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Electrochemical behaviour of a vanadium anode in phosphoric acid and phosphate solutions
Electrochimica Acta 51 (2006) 1990–1995
Electrochemical behaviour of a vanadium anode in phosphoric acid and phosphate solutions V. Alonzo a,∗ , A. Darchen a , E. Le Fur b , J.Y. Pivan b a
Laboratoire d’Electrochimie, Ecole Nationale Sup´erieure de Chimie de Rennes, Avenue du G´en´eral Leclerc, 35700 Rennes, France b Laboratoire de Chimie du Solide, UMR 6511, Ecole Nationale Sup´ erieure de Chimie de Rennes, Avenue du G´en´eral Leclerc, 35700 Rennes, France Received 6 April 2005; received in revised form 22 June 2005; accepted 29 June 2005 Available online 15 August 2005
Abstract Anodic polarisation of a vanadium electrode has been studied in H3 PO4 solutions and some phosphate solutions: LiH2 PO4 , NaH2 PO4 , KH2 PO4 and NH4 H2 PO4 . The anodic behaviour of a vanadium electrode showed similarities in weak concentrated H3 PO4 , in LiH2 PO4 and NaH2 PO4 solutions: the polarisation curve exhibited a current peak followed by current oscillations and then a current plateau. Concentrated H3 PO4 , 1 M KH2 PO4 and NH4 H2 PO4 solutions involved vanadium passivation with a very slight current density plateau. Yellow compound identified to VOPO4 ·2H2 O was obtained after controlled potential oxidation of vanadium in 5–10 M H3 PO4 . Green products were obtained in 1 M phosphate solutions and in 1–3 M H3 PO4 on vanadium anode after controlled potential electrolysis. All these vanadophosphate compounds contained the monovalent cation which was present in the solution. © 2005 Elsevier Ltd. All rights reserved. Keywords: Anodic electrodeposit; Current oscillations; Oxidation; Passivation; Vanadophosphate
1. Introduction Vanadophosphates or vanadium phosphorus oxides (VPO) are well-known catalysts for the mild oxidation of light alkanes [1]. These compounds and particularly lithium vanadophosphates are also of great interest as battery materials [2]. They are mesoporous compounds which are generally prepared by solid state synthesis with heating in air or by hydrothermal synthesis [3]. The general synthetic way starts from vanadium oxide in the presence of reducing metal powder or sheet. Among the vanadophosphates, VOPO4 ·2H2 O is a famous intercalation compound [4]. It is generally prepared by a prolonged boiling of V2 O5 in aqueous phosphoric acid [5]. It can also be prepared at room temperature by a sol–gel synthesis from alkoxide precursors [6] or by using an ultrasonic activation of an aqueous solution containing V2 O5 and H3 PO4 ∗
Corresponding author. Tel.: +33 2 23 23 80 26; fax: +33 2 23 23 81 99. E-mail address: [email protected] (V. Alonzo).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.06.030
[7]. We recently described a new route of synthesis by anodic oxidation of a vanadium sheet in a H3 PO4 solution [8]. The anodic behaviour of vanadium in acidic aqueous solutions has been sparsely studied [9–15]. Some passive or anodic oxide films were mentioned in aqueous solutions: for example VO2 [10], V2 O4 [11] V2 O5 [15], non-stoichiometric oxides or mixtures of oxides of varying valence [14] and also barium vanadate [12]. A few studies have been carried out in phosphoric acid solutions [16–19] but they did not shown the occurrence of vanadophosphate species. This new route of synthesis has been extended to the synthesis of alkali vanadophosphates, by anodic polarisation of vanadium in sodium [20], lithium, potassium and ammonium phosphate solutions. These compounds have been extensively studied and they are generally synthesized either by hydrothermal method [21] or by using reducing agent like iodide anion on VOPO4 ·2H2 O [4,22]. In both cases, the final products are obtained after large time reactions. The structure of these compounds shows VOPO4 layers with H2 O and intercalated cations between the layers. The electrochemical
V. Alonzo et al. / Electrochimica Acta 51 (2006) 1990–1995
1991
route seems to be a rapid way to obtain these compounds as electrodeposits on a vanadium anode. The electrochemical behaviour of a vanadium anode in phosphoric acid and various phosphate solutions is described in this article.
2. Experimental All electrochemical measurements were carried out in a conventional three-electrode electrochemical cell. The working electrode was a vanadium rod (1 mm diameter) purchased from Goodfellow (99.7% in purity) embedded in an insulating holder made of PTFE (Radiometer Analytical). A platinum wire was used as a counter electrode and the reference electrode was a saturated calomel electrode (SCE). All the potential values are reported herein with respect to the SCE. The solutions were prepared using H3 PO4 (85%, Aldrich), NaH2 PO4 (99%, Aldrich), LiH2 PO4 (99%, Aldrich), KH2 PO4 (>99%, Acros) or NH4 H2 PO4 (>99%, Fluka) and deionized water. Prior to experiments, the electrode was mechanically polished by a series of wet standings at different grit size (1200–4000) and rinsed with deionized water. All the experiments were performed at room temperature. The polarisation measurements were conducted with a 1280 Solartron potentiostat and the Corrware software (Scribner and Associates) was used to pilot the experiments from −0.7 to 6 V/SCE with a 10 mV s−1 scan rate. Controlled potential oxidation was carried out for a few minutes with a 0.25 mm thick and 1 cm large vanadium sheet (99.7%, Aldrich) whose 0.5 cm length was immersed in solution. The anodic deposits obtained on vanadium anode have been characterised by X-ray powder diffraction (Bragg–Brentano geometry, Rigaku–Geigerflex diffractometer). The diffraction pattern was scanned from 10◦ to 90◦ ˚ and a step length of 0.02◦ (2θ) using λ Cu K␣ = 1.54178 A (2θ) with 12 s counting time. After polarisation of vanadium sheets in phosphate solutions, Scanning Electron Microscopy (SEM) observations and Energy Dispersion X-ray Spectroscopy (EDS) analysis of anodic deposits were performed at the Scanning Electron Microscopy Centre of the University of Rennes (CMEBA Rennes). A Varian AA-10 apparatus was used to analyse lithium content in Li compounds by Atomic Emission Spectroscopy. Spectrophotometric determination of ammonium ions by the indophenol blue method [23] was performed with a Schimadzu 1600 spectrophotometer.
3. Results and discussion 3.1. Voltammetry in H3 PO4 solutions Various H3 PO4 concentrated solutions, from 1 to 14.74 M, were used for voltammetry experiments. Fig. 1 shows the potentiodynamic curves obtained for a vanadium microelec-
Fig. 1. Current–potential polarisation curves of the vanadium oxidation in H3 PO4 solutions at a stationary vanadium electrode. Scan rate = 10 mV s−1 .
trode in these solutions. Current increases with decreasing H3 PO4 concentrations. A passivation behaviour is observed for 6–14.74 M solutions. For 3 M or less concentrated solutions, the potentiodynamic curve exhibits an oxidation peak, before the current reaching a plateau with high current density. Lowering the scanning rate or using a rotating electrode involves oscillation apparition between the peak and the plateau, for the low concentrations [8]. The current oscillations are also observed during anodic oxidation performed at a controlled potential [8]. Electrochemical oscillations have been observed for a large number of metal anodic processes and particularly for copper, cobalt and iron oxidation in H3 PO4 [24–27]. Oscillations are generally explained in terms of competition between the growth and the chemical dissolution of metal anodic films. In the case of the anodic oxidation of vanadium, oscillations could result from formation/dissolution of a compound at the metal/solution interface, but in the actual state of our knowledge, we cannot assume it is the isolated vanadophosphate. In order to better understand the redox behaviour of the deposit, voltammetric studies were carried out with soluble ionic species of vanadium in phosphoric acid. A voltammetric study in a solution containing V2 O5 in 1 M H3 PO4 revealed on a carbon electrode a reversible reduction around +0.66 V/SCE, corresponding to the V(IV)/V(V) redox couple [8]. 3.2. Anodic oxidation of vanadium in H3 PO4 solutions Some yellow compounds were observed on the vanadium surface during the potentiodynamic measurement for 5–10 M H3 PO4 at potentials corresponding to the limiting current plateau. This yellow colour is characteristic of a V(V) species. In order to obtain a greater amount of product, a 1 cm2 immersed vanadium sheet was used as anode. The preparation of the solid was carried out at controlled potential: the sample was polarised at a potential corresponding to the current plateau and greater than 1 or 1.5 V/SCE according to the concentration. At 10 M concentration, a yellow pasty solid was
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V. Alonzo et al. / Electrochimica Acta 51 (2006) 1990–1995
Fig. 3. Powder X-ray diffraction patterns of the yellow deposit obtained on a vanadium anode polarised at +3 V/SCE in 6 M H3 PO4 (a) and of the green deposit obtained on a vanadium anode polarised at +3 V/SCE in 2 M H3 PO4 (b).
Fig. 2. SEM micrography of the anodic deposit on a vanadium anode polarised within 1 h at +4 V/SCE in 6 M H3 PO4 (a) and enlargement of the inside structure (b).
obtained. The deposit formed on the vanadium after polarisation at 3 V/SCE in 6 M H3 PO4 during 1 h was observed by SEM. It reveals a homogeneous deposit, with cracks due to retraction of the solid after drying (Fig. 2a). The inside structure of the deposit is made of platelets (Fig. 2b). Once the deposit was removed, the vanadium surface appeared on the eye scale to be shiny. The observation by SEM revealed a thin dissolution of grain boundaries, the interior of the grains being slightly etched. The vanadium surface after removal of the pasty deposit in 10 M H3 PO4 at 3 V/SCE showed the same aspect of the grain boundary but the interior of the grain was not etched. The pasty texture of the deposit obtained in 10 M H3 PO4 did not permit SEM observation. X-ray powder diffraction data analysis by DiffracAT software revealed that the yellow powder obtained in 6 M H3 PO4 (Fig. 3) was VOPO4 ·2H2 O, whose structure is described in quadratic system [28]. In 3 M H3 PO4 or less concentrated solutions, a green product was formed at the same applied potentials and fell in the solution while the initially colourless solution became blue.
A very small quantity of this product was recovered and was quickly rinsed since it dissolved in water. The green colour of the product indicated a mixed valence V(IV)/V(V) of the vanadium species and the blue coloration of the solution revealed the presence of a V(IV) ionic species. The occurrence of V(IV) at potential corresponding to V(V) species in voltammetric experiments could be explained by different mass transport conditions between a vertical plate and a 1 mm diameter disk. The surface of vanadium after 2 V/SCE polarisation in 2 M H3 PO4 revealed an important intergranular corrosion. Green powder of hydrated vanadophosphates with intercalated cations derive generally from the structure of VOPO4 ·2H2 O. Compounds with H+ intercalation were first described by Jacobson [29]. They were obtained by reduction of the yellow VOPO4 ·2H2 O with hydroquinone. More recently, the formula H0.2 VOPO4 ·2.33H2 O was attributed to the green compound obtained in hydrothermal conditions in alcohol medium [30]. The powder obtained in our study by anodic oxidation of vanadium in 2 M H3 PO4 (Fig. 3) was found to be close to this compound. 3.3. Voltammetry of vanadium in phosphate solutions Various phosphate solutions (NaH2 PO4 , LiH2 PO4 , KH2 PO4 and NH4 H2 PO4 ) were used for voltammetry experiments on vanadium. Fig. 4 shows potentiodynamic curves for a vanadium microelectrode. The behaviour of vanadium in 1 M NaH2 PO4 or 1 M LiH2 PO4 was similar to the anodic dissolution of vanadium in 1 M H3 PO4 . After the oxidation peak, the current slightly decreased before reaching a plateau. An oscillatory phenomenon was observed after the peak current maximum and before the current plateau. In NH4 H2 PO4 solution, oscillations were not observed and the current plateau was narrow: current density was 103 times smaller (6 × 10−4 A cm−2 ) than in LiH2 PO4 solution (about 0.6 A cm−2 ). In KH2 PO4 solution, peak current was very small (about 5 × 10−3 A cm−2 ) and no oscillation was observed, and the following current plateau was 102 times smaller (4 × 10−3 A cm−2 ) than in LiH2 PO4 solution. We
V. Alonzo et al. / Electrochimica Acta 51 (2006) 1990–1995
Fig. 4. Current–potential polarisation curves of the vanadium oxidation in 1 M phosphate solutions (stationary vanadium electrode; scan rate = 10 mV s−1 ).
don’t have any data about a lower conductivity of anodic deposit which could explain a better passivation in this case. Effect of LiH2 PO4 concentration is shown in Fig. 5. Oscillation domain increases with concentration whereas peak and plateau current are slightly smaller. In this particular case, lowering the scanning potential rate increases the oscillation frequency. Heating this solution also increases oscillations frequency and their current amplitude. Effect of rotating electrode rate is shown in Fig. 6: increasing rotating electrode amplifies oscillation domain. 3.4. Anodic oxidation of vanadium in phosphate solutions Some green compounds were observed on the vanadium surface during the potentiodynamic measurement for 1 M NaH2 PO4 or LiH2 PO4 at a potential corresponding to the limiting current plateau. The sample was polarised at 4 V/SCE for NaH2 PO4 and LiH2 PO4 solutions. The product formed on the vanadium after a few minutes fell in the solution. During the potentiostatic measurement, the current showed high amplitude oscillations appearing after an induction period
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Fig. 6. Current–potential curve for a vanadium rotating electrode (1000 rpm) in 1.5 M LiH2 PO4 solution (scan rate = 10 mV s−1 ).
Fig. 7. Current–time curve for vanadium foil (about 1 cm2 immersed) polarised at +4 V/SCE in 1 M NaH2 PO4 .
(Fig. 7). The manipulation was stopped when sufficient amounts of solid had been recovered in the cell for physicochemical characterisation. After filtration, the final products were washed with water and acetone, then dried in an oven for a few hours. A secondary reaction occurred during the electrolysis in some solutions. In LiH2 PO4 solution a white deposit precipitated on the counter electrode due to a localised increase of pH. This highly insoluble Li3 PO4 deposit contributed to lithium consumption from the solution. The NaH2 PO4 solution became a little turbid at the end of the polarisation. Table 1 gathers quantitative results for anodic product formation. Results indicate that yields are improved with Table 1 Electrosynthesis yields of vanadophosphate prepared in phosphate solutions
Fig. 5. Effect of phosphate concentration on the current–potential curves of the vanadium oxidation in LiH2 PO4 solutions (stationary vanadium electrode; scan rate = 10 mV s−1 ).
Solution
Concentration (M)
Temperature (◦ C)
Potential (V/SCE)
Farada¨ıc yield (%)
NaH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 KH2 PO4 NH4 H2 PO4
1 0.75 1 1 1.5 1.5 1.5 1.5 1 1
20 20 20 20 20 20 20 50 20 20
4 6 4 6 2 4 6 6 4 4
26 8 9 11 2 24 33 31 14 32
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increasing potential value. An increasing temperature did not involve a better yield. The electrolysis current was greater, but the solubility of the compounds seemed to be more important. The effect of concentration was studied with Li phosphate solutions: a better yield was obtained in 1.5 M solution. Similar potentiostatic polarisation of vanadium has been made in 1 M KH2 PO4 and 1 M NH4 H2 PO4 solutions. The electrolysis current was as high as in NaH2 PO4 and LiH2 PO4 solutions with the foil, whereas it was found to be 103 times smaller for the 1 mm embedded vanadium rod during potentiodynamic measurement in NH4 H2 PO4 solutions and 102 times smaller in KH2 PO4 solutions. Oscillations were observed in these conditions, whereas none was observed during potentiodynamic and potentiostatic measurements with the vanadium rod. The vertical position of the sheet certainly involves different mass transport conditions in comparison with the vanadium rod, which allows the formation of V(V)/V(IV) compounds. We did not observe significant different induction period in KH2 PO4 in comparison with those observed for the other solutions. In KH2 PO4 solution, a 4 V/SCE polarisation of vanadium sheet led to a mixture of green and yellow compounds. After rinsing with water and filtration, only the green powder was recovered. Considering the 1 M NH4 H2 PO4 solution, a green compound was also obtained. The remaining deposit on the vanadium surface polarised in lithium, sodium, potassium or ammonium hydrogen phosphate solutions revealed the presence of sphero¨ıds composed of sheets (Fig. 8) and the vanadium surface was greatly corroded. In addition to the observation, it was important to
analyze the deposits in order to establish a formula for these solids. The EDS analysis indicated in all cases a V/P ratio close to 1. This allowed us to propose a general formula Mx VOPO4 ·yH2 O for all the compounds (where M is the intercalated cation). A K/V ratio was found to be close to 0.45 for the KH2 PO4 solution and Na/V ratio close to 0.5 for the NaH2 PO4 solution. From the X-ray powder diffraction data (vide supra) a general formula Lix VOPO4 ·2H2 O for the lithium containing compound was proposed. The Li content was determined by Atomic Emission Spectroscopy and the analytical results gave a Li/V ratio of 0.5 resulting in the formula: Li0.5 VOPO4 ·2H2 O. Ammonium content was determined by spectrophotometric titration by indophenol blue method. NH4 + content was found to be in agreement with (NH4 )0.5 VOPO4 ·1.6H2 O. The diffractograms of the green powders obtained after a 4 V/SCE oxidation in 1 M NaH2 PO4 , KH2 PO4 , NH4 H2 PO4 or LiH2 PO4 are shown in Fig. 9. The first peak indicates in all the cases a decrease in the interlayer distance, which signifies the occurrence of a cation intercalation because this diminution is due to electrostatic interaction between the negatively charged layers and the interlayer cations. Data analysis using DiffracAT software revealed that the X-ray diagram for the green powder obtained in NaH2 PO4 solution (Fig. 9a) was very close to the one of the triclinic Na0.5 VOPO4 ·2H2 O, obtained from hydrothermal treatment [21]. The whole pattern of the lithium containing compound (Fig. 9c) was successfully indexed using the successive dichotomy method program ˚ DICVOL91 [31] on monoclinic unit cell (a = 7.284(7) A,
Fig. 8. SEM micrography of the anodic deposit on vanadium polarised within 15 min at +4 V/SCE in 1 M NH4 H2 PO4 .
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interlamellar spacing. The intercalated cations were found to verify a cation/vanadium ratio about 0.5, thus corresponding to a mixed +4/+5 vanadium oxidation state and a general formula M0.5 VOPO4 ·yH2 O for the anodically synthesized vanadophosphates.
References
Fig. 9. Powder X-ray diffraction patterns of the green deposits obtained on a vanadium anode polarised at +4 V/SCE in 1 M NaH2 PO4 (a), at +4 V/SCE in 1 M KH2 PO4 (b), at +6 V/SCE in 1.5 M LiH2 PO4 (c), and at +4 V/SCE in 1 M NH4 H2 PO4 (d).
˚ c = 6.242(6) A, ˚ β = 115.18(9)◦ , M20 = 20.9 b = 6.273(3) A, and F20 = 24.0(0.0167, 50)) [20]. X-ray diagram of compound obtained in NH4 H2 PO4 solution (Fig. 9d) was closely related to that of (NH4 )0.5 VOPO4 ·1.5H2 O, described by Do et al. [22]. X-ray powder diagram of compound obtained in KH2 PO4 solution does not exhibit a first single peak corresponding to the interlayer distance as usually observed (Fig. 9b). A minor peak is found in complement of the major one. The major one is related to K0.5 VOPO4 ·1.5H2 O [21]. The occurrence of a minor peak may be due to the presence of a second unknown minor phase while the dark green powder seems homogenous. This second phase may contain less potassium than the major one, because of the increasing interlayer spacing. This peculiar behaviour (occurrence of two phases) has already been mentioned by Zima et al. [32].
4. Conclusions The voltammetric experiments have shown the formation of an anodic deposit on a vanadium anode in phosphoric acid and in acidic phosphate solutions. The X-ray diffraction proved that the anodic oxidation of vanadium is a new route for the synthesis of mesoporous vanadophosphate compounds in mild conditions. In each case, by polarising vanadium at +4 or +6 V/SCE, a major crystallized phase was obtained in a few minutes, which is a great improvement in comparison with the other synthesis methods. The structure of these vanadophosphates is related to VOPO4 ·2H2 O, lamellar compound able to intercalate some cations in its
[1] J.C. Volta, C.R. Acad. Sci., Ser. IIc: Chim. 3 (2000) 717. [2] S. Whittingham, Chem. Rev. 104 (2004) 4271. [3] S. Boudin, A. Guesdon, A. Leclaire, M.M. Borel, Int. J. Inorg. Mater. 2 (2000) 561. [4] J. Kalousova, J. Votinsky, L. Benes, K. Melanova, Collect. Czech. Chem. Commun. 63 (1998) 1. [5] G. Ladwig, Z. Anorg. Allg. Chem. 338 (1965) 266. [6] S.A. Ennacri, C. R’Kha, P. Bardoux, J. Livage, Eur. J. Solid State Inorg. Chem. 30 (1993) 227. [7] N.G. Park, K.M. Kim, S.H. Chang, Electrochem. Commun. 3 (2001) 553. [8] V. Alonzo, A. Darchen, E. Le Fur, J.Y. Pivan, Electrochem. Commun. 4 (2002) 877. [9] R.D. Armstrong, M. Henderson, J. Electroanal. Chem. 26 (1970) 381. [10] R.G. Keil, R.E. Solomon, J. Electrochem. Soc. 115 (1968) 628. [11] R. Kammel, T. Kishi, T. Takei, H. Winterhager, Metalloberflaeche 24 (1970) 325. [12] S. Hornkjol, I.M. Hornkjol, Electrochim. Acta 36 (1991) 577. [13] M. Privman, T. Hepel, J. Electroanal. Chem. 382 (1995) 137. [14] F.M. Al-Kharafi, W.A. Badawy, Electrochim. Acta 42 (1997) 579. [15] W.D. Mackintosh, H.H. Plattner, J. Electrochem. Soc. 123 (1976) 523. [16] Z. El Abassi, S. Belcadi, J.J. Rameau, Electrochim. Acta 31 (1986) 1467. [17] M. Goto, D. Ishii, J. Electroanal. Chem. 36 (1972) 1. [18] C. Plazanet, M. Lamache, Electrochim. Acta 26 (1981) 45. [19] L.A. Kamel, S.M. Roshdy, R.M. Abou Shahba, A.S. Ibrahim, Model. Meas. Contr. 45 (1994) 55. [20] V. Alonzo, A. Darchen, E. Furet, R. Gautier, E. Le Fur, L. Le Polles, S. Messaoudi, J.Y. Pivan, J. Mater. Chem., submitted for publication. [21] S.L. Wang, H.Y. Kang, C.Y. Cheng, K.H. Lii, Inorg. Chem. 30 (1991) 3496. [22] J. Do, R.P. Bontchev, A.J. Jacobson, Inorg. Chem. 39 (2000) 3230. [23] C. Zadorojny, S. Saxton, R. Finger, JWPCF 45 (1973) 905. [24] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 132 (1985) 2872. [25] D. Sazou, M. Pagitsas, J. Electroanal. Chem. 323 (1992) 247. [26] D. Sazou, Electrochim. Acta 42 (1996) 627. [27] M.T. Koper, Adv. Chem. Phys. 92 (1996) 161. [28] H.R. Tietze, Aust. J. Chem. 34 (1981) 2035. [29] A.J. Jacobson, J.W. Johnson, Mater. Sci. Monogr. 28A (1985) 469. [30] B.G. Shpeizer, X. Ouyang, J.M. Heising, A. Clearfield, Chem. Mater. 13 (2001) 2288. [31] A. Boultif, D. Lou¨er, J. Appl. Crystallogr. 24 (1991) 987. [32] V. Zima, L. Benes, K. Melanova, J. Svoboda, J. Solid State Chem. 177 (2004) 1173.
Electrochemical behaviour of a vanadium anode in phosphoric acid and phosphate solutions V. Alonzo a,∗ , A. Darchen a , E. Le Fur b , J.Y. Pivan b a
Laboratoire d’Electrochimie, Ecole Nationale Sup´erieure de Chimie de Rennes, Avenue du G´en´eral Leclerc, 35700 Rennes, France b Laboratoire de Chimie du Solide, UMR 6511, Ecole Nationale Sup´ erieure de Chimie de Rennes, Avenue du G´en´eral Leclerc, 35700 Rennes, France Received 6 April 2005; received in revised form 22 June 2005; accepted 29 June 2005 Available online 15 August 2005
Abstract Anodic polarisation of a vanadium electrode has been studied in H3 PO4 solutions and some phosphate solutions: LiH2 PO4 , NaH2 PO4 , KH2 PO4 and NH4 H2 PO4 . The anodic behaviour of a vanadium electrode showed similarities in weak concentrated H3 PO4 , in LiH2 PO4 and NaH2 PO4 solutions: the polarisation curve exhibited a current peak followed by current oscillations and then a current plateau. Concentrated H3 PO4 , 1 M KH2 PO4 and NH4 H2 PO4 solutions involved vanadium passivation with a very slight current density plateau. Yellow compound identified to VOPO4 ·2H2 O was obtained after controlled potential oxidation of vanadium in 5–10 M H3 PO4 . Green products were obtained in 1 M phosphate solutions and in 1–3 M H3 PO4 on vanadium anode after controlled potential electrolysis. All these vanadophosphate compounds contained the monovalent cation which was present in the solution. © 2005 Elsevier Ltd. All rights reserved. Keywords: Anodic electrodeposit; Current oscillations; Oxidation; Passivation; Vanadophosphate
1. Introduction Vanadophosphates or vanadium phosphorus oxides (VPO) are well-known catalysts for the mild oxidation of light alkanes [1]. These compounds and particularly lithium vanadophosphates are also of great interest as battery materials [2]. They are mesoporous compounds which are generally prepared by solid state synthesis with heating in air or by hydrothermal synthesis [3]. The general synthetic way starts from vanadium oxide in the presence of reducing metal powder or sheet. Among the vanadophosphates, VOPO4 ·2H2 O is a famous intercalation compound [4]. It is generally prepared by a prolonged boiling of V2 O5 in aqueous phosphoric acid [5]. It can also be prepared at room temperature by a sol–gel synthesis from alkoxide precursors [6] or by using an ultrasonic activation of an aqueous solution containing V2 O5 and H3 PO4 ∗
Corresponding author. Tel.: +33 2 23 23 80 26; fax: +33 2 23 23 81 99. E-mail address: [email protected] (V. Alonzo).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.06.030
[7]. We recently described a new route of synthesis by anodic oxidation of a vanadium sheet in a H3 PO4 solution [8]. The anodic behaviour of vanadium in acidic aqueous solutions has been sparsely studied [9–15]. Some passive or anodic oxide films were mentioned in aqueous solutions: for example VO2 [10], V2 O4 [11] V2 O5 [15], non-stoichiometric oxides or mixtures of oxides of varying valence [14] and also barium vanadate [12]. A few studies have been carried out in phosphoric acid solutions [16–19] but they did not shown the occurrence of vanadophosphate species. This new route of synthesis has been extended to the synthesis of alkali vanadophosphates, by anodic polarisation of vanadium in sodium [20], lithium, potassium and ammonium phosphate solutions. These compounds have been extensively studied and they are generally synthesized either by hydrothermal method [21] or by using reducing agent like iodide anion on VOPO4 ·2H2 O [4,22]. In both cases, the final products are obtained after large time reactions. The structure of these compounds shows VOPO4 layers with H2 O and intercalated cations between the layers. The electrochemical
V. Alonzo et al. / Electrochimica Acta 51 (2006) 1990–1995
1991
route seems to be a rapid way to obtain these compounds as electrodeposits on a vanadium anode. The electrochemical behaviour of a vanadium anode in phosphoric acid and various phosphate solutions is described in this article.
2. Experimental All electrochemical measurements were carried out in a conventional three-electrode electrochemical cell. The working electrode was a vanadium rod (1 mm diameter) purchased from Goodfellow (99.7% in purity) embedded in an insulating holder made of PTFE (Radiometer Analytical). A platinum wire was used as a counter electrode and the reference electrode was a saturated calomel electrode (SCE). All the potential values are reported herein with respect to the SCE. The solutions were prepared using H3 PO4 (85%, Aldrich), NaH2 PO4 (99%, Aldrich), LiH2 PO4 (99%, Aldrich), KH2 PO4 (>99%, Acros) or NH4 H2 PO4 (>99%, Fluka) and deionized water. Prior to experiments, the electrode was mechanically polished by a series of wet standings at different grit size (1200–4000) and rinsed with deionized water. All the experiments were performed at room temperature. The polarisation measurements were conducted with a 1280 Solartron potentiostat and the Corrware software (Scribner and Associates) was used to pilot the experiments from −0.7 to 6 V/SCE with a 10 mV s−1 scan rate. Controlled potential oxidation was carried out for a few minutes with a 0.25 mm thick and 1 cm large vanadium sheet (99.7%, Aldrich) whose 0.5 cm length was immersed in solution. The anodic deposits obtained on vanadium anode have been characterised by X-ray powder diffraction (Bragg–Brentano geometry, Rigaku–Geigerflex diffractometer). The diffraction pattern was scanned from 10◦ to 90◦ ˚ and a step length of 0.02◦ (2θ) using λ Cu K␣ = 1.54178 A (2θ) with 12 s counting time. After polarisation of vanadium sheets in phosphate solutions, Scanning Electron Microscopy (SEM) observations and Energy Dispersion X-ray Spectroscopy (EDS) analysis of anodic deposits were performed at the Scanning Electron Microscopy Centre of the University of Rennes (CMEBA Rennes). A Varian AA-10 apparatus was used to analyse lithium content in Li compounds by Atomic Emission Spectroscopy. Spectrophotometric determination of ammonium ions by the indophenol blue method [23] was performed with a Schimadzu 1600 spectrophotometer.
3. Results and discussion 3.1. Voltammetry in H3 PO4 solutions Various H3 PO4 concentrated solutions, from 1 to 14.74 M, were used for voltammetry experiments. Fig. 1 shows the potentiodynamic curves obtained for a vanadium microelec-
Fig. 1. Current–potential polarisation curves of the vanadium oxidation in H3 PO4 solutions at a stationary vanadium electrode. Scan rate = 10 mV s−1 .
trode in these solutions. Current increases with decreasing H3 PO4 concentrations. A passivation behaviour is observed for 6–14.74 M solutions. For 3 M or less concentrated solutions, the potentiodynamic curve exhibits an oxidation peak, before the current reaching a plateau with high current density. Lowering the scanning rate or using a rotating electrode involves oscillation apparition between the peak and the plateau, for the low concentrations [8]. The current oscillations are also observed during anodic oxidation performed at a controlled potential [8]. Electrochemical oscillations have been observed for a large number of metal anodic processes and particularly for copper, cobalt and iron oxidation in H3 PO4 [24–27]. Oscillations are generally explained in terms of competition between the growth and the chemical dissolution of metal anodic films. In the case of the anodic oxidation of vanadium, oscillations could result from formation/dissolution of a compound at the metal/solution interface, but in the actual state of our knowledge, we cannot assume it is the isolated vanadophosphate. In order to better understand the redox behaviour of the deposit, voltammetric studies were carried out with soluble ionic species of vanadium in phosphoric acid. A voltammetric study in a solution containing V2 O5 in 1 M H3 PO4 revealed on a carbon electrode a reversible reduction around +0.66 V/SCE, corresponding to the V(IV)/V(V) redox couple [8]. 3.2. Anodic oxidation of vanadium in H3 PO4 solutions Some yellow compounds were observed on the vanadium surface during the potentiodynamic measurement for 5–10 M H3 PO4 at potentials corresponding to the limiting current plateau. This yellow colour is characteristic of a V(V) species. In order to obtain a greater amount of product, a 1 cm2 immersed vanadium sheet was used as anode. The preparation of the solid was carried out at controlled potential: the sample was polarised at a potential corresponding to the current plateau and greater than 1 or 1.5 V/SCE according to the concentration. At 10 M concentration, a yellow pasty solid was
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Fig. 3. Powder X-ray diffraction patterns of the yellow deposit obtained on a vanadium anode polarised at +3 V/SCE in 6 M H3 PO4 (a) and of the green deposit obtained on a vanadium anode polarised at +3 V/SCE in 2 M H3 PO4 (b).
Fig. 2. SEM micrography of the anodic deposit on a vanadium anode polarised within 1 h at +4 V/SCE in 6 M H3 PO4 (a) and enlargement of the inside structure (b).
obtained. The deposit formed on the vanadium after polarisation at 3 V/SCE in 6 M H3 PO4 during 1 h was observed by SEM. It reveals a homogeneous deposit, with cracks due to retraction of the solid after drying (Fig. 2a). The inside structure of the deposit is made of platelets (Fig. 2b). Once the deposit was removed, the vanadium surface appeared on the eye scale to be shiny. The observation by SEM revealed a thin dissolution of grain boundaries, the interior of the grains being slightly etched. The vanadium surface after removal of the pasty deposit in 10 M H3 PO4 at 3 V/SCE showed the same aspect of the grain boundary but the interior of the grain was not etched. The pasty texture of the deposit obtained in 10 M H3 PO4 did not permit SEM observation. X-ray powder diffraction data analysis by DiffracAT software revealed that the yellow powder obtained in 6 M H3 PO4 (Fig. 3) was VOPO4 ·2H2 O, whose structure is described in quadratic system [28]. In 3 M H3 PO4 or less concentrated solutions, a green product was formed at the same applied potentials and fell in the solution while the initially colourless solution became blue.
A very small quantity of this product was recovered and was quickly rinsed since it dissolved in water. The green colour of the product indicated a mixed valence V(IV)/V(V) of the vanadium species and the blue coloration of the solution revealed the presence of a V(IV) ionic species. The occurrence of V(IV) at potential corresponding to V(V) species in voltammetric experiments could be explained by different mass transport conditions between a vertical plate and a 1 mm diameter disk. The surface of vanadium after 2 V/SCE polarisation in 2 M H3 PO4 revealed an important intergranular corrosion. Green powder of hydrated vanadophosphates with intercalated cations derive generally from the structure of VOPO4 ·2H2 O. Compounds with H+ intercalation were first described by Jacobson [29]. They were obtained by reduction of the yellow VOPO4 ·2H2 O with hydroquinone. More recently, the formula H0.2 VOPO4 ·2.33H2 O was attributed to the green compound obtained in hydrothermal conditions in alcohol medium [30]. The powder obtained in our study by anodic oxidation of vanadium in 2 M H3 PO4 (Fig. 3) was found to be close to this compound. 3.3. Voltammetry of vanadium in phosphate solutions Various phosphate solutions (NaH2 PO4 , LiH2 PO4 , KH2 PO4 and NH4 H2 PO4 ) were used for voltammetry experiments on vanadium. Fig. 4 shows potentiodynamic curves for a vanadium microelectrode. The behaviour of vanadium in 1 M NaH2 PO4 or 1 M LiH2 PO4 was similar to the anodic dissolution of vanadium in 1 M H3 PO4 . After the oxidation peak, the current slightly decreased before reaching a plateau. An oscillatory phenomenon was observed after the peak current maximum and before the current plateau. In NH4 H2 PO4 solution, oscillations were not observed and the current plateau was narrow: current density was 103 times smaller (6 × 10−4 A cm−2 ) than in LiH2 PO4 solution (about 0.6 A cm−2 ). In KH2 PO4 solution, peak current was very small (about 5 × 10−3 A cm−2 ) and no oscillation was observed, and the following current plateau was 102 times smaller (4 × 10−3 A cm−2 ) than in LiH2 PO4 solution. We
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Fig. 4. Current–potential polarisation curves of the vanadium oxidation in 1 M phosphate solutions (stationary vanadium electrode; scan rate = 10 mV s−1 ).
don’t have any data about a lower conductivity of anodic deposit which could explain a better passivation in this case. Effect of LiH2 PO4 concentration is shown in Fig. 5. Oscillation domain increases with concentration whereas peak and plateau current are slightly smaller. In this particular case, lowering the scanning potential rate increases the oscillation frequency. Heating this solution also increases oscillations frequency and their current amplitude. Effect of rotating electrode rate is shown in Fig. 6: increasing rotating electrode amplifies oscillation domain. 3.4. Anodic oxidation of vanadium in phosphate solutions Some green compounds were observed on the vanadium surface during the potentiodynamic measurement for 1 M NaH2 PO4 or LiH2 PO4 at a potential corresponding to the limiting current plateau. The sample was polarised at 4 V/SCE for NaH2 PO4 and LiH2 PO4 solutions. The product formed on the vanadium after a few minutes fell in the solution. During the potentiostatic measurement, the current showed high amplitude oscillations appearing after an induction period
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Fig. 6. Current–potential curve for a vanadium rotating electrode (1000 rpm) in 1.5 M LiH2 PO4 solution (scan rate = 10 mV s−1 ).
Fig. 7. Current–time curve for vanadium foil (about 1 cm2 immersed) polarised at +4 V/SCE in 1 M NaH2 PO4 .
(Fig. 7). The manipulation was stopped when sufficient amounts of solid had been recovered in the cell for physicochemical characterisation. After filtration, the final products were washed with water and acetone, then dried in an oven for a few hours. A secondary reaction occurred during the electrolysis in some solutions. In LiH2 PO4 solution a white deposit precipitated on the counter electrode due to a localised increase of pH. This highly insoluble Li3 PO4 deposit contributed to lithium consumption from the solution. The NaH2 PO4 solution became a little turbid at the end of the polarisation. Table 1 gathers quantitative results for anodic product formation. Results indicate that yields are improved with Table 1 Electrosynthesis yields of vanadophosphate prepared in phosphate solutions
Fig. 5. Effect of phosphate concentration on the current–potential curves of the vanadium oxidation in LiH2 PO4 solutions (stationary vanadium electrode; scan rate = 10 mV s−1 ).
Solution
Concentration (M)
Temperature (◦ C)
Potential (V/SCE)
Farada¨ıc yield (%)
NaH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 LiH2 PO4 KH2 PO4 NH4 H2 PO4
1 0.75 1 1 1.5 1.5 1.5 1.5 1 1
20 20 20 20 20 20 20 50 20 20
4 6 4 6 2 4 6 6 4 4
26 8 9 11 2 24 33 31 14 32
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increasing potential value. An increasing temperature did not involve a better yield. The electrolysis current was greater, but the solubility of the compounds seemed to be more important. The effect of concentration was studied with Li phosphate solutions: a better yield was obtained in 1.5 M solution. Similar potentiostatic polarisation of vanadium has been made in 1 M KH2 PO4 and 1 M NH4 H2 PO4 solutions. The electrolysis current was as high as in NaH2 PO4 and LiH2 PO4 solutions with the foil, whereas it was found to be 103 times smaller for the 1 mm embedded vanadium rod during potentiodynamic measurement in NH4 H2 PO4 solutions and 102 times smaller in KH2 PO4 solutions. Oscillations were observed in these conditions, whereas none was observed during potentiodynamic and potentiostatic measurements with the vanadium rod. The vertical position of the sheet certainly involves different mass transport conditions in comparison with the vanadium rod, which allows the formation of V(V)/V(IV) compounds. We did not observe significant different induction period in KH2 PO4 in comparison with those observed for the other solutions. In KH2 PO4 solution, a 4 V/SCE polarisation of vanadium sheet led to a mixture of green and yellow compounds. After rinsing with water and filtration, only the green powder was recovered. Considering the 1 M NH4 H2 PO4 solution, a green compound was also obtained. The remaining deposit on the vanadium surface polarised in lithium, sodium, potassium or ammonium hydrogen phosphate solutions revealed the presence of sphero¨ıds composed of sheets (Fig. 8) and the vanadium surface was greatly corroded. In addition to the observation, it was important to
analyze the deposits in order to establish a formula for these solids. The EDS analysis indicated in all cases a V/P ratio close to 1. This allowed us to propose a general formula Mx VOPO4 ·yH2 O for all the compounds (where M is the intercalated cation). A K/V ratio was found to be close to 0.45 for the KH2 PO4 solution and Na/V ratio close to 0.5 for the NaH2 PO4 solution. From the X-ray powder diffraction data (vide supra) a general formula Lix VOPO4 ·2H2 O for the lithium containing compound was proposed. The Li content was determined by Atomic Emission Spectroscopy and the analytical results gave a Li/V ratio of 0.5 resulting in the formula: Li0.5 VOPO4 ·2H2 O. Ammonium content was determined by spectrophotometric titration by indophenol blue method. NH4 + content was found to be in agreement with (NH4 )0.5 VOPO4 ·1.6H2 O. The diffractograms of the green powders obtained after a 4 V/SCE oxidation in 1 M NaH2 PO4 , KH2 PO4 , NH4 H2 PO4 or LiH2 PO4 are shown in Fig. 9. The first peak indicates in all the cases a decrease in the interlayer distance, which signifies the occurrence of a cation intercalation because this diminution is due to electrostatic interaction between the negatively charged layers and the interlayer cations. Data analysis using DiffracAT software revealed that the X-ray diagram for the green powder obtained in NaH2 PO4 solution (Fig. 9a) was very close to the one of the triclinic Na0.5 VOPO4 ·2H2 O, obtained from hydrothermal treatment [21]. The whole pattern of the lithium containing compound (Fig. 9c) was successfully indexed using the successive dichotomy method program ˚ DICVOL91 [31] on monoclinic unit cell (a = 7.284(7) A,
Fig. 8. SEM micrography of the anodic deposit on vanadium polarised within 15 min at +4 V/SCE in 1 M NH4 H2 PO4 .
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interlamellar spacing. The intercalated cations were found to verify a cation/vanadium ratio about 0.5, thus corresponding to a mixed +4/+5 vanadium oxidation state and a general formula M0.5 VOPO4 ·yH2 O for the anodically synthesized vanadophosphates.
References
Fig. 9. Powder X-ray diffraction patterns of the green deposits obtained on a vanadium anode polarised at +4 V/SCE in 1 M NaH2 PO4 (a), at +4 V/SCE in 1 M KH2 PO4 (b), at +6 V/SCE in 1.5 M LiH2 PO4 (c), and at +4 V/SCE in 1 M NH4 H2 PO4 (d).
˚ c = 6.242(6) A, ˚ β = 115.18(9)◦ , M20 = 20.9 b = 6.273(3) A, and F20 = 24.0(0.0167, 50)) [20]. X-ray diagram of compound obtained in NH4 H2 PO4 solution (Fig. 9d) was closely related to that of (NH4 )0.5 VOPO4 ·1.5H2 O, described by Do et al. [22]. X-ray powder diagram of compound obtained in KH2 PO4 solution does not exhibit a first single peak corresponding to the interlayer distance as usually observed (Fig. 9b). A minor peak is found in complement of the major one. The major one is related to K0.5 VOPO4 ·1.5H2 O [21]. The occurrence of a minor peak may be due to the presence of a second unknown minor phase while the dark green powder seems homogenous. This second phase may contain less potassium than the major one, because of the increasing interlayer spacing. This peculiar behaviour (occurrence of two phases) has already been mentioned by Zima et al. [32].
4. Conclusions The voltammetric experiments have shown the formation of an anodic deposit on a vanadium anode in phosphoric acid and in acidic phosphate solutions. The X-ray diffraction proved that the anodic oxidation of vanadium is a new route for the synthesis of mesoporous vanadophosphate compounds in mild conditions. In each case, by polarising vanadium at +4 or +6 V/SCE, a major crystallized phase was obtained in a few minutes, which is a great improvement in comparison with the other synthesis methods. The structure of these vanadophosphates is related to VOPO4 ·2H2 O, lamellar compound able to intercalate some cations in its
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