shown in Fig. 7. A computer programme written by MacDonald [9] was used to fit the circuit to the data. R, and C, represent the resistance and capacitance associated with the electrolyte, whilst R,, represents a resistance due to the interfacial process and C,, represents the double layer capacitance. The following evidence supports this interpretation of the impedance data. Firstly, changing the DC potential influenced the size of the low frequency semicircle but not the high frequency semicircle. This suggested that the low frequency semicircle represents the electrode reaction and that the high frequency semicircle represents the bulk electrolyte response. Further proof that the low frequency semicircle was indeed due to the electrode process was obtained by measuring its corresponding capacitance. A value in the microfarad domain, which is typical of a double layer capacitance, was found. Second, a value of 1.12 nF was obtained for C,. While it is difficult to make a definitive interpretation of this number because the cell constant is unknown, due to the three-electrode geometry, it is typical of the capacitance expected for the grain boundaries in Na /?-alumina ceramics, sintered using the conditions in this paper, and certainly the value is too small for an interfacial process. Third, AC impedance measurements have been carried out on two-electrode cells consisting of platinum electrodes, between which was placed a Na /3-alumina pellet sintered using the conditions described in the experimental section. These measurements provided an electrolyte capacitance of 151 pF cm-‘. There seems little doubt that the high frequency semicircle arises from the electrolyte response. Finally it is important to note that the bulk resistance, corresponding to the diameter of the high frequency semicircle in Fig. 6, is approximately 4 k0. This value is comparable to that measured during the potential step experiments
Fig. 8 shows a slow sweep cyclic voltammogram obtained at a rate of 1.66 mV/s in UHV. The potential was swept from 0 to - 2.5 to + 2.5 V vs. Na, _,CoO,. In the cathodic region, the rise of current with potential corresponds to the production of sodium vapour from the source. The absence of an anodic wave indicates that, at this scan rate, all of the sodium produced has entered the gas phase since there are clearly no oxidisable species left on the cathode gauze after scanning cathodically. Mass spectra in Fig. 9a-f show the growth and decay of two peaks whilst the cyclic voltammogram was being recorded. These occurred at mass/charge (m/z) ratios of 23 and 39; the former being undoubtedly due to sodium. Besides these two peaks, there are four others found at mlz = 18, 28, 32 and 44, which remain stationary throughout the entire sweep. These are believed to be due, respectively, to the contaminants H,O, N,, 0, and CO, located within the chamber, and are considered not to be associated with gas evolution from the source. In order to ensure that this was indeed the case, a mass spectrum was recorded under identical conditions but in the absence of the three-electrode cell. The four peaks were still present alongside small contributions from the mlz = 23 and mlz = 39 peaks, which probably arose from minor quantities of sodium left over from the preceeding spectroelectrochemical
i&A
I
E vs.Nag.75CoO? (v)
Fig. 8. Slow-sweep cyclic voltammogram recorded toring of the evolved gases by mass spectrometry.
during moni-
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
8 5
dl
._z
Reverse Cathodic Sweep
l
-1.85 v
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(1996) 1371-1378
r Na
d e) Reverse Cathodic Sweep
-I .55v NaO
I-) Reverse Anodic Sweep +I33 mV -
fi
IO
0
20
30
Mass/Charge p=2sx
40
Ratio
IW71orr
T = 350°C
Fig. 9. Mass spectra recorded at several potentials on the cyclic voltammogram decrease during forward and backward cathodic scans respectively.
experiment and deposited on the walls of the UHV chamber. It therefore appeared that the cell generates sodium along with another species possessing a mass/ charge ratio of 39. An obvious interpretation would be that this peak arises from potassium. However, the levels of potassium present in the starting materials used to synthesise the cell components would seem to rule this out. Also, no such evidence of potassium was present in the LIMA experiments. Instead, we suggest that the m/z ratio of 39 corresponds to a NaO fragment, which arises from reaction between the electro-generated sodium and oxygen present in the chamber. This result reinforces
in Fig. 8. The peaks at m/z = 23 and 39 increase
and
our view that it is important to supplement electroanalytical data with simultaneous spectroscopic studies in order to gain a more precise understanding of the electrochemical mechanisms.
4. Conclusions In UHV it appears that Na a minority sodium containing Whether this second species electrochemical reduction or oxygen in the gas phase overpotentials, the kinetics
is generated along with species, possibly NaO. is generated directly by whether Na reacts with is not clear. At low of the electrochemical
1378
S.C. Roy, P.G. Bruce I Solid State Ionics 86-88
generation of vapour are observed. At overpotentials more negative than - 2.3 V, the current is limited by the electrolyte resistance only. The situation is more complex at higher pressures ( lop3 torr), with evidence for the nucleation and growth of sodium, and possibly other sodium containing compounds, at high overpotentials. Either the liquid sodium or the sodium containing species, which is directly electro-generated, may evaporate from the surface or take part in a following chemical reaction. There are several possible explanations for the differences observed at different pressure. For example, if the nucleation and growth does involve directly the electrochemical formation of a compound containing sodium with a contaminant from the gas phase (e.g. Na,O), this may be suppressed in UHV Alternatively, the rate of sodium evaporation at lo-* torr may ensure that as Na atoms are formed, they exist on the electrode surface for too short a time to form nuclei and grow into liquid-Na metal. Further work will be required to resolve which of these models is more appropriate.
Acknowledgments PGB is indebted award of a Pickering
to the Royal Society for the Research Fellowship. PGB and
(1996) 1371-1378
SCR are grateful to the EPSRC and the University St Andrews for supporting SCRs studentship.
of
References [l] Solid State Electrochemistry, ed. P.G. Bruce (Cambridge University Press, Cambridge, 1995). [2] P.G. Bruce, I. Abrahams, K.A. Prior and H. Stewart, Solid State Ionics 53 (1992) 1. 131 P.G. Bruce, S. Roy, H. Stewart and K.A. Prior, Mater. Res. Sot. Symp. Proc. 293 (1993) 93. [4] IS. Hauksson, S.Y. Wang, J. Simpson, M.R. Taghizadeh, K.A. Prior and B.C. Cavenett, Physica B 191 (1993) 124. [5] D.J. Dyson and W. Johnson, Trans. J. Brit. Ceram. Sot. 72 (1973) 49. [6] J.-J. Braconnier, C. Delmas, C. Fouassier and P. Hagenmuller, Mater. Res. Bull. 15 (1980) 1797. [7] Southampton Electrochemistry Group, Instrumental Methods in Electrochemistry (Ellis Horwood Limited, Chichester, 1985) p. 283. [8] W.I. Archer and R.D. Armstrong, in: Electrochemistry, Vol.7, ed. H.R. Thirsk (The Chemical Society, London, 1980) p. 188. [9] J. Ross Macdonald, Electrochim. Acta 38 (1993) 1883.